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Chemical quality of surface water

The water quality of the Senne, the Canal, and to a lesser degree the Woluwe, is subject to strong pressures due to human activity and its associated waste, which is typical of the urban environment and the industrial past of the Region. The presence of micro-pollutants which are harmful to the environment, and in particular ubiquitous pollutants (polycyclic aromatic hydrocarbons) and metal pollutants, compromise the possibility of achieving a good chemical quality in the Senne, the Canal and even the Woluwe.

33 priority substances (of which 15 are dangerous) and 5 other groups of pollutants under scrutiny

Micro-pollutants are chemical substances which are potentially toxic for ecosystems and even human health, even in very low concentrations. These pollutants are highly varied in nature and origin: pesticides, hydrocarbons, heavy metals, polychlorinated biphenyls (PCB), medicines and hormones, etc.

Of these substances, some are deemed to be of particular concern by the European Commission for the aquatic environment and are qualified as "priority" in the context of the Water Framework Directive due to their high concentrations in surface water or their particularly dangerous and persistent character (toxicity, persistence capacity and bioaccumulation in the environment). The list of 33 priority substances (or groups of substances) and 5 groups of additional pollutants (cfr. annexes 1 and 2 of the Government Decree of the Brussels-Capital Region of 2011) was extended to 12 new priority substances in 2013. The evaluation of the "chemical status" of surface water bodies is based on this list of pollutants, in accordance with the provisions of the Water Framework Directive (WFD).

133 other substances identified as "dangerous" must also be subject to monitoring within the Brussels-Capital Region and meet environmental quality objectives (cfr. annex 4 of the Governmental Decree of the Brussels-Capital Region of 2011).

Monitoring of micro-pollutants in the water but also in sediment and biota

Member States are obliged to carry out the monitoring of these pollutants - which are generally barely eliminated at the water treatment plants - and to take measures to gradually limit, or even prohibit, their discharge, emission and leakage. To this end, the Brussels Region has implemented monitoring programmes since 2001 for the chemical quality of its surface water, which are based on the analysis of concentrations of almost 200 parameters (see chapter 5 of the second water management plan). 5 sites have thus been monitored since the start of measurements: these relate to the entry and exit points to the Region for the three designated surface water bodies (Senne, Canal and Woluwe). The monitoring was extended in 2014 to new measuring sites: over the routes of these three water bodies but also over some of their tributaries. Since the data collected on these new locations is not yet representative of their quality, only data from the five "historical" sites will be analysed in this sheet.

Besides the water column, a tri-annual monitoring in the sediments (sludge) has been implemented since 2013 for almost 150 parameters. A previous measuring campaign in the sludge took place in 1993-1995: it only covered one third of the parameters analysed in 2013, but it allows nonetheless a comparison to be made between certain trends of the two campaigns. The objective of this monitoring is to ensure that there is no accumulation of certain lipophilic pollutants in the sludge (since the pollution retained in sediment is sometimes leached into the water column).

However, an annual monitoring of the presence of pollutants in the tissue of living aquatic organisms (biota) - predominantly fish and molluscs - has recently become obligatory (3 parameters since 2011 and 8 others from 2016 onwards). These pollutants have the particular feature of accumulating in biota due to their highly hydrophobic or lipophilic nature. Three parameters were analysed in juvenile carp in 2013: hexachlorobenzene, hexachlorobutadiene and mercury. It should be pointed out however that the methods of analysis are not always normalised or perfected: extreme prudence is therefore required in interpreting the results.

Environmental objectives in constant evolution

The quality objectives for pollutants in the water column and the biota which have been in force since 2011 are the Environmental Quality Standards (EQS). For some of these (metals, mineral oils), basic quality standards are applicable. These standards pertain to the annual averages of samples as well as, for certain dangerous priority substances, to maximum admissible concentrations for each sample (see methodological sheet).

In 2013, the EQS of 7 priority substances was revised (entering into force at the end of 2015). As previously indicated, 12 new priority substances (of which half were identified as "dangerous") were also designated and are subject to standards which will enter into force at the end of 2018. Additionally, the monitoring list for biota has been extended to 5 other pollutants.

Although the majority of the changes made by the 2013 directive are not applicable to the period covered by this edition of the State of the Environment, they need to be taken into account nonetheless in the context of future compliance with standards, and the identification of trends. This is why they have been analysed.

Satisfactory chemical quality for a large number of pollutants

Most of the 33(+5) European priority substances do not pose any problem to the three surface water bodies in the Brussels Region: the Canal, the Senne and the Woluwe. They are often not even detected in the water column, or the sludge, or the biota (cfr. chapter 1 of the second water management plan). In general, the Senne is the most contaminated waterway. Conversely, the Woluwe appears to be relatively well maintained, although with the exception of ubiquitous pollutants such as Polycyclic Aromatic Hydrocarbons (PAH).

