Baseline geochemical characterisation of a vulnerable tropical karstic aquifer; Lifou, New Caledonia

•Tap water from Lifou has a consistent isotopic composition similar to groundwater.•Rainwater isotopic composition was variable according to seasonal rainfall effects.•Nitrate loading in groundwater is low and reflect natural soil derived nutrients.•The aquifer is highly vulnerable to contamination from anthropogenic activity.•Critical to monitor future changes as population growth and climate change occurs.

Study regionLifou Island, near the main island of New Caledonia.Study focusStable oxygen and hydrogen isotopes of groundwater and rainfall were used to characterise baseline values for the main fresh water aquifer of Lifou Island and describe its recharge. Other stable isotope parameters (nitrates and DIC) were used to investigate the interaction between surface water (rainfall) and groundwater, including anthropogenic effects from human activities.New hydrological insights for the regionThis study represents the first baseline isotopic characterisation of Lifou Island’s groundwater aquifer composition and provides a reference for future investigative studies on groundwater quality and security. Groundwater sampled in June and October 2012 had nearly identical isotopic composition. Tap water sampled monthly between February 2012 and January 2014 also had a constant isotopic composition similar to the groundwater. Groundwater recharge was found to occur when monthly precipitation exceeded 140 mm, with the recharge cycle representing 20–30% of the annual rainfall. Relationships between HCO32− content, pH, soil δ13C DIC and satellite photo interpretation suggests a variance of soil pCO2, which is explained by different vegetation cover and higher water use efficiencies in forested areas (high pCO2, more negative δ13C isotope values). The δ15NNO3 values for most groundwater indicate they are uncontaminated with anthropogenic nitrates, although some samples taken in October (dry season) showed a slight denitrification, possibly of natural origin.