Identification of Groundwater Solute Sources Usin! Boron Isotopic Composition R . L . B A S S E T T , * , +P . M . B U S Z K A , t G. R. DAVIDSON,+ AND DAMARIS CHONG-DIAZt Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721, and US.Geological Survey, 5957 Lakeside Boulevard, Indianapolis, Indiana 46278
In a long-term evaluation of the injection of treated wastewater into an alluvial aquifer near El Paso, TX, the boron (8) concentration and B stable isotopic values were investigated as potential intrinsic tracers of recharge-water source. The B stable isotopic values (SllB) of background groundwater (14%0) and water from anthropogenic sources such as treated municipal wastewater (6-10%0) and irrigation-affected water (>40%0) were distinctly different. The B concentrations in combination with the B stable isotope values distinguished these water types and natural saline water as sources of recharge to groundwater. Boron is suited for use as a conservative tracer in groundwater because of its high solubility in aqueous solution (predominantly as boric acid), its natural presence in nearly all water, and the lack of effects by evaporation or volatilization, by oxidationreduction reactions, or by mineral precipitation or dissolution in all but extremely saline waters. The variability of any observed isotopic ratio in groundwater can be from several processes, such as variability in the B source, mixing, and partitioning reactions, including sorption, which cause isotopic fractionation. For rocks and minerals, a range in 611B values of more than 70%0 has been reported. Water sources sampled to date have a similarly broad range. In this injection test, B concentrations in all samples were below 1 mg/L, and differences between individual sources of salinity were indistinct based on analytical concentration alone. In contrast, a hypothetical, nonlinear mixture between solutions with different 611B values and different B concentrations demonstrated a detectable isotopic shift for small volume percent contributions of the source water with the largest boron concentration. The Hueco Bolson aquifer a t the injection site is geologically heterogeneous, and the free water surface is more than 90 m deep. The
0013-936X/95/0929-2915$09.00/0
1995 American Chemical Society
cost to establish a sufficient density of monitoring wells t o describe the flow of injected water in groundwater is prohibitive. For this and other sites, chemical and isotopic tracers such as B and 6”B may be used to identify the influence of various water sources with potentially adverse environmental consequences.
Introduction We report that at least three solute sources (or “watertypes”) to groundwater are distinguishable by their concentrations of boron (B) and its stable isotopes loB and llB, at a site near El Paso, ‘IX Anthropogenicwater types can have ratios of l1B:loB,or 611B values, that are different from that of native groundwater. Because B is commonly and naturally present in groundwater as a minor or trace constituent (11, it may be an excellent intrinsic tracer of solute sources in groundwater. If compositions of other natural B sources such as deep-aquifer brine or other saline water are considered, mixing between the different solute sources can be identified. This is possible because the difference in B concentration and B isotopic signature can be substantial for natural saline waters and most waters affected by contamination. The purpose of this paper is to describe the application of boron stable isotopes to the identification of anthropogenic and natural solute sources. The ubiquity of B derives from its high solubility as undissociated boric acid (H3B03;pK, = 9.2, ref 2). Boron is also found in concentrations of up to several tens of milligrams per liter (mglL) in environments such as saline lakes, geothermal waters, and sedimentary basin brines. Boron concentrations are not affected by oxidatiodreduction, volatilization, or evaporation and are only affected by mineral precipitation in extremely saline environments. Sorption on clay or oxyhydroxide surfaces removes B from solution in groundwater (3-5) and, consequently, is the only mechanism removing B from surface water or groundwater in non-evaporite environments. The natural variation in the 611B of whole rock and of rock-forming minerals has a range of at least 70700(Figure 1 in ref 6). The 611Brange for natural hydrologic samples such as ocean water, fumarole condensate, geothermal waters, native groundwater, and brines is similarly broad (6-16‘). For example, the isotopically lightest natural waters are from geothermal related sources, - 14.1%0( 1 7) and -9.3 to 4.4%0 (11);the isotopically heaviest waters are brines from the Dead Sea and Australian salt lakes, 57.4 and 59.2%0, respectively (15, 16). It is assumed in this discussion that the 611Bin native groundwater should reflect the rock type with which it was associated or the source waters from which it mixed or evolved. * Corresponding author e-mail address:
[email protected]. University of Arizona. +
* US. Geological Survey.
