A Geochemical and Geophysical Assessment of Coastal Groundwater Discharge at Select Sites in Maui and O’ahu, Hawai’i
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This chapter summarizes fieldwork conducted to derive new estimates of coastal groundwater discharge and associated nutrient loadings at select coastal sites in Hawai’i, USA. Locations for this work were typically identified based on pronounced, recent ecosystem degradation that may at least partially be attributable to sustained coastal groundwater discharge. Our suite of tools used to evaluate groundwater discharge included select U/Th series radionuclides, a broad spectrum of geochemical analytes, multi-channel electrical resistivity, and in situ oceanographic observations.
Based on the submarine groundwater discharge tracer 222Rn, coastal groundwater discharge rates ranged from about 22–50 cm per day at Kahekili, a site in the Ka’anapali region north of Lahaina in west Maui, while at Black Point in Maunalua Bay along southern O’ahu, coastal groundwater discharge rates ranged up to 700 cm per day, although the mean discharge rate at this site was 60 cm per day. The water chemistry of the discharging groundwater can be dramatically different than ambient seawater at both coastal sites. For example, at Kahekili the average concentrations of dissolved inorganic nitrogen (DIN), dissolved silicate (DSi) and total dissolved phosphorus (TDP) were roughly 188-, 36-, and 106-times higher in the discharging groundwater relative to ambient seawater, respectively. Such data extend our basic understanding of the physical controls on coastal groundwater discharge and provide an estimate of the magnitude and physical forcings of submarine groundwater discharge and associated trace metal and nutrient loads conveyed by this submarine route.
KeywordsDissolve Inorganic Nitrogen Dissolve Organic Nitrogen Groundwater Discharge Submarine Groundwater Discharge Total Dissolve Nitrogen
PWS gratefully acknowledges continued support for this work from the USGS Coastal Aquifer Project and the USGS Pacific Coral Reef Project, both of which are funded by the USGS Coastal and Marine Geology Program (CMGP). We would also like to thank Russell Sparks and Darla White (HI-DLNR/Division of Aquatic Resources) for invaluable logistical support, Chip Hunt (USGS-Honolulu) for early discussions that helped define original project objectives, and Susie Cochran (USGS-Santa Cruz) for expert GIS contributions. Technical reviews by Jessie Lacy and Nancy Prouty made substantial improvements to this paper.
CRG acknowledges support by a grant/cooperative agreement from the National Oceanic and Atmospheric Administration, Projects R/HE-2 and R/HE-17, which are sponsored by the University of Hawaii Sea Grant College Program, SOEST, under Institutional Grant No. NA09OAR4170060 from NOAA Office of Sea Grant, Department of Commerce. The views expressed herein are those of the author(s) and do not necessarily reflect the views of NOAA or any of its subagencies. UNIHI-SEAGRANT-BC-12-01. The use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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