Toepassing van LGMCAD voor de berekening van het nitraatgehalte in ruwwater op grondwaterpompstations in Twente en de Achterhoek. Tevens achtergronddocument voor de Nationale Milieuverkenning 2000-2030

For the 5th National Environmental Outlook 2000-2030 the breakthrough of nitrates is studied in groundwater abstraction wells located in Twente and the Achterhoek. In this report model choices and input data are described and discussed in detail. Model results indicate that at present nitrate concentrations are stable or slightly decreasing. The results also show that in 2030 the nitrate contents in the pumping stations located in the 'Achterhoek' will not exceed the EC drinking standard (50 mg L-1), while in 'Twente' several pumping stations will exceed the EC standard after 2030. Special attention is paid to denitrification. All calculations are performed twice: with and without denitrification. It appears that denitrification leads to concentrations one order of magnitude lower than when denitrification is not taken into account. The groundwater qualitity situation in 2000 is not sufficiently known to be used as an initial condition. Therefore, the year 1950 has been chosen as the start of the simulation period. Errors due to an incorrect estimate of the situation in 1950 are small, when predictions for 2000-2030 are considered. Simulated data for the period before 2000 and measurements collected since 1968 are compared. For the majority of pumping stations the model results appear to be higher than the measured values. This suggests that the predicted nitrate concentrations given for 5th National Environmental Outlook are probably too high. Several possible explanations are given and discussed.