Ground-water conditions in the eastern part of Michaud Flats, Fort Hall Indian Reservation, Idaho

The Michaud Flats study area occupies about 20 square miles on the Snake River Plain in southeastern Idaho: Ground-water resources are extensively developed for irrigation and industrial uses. The more productive wells are completed in the Pediment Gravel and the basalt flows of the Bighole Basalt and Starlight Formation and the sand and gravel of the Sunbeam Formation. A few shallow domestic wells are completed in the water-table aquifer in the Michaud Gravel. Historically, high levels of arsenic were observed in water from several wells completed in the shallow aquifer. In most wells and springs sampled during this study, levels of arsenic and other minor elements were within limits recommended by the U.S. Public Health Service. Heavy metal concentrations in one well completed in the shallow aquifer, and the concentration of arsenic exceeded the recommended limits. There is some evidence that arsenic, as well as several other minor elements, may occur in above-normal levels naturally in local ground water. Ground water used for irrigation generally has a low sodium-absorption ratio and can be used on almost all soils. All irrigation water sampled have a medium to high salinity hazard, and caution should be used when applying these waters on poorly drained soils. (USGS)