See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/237801016 Influence on groundwater quality of the Paleozoic Brabant Massif in Belgium due to overexploitat ion ARTICLE · JANUARY 1994 CITATION 1 READS 21 4 AUTHORS, INCLUDING: Kristine Walraevens Ghent University 87 PUBLICATIONS 533 CITATIONS SEE PROFILE Luc Lebbe Ghent University 87 PUBLICATIONS 658 CITATIONS SEE PROFILE Available from: Luc Lebbe Retrieved on: 05 February 2016
Groundwater Quality Management (Proceedings of the GQM 93 Conference held at Tallinn, September 1993). IAHS Publ. no. 220, 1994. 4 6 1
Influence on groundwater quality of the Paleozoic Brabant Massif in Belgium due to overexploitation
K. WALRAEVENS, L. LEBBE, M. DE CEUKELAIRE, E. VAN HOUTTE & W. DE BREUCK Laboratory for Applied Geology and Hydrogeology, University of Ghent, Krijgslaan 281 - S8, 9000 Gent, Belgium
F. MARRAS Instituto di Giacimenti Minerari Geofisica e Scienze Geologiche, Université di Cagliari, Piazza d'Armi, 09100 Cagliari, Italy
Abstract The overexploitation of the groundwater resources in the Paleozoic Brabant Massif in the western part of Flanders (Belgium) is causing quantitative problems since several years. The groundwater flow in the aquifer has been studied by means of mathematical modelling, both of the natural and the pumped situation. Hydrogeochemical studies have revealed that the regional groundwater quality distribution is still reflecting the natural flow pattern. However, locally, the overexploitation has led to a degradation of groundwater quality. In some areas, an anomalously high TDS-value results from the admixture of salt pore water, possibly from the overlying clay layers. In individual wells, the sulfate content has dramatically raised due to pyrite oxidation. The oxidation results from the excessive drawdown of the water level in the pumped well, thus allowing oxygen to enter in the aquifer. Regulatory measures are urgent in order to prevent the progressive degradation of groundwater quality.
GEOLOGICAL CONDITION
The Brabant Massif is situated in the western part of Flanders (Belgium) (Fig. 1). In the southwestern part of this area, it occurs mostly at a depth between 100 and 200 m. Its geology has been studied extensively by Legrand (1968).
The lithology of the Cambro-Silurian Brabant Massif consists of consolidated rocks (quartzites, phyllites, schists and volcanic rocks), the top of which is fractured. To the south of the outline in Fig. 1, it is overlain by the Devono-Carbonian Basin of Namur. Both belong to the Paleozoic Basement Complex.
The Paleozoic Basement is overlain by Cretaceous and Tertiary rocks (Fig. 2, Lebbe et al, 1988). The Cretaceous rocks are composed of chalk: the Turonian rocks, that are only present in the southern part, are fissured; the Campanian rocks are non-fissured. The overlying Tertiary rocks are unconsolidated, and consist of alternating sand and clay. The superficial Quaternary sediments (sand, silt and clay) are generally thin, except in the important river valleys (like the River Mandel in Fig. 2).
OVEREXPLOITATION
The fissured top of the Brabant Massif constitutes the main aquifer in the
462 K. Walraevens et al.
The Netherlands
0 50km •
Fig. 1 Location of the study area.
southwestern part of Flanders. Apart from its sometimes favourable discharges (specific capacity up to 400 m2 day 1 , Lebbe et al, 1987), the aquifer is preferred because of its groundwater quality: the total hardness is very low (in most cases less than 0.5 mmol H , Walraevens etal, 1989), and in general only traces of iron are present. Also the relatively high temperature (around 18 °C)
SW NE River Mandel
Q Quaternary sediments Yc Ypresian clay Be Bartonian clay Ls Landenian sand Le Ledian sand Lc Landenian clay Ps Paniseliansand Cc Cretaceous: Campinian chalk (non-fissured) Pc Paniselian clay Ct Cretaceous: Turanian chalk (fissured) Ys Ypresian sand
Fig. 2 Geological cross-section through the western part of Flanders (for location see Fig. 1).
Overexploitation and groundwater quality of the Brabant Massif, Belgium 463
is favourable. Therefore, the important industrial concentration in the southwestern part of Flanders (especially textile and food industries), is withdrawing large groundwater quantities from the aquifer (more than 16 millions of m3 in 1986, Lebbe etal, 1987).
The evolution of the piezometric level in the Brabant Massif is spectacular. Fig. 3 (De Ceukelaire et al., 1992) shows this evolution at Roeselare: whereas the piezometric level in 1910 was still at +17 m TAW (the Belgian reference level), in 1986 it was already below -100 m TAW. The piezometric evolution at Waregem (Fig. 3) shows that the sharp decline has started in the 1950's, although before that, the level had already dropped substantially. The piezometric map of the Brabant Massif in 1986 is shown in Fig. 4 (Lebbe et al, 1988). It is seen that in the area Roeselare-Tielt-Waregem an important depression cone has developed with hydraulic head minima at Ardooie and Wielsbeke (both -120 m TAW).
