THE FATE OF GEOTHERMAL ARSENIC IN THE MADISON AND MISSOURI RIVERS, MONTANA AND WYOMING

Geothermal As from Yellowstone National Park causes high As concentrat ions (10-370 mu g/L) in the Madison and Missouri Rivers in Montana and Wyoming. Arsenic transport is largely conservative in the upper basin as demonstrated by the near equivalence of dissolved and total-recove rable As concentrations, the constancy of As loads, and consistent rat ios of concentrations of As to conservative geothermal tracers. Diurna l cycling of As between aqueous and solid phases in response to pH-ind uced changes in sorption equilibria causes small variations of about 1 0-20% in dissolved As concentrations. HCl-extractable As concentration s in river and lake sediment in the upper basin are variable depending on position relative to the As-rich headwaters and geochemical and ph ysical processes associated with lakes. In the lower Missouri River, l arge quantities of suspended sediment from tributaries provide suffici ent sorption sites for substantial conversion of As from the aqueous p hase to the solid phase.