Wpływ budowy geologicznej zlewni Grzmiącej na odpływ podziemny z czwartorzędowych poziomów wodonośnych

Abstract

Influence of the Grzmiąca catchment geological structure on groundwater outflow from Quaternary aquifers (central Poland).S u m m a r y. The author concentrated on characteristic elements of the Grzmiąca basin geology determining its groundwater outflow. The study area is a small part of the Bzura drainage basin located NE of Łódź, in the northern margin of the Łódź Heights, which were formed during the Wartanian glaciation. Large thickness (50–110 m) of the fluvioglacially originated Pleistocene sands and gravels is the most important feature of this basin.. They are surrounded from N,W, and S with clays of the Odranian and Wartanian glaciations origin. The clays form a ring coinciding with the watershed zone of the basin. This structure is responsible for the high underground water capacity indicated by the analysis of the groundwater outflow changeability. Below the Pleistocene sediments the basin is sealed by Pliocene silts and clays, in practice precluding percolation of groundwaters below the drainage level. It was confirmed by the equalized water balance for 1996–1997 derived from field studies. High permeability of the surface sediments facilitates rainwater infiltration. It also concerns thawing waters, as shown by a comparing the river groundwater supply after and before thawing (March 1996). Fluctuations of the groundwater outflow are considerable but at the same time they show a very high runoff trend stability. The Grzmiąca catchment fills up with water for a long time and then is slowly drained. The other feature of the studied basin is its high spring yield which together with intensive linear drainage along the rivers results in a very high percentage of groundwater outflow in the total runoff (88.5%). This is due to the fact that the springs area and along the main river course there is a thick water-bearing horizon consisting of sands of the Odranian andWartanian age which are continuous, not separated with clays.