VOL. 29, NO. 12. 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
2915
106'223W
106'25'
106.20'
31'57'3U
31'55'
106°30'
106'00
EXPLANATION
32' 00
HUECO BOLSON RECHARGE PROJECT AREA
31' 30
WID
IKlECTlON WUL AND NUMBER
W7.
SAMPLED INJEmON WELL AND NUMBER
HB.7A
OBSERVATION WELL AND NUMBER
OB4A
SAMPLED OBSERVATION WELL
fdoDumbasin 62 0
320
AND
NUMBER
PRODUCTION WELL AND NUMBER SAMPLED PRODUCTION WELL AND NUMBER
2 MILES
0 0
1
2 KILOMEERS
FIGURE 1. Location of the Hueco Bolson Recharge Project in nollheast El Paso, TX. The location of the Palo Durn Basin is also depicted.
The 6"B signature should also reflect the anthropogenic modificationsmade to the native water (181. . . For examole, brines from oil fieldoperations, municipalwastewater,nonpoint agriculturally related recharge, coal-fired and geothermal power-plantwastewaters,and cooling-tower waste all have distinctive B concentrations and isotopic compositions (19). Davidson and Bassett have shown that B concentrations and 6llB values appear promising in determining source, mixing, and flow path for fly ash waste (19). The use of most stable isotopes as environmental or geochemical tracers is limited by potential fractionation along a flow path by processes such as precipitation1 dissolution, oxidationlreduction, volatilization, evaporation, orsolption. Fractionation-relatedchangesinthe mass of B stable isotopes maybe from the nearly 10%difference between the atomic mass of llB and 'OB. In a long-term evaluation of the injection of treated wastewater into an alluvial aquifer at the Hueco Bolson Recharge Project (HBRP) near El Paso, TX, the B concentration and B stable isotopic values were investigated as ~~~~
~~
2916 *ENVIRONMENTAL SCIENCE 5 TECHNOLOGY I VOL. 29. NO. 12.1995
tracers ofrecharge-water source. Atracer was needed to determine its subsurface extent andinfluenceonthefuturepotabilityofgroundwater near the HBRP. The investigation was conducted jointly in 1990 and 1991 by the US. Geological Survey (USGS), the El Paso Public Service Board (EPPSB), the US. Bureau of Reclamation, and the Texas Water Development Board at andnear theHBRP recharge facility (Figure 1).that is owned and operated by the EPPSB (20). Tertiary-treated wastewater has been injected into the alluvial sediments of the Hueco bolson aquifer through 10 wells at the HBRP from 1985until 1994to supplement available supplies of potable water. Theeffectivemanagement ofinjectedwaterat theHBRP requires that tracers be identified that indicate flow path and extent of mixing of injected water. The distribution of B and its stable isotopic ratio was evaluated as a tracer constituent with chloride and other more conventional tracer constituents such as bromide, chloride, nitrate, and stable isotopes of hydrogen and oxygen to determine the
extent to which the injected wastewater has mixed with ambient groundwater. This study emphasizes B concentrations and 611Bvalues because of their relatively greater differences among injected wastewater, background groundwater, and other sources of saline groundwater as compared with the other conservative tracers (20).
