Study and Modelling of Saltwater Intrusion into Aquifers. Proceedings 12th Saltwater Intrusion Meeting, Barcelona, Nov. 1992. CIHS. © CIMNE. Barcelona, 1993: 121-142.

GROUNDWATER QUALITY EVOLUTION IN THE BLACK­ SLUICE POLDER AREA AROUND ()

WALRAEVENS, K.*, BOUGHRIBA, M.** & DE BREUCK, W.

University of , Laboratory for Applied Geology and Hydrogeolo­ gy, Krijgslaan 281 - SB, 9000 GENT - BELGIUM.

(*) Research Associate of the National Fund for Scientific Re­ search (Belgium) (**) Present address : University Mohammed 1er, Faculty of Scien­ ces, Department of Earth Sciences, OUJDA-MAROCCO

SUMMARY

About two hundred gro~ndwater analyses have provided an insight into the hydrogeochemical evolution in the Black-Sluice Polder area, related to its salinization/desalinization history, and in the adjacent Woodland, which was protected from the sea by Count John's dike.

The study of hydrochemical profiles allowed to trace both salinization and desalinization phenomena, The older salinization phenomena, that are found in the deeper parts of the groundwater reservoir, can be attributed to the transgression of 1488, when the whole polder area was invaded by the sea. The salinization gradually progressed from the ground surface into the subsoil depth. Before complete equilibrium was reached in the groundwater reservoir, further salinization was stopped as a result of the reclamation of the polders. Fresh water in the shape of precipitation could now enter the subsoil, leadi~g tot desalinization phenomena. The combi­ nation of the salinization and the desalinization phenomena is leading to the groundwater quality actually observed.

Moreover, an important groundwater flow along ~he base of the Quaternary sediments is carrying fresh waters from the Woodland to the polders. This is reflected in the vertical distribution of water types in some borings in the polder area, just outside the "Wood­ land".

Some salinization was established in a part of the "Woodland". This is confirming the results from a geo-electric survey [1]. The salinization in this area is probably due to dike-bursts, which caused the sea water to enter the canals, where Holocene sediments were deposited.

1. INTRODUCTION

The Black-Sluice Polder area is situated in the north of Belgium, near the Dutch border, which forms the northern and eastern boundary limits (fig. 1). It is composed of 11 smaller polders, all bordered by dikes. 122 CHEMICAL ASPECTS

-;:;N

;.~ L. =- \,. . .~~ '.. · N '•' ,•_-

Fig. 1 - Localisation of the study area with indication of borings and investigated profiles. K. Walraevens et a!. 123

The Black-Sluice Polder is bordered to the south by Count John's Dike, separating it from the "Woodland •, "•hich was never flooded by the sea during Holocene times. This "Woodland" shows a somewhat higher topography than the adjacent polder area (more than +4 m TAWI compared to a level in between +2 and +4 m TAW). Besides, the topography of the "Woodland" generally is gently increasing in the southern direction. The villages of Assenede and Boekhoute are both located on the "Woodland's" edge. They are situated on two independent E-W directed Pleistocene sand ridges. In between Boek­ houte and Assenede, i.e. in between these sand ridges, the topogra­ phy of the "Woodland" is relatively lower. To the west, the Black­ Sluice Polder is bounded by the Harbour Dike, separating it from Isabel's Polder.

The final phase of the Dunkirk transgression (DIIIb) reached this area, completely flooding the present polders in 1488. The polders were gradually reclaimed. The last one was reclaimed in 1610: the north-south directed "Smalle Gelanden Polder", connecting the village of Assenede with the . This polder had served as the channel to Assenede-port. The remaining Vliet Brook is today the main drainage canal of the Black-Sluice Polder. After the reclamation, some dike-bursts occurred, causing floods. As a result of such a flood, the creek 'De Grote Kil" was formed in 1808.

2. GEOLOGY OF THE GROUNDWATER RESERVOIR

The geology of the considered groundwater reservoir has been represented by means of several lithostratigraphic cross-sections (fig. 2a up to 6a). The location of these cross-sections is indica­ ted in fig. 1. Profile FF' (fig. 2a) presents the most complete view on the lithostratigraphy.

