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

Salt water intrusion at Texel Introduction

Present ground surface in the Netherlands Introduction

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

Present phreatic water level in top layer Introduction 2000

Present chloride concentration in top layer Introduction 2000

 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

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

 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:

---> 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):

 Number of elements n x =80, n y =116, n z =23 total number of active elements: ~  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: m Model design Model parameters:

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

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

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

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

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

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

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

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

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