Occurrence of Nitrate in Groundwater--A Review

R. F. Spalding* and M. E. Exner

ABSTRACTThe results of federal, state, and local surveys, which included more

than 200 000 NOa-N data points, are summarized in this review ofNOn in groundwater in the USA. The levels of NO3-N are associatedwith source availability and regional environmental factors. In regionswhere well-drained soils are dominated by irrigated cropland, thereis a strong propensity toward the development of large areas withgroundwater that exceeds the maximum contaminant level of 10 mg/L NO3-N. Most of these areas are west of the Missouri River whereirrigation is a necessity. Aquifers in highly agricultural areas in thesoutheastern USA reportedly are not contaminated. Vegetative uptakeand denitrification in this warm, wet, C-rich environment are re-sponsible for the natural remediation of NO3 in shallow aquifers. Inthe Middle Atlantic states and the Delmarva Peninsula, localized con-tamination occurs beneath cropped, well-drained soils that receiveexcessive applications of manure and commercial fertilizer. Extensivetile drainage has for the most part prevented a NO~ problem in thegroundwater of the Corn Belt states. Throughout the USA there arerecurring themes. They include a decrease in NO3-N levels with depth;lower NO3-N levels in shallow wells (< 8 m); and a significant increasein NO~-N in older wells and in wells with poor construction. Thefactors affecting the distribution of NO3 in aquifers are complex andpoorly understood. Interdisciplinary studies using discrete depth sam-pling, hydrogeological indicators, isotopic tracers, and microbiologi-cal techniques are necessary to unravel the complex dynamics.

INGESTION of NO3 in drinking water has caused met-hemoglobinemia in infants under 6 mo of age andrecently caused the death of a South Dakota infant (John-son et al., 1987). The noncancerous acute toxicity ofNOn is the U.S. Environmental Protection Agencys(USEPA) basis for establishing a maximum contaminantlevel (MCL) for NOa-N in drinking water. Although acutetoxicity generally has been documented at concentrationsgreater than 50 mg/L NOn-N, the MCL has been set at10 mg/L. The documentation for the MCL recently wasreviewed during a reevaluation of the reference dose (RfD).The USEPAs findings reaffirmed the 1.6 mg/kg per dayRfD, which supports the present MCL of 10 mg/L NO3-N (Dourson et al., 1991). In rural community water sys-tems where NO3 levels exceed the MCL, the health con-cerns for this segment of the population have beenprotected by providing an alternative source of drinkingwater. Because the present technologies for removal ofNO3 from drinking water have serious limitations thatmake their application difficult to prescribe (Dahab, 1991),many communities have had to find new sources of water.Whether that source is surface water or groundwater froma deeper formation, it is an economic hardship for thesesmall communities to bring their water supply into com-pliance with the NO3 provision of the Safe Drinking WaterAct.

Additional research is necessary to confirm the rela-

R.F. Spalding, Water Center and Agronomy Dep. and M.E. Ex-ner, Conservation and Survey Div., Institute of Agric. and Nat.Res., Univ. of Nebraska, Lincoln, NE 68583-0844. Journal Se-ries no. 9975, Agric. Res. Div., Univ. of Nebraska. Received 19June 1992. *Corresponding author.Published in J. Environ. Qual. 22:392-402 (1993).

tionships between ingestion of NO3 in drinking water andhypertension (Malberg et al., 1978), increased infantmortality (Super et al., 1981), central nervous systembirth defects (Dorsch et al., 1984), and certain cancersincluding stomach cancer (Hill et al., 1973) and non-Hodgkins lymphoma (Weisenburger, 1991). Neither thenoncarcinogenic health effects of NO3 nor the carcino-genic effect of NO3 or N-nitroso compounds, producedwhen NO2 formed by bacterial reduction of NO3 reactswith nitrosatable substrates, have been proven conclu-sively. The experts (Weisenburger et al., 1991) agreethat the majority of evidence implicating NO3 as a caus-ative factor is based on correlation studies that provideonly weak evidence of an association and should not beused to establish a cause-and-effect relationship.

