Old data point to new sources of drinking water 

The landscape may be flat and relatively featureless, but beneath the upper layer of topsoil is a world of dramatic geological features. Deep-seated salt formations, ice-age deposits in buried meltwater valleys and a thick layer of compact clay typify the subsoil structure of the Danish island of Lolland - and make groundwater abstraction a significant challenge in much of the island.

Storstrøm County is presently involved in a mapping project aimed at improving knowledge of the geological conditions on the island. The project is part of a national initiative under which all counties are required to map their groundwater resources and draw up plans for safeguarding drinking water.

Rough outline

The Lolland mapping project is being carried out by COWI. By revisiting existing data on Lolland’s subsoil and reprocessing it in an entirely new way, COWI has been able to produce a rough outline of prevailing geological conditions. to aid the geologists. The new method enable the geologists to concentrate the field survey activities on the most interesting areas.

COWI project manager Henrik Olsen explains: "There is a lot of data available from hydrocarbon exploration on Lolland in the 1980s by oil companies. In addition there are old gravity measurements that show the gravitational pull. By combining the two types of data, we can fairly accurately gauge the composition of the top 200 metres of subsoil. And potable, or drinking, water is usually found in the top 50 metres."

New groundwater reservoirs

This exhaustive mapping project will serve multiple purposes. First, the upper part of the subsurface contains a number of buried valleys which were formed by glaciers and rivers of meltwater during the ice age. These valleys may well contain groundwater reservoirs.

Another purpose is to map the exact border of a very compact layer of clay that covers the whole of the southern part of Lolland. And finally, Lolland is experiencing problems with rising levels of salty groundwater, which is contaminating the drinking water. Therefore the depth to the surface of the salty groundwater must also be determined.

Storstrøm County geophysicist Ejner Nielsen explains: "In southern and north-western Lolland in particular, good groundwater is a limited resource and replacement resources are difficult to find. In order to plan abstraction areas for drinking water and how best to preserve drinking water in the future, we need a new and more detailed understanding of the prevailing geological conditions."

Calculations appear reliable

”The field surveys already carried out by COWI show the value of revisiting and reinterpreting old data,” adds Henrik Olsen. "Every time our calculations indicated the existence of a buried valley, we actually found one whose existence was not previously known.
 
By: Janne Toft Jensen, jaje@cowi.com
Published: 25.05.2004