Processes Controlling Hydrochemistry in a Claypan Soil Watershed in Missouri

Processes controlling hydrochemistry in a claypan soil watershed were elucidated using an array of chemical elements. Samples have been collected from rain water, streams, shallow subsurface water and groundwater since summer 2011 and analyzed for major ions and trace elements. Diagnostic tools of mixing models, which are based on principal component analysis, were used to determine conservative and reactive chemical elements subject to mixing in streams. Major elements in stream water, e.g., Ca, Mg, Na, K, S and Sr, were relatively conservative and primarily controlled by mixing of three endmembers, including rainwater, shallow subsurface water and groundwater. The mean electric conductivity (EC) in stream water was 240 μS cm^-1, while the mean EC value was 31, 102 and 687 μS cm^-1 in rain water, shallow subsurface water and groundwater, respectively. Mean Al and Fe concentrations in stream water (2496 and 2952 ppm, respectively) were mainly gained from soils as a result of chemical reactions. Concentrations of Cd, Cu, and Zn in stream water (23, 373 and 136 ppm on average, respectively) were primarily from atmospheric inputs (150, 5820 and 274498 ppm on average, respectively) but mostly lost in soils before they get to streams. These information help develop a conceptual understanding of contaminant transport in hydrological systems in claypan watersheds in the Midwest of U.S..