Scientific Investigations Report 2008–5089
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5089
Contaminant plumes of radiochemical and chemical constituents in the Snake River Plain aquifer at the INL are attributed to waste-disposal practices. Areal distribution of the plumes was interpreted from analyses of samples collected from a 3-dimensional flow system. Concentrations of these constituents represent samples collected during October 2005 from wells completed at various depths in the aquifer and differing well completions; for example, single and multiple screened intervals and open boreholes. No attempt was made to determine the vertical extent and distribution of these plumes. Radiochemical and chemical constituents analyzed for in ground-water samples collected from wells at the INL during 2002–05 include tritium, strontium-90, cesium-137, plutonium-238, plutonium-239, ‑240 (undivided), americium-241, gross alpha- and beta-particle radioactivity, chromium and other trace elements, sodium, chloride, sulfate, nitrate, fluoride, trace elements, volatile organic compounds, and total organic carbon. Physical properties of water measured during sampling events included specific conductance, temperature, and pH.
A tritium plume has developed in the Snake River Plain aquifer from discharge of wastewater at the INL since the 1950s. Tritium has a half-life of 12.3 years (Walker and others, 1989, p. 20). The primary sources of tritium in the aquifer have been the injection of wastewater through the disposal well at INTEC and the discharge of wastewater to the percolation ponds at the INTEC and RTC (fig. 4). Routine use of the disposal well at INTEC ended in February 1984; subsequently, radioactive wastewater has been discharged to the percolation ponds. About 31,620 Ci of tritium in wastewater was discharged to the well and percolation ponds from 1952 to 1998 (Bartholomay and others, 2000). Since 1993, tritium in wastewater at the RTC has been discharged to lined evaporation ponds, which should prevent migration to the aquifer. About 191 Ci of tritium were released in wastewater to the RTC lined evaporation ponds during 1999–2000 (Stoller Corporation, 2002a, 2002b). During 1996–99, no tritium was discharged to the ponds at the INTEC; during 2000, 0.03 Ci of tritium was discharged (Stoller Corporation, 2002a, 2002b) (fig. 9). Data are unavailable for the total amount of tritium in wastewater discharged during 2001–05.
In October 2005, reportable concentrations of tritium in ground water ranged from 0.51±0.12 to 11.5±0.6 pCi/mL and the tritium plume extended south-southwestward in the general direction of ground-water flow (fig. 14). In 1991, the area of the plume where concentrations exceeded the maximum contaminant level (MCL) of 20 pCi/mL (U.S. Environmental Protection Agency, 2001) was about 2.4 mi2 (Bartholomay and others, 1995). In 1995, five wells sampled by the USGS in different areas of the INL had concentrations of tritium that exceeded the MCL, but no plume representing values equal to or greater than the MCL was discernible (Bartholomay and others, 1997). By October 1998, concentrations of tritium in all water samples were less than the MCL. This trend continued through October 2005, when concentrations of tritium in water samples generally decreased and were all less than the MCL.
Long-term radioactive-decay processes and an overall decrease in tritium disposal rates contributed to decreased concentrations of tritium in water from most wells at the INL during 2002–05. Tritium concentrations in water from several wells southwest of the INTEC decreased or remained constant as they had during 1998–2001, with the exception of well USGS 47 (fig. 3), which increased a few picocuries per milliliter. Concentration decreases ranged from about 2.5 to 3.7 pCi/mL during 2002–05. Concentrations in water from well USGS 123 (fig. 6), southwest of the INTEC, decreased from 8.3±0.4 pCi/mL in April 2001 to 4.6±0.3 pCi/mL in October 2005. Concentrations in water from well USGS 114 decreased from 13.2±0.5 to 10.5±0.5 pCi/mL from July 2001 to October 2005. Concentrations in water from well USGS 77 decreased from 14.1±0.5 pCi/mL in April 2001 to 11.5±0.6 pCi/mL in October 2005. Concentrations in water from well CFA LF 3-9 decreased from 9.3±0.5 pCi/mL in October 2001 to 6.8±0.4 pCi/mL in October 2005. The decrease in tritium concentrations in water from wells south of the INTEC could be the result of decreased discharge of tritium to the percolation ponds since the early 1990s.
Near the southern boundary of the INL, tritium concentrations in water from wells USGS 103, 105, and 108 (fig. 5), exceeded the reporting level during 1983–85 (Pittman and others, 1988, p. 51; Mann and Cecil, 1990, p. 27). From 1985 to 1995, tritium concentrations in water from these wells were less than the reporting level (Bartholomay and others, 1997, p. 27). In October 1998, concentrations in water from well USGS 105, at the boundary, and from well USGS 124, south of the boundary, exceeded the reporting level and were 0.31±0.06 and 0.3±0.06 pCi/mL, respectively. These concentrations are similar to tritium concentrations reported by Busenberg and others (2000) from these two wells. Lower detection limits for tritium established by the RESL in the mid-1990s enabled the identification of smaller concentrations of tritium during 1996–98. During 1999–2005, concentrations of tritium in water from wells near the southern boundary of the INL (USGS 1, 103, 105, 108, 109, 110A) (fig. 5) and all wells sampled south of the INL boundary were less than the reporting level.
Tritium concentrations in water from wells USGS 83 and EBR 1 (fig. 14) within and near the tritium plume (fig. 14) were less than the reporting level during 2002–05. Well USGS 83 penetrates about 250 ft of the Snake River Plain aquifer and well EBR 1 penetrates about 490 ft of the aquifer. Most of the other wells in the tritium plume penetrate only the uppermost 50 to 200 ft of the aquifer. Tritium concentrations in water from wells USGS 83 and EBR 1 were less than the reporting level possibly because of dilution by water from deeper zones, a phenomena described by Mann and Cecil (1990, p. 18) for these wells.