The base of the considered groundwater reservoir is formed by heavy Eocene clay ( a3 : Member of Onderdijke). The groundwater reservoir is then composed of Oligocene and Quaternary sediments (table 1).

Table 1 - Stratigraphy of the considered groundwater reservoir

Quaternary Holocene PK

Pleistocene KZ

Tertiary Oligocene Member of Watervliet ZK Member of Bassevelde s3 ·- Eocene Member of Onderdijke a3

The Member of Bassevelde (s3) is occurring throughout the whole Black-Sluice Polder, as well as in the northern part of the

TAW Second General Leveling, the Belgian reference level corresponding to the low low sea level along the North Sea coast. F' t:3 ~N s~ """ I I Dutch-Belgian frontier Count John's dike ~ Polder boundary Dike Dike of Dike ··scheurhoekdij15, Maria's polder l "Groenendijk .. ~- I cJek "Grote Kil \Watercourse \Watercourse IJatercourse \Watercourse IJatercourse \Watercourse Leve~" l l I I I I Level !mTA\Ai) GDB10 [m TA\Ai) GDB11 BP30 GOBS - GDB7 GDB6 JP8 GLOf.4 I I I 8~9 I +10 ' ' +10

.t :::· n HOLOCENE ll ., :I: T ~r til v- .. ;.;. 0 ,. ~ 0 J~! . ~ PLEISTOCENE [~ f§ Q ATER-i;; r~ PLEISTOCENE ~ § . ;r.~ ~~~ ) ARY \ ~ ~~~ ~':::~ ------=- - !:"" -10 .- :: 10 - ·.·. .=:·:--; >CZl li\1.~~BER OF - 8~ . MEMBER OF BASSEVELDE --- )JATERVLlET- .------ril !Zk)...- - [53) -- - n -20 /. ------10 >--l .- ... .. CZl .- - MEMBER OF BASSEVELDE: '~ .- - MEMBER OF ONDERDIJKE-ADEGEM [ a3) FORMATION [53) - - -30 OF --- -30 ------.In -40 - --- MEMBER OF ONDER01JKE-ADEGEM (a3) FORMATION -sn OF THE -50 0 500 1000m

~loamy -sandy ~cla~ey ~sandy -loam -loamy -clay upeaty I r-.::-=~,_qsand·...... ·. sand loam sa d clay clay

Fig. 2a- Lithostratigraphy in profile FF'!Al Count John's Dike F F' N s POLDERS -~- "WOODLAND" l0.44 DB10 DB11 P30 DB8 DB7 DB6 P9 P8 P7 mTAW +10

0

?' -10 ~ ; -20 ~ "' ~

-30 ?'-

-40

-50 Legend:

piezometer with indication of filter screen II 500 1000m I ~ base of groundwater reservoir

Fig. 2b- Groundwater quality in profile FF'. t::J Vl A A' i::> 0\ NW SE POLDERS .. DB12 Creek "Rode Geur P13 DB9 DBS .. •2

-2

.. PLEISTOCENE

.. (J

-10 ::r:

-12 ~ _,. 8 -16 (IJ> -18 MEMBER OF BASSEVELDE (s3) til (J -20 ~ ...-02

-26

MEMBER OF ONDERDIJKE(a3) ~- ~"'' c:J£3 """"'-' ~ ..,..,_ ~ ..... , ..,

~ '""''"""' ~ """"'""" [2l2l """""""' Rg. 3a- Uthostratigraphy in profile AA'. rn-

~-· 500m ~~ = A A'

NW SE POLDERS Creek "Rode Geur mTAW .. DB12 P14 P13 P32 P31 DB8 ..