The same goes for the vast majority (more than 90%) of chemical substances deemed to be relevant at the Brussels-Capital Region level (cfr. annexes 3 and 4 of the Governmental Decree of the Brussels-Capital Region).

It should be noted that certain pollutants were not subject to an evaluation in some years (due to an absence of measurements), or to a comparison with the quality objectives, due to the inaccuracy of the methods of analysis used (detection threshold higher than the EQS).

The widespread problem of "PBT" substances, and PAH in particular

The European Union has produced a list of eight Persistent, Bioaccumulative and Toxic (PBT) substances, or substances behaving as such (cfr. article 8a of the directive 2013/39/EU). Among these are PAH, mercury, or other dioxins. Although they are not included on this list, polychlorinated biphenyls (PCB) also belong to the PBT family of pollutants. These substances are likely to be detected over the long term in the aquatic environment, despite the measures taken against them. Some of them are even able to be transported over long distances; in which case they are referred to as being ubiquitous or omnipresent since all environmental compartments are affected (water, air, soil, etc.).

Unsurprisingly, the Brussels Region is not spared from this contamination, which affects the vast majority of Member States of the European Union. The main problem in terms of the water column concerns PAH.

With the exception of naphthalene, all the PAH listed as priority or similar (i.e. anthracene, fluoranthene, benzo(a)pyrene, benzo(b)- and benzo(k)fluoranthene, benzo(ghi)perylene and indeno(123cd)pyrene) have exceeded the standards since 2001. All 3 waterways are affected (the Senne is the most exposed, the Woluwe the least). In recent years, the main incriminating PAH were fluoranthene and benzo(a)pyrene in the Senne, both in terms of annual averages or maximum admissible concentrations. If we take into account the future standards of the 2013 directive, the situation becomes more acute: both the list of pollutants and exposed waterways, and the list showing the frequency of exceedances, are extended.

However, 2 of the 8 PAH identified as "dangerous" are a cause for concern: acenaphthene and pyrene. Exceedances in acenaphthene were recorded in the Senne on certain years, but pyrene in particular has been recorded, sometimes in the Canal but very often in the Senne.

In general, compliance with standards pertaining to PAHs is part of a long and complex process to the extent that these pollutants essentially result from diffuse sources. According to the emissions inventory of 16 PAHs for 2010, 61% of the inflows result from diffuse sources: 48% from transport (tyre and road surface wear, incomplete combustion) and 13% from atmospheric deposition. Since PAHs are lipophilic, around 98% is retained in the sludge at water treatment plants. Their presence in the water results from discharges from stormwater overflows (35%), from wet weather treatment process at the water treatment plants (29%), and to a lesser degree, waste water from  areas which are connected to the sewge system but not to water treatment plants (15%). At the waterway level, the PAHs merge into sediment and do not degrade easily. This is demonstrated by their presence in sludge (whether during the campaign in the middle of the 1990s or the one of 2013): but their concentration remains stable.

Sources (to the left) and pathways (to the right) of net emissions of 16 PAHs in the surface water in 2010
Source: Brussels Environment, extract from the draft second water management plan (2016-2021), data produced by the inventory of emissions to surface water (2010)

Annual net emissions of 16 PAHs from the EPA list in 2010 to surface water: 133 kg.

As for polychlorinated biphenyls (PCB), excessive concentrations were frequently recorded in the water of the Senne and the Canal, both at the entry and the exit of the territory, as well as in the sludge, in 2013. This was in spite of the adoption of a regional plan for the elimination and decontamination of PCB-PCTs in 1999, and in 2005, the adoption of a programme of measures aiming to reduce this pollution. It is highly likely that these high concentrations result from historical pollution and a leaching of these pollutants, which are extremely persistent, from re-suspended contaminated sediment during heavy storms in particular.

Unfortunately, the inaccuracy of data does not allow all PBT substances in the water column to be evaluated: this is the case for example for brominated diphenyl ethers, compounds of tributyltin, for which detection limits are too high with regards to the standard. Although, given the specificities previously described, the probability that all of these pollutants are present in one or more environmental compartments at the regional level is high.

Other problematic substances: metals

The quantities of metals emitted into waterways and the Canal are far from negligible. Diffuse sources make an overwhelming contribution to zinc emissions (Zn) (65% by the corrosion of construction materials from buildings and structures and 21% by traffic) and to a lesser degree, to lead emissions (Pb). In contrast, businesses account for the bulk of inflows for other metals including nickel (Ni) and cadmium (Cd) (60% and 71% respectively).

Sources of net emissions of 4 metals in the surface water in 2010
Source: Brussels Environment, extract from the draft second water management plan (2016-2021), data produced by the inventory of emissions to surface water (2010)

Net emissions into the Senne are around 8 times greater than those into the Canal. Since metals are much less effectively treated or retained at water treatment plants, effluence from water treatment plants effectively represents the main pathway of metals into the Senne.