Tritium concentrations in water from wells south of the disposal well at INTEC (fig. 6) generally decreased during 1980–2005 (table 5) possibly in response to a decreased rate of tritium disposal from the INTEC and radioactive decay. Tritium concentrations in water from well USGS 59, near the INTEC percolation ponds (fig. 6), generally have decreased since 1980, but were unusually large in October 1983, 1985, 1991, and 1995 (table 5). The larger concentrations in 1983 and 1985 correlate with higher annual tritium discharge rates, however, annual tritium discharge was low in 1991 and 1995 (fig. 9). In 1986, perched water was detected outside the casing in well USGS 59. Following modifications to the well to prevent seepage of water into the well, a video log showed that some water from the perched zone was still seeping into the well. The larger concentrations in 1991 and 1995 could be the result of seepage from a perched zone. The larger concentrations also correlate with the use of the east infiltration pond and with disposal of tritium to the ponds. The smaller concentrations in water from well USGS 59 in 1989, 1993, 1994, and from 1996 to 2000 correlate with years in which little or no tritium was discharged to the percolation ponds (fig. 9). The slight increase in tritium concentrations in wells USGS 38, 47, 59, 77, and 111 between 2000 and 2001 (table 5), could have resulted from disposal of 0.03 Ci of tritium (Stoller Corporation, 2002b) to the INTEC percolation ponds and the lack of dilution by ground-water recharge because of low streamflows in the Big Lost River during 2000.
A strontium-90 plume developed in the Snake River Plain aquifer from wastewater disposal at the INL. Strontium-90 has a half-life of 29.1 years (Walker and others, 1989, p. 29). During 1952–98, about 24 Ci of strontium-90 was in wastewater injected directly into the aquifer through the disposal well and discharged to percolation ponds at the INTEC (Bartholomay and others, 2000). During this period, about 93 Ci of strontium-90 also was discharged to radioactive-waste infiltration and evaporation ponds at the RTC. During 1962–63, more than 33 Ci of strontium-90 in wastewater was discharged into a pit at the INTEC (Robertson and others, 1974, p. 117). During 1996–98, about 0.03 Ci of strontium-90 was discharged to percolation ponds at the INTEC (Bartholomay and others, 2000). During 1999, less than 0.001 Ci of strontium-90 was discharged at the INTEC or RTC (Stoller Corporation, 2002a, table 7-2); during 2000, 0.21 Ci of strontium-90/yttrium-90 was discharged at the RTC (Stoller Corporation, 2002b, table 6-2). No data are available for strontium-90 discharged during 2001–05.
In October 2005, 34 aquifer wells were sampled for strontium-90 throughout the INL. Concentrations of strontium-90 in water from 14 wells exceeded the reporting level. Concentrations ranged from 2.2±0.7 to 33.1±1.2 pCi/L. However, concentrations from most wells have remained relatively constant or decreased since 1989 (table 6). The area of the strontium-90 plume near the INTEC extended south-southwestward in the general direction of ground-water flow (fig. 15). The concentrations in water from wells USGS 37 and 45 have varied since 1980; the concentrations in water from well USGS 37 decreased during 2002–05, but increased slightly in water from well USGS 45 (table 6). Concentrations in water from well USGS 37 exceeded the reporting level in most years during 1980–2005, but were less than the reporting level in 1991 and 1995 (table 6). Concentrations in water from well USGS 45 were equal to or less than the reporting level for most years from 1984 to 2001, but exceeded the reporting level in 1990, 1991, 1993, 1998, in a replicate sample collected in 1995, and from 2002-05 (table 6). The October 1995 concentration of 76±3 pCi/L in water from well USGS 47 was larger than concentrations in most previous samples, but the quality-assurance replicate concentration of 47±2 pCi/L was similar to concentrations in most previous samples. Concentrations of strontium-90 in this well show an overall decrease since 1996. The concentrations in wells USGS 57 and 113 generally decreased from the 1980s to 2005, although concentrations in well USGS 57 increased during 2001–02 (table 6). The MCL for strontium-90 in drinking water is 8 pCi/L (U.S. Environmental Protection Agency, 2001).
Before 1989, strontium-90 concentrations in most wells had been decreasing likely because of factors including radioactive decay, diffusion, dispersion, changes in disposal methods, and dilution from natural recharge (Orr and Cecil, 1991, p. 35). The relatively constant concentrations in water from most of the wells sampled during 1992–95 could have resulted partly due to a lack of recharge from the Big Lost River. An increase in disposal of other chemical constituents into the percolation ponds also could have affected the exchange capacity of strontium-90 in the unsaturated zone (Bartholomay and others, 1997). The decrease of strontium-90 concentrations in water from some wells during 1999–2005 could be the result of the factors previously mentioned.
Strontium-90 has not been detected in the eastern Snake River Plain aquifer beneath the RTC partly because of the exclusive use of waste-disposal ponds and lined evaporation ponds rather than the disposal well for radioactive-wastewater disposal at the RTC. Sorption processes in sediments in the unsaturated zone beneath the radioactive waste-disposal pond could have minimized or prevented strontium-90 migration to the aquifer at the RTC. Additionally, the stratigraphy beneath the RTC includes more sediment than the stratigraphy beneath the INTEC (Anderson, 1991, p. 22–28).
In 1988, a DOE contractor was given the responsibility for monitoring areas around TAN and the TAN disposal well as part of the Environmental Restoration Program. The USGS collected samples from wells in the area for special studies in 1989, but the USGS has collected no samples in this area since December 1989. During 1988–96, four samples analyzed for strontium-90 from well USGS 24, just south of the TAN area, yielded one reportable concentration in 1990.
During 1952–93, about 438 Ci of cobalt-60 in wastewater was discharged to the RTC radioactive-waste infiltration ponds. Before 1974, the average disposal rate was about 18 Ci/yr; during 1974–88, the average disposal rate was 2.3 Ci/yr (Orr and Cecil, 1991, p. 35). During 1989–91, about 0.5 Ci of cobalt-60 was discharged to the ponds; during 1992–93, about 3.1 Ci of cobalt was discharged to the ponds. The half-life of cobalt-60 is 5.27 years (Walker and others, 1989, p. 25).
