Scientific Investigations Report 2006-5088
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5088
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. 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. This report presents an analysis of water-level and water-quality data collected from the wells in the USGS monitoring networks during 1999–2001.
Water in the Snake River Plain aquifer moves principally through fractures and interflow zones in basalt, generally flows southwestward, and eventually discharges at springs along the Snake River. The aquifer is recharged principally from infiltration of irrigation water, infiltration of streamflow, ground-water inflow from adjoining mountain drainage basins, and infiltration of precipitation.
In 2001, the altitude of the water table in the Snake River Plain aquifer at the INL was about 4,600 feet in the northern part and about 4,420 feet in the southwestern part. Water flowed southward and southwestward beneath the INL (fig. 9) at an average hydraulic gradient of about 4 feet per mile. From March–May 1998 to March–May 2001, water levels in INL wells increased in some areas and decreased in other areas. Increases ranged from about 3 feet in wells in the west-central part of the INL to about 1 foot in the northern part. Decreases ranged from 1 to 4 feet in the southwestern part of the INL.
About 191 Curies (Ci) of tritium were released in liquid effluent to the Reactor Technology Complex (RTC) lined evaporations ponds during 1999-2000. During 1999, no tritium was discharged to the ponds at the Idaho Nuclear Technology and Engineering Center (INTEC); during 2000, 0.03 Ci of tritium was discharged. Data for the total amount of tritium (in Ci) released in liquid effluent in 2001 are unavailable. Tritium concentrations in water samples decreased by as much as 8.3 picocuries per milliliter (pCi/mL) during 1999-2001 and ranged from 0.43±0.14 to 13.6±0.6 pCi/mL in October 2001. The tritium plume extended southwestward in the general direction of ground-water flow. During 1999–2001, concentrations of tritium in water from wells near the southern boundary of the INL and from all wells south of the INL boundary were less than the reporting level. Radioactive decay, decreased tritium disposal, dilution from recharge, and discontinuation of the use of the infiltration ponds at RTC and the disposal well at INTEC and subsequent wastewater discharge to infiltration ponds at INTEC contributed to decreased concentrations of tritium in ground water at the INL during 1999–2001.
During 1999, less than 0.001 Ci of strontium-90 was discharged at the INTEC or RTC; during 2000, 0.21 Ci of strontium-90/yttrium-90 was discharged at the RTC. Data are unavailable for the amount of strontium-90 discharged in 2001. Strontium-90 concentrations in water from wells at the INL decreased or remained constant during 1999–2001. In October 2001, concentrations of strontium-90 in water from 19 wells exceeded the reporting level. Concentrations ranged from 2.1±0.7 to 42.4±1.4 picocuries per liter (pCi/L) and the plume extended 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. 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 RTC.
During 1999–2001, concentrations of cesium-137, plutonium-238, and plutonium-239, -240 (undivided) in water from all wells sampled at the INL were less than the reporting level. The concentration of americium-241 in one sample was 0.003±0.001 pCi/L, the reporting level for that constituent. Cobalt-60 was not detected in any samples collected during 1999–2001.
Detectable concentrations of nonradioactive chemical constituents in water from the Snake River Plain aquifer at the INL were variable during 1999–2001.
During 1999–2001, water samples from several wells were analyzed for chromium. The concentration of chromium in water from well USGS 65 was 139 µg/L in October 2001, which exceeded the maximum contaminant level of 100 µg/L. However, this was less than the concentration of 168 µg/L measured in 1998. Water samples from the other wells contained from 6.7 to 21.3 µg/L of chromium, an overall decrease in concentrations since 1998.
In October 2001, concentrations of sodium in water samples from most of the wells in the southern part of the INL were larger than the background concentration of 10 mg/L, but were similar to or slightly less than October 1998 concentrations. The largest sodium concentration was 75 mg/L in water from well USGS 113.
In 2001, chloride concentrations in most water samples from the INTEC and the Central Facilities Area (CFA) exceeded background concentrations of 10 and 20 mg/L, respectively. In October 2001, chloride concentrations in water from wells USGS 113 and CFA 1, both south of the INTEC, were 175 and 103 mg/L, respectively. These concentrations were decreases from concentrations in October 1998. 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 81, 40, and 22 mg/L, respectively. Concentrations of chloride in all other wells near the RWMC were less than 19 mg/L.
During 2001, concentrations of sulfate in water from two wells near the RTC, two wells near the RWMC, and one well near the CFA exceeded 40 mg/L, the estimated background concentration of sulfate in the Snake River Plain aquifer at the INL.
In 2001, concentrations of nitrate in water from wells USGS 40, 43, 77, and CFA 1 were 16, 21, 16, and 14 mg/L, respectively. These generally were smaller concentrations 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.
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 background concentrations, which indicates that wastewater disposal has not had an appreciable affect on fluoride concentrations in the Snake River Plain aquifer near the INTEC.
During 1999–2001, water samples from 36 wells at and near the INL were collected and analyzed for purgeable organic compounds (POCs). Ten POCs were detected. Concentrations of from 1 to 5 POCs in water samples from 17 wells exceeded the minimum reporting level (MRL). 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. In October 2001, concentrations of 1,1,1-trichloroethane in water from four wells near the INTEC exceeded the MRL, and water from RWMC wells contained detectable concentrations of 1,1,1-trichloroethane, carbon tetrachloride, chloroform, trichloroethylene, and tetrachloroethylene. During 1999–2001, water samples from three wells near Test Area North (ANP 9, No Name 1, and PSTF Test) were analyzed for POCs and concentrations of POCs in water from these wells were all less than the MRL.
U.S.
Department of the Interior | U.S. Geological
Survey
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