DELAWARE GEOLOGICAL SURVEY REPORT OF INVESTIGATIONS NO, 45 EFFECTS OF AGRICULTURAL PRACTICES AND SEPTIC·SYSTEM EFFLUENT ON THE QUALITY OF WATER IN THE UNCONFINED AQUIFER IN PARTS OF EASTERN SUSSEX COUNTY. DELAWARE A FEET A' FEET CLUSTER 5 30 20 10 SEA LEVEL 0 -10 -20 -30 -40 -50 -60 -70 -80 ·90 LAND SURfACE WaTer 10,85 (D,30) 10.82 (0.32) 10.80 (O.32) 10.77 (0.34) 10.85 (0.32) CLUSTER 4 CLUSTER 3 Iffig<lliGll W.,.U 1'++-1'--'1---1- .. ? CLUSTER 2 10.66 (0.65) CLUSTER 1 10.61 (0.18J 30 20 10 o SEA LEVEL -10 ·20 -30 -40 -50 -60 -80 -90 -100 o I I I o I i 150 300 METERS BY JUDITH M. DENVER STATE OF DELAWARE UNIVERSllY OF DELAWARE NEWARK, DELAWARE JUNE 1989 ,.
Transcript
DELAWARE GEOLOGICAL SURVEYREPORT OF INVESTIGATIONS NO, 45
EFFECTS OF AGRICULTURAL PRACTICES AND SEPTIC·SYSTEMEFFLUENT ON THE QUALITY OF WATER IN THE UNCONFINED AQUIFER
IN PARTS OF EASTERN SUSSEX COUNTY. DELAWARE
A
FEET
A'
FEET
CLUSTER 530
20
10
SEA LEVEL 0
-10
-20
-30
-40
-50
-60
-70
-80
·90
LAND SURfACE
WaTer 1,,~1('
10,85(D,30)
10.82(0.32)
10.80(O.32)
10.77(0.34)
10.85(0.32)
CLUSTER 4 CLUSTER 3
Iffig<lliGll W.,.U
1'++-1'--'1---1- .. ?
CLUSTER 2
10.66(0.65)
CLUSTER 1
10.61(0.18J
30
20
10
o SEA LEVEL
-10
·20
-30
-40
-50
-60
-80
-90
-100oI I I
oI i
150 300 METERS
BY
JUDITH M. DENVER
STATE OF DELAWAREUNIVERSllY OF DELAWARE
NEWARK, DELAWAREJUNE 1989
,.
EFFECTS OF AGRICULT1IRAL PRACTICES AND SKl'TIC-SYSTEK EFFLUENT ON 1lIE
QUALITY OF VATER. III 1lIE UNCOliIFlllJID AQUIFER III PARTS OF
EASTERN SUSSEX COUNTY, DELAWARE
By
Judith K. DenverHydrologist, u.S. Geological Survey
Prepared by the United States Geological Surveyunder the Joint-Funded Prograa with the
water and surface water at the Fairmount site 542. Chemical analyses of water from wells affected
by septic-system effluent 633. Chemical analyses of water from wells affected
by agricultural practices 644. Chemical analyses of water from wells with
natural and nearly natural water quality 655. Conversion factors and abbreviations 66
iii
IUDSTRATIONS
Page
Figure 1. Map showing location of study area, the Fairmountsite, and wells sampled for chemical analysis ofwater affected by septic-system effluent........... ... 4
2. Map showing general distribution of soil associationsin eastern Sussex County, Delaware 10
3. Map showing the Fairmount site study area 13
4. Section showing the configuration of geologicunits and screened intervals of piezometersat the Fairmount site 15
5. Map showing land use and direction of ground-waterflow in the unconfined aquifer in the areasurrounding the Fairmount site 20
6. Map showing altitude of the water table atthe Fairmount site, (A) October 30, 1986 and(B) March 9, 1987..................................... 21
8. Profile showing flow in the unconfined aquifer atthe Fairmount site after 30 hours of irrigationpumping, July 18, 1985................................ 25
9. Graph showing relation of specific conductance tonitrate concentration for water in the unconfinedaquifer at the Fairmount site 27
10-12. Graphs showing relation of nitrate concentration to:
10. Concentrations of (A) calcium plusmagnesium and (B) alkalinity for waterin the unconfined aquifer at the Fairmountsite, September 1986............................ 28
11. Concentrations of (A) potassium and (B) chloridefor water in the unconfined aquifer at theFairmount site, September 1986 29
12. Concentrations of (A) sulfate and (B) sodiumfor water in the unconfined aquifer at theFairmount site, September 1986 30
iv
IUJJSTRATIONS - - Continued
PageFigures
13-15. Box plots showing:
13. (A) Nitrate concentrations and (B) calciumplus magnesium concentrations for water atdifferent depths in the unconfined aquiferat the Fairmount site......................... 33
14. (A) Potassium concentrations and (B) sulfateconcentrations for water at different depthsin the unconfined aquifer at theFairmount site................................ 34
15. (A) Sodium concentrations, (B) silicaconcentrations, and (C) pH for water atdifferent depths in the unconfined aquiferat the Fairmount site 35
16. Stiff diagrams showing concentrations of majorions resulting from agricultural fertilizers inwater from the cluster piezometers at theFairmount site, September 1987....................... 37
17. Profiles of specific conductance for water from thecluster piezometers at the Fairmount site, (A) May1985 and (B) January 1986... 39
18. Profiles of specific conductance for water from thecluster piezometers at the Fairmount site,(A) June 1986 and (B) September 1986................. 40
19. Graph showing specific conductance of water frompiezometers Ph13-03 and Ph13-16, February1986 through March 1987..................... ... ... ... 41
20. Map showing location of wells sampled with wateraffected by septic-system effluent, agriculturalfertilizers, and in water under natural and nearlynatural conditions in the unconfined aquifer..... .... 44
21. Boxplots showing comparisons of specificconductance in water from the unconfinedaquifer affected by septic-system effluent,agricultural fertilizers, and in water undernatural and nearly natural conditions................ 45
v
Page
Table 1. Average chemical composition of precipitation froma station located in Lewes, Delaware, August 1978through January 1983................................... 8
2. Chemical reactions occurring in the unconfined aquifer... 9
3. Schedule of water-quality sample collection at thecluster piezometers of the Fairmount site 14
4. Schedule of fertilization and planting at theFairmount site from January 1985 throughOctober 1986........................................... 19
5. Percentages of major ions in water from theunconfined aquifer under natural and nearly naturalconditions, affected by septic-system effluentand agricultural practices 46
vi
EFFECTS OF AGRICULnJRAL PRACTICES AND SEPTIC-SYSTEK EFFIJJElIIT ON THE
QUALITY OF WATER IN THE UlIlCOlilFIllllID AQUIFER IN PARTS OF
EASTERN SUSSEx: COUlIlTY. DELAWARE
By Judith K. Denver
ABSTRACT
The unconfined aquifer is a major source of water supply in easternSussex County, Delaware. It also is an important source of water forsurface-water bodies and deeper, confined aquifers. The aquifer consistsmainly of permeable sand and gravel; its shallow water table is susceptibleto contamination by nitrate and other chemical constituents associated withagricultural practices and effluent from septic systems.
Chemical analyses of ground-water samples and water levels frompiezometers with screened intervals at various depths around a 220-acreirrigated field known as the Fairmount site were used to study thesubsurface distribution and movement of nitrate and other fertilizercomponents. Land-use, soil, and aquifer characteristics at the Fairmountsite are similar to large parts of eastern Sussex County. There is a directrelation between concentrations of nitrate, the dominant anion, and specificconductance in water from the Fairmount site. Therefore, specific conductance was used to estimate nitrate concentrations in water samples and toindicate the degree of agricultural influence on water chemistry.
Factors including upgradient land use, ground-water withdrawal, ratesof fertilizer application, magnitude and timing of recharge, and aquiferproperties affect the distribution of agricultural chemicals in the aquifer.Concentrations of nitrate as nitrogen range from less than 2 mg/L(milligrams per liter) to greater than 40 mg/L and have a median concentration greater than 10 mg/L. Although agriculture has less of an influence indeeper parts of the aquifer than in shallower parts, nitrate (as nitrogen)concentrations are as high as 29 mg/L near the base of the aquifer, 80 feetbelow land surface. This degree of variability indicates that a watersample from an individual well in the unconfined aquifer is not an adequaterepresentation of overall quality.
Corn and soybeans generally are grown in alternate years at theFairmount site. More than 200 pounds per acre of nitrogen fertilizer areapplied to corn crops, and little or none is applied to grow soybeans.Consequently, variable amounts of excess nitrogen in the soil zone can beleached from the soil and percolate to the water table during the principalrecharge period that begins in late fall after the growing season ends. Theresult is stratification of nitrogen-bearing water in the flow system.
1
Ground-water withdrawal for irrigation promotes movement of chemicallystratified water into deeper parts of the unconfined aquifer. In parts ofthe aquifer not influenced significantly by ground-water pumping, land useupgradient from the Fairmount site controls much of the variability of waterquality with depth.
The chemical characteristics of ground water affected by septic-systemeffluent were investigated in samples from 11 wells in the study area. Themajor dissolved chemical components of septic-system effluent are organicand ammonia nitrogen, sodium, and chloride. Other components includecalcium, magnesium, potassium, sulfur, and phosphorus in lesser proportions,as well as organic compounds and components of detergents and cleaningagents. Although these chemicals were present in the water samples affectedby septic-system effluent, their presence cannot be used to identify thesource of water-quality problems uniquely because they also are present inmanures, fertilizers, and pesticides. Differences in the proportions ofdissolved constituents in ground water affected by septic-system effluentand by agricultural practices also were not distinct enough to identifysources of nitrate contamination.
The relative proportions of major dissolved constituents, expressed aspercentages of the total concentration of cations or anions, are differentin natural water and water affected by agricultural practices and septicsystem effluent. The median percentage of bicarbonate, the predominantanion in water under natural conditions in the study area, is 43 in waterunaffected by human activities and 7 in water affected by agriculturalpractices and septic-system effluent. The median percentages of nitrate are65 and 54 in water affected by agricultural practices and septic-systemeffluent, respectively, and 25 in water under natural conditions. Sodium,the predominant cation in water under natural conditions, has a medianpercentage of 61 in natural water and 29 in water affected by agriculturalpractices and septic systems where calcium is the predominant cation.
INTRODUCTION
Background
The unconfined aquifer is a principal source of agricultural, domestic,municipal, and industrial water supply in eastern Sussex County, Delaware.It also is an important source of water for surface-water bodies and deeperconfined aquifers. The area is primarily rural with resort developmentalong the Atlantic coast. Because of development, the demand for groundwater is increasing. Nitrate concentrations above the U.S. EnvironmentalProtection Agency (EPA) drinking-water standard of 10 mg/L (milligrams perliter) as nitrogen (U.S. Environmental Protection Agency, 1986) are commonin the unconfined aquifer.
2
The study area includes about 500 square miles of the DelmarvaPeninsula (Figure 1). The boundaries are the Sussex County line to thenorth, the Maryland-Delaware border to the south, the Chesapeake BayDelaware Bay and Chesapeake Bay-Atlantic Ocean drainage divides to the west,and the Atlantic Ocean to the east. Topography is relatively flat with landsurface elevations ranging from sea level near the coast to about 60 ft(feet) above sea level on the northern part of the drainage divide.
The unconfined aquifer is susceptible to contamination because thewater table generally is within 15 ft of the land surface and the soils aresandy. Water in the aquifer has a naturally low dissolved-solids contentand even small amounts of applied chemicals may cause major changes inground-water quality. Agricultural fertilizers and manure, and septicsystem effluent are the most widespread sources of contamination. Cropproduction accounts for about 50 percent of land use in eastern SussexCounty (Ritter, 1986). Areas with a high density of septic tanks are mostcommon in residential developments around Rehoboth, Indian River, and LittleAssawoman bays, also known as the Inland Bays. Unsewered residential areascomprise about 9 percent of the land use in the areas draining to these bays(Ritter, 1986).
