Submarine groundwater discharge in Northern Monterey Bay, California: Evaluation by mixing and mass balance models

Monterey Bay, California (CA) receives nutrients from multiple sources, including river discharge, upwelling of deep water, and submarine groundwater discharge (SGD). Here we evaluate the relative importance of these sources to Northern Monterey Bay with a mixing model that integrates radium isotopes (²²⁴Ra, ²²³Ra, ²²⁸Ra) and nutrient concentrations (SiO₄, NO₃, and PO₄). We also apply a radium isotope based mass balance model to determine SGD and associated nutrient fluxes to Monterey Bay at four sites. Our findings indicate that SGD is a relatively consistent source of nutrients across locations and seasons to Northern Monterey Bay, with fluid input on the order of 10-50 L min^-1 m^-1 of coastline, and the greatest impact of SGD fluxes is close to shore. In contrast, nutrient inputs from rivers and upwelling are more variable spatially and temporally. SGD nutrient fluxes are lower where seawater intrusion into coastal aquifers may limit flow of nutrient-rich meteoric groundwater into the coastal ocean.