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Rainy Summer Results in Higher Bacterial Counts Due to Stormwater Runoff

Updated: Sep 25, 2023

The WRWA has been sampling water quality at 19 sites along the Westport River and contributing streams for more than 20 years to evaluate the health of the river. We test the Westport River weekly from June-August. Bacteria levels can change by the day; each week’s results refer only to conditions the morning on which the samples were taken. We monitor a type of bacteria used for swimming and shellfishing standards, which have specific concentrations that are considered safe for humans, reported in cfu or colony forming units. Bacteria levels are typically elevated during wet weather events. A good precaution to take includes avoiding swimming right after a heavy rain. During dry weather, bacteria levels are typically low and the River is usually considered safe for swimming.

A fecal coliform test is used to determine whether water has been contaminated with fecal matter. The presence of fecal coliform indicates the possible presence of organisms that can cause illness. The EPA has set acceptable limits for fecal coliform in water based upon the use of the water. For example, drinking water cannot contain any fecal coliform but water for swimming may contain up to 200 fecal coliform colonies/ 100 ml.

By analyzing the 2023 data, we found that roughly 22% of our samples exceeded the safe swimming standards (primary contact recreation) this year. Compared against last summer, when we found only an 8% exceedance rate for swimming, this may seem like a concerning rise. However, the increase is not surprising given the large amount of rain this summer. This summer (June-August) we recorded 16.94 inches of rain at the River Center with some intense downpours. In 2022, we experienced a very dry summer, with about 6.2 inches of rain bringing less bacteria into the watershed via runoff. How do fecal coliforms get into rivers, streams and lakes? Large amounts of fecal coliform are released in the waste of animals and can be washed into streams by runoff from rain. To see the data check our River Monitoring webpage.

Paved areas contribute to fecal coliform contamination when wastes from dogs, cats, geese, and other animals are carried into storm drains, streams, rivers and lakes during storms. Over the years bacteria counts have diminished in the rivers due to Title V septic improvements and improved treatment and minimization of stormwater runoff – all of which the WRWA has advocated for using these data to make the case for reducing pollution sources.


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