Arbejdsrapport fra Miljøstyrelsen, 36/2005 – Scenarieberegning af nitratudvaskning

Scenarieberegning af nitratudvaskning

Summary and conclusions

This study evaluates scenarios in relation to implementing the coming EU Groundwater Directive by determining the extent of the farmed area that would exceed the nitrate concentration limits for water leaving the root zone of 75 and 150 mg nitrate/litre for sandy and clayey soils, respectively. With these limits, it is anticipated that a concentration of 50 mg nitrate/litre will not be exceeded in the groundwater due to the denitrification taking place below the root zone. The results are summarized at the municipality level as the average of the nitrate concentration leaving the root zone.

The analysis is based on the models and databases used by the Danish Institute of Agricultural Sciences for the final evaluation of the 2nd Action Plan for the Aquatic Environment (VMP II). The databases were updated with data on mineral fertiliser utilisation in year 2003. Additionally, an anticipated effect of the 3rd Action Plan for the Aquatic Environment (VMP III) on ground waters was included. It must be stressed that the method is based on a number of assumptions which cause some uncertainty in the results.

The model analysis showed that approx. 0.6% of Danish farmland could be expected to exceed the limits when VMP III is fully implemented. This figure is within the uncertainty levels of the calculation as only one of the two models used found a transgression of the limits.

Sensitivity analyses were performed on the value of the nitrate limit (on sandy soils only) and on the anticipated effect of VMP III. When the limit on sandy soils was changed from 62.5 to 87.5 mg nitrate/litre, the area exceeding the limit changed from 0.7 and 0.2 %. When the effect of VMP III was varied between 5 and 10 kg N/ha, the area exceeding the limit varied between 0.6 and 0.2%.

How much the nitrogen loss from Danish agriculture would have to be reduced by in order to avoid the above-mentioned transgressions was quantified. When the nitrate limit on sandy soils and the effect of VMP III were varied as mentioned above, the reduction in nitrogen losses required on a national level was between 11 and 211 tonnes of nitrogen.

The chosen conditions of the calculation are discussed. It is important to notice that the level on which calculations are summarised (here municipality level) has a considerable influence on the extent of transgression. This is due to the variation between farm types, soil types, climate, etc. If alternatively the nitrate leaching is down-scaled to farm level or smaller-scale groundwater bodies, there will be larger variations in the results and thus more transgressions. Similarly, calculations at county or regional level will lower the variation and reduce transgressions.