Variable infiltration and river flooding resulting in changing groundwater quality - A case study from Central Europe

The changes in groundwater quality occurring in a buried valley aquifer following a reduction in groundwater exploitation and enhanced infiltration due to extensive flooding of the Odra River in 1997 were investigated. Long-time series data for the chemical composition of groundwater in a large well field for drinking water supply indicated the deterioration of groundwater quality in the wells capturing water from the flooded area, which had been intensively cultivated since the 1960s. Infiltration of flooded river water into the aquifer is suggested by an elevated chloride concentration, although salt flushing from the rewatered unsaturated zone due to the enhanced recharge event is much more feasible. Concomitantly with chloride increases in the concentrations of sulphate, ferrous iron, manganese, and nickel imply the oxidation of pyrite (FeS ₂) which is abundant in the aquifer. The proton production resulting from pyrite oxidation is buffered by the dissolution of calcite, while the Ca:SO ₄ stoichiometry of the groundwater indicates that pyrite oxidation coupled with nitrate reduction is the dominant process occurring in the aquifer. The pyritic origin of SO42- is confirmed by the sulphur isotopic composition. The resultant Fe ²⁺ increase induces Mn-oxide dissolution and the mobilisation of Ni ²⁺ previously adsorbed to Mn-oxide surfaces. The study has a major implication for groundwater quality prediction studies where there are considerable variations in water level associated with groundwater management and climate change issues.