Wastewater injection, aquifer biogeochemical reactions, and resultant groundwater N fluxes to coastal waters: Kā'anapali, Maui, Hawai'i

We utilize N and C species concentration data along with δ15N values of NO3− and δ13C values of dissolved inorganic C to evaluate the stoichiometry of biogeochemical reactions (mineralization, nitrification, anammox, and denitrification) occurring within a subsurface wastewater plume that originates as treated wastewater injection and enters the coastal waters of Maui as submarine groundwater discharge. Additionally, we compare wastewater effluent time-series data, injection rates, and treatment history with submarine spring discharge time-series data. We find that heterotrophic denitrification is the primary mechanism of N loss within the groundwater plume and that chlorination for pathogen disinfection suppresses microbial activity in the aquifer responsible for N loss, resulting in increased coastal ocean N loading. Replacement of chlorination with UV disinfection may restore biogeochemical reactions responsible for N loss within the aquifer and return N-attenuating conditions in the effluent plume, reducing N loading to coastal waters.