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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5089

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Summary

Radiochemical and chemical wastewater discharged since 1952 to infiltration ponds and disposal wells at the Idaho National Laboratory (INL) has affected water quality in the Snake River Plain aquifer and in perched water zones at the INL. The U.S. Geological Survey, in cooperation with the U.S. Department of Energy, maintains aquifer and perched ground-water monitoring networks at the INL to determine hydrologic trends and to delineate the movement of radiochemical and chemical wastes in the aquifer and in perched water zones.

Water in the Snake River Plain aquifer primarily moves through fractures and interflow zones in basalt, generally flows southwestward, and eventually discharges at springs along the Snake River. The aquifer is recharged primarily from infiltration of irrigation water, infiltration of streamflow, ground-water inflow from adjoining mountain drainage basins, and infiltration of precipitation.

During March–May 2005, the altitude of the water table in the Snake River Plain aquifer was about 4,570 feet in the northern part of the INL, and about 4,400 feet in the southwestern part. Water flowed southward and southwestward beneath the INL at an average hydraulic gradient of about 4 feet per mile. From March–May 2001 to March–May 2005, water levels in INL declined in the area of the INL. Declines ranged from about 3 to 8 feet in wells in the southwestern part of the INL, about 10 to 15 feet in the west-central part of the INL, and about 6 to 11 feet in the northern part of the INL. Ground water moves southwestward from the INL and eventually is discharged to springs along the Snake River near Twin Falls, Idaho, about 100 miles southwest of the INL. Estimated discharge from the springs was about 3.54 million acre-feet per year for the 2005 water year.

Disposal of wastewater to infiltration ponds and infiltration of surface water at the RTC and INTEC resulted in formation of perched ground water in basalts and in sedimentary interbeds that overlie the Snake River Plain aquifer. Perched ground water beneath the RWMC formed from infiltration of snowmelt and rain and recharge from the Big Lost River and INL spreading areas. This perched water contains constituents leached from buried radioactive and organic-chemical wastes. Perched ground water is an integral part of the pathway for waste-constituent migration to the aquifer.

Data are unavailable for the total amount of tritium (in Curies) released in liquid effluent during 2002–05. A tritium plume developed in the Snake River Plain aquifer from discharge of wastewater at the INL since the 1950s. In October 2005, reportable concentrations of tritium in water from aquifer wells 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. In October 2005, concentrations of tritium in water samples generally decreased from the 1999–2001 reporting period and all concentrations were less than the maximum contaminant level of 20 pCi/mL.

At the RTC, wells completed in shallow perched ground water were sampled for tritium; all concentrations were less than the reporting level. Lack of available perched water to sample, and the history of non-reportable values of tritium in most of the shallow wells resulted in the decision to remove several wells from the sampling schedule at various times during 2002–05. Tritium concentrations in water from nine wells completed in deep perched ground water (PW 9, USGS 53, 55, 56, 61, 66, 70, 71, and 73) generally were greater than the reporting levels during at least one sampling event during 2002–05. Concentrations decreased in some wells and varied randomly in other wells. Tritium concentrations in water from six wells (USGS 60, 62, 63, 69, 72, and 78) were less than the reporting level during 2002–05. Tritium concentrations varied between reportable and nonreportable concentrations in water from three wells, PW 8, USGS 54 and 68, during 2002–05.

In July 2002, the tritium concentration in well SWP 8, completed in shallow perched water near INTEC was below the reporting level with a concentration of 0.16±0.14 pCi/mL. This well was dry during 2003 and sampling was discontinued due to lack of water in the well. During 2002–05, tritium concentrations in water from wells completed in deep perched ground water beneath the percolation ponds ranged from less than the reporting level in wells PW 1 and PW 5 to 1.8±0.2 pCi/mL in well PW 4, an increase in concentration from 2001 when all concentrations were less than the reporting level.

No data were available for strontium-90 discharged during 2002–05. A strontium-90 plume developed in the Snake River Plain aquifer from the disposal of wastewater at the INL. In October 2005, 34 aquifer wells were sampled for strontium-90. Concentrations of strontium-90 in water from 14 wells exceeded the reporting level. However, concentrations from most wells remained relatively constant or decreased since 1989. Concentrations ranged from 2.2±0.7 to 33.1±1.2 pCi/L and the area where strontium-90 was detected near the INTEC extended south-southwestward in the general direction of ground-water flow. Strontium-90 has not been detected within 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 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 is different from that beneath the INTEC; more sediment is present below the RTC than the INTEC.

During 2002, the concentration of strontium-90 in water from well SWP 8, completed in shallow perched ground water near INTEC had a concentration of 0.7 ± 0.7 pCi/L, below the reporting level. This well was dry during 2003, and sampling was subsequently discontinued. In April–October 2005, strontium-90 concentrations in deep perched ground water in wells closest to the percolation ponds were not sampled because of access problems, dry wells, or sampling was discontinued prior to 2005.

During 2002–05, concentrations of cesium-137 in water from all wells sampled by the USGS at the INL were less than the reporting level. Concentrations of plutonium-238, plutonium-239, -240 (undivided), and americium-241 in water from all wells sampled at the INL were less than the reporting level during 2002-05.

Detectable concentrations of nonradioactive chemical constituents in water from the Snake River Plain aquifer at the INL varied during 2002–05.

During 2002–05, water samples from several wells were analyzed for chromium. In April 2005, water from one aquifer well, USGS 65, south of RTC, equaled the MCL of 100 µg/L for total chromium. However, the concentration of chromium in water from that well decreased from 139 µg/L in October 2001. Concentrations in water samples from other aquifer wells ranged from 1.7 to 30.3 µg/L. Samples collected from well USGS 56 completed in deep perched water at the RTC, contained concentrations of chromium of 114 and 86 μg/L in 2004 and 2005, respectively.

