Scientific Investigations Report 2006-5088

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5088

Back to Table of Contents

Selected Radiochemical and Chemical Constituents and Physical Properties of Water in the Snake River Plain Aquifer, 1999–2001

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 were interpreted from analyses of samples collected from a 3-dimensional flow system. Concentra­tions of these constituents represent samples collected during October 2001 from various depths in the aquifer and with 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 at the INL during 1999–2001 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, purgeable organic compounds, and total organic carbon. 

Tritium

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 principal 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 infiltration ponds at the INTEC and RTC (fig. 5). Routine use of the disposal well at INTEC ended in February 1984; subsequently, radioactive wastewater has been discharged to the infiltration ponds. About 31,620 Ci of tritium in wastewater was discharged to the well and infiltration 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 Corp., 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 Corp., 2002a, 2002b) (fig. 7). Data for the total amount of tritium in wastewater discharged in 2001 are unavailable.

In October 2001, concentrations of tritium in water that exceeded the reporting level ranged from 0.43±0.14 to 13.6±0.6 pCi/mL and the tritium plume extended south-southwestward in the general direction of ground-water flow (fig. 12). The area of the plume in which concentrations exceeded the maximum contaminant level (MCL) of 20 pCi/mL (U.S. Environmental Protection Agency, 2001) was about 2.4 mi2 in 1991 (Bartholomay and others, 1995). In 1995, five wells sampled by the USGS located 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 2001, 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 of the wells at the INL during 1999–2001. Tritium concentrations in water from several wells south of the INTEC were smaller in 2001 than those measured in 1998. Concentrations decreased by as much as 8.3 pCi/mL during 1999–2001. Concentrations in water from well USGS 123 (fig. 5), southwest of the INTEC, decreased from 16.6±0.7 pCi/mL in April 1998 to 8.3±0.4  pCi/mL in April 2001. Concentrations in water from well USGS 114 decreased from 19.7±0.8 to 13.2±0.5 pCi/mL from July 1998 to July 2001. Concentrations in water from well USGS 77 decreased from 19.7±0.8 to 14.1±035 pCi/mL from April 1998 to April 2001. Concentrations in well CFA LF 3-9 (fig. 5), north of the Central Facilities Area, decreased from 14.8±0.6 to 9.3±0.5 pCi/mL from July 1998 to October 2001. The decrease in tritium concentrations in water from wells south of the INTEC could be the result of the decrease in discharge of tritium to the infiltration ponds since the early 1990s.

Near the southern boundary of the INL, tritium concentrations in water from wells USGS 103, 105, and 108 (fig. 4), 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 allowed for smaller concentrations of tritium to be identified during 1996–98. During 1999–2001, concentrations of tritium in water from wells near the southern boundary of the INL (USGS 1, 103, 105, 108, 109, 110A) (fig. 4) 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. 4) within and near the tritium plume (fig.  12) were less than the reporting level during 1999-2001. 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. 5) generally decreased during 1980–2001 (table 3) 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 infiltration ponds (fig. 5), generally have decreased since 1980, but were unusually large in October 1983, 1985, 1991, and 1995 (table 3). 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. 7). 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 infiltration ponds (fig. 7). The slight increase in tritium concentrations in wells USGS 38, 47, 59, 77, and 111 between 2000 and 2001 (table 3), could be a result of the disposal of 0.03 Ci of tritium (Stoller Corp., 2002b) to the INTEC infiltration ponds during 2000 and the lack of dilution by ground-water recharge because of low streamflows in the Big Lost River during that year.

Strontium-90

A strontium-90 plume has developed in the Snake River Plain aquifer from the disposal of wastewater 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 infiltration ponds at the INTEC (Bartholomay and others, 2000). During this time 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 infiltration 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 Corp., 2002a, table 7-2); during 2000, 0.21 Ci of strontium-90/yttrium-90 was discharged at the RTC (Stoller Corp., 2002b, table 6-2). Data are unavailable for the amount of strontium-90 discharged in 2001.