standard were analyzed by the separation process, yielding ameanratio of l1B/loBof4.04341 f0.0024. Stable isotopic ratios of B were reported as 611B in per mil units and computed as 11
l 1 ~ / l 0 ~ o f t h e s a m pie
6 B,= ("Bi'og of the standard
Methods and Results Chemical Analyses. Groundwater samples were collected after at least three well bore volumes were purged, and the following stability criteria were achieved: temperature within O.l"C,pH within 0.1 standard unit, dissolved oxygen within 0.1 mg/L, and specific conductance within 10 ,US/ cm. Samples of injected water were collected from a continuously flowing, full pipe after the same stability criteria were achieved. Water samples referenced by this paperwere collected, preserved, transported, and analyzed according to standard USGS procedures (20). Samples for B and B isotopic analysis were filtered through 0.45-pm membrane polycarbonate filters and stored in polyethylene bottles. All analyses of boron and chloride were performed at the USGS National Water Quality Laboratory in Arvada,
co.
B Isotopic Analysis. Water samples were prepared by the sequential (a) extraction of B with ion exchange, (b) evaporative concentration, (cl methyl borate distillation, (d) evaporation, and (e) resolubilization with the appropriate activator for loading on the filament. Several samples were prepared simultaneously. Recoveries of B from the procedure were 100%:no fractionation bias was introduced by the sample preparation. The method was developed using the National Bureau of Standards Standard Reference Material 951 (NBS SRM-9511, which is 99.9% boric acid (H3B03) (21). Boron isotopic analyses were performed at the Department of Hydrology and Water Resources, University of Arizona. Boron was extracted from water samples using an ionexchange column containing Amberlite IRA-743 resin, which is specific for aqueous B. The ion exchange process, however, was not a sufficient preparation step because of the interferences from concentrations of neutral aqueous species in groundwater and injected water samples. For example, concentrations of dissolved silica in the samples ranged from 25 to 31 mglL as Si02 in injected water and from 30 to 35 mg/L as Si in water from local observation and production wells (20). In the pH range of these water samples, from 7.2 to 8.1 pH units (201, silica is present principally as the neutral H4Si04species. Neutral aqueous species may be retained on the column along with B. These impurities were removed from the sample by the methyl borate distillation. The distillate was resolubilized in a sodium carbonate solution to produce a 1:l mass ratio of B to sodium (Na). The solution was then placed onto a tantalum filament, heated in a filament loader at 0.6 A to dryness, and then flashed to 1.OAuntil the bright white solid phase of sodium borate was visible. Mass spectral data were collected for B in the prepared samples as follows: baseline at atomic mass unit (amu) 80.5, the Na211B02+ and Na210BOzf ions at amu 89 and 88. Sample loads produced stable ion signals that ranged from 0.5 to 2.0 x A. An instrument warmup period of 8 min was used to produce a filament current of 1.0A. Data were then generally collected using each sample for either 50 or 100 values. Twenty subsamples of the NBS SRM-951
Using the reported ratio of l1B-loB for the NBS SRM-951 standard of 4.04362 f 0.00137 (211, this represents a standard deviation for 611B of 0.61%0 from the mean value. Hueco Bolson Recharge Project. El Paso is dependent on groundwater pumped from the Hueco bolson alluvial sediments for about 60% of its water supply (20). The El Paso Water Utility injected about 1.7million m3of tertiarytreated wastewater between 1985 and 1991 (22). Injected wastewater met or exceeded U.S. Environmental Protection Agency primary drinking-water standards. The treated water was injected through 10 wells (Figure 2) into the heterogeneous, unconsolidated alluvial sediments of the Hueco bolson aquifer at depths ranging from 90 to 250 m below land surface (20). Wells used for injection are completed and gravel packed through as much as 150 m of aquifer but are only perforated through freshwater zones in the most permeable sand and gravel deposits. Preliminary computer simulations of groundwater flow during injection operations indicated that the time required for injected water to travel through the aquifer to the adjacent production wells ranged from 2 to 6 years (22). The Hueco bolson aquifer has a heterogeneous distribution of hydraulic conductivity that is typical of alluvial deposits. The aquifer is composed of the upper Pliocene and lower Pleistocene coalescing alluvial fans and fluvial sediments of the ancestral Rio Grande. The deposits are composed of laterally-discontinuouslayers of gravel, sand, silt, and clay with individual layers ranging from 0.3 to 30 m in thickness (23). The saturated thickness of the freshwater zones of the aquifer near the recharge project is about 170 m. The vertically averaged hydraulic conductivity of sand and gravel layers in the aquifer near HBRP ranged from 6.4 mlday at well 31 to 27 m/day at well OB-5 (20).
