Arsenic (As) is a natural element in geothermal waters of Yellowstone National Park, and it is frequently present in concentrations above the national primary drinking water standard of 50 ug/L. The Firehole and Gibbon Rivers, tributaries to the Madison River, drain much of the geothermal water of the western part of the park. An estimated load of 272 kg/day of As is in the Madison River at West Yellowstone. It has been hypothesized that irrigation with As-laden Madison and upper Missouri River waters has resulted in As contamination of the thin alluvial aquifer near Three Forks, MT. However, in the upper Madison and upper Missouri River valleys, As concentrations in groundwater believed to be impacted by irrigation practices were below 10 ug/L.

     It has been shown that in oxic conditions, As is strongly adsorbed to soils. Paired irrigated and non-irrigated soils were identified in areas within the Madison and upper Missouri River valleys where groundwater suspected to be impacted by irrigation practices using Madison or upper Missouri River water. Soils were sampled to a depth of 3 meters, then were characterized for total and soluble As. Concentrations of total As in irrigated soils of the upper Madison River valley above Ennis reservoir and in soils of the upper Missouri River valley above Canyon Ferry reservoir were significantly higher than total As in non-irrigated soils within those locations, indicating As removal from irrigation water by sorption of soils. However, total and soluble As concentrations in the soils of the lower Madison River valley near Three Forks are orders of magnitude higher than concentrations observed in soils of the upper Madison and upper Missouri River valleys. Furthermore, total and soluble As concentrations were higher in non-irrigated soils than irrigated soils of the lower Madison River valley. This is a strong indication that the soils near Three Forks are inherently high in As either from a parent material or a historical depositional effect.

     Soluble As was monitored for an entire irrigation season in the soil profiles of two sites in the lower Madison River valley. No substantial increase in soluble As concentrations was observed following irrigation events, indicating irrigation is not a major mechanism for As contamination of the alluvial aquifer near Three Forks.

Keith, Kristin E., 1995. Arsenic in soils of the Madison and Upper Missouri River Valleys. M.S. Thesis, Land Resources and Environmental Sciences, Montana State University, Bozeman.

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