Flow and Distribution of Fresh and Salt Water in the Flemish Coastal Plain: a Comparative Study Between Saint-Pierre-Brouck (France) and Mannekensvere (Belgium)

FACULTY OF SCIENCES Master of Science in geology

Flow and distribution of fresh and salt water in the Flemish coastal plain: a comparative study between Saint-Pierre-Brouck (France) and Mannekensvere (Belgium)

Pieter Winters

Academic year 2013–2014

Master thesis submitted in partial fulfillment of the requirements for the degree of Master in Science in Geology

Promotor: Prof. Dr. L. Lebbe Tutor: G.J. Devriese Jury: Prof. Dr. S. Louwye, Mr. D. Vandevelde

SUMMARY

Historically, the coastal zones around the world have always been the most heavily populated regions; mainly due to the abundance of food and the ability to execute economic activities. The increase in population and the economic growth will ensure that these regions will come under even more pressure in the (near) future. On top of this, coastal aquifers are often very sensitive to salinization, with possible salt damage to crops and unsuitable surface water for irrigation as a result. The projected climate change, and the associated sea level rise, will only enhance this effect. To be able to offset these problems in the future, it is vital that we understand the processes involved in the salinization of coastal aquifers down to the smallest detail. It is self-evident that these salinity problems are not limited by national borders. Therefore it was chosen to perform a comparative hydrogeological study in two different areas, in two different countries. For that reason two density dependent groundwater flow models were formed. These models were created using the numerical code MOCDENS3D. This code incorporates two important and commonly used modules: (1) the three-dimensional computer code MOC3D which was adapted for density differences, and (2) MODFLOW-96, in which the groundwater flow calculations were performed. Two areas were chosen that exhibit strong similarities in terms of genesis on the one hand and in the field of hydraulic parameters on the other hand. The final choices were: (1) the Belgian village Mannekensvere, a district of the town Middelkerke, and (2) the French village Saint-Pierre-Brouck, located in the department called Nord (part of this second study area is located in the adjacent department, namely Pas-de-Calais). Both areas are located in the Flemish coastal plain, this region is a part of the North Sea coastal plain, stretching from Cap Blanc Nez in northern France to Skagen in Denmark. The coastal plain is a very complex and dynamic region that has formed through the many interactions between ocean and landscape forming processes throughout the Holocene. Although one was previously convinced that this Holocene infilling was only dependent on changes in sea level, nowadays one follows the idea that the complexity of the Holocene infilling, which has given shape to the coastal plain, is due to the following factors: (1) the geomorphological conditions and local topography of the base of the Holocene, (2) changes in sea level, by which the rate of sea level variations is also intended, (3) the supply of new sediment, (4) the changing influence of the various tidal channels, (5) the compaction of the deposited sediments and (6) the human influence. In the formation and the comparison between the two groundwater flow models, of Mannekensvere and Saint-

Pierre-Brouck, the focus was mainly on the fresh-to-salt interactions and the flow patterns of groundwater reservoir.

The first study in Mannekensvere, located on the Belgian side of the border, is a polder situated near the IJzer river. The IJzer river is the main river in the study area, and also the only navigable watercourse within the investigated area. By weirs, the IJzer river is shielded from possible tidal influences by the North Sea. The area is located at a distance of about 6 km from the coast. The formation of the numerical groundwater model of Mannekensvere went fairly smoothly. This is mainly because the different data sets that contain information about the subsurface of Flanders are collected centrally and what's more, they are freely accessible. The data sets that were used for the preparation of the groundwater model of Mannekensvere include, among others: (1) the literature, (2) different drilling reports, (3) the profile type map, (4) the base and permeability of the polder deposits, (5) the HCOV units in the area, (6) the salinity map (7) the Flemish Hydrographic Atlas and (8) a very detailed topographical map. Clear insights can be obtained from the groundwater and solute transport model that was formed. The most striking observation is the strong dependence of groundwater flow and solute transport in relation to the different rivers. First, there is the only navigable watercourse (the IJzer river). Since the water level (which is kept constant through the weirs) in the river is much higher than the groundwater level in the nearby groundwater reservoir, water will infiltrate from the IJzer river towards the groundwater reservoir. Since the water in the IJzer river is fresh water, there will be a locally thicker fresh water lens compared with the rest of the study area which is not characterized by infiltration. The opposite situation occurs at locations where unnavigable watercourses are present. In these unnavigable watercourses, the water level is systematically lower than the water table of the surrounding groundwater reservoir. Therefore, opposite to an infiltration process, a drainage process will take place towards the navigable watercourses. Since the groundwater reservoir is characterized by a high content of salt water from a certain depth, the unnavigable watercourses will attract salt water after a time. This induces a (strong) salinization of the water of the unnavigable watercourses. The hydraulic parameters of the substrate, and then mainly in the upper layers of the groundwater model, play the most important role in these interactions towards the rivers. This is explained by the fact that in areas where the soil is characterized by sediments with higher hydraulic permeability, the drainage towards the watercourses is much more effective. Because of the higher hydraulic conductivity of the subsurface, water will be able to flow much easier in the direction of the river, so that an

equilibrium will take place much faster between the local groundwater level and the water level in the river. This will result in a decrease of the head difference between the river and the reservoir, and thus less visible effects.

The second study, in the village of Saint-Pierre-Brouck, is located on the French side of the border. The study area is located about 17 km inland, between the cities of Calais and Dunkerque. The polder area is traversed by a navigable river, namely the river Aa. At present, this watercourse is strongly channelized. Within the study area a second navigable watercourse is present, namely the "Canal de Calais à Saint-Omer”. This watercourse is a branch of the river Aa. The coastal deposits that occur in this area correspond to a large extent with the deposits found in the neighbouring regions in Belgium. The formation of the groundwater model of this second study area went less smoothly than in the first study. The reason is that fewer freely available data about the subsurface of northern France was available. For this study following datasets were used: (1) literature, (2) drilling reports and (3) a topographical map of a relatively low resolution. In order to obtain the hydraulic parameters of the surface, a field study was performed. In this field study, the following measurements were performed: a pumping test, groundwater level and quality measurements and an EM39 survey. The results of these measurements were used subsequently in the formation and optimization of the numerical groundwater model. The most important test that was used to get an insight into the hydraulic parameters of the study was the pumping test. Through the interpretation of the response data from the pumping test, with the aid of the inverse numerical model HYDPARIDEN (or 'Hydraulic Parameter Identification'), it is possible to deduce the optimal values of the hydraulic parameters. It also returns the joint confidence interval calculated for each hydraulic parameter(group), as well as the deviation between the measured and calculated drawdowns. Resulting from the other measurements, the groundwater quality and the EM39 measurements, the main conclusion is that the studied groundwater is fresh water. Important to note is that this situation applies only to the sites where the field measurements were carried out. It is thus a difficult situation to draw conclusions about the entire area, with only some information that is based on the results of the field study. Due to the lack of data, which has been partially resolved by the execution of the field study, assumptions were made to circumvent these shortcomings. The main assumptions that have been made to build the model are: (1) the assumption that the lithological layers run horizontally across the study area and (2) the calculation of the initial groundwater level, the location of the fresh-to-salt transition zone and the permeability of the

III first two layers of the model directly from the topography. Additionally, certain characteristics from the groundwater model of Mannekensvere were adopted, examples are: the average sea level, the properties of the unnavigable watercourses, the drainage properties and the properties of the groundwater recharge. Even with these assumptions, an efficient groundwater model could be formed from the study area in Saint-Pierre-Brouck. From this model, some interesting conclusions can be drawn. The main observation is that the groundwater flow and solute transport in the model of Saint-Pierre-Brouck is dominated by the topography. Also, less infiltration and drainage processes were observed within the model. The principal groundwater flow process is a regional groundwater flow process from higher elevated areas toward lower areas, characterized by a high level of lateral flow. The strong association between the topography was explained by: (1) the assumptions that have been made in which many of the properties are directly depend on the topography and (2) the fact that the variation in topography over the study area is many times larger than the variation within the study area of Mannekensvere. However, because the hydraulic permeability of the substrate is relatively large, the influence of the rivers on the groundwater model is less visible. The situation over the entire model domain can be compared with the zones in the model of Mannekensvere where a high hydraulic conductivity was determined. The situation regarding the transition zone between fresh and salt water is totally different in the groundwater model of Saint-Pierre-Brouck. In contrast to the model of Mannekensvere, the major part of the groundwater reservoir consists of fresh groundwater. In these areas, a fresh water lens is present which is as thick or even thicker than the thickness of the model. Only two zones were observed where salt water is present; more specifically the two lowest zones within the study area. The difference can be explained on the basis of the law of GHYBEN- HERZBERG, where the variations in groundwater heads play an important role.

Both the groundwater models that were made serve as excellent models to get an initial understanding into the groundwater system. However, they should not be regarded as the final step in a detailed groundwater investigation. In a next stage, the results of these groundwater models can be used to search for the optimal location to perform further field based research. A summary was given as to which further research would provide added value in the case of both models. For the groundwater model of Mannekensvere the next logical step would be to validate the results by conducting a field survey. Depending on the objectives of the further investigation, the optimal location can be based on the results of the groundwater model. Due to the strong dependence of the rivers, it would be interesting to investigate this dependency

or to re-evaluate the parameters of the various streams in the field. For the groundwater model of Saint-Pierre-Brouck, three directions were indicated for further investigation. Firstly, the assumptions that were made to circumvent the lack of data could be analysed statistically. Herewith, the assumptions made to define the initial groundwater head and fresh-salt transition zone are intended. Secondly, a second field study could be carried out. From the results of the groundwater model, better locations could be chosen. Interesting places are particular the areas where salt water is present, in other words the lower regions of the study area. Thirdly, the values that were adopted from the groundwater model of Mannekensvere could be evaluated and if necessary adjusted for the groundwater model of Saint-Pierre- Brouck. Among these acquired values, we intend the average sea level, the characteristics of watercourses, the drainage properties and the properties that characterize the groundwater recharge.

SAMENVATTING

Historisch gezien zijn kustgebieden over de hele wereld altijd al de drukst bewoonde gebieden geweest; voornamelijk door de overvloed aan voedsel en de mogelijkheid om economische activiteiten uit te oefenen. De stijging van het bevolkingsaantal en de economische groei zullen er bovendien voor zorgen dat deze regio’s nog meer onder druk komen te staan in de (nabije) toekomst. Hier bovenop komt het feit dat grondwaterlagen die gelegen zijn aan de kust vaak zeer gevoelig zijn voor verzilting, met mogelijke zoutschade aan gewassen en ongeschikt oppervlaktewater voor irrigatie tot gevolg. De voorspelde klimaatsverandering, en de hiermee gepaard gaande zeespiegelstijging, zal dit effect alleen nog maar versterken. Om in de toekomst gerichte oplossingen te kunnen vormen tegen deze problemen, is het van primordiaal belang dat men de processen die betrokken zijn bij de verzilting van grondwaterlagen aan de kust tot in het kleinste detail begrijpt. Het is vanzelfsprekend dat de verziltingsproblemen in kustgebieden niet begrensd worden door landsgrenzen. Daarom werd er gekozen om een vergelijkende hydrogeologische studie uit te voeren in twee studiegebieden, in twee verschillende landen. Om tot een vergelijkende studie te komen werden er twee dichtheidsafhankelijke grondwaterstromingsmodellen gevormd. Deze modellen werden gemaakt met de numerieke code MOCDENS3D. Deze code integreert twee belangrijke en vaak gebruikte modules: (1) de driedimensionale computercode MOC3D die werd aangepast voor densiteitsverschillen, en (2) MODFLOW-96, waarin de berekeningen betreffende de grondwaterstromingen werden uitgevoerd. Er werden twee gebieden gekozen die sterke gelijkenissen vertonen op gebied van genese enerzijds en op gebied van hydraulische parameters anderzijds. De uiteindelijke keuze ging naar: (1) het Belgische dorpje Mannekensvere, een deelgemeente van de gemeente Middelkerke, en (2) het Franse dorpje Saint-Pierre-Brouck, gelegen in het departement Nord (een deel van dit tweede studiegebied ligt in het aangrenzende departement, namelijk Pas-de-Calais). Beide gebieden zijn gelegen in de Vlaamse kustvlakte, deze regio maakt deel uit van de Noordzee kustvlakte die zich uitstrekt van Cap Blanc Nez in het noorden van Frankrijk tot Skagen in Denemarken. Deze kustvlakte is een zeer complexe en dynamische regio die zich heeft gevormd door de vele interacties tussen oceaan- en landschapsvormende processen doorheen het Holoceen. Hoewel men er vroeger van overtuigd was dat deze opvulling enkel afhankelijk was van veranderingen in de zeespiegel, volgt men tegenwoordig het idee dat de complexiteit van de Holocene opvulling, die deze kustvlakte vorm heeft gegeven, te wijten is aan de volgende verschillende factoren: (1) de geomorfologische situatie en de lokale topografie van de basis

VII van het Holoceen, (2) de evolutie van de zeespiegel, waarmee men ook de snelheid van de zeespiegelvariaties bedoelt, (3) de aanvoer van nieuwe sedimenten, (4) de veranderende invloed van de verschillende getijdenkanalen, (5) de compactering van het afgezet sediment en (6) de invloed die rechtstreeks door de mens is veroorzaakt. Bij de vorming en de vergelijking tussen de twee grondwaterstromingsmodellen, van Mannekensvere en Saint- Pierre-Brouck, lag de focus vooral op de zoet-zout interacties en de stromingspatronen van het grondwater.

Het eerste studiegebied, in Mannekensvere, bevindt zich aan de Belgische kant van de grens en is gelegen in een polder nabij de IJzer. De IJzer is de voornaamste rivier in het studiegebied, en bovendien de enige navigeerbare waterloop binnen het onderzochte gebied. Door een sluizencomplex wordt de IJzer afgeschermd van de mogelijke getijden invloeden die de rivier zou kunnen ondervinden van de Noordzee. Het gebied ligt op een afstand van ongeveer 6 km van de kust. De vorming van het numerieke grondwater model van Mannekensvere verliep redelijk vlot. Dit komt vooral omdat de verschillende datasets die informatie bevatten over de ondergrond van Vlaanderen centraal verzameld worden, en daarenboven ook nog vrij toegankelijk zijn. De datasets die gebruikt werden voor het opstellen van het grondwater model van Mannekensvere zijn onder andere: (1) de literatuur, (2) verschillende boorverslagen, (3) de profieltypekaart, (4) de basis en doorlaatbaarheid van de polder afzettingen, (5) de HCOV eenheden in het gebied, (6) de verziltingskaart, (7) de Vlaamse Hydrografische Atlas en (8) een zeer gedetailleerde topografische kaart. Heldere inzichten kunnen bekomen worden uit het grondwater- en opgeloste stoffen transportmodel dat gevormd werd van Mannekensvere. De meest opvallende observatie is de sterke afhankelijkheid van de grondwaterstroming en het transport van opgeloste stoffen ten opzichte van de verschillende rivieren. Ten eerste is er de enige bevaarbare waterloop (de IJzer). Doordat het waterniveau, dat constant gehouden wordt door het sluizencomplex, in de IJzer een stuk hoger gelegen is dan de grondwaterstand in het nabijgelegen grondwaterreservoir, zal er water vanuit de IJzer in het grondwaterreservoir kunnen infiltreren. Doordat het water in de IJzer zoet water is, zal er hierdoor een plaatselijk dikkere zoetwaterlens ontstaan vergeleken met de rest van het studiegebied dat niet gekenmerkt wordt door infiltratie. De tegenovergestelde situatie vindt plaats op de locaties waar niet bevaarbare waterlopen aanwezig zijn. In deze, niet bevaarbare, waterlopen bevindt het waterniveau zich systematisch lager dan de grondwaterstand van het omringende grondwaterreservoir. Hierdoor zal er in plaats van een infiltratieproces, een drainageproces plaatsvinden richting de

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onbevaarbare waterlopen. Omdat het grondwaterreservoir vanaf een bepaalde diepte gekenmerkt wordt door een hoog gehalte aan zout water, zullen de onbevaarbare waterlopen na een tijd vooral zout water aantrekken. Dit zorgt voor een (sterke) verzilting van het water van de onbevaarbare waterlopen. De hydraulische parameters van de ondergrond, en dan voornamelijk in de bovenste lagen van het grondwater model, spelen de belangrijkste rol in deze interacties met de rivieren. Dit wordt verklaard door het feit dat in de zones waar de ondergrond wordt gekenmerkt door sedimenten met een hogere hydraulische doorlaatbaarheid, de afvoer door de waterwegen veel effectiever verloopt. Door de hogere hydraulische doorlatendheid zal het grondwater veel gemakkelijker stromen in de richting van de rivieren, waardoor er veel sneller een evenwicht zal plaatsvinden tussen de plaatselijke grondwaterstand van het grondwaterreservoir ten opzichte van het waterniveau in de rivieren. Dit zal resulteren in de afname van het verschil in stijghoogte tussen de rivier en het reservoir, en dus minder zichtbare effecten.

Het tweede studiegebied, in de gemeente Saint-Pierre-Brouck, is gelegen aan de Franse kant van de grens. Het studiegebied bevindt zich ongeveer 17 km landinwaarts, tussen de steden Calais en Dunkerque. Het poldergebied wordt er doorkruist door één bevaarbare rivier, met name de Aa. Op dit moment is deze waterloop sterk gekanaliseerd. Binnen het studiegebied bevindt er zich verder nog één bevaarbare waterloop namelijk het ‘Canal de Calais à Saint- Omer’. Dit is een zijtak van de Aa. De kustafzettingen die in het gebied voorkomen corresponderen in grote mate met de afzettingen die aangetroffen wordt in de aangrenzende Belgische gebieden. De vorming van een grondwater model bij dit tweede studiegebied verliep minder vlot dan bij het eerste studiegebied. De reden hiervoor is dat er voor het noorden van Frankrijk veel minder vrij toegankelijke data voorhanden is. Voor deze studie kon er gebruik gemaakt worden van de volgende datasets: (1) literatuur, (2) boorverslagen en (3) een topografische kaart van een relatief lage resolutie. Om toch een goed beeld te krijgen van de hydraulische parameters van de ondergrond, werd er geopteerd om een veldstudie uit te voeren. Binnen deze veldstudie werden de volgende metingen uitgevoerd: een pompproef, grondwaterstand- en kwaliteitsmetingen en een EM39 survey. De resultaten van deze metingen werden gebruikt in de verdere vorming en optimalisatie van het numerieke grondwater model. De belangrijkste test om een beter inzicht te krijgen in de hydraulische parameters van het studiegebied was de pompproef. Door het interpreteren van de responsgegevens van de pompproef, met behulp van het invers numerieke model HYDPARIDEN (of ‘Hydraulic Parameter Identification’), is het mogelijk om de optimale

IX waarden van de hydraulische parameters af te leiden. Hierbij wordt ook het gezamenlijk betrouwbaarheidsinterval berekend voor elke hydraulische parameter(groep), alsook de afwijking tussen de gemeten en berekende verlaging. Uit de overige metingen, de grondwaterkwaliteitsmetingen en de EM39 metingen, was de voornaamste conclusie dat het onderzochte grondwater zoet water is. Belangrijk om op te merken is dat deze situatie enkel geldt voor de locaties waar de veldmetingen werden uitgevoerd. Het is dus een moeilijke situatie om conclusies te trekken over het gehele gebied, met enkel informatie die gebaseerd is op de veldstudie. Door het gebrek aan gegevens, dat deels opgelost werd door het uitvoeren van de veldstudie, werden er ook nog aannames gemaakt om de tekortkomingen te omzeilen. De belangrijkste aannames die werden genomen om het model te bouwen zijn: (1) de veronderstelling dat de lithologische lagen horizontaal lopen over het studiegebied en (2) het berekenen van de initiële grondwaterstand, de ligging van de zoet-zout overgangszone en de doorlatendheid van de eerste twee lagen van het model rechtstreeks uit de topografie. Vervolgens werden er ook bepaalde eigenschappen overgenomen uit het grondwater model van Mannekensvere, voorbeelden zijn: het gemiddeld zeeniveau, de eigenschappen van de onbevaarbare waterlopen, de drainage-eigenschappen en de eigenschappen van de aanvulling van het grondwaterreservoir. Zelfs met deze aannames kon een goed functionerend grondwater model gevormd worden van Saint-Pierre-Brouck. Uit dit model kunnen een aantal interessante conclusies getrokken worden. De grondwaterstroming en het transport van opgeloste stoffen in het model van Saint-Pierre-Brouck wordt voornamelijk gedomineerd door de topografie. Er werden ook minder infiltratie- en drainageprocessen waargenomen binnen het model. Het voornaamste proces is een regionale grondwaterstroming van hoger gelegen gebieden in de richting van lager gelegen gebieden, gekenmerkt door een hoger gehalte aan laterale stroming. De sterke associatie tot de topografie werd verklaard door: (1) de aannames die gemaakt werden waarbij veel eigenschappen direct afhankelijk zijn van de topografie en (2) het feit dat de variatie in de topografie over studiegebied vele malen groter is dan de variatie binnen het studiegebied van Mannekensvere. Echter, doordat de hydraulische doorlatendheid van de ondergrond relatief groot is, is de invloed van rivieren op het grondwater model minder zichtbaar. De situatie over het gehele modelgebied kan vergeleken worden met de zones in het model van Mannekensvere waar een hoge hydraulische doorlatendheid werd vastgesteld. De situatie omtrent de transitiezone tussen zoet en zout water is ook totaal anders binnen het grondwater model van Saint-Pierre-Brouck. In tegenstelling tot het model van Mannekensvere, bestaat het grootste deel van het grondwater reservoir uit zoet grondwater. In deze gebieden vormt er zich een zoetwaterlens die even dik

of zelfs dikker is dan de dikte van het model. Er worden slechts twee gebieden waargenomen waar zout water aanwezig is; meer specifiek zijn dit twee lager gelegen zones binnen het studiegebied. Het verschil is vooral te verklaren aan de hand van de wet van GHYBEN- HERZBERG, waarbij de variaties in de grondwaterstand een belangrijke rol spelen.

Beide gevormde grondwater modellen fungeren als uitstekende modellen om een eerste inzicht te krijgen in het grondwater systeem. Ze moeten echter niet beschouwd worden als de finale stap in een gedetailleerd grondwater onderzoek. In een volgend stadium kunnen de resultaten van deze grondwater modellen gebruikt worden om optimale locaties te vinden voor verder onderzoek. Een duiding werd gegeven welk verder onderzoek een meerwaarde zou bieden in het geval van beide modellen. Voor het grondwater model van Mannekensvere zou de meest logische volgende stap zijn om de resultaten van het grondwater model te valideren door het uitvoeren van een veldonderzoek. Afhankelijk van de doelstellingen van het verder onderzoek, kan de optimale locatie gebaseerd worden op de resultaten van het grondwater model. Door de sterke afhankelijkheid ten opzichte van de rivieren zou het bijvoorbeeld interessant kunnen zijn om deze afhankelijkheid verder te onderzoeken, of de parameters van de verschillende waterlopen in het veld te re-evalueren. Bij het grondwater model van Saint-Pierre-Brouck werden er drie richtingen voor verder onderzoek geïndiceerd. Ten eerste zouden de aannames, die gemaakt werden om de datatekorten op te vangen, statistisch onderzocht kunnen worden. Hierbij worden de aannames bedoeld die gebruikt werden om de initiële grondwaterstand en de zoet-zout overgangszone te definiëren. Ten tweede zou er een tweede veldonderzoek uitgevoerd kunnen worden. Vanuit de resultaten van het grondwater model zouden dan betere locaties gekozen kunnen worden. Interessante plaatsen zijn vooral de zones waar zout water aanwezig is, met andere woorden de lager gelegen regio’s in het studiegebied. Ten derde zouden de overgenomen waarden uit het grondwater model van Mannekensvere geëvalueerd en indien nodig aangepast kunnen worden voor het grondwater model van Saint-Pierre-Brouck. Onder deze overgenomen waarden verstaan we: het gemiddeld zeeniveau, de eigenschappen van de waterlopen, de drainage- eigenschappen en de eigenschappen die de grondwater aanvulling karakteriseren.

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PREFACE

Completing this Master’s thesis would not have been possible without the support of many. Hereby I thank the University of Ghent, and in particular the Research Unit of Groundwater Modelling, for the admission to their facilities and the authorization to use their apparatuses. I would also like to thank Jasper Claus for making the use of his Python-scripts available. Subsequently, I would like to thank Professor Luc Lebbe. Not only for being my promoter, by which he is also responsible for the realization of this work, but also for the many hours of his time which he has invested in my cause. Hereby he did not only share his expertise on the field (and beyond), he also brightened up the lengthy car drives with the many fascinating stories that he has in his sleeve. Finally, but foremost I thank Gert-Jan Devriese for his immaculate guidance and assistance, both in the field and during every step in the realization of this work.

