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 exceed the Montana State human health standard (0.24 micromolar). In addition, ground water As concentrations in the northern portion of the Lower Madison River Valley are above the federal drinking water standard (0.67 micromolar) and correlate with high soluble As levels in overlying soils. The objectives of this study were to determine processes affecting As solubility in soils of the Madison and Upper Missouri River Basins and assess potential impacts of long-term irrigation with As-rich river water on As mobility. Sixteen irrigated (>20 yr of irrigation) and nonirrigated (never irrigated) soils were sampled in four major regions adjacent to the Madison and Upper Missouri Rivers. There were no significant differences (alpha = 0.05) in total or soluble As levels between irrigated and nonirrigated soils within any of the four regions. In addition, sorption coefficients (K(d) values) in six paired soils selected for additional chemical characterization were not significantly different (alpha = 0.05) between irrigated and nonirrigated soils. Ammonium oxalate extractable Fe and Mn concentrations were found to be positively correlated with K(d), and Olsen P concentration was found to be negatively correlated with K(d) based on a multiple linear regression (r(2) = 0.92). Sequential extractions performed for a subset of soils showed that labile As concentrations were similar between irrigated and nonirrigated soils. Soils that had been irrigated for >100 yr had additional As sorption capacities of at least 3 mg kg(-1) in saturated column studies following 30 yr of simulated irrigation. Based on this suite of data, it was determined that irrigation history has not significantly affected As solubility or attenuation capacities in soils of the study area.