Abstract. Geothermal As from Yellowstone National Park causes high As concentrations (10-370/•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-recoverable As concentrations, the constancy of As loads, and consistent ratios of concentrations of As to conservative geothermal tracers. Diurnal cycling of As between aqueous and solid phases in response to p H-induced changes in sorption equilibria causes small variations of about 10-20% in dissolved As concentrations. HCl-extractable As concentrations in river and lake sediment in the upper basin are variable depending on position relative to the As-rich headwaters and geochemical and physical processes associated with lakes. In the lower Missouri River, large quantities of suspended sediment from tributaries provide sufficient sorption sites for substantial conversion of As from the aqueous phase to the solid phase. This study was undertaken to determine the fate of As as it moves through more than 1140 km of river and several lakes in a large river system. We chose the Madison-Missouri River system in Montana and northwestern Wyoming because the high arsenic concentrations make it an excellent natural laboratory to examine the geochemistry of As on a basin-wide scale. The general goal of the study was to develop a more 3051
Water is our most valuable natural resource, and is used to support the demands of industry, agriculture, hydroelectric power generation, and municipalities. Water also sustains Montana’s booming recreation and tourism economy and maintains the diverse freshwater ecosystems that provide natural goods and services and promote human well-being. As our population continues to grow, and the collective demand for water increases, it is imperative that we carefully assess how our water is used, as well as how changes in water distribution, management, and governance are likely to influence its availability in the future. This is especially important in the context of a changing climate.
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