RESEARCH ARTICLE

Characterization of shallow groundwater qualityin the Lower St. Johns River Basin: a case study

Ying Ouyang & Jia-En Zhang & Prem Parajuli

Received: 16 January 2013 /Accepted: 23 May 2013 /Published online: 8 June 2013

Abstract Characterization of groundwater quality allows theevaluation of groundwater pollution and provides informationfor better management of groundwater resources. This studycharacterized the shallow groundwater quality and its spatialand seasonal variations in the Lower St. Johns River Basin,Florida, USA, under agricultural, forest, wastewater, and res-idential land uses using field measurements and two-dimensional kriging analysis. Comparison of the concentra-tions of groundwater quality constituents against the USEPA’swater quality criteria showed that the maximum nitrate/nitrite(NOx) and arsenic (As) concentrations exceeded the EPA’sdrinking water standard limits, while the maximumCl, SO4

2−,andMn concentrations exceeded the EPA’s national secondarydrinking water regulations. In general, high kriging estimatedgroundwater NH4

+ concentrations were found around theagricultural areas, while high kriging estimated groundwaterNOx concentrations were observed in the residential areaswith a high density of septic tank distribution. Our studyfurther revealed that more areas were found with high esti-mated NOx concentrations in summer than in spring. Thisoccurred partially because of more NOx leaching into theshallow groundwater due to the wetter summer and partiallybecause of faster nitrification rate due to the higher tempera-ture in summer. Large extent and high kriging estimated total

phosphorus concentrations were found in the residential areas.Overall, the groundwater Na and Mg concentration distribu-tions were relatively more even in summer than in spring.Higher kriging estimated groundwater As concentrations werefound around the agricultural areas, which exceeded the EPA’sdrinking water standard limit. Very small variations in ground-water dissolved organic carbon concentrations were observedbetween spring and summer. This study demonstrated that theconcentrations of groundwater quality constituents variedfrom location to location, and impacts of land uses on ground-water quality variation were profound.

Keywords Groundwater quality . Kriging . Spatialdistribution . Seasonal variation

Introduction

Groundwater pollution is a growing concern everywhere in theworld. Groundwater quality degradation in an aquifer is aresult of natural conditions and human activities. Natural con-ditions affect water quality in an aquifer by means of rechargeto and discharge from the aquifer, dissolution of minerals, andmixing of fresh groundwater with residential water or intrudedseawater (Canter 1996; Boniol 1996; Ouyang 2012). Humanactivities influence groundwater quality through the vadosezone leaching and ditch seepages of contaminants due toaccidental spill, leakage, and inappropriate application of con-taminants and fertilizers at the land surface; the upcoming ofwater with high dissolved solids from the deep zone due togroundwater withdrawals; and the introduction of irrigationwater from deep aquifers to surficial aquifers (Boniol 1996;Ouyang 2012).

Despite a need to understand groundwater quality statusin the Lower St. Johns River Basin (LSJRB), Florida, and itspotential adverse environmental impacts upon surface waterquality, there are few data sets that have comprehensively

Environ Sci Pollut Res (2013) 20:8860–8870DOI 10.1007/s11356-013-1864-x

# Springer-Verlag Berlin Heidelberg (outside the USA) 2013

Responsible editor: Hailong Wang

Y. Ouyang (*)USDA Forest Service, 100 Stone Blvd, Thompson Hall, Room 309,Mississippi State, MS 39762, USAe-mail: youyang@fs.fed.us

J.<E. ZhangDepartment of Ecology, South China Agricultural University,Wushan Road, Tianhe District, Guangzhou, China

P. ParajuliDepartment of Agricultural and Biological Engineering,Mississippi State University, Mail box 9632, Mississippi State,MS 39762, USA

summarized the shallow groundwater quality status. Boniol(1996) reported that the maximum concentrations of nitratewere 16.1 and 11.1 mg/L, respectively, in the Floridan aqui-fer and surficial aquifer of the LSJRB. In a preliminary study,however, we found that the maximum NOx concentrationcan be up to 43 mg/L in a residential septic tank disposalarea. Although these studies have provided good insightsinto the shallow groundwater N contamination status in theLSJRB, the spatial and seasonal variations of shallowgroundwater nutrients and other constituents and their po-tential adverse environmental impacts upon surface waterquality are still poorly understood. With an increased under-standing of the importance of groundwater resources forhuman consumption, agricultural and industrial uses, andecosystem health, there also is a greater need to evaluategroundwater quality.