In October 2001, concentrations of strontium-90 in water from 19 wells exceeded the reporting level. However, concentrations from most wells have remained relatively constant or decreased since 1989 (table 4). Concentrations ranged from 2.1±0.7 to 42.4±1.4 pCi/L and the area where strontium-90 was detected extended south-southwest­ward in the general direction of ground-water flow (fig. 13). The concentrations in water from wells USGS 37 and 45 have varied since 1980, but decreased between 1999–2001 (table 4). Concentrations in water from well USGS 37 exceeded the reporting level during 1980–2001, but were less than the reporting level in 1991 and 1995 (table 4). 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, and in a replicate sample collected in 1995 (table 4). 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. The 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 2001, although the concentration in well USGS 57 increased between 2000–2001 (table 4). 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 a combination of factors including changes in disposal methods, radioactive decay, diffu­sion, dispersion, 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 been partly a result of a lack of recharge from the Big Lost River. An increase in disposal of other chem­ical constituents into the infiltration 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 1999–2001 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. This can be explained partly by 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. In addition, the stratigraphy beneath the RTC is different from that beneath the INTEC in that more sediment is present below the RTC (Anderson, 1991, p. 22–28).

Cobalt-60

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.  5), south of the RTC, exceeded the reporting level through 1985 (Orr and Cecil, 1991, p. 35) but have not been detected since 1985. The decrease in discharge of cobalt-60 to the RTC radioactive-waste infiltration ponds, the change to 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. 4) exceeded the reporting level because of the discharge of radioactive wastewater to the well before 1972. The responsibility for monitoring the TAN disposal well was turned over to a DOE contractor in 1988 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 1999–2001.

Cesium-137

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 infiltration ponds (Stoller Corp., 2002a, table 7-2, footnote b; 2002b, table 6-2, footnote b). 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. 5) exceeded the reporting levels through 1985 (Orr and Cecil, 1991, p. 35) but have been less than the reporting level since 1985. The absence of detectable concentrations of cesium-137 probably is the result of the 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. 4) 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, the only samples collected by the USGS were in December 1989 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 1999–2001, concentrations of cesium-137 in water from all wells sampled by the USGS at the INL were less than the reporting level.

Plutonium

Monitoring of plutonium-238 and plutonium-239, -240 (undivided) in wastewater discharged to the Snake River Plain aquifer through the disposal well (fig. 5) 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 infiltration ponds at the INTEC (Bartholomay and others, 1997). During 1996–98, about 0.004 Ci of plutonium in wastewater was discharged to infiltration ponds at the INTEC. During 1999–2000, less than 0.001 Ci of plutonium was discharged (Stoller Corp., 2002a, table 7-2, footnote b; 2002b, table 6-2, footnote b). Discharge data for 2001 are unavailable.

Because of the radioactive wastewater discharged to the disposal well at INTEC, concentrations of plutonium isotopes in some samples from wells USGS 40 and 47 (fig. 5) 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. 4) 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 1999–2001, concentrations in water from all wells sampled by the USGS at the INL were less than the reporting level.

Americium-241

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. 5) and in water samples collected through 1988 from the TAN disposal well (fig. 4) exceeded the reporting level (Orr and Cecil, 1991, p. 38–39). During 1992–95, the concentrations of americium-241 in samples from each of 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 1996–2001, concentrations in all samples were less than the reporting level with the exception of one sample from the RWMC Production Well, which had a concentration of 0.003±0.001 pCi/L on April 12, 2001, equal to the reporting level.

Gross Alpha- and Beta-Particle Radioactivity

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. 4) 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 1999–2001, water from 56 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 as they were in October 1998. Thirty-one of the 56 wells sampled had concentrations of gross-beta particle radioactivity greater than the reporting level in at least one sample collected during 1999–2001, and ranged from 6±2 to 60±5 pCi/L. Well USGS 57 had a concentration of 60±5 pCi/L on July 6, 1999; however, this well had not previously been sampled for gross-beta particle activity. Gross-beta particle activity in most of the 31 wells showed steady or decreasing concentration trends during 1999–2001.

