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

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

Irrigation Produces Elevated Arsenic in the Underlying Groundwater of a Semi-Arid Basin in Southwestern Idaho

The shallow aquifer beneath the Western Snake River Plain (Idaho, USA) exhibits widespread elevated arsenic concentrations (up to 120 μg L−1). While semi-arid, crop irrigation has increased annual recharge to the aquifer from approximately 1 cm prior to a current rate of \u3e50 cm year−1. The highest aqueous arsenic concentrations are found in proximity to the water table (all values \u3e50 μg L−1 within 50 m) and concentrations decline with depth. Despite strong vertical redox stratification within the aquifer, spatial distribution of aqueous species indicates that redox processes are not primary drivers of arsenic mobilization. Arsenic release and transport occur under oxidizing conditions; groundwater wells containing dissolved arsenic at \u3e50 μg L−1 exhibit elevated concentrations of O2 (average 4 mg L−1) and NO3 (average 8 mg L−1) and low concentrations of dissolved Fe (μg L−1). Sequential extractions and spectroscopic analysis of surficial soils and sediments indicate solid phase arsenic is primarily arsenate and is present at elevated concentrations (4–45 mg kg−1, average: 17 mg kg−1) relative to global sedimentary abundances. The highest concentrations of easily mobilized arsenic (up to 7 mg kg−1) are associated with surficial soils and sediments visibly stained with iron oxides. Batch leaching experiments on these materials using irrigation waters produce pore water arsenic concentrations approximating those observed in the shallow aquifer (up to 152 μg L−1). While As:Cl aqueous phase relationships suggest minor evaporative enrichment, this appears to be a relic of the pre-irrigation environment. Collectively, these data indicate that infiltrating irrigation waters leach arsenic from surficial sediments to the underlying aquifer