Historically, the justification for surveys of NO3 levelsin groundwater and for research into NO3 behavior ingroundwater has been the adverse health effects attrib-uted to the ingestion of NO3 in drinking water. Now,however, there is growing environmental awareness thatNO3 is the limiting nutrient in nearshore environmentsand in several lakes (Ryther and Dunstan, 1971; Vinerand White, 1987). Concerns for nutrient levels in surfacewaters are fueling NO3 studies in the Chesapeake Bay(Glibert et al., 1991), the Mississippi Delta (Turner Rabalais, 1991), and the Aegean Sea (Ganoulis, 1991).Because groundwater is the major component of base-flow in creeks and rivers, an understanding of its con-tribution to the NO3 load in surface water is important.As a result there is a renewed effort to protect streamsfrom shallow groundwater inputs of NO3 as well as NO3,suspended sediments, P, and pesticides in storm runoff.Under certain flow and redox conditions, riparian zonesalong waterways have demonstrated a natural profi-ciency to intercept and denitrify NO3 in shallow ground-water (Gilliam, 1991; Schipper et al., 1991).

NITRATE IN GROUNDWATERGlobal Synopsis

Nitrate is the most ubiquitous chemical contaminantin the worlds aquifers and the levels of contaminationare increasing. Increased degradation of drinking waterand eutrophication of coastal waters in the EuropeanCommunity (EC) are consequences of steadily increasingNO3 levels in surface, ground, and coastal waters (Fried,1991). In areas of Belgium where agriculture has ad-versely affected groundwater quality, NO3-N concentra-tions range from ---4 to 11 mg/L. By 1995 more than 10million French, 20% of the population, will drink waterexceeding the EC limit of 11.3 mg/L NO3-N. Eight per-cent of the public waterworks in Denmark and 5% ofthose in the former Federal Republic of Germany supplygroundwater that exceeds the EC limit for NO3-N and inboth countries unprotected aquifers are contaminated withAbbreviations: USEPA, U.S. Environmental Protection Agency;MCL, maximum contaminant level; RFD, reference dose; EC,European Community; WATSTORE, Water Storage and RetrievalSystem; NAWV~S, National Alachlor Well Water Survey; NPS,National Pesticide Survey.

392

SPALDING & EXNER: OCCURRENCE OF NITRATE IN GROUNDWATER 393

NO3 (Fried, 1991). Increasing NO3 levels in the ground-water of the Netherlands eventually will effect 25% ofthe wellfields (Fried, 1991). Nitrate levels in many pri-vate wells in these three countries are unacceptable. Ineastern and central England, NO3 concentrations are in-creasing in the groundwater, which provides 30 to 50%of the drinking water (Chilton and Foster, 1991). If cur-rent inputs of N from agricultural land continue, NO3levels in the groundwater will be double the EC limit.Nitrate in the groundwater of several agricultural areasin southern Ontario, Canada, also exceeds 10 mg/L NO3-N (Gillham, 1991).

Nitrate contamination of groundwater is also a grow-ing problem in the Caribbean, Africa, the Middle East,Australia, and New Zealand. Agriculture has affectedgroundwater quality in Barbados two major catchments(Chilton, 1991). Although average NO3-N concentra-tions in the rural areas remained below the 10 mg/LWorld Health Organization guideline, nowhere in thecatchments were NO3 concentrations indicative of a pris-tine environment. Concentrations in the urban areas wereconsistently above 10 mg/L NO3-N and reflect N loadingfrom high density housing with unsewered sanitation.Faillat (1990) reported 3 contamination of ground-water beneath areas of the Ivory Coast that were de-forested and used for either crop production or settlements.Sewage effluent is an important source of water for ag-riculture in Israel and other semiarid areas with limitedground and surface water. In Israel, NO3 contaminationof shallow groundwater beneath sewage-irrigated landwas attributed to applications of fertilizer and sewageeffluent (Ronen and Magaritz, 1985). Nitrate-N levels one-third of the bores sampled on the Gambier Plain insoutheast South Australia exceeded 10 mg/L (Dillon etal., 1991). Leachates from leguminous pastures grazedby livestock were the source of most of the NO3 in thegroundwater, whereas NO3 in wastes from dairies, sale-yards, and milk and meat processing facilities was re-sponsible for the highest concentrations. In New Zealandunconfined aquifers often are extensively contaminatedby NO3 (Burden, 1982). The contamination is associatedwith intensive grazing on nonirrigated grass-clover pas-ture, grazing on irrigated pasture, and fertilization ofcrop and horticultural land.

