Environ. Sci. Technol. lQ93,27, 172-176
Application of Boron Isotopes for Identifying Contaminants such as Fly Ash Leachate in Groundwater Gregg R. Davldson" and Randy L. Bassett
Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721 The B isotopic ratio of a fly ash leachate can be very different from the B isotopic ratio of a natural groundwater. Mixtures of leachate and groundwater typically result in nonlinear B isotope mixing curves that enable identification and quantification of leachate contamination in a groundwater at much lower levels than possible using concentration analyses alone. Limits on B isotope use for contaminant quantification will exist for some environments such as landfills with multiple ash types, but B isotopic analysis may often remain the preferred method for qualitative identification.
Introduction Fly ash is a solid waste product of coal combustion. Nearly 50 million tons of fly ash is generated annually in the United States, 80% of which is discarded into landfills or disposal ponds (1). Rainfall, surface runoff, or utilityplant wastewater inevitably comes into contact with the ash, which often results in contaminant plumes beneath the disposal sites. These plumes become of special concern when they reach a water table where contaminants have the potential to travel great distances. Leachate from ash ponds is generally high in sulfate, boric acid, and total dissolved solids and may contain trace concentrations of other contaminants such as chromium and arsenic (2,3). A major environmental concern is the elevated dissolved B concentration. B is an essential nutrient in plants at microconcentrations, but becomes toxic to many plants above * 1 ppm ( 4 ) . B is concentrated in plant tissue, which results in high concentrations in coal (5). During coal combustion, part of the B content is lost in the flue gases, but the majority is concentrated in the ash in a highly soluble form (6, 7). B concentrations of groundwater beneath some fly ash disposal sites have been reported well over 30 ppm (3). A standard method for identifying offsite migration of fly ash leachate in a groundwater is the analysis of water samples along a suspected flow path for increases in the sulfate or B concentration (3). This method works well for samples that have been heavily contaminated by leachate with high sulfate or B concentrations, but it is less reliable at the edges of the plume, where the leachate is too dilute to significantly increase background concentrations, or when the initial concentration of the leachate is near that of the uncontaminated groundwater. We suspected that the use of B isotopes might provide a more sensitive method for determining the existence and extent of a leachate plume. If the B isotopic ratio (l1B/l0B) of the leachate is different from the ratio of the uncontaminated groundwater, then mixtures of the two waters will have ratios intermediate between the two. The presence and degree of leachate encroachment can then be calculated based on the magnitude of the shift in the ratios. Sulfur isotopic ratios (34S/32S)also have potential for this application, but interpretation of results is more complex because redox reactions in the ash ponds, soils, and groundwaters can alter the sulfur isotopic ratio beyond what would be expected from simple mixing of waters. B 172 Environ. Sci. Technol., Vol. 27, No. 1, 1993
is not involved in redox reactions at earth surface conditions, so isotopic ratio results are generally more easily interpreted. B is also a conservative tracer in most environments, though B is known to be retarded to some degree in high clay environments. Some care must be taken if dealing with regions of high clay content as boron isotopes may fractionate during adsorption exchanges (8, 9)* B isotopic investigation of terrestrial and lunar materials dates back several decades (10-12), but application of B isotopes to environmental problems has only been recently considered. The first study to be documented using B isotopes to identify contaminants in the environment was a result of an earlier phase of our research, completed in 1989 (13). At the time of that work, a thermal ionization mass spectrometer (TIMS) was used to measure B isotopic ratios using negative thermal ionization (NTI) to detect the BO2-molecular ion (11B02-/10B02-at amu 43 and 42, respectively). Subsequent evaluation of this method in our laboratory, however, suggested a strong relationship between the filament temperature and the isotopic results that was not observed in the earlier work. New values are reported in this paper for the same samples using positive thermal ionization (PTI) with much improved accuracy and precision. It is shown here that there can be large differences between the B isotopic ratios of leachate and groundwater. Substantially higher B concentrations in the leachates also mean that isotope mixing curves will be nonlinear, making leachate identifciation in a groundwater possible at much lower contamination levels than possible using B concentrations alone.
