Reconnaissance of arsenic in surface and ground water along the Madison and upper Missouri Rivers, southwestern and west-central Montana

Abstract

Geothermal waters in Yellowstone National Park contribute large quantities of arsenic to the headwaters of the Madison River. The Madison River flows for about 150 miles to the confluence of the Jefferson and Gallatin Rivers near Three Forks where the Missouri River begins. Arsenic concentrations in the Madison and upper Missouri Rivers and some ground water commonly exceed the State of Montana water-quality human-health standard of 18 micrograms per liter (jlg/L) as well as the U.S. Environmental Protection Agency Maximum Contaminant Level for drinking water of 50 p,g/L. This report describes the magnitude, extent, and possible sources of arsenic in surface andground water, describes the processes controlling arsenic in ground water, and assesses the effect of irrigation on ground-water quality.The median dissolved-arsenic concentration of the Madison River near West Yellowstone is 270 (ig/L, whereas the median dissolved-arsenic concentration of the Missouri River at Toston is 30 |ig/L. Arsenic concentrations decrease from the Madison River near West Yellowstone to the Missouri River at Toston because tributaries having much smaller arsenic concentrations dilute water in the Madison and upper Missouri Rivers. Downstream trends in arsenic loads and boron and lithium concentrations indicate that arsenic in the mainstem Madison and upper Missouri Rivers is largely chemically conservative.In the upper Madison River Valley, arsenic concentrations in irrigation-supply water range from <1 to 88.5 |0,g/L, whereas arsenic concentrations in water in drains and springs and seeps range from <0.7 to 25.8 |ig/L. Arsenic loads in water at selected surfacewater sites increase during the irrigation season, which likely indicates that some arsenic originates from irrigation. Most arsenic concentrations in water at these sites are less than about 20 |ig/L, and some arsenic from applied irrigation water is removed by sorption onto soils. Arsenic concentrations in ground water of the upper Madison River Valley range from 0.5 to 40 ^lg/L, with a median of 2.0 |Llg/L.