Proc. lOth SWIM, Ghent 1988, Natuurwet. Tijdschr. (W. DE BREUCK & L. WALSCHOT, eds) vol. 70, 1988 (published I 989) Ghent: Natuurwetenscbappelijk Tijdschrift 1989 p. 9-29, 14 figs , 5 tab. Hydrochemical evidence of fresh- and salt-water intrusions in the Coastal Aquifer System of the western P.J. STUYFZAND*

ABSTRACT

The spatial distribution of water types, classified according to STUYFZAND (1986a, 1986b), and datings of fresh groundwater have been used, together with geological and hydrological data, to elucidate the hydrological evolution of the coastal area of the Western Netherlands. Freshening, salinizing and equi­ librium water types are discerned in a longitudinal section of 100 km and in two cross sections. Freshening water types are found within the fresh-water lens mainly at three locations :

(a) the central base reflecting its downward growth in the period 1000-1600 A.D., upon the formation of the young dunes; (b) its inland side, mainly due to its continuing extension in that direction, following the reclamation of brackish lakes behind the dunes in the period 1550-1900 A.D.; and (c) elsewhere within and downstream of thick aquitards with high flow resistance, in which chloride inversions may still be present.

Salinizing water types occur mainly on the coastal face of the fresh-water lens due to lateral intrusion and below its central base due to upconing. This salinization is mainly due to excessive ab­ straction for water supply, the exploitation of sand, the creation of harbours and of the North Sea canal, and urbanization. Under the reclaimed lakes, east of the freshening water types, salinizing water occurs, probably in connection with the Holocene transgression and intrusion and upconing due to inten­ sive drainage as well as volumetric compensation for the expansion of the fresh-water lens. The exten­ sion of artificially recharged Rhine and polder waters must be considered as a salinization as well.

Fresh groundwaters have been dated using 3H, 14C, 180 and the breakthrough of recharged Rhine water. In the upper aquifer and aquitard (above 20 m-MSL) ages vary from 0-70 years, in the second aquifer (20-70 m-MSL) from 25 - 400 years. Below an aquiclude at 100 m-MSL, 800 years old dune water was found.

1. INTRODUCTION Many different ideas resulted, of which some of the exponents are listed in table I, together with Many hydrologists have studied the fresh/ salt other (hypothetical) sources of elevated Cl con­ history of the Western Netherlands (a.o. VER­ centrations in groundwater and their relevancy to SLUYS, 1916, 1918, 1931; RIBBIUS, 1925; GEIR­ the coastal district of the Western Netherlands. NAERT, 1973; MEINARDI, 1973, 1983; POMPER, 1977; ENGELEN, 1981, 1986; DE VRIES, 1981; In this contribution the results of recent hydro­ chemical research (STUYFZAND, 1985, 1987, WITT • WIT. 1982; l'viAAS I 1987). 1988a, in prep.) are combined with geological,

•Drs, The Netherlands' Waterworks' Testing and Research Institute KIWA Ltd., P.O. Box 1072, NL-3430 BB Nieuwegein, The Netherlands. 10 P.J. STUYFZAND

Table I. Possible sources of elevated Cl levels in groundwater and those relevant to the investigated area.

sources of elevated relevancy to coastal literature Cl contents groundwater area W. Netherlands

1. sedimentation (connate water) ++ De Sitter (1947) 2. convective transport (a) marine transgression ++ Versluys (1918), Geirnaert (density driven flow) (1973), De Vries (1981) (b) excessive pumping ++ Penninck (1915), Roebert (water supply, polders) (1972) (c) volumetric compensation ++ Engelen (1986) (d) sea spray + Stuyfzand (1985) (e) compaction ? Einsele (1978), Appelo (1977) (f) hydrothermal injection White (1957) (g) anthropogenic pollution + Stuyfzand (1986c)

3. dispersion + Meinardi (1973, 1983)

4. diffusion ? Volker (1961), Ranganathan l Hanor (1987)

5. evapo(transpi) ration + Stuyfzand (1984a)

6. hyperfiltration (geologic membranes) Kharaka • Berry (1973) Graf (1982)

7. cryoconcentration (ion exclusion on ? Pomper (1977) freezing; permafrost)

8. dissolution salt (a) natural deposits (evaporites) Glasbergen (1981) (b) anthropogenic deposits + Van Duyvenbooden l Kooper (waste dumps) (1981) 9. tectonic transport (uplift marine Maas (1987) aquitards)

++ = very relevant; + = relevant; - = not relevant; ? = uncertain.

geomorphological and hydrological information, to doned in the period 1948-1982 due to salt-water elucidate the hydrological evolution of the area in or were safeguarded by artificial recharge with terms of alternating and simultaneous fresh- and surface water from the rivers Rhine and Meuse or salt-water intrusions. from the polders behind the dunes (fig. 1 and table II). The studied area (fig. 1) contains a vast stock of fresh dune water which has been of paramount importance for the regional drinking-water supply since 1853. Many pumping stations were aban- II Fresh- and salt-water intrusions Western Netherlands

