Determination of Chlorofluorocarbon Apparent Ages in Groundwater from the West Side of the Island of Hawaii

Large quantities of nutrient-enriched groundwater are supplied to coastal west Hawaii via point-sourced and diffuse groundwater seepage sites as submarine groundwater discharge. These inputs are especially important given that they are the only freshwater source to the coastal zone due to the absence of perennial streams. It has been suggested that discharging groundwaters are less than 50 years old; yet, the apparent ages of these groundwaters have not been investigated, despite the copious water volumes discharging to the coastal zone. Apparent age data is crucial to fully understand nutrient transformations along groundwater flow paths and will help constrain groundwater residence times. Water samples from eighteen locations were collected in triplicate for apparent age determination. Sample chlorofluorocarbon concentrations were modeled using single, duel, and multiple water source models. The single water source model does not fit the data, given the area’s complex aquifer geometries and flow networks. The dual and multiple water source models fit the data and indicate that some water is greater than 50 years old, and that average residence times can vary greatly from location to location.