Surveys in the United StatesMadison and Brunetts (1985) mapping of NO3 con-

centrations in more than 87 000 wells was the first com-prehensive, nation-wide evaluation of the areal distributionof NO3 in groundwater (Fig. 1). Their database was the25-yr record of NO3 analyses in the U. S. GeologicalSurveys Water Storage and Retrieval System (WATS-TORE). Although the data, collected largely from spe-cial project areas and municipal wells, are not temporally,regionally, or vertically representative (Madison andBrunett, 1985), they are useful for identifying regionswith NO3 problems. Nitrate-N concentrations exceeded3 mg/L, Madison and Brunetts background level forNO3-N in U.S. aquifers, in agricultural areas of Maine,Delaware, Pennsylvania, central Minnesota, Wisconsin,western and northeastern Iowa, the Plains states of Texas,Oklahoma, Kansas, Nebraska, and South Dakota, east-ern Colorado, southeastern Washington, Arizona, andcentral and southern California. Lee and Nielsen (1989)

used Madison and Brunetts data together with infor-mation on aquifer vulnerability and N fertilizer usage todelineate areas where there is a potential for NO3 con-tamination of the groundwater. These refinements led tothe elimination of areas with elevated NO3-N concentra-tions in northern Maine and to the inclusion of areas inOhio, Indiana, and Illinois where WATSTORE data weresparse. The studies of Madison and Brunett (1985) andLee and Nielsen (1989) showed that nationally the pres-ence and the predicted occurrence of NO3 in groundwaterare nonuniform and are skewed toward the central andwestern USA. The 3 mg/L NO3-N background level canbe debated as being too conservative from an environ-mental perspective, because half the samples collectedacross the USA do not have detectable levels of NO3(USEPA, 1990). From a regulatory standpoint, however,it is more practical to delineate areas with higher NO3levels such as those that exceed the MCL.

An objective of both Monsanto Companys (1990) Na-tional Alachlor Well Water Survey (NAWWS), andUSEPAs (1990) National Pesticide Suyrvey (NPS) to estimate with statistical accuracy the proportion of thepopulation served by drinking water wells that exceedthe nitrate MCL. In the NAWWS 1430 domestic ruralwells in 89 counties in 26 states were sampled, whereas783 rural domestic wells in 90 counties in 38 states and566 community water systems in 50 states were sampledin the NPS. There are = 10.5 million rural domestic wellsand ~ 38 300 community water systems in the USA. Inthe NPS, the estimated incidence of contamination, de-fined in this paper as NO3-N levels in excess of the 10mg/L MCL, was lower in the high capacity communitywater systems (1.2%) than in the domestic wells (2.4%)(Fig. 2). This is to be expected. High capacity wellspump groundwater from a larger vertical interval of thesaturated zone than do domestic wells; consequently,concentrations of contaminants, which usually are higherat the top of the aquifer, are diluted. The discrepanciesin the results for the rural domestic wells in the twosurveys (Fig. 2) are related to land use. Because sam-piing sites in the NPS were selected without regard topesticide or land use, inclusion of nonagricultural areassignificantly lowered the number of wells that exceededthe MCL. The NAWWS, on the contrary, was restrictedto regions of alachlor use. Because these are areas thatare intensively cropped to corn (Zea mays L.) and soy-bean [Glycine max (L.) Merr.], N fertilizer, livestockwastes, and mineralized N from legumes and other or-ganic matter are potential sources of NO3 contamination.Consequently, the results of the NAWWS are a "worst-case" estimate of the incidence of NO3-contaminatedwells. From the NPS, USEPA (1992) estimates 4.5 mil-lion people including 66 000 infants under 1 yr of ageare served by community water systems or rural domes-tic wells that exceed the 10 mg/L NO3-N MCL.

In large-scale studies like the NPS, the relatively smallsample population in comparison to the total study areaprovides only minimal insight into the factors affectingthe occurrence and distribution of NO3. Many of theweak associations between NO3 concentrations and landuse and well construction identified in the NPS (USEPA,1992) have been thoroughly documented in smaller-scaleinvestigations. Parallel conclusions reached by the NPSand state and county surveys do, however, demonstrate

394 J. ENVIRON. QUAL., VOL. 22, JULY-SEPTEMBER 1993

Nitrate-nitrogen concentrationWater samples exceeded :5 mg/L in:~ 25 percent or more of sampled wells~ Fewer than 25 percent of sampled wells

Not determined~ Fewer than 5wells per county in data base

Fig. 1. Areal distribution of NOa-N concentrations in groundwater in the contiguous USA (Madison and Brunett, 1985).

that the findings of the latter are substantiated on a na-tional scale.

