DENSITY DEPENDENT GROUNDWATER FLOW AT THE ISLAND OF TEXEL, THE NETHERLANDS Gualbert H.P. OUDE ESSINK University of Utrecht; Inst. of Earth Sciences; Centre of Hydrology (ICHU); Dept. of Geophysics; P.O. Box 80021; 3508 TA Utrecht; The Netherlands; [email protected] ABSTRACT Salt water intrusion is investigated at Texel, which is a Wadden island in the northern part of The Netherlands with a surface area of approximately 130 km2. In this coastal groundwater system of Quatenary deposits, salinisation of the upper layers is taking place. At present, brackish water already occurs close to the surface of the low-lying polder areas at the eastern part of the island. Freshwater occurs up to –50 m M.S.L. in the sand-dune area at the western part. Density dependent groundwater flow in this system is modelled in three-dimensions by MOCDENS3D. The model is dimensioned 20 km by 29 km by 302 m depth, whereas about 125,000 active elements and one million particles simulate groundwater flow and salt transport during 500 years. The salinity in the top layer as well as the salt load at the surface of the polders will increase substantially during the next centuries. In addition, a relative sea level rise of 0.75 meter per century definitely intensifies the salinisation process, causing a further increase in salt load in the polders. As such, the increased salinisation of the top layer will affect the surface water system from an ecological as well as a socio-economical point of view. INTRODUCTION The lowest phreatic water levels can be measured in the so-called Prins Texel is the biggest Dutch Wadden Hendrik polder, with levels as low as island in the North Sea. It is often –2.0 m N.A.P.1 In addition, a dune called Holland in a nutshell (figure area called De Hooge Berg, which is 1a). The population of the island is situated in the southern part of the about 13,000, whereas in island in the polder area summertime, the number of people Dijkmanshuizen, has a phreatic can be as high as 60,000. A sand- water level of some +4.75 m N.A.P. dune area is present at the western The nature reserve ‘De Slufter’ in the side of the island, with phreatic water northwestern part of the island is a levels up to 4 metres above mean tidal salt-marsh. sea level. At the eastern side, four 1 low-lying polder areas with controlled N.A.P. stands for Normaal Amsterdams Peil. It water levels are present (figure 1b). roughly equals Mean Sea Level and is the reference level in The Netherlands. m Phreatic water level [m]: k < -4 m M.S.L. 0 . 0 -2.50 -4 - -2 - e -1.50 e 5 -2 - 0 nz . 2 e - -1.50 d - 0 - 2 Island of d -1.25 a 0 Texel . 2 -10 W s 5 -1.25 - - 10-20 -1.00 5 . element De Slufter -1.00 7 - 20-40 - ve -0.75 cti >40 0 . 1. Eijerland -0.75 0 ina a - 1 - -0.50 Se 5 North Sea . rth -0.50 2 - 1 0.00 Amsterdam - No 2. Waal en Burg/ 0 0.00 . Het Noorden - 5 0.50 1 - 0.50 Den Haag 5 lowest point: . 3. Dijkmanshuizen/ - 7 1.00 1 - -6.7 m M.S.L. a De Schans Rotterdam e 1.00 0 - ar . 1.50 0 De Hooge Berg e 2 Germany - n sea 1.50 u - d 5 . - 4. Prins Hendrik 2.00 2 d 2 ts - n polder 2.00 a - S 0 3.00 . 5 2 Belgium - 3.00 ive elemen - 4.00 5 . 7 inact 2 4.00 - - 0 25 50 75 100 km 6.50 highest point: 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 km a. +322 m M.S.L. b. Figure 1: a. map of The Netherlands: position of the island of Texel and ground surface of the Netherlands; b. map of the island of Texel: position of the four polder areas and sand-dune area as well as phreatic water level in the top aquifer at –0.75 m N.A.P. First, the computer code, which is by the MODFLOW module used to simulate variable density (McDonald and Harbaugh, 1988; flow in this groundwater system, is Harbaugh and McDonald, 1996). summarised. Second, the model of The advection-dispersion equation, Texel will be designed, based on which simulates the solute transport, subsoil parameters, model is solved by the MOC module using parameters and boundary the method of characteristics conditions. The numerical results of (Konikow and Bredehoeft, 1978; two scenarios of sea level rise are Konikow et al., 1996). Advective discussed in the next section, and transport of solutes is modelled by finally, some conclusions are drawn. means of the method of particle tracking and dispersive transport by means of the