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 (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 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 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 : 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 : 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 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. Westward the apparent resistivity values are diminished of greater L/2 values , interpreted as a rising interface between fresh and sal ine/brackish groundwater. At locat ion 7 and 12 the fresh water has disappeared and the apparent resistiv ity values are rather constant for all L/2 values . The soundings in the western part of this cross­ sect ion (Fig. 5) show a similar effect . Here again the sal ine water is rising towards the centre of the po I der • As can be seen from the graphs 18, 16 and 14 (Fig. 5) the interface in the western part 246 REINDER H. BOEKELMAN

0 14 aI 19 I I I :

3 /./ 4 II 5 'I 6

8 : 100 �i11 : 2 rises less compared to the eastern part of th is cross-sect ion. Another difference from the west to the east of the cross-section , are the apparent res ist ivity values at long electrode spacings wh ich are 3-4.Q m in the west indicat ing sal ine groundwater , whereas in the eastern part these values are 10rr m, indicat ing brackish water. At locat ion 20 and 19 there is no more indication of the presence of fresh water in

the subso i I • Cross-section II in the north of the pol der shows the same tendency (Fig. 6) . Again start ing at the east side the first few graphs do not show sal ine groundwater , but from graph 24 apparent resist ivity values for long electrode spacings tend to approx imate 3-4.R m, indicat ing sal ine water. Fig. 6 also shows the upconing of the brackish/sal ine groundwater as proven by west­ ward increasing res ist ivity values at continually shorter spacings . The west side of this cross-section shows again a less steep upconing (curves 36 to 27) as can be seen in Fig. 7. The sal ine groundwater comes closest to the surface at sounding 27. The soundings along the dike in the very north of the pol der do not differ essent ially from Geo-electrical survey in the polder "Groot M ijdrecht" 247

, � Geoelectrical sounding ...... curves of the eastern part m ....., , of cross-sect ion II. IT�I 4 mrrmli1t�?21 , , _p.inQm , , 11111111"1 1 3 6 B 1 3 6 B 2 4 5 10 2 4 5 I 1 100 ••••• 21 J If _--. - 24 E 2 25 -- 3 1\ ! - 27 ...lIN \1 c 1\ \ f \\\ 4 \ �. 65 B \\

10 �i\ \ 2 \

i � 3 : I 1/) 7 \\ / 7 6 / 5 . II : II I: B II" ' 7 100 /' II' 2 II · 1/ deeper part ions , as could be expected. Especially for small electrode spacings considerable changes in apparent specific re­ sistiv ity values can be not iced. This effect is due to the local situat ion of alternat ing peats, sands and clays of the Holocene layers. Secondly the apparent resistivity values at long electrode spacings in general approximate values between 3-451. m, indicat ing sal inegroundwa ter, CI- -content 4000 mg/I , with the exception of the soundings in the eastern part of the cross sections, where values of 10.n. m are found . The sal ine ground­ water could not be proved here due to the lim itat ions of the measuring device. Longer electrode spacings would have been necessary . To obtain more quant itat ive information the sounding curves have been interpreted using an 8-layer computer programm, the results of wh ich are shown for the first cross sect ion (Fig. 8) . More layers have been distinguished but because of simpl icity , only four layers are given here .

The Holocene top layer has a wide range of specific resistivity values (2.3 - 52 Q m) . Below a sandy layer with resistivity values between 35 and 40 Q m, containing fresh porewat er, can be distinguished . A layer with resistivit ies between 9 and 1 22 m re­ presents the brackish zone , whereas the zone with sal ine groundwater is indicated by 248 REINDER H. BOEKELMAN

£!Jc.2. Geoelectrical sounding curves of the western part {111111111!t!!!!.ltllll�!!i'IIL of cross-sect ion II.

_ Po in Qm 1 2 3 4 5 6 8 10 2 3 4 5 8 100 1 6

••••••• 36 i( I I 33 J E --- _.- -- 30 3 28 --'IN -_ 27 Lc- 1 4 \\ .. 5 .. 6 8 ..�\ � 10 \ .� 2 JN 3

- ' lL�� 4 L.l I V ill 5 o- 6 1.1 / l/ // 8 L/lL IL �o !r' 100 11,/ V.: 0' 0' I( (0 2 Iv r� specific res ist ivity values of 3.3 - 6R m. As can be seen in Fig. 8, the sal ine ground­ water reaches the surface between soundings 20 and 10. The zone with fresh and brackish pore-water is shal low « 20 m ) in the western part of the cross-section and disappears in the direction of the centre of the polder , whereas at the east side a very thick fresh water zone (-v 85 m ) and also a thick brackish zone is demonstrated . This can be ex­ plained by the small differences in pol der levels between the bounding polders in the western part and , on the contrary , great differences in the eastern part . Th is impl ies a great difference in groundwater flow from the bounding polders - est . 0.15 mm/day from the west , 3.1 mm/day from the east - wh ich consequently affect the thickness of the brack ish zone . It also explains the eastward decrease in specific res ist ivity values viz. 3.3 - 5.7 m. The specific res istivity cross-sections in the north show a similar tendency (Fig. 9) of a slow rising interface from the west towards the centre of the oR cross-section. At the border of the polder the interface shows up at /V 40 m below the surface , at sounding 27 at a minimum value of 22 m, here in the sandy Holocene top layer. The sal ine groundwater does not reach the surface in this cross-section. The slope of the interface is steeper from the north than the west , due to a greater difference in po lder levels. o