State-wide surveys of NO3 concentrations in rural wellshave been undertaken in Iowa (Kross et al., 1990), Kan-sas (Steichen et al., 1988), Nebraska (Spalding, 1991),North Carolina (Jennings, 1992, unpublished data), Ohio(Baker et al., 1989), and Texas (TSSWCB, 1991). though sampling sites in the North Carolina, Ohio, andTexas surveys were not selected by a statistically ran-domized design, the sheer number of samples in eachstudy negates most of the statistical bias introduced witha smaller sample. Nine thousand wells were sampled in

the North Carolina survey, 16 166 in the Ohio study,and 55 495 in the Texas survey. Figure 3 summarizesthe incidence of NO3 contamination in these surveys.The highest incidence of contamination occurs ingroundwater in the middle of the contiguous USA whereNO3 levels in =20% or more of the sampled wells inIowa, Nebraska, and Kansas exceed the MCL. In con-trast, the incidence of contamination is lower in Texas(8.2%) and very low in North Carolina (3.2%) and (2.7%).

Known or suspected NO3 contamination prompted in-tensive surveys encompassing several counties in Arkan-

Nitrate-Nitrogen Concentrations

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SPALDING & EXNER: OCCURRENCE OF NITRATE IN GROUNDWATER 395

sas (Arkansas CES, 1990), California (Anton et al., 1988),Delaware (Ritter and Chirnside, 1984), Pennsylvania(Pionke and Gilmeister, 1991), Washington (Ericksonand Norton, 1990), Minnesota (Ruhl, 1987), and SouthDakota (Goodman, 1985). The incidence of contami-nation reported in these studies is shown in Fig. 3. Inthe northeastern USA, NO3 contamination occurred fre-quently in the agricultural areas of Pennsylvania and Del-aware. Ten percent of the sampled wells in Pennsylvanias10 most agriculturally based counties exceeded the MCL(Pionke and Gilmeister, 1991). Excessive leachate fromfield application of animal wastes and fertilizers causedLancaster County to have some of the worst NO3 con-tamination in Pennsylvania. Well-drained soils and thepresence of several NO3 sources cause the groundwaterin recharge areas of the Delmarva Peninsula to be vul-nerable to NO3 contamination (Ritter and Chimside, 1984).The highest incidence of contamination was in the in-tensive broiler producing area of coastal Sussex Countywhere 37% of the wells exceeded the MCL. Leachatesfrom poultry manure appeared to be the major contrib-utor of NO3 to the groundwater in four of the five prob-lem areas. Leachates from commercial fertilizerapplications and septic systems were secondary sourcesof NO3 contamination.

The results from intensive state-wide monitoring andsmaller studies that have used a variety of tracers offeropportunities to understand the prevalence of NO3 con-tamination in the groundwater of some regions of theUSA rather than others. The intensive groundwater NO3surveys recently completed in North Carolina, Ohio, andNebraska, and ongoing groundwater research programsin these states are focused on understanding the man-agement, distribution, and persistence of NO3 in ground-water. Because these states represent three different regionsof the USA, the results of these investigations are stressedin this report. Data from surrounding states will be in-troduced to describe other factors that may be repre-sentative on a regional scale. It is recognized, however,that there are areas within these large regions that do notfit the generalities. Additional information from otherstates can be found in Power and Schepers (1989), Fed-kiw (1991), and Spalding and Exner (1991).

North Carolina and the SoutheastResults from the 3-year North Carolina Statewide Ni-

trate Survey of 9000 wells indicated that NO3 contami-nation is neither a widespread nor a severe problem(Jennings, 1992, unpublished data). Well depth was dominant factor affecting NO3 concentrations. Besidesshallow well depths, substandard well construction andthe siting of wells near potential sources of contamina-tion also were associated with higher NO3 concentra-tions. Although the majority of the wells sampled in thesurvey were from the intensively farmed Piedmont Pla-teau and the Coastal Plain, the incidence of contamina-tion was only 3.2%. The obvious question then is "Whyis the incidence of contamination so low?"

