FRESH- SALTWATER INTERACTION IN A MELIORATED COASTAL AREA OF THE BALTIC SEA, GERMANY

Parts of the coastal area of the Baltic Sea in Germany are influenced by melioration since the 19^th century. Especially in the 1970s a lot of areas were drained. One of these places is the region Darss, a small peninsula in Mecklenburg-Vorpommern (Germany).

Since 2003 the Hydrogeology Research Group of the Berlin Institute of Technology carries out investigations concerning the interactions between drainage, land use and drinking water supply. The results show a variation of the salt-freshwater interface influenced by rainfall, the Baltic Sea and brackish waters of the lagoon nearby. The present contribution characterizes a part of the hydrochemical and hydrodynamic aspects of the Darss.

In geological terms the Darss can be divided into three parts. The oldest, the Pleistocene Altdarss, consists of drift sediments of the Weichsel glacial period which form the basis of the aquifer followed by sandy deposits of the same period. Due to sea level changes various deposits of Holocene sands and marsh sediments were washed up in the northern part of the Darss which form the so called Neudarss. In the south you find Holocene alluvial deposits consisting of sands (Foredarss).

In the 1970s parts of the Darss, mainly the Foredarss was meliorated for agriculture. The area was drained using parallel orientated drainage channels. During wintertime with its high amount of precipitation the water in the channels is pumped into the lagoon, in summertime the evapotranspiration is dominant and water is pumped out of the lagoon into the channels to raise the water table.

The hydraulic connection between the melioration channels and the groundwater causes electric conductivities of the groundwater which extent up to 7.000 µS/cm. This high salinity of the water is responsible for the stall of flora in the neighboring forest areas.

The extraction of drinking water by the local waterworks leads to a reduction of the freshwater lens and benefits saltwater intrusion in the coastal areas. The additional pumping of brackish water into the channels causes a salinization of the groundwater at the Foredarss.

A water logging in parts of the drained area is planned. To describe the influence of this action monitoring wells were installed. A high resolution numerical model shows the sensitivity of the different influences on the fresh-saltwater interface.