Characterizing Groundwater Chemistry and Recharge in an Agricultural Claypan Watershed Using End Member Mixing Analysis

Soils with restrictive horizons near the surface, such as fragipans and claypans, are prone to surface runoff and erosion and cover approximately 2.9 million km² worldwide. In agricultural regions of the Midwestern United States, these soils extend over 16,000 km² and are vulnerable to loss of chemicals and nutrients from above and below the restrictive claypan horizon. In the 77 km² claypan-dominated Goodwater Creek Experimental Watershed in northern central Missouri, streams receive significant contributions of nitrate from groundwater sources. Given the importance of groundwater as a potential source of water and nutrients to surface waters in this agricultural region, we sought to characterize groundwater chemistry and recharge mechanisms and improve our understanding of subsurface water movement. Processes controlling groundwater chemistry, movement, and recharge were determined using natural abundances of geochemical tracers measured in groundwater from 2011 to 2017. Groundwater samples were collected from 25 wells ranging in depth from 2.7 to 15.7 m deep below the claypan and 7 piezometers placed above the argillic horizon. Stream water and precipitation samples were also collected at or near the catchment outlet. These water sources served as potential end members for mixing analyses. Results from the diagnostic tools of mixing models indicate that Mg²⁺, Na⁺, SO₄^2-, Cl^-, and NO₃^- behaved conservatively in groundwater in most wells, reflecting mixing of various source waters. Water in the deeper groundwater wells (+3 m from surface) primarily originated from groundwater at similar depths, often upslope or directly vertically above, with some contribution of shallow groundwater from within 3 m of the ground surface. Water chemistry in the shallow wells above the claypan horizon and within 3 m identified groundwater sources as a mixture of 2-3 end members, but it is obvious that chemical equilibrium was also controlling water chemistry in these more surficial soil layers. Our results suggest that preferential flow paths and upslope lateral drainage of deep groundwater are both important mechanisms of groundwater recharge and movement to streams in this claypan watershed.