ABSTRACT

Since 1952, radiochemical and chemical wastewater discharged to infiltration ponds (also called percolation ponds), evaporation ponds, and disposal wells at the Idaho National Laboratory (INL) has affected water quality in the eastern Snake River Plain aquifer and perched groundwater zones underlying the INL. The U.S. Geological Survey, in cooperation with the U.S. Department of Energy, maintains groundwater 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 groundwater zones. This report presents an analysis of water-level and water-quality data collected from aquifer and perched groundwater wells in the USGS groundwater monitoring networks during 2006–08.

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 primarily is recharged from infiltration of irrigation water, infiltration of streamflow, groundwater inflow from adjoining mountain drainage basins, and infiltration of precipitation.

From March–May 2005 to March–May 2008, water levels in wells generally remained constant or rose slightly in the southwestern corner of the INL. Water levels declined in the central and northern parts of the INL. The declines ranged from about 1 to 3 feet in the central part of the INL, to as much as 9 feet in the northern part of the INL. Water levels in perched groundwater wells around the Advanced Test Reactor Complex (ATRC) also declined.

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 2006–08. 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 April or October 2008, reportable concentrations of tritium in groundwater ranged from 810 ± 70 to 8,570 ± 190 picocuries per liter (pCi/L), and the tritium plume extended south-southwestward in the general direction of groundwater flow. Tritium concentrations in water from wells completed in shallow perched groundwater at the ATRC were less than the reporting levels. Tritium concentrations in deep perched groundwater exceeded the reporting level in 11 wells during at least one sampling event during 2006–08 at the ATRC. Tritium concentrations from one or more zones in each well were reportable in water samples collected at various depths in six wells equipped with multi-level Westbay^TM packer sampling systems.

Concentrations of strontium-90 in water from 24 of 52 aquifer wells sampled during April or October 2008 exceeded the reporting level. Concentrations ranged from 2.2 ± 0.7 to 32.7 ± 1.2 pCi/L. Strontium-90 has not been detected within the eastern Snake River Plain aquifer beneath the ATRC partly because of the exclusive use of waste-disposal ponds and lined evaporation ponds rather than using the disposal well for radioactive-wastewater disposal at ATRC. At the ATRC, the strontium-90 concentration in water from one well completed in shallow perched groundwater was less than the reporting level. During at least one sampling event during 2006–08, concentrations of strontium-90 in water from nine wells completed in deep perched groundwater at the ATRC were greater than reporting levels. Concentrations ranged from 2.1±0.7 to 70.5±1.8 pCi/L. At the Idaho Nuclear Technology and Engineering Center (INTEC), the reporting level was exceeded in water from two wells completed in deep perched groundwater. During 2006–08, concentrations of cesium-137, plutonium-238, and plutonium-239, -240 (undivided), and americium-241 were less than the reporting level in water samples from all wells and all zones in wells equipped with multi-level Westbay^TM packer sampling systems at the INL.

The concentration of chromium in water from one well south of the ATRC steadily decreased from 2006 to 2008 and was 93 micrograms per liter (μg/L) in 2008, just less than the maximum contaminant level (MCL). Concentrations in water samples from other wells ranged from 1.2 to 28.3 μg/L. During 2006–08, chromium was detected in one well completed in shallow perched groundwater at a concentration of 3 μg/L. Dissolved chromium was detected in water from 14 wells completed in deep perched groundwater at the ATRC during 2006–08.

Concentrations of sodium in water from wells south of the INTEC during 2006–08 generally were equal to or less than sodium concentrations detected in October 2005, with the exception of concentrations in water from well USGS 47 which was slightly higher in 2008 than in 2005. In October 2008, sodium concentrations in water from two wells near the Radioactive Waste Management Complex (RWMC) were 45 and 26 milligrams per liter (mg/L), slightly higher than the October 2005 concentrations. During 2006–08, analyses were not made for dissolved sodium concentrations in shallow perched groundwater at the ATRC. During April or October 2008, dissolved sodium concentrations in water from 16 wells completed in deep perched groundwater ranged from 6 to 23 mg/L; concentration in water from one well was 476 mg/L. The vertical distribution of sodium concentrations in wells equipped with Westbay^TM systems were fairly consistent with depth, with the exception of sodium concentrations in water from well USGS 132, which were much higher (26–30 mg/L) in the uppermost zone than in the deeper zones (9–12 mg/L).

