Changes to Nitrate Isotopic Composition of Wastewater Treatment Effluent and Rivers After Upgrades to Tertiary Treatment in the Narragansett Bay Watershed, RI

Due to nitrogen load reduction policies, wastewater treatment facilities (WWTFs) have upgraded to tertiary treatment - where denitrification reduces and removes nitrogen. Changes to the stable isotopic composition of nitrate inputs after upgrades or how it transfers to the estuary have not been assessed in Rhode Island. We investigate whether these upgrades impact the isotopic signature of nitrate inputs to Narragansett Bay. Samples from rivers and WWTFs discharging to Narragansett Bay characterize the anthropogenic source nitrate (NO3(-)) isotopic composition (δ(15)N-NO3(-) and δ(18)O-NO3(-)) and temporal variability. At one WWTF, tertiary treatment increased effluent nitrate δ(15)N-NO3(-) and δ(18)O-NO3(-) values by ~16‰. Riverine values increased by ~4‰, likely due to the combination of decreases in N and upgrades. Combined river and WWTF flux-weighted isotopic compositions showed enriched values and an amplitude reduction in monthly variability. When seasonal isotopic means are significantly different from other sources, δ(15)N-NO3(-) may be a useful tracer of inputs.