The Impact of Land Use on Nitrate-N Movement and Storage in the Vadose Zone of the Hastings’ WHPA

Nebraska has one of the largest agricultural economies in the United States and relies heavily on irrigation and fertilizer application to maintain crop yields. Over-irrigation and continuous application of nitrogen (N) in many areas has led to accumulation of nitrate-N in soils and sediments throughout the state’s vadose zone. Because nitrate-N is both persistent and mobile, groundwater concentrations in many areas of Nebraska and other agriculturally intensive states are increasing. Nitrate-N contamination of public and private drinking water supplies that utilize groundwater are of particular concern. Vadose zone sampling is an important method for rapidly assessing the effect of changing land use on potential groundwater contamination. In the current project, the occurrence and movement of nitrate-N was investigated using deep vadose zone soil cores collected from urban and irrigated farmland in the Hastings, NE Well Head Protection Area (WHPA) and compared to a previous study done at the same locations (R. Spalding & Toavs, 2011). Sampling previously collected sites allows for direct comparisons of current and historical nitrate-N profiles, potential movement, and can provide a method for evaluating effects of changing land use at the surface. Cumulative nitrate-N in the top 65 ft for urban irrigated lawns, pivot irrigated farmland, and gravity irrigated farmland had an average of 320, 540, and 700 total lbs-N/acre respectively. In farmland where irrigation changed from gravity to pivot application there was an average reduction of 170 lbs-N/acre in the top 55 ft of the profile over a five-year time span. This observation supports the use of sprinkler irrigation for more uniform water application, reducing potential leaching at the head and tail rows of gravity irrigated fields. While future studies are still needed, the importance of vadose zone monitoring in evaluating and protecting groundwater is beneficial in determining connections between surface activities and the underlying groundwater. Advisor: Daniel Sno