Stable isotopic compositions of groundwater and surface water were monitored during the period 1999–2001 to interpret mixing and flow paths in the saturated zone at the artificial recharge test site at Arrenaes, Denmark. The results from the stable isotopic investigation are compared to the Cl distributions. Surface water from the nearby Lake Arres0 is pumped into the recharge facilities and infiltrates in an uncon-fined aquifer with an overlying unsaturated zone of approximately 25 meters in a sequence of unconsolidated glaciofluvial-lacustrine sediments. The environmental tracers reveal that surface water has completely replaced groundwater beneath recharge areas whereas mixing in the vicinity of the test site ranges from 0% to 90% depending on the distance from recharge facilities, hydraulic gradient, travel time and abstraction strategy.