Pumping station: • active

0

®octiv~ + artificiaL recharge

Q 10km

D coastaL dun~ area [] younger dune sands rT,lJ.ijl recLaimed lake. E:::::3 beach bo"ier covered with LllTI1J dry in 187' E::3 oLder dune sands

Fig. 1. Location of the coastal dune area of the Western Netherlands, pumping stations for drinking-water supply (see also table II), a longitudinal section and two cross sections. 12 P.J. SWYFZAND

Table II. Data on the pumping stations for drinking-water supply discerned in fig. 1.

pumping station abstraction artificial recharge Cl-conc mg/1 I

before water total 1981 total 1981 onset close down supply 6 3 6 3 cq artif. nr. location comp. since stopped 10 m since 10 m abstraction recharge

I 1 Bergen PWN 1885 - 1,8 - - 31 67 I 2 Egmond a. Zee PWN 1915 1961 0,2 * - - 47 82 3 PWN 1924 - 21,2 1957 17,0 40 48 4 Wijk a. Zee PWN 1885 - 8,0 1975 6,4 38 69 5 IJmuiden-North WLZK 1916 - 0,6 - - 36 280 6 IJmuiden-West SVB 1899 - 0,5 - - 54 a 150 7 IJmuiden-South WLZK 1964 - 1,2 - - 39 d 55 8 Santpoort WLZK 1940 - 1,6 - - 42 42 9 WLZK 1904 - 1,0 - - 31 79 10 Overveen WLZK 1898 - 8,4 1975 1,8 32 43 11 Kraantje Lek WLZK 1964 - 0,9 - - 27 32 b 12 WLZK 1929 - 0,2 - - 29 168 13 Leiduin GW 1853 - 55,7 1957 45,5 30 57 14 Bennebroek PWN 1933 1948 0,7 * - - 55 87 15 Hillegom GWB 1925 1982 1,6 - - 28 191 16 GWN 1919 - 1,1 - - 29 62 17 Katwijk LDM 1878 - 20,5 1940 20,4 ? 33 18 Voorschoten GWVS 1909 1970 0,2 • - - 51 424 19 Wassenaar GWW 1928 1969 2,6 • - - 21 45 20 Scheveningen DWL 1874 - 43,9 1955 45,5 32 55 21 Voorhout GWVH 1929 1956 0,2 * - - 40 373 22 Voorburg DV 1898 1960 2,0 * - - 75 280 23 Monster WDM 1887 - 3,1 1970 3,0 86 c 186 i a = 1957; b = 1962; since then Cl lowered due to reduced abstraction; c = 1926; d = 1962; * = total during best years.

2. GEOLOGY AND HYDROGEOLOGICAL SUBDIVISION

For practical reasons, only Quaternary deposits to in fig. 4. The Holocene Westland Formation is a depth of 200 m-MSL (mean sea level) are consi­ further differentiated in fig. 5. dered. A longitudinal geological section is shown in fig. 2. The hydrogeological subdivision for A concise description of the geological history is the same section is presented in fig. 3. integrated in section 4 dealing with the hydrolo­ gical evolution. The aquifers and aquitards The approximate age and sedimentary environment discerned in fig. 3 are characterized in table III. of the discerned geologic formations is indicated Fresh· and salt-water intrusions Western Netherlands 13

Sou~h North Nonster The Hogue Katwljk Zondvoort 'lmuidr:n Caslricum Bergo:n Cumperduin

20 + + 20 10 + ... 10 MS.L M.S.L 10- -10 20 - - 20 30 - - 30 40 - - 40 50- - so .... 60 - ' .._.---.. _ ,.. """:"\.. - 60 70 - - 70 80 - - 80 90 - - 90 100- - 100 110- F.v. Tegel.en - 110 120- - 120 .,, F.v. Horderw;jk 130- ...... , -130 1.10- ,_ -140 150 - fl't- r ~DWI:: -150 -- m m Q 5/tlrl

Fig. 2. Longitudinal, geological section A (fig. 1), with formations, whose age and sedimentary environment are indicated in fig. 4. W2 = Westland Formation poor in lime; TW = Twente F.; U&S = Urk and Sterksel F.; F.v.Dr. = Drente F.

Soulh North Monster The Hague Kotwl'jk Zondlloorl. 'Jmuiden Costricum 20 + +20 10 + ... 10 MS.L M.S.L 10 - -10 20 - -20 30 - -30 40 - -40 50 - - so 60- -60 70- -70 80- -80 90 - -90 100- - 100 110- - 110 120- -120 130- _....sz._ grwl. -130 111 8 ...... silty 140 - -140 Osond [ -~sand 150 - 1!11111!fJsilty -150 m ~cloy ~peat m 0 51tm

Fig. 3. Longitudinal section A (fig. 1), with the hydrogeological subdivision. The aquifers numbered I-VI and aquitards lA-H, 2A-E, 3A-B, 4 and 5 are characterized in table III. 14 PJ. STUYFZAND

Krll• K,.fl.nlt•y•ll lit~WMUot! (II I O•O'fiDIIIC •.,..Jio N• Nw••.,_-t. Krl• KrllffMIMyol F-ol.ion(IIJ B • ••pofllt. N • • .poal,. of S • d.poai,. ol -IJI•rn rW.taiScMI.d •-It Norl.hf••mon ,;,.,. f~--:''%:)•11Lflclol (cot•p•rlodl I Eom l•fn£•'fllocilfl(-rmporio•J J701h opprDitlmot• flm• B. I' trri' yooral

Fig. 4. Quaternary formations in the coastal area of the Western Netherlands arranged according to age and genesis (based on information in ZAGWIJN • VAN STAALDUINEN, 1975; BREEUWER et al., 1979; ZAGWIJN, 1985).