The goal of this study was to characterize the shallowgroundwater quality under four major land uses, namely agri-cultural, forest, wastewater, and residential areas in the LSJRB.The specific objectives were to (1) determine the spatial andseasonal variations of the shallow groundwater constituentssuch as nutrients, cations, anions, heavy metals, and redoxpotential using ArcGIS geostatistic package in conjunctionwith field measurements and (2) evaluate the shallow ground-water quality using the EPA’s water quality criteria.

Materials and methods

Study area and sampling

In this study, water quality data collected from the shallowgroundwater system in the north area of the LSJRB duringspring and summer of 2005 and 2007 were used for theanalyses. The LSJRB is located in northeast Florida, between29° and 30° N and between 81.13° and 82.13° W (Fig. 1),with an area of about 7,192 km2. Land uses within the basinlargely consist of residential, commercial, industrial, mining,ranching, row crop, forest, and surface water. A series ofwater quality problems including point and nonpoint sourcepollutants such as nutrients, hydrocarbons, pesticides, andheavy metals (Campbell et al. 1993; Durell et al. 2001) havebeen identified and addressed since the 1950s.

Fifty-nine shallow groundwater wells (Fig. 1) wereinstalled or activated in 2003–2004 for the purpose of moni-toring groundwater quality under agricultural, forest, waste-water, and residential land uses in the LSJRB. Well casingdepths range from 4 to 7 m, which are considered shallowgroundwater wells in Florida. The groundwater samples werecollected seasonally and/or bi-weekly for a 3-year period by acontractor from 2005 to 2007. All sampling activities wereconducted in accordance with the standard operating proce-dures for the collection and analysis of water quality samples

and field data (SJRWMD 2010). These standard operatingprocedures are in compliance with the US EPA’s standardmethods for groundwater sampling and analysis. Statisticalanalysis was performed with SAS 9.0, and all of the experi-mental data were statistically evaluated at α=0.05.

Kriging analysis

Field data provide information on groundwater quality con-stituent concentrations at the specific sampled sites but donot provide the same information on other unsampled loca-tions. Therefore, the spatial and seasonal variations of theconstituents across the entire study area within the LSJRBcannot be thoroughly determined. Since it is difficult andexpensive to perform field measurements for every locationin the LSJRB, the kriging estimate was employed in thisstudy to provide a quantitative estimate of spatial and sea-sonal variations in groundwater quality constituent concen-trations. This information is useful for identifying the loca-tions of the highly contaminated spots in the study area.

Spatial distributions of groundwater quality constitu-ent concentrations in the north area of the LSJRB weredetermined by ordinary kriging estimation using the ArcGISgeostatistical analyst tool. The ordinary kriging is a weighted-linear-average estimator where the weights are chosen tominimize the estimated (kriged) variance. It uses data from asingle data type to predict values of that same data type atunsampled locations. The details for mastering the art ofkriging are published elsewhere (Cooper and Istok 1988;ASCE 1989; Isaaks and Srivastava 1989; Rouhani et al.1996; Goovaerts 1999; Ouyang et al. 2002).

Kriging procedures used in this study include: (1) prelim-inary data analysis, (2) data structural analysis, and (3) krigingestimation. Prior to kriging estimation, descriptive statisticswere performed to examine the groundwater quality datacollected from the LSJRB.Histogram plots of the data showedthat the groundwater quality constituents were somewhatabnormally distributed. In general, a normal distribution re-quirement in kriging analysis may not be so critical unless thedata set is too skewed or contains outliers. If that is the case,some kind of transformation is needed.

A data structural analysis was performed to determine thespatial correlation of the groundwater quality data, includingexperimental variogram, structural variogram model, andcross validation analyses. The experimental variogram isan inverse measure of the two-point covariance functionfor a stationary stochastic process. A variogram map wasconstructed to determine if the spatial correlation structure ofthe groundwater quality data is dependent upon direction.Since the spatial correlated distribution of the water qualitydata did not apparently depend on direction, an isotropicspherical model was selected to fit the experimentalvariograms. The model-fitting procedure was performed

Environ Sci Pollut Res (2013) 20:8860–8870 8861

graphically in order to find a structure that would be as closeas possible to the experimental variogram curves.

Cross validation is a general procedure that checks thecompatibility between a set of data and a structural model.The difference between the measured value and the crossvalidation estimated value is the estimation error, whichgives an indication of how well the data value fits into theneighborhood of the surrounding data values. The crossvalidation standardized errors between −2.5 and 2.5 repre-sent robust data and indicate that a model can correctlypredict the estimated values. The kriging domain used in thisstudy was 30 km×40 km, which encompassed the entirestudy area within the LSJRB.