Cobalt-60 concentrations in water from well USGS 65 (fig. 6), south of the RTC, exceeded the reporting level through 1985 (Orr and Cecil, 1991, p. 35); however, cobalt-60 has not been detected since 1985. The decreased discharge of cobalt-60 to the RTC radioactive-waste infiltration ponds, the use of lined evaporation ponds, and processes of radioactive decay and sorption in the unsaturated and perched ground-water zones could have contributed to the absence of detectable concentrations of cobalt-60 in ground water near the RTC since 1985.
Cobalt-60 concentrations in water from the TAN disposal well (fig. 5) exceeded the reporting level because of radioactive wastewater discharged to the well before 1972. In 1988, a DOE contractor was given the responsibility for monitoring areas around TAN and the TAN disposal well as part of the Environmental Restoration Program. Samples were collected by the USGS in 1989 for special studies but no samples have been collected since December 1989. Water from the TAN disposal well contained 170±40 pCi/L of cobalt-60 in December 1989.
During 1996–98, cobalt-60 concentrations in water from all wells sampled by the USGS at the INL were less than the reporting level. Cobalt-60 was not detected in any samples collected during 1999–2005.
From 1952 to 2000, about 138 Ci of cesium-137 in wastewater was discharged to the RTC radioactive-waste infiltration and lined evaporation ponds and about 23 Ci was discharged to the INTEC disposal well and infiltration ponds. During 1999–2000, about 0.009 Ci was discharged to the RTC lined evaporation ponds, and less than 0.001 Ci/yr was discharged to the INTEC percolation ponds (Stoller Corporation, 2002a, table 7-2, footnote b; 2002b, table 6-2, footnote b). No data are available for cesium-137 discharged during 2001–05. The half-life of cesium-137 is 30.17 years (Walker and others, 1989, p. 34).
Concentrations of cesium-137 in water from wells USGS 40 and 47 (fig. 6) exceeded the reporting levels through 1985 (Orr and Cecil, 1991, p. 35) but were less than the reporting level since 1985. The absence of detectable concentrations of cesium-137 probably resulted from discontinuation of wastewater discharge to the INTEC disposal well and to sorption processes in the unsaturated and perched ground-water zones.
Cesium-137 concentrations in water from the TAN disposal well (fig. 5) exceeded the reporting level because of wastewater discharge to the well before 1972. Because the responsibility for monitoring the TAN disposal well was turned over to a DOE contractor in 1988, samples collected by the USGS were in December 1989 only for special studies. The cesium-137 concentration at that time was 4,400±200 pCi/L (http://waterdata.usgs.gov/id/nwis/qw, accessed June 29, 2006).
During 2002–05, concentrations of cesium-137 were less than the reporting level in water from all wells sampled by the USGS at the INL.
Monitoring of plutonium-238 and plutonium-239, -240 (undivided) in wastewater discharged to the Snake River Plain aquifer through the disposal well (fig. 6) at INTEC began in 1974. Before that time, alpha radioactivity from disintegration of plutonium was not separable from the monitored, undifferentiated alpha radioactivity. The half-lives of plutonium-238, plutonium-239, and plutonium-240 are 87.7, 24,100, and 6,560 years, respectively (Walker and others, 1989, p. 46). During 1974–95, about 0.26 Ci of plutonium in wastewater was discharged to the disposal well and percolation ponds at the INTEC (Bartholomay and others, 1997). During 1996–98, about 0.004 Ci of plutonium in wastewater was discharged to percolation ponds at the INTEC. During 1999–2000, less than 0.001 Ci of plutonium was discharged (Stoller Corporation, 2002a, table 7-2, footnote b; 2002b, table 6-2, footnote b). No discharge data are available for 2001–05.
Because of radioactive wastewater discharged to the disposal well at INTEC, concentrations of plutonium isotopes in some samples from wells USGS 40 and 47 (fig. 6) through January 1987 exceeded the reporting level (Orr and Cecil, 1991, p. 37). Concentrations in samples collected from these wells since 1987 have been less than the reporting level.
Plutonium isotopes in water from the TAN disposal well (fig. 5) exceeded the reporting level because of radioactive-wastewater discharges before 1972. Because the responsibility for monitoring TAN disposal well was turned over to a DOE contractor in 1988, the only samples collected by the USGS since that time were collected in December 1989. The concentration of plutonium-238 in water from the TAN disposal well at that time was 0.26±0.04 pCi/L and the concentration of plutonium-239, -240 (undivided) was 0.71±0.06 pCi/L (Bartholomay and others, 1995).
During 2002–2005, concentrations of plutonium-238 and plutonium-239, -240 (undivided) in water from all wells sampled by the USGS at the INL were less than the reporting level.
Americium-241 is a decay product of plutonium-241 and plutonium isotopes have been detected in wastewater discharged to the Snake River Plain aquifer at the INL and are in wastes buried at the RWMC. The half-life of americium-241 is 432.7 years (Walker and others, 1989, p. 46). Concentrations of americium-241 in water samples collected between September 1972 and July 1982 from wells USGS 87, 88, 89, and 90 at the RWMC (fig. 6) and in water samples collected through 1988 from the TAN disposal well (fig. 5) exceeded the reporting level (Orr and Cecil, 1991, p. 38–39). During 1992–95, concentrations of americium-241 in samples from two wells were equal to the reporting level. On October 2, 1992, the concentration in water from well USGS 37 was 0.09±0.03 pCi/L; on April 20, 1993, the concentration in water from well USGS 120 was 0.06±0.02 pCi/L (Bartholomay and others, 1997). During 1996–2005, concentrations in all samples were less than the reporting level except one sample collected April 12, 2001, from the RWMC Production Well (RWMC PROD) with a concentration of 0.003±0.001 pCi/L, equal to the reporting level.