Consequently, as a result of the continuously falling groundwater head, the overexploitation induces groundwater quantity and quality problems.
hydraulic head (m TAW)
2 0 -
o-
-20 -
-40 -
-60 -
- 8 0 -
-100-
,
R o e s e l a r e
. ••
•
\ .
hydraulic head (m TAW)
4 0 -
20-
0 -
-20-
-40-
-60-
-80 -
-100-1 1 1 1 , 1 , , r — , 1 1 , 1 1 1 , 1 —
1910 1930 1950 1970 1990 1910 1930 1950 1970 1990
Fig. 3 Evolution of the hydraulic head in the Brabant Massif at Roeselare and at Waregem.
Fig. 4 Observed hydraulic head in the Brabant Massif in May 1986.
464 K. Walraevens et al.
GROUNDWATER FLOW
The groundwater flow in the Basement Complex (Brabant Massif + Basin of Namur) has been simulated by means of a Q3D mathematical model (Lebbe et al, 1987; Walraevens et al, 1990).
The calculated hydraulic head in natural conditions is shown in Fig. 5. The aquifer is replenished in the southern, relatively high region, where it is covered by a considerable thickness of sediments, among which more than 100 m of Ypresian clay. The recharge areas of the Basement Complex in natural conditions have been shaded in Fig. 6. The vertical inflow is, in most parts, between 0 and 1 mm year"1.
Fig. 5 Calculated hydraulic head in the Basement Complex in the natural situation.
Fig. 6 Calculated vertical flow between Basement Complex and overlying aquifer in natural conditions. The recharge area of the Basement Complex is shaded.
Overexploitation and groundwater quality of the Brabant Massif, Belgium 465
The horizontal flow within the Basement Complex occurs to the northwest in natural conditions (Fig. 5). Out of the recharge areas, it is gradually retarded due to an ascending vertical flow in the topographically lower regions (Fig. 6).
Groundwater withdrawal has substantially changed the flow pattern. The simulation of the situation in 1986 shows that recharge now occurs in the whole area (Fig. 7), except for two regions where the overlying aquifer is intensively exploited, resulting in an upward flow from the Basement Complex to this aquifer. Horizontal groundwater flow is now in the direction to the centres of the depression cones (Fig. 4).
I 1 vertical flow 0-5mm year1 (downward Slow ' > or recharge of Basement Complex)
TT^m vertical flow>5mm year 1 (downward flow A \ ' iHzi or recharge of Basement Complex) ^ C r ^
[ | vertical flow< Omm year1 (upward flow)
+ + Belgian frontier
Fig. 7 Calculated vertical flow between Basement Complex and overlying aquifer in the situation of 1986 with pumpings. The recharge area of the Basement Complex is shaded.
REGIONAL GROUNDWATER QUALITY
The extensive investigation of the groundwater quality in the Brabant Massif has shown its regional distribution to be still reflecting the natural flow pattern (Walraevens etal, 1989).
The starting point of the evolution of the groundwater quality is the marine environment that was reflected in the sediments before the end of the Tertiary age, when the whole area was transgressed by the sea. After the final marine regression, fresh water started to infiltrate in the recharge areas. This fresh water is resulting from precipitation, in which CaC03 has been dissolved. The natural groundwater flow regime was established. The infiltration of fresh water caused the marine conditions to be progressively expelled in the direction of groundwater flow. In the recharge areas (cf. Fig. 6), the overlying sediments, particularly the thick Ypresian clay, were gradually leached out. Within the Brabant Massif, marine conditions were pushed back to the northwest (cf. Fig. 5). Consequently, the chloride content is increasing in the same direction (Fig. 8). The same holds for the regional distribution of the sulfate content (Fig. 9).
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-100- line of equal chloride concentration (in mg I*1)
+ + Belgian frontier ^o ̂ S6! ^oidi
7 \
Brugge
600 -
Calais
è Lille
Fig. 8 Regional distribution of chloride content of the groundwater in the Brabant Massif.
-200-line of equal sulfate concentration (in mg I"1)
+ + Belgian frontier
Calais
è Lillo
Fig. 9 Regional distribution of sulfate content of the groundwater in the Brabant Massif.
DETAILED GROUNDWATER QUALITY INVESTIGATION
An area of detailed investigation (Fig. 1) was chosen around the centre of the depression cone between Tielt-Kortrijk-Waregem, in order to study the possible impact of the overexploitation on groundwater quality in the Brabant Massif (Marras, 1992). It is situated almost completely beyond the recharge area in natural conditions (cf. Fig. 6). But at present, the vertical inflow amounts to more than 5 mm year"1 in large parts of this area (cf. Fig. 7).
