Chlorofluorocarbon apparent ages of groundwaters from west Hawaii, USA

Kelly, Jacque; Glenn, Craig

Age and sources of groundwater in Kaloko-Honokohau NHP is studied. The volcanic coastal aquifers of west Hawaii supply drinking water to the area’s residents and nutrient-rich groundwater discharge to the nearby oligotrophic coastal waters. Despite the societal and ecological importance of the water in these aquifers, very little is known about the ages and recharge areas of the groundwater. We therefore determined aquifer recharge areas and groundwater residence times by sampling 18 locations for the oxygen and hydrogen isotopic composition of groundwater and chlorofluorocarbon (CFC) apparent groundwater ages. We sampled water supply wells, coastal wells, and coastal ponds. We applied a d18O/altitude gradient and well-established lapse rates to find that groundwater recharge predominantly occurs in the area’s maximum rainfall zone. Furthermore, the isotopic data suggest that fog drip contributes to aquifer recharge. A single-water source model yielded recharge years ranging from the mid-1960s to mid-1980s for ten samples (56% of samples). Alternatively, a simple binary mixing model, with one water source recharging before 1940 and the other after 1940, indicated that 14 samples (78% of samples) contained young water that recharged the aquifer between the mid-1970s and mid-1980s. We also find that CFCs can be used to distinguish between water originating from different aquifers in the area.