+2 ...... -10 ~ ~12· ~ -14 §"'

-16 Bs6-NaCI· i -18 ~

-20 ~ -22...... Bs7·CaCI·

legend: fig. 3b- Groundwater quality in profile M'. .. piezometer with indication of filter screen ~ • SOOm "\1\1 base of groundwater reservoir B B' I~ w Count John's Dike E "WOODLAND" POLDERS mTAW -~-e ... P44 DB1 Vfiet Brook DBS DB4 ......

..·2 ... .. PLEISTOCENE (] -10

-12 ~ -14 ~ -16 >Cl.l -18 "0 P5 -20 ..., Cl.l MEMBER OF BASSEVELDE (s3) -22

-24

-26 MEMBER OF ONDERDIJKE (a3)

.za ...... -32 CIJ - ~ loam ~ day

-34 c:Ia """"'""" ~ ""'~"~""" ~ oandyday ... ~ oandyloam ~ """"day EJa ""'~"~'""" ~- -38 ~ thdla ... {) --500m Fig. 4a - lithostraligrapiiy in profile BB'. ~- B B' w Count John's Dike E "WOODLAND" POLDERS mTAW -l- Vliet Brook .. P44 DB1 DBS DB4 .. ...

..-2 ... ..

-10 F3-CaHC03+ ('::

-12 ~=r -14 ~::> -16 "' -18 ~ ~ -20

-22

-24

-26

-28

-30

-32

Legend: -36 ft piezometer with indication of filter screen -38 filter screen of mini-piezometer so om Fig_ 4b- Groundwater quality in profile 88'_ ...... , - ~ base of groundwater reservoir t5 c C' w- Wtm ESE 0 POLDERS mTAW DBZ Vliet Brook Creek ·Grote Ki/" ..... 0810 DB3 <2 ...... PLEISTOCENE .. () ·10 ::r: ·12 ~ ·14 I:"'~ ·16 > MEMBER OF WATERVLIET (ZK) [/). ·18 til ·20 () ~ -22 ...... EiZI3 """' ~ loom ~day ... c:::I3 """""""" ~ """''loom ~ =ndydey ~ """'loom ~ loamy day CJ2l"""""""' -28 rn- ~llholls. -30 ~sr-I -32

-34 MEMBER OF BASSEVELDE (s3)

-36

-38

-40 Fig. Sa- Lithostratigraphy in profile CC'. ., SOOm -42 MEMBER OF ONDERDIJKE (a3) c c· WNW ESE POLDERS mTAW .. DBZ Vliet Brook 0810 Creek "Grote Ki/" DB3 .. ""' ..

-4

-4

4

-10 ?' -12 ~ -14

-16

-18 I ~ -20 f". ...-22· ...

legend: -30 i piezometer with indication of filter screen ... filter screen of mini-piezometer

-34 ~ base of groundwater reservoir -36...

-40 soom I~~- -42 ~ Fig. 5-Groundwaterqualityin profile CC'. ,_. N D D' "' N Count John's Dike s POLDERS "WOODLAND" mTAW ~- "" 085 - P45 P46 ......

-2 PLEISTOCENE

-4

-6 (j ::r: t!l -10 s:::

-12 t""~ -14 >CZl -16 til (j -18 Cil MEMBER OF BASSEVELDE (s3) -20

-22 -24 ...... -26 ITi3] _, 1§§3- ~""' -28 CJ:a """".... ESSf(l ""'"'­ ~-'""' ~-,- ~....., , [:]3 ..,., .... -30 .. MEMBER OF ONDERDIJKE (a3) ~- -32 ~ ...... • soom ~~ .... Rg. 6a - lithostratigraphy in profile DO'. D D' N Count John's Dike s POLDERS 'WOODLAND" mTAW -~- ..0-.. DB5 P45 P46 +2 ..

-4 .. .. ?" -10 Bs5·NaCJ· @ -12 ~ -14 ~ -16 a

-10 ~ ...-20 ......

-20 Legendi -30 a piezometer with indication of filter screen -32 filter screen of mini-piezometer

-34 ~ base of groundwater reservoir

Fig. 6b ·Groundwater quality in profile DD'. w -w 134 CHEMICAL ASPECTS

"Woodland". As a result of the north-north-eastern dip of the Tertiary layers, and of the relatively level topography, it is disappearing in the south (fig. 7). For the same reasons, the overlying Member of Watervliet (ZK) is only occurring in the extreme northern part of the area.

