Nitrate leaching contributes to groundwater contamination in many regions of the world. Agricultural activities are known to be the main source of nitrate affecting rural domestic and urban public water supply wells. Well-head water treatment for nitrate is expensive. Here we propose that reducing nitrate concentrations in groundwater may require changes in land-use, improvements in nutrient and irrigation management, or dilution with managed aquifer recharge in the agricultural landscape (“Ag-MAR”). We investigate the feasibility and long-term impacts of these farm best management practices (BMPs) through numerical modeling of nitrate fate and transport in a semi-arid, irrigated agricultural region overlying an alluvial groundwater basin. Practices considered include a) crop type change from an intense nitrate fertilizer crop to low-demand fertilizer crops, and b) distribution of high winter stream flows on agricultural land for recharge to groundwater during the wet winter season (Ag-MAR). Specifically, we investigate the spatial and dynamic effects of these BMPs on nitrate in an adjacent public supply well located in a rural, disadvantaged community.
We evaluate several scenarios: 1) business as usual (BAU), 2) continuous/discontinuous winter recharge (WR), 3) alternative low-impact crops (LIC), and 4) combination of low-impact crop with winter recharge (LICWR). The quickest and most enduring option is to implement both, agricultural nitrate load reduction by crop change and higher Ag-MAR in the source area. This scenario lead to more than 85% attenuation of nitrate in the supply well’s water and remediated ambient groundwater between the source area farmlands and the supply well. It also had the shortest required time to reach stable, low nitrate in the aquifer system and showed the most sustainable impact. Other combinations of crop changes and Ag-MAR led to some improvements. Recharge practices are shown to also be a tool for remediating groundwater in distance between public supply well and farm lands associated with its recharge zone.