Submarine Groundwater Discharge-Derived Nutrient Loads to San Francisco Bay: Implications to Future Ecosystem Changes

Abstract

Submarine groundwater discharge (SGD) was quantified at select sites in San Francisco Bay (SFB) from radium ( 223 Ra and 224 Ra) and radon ( 222 Rn) activities mea- sured in groundwater and surface water using simple mass balance box models. Based on these models, discharge rates in South and Central Bays were 0.3-7.4 m 3 day −1 m −1 .A l- though SGD fluxes at the two regions (Central and South Bays) of SFB were of the same order of magnitude, the dissolved inorganic nitrogen (DIN) species associated with SGD were different. In the South Bay, ammonium (NH4 + ) concentrations in groundwater were three-fold higher than in open bay waters, and NH4 + was the primary DIN form dis- charged by SGD. At the Central Bay site, the primary DIN form in groundwater and associated discharge was nitrate (NO3 − ). The stable isotope signatures (δ 15 NNO3 and δ 18 ONO3 )o f NO3 − in the South Bay groundwater and surface waters were both consistent with NO3 − derived from NH4 + that was isotopically enriched in 15 Nb y NH4 + volatilization. Based on the calculated SGD fluxes and groundwater nutrient concentrations, nutrient fluxes associated with SGD can ac- count for up to 16 % of DIN and 22 % of DIP in South and Central Bays. The form of DIN contributed to surface waters from SGD may impact the ratio of NO3 − to NH4 + available to phytoplankton with implications to bay productivity, phyto- plankton species distribution, and nutrient uptake rates. This assessment of nutrient delivery via groundwater discharge in SFB may provide vital information for future bay ecological wellbeing and sensitivity to future environmental stressors.