Previous studies (Miller, 1972; Robertson, 1977; Ritter and Chirnside,1982; and Denver, 1986) have delineated the extent of ground-water contamination problems and have identified areas with potential for contamination.These assessments were made by ground-water sampling and by relating thesampling results to land uses and soil characteristics. The studiesconcluded that fertilizer and poultry manure are the main sources of nitratecontamination and that effluent from septic tanks also contributes nitrateto the aquifer in some unsewered residential areas. Robertson (1977)identified several areas in eastern Sussex County with mean concentrationsof nitrate as nitrogen greater than 9.5 mg/L. The highest concentrationswere near Indian River Bay where soils are excessively well-drained andhighly permeable. Routine sampling of public water supplies by the DelawareDepartment of Health and Social Services also has documented elevatednitrate concentrations in eastern Sussex County.
Purpose and Scope
This report (1) adds to the existing knowledge of ground-water qualityin agricultural areas by describing the vertical and horizontal distributionand movement of nitrate and other chemical constituents around an irrigatedfield, and (2) describes the effects of effluent from domestic septicsystems on the quality of water in the unconfined aquifer. The chemistry ofwater affected by agricultural practices is compared to the chemistry ofwater affected by septic-system effluent to determine unique characteristicsof each that could be used to identify sources of ground-water contamination. These affected waters are also compared to water with natural andnearly natural chemistry.
A 220-acre field with a center-pivot irrigation system and a centrallylocated well, referred to as the Fairmount site (Figure 1), was instrumentedwith 31 piezometers to study the effects of irrigation on the subsurfacedistribution and movement of chemicals related to agricultural practices.
3
•EXPLANATION
10 KILOMETERS
5 10 MILES
Well sampled for chemicalanalysis of water affectedby septic-system effluent
I I ('1o 5
o
II "
,/-<-..
STUDY AREA
2t 3~M1LES
3~ I<ILOt.£TERS
10
I II I I
10 20o
o
PENNSYLVANIA
39°30'
39°00'
3.°30
3.°
00'
Figure 1. Location of study area, the Fairmount site, and wells sampledfor chemical analysis of water affected by septic-systemeffluent.
4
This site is representative of a large part of eastern Sussex County withsimilar land use, soil, and aquifer characteristics. Water-level and waterquality data were collected quarterly from April 1985 through March 1987.Water-quality analysis included laboratory determination of major ions andnutrients, and field measurements for specific conductance, pH, watertemperature, alkalinity, and dissolved oxygen. Continuous water-level datacollected from an observation well 75 ft from the pumped well were used torelate water-level changes during pumping to water quality, and to determineperiods of recharge to the aquifer. Vertical changes in water quality werestudied over time at 24 of the piezometers that were arranged in 5 clustersof 4 or 5 each with screens at different vertical intervals in the aquiferranging from 20 to 100 ft below land surface. Water quality in PhillipsBranch, a small stream adjacent to the field was compared to water qualityin a nearby part of the aquifer. Analysis of the data evaluates groundwater quality in relation to irrigation well pumping, the unstressed flowsystem, cropping and fertilization practices, soil characteristics, andrecharge.
Water samples from 11 wells (Figure 1) identified as having elevatednitrate concentrations resulting from septic-system effluent were collectedto evaluate the effects of the effluent on ground-water chemistry. Thewells selected were located downgradient from areas with closely spacedindividual septic systems where other land uses would not influence waterquality. Because wells potentially affected by septic systems frequentlyare located adjacent to or downgradient of agricultural land, or have beendeveloped on recently farmed areas, no additional wells could be found forsampling.
Ten additional wells with water quality affected by agriculturalpractices were selected (Appendix 3). Analyses from nine wells notsignificantly affected by agricultural practices, septic systems, or otherland uses were selected as indicators of natural or nearly natural waterquality (Appendix 4). Variations in water chemistry resulting from theseland uses and septic-system effluent are characterized using boxplots ofmajor-ion concentrations and relative proportions of ions.
Acknowledgments
This study was conducted under a Joint-Funded Agreement between theDelaware Geological Survey (DGS) and the U.S. Geological Survey (USGS). Thestaff of the DGS, under the direction of Robert R. Jordan, State Geologist,provided drilling and geophysical logging.
The Division of Public Health of the Delaware Department of Health andSocial Services is thanked for allowing access to their files and helping tolocate wells suitable for sampling.
Special thanks are extended to Townsend's, Inc., for allowingpiezometer installation and instrumentation on their property, and to thehomeowners, municipalities, and water companies who permitted their wells tobe sampled.
5
DESCRIPTION OF EASTERN SUSSEX COUNTY
Geologic Setting
Sussex County is located on the Delmarva Peninsula, which is a part ofthe Atlantic Coastal Plain physiographic province. The Coastal Plain isunderlain by a seaward-dipping wedge of unconsolidated to semiconsolidatedmarine and nonmarine sediments composed of gravel, sand, silt, and clay.The sediments range in age from Jurassic to Holocene and lie unconformablyon crystalline bedrock. The sediments thicken from about 5,000 to 10,000 ftfrom northwest to southeast across eastern Sussex County (Benson, 1984).The stratigraphic nomenclature in this report is that of the DGS and doesnot necessarily follow the usage of the USGS.
The units that comprise the unconfined aquifer in most of easternSussex County are the Beaverdam and Omar formations of the Columbia Group ofPliocene-Pleistocene age and sand units in the Chesapeake Group of Mioceneage where they subcrop the Columbia Group. Unnamed deposits of Holocene agethat overlie the Columbia Group along the shores of the Delaware Bay andAtlantic Ocean also are considered to be part of the unconfined aquifer.
The Beaverdam Formation of fluvial and possibly estuarine origin(Jordan and Talley, 1976) is the primary unit of interest to this study. Itgenerally is a medium to coarse quartz sand with variable amounts of finesand and gravel. Thin clay-silt beds are common throughout the formation.The Beaverdam ranges from 60 to 120 ft thick in the study area (Talley andAndres, 1987).
The Omar Formation of lagoonal deposition, with features of estuaries,marshes, and beaches (Jordan, 1962), overlies the Beaverdam Formation in thesouthern part of the study area. It is composed of alternating beds of sandand silt and averages 45 ft thick. The silt usually contains organic matterand is fossiliferous.
The sand units of the Chesapeake Group, a marine and marginal-marinesequence that underlies the Beaverdam Formation, include the Pocomokeaquifer of the Bethany formation and the Manokin aquifer of the Manokinformation. Both the Bethany and Manokin formations are considered informalunits as first proposed by Andres (1986a). The Bethany formation is thelithostratigraphic equivalent of the Pocomoke and Ocean City aquifers andtheir overlying, intervening, and basal confining units. The underlyingManokin formation is the lithostratigraphic equivalent of the Manokinaquifer. The thickness of the unconfined aquifer can be up to 200 ft inthese areas (Denver, 1983). These units are lithologically similar to theBeaverdam Formation, and although the Chesapeake Group sand tends to bebetter sorted than the Beaverdam, it commonly is difficult to distinguishbetween them.
Holocene sediments include estuarine, lagoonal, and barrier islandsequences deposited during a marine transgression (Kraft, 1971). Thesesediments may be up to 150 ft thick in buried stream channels (Talley andAndres, 1987).
6
Hydrologic Characteristics
The unconfined aquifer is a regional aquifer in eastern Sussex County.The saturated thickness of the water-bearing sand ranges from 10 to 170 ftand generally is greater than 80 ft (Andres, 1986b; Talley, 1987). Thewater table ranges from 1 to 20 ft below land surface and fluctuatesannually from 5 to 10 ft (Andres, 1986b).
The area receives average annual precipitation of 44 in. (inches), ofwhich approximately 14 in. recharges the unconfined aquifer. Most of theground-water recharge occurs from mid-October to early April when evapotranspiration is at a minimum (Johnston, 1973). The unconfined aquifergenerally is recharged in topographically high areas and discharges tosurface-water bodies and through downward leakage into underlying aquifersof the Chesapeake Group. A small amount of ground water also transpires andevaporates. Johnston (1973) calculated that pumpage from the unconfinedaquifer in Delaware represented only about 4 percent of the naturaldischarge. The regional ground-water divide is coincident with the westernedge of the study area (Figure 1). The Inland Bays, the Delaware Bay, theAtlantic Ocean, and larger streams are major discharge areas. Small streamsderive base flow from shallow, local flow systems.
Aquifer transmissivity varies locally because of lithologic changes andchanges in saturated thickness. Johnston (1977) estimated that the averagetransmissivity of the unconfined aquifer exceeded 10,000 ft 2/d (feet squaredper day). in the northern part of the study area on the basis of the resultsof a flow-model simulation. Values of transmissivity in the part of easternSussex County included in the model ranged from less than 5,000 to greaterthan 20,000 ft 2/d (Johnston, 1977). Values of storage coefficient wereestimated to range from 0.11 to 0.17, with an average of 0.14 (Johnston, 1973).
The flow system in the unconfined aquifer is complex because of thedissection of the land surface by numerous streams, anisotropy of thesediments, the channeled nature of the aquifer's base, and downward flowinto deeper aquifers. Johnston (1973, 1977) gives a detailed analysis ofthe hydrology of the unconfined aquifer.
Natural Ground-Water Chemistry
Naturally occurring concentrations of chemical constituents in water inthe unconfined aquifer are low, and specific conductance of the waterusually is less than 100 pS/cm (microsiemens per centimeter at 25 degreesCelsius). The aquifer is primarily composed of relatively insoluble quartzsand and the sediments generally are highly permeable, which reduces thecontact time for reactions to occur between water and aquifer materials.
The major dissolved constituents in natural water in the unconfinedaquifer are derived from atmospheric precipitation and weathering ofsilicate minerals, principally feldspars. Sodium and bicarbonate generallyare the dominant ions. Incongruent dissolution of the feldspars plagioclaseand orthoclase is a primary source of sodium, as well as potassium, calcium,bicarbonate, and silicic acid. Magnesium is contributed from weathering ofmicas and some heavy minerals. Natural sources of sulfate and nitrate are
7
precipitation and organic matter. Atmospheric precipitation is theprincipal source of chloride in the ground water and a primary source ofsodium, except in areas affected by saltwater intrusion. The chemicalconstituents in local precipitation and general silicate mineral dissolutionreactions are presented in Tables 1 and 2, respectively.
Oxidation and reduction reactions are an important control on concentrations of certain constituents in natural water--notably iron,nitrogen, sulfur, and bicarbonate (Table 2). Dissolved oxygen inprecipitation is consumed by oxidation of organic matter in the soil zone.All of the oxygen may be consumed in poorly drained soils that have highorganic matter content. Dissolved oxygen not consumed by reactions withorganic matter in the soil zone moves in recharge into the ground-watersystem where it is gradually consumed by oxidation of reduced species suchas ferrous iron l ammonium, and sulfide. Oxidation and reduction reactionsinvolving nitrogen and sulfur species are bacterially mediated.
Much of eastern Sussex County is covered by soils that are well orexcessively well-drained and contain very little organic matter. This isparticularly true of the soils of the Evesboro-Rumford association (Figure 2).Recharge water percolating through these soils contains relatively highconcentrations of dissolved oxygen. Along the western edge and southern partof the study area, soils generally are poorly drained and have large amountsof organic matter, and the water in the aquifer is under reducing conditions(Figure 2). Dissolved iron may be elevated in these areas.
TABLE 1. Average chemical composition of precipitation from a station
located in Lewes, Delaware, August 1978 through January 1983.
(Data provided by the MAP3S Precipitation Chemistry Network]
Constituent
Hydrogen, H+
Sodium, Na++ADmoniwn, NH4
Potassium, K+Calcium, Ca+2
Magnesium, Hg+2
Chloride. Cl--2
Sulfate, 804Nitrate, 003-
Total
8
Concentration
in milligrsms
per liter
0.051.22
.23
.OS
.13
.13
1.93
2.10
1.23
7.11
TABLE 2.. Chemical reactions occurring in the unconfined aquifer.
Cation-exchange reactions involving clay minerals also can modify waterchemistry in the unconfined aquifer. The significance of these reactionsvaries with the type of clay minerals present and the concentration ofcations in solution. Denver (1986) provides a detailed discussion of theseand other reactions controlling water chemistry in the unconfined aquifer.