Gross alpha- and beta-particle radioactivity is a measure of the total radioactivity given off as alpha and beta particles during the radioactive decay process. Gross alpha and beta measurements are used to screen for radioactivity in the aquifer as a possible indicator of ground-water contamination. Background concentrations of gross beta-particle radioactivity in the Snake River Plain aquifer in Idaho generally range from 0 to 7 pCi/L as cesium-137 (Knobel and others, 1992). Background concentrations of gross alpha particle radioactivity range from 0 to 3 µg/L as natural uranium (Knobel and others, 1992).
Before 1994, gross alpha- and beta-particle radioactivity in water from three wells west and south of the INL (wells USGS 8, 11, and 14, fig. 5) and four surface-water sites along the Big Lost River (fig. 1) were sampled. As part of the INL ground-water monitoring program adopted in 1994 (Sehlke and Bickford, 1993), the USGS expanded the number of wells at the INL used for sampling gross alpha- and gross beta-particle radioactivity.
During 2002–05, water from 54 wells was sampled for gross alpha- and gross-beta particle radioactivity. As in October 2001, concentrations of gross alpha-particle radioactivity were less than the reporting level in all samples. Concentrations of gross-beta particle radioactivity greater than the reporting level in at least one sample collected during 2002–05 were detected in water samples from 18 of the 54 wells and ranged from 6±2 to 44±4 pCi/L, a decrease in the number of wells with reportable concentrations and the maximum concentration from the 1999–2001 reporting period. A concentration of 60±5 pCi/L was detected in well USGS 57 on July 6, 1999; however, this well was not sampled for gross-beta particle activity during 2002–05. Gross-beta particle activity in most of the 18 wells showed steady or decreasing concentration trends during 2002–05.
During April or October 2005, water in 48 wells was sampled for gross alpha- and gross beta-particle radioactivity. Concentrations of gross alpha-particle radioactivity were less than the reporting level in all samples. Concentrations of gross beta-particle radioactivity in water from 4 of the 48 wells sampled in 2005 were greater than the reporting level and ranged from 6±2 to 25±3 pCi/L. Of the 48 wells sampled in 2005, the largest concentration was in well USGS 38, but the concentration decreased in water from this well since the 1999–2001 reporting period.
Wastewater from RTC cooling-tower operations contained an estimated 24,000 lb of chromium discharged to an infiltration pond during 1952–64 and an estimated 31,000 lb discharged to an injection well during 1965–72 (Mann and Knobel, 1988, p. 7). In October 1972, chromium used as a corrosion inhibitor in cooling-tower operations was replaced by a polyphosphate. No disposal of chromium to the subsurface at the RTC was reported after 1972. During 1971–83, about 265 lb of chromium in wastewater were discharged to the disposal well at INTEC and 720 lb of chromate were discharged at the Power Burst Facility (fig. 1) (Cassidy, 1984, p. 3). About 86 lbs of chromium were discharged to the INTEC percolation ponds during 1992–95 (Bartholomay and others, 1997) and 44 lbs during 1996–98 (Bartholomay and others, 2000). No information has been compiled on the total amount of chromium discharged during 1999–2005.
Background concentrations of chromium in the Snake River Plain aquifer range from 2 to 3 µg/L (Orr and others, 1991, p. 41). In April 2005, the MCL of 100 µg/L (U.S. Environmental Protection Agency, 2001) for total chromium in drinking water was equaled in water from one well, USGS 65, south of RTC (fig. 6). The concentration of chromium in water from that well was 100 µg/L, a decrease from 139 µg/L in October 2001 (Bartholomay and others, 2000). Concentrations in water samples from other wells ranged from 1.7 to 30.3 µg/L. The LRL for chromium ranged from 2 to10 µg/L during 2002–05; consequently, concentrations within that range were designated according to those LRLs as detections or nondetections during 2002–05.
During 1989–98, an estimated average annual 1.3 million lb/yr of sodium in wastewater were discharged at the INL (Bartholomay and others, 1995, 1997, and 2000). During 1996–98 about 708,000 lb/yr of sodium were discharged to the INTEC percolation ponds; about 58,000 lb/yr were discharged to the RTC chemical-waste infiltration pond; about 524,000 lb/yr were discharged to the NRF industrial-waste ditch; and about 5,000 lb/yr were discharged at CFA (Bartholomay and others, 2000) (fig. 1). The total amount of sodium discharged at the RTC was the amount of sodium ion estimated from the sodium hydrate solution discharged (Bartholomay and others, 2000). The total amount of sodium in wastewater discharged at individual facilities from 1999–2005 has not been compiled.
The background concentration of sodium in water from the Snake River Plain aquifer near the INL generally is less than 10 mg/L (Robertson and others, 1974, p. 155). In October 2001, concentrations in water from most wells in the southern part of the INL were greater than 10 mg/L.
Concentrations of sodium in water from wells near the INTEC generally have been variable since 1984 when disposal practices were changed from injection to the disposal well to discharge to percolation ponds (fig. 6; table 7, wells USGS 37, 40, 47, 57, 59, 111, and 113). During 1984–98, estimated discharge rates increased slightly at the INTEC, so the variability in concentrations in water from some wells could have resulted from this increase in discharge rates (Bartholomay and others, 2000). During 1999–2001, the larger concentrations of sodium were in water from wells at or near INTEC. During 2002–05, the largest concentration in water samples from aquifer wells at the INL was 76 mg/L in a sample from well USGS 113 (fig. 6, table 7), south of INTEC. Water from this well had the highest concentration of sodium of 76 mg/L in October 2002 but concentrations decreased through 2004 (table 7). Concentrations of sodium in water from other wells south of INTEC during 2002–05 generally were equal to or less than sodium concentrations detected in October 2001, with the exception of well USGS 40, which was slightly higher (table 7).
In 2004–05, sodium concentrations in water from wells USGS 88 and 120 (fig. 6), near the RWMC, were 41 and 24 mg/L. In March 2005, water from well MTR Test at the RTC (fig. 6), contained a sodium concentration of 10 mg/L, significantly less than the 1998 concentration of 42 µg/L and slightly less than the October 2001 concentration of 15 mg/L.
