ABSTRACT: Using a geographic information system and statistics, we evaluated spatial distributions of nitrate and chloride concentrations in groundwater in an area of north-central Texas with agricultural activity, in addition to oil and natural gas exploration and production. Data were compiled from 40 water wells sampled in 2007. Nitrate concentrations in three wells exceeded the maximum contaminant level (44 mg/L) for drinking water. The highest nitrate concentration was 149 mg/L, and concentrations were generally higher in shallower wells. Chloride concentrations exceeded the 250 mg/L secondary drinking water standard in two wells, with no significant association between chloride concentration and well depth. Results of this study suggest localized human impacts, especially for nitrate, and identify areas warranting future monitoring

P F Hudak

CiteSeerX

Int. J. Environ. Res., 6(3):663-668, Summer 2012
ISSN: 1735-6865
Received 22 Aug. 2011;                 Revised 20 Nov. 2011;                Accepted 27 Nov. 2011
*Corresponding author E-mail:hudak@unt.edu
663
Nitrate and Chloride Concentrations in Groundwater beneath a Portion of the
Trinity Group Outcrop Zone, Texas
Hudak, P. F.
Department of Geography and Environmental Science Program, University of North Texas, 1155
Union Circle #305279, Denton, TX 76203-5017, USA
ABSTRACT: Using a geographic information system and statistics, we evaluated spatial distributions of
nitrate and chloride concentrations in groundwater in an area of north-central Texas with agricultural activity,
in addition to oil and natural gas exploration and production. Data were compiled from 40 water wells sampled
in 2007. Nitrate concentrations in three wells exceeded the maximum contaminant level (44 mg/L) for drinking
water. The highest nitrate concentration was 149 mg/L, and concentrations were generally higher in shallower
wells. Chloride concentrations exceeded the 250 mg/L secondary drinking water standard in two wells, with no
significant association between chloride concentration and well depth. Results of this study suggest localized
human impacts, especially for nitrate, and identify areas warranting future monitoring.
Key words: Nitrate,Chloride, Groundwater,Trinity Aquifer,Texas
INTRODUCTION
A highly soluble complex ion, nitrate is a common
contaminant in groundwater worldwide (Spalding and
Exner, 1993). Exposure to high nitrate concentrations in
drinking water over long time periods may cause colon
and rectum cancers, methemoglobinemia in infants, and
non-Hodgkin’s lymphoma (Ward et al. , 1996;
Knobeloch et al., 2000; De Roos et al., 2003; Coetzee et
al., 2011). The standard, or maximum contaminant level
(MCL), for nitrate in drinking water is 44 mg/L (EPA
2009). Lots of studies are run regarding the groundwater
quality degradation through discharge of different
pollutants (Eneke et al., 2011; Belkhiri et al., 2011; Hudak,
2011;, Martinez-Paz and Perni, 2011; Romanelli et al.,
2011). There are many potential sources of nitrate in
groundwater, including fertilizers, septic tank effluent,
municipal sewage, animal feedlots, decaying
vegetation, and atmospheric deposition (Wilhelm et al.,
1996). In aerated soils, oxygen transforms nitrogen to
nitrate, which may percolate to groundwater. Annually,
U.S. farmers apply nearly 11 million metric tons of
nitrogen-bearing fertilizer to cropland and pastures
(Nolan et al., 1997); harvested crops remove only 40%
of that amount, leaving an excess in the environment
(Power, 1987). Animal manure contains another 6 million
metric tons of nitrogen, some of which is lost by
volatilization during storage and handling (Puckett,
1994). Other potential agricultural sources of nitrate
include organic nitrogen in plant detritus, bacterial
biomass, and soil constituents. For example, plowing
native vegetation releases nitrogen-bearing plant
detritus and aerates the soil, thus promoting nitrate
formation. Nonagricultural sources of nitrate include
septic systems, yard fertilizer, discharges containing
nitrogen-bearing effluent, and atmospheric deposition.