Experimental Methods New, acid-rinsed, Nalgene plastic bottles were used for the storage of all samples. Pyrex volumetric glassware was used only in the preparation of standards where contact time with the solution was minimal. The reservoir, delivery tube, and collection vessel for the methyl borate distillations were made entirely of Teflon. Column extractions were performed using Q g o n delivery tubes and 1.0-cm-i.d. polycarbonate columns. Deionized water was obtained by passing distilled tap water through a four-chambered Millipore Milli-Q purification system, with a resultant B concentration of less than 0.5 ppb. Leachate was prepared in the laboratory by adding 50 g of fly ash to loo0 mL of deionized water and shaking the resultant mixture for 20 min at room temperature. The slurry was allowed to settle for 10 min before filtering using a 0.45-pm cellulose acetate filter. B was extracted from aqueous solution using a methyl borate distillation procedure. A sample volume containing 20 pg of B (B concentrations determined using direct current plasma spectrophotometry) was added to a minimum of 50 mL of high-purity methanol. If the sample volume was greater than 3 mL, additional methanol was added to ensure a minimum methanol/sample ratio of 503. Distillate was collected in a Teflon beaker containing 50 mL of deionized water. Distillates were then evaporated to dryness in conical Teflon vials at 70 OC or less in a
0013-936X/93/0927-0172$04.00/0
0 1992 American Chemical Society
Table I. Physical and Chemical Description of the Fly Ash Samples
[BIc surface fly (pg/g area ash pH no. amb" hotbrc of solid) (m2/g) 150 152 157 159
12.6 4.3 9.3 10.9
12.8 4.3 8.1 11.2
2267 352 861 1739
5.0 7.2 1.0 0.7
Table 11. Average Major Element Composition of Fly Ash Samples"
fly ash element concn (mg/g) coal typec sub-bit. bit. bit. lignite
furnace config' front fired front fired cyclone tangential
"pH of water in contact with ash for 15 min at 20 'C. bpH of water in contact with ash for 24 h at 70 "C. Information obtained from EPRI.
element
150
A1 Ca Fe K Mg Na
67.2 126.6 43.0 8.2 17.8 30.3 14.8 217.0
S Si
152
157
159
125.8 14.2 112.2 18.4 4.8 4.1 8.4 208.7
99.6 19.6 172.4 21.0 4.9 6.2 9.5 204.1
93.4 70.4 19.7 14.8 18.6 4.8 4.5 232.0
Information provided by EPRI.
low-particulate, laminar-flow hood. Groundwater samples with low B concentrations required concentration prior to methyl borate distillation. B was extracted from solution using a B-selective ion-exchange resin, Amberlite IRA-743, produced by Rohm & Haas, Corp. The resin was prepared by passing the following sequence of solutions twice through a column filled with 10 mL of resin: 0.1 N HN03 to flush out residual B, 0.2 N NaOH to regenerate the resin, and deionized water to flush out excess base. A sample pH of 5 or higher is required for B exchange on the resin. B was collected from the column using 100 mL of 0.1 N "OB. Column flow rates were approximately 2 mL/min. The concentrated samples were further prepared using the methyl borate procedure. Isotopic analysis of standard solutions before and after use of the resin verified that no isotopic fractionation occurred during extraction as long as the capacity of the resin was not exceeded (-5 mg of B/mL of resin). Dried distillate samples were resolubilized in 4 pL of Na2C03solution (5 pg/pL Na). Samples were loaded onto single Ta filaments (0.5 X 0.025 mm) and heated to dryness under atmospheric pressure. Loaded filaments were warmed in the mass spectrometer at a source pressure of 1 X lo-' mbar, with a warmup to 1.0 A in 8 min. Isotopic ratios were obtained using positive thermal ionization with overall reproducibility better than f 2 % o (2a) in agreement with other studies using the sodium borate method (14,15). B was measured as the Na2B02+ molecular ion on a VG 336 TIMS with a 36-cm effective radius. Ion signals were generated between 0.6 X and 1.2 X A for a 20-pg B sample load with an accelerating voltage of 4 kV. NaZ1lBO2+at mass 89, and Na210B02+at mass 88 were measured simultaneously using dual Faraday collectors set at 1mass unit separation. A total of 50-100 ratios were collected for each sample Wich each ratio being measured over a 5-s cycle. The ion beam focus and intensity were monitored and corrected for drifts after every block of 10 ratios. There is not an established isotopic standard for B, but generally the National Bureau of Standards standard reference material 951 (NBS SRM-951),which is a boric acid, is used for instrument calibration. NBS SRM-951 has a reported l1B/loB ratio of 4.04362 f 0.001 37 (16). NBS SRM-951 standards were run each day with measured values within f l L of the reported value. No corrections were necessary for the samples.
