PS 43-79 - Not Green Enough? Assessing the efficacy of green and blue infrastructure in urban watersheds traversing New York City, NY

Background/Question/Methods

In New York City, New York (NYC), the large population and combined sewer overflow (CSO) system results in significant nutrient loading, particularly inorganic nitrogen (N), to the adjacent New York-New Jersey Harbor Estuary. Eutrophication, harmful algal blooms, and microbial pollution are of increasing concern in the estuary, leading to widespread green infrastructure initiatives (e.g., Million Trees Project, NYC DEP Green Infrastructure Plan) to reduce CSO contributions and blue infrastructure initiatives (e.g., the Billion Oyster Project and the NYC Parks Wetlands Management Framework) to reduce urban flooding and remove estuarine nutrients in situ. In this study, we used field sampling, existing datasets, and hydrologic modeling to construct N budgets for two NYC tidal creek watersheds. These were used to assess the primary sources and sinks of N, and the potential for green and blue infrastructure to remove N inputs to the estuary. Although a limited number of studies have assessed N removal capacity of this technology and infrastructure at the local scale, assessment has not been made on a watershed scale, nor are removal rates contextualized relative to N inputs.

Results/Conclusions

We found that millions of kg of N enter the creeks annually, with over half coming from treated wastewater via effluent and just under half coming from untreated wastewater entering via the CSO system. Green and blue infrastructure did not account for substantial N removal relative to total N inputs. In Newtown Creek, green infrastructure reduced N load by an estimated 2%, and bivalves were estimated to reduce in situ N by 0.1%. Interestingly, however, field measurements of N loading in Coney Island Creek demonstrated substantial N loss along flow pathways, suggesting additional N sinks not accounted for by the budgets. Our findings suggest that citywide efforts prioritizing green infrastructure construction may not contribute to as much N removal as previously thought. This may have important implications for how green and blue infrastructure projects are advertised and assessed, particularly in terms of ecological benefits. Blue and green infrastructure projects may increase environmental engagement and stewardship, which are important ecosystem service outcomes. However, it is important to be cognizant of the limitations associated with inferring the ecological benefits provided to the surrounding estuary by these infrastructure projects, as this assessment should be made within the context of total pollution inputs.