g r.' ". � : "1 .. , ! � = �-. - .. -u ...s., -5.9 '�=

'I . I

J 8000 ------��------��

• • ... Jacobohoe;;l I \ - n .S.4 j: ' _6.0 _ I I II· 11

___ =- cc;�CC:--�+�_�=: .....�� ... ,JI--�+-+-"':-��-�i--...... ;d!bo;.:",;:.... '"'h '=' _L_ - --:c,·-·o

� _-+-=--<

�

1STLA YER 2.3-52

7000 ---�-----'io""""; D 2N 35-40 .

AD . 3 9 -12 . . " RESISTIVITY PROFI TH ,:- 4 3.3 - 6 LE I

GEOELEKTRISCH ONDERZOEK 70 POLDER GROOT MIJDRECHT .8

80 METER , ,

11 000 -""':"'-"';"-MCI

.. 4

/

/ j 1/ I /J ' ' fl' _ /, xx . -SOUNDING .. �. o -u · 1 6 19 .1\.191 10 - 1.00 ----Ol �----....f� 17 42 ..... •

ll - 7.6 • RESISTI VITY PROFILES

33- 4.5 nm

GEOELEKTRISCH ONDERZOEK POLDER GROOT MIJDRECHT fig. 9 70

80 METER I I I I II, I I ..l 10 11 7 8 6 9 5 3 4 2 1 � I t -

,

SOUNDING 0 xx

10

20 I - 30 -4 I " 40 I 50

60

70 I-EAST o 100 200 300 400 500 METER - - 80 METER - - - GEOELEKTRISCH ONDERZOEK fig. 10 POLDER GROOT �JDRECHT Geo-electrical survey in the po l der "Groot M ijdrecht" 249

In the area , where from prev ious invest igat ions the flow towards the pol der is known to be max imal , a relat ive thick fresh water zone has been found (KRUIJK 1963) . In the eastern part a th ick fresh water zone of up to 90 m at the border is not iceable wh ich strongly decreases in western direction. Also a thick brackish zone can be ob­ served here. Although the thi ckness could not be determ ined accurately it is less than in the cross sect ion in the centre of the polder. An unexpected effect can be not iced in the east side of cross section I wh ich is shown in Fig. 10. Here the interpretat ions of several sound ings show an increase of specific resistivity with depth , whereas a decrease - due to higher sal inity - is expected , as the lithology is very likely to be uniform . This effect has also been noticed in previous in­ vest igat ions (DAM 1960) . A possible expl anat ion of this effect might be the occurrence of mixing of sal ine groundwater from the west side and water from the brack ish zone from the east side, wh ich is supported by the trend of the specific resistivity values as found. Another cause of this effect might be the method used . The interpretat ion of the sounding curves assumes 'electrically' hori zontal layers and espec ially in the area with a steep interface this assumpt ion is not correct .

5. DISCUSSIONS

For comparison results of WIT (1974) are shown in Fig. 11, wh ich presents chloride contour I ines as determ inated from samples of the upper groundwater. The results from our invest igat ions in the western part of the first cross-section fit well into this picture , with CI- -contents varying between 4000 and 5000 mg/I. However in the eastern part , accord ing to our results the contour I ines have to be moved westward . The lack of sampl ing po ints in this area can explain the difference. The cross-sections in the north also fit wel l. The presence of sal ine groundwater is de­ monstrated at sampl ing po int 8. Just north of this locat ion the geo-elect"r ical survey gave a maximum rise of the interface . The CI- -contents in the western and eastern part of the cross-section resemble our find ings. For deeper layers only a few data are available from a reg ional geo-electrical survey of Rijkswaterstaat (DAM 1960) . At location 47, at a depth of 33 m below m.s.I ., a CI- ­ content of 5180 mg/I was found , format ion resistiv ity 4.05< m. At a depth of 56 m below m.s.l. 5220 mg CI-/I , format ion resistiv ity 3.0n m. In the first cross-sect ion, at locat ion 4, a CI- -content of 5370 mg/ I showed up a a depth of 17.7 m below m.s.l, in a formation with a specific res istiv ity of 3.5n. m. These spec ific res ist ivity values are in good accordance with the values found in the present survey . The University of Le iden carried out investigat ions concerning ion-exchange in ground­ water (GEI RNAERT 1971). From these invest igat ions Fig. 12 and Fig. 13 are borrowed , wh ich give an east-west cross sect ion of the western part of Holland . 'Groot M ijdrecht ' is situated in the eastern part . In the figures different types of groundwater are distinguished , according to their origin:

- Ca (HC03) 2 - water (Ll.), which is actual ly infiltrated rai nwater ; 250 REINDER BOEKELMAN

H.