Agricultural Land Use
Corn and soybeans are the primary crops in eastern Sussex County. Theprincipal agricultural practices that influence ground-water chemistry areapplications of inorganic fertilizer, manure, and lime for improving cropproduction. The major nutrients in inorganic fertilizers are nitrogen,potassium, and phosphorus. Most of the nitrogen applied as inorganicfertilizer is in the form of ammonia compounds. Potassium is applied in theform of potassium chloride, and phosphorus usually is applied as phosphoruspentoxide. Nitrogen from manure is in the form of ammonia and organicnitrogen compounds. Poultry manure is the most common type used in easternSussex County. It also contains significant quantities of other majornutrients. Lime contributes calcium and magnesium for plant use and
Tidal marsh, salty-Coastal beach and dune landaswdatkm: Low areas that are regularly Ilo<xledby salt water, and areas of klose, saltybeacb and dunesands.
FallsingtOD-Pocomoke_Woodstown lI.5SOCiation: Verypoorly drained to moderately well drained soils thathale a moderately permeable subsoil ofsandy loamto sandyclayloarn.
Elkton-Matawan-Keyport associatioo: Poorlydrained and moderately\W:lI drained soils that ha..ea slowly petmeable, cla~ysubsoil.
Pocomoke-Fallsington-Elesboro aswciatiDn: Very poorlydrained and poorly drained soils that ba..e a moderatelypermeable subsoil ofsandy loam or sandy clay loam,and ellCl:ssi..ely drained soils that ha..e a rapidlypermeable sandysubsoil.
SassafraSoFalJsington lI.5SOCiatiDn: Well drainedand poorlydraiDed soils that ba..e a moderatelypermeable subsoil nfsandy loam to sandy clay
"'m.
~~
•
OAr
OElA WAitE
MARYLAND----- ----
38°50'
o 5 10 KILOMETERS
Figure 2, General distribution of soilSussex County,
associationsDelaware,
in eastern
10
bicarbonate which buffers soil acidity caused by nitrification (Denver,1986). Other fertilizer components that may affect ground-water chemistryare sulfate and micronutrients including small amounts of iron, manganese,zinc. boron, copper, chlorine, cobalt, and molybdenum. The amounts ofmicronutrients present probably do not significantly influence waterchemistry. Pesticides also are used to improve crop production. Theireffects on ground-water quality are not considered in this study.
Approximately 100,000 acres in eastern Sussex County are used for cropproduction (Ritter, 1986); about 6 percent of this acreage is irrigated.Much of the agricultural land is in areas with well- or excessively welldrained soils, which frequently have a low natural organic matter contentand low to moderate moisture capacity. Larger amounts of fertilizer andirrigation are needed to support crop production on these soils becausefertilizers are rapidly leached from the soil zone by percolating water.These factors increase the potential for nitrate contamination in the aquifer.
Corn and soybeans are the major crops in eastern Sussex County.Nitrogen requirements for corn production are 75 to 150 lb/acre (pounds peracre) without irrigation (Harris and Williams, 1981), depending on theability of the soil to retain nutrients. As much as 250 lb/acre of nitrogenare required for corn production on irrigated land. Nitrogen fertilizer isnot required for soybean production. Most crop nitrogen requirements aremet with inorganic fertilizers applied after planting and during the growingseason. As a result, nitrogen supplied through spreading of poultry manureis frequently in excess of crop needs. Manure generally is applied duringthe nongrowing season.
Septic Systems
Residential development is concentrated along u.S. Route 113, alongState Route 1 in the coastal areas, and in areas bordering Rehoboth Bay andIndian River Bay (Figure 1). Most of the towns in these areas are seweredand have centralized treatment plants that discharge to surface-waterbodies. Individual septic systems are used in the towns of Frankford,Dagsboro, Millville, Ocean View, and Ellendale (Figure 1). Other areas witha high density of individual septic systems are primarily located around theInland Bays. Many wells in these areas are screened in confined aquifers toavoid contamination by nitrates and saline water.
Effluent from septic systems discharges into tile fields or seepagebeds in or just below the subsoil. Nitrogen in the effluent is in ammoniaand organic forms. On the average, a septic system discharges about 7 lb ofnitrogen per person per year. Of this amount, 64 percent is estimated toreach the ground-water system and the remainder is lost by volatilization(Hall, R. L., U.S. Department of Agriculture, oral commun., 1986). Most ofthe nitrogen from the effluent is converted to nitrate in oxidizing zones ofthe soil and aquifer. Components of septic-system effluent are similar tothose in poultry manure. Detergents and other household chemicals also maybe present in the effluent.
11
IIETIIODS OF STUDY
Specific conductance, pH, alkalinity, water temperature, and dissolvedoxygen were measured in the field at each well or piezometer sampled forthis study. Prior to sampling, piezometers were purged using a suction pumpto remove silty water. They were then pumped with a submersible pump untilthe field measurements stabilized and samples could be collected. For wellswith permanent pumps, samples were collected upflow of any treatment systemsand water was allowed to run until the field measurements stabilized.Laboratory analysis of major ions, nutrients, and selected trace constituents was conducted by the USGS laboratory in Denver, Colo. Sampling andanalytical procedures used in this study are described by Skougstad andothers (1979). Laboratory analysis for nitrate is actually nitrate plusnitrite as nitrogen. This is generalized as nitrate as nitrogen, ornitrate, throughout this report because nitrite is usually present in muchlower concentrations than nitrate and is readily converted to nitrate;therefore, concentrations of nitrite are not reported separately.
Fairmount Site
Installation of Piezometers
Thirty-one piezometers were installed around the irrigated field atthe Fairmount site (Figure 3). Each piezometer was constructed using 2-in.inside-diameter, flush-threaded, polyvinyl chloride casing with a 5-ft-longslotted screen at the base. The piezometers bisecting the field arearranged in clusters of four or five each with screened intervals rangingfrom 20 to 25 ft below land surface to 97 to 103 ft below land surface(Figure 4). Each piezometer was installed in a separate hole. The piezometers installed on the perimeter of the field are between 20- and 25-ftdeep. A datum was established at each piezometer, and relative land surfaceelevations were established to within 0.02 ft.
Data Collection
Water levels were measured bimonthly in all piezometers from April 1985through March 1987, and at other selected times to evaluate drawdownassociated with irrigation. Continuous water-level records were collectedfrom November 1985 through May 1987 for an observation well located 75 ftfrom the irrigation well. The observation well is screened from 60 to 90 ftbelow land surface, which also is the screened interval of the irrigation well.
The schedule of field and laboratory sampling of ground water at theFairmount site is given in Table 3. Samples from Phillips Branch werecollected in June and October 1986 from an upstream and downstream siteduring base flow (Figure 3). A sample also was collected from the irrigation well. Results of the analyses are in Appendix 1.
Continuous records of specific conductance were collected from Februarythrough July 1987 in the 25-ft deep piezometer (Ph13-03) and the 45-ft deeppiezometer (Ph13-l6) in cluster 3 using USGS minimonitors. For most of theperiod of record, data on specific conductance agreed with field-determinedvalues made after pumping the piezometers for sampling.
12
COunty
"'"oa:
o
o 500 1000 FEET
I ' " , /--'-r',-+-'.o'li i ... I • I'300 METERS
39°39'00"
EXPlANATI ON
• Cluster Piezometers A 1 I A'® Water-level Piezometers
to. Stream Sampling Site Ph 13-02
location of geologicsection, shown on Figure 4
Well Number
Figure 3. Fairmount site study area.
13
TABLE 3. Schedule of water-quality sample collection at the clusterpiezometers of the Fairmount site.
[F, field analysis of specific conductance, dissolved oxygen, pH, alkalinity;L, laboratory analysis for major ions and nutrients and field parameters;ND, piezometer not developed for sampling; -, no sample collected]
Location Dateand
well number 4,5/85 7/85 10/85 1/86 3/86 6/86 9/86 4,5/87
Cluster 1Ph23-10 F F F L F L FPh23-12 F F L F L L FPh23-l3 F F L F L L FPh23-l4 F F L F L L F
Cluster 2.Ph13-04 LF F L L L L FPhl3-23 F F L L F L L FPhl3-24 F F L L F L L FPhl3-25 F F L F L L FPhl3-26 ND ND ND NO F L L F
Cluster 3Ph13-03 F F L L L L L FPh13-16 F F L L F L L FPhl3-17 F F L L F F L FPhl3-18 F F L L F F L F
Cluster 4Phl3-08 F F L L L L FPhl3-l9 F F L F L L FPhl3-20 F F L F L L FPh13-21 F F L F L L FPhl3-22 ND ND NO ND ND ND ND NO
Cluster 5Phl3-11 ND NO ND NO NO ND NO NDPhl3-28 F F L L L L FPhl3-l3 F F L F L L FPhl3-l4 F F L F L L FPhl3-15 ND ND NO ND ND ND ND F
Indicates uncertain correlationWell ro..rrtler and screellll'd imervalof piezometers and irrigation woll.
------? ?_----Lhconformily between
CoiLITbia Group andChesapeake Group
T.D. 150 FEET
Tolal depth ailed, in leel
Gamma log
Increasing Radiation
Voclical line indicates midpoint01 piezometer cluster
Figure 4. Section showing the configuration of geologic units and screenedintervals of piezometers at the Fairmount site.
15
Influence of Septic-System Effluent
Only 11 wells and piezometers (Figure 1) were sampled to study theeffects of septic-system effluent on ground-water chemistry as few wellscould be located with nitrate contamination that could be attributed solelyto septic-system effluent. Most areas with closely spaced septic tanks areadjacent to agricultural areas or occupy land formerly used for crop production. Water samples from wells were collected as near as possible to thewellhead. Three small-diameter piezometers were sampled using a peristalticpump. Field and laboratory analytical procedures were the same as thosewhich were used for the Fairmount site. Additionally, samples were analyzedfor boron and methylene blue active substance (MBAS).
EFFECTS OF AGRICULTURAL PRACTICES ON GROUND-VATER HYDROGEOLOGYAND GEOCHEIIISTRY UNDER AliI IRRIGATED FIKIJ)
Description of the Fairmount Site
The soil and aquifer characteristics and agricultural practices associated with the Fairmount site and surrounding area are typical of a largeportion of eastern Sussex County. The site is a 220-acre field with a centerpivot irrigation system and a centrally located supply well (Figure 3). Theirrigation system was installed in 1978. The closest surface-water feature isa perennial stream, Phillips Branch, that flows on the northern side of thefield. A drainage ditch on the southern and eastern sides of the fieldgenerally is dry except in the spring. Chicken houses are in use on thesouthern side of the field along Route 5. Agriculture is the predominant landuse upgradient from the Fairmount site.
Soils are primarily Evesboro loamy sand which is excessively drained andsandy with high permeability and generally low natural fertility (U.S.Department of Agriculture, 1974). Evesboro soils are part of the EvesboroRumford association (Figure 2) which is the most extensive association ineastern Sussex County. The southeastern corner of the field is covered byWoodstown soil, which is moderately well drained with moderate permeability.
The Columbia Group is 80 to 115 ft thick (Figure 4). The sediments arepredominantly quartz sand and coarsen with depth. The Columbia Group isunderlain by a clay layer at the top of the Bethany formation (informal unit,Andres, 1986a), except near cluster 2 (Figure 4) where no clay is present. Itis not clear whether the absence of the clay is due to an erosional unconformity related to deposition of the Columbia Group or a facies change withinthe Bethany formation. The uppermost sand unit in the Bethany formation, partof the Pocomoke aquifer, appears to be composed of coarse sand and gravelunder the southern half of the field and is siltier under the northern half(Figure 4). Areal variation in sand content is typical of this formationbecause of its deposition in a delta environment (Andres, 1986a).
17
Agricultural Practices
Corn and soybeans generally are grown in alternate years at the Fairmountsite. There was corn stubble on the field when the piezometers were installedin 1985. Soybeans were planted in 1985, the first year of water-quality-datacollection, and corn was grown in 1986. Table 4 summarizes the fertilizationand planting schedule.
The starter fertilizer used for corn and soybeans contains some nitrogen(Table 4). No additional nitrogen was applied to soybeans, but corn receiveda total of 179 Ibjacre through the center-pivot system during the irrigationseason. Sulfur also is applied periodically through the irrigation system inthe form of ammonium thiosulfate. In addition, boron was applied to corn, andboth crops received applications of the micronutrierits manganese and zincthrough the irrigation system (Table 4).
