Nitrate in shallow groundwater under agricultural land in sandy areas of the Netherlands, 1984-2003
The concentration of nitrate exceeded the EU standard in more than 20% of shallow groundwater in 2003. In 1995, this percentage was still 25%. In medium-deep groundwater, the number of monitoring sites where exceedances were found was 6%.
Changes in nitrate concentration
The percentage of exceedances of the EU value for nitrate (50 mg/l) in shallow groundwater (5-15 m depth) beneath sandy agricultural land between 1989 and 1995 was approximately 25%. There then seems to have been a fall to approximately 22%. This downturn may be the positive effect of the reduction of emissions from nitrogenous fertilisers at the end of the 1980s. In medium-deep groundwater (15-30 m), the number of monitoring sites exceeding the EU value remains about 6%. In the peat and clay-soil areas, the nitrate levels in groundwater at a depth of 5-30 metres are less than 50 mg/l.
Causes of the fall in the nitrate concentration
There are three reasons for the fall in the nitrate concentrations with depth. Firstly, groundwater needs time to flow downwards. Even in sandy soils where the downward flow is stronger, the water will take more than ten years to reach the indicated depth. If the figures are corrected accordingly, a clear link emerges between the use of fertilisers and nitrate in groundwater (Bronswijk and Prins, 2001). Secondly, there can be denitrification in the soil, causing nitrate to disappear. Finally some of the shallow groundwater (and nitrate) flows away horizontally in many areas to ditches, brooks, and so on.
Policy
The Netherlands aims to reduce nitrate levels in groundwater to less than 50 mg/l. The EU Nitrate Directive came into force at the end of 1991. The directive includes a limit of 170 kg/ha on the amount of nitrogen applied in manure. The Netherlands was implementing the Nitrate Directive in the mineral accounting system (MINAS) and a system of manure transfer contracts (MAOs). However, in October 2003, the European Court of Justice rejected the current Dutch fertiliser policy. MINAS will therefore be replaced from 2006 onwards by a system of use standards for both chemical fertilisers and manure.
Impact of nitrate in the uppermost groundwater
The run-off of nitrogen into groundwater threatens the quality of drinking water. A few water extraction wells in the east of the Netherlands have been shut down, deeper groundwater has to be extracted on occasion and extra costs are incurred for treatment purposes.
