Proc. lOth SWIM, Ghent 198M, Natuurwet. Ti jdschr. (W. DE BREUCK & L. WALSCHOT, eds) vol. 70. 1988 (published 1989) Ghent: Natuurwetenschappelijk Tijdschrift 1989 p. 359-375. 12 figs. Hydrogeological SWIM -excursion to the western coastal plain of Belgium L. LEBBE* K. WALRAEVENS* ''' 1. INTRODUCTION (Fresh-water supply of the western coastal plain) In the larger part of the Flemish coastal plain the In the phreatic aquifer, fresh water as well as possibilities of fresh-water exploitation are limi­ salt water occurs (W. DE BREUCK et al ._, 1974), ted. Three possibilities exist : exploitation of Under the young dunes, situated at the border the deep artesian aquifers, exploitation of the with the sea, the fresh-water pocket has reached shallow phreatic aquifer and collection of surface the "impermeable" clay substratum at a depth water in artificially created basins. between 20 and 40 m. North of the young dunes there is the shore with a salt-water lens above a Under the larger part of the Flemish coastal plain fresh-water tongue. South of the young dunes a thick clay layer of about one hundred metres there is the polder area where the larger part of thick separates the phreatic aquifer from the the phreatic aquifer is filled with fresh water. artesian aquifers. Under the clay there are two In this polder area there are a considerable artesian aquifers (W. DE BREUCK et al., 1987). number of fresh-water pockets only under old The first is located at a depth of 140 m and dune ridges and under some wide creek ridges. consists of silty very fine sand of about twenty The first water-catchment for public supply in metres thickness. The second at a depth of the western coastal plain was installed in such a about 250 m is located at the interface of the fresh-water pocket under the old dune ridge of chalk layer of Secondary age and the fractured Adinkerke-Ghyvelde. This water-catchment was shales of Primary age. Both aquifers have very started in 1926 by the IWVA with the help of the small transmissivities (between 10 and 40 m2/ d). wells installed by the British army during the As a consequence the installed wells in these first world war. The IWVA is an intermunicipal artesian aquifers have a very small specific water-supply company which was founded in 1924 capacity. The water of the artesian aquifers has (J. SPILLIAERT, 1960). The municipalities of a total salt content of between 2000 and 3000 Adinkerke, De Panne, Nieuwpoort, Oostduinkerke mg/1. It is a soft water which is rich in sodium and Veurne were the first share-holders. In cations and poor in calcium cations. The tempe­ spite of the small pumping rate this first water­ rature of the water is rather high, between 15 catchement very soon delivered water with high and 18 °C. Because of the depth of the aquifer, Cl--content. Therefore in 1933 plans were alrea­ the specific well capacity and the special water dy being made to construct a new water-catch­ quality, the artesian aquifers have until now only ment in the larger fresh-water pocket under the been used for small water catchments for the local young dunes, where the thickness of the aquifer industry, which has well defined water-quality ranges between twenty and thirty-five metre& demands. and where the transmissivity ranges between 200 "' Research Associate of the National Fund for Scientific Research (Belgium), Laboratory for Applied Geology and Hydrogeology, State University of Ghent, Krijgslaan 281, B-9000 Ghent, Belgium. ** senior Research Associate of the National Fund for Scientific Research (Belgium), Laboratory for Applied Geology and Hydrogeology, State University of Ghent, Krijgslaan 281, B-9000 Ghent, Belgium. 360 L. LEBBE & K. WALRAEVENS and 500 m2 /d. This water-catchment was situated on the territory of the municipalities of Oostduin­ kerke and Koksijde. Therefore the municipality of Koksijde also became in 1934 a share-holder of the IWV A. This second water-catchment of the IWVA for water-supply in the western coastal plain was started only after the second world war in 1946. Twenty years later, in 1966, a third water-catchment was started which is also situated in the young dune ridge at De Panne. A fourth water-catchment in the young dune ridge was already planned in the beginning of the seven­ ties. Until now this water-catchment has not yet commenced because of strong opposition to it. The quality of the pumped water of this shallow phreatic aquifer under the young dunes is good. The total salt content ranges between 500 and 800 mg/1. Until now the cC-content has been quite stable and is not so dependent on the discharge Fig. 1. The increasing water return of the rate like it was in the first