Poultry manure is applied to the field in the winter and spring. Duringthe period of data collection for this project, manure was applied only to thesouthern half of the field, and it was not considered part of the overallnutrient budget.
Soybeans, which require less water than corn, were irrigated during Julyand August. Corn was irrigated from planting in May through crop maturity inlate August. The center-pivot system rotates completely in 48 hours. Itdistributes approximately 1 acre-inch of water over the field for each rotation.
Ground-Water Flow System
Flow in the unconfined aquifer generally is from west to east across theFairmount site (Figure 5). Flow lines diverge from the ground-water divide,located west of the site, toward the major discharge area, Herring Creek. Themaximum amplitude of water-table fluctuation from April 1985 through March1987 was about 3 ft in the piezometers nearest Phillips Branch and 5 ft in thepiezometers farthest from the stream.
Before the piezometers were installed, flow in the unconfined aquifer wasthought to be toward Phillips Branch. This study has shown that flow paths inthe aquifer, however, basically parallel Phillips Branch. Water levels indicate that Phillips Branch influences the ground-water flow system (Figure 6).At times, the elevation of the streambed is higher than the water table andthe stream loses water to the aquifer (Figure 6A). The aquifer dischargesto the stream when ground-water levels are higher than the streambed (Figure 6B).
Water levels measured in the cluster piezometers show similar fluctuations at different depths in the aquifer, indicating a hydraulic connection.Vertical head differences in piezometers in a particular cluster generally areon the order of a few hundredths of a foot. Head relations vary, but the headusually was highest in the shallowest part of the aquifer and decreased withdepth to about 65 ft. Water-level change did not show a consistent patternbelow 65 ft of depth except in cluster 5, where the head in the deepest partof the aquifer was consistently higher than the head in the shallowest part.
18
TABLE 4. Schedule of fertilization and planting at the Fairmount site fromJanuary 1985 through October 1986.
Date Activity
1985 January
February
May 14
July 10August 17
October
1986 March 11
April 14-15
May 11July 30
September18-20
October 10
October 14
Lime broadcast on field, 1 ton per acre.
Potash (KCl) , 60 percent red granular broadcast,161 Ib/acre.
Intermittent irrigation, with a total of 179 Ib/acreliquid nitrogen and 25 Ib/acre sulfur applied throughirrigation system. Also applied 1 qt/acre 20 percentboron, 1 qt/acre 5 percent manganese, and 1 qt/acre9 percent zinc.
Harvest corn.
Lime broadcast on field, 1 ton per acre.
Plow field and plant winter wheat cover crop.
The unconfined aquifer is recharged primarily from mid-October to earlyApril, which is the nongrowing season. Normal rainfall during this period isabout 21 in., as measured from 1951 through 1980 at a station about 10 mi westof the Fairmount site (National Oceanic and Atmospheric Administration, 1987).There generally is a soil-moisture deficit during the remainder of the year,but heavy rains associated with tropical storms and thunderstorms occasionallyovercome the deficit and allow water to percolate to the water table.
19
'"o
BASE MODIFIED FROM U.S. GEOLOGICALSURVEY 1;24,000 FAIRMOUNT ANDHARBESON QUADRANGLES
o .5 1I ' , , , I ' , , , II I I , iii i I
o ,5 1 KILOMETER
MILE
FLOW LINE
... ..".,.
Represents general directionof ground-water flow.
Location of piezometer clusterand reference number.
.3
~
EXPLANATION
D Agricultural
II Wooded
Chicken house
~~20
WATER-TABLE CONTOUR
Number shows altitude of watertable in feet above sea level.
Contour interval 10 feet.
Figure 5. Land use and direction of ground-water flow in the unconfinedaquifer in the area surrounding the Fairmount site. (Watertable contours modified from Adams' and Boggess, 1964, andBoggess and others, 1964.)
A.,,',,'00'
39° 39' 30' 75·13' 00·
COunty
7S ·'2' 30·
EXPLANATION
12.32•
Location of piezometer.
Altitude of water table
in feet above sea level.
8 13 . 15
Altitude of stream bed
in feet above sea level.
'0---
WATER - TABLE CONTOUR
Lines show equal altitude
of water table above
sea level. Dashed where
approximately located.
Contour interval 1 foot.
B.,,',,'00'
COunry
~ 4.67
""""""II 14.50".""""
"===1~.15~
15 ·,2' 30'
o
o
Woods
500 1000 FEET
'I' ",'150 300 METERS
Figure 6, Altitude of the water table at the Fairmount site,(A) October 3D, 1986 and (B) March 9, 1987,
21
Changes in ground-water levels are shown for well Ph13-32 in Figurell.The water table rose less than 2 ft during the 1985 and 1986 water years whenprecipitation during the recharge period was 64 percent and 75 percent of thenormal, respectively (National Oceanic and Atmospheric Administration, 1985,1986, 1987). Precipitation was 117 percent of the long-term (1951-80) normalduring the 1987 water year recharge period, and the water table recovered morethan 4 ft.
Transmissivity of the unconfined aquifer at the Fairmount site isestimated to be about 18,000 ftld using Boulton's delayed-yield type curve forlate-time data (Lohman, 1979, Plate 8). The analysis is based on water levelsin well Ph13-32 recovering from drawdown caused by irrigation pumping. Theirrigation well and well Ph13-32 are screened from 60 to 90 ft below landsurface (essentially the bottom third of the aquifer). The storagecoefficient determined is 0.16, which is within the range given by Johnston(1973) for the unconfined aquifer.
Effects of Irrigation Pumping on Ground-Water Flow
The effects of irrigation pumping on the ground-water flow system at theFairmount site are shown in Figure 8. Near the irrigation well, there is alarge component of vertical downward flow from the water table toward thescreen. Water levels in well Ph13-32, located 75 ft from the irrigation well,rapidly declined about 4 ft at the onset of pumping (Figure 7). Thecontinuous decline of the water level in well Ph13-32 during irrigationpumping (Figure 7) shows that the ground-water-flow system does not reachsteady state during a 48-hour pumping period. Water levels in all piezometersdeclined during pumping, although the change was only a few hundredths of afoot, over a 48-hour pumping period, in the piezometers of clusters 1 and 5because of their greater distance from the pumped well.
Water-level changes in the cluster piezometers during pumping areinfluenced by the anisotropy of the sediments and the discontinuous confiningbed beneath cluster 2 (Figure 4). Stratification in the sediments comprisingthe unconfined aquifer results in a horizontal hydraulic conductivity thatgenerally is greater than the vertical hydraulic conductivity. Most of thewater derived from the pumped well is flowing horizontally toward the screenedinterval (Figure 8). The result is that drawdown is greater in thepiezometers screened nearest the screened interval of the pumping well, from60 to 90 ft below land surface, than in the shallower piezometers. Thedifferences in drawdown at different depths in the aquifer are particularlyemphasized in water levels from the cluster 3 piezometers adjacent to theirrigation well (Figure 8).
11 A water year is the 12-month period from October 1 through September 30.It is designated by the calendar year in which it ends.
22
6
"
'""
8
"
"
9
"
8
"
October 1 1984 - September 30 1985
October 1 1985 - September 30 1986
. .~
'/-\MlSSing RecordMissing AeCO,<l
(/~ r-
OCTOBER 1984 NOVEMBER DECEMBER JANUARY 1985 FEBRUARY MARCH APRIL MAY ""NE JULY AUGUST SEPTEMBER
, ,
~ "M"'ing Recordr r- .----'\
M'Hifig Record I' ,- 1./
"-OCTOBER 1985 NOVEMBER DECEMBER JANUARY 1986 FEBRUARY MARCH APRIL MAY JUNE JULY AUGUST SEPTEMBER
. ,, , ,
r~~~~r~ ~
//' "'" MISsing Record ~ ,---
r--~~.1iSSjngRecord
OCTOBEA 1986 NOVEMBER DECEMBER JANUARY 1987 FEBRUARY MARCH APRIL MAY JUNE JULY AUGUST SEPTEMBER
October 1 1986 - September 30 1987
8
'""""
W "U
"«II.a: ":::lIIIQZ 8
«...I
~ '"Q...I "Wm "...W "WII. "Z "...I "W>W...I
a: 6
W...« 8
~9
'"""""""
Figure 7. Water level in well Ph13-02 at the Fairmount site,November 1984 through September 1987.
23
30
20
A
FEET
CLUSTER 5
LAND SUR FACECLUSTER <1 CLUSTER 3
1"illaHoo Well
CLUSTER 2
A'
FEET
CLUSTER 1
30
20
10
SEA LEVEL 0
-10
-20
-30
-40
-50
-60
-70
-80
-90
Wale, lable
10.85(0.30)
10.82(0.32)
10.80(0.32)
10.77(0.34)
10.85(0.32)
10.61(0.18)
10
o SEA LEVEL
-10
-20
-30
-40
-50
-60
-70
-60
-90
-100J... ::-:- =:-::::: .L -100
o 500 1000 FEET
I
EXPLANAliONIScreened interval 01 irriga1ion well.
o 150 300 METERS
~ 10.63~ (0.61)
Screened interval of piezometer.Top number is hydraulic head,
in feet above sea level, on July 18, 1985.Bot1om number is decline in head from July 5, 1985.
Indicates uncertain correlation.
Direction of ground - water flow.
____ 8.0 _
Une of equal hydraulic head,in feet above sea level.
Contour interval 0.5 teet.
-----1 ?---------
Figure 8. Flow in the unconfined aquifer at the Fairmount site after30 hours of irrigation pumping, July 18, 1985.
25
Drawdown in each piezometer in cluster 2 during irrigation pumping isless than the drawdown in the corresponding piezometers in cluster 4, whichare at equal distances from the irrigation well (Figure 8). Water levelsapparently are influenced by an opening of unknown extent in the confining bedbeneath cluster 2 that allows interconnection between the unconfined aquiferand the Pocomoke aquifer of the Chesapeake Group. The head gradient nearcluster 2 indicates upward flow from the deeper sand through the openingduring irrigation pumping (Figure 8). This interconnection also acts as arecharge boundary and therefore limits the effect of pumping on water levelsin the piezometers of cluster 1.
Relation of Specific Conductance to Nitrate Concentrations
Nitrate is the dominant anion in oxidizing water in the unconfinedaquifer where influenced by agricultural practices because of the naturallylow ionic strength of the water. As a result, there is a direct correlationbetween nitrate concentration and specific conductance in ground water at theFairmount site (Figure 9). The Pearson's correlation coefficient is 0.98. Avalue of 1.00 indicates a perfect correlation and 0.00 indicates an absence ofcorrelation. Data for samples collected in September 1986 were used in theregression and calculation of the correlation coefficient, but data for allsamples are similarly correlated (Figure 9). Because of this close correlation, specific conductance can be used to estimate nitrate concentration whenlaboratory analysis for nitrate was not performed. Thus, specific conductanceis a general indicator of the degree of agricultural influence on ground waterat the Fairmount site. Later in this report, this relation is applied tostudy changes in water quality over time.
Trends in Maior·lon Concentrations
Positive correlations between nitrate concentration and concentrationsof other major ions related to lime and fertilizer applications, includingcalcium plus magnesium, chloride, potassium, and sulfate, also were observedin ground-water samples from the Fairmount site (Figures 10-12). Similarrelations between nitrate concentration and concentrations of calcium plusmagnesium, chloride, and potassium were observed in water samples collectedfrom irrigation wells in western Sussex County (Denver, 1986). Alkalinity andsodium concentrations do not show a positive correlation with nitrateconcentrations.
The close direct correlation between concentrations of nitrate andcalcium plus magnesium in the ground water indicates substantial agriculturalinfluence (Figure lOA). Calcium plus magnesium also correlates well withspecific conductance. Because divalent cations generally have strongeradsorption affinities than monovalent cations, and the concentration ofcalcium plus magnesium is high relative to other cations, calcium andmagnesium also probably dominate available clay-mineral exchange sites.Alkalinity produced by solution of the carbonate in lime is consumed inreactions that buffer the acidity produced by nitrification and, as a result,shows a weak negative correlation with nitrate concentration (Figure 10). Inwestern Sussex County, alkalinity concentrations in ground water influenced bynitrification were generally less than 10 mg/L (Denver, 1986). Ground waterfrom the Fairmount site follows this pattern.