The Member of Bassevelde (s3) consists of greyish green glau­ conitic fine sand. The Member of Watervliet (ZK) contains grey glauconitic sandy clay.

The Pleistocene deposits show a succession of mostly sandy and loamy sediments, in which sometimes an intermediate loamy layer in between two sandy layers may be obs·erved. The base of the Quater­ nary deposits is mostly relatively coarse.

In the polder area, the top layer with a thickness of about 0, 5 m, is constituted by Holocene brown loamy clay, the "polder clay". These Holocene deposits are absent in the "Woodland".

In the larger part of the area, the Pleistocene sediments are constituting one aquifer together with the Member of Bassevelde (s3). But in the north, where the less pervious Member of Watervliet is separating both layers, two aquifers can be distinguished. In the polder area, the Holocene top layer is less pervious.

3. GROUNDWATER FLOW

The groundwater flow in profile FF' has been studied by means of a mathematical model (fig. 8; [4]). In the same paper, also the hydraulic heads on the level -Z m TAW have been published (fig. 9).

This study has shown the fresh-water flow from the "Woodland" to the polder area to be mostly concentrated in the most permeable layer : the base of the Quaternary sediments. In the polder area shallow water cycles exist. These are related to very small variati­ ons in topographic elevation. For example, in profile FF', the main infiltration zones within the polder area are situated near to boring P30, and in between borings DBll and DBlO. The fresh water infiltrating in the polder area does not reach the base of the reservoir; the larger part is flowing horizontally through the most pervious layer in the direction of the drainage canals, where an important upward flow has developed. Below the base of the Quaterna­ ry sediments, the groundwater flow is strongly reduced.

4. HYDROGEOCHEMISTRY

4.1. Groundwater analyses

Numerous groundwater samples have been collected in the study area. They were mostly originating from ordinary piezometers, with a filter screen length of 1 m. These samples were takEn with a peri­ staltic pump.

In borings DBl, DB2, DB3, DB4 and DB5, also ~.lini-piezometers had been installed. These consist of very small diameter flexible K. Walraevens et a!. 135

. ' 0 "\' ,_,~· ~~-'--'~..__._, ::-_L __ , f"·

Fig. 7 - Geological map of the top of the Tertiary deposits. 136 CHEMICAL ASPECTS

N s

F POLDERS twoODlAND mT.AW

-10

-20

-30

500 1DDDm·

Fig. 8 - Calculated groundwater flow and quality distribution in profile FFT4J. K. Walraevens et a!. 137

Legend: line of equal fresh-water head in mTAW __ H _ (equidistance: 0,2m) on the level of -2mTAW 0 ' (measurements 9·10·11th April 1984)

" location of piezometer

I /

Fig. 9 -Fresh-water heads on the level of -2mTAW[4J, 138 CHEMICAL ASPECTS

tubes, attached at the outside of the ordinary piezometer· Every mini-piezometer is equipped with a filter screen of 0,10 m length. The arrangement of one mini-piezometer for every meter of increasing depth allows to obtain a very detailed view of the vertical distri­ bution of the groundwater quality. Sampling of the groundwater out of these mini-piezometers is performed by means of a bicycle-pump with reversed piston. This tool is producing a vacuum in a connected sampling bottle, that on its turn is connected to the flexible tube of the mini-piezometer. Groundwater from the mini-piezometer is thus sucked up into the bottle.

The groundwater analyses have been classified according to STUYFZAND [2).

4.2. Hydrogeochemical processes

Salinization and desalinization processes have determined the actual groundwater quality in the Black-Sluice polder area • The groundwater reservoir was subjected to salinization as a result of the Dunkirk III b-transgression. Subsequently, upon the reclamation of the polders followed desalinization. Later, dike-bursts may have caused a local and limited new salinization.

