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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 groundwater body of the Brusselian Sands is shallower and more directly linked to human activity, and suffers significant nitrate contamination, as well as contamination from pesticides and tetrachloroethylene (a pollutant which has been considerably highlighted since the last evaluation). Moreover, an upward trend has been observed for nitrates, certain pesticides and tetrachloroethylene.

Objective: achieving "good chemical status"

Environmental objectives concerning the groundwater present in the Brussels Region have been set out pursuant to the Water Framework Directive (WFD) and its ordinance, and the "daughter directive" concerning the protection of groundwater (2006/118/EC) and its decree for transposition. They relate to the "good quantitative and chemical status" of the 5 groundwater bodies in 2015 and up to 2021. Since the current chemical status of the Brusselian Sands groundwater body was evaluated as being "poor" (according to the nomenclature of the directive - see methodological sheet), an extension to the deadline (derogation) for good chemical status to 2027 was appealed to the European Commission.

Achieving a "good chemical status" implies meeting quality objectives (maximum concentrations of certain pollutants which must not be exceeded: quality standards for nitrates and pesticides and threshold values established for pollutants deemed to be a risk for groundwater – see the methodological sheet). The threshold values are established per groundwater body, and according to the water use (which in the Brussels Region is primarily drinking water supply, use by industrial activities or by the tertiary sector). The quality objectives are the same for all of the groundwater bodies, with the exception of threshold values for three metals (cadmium, lead and mercury), which are stricter for the Brusselian Sands groundwater body. The notion of "good chemical status" also refers to the absence of negative impacts on surface water and the land ecosystems which directly depend on this resource. In this context and in order to limit the risk of eutrophication of the ecosystems linked to the Brusselian Sands groundwater body, new threshold values will be established by July 2016 for two other pollutants: nitrites and total phosphorus/orthophosphates.

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 comprises 2 measurement campaigns per year:

  • Surveillance monitoring intended to reflect the overall status of each water body and any long-term trends and to detect any new pollutants. At the end of 2012, this monitoring took place at 23 monitoring sites spread across the 5 groundwater bodies. The monitoring concerns parameters that are relevant for the contamination of groundwater. The frequency of monitoring was biannual. In 2013, this frequency was reduced to an annual monitoring for the groundwater body of the Socle and Cretaceous period, and the groundwater body of the Landenian, given the knowledge acquired and the hydrogeological context of these water bodies;
  • Operational monitoring intended to follow the water bodies at risk of not achieving the "good chemical status" or those that show an upward trend of any pollutant. The operational monitoring shall also been undertaken to assess the impacts of the implementation of the prevention and protection programmes on the water bodies identified as being at risk. At the end of 2012, there were 10 monitoring sites spread across the Brusselian groundwater body, which were sampled twice a year and related to parameters identified as being at risk (in particular, nitrates, certain pesticides, tetrachloroethylene and a minimal list of polluting parameters deemed to be at risk).

For the shallow aquifers - in the alluvial grounds of the Senne 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 and identification of trends

Based on analysis of the results of the monitoring programmes from 2010 to 2012, the four groundwater bodies of the Socle and Cretaceous period, the Socle in the supply zone, the Landenian and the Ypresian (hill region) have been assessed as achieving the "good chemical status". Chlorides, iron and manganese observed in high concentrations at certain monitoring sites in the deep groundwater bodies would have resulted from the natural geochemical background of these aquifers. Given the trends calculated on the basis of the monitoring results from 2006 to 2012, these 4 groundwater bodies will probably meet the objective of good status in 2015 and 2021.

In contrast, the groundwater table of the Brusselian Sands - which is situated at a shallower depth in the subsoil - is more exposed to surface pollution. It was not evaluated as having "good chemical status" in 2012 and will not achieve this objective before 2015 or 2021. Exceedances of the quality standards have been observed at numerous monitoring sites, concerning nitrates, certain pesticides and tetrachloroethylene. The identification of trends based on the evolution of annual concentrations measured between 2006 and 2012 accounts for the considerable upward trend for nitrates, certain pesticides and tetrachloroethylene. In contrast, total pesticides show a slight downward trend.

Quality for the Brusselian Sands water body

Evolution of the chemical status of the water bodies of the Ypresian (Hill Region) and of the Brusselian Sands, based on the results of the monitoring programmes from 2010 to 2012
Source: Brussels Environment, dpt. Water, 2014

For nitrates, the exceedances can be observed essentially at the monitoring sites situated in highly urbanised areas. Conversely, low concentrations of nitrates were recorded in the south east area of the groundwater body at the Sonian forest, an area with very little anthropogenic pressure. A university study was carried out to identify the origin of the major sources of nitrate pollution (fertilisers, waste water infiltration, etc.), based on isotopic analyses on nitrogen and oxygen carried out between 2009 and 2011. The results of this study show that the pollution would have come from sewage effluents and from the decay of human or animal organic matter, at the monitoring sites which recorded the highest concentrations (>50 mg/l). The origin of this waste water still needs to be investigated: among the hypotheses put forward, notable explanations include leaks from the sewage system which is considered to be antiquated in certain areas (collectors were designed to drain away shallow groundwater in the past), the presence of leaching cesspools, etc. At the sites recording the lowest concentrations, the nitrates would have come from organic fertilisation and/or the decay of organic matter by micro-organisms in the soil.