Pieter Winters June 2014

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TABLE OF CONTENTS

1 General introduction ...... 1

2 Geological and hydrogeological setting: the Flemish coastal plain ...... 5

2.1 Pre-Holocene ...... 6

2.2 Genesis and evolution during the Holocene ...... 6 2.2.1 Old approach ...... 7 2.2.2 New approach ...... 10 2.2.3 Comparison between the two approaches ...... 14

3 Theoretical background ...... 17

3.1 Introduction to density dependent groundwater flow modelling ...... 18 3.1.1 Groundwater flow and solute transport ...... 18 3.1.2 Theoretical considerations ...... 19 3.1.3 MOCDENS3D ...... 22

3.2 Introduction to Hydraulic Parameter Identification for pumping tests ...... 24 3.2.1 Schematization of the numerical model ...... 25 3.2.2 Initial hydraulic parameters ...... 26 3.2.3 Optimal hydraulic parameters ...... 27 3.2.4 Interpretation based on the joint confidence region ...... 28

4 Preliminary research: Mannekensvere (BE) ...... 31

4.1 Site description ...... 32

4.2 Groundwater model ...... 33 4.2.1 Model setup ...... 33 4.2.2 Results of the numerical model...... 43

5 Main research: Saint-Pierre-Brouck (FR) ...... 51

5.1 Site description ...... 52 5.1.1 General ...... 52 5.1.2 Geology in Saint-Pierre-Brouck ...... 53

5.2 Field installation ...... 54 5.2.1 Research design ...... 54 5.2.2 Placing piezometers ...... 55 5.2.3 Positioning and levelling of the piezometers ...... 59

5.3 Field measurements ...... 60 5.3.1 EM39 ...... 60 5.3.2 Groundwater head ...... 65 5.3.3 Groundwater quality ...... 68

5.4 Pumping test ...... 73 5.4.1 Lay-out and implementation ...... 73 5.4.2 Subdivision of the subsurface ...... 74 5.4.3 Initial setup of the numerical model, and model calibration ...... 77 5.4.4 Results ...... 78 5.4.5 Interpretation ...... 84

5.5 Groundwater model ...... 87 5.5.1 Model setup ...... 87 5.5.2 Results of the numerical model ...... 97

6 Discussion ...... 107

6.1 Comparison of two numerical models ...... 108

6.2 Future research ...... 110

7 Conclusion ...... 113

8 References ...... 117

9 Annexes ...... 127

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LIST OF FIGURES

Figure 1: Curves illustrating depth under actual mean sea level versus the time before present (BP), after TERS (1973) and JELGERSMA (1979) (source: MRANI-ALAOUI, 2006)...... 8

Figure 2: Relative sea level curve for the study area with indication of the average rate of sea level rise (DENYS & BAETEMAN, 1995). (MHW: Mean High Water; MTL: Mean Tide Level) ...... 11

Figure 4: Illustration of two piezometers, one filled with fresh water and the other with saline water, both open to the same point in the aquifer (source: GUO & LANGEVIN, 2002)...... 21

Figure 5: Representation of the numerical model. R1 is the initial radius and A is a factor which is larger than 1 (source: LEBBE, 1999)...... 26

Figure 6: Joint confidence region for parameter(group)s hp and hp2. ith h p and h p 2 the optimal value, Fa.Sm the marginal confidence intervals, Fa.Sc the conditional confidence intervals, α the eigenvalues of the covariance matrix and β the orientation of the ellipsoid (source: LEBBE, 1999)...... 29

Figure 8: Localisation of the study area, the corners are indicated with the Lambert72 coordinates (source: Google earth/own research)...... 34

Figure 9: Grid of the groundwater model along with the topography of the study area (source: personal communication, G. DEVRIESE/own research)...... 34

Figure 10: Indication of the rivers in the study area, with the different categories of the rivers represented with different thicknesses (source: Google earth/own research)...... 36

Figure 11: Profile type map of the study area (source: own research)...... 37

XVII

Figure 12: Illustration of the recharge flow rate (q) in m³/d, and the recharge concentration in % salinity (source: own research)...... 38

Figure 13: Vertical cross-section through the groundwater model along the X-direction (y = 201000 m Lambert72) (source: personal communication, G. DEVRIESE)...... 39

Figure 14: Vertical cross-section through the groundwater model along the Y-direction (x = 41500 m Lambert72) (source: personal communication, G. DEVRIESE)...... 39

Figure 15: Horizontal hydraulic conductivity of the first layer in the groundwater model (source: own research)...... 40

Figure 16: Salinity map of DE BREUCK et al. (1974) at the study area (source: DE BREUCK et al., 1974/Databank Ondergrond Vlaanderen/own research)...... 42

Figure 18: Horizontal cross-section through layer 1 of the model, showing fresh water heads and isosurfaces (source: own research)...... 47

Figure 19: Vertical cross-section along row 100 of the model, parallel with the X-direction, showing fresh water heads and isosurfaces (source: own research)...... 47

Figure 20: Vertical cross-section along column 80 of the model, parallel with the Y-direction, showing fresh water heads and isosurfaces (source: own research)...... 48

Figure 21: Horizontal cross-section through layer 1 of the model, showing salinity percentage and isosurfaces (source: own research)...... 49

Figure 22: Horizontal cross-section through layer 2 of the model, showing salinity percentage and isosurfaces (source: own research)...... 50

Figure 23: Vertical cross-section along row 100 of the model, parallel with the X-direction, showing salinity percentage and isosurfaces (source: own research)...... 50

Figure 24: Vertical cross-section along column 80 of the model, parallel with the Y-direction, showing salinity percentage and isosurfaces (source: own research)...... 50

XVIII

Figure 26: Location of the placed piezometers within Saint-Pierre-Brouck (sources: Google earth/Google maps/own research)...... 55

Figure 27: Lithological summary of hand/flush drillings (source: own research). (IGN69: reference level for mainland France, 0 m IGN69 = 1,694 m TAW)...... 58

Figure 28: Field setup of the EM39 electromagnetic induction tool (source: VAN MEIR & LEBBE, 2003)...... 61

Figure 29: EM39 log of the different boreholes (source: own research)...... 64

Figure 30: Observed groundwater head and temperature values at piezometer 1A (source: own research)...... 67

Figure 31: Observed groundwater head values in piezometer 1A, and documented precipitation values from Bergues (source: own research/French Wunderground)...... 67

Figure 32: Piper diagram of the groundwater samples...... 70

Figure 33: Graphs showing the relationship between the electrical conductivity of the groundwater in function of the solute content...... 72

Figure 34: Location and lay-out of the pumping test (sources: Google earth/own research). (PP: pumping pit, MW: monitoring well)...... 74

Figure 35: Lithological description of the subsurface along with the subdivision of the groundwater reservoir in the numerical model (source: own research). (BGL: Below Ground Level)...... 76

Figure 36: Schematization of the different layers of the model, with the optimal values of the hydraulic parameters (D(i):thickness ; K(i):horizontal hydraulic conductivity ; C(i):hydraulic resistance ; SA(i):Ss(i):specific elastic storage ; S0:specific yield) (source: own research). ...81

Figure 37: Measured (crosses) and calculated (full line) drawdown in function of time and distance for layer 8 and 10 (source: own research)...... 82

XIX

Figure 39: Localisation of the study area, the locations of the piezometers are shown with red dots. An indication of the Lambert93 coordinates is shown at the lower left corner of the study area (source: Google earth/own research)...... 88

Figure 40: Grid of the groundwater model along with the topography of the study area (source: Institut National De L’Information Géographique et Forestière/own research)...... 88

Figure 42: Illustration of the horizontal hydraulic conductivity of the first and the second layer of the groundwater model (source: own research)...... 92

Figure 43: Vertical cross-section through the groundwater reservoir (source: own research). 94

Figure 44: Illustration of the initial concentration put into the model, here layer 3 is shown (source: own research)...... 96

Figure 45: Image of the study area; the red lines indicate the locations of the vertical cross- sections (y = 60th row; x = 20th and 130th column) (source: Google earth/own research). ... 98

Figure 46: Horizontal cross-section through layer 1 of the model, showing fresh water heads and isosurfaces (source: own research)...... 100

Figure 47: Vertical cross-section along row 60 of the model, parallel with the X-direction, showing fresh water heads and isosurfaces (source: own research)...... 100

Figure 48: Vertical cross-section along column 20 of the model, parallel with the Y-direction, showing fresh water heads and isosurfaces (source: own research)...... 101

Figure 49: Vertical cross-section along column 130 of the model, parallel with the Y- direction, showing fresh water heads and isosurfaces (source: own research)...... 101

Figure 50: Horizontal cross-section through layer 1 of the model, showing salinity percentage and isosurfaces (source: own research)...... 102

Figure 51: Vertical cross-section along row 60 of the model, parallel with the X-direction, showing salinity percentage and isosurfaces (source: own research)...... 103

Figure 52: Vertical cross-section along column 20 of the model, parallel with the Y-direction, showing salinity percentage and isosurfaces (source: own research)...... 103

Figure 53: Vertical cross-section along column 130 of the model, parallel with the Y- direction, showing salinity percentage and isosurfaces (source: own research)...... 103

Figure 54: Horizontal cross-section through layer 1 of the model, showing salinity percentage and isosurfaces (source: own research)...... 105

Figure 55: Vertical cross-section along row 60 of the model, parallel with the X-direction, showing salinity percentage and isosurfaces (source: own research)...... 105

Figure 56: Vertical cross-section along column 20 of the model, parallel with the Y-direction, showing salinity percentage and isosurfaces (source: own research)...... 106

Figure 57: Vertical cross-section along column 130 of the model, parallel with the Y- direction, showing salinity percentage and isosurfaces (source: own research)...... 106

XXI

LIST OF TABLES

Table 1: Summary of the transgression phases of Calais and Dunkerque, after HOUTHUYS et al. (1993) and JELGERSMA (1979) (source: MRANI-ALAOUI, 2006)...... 7

Table 2: Hydraulic resistivity towards the drainage system along with the maximal infiltration speed and the salinity percentage of the infiltrating water for the different profile types in the coastal and polder area (source: LEBBE et al., 2006)...... 37

Table 3: HCOV units which are present in the study area (source: MEYUS et al., 2000)...... 39

Table 4: Coordinates of the top of the piezometers (source: own research). (IGN69: reference level for mainland France, BGL: Below Ground Level)...... 59

Table 5: Filter intervals (source: own research). (AGL: Above Ground Level, BGL: Below Ground Level)...... 59

Table 6: Classification of fresh-to-salt water, based on TDS content (source: DE MOOR & DE BREUCK, 1969)...... 63

Table 7: Chemical analysis of the groundwater samples...... 68

Table 8: STUYFZAND classification of the different groundwater samples...... 70

Table 9: Initial estimated values of the hydraulic parameters. The different parameter groups are put in a coloured frame (source: own research)...... 78

Table 10: Summary of the output of the model, giving the reliability and the dependence of the various parameter groups (source: own research)...... 83

XXII

LIST OF ANNEXES

ANNEX 1: Description of drillings near Mannekensvere ...... 128

ANNEX 2: Drilling descriptions near Saint-Pierre-Brouck ...... 137

ANNEX 3: Output of the pumping test (part of LST-file) ...... 144

XXIII

XXIV

1 GENERAL INTRODUCTION

Chapter 1

Around the world, coastal areas have always been the most inhabited regions because of the abundance of food and the ability to run economic activities. In the future, these regions will renounce even more due to population rise and economic growth. On the other hand, coastal aquifers are often very sensitive to salinization. Possible effects of salinization are salt damage to crops and unfit surface water for irrigation. The predicted climate change and associated sea level rise will only magnify these effects to coastal aquifers (OUDE ESSINK, 200 ). This Master’s dissertation revolves around this burning issue. In order to find mitigating solutions to these problems, one must understand the processes that are involved in the salinization of coastal aquifers.

Since salinization problems of coastal areas are not bounded by borders, a comparative hydrogeological study is conducted between two regions in neighbouring countries. This trans-boundary study was executed by forming two density-dependent groundwater flow models. Two regions with high similarities in the genesis and the characteristics of the various deposits of the coastal plain were selected. For the first region, the Belgian village of Mannekensvere was chosen as a representative. In Flanders, data sets of the subsurface are centrally organized and maintained and in addition, these data are freely accessible, which made the formation of a groundwater model fairly straightforward. The data sets that were used are, among others: (1) literature, (2) drilling reports, (3) the profile type map, (4) the base and permeability of the polder deposits, (5) the base of the HCOV units (Hydrogeological Coding of the Subsurface in Flanders or ‘Hydrogeologische Codering van de Ondergrond Vlaanderen’ in Dutch), (6) the salinity map, the Flemish Hydrographic Atlas and (8) a highly detailed topographical map. The formation of a groundwater model in the second region on the other hand, chosen in the French village called Saint-Pierre-Brouck, was less straightforward. Here, no well-organized central database with easy-to-access data was available. We had to make do with some basic data: (1) literature, (2) drilling reports from borings in the neighbourhood of Saint-Pierre-Brouck and (3) a topographic map of a lower resolution. To get an idea of the subsurface properties, it was opted to conduct a field study, consisting of a pumping test, groundwater head and quality measurements and an EM39 survey. In this comparison, the main focus lies on the fresh-salt water interactions and the groundwater flow patterns.

This work consists of 4 main parts. The first part addresses the overall background of the

subject. The literature research into the study area, the Flemish coastal plain, is discussed (chapter 2), followed by an introduction into the theory behind density dependent

Chapter 2

groundwater flow modelling and hydraulic parameter identification of pumping tests (chapter 3). A second part describes the preliminary study to form the groundwater model of Mannekensvere (chapter 4). This chapter offers a clear description of the research area, followed by the setup and the results of the groundwater model of Mannekensvere. The third and principal part of this Master’s dissertation encompasses the main research to form the groundwater model of Saint-Pierre-Brouck (chapter 5). The following facets are discussed: the site description, the field installation, the field measurements, the pumping test and finally the groundwater model. A fourth and final part discusses the results from both numerical models (chapter 6).

Chapter 3

Chapter 4

2 GEOLOGICAL AND HYDROGEOLOGICAL SETTING:

THE FLEMISH COASTAL PLAIN

Chapter 5

In the following chapter, the geology of the Flemish coastal plain is discussed. This overview is primarily based on literature. The focus lies on the coastal region of northern France and western Belgium, more precisely within the valleys of two rivers: (1) the Aa in northern France and (2) the IJzer in western Belgium. These two regions show high similarities in the genesis and the characteristics of the various deposits of the coastal plain (MRANI-ALAOUI & ANTHONY, 2011).

2.1 Pre-Holocene Since the aim of this study is to characterize the groundwater in the phreatic aquifer, we concentrated the research to the Holocene deposits. However, it is important to have an idea of the distribution of the pre-Holocene substratum because this forms the lower boundary of the phreatic aquifer and thus of the groundwater model that was built.

The Ieper group, formed during the Ypersian (Paleogene), is the lowermost unit used in the pumping test of Saint-Pierre-Brouck and the groundwater models of Saint-Pierre-Brouck and Mannekensvere. It consists out of thick blue-green heavy clay and is therefore ideal as the aquitard, bounding the model. On top of this formation, sediments of the Pleistocene were deposited. Nevertheless, in the vicinity of the study area of Saint-Pierre-Brouck only a thin package of approximately 2 to 4m was deposited. The reason for this is the severe erosion during the glacial periods (sometimes up to the level of the Paleogene substrates). In northern France, the Pleistocene erosional basement dips strongly towards the actual coastline; cores and stratigraphic sections show a relatively shallow basement inland, and increasing sediment accommodation space towards the sea. The topography of this Pleistocene basement is characterized by deep cuts, corresponding with river valleys in the hinterland (MRANI- ALAOUI & ANTHONY, 2011; VAN DER WOUDE & ROELEVELD, 1985).

2.2 Genesis and evolution during the Holocene Two different approaches to the genesis and evolution of Holocene deposits have been described in the literature; these approaches will further be referred to as the ‘old’ and the ‘new’ approach. The oldest approach, proposed initially by DUBOIS ( 924), is based on the description of two stratigraphic units, the Calais and Dunkerque formations. These formations, which correspond with similar named transgressive cycles (see Table 1), are separated by a layer of 1 to 2 m of peat (called the surface peat) formed during a phase of stability and coastal progradation. The more recent approach ‘of the Belgian school’ is based

2 on the interpretation of the sedimentary environment, combined with the information of

Chapter 6

radiocarbon dating and the data from pollen records. Here, the speed of sea level rise is the only parameter that is connected to the sedimentary development of the coastal plain (MRANI-ALAOUI, 2006). What follows, is a discussion of these two approaches.

Table 1: Summary of the transgression phases of Calais and Dunkerque, after HOUTHUYS et al. (1993) and JELGERSMA (1979) (source: MRANI-ALAOUI, 2006).

Climatic Transgression Epoch Time BP1 Formation Sequence Phase 1150 BP - present Dunkerque III day Sub-Atlantic Formation of Dunkerque II 1700 - 1350 BP Dunkerque Dunkerque I 2550 - 2050 BP Dunkerque 0 Holocene 3450 - 2950 BP Sub-Boreal Surface peat Calais IV 4550 - 3750 BP Calais III 5250 - 4750 BP Formation of Atlantic Calais II 6250 - 5250 BP Calais Calais I 7950 - 6450 BP

2.2.1 Old approach The start of the Holocene corresponds with the end of the last glacial period (Weichsel glacial); this is the start of a global warming, which is associated with a rise in sea level. According to SOMME (1975), the melting of ice at the end of this glacial stage, had the consequence of rapid sea level rise leading to the current maritime plains and estuaries, hereby shaping the Picardy plain, Boulogne coast and also the Flemish coastal plain (MRANI-ALAOUI, 2006). This transgression, which goes on until the present day, is called the Flandrian transgression. A detailed description of this sea level rise in northern France has been detailed by two authors - TERS (1973) and JELGERSMA (1979) - and the results are summarized in Figure 1. The two curves show the sea level rise during the Holocene, yet their general appearance is different (MRANI-ALAOUI, 2006):

 TERS (1973): the first interpretation, which is based on information from the French coast, describes multiple oscillations of the sea level. However, these fluctuations could be caused by the large spread in the data that were used to plot the curve.  JELGERSMA (1979): the second interpretation, based on data from the coastal plain of the Netherlands and the Picardy maritime plain, can be described by a linear gradient within certain time periods. Overall, this linear curve matches a decreasing

1 BP: Before Present.

Chapter 7

rate of sea level rise. The sea level rise is approximately 1cm/year for the period of 10000 to 8000 BP and decreases gradually to 1,5 mm/year between 5000 and 3000 BP.

Figure 1: Curves illustrating depth under actual mean sea level versus the time before present (BP), after TERS (1973) and JELGERSMA (1979) (source: MRANI-ALAOUI, 2006). The first formation on top of the Pleistocene basement is the formation of Calais. At this time, fully marine conditions existed in the sea branches of the Aa and the IJzer (VAN DER WOUDE & ROELEVELD, 1985). The deposits of the Calais formation date from the Boreal until the Sub-Boreal (8000-4000 BP). The formation corresponds to deposits from successive transgressive phases (Calais I, II, III and IV). The formation is present over the total area of the coastal plain, with the exception of a few older beach ridges (e.g. dunes of Ghyvelde), over the total area of the coastal plain (HOUTHUYS, DE MOOR & SOMME, 1993). The formation is composed out of gray-blue tidal sandy sediments – called “sable pissard” – with some clay lenses observable in the upper parts (BAETEMAN, 1999; MRANI-ALAOUI, 2006). Where the Pleistocene basement was deeply incised, a thicker package is deposited.

On top of the formation of Calais, the so-called surface peat developed. A significant reduction of marine influence, generated at the seaside of the system, has been regarded as the

primary factor involved in the development of this surface peat (VAN DER WOUDE & ROELEVELD, 1985). This reduction of marine influence was due to the development of a

Chapter 8

beach-barrier complex. Starting in the late-Atlantic period, peat developed landwards of this border zone. Due to the reduction of marine activity it could spread seawards; thus converting the sea branches into fresh water areas (VAN DER WOUDE & ROELEVELD, 1985). Authors do not agree on whether the formation of peat occurred during a fall in sea level or during a period with a stable sea level (BAETEMAN, 1999). The base of the peat is dated at 3500-3400 BP (SOMME, 1979), with the highest deposition during the Sub-Boreal. It ceased to form at the beginning of the Sub-Atlantic. Nowadays, it is positioned at shallow depths (1 to 2 m BGL2) and generally does not exceed a thickness of 3 m (MRANI-ALAOUI, 2006; VAN DER WOUDE & ROELEVELD, 1985). The formation has a complex composition due to the periodic entering of the sea. Different kinds of peat can be differentiated within the surface peat: (1) reed marches, followed by (2) an open swamp forest and, lastly (3) peat moss at some places (VAN DER WOUDE & ROELEVELD, 1985; VAN HOUTTE, LEBBE & DE BREUCK, 1992).

On top of the surface peat (where present) and the formation of Calais, the formation of Dunkerque is deposited. It is the last deposition within the coastal plain, formed during four transgressive phases of unequal importance (Dunkerque 0, I, II and III) (SOMME, 1975). The deposition took place during the Sub-Boreal and the Sub-Atlantic. The second phase of the Dunkerque transgression is regarded as the most important event in the formation and development of the coastal plain (BAETEMAN, 1999). The sediments are composed out of sands, sandy-loams and marine clays. Compared to the formation of Calais, the deposits are sandier (MRANI-ALAOUI, 2006; VAN DER WOUDE & ROELEVELD, 1985). The clays were primarily deposited on mudflats while sand settled into incised creeks, whereby parts of the peat could have been eroded (BAETEMAN, 1999). At the present day, the thickness is around 1 to 2 m, except in the incised channels where the deposition reaches a greater thickness.

After the final retreat of the sea, relief inversion has taken place. This phenomenon, which is well known in the coastal plain because of the work of TAVERNIER (1947), occurs due to natural processes and human impact. Progressive manmade drainage of the coastal plain caused the surface peat to dry out and thus compact. This as opposed to the effect of old channels that are filled with sand, which do not compact as much while drying out (BAETEMAN, 1999; MRANI-ALAOUI, 2006). Therefore, the sandy channels - which first

2 BGL: Below Ground Level

Chapter 9

were depressions in the environment - are now elevated, compared to places which were dominated with peat and clay rich material. Manual extraction of peat reinforced this effect.

2.2.2 New approach Many studies carried out on the facies and development history of the coastal plain in northern France and western Belgium, have highlighted the complexity of the Holocene infill (BAETEMAN & VAN STRYDONCK, 1989; BAETEMAN & VERBRUGGEN, 1979; BAETEMAN, 1978, 1981, 1991, 1993, 1999, 2001, 2005; BAETEMAN, CLEVERINGA & VERBRUGGEN, 1981; BAETEMAN, SCOTT & VAN STRYDONCK, 2002; BEETS & VAN DER SPEK, 2000; BEETS, DE GROOT & DAVIES, 2003; DE CEUNYNCK, 1985; DE MOOR & MOSTAERT, 1993; DENYS & BAETEMAN, 1995; DENYS, 1985, 1989, 1993, 1999; KOHN, 1989; MOSTAERT & DE MOOR, 1989; MOSTAERT, 1985; THOEN & MILIS, 974; THOEN, 973, 978, 987; …). This complexity is due to a multitude of factors: (1) the geomorphic setting and basement topography, (2) the sea level history, (3) the sediment supply, (4) the changing influence of tidal channels, (5) the sediment compaction and (6) human intervention (MARGOTTA, 2013; MRANI-ALAOUI & ANTHONY, 2011). A schematic cross-section of this complex filling of the paleovalley, which started 10000 years ago after the last glacial period (Weichsel glacial), is given in Figure 2.

After the rise of the sea level during the pre-Boreal and Boreal time, tidal channels formed. The contact with the Pleistocene basement was erosive. The depositions in these tidal channels are composed out of fine-grained sands with shell fragments, muddy intertidal facies and some organic-rich laminations. Along with the continuing sea level rise, the piezometric groundwater level rose to readjust the balance. This resulted in the development of fresh water marshes in which peat could accumulate (BAETEMAN, 1999). This facies becomes finer upwards and represents the transition towards fresh water marshes (POSAMENTIER & VAIL, 1988; MARGOTTA, 2013). Initially the peat was deposited in the deepest parts of the Pleistocene basement, but with the continuing sea level rise and thus increasing water tables, the peat could develop higher and more landwards. This first formed peat package, right on top of the Pleistocene basement, is called the basal peat. Radiocarbon dating enabled us to determine that the oldest peat was deposited around 9500 years ago (BAETEMAN, 1999, 2006; MRANI-ALAOUI, 2006). It corresponds to ‘unité 1’ in Figure 3. By dating the basal peat at several depths - because the basal peat is a time-transgressive unit, shifting landward

and upward with the rising sea level - the speed of the sea level rise can be determined

Chapter 10

precisely (DENYS, 1993; DENYS & BAETEMAN, 1995). This principle has been used to form a relative sea level curve (see Figure 2).

Figure 2: Relative sea level curve for the study area with indication of the average rate of sea level rise (DENYS & BAETEMAN, 1995). (MHW: Mean High Water; MTL: Mean Tide Level) The continuing rise of the sea level resulted in an end of the fresh water marches. The environment soon changed to a tidal basin with channels and flats (BAETEMAN, 1999; MARGOTTA, 20 3). This tidal basin consisted out of mud flats and salt marches (‘slikke’ and ‘schorre’ in Dutch), which is a highly dynamic environment. Salt marshes, located above the normal high-water level, only flood at spring tide. Therefore vegetation has the ability to grow on the surface. Contrarily, mud flats flood almost daily, so that they remain largely bare. The system is thus in close relationship to the present sea level, and is highly sensitive to sea level changes. Changes in sea level can transform an environment which is mainly dependent on the sea, to an environment which is much less reliant to the sea and where fresh water marches can occur, and vice versa. During the genesis of the Holocene coastal plain, there were continuously shifts between both depositional environments (BAETEMAN, 2008).

