Framework

Groundwater produced in the coastal areas of the Netherlands for a major part is infiltrated surfacewater by artificial recharge. One of those areas are the dunes near Castricum. This artificial recharge scheme is operated by the watersupply company PWN.

In 1920 groundwater production from the fresh water body under the dunes has started. Then in 1957 the first artificial recharge scheme became operational. Surface water was and still is infiltrated from several elongated ponds. This scheme is called ICAS. In 1990 PWN started with deep well injection and recovery (DWAT).

Management of the scheme is aimed to maintain the fresh water body and is regarded as a strategic water storage. In total 24 Mm3/yr is infiltrated and recovered at ICAS. An additional 1 Mm3/yr is recovered from natural infiltration from precipitation excess. The deep well infiltration and recovery scheme DWAT infiltrates and recovers 4 to 5 Mm3/yr.

How the system is managed

Management of an artificial recharge scheme with a groundwater model can only be reliable when it is properly schematised. The way they are schematised depends strongly on their operation and management. Therefor first a short description of the scheme.

A water level of 2.8m NAP is maintained in the elongated basins with an allowed fluctuation of 20 cm. Around these basins a series of recovery wells are placed in the boundary section. In between the basins sections of recovery wells are present as well. Each section (12 in total) contains a series of wells. So the wells are clustered per section and managed by the abstraction rate of that particular section, the abstraction rate of each individual well may be different. These recovery wells abstract water from a depth of -6m NAP.

In the middle aquifer pumping wells recover the infiltrating water from the upper aquifer (from the basins). At a depth of -60m NAP the deep well injection and recovery scheme DWAT is present.

Results

Schematising the artificial recharge scheme in the groundwater model is carried out as follows.

The infiltration basins are schematised by defining the boundaries of the basins as infiltrating rivers (blue lines). These rivers have a constant waterlevel and an infiltration resistance. Each individual recovery well in the middle aquifer is incorperated in the model. The same is done for the wells of the DWAT deep well injection and recovery scheme.

The recovery wells of the upper aquifer are schematised in a different way. As described earlier these wells operate in sections. Each section has an abstraction rate and consists of a series of wells. In triwaco a system like this can be schematised by using a HOBO(HOrizontal BOring). A HOBO is a line element (river) representing the wells in a section. For each line element an abstraction rate can be defined. The model will calculate the water level for that particular section at the given abstraction rate. Because of the lowered water table the infiltration basins will react by infiltrating more water