Predicting denitrification in groundwater utilizing redoxcline depth and aquifer thickness

Area of Science:

• Environmental Science
• Hydrology
• Water Quality Management

Background:

• Flanders faces challenges with surface water quality due to high nitrate concentrations in agricultural watersheds.
• Understanding nitrate transfer from groundwater to surface water is crucial for effective water management.

Purpose of the Study:

• To estimate the fraction of groundwater nitrate load reaching surface water by assessing aquifer denitrification.
• To develop a predictive model for nitrate concentrations at catchment outlets based on aquifer properties.

Main Methods:

• Determined redoxcline depth using hydrochemistry data from multilevel groundwater wells.
• Calculated the ratio of oxidized to total aquifer thickness (Hooghoudt equivalent).
• Predicted catchment outlet nitrate concentrations using groundwater nitrate input and the Hooghoudt equivalent.

Main Results:

• The ratio of oxidized to aquifer thickness averaged 0.48 across 86 catchments.
• A model combining groundwater nitrate input (NEMO) and Hooghoudt equivalent showed encouraging performance (NSE=0.42) without calibration.
• The oxidized fraction significantly influenced surface water nitrate concentrations.

Conclusions:

• Aquifer denitrification, indicated by redoxcline depth, is a key factor controlling surface water nitrate.
• Surface water nitrate response to agricultural inputs is limited to areas where recharge water bypasses significant denitrification.
• Approximately 41% of the studied catchment areas in Flanders contribute to surface water nitrate loads without significant denitrification.