Modelling agricultural strategies to protect groundwater resources in the Hesbaye aquifer (Belgium)

Groundwater quality is spoiled by various substances resulting from human activities. In addition to pesticides, one of the most problematic substances is nitrate. Pumping prevention areas are strategic zones in terms of struggle against diffuse pollution by nitrate.

In order to reduce diffuse pollution of waters by nitrate, different strategies of agricultural practices can be implemented. Their impact can be evaluated either by monitoring water quality or using an agro-hydrological model. The advantage of modelling is to be able to test long term impacts of implemented measures and impacts of complementary measures.

Using EPICgrid distributed agro-hydrological model, we reproduced the current agricultural practices. We calculated nitrate leaching but also the nitrate stock in the root zone. These results were validated through a comparison with a lot of nitrogen stock measurements. (These measurements are compulsory in the Walloon context.) The nitrate concentration in pumping wells was also used to assess the efficiency of the model.

After this calibration phase, we modelled several scenarios of more water-protective agricultural practices. Indeed, the Hesbaye galleries represent an important source of drinking water in the Walloon region, it is therefore highly probable that further mitigation measures are going to be imposed in the pumping prevention area. An increase in the area cultivated with cereals, a strong limitation of mineral fertilisation as well as a conversion to grasslands were tested and combined.

The results of these simulations show how some agricultural practices scenarios can lead to an important decrease in diffuse pollution by nitrate. Prospective simulations taking into account possible future climate evolution (global change scenarios) are carried out to assess nitrate concentrations near the groundwater table for the deadlines of the WFD (2015, 2021 and 2027). Due to the transfer time through the vadose zone (more than 20 years in some subregions), it is showed that an increase in groundwater nitrate concentrations will occur, in some areas, until at least 2030 before new agricultural practices can impact positively the groundwater quality.