Sub •toge

Sub-

otltJnfit:

Sub­ boreal

AtltJntlc

1-----+1000 Bor~t

D • Dultltlrlul•patiu OD • Oc•..- flfln• -~~• C • Cal.al• tl.,... lu I • con•. c" y.at• - • lnl•rrupl.•'oll ( wll pita• tlllrlllfl tlull•lar111flli011 NS• Norl./1 S•o d-11•

Fig. 5. Holocene deposits in the coastal area of the Western Netherlands, arranged according to age and sedimentary environment (based on data of JELGERSMA et al. , 1970; ZAGWIJN • VAN STAALDUINEN, 1975; ZAGWIJN, 1986) . Fresh- and salt-water intrusions Western Netherlands 15

Table III. Global characteristics of the aquifers and aquitards discerned in fig. 3. Kh = median horizontal hydraulic conductivity; c = median vertical flow resistance; D = thickness~

AQUIFERS AQUITARDS nr. geol. unit description Kh nr. geol. unit description cv D (figs 3 • 4) (m/d) (figs 3 • 4) (d) Cm)

WestlYD I-III younger dune sand 12 Westland F. : I land OD I-III olHer dune sand 12 lA dune peat 100-15.000 0,05-1 F. NS I-II open marine sand 5-20 IB DI heavy estuarine clay 150.000 2 IC NS 1/II open marine, silty f"me sand 200 3 Twente F. eolian fine cover sand 5 ID CIII? silty clay 5.000 5 II Kr 1 • 2 nuvial sands ? IE CUI Bergen clay (silty) 50.000 12 Eem F. coarse marine sands 40 IF CII-IV, DO-III clay • peat ? ? U l SF. coarse nuvial sands 30 tidal-fiat sandy clay · 1 2 lG rllCl lagoonal clay 20.001l 2 Eem F. coarse marine sands 20-40 basal peat strongly compacted peat 0,4 IliA Drente F. fluvioglacial coarse sands 40 U l SF. coarse fiuvial sands 30 IH Twente F. silty clay ? 3 Enschede F. coarse fluvial sands n. 2A Eem F. marine sandy clay 500-40.000 2-30 Ills Kedichem F. fluvial sands 25 2B Eem F. continental fine sand • peat ? 10 Harderwijk F. coarse fluvial sands 50 2C Drente F. glaciolimnic varved cllly 100.000 20 2D Drente F. boulder clay 5.000 6 IV Tegelen F. fluvial (fine) sands 10-30 2E Kedichem F. loam, sandy clay • peat 100-2.000 1-15 v Tegelen F. fluvial (fine) sands 10-30 3A Enschede F. loam, sandy clay 1.000 6 Maassluis F. marine sands ? 3B Tegelen F. loam, sandy clay • peat 500-2.000 2-10 4 Tegelen F. loam, sandy clay ? 5-10 VI Maassluls F. marine fine sands 5-10 5 Maassluls F. marine sandy clay ? ?

F. = Formation; Kr = Kreftenheye; U • S = Urk and Sterksel. 16 P.J. STUYFZAND

3. HYDROCHEMISTRY

3.1. Classification of water types

In 1986 a hydrochemical classification of water _ most important cation ( Ca J and types was introduced ( STUYFZAND, 1986a, 1986 , /"'•b•YP• anion ( HC~ J, in mttq. C' b), which facilitates the identification of a.o. '--+/----separator cation exchange due to fresh- or salt-water / 11 alkalinity type 1 =1-2 mttq/L intrusion. Recently new options were added to 'Y"f (moderateLy tow J the classification and it was proposed to substi- 1 tute alkalinity for total hardness as the former is ~:=[:-,.,::-:--ChLoride < 0Dmn Ff'l!lsh) man yp• 3 • c' ( less influenced by chlorinity and cation exchange 1...... ------~ (STUYFZAND, 1988b). Fig. 6. The hierarchical structure of the Fig. 6 illustrates that a water type is determined classification system, with four levels of on the basis of : successively, Cl-content, alka­ subdivision, showing the coding of a water linity, the most important cat- and anion (after type in 10 positions.· The. example is called an almost essential preselection of the strongest a fresh calciumbicarbonate water with geohydrochemical cat- and anion family) and (Na (Na + K + Mg)-surplus and moderately low + K + Mg) corrected for sea salt. alkalinity. After S TUYFZAND ( 1988b) •

