Arsenic solubility and attenuation in soils of the Madison River Basin, Montana: Impacts of long-term irrigation

Arsenic concentrations in 850 km of the Madison and Upper Missouri Rivers e xceed the Montana State human health standard (0.24 mu M). In addition, gro und Water As concentrations in the northern portion of the Lower Madison Ri ver Valley are above the federal drinking water standard (0.67 mu M) and co rrelate with high soluble As levels in overlying soils. The objectives of t his study were to determine processes affecting As solubility in soils of t he Madison and Upper Missouri River Basins and assess potential impacts of long-term irrigation with As-rich river water on As mobility. Sixteen irrig ated (>20 yr of irrigation) and nonirrigated (never irrigated) soils were s ampled in four major regions adjacent to the Madison and Upper Missouri Riv ers. There were no significant differences (alpha = 0.05) in total or solub le As levels between irrigated and nonirrigated soils within ang of the fou r regions. In addition, sorption coefficients (K-d, values in six paired so ils selected for additional chemical characterization were not significantl y different (cy = 0.05) between irrigated and nonirrigated soils. Ammonium oxalate extractable Fe and Mn concentrations were found to be positively co rrelated with fid, and Olsen P concentration was found to be negatively cor related with Kd based on a multiple linear regression (r(2) = 0.92). Sequen tial extractions performed for a subset of soils showed that labile As conc entrations were similar between irrigated and nonirrigated soils. Soils tha t had been irrigated for >100 yr had additional As sorption capacities of a t least 3 mg kg(-1) in saturated column studies following 30 Jr of simulate d irrigation, Based on this suite of data, it was determined that irrigatio n history has not significantly affected As solubility or attenuation capac ities in soils of the study area.