Contaminant Sources in Stream Water of a Missouri Claypan Watershed

Elevated concentrations of nitrate-nitrogen and herbicides in stream water have degraded water quality and caused serious problems affecting human and aquatic ecosystem health in the Central Claypan Region of the US Midwest. However, the contribution of specific recharge sources to stream water is not well understood in claypan-dominated watersheds. The purpose of this study was to estimate the recharge sources to Goodwater Creek Experimental Watershed (GCEW) in north-central Missouri and investigate their importance to contaminant transport. Samples were collected from 2011 to 2014 from streams, piezometers, seep flows, and groundwater in GCEW and analyzed for major ions (including nitrate and nitrite), trace elements, stable H and O isotopes, total nitrogen (TN) and herbicides. Using an endmember mixing analysis based on conservative tracers, recharge contributions to stream flow were an average of 25% surface runoff, 44% shallow subsurface water, and 31% groundwater. TN concentrations were, on average, <0.05 ppm, 0.5 ppm, and 5 ppm in surface runoff, shallow subsurface water, and groundwater, respectively. Atrazine concentrations were, on average, <0.001 ppb, 0.052 ppb and <0.0001 in surface runoff, shallow subsurface water and groundwater, respectively. The data indicated that TN in stream water was primarily from groundwater, while shallow subsurface water was the dominant source of atrazine in stream water. An improved understanding of claypan hydrology and contaminant transport could lead to crop management practices that better protect surface water and groundwater in claypan-dominated watersheds.