Interrelations between river water and groundwater in water supply areas using environmental isotopes

Full text: In many densely populated regions the groundwater resources can no longer meet the demand for drinking water. As a result, many water catchment areas using bank filtration water are being installed near rivers. In special cases the groundwater supplies from such catchment areas can be increased by artificially induced infiltration of river water. In three case studies the infiltration rates and flow parameters of the bank storage in relation to the extracted groundwater are determined using environmental isotopes in combination with the usual hydrological methods. (1) The water supply area of about 2 km2 is situated in the confluence area of two rivers. The aquifer consists of sandy gravel layers of a thickness of 6-8 m. The 18O data from pumping and observation wells enabled the quantitative calculation of the relative proportions of infiltrated river water and their spatial distribution. The proportions of water in the pumping wells from each of the rivers were determined as a function of time, using the isotope curves for the pumping wells and for both rivers. Proportions of between 50 and 100% of specific river water have been found in the different pumping wells, with an estimated accuracy of ±5%. Correlation with results of pumping tests led to a localization of the infiltration areas. (2) The second case concerns a water catchment area on an approximately 0.3 km2 large river island. The changing 18O content of the river water is well suited for the determination of the water infiltrated into the tapping wells and its flow velocities. Assisted by the large number of observation wells, it was possible to draw a detailed map of the proportions of river water in the groundwater (40-100%) and to determine flow velocities of between 2 and 15 metres per day. The results obtained by isotope measurements were supported and confirmed by dye tracer tests and measurements of electrical conductivity. (3) In the case of an artificial groundwater enrichment by injected river. water the amount and transit time of infiltrated groundwater were estimated by means of 2H data. Using a simple mixing equation, a proportion of 90 ± 5% of artificially infiltrated water in relation to the autochthonous groundwater was calculated for a pumping well. Utilizing hydrological models together with the annual isotope variation, the mean transit time was determined to be between two and four months. (author)