Chloride concentrations in water from wells near the INTEC generally decreased since the late 1990s. During 2006–08, concentrations in most wells either generally were constant or increased slightly. Trends in concentrations in water from wells downgradient from the percolation ponds correlated with discharge rates into the ponds when travel time was considered. During 2008, chloride concentrations in water from wells USGS 88 and 89 at the RWMC were 91 and 41 mg/L. Concentrations of chloride in all other wells near the RWMC ranged from 11 to 25 mg/L. In 2008, concentrations in water from all wells at or near the ATRC ranged between 10 and 18 mg/L. During April 2008, dissolved chloride concentrations in shallow perched groundwater near the ATRC ranged from 11 to 13 mg/L; concentrations in deep perched groundwater during April or October 2008 ranged from 4 to 43 mg/L.

In 2008, sulfate concentrations ranged from 40 to 157 mg/L in water samples from nine aquifer wells in the south-central part of the INL, which exceeds the 40-mg/L background concentration of sulfate. The greater-than-background concentrations of sulfate in water from these wells probably resulted from sulfate disposal at the ATRC infiltration ponds. In October and April 2008, sulfate concentrations in water samples from two wells near the RWMC were greater than background levels and could have resulted from the well construction and (or) waste disposal at the RWMC. During 2007–08, sulfate concentrations from three wells southwest of the INTEC were 45, 47, and 46 mg/L. The maximum dissolved sulfate concentration in shallow perched groundwater near the ATRC was 399 mg/L in well CWP 1 in November 2006. During April–October 2008, the maximum concentration of dissolved sulfate in deep perched groundwater was 1,477 mg/L in well USGS 68, which is located west of the chemical-waste pond. Concentrations of sulfate in two wells completed in deep perched groundwater near the INTEC were 36 and 39 mg/L. Overall the vertical distribution of sulfate in water from wells equipped with Westbay^TM systems generally was consistent in most zones in wells during 2006–08.

The regional background concentration for nitrate (as N) is about 1 mg/L. In October 2008, concentrations of nitrate (as N) in water from most wells at and near the INTEC exceeded the background concentration and ranged from 2.2 to 5.97 mg/L. Near the ATRC, the concentration of nitrate (as N) in water from well USGS 65 was 1.5 mg/L. In 2008, concentrations of nitrate (as N) in water from wells USGS 88, 89, and 119 were 0.9, 1.7, and 1.4 mg/L, respectively. All concentrations measured in aquifer wells at the INL in 2008 were less than the MCL for drinking water of 10 mg/L as N, with the exception of a concentration of 18.9 mg/L from well USGS 50 at the INTEC.

During April or October 2008, fluoride concentrations in water samples from four aquifer wells ranged from 0.2 to 0.3 mg/L. These concentrations are similar to the background concentrations, which indicate that wastewater disposal has not appreciably affected fluoride concentrations in the Snake River Plain aquifer near the INTEC.

During 2006–08, water samples from 30 aquifer wells were collected and analyzed for volatile organic compounds (VOCs). Twelve VOCs were detected. Concentrations of from 1 to 10 VOCs were detected in water samples from 11 wells. Primary VOCs detected included carbon tetrachloride, trichloromethane, 1,1-dichloroethane, 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethylene.

During April or October 2008, water samples from 50 wells completed in the Snake River Plain aquifer at the INL were analyzed for total organic carbon (TOC); detected concentrations ranged from 0.43 to 1.9 mg/L.

Well USGS 92 is in the Subsurface Disposal Area (SDA) at the RWMC and is completed in a sedimentary interbed 214 ft below land surface. Perched groundwater in this well has moved through overlying sediments and basalt, and may contain waste constituents leached from radiochemical and organic chemical wastes buried in the SDA. During 2006–08, tritium concentrations in water samples from well USGS 92 exceeded the reporting level and ranged from 490±110 pCi/L in April 2006 to 300±80 pCi/L in June 2008. Water from well USGS 92 was sampled for VOCs in April 2007, and 9 VOCs were detected which was a decrease from the 15 compounds detected in 2002–03. Additionally, all VOC concentrations detected in 2007 were significantly lower than those detected during 2002–03, with the exception of toluene, which was not detected in 2002–03.

 

 

First posted September 29, 2010