Despite a 400% increase in fertilizer usage in NorthCarolina since 1945, NO3 levels in streams have notchanged (Jacobs and Gilliam, 1985). An analysis of yr of data collected by the Soil Science Department atNorth Carolina State University led Gilliam (1991) conclude that NO3 is not a problem in groundwaterdowngradient from properly fertilized fields. He, how-

40

StatisticallyRandomizedSurveys

30 KS

20 NE

StatewideSurveys

(5)CA

TX

VulnerableCounty

Surveys

(2)DE

Other ISurveys

SD

Fig. 3. Incidence of NO3-N contamination in large selectedsurveys (number in parentheses is number of countiessurveyed).

ever, qualified this statement by warning that N appli-cations above recommended rates have caused documentedincreases in groundwater NO3. Gilliam (1991) also ob-served that NO3 levels of 15 to 20 mg/L NOa-N occurredonly in soil water beneath fertilized cornfields on theCoastal Plain and were never encountered below 4 m.Gilliam and his colleagues have determined that suffi-cient dissolved organic matter percolates from the highorganic matter Coastal Plain soils to the shallow ground-water to provide the electrons for microbial reduction ofNO3 and that denitrification is largely responsible for theloss of most of the NO3 from agricultural land in theCoastal Plain (Daniels et al., 1975; Gambrell et al., 1975;Jacobs and Gilliam, 1985). In highly instrumented stud-ies, reductions in NO3 levels also have been documentedin shallow groundwater beneath riparian forests in Mary-land (Peterjohn and Correll, 1984; Jordan et al., 1993)and Georgia (Lowrance and Pionke, 1989; Lowrance,1992). The reduction dynamics are complex and can re-sult from either vegetative uptake, denitrification, or acombination of the two.

Hubbard and Sheridan (1989) reported that despiteheavy applications of fertilizer, abundant precipitation(=127 cm/yr), and rapid irrigation development of thesandy soils in the Piedmont and Coastal Plain of thesoutheastern USA, there are few instances of aquiferNO3 contamination. In a 10-county pilot study in Ar-kansas, only 3.2% of the 1232 wells sampled had NO3levels above the MCL, whereas 86% of the sampledwells had concentrations less than 3 mg/L NO3-N (Ar-kansas CES, 1990). High NO3 concentrations frequentlywere encountered in areas of intensive poultry and live-stock production. Steele and McCalister (1991) reportedthat even in areas receiving heavy applications of N frompoultry litter, NO3 concentrations in the groundwater av-eraged < 3 mg/L. It appears that the high temperatures,abundant rainfall, and the relatively high organic contentsoils in the Piedmont Plateau and Coastal Plain of thesoutheastern USA promote denitrification below the root

396 J. ENVIRON. QUAL., VOL. 22, JULY-SEPTEMBER 1993

zone and naturally remediate NO3 loading of the ground-water.

Ohio and the Corn BeltThe Corn Belt states of Iowa, Missouri, Illinois, In-

diana, and Ohio have some of the most productive andintensively cropped land in the world. The region is char-acterized by flat, fertile land; hot, humid summers; andample rainfall. The Corn Belt has both the largest pro-portion of cropland (58%) and the largest proportion corn cropland (30%) in the USA (U.S. Dep. of Com-merce, 1989).

Although Lee and Nielsens (1989) vulnerability as-sessment showed that groundwater in Ohio is vulnerableto NO3 contamination from agriculture, concentrationsexceeded the MCL in only 2.7% of the 14 478 domesticwells sampled in Baker et al.s (1989) state-wide surveyand exceeded 3 mg/L NO3-N in only 12.7% of the wells.The average concentration was 1.3 mg/L NO3-N. Giventhe low levels of NO3, Baker et al. (1989) assumed thatconcentrations less than 0.2 mg/L NO3-N representednatural background levels in the groundwater.

Documentation of numerous siting and constructionvariables that could be associated with NO3 contamina-tion in wells was an integral part of Baker et al.s (1989)study. High levels of NO3 occurred more often in olderwells, shallow (< 15 m) wells, and poorly constructedwells than in their counterparts. Poor construction refersto construction that is well below standards deemed ac-ceptable today. Denitrification can be a factor in thisstratification of NO3 with depth. Decreases in NO3 con-centrations with increasing depth also have been reportedin aquifers in Illinois (McKenna et al., 1988; Schock etal., 1992) and Iowa (Hallberg, 1989). The average 3concentration was higher in wells located within 60 mof cropland and feedlots; however, this was not true forwells within 60 m of septic systems. Interestingly, al-though there was an overall increase in NO3 levels inwells sited close to cropland, counties with the mostintense cropping had some of the lowest average NO3levels.