Results and discussion

General groundwater quality assessment

The shallow groundwater quality in the LSJRB can be char-acterized by chemical constituents and other properties. Thechemical constituents selected for this study include majorcations such as calcium (Ca), magnesium (Mg), manganese(Mn), and sodium (Na); major anions such as chloride (Cl),sulfate (SO4

2−), and carbonate alkalinity (HCO3−+CO3

−);

nutrients such as total nitrogen (TN), ammonium (NH4+),

nitrate (NO3−), nitrite (NO2

−), total phosphorus (TP), andphosphate (PO4

3−); and heavy metals such as arsenic (As)and lead (Pb). Other properties used for this study includedissolved organic carbon (DOC) and redox potential (ROP).

Analytical results show that concentrations of chemicalconstituents and values of other properties varied from loca-tion to location as well as from season to season. Table 1summarizes the descriptive statistics, including the number ofsamples; the minimum, maximum, and mean concentrations;and the standard deviations of the selected chemical constitu-ents and other properties. The US EPA’s water quality criteriaare also given in the table. Comparison of the concentrationsof groundwater quality constituents with EPA’s water qualitycriteria shows that the maximum NOx and As concentrationsexceeded the EPA’s drinking water standard limits, whilethe maximum Cl, SO4

2−, and Mn concentrations exceededthe EPA’s national secondary drinking water regulations(NDWRs). The NDWRs are nonenforceable guidelines regu-lating contaminants that may cause cosmetic effects or aes-thetic effects in drinking water. Table 1 further reveals that themaximum TN and TP concentrations exceeded the ambientwater quality criteria recommendations for rivers and streamsin nutrient ecoregion XII (southeastern area). Discharge of thisgroundwater with high TP and TN concentrations into the

0 30X, Easting (km)

150

20

40Fig. 1 Location of the studyarea in the Lower St. JohnsRiver Basin, Florida, showingthe shallow groundwatermonitoring wells (green circles)and land use codes

8862 Environ Sci Pollut Res (2013) 20:8860–8870

Lower St. Johns River (LSJR) would degrade the ambientwater quality.

Further analysis of the groundwater quality data showsthat there were 14 and 2 wells, respectively, with NOx and Asconcentrations exceeding the EPA’s drinking water limits,whereas there were 3, 4, and 59 wells, respectively, with theSO4

2−, Cl, and Mn concentrations exceeding the EPA’s sec-ondary drinking water regulations. However, it should bepointed out that the groundwater quality data were collectedfrom the shallow groundwater system, which is within 7 m indepth. Most of the Florida residents who use the well wateras a drinking water normally have wells with more than 33 min depth. However, contamination of shallow groundwaterwith these constituents could pose threats to deeper ground-water aquifer through percolation as well as to the adjacentsurface waters through discharge.

This study further reveals that there were 14 and 23 wells,respectively, with TN and TP exceeding the ambient waterquality criteria recommendations for rivers and streams innutrient ecoregion XII (Table 1), which could contaminatethe LSJR due to the groundwater discharge.

Spatial and seasonal variations of nutrients

Figures 2 through 5 show the spatial distributions of ground-water nutrient concentrations (in milligram per kilogram) inspring and summer. In general, higher kriging estimatedconcentrations of NH4

+ were found around the Julington

Creek (agricultural) area (Fig. 2). Within this area, thekriging estimated NH4

+ concentration was higher in summerthan in spring. For example, the estimated concentration ofNH4

+ was about 1.3 mg/L in summer around well HL-WM2,but it was about 1.1 mg/L in spring at the same location. Theformer was about 18 % higher than the latter. Figure 2 furtherreveals that the extent of the estimated NH4

+ with highconcentrations was larger in summer than in spring in thearea around well HL-WM2. Results indicate that seasonalvariations in groundwater NH4

+ concentrations are signifi-cant although the exact reasons remain unknown. A possibleexplanation of this phenomenon would be more NH4

+

leaching from the vadose zone into the shallow groundwaterdue to the wetter summer.

