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Nebraska is a large agricultural producing state with a heavy reliance on groundwater resources and nitrogen fertilizer application to maintain output. Poor management, such as excessive fertilizer application, improper timing, and over irrigation can lead to contamination of groundwater. Nitrate is the leading groundwater contaminant in Nebraska and chronic consumption at medium to high concentrations leads to adverse health effects. Nitrate, a highly soluble anion, is present in the root zone in numerous forms and undergoes biogeochemical transformations before being leached through the profile. In order to predict the timing and quantity of nitrate contamination in groundwater, researchers study the vadose zone, which is the soil and sediments from the land surface to the groundwater. The vadose zone acts as a storage compartment and transforms chemicals, such as nitrate, and acts as a filter for percolating water. A majority of research focuses on nitrate-nitrogen in the vadose zone, but an investigation of nitrogen storage in the vadose zone in the Central Platte region of Nebraska measured significant concentrations of ammonium, a cation expected to bind to clay particles in surface soils. In the Central Platte, overall, average nitrate storage in the vadose zone decreased by 10% in the past 30 years, with a large drop in surface loading, hinting that management practices have been effective. It was hypothesized that ammonium is a product of dissimilatory nitrate reduction to ammonium (DNRA), a biotic pathway that converts nitrate to ammonium. Selected samples were further analyzed to determine total Kjeldahl nitrogen (TKN), hot water extractable organic carbon (HWEOC), total organic carbon (TOC), and δ13C-OC to help evaluate potential chemical and microbial transformations of nitrogen to ammonium in the vadose zone. The hypothesis was rejected for site DH-32, where a spike of ammonium (3.56 μg/g), a drop in pH (5.92), slight increase in nitrate concentration (0.99 μg/g), and δ 13C-OC enrichment (-22.2 ‰) hint at an alternate process. The hypothesis was supported for site DH-36, where a spike in ammonium concentration (8.46 μg/g), drop in nitrate concentration (0.26 μg/, a negative gradient in pH (6.83), and no δ 13C-OC enrichment (-27.4 ‰) hint at DNRA.
Advisor: Daniel Snow