Groundwater quality changes due to iron sulphide oxidation in the Odra ice marginal valley – long-term process observations

In the year 1966 a well field supplying the city of Zielona Góra with water was opened in the ice-marginal valley of the Odra River, near the village of Zawada. After three years of water pumping from 22 wells tapping the shallow valley aquifer, a significant deterioration of water quality, especially involving the increase of concentrations of iron, sulphates, manganese and water hardness, as a result of the process of sulphide oxidation accumulated within the recent Holocene deposits, was observed. Despite drastic changes in quality (the increase of iron concentration to approximately 30 mg/l and sulphate concentration to above 300 mg/l), the well field was still operated as its water was used for purifying (coagulation) of the polluted surface water supplying the city of Zielona Góra. Based on the analysis of over 30-year-long period of water pumping, it was stated that the effect of the sulphide oxidation process on the quality of water was noticeable for the period of 19 years. The period of persisting very high concentrations of iron of above 15 mg/l amounted to approximately 6 years. It was also stated that the process of reducing the concentrations following the period of their maximum increase proceeded in two stages – the stage of rapid reduction of concentrations which lasted for about 4 years and the stage of slow changes which took about 10 years. The first one was related to the exhaustion of sulphides within the formations marked by good permeability, and the period of semi-decomposition of sulphides, which lasted for about 3 years, was determined based on the changes in iron concentrations. On the other hand, the reduction of concentrations at the stage of slow changes may be linked to the oxidation of sulphides occurring in the formations marked by weak permeability (silts, clays, peats).