The subdivision in main types on the basis of Cl has been adapted : the number of Cl-classes is reduced, whereas autochthonous dune water and infiltrated allochthonous surface waters are dif-·

Table IV. Division in main types and subdivision in types on the basis of resp. Cl and alkalinity ( ; HC0 + C0 ). 3 3 main types : chloride types : alkalinity (Alk) main type code Cl mg/1 type code Alk meq/1 fresh dune F < 300 very low * <0,5 fresh Rhine R < 300 low 0 0,5-1 fresh Meuse M < 300 moderately low 1 1-2 fresh polder r < 300 moderate 2 2-4 fresh waste water v < 300 moderately high 3 4-8 brackish B 300 - 10 4 high 4 8-16 salt s 10 4 - 2.104 very high 5 16-32 extremely high 6 32-64 extremely high 7 64-128 F, R, M, r, V : distinction on the basis of extremely high 8 128-256 18 3 natural tracers : 0 a. Cl (R); H • F (M); Br/Cl, extremely high 9 ~ 256 IBotCI or so /Cl (r); and Na/Cl (V) 4 Fresh- and salt-water intrusions Western Netherlands 17

ferentiated within the category "fresh water" during salt-water intrusion. Na, K and Mg do (table IV). not always ad- or desorb simultaneously during The subdivision into types on the basis of alka­ salt- r:spectively fresh-water intrusion. In linity equals the original version for total hard­ MgHC0 water for example, Mg and K are de­ 3 ness (table IV). sorbed, since the more easily desorbable Na has already been exchanged for Ca at an earlier stage The subdivision into subtypes is equal to the (WITT & WIT, 1982; STUYFZAND, 1985). During original version (STUYFZAND, 1986a, 1986b), salt-water intrusion the expelled Ca can "initially" whereas the subdivision into classes equals the drive Mg from the exchange complex (APPELO & second version (STUYFZAND, 1986b). The latter WILLEMSEN, 1987); and during fresh-water is summarized in table V and based on the follow­ intrusion Mg and some K can be adsorbed "initial­ ing empirical cation-exchange reaction (STUYF­ ly" when Na is desorbed at a very high rate ZAND, 1985) (BEEKMAN & APPELO, 1988). These deviations from reaction (1) are quantitatively insufficient to ++ Ca + (cxNa, IlK, (1-112 cx-1/2 ll)Mg)-clay ~ (1) influence the sign of (Na + K + Mg)-corrected, + + H (Ca)-clay + cxNa + IlK + (1-1/2 cx-1/2 ll)Mg which pleads for this classification parameter as an indicator of salinization or freshening. if clay forms the exchanger and with O:scx+a:s2.

Upon fresh-water encroachment, Ca expels the Under ideal circumstances, which prevail in formerly adsorbed Na-, K- and Mg-ions from the coastal aquifers in the North Sea basin, { Na + K exchanger. The above reaction then proceeds + Mg}corr thus determines the sign and in most from the left to the right, causing a significant cases quantifies the (total) cation-exchange reac- {Na+K+Mg} surplus. The reverse reaction occurs

Table. V. Subdivision of subtypes into classes according to (Na + K + Mg) corrected for sea salt. r = sum cations (meq/1); ra = sum anions. After STUYFZAND (1986b).

class code conditions holding for (Na + K + Mg) corr, in meq/1

(Na + K + Mg) deficit ( 1) < -11/2 Cl and < 1,5 (I:k - I:a) (Na + K + Mg) equilibrium (2) ii: -11/2 Cl and :!ii 11/2 Cl and §§ (Na + K + Mg) surplus ( 3 ) + > 11!2 Cl and > 1,5 (l:k - l:a)

( 1) = often pointing at a (former) salt-water intrusion (somewhere). ( 2 ) = mostly indicating sufficient flushing with constant quality water or, according to APPELO & WILLEMSEN (1987), stagnant conditions. ( 3 ) = often pointing at a (former) fresh-water encroachment (somewhere).

( I:k - I: a) §§ = I (Na + K + Mg) corr + ---- • 11/2 Cl I > 1,5 I l:k - l:a I . 1rk - ral (Na + K + Mg) corr = (Na + K + Mg) measured - 1,0716 Cl (meq/1) 18 P.J. STUYFZAND

tion, induced "by a (former) change in the posi­ Longitudinal Section A (fig. 7) tion of the fresh/salt interface. Non-ideal cir­ cumstances are discussed by STUYFZAND (1986a). A relatively thin fresh-water lens in this section running over the younger dunes, occurs at three It should be stressed that the exchange reaction locations. These are : - in a simpler form for the first time described by VERSLUYS (1916; 1931) - could also be fossile (1) the old tidal inlets with or without a narrow and might have taken place upstream of the dune belt near Monster, Katwijk and Cam­ present sampling site (RIBBIUS, 1925). The perduin, upconing of freshening water types into aquifers (2) a stagnant clay-rich zone between Castricum with very low CEC and containing equilibrium and Bergen and water types, may even be a precursor of salini­ (3) the industrial area of IJmuiden with a former zation. Cation-exchange phenomena must there­ heavy groundwater mining and since 1865 a fore be interpretated hydrologically with great strongly draining North Sea Canal. Below care. the fresh-water lens the brackish and salt water exhibit solinizing characteristics.

