Predicting denitrification in groundwater utilizing redoxcline depth and aquifer thickness

Due to high nitrate concentrations, Flanders has been struggling to achieve satisfactory surface water quality in agricultural watersheds. To improve understanding of nitrate transfer from groundwater to surface water, this study estimated the fraction of nitrate load from groundwater recharge that reaches the surface water by inferring the extent of denitrification in the aquifer based on the depth of the redoxcline. Nitrate concentrations at catchment outlets were predicted by multiplying nitrate concentrations in groundwater recharge by the ratio of the thickness of the oxidized zone to the aquifer thickness. The location of the redoxcline was determined from the hydrochemistry data available from a regional network of multilevel groundwater wells. Four scenarios were developed to compare the effect of differing assumptions on nitrate inputs and aquifer thickness on the catchment outlet nitrate predictions. The ratio of oxidized to entire aquifer thickness (Hooghoudt equivalent) for the studied 86 catchments varied from 0.07 to 1.0, averaging 0.48. The results indicated that combining groundwater nitrate input from a Nutrient Emission Model (NEMO) for Flanders and the Hooghoudt equivalent aquifer thickness provided the best predictions. Applying this model to 86 agricultural catchments (area: 0.4-37 km²) yielded a Nash-Sutcliffe model Efficiency (NSE) of 0.42 when comparing predicted to observed surface water nitrate concentrations. This is a reasonably encouraging model performance, given that this model did not involve calibration. We found that the variation in the oxidized fraction predominantly governed the nitrate concentration at the catchment outlets. Our results imply that nitrate loads in surface waters are only responsive to fertilizer and manure application in the fraction of the catchment area where recharge water will not undergo significant denitrification along its flow path to the surface water. Considering the variable catchment areas-, this fraction amounted 41 % of total area of the studied catchments in Flanders.