1 Quaternary Research Group (QRG), Department of Science and Environment, Roskilde University2 The Department of Environmental, Social and Spatial Change, Roskilde University
The regional, confined aquifer on the island of Zealand, in eastern Denmark, is the primary aquifer used for large-scale abstraction for the supplies of all larger cities, including Roskilde and the greater Copenhagen metropolitan area. Large-scale groundwater abstraction from this aquifer in the area near Lejre Denmark (approximately 15km to the SW of Roskilde) began in 1937, exporting approximately 18 million m3 of water per year to supply the city of Copenhagen. After abstraction began, streams in the area were observed to go dry after extended periods without precipitation, where as previously they never did. This study analyzes the changes in the groundwater potential between 1936 and 2006 in two stream catchments in central Zealand (Elverdam and Langvad) to assess how groundwater abstraction has affected the regional aquifers potential for contribution to base-flow in the streams, wetlands and lakes in the area. The results show that there was a significant impact on the regional groundwater aquifer in the Langvad river catchment, with groundwater as much as 17m lower in 1987 from 1936 (pre-abstraction). However, in the Elverdam river catchment, the levels remained virtually the same with very little impact on the groundwater divide between the two drainages. From 1987 to 2006, there was a recovery up to 8m in the Langvad drainage, with no significant changes elsewhere. The recovery was due to a reduction of approximately 8 million m3/year in groundwater abstraction. This is also reflected in the total area for groundwater discharge. In 1936, there was a potential spring discharge area of 19.1 km2, or 8.4% of the Langvad catchment. By 1987, this was reduced to 1.6 km2 (0.7%) in the Langvad catchment, showing the significant impact of groundwater abstraction. By 2006, the potential discharge area recovered slightly to 5.9 km2, approximately 1/3 of the total area before abstraction began. As expected, the potential discharge area in the Elverdam catchment has not significantly changed, with a potential discharge area of 9.8 km2 (16% of the catchment area) in 1936, decreasing slightly to 8.7 km2 by 1987, and then increasing to 10.1 km2 by 2006. This study shows that there has, and remains a significant impact on the potential discharge area and the baseflow in the Langvad catchment, though the recent reductions have helped the recovery. However, groundwater abstraction has not had a significant impact on the regional aquifer in the Elverdam catchment, with the potential discharge area remaining virtually the same from 1936 to 2006.