August CWMN Update

Our August water quality results show the harmful effect that polluted stormwater runoff has on the river.


This month’s water quality report shows the results from our August sampling, and the results were not as good as we would have liked. Water quality was affected due to a large amount of stormwater pollution that washed into the river the night before we sampled.

E. coli

In August, only 6 out of 39 sites met the EPA standard for boating, and only 2 sites met the standard for swimming.  Several of the sites were almost 100 times the level that is deemed safe for swimming. Below is a map of the watershed containing graduated symbols for each site sampled.  A green symbol indicates E. coli levels safe for swimming, a yellow symbol indicates E. coli levels safe for boating, and  a red symbol indicates E. coli levels exceeding safe standards.

While these numbers may look bad, we have to take into consideration that it rained heavily on our August sampling day.

We typically see a spike in E.coli bacteria when it rains because of stormwater runoff polluted with dog waste. We recommend staying out of the water for at least 48 hours after any storm.

Water quality in the Neponset River, when it hasn’t rained, is typically the opposite of what we saw in August, with almost all sites being safe for boating, and many more sites that are also safe for swimming.

Follow this link to a list of CWMN site descriptions: CWMN 2019 Site Descriptions

August E. coli Map

A map showing all CWMN sites sampled in August 2019 with graduating symbols indicating E. coli level according to the legend.  

August E. coli Levels vs. EPA Boating Standard

August data for E. coli levels by CFUs (colony-forming units). The Neponset River had very high levels of E.coli in the month of August

Phosphorus Results

Phosphorus is a key nutrient for plants. This is why people add fertilizer to their gardens and yards. Unfortunately, when too much fertilizer is applied it can get washed off when it rains and it ends up in our streams and ponds. Too much phosphorus can cause harmful algal blooms, cyanobacteria blooms, and fish kills.

Only 3 out of 34 streams and two out of 5 ponds tested had levels of phosphorus that are considered healthy.

While the Neponset has issues with too much phosphorus, the August results were not as good as we had hoped for. Again, this is largely due to stormwater pollution washing phosphorus off of lawns and into the river.

For tips on how to reduce your phosphorus impact on the river visit

August data for Total Phosphorus. The Neponset River experienced slightly higher than average Phosphorus levels in August. 

Dissolved Oxygen Results

Dissolved Oxygen has been referred to as the “air” fish breathe under water. If the concentrations of oxygen in the water get too low fish and other aquatic animals that can’t escape can suffocate and die.

Excess phosphorus is a key driver of low levels of dissolved oxygen, which is why we closely monitor both parameters. Dissolved Oxygen isn’t too bad for August with only 8 sites failing to meet a healthy DO level of above 5.00 mg/L.

August data for Dissolved Oxygen in mg/L.  The Neponset River experienced lower than average DO levels this month with most sites still meeting the standard.

Finally, we want to thank our dedicated volunteers for their participation over the past five months. We’re now in our 23rd year of monitoring the health of the Neponset River and couldn’t do it without their help.

Please note – the September and October sampling results will be published over the next month or so. For more information about water quality data or volunteer opportunities, please contact Kelly Di Stefano at

Kelly Di Stefano, Environmental Science Fellow, October 2019

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