During September-October 2001, water in 23 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 5 of the 23 wells sampled in October 2001 were greater than the reporting level and ranged from 6±2 to 57±5 pCi/L. Of the 23 wells sampled in October 2001, well USGS 38 had the largest concentration, and was the only well showing a generally increasing trend in gross-beta particle activity during 1999–2001. Well USGS 38 also is downgradient from well USGS 57 (fig. 3), which had the highest concentration of gross-beta particle activity for the 1999–2001 reporting period.

Chromium

Wastewater from RTC cooling-tower operations contained an estimated 24,000 lb of chromium that was discharged to an infiltration pond during 1952–64 and an estimated 31,000 lb that was discharged to an injection well during 1965–72 (Mann and Knobel, 1988, p. 7). In October 1972, chromium that was used as a corrosion inhibitor in cooling-tower operations was replaced by a polyphosphate. During 1971–83, about 265 lb of chromium in wastewater was discharged to the disposal well at INTEC and 720 lb of chromate was discharged at the Power Burst Facility (fig. 1) (Cassidy, 1984, p. 3). About 86 lbs of chromium was discharged to the INTEC infiltration 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 1999–2001.

Background concentrations of chromium in the Snake River Plain aquifer range from 2 to 3 µg/L (Orr and others, 1991, p. 41). In October 2001, the MCL of 100 µg/L (U.S. Environmental Protection Agency, 2001) for total chromium in drinking water was exceeded in water from one well, USGS 65, south of RTC (fig. 5). The concentration of chromium in water from that well was 139 µg/L, a decrease from 168 µg/L in October 1998 (Bartholomay and others, 2000). Concentrations in water samples from other wells ranged from 6.7 to 21.3 µg/L. The LRL for chromium ranged from 14 µg/L in October 1998 to 0.8 µg/L in October 2001; consequently, concentrations within that range were designated according to those LRLs as detections or nondetections during 1999–2001.

Sodium

During 1989–98, an average annual estimated value of 1.3 million lb/yr of sodium in wastewater was discharged at the INL (Bartholomay and others, 1995, 1997, and 2000). During 1996–1998 about 708,000 lb/yr of sodium was discharged to the INTEC infiltration ponds; about 58,000 lb/yr was discharged to the RTC chemical-waste infiltration pond; about 524,000 lb/yr was discharged to the NRF industrial-waste ditch; and about 5,000 lb/yr was discharged at CFA (Bartholomay and others, 2000) (fig. 1). The total amount of sodium discharged at the RTC is 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 to 2001 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 of the 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 increased slightly or remained constant since disposal practices were changed from injection to the disposal well to discharge to infiltration ponds in 1984 (fig. 3; table 5, wells USGS 37, 40, 47, 57, 59, 111, and 113). During 1984–98, estimated discharge rates increased slightly at the INTEC, so the increase in concentrations in water from some wells could be the result of 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 2001, the largest concentration in water samples from wells at the INL was 75 mg/L in a sample from well USGS 113 (fig. 5, table 5), south of INTEC. Water from this well had the highest concentration of sodium of 99 mg/L in 1999 but concentrations decreased through 2001 (table 5). In 2001, sodium concentrations in water from wells USGS 88 and 120 (fig. 5), near the RWMC, were 44 and 32 mg/L. In October 2001, water from well MTR Test at the RTC (fig. 5), contained a sodium concentration of 15 mg/L, significantly less than the 1998 concentration of 42 µg/L. Concentrations of sodium at INL during 1999–2001 generally were equal to or less than those in 1998, with the exception of well USGS 59 (table 5).

Chloride

About 2.3 million lb/yr of chloride in wastewater was discharged to infiltration ponds at the INL during 1996–98, which was 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 was discharged to the INTEC infiltration 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). Information on the total amount of chloride discharged in wastewater during 1999–2001 has not been compiled.