3.2. Spatial distribution of water types At the bottom of the fresh-water lens the predo­ minant water type is F +, pointing to a fresh­ 3 water encroachment. which preceeded the salini­ The spatial distribution of water types for the zation (see section 4). On top of the refreshe­ period · 1977-1987 is illustrated in the longitudinal ning F + waters, the equilibrium fresh dune section (fig. 7) and in the two cross sections waters \- • and F • are found. Only in those 2 3 (figs 8 and 9). Due to the strong reduction in dunes north of Bergen which are poor in lime, size of fig. 7, the water types had to be simpli­ the (very) low alkalinity dune waters F* -F do 1 fied : the subtypes have been omitted, leading to occur. A great depth of equilibrium dune wa"lers a short code and convenient association. marks the main hydrological recharge areas. This

Soulh Norlh Zon-rl t)mulden Ca.trieum Callll*fluln 20+ + ·20 10 + + 10 H.S.L M.S.L 10 - -10 20 - -20 30- -30 411- -411 50 - -50 60 - -60 70 - -70 80 - -10 90 - - 90 100 - - 100 110 - - 110 120 - - 120 130 - -130 11.0 - -1411 150 - -150 m m ~------~ . ------

Fig. 7. Longitudinal section A (fig. 1). with spatial distribution of water types for the period 1977-1987, based on ca. 500 samples from 100 piezometer tests. The classification outlined in section 3.1 has been stmplified by omission of subtypes. Examples : B 3 - = brackish salinizing water with moderately high alkalinity; F5 + = fresh, .freshening water with very high alkalinity; s2. = salt equilibrium water with moderate alkalinity. 19 Fre~h- and salt-water intrusions Western Netherlands

is especially true for the relatively low alkalinity due to excessive abstraction) and is therefore of F • waters at great depth, pointing at reduced the freshening kind CR +). 2 iJteraction with aquitards (which in general increase the alkalinity (STUYFZAND, 1985)). Cross sections Band C (figs 8 and 9) Artificially recharged allochthonous surface waters The sections B ·and C cross two different regions have penetrated into aquifer II, at least near The with respect to the lime content of the dunes (B Hague, Katwijk, Zandvoort and Castricum (in fig. decalcified), artifical recharge (C), a nearby 1 the pumping stations 20, 17, 13 and 3 resp.). deep polder (C, the reclaimed lake Haarlemmer­ Near Monster (pumping station 23) the artificial meer) and Cl-inversions. basin recharge started later than in the afore mentioned places and no recharged surface water From both sections further conclusions can be has reached aquifer II yet. drawn. Freshening waters are present especially on the Meuse water substituted the Rhine water near The inland side, where the eastward fresh-water flow Hague in 1976 and in 1983. the polder water near has been extended as a consequence of the recla­ Monster. The Rhine water in .aquifer II exhibit­ mation of marshland east of the dunes (fig. 10). ed, in general, salinizing characteristics (R -) The typical zoning of water types from the fresh­ 2 when it displaced dune water low in Na and Cl water intrusion front towards the hinterland, long (near Zandvoort and Castricum). Near The ago freed from salt, is s NaCl+ (often lacking), 3 Hague in aquifer II, the first Rhine water (in B NaCl+, BANaHC0..3 + (only where heavy clay is 4 1955) tends to displace brackish waters (originally involved), F NaHCU +, F MgHC0 +, F CaHC0 + 3 3 3 3 3 3 Dl_,.:e_ """-..,...... lltlen Sc,.,_,.,. : ~ ~:- !: '::,,.._ -;aliB •- mlfB mliB 20•

10 •

8 .§. -NaCL• 6 90- ._, 100-

110-

120-

130-

140-

150- m

Fig. 8. Cross section B (fig. 1) with the spatial distribution of water types for the period 1977-1987 - 20 P.J.STUYFZAND