In the period before 7500 BP, the sea level rise was relatively rapid (7 m/ka). The area which

is subject to tidal influences shifted land inwards, hereby depositing a package of clay and

Chapter 11

sands with a thickness up to 10 m. Peat was scarcely deposited, because the beginning vegetation on salt marches quickly became covered with clay from the shifting clay flats (BAETEMAN, 2008). No evidence was found of a lagoon environment, which suggests that the sediment accumulation kept in pace with the rate of sea level rise (BAETEMAN, 1999). After 7500 BP, the sea level rise started to decline (4 to 2,5 m/ka). This resulted in salt marches that were less sensitive to the tidal system. The sediment supply was now higher than the accommodation space which was created by the sea level rise, therefore the tidal basin was rapidly filled with sediment (BAETEMAN, 1999). On some salt marches, fresh water swamps could form, causing the deposition of peat. Even though the sea level rise started to decline, it was still fast enough to invade these salt marches after a while. Consequently, the peat layers formed during this period rarely exceed a few centimetres (BAETEMAN, 2008). Since tidal influence increases towards the sea, this effect - whereby salt marches are reinvaded by clay flats and tidal channels - is also more pronounced. Therefore, much less and also much thinner peat beds were deposited during this period near the coastline. Only one big peat package could be observed in the coastal plain of northern France and western Belgium (the surface peat, see previous chapter). This was the main reason to differentiate between the two stratigraphic deposits: (1) Calais and (2) Dunkerque, divided by the surface peat. The deposits of the coastal plain should not be presented as stacks of layers which are evenly spread over the entire area, but rather as complex and dynamic depositions under the influence of a lot more factors than just the sea level rise. The sea level rise continued to weaken (1 to 0,7 m/ka, after 5500 BP), therefore the landscape became even less dependent on the tidal system. Fresh water marshes could develop without regular floods. This has ensured that a thick layer of peat could form behind the cover of a beach-dune complex. A package of 1 to 3 m was deposited and it is this layer which is described in the old literature as the surface peat; separating the Calais from the Dunkerque formation. The deposition of this peat started in more inland areas around 6400 BP, while at the more seaward areas deposition started more than 1500 year later. By 4800 BP, most of the coastal plain was covered with this peat, except the area at the French-Belgian border called ‘De Moeren’ (Dutch) or ‘Les Moëres’ (French) (BAETEMAN, 2008). The sediments that were deposited since the start of the sea level rise (before 7500 BP) up to the peat formation during the sub- Boreal, correspond with ‘unité 2’ in Figure 3.

At the late sub-Boreal, after around 2000 to 3000 year of peat deposition, the environment

2 converted back to a tidal system. The exact period and cause of this drastic end of peat growth

Chapter 12

remain unclear (BAETEMAN, 2007, 2008). The curve of relative sea level rise reveals that there is no sudden rise in sea level at that period. Several hypotheses have already been established about a negative sediment balance (BEETS, VAN DER SPEK & VAN DER VALK, 1994) and the subsidence of peat resulting from human activity during the Roman period (VOS & VAN HEERINGEN, 1997). More recently, BAETEMAN (2005) discussed the possibility that the Holocene tidal channels were incised due to a large amount of water derived from inlands, after which the tides could penetrate the channels again. Two possible causes for this drainage were given: (1) climate change from relatively warm towards cold humid conditions which would induce a large amount of precipitation, or (2) anthropogenic activities, such as increased deforestation since the Iron Age (BAETEMAN, 2007, 2008). The penetration of this tidal system affected the coastal plain; tidal channels incised into the underlying layers. This cased deep vertical abrasions in the underlying Holocene and sometimes even Pleistocene deposits (BAETEMAN, 2007). Apart from these tidal channels, the deposits of the late Holocene consist out of a packet of 1 to 2m of homogeneous clays. Within the channels however, significant lithological differences are recorded (BAETEMAN, 2007). The thickness of the filling varies between 5 and 25 m, with a clear and abrupt erosive border. Two main sections can be distinguished: (1) a lower part and (2) an upper part. The lower part consists out of fine sands, horizontally and obliquely layered. Different erosive depositions of 5 to 15 cm thick can be detected. So the first package was deposited in an environment characterized by a relatively fast deposition with high energy conditions, alternated with incision phases (BAETEMAN, 2007). The upper part is characterized by a rapid alternation of centimetre thick wavy layers of clay and fine sands due to the tidal influences, with clear bioturbation. This deposition is typical for a low energetic deposition with a relatively low sedimentation rate. This upper part is not present in all channels; some channels are completely filled with sand up to the surface (BAETEMAN, 2007). The sediments, deposited during the late-Holocene, relate to ‘unité 3’ in Figure 3.

Human involvement in building dikes along the tidal channel has led to the dewatering and compaction of the upper deposits and a subsidence of the surface. When during a violent storm one of the dikes broke, the effect was often catastrophic. These floods, which were documented after 1000 BP, were previously described as the Dunkerque III transgression. However, they were caused by human activities (BAETEMAN, 2007).

Chapter 13

Figure 3: Schematic cross-section of the Holocene deposits in the coastal plain (source: MRANI-ALAOUI, 2006). (NMM: ‘Niveau Moyen de la Mer’: mean sea level; IGN69: the mean sea level reference used for mainland France) 2.2.3 Comparison between the two approaches Although the ‘old’ and the ‘new’ approach have quite a few similarities, and it is easy to understand why the tripartite - formation of Calais, formation of Dunkerque and intermediate surface peat - was introduced, it is clear that there are important differences. The following overview lists the new insights derived with the new approach (BAETEMAN, 1999; MRANI- ALAOUI, 2006):

 The characteristics of the sedimentary deposits and their succession are more complex than those described in the established stratigraphy.  The formation of the coastal plain is a complex function of: (1) the geomorphic setting and basement topography, (2) the sea level history, (3) the sediment supply, (4) the changing influence of tidal channels, (5) the sediment compaction and (6) human intervention.  The variation in the rate of sea level rise is the only regional factor, the other factors are based on local variations. This increases the complexity of the deposition.  Due to local variations (topography, sediment supply, …), the infilling of the Pleistocene paleosurface was not stable in time and the depositions know horizontal and vertical diversification.  The thick surface peat is not the only peat layer which has developed. Other intercalated layers were also developed in relationship to the rising sea level and the

2 tidal system.

Chapter 14

 The classic stratigraphic nomenclature based on the Calais/Dunkerque system is not correct and should be abandoned.  The different Dunkerque transgressions are not a correct representation of reality.

Although the authors, following the new approach, irremediably want the abandonment of the stratigraphic nomenclature Calais/Dunkerque, the subdivision is still used in new work which is often based on the classic literature instead of new field research. Moreover, the Dunkerque transgression-model stays an easy and simple scheme to discuss the evolution of the coastal plain; but it is not completely consistent with reality (BAETEMAN, 1999, 2007).

Chapter 15

Chapter 16

3 THEORETICAL BACKGROUND

Chapter 17

3.1 Introduction to density dependent groundwater flow modelling

3.1.1 Groundwater flow and solute transport Groundwater flow models are used, as the name indicates, to simulate the flow of groundwater in the saturated groundwater reservoir. The partial differential equation which describes this groundwater flow is based on the law of Darcy and the law of continuity. It is important that the difference of in- and out flowing water is equal to the change in storage of a certain elementary volume. Based on initial groundwater heads and hydraulic parameters, one can calculate the Darcian flow, the flow rates and the groundwater balances. Examples of the application of groundwater flow models are: (1) groundwater extraction, (2) simulation of regional groundwater flow, (3) calculation of groundwater flow near extraction or infiltration wells, (4) interactions between groundwater and surface water and (5) interactions of the groundwater system to a drainage system.

In comparison with head and flow modelling, it is far more complicated to model subsurface solute transport. One of the reasons is that the classic equation does not always effectively represent what is seen at field scale; thus, the used numerical model solves the wrong equation. It is not inconceivable that the classic solute transport equation is sometimes solved incorrectly in the numerical model because the mathematical properties of the transport equation vary depending on which terms in the equation are dominant. No single numerical method can anticipate to this, thus no method is optimal in every situation (KONIKOW, 2010). Another reason for why the representation tends to fail, is that it is necessary to consider the groundwater flow equation and the equation of solute transport simultaneously in order to describe solute transport; this also contributes to the complexity.

Due to the complexity of modelling solute transport, one should try to keep the model itself as simple as possible. The development and application of the model consist of steps in an evolutionary process: one should start simple and add increasing degrees of complexity so the effects of the added complexity (whether in processes, parameters, dimensionality, or boundary conditions) can be easily discerned. As stated by KONIKOW (2010): the secret to successful solute-transport modelling may simply be to lower expectations.

There are different field techniques, direct and indirect, that can be used to get information about solute transport (DE LOUW, EEMAN, SIEMON, VOORTMAN, GUNNINK, VAN

3 BAAREN & OUDE ESSINK, 2011), examples are: groundwater sampling, TEC (temperature

and electrical soil conductivity)-probe measurements, electrical cone penetration tests

Chapter 18

(ECPT), continuous vertical electrical soundings (CVES), electromagnetic surveys and the use of CTD-Diver®.

3.1.2 Theoretical considerations

3.1.2.1 Equation of state The density of water can vary due to the chemical composition. In addition, the pressure and temperature also play a role; however this effect is often neglected when modelling groundwater flow and will not be discussed in this work. Density dependent groundwater flow takes the density variation into effect. In normal groundwater models, with only fresh water, the density variations are usually neglected. The density is then taken as a constant of 1000 kg/m³. However, when modelling groundwater reservoirs where the seawater has an influence, this simplification is not valid. In those situations, the density varies from 1000 kg/m³ for fresh water up to 1025 kg/m³ for pure seawater.

The density difference between fresh water and seawater can be simplified as a function of the concentration of dissolved solids. In coastal regions, chloride is the predominant anion within these dissolved solids. Therefore, the density can be described as a function of chloride content. This relationship is described by the equation of state (OUDE ESSINK, 2001):

( ) ( ) ( 1 )

ith ρ(C) as the density of the groundwater at a given concentration C, ρf the density of fresh water ( 000 kg/m³), ρs the density of seawater (1025 kg/m³), α the relative density difference, also known as the buoyancy factor (0,025) and Cs the chloride concentration of seawater (18630 mg/l).

The value of the hydraulic conductivity also varies with varying densities, as can be deduced from the equation:

( 2 )

ith K the hydraulic conductivity, k the intrinsic permeability of the porous medium, ρ the density, g the acceleration due to gravity and μ the dynamic viscosity. However, the density

can only vary 2,5 percent, corresponding to the relative density difference α, which falls well

inside the uncertainty by which the hydraulic conductivity can be approximated.

Chapter 19

3.1.2.2 Adaption of Darcy’s law Normally, the law of Darcy is used to model groundwater flow. However, due to the addition of density dependent groundwater flow, this equation alone is no longer sufficient. The more general law for groundwater flow that accounts for density dependence of flow was first formulated with potentials that were expressed as pressures by BEAR (1972):

( ) ( 3 )

With components in x-, y- and z-direction:

( 4 ) ( )

With qx, qy and qz the specific discharge in the respective directions, k the intrinsic

permeability, μ the dynamic viscosity, P the water pressure, ρw the fluid density and g the acceleration due to gravity.

Using pressures makes it harder to gather data. It is far more common to formulate equations which are based on groundwater head. Also, most codes use groundwater head data as input, e.g.: MODFLO , MOCDENS3D, … However, density dependent groundwater has an influence on the hydraulic head; an equal measured pressure at the bottom of a piezometer will correspond with a higher fresh water column in the piezometer than in situations where the piezometer is filled with salt water. This principle is visualized in Figure 4. Therefore, the

fictional equivalent fresh water head, hf, was developed which takes this relationship into consideration, as suggested by LEBBE (1981). It is defined as (OUDE ESSINK, 2001):

( 5 )

With P the pressure of the water around the filter in the observation well, ρf the density of fresh water, g the acceleration of gravity and Z the location height of the filter of the observation well.

Chapter 20

Figure 4: Illustration of two piezometers, one filled with fresh water and the other with saline water, both open to the same point in the aquifer (source: GUO & LANGEVIN, 2002). By inserting equation 5 into equation 4, one can rewrite the law of Darcy so it can be used to model density dependent groundwater flow. The result is defined as:

( ( ) )

( ) ( ( ) )

( ) ( ) ( 6 )

With Kf,h the fresh water equivalent horizontal hydraulic conductivity, Kf,v the fresh water vertical hydraulic conductivity. One can deduce that the variable density does not influence

the horizontal flow components; the resulting equation equals the law of Darcy, with fresh 3

groundwater heads. However, for the vertical flow, the derivative of the elevation does not

Chapter 21

equal zero, resulting in an extra term. This term, (ρw - ρf)/ρf, is called the buoyancy term (OUDE ESSINK, 2001). The buoyancy term is accountable for the formation of fresh water lenses in a fresh-to-salt groundwater system.

3.1.2.3 Equation of solute transport To solve a solute transport model, the adjusted equation of Darcy needs to be solved simultaneously with the equation of solute transport. The following is a general form of the solute transport equation (LANGEVIN, THORNE, DAUSMAN, SUKOP & GUO, 2007; LANGEVIN, DAUSMAN & SUKOP, 2010; OUDE ESSINK, 2001):

( ) ( 7 ) ( ) [ ( ) ] ( )

k ith ρb the bulk density, the distribution coefficient of species k, θ the porosity, C the

concentration of species k, t the time, the molecular diffusion coefficient for species k, α

the dispersivity tensor, q the specific discharge, the source or sink concentration of species

k and a fluid source or sink with density ρs (density of the solid).

The storage term on the left side of the equation is prefixed with a retardation term. This retardation is caused by adsorption of solutes through the aquifer matrix material (LANGEVIN et al., 2010). Further inspection of the equation shows that the first term of the right side of the equation relates to molecular solute diffusion. This molecular diffusion is caused by concentration gradients. The second part of the right side of equation 1 corresponds with the solute dispersivity, caused by velocity variations in the flow path of the solutes. These variations can occur at micro scale (variations within the pore system) or at macro scale (variations due to subsurface heterogeneity). In solute transport, mechanical dispersion frequently dominates molecular diffusion. Also, solute dispersivity increases with the scale of the test (GELHAR, WELTY & REHFELDT, 1992; VANDENBOHEDE & LEBBE, 2002; VANDENBOHEDE & LEBBE, 2003; VANDENBOHEDE & LEBBE, 2006; VANDENBOHEDE, LOUWYCK & LEBBE, 2009). The last term of equation 1 indicates the change in concentration due to the injection or pumping of water with a certain concentration.

3.1.3 MOCDENS3D To build and run the density dependent groundwater flow model, the numerical code MOCDENS3D (LEBBE & OUDE ESSINK, 1999; OUDE ESSINK, 1998) was used. This

3 numerical code integrates two well-known modules: (1) the three dimensional computer code

Chapter 22

MOC3D (KONIKOW, GOODE & HORNBERGER, 1996), but adapted for density differences (OUDE ESSINK, 2001) and (2) MODFLOW-96 (MCDONALD & HARBAUGH, 1988). The equation describing the groundwater flow is solved in the MODFLOW-96 module, where the buoyancy term is added to cope with the variable density. The advection-dispersion, solute transport equation is solved in MOC3D, in two steps. First, advection transport is calculated by using particle tracking. Afterwards, the dispersive transport is calculated by the finite difference method (OUDE ESSINK, 2001). For a complete explanation of MOCDENS3D, we refer to the original literature of OUDE ESSINK (1998).

3.1.3.1 Modular layout of MOCDENS3D The numerical code consists of a main program and a large number of independent subroutines, called modules. In what follows, a short explanation is given for these modules, which correspond to a certain input file. The asterix (*) in the file names stands for the name of the problem. For more information we refer to the work of MCDONALD & HARBAUGH (1988).

 Infile.nam: file used to start the model, summarises all modules which will be used.

 *.bas: this file (basic package) is used to deal with the basic administrative tasks of the model. It consists of the dimensions (number of rows, columns and layers), the active and inactive cells, the boundary conditions, the initial heads and the discretization of the time (stress periods and the time steps).

 *.bcf: this file (block centred flow) comprises all hydrogeological information from the model. This includes the anisotropy, the width of the rows and columns, the indication if it is steady state or transient groundwater flow, the transmissivity of every cell and the ratio of the vertical hydraulic conductance over the thickness (or the inverse of the hydraulic resistance between the layers). When the model is defined as transient groundwater flow, then the storage terms are added (storage coefficients and specific yield).

 *.riv: in the river package, all information from the rivers is stored, so the exchange of water between the river and the groundwater reservoir can be simulated. This includes: the

3 location of every segment of the river (layer, row, column), the height of the water in the

Chapter 23

river, the conductance between the river and the groundwater reservoir (subject to the length, width and the properties of the less permeable layer) and the concentration.

 *.wel: in the well package, information is stored about the cells in which water is injected or pumped. Here, a discharge rate and a concentration are given. This package can also be used to simulate recharge due to percolation through the unsaturated zone, whereby every cell of the top layer is ‘injected’ with water.

 *.sip: this module (strongly implicit procedure) covers all information of the iteration, with - among others - the closing criterion and the maximum iterations.

 *.moc: this file encompasses all data from the simulations of solute transport. It covers the information about the dispersion, the decay and the diffusion. The initial concentrations are also given, along with the longitudinal and transversal horizontal and vertical dispersivity, the retardation, the thickness and the porosity. Finally, the initial number of particles per cell is defined.

 *.obs: this file (observation) defines in which cells the evolution of the head will be observed. These results are printed in the *.oba file.

 Grafile.out: output file which holds all data of the calculated fresh water heads, flow velocities in the x, y and z direction and the concentration after every stress period.

3.1.3.2 Visual MOCDENS3D Visual MOCDENS3D is a visualization and processing software developed for MOCDENS3D. It contains the following modules (VANDENBOHEDE, 2008): (1) a file manager module, (2) a grid builder module, (3) a module to run MOCDENS3D and visualize the output and (4) a module which uses particle tracking to calculate capture zones or stream lines. This software was used for this work, to visualize the results from the Grafile.out file.

3.2 Introduction to Hydraulic Parameter Identification for pumping tests HYDPARIDEN (LEBBE, 1999), or Hydraulic Parameter Identification, is the inverse numerical model used to interpret the data retrieved from a pumping test. From the

3 drawdowns measured during a pumping test, HYDPARIDEN is able to calculate the optimal

Chapter 24

values of the hydraulic parameters in an iterative way, along with their joint confidence areas and the deviation between the calculated and measured drawdowns (LEBBE & VAN MEIR, 2000). In what follows, a short introduction is given on how the numerical model works and how the results can be interpreted. For a more extensive and in-depth explanation, we refer to the manual: Hydraulic Parameter Identification, Generalized Interpretation Method for Single and Multiple Pumping Tests (LEBBE, 1999).

3.2.1 Schematization of the numerical model The subsurface is subdivided into a two-dimensional axial-symmetric model, composed out of: (1) different layers vertically and (2) different rings around a central axis horizontally. The different layers in the vertical dimension are based on different lithological layers and the amount of detail one requires. Based on literature and drilling descriptions, one can discretize the subsurface in different permeable and semi-permeable layers, which can be further subdivided to retrieve more detail from the model. The thickness of the different layers should be chosen to get the best results out of the model, thus thin layers close to the pumped layer, and gradually thickening layers away from the pumped layer. This is because the influence of the pumping decreases with increasing distance to the pumped layer. The groundwater reservoir is schematized in an alternation of pervious and/or semi-pervious layers, presumed to be laterally continuous with an infinite homogeneous extension. The lowermost layer is always bounded by an impermeable boundary, while the uppermost layer is bounded by the water table. The layers are numbered from the bottom upwards.

The layers are subdivided into a number of coaxial rings around the pumping well as central axis. The inner and outer radii of the rings increase logarithmically, allowing the calculation of drawdowns within the same order of accuracy at distances from the pumped well (which are in the order of centimetres, decimetres, meters, decametres and hectometres) (LEBBE, 1999). Figure 5 shows the representation of the numerical model, and also the logarithmic increase of the inner radii (R(I-1/2)), the nodal radii (R(I)) and the outer radii (R(I+1/2)) of the (I,J)th ring. Here, R1 is defined as the inner radius of the first ring and A as the factor by which the radius of every ring increases. The outer radii of every ring are equal to the inner radii of the subsequent ring. The nodal radii are defined as the radii of the nodal circle, which runs through the half height of every layer and through the middle of every ring. The drawdowns are calculated in these nodal circles at different time steps, and because of the

radial flow towards the pumping well this value is equal at every point of a nodal circle. As a

Chapter 25

result, three-dimensional flow can be simplified to a two-dimensional problem. The two- dimensional grid is also shown in Figure 5.

Figure 5: Representation of the numerical model. R1 is the initial radius and A is a factor which is larger than 1 (source: LEBBE, 1999). The time is subdivided in different time steps. The first considered time steps are very small, and these time steps increase logarithmically with the same factor A. This makes it possible to calculate the drawdowns within the same order of accuracy after seconds, minutes, hours, days, weeks and months of starting the pump. The formula which describes the nth time step of the logarithmic series is defined as:

( 8 )

With T1 the initial time step and A the factor by which each time step increases, which is equal to factor A used to increase the radii of the circles.

3.2.2 Initial hydraulic parameters After the discretization of the subsurface, an initial value should be given for the hydraulic parameters. From these values, the iteration process searching for the optimal values can start. Every layer should get a horizontal conductivity, a hydraulic resistance and a specific storage

assigned and also, a storage coefficient at the water table should be specified. These initial

values can be based on literature, drilling reports or from direct or indirect measurements,

Chapter 26

combined with a dose of well-educated guesswork. It is important to make a good first estimate; this shortens the run-time needed to come to optimal parameters.

Other initial parameters which should be provided, are the parameters associated with well loss. The well loss, along with the aquifer loss, affects the drawdown in the pumping well. The aquifer loss is defined as the drawdown at the depth of the well screen in the theoretical case that the unaltered sediments are immediately behind the well screen. However, in reality there is the inserted gravel pack between the well screen and the borehole wall. By utilizing the pumping well, the layer outside of the borehole wall gets altered and finer particles will be removed; this will have a result on the permeability of the sediments in the vicinity of the well screen. The well loss is thus defined as the difference between the measured drawdown and the aquifer loss, and can be described as:

( 9 )

With C a constant of the well loss function, N a constant which is greater than one (usually defined as 2) and Q the discharge rate.

The well loss can be positive or negative, depending on the sign of the C-constant. With a positive C value, the measured drawdown is higher than the aquifer loss, hence the average permeability around the well screen will be lower than the permeability of sediments situated at the same level of the well screen. A negative C value is the result of a lower drawdown than calculated solely with the aquifer loss, hence there is a higher permeability around the well screen, for example due to well development.

3.2.3 Optimal hydraulic parameters

3.2.3.1 Residuals and sensitivities After implementing the model, new hydraulic parameters will be calculated. The deviations between the subsequently calculated drawdowns and the measured drawdown are called the residuals, and are defined as:

( 10 ) ̂

̂ Where r are the residuals, S* equals the measured drawdown, and the calculated

drawdowns.

Chapter 27

It is also important to get an idea of the sensitivity of the drawdowns in function of a certain hydraulic parameter(group). This relationship is defined as:

̂ ̂ ( ) ( 11 )

th With Jij the sensitivity, ̂ the calculated drawdown corresponding with the i observation, hpj the jth hydraulic parameter(group), sf the sensitivity factor. The logarithms are used because of the large variation of the hydraulic parameters. In this way, the sensitivity is calculated for each parameter(group).

3.2.3.2 Adjustment of parameter values The aim of the model is to find the most optimal solution to the problem. In an iterative manner, the hydraulic parameters are adjusted, with adjustment factors. Therefore the calculated residuals and sensitivities are used. With every iteration, the old hydraulic parameters are multiplied with their corresponding adjustment factors to obtain the new hydraulic parameters:

( 12 )

th th With the j hydraulic parameter and the m iteration and the logarithm of the adjustment factor of the jth parameter during the mth iteration. Normally after every iteration the adjustment factor decreases until optimal values are achieved, at this point the adjustment factor is quasi equal to 1 and the sum of the squared residuals reaches a minimum value. At this point the calculated drawdowns match the measurements as much as possible, indicating that the obtained hydraulic parameters provide a situation which could correspond well with reality. The word ‘could’ is used specifically to indicate that hydraulic parameters which provide good results are not necessary the only set of parameters that provide decent results. It is therefore essential to verify these hydraulic parameters with other measurements.

3.2.4 Interpretation based on the joint confidence region The accuracy of the optimal hydraulic parameters can be examined with a joint confidence region; this is when a statistical region is created as a multi-dimensional generalization of the confidence interval. The joint confidence region of n-parameter(group)s can be approximated

by an n-dimensional ellipsoid. The confidence region will include the value of a

parameter(group) with a given percentage of possibility. Usually the confidence regions of

Chapter 28

90,00 %, 99,00 %, 99,90 % and 99,99 % are used. Figure 6 shows the 2 dimensional confidence region for a situation with 2 parameter(group)s.

Figure 6: Joint confidence region for parameter(grou s and i and e o imal value, Fa.Sm the marginal confidence intervals, Fa.Sc the conditional confidence intervals, α the eigenvalues of the covariance matrix and β the orientation of the ellipsoid (source: LEBBE, 1999). The elliptical shape of the confidence region is based on the following elements: (1) the eigenvalues and eigenvectors of the covariance matrix, (2) the marginal and conditional standard deviations and (3) the condition indexes. The central point equals the optimal value of the two parameter(group)s, then the length and orientation of the main axes of the ellipsoid correspond with the square root of the eigenvalues of the covariance matrix. This implies shorter axes correspond with more accurate parameters. The shape of the ellipsoid is based on the marginal standard deviation Sm and the conditional standard deviation Sc. Sm corresponds with the maximal possible deviation within the confidence region, while Sc corresponds with the maximal possible deviation with all other parameters at optimal values. The conditional standard deviation is almost always smaller than the marginal standard deviation. The difference between Sm and Sc in one parameter is also an indication of the dependence of this parameter to the other parameter(group)s. A big difference will stretch out

the ellipsoid, thus the parameters will be more dependent of each other. This dependence between the different parameter(group)s is also shown in the correlation matrix. Lastly, the

Chapter 29

condition index - which is the ratio of an axis of the ellipsoid over the smallest axis - shows the dependence of the different parameters.