Salinization starts with a rising concentration, due to the admixture of seawater, which is quickly followed by cation exchange on the clay surfaces, liberating ca2•-ions, and adsorbing marine cations (Na+, K+, Mg2+). The succession of groundwater types resul­ ting from salinization can be schematized as follows :

F-CaHC03¢ ~ Fb-CaHC03¢ ~ B-CaHC03- ~ B-CaMix- ~ B6 -CaCl- ~ B.-NaCl­ ~S-NaCl- ~s-NaCl¢

When groundwater flow is vertical and downward, these succee­ ding watertypes will gradually penetrate deeper, while the most saline water qualities will be found near the ground surface.

Desalinization starts with dilution, due to admixture of fresh water, which is quickly followed by cation exchange, where marine cations are desorbed and ca2•-ions are adsorbed by the clay surfaces [ 3).

Desalinization can occur in conditions where salinization has been completed. The succession of groundwater types can then be generalized as follows : S-NaCl¢ ~ B.-NaCl¢ ~ B.-NaCl+ ~ B.-NaMix+ ~ B.-NaHC03t ~ B-NaBC03+ ~ Fb-NaHC03+ ~ Fb-MgHC03+ ~ F-MgHC0 3+ ~ F-CaHC03+ ~ F-CaHC03¢ However, when salinization was not completed yet, the succes­ sion of groundwater types resulting from desalinization may be different. For example, when the cation-exchange complex had not yet been restored in equilibrium during the salinization, desalinization will start from S-NaCl- :

S-NaCl- ~ B8 -NaCl- ~ B.-NaCl¢ ~ B6 -NaCl+ ~ B6 -NaMix+ ~ B6 -NaHC03 + ~ B-NaHC03+ ~ Fb-MgHC03+ ~ F-CaHC03+ ~ F-CaHC03¢ When only the B.-CaCl--type was reached during salinization, desalinization will cause the following succession to occur :

B.-caCl- ~ B.-CaCl¢ ~ B6 -CaCl+ ~ B-CaCl+ ~ B-CaHC03+ ~ Fb-CaHC0 3 + K. Walraevens et al. 139

As such, we can encounter the successive groundwater types representing desalinization on top of the succession representing a previous (completed or not) salinization (in the case of vertical and downward groundwater flow).

4.3. Hydrogeochemical profiles

The hydrogeochemical profiles have been represented in fig. 2b to 6b. The hardness codes in the watertype names, reported on the profiles, are just indicative. Indeed, this characteristic presents somewhat more variation than the other parts do.

4.3;1. Profile FF' (fig. 2b)

This north-south cross-section passes from the polders to the "Woodland". It is parallel to the fresh-water flow from the 'Wood­ land' to the polder area.

The groundwater quality in the "Woodland' is characterized by the F-CaHC03-type, which is often on the top passing to the F-CaMix­ type with increased sulphate concentration (pyrite oxidation in unsaturated zone).

On the boundary between the 'Woodland' and the polders, a typical fresh/salt-water interface is observed. The important fresh­ water flow from the 'Woodland" to the polders along the highly permeable base of the Quaternary deposits, is shown in DB7, where in this layer fresh water occurs below the fresh-brackish type.

In the polder area, fresh to saline groundwater occurs. Gene­ rally, the salinity increases with depth. The upper fresh water layer is thicker in the infiltration areas of the shallow water cycles. In the outflow areas to canals and creeks and in the areas of low flow velocities, relatively high salt concentrations may be preserved in the Quaternary sediments. This Qua ternary layer is predominantly subjected to desalinization. But in DBB, only an upper thin layer shows desalinization phenomena. This is due to the location near a drainage canal, where a considerable upward flow from the underlying Tertiary sediments exists. This causes brackish water to reach the canal.

The Tertiary layers (s3 and ZK) mostly bear the S7-NaCl­ -ground watertype. The groundwater flow in these layers is strongly reduced. Therefore, the old marine water is prese::ved below the shallow water cycles. This salt groundwater still bears witness to the salinization that was provoked by the Dunkirk III b-transgressi­ on. This salinization has not been completed in this part of the reservoir the cation-exchange equilibrium has not yet been reached.

