Monthly E.coli concentration at Watertown dam footbridge

Of the 165 samples, 64 (39%) fell within the safe to swim (0-126 cfu/100 mL) limit, 80 (48%) within the safe to boat but not to swim (127-630 cfu/100 mL) limit, and 21 (13%) within the not safe to swim or boat (above 630 cfu/100 mL) limit (MA state water quality standards). Large E. coli concentrations are usually explained by increased runoff after rain events.

Note that the y-axis has a logarithmic scale. Usually, one sample is collected from the Charles River per month. For months where several samples were collected, the mean value is plotted.
Chart: François Delavy Source: CRWA, own computation Get the data