Geophysical contributions to the multidisciplinary reconstruction of the “Bleichesee” in a floodplain near Nördlingen, Southern Germany

The town of Nördlingen is one of the few remaining medieval towns in Germany. In the Middle Ages, Nördlingen was a centre of tanning and dyeing, which shaped hydraulic engineering along the river Eger and thus had a considerable impact on the floodplains close to the town. The DFG funded SPP 2361 ‘On the way to the fluvial anthroposphere’ focusses on investigating such pre-industrial floodplains in Central Europe and their development. Within the sub-project ‘Local Pathways to the Fluvial Anthroposphere at Echaz (Rhine) and Eger (Danube)’, the focus is on the multidisciplinary reconstruction of the land use of the floodplains and the reconstruction of the effects of urban crafts and waste disposal on floodplain pollution. To this end, we use multidisciplinary approaches, including the digitisation of historical maps, the integration of digital terrain models, geophysical investigations and the analysis of sediment cores from the Eger floodplain.

Here, we focus on the results of near-surface geophysical investigations carried out as part of the comprehensive floodplain exploration at the Bleichesee (a former bleaching lake), in which we used (1) electromagnetic induction (EMI) for area-wide mapping and (2) electrical resistivity tomography (ERT) for transect-wise mapping. With this combined approach, we were able to delineate the gravel bodies of the river Eger, which are characterised by a coarse-grained sediments, and identify regions with fine-grained alluvial deposits and anthropogenic backfills. Based on these results, sites were selected for driving core and hand drillings for detailed sediment analysis. In addition, direct push-based investigations can provide high-resolution vertical information on various subsurface properties (electrical conductivity, colour spectrum, hydraulic conductivity, etc.), whereby we focused on colour profiles at the Nördlingen site when investigating the Bleichesee lake. These were logged along a transect at intervals of 25 centimetres and thus provide impressive insights into the deposits and filling of the Bleichesee.

At present, the results of the geophysical measurements and in-situ descriptions by means of driving core and hand drillings as well as the extensive laboratory analyses of the sediment samples are being compiled. The aim is the chronostratigraphic description of the Eger floodplain and its history of pollution. In this respect, geophysical proxies can provide valuable support.