26
450o 0
10 15 20 25 30 35NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER
Pearson's correlation coefficient:: 0.98
4540
o
EXPLANATION
a All samples collectedat cluster piezometers
Sample collected at• cluster p~ezometer
in September 1986
o
00
80 0o 00
.00 ••00 00
i·o ••0 @g
o
o
•
5o50
150
100
350
200
400
250
300
<Jl <JlZ ::>w u;::;;
...JW Wu; ()
0<Jla:
() ww
::;; a:
~Clw0
W"'() N
Z
'" >->- '"() a:::> w0 >-Z w0 ::;;() >=() Z
w:: w()
() a:w0- W<Jl 0-
Figure 9. Relation of specific conductance to nitrate concentration forwater in the unconfined aquifer at the Fairmount site.
27
A. 60
•50
::;: c::::> w • •(f) >- •w --' 40 •z c::C) w I. •« ll. •::;: ••(f) •(f) ::;: 30::> «--' c::ll.
C) •::;: --' 20::> --'0 ::;: ••--' •« z •0 10 • Pearson's correlalion.. coefficient::;: 0.97
0 ,
0 5 10 15 20 25 30 35 40NITRATE AS NITROGEN, IN MILLIGRAMS PER LITER
Figure 10. Relation of nitrate concentration to concentrations of(A) calcium plus magnesium and (B) alkalinity for waterin the unconfined aquifer at the Fairmount site,September 1986.
Figure 11. Relation of nitrate concentration to concentrations of(A) potassium and (B) chloride for water in the unconfinedaquifer at the Fairmount site, September 1986,
Figure 12. Relation of nitrate concentration to concentrations of(A) sulfate and (B) sodium for water in the unconfinedaquifer at the Fairmount site, September 1986.
30
Potassium concentration generally is greater at higher nitrateconcentrations (Figure llA). However, potassium concentration is lower thanconcentrations of other cations, and it does not correlate closely withnitrate concentrations because potassium ions are strongly attracted tocertain clay minerals and are fixed at cation-exchange sites in the claylayers. In contrast, chloride, applied in conjunction with potassium, is anonreactive ion. Hence, chloride concentration increases more rapidly thanpotassium concentration for a given increase in nitrate concentration, and itis therefore more strongly correlated with nitrate (Figure 11).
Anomalously high potassium concentrations were measured in samples fromthe shallowest piezometer of cluster 3, Ph13-03 (shown as 20 mg/L in figurellA). This piezometer is adjacent to the irrigation well, and, although thecause of the elevated concentrations are unknown, they may result from leaksor spills of ammonia-based fertilizer near the tank that feeds liquidfertilizer into the irrigation system. Ammonium ions and potassium ions havesimilar adsorption affinities, and the excess ammonium ions from spills couldbe occupying the exchange sites on clay usually available to potassium.
Sulfate also is applied through the irrigation system. Sulfate concentration generally increases with increasing nitrate concentration (Figurel2A) , but because sulfate is absorbed by plants and microorganisms andtherefore retained in the organic fraction of the soil, sulfate concentrationsdiffer widely.
Sodium is not a major fertilizer component, although some is availablefrom sodium chloride in poultry manure. As a result, there is no linearcorrelation between sodium and nitrate concentrations (Figure l2B). Thesodium concentration in ground water is primarily from natural sourcesincluding dissolution of feldspars and precipitation. Sodium ions that enterthe ground water at the Fairmount site are essentially conservative because ofthe greater adsorption affinity of calcium and magnesium on clay mineralexchange sites.
Variations in Water Chemistry
The distribution of agricultural chemicals in the unconfined aquifer isinfluenced by many factors including land use, irrigation, ground-waterwithdrawal, varying type and rate of fertilization, recharge, heterogeneity ofaquifer sediments, and the texture and organic matter content of the soils.As a result of these factors, fertilizer components do not leach into groundwater at uniform rates and the aquifer has zones of differing water quality.For example, nitrogen and other mobile fertilizer components that are appliedin excess of crop requirements during the growing season when there is a soilmoisture deficit migrate into the aquifer with recharge after evapotranspiration declines in the fall. Heavy thunderstorms also could cause largerquantities of agricultural chemicals to leach into the aquifer. In alternateyears when soybeans are grown, nitrogen is not applied during the growingseason and less nitrogen is available to leach from the soil zone into theaquifer.
31
At the Fairmount site, ground-water flow generally is downward undernatural conditions and agricultural chemicals reaching the water table aregradually carried down into the flow system. The horizontal velocity of waterin the aquifer, excluding the influence of irrigation pumping, is estimated tobe between 240 and 380 feet per year. This is based on a measured hydraulicgradient of 0.0013, a horizontal hydraulic conductivity of 200 feet per day,and a porosity of 25 to 40 percent. Variations in the relative amounts ofclay, silt, sand, and gravel cause flow at different velocities in theaquifer. The stratification of the sediments also influences chemicalstratification in the water. Irrigation pumping significantly increases thehydraulic gradient near the pumped well and, therefore, the downward movementof water and dissolved fertilizer components near the well is greater thanunder natural flow conditions.
Spatial Variations
Concentrations of nitrate and other ions introduced by agriculturalpractices vary considerably throughout the unconfined aquifer (Figures 13 and14). However, agricultural influence is significant even near the base of theaquifer where the median nitrate concentration exceeded 10 mg/L (Figure 13A).
Although concentrations of ions introduced by agricultural practicesgenerally are higher near the surface of the aquifer and decrease with depth,the median concentrations of nitrate (Figure 13A) and calcium plus magnesium(Figure 13B) were greater in the depth interval from 40 to 45 ft than in theinterval from 20 to 25 ft. This distribution could be caused by variations infertilizer-application rates or differences in the rate of fertilizer leachingbecause of the timing and amount of recharge, or both. In contrast, potassiumand sulfate concentrations generally do decrease with depth (Figure 14).These ions are less mobile than nitrate, calcium, or magnesium ions because ofthe greater fixation of potassium on cation-exchange sites in illitic clay andabsorption of sulfate by organic material in the shallow subsurface.
Concentrations of sodium and silica, and pH values generally increasewith depth in the aquifer (Figure 15). Water in deep parts of the flow systemis in contact with aquifer materials for a longer time than water in shallowflow systems; this increased contact time increases the opportunity forsilicate-mineral dissolution, which is indicated by increased concentrationsof sodium and silica. In water under natural conditions, pH increases as aresult of mineral dissolution. However, nitrification produces hydrogen ions(Table 2, reaction 5) and, therefore, is probably the major control on pH atthe Fairmount site because of the predominance of nitrate in the water.
Some of the variability in water chemistry with depth may result fromground-water pumping. A large component of downward flow near the irrigationwell during pumping (Figure 8) promotes movement of agricultural chemicalsinto the aquifer. This results in increased concentrations of ions associatedwith agricultural practices in water nearest the pumped well. Irrigationpumping cannot be directly attributed to the distribution of agriculturalchemicals related to agriculture in other parts of the aquifer because of thelimited extent of pumping influence and the complexity of other factorsinfluencing the water chemistry.
32
A.a: 50.0
i" Nitrate as nitrogen:::0
APPROXIMATE SAMPLED DEPTH RANGE, IN FEET BELOW LAND SURFACE
APPROXIMATE SAMPLED DEPTH RANGE, IN FEET BELOW LAND SURF ACE
Figure 13. (A) Nitrate concentrations and (8) calcium plus magnesiumconcentrations for water at different depths in theunconfined aquifer at the Fairmount site.
APPROXIMATE SAMPLED DEPTH RANGE, IN FEET BELOW LAND SURFACE
Figure 15. (A) Sodium concentrations, (B) silica concentrations, and (C) pHfor water at different depths in the unconfined aquifer atthe Fairmount site.
The degree of agricultural influence in the unconfined aquifer is quitevariable across the Fairmount site. Nitrate concentrations ranged from lessthan 2 mg/L to greater than 40 mg/L. A profile of Stiff diagrams representingthe net concentration of major ions resulting from agriculture illustratesthese variations (Figure 16). Concentrations of ions in water from the twodeepest piezometers in cluster 5 were very similar and did not reflect agricultural influence. The water in these piezometers was assumed to representnatural water. The profile was produced by subtracting the concentrations ofions in natural water from these piezometers from concentrations of ions inwater from the other piezometers.
Water from the piezometers in cluster 1 is affected significantly byagricultural practices. Nitrate concentration in water from the deepestpiezometer was 29 mg/L in September 1986. In contrast, below a 40-ft depth,water from piezometers in cluster 5 does not appear to be affected byagricultural practices. Neither cluster is significantly affected byirrigation pumping and the chemical variability between them is attributedprimarily to land uses directly upgradient from the field. Regional groundwater-flow directions and upgradient land uses are shown in Figure 5.
Agriculture is the predominant land use upgradient from the Fairmountsite. In addition, several large chicken houses were previously located upgradient of cluster 1 (Figure 5). Except for the existing chicken houses onthe southwestern corner of the Fairmount site, the other chicken houses havebeen removed and the land area has been converted to crop production.Previous storage and spreading of manure probably still influences groundwater quality, and contributes to the agricultural effects on the aquifer nearcluster 1. In contrast, the degree of agricultural influence in water fromthe two deepest piezometers of cluster 2 is much less than in the adjacentdeep piezometer in cluster 1 (Figure 16). Upward flow of water from thePocomoke aquifer through the break in the confining bed during irrigationpumping (Figure 8) apparently influences water quality in these piezometers.
The wooded area adjacent to Phillips Branch immediately upgradient fromthe area near cluster 5 apparently influences water quality in the deeperparts of the aquifer. Water samples from Phillips Branch did not showcharacteristics of agricultural influence, but were very similar to those fromdeeper piezometers in cluster 5 (Appendix 1).
Some chemical processes in the unconfined aquifer are shown by the Stiffdiagrams in Figure 16. Under natural conditions, bicarbonate usually is themajor anion in the unconfined aquifer. However, bicarbonate concentrations inseveral samples are below those in natural water because of reaction with acidproduced by nitrification. Bicarbonate produced by solution of lime also hasbeen depleted in the buffering reactions.
The proportion of calcium plus magnesium to nitrate is relativelyconstant in water from the Fairmount site (Figure IDA). The proportion ofcalcium to magnesium, however, is not constant (Figure 16). This distributionprobably results from cation exchange between calcium and magnesium, but thiscannot be quantified further without information on clay mineralogy and thetype and quantities of exchangeable cations in the clay.
36
-90
-30
-20
-80
-40
-60
a SEA LEVEL
20
-50
10
-70
-10
A'FEET
30CLUSTER 1
VERTICAL EXAGGERATION X 20
Land Surface
CLUSTER 2CLUSTER 3CLUSTER 4
(NATURAL
WATER)
(NATURALWATER)
CLUSTER 5
o 500 1000 FEETI '" I , , , , !i • i 0 , I
o 150 300 METERS, SCALE IS APPROXIMATE I -100
DATUM IS SEA LEVEL
AFEET
30
20
10
SEA LEVEL 0
-10
-20
-30
-40
-50
-£0
w -70...,-80
-90
-100
Cations Anions
~r':' ':' j '.' ':'1'f ..,Mmiequivalents per liter (megll)(Note: For NOll +N03 ,
1 meq/l= 14 milligrams per liter.>
~
EXPLANATION
Concentrations of ionsin natural water fromcluster 5 piezometers
81111 diagrams are positioned over the approximate screenedIntervals of the cluster piezometers. Each diagram representsthe net concentration of Ions In water after subtraction ofthe concentration of Ions In natural water from piezometersin cluster 5. The solid black pari of the diagrams represent
depletion of bicarbonate. Concentration of bicarbonate Isgreater in natural water than in water Influenced by nitrification.
Figure 16, Concentrations of major ions resulting from agricultural fertilizers in water from the cluster piezometers at the Fairmountsite, September 1987.