Chapter 30

4 PRELIMINARY RESEARCH: MANNEKENSVERE (BE)

Chapter 31

4.1 Site description

Figure 7: Localisation of the study area within Belgium and more precisely within the Belgian coastal plain. The IJzer is also indicated. Mannekensvere is indicated with a green dot (sources: BAETEMAN, 2007/Wikipedia/own research). The implementation of this study, at the Belgian side of the border, is carried out in a polder in the vicinity of the river IJzer. This polder is located in the village of Middelkerke, more precisely in the district of Mannekensvere, in the province of West Flanders (see Figure 7). The study area is located roughly 6 km inlands. The genesis of this polder area is explained in chapter 2. The overall height of the polder varies from +2 up to +5 m TA (‘Tweede Algemene aterpassing’, the Belgian reference level). This means that the situated part of the coastal plain lies below the high tide level (BAETEMAN, 2007).

The polders are drained by one major river: the IJzer. This 78 km long river runs from Lederzeele (FR) to Nieuwpoort (BE), where is flows into the North Sea. At the North Sea, a weir shields the IJzer from the tidal fluctuations, making discharge possible only at low tide. The total catchment area is around 1101 km², with approximately 35 % polders. A ridge separates the western part of the catchment area of the IJzer from the eastern part of the catchment area of the Aa river (HEYLEN, 1997).

A selection of drillings with close proximity to Mannekensvere was made; these can be retrieved in annex 1. Based on these data, an assumption of the geology was made. As the interest in this study only goes to the unconfined aquifer, with the lower boundary fixed at the Ieper group, no interpretation will be made of the underlying formations. The formation of Kortrijk and the formation of Tielt are both part of the clay rich Ieper group. These formations

were observed in drilling reports 1, 3, 4 and 5. The depth varies from 11,65 m BGL up to

4 22,41 m BGL. On top of the Ieper group, Pleistocene deposits were detected; nevertheless not

Chapter 32

all drillings contained Pleistocene deposits. The thickness of these deposits varies from 3,71 m (drilling 1) up to 7,85 m (drilling 2). The rest of the deposits are formed out of Holocene sediments; with a thickness of 7,87 m up to 22,70 m. In most drillings, except for drilling 2, this formation is described as peat and clay rich deposits; significant peat horizons can be observed. In drilling 2, the deposits are primarily composed out of sandy material. This drilling is probably located within a tidal channel which formed after the so called surface peat. Because only drilling report 2 shows characteristics of a filled tidal channel, while all others lack all features, one can deduce that the lateral spread was (locally) not very extensive.

4.2 Groundwater model

4.2.1 Model setup This section describes all properties that were used for the groundwater model of Mannekensvere. These properties mostly originate from a model building tool, called MAK3DZIL. The program uses, among others, the profile type map, the base and permeability of the polder deposits, the HCOV units, the salinity map and the topographical map.

4.2.1.1 Grid properties This groundwater model, which is located in Mannekensvere (Belgium), is modelled in 3 dimensions. The length of the model is 4 km and corresponds with the X-Lambert723 coordinates extending from 39500 m to 43500 m. The width of the model, which is 3 km, corresponds with the Y-Lambert72 coordinates ranging from 200500 m to 203500 m. The model is oriented exactly east-west. The thickness of the model is 35 m. The base of the model is located at -30 m TAW, whereas the top of the model is formed by the groundwater table. The mesh consisted of 160 columns of 25 m, 120 rows of 25 m and 14 layers of 2,5 m. In the upper layer, this thickness can vary. The base of the upper layer is located at 2.5 m TAW, while the top is formed by the groundwater table. This gives a total of 268800 cells within this groundwater model. Figure 8 shows the location of the study area, while Figure 9 displays the topography of the study area.

3 Lambert72 is the geographic projection and coordinates system used in Belgium.

Chapter 33

Figure 8: Localisation of the study area, the corners are indicated with the Lambert72 coordinates (source: Google earth/own research).

Figure 9: Grid of the groundwater model along with the topography of the study area (source: personal communication, G. DEVRIESE/own research). 4.2.1.2 Time properties The total simulation time spanned a period of ca. 40 years. This selected simulation period

4 was chosen because the simulation of the present day flow and distribution is modelled

Chapter 34

starting from the salinity map of DE BREUCK, DE MOOR, MARECHAL & TAVERNIER (1974). This map will be discussed later, however it is important to know that the initial conditions are based on data retrieved from the period 1963 to 1973. To obtain the present day situation, the model should therefore run approximately 40 years. The total time was subdivided into 9 different periods, for each of these periods the boundary conditions stayed constant. For the first period, a break-in period was chosen with a length of 3,65 days. This period was executed in 25 steps. The following 8 periods have a period length of 1826,25 days (= 5 years) and are subdivided in 50 time steps. For all time steps, the time step multiplier (TSMULTI) was taken equal to 1, so that all time steps within a period have an identical length.

4.2.1.3 River properties The rivers are implemented through the MODFLOW river package. Figure 10 shows the rivers as they are put into the model. Based on data from DOV, the rivers are put in different categories. Two major groups are distinguished: (1) the navigable watercourses and (2) the unnavigable watercourses. The unnavigable watercourses are further subdivided, based on the authority responsible for the maintenance:

 First category: maintenance by the Flemish government  Second category: maintenance by the provinces  Third category: maintenance by the villages

These 4 groups are represented in Figure 10 with different thicknesses. The only navigable watercourse, the IJzer river, is shown as the widest line central in the figure. The different categories of unnavigable watercourses are shown as increasingly thinner lines. Each group has other river properties assigned, which are stored in the *.riv file. These properties are based on the Flemish Hydrographic Atlas (VHA: ‘Vlaamse Hydrografische Atlas’ in Dutch), the topography and the HCOV-database. We presume here the following properties: (1) the navigable watercourse (the IJzer river) has a width of 36 m and a water level in the river equal to 3,14 m TAW, (2) the first category of the unnavigable watercourses is characterized by a width of 10 m and a water level equal to 1,0 m BGL, (3) the second category of unnavigable watercourses is modelled with a width of 4 m and a water level equal to 0,9 m BGL and finally (4) the third category of the unnavigable watercourses is characterized by a width of 2

m and a water level in the river of 0,8 m BGL. The concentration of the water in the rivers in

Chapter 35

all categories is equal to 1 % of the salinity of North Sea water. Finally, the hydraulic resistance towards the ditch was set to 3 days for all river categories.

Figure 10: Indication of the rivers in the study area, with the different categories of the rivers represented with different thicknesses (source: Google earth/own research). 4.2.1.4 Drainage properties The coastal plain in Belgium is historically well drained. To model the drainage, the MODFLOW river package is used once more. Therefore, the data have to be written conform the *.riv file. Every uppermost active cell in the model area where no actual river is, receives a contact factor, a water level, a maximum rate of infiltration and a salinity percentage in function of the soil type. In this study area, 5 different profile types are present (shown in Figure 11), each with its own set of parameters shown in Table 2 (LEBBE, VANDENBOHEDE & WAEYAERT, 2006). The hydraulic resistivity in this table is used to calculate the contact factor.

Chapter 36

Maximal Hydraulic infiltration Salinity Profile type Code resistivity speed (%) (d) (qmax in m/d) Poolground 19 667 5.0 x10-4 2

Salt march polder 20 2500 5.0 x10-4 2

Channel 21 667 5.0 x10-4 2

Creekridge 22 83 4.0 x10-4 1 Claycovered 23 2500 5.0 x10-4 2 polder

Figure 11: Profile type map of the study area (source: own research). 4.2.1.5 Recharge properties

The recharge properties are based on the sediments in the underground, more precisely on the

profile type map shown above. Two different zones can be observed: (1) a zone characterized

Chapter 37

by a recharge flow rate of 0,06 m³/d and a concentration of 1,0 % salinity, and (2) a zone with a recharge flow rate of 0,25 m³/d and a concentration of 0 % salinity. The second zone, with the higher recharge flux, corresponds with the Creekridge profile type. It was opted to simulate the recharge with the MODFLOW well package. Here every upper active cell of the model is filled with well-cells in order to let the flux, which is kept constant throughout the simulation period, get into the model. The two different zones, with corresponding flow rate and concentration, are visualized in Figure 12.

Figure 12: Illustration of the recharge flow rate (q) in m³/d, and the recharge concentration in % salinity (source: own research). 4.2.1.6 Properties of the subsurface The discretization of the groundwater reservoir is based on data from the HCOV database. HCOV stands for the Hydrogeological Coding of the Subsurface in Flanders (HCOV: ‘Hydrogeologische Codering van de Ondergrond Vlaanderen’ in Dutch). The local geology is concisely discussed in the previous chapter, based on a few drilling descriptions in DOV. However, the model building tool MAK3DZIL combines much more information to define the subsurface. In Figure 13 and Figure 14 a horizontal and vertical cross-section through the groundwater model is shown. All HCOV units are characterized by different hydraulic properties. Table 3 summarizes these units (MEYUS, BATELAAN & DE SMEDT, 2000). In what follows, the hydraulic parameters of each HCOV unit will be discussed (source: personal communication, G. DEVRIESE).

Chapter 38

Figure 13: Vertical cross-section through the groundwater model along the X-direction (y = 201000 m Lambert72) (source: personal communication, G. DEVRIESE).

Figure 14: Vertical cross-section through the groundwater model along the Y-direction (x = 41500 m Lambert72) (source: personal communication, G. DEVRIESE).

Table 3: HCOV units which are present in the study area (source: MEYUS et al., 2000).

Main unit Sub-unit 0130 Polder deposits 0100 Quaternary Aquifer System 0150 Cover layers 0160 Pleistocene deposits 0910 Silt of Kortemark

0900 Ypresian Aquitard System

0920 Kortrijk deposits

Chapter 39

 0100 Quaternary Aquifer System

0130 Polder deposits The hydraulic conductivity within the polder deposits are characterized by big lateral and vertical variations, therefore the HCOV 0130 unit is further subdivided in five smaller subgroups, based on the profile type map (analogue to the principle explained in the drainage properties) these subgroups get different hydraulic parameters. These subgroups are:

 0130a: Upper Polder deposits  0130b: Upper middle Polder deposits  0130c: Middle Polder deposits  0130d: Lower middle Polder deposits  0130e: Lower Polder deposits

Figure 15 shows the horizontal hydraulic conductivity through the first layer. Especially the Creekridge material stands out, with a significantly higher hydraulic conductivity. The effective porosity is equal to 0,38.

Figure 15: Horizontal hydraulic conductivity of the first layer in the groundwater model (source: own

research).

Chapter 40

0150 Cover layers The horizontal hydraulic conductivity of HCOV unit 0150 was set to 2 m/d, with the anisotropy (ratio of horizontal hydraulic conductivity to vertical hydraulic conductivity) equal to 200. The vertical hydraulic conductivity equals 0,01 m/d. This unit has an effective porosity of 0,38.

0160 Pleistocene deposits The horizontal hydraulic conductivity of the Pleistocene deposits was set to 7 m/d, with the anisotropy set to ca. 11 to get to a vertical hydraulic conductivity of 0,65 m/d. Furthermore, the unit is characterized by an effective porosity of 0,38.

 0900 Ypresian Aquitard System

0910 Silt of Kortemark The horizontal hydraulic conductivity of HCOV unit 0910 is set to 0,025 m/d and the vertical hydraulic conductivity is set to 0.000321 m/d. This results in an anisotropy of 78. The effective porosity of this layer is equal to 0,38.

0920 Kortrijk deposits For this unit, the same values are used as in HCOV unit 0910: a horizontal hydraulic conductivity of 0,025 m/d, a vertical hydraulic conductivity of 0,000321 m/d and an effective porosity of 0,38. This package and the Silt of Kortemark package are characterized by a very low conductivity. These low permeable sediments form the base of the groundwater model.

4.2.1.7 Fresh water head properties As described in the theoretic background, the groundwater heads will be expressed in fresh water heads. The initial heads were based on the topography (shown in Figure 9). The proximity to the reality of these head values is related to the time needed to find a solution for the model. However, the initial values have limited influence on the end results because permanent groundwater flow is assumed.

4.2.1.8 Solute transport properties We opted to use the salinity percentage to indicate the salt content in the model; other alternatives are the Cl--content as generally used in the Netherlands or the Total Dissolved Solids (TDS in mg/l). Hereby, fresh water with a TDS value lower than 800 mg/l has a

salinity percentage of 0, whereas seawater with a TDS value higher than 28000 mg/l has a 4

Chapter 41

salinity percentage of 100. All values between these two end members can be calculated with the following equation:

( ) ( 13 )

( )

For the main parameters in solute transport, the following values were adopted: the longitudinal dispersivity was set to a value of 0,3 m, the horizontal transverse dispersivity to a value of 0,05 m and the vertical transverse dispersivity was set to 0,03 m. The retardation factor was kept simple to 1.

The initial groundwater concentration is based on the salinity map of DE BREUCK et al. (1974), which is shown in Figure 16. This map shows the depth to the transition zone between fresh and salt groundwater, in m BGL, for the coastal area of Belgium. The fresh-to-salt transition zone was defined by DE BREUCK et al. (1974) at a TDS of 1500 mg/l. One must consider that this salinity map was produced in 1974. So the present day situation is obtained after a simulation period of approximately 40 years. The subdivision of the time periods is based on this principle. From Figure 16 we learn that the depth of the transition zone between fresh and salt water varies from less than 2 m BGL to maximum 10 m BGL.

4 Figure 16: Salinity map of DE BREUCK et al. (1974) at the study area (source: DE BREUCK et al., 1974/Databank Ondergrond Vlaanderen/own research).

Chapter 42

4.2.1.9 Boundary conditions At the bottom of the model, no-flow boundary conditions are retained. Because little is known about the actual situation, the 4 sides are simplified to no-flow boundaries. By doing so, we assume that the vertical boundaries are impermeable. This is not a problem because we are mostly interested in the interactions close to the IJzer river, and the boundaries are located far enough (to have no influence).

Inflow is possible due to natural recharge and due to interactions with rivers. The concentration of this inflow is equal to 0 % or 1 % salinity for the recharge water and 1 % salinity for the river water. Water can leave the system by interaction with the rivers of the model (in reality the watercourses and the drainage system).

4.2.2 Results of the numerical model This chapter summarizes the results that came out of the last model run, incorporating groundwater flow and solute transport. The results show the present day flow and distribution of fresh and salt water obtained after the simulation of a period of approximately 40 years, starting with the fresh-salt water distribution derived from the salinity map of DE BREUCK et al. (1974). Since the model was simplified to steady state flow with constant boundary conditions, the results will be a representation of the mean annual state without seasonal variations. To interpret the results of both groundwater flow and solute transport, horizontal and vertical cross-sections through the model were used. For the horizontal cross-section it was opted to choose the horizontal cross-section corresponding with layer 1 of the model (2,5 m TAW to 0 m TAW). The shown contour lines of the fresh water heads are a good approximation of the location of the water table. For the interpretation of solute transport, the horizontal cross-section of layer 2 is also shown (0 m TAW to -2,5 m TAW). The obtained results are also shown in two selected vertical cross-sections through the modelled area. The same location of cross sections are chosen as the ones in which the HCOV units are shown (see previous chapter). In this way it is easy to compare the flow and distribution of fresh and salt water with the areal distribution of the hydrogeological units. The first vertical cross- section is located along row 100 of the model (X-direction at 201000 m Lambert72), and the second vertical cross-section along column 80 (Y-direction at 41500 m Lambert72). Figure 17 shows the position of these cross-sections.

Chapter 43

Figure 17: Image of the study area; the red lines indicate the used vertical cross-sections (y = 201000 m Lambert72 or 100th row; x = 41500 m Lambert72 or 80th column) (source: Google earth/own research). 4.2.2.1 Fresh water head Figure 18 shows the horizontal cross-section through the uppermost layer of the model. What stands out first is the strong dependence of the rivers; both the highest and lowest heads can be observed at the position of rivers. The only navigable watercourse, the IJzer river, corresponds with the highest fresh water heads in the study area. The IJzer river is modelled with a constant head, which is realistic because the level is kept constant with the application of a shielding weir. Because this (constant) level is higher than the surrounding fresh water heads, water will infiltrate from the IJzer river towards the surrounding groundwater reservoir. This is in high contrast with the unnavigable watercourses; here the fresh water head is significantly lower than the fresh water head in the direct proximity. This implicates that the unnavigable watercourses drain water from the groundwater reservoir. This effect is the most pronounced in category 1 of unnavigable watercourses and systematically loses its efficiency with the higher categories. This can simply be explained by the fact that the constant head value of the unnavigable watercourses varies with every category: (1) 1 m BGL for category 1, (2) 0,9 m BGL for category 2 and (3) 0,8 m BGL for category 3. This

subdivision in an infiltrating navigable watercourse (IJzer river) on the one hand versus

4 draining unnavigable watercourses on the other hand can also be observed in the vertical

Chapter 44

cross-sections (Figure 19 and Figure 20). In the vertical cross-section along row 100 of the model, parallel with the X-direction, the IJzer river is located between 2250 m and 2500 m (see Figure 19). At the surface, the fresh water head value is slightly higher than 3 m TAW, this value lowers laterally and vertically up to a certain depth. This implies that water from the IJzer river can infiltrate up to a certain depth, and will flow radially away from the centre. In Figure 20 where the vertical cross-section along column 80 of the model is shown, parallel with the Y-direction, the IJzer river is located at around 1750 m. The opposite story applies for the unnavigable watercourses (not so clear in Figure 19 nor Figure 20), where groundwater is radially attracted and drained, and some upwards flow is also present. The main groundwater flow interactions are: (1) the infiltration of water from the IJzer river towards the surrounding groundwater reservoir system, both through vertical and lateral flow, (2) the infiltration of recharge water within parcels, primarily due to lateral flow and (3) the drainage towards the unnavigable watercourses, primarily through lateral groundwater flow.

A second important observation in the horizontal cross-section is that the rivers east of the IJzer river are more pronounced than the rivers west of it. In the eastern zone the head values of the rivers are significantly lower than the surrounding head values, while in the west no clear distinction can be found in the head values between the river and the surroundings. This classification into two zones, east and west of the IJzer river, is a bit of a simplification. For instance, one can see that there is a zone east of the IJzer river in the middle of the study area where rivers are less pronounced, while another river at the western extremity of the model is very pronounced. Nevertheless, in what follows the explanation is done based on these two zones. The zones can be explained by the different profile types in the first formation of the model, the Polder deposits (HCOV 0130) and, stemming from this, the different hydraulic parameters. One should compare the fresh water head of layer 1 (Figure 18) with the profile type map (Figure 11) or with the figure showing the horizontal hydraulic conductivity of the first layer of the model (Figure 15). The zones where the rivers are less pronounced, mainly west of the IJzer river, coincide with the Creekridge profile type; which is characterized by a higher hydraulic conductivity. All other profile types, characterized by a much lower hydraulic conductivity, are more present east of the IJzer river. Consequently, there is a clear relationship between the hydraulic conductivity of the first formation of the model (HCOV 0130, Polder deposits) and the fresh water head variation due to the presence of rivers: a higher hydraulic conductivity results in a lower difference in hydraulic head and thus less

4 pronounced rivers in Figure 18, and vice versa. This can be explained by the fact that in a

Chapter 45

zone, where the subsurface is characterized by a higher hydraulic conductivity, the drainage due to the unnavigable watercourses is more effective. Due to the higher hydraulic conductivity, groundwater will flow more easily into the rivers, hereby locally lowering the fresh water head of the reservoir. This makes the difference in fresh water head between the river and the reservoir decline sharply, hence the watercourses are less pronounced in Figure 18. If one looks more closely to the profile type map (Figure 11) and the distribution of the fresh water head (Figure 18), one can notice that this relationship also works on a smaller scale. The best examples here are the two spots within the IJzer river where the difference in fresh water head, between the IJzer river and the direct surroundings, is almost nullified. These spots are positioned exactly where the IJzer river runs through the deposits of the Creekridge. Due to the larger fresh water head variations over a small area in the zone with more pronounced rivers, mostly east of the IJzer river, these zones will also be more subject to lateral groundwater flow.

A final observation can be made from the vertical cross-sections. In both figures (Figure 19 and Figure 20), the fresh water head rises with increasing depth. From a certain depth, approximately 18 to 22 m BGL, the fresh water head is higher than 3 m TAW. Below this transition zone, the lateral component of the groundwater flow diminishes, while the vertical gradient becomes more important. One could say that the groundwater flows from the lower part of the model upwards. Nevertheless, this upwards flow is negligible because below the transition zone the deposits from the Ypresian Aquitard System are deposited. And the low hydraulic conductivity will reduce the actual groundwater flow up to a negligible value.

Chapter 46

Figure 18: Horizontal cross-section through layer 1 of the model, showing fresh water heads and isosurfaces (source: own research).

Figure 19: Vertical cross-section along row 100 of the model, parallel with the X-direction, showing fresh water heads and isosurfaces (source: own research).

Chapter 47

Figure 20: Vertical cross-section along column 80 of the model, parallel with the Y-direction, showing fresh water heads and isosurfaces (source: own research). 4.2.2.2 Salinity Figure 21, Figure 23 and Figure 24 show the same cross-sections as in the previous part, yet this time the salinity percentage is visualized. The main principles that were discussed in the previous part, can also be deduced from the salinity figures. Firstly there is the observation that the navigable watercourse, the IJzer river, falls under an infiltrating regime. Due to this infiltration, fresh water from the IJzer river can infiltrate deeper in the groundwater reservoir. This can be seen in the horizontal cross-sections (Figure 21 and Figure 22): the area covered by the IJzer river clearly stands out, especially in the second layer where the surroundings are already salinized, by its low salinity percent. This effect is even more visible in the vertical cross-sections (Figure 23 and Figure 24), from which we can deduce that fresh water from the IJzer river can infiltrate to a depth of approximately 12 m BGL. This is - again - in sharp contrast with the unnavigable watercourses, which drain the groundwater reservoir. This effect is best displayed in Figure 21, where the horizontal cross-section through layer 1 of the model is shown. In this figure, the unnavigable watercourses (especially east of the IJzer river) are characterized by a higher salinity compared with the direct lateral surroundings. This is due to the drainage of the groundwater reservoir; these rivers attract water with a higher salinity from deeper in the reservoir, thus salinizing the water. In the vertical cross- sections this is shown by the up-coning of salinity below unnavigable watercourses. Over the total area, except for the area where the IJzer river is located, the fresh-to-salt transition zone

lies between circa 2 m BGL and 8 m BGL.

Chapter 48

The second principle, a higher fresh water head variation in the proximity of the rivers where the top formation is characterized by a lower hydraulic conductivity, is also apparent in the figures illustrating the salinity. In zones where the subsurface is characterized by a lower hydraulic conductivity (mostly east of the IJzer river), fresh recharge water can infiltrate between the rivers, while only the rivers show a higher salinity content due to the drainage of deeper groundwater (with a higher salinity). In contrast, in Creekridge zones with a higher hydraulic conductivity, the fresh water head of the entire area is lower (as explained earlier). These areas are characterized by a slight upwards flow, compared with adjacent zones where the fresh water head lies higher. This causes the salinization of the total Creekridge zone and in consequence not only the crosscutting rivers, while the adjacent zones are characterized by infiltrating fresh water.

Figure 21: Horizontal cross-section through layer 1 of the model, showing salinity percentage and isosurfaces (source: own research).

Chapter 49

Figure 22: Horizontal cross-section through layer 2 of the model, showing salinity percentage and isosurfaces (source: own research).

Figure 23: Vertical cross-section along row 100 of the model, parallel with the X-direction, showing salinity percentage and isosurfaces (source: own research).

Figure 24: Vertical cross-section along column 80 of the model, parallel with the Y-direction, showing salinity percentage and isosurfaces (source: own research).

Chapter 50

5 MAIN RESEARCH: SAINT-PIERRE-BROUCK (FR)

Chapter 51

5.1 Site description

5.1.1 General

Figure 25: Localisation of the study area and indication of the Aa river valley (right), Saint-Pierre-Brouck is indicated with a green dot (sources: Map of France/MARGOTTA, 2013/own research). This part of the study was carried out in Northern France, more precisely in the village of Saint-Pierre-Brouck in the departments Nord and Pas-de-Calais (see Figure 25). Saint-Pierre- Brouck is located in the Flemish coastal zone. This is an extremely dynamic area in which the interactions between oceanic and terrestrial processes shaped the (sub)surface (CARTER, 1988; MARGOTTA, 2013).

The Flemish coastal plain is part of the North Sea coastal plain, which occupies a 10 to 20 km wide embayment that stretches from Cap Blanc Nez in northern France up to Skagen in Denmark (BAETEMAN, 2008; MRANI-ALAOUI & ANTHONY, 2011; VAN DER WOUDE & ROELEVELD, 1985). The study area, located roughly 17 km inlands between the cities of Calais and Dunkerque, lies in the south-western extremity of the elongated North Sea coastal plain. The plain is characterised by Holocene tidal to non-tidal deposits that have been developed in a variety of geomorphic settings, ranging from estuaries to embayments (ALLEN, 2000; MRANI-ALAOUI & ANTHONY, 2011). Through human intervention and due to the embankment of the coastal plain since the Middle Ages, the area converted to a polder (BAETEMAN, 2008). The modern landscape is a mosaic of cultivated and build up areas (GANDOUIN, PONEL, ANDRIEU-PONEL, GUITER, DE BEAULIEU, DJAMALI, FRANQUET, VAN VLIET-LANOE, ALVITRE, MEURISSE, BROCANDEL & BRULHET, 2009).