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 2001, concentrations of chloride in most water samples from the INTEC and south to the CFA (fig. 14) 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 infiltration ponds in 1984 (fig.  3; table 6, wells USGS 37, 40, 47, 57, 59, 111, and 113). Trends in concentrations in water from wells downgradient from the infiltration ponds correlated with discharge rates into the ponds when traveltime was considered. For example, chloride concentrations in water from wells USGS 57 and 37 were smallest in 1985, the year during 1984–98 in which the smallest amount of chloride was discharged into the ponds (fig. 15). Water from well USGS 37 had a smaller concentration of 27.2 mg/L in April 2001; however, disposal data are unavailable for 1999–2001. This small value may indicate decreased disposal rates at some period of time prior to collection of the sample. Con­centrations in water from well USGS 37 generally correlated with discharge rates into ponds when longer traveltime was considered (fig. 15). 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. Chloride concentrations in water from USGS 59, near the INTEC infiltration ponds, were variable during 1984–2001; concentra­tions were unusually large in October 1991 and 1995 (table 6). 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 infiltration ponds were about 270 mg/L in 1991 and 1995 (Bartholomay and others, 1997). In October 2001, chloride concentrations in water from wells USGS 113 and CFA 1, south of the INTEC, were 175 and 103 mg/L, respectively (table 6). These concentrations were decreases from concentrations in October 1998.

In October 2001 at the RTC, the chloride concentration in water from well USGS 65 was 19 mg/L. Chloride concentrations in water from all other wells completed in the Snake River Plain aquifer at or near the RTC were less than the concentration of 13.4 mg/L in water from well USGS 79. At the RWMC, chloride concentra­tions in water from wells USGS 88, 89, and 120 were 81, 40, and 23 mg/L, respectively. Concentrations of chloride in all other wells near the RWMC were less than 19 mg/L. The secondary MCL for chloride in drinking water is 250 mg/L (U.S. Environmental Protection Agency, 2001).

Sulfate

Compiled data for the amount of sulfate in wastewater discharged during 1999–2001 is unavailable. During 1996–1998, about 0.8 million lb/yr of sulfate in wastewater was discharged at the INL, which was 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 was discharged to infiltration ponds at the RTC; 146,000 lb/yr was discharged to infiltration ponds at the INTEC; and 45,000 lb/year was discharged to the NRF industrial-waste ditch (Bartholomay and others, 2000). The 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).

Because of the disposal history of sulfate at the various facilities, water-sample collection for sulfate analyses at several wells was added to the water-quality monitoring network in 1995. In 2001, sulfate concentrations in water samples from five 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 and USGS 65, collected in October 2001 (fig. 5) were 64 and 154 mg/L, respectively, a slight increase in concentrations from samples collected in October 1998. The larger-than-background concentrations in water from these wells probably resulted from sulfate disposal at the RTC infiltration ponds. In October 2001, sulfate concentrations in water samples from USGS 120 and USGS 88 (fig. 5), near the RWMC, were 42 and 63 mg/L, respectively. The concentration of sulfate in well USGS 120 represented a slight decrease in concentration from the October 1998 value of 53 mg/L. The concentration of sulfate in well USGS 88 indicated a slight increase in concentration from the October 1998 value of 59 mg/L. 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). The sulfate concentration in well CFA 2 (fig. 4), 47 mg/L, also exceeded the background concentration. This was a slight increase from the October 1998 concentration of 44 mg/L. The secondary MCL for sulfate in drinking water is 250 mg/L (U.S. Environmental Protection Agency, 2001).

Nitrate

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 infiltration ponds after February 1984 (Orr and Cecil, 1991). About 260,000 lb of nitrate was discharged to the INTEC infiltration ponds during 1996–98, of which 220,000 lb was discharged during February 1996 (Bartholomay and others, 2000). The average annual discharge rate during 1996–98 was about 86,700 lb, which is 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 1999–2001 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 in this report. This conversion was made because (1) comparisons between concentrations given in this report and in previous reports then can be made and (2) nitrite analyses indicate that almost all the nitrite plus nitrate concentration is 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 infiltration 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. 4 and 5) 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). The decreases could have resulted from dilution by recharge from the Big Lost River and long-term decreases in discharge rates.

The original nitrate concentration calculated for a water sample collected on October 22, 2001, from well USGS 43 near the INTEC, was 91 mg/L (as nitrate). This value was verified by the NWQL, but because of the discrepancy between this concentration and historically lower concentrations in water from this well, the concentration of 21 mg/L (as nitrate), calculated for a quality assurance replicate sample collected on the same date from well USGS 43, was used in this report. In October 2001, concentrations of nitrate in water from wells USGS 40, 43, and 77 near the INTEC, and in well CFA 1 were 16, 21, 16, and 14 mg/L (as nitrate), respectively. These were generally smaller in concentration than those in 1998, with the exception of the concentration in water from well USGS 40 which had slightly increased. However, since 1981, there has been an overall decrease in nitrate concentration in water from these wells.