Temporal Variations
Specific conductance is used to indicate temporal changes in waterquality because of its close relation to concentrations of nitrate (Figure 9).Spatial variability in specific conductance is shown for four time periodsfrom May 1985 through September 1986 (Figures 17 and 18). Changes in specificconductance in water from individual piezometers varied from 6 ~S/cm togreater than 80 ~S/cm. Differences in specific conductance with depth in aparticular cluster were greater than 300 ~S/cm in some instances. Thisdegree of variability indicates that a water sample from an individual well inthe unconfined aquifer is not an adequate representation of overall quality inin the aquifer. Specific conductance of water from the irrigation well(Figure 18B) represents a mixture of water over a larger depth interval of theaquifer and is a better indicator of average water quality.
There is no regular pattern of change in specific conductance over timeat different depths because of the complexity of the flow system, the influence of irrigation, and the chemical variability of water recharging theaquifer. Several changes in specific conductance in water from thepiezometers of cluster 3 appear to reflect the effects of irrigation pumping(Figures 17 and 18). For example, the increase in specific conductance fromMay 1985 to January 1986 in the three piezometers screened shallower than theirrigation well shows that higher-conductivity water has been drawn toward thewell during the 1985 irrigation season.
Temporal changes in specific conductance were recorded continuouslyfrom February 1986 through March 1987 in the 25- and 45-ft-deep piezometers ofcluster 3, Ph13-03 and Ph13-16, respectively (Figure 19). These data wereverified by periodic field measurements and therefore are believed toaccurately represent water-quality conditions over time. Temporal variationsin water quality can be related to the movement of water with differentdegrees of agricultural influence into deeper parts of the flow system byirrigation pumping and by recharge.
When the continuous monitors were first installed, the specific conductance of water in piezometer Ph13-03 was lower than that in piezometerPh13-16. During the 1986 irrigation season, the lower-conductivity water wasdrawn deeper into the aquifer with each pumping cycle and the specificconductance of water in piezometer Ph13-16 gradually decreased (Figure 19).As the irrigation season progressed, from May through July 1986, the specificconductance of water in piezometer Ph13-16 began to resemble the specificconductance of the water initially in Ph13-03, the shallower piezometer.Specific conductance changed rapidly with water-level changes associated withirrigation pumping. In piezometer PhI3-03, specific conductance increasedwhen the irrigation well was pumping, implying that water in the aquifer abovethe screened interval had a higher conductivity. The change in specificconductance was less dramatic than that in piezometer Ph13-16, but drawdownwas also relatively less than in piezometer Ph13-16 (Figure 8).
38
CLUSTER 4CLUSTER 5
SEA LEVEL
2010
o-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
A'FEET 30
CLUSTER 1
NS
CLUSTER 2
land Surface
VERTICAL EXAGGERATION X 21
- - - --
375 380
386 364
370 33330
109 233 ~ 333
<b" 00
Irrigation Well
CLUSTER 3
Water table--0'
o~
NS
270
225
397
NS
NS
200\136
64
"°0 g
313300
DATUM IS SEA lEVEL
AFEET
30
20
10
o-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
IA.)May1985
SEA LEVEL
Number is specific conductance in microsiemens
per cenlimeler at 25 degrees Celsi us.
NS means not sampled.
VERTICAL EXAGGERATION X 21
SEA LEVEL
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
20
10
o
A'FEET
30CLUSTER 1
SPECIFIC CONDUCTANCE CONTOUR
.------ ---200
Shows specilie conductance
in microsiemens at 25 degrees Celsius. </AS/em).
Contour interval 100jtS/cm.
CLUSTER 2
land Surface
373 370
360 392
338 307300
200
178,01°0321
'00 "00NS
CLUSTER 3
Water Table
EXPLANATION
Irrigation Well
o 500 1000 FEET1-1+1+1-t1-i:~I4-1 ,I-'f+i-I,'o 150 300 METERS
oNS g
CLUSTER 4
"oo
NS
1iil235
NS
70
SCREENED INTERVAL
345 0--1=..:.....;...._.0
CLUSTER 5
DATUM IS SEA lEVEL
20
10
o
AFEET
30
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
SEA LEVEL
lB.)January
1986
Figure 17. Specific conductance for water from the cluster piezometersat the Fairmount site, (A) May 1985 and (B) January 1986.
-90 -90DATUM IS SEA LEVEL VERTICAL EXAGGERATION X 21
-100 -1000 500 1000 FEET
I , I , I I " , , I( I , , I
0 150 300 METERS
~ 235
SCAEENEDINTERVAlNumber is specific conductance in microsiemens
per cenlimeter at 25 degrees Celsius.
NS means not sampled.
EXPLANATION.. --200~
SPECIfiC CONDUCTANCE CONTOUR
Shows specilic conductance
in microsiemens at 25 degrees Celsius. tIlS/cm).
Contour interval 100i-lS/cm.
Figure 18. Specific conductance for water from the cluster piezometers atthe Fairmount site, (A) June 1986 and (B) September 1986.
40
'00 500
w m m0 Irrigation
~ Seasollwu
0'SO <5,
,~
< •~
~ m mw,~zwu
'00 '00~
w•wzw
'"~ on,a•u
" '50 '50
•Wuz< '" 325~
u0azau
000 000u •"uw•w
275 '"
250FEBRUARV MARCH APRil
1986
JUNE J(Jt. v AUGUST SEPTEMfIfR OCTOBER· NOVEt.IlER OECEMllEA.w.uARY FEffiUARY MARCH
1987
EXPLANATION
Ph13-03. 25 leet deep
PhI3-16, 45 tee I deep
---E) ~ Missing Record
Mean dally specific conductance01 waler measured by sensors inthe screened inlervals 01 piezometers.
• Ph13-03
• Ph13-16
Field measurement of specificconductance from waler collecledduring waler - quality sampling_
Figure 19. Specific conductance of water from piezometers Ph13-03 andPh13-16, February 1986 through March 1987.
41
During December 1986 through March 1987, water levels in the unconfinedaquifer near cluster 3 (Figure 7) recovered about 4 ft. Water with higherspecific conductance migrated deeper into the aquifer and the specific conductance in piezometer Ph13-03 increased (Figure 19). The specific conductance of water in piezometer Ph13-16 decreased over the same period aslower-conductivity water moved into that zone of the aquifer. Both of thesewater-quality trends are consistent with downward flow during irrigationpumping. Water with the lowest specific conductance, about 290 pS/cm, whichcorresponds to greater than 20 mg/L of nitrate as nitrogen, still is affectedsignificantly by agriculture.
EFFECTS OF SEPTIC-SYSTEK EFFLUENT ON GROUND-VATER CHEMISTRY
Organic and ammonia nitrogen, sodium, and chloride are the major chemicalcomponents of septic-system effluent. Other minor components include calcium,magnesium, potassium, sulfur, and phosphorus, as well as organic compounds andcomponents of detergents and cleansing agents.
Concentrations of organic and ammonia nitrogen range from 8 to 35 mg/Land from 12 to 50 mg/L as nitrogen, respectively, in average domestic septicsystem effluent (Eastburn and Ritter, [1985J). In aerobic soils, most of thenitrogen is converted to nitrate by the nitrification process. Under anaerobic conditions, ammonia is the dominant nitrogen species.
Sodium and chloride are major components of septic-system effluentbecause of salt in the human diet. Both of these components and nitrate,under aerobic conditions, do not react significantly with aquifer materialsand are relatively mobile in ground water. Concentrations of nitrate, sodium,and chloride above background levels could indicate contamination by septicsystem effluent. However, manure spreading also may be a source of theseconstituents.
Boron and phosphorus have been used to identify septic-system effluent inground water (1£ Blanc, 1982). Because boron and phosphorus are also used asfertilizers, the source of contamination could not be distinguished based onthe presence of these ions in areas with both septic system and agriculturalland uses. Boron is relatively mobile in oxidized ground water, but phosphorus forms complexes with other ions and is fixed by silicate clay andhydrous oxides of iron, manganese, and aluminum. Concentrations of phosphorusin water from the unconfined aquifer are usually below the detection limit.
Detergents are manmade chemicals and their presence in ground water maybe used as an indicator of wastewater presence. Prior to 1964, the mostcornmon surfactant in detergents was alkyl benzene sulfonate (ABS) , which isnot biodegradable (LeBlanc, 1982). To reduce the detrimental environmentaleffects of detergents, ABS was replaced by the biodegradable surfactant linearalkyl sulfonate (LAS). Concentrations of both surfactants in water aremeasured by the MBAS (methylene blue active substances) method which does notdistinguish between them. Surfactants are also used in manufacturing of somepesticides and, therefore, MBAS may also be present in water affected byagricultural practices.
42
The principal components of septic-system effluent are common toagricultural chemicals and manures, and levels of these components abovebackground concentrations do not necessarily indicate contamination bydomestic wastewater. As a result, previous workers in the study area (Ritterand Chirnside, 1982; Robertson, 1977) identified areas with ground-watercontamination from septic systems primarily on the basis of local land use.This also was the general procedure used in this study to initially selectwells with known or suspected elevated nitrate concentrations resulting fromseptic-system effluent. The wells sampled in this study are shown in Figure20. Six of the wells sampled (Me24-06, Me14-2l, Oi35-30, Oi23-08, Qh4l-08,and Qh52-03) are in residential areas; four (Nf22-0l, Nf22-02, Ng4l-03, andPh35-l7) are supply wells for mobile-home parks located in wooded areas; andone (Ph42-0l) is a supply well located downgradient of a mobile-home park withindividual septic systems in a predominantly agricultural area. Chemicalanalyses of water from these wells (Appendix 2) show elevated concentrationsof ions associated with septic-system effluent. However, because these arenot unique to septic-system effluent, the source cannot be positivelyidentified.
COHPARISOIil OF lIIA11JRAL WATER CHEKISTRY AND CHEKISTRY OF WATERAFFECTED BY SEPTIC-SYSTEM EFFLUEIlT AND AGRICUL11JRAL PRACTICES
One of the objectives of this study was to identify unique chemicalcharacteristics of water affected by septic-system effluent to help differentiate between sources of nitrate in the unconfined aquifer. Although thechemical composition of septic-system effluent and agricultural leachate issimilar, the proportions of the components entering the aquifer from thedifferent sources would be expected to differ because the amounts and forms ofchemicals in each are not the same.
Appendices 2 and 3 show analyses of ground water affected by septicsystem effluent and agricultural practices. Analysis of water from theirrigation well at the Fairmount site, Ph13-02, is included in Appendix 2.Appendix 4 presents data from wells with natural and nearly natural waterquality. Even though water with nearly natural water quality may be slightlyaffected by human activities, nitrate concentrations are less than 3 mg/L, andnitrification is not a dominant chemical process affecting water chemistry.Figure 20 shows the location of wells in each of these tables. Comparisonsare shown by boxplots of the specific conductance (Figure 21) and by themedian relative percentages of major ions in the water samples (Table 5).
Water affected by septic-system effluent has a higher median specificconductance (172 ~S/cm) than water affected by agricultural practices(134 ~S/cm) (Figure 2lA). Both are considerably higher than the conductivity of natural water, which has a median value of 60 ~S/cm. Most of thesamples from agricultural areas are from irrigation wells that generally aredeeper than the wells affected by septic-system effluent, and specificconductance is generally less in deep parts of the aquifer than in shallowparts. Irrigation wells generally have longer screened intervals than thewells affected by septic-system effluent, which probably accounts for thewider range of ion concentrations in water from agricultural areas than wateraffected by septic-system effluent.
43
DELA WARE
BAY
...
..(")
on
'"
~ Dagsboro
Oe31-02 "i'~
~F,ankfO'd
Rd35-05 DELAWARE
~-=-I.-+--~~~~~------.......~MARYLAND
BASE FROM U.S. GEOLOGICAL
SURVEY 1:500,000 STATE MAP10 MILESI
I10 KILOMETERS
5I
oI ' .., i fi j
o 5
EXPLANATION
Ph42-01
••A
Well Number
Water intluenced by septic-system eftluent.
Water influenced by agricultural actvities.
Water under natural and nearly natural conditions.
Figure 20. Location of wells sampled with water affected by septic-systemeffluent, agricultural fertilizers, and in water under naturaland nearly natural conditions in the unconfined aquifer.
Figure 21. Comparisons of specific conductance in water from the unconfinedaquifer affected by septic-system effluent, agricultural fertilizers, and in water under natural and nearly naturalconditions.