Chapter 52

In the study area, one major river crosses the plain: the river Aa. This 80 km long river runs from Bourthes to Gravelines where it flows into the North Sea. The 560 km² river catchment is predominantly located in a zone of subsidence (GANDOUIN et al., 2009; MANSY, MANBY, AVERBUCH, EVEREARTS, BERGERAT, VAN VLIET-LANOE, LAMARCHE & VANDYCKE, 2003). Our study area lies in the lower reaches of the river Aa. A single narrow outlet (about 1 km wide near Watten) connects a large catchment basin (St-Omer basin, 40 km²) to the coastal zone, forming the Aa valley plain (see Figure 25). In the bordering hills upstream of the Aa valley, bounding the study area landwards, Cretaceous chalks, Tertiary sands and clays and Pleistocene loams are outcropping (SOMME, MUNAUT, EMONTSPOHL, LIMONDIN, LEFEVRE, CUNAT-BOGE, MOUTHON & GILOT, 1994; MRANI-ALAOUI & ANTHONY, 2011; VAN DER WOUDE & ROELEVELD, 1985). Within the lower reaches of the river Aa, the river gradient is very moderate (0,1 m/km in the St-Omer Basin), so that the river mouth was originally characterized by a wide estuary with inland marshes (GANDOUIN et al., 2009). The catchment area of the river Aa is separated from the catchment area of the river IJzer by a ridge (HEYLEN, 1997). Nowadays, the plain is completely reclaimed and protected seawards by a continuous 100 to 600 m wide coastal dune barrier, dikes and port defences. The surface of the plain lies at an elevation ranging from −2 to +4 m IGN69, the mean sea level reference used for mainland France. As a reference, 0 m IGN69 corresponds to 1,694 m TAW, the national reference level for Belgium. The coast experiences mean spring and neap tidal ranges of about 5,5 and 3,6 m. Therefore much of the coastal plain lies below the level of mean high water and is susceptible to marine flooding (MRANI-ALAOUI & ANTHONY, 2011). At present, the river Aa is canalized and connected in Arques to the Canal de Neufossé.

MRANI-ALAOUI and ANTHONY (2011) mention that the various facies that characterize the coastal plain deposits in northern France, correspond - with high similarities - with those described in neighbouring Belgian parts of the coastal plain. This is beneficial for the comparison of the study area in Saint-Pierre-Brouck with the modelled area of Mannekensvere.

5.1.2 Geology in Saint-Pierre-Brouck It is possible to make an assumption of the Holocene deposits in the study area based on information derived from literature (discussed in chapter 2), combined with data retrieved

from 5 flush drillings that were made (will be discussed further) and 9 reports that were found of drillings at close proximity to the study area (see Annex 2). This information is also used to

Chapter 53

construct the initial setup of the pumping test and of the groundwater model. As the interest in this study only goes to the unconfined aquifer, with the lower boundary fixed at Ieper group, no interpretation was made of the underlying formations.

In the deeper drilling reports (2, 3, 4, 5) the top of a clay rich package, interpreted as Ypersian, varies from 15,3 m to 20,8 m. On top of this boundary, silts and sands can be found. It is striking that in all drilling reports, with the exception of drilling 6, no peat was found in the boreholes. This corresponds to the observations made during all 5 flush drillings. Here peat fragments were found, yet no clear peat layer was encountered; especially not a layer of 1 to 3 m thick, corresponding to the surface peat. The exception is drilling 6, positioned a few kilometres east of Saint-Pierre-Brouck. Here, a 1 m thick peat layer was observed. This matches the information derived from talking with a local farmer, who recalled that clay and peat packages are present east of our study area. Based on all this information, it is safe to assume that our study area is located within a tidal channel which formed after deposition of the so-called surface peat. Therefore eroding through the surface peat layer, which explains the peat fragments that were detected in the flush drillings. It is impossible to have an exact idea of the depth of this tidal channel based on this information alone, because the deepest performed flush drilling was only 9 m BGL.

5.2 Field installation

5.2.1 Research design A research design was elaborated with the following research objectives in mind: (1) to perform a pumping test and (2) to create a regional groundwater model. A total of 5 piezometers were installed in the southern part of Saint-Pierre-Brouck, at three different locations. These locations were chosen perpendicular to the river Aa, to best meet the requirements of building a regional groundwater model. The resulting locations, however, do not lie perfectly perpendicular to the river, because a permission had to be provided and that turned out to be not so evident. The first location, closest to the river Aa, is on one of the parcels of farmer Mr. Faveeuw which are used to grow corn, the second location is on a pasture of farmer Mr. Pas and the third location is on a grass field along the public road, near the Denna watercourse.

In order to meet the requirements of the pumping test, two extra piezometers were installed at

5 the first location. To eliminate the influence of the river Aa on the pumping test, it was chosen

to place the set of piezometers parallel to the river. The distance between the set of

Chapter 54

piezometers and the river Aa is approximately 130 m. The last two piezometers were positioned at 4 m and 17 m respectively from the former installed piezometer. In the continuation of this work, the five piezometers will be referred to as: 1A, 1B, 1C, 2 and 3. Figure 26 shows the exact location of the different piezometers.

Figure 26: Location of the placed piezometers within Saint-Pierre-Brouck (sources: Google earth/Google maps/own research). 5.2.2 Placing piezometers

th th

All five piezometers were installed on the 18 and 19 of February 2014. From the surface up 5

to the water table, a Dutch-type hand auger drilling was performed. This is because the

Chapter 55

borehole stays open above the water table, yet below the water table some kind of support is needed to keep the borehole from collapsing. Therefore, below the water table, the transfer was made to a direct flush drilling. This technique works as follows: (1) the drilling head is rotated gently to and fro, while water is injected through a hose attached to the drilling head; (2) through the rotating force and the erosive power of the water, sediment is released from the subsurface and transported upwards, out of the borehole; (3) the over-pressure which is created by the injected water ensures that the borehole does not collapse. The major advantage of this method is that the borehole can be drilled without being cased immediately. Direct flushing is generally only used for drilling small diameter boreholes, due to the relatively large quantities of flushing agent required.

Although it is practically impossible to pinpoint the exact location of lithological transitions with the use of a flush drilling, because the upcoming sediment is diluted and mixed before it reaches the surface, it still gives a reasonable indication of the subsurface. Following here, is a brief description of the different drillings. Figure 27 displays a summary of these data. The same main soil types occur in the different boreholes: (1) a first zone, up to 0,1 m, consisting of vegetation (grass) and adjacent root zone, (2) 0,5 to 0,7 m of dark, clay rich, humic sands which make up the top-soil and (3) the rest of the drillings, consisting primarily out of grey sands. While performing the flush drilling within the grey sands, some peat fragments emerged from the borehole. The sections where most peat fragments were coming forth, are indicated on the figure. Nonetheless, these sections may best be looked at with a degree of uncertainty, because the drilling head could have been at another location by the time the peat fragments reached the surface. It is also difficult to assess whether these fragments originate from thicker packages of sand, intercalated with peat fragments, or from small individual layers in the subsurface. However, the prevailing idea during the field study was that the peat fragments are intercalated within the sand package. Another observation which was made - around 2,3 m BGL in 1A and around 3,3 m BGL in 1C - was a narrowing of the borehole during the flush drilling. Above these constrictions, the flush drilling hosted a bigger discharge, corresponding with less permeable sediments surrounding the borehole. Below the constrictions the discharge dropped, indicating more permeable material. These constrictions could correspond with thin clay or peat layers, with less permeable sands on top and more permeable sands below. One possible, yet not exclusive, hypothesis is that peat forms more easily on more permeable sediments. Furthermore, within the drilling of location 2, more

5 reduction spots were observed in the vicinity of the water table, which may be an indication of

Chapter 56

a higher iron content. During the drilling at location 3, a lot of gas bubbles were observed when the drill head was around 5,4 m BGL, thus just below the estimated peat-rich sands. The composition of these gas bubbles is unknown (due to the lack of equipment to determine them), however the lack of colour and odour are indications that it may have been methane seepage. If so, this could be remnant from the reduction zone, though it is precarious to draw a conclusion from so few observations. After completing the drillings, piezometers were installed within the boreholes. These piezometers consist of two types of polyvinylchloride (PVC) tubes: (1) a regular closed tube and (2) a filter tube with equally spaced vertical slots, so the groundwater can enter the PVC tubes. The angular space between the PVC tube and the borehole was filled up with calibrated sand with a clay seal (bentonite plug) on top. This clay seal prevents the transport of water to the filter from overlying layers. Table 5 shows the filter interval of all the installed piezometers.

Chapter 57

-5 0 5 mIGN69 sand s sand ragment + f peat sand s sand ragment + f peat sand able ert wat sand clayey ion at veget A n f end drilling of LOCATION1 sand sand clayey ion at veget B a r able ert wat n f end drilling of sand s sand ragment + f peat sand s sand ragment + f peat sand s sand ragment + f peat sand sand clayey ion at veget C a r able ert wat n f end drilling of LOCATION2 s sand s sand ragment + f peat sand sand clayey ion at veget a r able ert wat n f end drilling of LOCATION3 sand s sand ragment + f peat sand sand clayey ion at veget a r able ert wat n f end drilling of

5 Figure 27: Lithological summary of hand/flush drillings (source: own research). (IGN69: reference level

for mainland France, 0 m IGN69 = 1,694 m TAW).

Chapter 58

5.2.3 Positioning and levelling of the piezometers After installing the piezometers, a GPS (GARMIN eTrex 30) was used to retrieve the exact location and a levelling campaign was performed to retrieve the exact height of all piezometers. The intent of the levelling campaign is to express certain points in the landscape in respect to a reference plane. For mainland France the NGF – IGN69 is used, this stands for the General Levelling of France (Nivellement general de la France) overseen by the Institut Géographique National and is defined by the mean sea level at the old harbour of Marseille. Compared to the Belgian zero level, 0 m IGN69 corresponds to 1,694 m TAW (Tweede Algemene Waterpassing). With the levelling devise it is possible to measure the height difference between a piezometer and a bench mark with a known height (Repère de nivellement). Fortunately there were several existing benchmarks in the area; therefore the levelling campaign could be performed with a limited number of intermediate steps. As a result, an overview of the different coordinates is given in Table 4 along with the filter interval of the different piezometers in Table 5.

Table 4: Coordinates of the top of the piezometers (source: own research). (IGN69: reference level for mainland France, BGL: Below Ground Level).

LAT/LON coordinates LAMBERT93 coordinates z name latitude longitude easting northing (mIGN69) 1A 50°52’37’’ 2° ’55’’ 643482,626 7087032,908 2,987 1B 50°52’37’’ 2° ’2 ’’ 642816,301 7087008,832 2,769 1C 50°52’37’’ 2° ’2 ’’ 642816,301 7087008,832 2,787 2 50°52’45’’ 2° ’55’’ 643485,141 7087280,695 2,283 3 50°52’34’’ 2° 2’24’’ 644049,818 7086934,32 2,491

Table 5: Filter intervals (source: own research). (AGL: Above Ground Level, BGL: Below Ground Level).

Filter interval Top filter name Top Bottom Top Bottom (mAGL) (mBGL) (mBGL) (mIGN69) (mIGN69) 1A 0,25 3,75 5,75 -1,013 -3,013 1B 0,19 2,3 3,3 0,279 -0,721 1C 0,18 2,75 4,75 -0,143 -2,143 2 0,17 3,83 4,83 -1,717 -2,717 3 0,19 3,81 5,81 -1,509 -3,509

Chapter 59

5.3 Field measurements This chapter provides an overview of the used field measurements, along with the results from these measurements. First the EM39 induction probe is discussed, followed by the clarification of fresh water head measurements and finally the groundwater quality study is explained. The pumping test is also a field measurement, though the extent and the importance of this pumping test ensured it to be discussed in a separate chapter.

5.3.1 EM39 With the EM39 probe, electromagnetical induction measurements were made in the 5 boreholes. These geophysical borehole measurements provide a better insight in the structure of the groundwater reservoir.

5.3.1.1 Method The focussed electromagnetic induction tool (EM39, Geonics©) is used to get a detailed vertical log of the lithology and the water quality. This is done by measuring the electrical conductivity of the sediments and the pore water. It is designed for the use in wells, which are encased with non-conductive materials (VANDENBOHEDE, LEBBE, GYSENS & DE WOLF, 2008). The method can be employed in many fields of study: (1) general geological surveys, (2) hydrostratigraphic studies and (3) contamination studies. In this study, it was used in a hydrostratigraphic study to identify fresh and saline water in the aquifer, where the salt water will influence the electrical conductivity properties of the water. The main advantage here is that a complete vertical salinity profile can be built, instead of just a screened interval (VAN MEIR & LEBBE, 2003).

The probe consists of two primary coils (see Figure 28). The first coil is a small internal transmitter coil (Tx), which induces a primary magnetic field by sending out an alternating current (39,2 kHz). This primary magnetic field will induce secondary currents around the well, called eddy currents (VANDENBOHEDE et al., 2008). Resulting from these eddy currents, an alternating secondary magnetic field is induced, which can be observed by the second coil in the EM39 probe; the receiver coil (Rx).

Chapter 60

Figure 28: Field setup of the EM39 electromagnetic induction tool (source: VAN MEIR & LEBBE, 2003). Within the EM39 probe, the transmitter (Tx) and receiver (Rx) coils are positioned 50 cm apart. This arrangement provides a large lateral range and a high degree of vertical resolution. However, due to the short distance between the different coils, the direct environment around the borehole - thus the drilling mud and lining of the well - will have a high influence. Therefore, a centrally located focusing coil is incorporated to solely measure the influence of the conductive surrounding of the borehole (VANDENBOHEDE et al., 2008). With this method, the device can be calibrated so that the response from the surrounding of the borehole is neglected; the resulting signal will correspond with 0,3 to 1 m around the centre of the well (VAN MEIR & LEBBE, 2003). As a result, the instrument will be blind for the conductivity of fluids in the borehole and for disturbances around it.

As long as the measured magnetic field is lower than 500 mS/m, it will be linearly proportional to the electrical conductivity of the material surrounding the well (VANDENBOHEDE et al., 2008). Above 500 mS/m, the relationship becomes non-linear and a correction curve needs to be used to get the effective conductivity of the surrounding (VAN MEIR & LEBBE, 2003). During this study the measurements never surpassed 25 mS/m, consequently no extra precautions needed to be taken. By calibrating the device, the electrical

conductivity of the sediment and pore water at the study area could be measured directly.

Chapter 61

Thus with the EM39 method, it is possible to form a detailed characterization of the surrounding of the boreholes. However for this study, it would be more interesting to get an idea of the salinization of the groundwater, surrounding the borehole. VAN MEIR and LEBBE (2003) developed a formula to seclude the information of the pore water from the bulk electrical conductivity of the surrounding, in a way that a first estimate can be made of the total dissolved solids (TDS) in the pore water. This formula, which is valid at a temperature of 10-11 °C, is as follows:

( 14 ) ( )

ith F the formation factor and σb the measured electrical conductivity in mS/m. The formation factor can be expresses as (VAN MEIR & LEBBE, 2003):

( 15 )

ith ρb the bulk resistivity and ρw the pore water resistivity. Based on empirical studies, the typical value of the formation factor for sandy deposits of the Belgian coastal plain equals 4 (LEBBE & DE BREUCK, 1994; VANDENBOHEDE & LEBBE, 2008; VAN MEIR & LEBBE, 2003). From placing the piezometers we know that the subsurface consists primarily out of sandy material, and there are no indications that the formation factor would be different at the coastal plain of northern France. Therefore, the relation between the total dissolved solids (TDS) and the measured electrical conductivity can be rewritten as:

( 16 ) ( )

To differentiate between the TDS values, DE MOOR and DE BREUCK (1969) defined different water classes. This classification (see Table 6) will be used to interpret the data from the EM39 measurements.

Chapter 62

Table 6: Classification of fresh-to-salt water, based on TDS content (source: DE MOOR & DE BREUCK, 1969).

TDS content Type of water (mg/l) Extremely fresh < 200 Fresh 200 - 400 Moderately fresh 400 - 800 Weakly fresh 800 - 1600 Moderately brackish 1600 - 3200 Brackish 3200 - 6400 Extremely brackish 6400 - 12800 Moderately salty 12800 - 25600 Extremely salty >25600

5.3.1.2 Implementation and results The execution of the EM39 measurements was performed on the 12th of March 2014 for the boreholes at location 2 and 3, and on the 13th of March 2014 for the three boreholes at location 1. During the execution, the probe was lowered into the borehole, and every 20 cm a value for the electrical conductivity was recorded. This was done from 90 cm below the top of the piezometer up to the deepest point of the borehole. The reason why no measurements were taken in the first 90 cm, is because of the distance from the receiver coil to the beginning of the EM39 probe (90 cm). The results of the measurements are shown in Figure 29, where the electrical conductivity is plotted versus the depth (in IGN69, the French reference level).

Chapter 63

EM39

1A 1B TDS (mg/l) TDS (mg/l) 200 400 600 800 200 400 600 800 3 3

2 2

1 1

0 0 mIGN69 mIGN69 -1 -1

-2 -2

-3 -3 5 10 15 20 5 10 15 20 Electrical conductivity (mS/m) Electrical conductivity (mS/m)

1C 2 TDS (mg/l) TDS (mg/l) 200 400 600 800 200 400 600 800 3 3

2 2

1 1

0 0 mIGN69 mIGN69 -1 -1

-2 -2

-3 -3 5 10 15 20 5 10 15 20 Electrical conductivity (mS/m) Electrical conductivity (mS/m)

3 ALL TDS (mg/l) TDS (mg/l) 200 400 600 800 200 400 600 800 3 3

2 2

1 1

0 0 mIGN69 mIGN69 -1 -1

-2 -2

-3 -3 5 10 15 20 5 10 15 20

Electrical conductivity (mS/m) Electrical conductivity (mS/m)

Figure 29: EM39 log of the different boreholes (source: own research).

Chapter 64

5.3.1.3 Interpretation The most striking observation made from these results, is that there is an absence of any salinity. The TDS values range from 316 to 832 mg/l, which corresponds to fresh to moderately fresh water (DE MOOR & DE BREUCK, 1969). Only one value surpasses 800 mg/l and can be described as weakly fresh water. This corresponds with the results from the groundwater samples, which will be discussed further. There is also a noticeable difference between the results from the three boreholes at location 1 and the results from locations 2 and 3. At location 1, the pore water shows a lower TDS content, hardly exceeding 600 mg/l. Also, the measurements at increasing depth do not show a higher salinity. We could even say that the TDS content displays a slight lowering below -1 m IGN69, nevertheless this effect is negligible. At locations 2 and 3 on the other hand, the TDS content shows a significant trend. The TDS in both locations rises with increasing depth, both up to around 800 mg/l at around - 2 or -3 m IGN69. It is unfortunate that the depth of the installed piezometers is limited, because this could be the upper zone of the fresh-to-salt transition zone, and it would have been very interesting to get direct measurements from deeper in the subsurface. In Figure 29, showing an overview of all EM39 measurements, no real trends may be inferred, except for the ones explained above. However, one must remember that the electrical conductivity depends not only on the composition of the groundwater (which is fresh water in the studied area), but it also depends on the composition of the sediments. The variation in electrical conductivity could be explained mainly by the variations in two characteristics of the sediments: (1) the porosity and (2) the electrical conductivity of the soil matrix. Because sands are characterized by a low electrical conductivity, and clays or peat by a high electrical conductivity, one explanation of the variation of the electrical conductivity could be that it is a result of the presence of peat fragments, along with a higher silt fraction, within the sand package.

5.3.2 Groundwater head To measure the groundwater heads automatically in the installed piezometers, Divers® were used. The Divers® measured the (water)pressure and temperature in the piezometers. The measurements were recorded every 10 minutes from the 13th of March until the 15th of April in piezometer 1A and 2. Unfortunately, something went wrong in the data storage of the pressure sensor in piezometer 2, so these data were not useable. The results, retrieved from piezometer 1A, are shown in Figure 30 and Figure 31.

Chapter 65

In Figure 30, the measured groundwater heads, in function of the French reference level (IGN69), are shown along with the measured temperature. During these 34 days, the groundwater head values varied from 1,596 m IGN to 1,945 m IGN (thus only 34,9 centimetre), which is relatively constant. With the ground level at location 1A around 2,737 m IGN, the groundwater table fluctuated slightly around 1 meter below the surface. The water temperature follows a positive trend over time, which is natural because the ground starts to warm up after winter. This temperature rise is limited and is ca. 0,6 degrees. It is unclear why the variations within the temperature measurements are so high before the 21st of March, compared with the results after this period.

In Figure 31, the measured groundwater heads are shown along with the daily precipitation. The meteorological data were retrieved from a weather station in the neighbourhood of the study area, more precisely from Bergues (approximately 20 km from Saint-Pierre-Brouck). Firstly, what should be mentioned is that very little precipitation occurred during the 34 days that the groundwater head values were measured. The measurements from Bergues indicate that in total only 4,53 mm of rain fell during this period on this location (this can be slightly different for the precipitation in Saint-Pierre-Brouck itself). The rain primarily fell during two periods: a first period from 21 until 28 March with 3 peaks during that period, and a second period from 6 to 8 April with one main peak. In the measured groundwater heads, one can see a relationship with the periods of precipitation, especially after the second period. A few hours after the precipitation, the groundwater table rises. The first period is characterized by a broader peak in the change of the water table over time, whereas it changed more abruptly in the second period. It is hard to define the exact time of the delay of the water table, because the precipitation values are summed up per day. The canalised Aa, in the neighbourhood of piezometer 1A, could also have an effect on the measurements. Unfortunately something went wrong with the measurements in piezometer 2, therefore is it impossible to deduce a relationship to the canal.

Chapter 66

piezometer 1A 2 11

1,9 10,8

1,8 10,6

1,7 10,4

C) 1,6 10,2 °

1,5 10

1,4 9,8 Temperature(

1,3 9,6 Groundwaterhead(mIGN) 1,2 9,4 Head 1,1 9,2 Temperature 1 9 13/03 17/03 21/03 25/03 29/03 2/04 6/04 10/04 14/04 Figure 30: Observed groundwater head and temperature values at piezometer 1A (source: own research).

piezometer 1A 2 4

1,9 3,5

1,8 3 1,7 2,5 1,6 Head 1,5 2 Precipitation 1,4 1,5

1,3 Precipitation(mm/d)

Groundwaterhead(mIGN) 1 1,2 0,5 1,1

1 0 13/03 17/03 21/03 25/03 29/03 2/04 6/04 10/04 14/04 Figure 31: Observed groundwater head values in piezometer 1A, and documented precipitation values from Bergues (source: own research/French Wunderground).

Chapter 67

5.3.3 Groundwater quality To get a better insight in the evolution of the groundwater quality, 4 samples were taken to be analysed chemically. One sample was chosen at every location (more precisely at 1A, 2 and 3), and at location 2 a second sample was taken because the owner of the land had a deeper pumping well at approximately 7 m depth. During the sampling, prior to the entry of oxygen which disturbs the original state, the electrical conductivity was measured. These samples were then analysed in the laboratory of soil science at Ghent University; here the ionic content of the different samples was measured. The samples were tested on the content of: (1) calcium, (2) potassium, (3) magnesium, (4) sodium, (5) chloride, (6) sulphate, (7) nitrate, (8) nitrogen dioxide and (9) bicarbonate. The results are shown in Table 7. The results of the nitrate and nitrogen dioxide are not shown in the table, because in most samples the detection limit was not reached. The only exception was a nitrate content of 2,8722 ppm detected in sample 2 (7 m BGL). This value could be explained by the way the sample was taken; the groundwater was directly pumped into a container used to store the water from which the water sample was taken. This water from the container is used for farming purposes, so it is to be expected that the container comes in contact with man-made fertilizers. By staying in this container, the water could have been contaminated which could explain the nitrate content. Overall the samples are characterized by a very low content of TDS (total dissolved solids), thus little mineralization. Based on the classification of DE MOOR and DE BREUCK (1969), shown in Table 6, the water samples are moderately fresh to weakly fresh. The dominant cation is calcium and the dominant anion bicarbonate, still the concentrations are very low.

Table 7: Chemical analysis of the groundwater samples.

Electrical Ionic content (ppm) TDS conductivity (ppm) (mS/m) Ca K Mg Na Cl SO4 HCO3

1A 119,40 113,95 13,14 9,68 39,94 43,42 35,82 427,39 683

2 208,40 263,64 35,61 10,84 38,70 43,73 280,27 576,77 1249

2 (7m BGL) 166,50 214,34 15,57 7,28 27,36 59,46 141,02 516,06 981

3 166,70 227,44 2,87 10,20 22,34 61,44 145,39 521,62 991 detection / 5,80E-02 1,20E-02 6,00E-03 1,10E-02 1,20E-01 1,20E-01 1,0E+00 / limit

Based on the results from the chemical analysis as shown in Table 7, the type and evolution of

5 the groundwater can be determined. Two methods are used for this: (1) the STUYFZAND

(1986) groundwater classification system and (2) the piper diagram. The STUYFZAND

Chapter 68

classification combines many aspects of the groundwater to get to a 4 symbol classification, namely: (1) the chloride content, (2) the water hardness, (3) the dominant cation(s) and anion(s) and (4) whether or not cation exchange has taken place. It is a method which is particularly useful where groundwater salinization or freshening has taken place. Table 8 shows the STUYFZAND classification of the different samples. On the other hand, the piper diagram is used to represent all 4 samples in one diagram, based on the cation and anion content. The position of the samples in the rhombic plane can indicate whether mixing has taken place between two other water types, and whether chemical reactions have occurred. This is especially true when different samples are taken at different evolution stages of the water, unfortunately the analysis of the 4 samples is very similar. The results of the piper diagram are shown in Figure 32.

