Identification and quantification of submarine groundwater discharge in the Hawaiian islands

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Kelly, Jacque Lynn
submarine groundwater discharge; Oahu; thermal infrared; radon; dissolved inorganic nutrients; Pearl Harbor
thesis / dissertation description
Submarine groundwater discharge (SGD) is a fundamental pathway for nutrient and contaminant entry to the world's coastal zones from terrestrial aquifers. The distribution and scale of SGD vary spatially and temporally, necessitating use of multiple methodologies for its study. High-resolution aerial thermal infrared (TIR) remote sensing was employed to map the distribution of cold SGD around much of Oahu, Hawaii, and a dual infrared thermography and in situ tracer (222Rn, temperature, salinity, and dissolved inorganic nutrient) study in Pearl Harbor was conducted to determine locations and fluxes of groundwater discharge to the harbor. Surface water surveys of the natural tracer 222Rn in Pearl Harbor confirmed point source and diffuse seepage areas identified in SST maps. Conservative estimates of groundwater fluxes derived from 222Rn inventories of surface water indicate that between 119,400 and 322,030 m3/d of groundwater enters the harbor along its shoreline. Recently discharged groundwater contributes at least 51,600 mol/d of nitrate, 4,500 mol/d of phosphate, and 835,000 mol/d of silica to the harbor. Isotopic analyses of dissolved nitrate suggest that multiple water sources exist in the harbor and that these sources mix within the aquifer. Chlorofluorocarbon groundwater apparent ages in Kona Hawaii were investigated to determine groundwater residence times and potential implications of the residence times on discharging groundwater. A single water-source model indicates that groundwater recharged four supply wells during the mid-1960s and mid-1970s. Recharge occurred between the mid-1970s and mid-1980s for several coastal wells and ponds using the same model. Alternately, a simple binary mixing model, with one water source recharged prior to 1940 (young water) and the other after 1940 (old water), indicates recharge of the young water fraction from between the mid-1970s to mid-1980s for several wells and coastal ponds. Water supply wells contain greater proportions of relatively old groundwater than coastal wells and ponds, consistent with sampling depths, complex aquifer geometries, and varied flow networks that cause mixing of old water with younger water within the aquifers. Furthermore, CFCs may be used to identify water from different aquifers.