The low incidence of NO3 contamination, the low av-erage concentrations, and the associations between con-centration and land use in Ohio suggest that significantquantities of surface-applied N are not reaching thegroundwater. Logan et al. (1980) showed that leachatesfrom cropped land were intercepted by tile drains anddischarged to surface waters. They measured losses of20 to 100 kg NO3-N/ha per yr from tile-drained fields atnumerous sites in several North-Central states. Theselosses also explain the occurrence of NO3-N in concen-trations above the MCL in the Scioto River at Columbus,OH, during the spring. Power and Schepers (1989) hy-pothesized that the diversion of NO3-contaminated re-charge by tile drains and its subsequent discharge to surfacewater is the major fate of surface-applied N in severalstates in the eastern Corn Belt.

Iowa, the state with the highest N fertilizer use, ap-pears to be a transition zone between relatively minorgroundwater NO3 problems in the Corn Belt states to theeast and extensive NO3 contamination in the ground-water of the Plains states. The results of Iowas statis-tically representative State-Wide Rural Well-Water Survey(Kross et al., 1990) showed that the regional distribution

of NO3 concentrations in excess of the MCL was notuniform and was skewed. North central Iowa had thelowest incidence of contamination (5.6%). Contamina-tion occurred more frequently in south-central Iowa whereNO3 levels exceeded the MCL in 28.1% of the wells,whereas the highest incidents of contamination were inthe glaciated areas of southwest and northwest Iowa where31.4 and 38.2%, respectively, of the wells exceeded theMCL. A major difference between the areas with high(> 25%) and low (< 25%) incidents of contamination related to well depth and well construction. Most (63 73%) of the wells in northwest, southwest, and south-central Iowa are large diameter seepage wells that act assumps for the collection of infiltrate. By design they areshallow wells without a watertight casing; consequently,they are very vulnerable to surface contamination a factalso borne out by the very high incidence (83%) of totalcoliform contamination (Kross et al., 1990). Becauseresults from areas with large numbers of poorly con-structed wells are not representative of aquifer conditionsand tend to overestimate the problem, the contibutionfrom nonpoint sources in these regions is difficult toassess.

Nebraska and the WestThe topography and climate of Nebraska, centered in

the Great Plains, cause agricultural practices to be quitedifferent from those in the Corn Belt and the southeasternUSA. Irrigation and an abundant supply of easily acces-sible groundwater enable Nebraska to irrigate about 2.3million ha of cropland (U.S. Dep. of Commerce, 1989).Only California has more irrigated cropland (= 3.1 mil-lion ha). Nebraska is consistently one of the three leadingcorn-producing states; the others are Iowa and Illinois.

The impact of NO3 from all potential sources appearsmore direct in the groundwater of Iowa, Nebraska, andthe western states. Commercial fertilizer and mineralizedN from both natural accumulations of organic matter andcrop residues are dispersed sources of NO3 that havecontaminated groundwater. Both cultivation and irriga-tion, especially when excessive irrigation occurs on well-drained soils, exacerbate the vertical transport of NO3from these sources. Mineralized natural organic N is asource of NO3 contamination in the Montana saline seeparea (Miller et al., 1981) and a potential contaminant southwestern Nebraska (Boyce et al., 1976). In the SandPlain Aquifer region of Minnesota where 20% of thewells exceeded the MCL (Ruhl, 1987), the incidence contamination in the irrigated cropland area approached50%. Concentrations averaged 17 mg/L NO3-N in theirrigated areas and 5.4 mg/L in the nonirrigated culti-vated areas (Anderson, 1989). The sand plain region central Wisconsin is another area where the groundwateris contaminated by leachates from irrigated agriculture(Saffigna and Keeney, 1977). Power and Schepers (1989)point to the association of irrigation with the large arealNO3-N contaminated areas in the states of Arizona andCalifornia. Historically the groundwater beneath the in-tensely farmed and irrigated basins in central and south-ern California has had high concentrations of NO3 (Ward,1970). In six southern California counties NO3 levels in= 12% of the municipal wells exceeded the MCL (Antonet al., 1988). Primarily as a consequence of exceedingthe MCL, ~-4% of the domestic groundwater supply is

SPALDING & EXNER: OCCURRENCE OF NITRATE IN GROUNDWATER 397

lost annually. In contrast, less than 0.5% of the supplyhas been lost to organic chemical contamination. Fertil-izer leachates from lawns also are a potential threat togroundwater contamination (Exner et al., 1991). Good-man (1985) reported that livestock containment areas andaccidental fertilizer releases were two point sources re-sponsible for much of the NO3 contamination in the BigSioux aquifer in South Dakota. Poor well constructionand improper siting of wells compound the contamina-tion. In Washington, additional point sources of NO3contamination are lagoon seepage and illegal dischargesin dry wells (Spalding et al., 1982). A full treatise sources of NO3 contamination is given in Keeney (1986).

