Modeling Legacy Nitrate Leaching Potential during Agricultural Managed Aquifer Recharge

Abstract:

Agricultural intensification in California has led to the accumulation of legacy nitrate in the underlying vadose zone. Additionally, dependence on groundwater for increased irrigation demands has resulted in the widespread depletion of groundwater resources. Legislature like the Sustainable Groundwater Management Act in California puts into action plans to reach groundwater use sustainability by 2040, partly achieved through the implementation of long-term groundwater recharge programs. Agricultural managed aquifer recharge (AgMAR) has emerged as a promising groundwater replenishment opportunity in California; AgMAR is a form of managed aquifer recharge where farmland is flooded during the winter using excess surface water in order to recharge the underlying groundwater. However, questions remain as to how AgMAR could be implemented on fertilized agricultural fields such that nitrate leaching from the root zone is minimized. Specifically, we are interested in understanding how AgMAR impacts the biogeochemical dynamics of nitrogen cycling, and potential best management practices for future AgMAR implementation. Building on previous field and laboratory experiments, a calibrated HP1 (HYDRUS-1D and PHREEQC) model is presented. HYDRUS-1D models variably saturated water flow and solute transport by solving the Richards’ equation and Fickian-based advection dispersion equations. PHREEQC implements a nitrogen cycling model representing mineralization, immobilization, nitrification and denitrification dynamics using conditional, kinetically controlled reactions and surface complexation modeling. Paired together using the HP1 code, we have recreated previous AgMAR field experiments, investigating the impact that field scale heterogeneity of legacy nitrate has on nitrate leaching potential under AgMAR. We also present a multi-scenario analysis which considers the effects of soil texture, flooding frequency and flooding magnitude on the nitrate leaching potential during AgMAR project implementation. Together these results will allow developing best management practices for the joint use of agricultural lands for groundwater recharge and crop production.