Results and Discussion Isotopic ratios of samples (spl) are reported as per mil (%o) shifts from NBS SRM-951 (std) using the following equation:
Table 111. B Concentrations and 6"B Values of Fly Ash Leachates and Groundwater Samples
sample
P I (PPm)
P B (%)
14.2 3.3 10.1 11.6
+15.8 -19.2 -4.1 -7.9
leachate no. 150 152 157 159
groundwaters from Wisconsin Arizona (17) Arizona (17) Texas (18)
0.03 0.35 0.30 0.09
+1.8 +17.1 +31.2 +14.7
Four fly ash samples were provided by the Electric Power Research Institute (EPRI) representing a variety of coal types, furnace configurations, and leachate pH. Chemical and physical characteristics of the fly ash samples are given in Table I. The major element composition of each ash is tabulated in Table 11. Four groundwater samples were obtained from different parts of the United States. All samples came from aquifers located in heterogeneous, unconsolidated sediments. The Wisconsin site is a glacial till deposit, while the Texas and Arizona sites are all located in alluvial basins. The results of B isotopic analyses of the fly ash leachates and groundwater samples are reported in Table 111. It is apparent that fly ash leachates do not share a common P B value, nor do all groundwaters. It is equally apparent, however, that there is a significant potential for a large difference to exist between a groundwater and an infringing leachate. The P B of a solution that is a mixture of leachate and background water is a function of both the P B and the B concentration of each independent water and the volume percent that each is contributing to the new solution. The equation used to calculate P B for a mixed water is
where C is the B concentration, % is the percent by volume, and the subscripts m, 1, and b are for mixed, leachate, and background water, respectively. Figures 1and 2 compare the usefulness of B isotopes and B concentrations for identifying leachate contamination in a groundwater. A hypothetical leachate and groundwater have been used for simplicity. In each case, a concentration of 10 and 0.5 ppm B is used for the leachate, and a concentration of 0.1 ppm B for the groundwater. P B values of -5760 and +5%0are used for leachate and groundwater, respectively. Figure 1illustrates the P B of a leachate/groundwater mixture as a function of percent leachate by volume. If Environ. Sci. Technol., Vol. 27, No. 1, 1993
173
a conservative detection limit of f2%0is assumed, a leachate with 10 ppm B in this system can theoretically still be detected using B isotopic ratios in a water containing less than 0.3% leachate. If the leachate B concentration is lowered from 10 to 0.5 ppm (100 times the background lowered to 5 times the background), the leachate is still detectable in a water containing - 5 % leachate. In contrast, Figure 2 illustrates the B concentration of a leachate/groundwater mixture also as a function of percent leachate. If the leachate concentration is 10 ppm B, the plume may be detectable down to - 5 % leachate. Below 5% leachate, natural fluctuations in the background B concentration and analytical uncertainty make it difficult to differentiate between contaminated and background samples. If the B concentration of the leachate is lowered to 0.5 ppm, as in Figure 1,contaminant identification using only B concentrations becomes virtually useless for samples containing less than 50% leachate. Fluctuations in the background B concentration will also have an effect on the use of B isotopes in identifying fly ash leachate, but to a much smaller degree than when B concentrations alone are used. In the previous example, if the 0.5 ppm B leachate were mixed in a localized area containing background B as high as 5 times the normal (0.5 ppm B) so as to be equal to the concentration of the leachate, it would still be apparent that leachate was present with as little as 20% leachate in the sample. Attempts to quantify the percent leachate present would result in underestimates, but the results would still be qualitatively accurate. Using B concentrations under the same circumstances, even qualitative results would not be meaningful. To apply the previous discussion to actual fly ash leachate and groundwater samples, the P B of leachate/ groundwater mixtures are again plotted as a function of percent leachate in Figure 3. Mixing curves are shown here for each of the four fly ash leachates with the Wisconsin groundwater sample. For this groundwater, leachate contamination could theoretically be identified by the time a water sample contained between 0.04% and 0.2% leachate, using leachates 150 and 157 as best and worst cases, respectively. What is possibly more significant, however, is that in each case the P B of the contaminated water is already close to the pure leachate value by the time a sample contains only 2% or 3% leachate. The potential for early detection of a leachate plume will increase still more if an analytical method with higher precision is used, such as PTI using dicesium metaborate (19). Figure 4 provides a generalized comparison of 611B and B concentration values as they might appear