According to the classification system of STUYFZAND (1986), the groundwater samples are fresh, hard to very hard, calcium bicarbonate water. The samples at location 2 show a (Na+K+Mg)-equilibrium, while the samples at locations 1 and 3 are characterized by a (Na+K+Mg)-surplus. In the piper-diagram the samples are positioned at the composition of fresh water (left corner), which also indicates calcium bicarbonate water. Based on these results, combined with what is known about the hydrological setting, one can assume that the groundwater is formed under freshening conditions, thus in the transition from an initial marine environment towards fresh water conditions. Initially the water type would have been S-NaCl0 (based on STUYFZAND) with as major ions: chloride, sodium, magnesium and sulphates. The absorbed cations would have been in equilibrium with the groundwater, indicated by a cation exchange code of 0. When a new fresh water (F-CaHCO30) member intrudes into the initial seawater saturated aquifer, both end members move along by advection. At their interface, mixing occurs due to hydrodynamic dispersion. Hereby, the chloride concentrations reduce proportionally. At the interface, the main chemical reaction is cation exchange, resulting in a surplus of marine cations and a deficit of the fresh water cations (especially calcium). According to the affinity sequence (sodium < potassium < magnesium), sodium will be released first by the clay minerals, then follows potassium and finally magnesium is exchanged, all against the fresh water cation calcium. This results in the formation of first NaHCO3+ type water, then MgHCO3+ and then CaHCO3+ type water. Finally, the groundwater evolves into an equilibrium state with the absorbed cations, and is classified as CaHCO30. Thus, the groundwater samples in this study correspond to the last

5 stage in the freshening of initial marine conditions, towards fresh water conditions.

Chapter 69

Table 8: STUYFZAND classification of the different groundwater samples.

STUYFZAND classification

1A F 2 Ca HCO3 + 2 F 3 Ca HCO3 0 2 (7m BGL) F 3 Ca HCO3 0 3 F 3 Ca HCO3 +

Figure 32: Piper diagram of the groundwater samples. In addition to the discussion of the type and genesis of the groundwater, the relationship between the ionic content and the electrical conductivity can be investigated. This relationship is presented in Figure 33.

First of all, one can observe that the fit of the electrical conductivity to the various ions varies considerably. The coefficient of determination (R²), indicating how well the data points fit the linear curve, ranges from a very poor fit (R²: 0,0031) to a very good fit (R²: 0,9858). Especially where the fit is good, a positive linear relationship exists; more ions in solution correspond to a higher electrical conductivity. The best fit (R²: 0,9319 – 0,9858) clearly corresponds with ions that are more abundant, namely: calcium, bicarbonate and sulphate. The calcium and bicarbonate content can be explained by the presence of fresh water. On the

5 other hand, the concentration of sulphates shows that the samples were taken in a zone where

Chapter 70

the oxidation of sulphide minerals (e.g. pyrite) has occurred, and probably not in an area where sulphates were reduced to sulphides. The worst fit (R²: 0,0031 – 0,0102) corresponds to sodium and chloride; ions that are typical for salt water. The coefficient of determination of magnesium and potassium lies somewhere in between.

Chapter 71

Electrical conductivity in function of the solute content

300 40 y = 1,701x - 76,241 35 250 R² = 0,9319 y = 0,2413x - 23,085 30 (ppm) Potassium 200 R² = 0,4108 25 150 20 15 100

10 Calcium(ppm) 50 5 0 0 0 50 100 150 200 250 0 50 100 150 200 250 Electrical conductivity (mS/m) Electrical conductivity (mS/m)

12 70

10 60

Chloride (ppm) Chloride

8 50 40 6 y = 0,012x + 7,5137 30 4 R² = 0,0792 y = 0,0151x + 49,519

20 2 R² = 0,0031

10 Magnesium(ppm) 0 0 0 50 100 150 200 250 0 50 100 150 200 250 Electrical conductivity (mS/m) Electrical conductivity (mS/m)

45 300 40 y = 2,7316x - 300,78 250

35 R² = 0,9833 (ppm) Sulphate

30 200 25 150 20 15 100 y = -0,0239x + 36,042

10 Sodium(ppm) R² = 0,0102 50 5 0 0 0 50 100 150 200 250 0 50 100 150 200 250 Electrical conductivity (mS/m) Electrical conductivity (mS/m)

700 600

500 400 300 y = 1,6868x + 231,71 200 R² = 0,9858 100

Bicarbonate(ppm) 0 0 50 100 150 200 250 Electrical conductivity (mS/m)

Figure 33: Graphs showing the relationship between the electrical conductivity of the groundwater in function of the solute content.

Chapter 72

5.4 Pumping test In order to have better insights in the hydraulic parameters of the study area, a pumping test was executed. The response data from the pumping test are used to estimate the hydraulic properties of the subsurface in location 1; this was done by means of the inverse numerical model HYDPARIDEN. These results were then used, in association with other data, to build the regional groundwater model of Saint-Pierre-Brouck.

This chapter describes the execution and interpretation of the pumping test. First, the location and the setup of the pumping test are discussed, along with a clarification of the execution. Subsequently, the subdivision of the groundwater reservoir in the numerical model is explained, followed by a discussion on the initial setup and a brief description of how the model is calibrated. Finally, the results and interpretation of the model are given.

5.4.1 Lay-out and implementation The setup for the pumping test consisted of a gasoline pump, a pumping pit, two monitoring wells and a set of (Schlumberger) pressure sensors. The pumping test was performed in location 1, where three piezometers were installed. It was opted to install the piezometers in a line parallel with the river Aa, so that the influence of the river is equal in all three the piezometers, and thus eliminable. For the test, piezometer 1A was used as the pumping pit. The two other piezometers at location 1, 1B and 1C, acted as monitoring wells. From pumping well 1A, the distance to 1B is 4 m and 17 m to 1C. The location and setup of the pumping test is visualized in Figure 34; and for a detailed overview of the filter intervals and the exact coordinates of all piezometers, we refer to Table 4 and Table 5.

The pumping test started on the 12th of March 2014 at 11:40:50 and ended the same day at 16:28:50, so the test was performed for 4 hours and 48 minutes. During the pumping test, the lowering of the groundwater table was measured every 15 (1A – 1C) or 30 (1B) seconds with the use of (Schlumberger) pressure sensors. These pressure sensors, or Divers®, measured the (water)pressure, temperature and - in the pumping well - electrical conductance (EC). However, only the measured (water)pressure was used for the interpretation of the pumping test. To estimate the discharge of the pumping pit, the time needed to fill a bucket was recorded. This was done frequently during the pumping test and resulted in an estimated constant discharge of 86,4 m³/d during the pumping test.

Chapter 73

Figure 34: Location and lay-out of the pumping test (sources: Google earth/own research). (PP: pumping pit, MW: monitoring well). 5.4.2 Subdivision of the subsurface The subdivision of the groundwater reservoir into the numerical model is based on the local lithology. For a detailed overview of the local lithology, we refer back to chapter 2 where the geology in Saint-Pierre-Brouck is discussed. Because of the scarcity of the data, compared to the thorough Belgian database (DOV), this implies a definite uncertainty. The schematization of the numerical model also assumes homogeneous and anisotropic layers, which could conflict with reality.

For the interpretation of the pumping test, the groundwater reservoir - and thus the numerical model - was subdivided into 12 layers. The base of the numerical model is located at a depth of 20 m BGL. This depth corresponds with the base of the Pleistocene silty sand/silt deposits or the top of the clay of the Ieper group. The clay of the Ieper group serves as the bounding aquitard for the groundwater reservoir/ numerical model. On top of the Ieper clay, two layers with a (presumably) Pleistocene age are present. The first one (layer 1) has a thickness of 2 m and is composed of silty sand, whereas the second one (layer 2) also has a thickness of 2 m and is composed out of silt. The presumed presence of these 2 layers in the study area is based on their presence in a drilling report (drilling report 2, annex 2) from the neighbourhood of the study area, as the flush drillings executed in the framework of this master thesis do not reach this depth. On top of these two layers, the infilling of a late Holocene tidal channel is modelled. This is rectified based on the literature review and the observations made during the

5 field measurements. The late Holocene tidal channel mainly consists of (moderately) coarse

Chapter 74

sands, with peat fragments and loam admixture near the top of the groundwater reservoir. To go into more detail, layers 3 until 8 (ca. -3.7 to -16 m BGL) are all composed out of moderately coarse sand. The thickness of the layers varies from 0.46 m to 4 m. On top of these layers, 2 layers of coarse sands are present (layers 9 and 10). These layers have a thickness of 0.44 m and 1 m respectively and correspond with the zone of (very) high permeability which was encountered during the installation of the wells. On top of these (very) permeable layers, two layers with a lower permeability are present. These layers (layers 11 and 12) have a thickness of 0.1 m and 1 m respectively and consist of peat-rich silty sand. They correspond with the zone of lower permeability and the presence of peat fragments encountered during the execution of the flush drillings. The top of layer 12 corresponds with the groundwater table. The subdivision of the layers in the numerical model was made in such a way that the screen of pumping well (piezometer A) and the closest observation well (piezometer B) coincide with layer 8 and 10 respectively. The observation well at 17 m from the pumped well (piezometer C) is located in three layers (layer 8 - 10), but has been assigned to layer 8 for the interpretation of the pumping test. This is rectified, as layer 8 takes the largest part of the filter interval and the distance towards the pumping well is already rather far. The thickness of the different layers was chosen in order to receive the best results out of the model; with thin layers close to the pumped layer and gradually thickening layers away from the pumped layer. This is because principally vertical flow to the pumped layer decreases with increasing distance to this layer. The lithological description of the subsurface is presented in Figure 35 along with the corresponding schematization in the numerical model.

Chapter 75

mBGL A B C 0 sand watertable Layer

peat rich/loamy sand 12 11

coarse sand 10 9

8 5

moderately coarse sand 10 4

silt 2

silty sand 1

20 ////////////////////////////////////////////////////////// clay

Figure 35: Lithological description of the subsurface along with the subdivision of the groundwater reservoir in the numerical model (source: own research). (BGL: Below Ground Level).

Chapter 76

5.4.3 Initial setup of the numerical model, and model calibration The initial values of the hydraulic parameters were estimated based on the lithological data. These parameters were then calibrated using the HYDPARIDEN software package (see Table 9 for the initial values of the parameters). It is important to mention that the hydraulic resistance, which is noted as C(i), indicates the hydraulic resistance between layer i and the layer on top, i+1.

For the proper functioning of the model, it is important to group parameters which have a similar effect within the system. Parameters within a single group are adjusted in the same way during every iteration. Not all parameters are put in parameter groups, because some parameters are too sensitive, and it may cause the crash of the used software. After some iterations with the invers model combined with a number of sensitivity analyses, four groups were formed within the hydraulic parameters; these are displayed in Table 9.

Group 1: The first group of parameters consists of the horizontal hydraulic conductivity and the hydraulic resistance of the first 8 layers. Layer 1 and 2 are included in this group because they are positioned the furthest from the pumped layer and so they don’t have a huge effect. By considering these parameters as one group, the number of parameter groups to optimise is limited. The principal reason for this larger grouping is that it enhances the sensitivity of the parameter group. Nevertheless, the initial values for layer 1, 2 and 3-8 are different. The hydraulic conductivity and the hydraulic resistance are put in the same group because they evolve similarly, but inversely: an increase of the horizontal hydraulic conductivity will result in a decrease of the hydraulic resistance and vice versa. So within this group Kh(1-8) and C(1-7) are gathered. Group 2: Here the horizontal hydraulic conductivity and the hydraulic resistance of the two coarse sand layers are grouped together, thus from layers 9 and 10. This relates to Kh(9-10) and C(8-9). Group 3: This group is analogue to group 2, yet now the horizontal hydraulic conductivity and the hydraulic resistance of the two peat-rich sand layers are combined, corresponding to layer 11 and 12. This corresponds to Kh(11-12) and C(10-11). Group 4: This last group combines the specific elastic storage (Ss) of all layers, because

these values will know a similar evolution. The initial values are calculated with the

Chapter 77

formula of Van der Gun (see LEBBE, 1999 on page 46), which uses the depth interval. Thus this group is composed of Ss(1-12). The model was calibrated in two different manners used simultaneously: (1) manually by adjusting the parameters based on the interpretation of the fit between the simulated and measured values, and (2) by using the iterative process within HYDPARIDEN software where the parameter groups are adjusted in order to minimize the discrepancies between the model-generated data and the corresponding measurements. When the inverse process converges, the process is continued until an optimal set of parameters is found. As this does not automatically take into account optimal interpretation (e.g. a good model fit can provide physically unrealistic parameters), the combination of HYDPARIDEN and manual calibration turned out to be a great solution.

Table 9: Initial estimated values of the hydraulic parameters. The different parameter groups are put in a coloured frame (source: own research).

horizontal specific specific well loss layer thickness hydraulic anisotropy resistance elastic yield coefficient conductivity storage Kh(i)/ i D(i) (m) Kh(i) (m/d) - C(i) (d) Ss(i) - S0 - C - Kv(i) 1 2,00 10 3,00 0,9000 3,48285E-05 0,02 0,0001

2 2,00 5 3,00 0,7000 3,75175E-05

3 4,00 25 1,25 0,1750 4,31373E-05

4 3,00 25 1,25 0,1250 5,23591E-05

5 2,00 25 1,25 0,0700 6,27830E-05

6 0,80 25 1,25 0,0315 7,10740E-05

7 0,46 25 1,25 0,0615 7,60167E-05

8 2,00 25 1,25 0,0610 9,13444E-05

9 0,44 25 1,25 0,0360 1,09380E-04

10 1,00 25 1,25 0,1250 1,30232E-04

11 0,10 5 10,00 1,1000 1,59905E-04

12 1,00 5 5,00 - 2,01602E-04

5.4.4 Results

The results of the numerical model of this pumping test are shown in the output of the model,

this info is summarized in: Figure 36, Figure 37, Table 10, Figure 38 and annex 3. Due to this

Chapter 78

large output quantity, it is possible that one does not see the wood for the trees anymore. Therefore a short explanation is given here of this output:

 Figure 36: this figure shows a schematization of the numerical model, on which the optimal values of the hydraulic parameters are displayed. These optimal values are the result of multiple iterations of the numerical model, as explained in the model calibration.  Figure 37: on this figure, the observed and calculated drawdowns are displayed in function of time since the start of the pumping on the one hand and in function of distance from the pumping well on the other hand. The observed drawdowns correspond to 70 observations, which are derived data recorded with the Divers®, more precisely from the Diver® within the pumping well 1A (25 observation), and the observation wells 1B (22 observations) and 1C (23 observations).  Table 10: this table gives an overview of the output of the numerical model with regard to the confidence and dependence of the parameter groups. The parameter groups are numbered as described in the previous chapter. It is important to mention that the optimal value of a parameter group corresponds to the optimal value of the first hydraulic parameter within this group, thus not with an average for the entire group. For groups 1, 2, 3 and 4 this relates to: the horizontal hydraulic conductivity of layer 3 ‘Kh(3)’, the hydraulic resistance between layers 9 and 0 ‘C(9)’, the hydraulic resistance between layers 0 and ‘C( 0)’ and the specific elastic storage of layer 1 ‘Ss( )’ respectively.  Figure 38: this figure displays the cross sections through the joint confidence area around the optimum values of the hydraulic parameter groups. Because we worked with 4 parameter groups, a 4-dimensional joint confidence ellipsoid can be shaped. To be able to visualize this on paper, 6 cross-sections are made through this 4- dimensional ellipsoid. Each of these cross-sections then considers two parameter groups, while the other two parameter groups stay constant at their optimal value. The centre of each cross-section matches with the optimal values of the considered parameter groups. The same comment applies to this figure as for Table 10 that the optimal value of the parameter group corresponds with the other optimal value of the first hydraulic parameter within this group.

 Annex 3: presents part of the .lst file, one of the output files of the model. This .lst or

LIST file summarizes: (1) model parameters and optimal hydraulic parameters, (2) the

Chapter 79

observed drawdowns at certain times from the start of the pumping test, used to calibrate the model, (3) the drawdown in every ring around the pumping well per time step and (4) the comparison of the observed and calculated drawdown along with the mean and standard deviation. Except for (3) the drawdown in every ring around the pumping well, all data are given in annex 3.

Chapter 80

Chapter 81

Figure 37: Measured (crosses) and calculated (full line) drawdown in function of time and distance for layer 8 and 10 (source: own research).

Chapter 82

Table 10: Summary of the output of the model, giving the reliability and the dependence of the various parameter groups (source: own research).

MARGINAL AND CONDITIONAL STANDARD DEVIATION Marginal standard Conditional standard Par. gr. Optimal value Unit deviation deviation 1 4,289711923 m/d 0,0085 0,0049 4 0,000040651 m-1 0,0245 0,0201 2 0,033580905 d 0,0199 0,0172 3 3,579137233 d 0,1073 0,0574

MATRIX OF CORRELATION COEFFICIENTS Par. gr. 1 4 2 3 1 1,0000 0,7686 0,0086 -0,2797 4 0,7686 1,0000 0,1043 -0,5259 2 0,0086 0,1043 1,0000 0,2832 3 -0,2797 -0,5259 0,2832 1,0000

CONDITION INDEXES OF SENSITIVITY MATRIX 1,0 3,4 4,8 22,1

MATRIX OF MARGINAL VARIANCE-DECOMPOSITION PROPORTIONS 1 0,2937 0,2233 0,3944 0,0886 4 0,0042 0,1188 0,5805 0,2965 2 0,0003 0,4863 0,4322 0,0811 3 0,0000 0,0003 0,0003 0,9994

INVERSE CONDITION INDEXES OF COVARIANCE MATRIX 22,1 4,8 3,4 1,0

MATRIX OF INVERSE CONDITIONAL VARIANCE-DECOMPOSITION

PROPORTIONS 1 0,0000 0,0007 0,0002 0,9991

Chapter 4 0,0018 0,4622 0,4428 0,0932 83 2 0,0015 0,1346 0,7106 0,1533 3 0,2499 0,6200 0,0658 0,0642

Figure 38: Cross-sections through the joint confidence area around the optimum values of the hydraulic parameter groups. The axes indicate the variations of two parameter groups, while two other groups stay at optimal values (K1-8: group 1 ; C8-9: group 2 ; C9-10: group 3 ; S1-12: group 4) (source: own research). 5.4.5 Interpretation In Figure 37, which illustrates the measured and the calculated drawdown in function of time and distance, one can observe that the fit is pretty decent. This is backed up by a small sum of squares of the residuals, equal to 0,09770 for the 70 observations.

The optimal values of the hydraulic parameters according to the subdivision of the groundwater reservoir and the assumption made during the parameterization, results in a close match between the observed and the simulated drawdowns. Layers 1 and 2, which correspond

with silts and silty sand, present a horizontal hydraulic conductivity of 1,72 m/d and 0,858

m/d respectively. On top of this, the main package of moderately coarse sands (layers 3-8),

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corresponds with a slightly higher horizontal hydraulic conductivity of 4,29 m/d. The layers descripted as peat-rich sands (layers 11-12) have a significantly lower horizontal conductivity of 0,14m/d, due to the low permeable peat-fragments (and possibly the higher silt content) within the package. The coarse sands directly below (layers 9-10) have a significantly higher horizontal hydraulic conductivity of 24,2 m/d. This highly matches the observations made during the flush drilling, and apparently contributed to a good fit of the calculated to the observed drawdowns.

The same differentiation - (1) silt and silty sand, (2) moderately coarse sands, (3) coarse sands and (4) peat-rich sands - is retrieved from the optimal values of the hydraulic resistance. A high resistance in the silt and silty sands and in the peat-rich sands indicates a lower vertical hydraulic conductivity. Especially the uppermost layer, with a hydraulic resistance of 75,2 d, has a low permeability. The biggest package of the moderately coarse sands has a significantly lower hydraulic resistance and thus a higher permeability, it corresponds to a vertical hydraulic conductivity in the package of 3,43 m/d. Ultimately, the coarse sands below the peat-rich layer show the lowest hydraulic resistance. It is important to mention that the hydraulic resistance results cannot be interpreted directly, one must keep in mind that these values are also subject to the thickness of the layers.

From annex 3, where the optimal values of the hydraulic parameters are given, we learn that the C-value of the well loss equals -0,000315 m-5d2. Especially noteworthy is the fact that the sign is negative. Therefore this is a pumping well which is overly developed, the pumping efficiency is greater than 100 %. Due to the pumping of water, fine particles are extracted from the sediments in close proximity of the screen, resulting in a zone with a higher conductivity than the conductivity of the unaltered sediments. The horizontal hydraulic gradient in the vicinity of the pumping well is smaller than in the case of unaltered sediments. This implies that the drawdown observed in the pumping well, is smaller than would be expected. Based on the equation which describes the well loss (described in chapter 3), the total lowering can be quantified. In this situation, the drawdown in the pumping well is 2,35 m lower than would be expected if the surrounding of the pumping well was unaltered. This explains why in Figure 37, in layer 8, the observed and modelled drawdown of the pumping well (1A) correspond with approximately the same value, at time zero. Also, the abrupt jump in the drawdown over distance in layer 8 is a result of the well loss.

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From Table 10 and Figure 38, one gets a better insight in the statistics of the parameter groups. Parameter group 1 (Kh1-8 & C1-7) has the greatest accuracy, characterized by the smallest marginal and conditional standard deviation. However, the marginal standard deviation is significantly higher than the conditional standard deviation, thus the parameter group has certain dependence towards other parameter groups. From the matrix of correlation coefficients one can deduce that the main dependency relates to parameter group 4 (Ss1-12). Corresponding to the large marginal and conditional standard deviation, one identifies that parameter group 3 (Kh11-12 & C10-11) is derived with the smallest accuracy. Here also, the marginal standard deviation is significantly greater than the conditional standard deviation, with a high dependency to parameter group 4. The rest of the parameter groups show a small variation between the marginal and conditional standard deviation, and rather small correlation coefficients, which indicate that dependency to other parameter groups is limited.

The largest condition index points to the fact that the largest principal axis of the ellipsoid is 22,1 times larger than the smallest principal axis. The other condition indexes show us that the second and third largest principal axes are 4,8 and 3,4 times the smallest axis respectively. There is thus a weak dependence between the parameter groups. However, the condition number (largest condition index) is responsible for 99,94 % of the marginal variance of parameter group 3, yet it has a far smaller effect on the other parameter groups. This information parallels the information from the marginal and conditional standard deviation, where it was also shown that parameter group 3 has the smallest accuracy. This is also reflected in Figure 38, where all cross-sections which visualize variations in parameter group 3 show a large spread. Thus the hydraulic conductivity and resistance of the top layers are the least truthful in the numerical model.

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5.5 Groundwater model

5.5.1 Model setup This section describes all properties that were used in the groundwater model of Saint-Pierre- Brouck, as they originated from field measurements, literature and the interpretation of the pumping test.

5.5.1.1 Grid properties This groundwater model, located in Saint-Pierre-Brouck, is modelled in 3 dimensions. The length and width of the model were taken equal to the groundwater model of Mannekensvere: the length of the model is 4 km and the width is 3 km. The lower left corner corresponds with 641576,0 m east and 7085632,0 m north in the Lambert934 coordinates system. The upper right corner corresponds with 644664,0 m east and 7089565,0 m north. As can be seen in Figure 39, which shows the location of the study area in Saint-Pierre-Brouck, the model is not perfectly east-west oriented. The rows of the model have an angle of 75 ° with respect to the north, counting in clockwise direction.

For the thickness of the model 20 m is adopted. The base of the model is located at -17,5 m IGN, while the top of the model is formed by the groundwater table, which varies between 0 and 4 m IGN. The mesh consists of 160 columns of 25 m, 120 rows of 25 m and 8 layers of 2,5 m. In the upper layer, this thickness can vary due to the position of the water table (the base is always located at 0 m IGN, while the top is formed by the water table). This gives a total of 268800 cells within this groundwater model. In Figure 40, the topography of the study area is shown. Based on this topography, the initial fresh water head and - as of here forth - also the initial salinity are demarcated. This topography was retrieved online from the national institute of geographical and forestry information (IGN ‘Institut National de l’Information Géographique et Forestière’ in French). Unfortunately, the resolution of the retrieved raster file was only 75x75 m, which is a bad resolution compared to the raster file used in the groundwater model of Mannekensvere. This difference is clearly visible in the figure.

4 Lambert93 is the geographic projection and coordinates system used in France.

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Figure 40: Grid of the groundwater model along with the topography of the study area (source: Institut Na ional De L’Informa ion Géogra ique e Fores ière/own research).

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5.5.1.2 Time properties The total simulation time has been taken the same as in the groundwater model of Mannekensvere, thus a time period of ca. 40 years. Since we started from the hypothetical salinity map of the present situation (more on this later), the model ends in 2054. The total time was subdivided into 13 different periods, and for each of these periods the boundary conditions stayed constant. For the first period, a break-in period with a length of 3,65 days was chosen. This period was executed in 50 steps. The following 5 periods have a period length of 365,25 days (= 1 year) and were also divides in 50 time steps. These relatively short periods are chosen so that the results can be retrieved frequently in the first years. The following 7 periods have a longer period length of 1826,25 days (= 5 years) and are again subdivided in 50 time steps. For all time steps, the time step multiplier (TSMULTI) was taken equal to 1, so that all time steps within a period have the same length.

5.5.1.3 River properties The rivers are implemented through the MODFLOW river package. Figure 41 shows the rivers as they are put into the model. Because only little information was found on the classification of the different types of rivers, it was chosen to use the same subdivision as in the model of Mannekensvere: (1) the navigable watercourses and (2) the unnavigable watercourses, with the latter again subdivided in three other categories.

