Formowanie siȩ skŁadu chemicznego wód podziemnych plejstoceńskiego poziomu wodonośnego w rejonie kopalni piasku kotlarnia

The paper presents factors and processes determining chemical composition of groundwater from the Pleistocene aquifer in the area of Kotlarnia sand pit in the conditions of maximal lowering of water table. Diagnostic surface- and groundwater monitoring, field and laboratory research were carried out in November 2009. Hydrogeochemical processes occurring in investigated aquifer were identified on the basis of geochemical modeling performed by using PhreeqC codes. Water from a great part of the researched Pleistocene aquifer is rich in SO₄^2-, characterized by diversified mineralization (TDS from 100.76 to 277.28 mg/1) and diversified pH, often lowered to the value of 5.09. Locally, there occurs highly mineralized (TDS up to 7495.71 mg/1) groundwater of Cl-Na type. One of the most important factors influencing changes in the chemical composition of groundwater are loosing of polluted water from the Bierawka river and longlasting lowering of the groundwater table by mining drainage. Pyrite occurring in the unsaturated zone of increased thickness is oxidized either by O₂ or NO₃^-. Aforementioned process is responsible for both decreased pH value (below 6.0) and increased concentration of SO₄^2- in the examined groundwater. Secondary processes connected with pyrite oxidation, such as ion exchange or gypsum precipitation, have also been recognized.