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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
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Salt water intrusion at Texel Introduction
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Present ground surface in the Netherlands Introduction
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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
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Present phreatic water level in top layer Introduction 2000
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Present chloride concentration in top layer Introduction 2000
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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
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Groundwater flow equation (MODFLOW, 1988) Darcy Continuity Freshwater head Advection-dispersion equation (MOC3D, 1996) Equation of state: relation density & concentration Computer code
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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:
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---> 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):
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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:
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Calculated present seepage and infiltration at -1.5 m M.S.L. Discussion 2000
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Calculated present salt load at -1.5 m M.S.L. Discussion 2000
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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
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A. Change in concentration in top layer Scenario I: present mean sea level during 200 years 20002200
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Scenario II: relative sea level rise of 0.75 m/c during 200 years 2000 A. Change in concentration in top layer 2200
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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
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Scenario II: relative sea level rise of 0.75 m/c during 200 years B. Change in seepage 22002000
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Scenario II: relative sea level rise of 0.75 m/c during 200 years C. Change in salt load 22002000
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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
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