Upon reaching the saturated zone, nitrate tends to
stay in solution, rather than adsorb or precipitate onto
aquifer solids (Hem, 1985). Most public and private
water treatment systems do not remove nitrate from
well water.  Determining sources of nitrate in
groundwater can be difficult; sometimes, associations
between nitrate concentrations and other parameters,
such as well depth and chloride concentrations, may
be useful for this purpose. Inverse associations
between nitrate concentration and well depth suggest
a shallow source, and direct associations between
nitrate and chloride concentrations may indicate a
common source.  A soluble constituent of various
minerals, chloride resides in virtually all freshwater,
usually at low concentrations (Hem, 1985). Chloride
compounds may accumulate in cropland from
evaporated irrigation water and fertilizer. In areas with
oil and gas production, brine may also be a source of
664
Hudak, P. F.
chloride in surface soils or groundwater. Texas oilfield
brine averages 50,000 mg/L chloride (TWC, 1989).
Historically, oilfield brine produced in the U.S. was
discharged to surface excavations, natural depressions,
or gullies, and sprayed onto dirt roads. Presently, most
oilfield brine is injected into deep disposal wells;
however, both present and historical brine disposal
practices may contaminate groundwater (TDA, 1985).
High chloride concentrations may adversely impact
drinking water or irrigation water. The U.S. secondary
drinking water standard for chloride is 250 mg/L (EPA,
2009). In irrigation water, chloride concentrations
above 140 mg/L may damage sensitive crops; values
exceeding 360 mg/L may cause severe problems (Bower,
1978). Previous investigations suggested regions
within Texas susceptible to high nitrate concentrations
in groundwater (TWC, 1989; Hudak, 2000). This article
documents the occurrence and possible sources of
nitrate and chloride concentrations in groundwater
beneath agricultural land overlying a shallow aquifer
and deeper oil and gas reservoirs in north-central Texas,
USA (Fig. 1).
MATERIALS & METHODS
The study area comprises five counties—Brown,
Comanche, Eastland, Erath, and Mills—in a rural part
of north-central Texas (Fig. 1, Table 1). Throughout
this area, the Cretaceous Trinity Group provides
groundwater for several purposes. The aquifer is
unconfined in this area and vulnerable to
contamination from sources originating at or near the
land surface.
Rainfall, irrigation, and influent surface water
recharge the Trinity Aquifer. On average, the study
area receives approximately 76 cm of precipitation per
year; approximately 2.5 cm recharges the aquifer (Baker,
1990). Average depth to groundwater in the study area
is about 15 m, and groundwater flows generally
southeastward. Water-bearing sand formations of the
Trinity Group outcrop within the study area and dip
underground to the east, where the aquifer becomes
confined (Baker, 1990). From bottom to top, the Trinity
Table 1. Population and Land Use
 
County 
Total Oil Well 
Counts 
Total Gas 
Well Counts 
 
Population  
Percent Land in  
Farms 
Brown 1,237 701 38,088 93 
Comanche 121 244 13,559 97 
Eastland 1,418 1,177 18,167 88 
Erath 28 515 36,184 90 
Mills 1 3 4,994 99 
 Sources: Railroad Commission of Texas; U.S. Census Bureau
Group includes the Twin Mountains, Glen Rose, and
Paluxy Formations. The Twin Mountains Formation
consists of sand, shale, clay, and a basal gravel and
conglomerate (Peckham et al., 1963). A confining unit,
the Glen Rose Formation includes limestone, marl,
shale, and anhydrite. The Paluxy Formation consists
mainly of sand and shale. Thickness of water-bearing
sand zones within the Trinity Group ranges up to
approximately 15 m (Ulery and Brown, 1995). Farms
account for over 90% of the study area, with pasture
and cropland the predominant land cover (Table 1).
Forage (hay), pecans, wheat, and sorghum are major
crops grown in the study area. Farmers also raise
various livestock, including cattle, goats, quail, and
sheep. Patches of remnant oak forest, suited to well-
drained soils, also occupy portions of the study area.
Currently, there are more than 5,000 oil and gas wells
(active, inactive, and injection) tapping Paleozoic
reservoirs beneath the study area (Table 1); mainly
these wells occupy the northwest half of the area. Well
depth, nitrate, and chloride concentrations were
compiled for 40 water wells in the Groundwater
Database of the Texas Water Development Board.