Changes in kriging estimated NOx concentrations in springand summer are shown in Fig. 3. Unlike the case of NH4

+,high estimated NOx concentrations were observed at theStrawberry Creek and Red Bay Branch areas. These werethe residential areas with a high density of septic tank distri-bution. The high concentration and large extent of NOx distri-bution in the areas were presumably attributed to the leakageof NOx from the septic tanks into the shallow groundwater.Under aerobic conditions, NH4

+ can be oxidized into NOx bycertain microorganisms in the soil. This negatively chargedNOx would leach through the vadose zone into the shallowgroundwater. Septic tank system is the most common form ofon-site wastewater management system. It has been reportedthat among all of the groundwater pollution sources, septic

Table 1 Statistical data of groundwater quality constituents in the study area within the LSJRB

Parameter Minimum Maximum Average Number of samples Standard deviation Water quality criteria

TKN-D (mg/L) 0.00 33.74 1.29 366 5.16 0.90a

NH4-D (mg/L) 0.00 35.18 1.00 366 5.10

NOx-D (mg/L) 0.00 43.70 4.92 373 6.63 10.00b

As-D (μg/L) 0.00 44.20 0.83 366 4.92 10.00b

PO4-D (mg/L) 0.00 3.55 0.16 375 0.58

TP-D (mg/L) 0.00 3.63 0.18 366 0.62 0.04a

ROP (mV) −302.00 401.00 149.93 352 148.29

DOC (mg/L) 0.00 124.80 5.92 369 11.98

Sr-D (mg/L) 7.05 7,397.42 678.64 366 1,019.43

pH 3.62 8.68 5.57 524 0.79 6.5–8.5c

Na-D (mg/L) 2.56 418.31 30.87 366 37.62

CL (mg/L) 1.00 1,022.63 61.29 565 114.80 250.00c

Alkalinity (mg/L) 0.50 332.03 66.33 159 75.59

SO4 (mg/L) 0.00 808.25 61.93 555 69.20 250.00c

Ca-D (mg/L) 0.39 281.37 40.48 555 34.83

Mg-D (mg/L) 0.67 145.41 11.02 366 13.09

Mn-D (mg/L) 0.26 732.40 31.22 366 64.17 0.05c

a EPA ambient water quality criteria recommendations for river and streams in nutrient ecoregion XII (EPA 822-B-00-021, December 2000)b EPA drinking water standard limits (http://www.epa.gov/safewater/mcl.html)c EPA national secondary drinking water regulations (http://www.epa.gov/safewater/mcl.html)

Environ Sci Pollut Res (2013) 20:8860–8870 8863

tank systems discharge the greatest total volume of wastewa-ter directly into soils overlaying groundwater and are thesecond largest source of groundwater nutrient contaminationin the USA (Ouyang and Zhang 2012).

Considerable variations in groundwater NOx concentra-tion distribution pattern were also observed between springand summer. It is apparent from Fig. 3 that more areas werefound with high kriging estimated NOx concentrations insummer than in spring. This occurred partially because ofmore NOx leaching into the shallow groundwater due to thewetter summer and partially because of faster nitrificationrate due to the higher temperature in summer.

Kriging estimated TN concentrations showed a similartrend in both spring and summer (Fig. 4). It further appearsthat high estimated TN concentrations were located in theareas near both ends of the Buckman Bridge. This distribution

pattern is similar to that of NH4+ in summer, indicating that

TN and NH4+ may come from similar sources and were most

likely from the chemical fertilizers in these residential areaswith a low density of septic tank distribution.

Figure 5 shows the spatial distribution of groundwater TPconcentrations in spring and summer. In general, large extentand high kriging estimated TP concentrations were foundfrom Cedar River to Trout Creek (residential) areas. It is alsoevident that variations in estimated TP concentrations betweenspring and summer were discernable as shown in Fig. 5.

Spatial and seasonal variations of cations and DOC

Spatial variations of the kriging estimated groundwater Asconcentrations and DOC contents in spring and summer areshown in Figs. 6 and 7. Arsenic is a ubiquitous trace metal

Spring

Summer

0

20

40

0

20

40

0 3015X, Easting (km)

