ISOTOPIC AND GEOCHEMICAL INVESTIGATION INTO THE SOURCE OF ELEVATED URANIUM CONCENTRATIONS IN THE WESTERN SNAKE RIVER PLAIN AQUIFER

In light of the Environmental Protection Agency’s recent establishment of a maximum contaminant level for uranium (U) in public drinking water systems (30 µg/L), there has been an increase in regulatory efforts to characterize the occurrence of U in groundwater. Within the highly populated Treasure Valley potion of Idaho’s Western Snake River Plain Aquifer, existing data show that locations of elevated groundwater U are spatially widespread yet fairly isolated in their occurrence. There is also little evidence of a trend between aqueous U concentrations and depth. By using both U and strontium (Sr) isotopic ratios of water as tracers of its history of geologic weathering, sources of naturally occurring U in groundwater can be inferred.

Within the Treasure Valley, the Boise River represents the primary source of water to the underlying aquifer. Analysis of surface water from the Boise River and its tributaries, as well as from four other regional rivers, was conducted to characterize the source water for the aquifer and observe its relationship to other regional surface waters. Results show that rivers emanating from catchments with contrasting geologies yield significantly different aqueous U concentrations and distinct U and Sr isotopic signatures. Among surface waters analyzed, several samples collected within the Treasure Valley exhibited among the highest U concentrations within the region.

Among eight spatially distributed groundwater well samples, the U concentrations varied widely (8-74 µg/L). An array of unique isotopic signatures ranged from those resembling the input surface water end member to those more heavily influenced by end members representing weathering products of aquifer materials. To constrain which geologic materials might be contributing U to the groundwater, surficial sediments were leached with a series of selective extractants. Analysis of the leachates reveals several samples with both high U concentrations and isotopic signatures representative of the groundwater samples. Although the bulk U contents of the surficial sediments were not particularly elevated, isotopic evidence suggests that they may still be important sources of natural U to the Treasure Valley Aquifer.