In Aquifer Storage and Recovery (ASR) schemes, geochemical and biogeochemical reactions play an important role, impacting on aquifer matrix integrity and recovered water quality. In order to quantify the geochemical reactions occurring, it is necessary to trace the injectant. The use of chloride and the stable isotopes of the water molecules as conservative tracers was investigated in an ASR trial at Bolivar, South Australia where reclaimed effluent was injected into a brackish aquifer. Chloride is a primary indicator of mixing of injectant and native groundwater. The injection period spanned 18 months and the quality of the injectant varied with seasons leading to potentially large errors in calculating the proportion of injectant in the sampled mixture. Stable isotopes of the native groundwater and the injectant were also significantly different, allowing identification of injectant breakthrough. The injectant isotopic signature also showed seasonal variations. Hence a combined approach using chloride and stable isotopes assists in more accurately quantifying the mixing fraction and the travel time of water in the aquifer between injection and observation wells.