IRRIGATION RELATED ARSENIC CONTAMINATION OF A THIN, ALLUVIAL AQUIFER, MADISON RIVER VALLEY, MONTANA, USA

The arsenic concentration in 13 water samples from wells in the alluvial aquifer of the Madison River Valley, Montana ranged from 26 to 150 mu g/l. The Madison River, which originates in Yellowstone National Park, has a mean arsenic concentration of 51 mu g/l ( sigma equals 26 mu g/l), based upon limited sampling during the main irrigation period. Groundwater arsenic concentration increases down the valley can be best correlated with the intensity of ditch irrigation in this semiarid area. No other sources of dissolved arsenic as concentrated as that of the river water have been identified. Evaporative concentration of river-derived irrigation water is believed to have been the overwhelming factor in the contamination of this aquifer.