Mechanism of Nitrate-N Transport in Surface Water and Groundwater in a Missourian Claypan Watershed

Nitrogen (N) fertilizer applications have resulted in widespread surface water and groundwater nitrate-N (NO3-N) contamination in the U.S. Corn Belt. However, the processes that control NO3-N concentrations in both surface water and groundwater were not well understood, particularly for claypan-dominated watersheds. The objective of this study was to understand the processes that control the transport of NO3-N in/from surface water and/to groundwater in Goodwater Creek Experimental Watershed (GCEW), a claypan-dominated watershed in north-central Missouri. The mean nitrate-N concentration in groundwater determined from more than 2000 samples collected during 1991 - 2004 was approximately three times that in stream water at GCEW. Despite the low hydraulic conductivity of the claypan soils, considerable NO3-N contamination of the glacial till aquifer occurred, with 38% of the wells exceeding 10 mg l-1. Groundwater recharge by preferential pathways through the claypan appeared to be the primary mechanism for NO3-N movement to the aquifer. In comparison, only 19-23% of sampled wells in non-claypan agricultural watersheds over the continental U.S. exceeded 10 mg l-1, suggesting that groundwater in GCEW appears to be more susceptible to NO3-N contamination than non-claypan watersheds. Using endmember mixing analysis based on conservative tracers, stream flow during the relatively low flow periods was primarily from surface runoff, shallow subsurface water and groundwater, with an average contribution of 25%, 44% and 31% from 2011 to 2014, respectively. Stable isotopic values of nitrate (d15N, d18O and D17O) also demonstrated that groundwater exerted a major control on nitrate concentrations in stream water. This information greatly improves our understanding of the processes that control NO3-N transport to the groundwater system of claypan-dominated watersheds.