Geochemical Dynamics of Nearshore Submarine Groundwater Discharge: Maunalua Bay, Oahu, Hawai‘i

Abstract

Using a multi-disciplinary approach, wastewater signatures were examined in groundwater emanating from three coastal regions (Black Point, Kawaikui, and Wailupe) located on the southeastern shore of Oʻahu in order to quantify the effects of on-site sewage disposal system (OSDS) effluent on groundwater quality. Black Point’s aquifer, Waialae West, contained 328 OSDS, primarily cesspools, and nearly 120 of these were within 1 km of the coast. For comparison, the aquifer feeding both Kawaikui and Wailupe contained a total of just 51 units. NO3- concentrations (166 – 171 µM) and δ15N–NO3- values (10.4 – 10.9‰) were elevated in groundwater discharging at Black Point relative to groundwater discharging at Kawaikui and Wailupe. Mixing relationships between up-gradient terrestrial groundwater and coastal groundwater in the Waialae West Aquifer indicated simple two-component mixing between a high NO3-, high δ15N–NO3- value wastewater source and low NO3-, low δ15N–NO3- value groundwater endmember. A spatial wastewater-recharge model was also used to corroborate the δ15N–NO3- source designation and to estimate the relative volumetric contribution of wastewater effluent and associated N and P loads to terrestrial groundwater in the Waialae West Aquifer.The results from this model aligned with the groundwater geochemical data and indicated that wastewater composes up to 13% of the Waialae West Aquifer’s submarine groundwater discharge. In light of these findings, the relationships between coastal groundwater discharge and reef biogeochemistry at Black Point and Wailupe were investigated to better understand how differences in groundwater geochemistry may drive changes in proximal marine carbonate chemistry. An in situ sampling station was set up adjacent to the location of the primary groundwater spring at each location to monitor salinity, water temperature, pH, dissolved oxygen (DO), and submarine groundwater discharge over 30 days in August 2015 at Black Point and 25 days in September 2015 at Wailupe. An additional high-resolution 24-hour sampling event was completed at four sites at each location, including the primary groundwater discharge point, to monitor changes in salinity, inorganic nutrients, dissolved inorganic carbon (DIC), total alkalinity (TA), and δ13C-DIC values. Groundwater inputs had variable DIC (1780 – 3038 µM), low pHT (7.4 – 7.5), and high NO3- (76 – 162 µM) levels relative to ambient seawater. Sites at both locations experienced significant variability in physicochemical parameters (e.g. Salinity, water temperature, pH, DO, DIC, TA, and nutrients) with the greatest variability occurring at sites closest to the groundwater seeps. These groundwater-induced physicochemical ranges aligned with trends in net community calcification (NCC) and production (NCP) rates, although community responses to these groundwater-induced physicochemical gradients were location- specific. Overall, sites closest to the groundwater inputs showed reduced daytime calcification rates and elevated dissolution rates at night relative to sites further from the groundwater discharge point at both reefs. NCC and NCP rates at Black Point were generally elevated relative to those at Wailupe, potentially as a result of the aforementioned wastewater-derived nutrient inputs. Further, coastal groundwater discharge was a major source of CO2 to the atmosphere at our study locations, with groundwater-driven air-sea CO2 exchange rates reaching up to 82 mmol C m-2 d-1, vastly exceeding air-sea CO2 fluxes from biological processes.