New measures, planned in the context of the second water management plan, would improve the knowledge about the origin and temporal variability of the observed nitrate concentrations. These include extending the monitoring network to new sites, the continuation of isotopic analyses, the analysis of new indicator parameters for pollution, and field surveys on waste water management, or agricultural or related practices.

The pesticides that are present in the Brusselian groundwater body in significant amounts are herbicides: atrazine and its degradation products, as well as 2.6 dichlorobenzamide (BAM). The exceedances of the standards were primarily observed in the western half of the water body, particularly at drinking water catchments at the Bois de la Cambre and the Sonian forest, as well as in a low-urbanised area of Uccle. The contamination by atrazine and its derivatives suggest historical pollution (atrazine has been banned from use since 2004), or the prohibited use of old stocks of products. Other herbicides were also observed locally and occasionally. The pesticides identified are predominantly intended for household, private or public use (for maintaining gardens, streets, green spaces, cemeteries, etc.).

The regulatory provisions pertaining to the sale and withdrawal of accreditation of certain pesticides, which prevent their use by private individuals and public authorities, appear to have had a positive impact on the improvement in the groundwater quality. However the Brusselian groundwater body will not achieve "good status" by 2021 given the high stability of some pesticides present in the environment, the very slow and complex migration process of pesticides through the soil and sub-soil (adsorption/desorption process on soil particulates) and the slow renewal of groundwater.

The ordinance of 20 June 2013 on pesticide management which is compatible with sustainable development as well as the accompanying regional programme for pesticide reduction 2013-2017, reinforce the requirements and conditions concerning the use of pesticides. Since 20 June 2013 (unless there is a derogation, valid until 1 January 2019 at the latest), public officials can no longer use pesticides in public spaces (parks, squares, streets, sports fields, etc.). Since 1 March 2014, it is prohibited for anyone to use them in sensitive areas which are susceptible to increased risk (see the list in the ordinance) and from 1 January 2016, the prohibition will be extended to type III protection zones for water catchment intended for human consumption. These measures should contribute to reducing concentrations even further in groundwater.

Additionally, surveys into the use of pesticides have been carried out within the scope of the regional programme. They help to better understand the behaviour of citizens, public authorities and businesses with regard to the use of these substances. A survey carried out among 800 households at the start of 2015 revealed that private individuals handle pesticides to such an extent that the upkeep allocated to the surface area concerned is considerable (Sonecom, 2015). Four types of exterior space for increasing upkeep were compared: pathways and pavements, lawns, flower beds and/or boxes, vegetable plots and/or orchards. The use of pesticides ranged from 8% of the households surveyed who had a pathway or pavement, to 26% for those who had an orchard and/or vegetable plot.

Tetrachloroethylene is present in significant amounts at certain sites of the Brusselian groundwater body, in its western part which is highly urbanised. This volatile organic halogen compound is a solvent which is used in industry (e.g. in dry-cleaning, painting, scouring of metallic surfaces, etc.). The exceedances observed result from past (polluted sites) and current industrial activity.

The second water management plan intends to identify these point sources of pollution by studying the correlation between the environmental permits of the end-user sectors for this substance, the inventory of the contaminated soils and the high concentrations measured in the groundwater.

Other pollutants (ammonium, sulphates, chlorides, chlorates, etc.), that result from specific surface activities, have also been measured locally and/or occasionally at certain monitoring sites.

Pursuant to the WFD, a programme of actions aiming to achieve good chemical status for the Brusselian groundwater body was drawn up at the end of 2009 and the end of 2012. Its implementation is supposed to continue until 2021, as anticipated by the second water management plan. Its success has turned out to be particularly delicate due to the multitude of potential pollution sources, both point sources and diffuse sources, as well as the complexity of the transfer dynamic of pollutants in the soil and sub-soil, the inertia of water bodies, and the cross-border aspect of groundwater.

Date de mise à jour: 30/05/2020



7. Eaux souterraines (2005) (.pdf, in French and Dutch only)

State of the Environment’s sheet(s)

Focus : Quantitative status of groundwater (edition 2007-2010) (in French and Dutch only)

Study(ies) and report(s)

EARTH AND LIFE INSTITUTE – UNIVERSITE CATHOLIQUE DE LOUVAIN (UCL), De Coster A., Vanclooster M., March 2013. « Etude relative à la pollution de la masse d’eau du Bruxellien par les nitrates dans la Région de Bruxelles-Capitale : Etat des lieux et essai d’identification des sources de pollution ». Study performed on behalf of Brussels Environment. 87 pp. Restricted (.pdf, in French only)

SONECOM, May 2015. « Sondage sur le comportement des ménages en matière d’achat et d’utilisation de pesticides dans la Région de Bruxelles-Capitale et dans les zones de captage ». Study performed on behalf of Brussels Environment. 78 pp. Restricted (.ppt, in French only)