Water samples were collected in 2007. Sampled wells
served various uses, but primarily domestic (27 wells)
and public (7 wells) water supply. Other wells in the
dataset were used for irrigation (3 wells) and stock (2
wells), and one well was unused.
Depths of sampled wells ranged from 44 ft (13 m)
to 460 ft (140 m), with a median depth of 160 ft (49 m)
(Table 2, Fig. 2). Sampling and analysis followed
standard procedures described in TWDB (2003). After
measuring water depth, wells were pumped until
temperature, conductivity, and pH stabilized. Samples
were taken directly from each well, filtered, preserved,
and delivered to an analytical laboratory. Analyses
were completed using automated colorimetry or ion
chromatography.
MINITAB (Minitab, State College, Pennsylvania)
was used to compute descriptive statistics and rank
correlations among well depth and solute
concentrations. Rank correlations were computed
Int. J. Environ. Res., 6(3):663-668, Summer 2012
665
Fig. 1. Five-county study area and sampled wells (x)
Fig. 2. Well depths; from smallest to largest, circles represent 44-100, 101-200, 201-300, and
301-460 ft; 1 ft = 0.31 m
666
Nitrate and Chloride Concentrations in Groundwater
because the data were non-normally distributed. Solute
concentration and well depth categories were mapped
with the ArcView (Environmental Systems Research
Institute, Redlands, California) geographic information
system.
RESULTS & DISCUSSION
`Observed nitrate concentrations ranged from non-
detected to 149 mg/L (Table 2, Fig. 3). Twenty-six of 40
samples had detectable nitrate levels; however, the
median concentration was only 2 mg/L (Table 2). Eleven
of 40 nitrate observations exceeded 10 mg/L, but only
three of those observations also exceeded the MCL of
44 mg/L.
Table 2. Summary of Solute Concentrations and Well Depths*
Parameter M inimum Median Maximum 
Nitrate (mg/L) <0.44 2 149 
Chloride (mg/L) 2 36 595 
Well depth (ft) 44 160 460 
 *1 ft = 0.31 m
Fig. 3. Nitrate concentrations; from smallest to largest, circles represent <0.44, 0.44-10, 11-44,
and 45-149 mg/L
A statistically significant, inverse association
between nitrate concentration and well depth (Table
3) is consistent with nitrate originating from
agricultural sources at or near the land surface. From a
planning perspective, an inverse association between
nitrate concentration and well depth suggests that
deeper wells may reduce the threat of pollution in
vulnerable parts of the study area. Agricultural
practices, especially fertilizer applications, probably
account for nitrate observations in the highest mapped
categories.
Only two chloride observations surpassed the
secondary MCL of 250 mg/L. The highest chloride
Table 3. Rank Correlations
 Depth  Chlor ide  
Chlor ide -0.159  
Nitrate -0.626* 0.265 
 *Statistically significant at significance level of 0.01
667
Int. J. Environ. Res., 6(3):663-668, Summer 2012
concentrations were in relatively shallow wells, less
than 100 ft (31 m) deep; these wells may have been
impacted by a contaminant source near the land
surface, for example, oil and gas activity or irrigation
return flow. However, over the entire study area, there
was no significant association between chloride
concentration and well depth. While natural
constituents of the Trinity Group likely exert a major
control on observed chloride concentrations,
anomalous concentrations (above 250 mg/L), and a
tendency for higher chloride concentrations in the
northwestern part of the study area (with much heavier
oil and gas activity), suggest possible human impacts.
Figs 3 and 4 show locations within the study area with
relatively high nitrate and chloride concentrations,
which have the potential to produce high
concentrations in present and future water wells.
Groundwater in these areas especially should be
periodically monitored to determine whether nitrate,
chloride, and other solute concentrations are suitable
for drinking, irrigation, or other intended uses. Both
concentration levels and water use dictate appropriate
filtration measures; for example, reverse osmosis could
remove nitrate, chloride, and other solutes from
drinking water, though this process is relatively
expensive. Less costly options may include treating
only drinking water, or consuming bottled water.