finite difference CHARACTERISTICS OF THE method. A so-called freshwater head NUMERICAL MODEL If is introduced to take into account differences in density in the MOCDENS3D (Oude Essink, 1998), calculation of the head: which is the three-dimensional p computer code MOC3D (Konikow et I f z (1) al.,1996) but adapted for density U g differences, is used to simulate the f transient groundwater system as it where If is the freshwater head [L], occurs on the island of Texel. The Uf is the reference density, usually groundwater flow equation is solved the density of fresh groundwater at reference chloride concentration C0 convergence criterion for the [M L-3], p is the pressure [M L-1 T-2], groundwater flow equation and z is the elevation head [L]. See (freshwater head) is equal to 10-5 m. Oude Essink (1998, 1999, 2000) for The total simulation time is 500 a detailed description of the years. adaptation of MODFLOW to density differences. A linear equation of The groundwater system consists of state couples groundwater flow and permeable aquifers, intersected by solute transport: loamy aquitards and aquitards of U (C) U [1 E (C C )] (2) clayey and peat composite. The f C 0 system can be divided into six main where U(C) is the density of -3 subsystems. The top subsystem groundwater [M L ], C is the chloride (from 0 m to –22 m N.A.P.) and the -3 concentration [M L ], and EC is the second subsystem (from –22 m to – volumetric concentration expansion 62 m N.A.P.) have hydraulic gradient [L3 M-1]. MOCDENS3D conductivities kx of approximately 5 takes into account hydrodynamic m/d and 30 m/d, respectively. The dispersion. For a conservative solute third subsystem is an aquitard of 10 as chloride, the molecular diffusion m thickness and has hydraulic for porous media is taken equal to conductivities k which varies from -9 2 x 10 m /s. 0.01 to 1 m/d. The fourth subsystem (from –72 m to –102 m N.A.P.) and For the numerical computations the fifth subsystem (from –102 m to – following parameters are applied. 202 m N.A.P.) have hydraulic The groundwater system consists of conductivities kx of some 30 m/d and a 3D grid of 20.0 km by 29.0 km by only 2 m/d, respectively. The lowest 302 m depth. Each element is 250 m subsystem, number six, has a by 250 m long. In vertical direction hydraulic conductivity kx of the thickness of the elements varies approximately 10 m/d to 30 m/d. from 1.5 m at the top layer to 20 m Note that the first, second and fourth over the deepest ten layers. The grid subsystems are intersected by contains 213440 elements: nx=80, aquitards. ny=116, nz=23, where ni denotes the number of elements in the i direction. The following subsoil parameters are Due to the rugged coastline of the assumed: the anisotropy ratio kz/kx system and the irregular shape of equals 0.4 for all layers. The the impervious hydrogeologic base, effective porosity ne is 0.35. The only 58.8 % of the elements longitudinal dispersivity DL is set (125,554 out of 213,440) is equal to 2 m, while the ratio of considered as active elements. Each transversal to longitudinal element contains eight particles to dispersivity is 0.1. Note that no solve the advection term of the numerical 'Peclet' problems occurred solute transport equation. As such, during the simulations (Oude Essink some one million particles are used and Boekelman, 1996). On the initially. The flow time step 't to applied time scale, the specific recalculate the groundwater flow -1 storativity Ss [L ] can be set to zero. equation equals one year. The The bottom of the system as well as corresponding density of that saline the vertical sea-side borders are groundwater equals 1024.1 kg/m3. considered to be no-flux boundaries. At the top of the system, the mean DISCUSSION sea level is –0.10 m N.A.P. and is constant in time in case of no sea Numerical simulation of the period level rise2. A number of low-lying 1990-2000 AD areas is present in the system with a total area of approximately 124 km2. Density dependent groundwater flow The phreatic water level in the polder is not only determined by head areas differs significantly, varying differences but also by density from –2.05 m to +4.75 m N.A.P. (De differences. Variable density Hooge Berg) (figure 1b), and is kept groundwater flow simulated with a constant in time.