Quantification of point-source groundwater discharges to the ocean from the shoreline of the Big Island, Hawaii

Aerial thermal infrared imaging has revealed that submarine groundwater discharge (SGD) along the western coast of the Big Island of Hawaii is often focused as point-source discharges that create buoyant groundwater plumes that mix into the coastal ocean. We quantified the SGD fluxes associated with several plumes using natural geochemical tracers. Offshore transects of [sup.222]Rn and [sup.224]Ra show elevated activities and corresponding low salinities in the nearshore waters within the plumes, indicating that these naturally occurring radionuclides can be useful tracers of groundwater inputs in this area. Using a series of simultaneous mass balance equations for water, salt, and radon, we determined the groundwater fluxes of six plumes near Kona, Hawaii. The average SGD fluxes ranged from 1100 [m.sup.3] [d.sup.-1] to 12,000 [m.sup.3] [d.sup.-1] of total (fresh + saline) SGD. The fresh (meteoric) groundwater equivalents for the same flows, modeled by adjusting the groundwater end member to reflect freshwater rather than brackish groundwater composition, ranged from 630 [m.sup.3] [d.sup.-1] to 8600 [m.sup.3] [d.sup.-1]. These fluxes are in general agreement with earlier results obtained by hydrological water budgets and physical oceanographic analyses of fresh SGD rates in this region.