For the efficient regulation of agricultural practices aimed at minimising nitrate influxes to streams, it is essential to identify potential zones where nitrate is naturally removed by denitrification prior to reaching the stream network. Large-scale distributed hydrological models used for this purpose often do not include lakes and smaller stream tributaries. This study investigates the influence of the resolution of the surface water system representation on simulated flow, nitrate transport and potential for removal by denitrification in groundwater.
A regional (160 km2) 3D numerical groundwater model is set up including a lake-rich area in Western Denmark. Four scenarios with different spatial resolution of the surface water system are evaluated. The most complex model in regards to the surface water system has a 56-km stream network and 13 lakes, while the simplest model only represents 19 km of the stream network and none of the lakes. When the surface water system is not fully represented in a groundwater model, the option for removing water to match the observed water table is either to incorporate drains or increase the aquifer hydraulic conductivity. The four groundwater-surface water flow systems were evaluated using particle tracking. The fraction of the total travel time that the particles spend below the groundwater redox boundary prior to discharging to the surface water, was recorded to determine the potential for denitrification (due to pyrite oxidation).
When removing surface water bodies and increasing hydraulic conductivity, capture zones increase and the potential for removing nitrate decrease. Regulations on agriculture based on such models would therefore affect a large area but not necessarily reduce environmental impact. Including drains instead of parts of the surface water system has the opposite effect on the capture zone, which would potentially lead to inefficient regulations.