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

• Submarine groundwater discharge (SGD) nutrient fluxes are calculated in Monterey Bay.
• SGD fluxes are compared to river and deep water input.
• SGD is the same throughout the year while other sources are seasonal.
• SGD influence is limited to the near shore ocean.

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.