Phosphorus by the Numbers: Boston’s Charles River

It seems like everywhere we turn, there’s too much phosphorus in our waters. The resulting algal blooms are unsightly and frequently toxic. And of course, when the algae die and decompose, offensive odors and dead fish ensue. But you knew that.

Last week at the 2010 Marquette University Law School Conference on “Water and People” 300 of us had the pleasure of listening in on a presentation on how Boston can best combat the phosphorus problem in its beloved Charles River. Bob Zimmerman, Executive Director of the Charles River Watershed Association, led us through their analysis of phosphorus loadings in the Upper Charles River.

Their detailed TMDL included an analysis of phosphorus inputs by eight land use types. In addition to land use types, they included inputs from the five small to medium sewage treatment plants (POTWs) in the Upper Charles, as well as combined sewer overflows (CSOs). By comparing phosphorus loadings by land use type with the relative cover of that land use type, they were able to extrapolate which types were the biggest contributors in the watershed.

Agriculture is a small component (3%) of the watershed, and apparently contributes only 2% of the phosphorus loading in the watershed making it an average (not egregious) source. However, three land use types – commercial, industrial, and high density residential – contributed disproportionately high loadings to the river. For instance, commercial properties were only 3% of the land use, but contributed 10% of the phosphorus loadings. Industrial and high density residential properties were 5% and 12% of the land use but 14% and 26%, respectively, of the loadings. Other significant sources were from CSOs (6%) and POTWs (17%).

Knowing the sources of pollution gives the region a fact-based platform for improving its water quality. Combining these numbers with the cost for various practices that can remove nutrients from the surface waters, CRWA and its community partners can recommend the most cost-effective practices, and even has the platform for offering pollution trading schemes. Even better is that the green infrastructure practices that they recommend come with many co-benefits such as ground water recharge, reduced flooding, more attractive communities, etc.

The other interesting twist that we’ll be seeing unfold in the Charles River watershed is the implementation of “Residual Designation Authority” which is made possible by section 402 of the Clean Water Act. EPA Region 1 will be piloting a program in a few select sites in New England, including the Charles, where they will be requiring owners of existing impermeable sites greater than two acres to retrofit the site to remove 65% of the phosphorus from the site. I’m very curious to see how this unfolds, and whether other parts of the country will adopt a similar approach.

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