The Importance of Groundwater in Regulating Stream Water Quantity and Quality in a Claypan Watershed in Missouri

Abstract:

Claypans are a sub-soil horizon of smectitic mineralogy that impedes percolation and promotes surface runoff. These soils, which encompass 33,000 km² in portions of Missouri and Illinois, have long been thought to restrict contaminant transport into/from groundwater. However, recent studies have shown that claypan watersheds are vulnerable to the loss of agricultural chemicals, particularly nitrate. The purpose of our study was to understand the role of groundwater in regulating stream water quantity and quality in Goodwater Creek Experimental Watershed (GCEW) in northern central Missouri. Samples have been collected since 2011 from precipitation, stream water at three locations along the main stream, groundwater from twenty five wells with depths ranging from 3 to 15 m, and soil water from seven piezometers above the claypan. Mean nitrate concentrations were 4.1 ppm in stream water at the watershed outlet, 1.0 ppm in precipitation, 3.2 ppm in soil water at piezometers, and 18.9 ppm in groundwater. Using diagnostic tools of mixing model and end-member mixing analysis, three natural tracers were determined to be conservative – electric conductivity (EC), Mg²⁺ and Na⁺ and three end-members were identified to control stream water – surface runoff (chemically characterized by precipitation), interflow above the claypan (characterized by soil water in piezometers), and groundwater. Interflow and groundwater contributed, on average, 26% and 12% of stream flow at the watershed outlet, respectively. During low flows, however, the contribution of groundwater increased to 30-40%. Also, the mean contribution of groundwater increased to 16% at middle course and 19% at the headwaters. Nitrate concentrations in stream water were dominated by the contributions from groundwater during low flows, ranging from 50 to 90% at all stream sampling locations. This study highlights the vulnerability of groundwater to nitrate contamination, even in runoff-prone watersheds, and demonstrated its importance in regulating stream water quantity and quality in claypan watersheds.