A new compliance checking level for nitrate in groundwater : modelling nitrate leaching and the fate of nitrogen in the upper 5 meter of the groundwater system

Research was conducted on the implications of a possible lowering of the sampling depth for the nitrate compliance checking level in the Netherlands. The STONE model was used to simulate nitrate concentrations and nitrogen balances for the three main sand districts (North, Central, South) and for three groundwater classes (dry, moderate, wet). Results of sediment samplings were evaluated to refine the model input and to obtain information for model validation. Water quality data of regular monitoring networks were processed for an assessment of the model performance. In the dry sand districts the current nitrate concentration does not decrease with depth but the future concentrations are expected to decrease with depth due to denitrification. The current pattern is confirmed by field observations and is explained by the combination of the decreasing trend of the historical nitrate loading of the groundwater and denitrification. Despite the reduction of future nitrate concentrations, the drinking water standard of 50 mg l-1 will still be exceeded at 5 meter below the groundwater level in the dry sandy soils. The dry sandy soils show a nitrate load on surface waters which should be taken into account for the evaluation of Water Framework Directive related targets and progress