Factors Controlling the Concentration of Arsenic in the Treasure Valley Shallow Aquifer, Idaho

Publication Date


Type of Culminating Activity


Degree Title

Master of Science in Hydrologic Sciences



Major Advisor

Shawn Benner


A study has been undertaken to elucidate the geochemical mechanisms by which arsenic is being stored, released and transported in the shallow sedimentary aquifer beneath the Western Snake River Plain, Idaho where groundwater arsenic concentrations exceed 100 μg L-1. While semi-arid, this region is extensively irrigated for agriculture. We have evaluated the effects of the infiltration of irrigation waters from surface and subsurface sources on the release of arsenic to the aquifer system in this arid region. Analyses of groundwater chemistry indicate the highest aqueous concentrations occur in conjunction with high dissolved oxygen and nitrate and low iron and manganese concentrations near the water table. Sequential extraction and batch experiments performed on un-irrigated shallow sediments from the basin show solid phase arsenic near average sedimentary alluvium and rock abundances (4 to 45 mg kg-1, average: 17 mg kg-1). The highest aqueous concentrations were associated with surficial aeolian material and iron oxide coatings and concretions. Batch leaching experiments on soils and iron (oxy)hydroxide coated sediments produce aqueous arsenic concentrations approximating those observed in the shallow aquifer (up to 152 μg L-1). Analysis of flood irrigation waters indicate limited arsenic release to surface runoff but waters from seeps proximal to the irrigated arsenic affected zone contain arsenic to 75 μg L-1. These data plot along evaporative trends. Collectively these data suggest that leaching from surficial and vadose sediments, facilitated by evaporative enrichment and ionic competition, releases arsenic to infiltrating waters and that interaction with metal (oxy)hydroxides in the vadose zone and shallow aquifer may attenuate arsenic levels in the shallow aquifer.

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