and finally F CaHC0 . or F.3CaHC0 • Salinizing Chloride inversions occur in section B across the 3 3 brackish watJrs are found m front of the east­ Bergen clay (aquitard 1E) and Eemian clay (aqui­ wardly extending dune system and reflect proba­ tard 2A). In the approx. 5.000 years old, ble salt-water intrusion during the Holocene marine Bergen clay (DE MULDER i BOSCH, 1982; transgression coupled with salinization due to JELGERSMA, 1983) salt connate water is found volumetric compensation for growing fresh-water with Cl up to 15.000 mg/1 (STUYFZAND, in lenses and salinization due to the draining effect prep.). On less stagnant sites under the youn­ of the (deep) polders. ger dunes (fig. 7), a slow freshening of the Bergen clay is taking place. In section C a Intrusion of North Sea water with a Cl content of chloride inversion is present in the glacial clays 16.000 - 17.000 mg/1 can be seen clearly below constituting aquitard 2C and 2D, where a maxi­ the western part of the fresh-water lens in mum Cl-level of 9.000 mg/1 is found (STUYF­ section C (figs 9 and 10). In section B (fig. 8) ZAND, 1988a). this intrusion has proceeded further, probably because an aquitard just below the fresh-water Other Cl-inversions not shown, occur in shallow lens is lacking. Holocene clays adjacent to the dunes, in aquitard 1D near , in aquitard 2E under the glacial In the shallow polders between the dunes and basin of Haarlem and close to the shore, where deep polders (reclaimed lakes), polder water salt water intruded laterally in the period 1900- inilltrates, which contains a high proportion of 1960. Rhine water and admixed dune water or brackish seepage water. -~ --­.,.... - -­... - J!!! JS. Jlli. .m. JIG. .mo. ...ns. 117,1ft, 111 ,. zn u1 JU 'J' JJ 10 + Ul•

N.A.P

10 -

20 -

30-

40 -

50 -

110 -

70 - eo -

90 -

100 - S2 NoCl- 110 -

120 -

130 -

140 -

150 - m ..

Fig. 9. Cross section C (fig. 1) with the spatial distribution of water types for the period 1977-1987. Fresh- and salt-water intrusions Western Netherlands 21

-·· .., .. ,..., ,.,..,., ..~.,. ,.,.., ,..,.,;.....,_ - 1!.!! .!!£ .ill. ~ lK .ill. .ill. M .ill. 4U 111,1ft, 150 l.H 2U 231 lU 107 ua n • ,. 10 + ' Jlol N.AP

10 -

20-

30 - 1:11 f-15881 40- I

50-

60 -

70- eo -

90 -

100 -

110 -

120-

130 -

140 -

150 - m

Fig. 10. Cross section C, with piezometric heads in em +NAP (NAP = 0,15 m + mean sea level), with equipotential lines for fresh water and intruded North Sea water separately. Mean situation 1981 showing the effects of reclamation of the lake .

3.3. Dating (STUYFZAND, 1984b). which corresponds with 1963-1965 atmospheric H-bomb testing. Less than 2 T. U. indicates that water infiltrated before Fresh groundwater has been dated in great detail 1953. in the area south of Zandvoort, using tritium, radiocarbon, oxygen-18 and the breakthrough of artificially recharged Rhine water since 1957. Results are shown for a cross section only 2 km north of SP.ction C, in fig. 11. The dating by means of 14 C (t 1/2 = 5730 years) is suffering from many geochemical complications. These can be circumvented under certain condi­ tions with the approach of PEARSON • HANSHAW 3 With a half life of 12,3 years H can be used (1970), here below 25 m-MSL in aquifer II. In adequately in the upper 25 metres of the aquifer the upper part of this aquifer 14C-levels of 88±2 3 system. In the dunes a H-peak of ·Ca. 100 T.U. pmc prevail, the water is free of 3 H and calcula­ was often found at a depth of 15 m-MSL ted to be 70 years old with a concentration of 22 P.J. S1UYFZAND

D!i- !!£ !!£ i!£~ !!£ !!£ .!!£ II " H IIIJZII QO II -- 117• "'

10 •

1.0-

50-

60-

70 -

110 -

90 -

100 -

I 110 - I I I 1211 - I1 1 :I Rhine water

130 - ~==~ palderwater 1850 1 ~ "' - 125 age (years} fresh-btTJckish. ·+·-· -·-·--- terface (300 mg Cl/ L J 150 - I.S -G71 0fV-SMOW} 1981 in 1850 and 1981 resp. ,.,00./ i sochrane (age • 700 y J ··························

Fig. 11. Age distribution in a cross section over the fresh-water lens south of Zandvoort in 1981, based on 3H, 14 C, lBo and the breakthrough of Rhine water infiltrated since 1957. The fresh/brackish interface around 1850 illustrates a general salinization in the west and freshening in the east.

Total Inorganic Carbon (TIC) of 5-7 mmol/1 TIC = measured TIC (mmol/1); depending mainly on the thickness of aquitard lD TICm = TIC in upper part of aquifer II, i.e. 5-7 (STUYFZAND, 1988a). Higher TIC-levels origina­ mmoY/1. te from interaction with sediments older then 80. 000 years and can now be judged to dilute only, so that Oxygen -18 ( 180)

