Stable isotope and hydrogeological measurements: Implications for transit time and mixing ratio in a riparian system of the Danube River

• Most complete stable isotope data of the Danube River from Budapest (Hungary) area. • Systematic difference is found in δ 18 O values of Danube between Vienna and Budapest. • Transit time distributions of different production wells were determined. • Wells revealed distance dependency in various parameters. • Mixing ratios was estimated for the exploited water in the production wells. Riverbank filtration has a very important role in the supply of drinking water globally. However, this water source is vulnerable, because rivers are easily contaminated. To assess its vulnerability, it is essential to understand the properties of the aquifer including the sources of the water and the transit time. Therefore, a comprehensive investigation of the Hungarian Danube River-‘riverbank’ system was conducted on two of its islands (Szentendre and Csepel). Water level, temperature and electrical conductivity were monitored in the wells located in the riverbank, and water samples were collected regularly from the wells and the river for stable hydrogen and oxygen isotope analyses over a year. It was found that the wells located at different distances from the Danube River have various responses to the hydrological changes occurring in the river. Transit time distributions (TTD) belonging to different production wells were determined by the lumped parameter method, based on the convolution integral. Differences in the TTDs of the wells are explained by their distance from the Danube River, spatial variations in the aquifer hydraulic properties and the varying pumping rates. Additionally, the estimated mixing ratio of different water sources (i.e., Danube River and background shallow groundwater infiltrated from precipitation) to the exploited water suggests that the contribution of precipitation is less than 20 %. Furthermore, the stable water isotope data of the Danube River in the Budapest area measured from 1998 to 2021 was gathered and evaluated. It appears that the δ 18 O values of the Danube River at Budapest show systematically higher values than those from Vienna, indicating lowland water contribution and some evaporation effect.