45
TABLE 5. Percentages of major ions in water from the unconfined aquifer under natural and nearly natural
conditions, affected by septic-system effluent and agricultural practices.
[Percentages of reacting values are calculated from milliequivalents per liter of each constituent]
Natural and nearly Septic-system Apricultural
Constituent natural effluent practices
Median Maximum Minimum Median MaxilDUlD Minimum Median Maximum Minimum
The most significant difference between water affected by septic-systemeffluent and water affected by agricultural practices is the relativeproportion of nitrate to other anions. The median percentage of nitrate was65 in samples from agricultural areas and 54 in areas affected by septicsystems (Table 5). The Kruskal-Wallis test and Fisher's least significantdifference procedure indicate these differences are statistically significantat " - 0.05.
Sulfate is the only other major ion that shows a significant differencein concentration depending on its source (Table 5). Higher proportions ofsulfate in water affected by septic systems than in water affected byagricultural practices and natural water probablY result from oxidation oforganic sulfur in the effluent.
Bicarbonate, the primary contributor to alkalinity, is the major anion innatural water. Alkalinity concentrations and proportions (Table 5) aresimilar in water affected by the septic systems and agricultural practices.Both are considerably lower than concentrations in natural water becausebicarbonate is consumed in reactions with acid produced during nitrification(Table 2). Values of pH are affected similarly (Appendices 2, 3, and 4).
Concentrations of sodium, chloride, calcium, magnesium, and potassium inwater affected by septic-system effluent and agriculture are elevated abovethose in the natural and nearly natural water samples, but they are notsignificantly different from each other (Appendices 2, 3, and 4). Sodium isthe major cation in water under natural and nearly natural conditions, whereascalcium predominates in water affected by septic-system effluent andagricultural practices (Table 5).
46
Concentrations of MBAS range from 0.03 to 0.15 mg/L in water affected byseptic-system effluent (Appendix 2). Analysis of MBAS was available only forwater from 3 wells with natural quality, but the MBAS concentrations in thesewells range from 0.03 to 0.04 mg/L. Thus, MBAS is slightly elevated in wateraffected by septic-system effluent. Water from 3 piezometers affected byagricultural practices at the Fairmount site also was analyzed for MBAS(Appendix 3). Concentrations of MBAS were 0.13 mg/L, 0.21 mg/L, and 0.23mg/L. The source of MBAS in water affected by agricultural practices probablyis surfactants in pesticides.
In summary, comparison of water affected by septic-system effluent towater affected by agricultural chemicals shows different proportions ofnitrate and sulfate. Neither these nor other ions, however, show differencesin proportions that are distinct enough to allow determination of nitratesource based on major-ion concentrations for an individual water sample.
SOMKARY AND CONCUJSIONS
This report describes the distribution and movement of nitrate and otherchemical constituents in ground water under an irrigated field, and identifiesthe effects of domestic septic-system effluent on the quality of ground water.The chemistry of water affected by agricultural practices is compared to thechemistry of water affected by septic-system effluent to look for uniquecharacteristics of each that could be used to identify sources of ground-watercontamination. Both types of water also are compared to water with naturaland nearly natural chemistry.
Nitrate and other chemical constituents from anthropogenic sourcessignificantly affect the chemical character of water in the unconfined aquiferbecause of the naturally low concentrations of dissolved ions (specificconductance is generally less than 100 ~S/cm). Fertilizer and manureapplied to cropland are the major sources of nitrate. Approximately 50percent of the land (about 100,000 acres) in eastern Sussex County is used forcrop production; about 6 percent of this acreage is irrigated. Effluent fromdomestic septic systems is another significant source of nitrate in theunconfined aquifer in unsewered residential areas, which occupy about 9percent of the land in eastern Sussex County.
Thirty-one piezometers were installed in an irrigated field, referred toas the Fairmount site, to study the distribution and movement of nitrate andother chemical constituents related to ~ertilizer and manure applications.Soil and aquifer characteristics at the Fairmount site are typical of a largeportion of eastern Sussex County. Clusters of 4 or 5 piezometers each wereinstalled from north to south across the field; the central cluster waslocated about 75 ft from the irrigation well. The piezometers in each clusterwere screened at different vertical intervals ranging from 20 to 100 ft belowland surface. Water levels and water-quality data were collected from April1985 through March 1987.
47
Nitrate is the dominant anion in water affected by agriculturalpractices, and there is a close correlation between nitrate concentrations andspecific conductance in water samples from the Fairmount site. Specificconductance can be used to estimate nitrate concentration, and it is a goodindicator of the degree of agricultural influence. Nitrate concentrationsalso correlate positively with concentrations of other major ions associatedwith fertilizer application including calcium plus magnesium, potassium,chloride, and sulfate. Bicarbonate, produced by solution of lime, is consumedin buffering reactions with acid that is produced during nitrification, and,as a result, there is a weak negative correlation between concentrations ofbicarbonate and nitrate.
Factors that influence the distribution of agricultural chemicals in theunconfined aquifer include ground-water withdrawal, land use, irrigation,variable rates and timing of fertilization, timing and intensity of recharge,heterogeneity of aquifer sediments, and the texture and organic matter contentof the soils. Concentrations of nitrate as nitrogen ranged from less than2 mg/L to greater than 40 mg/L (with a median concentration greater than10 mg/L) in water samples from the cluster piezometers at the Fairmount site,indicating a large spatial variation in agricultural influence. Althoughnitrate concentrations were normally lowest in samples from the deepestpiezometers in each cluster, concentrations as high as 29 mg/L indicate thatagricultural influence is significant near the base of the aquifer.
Land use upgradient from the Fairmount site controls much of thevariation in water quality with depth. Land upgradient from cluster 5, on thenorthern side of the site, is primarily wooded. Water samples from PhillipsBranch, which flows through the woods adjacent to cluster 5, had concentrations of nitrate as nitrogen below 2 mg/L and were very similar in quality towater samples from the deeper cluster 5 piezometers. In contrast, watersamples from a cluster 1 piezometer screened near the base of the aquifer, onthe southern side of the site, had the highest nitrate concentrations measurednear the base of the aquifer at the Fairmount site. Corn and soybeanproduction is the major upgradient land use, and older maps also show severallarge chicken houses in the area which have been removed. Nitrogen leachedfrom manure piles and spreading of manure probably contributes significantlyto the elevated nitrate concentrations at depth in the aquifer.
Variable leaching of chemicals into the aquifer results in widely differing zones of water quality which change over time. The range in specificconductance of water from individual piezometers ranged from 6 pS/cm togreater than 80 pS/cm over the period of study. In a single samplingperiod, the specific conductance in water samples from a single clusterdiffered by more than 300 pS/cm at different depths. This indicates that awater sample from an individual well in the unconfined aquifer is not anadequate representation of overall quality in the aquifer.
Recharge to the unconfined aquifer generally occurs after evapotranspiration declines in the fall and continues until the growing seasonbegins in the spring. Different amounts of nitrate that have accumulated inthe soil zone over the growing season during corn production, for which over200 Ib/acre of nitrogen fertilizer is applied, are available to leach into theaquifer than from soybean production, which receives little or no nitrogen
48
fertilizer. As a result, water of variable quality reaches the water tableand stratification of nitrate-bearing water occurs. This stratification isshown by temporal changes in specific conductance in water from two shallowpiezometers nearest to the irrigation well where downward flow is greatestduring irrigation pumping. Because of the complexity of factors thatinfluence nutrient leaching and movement in the aquifer, there is no clearlydefined pattern in areal water-quality trends in samples from the clusterpiezometers as a result of irrigation.
The major components in septic-system effluent are organic and ammonianitrogen, sodium, and chloride. Other minor components include calcium,magnesium, potassium, sulfur, and phosphorus, as well as organic compounds andcomponents of detergents and cleansing agents. Concentrations of thesecomponents were elevated above background levels in the water samplescollected from 11 wells that were assumed to be affected by septic-systemeffluent. However, these components also are common in agricultural applications of manures, fertilizers, and pesticides. As a result, sources ofwater-quality problems cannot be based on the presence of these components inground water in eastern Sussex County. The proportions of major ions inground water from different land uses were also not distinct enough toidentify sources of nitrate contamination.
Concentrations of chemicals associated with agricultural practices andseptic-system effluent tend to mask the natural chemistry of the water becauseionic concentrations in water under natural conditions are low. Alkalinity(as bicarbonate) constitutes a median of 7 percent of the major anions inwater affected by agricultural practices and septic-system effluent as opposedto 43 percent in water with natural and nearly natural water quality underoxidizing conditions. The median percentages of nitrate are 65 and 54 inwater affected by agricultural practices and septic-system effluent, respectively, and 25 percent in water with natural and nearly natural water quality.The predominant cation in natural water is sodium, with a median of 61 percent.In water affected by agricultural practices or septic-system effluent, themedian is about 29 percent for sodium, and calcium is the major cation.
49
REFERENCES
Adams, J. K., and Boggess, D. H., 1964, Water-table, surface-drainage, andengineering soils map of the Harbeson quadrangle, Delaware: U.S.Geological Survey Hydrologic Investigations Atlas HA-l08, 1 sheet, scale1:24,000.
Andres, A. S., 1986a, Stratigraphy and depositional history of the postChoptank Chesapeake Group: Delaware Geological Survey Report ofInvestigations No. 42, 39 p.
Andres, A. S., 1986b, Geohydrology of the northern coastal area, Sheet 1Basic geohydrologic data: Delaware Geological Survey Hydrologic MapSeries, No.5, 1 sheet, scale 1:24,000.
Benson, R. N., 1984, Structure contour map of Pre-Mesozoic basement,landward margin of Baltimore Canyon Trough: Delaware Geological SurveyMiscellaneous Map Series, No.2, 1 sheet, scale 1:500,000.
Boggess, D. H., Adams, J. K., and Davis, C. F., 1964, Water-table, surfacedrainage, and. engineering soils map of the Rehoboth Beach area,Delaware: U.S. Geological Survey Hydrologic Investigations Atlas HA109, scale 1:24,000.
Denver, J. M., 1983, Configuration of the base and thickness of the unconfinedaquifer in southeastern Sussex County, Delaware: Delaware GeologicalSurvey Open-File Report No. 20, 12 p.
1986, Hydrogeology and geochemistry of the unconfined aquifer, westcentral and southwestern Delaware: Delaware Geological Survey Report ofInvestigations No. 41, 100 p.
Eastburn, R. P., and Ritter, W. F., [1985], Denitrification and on-sitewastewater treatment systems: State of Delaware Department of NaturalResources and Environmental Control, 104 p.
Harris, J. R., and Williams, T. H., 1981, Fertigating with ground waternitrogen: Newark, Delaware, University of Delaware CooperativeExtension Service, Fact Sheet, 2 p.
Johnston, R. H., 1973, Hydrology of the Columbia (Pleistocene) deposits ofDelaware: An appraisal of the regional water-table aquifer: DelawareGeological Survey Bulletin No. 14, 78 p.
1977, Digital model of the unconfined aquifer in central andsoutheastern Delaware: Delaware Geological Survey Bulletin No. 15, 47 p.
Jordan, R. R., 1962, Stratigraphy of the sedimentary rocks of Delaware:Delaware Geological Survey, Bulletin No.9, 51 p.
Jordan, R. R., and Talley, J. H., 1976, Guidebook: Columbia deposits ofDelaware: Delaware Geological Survey Open-File Report No.8, 49 p.
50
Kraft, J. C., 1971, Sedimentary facies patterns and geologic history ofa Holocene marine transgression: Geological Society of AmericaBulletin, v. 82, p. 2131-2158.
Le Blanc, D. R., 1982, Sewage plume in a sand and gravel aquifer, Cape Cod,Massachusetts: U.S. Geological Survey Open-File Report 82-274, 35 p.
Lohman, S. W., 1979, Ground-water hydraulics: U.S. Geological SurveyProfessional Paper 708, 70 p.
Miller, J. C., 1972, Nitrate contamination of the water-table aquifer inDelaware: Delaware Geological Survey Report of InvestigationsNo. 20, 36 p.
National Oceanic and Atmospheric Administration, 1985-87, Climatologicaldata annual summary, Maryland and Delaware 1984-86: Department ofCommerce, Asheville, North Carolina, National Climatic Data Center(published annually).