These four groups are represented in Figure 41 with different thicknesses. The only navigable watercourses are the river Aa and a side-branch, the ‘Canal de Calais à Saint-Omer’. The different categories within the unnavigable watercourses are shown as increasingly thinner lines. Each group has other river properties assigned, which are stored in the *.riv file. Except for the river Aa and the side-channel, where the width and depth of the river were measured in the field, no information of the actual hydraulic properties of the different (unnavigable) rivers in the north of France was known. Hence, the same properties as in the groundwater model of Mannekensvere were used. The applied properties are: (1) the navigable watercourses have a width of 21 m and a water level in the river equal to 2,2 m IGN, (2) the first category of the unnavigable watercourses is characterized by a width of 10 m and a water level equal to 1,0 m BGL, (3) the second category of unnavigable watercourses is modelled with a width of 4 m and a water level equal to 0,9 m BGL and finally (4) the third category of the unnavigable watercourses is characterized by a width of 2 m and a water level in the river

of 0,8 m BGL. Clearly, these values are strongly dependent on the topography, and consequently this application creates an immediate decrease in accuracy because the

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topography maps are not very detailed. The overall concentration of the water in the rivers is equal to 1 % salinity. The hydraulic resistance towards the ditch was set to 3 days for all river categories.

Figure 41: Indication of the rivers in the study area, the different categories of the rivers are visualized with different thicknesses, the locations where piezometers were installed are shown in red (source: Google earth/own research). 5.5.1.4 Drainage properties In this groundwater model, the drainage was also included in the MODFLOW river package. Every uppermost active cell in the model area where no actual river is, receives a contact factor, a water level, a maximum rate of infiltration and a salinity percentage in function of the soil type. No direct measurements of these properties were made, therefore we tried to use the same profile types as in the model of Mannekensvere. Based on the executed field measurements, the properties of the top layer show the most similarities with profile type (22) ‘Creekridge’; primarily sand facies which are deposited while filling old channels. The hydraulic resistivity towards the drainage system is set to 83 days, the maximal infiltration speed to 4.0x10-4 m/d and the infiltration salinity to 1 %. The given hydraulic resistivity is used to calculate the contact factor.

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5.5.1.5 Recharge properties One set of recharge properties is estimated for the total study area. The recharge flow rate is set to 0,25 m³/d, with a concentration of the recharge water equal to 0 % salinity. It was opted to simulate the recharge with the MODFLOW well package; here every upper active cell of the model is filled with well-cells in order to let the flux, which is kept constant throughout the total simulation period, get into the model.

5.5.1.6 Properties of the subsurface The discretization of the groundwater reservoir is based on data from the drilling reports, combined with the results from the pumping test. Because laterally, not much information is known about the subsurface, it was chosen to discretize the subsurface in homogeneous, anisotropic layers which are placed horizontally. It is possible that this simplification conflicts with reality. The base of the numerical model is located at a depth of 20 m BGL or – 17,5 m IGN. This boundary corresponds with the transition of the Pleistocene silty sand/silt deposits towards the clay of the Ieper group. The clay of the Ieper group forms the bounding aquitard. On top of this boundary, three main packages are modelled: (1) a Holocene peat rich/loamy sand package, (2) a Holocene coarse sand package and (3) the Pleistocene silty sand/silt package. In what follows, the hydraulic parameters of these three packages are discussed. An overview is given in Figure 43.

 Holocene peat rich/loamy sands From the pumping test we know that the sandy deposits with peat and loam fragments are characterized by a lower hydraulic permeability. It is much harder to extrapolate this layer laterally over the model area. Based on the observations made during the placement of the piezometers (see Figure 27 in chapter 5.2.2), we defined the first 5 m as Holocene peat rich/loamy sands. This corresponds with the first two layers of the model, or from level 2,5 m IGN to -2,5 m IGN. Although the hydraulic parameters of the other packages are based on the results from the pumping test, another principle was used while assigning properties to the Holocene peat rich/loamy sands package. The principle of relief inversion was applied: the sandy channel deposits - which were historically depressions in the environment - are now elevated, as compared to places dominated by peat and clay rich material (because these deposits compacted much more). By combining the idea of relief inversion with the common knowledge that sandy material has a relatively high hydraulic conductivity, while peat and

5 clay rich material is characterized by a relatively low hydraulic conductivity, a link is formed between the hydraulic properties and the topography. This link was used to set the hydraulic

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parameters of the first two layers of the groundwater model. For every 25x25 m cell, the hydraulic conductivity and the hydraulic resistance are based on the topography; a higher topography corresponds with a high hydraulic conductivity (sandy material), where a lower topography has a lower hydraulic conductivity (peat or clay rich material). The result of this principle for the horizontal hydraulic conductivity is shown in Figure 42. The horizontal hydraulic conductivity ranges from 0,36 m/d to 4,06 m/d, while the vertical hydraulic conductivity ranges from 0,29 m/d to 3,30 m/d. The porosity of the package is set to 0,38. The results correspond with the idea that peat is present in the eastern part of the modelled area, where unfortunately no field measurements were performed. Here again, we must emphasize that the resolution of the topography raster is rather low, producing less accurate results.

Figure 42: Illustration of the horizontal hydraulic conductivity of the first and the second layer of the groundwater model (source: own research).

 Holocene coarse sands A total thickness of 11 m, or from -2,5 m IGN to -13,5 m IGN, of Holocene coarse sands was modelled. The hydraulic parameters of this package are based on the results from the pumping test, more precisely from the similarly named coarse sands. The horizontal hydraulic conductivity was set to 4,3 m/d, with the anisotropy equal to 1,254. The vertical hydraulic

conductivity equals 3,43 m/d. This unit has a porosity of 0,38.

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 Pleistocene silty sands/silts The last package, with a total thickness of 4 m, is modelled from -13,5 m IGN up to the lower boundary at -17,5 m IGN. For this package, the hydraulic parameters are also based on the results of the pumping test. This time, the values are set to the average of the two Pleistocene layers: (1) the silt layer and (2) the silty sand layer. The horizontal hydraulic conductivity of the Pleistocene deposits was set to 1,29 m/d, with the anisotropy set to 3,14 to get to a vertical hydraulic conductivity of 0,41 m/d. Furthermore, the unit is characterized by a porosity of 0,38.

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mIGN Layer 2,5

Kh and Kv in function of the Holocene peat rich topography /loamy sands porosity = 0,38

-2,5

-7,5 Kh = 4,4 m/d Holocene coarse sands Kv = 3,43 m/d porosity = 0,38 5

-12,5

Kh = 1,29 m/d Pleistocene silty Kv = 0,41 m/d sands/silts porosity = 0,38 8

-17,5 ////////////////////////////////////////////////////////// clay of Ieper group

Figure 43: Vertical cross-section through the groundwater reservoir (source: own research).

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5.5.1.7 Fresh water head properties The groundwater heads in the numerical model will be expressed in fresh water heads. The initial heads were based on the topography (shown in Figure 40). Even though the initial head values have a limited influence on the end results, it is still very important that these values are well estimated. This is because in this groundwater model, the initial salt concentration is based on these initial head values (more on this in the next section). The initial head value was calculated for every finite difference cell with the following simple formula:

( ) ( 17 )

Where HEAD is equal to the initial head value in every cell and TOPO is the z-value of the topography in m IGN. By applying this formula, the initial head values closely resemble the topography, however they are slightly smoothed out. For every layer, the initial head values are equal. From the topography - which ranges from 0,3 m IGN to 4,5 m IGN - one can deduce that the initial head values vary from -0,775 m IGN to 2,375 m IGN.

5.5.1.8 Solute transport properties The main solute transport parameters are set to equal the groundwater model of Mannekensvere, with the following values: the longitudinal dispersivity was set to a value of 0,3 m, the horizontal transverse dispersivity to a value of 0,05 m and the vertical transverse dispersivity was set to 0,03 m. The retardation factor was retained simple to 1.

Unfortunately, the salinity map of DE BREUCK et al. (1974) ends at the Belgian border, therefore another method had to be used to set the initial groundwater concentration in the model. We opted to use the salinity percentage again, to maintain simplicity while comparing the two groundwater models. In order to retrieve the initial groundwater concentration, the law of GHYBEN-HERZBERG (GHYBEN, 1889; HERZBERG, 1901) was applied. This law expresses the depth of the salinity level as a function of the groundwater table.

( ) ( 18 )

With D the depth to the salinity level below mean sea level, H the height of the groundwater table above the mean sea level, ρf the density of fresh water and ρs the density of sea water.

In the study area, the density of salt water is approximated as 1020 kg/m³ and the density of fresh water as 1000 kg/m³. The mean sea level is set to 0,65 m IGN, which corresponds with

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2,35 m TAW at the Belgian coast (LEBBE et al., 2006). So based on the law of GHYBEN- HERZBERG, we know that for every place where the groundwater table lies a certain value higher than the mean sea level (0,65 m IGN), the salinity level lies 50 times this value below the mean sea level. By using the initial fresh water head values to retrieve the initial groundwater concentration, we ensure that the initial concentration depends on the topography. The top of the transition zone between fresh and salt water is defined at a TDS of 1500 mg/l. Figure 44 shows the initial groundwater concentration for layer 3. It is clearly shown that the east of the study area is characterized by mainly salt water (100 % salinity), corresponding with lower situated region, while the western side - which is situated higher - contains only fresh water (0 % salinity). This initial approximation does not correspond completely with the observations made during the field measurements. Chemical analysis of all groundwater samples (from all piezometers) showed it to be fresh water. This can also be seen in Figure 44, where locations 1 and 2 lay within the fresh water region. However, location 3 lays within the salt water region, which does not correspond with the chemical analysis. This is a very interesting point to keep track of while running the groundwater model. It is also possible that the position of the transition zone between fresh and salt water is not entirely accurate, mainly due to the many assumptions that were made.

5 Figure 44: Illustration of the initial concentration put into the model, here layer 3 is shown (source: own research).

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5.5.1.9 Boundary conditions On the bottom of the model, no-flow boundary conditions were retained. The northern boundary was placed on the main road connecting Saint-Pierre-Brouck with Capelle Brouck. Since this road lies the highest locally, with farmland on the other side, we assumed that this boundary would act as a water divide. Little is known about the other boundaries, hence these boundaries were modelled as constant head boundaries.

Inflow is possible as recharge and due to interactions with the rivers. The concentration of this inflow is equal to 0 % salinity for the recharge water and 1 % salinity for the river water. Water can leave the system by interaction with the rivers.

5.5.2 Results of the numerical model In this chapter the results of the numerical model, incorporating groundwater flow and solute transport, will be delivered and discussed. Since the model area is not located perfectly east- west, we opted to discuss the results with the use of left, right, upper and lower, relative to the model grid. For the initial setup, it was chosen to estimate the present day situation. However, equivalent to the groundwater model of Mannekensvere, the model was simulated for 40 years. Since permanent groundwater flow is assumed, the variation of the groundwater flow and hydraulic heads throughout the simulation is negligible; even after the first simulation period (3,65 days) the groundwater flow and hydraulic heads were in equilibrium. This is not valid when we look at the solute transport; here the salinity distribution shows a large change throughout the simulation period. Consequently, the results are shown for the initial and the equilibrium situation:

 Initial situation: situation after the 1st simulation period (3,65 days)  Equilibrium situation: situation where the solute transport reaches a dynamic equilibrium state, which is after the 11th simulation period (10961,15 days or approximately 30 years)

Since the model was simplified to steady state flow, with constant boundary conditions, the results will be a representation of the mean annual state without seasonal variations. For a second time, horizontal and vertical cross-sections through the model were used to interpret the results of both groundwater flow and solute transport. For the horizontal cross-section, it was opted to choose the cross-section corresponding with the 1st layer of the model (2,5 m IGN to 0 m IGN), because here interactions of the groundwater reservoir with the rivers are

5 most unblemished. For the vertical cross-sections, a distinction was made between the vertical cross-sections along the rows and along the columns. Only one vertical cross-section was

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made along the rows in the middle of the model. The cross-section is taken at row 60. This cross-section lays perpendicular to the main features in the model: (1) the river Aa and (2) the topographic variation (gradually lowering topography from the left side of the model towards the right side). For the vertical cross-sections along the columns, two locations were chosen: (1) at the 20th column which is located left of the river Aa and into the higher located landscape and (2) at the 130th column which is located right of the river Aa and into the lower regions. Figure 45 shows the position of these cross-sections.

Figure 45: Image of the study area; the red lines indicate the locations of the vertical cross-sections (y = 60th row; x = 20th and 130th column) (source: Google earth/own research). 5.5.2.1 Fresh water head In Figure 46 the fresh water heads of layer 1 are shown. These heads can be considered as a good approximation of the location of the water table. The strong dependence to the topography is most pronounced. The distribution of the fresh water head of layer 1 reflects the topography in almost the entire area. There is a gradual lowering of the fresh water head from the left to the right of the model area, corresponding with the local topography. Only one area interrupts this gradual lowering from left to right. In the centre of the model a lower region is located. This area is again a direct effect from the topography. Only the navigable

watercourses (the river Aa and its side-channel) stand out in the figure. Here, the fresh water head is slightly higher than in the immediate surroundings, thus some water will infiltrate

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from the river Aa and the side-channel towards the groundwater reservoir. This difference in fresh water head between the navigable watercourses and the direct surroundings, even though it is observable, is truly minor. The reason why the fresh water head difference between the rivers and the direct surroundings is so small, is because the hydraulic conductivity of the deposits where these rivers run through is relatively high. Due to the higher hydraulic conductivity, groundwater will flow more easily into or out of the rivers, hereby the local fresh water head of the reservoir will lower or rise. As a result, the difference in fresh water head between the river and the reservoir will decline severely, resulting in less pronounced watercourses in Figure 46. If one looks in more detail, the lower right part of the model also shows some dependency to river heads. This fresh water head difference between river and surroundings, even though the difference is very limited, could be explained by the direct relation between the topography and the hydraulic conductivity. This relation implies that the low-lying region corresponds with a lower hydraulic conductivity, hence the fresh water head difference between rivers and the direct surroundings is more pronounced.

The vertical cross-sections give more information about the groundwater flow regime. A clear distinction can be made between flow in the direction of the rows and flow along the columns. First the distribution of the fresh water heads in the direction of the rows will be discussed (see Figure 47). Here the gradual decrease in fresh water head from left to right is very clear, with a single region characterized by lower fresh water head values in the centre (around 2000 m). The location of the river Aa is also observable around 1500 m, though one can observe that the fresh water head differences between the river Aa and the surroundings are very limited. As shown by the almost vertical isosurfaces, primarily lateral groundwater flow is present (from left to right). However, the lower fresh water head values in the centre and the right of the cross-section will result in the drainage of the groundwater reservoir. At these locations the isosurfaces are less pronounced vertically, thus beyond lateral flow also vertical upwards flow will take place.

The vertical cross-sections that visualize the distribution of fresh water head values along the columns show other relations. From the cross-section along column 20 (Figure 48), taken through the high-lying parts of the model, one main interpretation can be deduced: approximately no groundwater flow takes place in the direction of the columns. This is because, with isosurfaces shown every 0,125 m, the fresh water head difference within the

vertical cross-section along column 20 is lower than 0,125m over a distance of 3 km. The groundwater flow that does occur, is primarily lateral groundwater flow. The second cross-

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section along column 130 (Figure 49), taken through the low-lying parts of the model, shows a different story. Here the fresh water head difference within the cross-section is higher; hence some lateral (left to right) and vertical (upwards) flow will take place.

Figure 46: Horizontal cross-section through layer 1 of the model, showing fresh water heads and isosurfaces (source: own research).

Figure 47: Vertical cross-section along row 60 of the model, parallel with the X-direction, showing fresh water heads and isosurfaces (source: own research).

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Figure 48: Vertical cross-section along column 20 of the model, parallel with the Y-direction, showing fresh water heads and isosurfaces (source: own research).

Figure 49: Vertical cross-section along column 130 of the model, parallel with the Y-direction, showing fresh water heads and isosurfaces (source: own research). 5.5.2.2 Salinity

 Initial situation Compared to the discussion of the fresh water head distribution, the same cross-sections are shown, however this time the salinity percentage of the initial situation is visualized. Figure 50 shows the horizontal cross-section of the first layer of the model. In this figure, the salinity distribution within the area is shown. The largest part of the modelled area is completely fresh, while the right side is characterized by 100% saline water. The jagged border between the fresh and salt water is closely related to the initial fresh water heads and thus also to the topography by the law of GHYBEN-HERZBERG (GHYBEN, 1889; HERZBERG, 1901). This border corresponds with the places where the initial fresh water head equals 0,65 m IGN. The groundwater reservoir will be completely composed out of salt water where the fresh water head is below this limit. On the other hand, a thick fresh water lens is quickly formed where the fresh water head is higher than 0,65 m IGN. For example, if the fresh water head

lies 40 cm above the limiting value (0,65 m IGN), then the law of GHYBEN-HERZBERG

5 suggests that a fresh water lens of 20 m thick is present below this place (that would already

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be more than the entire thickness of the model). This is confirmed in the vertical cross- sections. The vertical cross-section along row 60 (Figure 51) shows the same distribution: (1) the left side is characterized by almost completely fresh groundwater, while (2) the groundwater reservoir at the right side is completely composed out of saline groundwater. The cone of saline groundwater in the middle of the cross-section corresponds with the low-lying region in the central part of the model. For the second vertical cross-section (Figure 52), showing the salinity percentage along column 20 of the model, the interpretation is short and simple: the total area is composed out of fresh water. Almost the complete opposite situation is present in the vertical cross-section along column 130 (Figure 53). Here, the total area - apart from a small area between 0 m and 700 m - is composed out of saline groundwater.

Figure 50: Horizontal cross-section through layer 1 of the model, showing salinity percentage and isosurfaces (source: own research).

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Figure 51: Vertical cross-section along row 60 of the model, parallel with the X-direction, showing salinity percentage and isosurfaces (source: own research).

Figure 52: Vertical cross-section along column 20 of the model, parallel with the Y-direction, showing salinity percentage and isosurfaces (source: own research).

Figure 53: Vertical cross-section along column 130 of the model, parallel with the Y-direction, showing salinity percentage and isosurfaces (source: own research).

 Equilibrium situation Once more, the same cross-sections are shown, and this time the salinity percentage after a simulation period of ca. 30 years is presented. The main distribution of the salinity stays more or less the same: with the transition of fresh-to-salt water from left to right and in the middle a zone with a higher saline content. In this chapter only the evolution from the initial situation is discussed. Three main processes can be deduced from the results.

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Firstly, there is the salinization in the centre of the model. This is shown in Figure 54, the horizontal cross-section through layer 1 and in Figure 55, the vertical cross-section along row 60. In the centre of Figure 54 the same elongated shape emerges as can be seen in the distribution of the fresh water head values. The lower fresh water head generates an upwards flow, and due to the presence of salt rich groundwater in the underground, this upwards flow will transport groundwater with a higher salinity to the surface. In Figure 55 this process is shown by the expansion of the cone-shaped saline groundwater towards the surface. Secondly, the opposite process occurs: freshening of the zone characterized by pure saline water. This process is shown in Figure 54, the horizontal cross-section through layer 1; in Figure 57, the vertical cross-section along column 130 and to a smaller degree in Figure 55, the vertical cross-section along row 60. Even though this region is characterized by upwards groundwater flow, whereby the same salinization would occur as described in the first process, the top of the groundwater reservoir still freshens. This is due to the added fresh recharge water. This fresh water will infiltrate up to a certain depth, at some places up to approximately 7 m BGL, according to Figure 57. Then thirdly, the drainage of the groundwater reservoir towards the rivers can be observed in Figure 54, solely in the freshened zone. Even though the horizontal cross-section through the first layer of the model did not give a clear image when displaying the fresh water head distribution (Figure 46), a clear image emerged when displaying the salinity distribution (Figure 54). The rivers are clearly characterized by a higher salinity than the surrounding groundwater reservoir. This implies that the watercourses drain salt water from the deeper groundwater reservoir. Finally, one can observe that the vertical cross-section along column 20 of the model (Figure 56) did not change as compared to the initial situation. This cross-section is still purely comprised out of fresh groundwater.

As cited above in the model setup, the chemical result of groundwater from the piezometer at location 3 does not correspond with the initial salinity distribution (for locations 1 and 2 the chemical results of the groundwater samples do correspond with the initial salinity). Therefore it was important to keep track of this point throughout the simulation. It was already indicated that location 3 lies very close to the boundary between fresh and salt water in the initial situation. After reaching an equilibrium state, the fresh water zone shifted a lot towards location 3. Even though Figure 54 still shows that location 3 is positioned in a zone where high salinity groundwater is present, it is realistic that fresh water is locally present.

5 Moreover, the result of the EM39 test at location 3, where a trend emerged (increasing salinity

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with increasing depth), is another argument for the hypothesis that the fresh-to-salt transition zone at this location was very near. As previously mentioned, it is very unfortunate that no deeper drilling was made here, or another drilling more to the right of the model.

Figure 54: Horizontal cross-section through layer 1 of the model, showing salinity percentage and isosurfaces (source: own research).

Figure 55: Vertical cross-section along row 60 of the model, parallel with the X-direction, showing salinity percentage and isosurfaces (source: own research).

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Figure 56: Vertical cross-section along column 20 of the model, parallel with the Y-direction, showing salinity percentage and isosurfaces (source: own research).

Figure 57: Vertical cross-section along column 130 of the model, parallel with the Y-direction, showing salinity percentage and isosurfaces (source: own research).

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6 DISCUSSION

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6.1 Comparison of two numerical models In this section a comparison is made between the groundwater models of Mannekensvere (Belgium) and Saint-Pierre-Brouck (France). Therefore primarily, yet not exclusively, the setup and results discussed in chapter 4.2 for the model of Mannekensvere and in chapter 5.5 for the model of Saint-Pierre-Brouck will be used. In this discussion, the focus is placed on the following 4 main topics: (1) the data availability, (2) the interactions between the groundwater reservoir and the rivers, (3) the relationship of the groundwater reservoir to the topography and (4) the fresh-to-salt water distribution.

 Data availability For the groundwater model of Mannekensvere we were able to make use of a multitude of data-sets, e.g.: literature, drilling reports, the profile type map, the base and permeability of the polder deposits, the HCOV units, the salinity map and a highly detailed topographical map. The easy accessibility of these data made it possible to form a relatively detailed groundwater model, without even performing a field survey. There is - of course - a level of accuracy lost when no field survey is conducted; however this situation is perfect to form an initial idea before executing an expensive field study. In this way one can determine in advance where detailed measurements would be most interesting. This is in contrast with the draw up of the groundwater model of Saint-Pierre-Brouck. Here we could only count on some basic data, e.g.: literature, drilling reports from borings in the neighbourhood of Saint-Pierre- Brouck and a topographic map of a lesser resolution. This made it mandatory to conduct a field study to get an idea of the properties of the subsurface. This was done by performing a pumping test, fresh water head and quality measurements and an EM39 survey. Because little was known about the study area, the location of these field measurements was picked semi- randomly, which proved less efficient and thus more expensive. And even with this field study, the level of detail one gets from, for example, the salinity map of DE BREUCK, DE MOOR, MARECHAL and TAVERNIER (1974) could not be approached. This is why a set of assumptions was made to construct the groundwater model. The main assumptions were: (1) assuming that the lithological layers run horizontally across the entire study area and (2) computing the initial fresh water head, the fresh-salt transition zone and the hydraulic conductivity of the first two layers directly from the topography. These assumptions are the weak chains in the model, although they did make it possible to construct the groundwater model in the first place.

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 Interactions towards the rivers The major part of the groundwater model of Mannekensvere shows a clear relationship to the rivers present. There is the navigable watercourse, the IJzer river, where fresh water can infiltrate on the one hand and the unnavigable watercourses, that drain salt-rich water from the groundwater reservoir on the other hand. As clarified, this relationship is less pronounced in zones where Creekridge material is defined, thus where the hydraulic conductivity of the subsurface is higher. In the model of Saint-Pierre-Brouck, this relationship manifests hardly anywhere. Only some, very limited, infiltration at the navigable watercourses (the river Aa and its side-channel) can be observed, along with slightly draining unnavigable watercourses in the lowest part of the study area. This difference between the two models is mainly caused by the difference in hydraulic properties of the first layers of the model. If one would compare the observed ground from Saint-Pierre-Brouck with the different profile types from the soil type map of Mannekensvere, then one would conclude that it is best comparable with the Creekridge material. At all three locations, mainly sandy material with a high hydraulic conductivity was found. This explains why the interactions from the groundwater reservoir towards the rivers are much less pronounced in the groundwater model of Saint-Pierre- Brouck.

 Relationship to the topography Although the groundwater flow is dependent on the topography, there is still a big difference between the two models. For example in the model of Mannekensvere, there are two zones which lay significantly higher than the surroundings: in the left and right corner of the model area, corresponding with a farm or a village situated at a local high spot. These zones are also apparent in the horizontal cross-sections of the distribution of fresh water head and salinity percentage in the first layer, due to the higher fresh water head and the lower salinity (result of infiltration). Consequently, there are undoubtedly some processes which are closely related to the present topography. In the model of Saint-Pierre-Brouck these processes are extremely dominant. Much more than as a result of the interactions towards rivers, the fresh water head and salinity distribution is shaped due to topography related processes. This strong relationship can be explained partially by the assumptions that were made whereby many properties directly originate from the topography. As cited above, these assumptions reduce the accuracy of the model, especially because only a topographic map of a lesser resolution was found. Another factor that should be taken into account is that the variation in the

6 topography throughout the model of Saint-Pierre-Brouck if far greater than in the model of Mannekensvere.

