1. Density dependent groundwater flow at the island of Texel, The Netherlands  Introduction  Computer code  Model design  Discussion  Conclusions Gualbert Oude Essink Earth Sciences Utrecht University The Netherlands

2. Salt water intrusion at Texel Introduction

3. Present ground surface in the Netherlands Introduction

4. The island of Texel Introduction  Tourist island in summer time  Land surface: 130 km 2  Polder areas: 1. Eijerland 2. Waal en Burg 2. Waal en Burg 3. Dijkmanshuizen 3. Dijkmanshuizen 4. Hendrik polder 4. Hendrik polder  Sand-dune area at western side  ‘De Slufter’ is a tidal salt-marsh  North Sea surrounds the island

5. Present phreatic water level in top layer Introduction 2000

6. Present chloride concentration in top layer Introduction 2000

7.  density dependent groundwater flow Darcy Darcy continuity continuity  solute transport advection advection hydrodynamic dispersion hydrodynamic dispersion  displacement of fresh, brackish and saline groundwater  linear relation between density & concentration Computer code MOCDENS3D = MOC3D (Konikow et al., 1996) but adapted for density differences

8. Groundwater flow equation (MODFLOW, 1988) Darcy Continuity Freshwater head Advection-dispersion equation (MOC3D, 1996) Equation of state: relation density & concentration Computer code

9.  Effective porosity: 0.3  Anisotropy: 0.4  Hydrodynamic dispersion:  L =2 m,  TH =0.2 m,  TV =0.2 m  L =2 m,  TH =0.2 m,  TV =0.2 m molecular diffusion=10 -9 m 2 /s molecular diffusion=10 -9 m 2 /s  Density groundwater: fresh  f =1000 kg/m 3, saline  s =1024 kg/m 3 fresh  f =1000 kg/m 3, saline  s =1024 kg/m 3 Boundary conditions:  No flow at sea side  Neumann in dunes: natural recharge of 1 mm/day  Dirichlet in polder area: constant phreatic water level Model design Subsoil parameters:

10. ---> aquifer 1: k h =~5 m/day (intersected by aquitards) ---> aquifer 1: k h =~5 m/day (intersected by aquitards) ---> aquifer 2: k h =~30 m/day (intersected by aquitards) ---> aquifer 2: k h =~30 m/day (intersected by aquitards) ---> aquitard 1: k h =0.01 to 1 m/day ---> aquitard 1: k h =0.01 to 1 m/day ---> aquifer 3: k h =~30 m/day (intersected by aquitards) ---> aquifer 3: k h =~30 m/day (intersected by aquitards) ---> aquifer 4: k h =2 m/day ---> aquifer 4: k h =2 m/day ---> aquifer 5: k h =10 to 30 m/day ---> aquifer 5: k h =10 to 30 m/day Model design Subsoil composition (simplified):

11.  Number of elements n x =80, n y =116, n z =23 total number of active elements: ~125000  Sizes of elements:  x=250 m,  y=250 m,  z=1.5 to 20 m  Particles per element: 8  Flow time step: 1 year  Convergence criterium: 10 -5 m Model design Model parameters:

12. Calculated present seepage and infiltration at -1.5 m M.S.L. Discussion 2000

13. Calculated present salt load at -1.5 m M.S.L. Discussion 2000

14. Modelling of two sea level rise scenarios: I. Present mean sea level during 200 years II. Relative sea level rise of 0.75 m/century during 200 years Interest is focused on: A. Change in concentration in top layer B. Change in seepage in polders C. Change in salt load in polders Discussion

15. A. Change in concentration in top layer Scenario I: present mean sea level during 200 years 20002200

16. Scenario II: relative sea level rise of 0.75 m/c during 200 years 2000 A. Change in concentration in top layer 2200

17. Scenario II: relative sea level rise of 0.75 m/c during 200 years A. Change in concentration in row 76: East-West profile 2000 2200

18. Scenario II: relative sea level rise of 0.75 m/c during 200 years B. Change in seepage 22002000

19. Scenario II: relative sea level rise of 0.75 m/c during 200 years C. Change in salt load 22002000

20. Conclusions:  numerical dispersion is limited (no Peclet number problems)  initial density distribution is difficult to determine  present situation is not in a dynamic equilibrium  salinisation during coming 200 years is significant due to: the present difference in polder level and sea level the present difference in polder level and sea level t he sea level rise t he sea level rise  effect of sea level rise: accelerates the salinisation process accelerates the salinisation process salt load and seepage in polders increases substantial salt load and seepage in polders increases substantial