Ritter, W. F., 1986, Nutrient budgets for the Inland Bays: Newark, Delaware,University of Delaware Agricultural Engineering Department, 70 p.
Ritter, W. F., and Chirnside, A. E. M., 1982, Ground-water quality in selectedareas of Kent and Sussex counties, Delaware: Newark, Delaware, Universityof Delaware Agricultural Engineering Department, 229 p.
potential problemsNewark, Delaware,
Robertson, F. N., 1977, The quality andin coastal Sussex County, Delaware:Delaware Yater Resources Center, 58 p.
of the ground-waterUniversity of
Skougstad, M. W., Fishman, M. J., Friedman, L. C., Erdman, D. E., and Duncan,S. S., editors, 1979, Methods for determination of inorganic substances inwater and fluvial sediments: U.S. Geological Survey Techniques of WaterResources Investigations, Book 5, Chapter Al, 626 p.
Talley, J. H., 1987, Geohydrology of the southern coastal area, Sheet 1 Basic geohydrologic data: Delaware Geological Survey Hydrologic MapSeries, No.7, 1 sheet, scale 1:24,000.
Talley, J. H., and Andres, A.Sussex County, Delaware:No. 14, 101 p.
S., 1987, Basic hydrologic data for coastalDelaware Geological Survey Special Publication
U.S. Department of Agriculture, 1974, Soil survey of Sussex County, Delaware:U.S. Department of Agriculture, Soil Conservation Service, 74 p.
U.S. Environmental Protection Agency, 1986, Maximum contaminant levels(subpart B of part 141, National interim primary drinking-waterregulations): U.S. Code of Federal Regulations, Title 40, Parts 100 to149, revised July 1, 1986, p. 524-528.
51
52
APPF.RDlCES
53
APPENDIX 1. Well records and chemical analyses of ground water and surface water at the Fairmount site.
[~S/cm, microsiemens per centimeter at 25 degrees Celsius; des C, degrees Celsius; <, less than;mg/L, milligrams per liter: PS/L, micrograms per liter; ft, feet; min, minutes;
WB WAT. whole water; --, no analysis]
DEPTH DEPTH ELEVATION PUMPTO TOP TO BOT- OP LAND OR FL~ SPE-
OP TQ1 OF SURFACE PERIOD CIFICSAMPLE SAMPLE DATUM PRIOR TEMPER- CON- OXYGEN, pHINTER- INTER- (!t TO SAM- ATURE DUCT- DIS- (stand-
DGS DATE VAL VAL above FLING WATER ANCE SOLVED ardWELL NUMBER (ft) (ft) sea level> (min) (deg C) (l£S!cm) (mg/L) tmits)
SOLVED SOLVED SOLVED SOLVED SOLVED (mg/L SOLVED SOLVED SUB-(mg/L) (mg/L (mg/L (mg/L (mg/L a. (.8/L (~8o/L STANCEas Mg) as Ha) as K) as Cl) as S04) Si02) as B) as Fe) (mg/L)
APPENDIX 3. Chemical analyses of water from ....ells affected by agricultural practices.
(~S/cm, microsiemens per centimeter at 25 degrees Celsius; deg C, degrees Celsius; <. less than; >. greater than;me/l, milligrams per liter; ~/l, micrograms per liter; ft, feet; min, minutes;
we WAT - whole ....ater; --, no analysis]
DGSWELL NUMBER DATE
DEPTHto TOP
OFSAMPLEINTERVAL(ft)
DEPTHTO BOT~OF
SAMPLEINTERVAL(ft)
DEPTHOF
BOLE,roTAL(ft)
ELEVATIONOF LAIIDSURFACE
DATUM(ft
abovesea level)
PDMPOR FLOWPERIOD
PRIOR TEHFERTO SAMP- ATURE
LING WATER(min) (deg C)
SPECIFIC
CONDUCT
ANCE(pS/em)
OXYGENDIS
SOLVED(mgIL)
Ob1q-02Ob35-030025-04Od23-07Pb35-02
Pd33-03Pdq5-03Ph13-02Qe31-02R022-0S
07-H-S3OS-lS-S309-13-8207-H-S306-211-62
10-011-6309-09-82.10-22-8607-1S-6309-16-83
.059566933
.2SO6323.0
8072969989
47100
938345
8090969991
82101
938345
51.048.045.041.030.0
35.040.022.035.044.0
>6055
>60>60>60
95>60
1380>60
80
15.017.016.014.015.5
19.514.515.016.015.0
716486
180123
235226230134166
7.55.5
11.29.09.2
4.811.9
8.74.1
llGSWELL NUMBER
Ob14-02Ob3S-03Oe25-04Od23-07Pb35-02
Pd33-03Pd4S-03Ph13-02Oe31-02Re22-0S
pH(stand
ardunits)
5.605.606.005.004.20
5.206.105.565.204.90
pHLAB
(standerd
units)
4.905.006.304.806.00
5.205.906.005.504.90
ALKALINITYwe HATTOTALFIELD
(mall asCaC03)
56721
37632
AlKALINITY
LAB(mglL
asCaC03)
5.06.07.03.04.0
7 .06.07 .03.06.0
NITRONITROGEN
AIKlNIADIS
SOLVED(mglLas N)
<0.0100.0500.1100.0600.670
0.0100.040
<0.010<0.010
0.010
GEN, AMtt'JNIA+ORGANIC
DIS-SOLVED(mgIL)as N)
<0.100.201.3
<0.101.2
<0.100.300.80
<0.100.30
NITROGEN,
NOW+N03DIS
SOLVEO(mglLas N)
5.005.506.50
15.010.0
20.019.018.012.014.0
PBOSPHORUS.
DISSOLVED("""Las P)
0.0200.0400.0200.0200.010
<0.010<0.010
0.0100.0300.010
DGSWELL NUMBER
Ob14-02Ob3S-03Oc2S-04Od23-07Pb35-02
Pd33-03Pd45-03Ph13-02Qe31-02Re22-0S
BARONESS
("",'L)..CaC03
1010164735
58657736SO
CALCIUMDIS
SOLVED("""L
as Cal
2.42.54.17.95.6
1517179.4
10
64
1lAGNESlUM,
DISSOLVED(mglLas Hg)
1.00.831.86.55.0
4.85.38.43.55.9
SODIUMDIS
SOLVED(mglLas Na)
6.48.98.58.85.7
2011117.97.8
POTASSIUM,DIS
SOLVED(mglLas K)
2.02.23.13.92.8
4.13.42.22.34.0
CBLORIDE,
DISSOLVED(mglLas el)
8.14.98.8
139.3
1818191116
SILICADIS
SOLVED("""Las Si02)
1919181114
1817181414
IRONDIS
SOLVED(~IL
as Fe)
<313'8<327
35<311
914
APPENDIX 4. Chemical analyses of water from wells with natural and nearly natural water quality.
[~S/cm. microsiemens per centimeter at 25 degrees Celsius; deg C, degrees Celsius; <, less than; >. greater than;mg/L, milligrams per liter; ~/L, micrograms per literj ft, feet; min, minutes;
WH WAT - whole "aterJ
DGSWELL NUMBER
DATE
DEPTHTO TOP
OFSAMPLEINTERVAL(ft)
DEPTHTO BOTTa! OFSAMPLEINTERVAL(ft)
DEPTHOF
HOLE,TOTAL(ft)
ELEVATIONOF LAlIDSURFACEDATUM(ft
abovesea level)
PUMPOR FLOWPERIODPRIORTO SAMPLING(min)
T»<PERATUREWATER
(deg C)
SPECIFICCONDUCTANCE
(p.S/cm)
OXYGEN,DIS
SOLVED(mg/L)
pH(stand
ardunits)
Md55-02Nd2S-05Nd25-06NeS4-03Ph13-29
OfU-OlPh13-14Qc52-06Rd35-05
09-09-8310-18-8301-07-8303-14-8304-14-87
02-09-8301-16-8609-08-8207-29-83
40493872
97706029
5054438245
11775
12099
505443
E16S45
120
12099
56.050.050.050.06.0
48.018.042.055.0
3045a5
11015
3030
>60>60
15.017.514.013.515.0
15.516.014.515.5
4056503966
59704353
3.43.61.62.6
3.1
7.47.2
5.805.905.905.605.02
6.005.705.405.50
DGSWELL NUMBER
Md55-02Nd2S-05Nd2S-06NeS4-03Ph13-29
Of41-01Ph13-14Qc52-06Rd35-0S
DGSWELL NUMBER
Md55-02Nd2S-05Nd25-06Ne54-03Ph13-29
OfU-OlPh13-14Qc52-06Rd35-0S
pHLAB
(standard
units)
S.105.705.805.306.10
5.806.205.905.50
MAGNESIUM,DIS
SOLVED(mg/Las mg)
0.220.450.340.350.66
0.590.700.971.4
ALKALINITYWH WAT
TOTALFIELD
mg/L asCaC03
13101310
9
159
156
SODIUM,DIS
SOLVED(mg/Las Na)
5.47.26.84.48.9
6.78.74.85.0
ALKALINITY
LAB(mg/LasCaC03)
1614149.0
11
129.09.07.0
POTASSIUM,DIS
SOLVED(mg/Las K)
0.400.900.801.21.3
1.31.31.61.8
65
NITROGEN,
AlHJNIADIS
SOLVED(mg/Las N)
<0.010<0.010
0.050<0.010<0.010
<0.0100.0300.020
<0.010
CHLORIDE,DISSOLVED(mg/Las Cl)
4.24.74.15.0
11
5.68.56.07.3
NITROGEN AMl"l)NiA +
ORGANICDIS
SOLVED(mg/Las N)
0.200.20
<0.100.50
0.500.500.700.10
SULFATEDISSOLVED(mg/L
85 S04)
0.60.4
<1.00.90.4
<5.01.12.00.4
NITROGEN,
N02+oo3DIS
SOLVED(mg/Las N)
2.301.20
<0.1002.80
1.802.102.401.90
SILICA,DISSOLVED(mg/Ls.
S102)
3023241421
28221614
PHOSPHORUS.
DISSOLVED(mg/Las P)
<0.010<0.010
0.010<0.010
0.010
<0.010<0.010<0.010
0.050
BORON,DIS
SOLVED(#8/Las B)
<10
HARDNESS(mg/LasCaC03)
6874
11
101011
8
IRON,DIS
SOLVED(p.g/L
as Fe)
3716<3
3005
6722<3<3
CALCIUMDISSOLVED(mg/Las Cal
2.02.52.21.23.3
2.82.82.60.98
METHYLENEBLUE
ACTIVESUB
STANCE(mg/L)
0.01
APPENDIX 5
CONVERSION FACTORS AND ABIlREVIATIONS
For those readers who may prefer to use metric (International System)units rather than the inch-pound units used in this report, values may beconverted using the following factors:
Multiply inch-pound unit
foot (ft)foot per day (ft/d)foot squared per day (ft2/d)gallon per minute (gal/min)inch (in.)mile (mi)square mile (mi2)acreacre-inchpound (lb)pound per acre (lb/acre)quart per acre (qt/acre)ton per acre
!2v
0.30480.30480.09290.06308
25.41.6092.5900.40471.0280.45361.122.3382.242
To obtain metric unit
meter (m)meter per day (m/d) 2meter squared per day (m /d)liter per second (L/s)millimeter (rom)kilometer (km)square kilometer (km
2)hectare (ha) 3.cubic centimeter (cm )kilogram (kg)kilogram per hectare (kg/ha)liter per hectare (L/ha)metric ton per hectare
Chemical concentration, water temperature, and specific conductance aregiven in metric units. Chemical concentration is expressed in milligrams perliter (mg/L), micrograms per liter (~g/L), or milliequivalents per liter(meq/L). Water temperature in degrees Celsius (oC) can be converted todegrees Fahrenheit (oF) by using the following equation:
Specific conductance is expressed in microsiemens per centimeter(~S/cm) at 25 degrees Celsius.
Sea level: In this report "sea level" refers to the National GeodeticVertical Datum of 1929 (NGVD of 1929)--a geodetic datum derived from a generaladjustment of the first-order level nets of both the United States and Canada,formerly called "Sea Level Datum of 1929."
