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

Trends in nitrogen isotope ratios of juvenile winter flounder reflect changing nitrogen inputs to Rhode Island, USA estuarine systems

“…Because of this there has been a large north-south gradient in surface seawater nitrogen concentrations in the Bay (Oviatt et al 2002;Pruell et al 2006;Thornber et al 2008). Since δ 15 N values of sewage NO 3 as high as 28 ‰ were reported for WWTFs in the Narragansett Bay watershed (Schmidt et al 2016) and because the oceanic δ 15 N endmember is generally considered to be in the range of 5 to 8 ‰ for NO 3 - (Fry 2002;Dahnke et al 2008), a large gradient in δ 15 N values would be expected along the Bay gradient. However, the δ 15 N trends observed in winter flounder collected along this gradient between 2002 and 2004 did not show a decrease with distance down the estuary.…”

“…The second possible reason for the increase in δ 15 N values is that upgrades of wastewater systems to tertiary treatment can increase in the δ 15 N of the NO 3 released by the facility due to the fractionation associated with the microbial denitrification employed in the tertiary treatment process (Savage 2005;Schmidt 2014). In fact, Gaston et al (2004) reported that eastern hula fish (Trachinops taeniatus) collected near secondary and tertiary WWTFs had higher δ 15 N values when compared with those collected near primary facilities.…”

“…In fact, Gaston et al (2004) reported that eastern hula fish (Trachinops taeniatus) collected near secondary and tertiary WWTFs had higher δ 15 N values when compared with those collected near primary facilities. After the upgrade of several WWTFs within the Narragansett Bay watershed to tertiary treatment, Schmidt et al (2016) reported that there was a 7-8 ‰ increase in sewage NO 3 --δ 15 N values and a 2 ‰ increase in the δ 15 N value of NO 3 in the Providence River region of Narragansett Bay. Therefore, this may have contributed to the increase observed in the δ 15 N values of winter flounder collected at the PCM station.…”

Establishing an Anthropogenic Nitrogen Baseline Using Native American Shell Middens

“…values between 2005 and 2015 reflected the change in δ 15 N value of the source N, despite changes in the quantity of N inputs in recent years. When the sewage treatment plants upgraded to tertiary treatment in the late-2000s, the N concentration in effluent decreased by more than half, but the δ 15 N values of nitrate in the remaining effluent increased from about 7 (2009) to 13.4 (2012) (Chaves, 2004;Schmidt et al, 2016). Thus, the source decreased but the δ 15 N value of that source increased and was recorded in the shell organic matter of local clams.…”

“…Between 2006 and 2008, the δ 15 N values in macroalgae in the Providence River Estuary and upper Narragansett Bay increased by about 1 , potentially associated with the upgrade from secondary to tertiary treatment. A recent study has demonstrated that, while concentrations of bioavailable N in tertiary treated effluent has been greatly reduced, the denitrification process has increased effluent δ 15 N values by about 7 (Schmidt et al, 2016). Thus, while N inputs have been reduced, the signature of that N has increased substantially.…”