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Chemical status of groundwater

According to the monitoring results of the groundwater quality, 4 out of 5 groundwater bodies in the Brussels Region have a "good chemical status". In contrast, the water body of the Brusselian sands, which is of limited depth and in direct contact with human activities, shows concentrations that exceed the standards for a number of parameters: nitrates, pesticides and several other pollutants.

Objective: realising "good chemical status"

Environmental objectives have been set for the groundwater in the Brussels Region that correspond to the Water Framework Directive and the Brussels Ruling for Water (WFD and Kaderordonnantie Water or KOW). These aim to achieve the "good quantitative and chemical status" for the 5 groundwater bodies by 2015. Given the fact that the current chemical status of the Brusselian Sands was reported as inadequate, the European Commission has been asked for a postponement until 2027 with respect to good chemical status. 
Achieving "good chemical status" implies conformity with specific quality objectives (such as not to be exceeded, maximum concentrations of a number of pollutants) and the absence of negative effects on the depending aquatic and terrestrial ecosystems.

Monitoring the quality of groundwater bodies

The monitoring of the chemical status of these 5 groundwater bodies that started in 2004, involves taking samples which is mainly carried out at active catchments and at a few sources. It actually regards 2 individual monitoring programmes which each comprise 2 measurement campaigns per year:

  • monitoring checks intended to reflect the overall status of each water body and to detect any long-term trends and any new pollutants. At the end of 2012, these checks took place at 23 monitoring sites spread across the 5 bodies of groundwater. The checks concern parameters that are relevant for the contamination of groundwater;
  • •operational checks intended to follow the water bodies at risk of not attaining a "good chemical status" or those that seem to be showing an increased susceptibility to a specific pollutant. The operational checks also allow to evaluate the effects of the prevention and protection programmes on the at-risk water bodies. At the end of 2012, these checks concerned 10 monitoring sites spread all over the Brusselian waterbody and were related to the risk parameters (more specifically, nitrates, pesticides and other pertinent derivatives).

For the superficial aquifers - in the alluvial grounds of the Zenne valley and adjacent valleys, as well as in the sediments of the Quaternary - there is currently no systematic, qualitative monitoring.

Chemical status of groundwater bodies

The analysis of the 2012 results from the monitoring programmes indicate that the water bodies of the Socle and the Cretaceous period, of the Socle in the supply area, of the Landenian and the Ypresian (Hill region) have a good chemical status. Chlorides, iron and manganese as observed in high concentrations at specific monitoring sites in deep water bodies should result from the natural geochemical backgrounds of these aquifers. If the tendencies that were observed for the period 2004 to 2012 persist, expectations are that these 4 water bodies will realise the good condition by 2015.
In contrast, the phreatic water table of the Brusselian Sands - present at a limited depth in the underground and without an impermeable geological formation on top of it - is more severely exposed to surface contamination. The chemical status of the layer was found to be inadequate in 2012 and this will also be the case in 2015. The quality standards for both nitrates and specific pesticides are being exceeded. The measured concentrations show a generally upward tendency for nitrates but a downward trend for pesticides.

Current quality and trends for the Brusselian Sands water body

The excess measurements for nitrates are primarily observed at the checkpoints located in very urbanised areas. The low nitrate concentrations are, by contrast, measured in the south-eastern section of the water body, in the area that corresponds to the Sonian Forest and is not really exposed to human activity. A university study was conducted in order to determine whether the origin of the nitrate pollution was organic or mineralogic (infiltration of waste water, churchyards, manure...). The investigation, which was carried out between 2009 and the end of 2011, was based on the isotopic analysis of the nitrogen and oxygen. The results of this study indicated that the high concentrations of pollution at the monitoring sites (>50mg/l) originated from discharged waste water. The origin of the waste water must be further examined: one of the hypotheses involved leaks in sewerage systems (at some places, these are ruined and sewerage systems were sometimes designed to drain away rising groundwater), cesspits, etc. At other sites, the nitrates could come from organic fertilisation and/or the breakdown of organic material by micro-organisms in the soil. 
The 2nd water management plan considers to implement new measures that would improve the knowledge about the origin of the nitrate concentrations and their variation over time. This involves the expansion of the monitoring to include new sites, the continuation of isotopic analyses as well as research into the management of waste water or agricultural and related practices.
The pesticides that are present in the Brusselian water body in significant quantities are atrazine and its decomposition products as well as 2.6 dichlorobenzamide (BAM). The exceedances of the standards related to these substances were primarily observed in the western half of the water body, more specifically in the vicinity of catchments in Ter Kamerenbos and Sonian Forest, as well as in the non-urbanised Ukkel zone. Other herbicides are also found occasionally and locally. The detected pesticides are primarily intended for household use in both private households and public spaces (garden maintenance, avenues, green spaces, burial grounds...).
It seems that regulations that relate to the trading in and the recognition of certain pesticides, meaning that they could no longer be used by private individuals and public bodies, have a positive impact on improving the quality of water layers. The status of the Brusselian water body will nevertheless remain inadequate until 2021 as a result of the high stability of certain pesticides encountered in the environment, the very slow and complex migration process of pesticides in the soil and subsoil (adsorption/desorption processes on soil particulates) and the fact that groundwater renews itself very slowly.
The ruling for water of 20 June 2013 that bans the use of pesticides in sensitive areas and in public spaces (from 2019) as well as the corresponding regional programme for the reduction of pesticides 2013-2017, reinforce the requirements and conditions for the use of pesticides and should also reduce the concentrations in the groundwater.
Other contaminating substances (tetrachloroethylene, ammonium, sulphates, chlorides, chlorates, …) that originate from specific surface-based activities, were also locally and/or occasionally detected at some monitoring sites. The presence of tetrachloroethylene was deemed to be significant in the Brusselian water body.
In the context of the application of the WFD, a programme of measures was set up at the end of 2009 in order to realise the good chemical status for the Brusselian water body, the status of which is inadequate. This programme must be maintained until 2021, as set forth in the 2nd water management plan. The chances of success are however not guaranteed due to the various potential local and diffuse contamination sources, the complicated transfer dynamic of pollutants in soil and subsoil, the inertia of the water body or even the cross-border nature of the water-transporting layers.

Assessment of the chemical status of the Ypresian (Hill Region) and Brusselian water bodies according to the results from the 2004 to 2012 monitoring programmes
Source: Bruxelles Environnement-Leefmilieu Brussel, Water Strategy department

Assessment of the chemical status of the Ypresian (Hill Region) and Brusselian water bodies according to the results from the 2004 to 2012 monitoring programmes

 

 

Date de mise à jour: 23/01/2018