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General physico-chemical quality of surface water

A good physico-chemical quality of water is an essential prerequisite for the survival and development of aquatic life. A positive and generalised evolution of the three surface water bodies has been observed for many years for their dissolved oxygen content. The Woluwe body has good physico-chemical quality, which is stable over time. The Canal body has a good physico-chemical quality in general, but conductivity still remains too high. Despite a clear improvement, the Senne body shows deteriorated quality, especially at the exit to the Region. Exceedances of standards persist for certain parameters (among others, conductivity, BOD, suspended matter).

Target objective: the basic quality standards

There are specific quality objectives for the parameters which determine the general physico-chemical quality of water: the basic quality standards (in force since 2011 - see methodological sheet). Since the Woluwe body is situated in a Natura 2000 zone, stricter standards will soon be applicable for 4 physico-chemical parameters (see the methodological sheet and chapter 4 of the second water management plan). In the context of the drafting of the second water management plan, a selection of 9 parameters (out of the 17 listed in the decree) have been carried out (see chapter 4 of the second water management plan). The selected parameters are:

  • temperature,
  • acidity (pH level),
  • conductivity,
  • the dissolved oxygen content: essential for aquatic life and for the degradation of biodegradable pollutants, facilitating self-purification
  • the organic load (the Biochemical Oxygen Demand (BOD) - pollution benchmark for biodegradable organic matter for which the degradation consumes dissolved oxygen, the Chemical Oxygen Demand (COD))
  • the turbidity: suspended matter
  • and the nutrients (total nitrogen and total phosphorus).

These physico-chemical parameters make a contribution to the ecological quality of waterways. Given that the physico-chemical quality serves to support aquatic life, it is reflected indirectly in the status or the ecological potential of surface water (see "Biological quality of the main waterways and ponds").

Taking into account these environmental objectives resulting from the Water Framework Directive, this sheet focuses specifically on the three surface water bodies in the Brussels Region (Woluwe, Canal and Senne), upstream and downstream of the territory. It should be noted however that the monitoring network was recently extended (in early 2014) to intermediate measuring points along the route, as well as at another Brussels waterway (the Neerpedebeek). Given the too-short series of measurements available, these results are not yet representative.

The Woluwe: very good and stable physico-chemical quality

The water of the Woluwe is of a very good quality: the organic load is very low (BOD of 2 mg/l over the period 2001-2014), the water is clear (suspended matter around 20 mg/l) and has a low nutrient concentration (in the order of 3 mg/l for total nitrogen and 0.2 mg/l for total phosphorus). The quality standards for the 9 targeted physico-chemical parameters are also respected here. The stricter standards which will soon be applicable have already been applicable for the temperature, dissolved oxygen and COD parameters since 2011; in contrast, the stricter standard for suspended matter has been exceeded on two occasions since 2011 (in 2011 and 2014). The dissolved oxygen content even shows an upward trend since 2009 with an increase of more than 2 mg/l compared with the start of measurements in 2001: its concentration reached almost 10 mg/l in 2014. The trend for the other parameters is very stable over time, with little in the way of fluctuation. This can be explained by the fact that the Woluwe is primarily supplied by water sources originating in the Sonian forest, and hardly receives any polluting discharges during its course through Brussels.

The Canal: a water body of good quality overall, but with too high levels of conductivity and suspended matter.

In general, the quality of the Canal remains consistent from the start to the end of its course through the regional territory. However, three parameters show interesting differences. So, during its course through the Brussels Region, the water temperature of the Canal increases by 2°C on average (since the start of measurements), which is reflected by a decrease in the dissolved oxygen concentration (of around 2 mg/l). The water of the Canal is generally more turbid at the entry to the Region than at the exit (an average difference of 10 to 15 mg/l is observed for suspended matter), even if the difference has tended to narrow over the last 4 years.

On the whole, the water of the Canal is of good quality. It shows low organic pollution (BOD of around 2 mg/l and COD of around 25 mg/l). It is also characterised by a relatively low nutrients load (almost 6 mg/l on average for total nitrogen and 0.4 mg/l for total phosphorus). The average annual concentrations of total nitrogen even show a downward trend since the start of measurements. As for concentrations of total phosphorus, they appear to have stabilised. The dissolved oxygen content increased very significantly both at the entry and the exit of the Region (from 2 to 8 mg/l between 2001 and 2014 for this last measuring point).

As regards the Canal, few exceedances of the basic quality standards have been observed. The dissolved oxygen for which the content was still insufficient in the 2000s at the exit to the territory, has met the standard since 2009, showing a favourable trend. Suspended matter for which the concentrations systematically exceeded the standard at the entry to the territory between 2007 and 2011 appear to show a real improvement over the last three years, and the standard has been respected since 2012 (values approaching 30 mg/l). Nonetheless, the large inter-annual variability of annual averages should prompt prudence with regard to this parameter. Finally, conductivity flirts constantly with the standard, leading to exceedances roughly every other year. The high stability of concentrations measured leads one to suspect moreover that this situation could persist.

The origin of the high turbidity values has not been clearly identified at the present time. The Canal is subject to various potential sources of pollution across the regional territory including, in particular, the direct inflow of poor quality water from Neerpedebeek, Broekbeek and, via pumping, from the Senne, overflows from the collectors or from the Senne during heavy rainfall, some occasional direct discharge from polluted water, pollution due to waterway traffic, or even the re-suspension of pollutants present in sediment (dredging, eddies).

The flow of the Senne is strongly influenced by discharge from water treatment plants

Depending on the conditions, half or even two thirds of the average daily flow of the Senne at the exit of Brussels is made up of flows of discharge water from the North water treatment plant. Additional measurements taken along the route of the Senne in 2014 showed a diluting effect for certain pollutants, after the discharge from the water treatment plant. Moreover, this "warmer" discharge water could be the reason for the increase in temperature of the Senne during its course through Brussels (around 1°C since 2007).

