The study of the interactions between groundwater and Sava River water in the Ljubljansko polje aquifer system (Slovenia)

River basin aquifers are common sites for drinking water wells as bank filtration can be a cost effective pretreatment technology. A groundwater vulnerability to pollution depends on a groundwater mean residence time and on a relative contribution of river water versus local precipitation to groundwater. Environmental isotopes of oxygen and hydrogen (delta O-18 and delta H-2), tritium (H-3) and concentrations of nitrate (NO3-) were used to investigate hydrological pathways, mean residence time and interactions between surface water and groundwater in the Ljubljansko polje aquifer system in Slovenia. (delta O-18 and delta H-2 values indicate a spatial variability of the influence of individual groundwater sources inside the aquifer - local precipitation and the Sava River water. Fractions of river water in groundwater depend on the depth of perforated screens in the pumping wells and their distance from the Sava River. It was estimated that groundwater at wells Klece 11, Hrastje 3, and Hrastje 8 is mostly composed of recently infiltrated local precipitation, while the Sava River is the dominant source of groundwater at the well jarski prod 1. Groundwater at wells Klece 8, Klece 12, and jarski prod 3 contains on average between 41% and 48% of the Sava River water. The H-3 and H-3/He-3 methods indicate short mean residence time of groundwater present at jarski prod (2-7 years) and Hrastje (7-8 years). A small fraction (<10%) of old groundwater is present at Klece. Furthermore, infiltration of local precipitation influenced the levels of NO3- at Hrastje. These data extend our understanding of groundwater flow in the Ljubljansko polje aquifer system, interactions between the Sava River water/local precipitation and groundwater, and the utility of isotope tracers in evaluating the spatial distribution of groundwater vulnerability to pollution. (C) 2017 The Authors. Published by Elsevier B.V.