About 2.3 million lb/yr of chloride in wastewater was discharged to infiltration ponds at the INL during 1996–98, an increase from the estimated 1.5 million lb/yr discharged during 1992–95 (Bartholomay and others, 1997, p. 36). Of the 2.3 million lb/yr discharged during 1996–98, about 1.17 million lb/yr were discharged to the INTEC percolation ponds (fig. 3; Bartholomay and others, 2000), which was about the same amount discharged during 1986–95 (Orr and Cecil, 1991, p. 40; Bartholomay and others, 1995, p. 31; Bartholomay and others, 1997, p. 36). No information has been compiled for the total amount of chloride discharged in wastewater during 1999–2005.
The background chloride concentration in water from the Snake River Plain aquifer at the INL generally is about 15 mg/L (Robertson and others, 1974, p. 150); the ambient chloride concentration near the INTEC is about 10 mg/L and, near the CFA, about 20 mg/L. In 2005, concentrations of chloride in most water samples from wells closest to the INTEC and the CFA (fig. 16) exceeded 20 mg/L.
Chloride concentrations in water from wells near the INTEC generally have increased or remained constant since disposal practices were changed from injection to the disposal well to discharge to percolation ponds in 1984 through about 2001. During 2002–05, concentrations decreased in some wells, and increased in others (fig. 6; table 8, wells USGS 37, 40, 47, 57, 59, 111, and 113). Trends in concentrations in water from wells downgradient from the percolation ponds correlated with discharge rates into the ponds when travel time was considered. For example, chloride concentrations in water from wells USGS 37 and 57 were smallest in 1985, during the period (1984–98) when the smallest amount of chloride was discharged to the ponds (fig. 17). Water from well USGS 37 had smaller concentrations of 27 mg/L in April 2001, 24 mg/L in October 2004, and 22 mg/L in October 2005; however, no disposal data are available for 1999–2005. These small values may indicate decreased disposal rates at some time prior to collection of the sample. Concentrations in water from well USGS 37 generally correlated with discharge rates into ponds when longer travel time was considered (fig. 17). Concentrations of chloride in water from well USGS 57 increased as discharge rates increased through 1993; concentrations then decreased through 1995, increased in 1996, and decreased again in 1997 and 1998. Concentrations continued decreasing through October 2000, and then increased through October 2001. Since 2001, concentrations have steadily decreased in both wells. Chloride concentrations in water from USGS 59, near the INTEC percolation ponds, were variable during 1984–2005; concentrations were unusually large in October 1991, 1995, and 2002 (table 8). The larger concentrations probably were caused by seepage down the well from the perched ground-water zone, in which chloride concentrations in perched water wells near the percolation ponds were about 270 mg/L in 1991 and 1995 (Bartholomay and others, 1997). In April 2004, the chloride concentration in water from well USGS 113, south of the INTEC, was 127 mg/L, (table 8), a decrease from the concentration of 175 mg/L in October 2001. In April 2005, water from well CFA 1, also south of INTEC had a slight increase in concentration since October 2001, at 114 mg/L.
In April 2005, the chloride concentration in water from well USGS 65 near the RTC was 19 mg/L. Chloride concentrations in water from all other wells completed in the Snake River Plain aquifer at or near the RTC ranged between 9 and 12 mg/L during 2002–05. During 2002–05, chloride concentrations in water from wells USGS 88, 89, and 120 at the RWMC were 86, 41, and 20 mg/L, respectively, nearly the same as the 1999–2001 reporting period. Concentrations of chloride in all other wells near the RWMC were less than 13 mg/L. The secondary MCL for chloride in drinking water is 250 mg/L (U.S. Environmental Protection Agency, 2001).
During 1996–98, about 0.8 million lb/yr of sulfate in wastewater were discharged at the INL, a decrease from the 1.05 million lb/yr discharged during 1992–95 (Bartholomay and others, 2000). Of the 0.8 million lb/yr discharged during 1996–98, about 610,000 lb/yr were discharged to infiltration ponds at the RTC, 146,000 lb/yr were discharged to percolation ponds at the INTEC, and 45,000 lb/year was discharged to the NRF industrial-waste ditch (Bartholomay and others, 2000). Background concentrations of sulfate in the Snake River Plain aquifer in the south-central part of the INL range from about 10 to 40 mg/L (Robertson and others, 1974, p. 72). No compiled data were available for sulfate in wastewater discharged during 1999–2005.
Because of the sulfate disposal history at the various facilities, water-sample collection for sulfate analyses at several wells was added to the water-quality monitoring network in 1995. In 2005, sulfate concentrations in water samples from nine wells in the south-central part of the INL exceeded the 40-mg/L background concentration of sulfate. Concentrations in water samples from MTR Test decreased from 64 mg/L in October 2001 to 23 mg/L in March 2005. A water sample was collected from well USGS 65 with a concentration of 155 mg/L (similar to the October 2001 concentration). The larger-than-background concentrations in water from these wells probably resulted from sulfate disposal at the RTC infiltration ponds.
In October 2005, sulfate concentrations in water samples from USGS 88 and USGS 119 (fig. 6), near the RWMC, were 53 and 40 mg/L, respectively. The concentration of sulfate in well USGS 88 represented a slight decrease in concentration from the October 2001 value of 63 mg/L. The concentration of sulfate in well USGS 119 indicated a slight increase in concentration from the October 2001 value of 34 mg/L, bringing it above background. The larger-than-background concentration in water from these wells could have resulted from the well construction and (or) waste disposal at the RWMC (Pittman and others, 1988, p. 57–61). In October 2004, the sulfate concentration in well CFA 2 (fig. 5), 42 mg/L, also exceeded the background concentration. This was a slight decrease from the October 2001 concentration of 47 mg/L. In 2005, concentrations were 42, 46, and 46 mg/L in water from wells USGS 34, 35, and 39, respectively, southwest of INTEC. Historically, concentrations in these wells were at or just below background. The secondary MCL for sulfate in drinking water is 250 mg/L (U.S. Environmental Protection Agency, 2001).
