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De freatische waters in het oostelijk kustgebied en in de Vlaamse vallei
De Moor, G.; De Breuck, W. (1969). De freatische waters in het oostelijk kustgebied en in de Vlaamse vallei. Natuurwet. Tijdschr. 51(1-2): 3-68, + 8 annexes
In: Natuurwetenschappelijk Tijdschrift. L. Walschot/Natuur- en Geneeskundige Vennootschap: Gent. ISSN 0770-1748, more
Peer reviewed article  

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Keywords
    Coastal zone; Water table; ANE, Belgium [Marine Regions]; Brackish water; Fresh water

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Abstract
    A study of the characteristics and the hydrochemistry of the water table aquifer in northwest Belgium has been conducted between August 1965 and January 1968.Two areas of about 450 km² each have been studied in detail: the Eastern Coastal Area north of Bruges and the Flemish Valley north of Ghent. The topography only shows a microrelief and the water table is usually found at a depth of 1 to 2 m below the surface. The aquifer is being supplied by precipitation and run-off from the hinterland. In both areas the reservoir rock is formed mainly by Quaternary sands and sometimes reaches a thickness of more than 35 m. These sands cover an erosion relief on top of the Tertiary strata, which are dipping gently in a NNE direction. The reservoir is reaching a thickness of more than 60 m where the Tertiary substratum consists of sand. In the main part of the Flemish Valley Weichselian sands are found at the surface and Holocene alluvium along the water courses. In the polder land in the northern part of the Flemish Valley and in the Coastal Area the surface sediments are made up by Subboreal peat and clayey and sandy Dunkerquian sediments. The study is founded upon a survey of 1500 resistivity soundings, in Wenner arrangement up to an electrode spacing of 100 m. The interpretation of the resistivity data has provided a map showing the depth of the main electrical contact surface. By further interpretation of the resistivities and the contact depths several hydrological units have been outlined. In two of these units, where brackish water is present in the subsoil, the fresh-/brackish-water boundary has been mapped. To control the geo-electrical survey 90 dry borings ranging in depth from 7 to 35 m have been performed. In every drilling site observation wells have been constructed. One-meter-long screens have been placed at several depths. Thus unmixed water samples have been taken at known depths. Chemical analysis of 150 samples has been made. Waters have been classified after their total dissolved-solids content, their relative ion distribution (CO3 + HCO3, SO4, Cl; Na + K, Ca, Mg), their magnesium-calcium ratio and their sulphate-chloride ratio. Each of these data has been represented by a letter or a number forming together a symbol for a water type. The different waters have been grouped into nine classes. One of these groups correspond with polluted waters. The vertical and horizontal distribution of these waters is shown on a map and on schematic sections. They show a geographical zoning and a vertical succession which give a clue to the explanation of their genesis, apparent also from the relationship of the different ion distributions in a single Piper diagram. The occurrence, the origin, the genesis and the evolution of the different waters is being explained in the light of the Quaternary geological evolution of the area, the characteristics of the reservoir rock and the physico-chemical processes in the water. The salt-water encroachment area has been outlined and the main groundwater currents are determined as concerns their orientation and their age. Thus the marine Eemian sediments in the Flemish Valley have been thorougly flushed at the beginning of the Weichselian and the composition of the Dunkerquian sediments has played a very important role in the fresh-/brackish-water distribution in the polder land.The comparison of the conductance of the pore water and the resistivity of the water layer in which the water sample has been taken has shown the hydrochemical meaning of the resistivity data and the geo-electrical fresh-/brackish-water boundary. This boundary seems to coincide with the transition between fresh water and water with more than 1500 mg/l total dissolved solids. This limit determines the potability and the irrigation suitability of the water. Hence the resistivity survey seems to be a very useful and inexpensive tool to map the fresh-water bodies in a brackish aquifer. Hence within certain limits the conductance of the water can be deduced from the measured resistivity and the conductance can be transformed in terms of chloride content, sulphate content and total hardness.

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