Nitrate observations in this study are well within
the range reported in other parts of the world. For
example, Kolpin et al. (1994) reported nitrate
concentrations above 45 mg/L in six percent of 303
wells tapping shallow aquifers (less than 15 m deep) in
Fig. 4. Chloride concentrations; from smallest to largest, circles represent 2-20, 21-100, 101-250, and
251-595 mg/L
corn- and soybean-producing areas of the midwestern
United States. Moreover, Strebel et al. (1989) and Fried
(1991) reviewed nitrate pollution of groundwater in
Europe; highest nitrate concentrations, above 50 mg/
L, occurred in sandy soils beneath intensely managed
crops and grazed grassland. Furthermore, Zhang et al.
(1996) reported that more than 50% of sampled water
wells in a fertilized region of northern China had nitrate
levels above 50 mg/L. Very high concentrations, up to
300 mg/L, were found in groundwater beneath
vegetable-producing areas, small cities and towns, and
farmers’ yards. Chloride concentrations observed in
this study are low relative to those observed in some
areas with intensive agriculture and oil and gas
production. For example, Hudak (2003) compiled
samples from 198 water wells in the Edwards-Trinity
Plateau Aquifer of west-central Texas; 49 samples had
chloride concentrations above 250 mg/L, 22 samples
had greater than 500 mg/L chloride, and nine samples
exceeded 1,000 mg/L chloride.
CONCLUSION
In conclusion, out of 40 samples, only three nitrate
observations and two chloride observations exceeded
the respective drinking water standards. Shallower
wells within the study area tend to be more vulnerable
to nitrate contamination. A few (relatively shallow) wells
produced samples with high chloride concentrations;
however, there was no significant association between
chloride concentration and well depth across the study
area. Continued monitoring of locations experiencing
elevated nitrate and chloride concentrations within the
668
Hudak, P. F.
study area would help identify contaminant sources,
assess impacts to groundwater, and facilitate
appropriate source control measures.
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Evaluation of water resources in part of north-central Texas.
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De Roos, A. J., Ward, M. H., Lynch, C. F. and Cantor, K. P.
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EPA, (2009) United States Environmental Protection Agency,
National primary drinking water regulations. United States
Environmental Protection Agency, Washington, D.C.
Fried, J. J. (1991). Nitrates and their control in the EEC
aquatic environment. p. 3-11. In I. Bogardi and R. D. Kuzelka
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Hudak, P. F. (2003). Oil production and groundwater quality
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Outcrop Zones along the Brazos River, East-Central Texas.
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H. (2000). Blue babies and nitrate-contaminated well water.
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(1997). Risk of nitrate in groundwaters of the United States
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Peckham, R. C., Souders, V. L., Dillard, J. W. and Baker, B.
B. (1963). Reconnaissance investigation of the ground-water
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Nitrate pollution of groundwater in western Europe.
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Nitrate and Chloride Concentrations in Groundwater beneath a Portion of the Trinity Group Outcrop Zone, Texas

This article discusses a study in which the authors evaluated spatial distributions of nitrate and chloride concentrations in groundwater in an area of north-central Texas with agricultural activity, in addition to oil and natural gas exploration and activity

Hudak, Paul F.

