Use of Design Projects GEM cable in the Berkheide Well Recharge System
Dune Water Company of South Holland, The Hague, Netherlands
The dunes in the Netherlands have been used for more than a century for drinking water. Initially only groundwater was abstracted. Then, as the demand for drinking water continued to rise, the capacity was increased by means of artificial recharge with spreading ponds. However, because of the impact this method had on the environment, use is being made today of a new recharge and recovery method that employs deep wells below natural layers of clay.
The hydro-geological study into the possibility of a deep well recharge system n the area started in 1987. Given the specific local hydro-geological conditions, deep well recharge seemed feasible in this location. The aquifer held freshwater and above and below it was a layer of clay.
Figure 1. Hydro-geological cross section
Figure 1. shows a cross section of the area at right angles to the coast. The geological structure can be viewed as two major aquifers.
Environmental impact plays an important role and a maximum excess recharge of 10% was permitted to prevent any rising of salt groundwater. However, the higher the percentage the greater the ecological and agricultural impacts.
Monitoring focuses on the quantity and quality of the recharge water and the recovered groundwater, as well as on the impacts of the projects on the groundwater tables and the groundwater quality (Koopman 1995, Looy 1997). This later aspect mainly concerns possible ecological and agricultural damage.
For the purpose of monitoring the groundwater, an extensive measurement network has been installed. This comprises observation wells with 1m long screens at different depths. At regular intervals samples are taken and the hydraulic head measured.
Practically all the observation wells are fitted with a geo-resistivity measuring cable (GEM cable). This type of cable allows the measurement, at various depths, of the conductivity of the soil and the groundwater at the same time. Based on these measurements, the shifting of salt groundwater may be deduced.
The movements of the interface between fresh and salt groundwater over a period of less than a year allows only very general conclusions to be made. Below the recharge wells, the interface moves clearly downwards. In the neighbourhood of the recovery wells, there is virtually no movement at all.
In the future more attention will be paid to the further movement of the interface fresh and salt groundwater and the dispersion of recharge water outside the well recharge systems.
Design Projects would like to thank Mr M W Kortleve and Mrs M Poppelier for co-operation in allowing us to use these extracts from the original article.