Measurement of submarine groundwater discharge in Kahana Bay, O‘ahu, Hawai‘i

Submarine groundwater discharge (SGD) is neither well understood nor commonly investigated in Hawai‘i, but it is recognized as a potential pollution source to coastal environments. Between 1998 and 2000, this study located and quantified both total SGD and the terrestrial SGD fraction ( ftgw) in Kahana Bay, O‘ahu. CTD casts were used to profile the water structure and identify potential areas of SGD impact in the bay. Lee-type seepage meters were used to measure SGD rates and collect samples of SGD directly. Radon-222, Si, Cl, and total alkalinity (Ta) were used as natural tracers to measure the terrestrial groundwater fraction within SGD. Nutrient concentrations were also measured to calculate total nutrient fluxes into the bay via SGD. Ninety percent of the SGD in Kahana Bay occurs in the inner bay within 1 km of the shoreline. The average total SGD flux measured was 90 3 106 L d21, 16% of which was terrestrial groundwater. By comparison, the average annual surface runoff from Kahana River is 90.7 3 106 L d21. Estimated fluxes of total dissolved phosphorus and nitrogen by SGD to the bay were 500 and 200% greater than fluxes via surface runoff, respectively. Thus, SGD in Kahana Bay has proved to be a significant source of both freshwater and total nutrient input comparable to that from the surface runoff of Kahana River. Submarine groundwater discharge (SGD) is the seepage of any fluids from coastal submarine sediments into the overlying coastal ocean. In this respect, coastal submarine sediments act as a subterranean estuary, an aquifer separate unto itself with a unique chemistry distinguishing it from either the meteoric or marine hydrologic systems (Moore 1999). SGD is neither well understood nor commonly investigated in Hawai‘i. However, SGD has recently become recognized as a potential pollution source because it can include meteoric groundwater anywhere a terrestrial aquifer with positive head is in contact with the shoreline. In 1997, the Scientific 1 To whom correspondence should be addressed. Present address: Department of Earth and Space Sciences, University of Washington, Seattle, Washington ([email protected]).