UNT Digital Library

Int. J. Environ. Res., 6(3):663-668, Summer 2012ISSN: 1735-6865Received 22 Aug. 2011;                 Revised 20 Nov. 2011;                Accepted 27 Nov. 2011*Corresponding author E-mail:hudak@unt.edu663Nitrate and Chloride Concentrations in Groundwater beneath a Portion of theTrinity Group Outcrop Zone, TexasHudak, P. F.Department of Geography and Environmental Science Program, University of North Texas, 1155Union Circle #305279, Denton, TX 76203-5017, USAABSTRACT: Using a geographic information system and statistics, we evaluated spatial distributions ofnitrate and chloride concentrations in groundwater in an area of north-central Texas with agricultural activity,in addition to oil and natural gas exploration and production. Data were compiled from 40 water wells sampledin 2007. Nitrate concentrations in three wells exceeded the maximum contaminant level (44 mg/L) for drinkingwater. The highest nitrate concentration was 149 mg/L, and concentrations were generally higher in shallowerwells. Chloride concentrations exceeded the 250 mg/L secondary drinking water standard in two wells, with nosignificant association between chloride concentration and well depth. Results of this study suggest localizedhuman impacts, especially for nitrate, and identify areas warranting future monitoring.Key words: Nitrate,Chloride, Groundwater,Trinity Aquifer,TexasINTRODUCTIONA highly soluble complex ion, nitrate is a commoncontaminant in groundwater worldwide (Spalding andExner, 1993). Exposure to high nitrate concentrations indrinking water over long time periods may cause colonand rectum cancers, methemoglobinemia in infants, andnon-Hodgkin’s lymphoma (Ward et al. , 1996;Knobeloch et al., 2000; De Roos et al., 2003; Coetzee etal., 2011). The standard, or maximum contaminant level(MCL), for nitrate in drinking water is 44 mg/L (EPA2009). Lots of studies are run regarding the groundwaterquality degradation through discharge of differentpollutants (Eneke et al., 2011; Belkhiri et al., 2011; Hudak,2011;, Martinez-Paz and Perni, 2011; Romanelli et al.,2011). There are many potential sources of nitrate ingroundwater, including fertilizers, septic tank effluent,municipal sewage, animal feedlots, decayingvegetation, and atmospheric deposition (Wilhelm et al.,1996). In aerated soils, oxygen transforms nitrogen tonitrate, which may percolate to groundwater. Annually,U.S. farmers apply nearly 11 million metric tons ofnitrogen-bearing fertilizer to cropland and pastures(Nolan et al., 1997); harvested crops remove only 40%of that amount, leaving an excess in the environment(Power, 1987). Animal manure contains another 6 millionmetric tons of nitrogen, some of which is lost byvolatilization during storage and handling (Puckett,1994). Other potential agricultural sources of nitrateinclude organic nitrogen in plant detritus, bacterialbiomass, and soil constituents. For example, plowingnative vegetation releases nitrogen-bearing plantdetritus and aerates the soil, thus promoting nitrateformation. Nonagricultural sources of nitrate includeseptic systems, yard fertilizer, discharges containingnitrogen-bearing effluent, and atmospheric deposition.Upon reaching the saturated zone, nitrate tends tostay in solution, rather than adsorb or precipitate ontoaquifer solids (Hem, 1985). Most public and privatewater treatment systems do not remove nitrate fromwell water.  Determining sources of nitrate ingroundwater can be difficult; sometimes, associationsbetween nitrate concentrations and other parameters,such as well depth and chloride concentrations, maybe useful for this purpose. Inverse associationsbetween nitrate concentration and well depth suggesta shallow source, and direct associations betweennitrate and chloride concentrations may indicate acommon source.  A soluble constituent of variousminerals, chloride resides in virtually all freshwater,usually at low concentrations (Hem, 1985). Chloridecompounds may accumulate in cropland fromevaporated irrigation water and fertilizer. In areas withoil and gas production, brine may also be a source of664Hudak, P. F.chloride in surface soils or groundwater. Texas oilfieldbrine averages 50,000 mg/L chloride (TWC, 1989).Historically, oilfield brine produced in the U.S. wasdischarged to surface excavations, natural depressions,or gullies, and sprayed onto dirt roads. Presently, mostoilfield brine is injected into deep disposal wells;however, both present and historical brine disposalpractices may contaminate groundwater (TDA, 1985).High chloride concentrations may adversely impactdrinking water