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

Abstract Study region Lifou Island, near the main island of New Caledonia. Study focus Stable 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 region This 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 HCO 3 2− content, pH, soil δ 13 C DIC and satellite photo interpretation suggests a variance of soil pCO 2 , which is explained by different vegetation cover and higher water use efficiencies in forested areas (high pCO 2 , more negative δ 13 C isotope values). The δ 15 N NO3 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.