The distribution of the chloride content largely corresponds with the regional pattern (Fig. 10). In general, the chloride content increases from 40 mg l"1
Overexploitation and groundwater quality of the Brabant Massif, Belgium 467
Fig. 10 Distribution of chloride content of the groundwater in the Brabant Massif in the area of detailed investigation.
in the southeast to above 200 mg l"1 in the northwest. Around Tielt, groundwater is somewhat less saline (< 200 mg H), corresponding to a small part of the recharge area in natural conditions (cf. Fig. 6).
But a particular situation can be observed between Wielsbeke and Waregem, where the chloride content increases remarkably in several wells, with maxima above 700 mg l"1. This increased chloride content is accompanied by higher sodium, potassium, calcium and magnesium contents, resulting in a strongly increased TDS-value. This situation can be ascribed to the admixture of more saline pore water from the sediments overlying the Brabant Massif, especially from the Ypresian clay, in an area where no recharge took place in natural conditions. Consequently, these sediments have not been leached out. But as a result of groundwater abstraction, recharge is now occurring also in this area, entailing the seepage of salt water into the Brabant Massif. In the areas of strong overexploitation, this has apparently already altered the quality of the groundwater extracted from the Brabant Massif. Probably, the same explanation holds for the high chloride content (> 800 mg H) in some wells to the west of Roeselare, observed during the regional quality investigation (Fig. 8). Despite the overexploitation, the absence of an augmented mineral content between the two regions can be explained as a result of its correspondance to the natural recharge area. On the other hand, it is also possible that the more saline water originates from deeper, less fissured parts of the Brabant Massif itself. More research is necessary to give a decisive answer to this question.
Interesting conclusions can also be drawn from a survey of the sulfate content (Fig. 11). In general, the distribution corresponds to the regional pattern. Sulfate increases from less than 50 mg H in the south-east to more than 300 mg l"1 in the north. But locally, always in individual wells, exceptionally high values occur. The maximum amounts to 535 mg l"1. These
468 K. Walraevens et al.
Fig. 11 Distribution of sulfate content of the groundwater in the Brabant Massif in the area of detailed investigation.
values are associated with a slightly lowered pH, while the chloride content is not altered. They can be ascribed to the oxidation of pyrite. It is well-known that the rocks of the Brabant Massif contain veins with a high content of this mineral. As a result of an excessive drawdown of the hydraulic head in the pumped well, below the casing, air is allowed to enter in the aquifer. When the pumping is interrupted, the water level in the well is raised, preventing the air to escape again. The oxygen can then attack the pyrite, according to the reaction:
This results in high sulfate concentrations in the immediate surroundings of the well, explaining the isolated appearances of the phenomenon.
CONCLUSION
Even though the regional groundwater quality in the Brabant Massif is still reflecting the natural flow pattern, there are local anomalies in the groundwater composition. These clearly result from overexploitation of the aquifer. At one hand, there are areas in which the TDS-value, and particularly the chloride content, has been raised by the admixture of more saline groundwater from the overlying sediments (or possibly the underlying rocks). On the other hand, single wells show anomalously high sulfate concentrations, due to oxygen entering the aquifer.
Regulatory measures are urgent in order to prevent the progressive degradation of groundwater quality. The problem of the raised sulfate contents can be prevented by avoiding to drawdown the water level in the pumped well
Overexploitation and groundwater quality of the Brabant Massif, Belgium 469
below the casing. The depth at which the submersible pump is installed should be adapted in order to achieve this aim. This condition can be stipulated in the exploitation allowance, which is granted by the Administration. It should be firmly controlled.
The increased TDS-value constitutes a greater problem. The admixture of more saline groundwater will continue as long as the recharge area is enlarged with respect to the natural situation. Of course, the amount of admixture will depend upon the magnitude of the vertical inflow in these new recharge areas. In order to mitigate this inflow, the drawdown of the hydraulic head with respect to the natural situation should be diminished. This can be achieved by significantly reducing the groundwater withdrawal from the Brabant Massif. Another possibility could consist in the artificial injection of water in the Brabant Massif. This has been simulated with the help of a mathematical model (Lebbe et al., 1987; Walraevens et al, 1990). A total injection of 4.4 millions of m3 per year, in the centres of the depression cone, would lead to a rise of the hydraulic head by 10 m at Bruges and Ghent. The influence of this rise on groundwater quality in the Brabant Massif, and the preferable quality of the injected water, still remain to be studied.
Acknowledgements The National Fund for Scientific Research (Belgium) supported this study by confering research appointments to K. Walraevens and L. Lebbe. The study benefited by project orders from the Ministry of the Flemish Community. The grant for ERASMUS-project AGRICOLA (ICP-91-B-l 183) from the European Community was helpful in order to allow F. Marras to participate in the work.
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