Major local sources of recharge to the aquifer near the HBRP, other than from the injection wells, were (a) about 3.3 million m3 from irrigation of a former dairy farm north of the site from 1965 to 1975 (241, (b) from unlined ponds at the water treatment plant about 1.6km east of the HBRP from about 1963 to 1991 (241, and (c) from waste coolingwater ponds at a natural gas processing plant about 1.6km north of the HBRP. The historical potentiometric gradient in the aquifer is toward the Rio Grande (23). The direction of groundwater flow is now toward the southwest, as controlled by regional drawdown (Figure 2). Comparative Data. The pertinent chemical analyses and the B isotopic values are given in Table 1 for injected water and for samples from production wells in the Hueco bolson aquifer: background groundwater, irrigation-affected water, and naturally occurring saline water. Samples of the deeper brine from the Hueco bolson sediments were unavailable because of the lack of well completions below the freshwater zones of the aquifer. For comparison, analyses of samples obtained from brine-bearing aquifers in the Palo Duro basin are discussed later in this paper. The Palo Duro basin is in the southern part of the panhandle VOL. 29. NO. 12. 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY m 2917
TABLE 1
Concentrations of Boron and Chloride and Stable Isotopic Values of Boron in Injected Water and Groundwater from the Hueco Bolson Recharge Project Area, El Paso. TX well no.
date sampled
boron
chloride
ImgRI
ImgILl
8% 1x01
Injected-Water Samples RW-7
RW-8
8-27-91 8-31-90 8-25-91
0.27 0.30 0.28
150 180 140
6.3 10.1
400 160
43.3 30.1
980
8.3
140 140 93 140 140
14.7 13.3 14.9 15.6 16.7
160 160 160 150 150 140 140 130 170 140 150 160 110 130 120 110
12.9 15.1 5.3 9.1 25.5 24.9 14.4 15.3 10.9 9.5 14.2 9.7 18.2 24.4 19.6 22.2
90 84 190 130 130 51 54 80 240 290 260
18.2 19.9 25.5 46.1 49.0 19.6 21.5 19.1 28.3 15.4 20.5 18.5 23.4
6.0
Irrigation-AllectedWater 35
8-22-90 9-10-91
53
9-07-91
29
8-31-90 8-27-91 8-23-90 8-30-90 8-23-91
OB-1
8-28-90 8-30-91 6-04-90 6-04-90 8-29-91 8-29-91 8-28-90 8-30-91 8-27-90 8-27-91 8-27-91 8-27-91 8-27-90 8-28-91 8-27-90 8-28-91
0.68 0.12
Saline Water 0.08
BackgroundWater 0.10 0.11 0.09 0.11 0.12
ObservationWells OB-4A 1131 rnl OB-4A 1194 m) 0 8 - 4 8 1131 m) OB-4B I210 m) OB-5 OB-7A 1131 ml OB-7A 1184 ml 08-78 1131 m) 0 8 - 7 8 1184 rnl
0.21 0.25 0.30 0.29 0.17 0.17 0.08 0.09 0.26 0.29 0.26 0.28 0.12 0.12 0.12 0.12
ProductionWells 32
I
ll.59
/
0
lKiECnONn€u
I
2 UlES O
0
L
S
I
-
7
34 36
-
2mm-
FIGURE 2. Maps showing (A) potentiometric surface of the HueCO bolson aquifer, January 1990, ( 6 )chloride isopleths. and (C) 8 ' 6 value isopleths.
of Texas and several hundred kilometers to the northeast of El Paso (Figure 1). Brine samples were collected from deep-basin aquifers in the Palo Duro basin in carbonate and sandstone lithologies that are similar to those which eroded to form the unconsolidated deposits of the Hueco bolson. The well locations, sampled depths, brine chemical compositions, and hasin hydrogeology of the brines are reported in previous studies (25, 26). and the 6"B values of the Palo Duro basin brines are shown later in Figure 6.