or irrigation water. The U.S. secondarydrinking water standard for chloride is 250 mg/L (EPA,2009). In irrigation water, chloride concentrationsabove 140 mg/L may damage sensitive crops; valuesexceeding 360 mg/L may cause severe problems (Bower,1978). Previous investigations suggested regionswithin Texas susceptible to high nitrate concentrationsin groundwater (TWC, 1989; Hudak, 2000). This articledocuments the occurrence and possible sources ofnitrate and chloride concentrations in groundwaterbeneath agricultural land overlying a shallow aquiferand deeper oil and gas reservoirs in north-central Texas,USA (Fig. 1).MATERIALS & METHODSThe study area comprises five counties—Brown,Comanche, Eastland, Erath, and Mills—in a rural partof north-central Texas (Fig. 1, Table 1). Throughoutthis area, the Cretaceous Trinity Group providesgroundwater for several purposes. The aquifer isunconfined in this area and vulnerable tocontamination from sources originating at or near theland surface.Rainfall, irrigation, and influent surface waterrecharge the Trinity Aquifer. On average, the studyarea receives approximately 76 cm of precipitation peryear; approximately 2.5 cm recharges the aquifer (Baker,1990). Average depth to groundwater in the study areais about 15 m, and groundwater flows generallysoutheastward. Water-bearing sand formations of theTrinity Group outcrop within the study area and dipunderground to the east, where the aquifer becomesconfined (Baker, 1990). From bottom to top, the TrinityTable 1. Population and Land Use County Total Oil Well Counts Total Gas Well Counts  Population  Percent Land in  Farms Brown 1,237 701 38,088 93 Comanche 121 244 13,559 97 Eastland 1,418 1,177 18,167 88 Erath 28 515 36,184 90 Mills 1 3 4,994 99  Sources: Railroad Commission of Texas; U.S. Census BureauGroup includes the Twin Mountains, Glen Rose, andPaluxy Formations. The Twin Mountains Formationconsists of sand, shale, clay, and a basal gravel andconglomerate (Peckham et al., 1963). A confining unit,the Glen Rose Formation includes limestone, marl,shale, and anhydrite. The Paluxy Formation consistsmainly of sand and shale. Thickness of water-bearingsand zones within the Trinity Group ranges up toapproximately 15 m (Ulery and Brown, 1995). Farmsaccount for over 90% of the study area, with pastureand cropland the predominant land cover (Table 1).Forage (hay), pecans, wheat, and sorghum are majorcrops grown in the study area. Farmers also raisevarious livestock, including cattle, goats, quail, andsheep. Patches of remnant oak forest, suited to well-drained soils, also occupy portions of the study area.Currently, there are more than 5,000 oil and gas wells(active, inactive, and injection) tapping Paleozoicreservoirs beneath the study area (Table 1); mainlythese wells occupy the northwest half of the area. Welldepth, nitrate, and chloride concentrations werecompiled for 40 water wells in the GroundwaterDatabase of the Texas Water Development Board.Water samples were collected in 2007. Sampled wellsserved various uses, but primarily domestic (27 wells)and public (7 wells) water supply. Other wells in thedataset were used for irrigation (3 wells) and stock (2wells), and one well was unused.Depths of sampled wells ranged from 44 ft (13 m)to 460 ft (140 m), with a median depth of 160 ft (49 m)(Table 2, Fig. 2). Sampling and analysis followedstandard procedures described in TWDB (2003). Aftermeasuring water depth, wells were pumped untiltemperature, conductivity, and pH stabilized. Sampleswere taken directly from each well, filtered, preserved,and delivered to an analytical laboratory. Analyseswere completed using automated colorimetry or ionchromatography.MINITAB (Minitab, State College, Pennsylvania)was used to compute descriptive statistics and rankcorrelations among well depth and soluteconcentrations. Rank correlations were computedInt. J. Environ. Res., 6(3):663-668, Summer 2012665Fig. 1. Five-county study area and sampled wells (x)Fig. 2. Well depths; from smallest to largest, circles represent 44-100, 101-200, 201-300, and301-460 ft; 1 ft = 0.31 m666Nitrate and Chloride Concentrations in Groundwaterbecause the data were non-normally distributed. Soluteconcentration and well depth categories were mappedwith the ArcView (Environmental Systems ResearchInstitute, Redlands, California) geographic informationsystem.RESULTS & DISCUSSION`Observed nitrate concentrations ranged from non-detected to 149 mg/L (Table 2, Fig. 3). Twenty-six of 40samples had detectable nitrate levels; however, themedian concentration was only 2 mg/L (Table 2). Elevenof 