Subterranean estuaries (STEs) form at the land-sea boundary where groundwater and seawater mix. These biogeochemically reactive zones influence groundwater-borne nutrient concentrations and speciation prior to export via submarine groundwater discharge (SGD). We examined a STE located along the York River Estuary (YRE) to determine if SGD delivers dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to the overlying water. We assessed variations in STE geochemical profiles with depth across locations, times, and tidal stages, estimated N removal along the STE flow path, measured hydraulic gradients to estimate SGD, and calculated potential nutrient fluxes. Salinity, dissolved oxygen (DO), DIN, and DIP varied significantly with depth and season (p < 0.05), but not location or tidal stage. Ammonium dominated the DIN pool deep in the STE. Moving toward the sediment surface, ammonium concentrations decreased as nitrate and DO concentrations increased, suggesting nitrification. Potential sediment N removal rates mediated by denitrification were <8 mmoles N m(-2) d(-1). The total groundwater discharge rate was 38 +/- 11 L m(-2) d(-1); discharge followed tidal and seasonal patterns. Net SGD nutrient fluxes were 0.065-3.2 and 0.019-0.093 mmoles m(-2) d(-1) for DIN and DIP, respectively. However, microbial N removal in the STE may attenuate 0.58% to >100% of groundwater DIN. SGD fluxes were on the same order of magnitude as diffusive benthic fluxes but accounted for <10% of the nutrients delivered by fluvial advection in the YRE. Our results indicate the importance of STE biogeochemical transformations to SGD flux estimations and their role in coastal eutrophication and nutrient dynamics. Plain Language Summary At the coastline, groundwater is released to coastal waters and at this land-sea boundary, groundwater and seawater meet and mix in subterranean estuaries (STEs). Groundwater can be a source of nutrients, trace metals, and carbon to the overlying water. Within the STE, groundwater-borne nutrients may be consumed or changed, altering what is released to overlying water. In this study, we examined a STE located in the Chesapeake Bay to characterize groundwater constituents with depth, measure nitrogen removal rates, and estimate groundwater nutrient export to coastal waters. Groundwater salinity, dissolved oxygen, and nutrients varied across space and time. Nitrogen removal was measured in sediment incubation experiments, revealing that sedimentary microbes in STEs can reduce some or all the nitrogen accumulated in groundwater. Groundwater discharge exported <10% of the nutrients flowing downstream in surface water and microbial nitrogen removal within the STE lowers nitrogen transport by groundwater. Our results highlight the importance of nutrient cycling in STEs and how these zones influence the role of groundwater as a nutrient source and, therefore, coastal water quality.
