Distribution of selected radiochemical and chemical constituents in perched ground water, Idaho National Engineering Laboratory, Idaho, 1992-95

Abstract

Radiochemical and chemical wastes generated at facilities at the Idaho National Engineering Laboratory (INEL) have been discharged to infiltration ponds at the Test Reactor Area (TRA) and the Idaho Chemical Processing Plant (ICPP) and buried at the Radioactive Waste Management Complex (RWMC) since 1952. Disposal of wastewater to ponds and infiltration of surface water at waste-burial sites have resulted in formation of perched ground water hi basalts and in sedimentary interbeds above the Snake River Plain aquifer. Perched ground water is an integral part of the pathway for waste-constituent migration to the aquifer. The U.S. Geological Survey, in cooperation with the U.S. Department of Energy, maintains a continuous monitoring network at the INEL to determine hydrologic trends and to monitor the movement of wastewater discharged from facilities. This report presents an analysis of water-level and water-quality data collected from perched ground water at the INEL during 1992-95. During 1992-95, tritium concentrations in water from wells completed in deep perched ground water at the TRA generally decreased or were variable. During July-October 1995, concentrations ranged from less than the reporting level to 158±5 picocuries per milliliter (pCi/mL). The maximum tritium concentration in the shallow perched ground water at the TRA during 1992-95 was 3,940±60 pCi/mL in January 1992. By October 1995, the tritium concentration in water from the same well had decreased to 22.4±0.9 pCi/mL. Tritium concentrations in water from wells at the TRA were affected by distance of the well from the radioactive-waste ponds, depth of the water below the ponds, monthly variations in the amount of tritium discharged, discontinued use of the radioactive-waste ponds, radioactive decay, and dilution from nonradioactive water. During 1992-95, strontium-90 concentrations in water from wells completed in deep perched ground water at the TRA were variable. During October 1995, concentrations were from 6.4±0.9 to 143±5 pCi/L. Cesium-137, chromium-51, and cobalt-60 all were detected in water from a shallow well near the leaky radioactive-waste pond retention basin. Dissolved chromium concentrations in perched ground water at the TRA during 1995 were from less than 5 to 590 micrograms per liter. The largest concentrations were in water from wells north and west of the radioactive-waste ponds. Dissolved sodium concentrations were from 7.1 to 1,200 milligrams per liter (mg/L) in 1995. Dissolved sulfate concentrations were from 18 to 3,900 mg/L. The largest concentrations of sodium and sulfate were in water from a well near the chemical-waste pond. During 1992-95, tritium concentrations in water from wells completed in deep perched ground water near the ICPP infiltration ponds generally decreased because of decreased disposal; strontium-90 concentrations were variable. In October 1995, tritium concentrations ranged from less than the reporting level to 1.0±0.2 pCi/mL; strontium-90 concentrations were below the reporting level in all wells. During 1992-95, concentrations of sodium, chloride, sulfate, and nitrate in water from wells completed in perched ground water near the ICPP infiltration ponds were similar to the concentrations of the constituents in the wastewater discharged. During 1992-94, concentrations of americium241 and plutonium-238 were above the reporting level in one sample each from a well completed in perched ground water at the RWMC. Other radionuclides had concentrations below the reporting levels.