Submarine Groundwater Discharge on a Semi-Arid Island (Santa Catalina, California): Assessing Radon/Salinity Mass Balance Method Uncertainty and Utility for Groundwater Recharge Estimation

Abstract

Quantifying the freshwater component of submarine groundwater discharge (SGD) is critical in the analysis of terrestrial influences on marine ecosystems and in assessing the water budget of coastal aquifers. In semi-arid to arid settings, this quantification is difficult because low SGD rates translate into low concentrations of groundwater solutes in coastal waters. In this study, fresh SGD (FSGD) was quantified for Toyon Bay on Catalina Island, California, for wet and dry seasons using a combination of radon and salinity mass balance models, and the results were compared to the watershed-specific groundwater recharge rates obtained from a soil water balance (SWB) model. Calculated FSGD rates vary with season (high in spring, low in fall) and fall short of the modeled recharge rates by a factor of up to 7. While sensitivity analyses revealed FSGD estimates to be significantly influenced by uncertainties in geochemical variability of the groundwater end-member and fluctuations of water depth, the results of this study do not support the SWB model-based recharge rates. The findings of this study highlight the utility of the radon and salinity mass balance-based FSGD estimates as groundwater recharge calibration targets, which may aid in the establishment of more refined sustainable groundwater yields.