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There are a variety of sources that contribute bacteria to our surface waters. These include:
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E. coli bacteria live in the digestive systems of humans and other warm blooded animals. Therefore, they are found in sewage and other wastewater. Most strains are not dangerous, but they can indicate the presence of other disease-causing bacteria.
Swimming in unsafe water may result in minor illnesses such as sore throats or diarrhea. It may also result in more serious illnesses that can last a longer period of time. Children, the elderly and persons with weakened immune systems are at greater risk for illness when they come into contact with contaminated water.
Beaches are sampled a minimum of five times (or "events") per month. Each sampling "event" consists of three samples taken at representative locations within the bathing beach area. The water samples are collected one foot below the surface in three to six feet of water. The samples are then taken to a contracted certified laboratory to be analyzed for E. Coli bacteria.
A beach is closed because monitoring conducted by Washtenaw County Environmental Health determined that, at the time of testing, levels of bacteria exceeded the limits set by the Michigan Public Health Code. The geometric mean of the results from one sampling "event" cannot exceed 300 E. Coli per 100 milliliters water. In addition, the 30-day geometric mean cannot exceed 130 E. Coli per 100 milliliters water.
Water is a hostile environment to the bacteria, so they generally do not live long in water. Factors such as wind and wave action, as well as UV light from the sun will help to reduce the level of bacteria. The length of time this takes in unpredictable, however it is usually less than 48 hours. It is important to note that bacteria levels may remain high if a continuous source of pollution is impacting the area.
The Washtenaw County Environmental Health Division will continue monitoring the beach water quality and will permit a beach to re-open for swimming when bacteria levels fall back to acceptable levels. Check with the park or beach authority as recreational activities other than swimming are usually still available.
You are the best person to decide when and if it is safe to swim at a particular location. Our changing weather means that swimming conditions can vary throughout the day and from day to day. It is a good idea to avoid swimming in lakes and rivers for 48 hours following a heavy rain. Surface runoff from storm sewer systems can carry high levels of pollution such as residue from vehicle exhaust, fertilizer, pesticides, oil, and waste from urban pets and rural barnyards into the water. Overflowing storm systems can also carry untreated sewage into rivers and lakes. Here are a few recommendations for deciding when and where to swim: