Framework

Water supply company Hydron and the Provence of Utrecht have reached an agreement to reduce the groundwater production on the Ridge of Utrecht. This reduction scenario is valid until 2006, by then the benefit for nature as a result of the reduced production
and alternative locations for replacement of existing production capacity should be sorted out.

Water supply company Hydron therefore initiated a project in search for so called 'white areas' in the Provence of Utrecht. White areas are, in this project, defined as areas in the Provence of Utrecht where production of groundwater has a negligible effect on both nature reserves sensitive to a lowering of the groundwater table and aquatic nature. A negligible effect is defined as a reduction of seepage (upward flux) less than 0.2 mm/day. So a 'white area' is an optimum location for replacement of existing production capacity.

The ultimate objective of the project was to obtain a reliable map of the so-called 'white areas', whereby distinction is made between depth and production capacity.

How it was done

To determine the white areas the search can be aproached either from nature reserves or from the search area. The approach is essential to how the modelling is carried out. The advantage of aproaching the search from nature reserves is that fewer simulation runs are necessary. But this is only valid when the system reacts linear, which is not the case.

The approach from the search area is always valid. This means that, with existing models, the solution with the optimum accuracy is achieved. A disadvantage is that more simulation runs will be necessary, more than a 100.000 runs to be more specific. When these simulation runs are executed automatically only a limited effort is asked from of the modeller. For this reason the program 'walking well field' was developed. This program defines a production well on node 1 only, carries out a simulation run, saves results and defines a well on node 2 only, ..., until all nodes have been a well. After every simulation run the result is compared to the conditions of the 'white areas'.

The simulation is carried out in six simulation sessions. During the first session all possible wells in the search area for the smallest extraction and lowermost aquifer are calculated. In every following session the search area is reduced by rejecting wells from the previous session that effect nature reserves. This enormously reduces the time for a simulation run. After each session the results are validated and the search area is redefined (reduced).

Results

This project has resulted in a map with all possible locations suitable for replacement of existing production capacity for the two investigated aquifers. The map presented here shows the results for the uppermost aquifer.

The methology 'walking well field' decribed here can also be used for other type of projects. Nature reserves used in this project for instance can be replaced by contaminated areas to carry out a risk assessment for production sites or a feasablity studie for new sites. It can even be used to decide the necessity for cleaning up contaminated areas. Another type of project could involve the search for optimum controlled water levels (optimum groundwater level and seepage intensity) for nature. In fact any intervention of influence on the water system (quantitative or qualitative) can be compared and validated to any specified object (nature reserves, rural areas, water courses, contaminated areas, etc.) obtaining a spatial optimum for the specified situation. The result is a map which can directly be used to set out policies and decision making.