Am The phreatic level in the dunes south of Zandvoort Age = 70 - 8270 ln {--- [years] dropped several metres, approximately 100 years 88 ago due to abstraction (STUYFZAND, 1988a). ( 2 ) This stopped the winter outflow of surface water where Am = 1'+c activity measured (pmc); from the area by brooks, as well as the open wa- F~>h· and salt·water intrusions Western Netherlands 23

ter evaporation. Consequently, 180-levels drop­ supply accurate datings (STUYFZAND, 1985). ped from -6,5±0,2 V-81\JOW to -7 ,2±0,2 °f 00 °f 00 V-SMOW. This explains at least the 180-pattem Discussion ofresults in fig. 11 shown in fig. 11. lSQ was also used to recognize infiltrated Rhine water (-9,4±0,2 °100 V-SMOW and inflltrated polder water ( -5, 7±0 ;·3 °I 00 V-SMOW). Differences in the position of the .fresh/brackish interface (300 mg Cl/1) between 1850 and 1981 can be ascribed to salinization in the western part due to excessive abstraction for drinking-water Breakthrough ofRhine Water supply in the period 1880-1958, and to freshening in the eastern part after reclamation of the Haar­ lemmermeer Lake in 1852. The deep fresh water The artificial recharge with Rhine water through at 100 m-MSL below heavy glacio-lacustrine clay, spreading basins, started in the dunes south of was cut off from the fresh-water lens probably in Zandvoort in 1957. The breakthrough and spatial the period 1900-1935. It contains the oldest dune distribution of Rhine water in the subsoil give water (800 years or more) detected in this area. therefore a clue of age (STUYFZAND, 1988a). Also years with extreme Cl-levels of Rhine water With the analytical formula of BRAKEL (1968) can be traced back in monitoring wells and thus optimized by BAKKER (1981) and extended to in-

Cllronoatrvti­ Sao lion Wtiells1> • u,., Sub- 1/pflw' Sub- groplly lion ai.IGntie atlantic

5 Time 2.ro' 1.1.10 1.2. ro• 2. ro' • .ro' /ynrs B.PJ P.M.S.L

50-

100- F F F F F F Q.4: 150- .a

1 200-

250- Flow ctmdit1011s salt wulerdepa· odi,.t:in:u. dHpp.rma­ salt water shaiiGw sottllradtish ltnllening anthropogenic u~r lOOm sit1on& IGtion lr•sh lrost & deposition p.rmofrost wut.rdeP9· salt wutet" s.m1stagnont wot.r odilf• ein:u­ and intru­ I frnhrl· siUon and eneroochm.nt situation totion l,.sll sion ning ifltrusion wat.r

fp•rilmorine eontin.ntol ., .. S.L • l'resenf H~fl E F G et e • ad_im~n tory 0 --ll.- ~ N.S. L. D Mposits deposits ,-.,.. S~ UHL • • • .,.,,ronment

E • ,;..aJ.ian F " ftulfiol G • giGeial L. IOfiOOnat ; H • morifle ; TF • tidal 1/.Qt • •stuarin•

Fig. 12. Scheme of the geological and hydrological evolution of the coastal district of the Western Netherlands during the Quaternary period, with emphasis on sedimentary environments and flow conditions in the upper 100 metres. 24 P.J. STUYFZAND j; I elude effects of anisotropy by STUYFZAND given below. I ( 1985), it was calculated that the fresh-water lens of 1850, took approximately 450 years to reach 99% of its (much larger) equilibrium size since the ThePieistocene broadening of the main dune ridge when the younger dunes formed around 1000 A.D. (STUYF­ ZAND, 1988a). The prolonged deposition of clay-rich sediments in a marine and coastal environment during the Upper Tertiary, continued during the Early 4. FRESH- AND SALT-WATER INTRUSIONS Pleistocene. The resulting Oosterhout Formation IN THE QUATERNARY PERIOD and Maassluis Formation contained and probably still contain connate salt water, entrapped under semistagnant conditions. The hydrological evolution of the studied area during the Quaternary period, in terms of alter­ A significant westward shift of the coastline nating and simultaneous fresh- and salt-water occurred during the Upper Tiglian, some 1,8.106 intrusions is shown on a long time scale in fig. 12 years ago. A substantial fluvial complex formed and in detail for the past 6. 000 years in fig. 13. in the subsequent 1,6.106 years, during which A brief description of the discerned stages is fresh water may have circulated actively and

t.100 B.C. 3000B.C. 500 A.D.

0 :A ,\,'lo:UUUUSISS'v. s

F• 8· 50- 50-

100- 8·

J, 150- 150- m m m s ___.....

1600A.D. 1955 A.D. 1987 A .D.

Fig. 13. Scheme of the geological and hydrochemical evolution of the coastal dune area of the Western Netherlands during the past 6000 years. R = artificially recharged Rhine water; r = polder water, mainly composed of Rhine water; F = fresh; B = brackish (300 < Cl < 10000 mg/1}; S = salt; + = freshening; - = salinizing; + nor - = equilibrium. Fresh- and salt-water intrusions Western Netherlands 25

