Field Estimation of Groundwater Nitrate Exported from an Individual Field by Combining Passive Diffusion Profiling with Darcy's Flux.

Direct groundwater measurement of the magnitude and timing of nitrate leaching relative to crop and soil management is critical to optimizing BMPs. Some studies have measured groundwater nitrate at or near the water table to estimate nitrate leaching. However, detailed vertical profiles of solutes are difficult to obtain for deep water tables, particularly when water table elevations fluctuate significantly. We combined detailed vertical concentration profiles obtained by passive diffusion sampling and discharge measured using Darcy’s Law to estimate monthly groundwater nitrate flux from a single intensively managed agricultural field overlaying a shallow, permeable, unconfined aquifer. Multilayer passive diffusion samplers were spaced at 10 cm vertical increments and deployed in fully screened wells located up-gradient and down-gradient of the field edges. Nitrogen was not applied directly up-gradient of the field during the study period and nitrate concentrations at the water table were low (< 4 mg nitrate-N/L). The down-gradient nitrate concentrations were high with maximum monthly concentrations reaching 80 mg nitrate-N/L. Hydraulic conductivities (K) were estimated using Hazen’s empirical formula, with grain size analyses of core samples obtained by sonic drilling. Darcy fluxes were estimated using depth-appropriate K values and a single horizontal hydraulic gradient for each sampling period. The nitrate mass flux for each depth increment was estimated for a vertical cross-sectional area of unit width centered around each diffusion cell. Nitrate mass fluxes for the sampling increments coming off the field were summed to estimate the monthly nitrate flux (in kg N/ha equivalent) leaving the field property. Monthly nitrate export values ranged from 2 to 33 kg N/yr equivalent, with equivalent annual nitrate flux ranging from 77 to 110 kg N/ha.