Changes to nitrate isotopic composition of wastewater treatment effluent and rivers after upgrades to tertiary treatment in the Narragansett Bay watershed, RI

• Published in: Marine pollution bulletin Vol. 104; no. 1-2; pp. 61 - 69
• Main Authors: Schmidt, Courtney E., Robinson, Rebecca S., Fields, Lindsey, Nixon, Scott W.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 15.03.2016
• Subjects: anthropogenic activities; Bays; Brackish; Denitrification; Environmental Monitoring; estuaries; issues and policy; Marine; Narragansett Bay; Nitrate; nitrates; Nitrates - analysis; Nitrogen; Nitrogen and oxygen isotopes; Nitrogen Isotopes - analysis; Nitrogen Oxides; Rhode Island; rivers; Rivers - chemistry; temporal variation; Tertiary treatment; Waste Water - chemistry; Wastewater treatment; Water Pollutants, Chemical - analysis; water pollution; watersheds; Rhode Island; ANW, USA, Rhode Island, Narragansett Bay; Denitrification; Nitrate; Narragansett Bay; Nitrogen and oxygen isotopes; Wastewater treatment; Tertiary treatment
• ISSN: 0025-326X; 1879-3363
• DOI: 10.1016/j.marpolbul.2016.02.010

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 (δ15N–NO3− and δ18O–NO3−) and temporal variability. At one WWTF, tertiary treatment increased effluent nitrate δ15N–NO3− and δ18O–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, δ15N–NO3− may be a useful tracer of inputs. [Display omitted] •We analyzed δ15N–NO3− and δ18O–NO3− of riverine and WWTF inputs to Narragansett Bay.•Denitrification added at a WWTF increased δ15N–NO3− and δ18O–NO3− by 16–17‰.•Riverine δ15N–NO3− and δ18O–NO3− increased 4‰.•Riverine increase likely due to decrease of N and added denitrification.•Combined WWTF and river discharges δ15N–NO3− and δ18O–NO3− averaged >8‰.