freshened the Upper-Maassluis Formation to some of the younger dunes, which were blown over the extent (APPELO II GEIRNAERT, 1983). During older dune ridges and over the interspaced beach the Saalian glacial a deep permafrost dominated plains rich in peat. Consequently many small the most severe episodes, whereas the fresh hydrological dune systems were united in one circulation continued during less cold episodes. broad dune system, in which the fresh-water lens expanded. This growth took approximately 450 The onset of the Eemian interglacial caused a new years, according to calculations discussed in transgression of the North Sea some 1,3.105 years section 3. 3 and geological evidence from dated ago, first through deeply incised valleys and peat layers reflecting the rise in the water table former glacier basins. Consequently salt water (ZAGWIJN, 1984). was included in the Eemian Formation and invaded the underlying deposits by its higher density. The expanding fresh-water lens generated freshe­ ning water types within the lens and salinization The Weichselian glacial resulted again in a drop of along its borders due to volumetric compensation the sea level (ca. 80 m), in shallow permafrost (fig. 13, 1600 A.D.). and freshening. The Eemian salinization was washed out probably completely in this period, which lasted some 90. 000 years. Only in very The Holocene from 1600 til/1957 A.D. heavy glaciolacustrine varve clay and thick Ee­ milln clay basins some traces could have been preserved. The first important anthropogenic deformations of the fresh-water lens in the dunes occurred upon reclamation of brackish lakes behind the dunes (fig. 1), and locally upon exploitation of dune The Holocene til/1000 A.D. sand. The first caused an eastward expansion of the fresh-water lens not only by lowering the eastern base level with 2-6 metres but also by Sea level rose with the melting of the Weichselian broadening of the inf'lltration area with shallow inland ice. The so-called "Basal peat" was form­ polders bordering the dunes. The lower inland ed in a landward shifting, fresh coastal swamp base level also enhanced the intrusion of North environment. In the early Atlantic (fig. 5) the Sea water. The excavation of dune sand caused area was drowned by a shallow brackish lagoon. a narrowing or lowering of the dunes often in Gradually the area changed into a tidal flat with combination with a more efficient drainage, · so protection from the open sea by a landward that the fresh-water lens shrunk locally. migrating, discontinuous coastal barrier (fig. 13, Groundwater extraction for drinking-water supply 4100 B.C.). Brackish water intruded mainly since 1853 (fig. 1 and table II) increased till 1957 thro11gh deep tidal gullies, sea-water intrusion and caused large scale salinization especially in took advantage of marine erosion of most basal the western part of the fresh-water pocket. The peat and heavy lagoonal clay. Residual deep digging of the North Sea canal (completed in fresh water was forced out of the system or 1876) and its low surface water levels, caused a mixed with the invaded brackish water. Due to a spec::tacular salinization, aided by heavy pumping slowing down of the rise in sea level, an increase for industrial water supply and the extension of in the affluence of sediment or a lowering of the the harbour area (STUYFZAND, 1987). Another tidal range 3000 B.C., coastal barriers were cause of salinization, often overlooked, is the formed now seaward of the former one. This led decrease in recharge by the enormous urbaniza­ to the preservation of several barriers, each with tion of the coastal dune area with its export of its own small fresh-water lens (fig. 13, 500 rainfall by sewers, installed in the 20th century. A.D.}. Changes in the position of the coast-line, vegeta­ tion cover and drainage of adjoining polders were of minor importance (BAKKER, 1981). The Holocene from 1000 tiH 1600 A.D.

Around 1000 A.D., marine erosion resulted in an eastward shift of the coast-line, a steepening of the sea floor and the formation in ca. 200 years 26 P.J. STUYFZAND

Fig. 14. Salinization in the bordering areas of brackish upconings by volumetric compensation, upon close down of deep wells. A: without second aquitard and without lateral salt-water intrusion. B: contrary to A.

The Holocene after 1957 A.D. 5. GENERAL CONCLUSIONS

This period is characterized by a large scale Hydrochemistry is a powerful tool in the study of artificial recharge of the dunes by basins with the hydrology and hydrological evolution of surface water from the river Rhine or Meuse or coastal aquifer systems, in particular by from the polders. The locations are indicated in fig. 1 and described in table II. A general (a) the determination of actual and previous freshening is proceeding there, most spectacular changes in the position of the fresh/salt however in the infiltration area of The Hague due interface, from a snapshot survey of cation­ to a high recharge/abstraction-ratio and a low exchange phenomena; vertical flow resistance (fig. 7, table II). (b) the dating of fresh groundwater by various methods and mapping isochrones; and The artificial recharge and shallow recollection (c) the recognition of different sources of ele­ systems made many deep dune-water abstraction vated Cl-levels. wells superfluous. When these wells were aban­ doned, brackish upconings collapsed clearly The discordant position of freshening on top of, (ENGELEN • ROEBERT, 1974; STUYFZAND, or adjacent to salinizing water types in the du­ 1988a) and - by volumetric compensation - gave nes, indicates how careful cation-exchange pheno­ rise to salinization in the bordering areas (fig. mena must be interpreted hydrologically. This 14). On a larger scale volumetric compensation of situation in the central dune area has been caus­ surface-water expansion in the subsoil caused ed by recent salinization mainly due to abstrac­ some salinization in the bordering areas (STUYF­ tion, after a freshening of the deep aquifers and ZAND, 1985 ; 1988a) . aquitards since the broadening of the dunes 1000 Fn:sb- and salt-water intrusions Western Netherlands

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