ESTIMATING THE NITROGEN LOAD TO GROUNDWATER BENEATH AN AGRICULTURAL FIELD

Nitrate leaching from agricultural fertilizers is a significant source of contamination to the world's shallow aquifers. This field-scale study of nitrate transport in an unconfined, glacial outwash aquifer is co-sponsored by Wisconsin’s Department of Natural Resources and members of the agricultural community, who share a common goal of reducing nitrate loss from row-crop production. Early phases of this work focused on developing a groundwater monitoring network, sampling methods, and the use of injected groundwater tracers to quantify nitrate mass leaving the field through the groundwater system.

Instrumentation at the 65-acre field site includes eight monitoring wells to measure hydraulic gradients and assess seasonal variation in the magnitude and direction of groundwater flow. Multilevel monitoring systems at the field edge are used to sample groundwater quality at 1.5-foot intervals from the water table, from about 7 feet below ground surface, to a depth of 35 feet. We developed a method to estimate the annual nitrate load to the water table that relies on a bromide tracer introduced to the water table beneath the cropped area of the field. The first tracer injection, immediately prior to planting in April, 2016, marks the location of groundwater that recharged at the time of spring planting. A second tracer injection just prior to spring planting in 2017, marks the end of recharge for the 2016 growing season and recharge year. The field-edge monitoring system is sampled monthly for profiles of nitrate and bromide with depth. Taken together, these data support an estimate of nitrate mass in groundwater that flows through a control plane at the down-gradient field edge over the 2016 growing year. We applied a Monte Carlo technique to the mass loading calculation to account for measurement error of nitrate in groundwater samples and uncertainty related to heterogeneity in hydraulic conductivity. This method results in an annual nitrate loading rate of 95 lb-N/acre, with a standard deviation of 14 lb-N/acre. This is on the order of half of the nitrogen applied to the field in 2016. This work highlights the potentially significant economic and environmental benefits that could result from implementing agricultural practices that improve fertilizer utilization by crops.