Agricultural emission reduction policy and its effect on groundwater quality in nature areas in The Netherlands over the past 25 years
National Institute for Public Health and the Environment RIVM
Esther Wattel, Leo Boumans, Eric van der SwaluwNational Institute for Public Health and the Environment RIVMPO Box 13720 BA BilthovenThe NetherlandsAir pollution is a transboundary phenomenon. In the Convention on Long-Range Transboundary Air Pollution (CLRTAP) as well as in the European Union’s Directive on National Emission Ceilings (NEC), international agreements have been reached on the reduction of emissions. Excessive deposition of nitrogen and sulphur induces acidification and eutrophication of the soil and its groundwater. To reduce air pollution and its negative effect on ecosystems, several policy measures were taken in The Netherlands, such as the obligation to reduce N emissions when spreading manure and the installation of air scrubbers on animal houses.The Dutch National Acidification Trend Monitoring Network (TMV) was established in 1989.The purpose of the network is to study the impacts of national emission reduction policies on the quality of groundwater in the Netherlands. It specifically monitors the quality of the upper meter of the groundwater under nature areas (forest and heath) with sandy soils.The groundwater under these areas is not affected by any notable pollutants other than atmospheric deposition. In addition, sandy soils have a limited capacity to neutralize the impacts of acidification. For these reasons, the impacts of atmospheric deposition on groundwater quality are most clearly detected under nature areas with sandy soils. In other monitoring networks, the effects of atmospheric deposition are difficult or impossible to discern from other sources of pollution. In agricultural areas, for example, the impacts of fertilizer application on groundwater quality obscure those of other sources of pollution.In addition to TMV, we used data on rainwater quality from the Dutch national monitoring network on Air Quality (LML), to quantify the amount of wet deposition. We also used the National Groundwater Quality Monitoring Program for data on the quality of groundwater at 10m depth (LMG).Our analysis of the measurements shows that rainwater quality and groundwater quality in nature areas have improved significantly over the past 25 years. The impacts of decreased emission of nitrogen are found in rainwater, shallow groundwater and groundwater at 10 m below the surface. In 2014 the median N concentration in rainwater had decreased by 44% compared to 1988. The median concentration of N in the upper groundwater decreased by 61% between 1988 and 2014.A location-wise comparison of the observation data of TMV from 2014 and 1989, using a paired samples t-test, revealed that the pH was significantly higher in 2014 than in 1989, while the nitrate, sulphate and aluminium concentrations were significantly lower (a < 0.05). The analysis of the combined observations shows that the (inter)national measures taken to reduce emissions have resulted in less acidification and eutrophication in nature areas with sandy soil.