The subterranean flow of fresh and salt water underneath the western Belgian beach

On the gently sloping sandy runnel and ridge beach thirty holes were drilled through the unconfined aquifer. The unconfined aquifer consists of a complex of permeable and semi-permeable layers with a thickness of about 30 m. In each of the boreholes a resistivity logging was performed. The resistivity logging thus obtained provide a fairly good idea of the fresh, brackish and salt water distribution underneath the beach. Five resistivity profiles perpendicular to the shore line were drawn. At one of these profiles the hydraulic head pattern has b pen measured continuously inthe upper and the lower part of the aquifer. From these piezometers groundwater has been sampled for determining the mean anion- and kation concentrations. Based on these data a mathematical model, which describes the flow of fresh and salt water has been developed

Density dependent groundwater flow model of the shore and dune area of the Westhoek nature reserve (Belgium)

The Westhoek nature reserve is a dune area situated along the French-Belgian border. Below the dunes a fresh-water lens is found. A particular distribution of salt-water occurring above fresh-water is found under the adjacent shore. This less known water quality distribution is in dynamic equilibrium. A 2D density dependent groundwater flow model was made using the MOCDENS3D code. First, the groundwater flow and water quality evolution under the shore and in the dunes are modeled. Then the possible impact of sea level rise is simulated for a number of different scenarios. These scenarios reflect different reactions of coastal morphology and human intervention on the sea level rise. Depending on the scenario, the extent of the shore's salt-water lens can increase, decrease or even completely disappear. Simultaneously, the extent of the dune's fresh-water lens can significantly be altered. The simulations illustrate also, besides the effects of sea level rise, that changes in boundary conditions (drainage levels, shore morphology, sea water level), either natural or human induced, can importantly alter the water quality distribution. Because of the high ecological value of the area and the dune's importance for drinking water production, these changes should be well studied beforehand

Hydrogeologie van het duingebied tussen Koksijde en Oostduinkerke

The lithology of the unconfined aquifer between Koksijde and Oostduinkerke have been deduced from well logs. From a pumping test a value of 340 m2/d was obtained for the transmissivity and 3.10-3 for the elastic storage coefficient. Analyses of water samples indicated that the groundwater contains more salt near the water catchment than to north of it. Geo-electric well logging has shown the presence of fresh water over the entire thickness of the aquifer at the high-tide line. The groundwater currents have been computed by means of a mathematical model for two cases of a groundwater extraction of 1,5 million m3 per year

Een matematisch model van de grondwaterstromingen in het kwartaire reservoir ten oosten van De Haan

A mathematical model has been developed to simulate groundwater flow in the Quaternary aquifer system east of De Haan (Belgium). The estimated lateral variations of the hydraulic parameters have repeatedly been adjusted by comparing the calculated hydraulic-head configuration to the observed one. After this calibration of the model was achieved, the influence of groundwater extraction has been examined. Even at pumping rates as low as 250.000 m³/year, artificial replenishment of the aquifer system will be necessary in order to avoid flow of salt water towards the wells

Modelling of density dependent groundwater flow in the south-western Belgian coastal plain

The study area is situated at the landward border of the Belgian western coastal plain at the right bank of the river Ijzer. The phreatic aquifer is of quaternary age. Until about the 12th century, the hydrology of the area was influenced by the North Sea. The area was criss-crossed by tidal channels and gullies and the aquifer was mainly filled with salt water. From about 1100 AD, the area was gradually reclaimed, halting the influence of the sea. The channels silted up. Fresh water started to infiltrate replacing the salt water. This leads to the actual distribution of fresh and salt water. Fresh water lenses are found below the creek ridges, the former tidal channels where the aquifer consists mainly of sandy sediments. In the adjacent area the phreatic aquifer consists of less permeable sediments (clay, silt and peat) and is still mainly filled with salt water. In these areas where the water table is slightly lower than in the creek ridges, only a thin lens of fresh water occurs above the salt water. A preliminary field survey was performed to compliment the existing data. In these field survey a number of drillings were executed and observation wells were installed. The variation of the salt content with depth was inferred from electrical conductivities measured with the focused electromagnetic induction method. A pumping test was performed to derive the hydraulic conductivity of the channel sediments. All these data were integrated in a 3D density dependent model, using the MOCDENS3D code to simulate the evolution of the water quality distribution from 1100 AD until a few centuries in the future. The simulations demonstrate that the fresh water lenses under the channels are formed in about 400 years. These fresh water lenses can only be considered as a limited water resource in this area. The influence of this drainage on the groundwater flow and water quality distribution is illustrated by the model. The mean drainage level is below mean sea level. In the creek ridges there is an important upward flow of fresh water under the drainage canals. In the adjacent areas with low permeable sediments the drainage and flow is very small. The salt water in these sediments is trapped since the land reclamation in the 12th century and will stay there for many centuries