The Effect of Stormwater on Water Quality

052213 Bacteria Map

Map illustrating bacteria levels at Neponset sampling locations from May, 2013, sampling results.

In May, we had our second watershed-wide sampling event. You can view previous results from April, as well as previous years, for some comparison, but the big takeaway message from the results in May is the impact that stormwater can have on water quality.

For each sampling event, we pay particular attention to how much precipitation has occurred before the event to help us understand the effects that stormwater may be having throughout the watershed. In April, the sampling event had barely enough precipitation preceding it to be classified as a wet weather event. However, due to the small amount of precipitation and lower water temperatures, we had only a few problem spots that failed water quality standards for bacteria. In May, we had more significant rains preceding sampling, 0.07″ in April vs. 0.30″ preceding May’s sampling. Throw in increased water temperatures and you are likely to see an increased contribution from stormwater on the overall results.

In April, there were only two locations that failed to meet water quality standards for bacteria. In May that number jumped to 22. Sampling locations from headwater areas, such as MIB030, on down to portions of the lower mainstem, such as sites NER185 and NER200, all saw increased levels of bacteria above the State standard of 235 MPN/100mL for single samples. Eight of these sites had levels of bacteria over 1000 MPN/100mL. It might be easier to see the contribution that stormwater has on water quality conditions visually, by looking at the map on this page. All of the locations with orange or red circles exceeded the State water quality criteria for bacteria.

In addition to problems with bacteria, stormwater also can elevate nutrient inputs to rivers and streams. While total phosphorus levels had some problem areas throughout the watershed, it was the levels of ortho-phosphate, the form of phosphorus in water that is most readily available for uptake by aquatic organisms and limiting nutrient in freshwater systems, that illustrated stormwater’s influence the best (See May results). High phosphorus levels can lead to excessive algal growth that can lower dissolved oxygen levels and make it tough to near impossible for other aquatic organisms to survive. While the levels in the Neponset are not at that threshold, they are most certainly worth monitoring long term in hopes of preventing those types of scenarios before they can take hold and become persistent.

It is because of the great influence that stormwater can have on water quality, that we here at NepRWA have spent a lot of our time the last few years working to improve stormwater rules and controls in our watershed towns. By focusing on procedures and infrastructure that treat stormwater before it enters the river, we hope to see decreases in the influence it has currently, and help to improve water quality long term in the Neponset watershed.

You can read more about our work to clean up stormwater on our BMP survey page.

For more information on our water quality monitoring program and our sampling results please visit our CWMN data page.

William Guenther, Environmental Scientist, October 2, 2013



One response to “The Effect of Stormwater on Water Quality”

  1. Water Tester says:

    Great to see the water testing results online. I’m impressed how many stations violated the standards with such a small amount of rain.

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