The Senne: a striking evolution

If the water of the Woluwe appears to be of very good quality, and that of the Canal relatively unpolluted, the same cannot be said of the Senne. However the analyses highlight a very important overall improvement in the general physico-chemical quality of the Senne's water, at the exit to the regional territory. This trend is illustrated by the following chart. In order to smooth out inter-annual variations, the moving averages over 3 consecutive years have been used. And to enable a comparison of the parameters between themselves, a dimensionless representation has been used (2001 is the reference year).

Relative evolution of the moving average concentrations over 3 years of certain physico-chemical parameters of the Senne at the exit to Brussels (2001-2014) compared with 2001
Source: Brussels Environment, dpt. Water, 2015

The spectacular decrease of the organic load (BOD, COD), of suspended matter, and also nutrients (total nitrogen and phosphorus) between 2001 and 2014 logically went hand in hand with an increase in dissolved oxygen concentrations. The positive trend in terms of parameters other than dissolved oxygen was especially pronounced up until 2009; since then, it appears to have stabilised.

This positive evolution results from the commissioning of the South and North regional water treatment plants (only the latter is equipped with an effective treatment for eliminating nitrogen and phosphorus), progressive connections of the existing sewers to these stations (which explains the rather gradual trend instead of a tiered trend) as well as the improvement in the quality of the Senne when it enters the Region (which has also tended to improve since 2003-2005, in connection with the enhanced treatment of water upstream of the Region).

Other explanatory factors could be put forward, such as the gradual reduction in the use of phosphates in detergent products, the reduction in atmospheric depositions of nitrogen, or even the reduction in the input of nitrogen by agriculture and farming.

The recent improvement in the water quality of the Senne is already having beneficial effects on the aquatic life present in this waterway, both upstream and downstream of the Region. In the Brussels Region, a slight positive trend seems to be taking shape, although it still needs to be confirmed in the future (see "Biological quality of the main waterways and ponds").

But water quality is still degraded

Evolution of the physico-chemical quality of the Senne (2001-2014)
Source: Brussels Environment, dpt. Reporting and environmental impacts, 2015

One of the consequences of this positive evolution in the Senne is that, gradually, the water quality at the exit to the Region approximates the level it is at the entry to the Region. Another consequence is increased compliance with standards, both at the entry and exit to the Region, in particular for:

  • the average content of dissolved oxygen: since 2007 at the entry to the Region and since 2011 at the exit to the Region; the concentrations were almost 2.5 to 3 times higher in 2014 compared with 2006; nonetheless it is useful to qualify these positive results by occurrences of decreases in dissolved oxygen (during heatwaves or overflows during rainfall) to below the threshold of 3 mg/l, which is deemed to be critical for fish life, even if these occurrences only last a few hours or days.
  • The COD: at the entry since 2005 (except in 2010) and at the exit since 2012;
  • Nutrients: the standards for total nitrogen and total phosphorus have been respected respectively since 2008 and 2010 at the two measuring points. The measurements for total phosphorus sometimes experience occasional peaks in concentrations, which prompt strict vigilance with regards to this parameter.

In contrast, for BOD, although the standard has been respected at the entry since 2005, this has only been the case at the exit for 3 years out of 5 between 2010 and 2014. Moreover, probably in connection with the persistently high organic load, the very high conductivity values (almost 25% above the standard) result in a systematic exceedance of the standard. Although conductivity is lower at the entry, it still frequently exceeds the standard.

Another parameter which is the cause of frequent exceedances in the Senne: suspended matter. Even if the improvement compared to the start of the 2000s is undeniable in the north of the Region (concentrations at the time were more than double the standard), the measurements show important fluctuations which are reflected in the high variability of annual averages and between the two measuring points. Since 2009, the standard has after all been respected on 2 occasions at the entry to the Region (in 2012 and 2014) and once at the exit (in 2013).

Vigilance points for the future

The number of downgraded parameters has decreased for 4 years. However, efforts should continue both in the Brussels Region and upstream, in order to achieve all of the basic quality standards which have been in force since 2011. Conductivity and suspended matter still pose a problem for the Canal and the Senne, as does BOD in the case of the Senne. We should also remain vigilant with regards to certain parameters whose fluctuations occasionally cause peaks or falls in concentration (such as total phosphorus and dissolved oxygen in the case of the Senne): these changes could lead to an exceedance of the standards if they reoccur several times throughout the year and threaten fish life.

In 2016, the Brussels Region plans to lower the basic quality standards for a certain number of parameters, with a view to harmonising itself with those in force in Flanders and Wallonia (see second water management plan) and in the particular case of the Woluwe, with the aim of protecting habitats and species of the Natura 2000 network. In which case, several parameters risk becoming downgraded (again).

Date de mise à jour: 30/05/2020


Other publications from Brussels Environment

Fact Sheets of the physical-chemistry of the Brussels surface water (2001-2012), September 2015. 118 pp. Intern use only (.pdf, in French only)

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)

BDB, 2014. “Controle van de fysisch-chemische oppervlaktewaterkwaliteit in het Brussels Hoofdstedelijk Gewest – Analyseresultaten van het jaar 2013”. Study performed on behalf of Brussels Environment. Restricted (.xls, in Dutch only)

EUROFINS, 2015. “Controle van de fysisch-chemische oppervlaktewaterkwaliteit in het Brussels Hoofdstedelijk Gewest –Analyseresultaten van het jaar 2014”. Study performed on behalf of Brussels Environment. Restricted (.xls, in Dutch only)