Julington Creek

Julington Creek

HL-MW2

HL-MW2

Fig. 2 Spatial distribution ofgroundwater NH4

concentrations in spring andsummer

8864 Environ Sci Pollut Res (2013) 20:8860–8870

found in environments throughout the world. The majorsources of As pollution are anthropogenic and natural inputs.Anthropogenic sources include mining and smelting of met-alliferous ores, municipal waste, landfill leachates, fertil-izers, pesticides, and sewage (Forstner 1995; Rio et al.2002). Natural sources of As pollution include As-rich par-ent materials as As easily substitutes for Si, Al, or Fe insilicate minerals, volcanic activities, wind-borne soil parti-cles, sea salt sprays, and microbial volatilization (Nriagu1994; Bhumbla and Keefer 1994). Long-term exposure toAs can lead to a variety of skin, neurological and peripheralvascular disorders, and cancers of the skin, bladder, liver,lung, kidney, and colon. In addition, diabetes, ischemic heartdisease, reproductive effects, and impairment of liver func-tion have also been linked to As exposure (Bhumbla andKeefer 1994). Figure 6 shows the spatial distribution of

groundwater As concentrations in spring and summer. It isapparent from this figure that higher groundwater As con-tents were found around the Julington Creek and PetersBranch (agricultural) areas with concentrations exceedingthe EPA’s drinking water standard limit. Although the exactsources of the arsenic contamination in this area remain to beinvestigated, the possible sources would be from chromatedcopper arsenate (CCA), geologic sources (phosphate rockand limestone mining), and arsenic herbicide (monosodiummethylarsonate). Solo-Gabriele et al. (2003) performed acomprehensive investigation on arsenic sources within thestate of Florida. These authors found that among the arseniccontamination sources, about 70 % is associated with theproduction of CCA-treated wood, 20 % is associated withgeologic sources, and 4 % was associated with the arsenicalherbicide monosodium methylarsonate. CCA is a chemical

Spring

Summer

0

20

40

0

20

40

00

3015X, Easting (km)

Red Bay Branch Strawberry Creek

Red Bay Branch Strawberry Creek

Fig. 3 Spatial distributionof groundwater NOx

concentrations in spring andsummer

Environ Sci Pollut Res (2013) 20:8860–8870 8865

used for wood preservative treatment in the state of Floridaand has been accumulated in the surface reservoirs due to thein-service use of the wood product and due to the disposalof CCA-treated wood (Solo-Gabriele et al. 1998). TheJulington Creek area has been used for the CCA-treatedwood transportation. In addition to the application of arsen-ical herbicide for agricultural practices, arsenic trioxide hasbeen used to eradicate the cattle tick which carried a microbe(Boophilus annulatus) that caused cattle fever, an illnesswhich resulted in weight loss, reduced milk production,and weakness among cattle. These herbicide and pesticidecould be the sources for Peters Branch. While the shallowgroundwater is not used as the drinking water by the localresidents, further study is warranted to identify the sourcesof As in the areas. The spatial distribution of groundwater

As concentrations was relatively more even in summer thanin spring.

Naturally occurring DOC is an important feature ofstream water quality. It contributes significantly to the acid-ity of natural waters through organic acids, biological activ-ities through the absorption of light, and water chemistrythrough the complexation of metals and production of carci-nogenic compounds with chlorine. In addition, by formingorganic complexes, DOC can influence nutrient availabilityand control the solubility and toxicity of contaminants. DOCcan also increase the weathering rate of minerals and in-crease the solubility and thus the mobility and transport ofmany metals and organic contaminants (Ouyang 2003).Figure 7 shows the spatial distributions of groundwaterDOC concentrations in spring and summer. In general, very

Spring

Summer

0

20

40

0

20

40

00

3015X, Easting (km)

BuckmanBridge

BuckmanBridge

Fig. 4 Spatial distributionof groundwater TKNconcentrations in spring andsummer

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small variations in groundwater DOC concentrations wereobserved between spring and summer. The lower DOC con-centrations were found around the Browns Creek area,whereas the high DOC concentrations were observed aroundthe Doctors Lake area (more trees coverage).

In general, the groundwater Na concentration distributionwas relatively more even in summer than in spring (figurenot shown). We attributed this discrepancy to the dilutioneffects of rainwater during the wetter summer. Differences ingroundwater Mg concentrations developed between springand summer (figure not shown). In other words, the spatialdistribution of groundwater Mg concentrations was relative-ly more even in summer than in spring. There was a largespot with a higher Mg concentration in the south of well RV-MW3 in spring as compared to that in summer. A wetter

summer could explain the phenomenon. As more rainwaterleached into the groundwater, more Mg was diluted andresulted in more even distribution of Mg.

Conclusion

Field data analysis shows that concentrations of groundwaterquality constituents varied from location to location as well asfrom season to season. Comparison of the concentrations ofgroundwater quality constituents with EPA’s water qualitycriteria shows that the maximum nitrate/nitrite (NOx) and Asconcentrations exceeded the EPA’s drinking water standardlimits, while the maximum Cl, SO4

2−, and Mn concentrationsexceeded the EPA’s NDWRs).

