A Summary of the Submarine Groundwater Discharge (SGD) in Kahana Bay: Spatial and Intra-Daily Variability

Abstract

From global to local scales, submarine groundwater discharge (SGD) is a significant conduit through which nutrients and contaminants may be transported to the coastal ocean. Therefore, it is important that SGD inputs to coastal marine environments be located, quantified, and understood fully. This study utilizes a combination of different SGD research techniques to establish a comprehensive research methodology for SGD studies that can provide a regional-scale understanding of SGD, quantify its flux to coastal waters, and the nutrient fluxes derived from SGD inputs. Kahana Bay, a semiprotected embayment on the northeastern shore of O`ahu, Hawai`i, was selected as the field site for this study because its watershed is an important hydrologic resource, and the bay itself is a known conduit of this watershed’s SGD. Due to the relatively cool temperature of groundwater with respect to overlying seawater, the surface water expression of SGD in Kahana Bay was first mapped using high-resolution airborne thermal infrared (TIR) remote sensing. After identifying potential SGD in the TIR map, an in situ survey was conducted of the surface waters around the perimeter of the bay for 222Rn activities – a geochemical tracer for SGD. Based on a coastal 222Rn mass balance, we calculated a total SGD flux to Kahana Bay of 62,736 m3/day. Using piezometer data from coastal sediments, we also determined that about 30% (18,820 m3/day) of the SGD is freshwater. Huilua Fishpond, on the eastern edge of Kahana Bay, is calculated here to account for ~63% of the bay’s total SGD. Time series data from the eastern shore of Kahana Bay were used to determine the intra-daily variability of SGD with respect to tidal fluctuations and the effects this variability has on coastal salinity and pH. Average dissolved nutrient concentrations in groundwater samples were multiplied by the SGD flux to estimate the SGD-derived nutrient fluxes to Kahana Bay. Dissolved nutrient data were also collected from Kahana Stream and ratios of nutrients derived from SGD:stream were calculated to be approximately 4:1 for TDN, 5:1 for TDP, and 1:1 for silica (Si). Both the volumetric SGD and nutrient fluxes calculated in this study are in agreement with previous 222Rn/nutrient findings from the benthic seepage meter studies of Garrison et al. (2003).