2'J 266 J55 462 426" 9 249 8�--ri�--- 4 5 302 6 '42 t 7, '" " " , ,

� sa!!1P1ing point \ ' , '. , 5JJ, 5 2J4 Cl contents mg/ l •, , \ 5006 2 2J ) "01 , groundwater resp.2, 't�b '. contourl ine 500 mg/:ROl ,'22\ upper groundwater •, 4 'z , 2 6 ,,500" 604�.. � '8��9 \�, 2 3 \\ \ I' \

' I \ , , , I " � 33 � � , I ",' , , �'/J905 ,II . �B08 '"Zuid�rwoar I " '"II, I " '",It I I 41 " I , I 89 , I I 479.) , 160 , , , , , 39 365'5 ',37 , f 40 I "II /:/ :� I I I I I I " "

,

Fig. 11 Chlor ide contents ( mg/ I ) in the groundwater near surface . Contour lines after WIT (1974) .

--- - NaH C03 - water (0) , found in marine sedi ments . If the sal ine water is superseded by fresh water, or if sal ine water has intruded in continental sedi ments and then again has been superseded by fresh water ; - NaCI - water ( 0 ) , water of seawater composition , e.g. with high chloride content ; - CaCI 2 - water ( 0 ) , formed if sal ine water intrudes in sediments, which have been in contact with freshwat er; - mixed - water (x ) , mixture of the first two with the latter two types of water. Originally Ca(HC03) 2 - water was found near the infiltrat ion area , the hills of Utrecht and Veluwe and in the direct ion of flow towards the sea NaHC03 - water was formed , Geo-electrical survey in the polder "Groot M ijdrecht" 251

VIN«!VUN V..hI lElDEN '.AO ""UOI!C"' ...... "OUX IIH--r- CON1 ...... , "0I,x1l" o w

30

. ,.I r l· \ Ca tHCOJ)J r' \ \ \ km [ \ \ \ \ \ ++ \ \.

Fig. 1 2 Ground-water types of western Holland , E-W cross-section (after GE IRNAERT 1 971).

Fig. 13 Location of the cross-section of Fig. 12 (after GEIRNAERT 1971 252 REINDER H. BOEKELMAN

where the pore water of marine sediments was superseded by fresh water. After the reclamation of 'Groot Mijdrecht ' the salt water was coning up as Fig. 12 shows , thus spl itting the NaH C03 - zone . At the eastern and western borders of the pol der mixed NaCI- and Ca(HC03 ) 2-waters are found . At the west side appeared to be rather narrow , whereas at the east side of the pol der a wide zone with mixed water was observed . These findings also support the re­ sults of the geo-electrical survey .

6. CONCLUSIONS

For inventory investigations concerning sal inizat ion geo-electrical sounding can be very helpful . The results of the survey in 'Groot M ijdrecht ' so far show a strong sal inizat ion in the centre of the pol der, with a sharp rising interface between fresh and brackish/ sal ine groundwater in the east . Also a thick transitional zone is demonstrated . In the west the interface is rising slowly and a shal low transitional zone is found . In the north the situation I ies between those extremes. In order to predict a possible steady state situation , the survey should be repeated . An interest ing subject for further research is to invest igate the composition and origin of the transitional zones e.g. by wel l-logging andus ing isotope techniques.

7. REFERENCES

DAM , J.C. VAN (1960) : Het kwelonderzoek in de Horstermeerpolder. - R ijkswater­ staat j Den Haag . (1964) : Geo-elektrisch onderzoek Randgeb ied Zuid Flevoland . - Rij kswaterstaat j Den Haag .

(1976) : Partial deplet ion of sal ine groundwater by seepage . - J.Hydro l ., 29 , 3/4 : 315 - 339. GEIRNAERT, W. (1971):Het optreden van kath ionenuitwisseling in groundwater. - H 20, §.: 118 - 127. HOORN, W .H.C. TEN (1981): Some calculat ions concerning the fresh water salt water interface in the subsoil - Geol .Jb. , C 29j Hannover. - (This paper) KRUIJK, K.H. (1963) : Groot Mijdrecht , Hydrologisch onderzoek . - Provinciale Water­ staat j Utrecht. LEENEN , J.D. (1978) : Geo-elektrisch onderzoek naar de verzilting van de polder Groot Mijdrecht . - Engin.Thes . Univ.Technol .j Delft. MEINARDI, C.R. (1974) : The origin of brackish groundwater in the lower parts of the Netherlands . - Dutch nat.lnst . Water Supplyj Le idschendam . Geo-electrical survey in the pol der "Groot M ijdrecht" 253

R IJKS GEOLOGISCHE DIENST (1975) : Geologie Provincie Utrecht . - R ijks geol . Dienst ; Haarlem . VOLKER , A. (1961) : Source of brackish groundwater in Pleistocene format ions beneath the Dutch Polderland . - Econ.Geol . WIT, K.E. (1974) : Hydrologisch onderzoek in Midden-West-Nederland . - Inst itute for Land and Water Management Research (I.C.W.), Nota 792 , I.C.W; Wageningen .