Hydrochemistry at the HBRP site is complex because of geological heterogeneity and multiple water sources. The B and chloride (CI) composition of groundwater can he attributed to mixingfrom at least three known water types: native groundwater,injected water, and irrigation-affected water from the former dairy farm that was upgradient from 2918. ENVIRONMENTAL SCIENCE &TECHNOLOGY IVOL. 29. NO. 12,1995
40 42 44 56. 59 404
8-21-90 8-25-91 8-22-90 8-30-90 8-26-91 9-01-90 9-07-91 8-23-90 9-01-90 9-01-90 9-07-91 9-09-91 9-06-91
0.12 0.12 0.12 0.23 0.27 0.09 0.10 0.11 0.09
0.08 0.09 0.08 0.08
77n
290
theHBRP (Table 1andFigure2). Rechargeofwaste-cooling water from the natural gas processing plant that is upgradient from the site may also affect the boron and chloride composition of groundwater. The boron and chloride analysesfrom the wells indicate that background water has among the lowest boron and chloride concentrations (Figure 3). The CI composition of the municipal wastewater, which has been routinely monitored since injection began, has been relatively uniform in chemical composition during the injection period (27). The original chemical composition of the inigationwateratthefonnerdajifarmwasnot determined because irrigation was discontinued about 17years before this study began. Irrigation at the former dairy farm was used to grow crops for animal feed.
0 . 8 1 " ' "
"
"
"
"
"
M W I J I I BElwEw INJECTEDWATER AND INTEWEDUTEMDCNRB OF BACKCROUND AND M!A 36 WATWS
"
"
"
"
" ' I
I
MmE BEIyIIEp( BACKQROUND WATER AND WATER FROM 36
I
0"'"' 0
"
10
"
"
"
"
"
30
20
8''
"
40
"
"
"
m
I
" " Bo
8, IN PER MIL
FIGURE 3. Plots of selected constituents from the sampled wells: (A) boron and chloride concentrations; ( 6 ) boron concentrations and 6 W values.
The approximate geographic origin of these water types is known; consequently, a C1 isopleth map is instructive (Figure2). A plume of injected water with C1concentrations between about 150 and 200 mg/L surrounds the recharge wells. A source of high C1 concentrations is located to the north of the project site near the former dairy farm. Low, background C1 concentrations that are less than about 150 mg/L were determined in groundwater from the margins of the study area. White (24)defined a plume of irrigationaffected water, oriented toward the south through the eastern half of the HBRP area. The water-table gradient before injection of treated water by the HBRP was responsible for the orientation of the plume. The remnant of this plume of irrigation-affected water is approximatelydefined as the parts of the aquifer with more than 200 mg/L C1-about the eastern third of the HBRP (Figure 2). A cross plot of C1 and B indicates that injected water, background water, irrigation-affected water, and saline water differ in their chemical composition (Figure 3A). The background water composition is represented by samples from wells 29a, 31, and 41, which are distant from known sources of anthropogenicallyaffected recharge (Table 1 and Figure 2). The most representative sample of irrigationaffected water is considered to be the 1990 sample from well 35. The well is the closest downgradient well from the former dairy farm (Figure 2) and has concentrations of chloride and other constituents that are similar to those observed since irrigation ceased (El Paso Public Service Board, unpublished data, 1993). Saline water is represented as the chemical composition of water from well 53.
