Unsaturated Zone Profiles Linking Land Surface Applications and Groundwater Nitrate Contamination: Case Studies Seymour Aquifers, Texas

An evaluation of the source and cycling of nitrogen related to current land use and past changes in land use was conducted to elucidate the high nitrate-N contamination in the Seymour alluvial aquifer, Rolling Plains, Texas. This study focuses on the largest alluvial pod of the aquifer, in Haskell and Knox counties (1100 square km), where high nitrate-N concentrations have consistently exceeded the EPA Maximum Contaminant Level of 10 mg/L in up to 75% of the wells since the early 1950's. The region is highly cultivated (71%) and is particularly vulnerable to contamination from surface sources because it is unconfined, has a median water table depth of 8 m, and mostly (40%) consists of sandy soils. Unsaturated zone profiles (36) from 3 different land use settings (5 rangeland, 11 rainfed, and 17 irrigated) were used to quantify subsoil nitrate reservoirs and to link land surface source data with groundwater nitrate levels in the aquifer. Subsoil nitrate reservoirs in rangeland settings are low (1-6 kg N/ha/m, median: 1 kg N/ha/m). Mobilizing the entire reservoir into the underlying aquifer (30% porosity, 10 m saturated thickness) would result in a maximum value of only 1.5 mg nitrate-N/L, far less than the levels in the aquifer prior to use of N based fertilizers in the late 1950s (1-37 mg nitrate-N/L, median: 12.2 mg nitrate-N/L). Cultivated areas show higher subsoil nitrate reservoirs with rainfed settings ranging from 1-148 kg nitrate-N/ha/m (median: 40 kg nitrate-N/ha/m) and irrigated settings ranging from 1-184 kg nitrate-N/ha/m (median: 43 kg nitrate-N/ha/m). Current high groundwater nitrate-N levels (1-80 mg nitrate-N/L; median: 15.5 mg nitrate-N/L) are likely to persist as the nitrate reservoirs under cultivated areas are mobilized into the aquifer