Comparison of Two AEM Groundwater Mineralisation Surveys in the Werra River Valley, Germany

The dissolution of Zechstein salt and the discharge of saltwater of the potash salt producing industry into the river Werra near the Hessian-Thuringian border in central Germany has led to a considerable mineralisation of ground and surface waters. In order to reduce the amount of saline water emissions directly into the river, the injection of waste water into the so called Plattendolomit was introduced. About 1000 million cubic meters have been stored in this porous and karstic limestone and dolomite bed of some ten metres thickness at about 500 meters depth. The waste water displaced the formation water upwards and in areas where fault zones exist saltwater rising occurred. Contracted by K+S KALI GmbH, BGR conducted a regional (1128 square kilometers) airborne geophysical survey in 2008 in addition to smaller survey (576 square kilometers) flown in 1996/97, both at 200/2000 meters line/tie-line spacing. Of particular interest was an area where a test disposal of saline waste water (9.5 million cubic meters) in the Gerstungen syncline took place between 1999 and 2007. Although complicated by the use of different helicopter-borne frequency-domain electromagnetic (HEM) systems (Dighem III and Resolve), it has been possible to quantitatively compare the results of both HEM surveys in order to map and outline changes of the near-surface groundwater mineralisation. Taking into account the challenging survey conditions, no significant increase in groundwater mineralisation could be substantiated within the achievable exploration depths of about 20 meters (saltwater ponds) and 200 meters (hard rock).