The plot of B and C1 concentrations in Figure 3 indicates a major mixing trend between background water and either injected or irrigation-affected water and a minor mixing trend between background water and saline water. The effect of these mixtures on aquifer hydrochemistry is also indicated by the large amount of recharge from injected water and irrigation-affected water and by descriptions of groundwater quality degradation by irrigation-affected and saline waters (24). The relatively large difference of the C1 concentrations between the saline water type and samples from wells 35,44,56,59,and 404 implies a small contribution from this water type to their hydrochemistry (Table 1). The plot of B and C1data does not indicate however whether injected water or irrigation-affected water affect the hydrochemistry of water from observation wells and from well 36. Isotopic data further distinguish the chemistry of B in the water samples (Figure 2) and provide a "label" for the source of water in the major and minor mixing trends. Boron Isotopic Signatures. The distribution of 611B values is areally associated with that of C1 (Figure 2). Water samples with similar concentrations of B however have substantially different isotopic signatures (Figure 3). The mixing processes that best explain the B concentration and 6 llB data do not substantially involve the saline water type (Figure 3A). In this figure, the saline water sample is similar to background water and does not influence the B composition of groundwater samples. In addition, the 6 I l B ofwater from observation wells near the injection wells appears to reflect amixture between injected water, background water, and water of a compositionsimilar to that ofwell 36. Well 36 is downgradientfrom the northern half of the former dairy farm and the natural gas processing plant (Figure 2). The hydrochemistry of water from well 36 represents the combined influences of cooling water from metal plating at the natural gas processing plant ("cooling water") and irrigation-affected water. The cooling water influence is indicated by persistent detections of chromium, ranging from 35 to 9Opg/L, in samples collected from 1985 to 1989 (EPPSB, unpublished data, 1991). In contrast, chromium concentrations from other wells near the HBRP, including well 35, are all generally less than 10 pg/L. The influence of irrigation-affectedwater on the hydrochemistrysampled from well 36 was identified by White as increases in C1, and dissolved solids concentrations from 1975 to 1981 as irrigation-affected water reached its largest extent (23). Mixing between injected water and background water is not evident in the other production wells sampled; rather mixing dominion between injected water and irrigationaffected water or cooling water is most evident. Most significantly, the balance of the water compositions compose a nonlinear trend between the end member of the background water composition and the cooling water represented by water from well 36. The confidence in the end-member composition that represents irrigation-affected water, well 35, merits scrutiny. The value with the largest B concentration and 6 l l B value in Figure 3 was collected in August 1990, after the well had been producing water for 5 days. The second analysis was collected in September 1991, after the well had been producing about 500 gal/min for2 h. The 1991 sample was collected despite the lack of stable pH and specific conductance values because of mechanical difficultieswiththe pump. The 1991 analysis, however, conformed with the nonlinear trend between background water and water from well 36. The VOL. 29. NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2919
a a
1.5 r---
1
1
141 13L
lor -I 5 091 z 08.
$
[8 z
B
8
iI
t CURVE 3;
I
i
I
07, 08C
,'
-I
// 4 f
2
t~
03t 02-
I
4
CURVE 1
01-
1 -L-1L b
10
0 0
20
30
50
40
d
Bo
8'b, IN PER MIL FIGURE 4. Example of volume-based, linear, and nonlinear, twocomponent mixing involving three hypothetical end-member compositions with identical boron concentrations and different 6"B values. Curve numbers are discussed in the text. a w
IEND-MEMBER MIXING LINES OBSERVED DATA HYPOTHETICALDATA
5
z -
15
Em
y
END-MEMBER COMPOSITIONS B
sf
INJECTEDWATER BACKGROUNDWATER
"O 0.5
0 0 2
9, 10
20
30
8"
40
50
60
70
B, IN PER MIL
FIGURE 5. Mixing scenarios for volume-based mixing in a threecomponent system and a mixing possibility for end-member waters at the Hueco Bolson Recharge Project site.