Historically, nitrate concentrations in water from wells near the RWMC have 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. Near the RTC, the concentration of nitrate in water from well USGS 65 was 8 mg/L, a slight increase from the 1998 value of 7 mg/L (as nitrate). Figure 16 shows the generalized distribution of nitrate concentrations in water samples collected in October 2001. All concentrations measured in 2001 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)].

Fluoride

About 39,710 lb of fluoride in wastewater was discharged to infiltration ponds at the INTEC during 1971–98 (Bartholomay and others, 2000). The 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 1999–2001, water samples from 12 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, and changed to 0.11 mg/L on October 1, 2001. The previous MRL was 0.1 mg/L.

Trace Elements

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 1999-2001. 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 1999–2001.

Beginning in 1998, the NWQL began implementing new reporting procedures based on 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 being detected during 1999–2001, although historically the concentrations were less than the MRL. A summary of concentration ranges in water samples analyzed for each trace element during 1999–2001 by the USGS follows.

Aluminum—About 117 lb of aluminum in wastewater was discharged at the INTEC during 1995–98. There are no other recorded discharges of aluminum at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from 17 wells completed in the Snake River Plain aquifer at the INL were analyzed for aluminum; concentrations ranged from 1 to 18 µg/L. The MRL for aluminum during this period was 1 or 10 µg/L; a new LRL was not set.

Antimony—There are no recorded discharges of antimony in wastewater at the INL (Bartholomay and others, 2000). During 1996–98, water samples from nine wells completed in the Snake River Plain aquifer at the INL were analyzed for antimony; concentrations in all samples were less than the MRL of 1 µg/L (Bartholomay and others, 2000). The LRL for antimony was changed to 0.048 µg/L beginning October 1, 2000. Detected concentrations of antimony in water samples from nine wells after October 1, 2000, ranged from 0.09 to 0.24 µg/L. Concentrations reported in samples analyzed prior to the change in reporting conventions were all less than the MRL of 1 µg/L.

Arsenic—About 11 lb of arsenic in wastewater was discharged at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from 17 wells completed in the Snake River Plain aquifer at the INL were analyzed for arsenic; detected concentrations ranged from less than 1 to 3.7 µg/L. The LRL for arsenic was set at 2 µg/L beginning October 1, 1999. The previous MRL was 1 µg/L.

Barium—About 4,740 lb of barium in wastewater was discharged at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from 17 wells completed in the Snake River Plain aquifer at the INL were analyzed for barium; detected concentrations ranged from 16 to 142 µg/L. The MRL for barium of 1 µg/L did not change.

Beryllium—Less than 1 lb of beryllium in wastewater was discharged at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from nine wells completed in the Snake River Plain aquifer at the INL were analyzed for beryllium; all concentrations were less than the LRL of 0.06 µg/L implemented October 1, 2000. The previous MRL was 1 µg/L.

Cadmium—About 22 lb of cadmium was discharged in wastewater at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from nine wells completed in the Snake River Plain aquifer at the INL were analyzed for cadmium; detected concentrations ranged from 0.04 to 0.19 µg/L. The LRL for cadmium was set at 0.037 µg/L beginning October 1, 2000. The previous MRL was 1 µg/L.

Cobalt—No discharges of cobalt in wastewater are recorded at the INL (Bartholomay and others, 2000). During 1999–2001, water samples from nine wells completed in the Snake River Plain aquifer at the INL were analyzed for cobalt; detected concentrations ranged from 0.04 to 0.25 µg/L. The LRL for cobalt was set at 0.015 µg/L beginning October 1, 2000. The previous MRL was 1 µg/L.

