Rapporten - Søren Højmark Rasmussen

English summary
The municipality of Aalborg is assessing the possibility of extracting 1 mil. m 3
groundwater pr. year from Volsted forest, 15 km south of Aalborg city. Before the
extraction can start an environmental assessment of the consequences on the
surrounding area has to be made. Close by Volsted forest there are several areas
protected by the EU Habitat Directive (Natura 2000), among them 5 rich fens. The
main objective of this paper is modelling and assessing the effect of the groundwater
extraction on the rich fens.
The peat soil in the rich fens is constantly water saturated, the groundwater is
alkaline because it originates from the regional limestone aquifer, and the soil
contains few nutrients. When these conditions are present the special rich fen
vegetation grows. To conserve the rich fen vegetation it is crucial to maintain the
constant water saturation.
The aim of this paper is to examine how the hydrological conditions will change as a
result of lower groundwater potential after the extraction is initiated and the effect
this will have on the rich fen vegetation. To examine the groundwater table
drawdown a dynamic groundwater model was established using Mike SHE. The
hydrological conditions were simulated before and after the groundwater extraction.
Using the simulated dynamics in the water table a “safe zone” was presented. Within
this zone the water table can fluctuate without compromising the conditions for the
rich fen vegetation.
The results show that the groundwater table is stable throughout the year in the peat
soil, and that it responds more to wet and dry years than to seasonal changes. After
the groundwater extraction the groundwater table will be drawn down 8 cm on
average in the rich fen. The “safe zone” for the groundwater table is 24 cm below
ground level. During dry conditions the drawdown exceeds 24 cm, and reaches 33
cm. However, the capillary rise in the peat soil is > 30 cm, and will secure the
continued water saturation in the rich fen. In relation to the groundwater table
drawdown the groundwater extraction in Volsted forest can be implemented.
With the groundwater extraction the water flow velocity in the peat will be reduced
by 7-17 % which can affect pH and nutrient composition. The effect on pH and the
nutrient composition in the rich fen due to the groundwater extraction is beyond the
aim of this paper to investigate. The results can be used for further investigations on
the effect of the groundwater extraction.