A recent compilation (Exner and Spalding, 1990) NO3 data from 5826 wells sampled in Nebraska between1984 and 1988 indicated that NO3-N concentrations ex-ceeded the MCL in more than 20% of the wells. Slightlymore than half of the wells that exceeded the MCL werein areas highly vulnerable to leaching (Fig. 4). Theseareas are characterized by fence-row to fence-row irri-gated corn grown on well to excessively well-drainedsoils and a vadose zone less than 15 m thick. Becausemuch of the data was from site investigations, it is con-centrated in areas with elevated NO3 concentrations. Themore than 202 000 contiguous hectares underlain by NO3-contaminated groundwater in the central Platte region(Buffalo, Hall, and Merrick counties in Fig. 4) are thelargest areal expanse of NO3-contaminated groundwaterin Nebraska. Nitrate levels in this groundwater have in-creased at rates of 0.4 to 1.0 mg NO3-N/L per yr (Exner,1985). Studies measuring NO3 and pesticide concentra-tions and N stable isotope ratios in groundwater fromboth existing wells (Exner and Spalding, 1976; Gormly

and Spalding, 1979; Spalding et al., 1979, 1980) andupgradient and downgradient nested monitoring wells(Junk et al., 1980; Spalding and Exner, 1980) and in thevadose zone (unpublished data) showed the NO3 contam-ination in these three counties is predominantly from anonpoint source and that the primary source is N fertil-izer applied to cropland. Excessive N fertilization andirrigation exacerbate the contamination (Schepers et al.,1991). Nonpoint NO3 contamination also has been iden-tified in northern Holt County, in eastern Hall County,at Sidney in Cheyenne County, and near Oshkosh inGarden County (Fig. 4). The sources of the contamina-tion in these areas also are agronomic and include ap-plied commercial fertilizer (Exner and Spalding, 1979;Exner, 1990), injected sewage sludge (Spalding et al.,1993), and applied manure (Bryda, 1988).

Between 1985 and 1989 the Nebraska Department ofHealth conducted a study statistically designed to esti-mate the population at risk of ingesting NO3-contami-nated water from rural domestic wells. Nitrateconcentrations in 2195 rural domestic wells from Ne-braskas 93 counties averaged 6.6 mg/L NO3-N and ex-ceeded the MCL in 17.4% of the sampled wells (Spalding,1991). This incidence of contamination was very similarto the --21% obtained by Exner and Spalding (1990) their compilation of nonrandom data. The weighted meanexposure level for the rural population using privatedrinking water wells was 7.5 mg/L NO3-N

, whereas the

weighted frequency of exposure to NO3 levels exceedingthe MCL was 20.4%.

As in the Ohio study (Baker et al., 1989), the levelof NO3 in the well was not associated with the distancebetween the well and septic system (Spalding, 1991).

L

County KeyWell use 1- Buffalo County

Doesticmeo

~~

~ 2- Hall countyIrrigation Stock Areas where the depth to water >- 10% of county area is cropped 3- Merrick County

~, is less than 50 feet and there is to irrigated corn 4- Douglas CountyPublic Supply Monitoring irrigated agriculture 5- Holt County6- Cheyenne County

7- Garden County

Fig. 4. Distribution of NO3-N concentrations greater than 10 mg/L in Nebraskas groundwater (Exner and Spalding, 1990).

398 J. ENVIRON. QUAL., VOL. 22, JULY-SEPTEMBER 1993

Nitrate-NitrogenContinuous

ConcentrationsCasing

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SPALDING & EXNER: OCCURRENCE OF NITRATE IN GROUNDWATER 399

tions and total coliform levels. This area of northeastNebraska is adjacent and directly west of the Iowa area,which had the highest incidence of both poor well con-struction and NO3 contamination in that state (Kross etal., 1990). In northwest Iowa it is difficult to obtainadequate groundwater; consequently, large diameter, open-jointed wells have been constructed to collect and storeperched vadose zone water. In northeast Nebraska, how-ever, the wells generally intersect the water table of theregional groundwater reservoir. The large diameter, open-jointed wells are typical of well construction in areaswith fine-textured vadose zone sediments derived fromloess where caving was not a problem.

