The areas of brine leakage to freshwater aquifers were mapped with the use of over 20,000 archival chemical analyses from the data bank of the Polish Geological Institute. Such areas were named chloride anomaly zones, and were defined by groups of wells with concentration of Cl- ion in groundwater exceeding the value 60 mg/dm3 - the upper limit of the hydrogeochemical background.
The next stage of research was focused on specific chloride anomaly zones, and on intakes affected by groundwater salinization. It consisted of chemical analyses, including rare earth elements (REE) and isotopic (δ2H, δ18O) analyses of groundwater samples. The correlation of hydrochemical, geophysical and drilling data confirmed that, chlorides anomalies have been predominantly developed in geologic situations, enabling the inflow of salt waters from deeper parts of the Mesozoic complex into the shallow freshwater aquifers. These are: 1) tectonic fault and fracture zones, 2) hydrogeological windows above uplifted crests of salt anticlines and elevated tectonic blocks, 3) mature salt diapirs.
The natural process of brine migration was accelerated in some areas due to groundwater exploitation and this led to more intense mixing with fresh, drinking waters. The inflow of saline waters, 'older’ than modern infiltration waters (supposedly diluted Mesozoic brines), into freshwater aquifers, was proved by chemical and isotopic tests (δ2H and δ18O).
Brine leakage is responsible for a significant decline in groundwater quality, which has been reported from numerous intakes working in the indicated chloride anomaly zones. This is reflected through high concentration of chlorides (>250 mg/dm3) and sodium (>200 mg/dm3) ions, in some cases associated with elevated contents of sulfate (>250 mg/dm3) and ammonium (>0,5 mg/dm3) ions. Such waters are often characterized also by elevated K, Sr, B, Mg and Ba ion concentrations.
An analysis of the content of rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and yttrium (REY) was used as an additional tool supporting the efforts to recognize the nature of salinization processes which have been affecting the examined freshwater aquifers. REY patterns (describing mutual relations between European Shale normalized concentrations of individual elements) were determined for the groundwater samples from selected communal intakes. Afterwards they were compared to REY patterns determined for brines samples taken from neighboring Spa or geothermal wells. In case of some wells located in area of the Baltic Sea coast, REY patterns calculated for seawater samples were also considered.
Acknowledgements: This work was supported by the National Science Centre, Poland, under Project no. UMO-2015/17/B/ST10/03295