The effects of ascent recharge on deep exploitable aquifers on the North European Plain (a case study of the Rogoźno salt anticline, Poland)

In the paper, we analyse the impact of ascending brines from the Mesozoic basement on the formation of hydrogeological conditions of deep exploitable aquifers. In the North European Plain, there are numerous salt structures with associated tectonic deformations, which form migration zones of saline waters from the Mesozoic basement to the Cenozoic cover. This creates a geogenic threat to exploitable aquifers, especially in terms of groundwater exploitation conditions. Previous studies of these phenomena had been conducted on a regional scale and focused mainly on the distribution of chlorides in receiver aquifers without detailed analysis of hydrodynamical and hydrogeochemical effects of the ascent. As part of research into the topic, it is necessary at this point to develop the methodology of assessment of geogenic risks for groundwater, for example for reasons of protection of fresh groundwater resources. As the research area, a halotectonic anticline region was chosen, where the Mesozoic basement is characterized by strong tectonic deformation, and the deepest Cenozoic aquifer has a regional spread. Some simple analyses allowed to clearly identify the zones of saline groundwater ascent along tectonic discontinuities of the Mesozoic bedrock. This phenomenon causes the appearance of waters with a mineralisation up to 2000 mg/L in the exploitable aquifer. Ascent recharge through active faults causes the presence of zones with anomalously high piezometric pressure in the cover of the fault overlay. The shape of these zones is correlated with the course of tectonic deformations. Influence of geogenic pollution is greater in the marginal zones of the anticline than above it. For the full assessment of ascent impact on hydrogeological conditions of the receiver, it is necessary not only to analyse chloride concentrations, but also other groundwater components – i.e. sulphates. Hydrogeochemical analysis may allow for identification of the shallower and deeper ascent recharge zones from the Mesozoic basement. Comprehensive identification of these factors, even in the case of poor tectonic control, can contribute to optimization of groundwater exploitation and protection conditions in the areas potentially and actually threatened by ascent.