analysis from 1990 has been duplicated and is much more saline than the 1991 sample. The 1991 sample apparently produced groundwaterthat was influencedby coolingwater due to the short purging time of the well before it was sampled. The nonlinear trend observed between the irrigation water and the background water is expected for bimodal, volume-basedmixing of waters with different isotopic ratios. A hypothetical, volume-based binary mixing of two waters with the same elemental concentrations but different isotopic ratios is a linear relationship (Figure 4, curve 1). In contrast, aplot of elemental concentrationswith different elemental concentrations and isotopic values is nonlinear because 6 lB is a ratio (eq 1 and Figure 4, curves 2 and 3). Nonlinear mixing is useful in identifying even low percentages of contaminated water when the elemental concentration of the isotopic tracer is greater in the contaminant than in background water (19). Using the relationship shown below, it is also possible to evaluate mixing of three or more components, even if the elemental concentrations differ over a relatively small range, if the 611B values also differ (Figure 5). 2920
i
1
f2 ::i 5
dl'B, =
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 12. 1995
where CCandVFi are the concentration and volume fraction of component i, respectively; 611Bi is the stable isotopic ratio of B of component i; rn is mixed water; and i is component waters B, R, C, C', and C"contributing to mixed water (Figure 5). For example, the curves in Figure 5 are computed for mixing between three water types. Water type B is considered an example of background water, water type R represents injected water, and water type C represents irrigation-affected water. The hypothetical water types C' and C" in Figure 5, represent the boron concentrations and dllB values if irrigation-affected water originally had a larger B concentration (C') and if cooling water had a larger 8'B value (C'3 than water type C. The large difference in dl1B values continues to be significant even where the difference in B concentrations between background and the other water types are small. One could easily lose confidence in the analytical difference in B in the basal regions of any of the curves; however, the variation in lB is still easily measurable and definitive. The hypothetical mixing curves from C' and C" to B and R indicate that the actual end-member compositions for cooling water and irrigation-affected water may have larger 611B values and B concentrations than any water collected in this study. In general, a study of the regional variation in B isotopic signature is probablyof limited use because of the numerous natural and human-affectedsources ofboron; the B isotopic signature is of greater value in a local investigation with definable initial compositions. Substantial differences in 611B values exist at the HBRP between background groundwater and other water types. On a local scale, the 611B values should be more useful than isotopes of most elemental constituents in tracing groundwaterflow because of boron's high solubility, its ubiquity in both rocks and water, and its range of isotope values. It is essential that the sources of the B and the processes affecting its isotopic value be considered. Processes Affecting B Isotopic Composition. The systematics of 611B variability in groundwater can be best understood in terms of three mitigating processes: (1) variability of the source of B, (2) fractionation during sorption, and (3) fractionation during precipitation or volatilization. Oxidation or reduction reactions involving B species can be excluded from consideration in natural process because of the large potential required to cause the reactions. The more important cause of 611B variability in waters of environmental significance is probably from the variety of anthropogenic sources. Discharge of waters from anthropogenic sources may create zones of water labeled with isotopic signatures distinct from the native water. For example, Gellenbeck (17) found a 611B range from -12 to +45% in water sampled from 54 wells in the Phoenix, AZ, area. Most of these locations contained native groundwater whose quality had been degraded by nitrate from many different water uses. Municipal wastewater, one of the largest sources of human-affected water, contains a 611B signature related to the B in soaps and cleaners. Most soap manufacturers
FIGURE 6. Ollferent water lypes separable by a three-dimensional group of the logarithms of boron and chloride concentrations and 8%values. obtain their B from evaporite minerals derived from
