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Surface water-groundwater exchange in streams is influenced by hydrologic conditions in the aquifer, hydraulic conditions in the channel, and the spatial distribution of permeability under and around the channel. The exchange with adjacent aquifers is less well understood in large rivers because tracer experiments are much more difficult due to water depth and higher flows than in small streams where they have been used. In this study we ask whether groundwater discharge along a 75 km reach of the Columbia River near the Hanford Site in eastern Washington is determined more by the large scale geologic units in the area or by finer scale riverbed interface sediment deposits. In 2021 we collected water quality data in February (~146 kcfs) and July (~117 kcfs). In July we also collected river bed geophysical data using a floating transient electromagnetic system (FloaTEM). We classified water quality anomalies by depth, temperature, and electrical conductivity (EC). Anomalies (though not always the same type) were identified at many of the same locations in February and July. The majority of the anomalies (69% in February and 52% in July) were high EC and high temperature compared to the river. High temperature inflows with a similar EC to river water were identified in July, but not in February. Though high temperature summer anomalies were unexpected, we found that the average temperature in wells in the area in July (about 22 C) was higher than the river temperature (about 20 C). Preliminary analysis of the geophysical data suggests that areas with high resistivity (which indicates more permeable sand and coarse-grained material) aligned with water quality anomalies – water quality anomalies not aligned with resistivity shifts are likely associated with surface controls (such as irrigation return flows).

Margaret.Digiorno@colorado.edu

Civil, Environmental and Architectural Engineering Graduate Student, Â鶹Ãâ·Ñ°æÏÂÔØBoulder