Treated Wastewater and Nitrate Transport Beneath Irrigated Fields: A Kansas water reuse project

There is concern that the use of treated wastewater south of Dodge City, Kansas, which consists of mainly silty clay loam soils, has resulted in high nitrate-nitrogen (N) concentrations (10 – 50 mg/kg) in the subsurface upper 15-m vadose zone, and also in the underlying deep (20-45 m) ground water. The goal of this two-year field monitoring project was to assess how and under what circumstances N nutrients under secondary-treated wastewater-irrigated corn can reach the deep ground water of the underlying High Plains aquifer, and what can realistically be done to minimize this problem. We collected deep cores for physical and chemical properties characterization; installed neutron moisture probe access tubes and suction lysimeters for periodic measurements; sampled area monitoring, irrigation, and domestic wells; performed dye tracer experiments to examine soil preferential flow processes through macropores; and obtained climatic, crop, irrigation, and N application rate records. These data and additional information were used in the comprehensive Root Zone Water Quality Model (RZWQM2) to identify key parameters and processes that influence nitrogen losses in the study area. We demonstrated that nitrate-N transport processes result in significant accumulations of N in the thick vadose zone. We also showed that nitrate-N in the underlying groundwater is increasing with time. RZWQM2 simulations indicated that macropore flow is generated particularly during heavy rainfall events, but during our 2005-2006 simulations the total macropore flow was only about 3% of precipitation. Our calibrated model indicated that reducing current levels of corn N fertilization by more than half to the level of 170 kg/ha substantially increases nitrogen use efficiency and achieves near maximum crop yield. Combining such measures with a crop rotation that includes alfalfa should further reduce the amounts of residual N in the soil.