During 2002–05, the largest concentration of sodium in water samples from aquifer wells at the INL was 76 mg/L in a sample collected from well USGS 113, south of INTEC, but concentrations decreased through 2004.

During 2002–05, no analyses were made for dissolved sodium concentrations in shallow perched ground water at the RTC. Dissolved sodium concentrations in water from 16 wells completed in deep perched ground water at the RTC were determined. During April–October 2005, dissolved sodium concentrations ranged from 6 to 27 mg/L in all wells except well USGS 68, (370 mg/L). At the INTEC, water from well SWP 8, completed in shallow perched ground water, contained a concentration of 120 mg/L of dissolved sodium in July 2002. Dissolved sodium concentrations in deep perched ground water in wells closest to the infiltration ponds (PW 2 and 4) ranged from 106 mg/L in well PW 2 in October 2002 to 83 mg/L in well PW 4 in October 2003.

Chloride concentrations in water from aquifer 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, chloride concentrations decreased in some wells and increased in others. Trends in concentrations in water from wells downgradient from the percolation ponds correlated with discharge rates into the ponds when travel time was considered. In April 2005 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 ranged between 9 and 12 mg/L during 2002–05. At the RWMC, chloride concentrations in water from aquifer wells USGS 88, 89, and 120 were 86, 41, and 20 mg/L, respectively, nearly the same as the 1999–2001 reporting period.

During April–October 2005, dissolved chloride concentrations in shallow perched ground water at the RTC ranged from 10 mg/L in well CWP 3 to 32 mg/L in well CWP 1. Dissolved chloride concentrations in deep perched ground water ranged from 3 mg/L in well USGS 78 to 35 mg/L in well USGS 68.

The dissolved chloride concentration in well SWP 8 at the INTEC was 145 mg/L in July 2002, similar to the July 2001 concentration of 153 mg/L. No samples were collected from well SWP 8 during 2003–05 because the well was dry. During 2002–05, dissolved chloride concentrations in deep perched ground water in wells closest to the percolation ponds (PW 1 through 5) ranged from 118 to 322 mg/L in well PW 4. The variability and values of concentrations of dissolved chloride in this well are similar to the 1999–2001 reporting period.

In 2005, sulfate concentrations in water samples from nine aquifer wells in the south-central part of the INL exceeded the 40 mg/L background concentration of sulfate. The maximum dissolved sulfate concentration in shallow perched ground water at the RTC was 396 mg/L in well CWP 1 in October 2005. Concentrations of dissolved sulfate in this well varied between 26 to 396 mg/L during 2002–05. The higher concentrations are attributed to sulfate disposal to nearby cold-waste ponds. The most recent detected concentrations of dissolved sulfate in water from wells USGS 54, 60, 63, 69, and PW 8, completed in deep perched ground water near the cold-waste ponds, ranged from 66 to 276 mg/L during April–October 2005. These large concentrations indicate that water in the wells also was affected by discharge into the cold-waste ponds. During April–October 2005, the maximum concentration of dissolved sulfate in deep perched ground water was 951 mg/L in well USGS 68, west of the chemical-waste pond. This dissolved sulfate concentration has steadily decreased from 1,409 mg/L in October 2001.

The dissolved sulfate concentration in shallow perched ground water at the INTEC from well SWP 8 was 49 mg/L in July 2002. Dissolved sulfate concentrations in water from wells completed in the deep perched ground water closest to the INTEC infiltration ponds (PW 2 and 4) were 35 mg/L in October 2002. After 2002, no samples were collected from wells PW1 through 3 and PW5 because of access problems, dry wells, or sample collection discontinued during 2002-05. Historically, dissolved sulfate concentrations in these wells have fluctuated between about 22 and 41 mg/L. Concentrations of dissolved sulfate in samples from well USGS 50 ranged from 26 to 40 mg/L during 2002–04. Historically, dissolved sulfate concentrations in water from well USGS 50 have fluctuated around these values.

Concentrations of nitrate in ground water not affected by wastewater disposal from INL facilities generally are less than 5 mg/L (as nitrate). In October 2005, concentrations of nitrate in water from aquifer 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 aquifer wells near the RWMC slightly exceeded the regional background concentration of about 5 mg/L (as nitrate) or 1 to 2 mg/L as nitrogen. 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 aquifer well USGS 65 was 7 mg/L, a slight decrease from the 2001 value of 8 mg/L (as nitrate). All concentrations measured in aquifer wells during 2005 were less than the MCL for drinking water of 44 mg/L (as nitrate, or 10 mg/L as nitrogen).

During April to October 2005, water samples from five aquifer wells were analyzed for fluoride; detected concentrations ranged from 0.2 to 0.3 mg/L. These concentrations are similar to background concentrations, indicating that wastewater disposal has not had an appreciable affect on fluoride concentrations in the Snake River Plain aquifer near the INTEC.

During 2002–05, water samples from 30 aquifer 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.

During 2002–05, attempts were made each year to sample well USGS 92, completed in perched water at the RWMC; however, lack of water in the well precluded obtaining an adequate sample during most sampling events. Concentrations of 16 VOCs were detected during 2002–03. Most of the same VOCs except chloroethane that were detected during 1999–2001 were detected during 2002–03; additionally, bromodichloromethane was detected. Most VOCs fluctuated through time and show no distinct trend.

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