Freshwater - State and impacts (Belgium)

In rural areas, the degradation of groundwater quality is largely the result of excessively high concentrations of nitrate which usually appear when the amount of nitrogen fertilisers applied go beyond the needs of the crops. In urban areas, contamination of groundwater by nitrates derived mainly from the lack of sewers or leaks in the sewage network of residual urban waste water.

Generally, the highest levels of contamination (more than 50 mg NO₃^-/l) are observed in the northern part of Belgium (Flemish Region), especially in the less deep phreatic groundwater bodies (e.g. Quaternary sands, Tertiary Brusselian and Yperian sands, Quaternary Meuse-Rhine deposits and Terrace sediments…), in areas where agricultural activity and/or the density of population is particularly high.

Due to a change of monitoring system and monitoring practices between 2003 and 2004, no long-term trends can be determined in the three Belgian regions. For the Flemish Region, a complete new groundwater monitoring network consisting of 2.100 multi-level wells (located in agricultural areas) has been operational since 2004. For the Walloon Region, the application of the WFD required a restructuring and a redeployment of the existing groundwater monitoring network in 2006, in order to improve its representativeness by taking into account the less characterised water bodies and the non-exploited resources. The Walloon WFD network includes +/- 600 sites of control.

The nitrate concentrations in groundwater throughout Belgium have increased in recent years: the proportion of monitoring points with average nitrate concentrations higher than 25 mg/l went from 46,4 % (in 2004) to 49,8 % (over the period 2007-2008). This trend is not necessarily linked to the current development of agricultural practices, which goes towards a reduction in the application of (organic and mineral) nitrogen fertilisers. Next to the availability of external N-sources the degree of contamination of the groundwater bodies depends also on other factors such as rainfall, transfer time to aquifers, local manure, and the vulnerability of the shallow aquifers or the quantity of nitrogen still present in the soils. Groundwater bodies are ‘slow-response’ systems in general so that, due to the travel times of nitrate bearing infiltration water, effects of measures in shallow groundwater are visible earliest after several months, usually after years and locally after decennia.