C29 Geo-Electrical Survey in the Polder 'G Root Mijdrecht
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Geol .Jb. C29 24 1 - 253 13 Fig. Hannover 1981 Geo-electrical survey in the polder 'Groot Mijdrecht' REINDER H. BOEKE LMAN Geo-electrical su rvey, groundwater, salinization, uplift, interface freshwater I saltwater, pollution, chloride so il, influence, agriculture Netherlands (Groot-Mijdrecht polder) A b s t r act: The salinization of the subsoil especially in the western part of the Netherlands has a detrimental influence on agriculture and horticulture. From previous investigations it is known that 75 % of the chloride load of polders in western Holland is due to seepage from the subsoil. In this report the results of fieldwork in the polder 'Groot Mijdrecht' are presented . Groot Mijdrecht is a low polder - polder level 6.5 m below M.S.L. - causing a strong upconing of saline groundwater. By means of geo-electrical investigations the position of the interface (zone) between fresh and saline groundwater has been determined. The first results show a small grad ient of the interface on the west side and a rather steep course in the eastern part of the polder. The latter is caused by a higher polder level of the bounding 'Vinkeveense polder'. It has also been proved that saline groundwater is reaching the surface in the middle of the polder. [Geoelektrische Untersuchung des Polders 'Groot Mijdrecht'] Die Grundwasserversalzung insbesondere im westlichen Teil der Niederlande wirkt sich nachteilig auf die Landwirtschaft und auf den Gartenbau aus. Von friiheren Beobachtungen ist bekannt, daB ca. 75 % der Chloridbelastung in den Poldern Westhollands auf den Aufstieg versalzten Gru ndwassers zu riickzufiihren ist. In diesem Beitrag werden die Ergebnisse von Felduntersuchungen im Bereich des Polders "Groot Mijdrecht" dargelegt. Groot Mijdrecht ist ein tiefl iegender Po lder - mittlerer Wasserspiegel bei NN - 6,50 m -, der einen starken Aufstieg versalzten Grundwassers verursacht. Mit Hilfe von geoelektrischen Untersuchungen konnte die Lage der Obergangszone zwischen siiBem und versalztem Grundwasser ermittelt werden. Die ersten ErgebniSse zeigen einen geringen Grad ienten der Obergangszone auf der westlichen und einen vergleichsweise steilen Abfall der Obergangszone auf der ostlichen Seite des Polders. Letzteres ist bedingt durch den benachbarten "Vinkeveense Po lder", der einen hoher liegenden Wasserspiegel besitzt. Dariiberhinaus konnte nachgewiesen werden, daB das versalzte Gru ndwasser in der Mitte des Polders die Erdoberfliiche erreicht. *) Author's Address: R.H. BOEKELMAN, Technische Hogeschool Delft, Postbus 5048, NL-2600 GA, Delft, The Netherlands. 242 REINDER H. BOEKELMAN TABLE OF CONTENTS Page 242 1. I ntroduct ion •••••••••••••••••••••••••••••••••••••••••••••••••••••• 243 2. Geology of the Survey Area ...... ••• •• •••• •• • ••• ••••••••••• •••••••• 3. Invest igat ions .................................................... 244 4. Results ......................................................... 245 5. Discussions 249 6. Conclus ions 252 7. References ...................................................... 252 1. INTRODUCTION Sal inizat ion of the subsoil in the Netherlands especially in the low Western part has a detrimental effect on agri- and hort iculture . Th is effect is enforced by all kinds of measures of man , i.e. reclamation of lakes , necessitat ing lowering of the water level and thus , the potential difference between deeper sal ine groundwater and polder level initiates seepage of sal ine groundwater. Sandpits are another cause of sal inizat ion as the impermeable l ayers are disturbed or taken away causing a strong sal inization of the surface water. Also wells for industrial use or for drinking water supply are causing upconings of sal ine groundwater. In order to be able to predict changes in the groundwater system and to prevent unwanted effects caused by the measures mentioned before , the Water Manage ment Group of the University of Technology started to develop models (DAM 1976 , HOORN 1979) . � Locat ion of Groot Mijdrecht in the Western part of The Netherlands o POLDER-LEVELS ; PROFILES DELFT UNIVERSITY OF T�NOLOGY (--) WATERMANAGEMENT GROUP fig. 2 Location map of polder Groot Mijdrecht , cross sections I and II and polder levels (m below m.s.I.) Geo-electrical survey in the pol der "Groot M ijdrecht" 243 Apart from this also fieldwork was initiated to establ ish the present state of sal ini zat ion in a certain area and if possible to compare th:s with results obtained by model calculat ions. The area wh ich was chosen is a deep pol der in the Western part of Holland , Groot Mijdrecht (Fig. 1). The pol der is situated about 20 km south of Amsterdam and is about 20 km2 large . Reclamation of this former lake took place a century ago . The pol der level is 6.50 m m.s.l. Figure 2 shows pol der Groot Mij drecht with surrounding pol ders and pol der levels. In the East : Vinkeveen 2.01 m below m.s.l. I n the North : Ronde Hoep 2.47 m below m.s.l. In the West : 1 st , 2nd , 3rd po I ders of M i j drecht 5.65 m below m.s.l. I n the South : Wilnis 5.97 m below m.s.l. Especially from the east side a strong seepage can be expected because of a 4.5 m dif ference in pol der levels. Also because of the lower pol der level the sal ine groundwater is coning up accord ing to the Badon Ghijben/Herzberg relat ion , causing sal ine seepage to the pol der , wh ich can be demonstrated by groundwater samples. 