Groundwater chemistry and hydrogeochemical processes in a soil catena of the Poznan Lakeland, central Poland

An attempt has been made to identify the chemical processes that control the hydrochemistry of groundwater in the catena of Retisols/Luvisols and Gleysols/Phaeozems within the Opalenica Plain, central Poland. Groundwater samples were collected from 6 representative piezometers. The quantity of dissolved components in groundwater was connected with the location of piezometers in the relief studied and the properties of the soil’s parent materials. The groundwater of soil located higher in the soil catena was characterised by smaller concentrations of the components when compared with the composition of water from the lowest sites on the slope. The dominant hydrochemical facies are Ca,Mg–HCO3,SO4, Ca,Mg-mixed or Ca,Mg–HCO3 types in the groundwater from the higher part of the slope and Mg–HCO3,SO4 and Mg–HCO3 types in the water from the lower part of the catena. The chemistry of groundwater is the product of the weathering of carbonate minerals and silicate. The Ca2+/alkalinity molar ratio in the groundwater samples from piezometers located higher in the soil catena indicate that the weathering of both calcite and dolomite can occur there, while only dolomite weathering takes place in the groundwater of the catena pediment. The Ca2+/Mg2+ molar ratio in the water samples from the footslope shows an excess of magnesium over calcium, which could be a result of water interacting with Mg-rich silicate minerals, as an additional process to dissolution of dolomite. Furthermore, data for a few water samples from the footslope cover the evaporation zone of the Gibbs plot. This suggests that while contamination could affect the amount of dissolved components, evaporation can modify the groundwater chemistry of footslope. Cluster analysis grouped groundwater samples into two groups with similar characteristics, indicating the influence of the location of a piezometer in the soil catena on the groundwater chemistry.