Variability in Submarine Groundwater Discharge Composition and the Fate of Groundwater Delivered Nutrients at Kīholo Bay and Honokōhau Harbor, North Kona District, Hawai‘i

Abstract

Submarine groundwater discharge (SGD) on the leeward coasts of Hawaiʻi is the largest natural conveyor of terrestrially-derived nutrients to the ocean. However, not much is known as to how these groundwater discharges change, chemically or volumetrically, with time and between wet and dry seasons. SGD flux occurrence, magnitude, and relationship to primary productivity were studied at a relatively pristine site, Kīholo Bay, and a manmade, anthropogenically-influenced site, Honokōhau Harbor, on the leeward (“Kona”) coast of Hawaiʻi, from 2010-2014. Salinity and 222Rn were utilized as natural, conservative tracers to quantify fluxes of groundwater discharge to the coastal ocean. At Kīholo Bay, mean total SGD flux to the coast was 2,300 m3/d (or 8 m3/m/d), of which, between 20-60% was fresh (salinity 0.1). At Honokōhau Harbor, mean total SGD flux was 11,600 m3/d (18 m3/m/d), and 10-40% of this total was fresh. Though total groundwater flux volumes did not differ between wet and dry seasons at these sites, the fraction of total SGD flux that was fresh was 3-7% greater, and the thickness of the buoyant, groundwater plume within the coastal zone was 0.5-2.0 m deeper, during the wet season. Analysis of the timing and quantity of total, fresh, and recirculated seawater fluxes suggests that in addition to limiting the volume of recirculated seawater recharged to the aquifer, the physical force of discharging freshwater pushes most of the high tide recharged recirculated seawater out of the aquifer, every low tide. Thus, a significant volume of recirculated seawater has a very short residence time within the coastal aquifer (less than one tidal cycle). Variations in the composition of total groundwater flux have historically been explained through hypothetical (though probable) variances in recharge through precipitation and differences in the physical pathways of the groundwater. And while this study does not diminish the significance of the two controls on total SGD flux, it also suggests that the composition of total SGD flux, comprised mostly of recirculated seawater (~60%) in Kona, is determined by the force and spread of freshwater exiting the aquifer and the subsequent limitation this force imposes on saltwater recirculation. The loss of this freshwater flux, via pumping of groundwater wells or decreased recharge, has the potential to increase the volumes of recirculated seawater pumped into the aquifer on tidal-scale timelines. This has implications for geochemical cycling within the subterranean estuary and the coastal aquifer. Fluxes of bioavailable N and P at the anthropogenically influenced site, Honokōhau Harbor, are up to 10 times the magnitude of those measured at the pristine site, Kīholo Bay. Conservative mixing lines plotted through Hawaii Expansion wells, that are located between the local Kealakehe Wastewater Treatment Facility, and the marine endmember suggest that groundwaters at Honokōhau Harbor receive nutrients from these wells. Large dissolved loads of inorganic nitrogen (DIN) and phosphorus (DIP) are delivered to coastal waters at Kīholo Bay (up to 12 mol DIP/d and 950 mol DIN/d) and Honokōhau Harbor (up to 190 mol DIP/d and 4,190 mol DIN/d). While the average DIN and DIP fluxes changed little during all seasons (wet v. dry), the greatest fluxes of DIN and DIP at both sites occurred during the dry season month of March 2011. Chlorophyll-a concentrations measured in surface water samples from Kīholo Bay and Honokōhau Harbor were consistent with the lowest values measured globally and ranged from 0.01-0.65 μg/L and 0-0.3 μg/L, respectively. The highest concentrations of Chl-a were measured ~100-350 m away from groundwater outlets. Low primary productivity at both sites may be attributed to short residence time of groundwater derived nutrients, calculated via the natural geochemical tracers, 223Ra and 224Ra, or environmental conditions (low temperature/salinity) that are inhospitable to phytoplankton growth, dependent upon species present, within the groundwater plume. Consequently, increases in SGD temperature and salinity, as could happen if the volume of recirculated seawater in total SGD increases should promote phytoplankton growth closer to shore at both sites.