Copper—About 81 lb of copper in wastewater was discharged at the INTEC during 1995–98. There are no other recorded discharges at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from nine wells completed in the Snake River Plain aquifer at the INL were analyzed for copper; detected concentrations ranged from 0.23 to 2.2 µg/L. The LRL for copper was set at 0.23 µg/L beginning October 1, 2000. The previous MRL was 1 µg/L.

Iron—About 752 lb of iron in wastewater was discharged at the INTEC during 1995–98. There are no other recorded discharges at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from eight wells completed in the Snake River Plain aquifer at the INL were analyzed for iron; detected concentrations ranged 12 to 50 µg/L. The LRL for iron was set at 10 µg/L beginning October 1, 1998. The previous MRL also was 10 µg/L .

Lead—About 556 lb of lead in wastewater was discharged at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from 17 wells completed in the Snake River Plain aquifer at the INL were analyzed for lead; detected concentrations ranged from 0.1 to 13 µg/L. The LRL for lead was set at 0.08 µg/L beginning October 1, 2000. The previous MRL was 1 µg/L.

Lithium—No discharges of lithium in wastewater are recorded at the INL (Bartholomay and others, 2000). During 1999–2001, water samples from eight wells completed in the Snake River Plain aquifer at the INL were analyzed for lithium; detected concentrations ranged from 4.6 to 5.5 µg/L. The LRL for lithium was set at 0.3 µg/L beginning October 1, 2000. The previous MRL also was 0.3 µg/L.

Manganese—About 44 lb of manganese in wastewater was discharged at the INTEC during 1995–98. There were no other recorded discharges of manganese at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from 17 wells completed in the Snake River Plain aquifer at the INL were analyzed for manganese; detected concentrations ranged from less than 0.16 to 10.5 µg/L. The LRL for manganese was set at 0.1 µg/L beginning October 1, 2000. MRLs used during 1999–2000 for manganese were 1, 2, 2.2, or 3.2 µg/L.

Mercury—About 141 lb of mercury in wastewater was discharged at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from 24 wells completed in the Snake River Plain aquifer at the INL were analyzed for mercury; detected concentrations ranged from 0.51 to 0.53 µg/L. The LRL for mercury was set at 0.011 µg/L beginning October 1, 2001. MRLs used during 1999–2001 were 0.23, 0.1, and 0.01 µg/L.

Molybdenum—No discharges of molybdenum in wastewater are recorded at the INL (Bartholomay and others, 2000). During 1999–2001, water samples from 17 wells completed in the Snake River Plain aquifer at the INL were analyzed for molybdenum; detected concentrations ranged from 1 to 6 µg/L. The LRL for molybdenum was set at 0.2 µg/L beginning October 1, 2000. The previous MRL was 1 µg/L.

Nickel—No discharges of nickel in wastewater are recorded at the INL (Bartholomay and others, 2000). During 1999–2001, water samples from nine wells completed in the Snake River Plain aquifer at the INL were analyzed for nickel; detected concentrations ranged from 0.15 to 3 µg/L. The LRL for nickel was set at 0.06 µg/L beginning October 1, 2000. The previous MRL was 1 µg/L.

Selenium—About 9 lb of selenium in wastewater was discharged at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from six wells completed in the Snake River Plain aquifer at the INL were analyzed for selenium; detected concentrations ranged from 1.2 to 2.6 µg/L. The LRL for selenium was set at 2.4 µg/L beginning October 1, 1999. The previous MRL was 1 µg/L.

Silver—About 190 lb of silver in wastewater was discharged at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from nine wells completed in the Snake River Plain aquifer at the INL were analyzed for silver; concentrations were less than 1 µg/L. The MRL for silver did not change.

Strontium—No discharges of stable strontium isotopes in wastewater are recorded at the INL (Bartholomay and others, 2000). During 1999–2001, water samples from eight wells completed in the Snake River Plain aquifer at the INL were analyzed for stable strontium; detected concentrations ranged from 150 to 315 µg/L. The LRL for strontium was set at 0.08 µg/L beginning October 1, 2000. The previous MRL was 0.2 µg/L.

Thallium—No discharges of thallium in wastewater are recorded at the INL (Bartholomay and others, 2000). During 1999–2001, water samples from five wells completed in the Snake River Plain aquifer at the INL were analyzed for thallium; all concentrations were less than the LRL of 0.04  µg/L implemented October 1, 2000, and the previous MRL of 0.5 µg/L.

Uranium—No discharges of uranium in wastewater are recorded at the INL (Bartholomay and others, 2000). During 1999–2001, water samples from nine wells completed in the Snake River Plain aquifer at the INL were analyzed for uranium; detected concentrations ranged from 1.4 to 2.6 µg/L. The LRL for uranium was set at 0.018 µg/L beginning October 1, 2000. The previous MRL was 1 µg/L.

Vanadium—No discharges of vanadium in wastewater are recorded at the INL (Bartholomay and others, 2000). During 1999–2001, water samples from six wells completed in the Snake River Plain aquifer at the INL were analyzed for vanadium; detected concentrations ranged from 3.6 to 14.1 µg/L. The LRL for vanadium was set at 0.2 µg/L beginning October 1, 2000. The previous MRL was 1 µg/L.

Zinc—About 5,240 lb of zinc in wastewater was discharged at the INL during 1971–98 (Bartholomay and others, 2000). During 1999–2001, water samples from 17 wells completed in the Snake River Plain aquifer at the INL were analyzed for zinc; detected concentrations ranged from 1.4 to 554 µg/L. The MRL of 1 µg/L did not change.

Purgeable Organic Compounds

Purgeable organic compounds (POCs) are present in 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 POCs as part of a reconnaissance sampling program (Mann and Knobel, 1987). Analyses indicated that concentrations of from 1 to 12 POCs 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 POCs, 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 POCs, 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 POCs, 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 POCs, 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 POCS. Ten POCs were detected. Concentrations of from 1 to 5 POCs, 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 POCs was changed from 0.2 to 0.1 µg/L during 1998–2001, a change that resulted in detections of smaller concentrations than in previous years.

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 of the wells near the INTEC that previously had water containing concentrations of 1,1,1-trichloroethane exceeding the MRL. Concentrations in water from 8 of the 10 wells still exceeded the MRL (Bartholomay and others, 1997). Water samples from three of the wells near INTEC that previously had water containing concentrations of 1,1,1- trichloroethane exceeding the MRL were collected during 1996–98, and concentrations in water from all three of the wells exceeded the MRL.

During October 2001, concentrations of 1,1,1‑trichloroethane in water from wells USGS 34, 38, and 77, and a newer well, USGS 128, south of the INTEC, exceeded the MRL. The concentrations in water from wells USGS 34 and 77 were 0.14 and 0.22 µg/L, respectively. The concentrations in water in wells USGS 38 and 128 were 0.15 and 0.16 µg/L, respectively. The detection of these concentrations resulted from the lowering of the MRL from 0.2 to 0.1 µg/L effective October 1, 1998. All 1,1,1‑trichloroethane concentrations were less than the MCL for drinking water of 200 µg/L (U.S. Environmental Protection Agency, 2001). Concentrations of 0.14 and 0.12 µg/L of 1,1 dichloroethylene also were detected in 2001 in two samples from well USGS 77. The detection of these concentrations resulted from the change in the MRL from 0.2 to 0.1 µg/L effective May 12, 2001.

During 1996–98, concentrations of POCs 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 October 2001, water from the RWMC Production well yielded detections for the same POCs, however, all concentrations had decreased from those of October 1998. Reported concentrations were 3.6 µg/L of carbon tetrachloride, 0.5 µg/L of chloroform, 0.4 µg/L of 1,1,1-trichloroethane, 1.6 µg/L of trichloroethylene, and 0.17 µg/L of tetrachloroethylene.