In DB7, the salinization in the s3-layer is even more incom­ plete : the groundwater type is S7-CaCl-. This borin3 is situated in the polders, but very close t.o Count John's Dike. The marine influ­ ence in this place will have been smaller compared to the remaining polder area. Also the adjacent boring DB6, situated just behind 140 CHEMICAL ASPECTS

Count John's Dike, is showing the CaCl-waterquality in s3. But here, the salt content is much lower, as was expected for a locality within the "Woodland".

4.3.2. Profile AA' (fig. 3b)

This profile has a NW-SE orientation. It is situated in the polders. It is approximately parallel to Count John's Dike. In its central part, it crosses the Vliet Brook.

The groundwater quality in the Quaternary sediments is fresh to saline. The shallow water cycles, characteristic for the polder area, can be clearly recognized. The upconing of salt water under the drainage canals is obvious. Especially the important Vliet Brook exerts a large influence. The desalinization is more pronounced in the infiltration areas of the. shallow water cycles.

The Tertiary sediments are predominantly filled with ground­

water of the S6-NaCl--type. But in DB12, the B8 7-CaCl--type has been observed, giving evidence from an incompleted salinization. This can be ascribed to the presence of the clayey Member of Watervliet (ZK), retarding the salinization of the underlying sediments. In the other borings of the cross-section, ZK is not present.

4.3.3. Profile BB' (fig. 4b)

This west-east profile starts in the "Woodland", and crosses Count John's Dike. At first, the polders are crossed parallel and very close to Count John's Dike (near to DB!). This part of the profile is perpendicular to the fresh-water flow from the "Woodland" to the polders. Further on, the profile is intersecting the Vliet Brook, and is continuing within the polder area.

The groundwater quality recognized in the 'Woodland" belongs to the F-CaHC03t-type. The transition from "Woodland' to polders corresponds to a fresh/salt-water interfa.ce.

In DB!, we notice the fresh-water flow from the "Woodland" to the polders, normally along the permeable base of the Quaternary sediments, but participating in the upward flow to the Vliet Brook.

In the polder area, the salinity of the groundwater in· the Qua ternary sediments is highly variable, as a result of different measures of desalinization. In the Tertiary sediments, the marine influence has largely remained unaffected.

4.3.4. Profile CC' (fig. 5b)

This WNW-ESE profile is situated in the polder area. It is crossing the Vliet Brook and the creek "De Grote Kil". The eastern half of the profile is dominated by this creek.

Throughout the whole cross-section, the clayey Member of Watervliet (ZK) is present at the top of the Tertiary sediments.

Both in DB2 and DBlO, the normal succession of groundwater types in the Quaternary sediments is observed from fresh to K. Walraevens et a!. 141

brackish-salt and salt groundwater. But in DB3, the F2-CaHC0 3+-type is followed in depth by B0 7-CaC1--groundwater. The local presence of this watertype, representing incomplete salinization, in the Quater­ nary sediments, above the Member of Watervliet (ZK), is likely to be due to the flood of 1808, when the creek "De Grote Kil' was formed. In the meantime, desalinization has converted the groundwater of the upper 10 m into fresh water again.

In the Tertiary deposits, both in the Member of Watervliet (ZK) and the Member of Bassevelde (s3), the groundwater belongs to the S7-NaCl-type, reflecting the older salinization phase (Dunkirk IIIb).

4.3.5. Profile DD' (fig. 6b)

This north-south profile extends from the polders to the 'Woodland". It is parallel to the fresh-water flow from the 'Wood­ land" to the polder area. This fresh-water flow, (F-CaHC03+) is clearly observed in DB5, where it is splitting up the F-NAHC03+­ waterquality. On a greater depth, the groundwater in the Quaternary

sediments becomes more saline (up to B0 -NaCl-). Also the Tertiary Member of Watervliet (ZK) has been salinized (B.-NaCl-). The situa­ tion in DB5 is as could be expected for a location in the polder area.

