42-10 A Comparison of Chemical, Physical, and Isotopic Baseflow Separation Methods to Investigate Groundwater and Surface Water Interactions in the Chippewa River (WI)

Abstract:

Streams and rivers integrate water from multiple sources that possess distinct geochemical and isotopic signatures. Characterizing these endmembers is essential for baseflow separation. A better understanding of groundwater-surface water interactions improves our ability to evaluate contaminant transport and safeguard aquatic ecosystem health in Midwestern river systems. This study investigates the Chippewa River, which drains the second largest watershed in Wisconsin. The downstream portion of the Chippewa River supports a greater number of rare species and more native prairie than any area of comparable size in Wisconsin. Despite the regional importance of the Chippewa River, studies of source contributions to streamflow are sparse. In this study we conducted temporal hydrological sampling following snowmelt (i.e., late spring) through the variable summer season to capture a broad range of hydrologic conditions, including periods of low discharge dominated by baseflow and high-flow episodes driven by storm events. Sampling locations included the Chippewa River near Eau Claire (WI), a small tributary, a hillslope seep, a nearby spring, and several wells to represent potential streamflow endmembers. We utilized a multi-tracer approach combining field geochemistry (e.g., specific conductance, pH) with laboratory measurements of chloride, bromide, nitrate, water stable isotopes, and Radon-222. Streamflow records were retrieved from local USGS gauging stations on the Chippewa River.

Preliminary results from the Chippewa River reveal an inverse exponential relationship between Radon-222 concentration and stream discharge. Spatial and temporal patterns of nitrate and chloride among endmembers provide insight into watershed-wide anthropogenic influences on water quality. Future analyses of water stable isotopes will provide an additional constraint on streamflow endmember contributions. Pending work will compare temporal baseflow contributions derived from water stable isotopes, Radon-222, and specific conductance with results derived from stream hydrograph baseflow separation methods. Together, these findings will advance understanding of local watershed hydrology and provide valuable information for water resource management.

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