Modeling the effects of agricultural management practices on groundwater in Shelton, USA Modelación de los efectos de prácticas de manejo agrícola en el agua subterránea en Shelton, EEUU

An integrated methodology was developed to assess the impact of recharge rates, nitrate leaching from crop root zones, and irrigation pumping rates on groundwater quality and quantity. Monthly values for deep percolation of water, irrigation pumping and leaching of nitrate-nitrogen were calculated with a soil-water-plant model. An Intermediate Vadose Zone Model (IVZM) was developed and used together with models for groundwater flow (MODFLOW) and solute transport (MT3D) to simulate the movement of water and nitrate-nitrogen. The IVZM generates recharge files used by MODFLOW andJMT3D. The methodology was applied to an area near the town of Shelton, Nebraska, where a shallow sand-gravel aquifer is highly contaminated by nitrate-nitrogen. MODFLOW was calibrated using historic groundwater level data from 1981 to 1996. Simulation results suggest that concentration reductions of 10% in the upper third of the saturated zone are possible after 10 years of improved irrigation and nitrogen fertilizer management. Groundwater quality changes slowly propagate down-gradient from the field and towards the lower part of the aquifer. A particle tracking technique showed that water from a well does not provide a good indication of the influence of improved management in the adjacentfield. The quality ofwater pumpedfrom a well represents the effect of management over 15 years and from more than 1.5 km up-gradient from the well. Regional groundwater quality depends on local management practices over a long period. Regulatory programs to improve water quality require therefore, widespread adoption and substantial time to provide significant water quality improvements. Se desarrolló una metodología integrada para evaluar el impacto en la calidad y cantidad de agua subterránea ocasionado por la tasa de recarga, infiltración de nitrato desde zonas de cultivos, e intensidad de bombeo para irrigación. Se calculan valores mensuales de percolación profunda, irrigación por bombeo e infiltración de nitrato-nitrógeno con un modelo de suelo-agua-planta. Un modelo de zona vadosa intermedia (IVZM) fue desarrollado y usado junto con modelos de flujo de agua subterránea (MODFLOW) y transporte de soluto (MT3D) para simular el movimiento del agua y nitrato-nitrógeno. IVZM genera archivos de recarga usados por MODFLOW y MT3D. La metodología fue aplicada a un área próxima a la localidad de Shelton, Nebraska, donde un acuífero somero de arena y grava está altamente contaminado por nitrato-nitrógeno. MODFLOW fue calibrado usando registros históricos de nivel de agua de 1981 a 1996. Resultados de