Submarine Groundwater Discharge in a Stream-Dominated Embayment: Kahana Bay, Oahu, Hawaii

Located on the northeastern shore of O`ahu, Hawai`i, Kahana Valley is one of the island’s wettest watersheds. It has a continuously flowing stream, which introduces some challenges when conducting a submarine groundwater discharge (SGD) study. This study utilized a combination of established SGD research techniques, including high-resolution airborne thermal infrared (TIR) remote sensing and in-situ surveys for groundwater tracers (e.g. salinity, temperature, and 222Rn activities). This combination of techniques provided a quantitative understanding of the regional-scale and intra-daily variability of SGD in Kahana Bay. The total SGD flux was calculated to be 62,736 m3/day, approximately 63% of which was derived from a traditional Hawaiian fishpond on the bay’s eastern shore. This fishpond also had the highest nutrient concentrations in the bay. Remarkably, the SGD-derived nutrient fluxes to the bay were calculated be greater than or equal to those from Kahana Stream, ranging from ratios of 5:1 for TDP, 4:1 for TDN, and 1:1 for silica. Submarine groundwater discharge, therefore, has a significant influence on the bay’s geochemical budgets.