Spring

Summer

0

20

40

0

20

40

00

3015X, Easting (km)

Cedar River

Trout Creek

Trout Creek

Cedar River

Fig. 5 Spatial distribution ofgroundwater TP concentrationsin spring and summer

Environ Sci Pollut Res (2013) 20:8860–8870 8867

It is also apparent that the maximum TN and TP concen-trations exceeded the ambient water quality criteria recom-mendations for rivers and streams in nutrient ecoregion XII(southeastern area). Discharge of this groundwater with highTP and TN concentrations into the LSJR would degrade theambient water quality.

Further analysis of the groundwater quality data revealsthat there were 14 and 2 wells, respectively, with NOx and Asconcentrations exceeding the EPA’s drinking water limits,whereas there were 3, 4, and 59 wells, respectively, withSO4

2−, Cl, and Mn concentrations exceeding the EPA’s sec-ondary drinking water regulations. Furthermore, there were14 and 23 wells, respectively, with TN and TP concentra-tions exceeding the ambient water quality criteria recommen-dations for rivers and streams in nutrient ecoregion XII, which

would potentially contaminate the LSJR due to the ground-water discharge.

In general, high kriging estimated groundwater NH4+

concentrations were found around the Julington Creek agri-cultural area. Within this area, the estimated NH4

+ concen-trations were higher in summer than in spring. Results indi-cate that seasonal variations in groundwater NH4

+ concen-trations were significant although the exact reasons remainunknown. A possible explanation of this phenomenon wouldbe more NH4

+ leaching from the vadose zone into the shal-low groundwater due to the wetter summer.

Unlike the case of NH4+, high kriging estimated groundwater

NOx concentrations were observed at the Strawberry Creek andRed Bay Branch areas. These were the residential areas with ahigh density of septic tank distribution. The high concentration

Spring

Summer

0

20

40

0

20

40

00

3015X, Easting (km)

Julington Creek

Julington Creek

Peter Branch

Peter Branch

Fig. 6 Spatial distributionof groundwater arsenicconcentrations in spring andsummer

8868 Environ Sci Pollut Res (2013) 20:8860–8870

and large extent of NOx distribution in the areas were presum-ably attributed to the leakage of NOx from the septic tanks intothe shallow groundwater. It has been reported that among all ofthe groundwater pollution sources, septic tank systems dischargethe greatest total volume of wastewater directly into soils over-laying groundwater and are the second largest source of ground-water nutrient contamination in the USA.

This study further reveals that more areas were found withhigh estimated NOx concentrations in summer than in spring.This occurred partially because of more NOx leaching into theshallow groundwater due to the wetter summer and partiallybecause of faster nitrification rate due to the higher tempera-ture in summer.

It appears that high estimated TN concentrations were lo-cated in the areas near both ends of the Buckman Bridge. Thisdistribution pattern was similar to that of NH4

+ in summer,

indicating that TN and NH4+ could come from similar sources

and were most likely from the chemical fertilizers in theseresidential areas with a low density of septic tank distribution.

Large extent and high kriging estimated TP concentrationswere found from the Cedar River to the Trout Creek areas. It isalso evident that variations in kriging estimated TP concen-trations were discernable between spring and summer.

Higher kriging estimated groundwater As concentrationswere found around the Julington Creek and Peters Branchareas, which exceeded the EPA’s drinking water standardlimit. Although the shallow groundwater is not used asdrinking water by the local residents, further study iswarranted to identify the sources of As and potential migra-tion of As from shallow groundwater into the deep aquifer inthe areas. Very small variations in groundwater DOC con-centrations were observed between spring and summer.

Spring

Summer

0

20

40

0

20

40

00

3015X, Easting (km)

Doctors Lake

Browns Creek

Doctors Lake

Browns Creek

Fig. 7 Spatial distributionof groundwater DOCconcentrations in spring andsummer

Environ Sci Pollut Res (2013) 20:8860–8870 8869

It should be pointed out that in Florida, shallow groundwateris not recommended as drinking water. Our findings (e.g.,shallow groundwater is contaminated by nutrients and heavymetals) further strengthen this recommendation. Contaminationof shallow groundwater with such pollutants could pose threatsto the deeper groundwater aquifer through percolation as wellas to the adjacent surface waters through discharge.

Further study is warranted to estimate the discharges of theshallow groundwater quality constituents (with concentrationsexceeding the EPA’s water quality criteria) into the LSJR andtheir potential adverse impacts upon the river water quality.

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