A plot of carbon tetrachloride concentrations in water from the RWMC Pro­duction well (fig. 17) indicates that concentration trends have generally increased with time, although the October 2001 sample concentration was slightly less than the October 1998 concentration. In October 2001, concentrations of carbon tetrachloride, 1,1,1‑trichloroethane, trichloroethylene, and chloroform in water from well USGS 87 (fig. 5) exceeded the reporting levels. The detection of chloroform in 2001 and not in 1998 could have resulted from the lowering of the MRL for chloroform from 0.2 to 0.1 µg/L on October 1, 1998. Concentrations of chloroform and trichloroeth­ylene and estimated values of carbon tetrachloride and 1,1,1‑trichloroethane in water from well USGS 88 (fig. 5) also exceeded the reporting levels in October 2001. The concentrations for chloroform and trichloroethylene were the same or slightly less than October 1998 concentrations. Concentrations of carbon tetrachloride, chloro­form, tetrachloroethylene, 1,1,1-trichloroethane, and trichloroethylene in water from well USGS 120 (fig.  5) exceeded the reporting level; concentrations were the same or slightly less than October 1998 concentrations. Concentrations of carbon tetrachloride, 1,1,1-trichloroethane, trichloroethylene, chloro­form, and tetrachloroethylene in a September 2001 sample from well RWMC M7S (fig. 3) exceeded the MRLs.

During 1987–89, concentrations of from 1 to 15 POCs 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 POC concentrations exceeded their reporting levels. In addition, water from well USGS 24 (fig. 2) was analyzed in 1996 and con­centrations of nine POCs exceeded their reporting levels. Concentrations of two of these POCs, 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 1999–2001, water samples from three wells near TAN (ANP 9, No Name 1, and PSTF Test) (fig. 2) were sampled for POCs. Concentrations of POCs in water from these wells were all less than the MRL.

Total Organic Carbon

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 September-October 2001, water samples from 22 wells completed in the Snake River Plain aquifer at the INL were analyzed for TOC; detected concentrations ranged from 0.6 to 3.1 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.

Specific Conductance, Temperature, and pH

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 infiltration ponds and extended downgradient from the INTEC to the CFA (fig. 18). The specific conductance of water from several wells within this plume had increased from about 500 µS/cm in 1985 (Pittman and others, 1988, p. 64) to more than 1,000 µS/cm in 1998 but then decreased to about 960 µS/cm by 2001 in water from well USGS 51. Specific conductance of water from well USGS 113 (fig. 5) was 1,080 µS/cm in October 1998, and then decreased to 937 µS/cm in October 2001.

The specific conductance of water from several wells at the RTC exceeded 400 µS/cm in 2001. Specific conductance of water from well USGS 65 (fig. 5), downgradient from the infiltration ponds at the RTC, was 628 µS/cm in October 2001. Maximum specific conductance of water from wells USGS 88 and 120 (fig. 5), near the RWMC, were 581 and 452 µS/cm, respectively, in October 2001.

In 1999, the specific conductance of water from 121 wells ranged from 273 to 1,070 µS/cm; the median specific conductance was 405 µS/cm. In 2000, the specific conductance of water from 127 wells ranged from 260 to 991 µS/cm; the median specific conductance was 404 µS/cm. In 2001, the specific conductance of water from 126 wells at the INL ranged from 262 to 960 µS/cm; the median specific conductance was 402 µS/cm.

During each year, 1999-2001, water temperature and pH were measured in water from 121, 127, and 126 wells at the INL, respectively. Well P&W 2 (fig. 4) consistently had the lowest water temperatures ranging from 7.6 to 8.2°C. Well USGS 22 consistently had the highest water temperatures, ranging from 19.5 to 20.1°C. The median water temperature for all wells sampled was 13.0°C in each year, 1999–2001, a slight increase from 1998. In 1999, the pH ranged from 7.4 in well USGS 57 to 8.5 in well USGS 119 (fig. 3). In 2000, the pH ranged from 7.4 in water from well USGS 39 to 8.8 in water from well USGS 51 (fig. 3). In 2001, the pH ranged from 7.5 in water from wells Highway 3 (fig. 4) and TRA 4 (fig. 5) to 8.4 in water from well USGS 88 (fig. 5). The median pH in water from all wells was 8.0 in each year, 1999–2001, the same as in 1998.

Back to Table of Contents


AccessibilityFOIAPrivacyPolicies and Notices

Take Pride in America home page.FirstGov buttonU.S. Department of the Interior | U.S. Geological Survey
Persistent URL: https://pubs.water.usgs.gov/sir20065088
Page Contact Information: Publications Team
Page Last Modified: Thursday, 01-Dec-2016 19:11:29 EST