OXYGEN ISOTOPE AND GEOCHEMICAL VARIATIONS ALONG THE MISSOURI RIVER

The Missouri River basin occupies a significant portion of the continental United States and provides an excellent opportunity to study important water quality and supply issues. The geochemical character of the Missouri River varies greatly along its length, and exhibits several unusual features such as a downstream decrease in TDS in the lower basin. These effects are explicable in terms of geographically diverse sub-basins that contribute waters with distinct chemical characteristics and distinct oxygen isotopic signatures, which can be exploited to identify solute source regions. Average oxygen isotope delta values in the Missouri River main stem systematically increase from less than -17‰ in the headwaters to about -9‰ in the lower basin (Winston and Criss, Env. Geo. 2003; Coplen and Kendall, USGS 2000). This trend is due to geographic differences in the isotopic composition of source precipitation and to the integration of waters from upstream regions. Seasonal variations are small in the upper basin due to snowmelt effects, are even smaller in the middle basin due to the long residence times (several years) in six huge reservoirs, and are largest in the lower basin, where the dominant regional source of the river water changes seasonally. Criss, Davisson and Kopp (JAWWA 2001) recognized strong relationships between chemical and oxygen isotope concentrations and streamflow in the lower Missouri River, which could be deconvolved to identify source regions by using discharge data from gauging stations. For a given reach, discharge values at the upstream and downstream stations are used to calculate a daily “within reach fraction” representing the proportion of flow derived from regions between the stations. Correlations between the within reach fraction and various chemical parameters reveal source regions for some constituents. Sodium and sulfate originate predominantly from the basin above Sioux City, Iowa while nitrate is largely derived from agricultural regions below Sioux City. Average water chemistry values are also successfully predicted for the upstream stations of two reaches on the lower Missouri River based on changes in discharge along each reach and water quality measurements collected at the downstream stations.