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 Fresh-to-salt distribution The situation of the fresh-to-salt transition zone is totally different between the two models. In the model of Mannekensvere, the subsurface is characterized by its high salinity. Only the uppermost region of the groundwater reservoir is composed out of fresh groundwater (up to 12 m BGL at certain positions). This fresh water lens is formed by the infiltration of fresh water, derived from the rivers or the recharge. On the other hand, the major part of the groundwater reservoir in Saint-Pierre-Brouck is composed out of fresh groundwater. There are only two regions where salt water is present: (1) a cone shaped salt water body in the centre of the model and (2) a larger salt-rich zone, somewhat comparable with the saline groundwater zone in the model of Mannekensvere, in the topographical lower regions of the model. The difference between the two models can be explained by the fresh water head variations and distribution. If one looks at the difference between the global fresh water head values and the mean sea level used in the law of GHYBEN-HERZBERG (GHYBEN, 1889; HERZBERG, 1901), 2,35 m TAW at the Belgian coast (0,65 m IGN), then the following can be deduced. In the model of Mannekensvere, the global fresh water head varies around 2,45 m TAW, with very diminutive deviation. Thus, the difference between the mean fresh water head and the mean sea level is approximately 0,10 m. Based on the law of GHYBEN- HERZBERG, this would imply a fresh water lens of 5,1 m. The small variation of the fresh water head implies that this situation is more or less present over the total area. This approximation fits the model results. The situation is totally different in the model of Saint- Pierre-Brouck. Here, the variation of the fresh water head is much greater, going from more than 2 m IGN up to less than 0 m IGN. The difference between mean fresh water head and mean sea level varies from approximately -0,65 m to 1,35 m. Compared to the 0,10 m difference in the model of Mannekensvere, these values are much larger. Based on the law of GHYBEN-HERZBERG, this would imply that there are regions with a fresh water lens of approximately 69 m thick, whereas other regions are characterized by no fresh water lens. The severe variation in fresh water head values ensures that over the total area all possible situations could occur between these two approached extremities.

6.2 Future research Both groundwater models that were formed are good models to gain an initial understanding of the system. However they should both be seen as the initial step in forming a detailed study of a particular area. Based on the results, the optimal locations to perform a field study can be

6 deduced. This deduction should follow the main rule to retrieve the best possible calibration

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of the model by applying as little fieldwork as possible, primarily because fieldwork (depending of course on what kind of fieldwork) is the most expensive part of a study. One should always reflect on whether the investments are worth the increase in detail. In what follows, the most logical proceeding steps for both studies are discussed.

For the groundwater model of Mannekensvere, the most logical next step would be to validate the input and results from the groundwater model by conducting a field survey. The optimal locations to perform this field survey should be based on the results and the further interests. This model showed a clear relationship towards the different rivers, therefore - depending on the amount of detail that is required - the hydraulic properties of the different river categories could be investigated, and if necessary revised.

Possible proceeding steps to enhance the model of Saint-Pierre-Brouck are more straightforward than for Mannekensvere. Three steps are considered the most important: (1) analysing the assumptions that were made, (2) conducting a second field study and (3) evaluating the properties that were adopted from the model of Mannekensvere.

Firstly, the analysis of the assumptions (the approximation of the initial fresh water heads and salinity percentages that were made directly from the topography) will be discussed. As explained previously, these assumptions are the weak chains in the model. It would be really interesting to make a statistical description of the sensitivity of each of these assumptions. More concrete, the following analyses are proposed:

 Analysis of the formula which is used to define the initial head values [HEAD=(TOPO.0,75)-1], as described in chapter 5.5.1.7. For example, the value 0,75 in the formula was selected based on experience in other modelling situations, thus a sensitivity analysis of this value could prove very fascinating. This is especially true since the head values, which are usually of limited importance, were used to approximate the initial salt concentration.  A similar exercise can be carried out for the formula used to define the initial salt water content, the law of GHYBEN-HERZBERG (GHYBEN, 1889; HERZBERG, 1901). Except for the initial head values, which would be checked during the previous analysis, the mean sea level also has an important role. This value, set to 0,65 m IGN, has a direct influence on the distribution of fresh and salt water in the model. We can

presume that this value, adopted from the mean sea level at the Belgian coast, will not differ considerably. However, even a few centimetres difference will have a severe

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effect on the distribution of the fresh-to-salt transition zone. For example, a slight lowering of this value would ensure that the observations at location 3, merely fresh groundwater, would correspond with the calculated values from the model.

Secondly, a subsequent field study should be performed. Due to the model results, and the results of the already performed field measurements, this second study can be much more targeted. New piezometers could be installed in the lower regions, where salt groundwater is expected. It would also be interesting to install a new piezometer in the centre of the model area, where a cone of salt groundwater is assumed to be present. And perhaps it would also be interesting to install deeper piezometers at locations 2 and 3, where a clear trend of increasing salinity was observed in the EM39 results. At these new locations, it would be interesting to perform new EM39 measurements, preferably in deeper piezometers. Likewise, groundwater quality measurements would be interesting at these new locations. If a second pumping test would be executed, it would be most interesting to perform this one in the lower region, where we assumed that the hydraulic conductivity is the smallest and where local people indicated the presence of peat.

Thirdly, the values that are adopted from the model of Mannekensvere should be evaluated, and if necessary adjusted. The assumption of the mean sea level, adopted from the Belgian coast, is already discussed. Besides this parameter, also the river properties, the drainage properties and the recharge properties should be assessed. These properties could be determined better with the use of specific research studies.

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7 CONCLUSION

Chapter 113

For this trans-boundary Master’s dissertation, the objective was to form two density- dependent groundwater flow models of two characteristically equivalent regions; one in the Belgian coastal plain in and around the village of Mannekensvere, and another in the coastal area of northern France, more precisely in the village of Saint-Pierre-Brouck. In the comparison of these two numerical models, we mainly focused on the fresh-salt water interactions and the groundwater flow patterns.

The development of the groundwater model of Mannekensvere is based on a large amount of available data, from which the model setup was distilled. The data sets that were used are: (1) literature, (2) drilling reports, (3) the profile type map, (4) the base and permeability of the polder deposits, (5) the HCOV units, (6) the salinity map, (7) the Flemish Hydrographic Atlas and (8) a highly detailed topographical map. Since less data were freely available with regards to the model area of Saint-Pierre-Brouck, it was opted to conduct a field study. In the study area a multitude of measurements were performed: (1) a pumping test, (2) fresh water head and quality measurements and (3) an EM39 survey. These measurements were used to develop the numerical model. With a lack of data for the model setup present, assumptions were made to get round these shortcomings. The main assumptions that were taken to build the model are: (1) assuming that the lithological layers run horizontally across the entire study area and (2) computing the initial fresh water head, the depth of the fresh-to-salt transition zone and the hydraulic conductivity of the first two layers directly from the topography. Another method that was used to avoid data shortcomings is to adopt certain properties, there were no difference is expected, from the groundwater model of Mannekensvere. Examples are the mean sea level, the river properties, the drainage properties and the recharge properties. Even with these shortcomings, some interesting conclusions about both groundwater models can be drawn at the end of this work.

First off, both models give clear insights in how groundwater and solutes flow within the study areas. In the groundwater model of Mannekensvere, the flow of groundwater and solutes is mainly dominated by the rivers. There is the navigable watercourse, the IJzer river, where fresh water can infiltrate, whereas the unnavigable watercourses drain salt rich water from the subsurface. The hydraulic parameters, primarily in the uppermost layers, play a primordial part in the groundwater flow interactions with rivers. This is explained by the fact that in zones where the subsurface is characterized by a higher hydraulic conductivity, the

drainage due to watercourses is more effective. Due to the higher hydraulic conductivity, groundwater will flow more easily into the rivers, hereby locally lowering the fresh water

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head of the reservoir. This will result in the decline of the difference in fresh water head between the river and the groundwater reservoir. On the other hand, groundwater flow and solute transport in the model of Saint-Pierre-Brouck are mainly dominated by the topography. This strong association is explained by: (1) the assumptions that were made whereby many properties directly originate from the topography and (2) the fact that the variation in the topography throughout the model of Saint-Pierre-Brouck is far greater than in the model of Mannekensvere.

The situation of the fresh-to-salt transition zone is different between the two models. In the model of Mannekensvere, the groundwater reservoir is mainly composed out of saline groundwater, with only a slender fresh water lens in the uppermost layers. In the model of Saint-Pierre-Brouck, the major part of the groundwater reservoir is filled with fresh groundwater, with only two regions where salt water is concentrated. The difference was explained by the law of GHYBEN-HERZBERG, whereby the fresh water head variations and distribution play an important role.

What is important to keep in mind is that both groundwater models are good initial models in the design of a detailed study; however they should not be mediated as a final product. Some further research directions were denoted, building forth on these models. For the groundwater model of Mannekensvere, the best possible improvement would be to conduct a fieldwork campaign in order to validate the input and results. For the groundwater model of Saint- Pierre-Brouck three specific proceeding steps were proposed: (1) a statistical analysis of the assumptions that were made, (2) a focussed second field study and (3) a thorough evaluation of the properties that were adopted from the model of Mannekensvere.

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Chapter 116

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Websites  Institut National De L’Information Géographique et Forestière. URL consulted on

12/03/2014. (www.professionnels.ign.fr)

 Infoterre, le visualiseur des données scientifiques. URL consulted on 19/04/2014.

(www.infoterre.brgm.fr)

 Map of France. URL consulted on 14/03/2014. (www.Map-France.com)

 Databank Ondergrond Vlaanderen. URL consulted on 23/03/2014.

(www.dov.vlaanderen.be)

 Google maps. URL consulted on 22/04/2014. (www.maps.google.com)

 Google earth. URL consulted on 22/04/2014. (www.earth.google.com)

 GPS Visualizer: Do-It-Yourself Mapping. URL consulted on 22/04/2014.

(www.gpsvisualizer.com)

 Wikipedia, De vrije encyclopedie. URL consulted on 26/04/2014.

(http://nl.wikipedia.org/wiki/Mannekensvere)

Chapter 126

9 ANNEXES

Chapter 127

ANNEX 1: DESCRIPTION OF DRILLINGS NEAR MANNEKENSVERE

(source: Databank Ondergrond Vlaanderen)

Chapter 128

Chapter 129

Chapter 130

Chapter 131

Chapter 132

Chapter 133

Chapter 134

Chapter 135

Chapter 136

ANNEX 2: DRILLING DESCRIPTIONS NEAR SAINT-PIERRE-BROUCK

(source: Google earth/Infoterre/GPS Visualizer)5

5 It is important to mention that, even though it is not reliable with reality, some of the drilling reports use the classic stratigraphic nomenclature of Calais/Dunkerque.

Chapter 137

Chapter 138

Chapter 139

Chapter 140

Chapter 141

Chapter 142

Chapter 143

ANNEX 3: OUTPUT OF THE PUMPING TEST (PART OF LST-FILE)

(source: own research)

Model parameters and optimal hydraulic parameters

RADIUS OF WELLSCREEN,R,IN M,------0.005 DISCHARGE OF PUMPED WELL,Q,IN M3/DAY,------86.400 INITIAL TIME,T1,IN MIN,------0.010 LOGARTMIC INCREASE OF TIME AND OF RADIUS OF RINGS LOGA,------0.100 LATEST CALCULATED TIME,T2,IN MIN,------325. NUMBER OF LAYERS,N,------12 NUMBER OF RINGS,M,------52 THE WELLSCREEN SITUATED IS SITUATED IN LAYER------8 THICKNESS OF THE SUCCESSIVE LAYERS,IN M NUMBERED FROM LOWER TO UPPER THICKNESS OF LAYER 1,IN M,------2.000 THICKNESS OF LAYER 2,IN M,------2.000 THICKNESS OF LAYER 3,IN M,------4.000 THICKNESS OF LAYER 4,IN M,------3.000 THICKNESS OF LAYER 5,IN M,------2.000 THICKNESS OF LAYER 6,IN M,------0.800 THICKNESS OF LAYER 7,IN M,------0.460 THICKNESS OF LAYER 8,IN M,------2.000 THICKNESS OF LAYER 9,IN M,------0.440 THICKNESS OF LAYER 10,IN M,------1.000 THICKNESS OF LAYER 11,IN M,------0.100 THICKNESS OF LAYER 12,IN M,------1.000 ------NUMBER OF HYDRAULIC PARAMETER ---/NR./------HYDRAULIC CONDUCTIVITY,K( 1),IN M/DAY,------/ 1/-- 1.716 HYDRAULIC CONDUCTIVITY,K( 2),IN M/DAY,------/ 2/-- 0.858 HYDRAULIC CONDUCTIVITY,K( 3),IN M/DAY,------/ 3/-- 4.290 HYDRAULIC CONDUCTIVITY,K( 4),IN M/DAY,------/ 4/-- 4.290 HYDRAULIC CONDUCTIVITY,K( 5),IN M/DAY,------/ 5/-- 4.290 HYDRAULIC CONDUCTIVITY,K( 6),IN M/DAY,------/ 6/-- 4.290 HYDRAULIC CONDUCTIVITY,K( 7),IN M/DAY,------/ 7/-- 4.290 HYDRAULIC CONDUCTIVITY,K( 8),IN M/DAY,------/ 8/-- 4.290 HYDRAULIC CONDUCTIVITY,K( 9),IN M/DAY,------/ 9/-- 24.185 HYDRAULIC CONDUCTIVITY,K(10),IN M/DAY,------/ 10/-- 24.185 HYDRAULIC CONDUCTIVITY,K(11),IN M/DAY,------/ 11/-- 0.140 HYDRAULIC CONDUCTIVITY,K(12),IN M/DAY,------/ 12/-- 0.140 HYDRAULIC RESISTANCE,C( 1),IN DAY,------/ 13/-- 5.252 HYDRAULIC RESISTANCE,C( 2),IN DAY,------/ 14/-- 3.939 HYDRAULIC RESISTANCE,C( 3),IN DAY,------/ 15/-- 1.020 HYDRAULIC RESISTANCE,C( 4),IN DAY,------/ 16/-- 0.728 HYDRAULIC RESISTANCE,C( 5),IN DAY,------/ 17/-- 0.408 HYDRAULIC RESISTANCE,C( 6),IN DAY,------/ 18/-- 0.184 HYDRAULIC RESISTANCE,C( 7),IN DAY,------/ 19/-- 0.358 HYDRAULIC RESISTANCE,C( 8),IN DAY,------/ 20/-- 0.034 HYDRAULIC RESISTANCE,C( 9),IN DAY,------/ 21/-- 0.027 HYDRAULIC RESISTANCE,C(10),IN DAY,------/ 22/-- 3.579 HYDRAULIC RESISTANCE,C(11),IN DAY,------/ 23/-- 75.162 SPECIFIC ELASTIC STORAGE,SA( 1),IN M-1,------/ 24/-- 0.407E-04 SPECIFIC ELASTIC STORAGE,SA( 2),IN M-1,------/ 25/-- 0.441E-04 SPECIFIC ELASTIC STORAGE,SA( 3),IN M-1,------/ 26/-- 0.500E-04 SPECIFIC ELASTIC STORAGE,SA( 4),IN M-1,------/ 27/-- 0.604E-04

9 SPECIFIC ELASTIC STORAGE,SA( 5),IN M-1,------/ 28/-- 0.732E-04

SPECIFIC ELASTIC STORAGE,SA( 6),IN M-1,------/ 29/-- 0.825E-04 SPECIFIC ELASTIC STORAGE,SA( 7),IN M-1,------/ 30/-- 0.883E-04

Chapter 144

SPECIFIC ELASTIC STORAGE,SA( 8),IN M-1,------/ 31/-- 0.106E-03 SPECIFIC ELASTIC STORAGE,SA( 9),IN M-1,------/ 32/-- 0.127E-03 SPECIFIC ELASTIC STORAGE,SA(10),IN M-1,------/ 33/-- 0.151E-03 SPECIFIC ELASTIC STORAGE,SA(11),IN M-1,------/ 34/-- 0.186E-03 SPECIFIC ELASTIC STORAGE,SA(12),IN M-1,------/ 35/-- 0.235E-03 STORAGE COEFFICIENT AT THE WATERTABLE,S0,----/ 36/-- 0.1400000 C-VALUE OF WELL LOSS IN M**(1-3N)D**N,------.0003150 N-POWER OF WELL LOSS ------2.0000

List of observed drawdowns, used to calibrate the model

N-POWER OF WELL LOSS ------2.0000 OBS.WELL 1 IN LAYER 8 AT 0.0M OF PUMPED WELL HAS 25 OBSERVATIONS ------TIME (MIN) 1.0 1.3 1.6 2.0 2.5 3.2 4.0 5.0 6.3 7.9 DRAWDOWN(M) 3.120 3.270 3.300 3.310 3.340 3.380 3.370 3.400 3.410 3.430 TIME (MIN) 10.0 12.6 15.8 19.9 25.1 31.6 39.8 50.1 63.1 79.4 DRAWDOWN(M) 3.460 3.480 3.460 3.460 3.520 3.510 3.520 3.520 3.520 3.530 TIME (MIN) 100.0 125.9 158.0 199.5 251.2 DRAWDOWN(M) 3.510 3.540 3.560 3.540 3.540 ------OBS.WELL 2 IN LAYER 8 AT 17.0M OF PUMPED WELL HAS 22 OBSERVATIONS ------TIME (MIN) 2.0 2.5 3.2 4.0 5.0 6.3 7.9 10.0 12.6 15.8 DRAWDOWN(M) 0.030 0.040 0.050 0.070 0.080 0.100 0.110 0.130 0.140 0.150 TIME (MIN) 19.9 25.1 31.6 39.8 50.1 63.1 79.4 100.0 125.9 158.0 DRAWDOWN(M) 0.160 0.170 0.180 0.180 0.190 0.200 0.200 0.210 0.220 0.220 TIME (MIN) 199.5 251.2 DRAWDOWN(M) 0.230 0.230 ------OBS.WELL 3 IN LAYER10 AT 4.0M OF PUMPED WELL HAS 23 OBSERVATIONS ------TIME (MIN) 1.6 2.0 2.5 3.2 4.0 5.0 6.3 7.9 10.0 12.6 DRAWDOWN(M) 0.180 0.240 0.270 0.300 0.330 0.350 0.380 0.400 0.410 0.430 TIME (MIN) 15.9 20.0 25.1 31.6 39.8 50.1 63.1 79.4 100.0 125.9 DRAWDOWN(M) 0.450 0.460 0.470 0.480 0.490 0.490 0.500 0.510 0.520 0.520 TIME (MIN) 158.5 199.5 251.2 DRAWDOWN(M) 0.530 0.530 0.540 ------

Chapter 145

Comparison of observed and calculated drawdown after model run with optimal values

OBSERVATION WELL 1 IN LAYER 8 AT 0.0M OF PUMPED WELL OBSERVATION TIME(MIN) LOG. CALCUL. LOG. OBSERVED LOG. DIF. NUMBER OBSERVATION DRAWDOWN(M) DRAWDOWN(M) DRAWDOWN 1 1.00 0.4853 0.4942 -0.0089 2 1.30 0.5394 0.5145 0.0248 3 1.60 0.4970 0.5185 -0.0216 4 2.00 0.5461 0.5198 0.0263 5 2.50 0.5040 0.5237 -0.0197 6 3.20 0.5454 0.5289 0.0165 7 4.00 0.5111 0.5276 -0.0165 8 5.00 0.5476 0.5315 0.0161 9 6.30 0.5169 0.5328 -0.0159 10 7.90 0.5483 0.5353 0.0130 11 10.00 0.5228 0.5391 -0.0163 12 12.60 0.5502 0.5416 0.0087 13 15.80 0.5284 0.5391 -0.0107 14 19.90 0.5515 0.5391 0.0124 15 25.10 0.5332 0.5465 -0.0133 16 31.60 0.5535 0.5453 0.0082 17 39.80 0.5378 0.5465 -0.0088 18 50.10 0.5551 0.5465 0.0086 19 63.10 0.5418 0.5465 -0.0048 20 79.40 0.5567 0.5478 0.0090 21 100.00 0.5455 0.5453 0.0002 22 125.90 0.5582 0.5490 0.0092 23 158.00 0.5487 0.5514 -0.0028 24 199.50 0.5594 0.5490 0.0104 25 251.20 0.5511 0.5490 0.0021 MEAN OF DEVIATIONS TO OBSERVATIONS IN WELL 1 OF 16 OBSERVATIONS BEFORE 31.6 MIN. AFTER START OF PUMPAGE 0.0002 STANDARD DEVIATION ------0.0170 MEAN OF DEVIATIONS TO OBSERVATIONS IN WELL 1 OF 9 OBSERVATIONS AFTER 31.6 MIN. AFTER START OF PUMPAGE 0.0026 STANDARD DEVIATION ------0.0071 MEAN OF DEVIATIONS TO ALL OBSERVATIONS OF WELL 1 ------0.0010 STANDARD DEVIATION ------0.0141

OBSERVATION WELL 2 IN LAYER 8 AT 17.0M OF PUMPED WELL OBSERVATION TIME(MIN) LOG. CALCUL. LOG. OBSERVED LOG. DIF. NUMBER OBSERVATION DRAWDOWN(M) DRAWDOWN(M) DRAWDOWN 1 2.00 -1.4476 -1.5229 0.0753 2 2.50 -1.3535 -1.3979 0.0444 3 3.20 -1.2624 -1.3010 0.0386 4 4.00 -1.1901 -1.1549 -0.0352 5 5.00 -1.1259 -1.0969 -0.0290 6 6.30 -1.0664 -1.0000 -0.0664 7 7.90 -1.0125 -0.9586 -0.0539 8 10.00 -0.9614 -0.8861 -0.0753 9 12.60 -0.9165 -0.8539 -0.0627 10 15.80 -0.8768 -0.8239 -0.0529 11 19.90 -0.8403 -0.7959 -0.0444 12 25.10 -0.8050 -0.7696 -0.0354 13 31.60 -0.7741 -0.7447 -0.0294 14 39.80 -0.7462 -0.7447 -0.0015 15 50.10 -0.7216 -0.7212 -0.0003 16 63.10 -0.6994 -0.6990 -0.0004

9 17 79.40 -0.6781 -0.6990 0.0209 18 100.00 -0.6599 -0.6778 0.0179 19 125.90 -0.6440 -0.6576 0.0136

Chapter 146

20 158.00 -0.6306 -0.6576 0.0269 21 199.50 -0.6188 -0.6383 0.0194 22 251.20 -0.6083 -0.6383 0.0300 MEAN OF DEVIATIONS TO OBSERVATIONS IN WELL 2 OF 13 OBSERVATIONS BEFORE 31.6 MIN. AFTER START OF PUMPAGE -0.0251 STANDARD DEVIATION ------0.0473 MEAN OF DEVIATIONS TO OBSERVATIONS IN WELL 2 OF 9 OBSERVATIONS AFTER 31.6 MIN. AFTER START OF PUMPAGE 0.0140 STANDARD DEVIATION ------0.0121 MEAN OF DEVIATIONS TO ALL OBSERVATIONS OF WELL 2 ------0.0091 STANDARD DEVIATION ------0.0415

OBSERVATION WELL 3 IN LAYER10 AT 4.0M OF PUMPED WELL OBSERVATION TIME(MIN) LOG. CALCUL. LOG. OBSERVED LOG. DIF. NUMBER OBSERVATION DRAWDOWN(M) DRAWDOWN(M) DRAWDOWN 1 1.60 -0.5526 -0.7447 0.1921 2 2.00 -0.5309 -0.6198 0.0889 3 2.50 -0.5089 -0.5686 0.0598 4 3.20 -0.4878 -0.5229 0.0351 5 4.00 -0.4702 -0.4815 0.0113 6 5.00 -0.4540 -0.4559 0.0020 7 6.30 -0.4380 -0.4202 -0.0178 8 7.90 -0.4215 -0.3979 -0.0235 9 10.00 -0.4059 -0.3872 -0.0187 10 12.60 -0.3916 -0.3665 -0.0251 11 15.90 -0.3780 -0.3468 -0.0312 12 20.00 -0.3653 -0.3372 -0.0281 13 25.10 -0.3520 -0.3279 -0.0241 14 31.60 -0.3398 -0.3188 -0.0210 15 39.80 -0.3286 -0.3098 -0.0187 16 50.10 -0.3181 -0.3098 -0.0083 17 63.10 -0.3085 -0.3010 -0.0075 18 79.40 -0.2987 -0.2924 -0.0063 19 100.00 -0.2900 -0.2840 -0.0060 20 125.90 -0.2823 -0.2840 0.0017 21 158.50 -0.2755 -0.2757 0.0002 22 199.50 -0.2695 -0.2757 0.0062 23 251.20 -0.2638 -0.2676 0.0038 MEAN OF DEVIATIONS TO OBSERVATIONS IN WELL 3 OF 14 OBSERVATIONS BEFORE 31.6 MIN. AFTER START OF PUMPAGE 0.0143 STANDARD DEVIATION ------0.0630 MEAN OF DEVIATIONS TO OBSERVATIONS IN WELL 3 OF 9 OBSERVATIONS AFTER 31.6 MIN. AFTER START OF PUMPAGE -0.0039 STANDARD DEVIATION ------0.0077 MEAN OF DEVIATIONS TO ALL OBSERVATIONS OF WELL 3 ------0.0072 STANDARD DEVIATION ------0.0495

MEAN OF DEVIATIONS TO ALL OBSERVATIONS ------0.0001 STANDARD DEVIATION ------0.0376

MEAN OF DEVIATIONS OF 47 OBSEVATIONS IN LAYER 8 ------0.0037 STANDARD DEVIATION ------0.0302

MEAN OF DEVIATIONS OF 23 OBSEVATIONS IN LAYER 10 ------0.0072 STANDARD DEVIATION ------0.0495

Chapter 147