along the flow path of a groundwater contaminant plume. A cross section of a hypothetical fly ash disposal site is depicted with a leachate plume mixing and moving with a groundwater. A B concentration of 10 ppm is used for pure leachate, and 0.1 ppm B for uncontaminated groundwater as in Figures 1 and 2. The upper dispersion curve is a solution to a simple one-dimensional flow equation from Ogata (20),but the exact values used to generate the curve are not important. Any dispersion curve could be used to compare P B and B concentration values along a flow path. The lower profile in Figure 4 represents the B isotope values expected at each point along the flow path based on the B concentrations observed in the upper profile. In this example, the end-member P B and B concentration values are also the same as in Figures 1 and 2 (leachate, 10 ppm B, P B = -5%0; groundwater, 0.1 ppm B, P B = +5%0). 174
Envlron. Scl. Technol., Vol. 27,
No. 1, 1993
In this system, the edge of the plume cannot be confidently defined using B concentrations because there is a significant region where the measured B concentrations of background and plume samples are indistinguishable. In this same region, however, the P B of the plume remains measurably distinct from the 611B of the background water down to extremely small levels of contamination. There are a few potential limitations to this method that need to be considered. One involves the fact that different fly ashes can have different P B and B concentration values. In order to accurately determine the degree of contamination present at a sampling site, it must be known that only one type of fly ash has been disposed in that area. When more than one type of fly ash has been disposed at a site, isotopic shifts away from the background groundwater values will still make sensitive qualitative identification of contamination possible, but precise determination of the degree of contamination cannot be made. The use of clay liners beneath disposal ponds must also be considered. If isotopic fractionation of boron isotopes occurs as leachate seeps through a clay liner, the P B of a laboratory-leachedsample of ash from that site could not be used as an end-member value. In this case, water samples taken from the unsaturated zone beneath the site would give a more accurate end-member 611B value. If such samples are not available, accurate estimates of groundwater contamination may still be possible. Other methods such as sulfate and boron concentration analyses may allow the degree of contamination to be determined where leachate encroachment has been heavy. Once the percent contribution made by the leachate is known for a specific sample, measurements of P B and B concentrations of the contaminated sample and of the background groundwater will allow a 611Band B concentration value to be estimated for the pure leachate. These values can then be used to identify and quantify leachate contamination at the edges of a contaminant plume where the use of sulfate or boron concentrations alone is no longer sufficient. Further study will need to be done to determine the effect of incremental removal of soluble B, as might be expected during a period of successive light rainfalls. Rayleigh fractionation caused by incremental removal of adsorbed B could result in discrete pulses of leachate reaching the groundwater, each with a different isotopic signature. If such fractionation were occurring and dispersion within the unsaturated zone were not sufficient to homogenize the resulting P B values, the impact on leachate identification and quantification in the groundwater would be virtually the same as when ashes of different types are disposed at the same site. Sensitive qualitative identification of leachate would still be possible, but accurate determination of the degree of contamination would not. Conclusions It is demonstrated here that the potential is high for there to be a large difference between the B isotopic ratio of a groundwater and of an infringing fly ash leachate. Typically higher B concentrations in leachate result in nonlinear mixing curves that enable identification and quantification of leachate in a groundwater at much lower levels than possible using B concentrations alone. The use of B isotopes for determining the concentration of leachate in a groundwater will be limited to environments where the leachate is known to have a unique B isotopic value. Although quantitative measurements may not be possible in environments such as landfills holding
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Flgure 2. B concentratlon of a leachate/groundwater mixture as a function of percent leachate by volume: leachate, [B] = 10 and 0.5 ppm; groundwater, [B] = 0.1 ppm. -20
Leachate 152
Leachate 157
$
Wisconsin ground water o , ; A B = 0 . 0 3 p p m , 611B=t1,8%0
15 20
Flgure 4. Cross section of a hypotheticalfly ash contaminatlon site. The graphs compare B concentrations along the flow path with the corresponding 611B values: leachate, [B] = 10 ppm, 6"B = - 5 % ~ ; groundwater, [B] = 0.1 ppm, 611B = +5%0.
