Isotope-hydrological and hydrochemical study to investigate the penetration of pollution into a deep aquifer

Within the city area of Munich the Younger Tertiary aquifer of the Obere Suesswassermolasse (OSM) is rich in water and is tapped by numerous wells reaching down to depths of about 250 m. The deep groundwater generally has the quality of drinking water and is used directly, for instance, by the food industry and as a public emergency water supply. The groundwater in the overlying Quaternary aquifer has become increasingly polluted. The groundwater in the deeper aquifer is more or less separated from the Quaternary gravels by a low-permeable clay and silt layer up to 40 m thick, which contains intermediate sand lenses. Within the city area of Munich the groundwater of the younger Tertiary aquifer is predominantly recharged by leakage through this layer from the Quaternary aquifer due to depression of the groundwater table caused by pumping. Combined isotope-hydrological and hydrochemical investigations were performed and, taking into account the hydrogeological and hydraulic conditions, used to clarify to what extent the clay and silt layer is able to protect the underlying aquifer from penetration of pollutants with different physical and chemical properties, in particular chlorinated hydrocarbons. Water samples were taken, in some cases repeatedly, from about 50 wells tapping the shallow and deep groundwater aquifer in selected sub-areas of the city of Munich, and the concentrations of 2H, 3H, 18O, 13C, 14C, and in some cases 3He, were measured, together with the usual hydrochemical parameters and concentrations of chlorinated hydrocarbons. The analyses of environmental isotopes made it possible to classify the sampling points with respect to the mean residence time of the groundwater. An attempt was also made to correlate the dispersal of pollutants in the Younger Tertiary aquifer with the residence time. The results obtained show that under hydraulic stress of the aquifer young tritium-containing groundwater has in general penetrated down to depths not greater than 60 m. As in general the effective depth range of inflow into the well is unknown, the measured isotope and chemical concentrations are referred to the upper edges of the filter tubes or the lower edges of the casing or covering layers, respectively. The groundwater model ages decrease from Recent values in the Quaternary aquifer to about 15 000 years, b.p. at a depth of approximately 200 m. Chlorinated hydrocarbons, chlorides, nitrates and sulphates were detected in some places at comparable depths to tritium. Sulphate concentrations around 10 mg/L in the deep groundwater are of geogenic origin. In isolated cases relatively high tritium and/or chlorinated hydrocarbon concentrations were measured in deep wells. These are possibly due to leaky borehole constructions. Laboratory tests showed that some adsorption of chlorinated hydrocarbons takes place in clay and silt samples. The effect is reversible and, as is known, can lead to retardation effects. The investigation shows clearly that it is necessary to introduce measures for the protection of the deep groundwater. Consumption should be limited to 107 m3/ year and the deep groundwater should only be used for purposes which require drinking water quality. Groundwater quality should be secured by technical requirements for well construction, improvement of quality of the polluted shallow groundwater, and improved protection against further pollution. The combined isotope-hydrological and hydrochemical observation of the groundwater should be continued at regular intervals