The present study aims to provide insight into the characteristics of carbonate aquifers supplying springs used by local households in the small coastal Larawali and Kadahang catchments of East Sumba. It is thought that the aquifers are predominantly recharged by rapid flow through fractures and that rain water and ground waters would have similar isotopic signatures. The main objective of this study was to test this assumption by analysing the stable isotope compositions of springs, shallow wells and rainfall within the catchments.
Wells, springs, rainfall collection sites and one surface stream were sampled multiple times during the wet season of 2016-17. Samples were extracted from wells and streams using a top-weighted Hydrasleeve no-purge sampler and analysed by cavity ring-down spectrometry (Picarro L2130-i) for δ2H and δ18O at the Environmental Chemistry and Microbiology Unit, Charles Darwin University, Darwin.
Results did not consistently fall along meteoric water lines, suggesting potentially different water sources or that groundwater samples represent a mixture of sources. Rainfall samples were depleted in δ18O relative to the local meteoric water line (LMWL), indicating they may have originated from high intensity rainfall events that were depleted in the heavy isotopes (influenced by the “amount effect”). Streamwater was significantly more enriched in both isotopes than rainfall, indicating the probable effect of evaporation. By contrast early wet season well and spring samples fell on the global meteoric water line. This could indicate a different source or origin of these waters.
Stable isotope analyses in isolation cannot give us a good understanding of recharge processes. Repeated field campaigns for stable isotopes analyses with the addition of major ion analyses and geological survey will provide further insight into the characteristics of these wet-dry tropical karst aquifers.