for precisely identifying the extreme limits of a plume, and for situations where the B or sulfate concentrations of the leachate are not significantly higher than the background. Further study in this field should demonstrate the usefulness of B isotopes for identifying contaminants from any source that is known to have B concentration levels near or higher than the local groundwater. Example contaminant sources may include intruding seawater, runoff from feedlots, and seepage from effluent ponds from municipal treatment facilities, paper mills, or mining operations. In addition, it may also prove possible to identify the proportions of two freshwaters mixing along a flow path if their original B isotopic ratios are unique. Such a circumstance may be very likely if the origins of the two waters are located in different geologic environments.
Literature Cited 1989 Coal Combustion By-product-Production
-
Leachate 159
U
'.
100
0.5ppm B Leachate I
20
0
Water Table
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I
60
Flgure 1. 6"B of a leachate/groundwater mixture as a function of percent leachate by volume: leachate, [B] = 10 and 0.5 ppm, 6% = -5%~; groundwater, [B] = 0.1 ppm, 6' B = +5%0.
0
-
-
Leachate 150 I
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and Consumption; American Coal Ash Association, Inc.: Washington, DC, 1991. Leaching Studies on Utility Solid Wastes: Feasibility Experiments. Electr. Power Res. Znst. [Rep.]1985, EPRZ EA-4215. Groundwater Data Analyses at Utility Waste Disposal Sites. Elec. Power Res. Znst. [Rep.] 1985, EPRI EA-4165. Ayers, R. S.; Westcot, D. W. Water Quality for Agriculture. F A 0 Irrig. Drain. Pap. 1985, No. 29, 82. Churey, D. J.; Gutenmann, W. H.; Kabata-Pendias, A,; Lisk, D. J. J. Agric. Food Chem. 1979,27,910-911. Gladney, E. S.;Wangen, L. E.; Curtis, D. B.; Jurney, E. T. Environ. Sei. Technol. 1978, 12, 1084-1085. Cox, J. A.; Lundquist, G. L.; Przyjazny, A.; Schmulbach, C. D. Environ. Sei. Technol. 1978,12, 722-723. Goldberg, S.; Glaubig, R. A. Soil Sci. Am. J. 1985, 49, 1374-1379.
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Kanzaki, T.; Yoshida, M.; Nomura, M.; Kakihana, H.; Ozawa, T. Geochim. Cosmochim. Acta 1979,43,1859-1863. Envlron. Sci. Technol., Vol. 27, No. 1, 1993
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(16) Cantanzaro, E. J., Champion, C. E.; Garner, E. L.; Marinenko, G.; Sappenfield, K. M.; Shield, W. R. NBS Spec. Publ. (U.S.) 1970, NO.260-1 7. (17) Lerner, L. M.S. Thesis, University of Arizona, Tucson, AZ, 1992. (18) Bassett, R. L.; Buszka, P.; Davidson, G. R.; Diaz-Chong,
176 Envlron. Sci. Technol., Vol. 27, No. 1, 1993
D. Geochim. Cosmochim. Acta, submitted. (19) Spivack, A. J.; Edmond, J. M. Anal. Chem. 1986,58, 31. (20) Ogata, A. U.S. Geol. Surv. Prof. Pap. 1970, No. 411-I. Received for review March 10,1992. Revised manuscript received September 4, 1992. Accepted September 17, 1992.