Although a downward evolution has been observed in the sludge in the time between the two campaigns (Cd, Pb, Hg and Ni), the concentrations measured in the water often exceed the basic quality standards, especially for dissolved compounds (similarly to dissolved zinc, dissolved arsenic and dissolved copper), and even the environmental quality standards. And the problem will only get worse since some EQS will become stricter from the end of 2015: compliance with the standards will be problematic for the Senne and the Canal, and even for the Woluwe. However, these results are subject to reservation as to the quality and the accuracy of the analyses, in view of the differences observed with the values measured in Flanders, and upstream and downstream of the Region.

Other pollutants deserving particular attention

  • With regards to the pesticides on the list of priority substances and on the list of other pollutants, the standards relating both to annual averages and maximum admissible concentrations have always been respected. Temporary measures beyond the threshold concentration established for the annual average were just observed in 2008 for diuron (Senne "in" and "out", Canal "in") and isoproturon (Senne "in" and "out"). The observed contamination by pesticides in the groundwater of the Region does not therefore hold true for surface water (see "Chemical status of groundwater").
  • The annual average concentrations of DEHP (a type of phtalate used as a plasticiser) only exceeded the standard in one year (in 2007: Senne "in" and "out", Canal "in") out of the 6 measurement years where the evaluation of this pollutant was possible and representative. It appears that improvement continued in the Senne and the Canal between 2012 and 2014.
  • Mineral oils, although not subject to compliance with EQS, require particular attention since they are emitted in large quantities in the regional territory (32.8 gross emission tonnes of which 8.7 tonnes into the Senne as net emission and 1.37 tonnes into the Canal), by road and rail traffic (oil used for switches). A significant source for the Senne is the Schaerbeek-Formation site. The oils flow into the Canal via stormwater overflows (60%) and direct discharges (34%)
  • Concentrations of brominated diphenyl ethers in the water column cannot currently be defined with sufficient accuracy, but their presence in sludge has been recorded. These substances probably arrived there by discharge from the sewage system (up until the 2000s, they were present in domestic waste water) and by leakage during storms.
  • Nonylphenols are substances which have only recently become a cause for concern, and showed an exceedance of the annual average authorised for the Canal ("out") in 2013 and 2014, and for the Senne ("out") in 2014, as well as an exceedance of the maximum admissible concentration for the Senne ("out") in the same year.
  • Other parameters also deserve particular attention, including chlorides (whose high concentrations recorded in the Senne, especially at the exit, even resulted in an exceedance of the standard in 2014), or non-ionic surface-active substances which cause exceedances in some years. Moreover, despite the inaccuracy of the measurements, concentrations of cyanides appear to be particularly high in the Senne and exceed the standards quite regularly at the exit to the Region.

How can this pollution be combated effectively?

A large number of preventive and remedial measures contained within the water management plan 2010-2015 and the future plan 2016-2021 have been, or will be taken to reduce the chemical pollution in the environment or in waterways: the management of environmental permits (discharge standards, using the best available techniques, etc.), regulation on volatile organic compounds, the take-back obligation for used solvents, the restriction followed by the prohibition of the use of pesticides in public spaces from 2019 onwards, the dredging of waterways and ponds, information and awareness-raising regarding the use of certain products, the limitation of water discharge and pollution-loaded run-off into water bodies, etc. The improvement in the quality of surface water in the Brussels Region also depends on the efforts made upstream of the Region.

Nonetheless, the fight against ubiquitous substances such as PAHs cannot be limited to water policy alone. Only comprehensive management at the European level (or even global level) combined with a cross-disciplinary approach with the various other environmental policies (water, air, soil, etc.) will be enough to contain this pollution.

Date de mise à jour: 30/05/2020


Study(ies) and report(s)

Technical reports presenting the results of annual measurement campaigns of monitoring of the physico-chemical quality of surface water, various years (.pdf, in French or Dutch only)

VITO, December 2013. «Inventarisatie van de emissies naar water in het Brussels Hoofdstedelijk Gewest» Post 1: voorstudie. Study performed on behalf of Brussels Environment. 78 pp. (.zip, in Dutch only)

VITO, June 2014. «Inventarisatie van de emissies naar water in het Brussels Hoofdstedelijk Gewest» Post 2: analyserapport en factsheets. Study performed on behalf of Brussels Environment. 346 pp. (.zip, in Dutch only)

VITO, January 2014. «Technische nota – Transport naar het oppervlaktewater binnen WEISS voor Brussels Hoofdstedelijk Gewest». Study performed on behalf of Brussels Environment. 32 pp. (.zip, in Dutch only)