Wastewater containing nitrate was injected into the Snake River Plain aquifer through the INTEC disposal well from 1952 to February 1984 and discharged to the INTEC percolation ponds after February 1984 (Orr and Cecil, 1991). About 260,000 lb of nitrate were discharged to the INTEC percolation ponds during 1996–98, 220,000 lb of which were discharged during February 1996 (Bartholomay and others, 2000). The average annual discharge rate during 1996–98 was about 86,700 lb, about 50 percent of the discharge rate during 1986–88 and 30 percent of the rate during 1979–85 (Bartholomay and others, 2000). Annual discharge rates of nitrate for 1999–2005 have not been compiled. Concentrations of nitrate in ground water not affected by wastewater disposal from INL facilities generally are less than 5 mg/L (as nitrate) (Robertson and others, 1974, p. 73).
Concentrations of nitrite plus nitrate reported by the NWQL as nitrogen in milligrams per liter have been converted to nitrate in milligrams per liter because (1) nitrate concentrations for aquifer wells are reported as nitrate in this report so that comparisons between plume maps in this report and in previous reports can be made and (2) nitrite analyses indicate that almost all nitrite plus nitrate concentrations in water are nitrate at and near the INL.
Nitrate concentrations at the INL have changed in response to reduced disposal rates and the transition from injection of wastewater to the INTEC disposal well to percolation ponds in 1984. In 1981, the maximum nitrate concentration for wells near the INTEC was 62 mg/L (as nitrate) in water from well USGS 43 at the INTEC (Lewis and Jensen, 1985). By 1985, maximum concentrations in wells near the INTEC ranged from less than 5 to 27 mg/L (as nitrate) (Pittman and others, 1988, p. 61). By 1995, concentrations in wells near the INTEC ranged from less than 5 to 49 mg/L (as nitrate). In 1998, nitrate concentrations in samples from wells CFA 1, USGS 40, 43, and 77 (figs. 5 and 6) were 17, 14, 31, and 18 mg/L (as nitrate), respectively (Bartholomay and others, 2000). The 1998 concentrations represent either a continuation of or a decrease in concentrations from those reported in 1995 (Bartholomay and others, 1997, p. 41). In October 2001, concentrations in samples from these wells were 14, 16, 21, and 16 mg/L (as nitrate), respectively, generally similar to or less than the 1998 concentrations. The decreases could have resulted from dilution by recharge from the Big Lost River and long-term decreases in discharge rates.
In October 2005, concentrations of nitrate in water from wells USGS 41, 43, 45, 47, 52, 57, 67, 77, 112, 114, 115 near the INTEC, exceeded 5 mg/L (as nitrate) and concentrations ranged from 6 mg/L in well USGS 45 to 34 mg/L in well USGS 43.
Historically, nitrate concentrations in water from wells near the RWMC slightly exceeded the regional background concentration of about 5 mg/L (as nitrate) (Orr and Cecil, 1991) or 1 to 2 mg/L as nitrogen (Knobel and others, 1992). In 1998, nitrate concentrations in water samples from wells USGS 88, 89, and 119, near the RWMC, exceeded the expected background and were 7, 9, and 6 mg/L, respectively (as nitrate) (Bartholomay and others, 2000). In 2001, the concentrations of nitrate in water from wells USGS 88, 89, and 119 were relatively unchanged at 7, 8, and 6 mg/L (as nitrate), respectively. In 2005, concentrations of nitrate in water from wells USGS 88, 89, and 119 also remained relatively unchanged at 4, 8, and 7 mg/L (as nitrate), respectively. Near the RTC, the concentration of nitrate in water from well USGS 65 was 7 mg/L, a slight decrease from the 2001 concentration of 8 mg/L (as nitrate). Figure 18 shows the generalized distribution of nitrate concentrations in water samples collected in October 2005. All concentrations measured in 2005 were less than the MCL for drinking water of 44 mg/L [as nitrate, or 10 mg/L as nitrogen (U.S. Environmental Protection Agency, 2001)].
About 39,710 lb of fluoride in wastewater was discharged to percolation ponds at the INTEC during 1971–98 (Bartholomay and others, 2000). Background concentrations of fluoride in the Snake River Plain aquifer in the southwestern part of the INL range from about 0.1 to 0.3 mg/L (Robertson and others, 1974, p. 75). Amounts of fluoride discharged since 1998 have not been compiled.
As part of the INL ground-water monitoring program adopted in 1994, the USGS began analyzing samples collected near the INTEC for concentrations of fluoride. During April through October 2005, water samples from five wells were analyzed for fluoride; detected concentrations ranged from 0.2 to 0.3 mg/L. These concentrations are similar to the background concentrations reported by Robertson and others (1974), which indicates that wastewater disposal has not had an appreciable affect on fluoride concentrations in the Snake River Plain aquifer near the INTEC. The LRL for fluoride was set at 0.16 mg/L beginning October 16, 2000, revised to 0.11 mg/L on October 1, 2001, and to 0.10 mg/L on October 1, 2004. The previous MRL was 0.1 mg/L.
As part of the INL ground-water monitoring program adopted in 1994 and several special sampling programs, water samples from several wells were collected and analyzed for various trace elements during 2002–05. These trace elements were aluminum, antimony, arsenic, barium, beryllium, cadmium, cobalt, copper, iron, lead, lithium, manganese, mercury, molybdenum, nickel, selenium, silver, strontium, thallium, uranium, vanadium, and zinc. A summary of background concentrations of selected constituents in Snake River Plain aquifer water samples is presented in Knobel and others (1992, p. 52). Because the amounts of each constituent in wastewater discharged from INL facilities have not been compiled annually from monitoring data since 1998, these amounts are unavailable for 2002–05.
