Salt Water Intrusion at the Island of Texel, the Netherlands: a Numerical Study

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First International Conference on Saltwater Intrusion and Coastal Aquifers— Monitoring, Modeling, and Management. Essaouira, Morocco, April 23–25, 2001 Salt water intrusion at the island of Texel, The Netherlands: a numerical study

G.H.P.Oude Essink

Utrecht University, Faculty of Earth Sciences, P.O.Box 80021, 3508 TA Utrecht, The Netherlands

ABSTRACT

Salt water intrusion is investigated in the groundwater system of Texel, which is a Wadden island in the northern part of The Netherlands with a surface area of ~130 km2 (Figure 1). In this coastal groundwater system of Quatenary deposits, salinisation of the upper layers is already taking place. At present, brackish water occurs close to the surface of the low-lying areas at the eastern part of the island, whereas freshwater occurs up to -50 m M.S.L. under the sand-dunes at the western part. In addition, a freshwater lens with a thickness of fifty meters has evolved in the sand-dune called De Hooge Berg, which is situated in the southern part of the island. Density dependent groundwater flow is modeled in three-dimensions with MOCDENS3D (Oude Essink, 1998). This code is in fact MOC3D (Konikow et al., 1996) but adapted for density differences. The geometry of the model is 20 km by 29 km by 302 m depth. The areal size of an model cell is 250 m by 250 whereas the thickness varies from 1.5 m to 20.0 m. Due to the irregular shape of the island, only some 60 % of the cells in the grid is active. About 125,000 active cells and one million particles model simulate groundwater flow and salt transport during 500 years. The salinity in the top layer as well as the salt load to the surface of the polders will increase substantially during the next centuries. In addition, a relative sea level rise of 0.75 m per century intensifies the salinisation process seriously, causing a further increase in salt load in the polders and a decrease of the volume of the freshwater lenses. 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. Measures to compensate salt water intrusion are suggested, though most of them appear to be ineffective.

References Konikow, L.F., Goode, D.J. and G.Z. Hornberger, A three-dimensional method-of- characteristics solute-transport model (MOC3D), U.S. Geol. Surv. Water-Res. Invest. Rep. 96-4267, 87 pp., 1996.

Oude Essink, G.H.P., 1998, MOC3D adapted to simulate 3D density-dependent groundwater flow, In: Proc. of the MODFLOW'98 Conference, October 4-8, 1998, Golden, Colorado, USA, Volume I, pp. 291-303.

Keywords: 3D numerical modeling, variable-density flow, MOCDENS3D, salt water intrusion, island of Texel

Corresponding author: Gualbert H.P.Oude Essink, Assistant Professor, Utrecht University, Faculty of Earth Sciences, Budapestlaan 4, 3584 CD Utrecht, The Netherlands. Email: [email protected]. Web: http://www.geo.uu.nl/~goe/

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0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 km 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 km 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 km Concentration [mg Cl -/l] 0 50 150 300 1000 2500 5000 7500 10000 12500 15000 ------50 150 300 1000 2500 5000 7500 10000 12500 15000 18630 Figure 2 Chloride concentration in the top layer at -0.75 m N.A.P. for the years 2000, 2200 and 2500 AD. The relative sea level rise is 0.75 m per century.