40 nitrate observations exceeded 10 mg/L, but onlythree of those observations also exceeded the MCL of44 mg/L.Table 2. Summary of Solute Concentrations and Well Depths*Parameter M inimum Median Maximum Nitrate (mg/L) <0.44 2 149 Chloride (mg/L) 2 36 595 Well depth (ft) 44 160 460  *1 ft = 0.31 mFig. 3. Nitrate concentrations; from smallest to largest, circles represent <0.44, 0.44-10, 11-44,and 45-149 mg/LA statistically significant, inverse associationbetween nitrate concentration and well depth (Table3) is consistent with nitrate originating fromagricultural sources at or near the land surface. From aplanning perspective, an inverse association betweennitrate concentration and well depth suggests thatdeeper wells may reduce the threat of pollution invulnerable parts of the study area. Agriculturalpractices, especially fertilizer applications, probablyaccount for nitrate observations in the highest mappedcategories.Only two chloride observations surpassed thesecondary MCL of 250 mg/L. The highest chlorideTable 3. Rank Correlations Depth  Chlor ide  Chlor ide -0.159  Nitrate -0.626* 0.265  *Statistically significant at significance level of 0.01667Int. J. Environ. Res., 6(3):663-668, Summer 2012concentrations were in relatively shallow wells, lessthan 100 ft (31 m) deep; these wells may have beenimpacted by a contaminant source near the landsurface, for example, oil and gas activity or irrigationreturn flow. However, over the entire study area, therewas no significant association between chlorideconcentration and well depth. While naturalconstituents of the Trinity Group likely exert a majorcontrol on observed chloride concentrations,anomalous concentrations (above 250 mg/L), and atendency for higher chloride concentrations in thenorthwestern part of the study area (with much heavieroil and gas activity), suggest possible human impacts.Figs 3 and 4 show locations within the study area withrelatively high nitrate and chloride concentrations,which have the potential to produce highconcentrations in present and future water wells.Groundwater in these areas especially should beperiodically monitored to determine whether nitrate,chloride, and other solute concentrations are suitablefor drinking, irrigation, or other intended uses. Bothconcentration levels and water use dictate appropriatefiltration measures; for example, reverse osmosis couldremove nitrate, chloride, and other solutes fromdrinking water, though this process is relativelyexpensive. Less costly options may include treatingonly drinking water, or consuming bottled water.Nitrate observations in this study are well withinthe range reported in other parts of the world. Forexample, Kolpin et al. (1994) reported nitrateconcentrations above 45 mg/L in six percent of 303wells tapping shallow aquifers (less than 15 m deep) inFig. 4. Chloride concentrations; from smallest to largest, circles represent 2-20, 21-100, 101-250, and251-595 mg/Lcorn- and soybean-producing areas of the midwesternUnited States. Moreover, Strebel et al. (1989) and Fried(1991) reviewed nitrate pollution of groundwater inEurope; highest nitrate concentrations, above 50 mg/L, occurred in sandy soils beneath intensely managedcrops and grazed grassland. Furthermore, Zhang et al.(1996) reported that more than 50% of sampled waterwells in a fertilized region of northern China had nitratelevels above 50 mg/L. Very high concentrations, up to300 mg/L, were found in groundwater beneathvegetable-producing areas, small cities and towns, andfarmers’ yards. Chloride concentrations observed inthis study are low relative to those observed in someareas with intensive agriculture and oil and gasproduction. For example, Hudak (2003) compiledsamples from 198 water wells in the Edwards-TrinityPlateau Aquifer of west-central Texas; 49 samples hadchloride concentrations above 250 mg/L, 22 sampleshad greater than 500 mg/L chloride, and nine samplesexceeded 1,000 mg/L chloride.CONCLUSIONIn conclusion, out of 40 samples, only three nitrateobservations and two chloride observations exceededthe respective drinking water standards. Shallowerwells within the study area tend to be more vulnerableto nitrate contamination. A few (relatively shallow) wellsproduced samples with high chloride concentrations;however, there was no significant association betweenchloride concentration and well depth across the studyarea. 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Nitrate and Chloride Concentrations in Groundwater beneath a Portion of the Trinity Group Outcrop Zone, Texas

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