Borings P45 and P46 are situated in the "Woodland". We would expect no salinization • However, brackish-salt and brackish ground­ water are observed respectively. These observations are confirming the results of a geo-electric survey' [1], establishing salinization in that certain part of the 'Woodland'. An explanation is handed by HEYSE (pers. comm.) the geomorphologic mapping of this area has proven the presence of Holocene deposits in a gully system, in which the seawater would have entered the 'Woodland". Indeed, the two borings are situated in a relatively low part of the "Woodland', in between the Pleistocene sand ridges of Boekhoute and Assenede.

5. CONCLUSION

The groundwater quality in the Black-Sluice Polder has been mainly determined by the marine Dunkirk IIIb-transgression, causing the salinization of the polder area. This marine influence has largely remained unaffected in the Tertiary sediments.

The Quaternary sediments have further been subjected to de­ salinization, following on the reclamation of the polders. The groundwater flow pattern in this new situation consists of the fresh-water flow from the "Woodland' to the polders, and of shallow water cycles in the polder area. Both are essentially limited in depth by the very pervious base of the Quaternary sediments. As a result, the marine water has been preserved in the Tertiary depo­ sits.

The study of several cross-sections has confirmed that salinization conditions still prevail in the Tertiary layers. In the Quaternary sediments, mostly desalinization phenomena are met with. The desalinization has progressed deeper in the recharge areas of the shallow water cycles. However, in the areas of upward flow to 142 CHEMICAL ASPECTS

creeks and drainage canals, salinization conditions may persist at some depth in the Quaternary deposits, and brackish water may reach the canals.

The fresh-water flow from the "Woodland" to the polders, was observed in the profiles crossing both "Woodland" and polder area. It is causing a desalinization.

The B. 7-CaCl--groundwatertype, characteristic for an incom­ plete salinization, was encountered in two different conditions. In profile AA' it was observed in the Tertiary Member of Bassevelde ( s3), below the clayey Member of Watervliet ( ZK), the presence of which was responsable for the retardation of the salinization of s3. In profile CC', the same watertype is present in the Quaternary sediments, above the Member of Watervliet, in the area of the creek "De Grote Kil". The flood of 1808, causing the development of this creek, has locally led to a recent limited salinization phase, explaining the presence of the B.7-CaCl--water.

In the "Woodland", normally no salinization is observed. An exception is represented in profile DD', where in the relatively low region between the Pleistocene sand ridges of Boeklwute and Assene­ de, salinization phenomena are met with. These are associated with a Holocene gully system, through which the marine influence could penetrate into the "Woodland".

ACKNOWLEDGEMENTS

This study was supported by the National Fund for Scientific Research (Belgium), by the Institute for Scientific Research in Industry and Agriculture (Belgium), by the Belgian Geological Service and by the Belgian Administration for Development Cooperation.

REFERENCES

1. BOUGHRIBA, M. (1992). La selinisetion du syst~me aquif~re Oligo­ Pl~istoc~ne dens le r~gion de Boekhoute et d'Assenede (Belgique). 248 p. Ghent : State University (Doctor's thesis).

2. STUYFZAND, P.J. (1986). A new hydrochemical cll'.ssification of watertypes Principles and application to the coastal dunes aquifer system of the Netherlands. Proceedings of the 9th Salt­ Water Intrusion Meeting, Delft, 641-655.

3. WALRAEVENS, K. (1987). Hydrogeologie en hydrochemie van het Ledo­ Peniseliaen in Oost- en West-Vlaenderen, 350 p., 74 pl., 102 fig., 4 ann. Ghent : State University (Doctor's thesis).

4. WALRAEVENS, K., LEBBE, L. & PEDE, K. (1988). Hydrogeological SWIM-excursion to the Black-Sluice Polder area in the Flemish Valley of Belgium. lOth Salt-We ter Intrusion Meeting, Ghent, 1988. Netuurwetenscheppelijk Tijdschrift 70, 376-595.