The incidence of NO3 contamination was very high inwells with open-jointed construction regardless of theage of the well (Fig. 7). This association also has beenfrequently reported (Steichen et al., 1988; Baker et al.,1989; Kross et al., 1990). Regardless of the water tight-ness of the casing, NO3 as well as total coliform con-tamination were encountered slightly less often in newer(< 20 yr old) wells than in wells more than 40 yr old.Similar associations between well construction, depth,age, and siting and the incidence of NO3 contaminationin studies in Iowa (Kross et al., 1990) and Ohio (Bakeret al., 1989) support the widely held belief that modernwell construction practices provide an effective barrierto surface contamination and can reduce the incidenceof NO3 contamination in domestic rural wells.

A highly suspected source of the NO3 contaminationin the wells in glaciated northeastern Nebraska is manureleachate. Leachate from the dissolution of manure inbarnyards and corrals was the source of most of the NO3contamination in domestic and stock wells in glaciatedsoutheastern Nebraska (Exner et al., 1985). The poten-tial for development of cracks is much higher in thesmectite-containing loess soils and unsaturated zone inglaciated eastern Nebraska than it is to the west. Nitrateand bacteria from the manure probably are transportedin the unsaturated zone by preferential flow in desicca-tion cracks that develop in the fine-textured soils sur-

rounding the well. These cracks can be more than 3 mdeep and extend radially more than 20 m. The fine-textured soils and unsaturated zone of glaciated easternNebraska appear to form a generalized line of demar-cation between this point-source area with wells con-taining very high NO3 concentrations and nonpoint sourceareas to the west.

CONCLUSIONSThe skewed occurrence of large areas of NO3-contam-

inated groundwater in the USA indicates that the ground-water in many intensely agricultural regions is notparticularly vulnerable to NO3 contamination. This dis-tribution pattern is a consequence of both natural andman-induced factors. In the southeastern USA, abundantrainfall, high temperatures, and riparian soils with highorganic C cause relatively rapid uptake of NO3 and/ordenitrification. Denitrification may also occur in the richsoils and subsoils of the Corn Belt but tile drainage ap-pears more important in intercepting the downwardmovement of NO3. The incidence of NO3 contaminationtends to increase in areas west of central Iowa and NO3concentrations are high in many sections of the GreatPlains. Groundwater beneath irrigated, row-cropped areaswith well-drained soils and permeable vadose zones inMinnesota, Washington, Arizona, California, and Ne-braska is most impacted by dispersed-source NO3 con-tamination.

A recurring theme throughout this review was thatsubstandard well construction and improper siting of wellswere associated with anomalously high NO3 levels. Manyof these wells were old dug wells. Samples from thesewells are not representative of aquifer conditions andshould not be used to assess dispersed-source contami-nation. These wells are highly vulnerable to surficial NO3contamination, because cracks in the fine-textured loesssoils radiate from them during dry periods and providepreferential flow paths for contaminant transport to openjoints in their casings.

52.6

Nitrate-Nitrogen ConcentrationsI []

400 J. ENVIRON. QUAL., VOL. 22, JULY-SEPTEMBER 1993

Although agrichemical contamination of groundwateris a growing concern both nationally and internationally,it must be kept in perspective. In most states the majorareas of dispersed source, NO3-contaminated ground-water have been delineated and are small in proportionto the total area of the state. As an example the estimated300 000 ha underlain by nonpoint NO3-contaminatedgroundwater in Nebraska comprise only 1.5% of the state'sarea. Although elevated NO3 concentrations in wells inother areas of Nebraska could result from agrichemicalcontamination, there certainly is a strong case for point-source contamination in the poorly constructed and poorlysited wells in eastern Nebraska and the adjoining area ofwestern Iowa.

The distribution of NO3 in groundwater is controlledby a number of factors. They include source availability,thickness and composition of the vadose zone, precipi-tation, irrigation, vertical flow, aquifer heterogeneity,dissolved oxygen concentrations and electron donoravailability, dispersion, and saturated thickness. Re-search must focus on the dynamics of groundwater NO3.It is suggested that future assessments of the relativeimportance of vegetative uptake, denitrification, andgeohydrology in limiting NO3 contamination in deep andvery shallow groundwater systems include measure-ments of dissolved oxygen, dissolved organic C, Fe,stable N isotopes, and microbial activity in discrete ver-tical intervals. Mariotti et al. (1988), Smith et al. (1991),and Spalding et al. (1993) are examples of such assess-ments.

SPALDING & EXNER: OCCURRENCE OF NITRATE IN GROUNDWATER 401

402 J. ENVIRON. QUAL., VOL. 22, JULY-SEPTEMBER 1993

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