SPATIAL AND TEMPORAL VARIATION OF NITRATE CONCENTRATIONS IN GROUNDWATER IN A CLAYPAN WATERSHED IN MISSOURI

Spatial and temporal variations of nitrate concentrations in groundwater at Goodwater Creek Experimental Watershed were examined to understand the controls of nitrate transport in groundwater. The 75-km² watershed is dominated by a continuous, restrictive clay layer 20-50 cm beneath the top soil. The mean nitrate-N concentration of the 94 wells located throughout the watershed was 12 mg L^-1 from 1991 to 2004, which is comparable to the mean value in shallow groundwater at agricultural watersheds with well-drained soils throughout the US. The temporal variation of nitrate-N concentrations from 1991 to 2004 was dominated by three patterns among 94 wells: no obvious trend, an increasing trend and a decreasing trend. The variation of nitrate-N concentrations with well depth was dominated by a bulge-shape: increasing with depth above 10 m but decreasing below it. This threshold depth is consistent with a continuous glacial till layer located at the same depth, which has much higher hydraulic conductivity than the deposits above and below it. It is believed that those patterns are primarily caused by a combination of the geologic deposits and the land use history, with water below 10 m mainly from recharge occurred a century ago when farming has not started in the area but above it from recent recharge particularly after 1940s when using of inorganic fertilizers has intensified. This information is helpful for groundwater quality management at claypan watersheds in the US Midwest.