terrestrialevaporite lakes such as Searles Lake in California. B-containingminerals suchas boraxand tincalconitefrom these sources have measured GLIBthat range from -1 to +?%a (181. Wastewater samples from El Paso measured to date havesimilarvalues (Table 1);however, theassociation is circumstantial and tentative with the presently small database. Similarly, B is readily derived from leaching of fly ash near coal-fired power plants, and the resultant leachate maybe found in groundwater. The resultant 6"B value depends on the source of the coal and the pH of the leachate (191. Reporteddl'Bvalues haverangedover23%0 and, depending on the native groundwater signature, may be a distinct marker of this anthropogenic water source. Values of 6ILBof fly ash leachate and wastewater cannot presently be predicted because of the small database. An obvious application is the detection of contamination from deep-aquifer brine or oil-field brine disposal. Most oil-field brines are marine related, and consequently the d"B values are usually isotopically heavy. Consequently, the B isotopic signature will serve as an additional intrinsic tracer confirming the water as a marine-derived brine source. In many regions of oil and gas production, such as the petroleum provinces of west Texas, the local groundwater often contains concentrations of dissolved solids that render it unusable for public supply because of the semiarid environment and the presence of evaporates near the surface (28). These shallow aquifers can often contain bigb dissolved solids content and mixing with oilfield brine sources can be indistinct. The B isotopic signature can be used as a supplementary tool to establish the presence of oil-field related water. Figure 6 is a display ofthe6"B forwatercompositionsfromavarietyofsources, plotted with respect to both total B and total CI concentration. The data displayed in this figure are for water samples limited to the west Texas area in which we have conducted recent investigationsand include shallowaquifers,oil-field brines, municipal wastewater from El Paso, and groundwater affected by leachate from agricultural sources. For theseexamples,thewatertypes produceadistinctsignature clearly within the precision of the analytical methods for eachcomponent and demonstrate thevariability ofsource waters.
Field Limitations. The interpretation of the hydrochemistryin complexhydrologicfield projects requires the use of numerous chemical, hydrologic, geophysical, and analytical tools. This portion ofthe HBRP study illustrates the potential use of an intrinsic isotopic tracers 6'IB as a supplementary tool to determine the presence of mixed waters. Amore detailed monitoring of injected-water flow in the aquifer would require numerous additional wells COUStruCted to sample each transmissive interval and to determine the spatial variability of background water chemistry. The area of interest near the HBRP is almost 50 km2,and the well density is about 1 we11/2 !anzfor an aquifer with a permanent water table deeper than 90 m. This low well density is insufficient to monitor the movement of recharged water in detail. Interpretation of water movement and solute transport in field environments such as this are often severely hampered by the scale of the project. the aquifer heterogeneity, the required capital investment, and the multiple anthropogenic fingerprints in the aquifer. The principal mixing components can be deconvoluted however if endmember water types are identified. Further development of '"forensic"tools such as described here can significantly improve our interpretive capabilities in environmental hydrogeology.
Conclusions The B concentration and d"B values in municipal wastewater, when significantly different from the background groundwater, can be used as an excellent intrinsic tracer. A plot of B Concentration and 6'IB values can result in a nonlinear mixing curve that additionally defines the mixing process and provides a more definitive display than is possible simply by observing concentration differences alone. This additionally provides an avenue for the identification of small percentages of the higher concentration solution in the mixture. Boron isotopic signatures of some anthropogenically derivedwatersourcesmaybe distinguishableandmaysewe as fingerprints of the source, at least on a local scale. In this study, injected, treated wastewater, with a S"B that ranged from 6 to 10%0,had a distinguishable signature from background groundwater (about 14%0) and irrigationaffected water (greater than 4494. The d"B value for injected water may actually be characteristic ofwastewater because of the use of similar soaps and cleansers that contain B. When the isotopic value is combined with the dissolved B and Cl concentration, water types can be distinguished by displaying the data in a three-dimensional representation. Thisdisplayusingthe6"Bd be generally applicable because B is present in both uncontaminated and contaminated water sources and is highly soluble and mobile in these environments.
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Received for review J u n e 27, 1994. Revised manuscript received March 1, 1995. Accepted September 8, 1995.@
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ES9404025 @Abstractpublished in Advance ACS Abstracts, October 1, 1995.