water-catchment. I.W.V.A. 1926 to 1984 (Technical annual Although the quality of the pumped water in the report, 1984). young dunes does not change much and the water-supply of the young dunes seems until now unlimited, the danger of salt-water encroachment is a reality. Too heavy a local pumping of water After this short review of the different water­ in the young dunes may attract salt water from the shore or from the polder area, although this catchment possibilities of the western coastal plain,- it becomes clear that the fresh-water lenses undesirable effect will only be observable in the are very important for the water-supply of the pumped wells after several decades of pumping. visited area. Besides the danger of salt-water flow to the water-catchments, there is also a danger of salt-water seepage in some low-lying From its foundation on, the IWV A has delivered places in the polder area. water to the town of Veurne and to its share-hol­ ding municipalities at the coast. Because of the For the management of the fresh groundwater a expansion of coastal tourism especially after the thorough knowledge of the distribution and the second world war, the water demand has increa­ flow of fresh and salt water is necessary. In sed continuously (fig. 1). A second, although this guide of the hydrogeological SWIM-excursion minor reason for the steady growth of the water a short review of the hydrogeological studies in demand is the connection of all the rural munici­ the western coastal plain will be given. Some of palities west of the river Yser with the IWVA the results will be demonstrated in the field and public water-supply system from 1961 on. described in the last part of the guide. The third possibility of fresh-water exploitation in 2. REVIEW OF HYDROGEOLOGICAL STUDIES IN the coastal area is the interception of fresh water THE WESTERN COASTAL PLAIN OF BELGIUM from small rivers flowing towards the coastal plain, in artificially created basins. Since 1974 the NMW has exploited such a water-catchment. R. d'ANDRIMONT (1902) described for the first It supplies water to the region south of the time the groundwater flow and quality distribution coastal plain during the whole year and to the in the Belgian coastal plain. An attempt was industrial zone of Veurne during the summer made to apply the new principle of B. HERZBERG months when there is water shortage. The NMW (1901). Because of a wrong estimation of the was the national public water-supply company of mean groundwater-level in the polders with res­ Belgium. In 1987 it was turned into the regional pect to the mean sea-level, he concluded that the water-supply company of Flanders, VMW. phreatic aquifer under the whole Belgian coastal VERKLARING · LtGENDE • LEGEND Zaut ~ater f • 1500 ppm) op ..n di~c. v•n: Eu lilh ( • 1500 ppm) I un• profond1ur de : 20 ·25m ~'Ill ,..~ I ,. 1500 ppm) •t • depth af : 25·30m c 2m - 30 ·35m .... __ __ _ 5 · 10m - -lono ...- - ....., ...... .,_..... ..... 10 ·15m U~ - do ·- - hot Food1 - KaiiKUol Fundamontool Ondonaok Fig. 2. Depth of salt-/fresh-water interface in the phreatic aquifer under the western Belgian coastal plain (investigation 1963-1973) (W. DE BREUCK et al., 1974). 362 L. LEBBE & K. WALRAEVENS Hydrogeological study of 'w'esthoek area __.._ fl ,...,., •• IL.IftK "1111 ~c.aa.~~~• .,... ...., ._. • 1 • t•v ..... l.o\P'II&. ·na .......... .;.. .., ,..,,.. N ~ 0 0 I 0 ·------~----~··· Fig. 3. Situation of the hydrogeological study in the Westhoek area with calculated lines of equal hydraulic head and the location of the resistivity profiles in fig. 4 (L. LEBBE, 1981). plain must be completely filled with fresh water. carried out on a withdrawn sample to date the This conclusion provoked a discussion with 0. sea-water encroachment near the Eocene clay van ERTBRON (1903) who stated that only small substratum in the polder area just behind the amounts of good water were available in the young dune ridge (W. DE BREUCK r. G. DE polder areas. MOOR, 1974). H. THIELE ( 1943) was the first man who tried to During the period 1974-1978 a hydrogeological map the depth of the fresh-/salt-water interface study was carried out in De Panne (L. LEBBE, under the polder area of French- en West-Flan­ 1978). It provided a detailed lithostratigraphical ders. Only a small amount of his data was fed description of the groundwater reservoir. The back. The depth of the fresh-/salt-water inter­ hydraulic parameters of the different layers were face of the coastal plain of West-Flanders was deduced from pumping tests and from observation finally mapped by W. DE BREUCK et al. (1974) of seasonal and tidal fluctuations of the hydraulic and published in the proceedings of the 4th head.