Focus: quantitative status of groundwater

The quantitative status of the five groundwater bodies was deemed to be good and should remain so up to 2021. In the case of the Brusselian Sands groundwater body, groundwater abstraction is evolving in a downward trend, and climate change could positively influence the groundwater recharge. Nonetheless, the variation trends of the piezometric level at certain measuring points do not follow the overall trend observed for the majority of the monitoring sites. There is therefore a potential risk and doubt as to whether this groundwater body can achieve good quantitative status in the coming years.

Target objective: achieving "good quantitative 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" on the protection of ground water (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. Good quantitative status equates to the sustainable management of the water resource, taking into account the water abstraction and the recharge of aquifers.

The characterisation of the good status is based primarily on the analysis of the evolution of groundwater levels, taking into account the volumes abstracted. The evolution of the rainfall and of the sealed surfaces which reduce infiltration into water tables are also factors which influence the recharge of groundwater, and thus the quantitative status of groundwater.

A digital model of the Brusselian Sands groundwater body has been available since the end of 2015. It will enable a better understanding of the trend evolution of the quantitative status of this water table, by testing the impacts of extreme climate change scenarios and its sensitivity with regards to catchments.

Volumes of water abstracted

About a hundred catchments, distributed across the various groundwater bodies, are subject to authorisation. In 2013, 2.5 million m3 of water was abstracted from the different groundwaters, of which around three quarters were from the Vivaqua catchments situated in the Bois de la Cambre and the Sonian forest (Brusselian Sands groundwater body), which was intended for the production of drinking water (bearing in mind that this only covers 3% of requirements - see "Supply and consumption of tap water"). The remaining quarter is devoted to industrial or tertiary use. The volume allocated to the agricultural sector is negligible, given that this sector is hardly present in the Brussels Region.

Since 2003, a very clear downward trend has been observed in terms of the volumes harnessed which are subject to authorisation, for all groundwater bodies and all uses. This general reduction is explained by lower levels of abstraction for drinking water needs between 2003 and 2011, and by the tertiarisation of the Brussels economy: abstraction for industrial use continues to decline both in number and in volume. The decrease observed for the tertiary sector despite this tertiarisation could indicate that the tertiary sector resorts less and less to groundwater catchment as an alternative to drinking water.

Forecasts are either at the status quo or at a lower level concerning abstraction for the industrial and tertiary sectors, and a stabilising of demand for drinking water by households (see "Domestic consumption of drinking water").

In addition to these catchments which are subject to authorisation, temporary pumping is carried out during works on construction sites in order to lower the level of the groundwater and enable the dry construction of building foundations, or during remediation works on contaminated soil. Permanent catchments are also realised in order to prevent floods in the underground metro infrastructure, or for hydrothermic use of the groundwater. The groundwater used comes from the Brusselian Sands and water tables from the quaternary period. The volumes involved are not known precisely, but they can be considerable.

Soil sealing

The study on the evolution of soil sealing (IGEAT, 2006) highlighted a growing proportion of sealed surfaces in the Brussels Region between 1955 and 2006: from 27% to 47%. A considerable amount of surfaces which potentially limit infiltration to groundwater. Given the increasing urbanisation of the Region, this rate was likely to increase and will continue to do so. The challenge with regard to maintaining groundwater recharge is therefore to compensate the loss of unsealed surfaces by infiltration works and/or to maintain natural infiltration zones. The precise impact of the decrease in unsealed surfaces on the groundwater recharge has not been quantified to date.

What impact will climate change have on the groundwater level?

If we want to examine the influence of the climate on groundwater, we should focus in particular on the period which is deemed to be favourable for the groundwater recharge. According to two studies, this period, referred to as the effective recharge period, extends from September-October to February-March for the Brussels-Capital Region (RMI, 2014 and study on the simulation of the Brusselian, 2015). In spring and summer, infiltration from rainfall is limited, since water is absorbed for plant growth.

The monthly totals of rainfall at Uccle during the recharge period, between September and March, show that since 1901, there has been strong inter-annual variability and a slight upward trend (around +10%) (RMI, 2014). Nevertheless, taking into account shorter time steps, a strong fluctuation of the trends can be observed.

According to the climate scenarios presented in the study on the Brussels Region's adaptation to climate change (Factor-X, Ecores, TEC, 2012), climate change should be reflected by a change in rainfall patterns, which varies according to the season (more humid winters but, on the other hand, less rainy springs and summers). Since the effective infiltration of rainfall essentially takes place in winter, climate change could therefore potentially have a positive effect on groundwater recharge. The digital model of the groundwater body should enable this impact to be quantified.

Monitoring of the quantitative status of groundwater

Three monitoring networks ensure the monitoring of the piezometric level of groundwater: the first encompasses the 5 groundwater bodies defined in the Brussels Region (which included 48 measuring points at the end of 2012), the second encompasses the quaternary sediments and the shallow alluvial aquifers (3 measuring points) and the last one is specific to the protection zone for water catchment intended for human consumption (10 measuring points).

The piezometric monitoring has been extended since October 2012 in line with the flow from eleven sources, emerging from the Brusselian groundwater.

Characterisation of the quantitative status of groundwater

Currently, given the evolution of piezometric levels - available at certain monitoring sites for more than 25 years - , the 5 groundwater bodies are considered to have good quantitative status. They will probably remain so up to 2021 to the extent that the trends related to the current abstractions and the water supplying the aquifers remain unchanged.

The 4 groundwater bodies of the Socle and Cretaceous period, the Socle in the supply zone, the Landenian and the Ypresian (Hill Region) have been declared as likely to achieve good quantitative status in 2021. In fact, the level of these groundwaters has experienced an overall upward trend since 1996 at different sites (followed by a stabilising since 2004 in the case of the Ypresian (Hill Region)) and the pressure from abstraction has continued to decline.

With regards to the Brusselian groundwater body, good status should also be achieved by 2021. However, there is widespread doubt as to the evolution beyond this timescale. The historical piezometric data show strong temporal and spatial variability depending on the measuring points in question. This variability is illustrated in the following chart.

Evolution of the piezometric level of the Brusselian groundwater body at two measuring points (371 and 397)
Source: Brussels Environment, 2015
 

Given the relatively shallow depth of the groundwater and its free character, the piezometric level is directly influenced by rainfall: the level fluctuates according to recharge or discharge episodes in the groundwater. But these fluctuations are not identical or synchronous at every measuring point. As illustrated by the charts above, the cycle can be seasonal (like at point 371) or multi-annual (like at point 397). However, concerning the multi-annual trends, the trend reversals do not necessarily occur at the same date and the evolutions observed do not necessarily move in the same direction (the recent multi-annual trend since 2007 is downward overall at point 397, but upward at point 371).

Several possible factors can be put forward to explain this variability observed at the Brusselian groundwater body: the environment of the measuring site (urbanisation, micro-climate, etc.), its location in relation to the limits of the aquifer, the depth of the groundwater at the measuring point, the impact of pumping stations located in the Brussels and/or Flemish Region, the buffering effect of the catchments intended for the production of drinking water from the Bois de la Cambre and the Sonian forest, the lithology of the geological formations of the non-saturated zone crossed by infiltration water, etc. Nevertheless, it is difficult to estimate the significance of these factors and the future evolution of levels without supplementary analysis. The digital model of the groundwater body should provide valuable inputs to answers in this respect.