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

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Abstract

Radiochemical and chemical wastewater discharged since 1952 to infiltration ponds, evaporation ponds, and disposal wells at the Idaho National Laboratory (INL) has affected water quality in the Snake River Plain aquifer and perched-water zones underlying the INL. The U.S. Geological Survey, in cooperation with the U.S. Department of Energy, maintains 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. This report presents an analysis of water-level and water-quality data collected from aquifer and perched-water wells in the USGS ground-water monitoring networks during 2002–05.

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.

From March–May 2001 to March–May 2005, water levels in wells declined throughout the INL area. The declines ranged from about 3 to 8 feet 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. Water levels in perched water wells declined also, with the water level dropping below the bottom of the pump in many wells during 2002–05.

For radionuclides, concentrations that equal 3s, wheres s is the sample standard deviation, represent a measurement at the minimum detectable concentration, or “reporting level.” Detectable concentrations of radiochemical constituents in water samples from wells in the Snake River Plain aquifer at the INL generally decreased or remained constant during 2002–05. Decreases in concentrations were attributed to decreased rates of radioactive-waste disposal, radioactive decay, changes in waste-disposal methods, and dilution from recharge and underflow. In October 2005, reportable concentrations of tritium in ground water ranged from 0.51±0.12 to 11.5±0.6 picocuries per milliliter and the tritium plume extended south-southwestward in the general direction of ground-water flow. Tritium concentrations in water from several wells southwest of the Idaho Nuclear Technology and Engineering Center (INTEC) decreased or remained constant as they had during 1998–2001, with the exception of well USGS 47, which increased a few picocuries per milliliter. Most wells completed in shallow perched water at the Reactor Technology Complex (RTC) were dry during 2002­­–05. Tritium concentrations in deep perched water exceeded the reporting level in nine wells at the RTC. The tritium concentration in water from one deep perched water well exceeded the reporting level at the INTEC. Concentrations of strontium-90 in water from 14 of 34 wells sampled during October 2005 exceeded the reporting level. Concentrations ranged from 2.2±0.7 to 33.1±1.2 picocuries per liter. However, concentrations from most wells remained relatively constant or decreased since 1989. 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. At the RTC, strontium-90 concentrations in water from six wells completed in deep perched ground water exceeded the reporting level during 2002-05. At the INTEC, the reporting level was exceeded in water from three wells completed in deep perched ground water. During 2002–05, concentrations of plutonium-238, and plutonium-239, -240 (undivided), and americium-241 were less than the reporting level in water samples from all wells sampled at the INL. 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.

Changes in detectable concentrations of nonradioactive chemical constituents in water from the Snake River Plain aquifer at the INL varied during 2002–05. In April 2005, water from well USGS 65, south of the Reactor Technology Complex (RTC) [formerly known as the Test Reactor Area (TRA)], contained 100 micrograms per liter (µg/L) of chromium, a decrease from the concentration of 139 µg/L detected in October 2001. Other water samples contained from less than 1.7 to 30.3 µg/L of chromium. Chromium was detected in water from 2 wells completed in shallow perched ground water, and in 17 wells completed in deep perched water. During 2002–05, the largest concentration of sodium in water samples from aquifer wells at the INL was 76 milligrams per liter (mg/L) in a sample from well USGS 113, south of INTEC. During April–October 2005, dissolved sodium concentrations in deep perched water at the RTC ranged from 6 to 27 mg/L in all wells except well USGS 68 (370 mg/L). No analyses were made for sodium in shallow perched ground water at the RTC during 2002–05. Dissolved sodium concentrations in water from 16 wells completed in deep perched water at the RTC were determined. At the INTEC, sodium concentrations were determined from one well completed in shallow perched ground water, and from two wells completed in deep perched ground water. In 2005, chloride concentrations in most water samples from the INTEC and the Central Facilities Area (CFA) exceeded ambient concentrations of 10 and 20 mg/L, respectively. Chloride concentrations in water from wells near the RTC were less than 20 mg/L. At the Radioactive Waste Management Complex (RWMC), chloride concentrations in water from wells USGS 88, 89, and 120 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. During April to October 2005, chloride concentrations in shallow perched ground water from three wells at the RTC ranged from 10 to 32 mg/L and from 3 to 35 mg/L in deep perched ground water. At the INTEC, dissolved chloride concentrations in deep perched ground water in wells closest to the percolation ponds ranged from 118 to 332 mg/L. In 2005, sulfate concentrations in water from aquifer wells USGS 34, 35, and 39, southwest of INTEC, were 42, 46, and 46 mg/L, respectively. Historically, concentrations in these wells have been at or just below 40 mg/L, the estimated background concentration of sulfate in the Snake River Plain aquifer at the INL. The maximum sulfate concentration in water from wells completed in shallow perched ground water at the RTC was 396 mg/L. During April to October 2005, concentrations of dissolved sulfate in water from wells completed in deep perched ground water at the RTC ranged from 66 to 276 mg/L. Concentrations of dissolved sulfate in water from two wells completed in deep perched ground water at the INTEC were 35 mg/L.

In October 2005, concentrations of nitrate in water from wells USGS 41, 43, 45, 47, 52, 57, 67, 77, 112, 114, and 115 near the INTEC, exceeded the regional background of 5 mg/L (as nitrate) and concentrations ranged from 6 mg/L in well USGS 45 to 34 mg/L in well USGS 43. However, since 1981, nitrate concentrations have decreased overall in water from these wells.

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 the background concentrations, which indicate that wastewater disposal has not had an appreciable affect on fluoride concentrations in the Snake River Plain aquifer near the INTEC.

During 2002–05, 12 volatile organic compounds (VOCs) were detected in water from aquifer wells at the INL. Concentrations of from 1 to 9 VOCs were detected in water samples from 13 wells. 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. Most of the same VOCs except chloroethane that were detected during 1999–2001 were detected during 2002–03; additionally, bromodichloromethane was detected. Concentrations of 16 VOCs were detected during 2002–03. Most VOCs fluctuated through time and show no distinct trend.

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