Characterizing the variability of phosphorus export from urban stormwater for potential treatment strategies

Abstract

The impairment by eutrophication and recent escalating awareness of improving the eutrophication situation of Charles River by has led to regulations aimed at reducing phosphorus loading to the river. To address phosphorus loadings of stormwater runoff from dense industrial, commercial, and residential lands, as per regulations, the City of Cambridge, MA, must reduce its annual phosphorus export over a period of years based on incremental reductions. One potential treatment approach for areas with separated sewers is to divert stormwater to some type of treatment. However, it is not possible to treat all stormwater. To develop an optimized diversion and treatment strategy, a collaborative study between the City of Cambridge, Stantec, and Northeastern University is conducting stormwater sampling to understand the variability in phosphorus export from different urban landscapes. Specifically, we are investigating the phosphorus loading associated with different particle size fractions during runoff events from different land uses. The focus on particle size provides a connection to flow velocities required for transport (i.e., shear stress), which can be modeled and may be used to trigger flow diversions. Thus, knowledge of the phosphorus loadings over a range of particle size fractions is needed to characterize the potential export reductions based on flow velocity thresholds. In this study, 1-hour composite stormwater samples for six storm events were collected from four different catchments within the City of Cambridge, MA. The stormwater samples were collected using ISCO 6712c autosamplers. The samples were divided into six subsamples based on particle size- unfiltered, filtered through 250, 100, 50, 25, and 10 micron filters. The subsamples were then analyzed for total phosphorus and total solids. The analysis of these high-resolution stormwater samples shows that the phosphorus is not uniformly distributed between the particle size fractions and differs from total solids distributions, which supports potential diversion and treatment approaches based on flow velocity. However, the event loading distributions are also highly dependent on rainfall and landcover characteristics. Another key finding that will be highlighted is the effect of street sweeping on controlling the phosphorus export.