Pollution sources to the lower Charles include
upstream community wastewater treatment plants, contaminated stormwater
runoff, leaking septic systems, and combined sewer overflow.
In the Charles, most fecal coliform bacteria counts do not exceed the
state standard of 200 colonies per 100 milliliters.
The area between the Watertown dam and the Arsenal Mall
(upstream of any CSO discharges) is the most consistently
contaminated section surveyed; and the Charles River
Basin, the important recreational area between Cambridge
and Boston, had the best bacterial water quality.