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 (δ18O and δ2H), tritium (3H) 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. δ18O and δ2H 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 Kleče 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 Jarški prod 1. Groundwater at wells Kleče 8, Kleče 12, and Jarški prod 3 contains on average between 41% and 48% of the Sava River water. The 3H and 3H/3He methods indicate short mean residence time of groundwater present at Jarški prod (2-7 years) and Hrastje (7-8 years). A small fraction (<10%) of old groundwater is present at Kleče. 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.