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. - Highlights: • We examine the N biogeochemistry of injected wastewater transport on Maui, Hawai'i. • Wastewater effluent enters coastal waters as submarine groundwater discharge. • Denitrification is responsible for the majority of N attenuation during aquifer transport. • Pathogen chlorination of injected wastewater likely suppresses N-reducing bacteria. • Replacing chlorination with UV disinfection may reduce N-loading to coastal ocean.