Beginning in 1998, the NWQL began implementing new reporting procedures based on long-term method detection levels (LT-MDLs) for some analytical methods (Childress and others, 1999). This change in LRLs (as opposed to MRLs) for some elements accounts for concentrations of some elements detected during 1999–2005, although historically the concentrations were less than the MRL. Table 9 presents a summary of disposal data, disposal periods, and trace element concentration ranges in water samples analyzed during 2002–05 by the USGS.
Volatile organic compounds (VOCs) are present in water from the Snake River Plain aquifer because of waste-disposal practices at the INL. In 1987, water samples from 81 wells completed in the Snake River Plain aquifer at and near the INL were analyzed for 36 VOCs as part of a reconnaissance sampling program (Mann and Knobel, 1987). Analyses indicated that concentrations of from 1 to 12 VOCs in samples from 45 wells exceeded their reporting levels. The prevalent compounds were trichloroethylene, 1,1,1-trichloroethane, toluene, tetrachloroethylene, carbon tetrachloride, chloroform, 1,1-dichloroethylene, and dichlorodifluoromethane. In 1988 and 1989, water samples were collected from 38 wells as a continuation of the 1987 study (Mann, 1990). Concentrations of from 1 to 19 VOCs, primarily carbon tetrachloride, 1,1,1-trichloroethane, and trichloroethylene, in water samples from 22 wells exceeded the MRLs. In 1990 and 1991, water samples were collected from 76 wells for various water-quality studies at or near the INL (Liszewski and Mann, 1992). Concentrations of from 1 to 14 VOCs, primarily carbon tetrachloride, 1,1,1-trichloroethane, and trichloroethylene, in water samples from 31 of these wells exceeded the MRLs. During 1992–95, water samples were collected from 54 wells at or near the INL for various water-quality studies (Greene and Tucker, 1998). Concentrations of from 1 to 14 VOCs, primarily carbon tetrachloride, 1,1,1-trichloroethane, trichloroethylene, and chloroform, in water samples from 23 of these wells exceeded the MRLs. During 1996–98, water samples were collected from 44 wells at or near the INL for various water-quality studies. Concentrations of from 1 to 12 VOCs, primarily carbon tetrachloride, 1,1,1-trichloroethane, trichloroethylene, chloroform, and tetrachloroethylene, in water samples from 15 of these wells exceeded the MRLs (Bartholomay and others, 2000). During 1999–2001, water samples from 36 wells at and near the INL were analyzed for VOCs. Ten VOCs were detected. Concentrations of from 1 to 5 VOCs, primarily carbon tetrachloride, 1,1,1-trichloroethane, trichloroethylene, chloroform, and tetrachloroethylene, in water samples from 17 of these wells exceeded the MRLs. The MRL for some VOCs was revised from 0.2 to 0.1 µg/L during 1998–2001, a change that resulted in detections of smaller concentrations than in previous years.
During 2002–05, water samples from 30 wells were collected and analyzed for VOCs. Twelve VOCs were detected. Concentrations of from 1 to 9 VOCs were detected in water samples from 13 wells. The primary VOCs detected included carbon tetrachloride, chloroform, 1,1-dichloroethane, 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethylene.
A plume of 1,1,1-trichloroethane, a solvent used in industrial cleaning processes (Lucius and others, 1989, p. 450), has developed in the Snake River Plain aquifer near the INTEC because of waste-disposal practices (Bartholomay and others, 1995). During 1992–95, water samples were collected from 10 wells near the INTEC that previously contained water with concentrations of 1,1,1-trichloroethane exceeding the MRL. Concentrations in water from 8 of the 10 wells exceeded the MRL (Bartholomay and others, 1997). During 1996–98, water samples were collected from three wells near INTEC that previously contained water with concentrations of 1,1,1-trichloroethane exceeding the MRL; concentrations in water from all three of the wells exceeded the MRL.
During 2004–05, concentrations of 1,1,1-trichloroethane in water from wells USGS 34, 38, 65, and 77, south of the INTEC, exceeded the MRL. The concentrations in water from wells USGS 34 and 38 were 0.10 and 0.11 µg/L, respectively. The concentrations in water in wells USGS 65 and 77 were 0.14 and 0.2 µg/L, respectively. The MRL for 1,1,1-trichloroethane varied between 0.2 to 0.1 µg/L during 2002–05. The detection of these small concentrations resulted from the lower MRL. All 1,1,1-trichloroethane concentrations were less than the MCL for drinking water of 200 µg/L (U.S. Environmental Protection Agency, 2001). Water from wells USGS 65 and 77 also contained concentrations of 1,1-dichloroethylene ranging from 0.10 to 0.18 µg/L during 2002–05.
During 1996–98, concentrations of VOCs in water samples from several wells at and near the RWMC exceeded the reporting levels (Bartholomay and others, 2000). For example, in October 1998, water from the RWMC Production Well contained 4.5 µg/L of carbon tetrachloride, 0.8 µg/L of chloroform, 0.5 µg/L of 1,1,1-trichloroethane, 2.1 µg/L of trichloroethylene, and 0.18 µg/L of tetrachloroethylene (Bartholomay and others, 2000). In December 2005, 9 VOCs were detected in water from the RWMC Production Well. Reported concentrations were 6.3 µg/L of carbon tetrachloride, 1.7 µg/L of chloroform, 0.52 µg/L of 1,1,1-trichloroethane, 3.2 µg/L of trichloroethylene, and 0.28 µg/L of tetrachloroethylene. Concentrations of all these VOCs increased since October 2001. A plot of carbon tetrachloride concentrations in water from the RWMC Production Well (fig. 19) indicates that concentration trends generally have increased with time. Water from the RWMC Production Well also yielded detections of 0.20 µg/L of xylene, 0.33 µg/L of bromodichloromethane, 0.73 µg/L of dibromochloromethane, and 1.6 µg/L of tribromoethane.
