Determining hydrologic pathways of streamflow using geochemical tracers in a claypan watershed

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College of Forest Resources and Environmental Science


Despite the low permeability of claypan soils, groundwater has been heavily contaminated by nitrate in agricultural watersheds dominated by claypan soils. However, it is unclear how nitrate concentrations in groundwater affect stream water quality. In this study, streamflow pathways were investigated using natural geochemical tracers in the 73‐km2 Goodwater Creek Experimental Watershed in northeastern Missouri. Samples were collected from 2011 to 2017 from stream water (weekly‐biweekly), precipitation (event‐based), groundwater in 25 wells with screened depths varying from 2 to 16 m (bimonthly–seasonal) and interflow above the claypan in 7 shallow piezometers (weekly–monthly). The results of endmember mixing analysis using major ions indicate that streamflow was dominated by near‐surface runoff (59 ± 20%), followed by interflow (25 ± 16%) and groundwater (16 ± 13%). Analysis of endmember distances using the mixing space defined by stream water chemistry suggests that groundwater contributions to streamflow came primarily from the intermediate to deep glacial till aquifer near and below 8 m. Near‐surface runoff was persistent and dominant even after isolated precipitation events during a prolonged dry period. It is hypothesised that the alluvial aquifer near stream banks acts as a mixing zone to receive and store various source waters, resulting in persistent delivery of runoff to the stream. Groundwater, even though its contribution was limited, plays a significant role in regulating streamflow NO3− concentrations. This study significantly improves our understanding of claypan hydrology and will lead to the development of models and decision support tools for implementation of management practices that improve groundwater and stream water quality in restrictive layer watersheds.

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Hydrological Processes