THE EFFECTS OF MIXING UNDERGROUND AND SURFACE WATER IN THE CONDITIONS OF WATER DISCHARGE FROM THE EXCAVATION DRAINAGE – CASE STUDY OF THE POTOK SŁUŻEWIECKI IN WARSAW

The paper presents a typical problem for agglomeration areas, i.e. the discharge of drainage water from the excavation pit to the watercourse. In case of the Potok Służewiecki, the most important problem is a significant difference in the chemical composition of surface and groundwater. An important part of the Potok Służewiecki water originates from polluted water discharged from rainwater sewage collectors. Discharged groundwater is mostly categorized as the quality class II, but significant concentrations of Fe and Mn reduce its categorization to quality class III. In order to determine the extent of changes in the chemical composition of the Potok Służewiecki water due to water mixing process, geochemical modelling in various quantitative ratios was carried out. Obtained results show lowering of the pH and almost complete transition of all Fe speciations into the Fe3+ form, regardless of the mixing ration of ground and surface water. This implies the appearance of coloured water in the Potok Służewiecki. Therefore, assuming the use of additional aeration measures, the oxygen amount needed for complete removal of iron from groundwater before its discharge to the Potok Służewiecki was calculated. Possible impact of the Potok Służewiecki on the rock matrix before and after mixing with discharge water was also presented in the article.