2. GEOLOGY OF THE SURVEY AREA The deepest format ions known from borings are marine layers of Tert iary age , the so called 'Oosterhout ' format ion , consisting of fine silty sands with shells intersected by clay layers .(Fig. 3) . On top of these Pliocene layers , the oldest Pleistocene format ion - the Maassluis formation - was deposited , also of marine origin, consist ing of rather fine sands with clay layers . The first Major non-marine formation, the 'Tegelen ' format ion was deposited by the rivers Rhine and Meuse . It consists mainly of sand with occasional clay layers . The most important aquifer in the area are the coarse sands of the Harderwijk format ion with a thickness of about 50 m. In the southern part of the pol der it is overlain by the Kedichem format ion , rather thick clay layers and fine sands . In the northern part the format ion of 'Urk' and 'Sterksel ' covers the Harder wijk format ion, coarse sands with occasional loam and fine sand layers . During the Saale glacial period the 'Drenthe ' format ion was depos ited , consisting of rather coarse sands. During this glacial period also the ice-pushed rigs were created , with material from the 'Urk' and 'Sterksel ' format ion , wh ich nowadays acts as an important infil trat ion area . In the 'Weichsel ian ' glacial period layers of fine sand of eol ic origin were depos ited , some 10 m thick. The top of the subsoil is of Holoceneage (thickness 5-10m) and consists of clay, sand and peat layers of marine origin. The distribut ion of sal ine and brack ish water in the subsoil is not only due to the geolo gical history , but is also affected by the flow of fresh groundwater from the infiltrat ion areas - hills of Utrecht and Veluwe - which causes hydrodynam ical dispersion. Mole cular diffusion of chloride ions is an other important process wh ich can explain the present distribut ion of sal ine and brack ish groundwater (MEINARD I 1974 , VOLKER 1961). 244 REINDER H. BOEKELMAN HoLocene Twente Drenthe � Schematic geological sect ion of the underground in the U rk / SterkseL Groot M ijdrecht area . Harderwijk Teg elen Pleistocene MaassLuis (marine) . Pliocene Oosterhout (marine) 300m 3. INVESTIGATIONS A geo-electrical survey was carried out in order to obtain an impression of the present state of the sal inizat ion of the subsoil of 'Groot Mij drecht '. At first a density of 5 soundings/km2 was adopted , but already from prev ious invest igat ions it was known that a higher dens ity was necessary , especially in the eastern part of the pol der be cause of a rather steep fresh-water/salt-water interface . Three cross-sections have been invest igated (Fig. 2) , in all some 45 soundings. The first one was carried out by a student with technical assi stance of the Water Manage ment Group (LEENEN 1978) . During the autumn of 1978 the cross sections in the North were investigated and by the end of 1981 enough material will be available to fin ish the inventory investigat ions. For the geo-electrical survey a 400 Watt D.C. instrument has been used . The soundings were carried out according to the Schlum berger arrangement , with current electrode spacings ranging from 3 - 600 m. In order to diminish the effects of - electrically - non-hori zontal layers , e.g. a steep fresh water/sa I t -water interface , the line a long wh i ch the electrodes were spaced was kept as much as possible perpendicular to the expected slope of the interface . Geo-electrical survey in the polder "Groot M ijdrecht" 245 4. RESULTS The first corss-sect ion investigated is situated in the middle narrow part of the polder (Fig. 2) . The sounding curves (Fig. 4) represent the apparent specific res istivit ies as a function of ha I f of the current electrode spac i ngs • 11111111rl l tmlr� � Geoelectrical sounding curves of the eastern part of cross-sect ion 1. Pain Qm 1 23456810 2 3 4 5 6 8 100 1 I _ I ...••••• 2 \ --I- 3� I _ ! . - 5 1 2 j --6 E ! -- 7 - .. 3 i - 12 ! /\ .!: 4 V \. 5 I '. 6 \. I 8 1\ 10 �' . \\\. y .. 2 J 3 \ 1\ I .� 4 1 1 11 5 1\ \ I r: : II' II· 6 II 8 1\ \ / : / 100 � � / V ./ .. 2 1\ 1L �'-- At the east side of the pol der (curve 2) rather high resistiv ity values are found even at long spac ings.