In April 2005, concentrations of carbon tetrachloride, 1,1,1-trichloroethane, tetrachloromethane, trichloroethylene, and chloroform in water from well USGS 87 (fig. 6) exceeded the reporting levels. Concentrations of carbon tetrachloride, trichloroethylene and chloroform in water from well USGS 88 (fig. 6) also exceeded the reporting levels in October 2005. In April 2004, concentrations of carbon tetrachloride, 1,1,1-trichloroethane, trichloroethylene, and chloroform in water from well USGS 120 (fig. 6) exceeded the reporting level. In April 2002, concentrations of carbon tetrachloride exceeded the reporting level in water from well USGS 119, south of the RWMC.
During 1987–89, concentrations of from 1 to 15 VOCs in water from 10 wells near the TAN exceeded their reporting levels (Mann and Knobel, 1987; Mann, 1990). Water samples from TAN wells were not collected by the USGS during 1990–93 because the wells were not part of routine sampling. During 1994–95, samples from six wells near the TAN were collected and analyzed as part of the INL ground-water monitoring program (Sehlke and Bickford, 1993). One sample from the well No Name 1 (formerly called TAN Expl. Well; fig. 2) contained 1.1 µg/L of isopropylbenzene. One sample from well ANP 9 (fig. 2) contained 11 µg/L of toluene (Bartholomay and others, 1997). During 1996–98, samples were collected from five wells near TAN as part of the USGS ground-water monitoring program. No VOC concentrations exceeded their reporting levels. Additionally, water from well USGS 24 (fig. 2) was analyzed in 1996 and concentrations of nine VOCs exceeded their reporting levels. Concentrations of two of these VOCs, 990 µg/L of trichloroethylene and 46 µg/L of tetrachloroethylene, exceeded their respective MCLs of 5 µg/L for drinking water (U.S. Environmental Protection Agency, 1998; Bartholomay and others, 2000). During 2002–05, water samples from three wells near TAN (ANP 9, No Name 1, and PSTF Test) (fig. 2) were sampled for VOCs. Concentrations of VOCs in water from these wells were all less than the MRL with the exception of chloroform, detected in all three wells. Concentrations ranged from 0.14 to 0.2 µg/L.
Analyses of total organic carbon (TOC) are used to screen for organic compounds in the aquifer as a general indicator of ground-water contamination. As part of the INL ground-water monitoring program adopted in 1994, the USGS began collecting and analyzing water from several wells at the INL for TOC. During October 2005, water samples from 21 wells completed in the Snake River Plain aquifer at the INL were analyzed for TOC; detected concentrations ranged from 0.44 to 8.0 mg/L. The LRL for TOC was set at 0.27 mg/L beginning October 1, 1999, and revised to 0.6 mg/L beginning October 20, 2000. The previous MRL was 0.1 mg/L. The MRL was set to 0.4 mg/L in October 2002.
Specific conductance is a measure of the electrical conductivity of water and is proportional to the quantities of dissolved chemical constituents in the water. Dissolved chemical constituents such as chloride, sodium, and sulfate in wastewater discharged to disposal wells and infiltration ponds at INL facilities generally have increased the specific conductance of ground water through time.
The general increase in specific conductance in ground water attributed to wastewater discharged to the aquifer since the mid-1980s is apparent in ground water downgradient from INL facilities. A plume of increased specific conductance originated from the INTEC percolation ponds (fig. 6) and extended downgradient from the INTEC to the CFA (fig. 20). The specific conductance of water from several wells within this plume increased from about 500 µS/cm in 1985 (Pittman and others, 1988, p. 64) to more than 1,000 µS/cm in 1998, but decreased to about 960 µS/cm by 2001 in water from well USGS 51. This decreasing trend continued during 2002–05; the maximum specific conductance measured in well CFA LF 3-9 (fig. 6) was 819 µS/cm in October 2005. Specific conductance of water from well USGS 113 (fig. 6) was 1,080 µS/cm in October 1998, decreased to 937 µS/cm in October 2001, and continued to decrease to 774 µS/cm by April 2004. No measurements are available from well USGS 113 for 2005 because the well was out of service for pump maintenance.
The specific conductance of water from several wells at the RTC exceeded 400 µS/cm in 2005. Specific conductance of water from well USGS 65 (fig. 6), downgradient from the infiltration ponds at the RTC, was 607 µS/cm in April 2005. Maximum specific conductance measured in water from well USGS 88, near the RWMC, was 570 µS/cm in October 2005, a slight decrease from the October 2001 measurement of 581 µS/cm.
In 2005, the specific conductance of water from 126 wells ranged from 234 to 819 µS/cm; the median specific conductance was 389 µS/cm. This represents a decrease in overall specific conductance values since 2001 when the range for 126 wells was 262–960 µS/cm and the median specific conductance was 402 µS/cm.
During each year, 2002–05, water temperature and pH were measured in water from 127, 130, 128, and 126 wells at the INL, respectively. The lowest water temperatures were consistently in well P&W 2 (fig. 5), ranging from 7.7 to 8.2°C. The highest water temperatures were consistently in well USGS 7, ranging from 18.9 to 19.5°C. The median water temperature for all wells sampled each year, 2002–05 was 12.9, 12.8, 12.8, and 12.9°C, respectively, a slight decrease from the 1999–2001 reporting period when the median was 13.0°C for each year.
In 2002, pH ranged from 7.5 in well USGS 4 (fig. 5) to 8.5 in well USGS 88 (fig. 6). In 2003, pH ranged from 7.2 in water from the RWMC Production Well to 8.4 in water from well USGS 119 (fig. 6). In 2004, pH ranged from 7.4 in water from the RWMC Production Well to 8.4 in water from well USGS 89 (fig. 6). In 2005, the pH ranged from 7.1 in well USGS 4 (fig. 5) to 8.8 in well USGS 119 (fig. 6). The median pH in water from all wells for each year 2002–05 was 8.0, 7.9, 7.9, and 7.6, representing a slight decrease in pH from the 1999-2001 reporting period when median pH was 8.0 each year.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2008/5089
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