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

Abstract

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 222 Rn and 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 3 d 21 to 12,000 m 3 d 21 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 3 d 21 to 8600 m 3 d 21 . 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.

haracterizing the Effects of Two Storms on the Coastal Waters of O‘ahu, Hawai‘i, Using Data from the Pacific Islands Ocean Observing System

“…Submarine groundwater discharges into the coastal ocean can be significant in Hawai'i (Peterson et al 2009), but we do not know the magnitude of this process in our study area. (Hoover et al 2006).…”

Chemical weathering fluxes from volcanic islands and the importance of groundwater: The Hawaiian example

“…SGD fluxes from the Hawaiian Islands are poorly quantified, having been modeled in a few discrete locations only (Johnson et al, 2008;Knee et al, 2008;Street et al, 2008;Peterson et al, 2009), but none of these efforts have targeted the total SGD flux from an entire island or the archipelago as a whole. The USGS has performed whole-island water budget studies for all the major Hawaiian Islands except for the island of Hawai'i, where the water budget has been described in several regional studies (performed by the USGS, the State of Hawai'i and private companies), which together cover the entire island.…”

“…For example, SGD from the island of Hawai'i has a temperature of ~19-21°C (Johnson et al, 2008;Peterson et al, 2009), andConrad et al (1997) showed that groundwater in deep drill holes on the east flank of Kilauea volcano is the same temperature from sea level down to ~500 m below sea level -~20-25°C. The results from Rad et al (2007) are therefore not directly comparable with our work.…”

“…Given the importance of SGD in Hawai'i (e.g. Juvik and Juvik, 1998;Peterson et al, 2009), a total of 12 groundwater wells were also sampled on the island of Hawai'i in 2005 and 2006. Chemical data for five more groundwater wells on Maui, O'ahu and Kaua'i was obtained from the USGS online database (http://nwis.waterdata.usgs.gov/usa/nwis/qwdata).…”

“…Several studies provide evidence that SGD is widespread in Hawai'i as well. Street et al (2008) used multiple chemical tracers (salinity, dissolved silica and Ra-isotopes) to quantify SGD in several locations in Hawai'i, Johnson et al (2008) used thermal infrared imagery to demonstrate the presence of large, cold freshwater plumes along the west coast of the island of Hawai'i and Peterson et al (2009) quantified the discharge via these plumes using salinity and Ra-isotopes. SGD is not only an important pathway for the delivery of water from land to ocean; it also transports significant amounts of dissolved solids from weathering on land directly to the global ocean (Rad et al, 2007;Georg et al, 2009).…”