Origin of bank filtered groundwater on the Csepel Island (below Budapest)

The drinking water for the Hungarian capitol is mainly covered from bank filter wells of the riparian aquifer system of the River Danube. A part of these wells are located on the Csepel Island (south of Budapest). The aquifer system is highly vulnerable for pollution coming either from the Danube or from the background shallow groundwater. Budapest contaminates continuously the Danube water with communal and industrial wastewater. The contamination can have an effect on the quality of the bank-filtered water on the Csepel Island. The knowledge of the flowing system in the alluvial aquifer is very important for providing high quality drinking water and for protecting the drinking water resources. The aims of this study are to determine the origin of the exploited water and the shallow groundwater, the ratio Danube water/background water, and the average transit time of the infiltrated Danube water for the observed wells. For reaching of these purposes stable oxygen and radioactive tritium isotope measurements, water chemical and hydraulic data have been applied. These data are compared and built together with the hydraulic data in the modeling process that was made with the 3 dimensional variation of the model program MODFLOW. The origin of the water from production or observation wells can be determined on the basis of the significant difference between the δ18O values of the Danube water and the shallow groundwater (recharging from the infiltrating precipitation). The mean δ18O values in Hungary: -11.0 [per mille]VSMOW for the Danube water, -9.3 ± 0.4 [per mille]VSMOW for the locally infiltrated phreatic groundwater and -11.8 ± 0.8 [per mille]VSMOW for the deep old groundwater. From 1998 till 2002 in every quarter a year water samples were taken from several observation wells in the northern part of the island for stable oxygen isotope measurements. On the basis of the isotopic significance we could separate the middle area of the island, where the measured δ18O values correspond to the mean of the fresh shallow groundwater, so the Danube water component cannot be detected. From the riverbank towards the middle of the island we could observe the decreasing of the river water component and determine the ratio of the river water. On the basis of the measured δ18O values we could separate another area where the shallow groundwater (originated from the infiltrated precipitation) mixed with a groundwater characterized with more positive stable oxygen isotope composition. This groundwater has enriched on the surface before the infiltration. Nowadays we can find such water in the 'Kavicsos' Lake. During the observation period in the case of an observation well, the measured δ18O values were always more negative than the mean of the Danube water (-11.0 [per mille]). The measured tritium concentrations were always lower (6-12 TU) than that of the Danube water (15-50 TU) and of the phreatic groundwater (15-20 TU). From these data we can infer that in the surroundings of the F23 well the phreatic groundwater and the Danube water have mixed with upwelling old groundwater. This tritiumless old groundwater, based on measurements on wells exploiting deeper groundwater in this area, can be characterized with -12.0 - 13.8 [per mille] δ18O values and its radiocarbon age is between 24000 and 31000 years