Water Quality: Bacteria

Later this month, we’ll begin working with ShoreRivers to monitor bacteria levels as part of Swimmable ShoreRivers. This program begins the Thursday before Memorial Day and entails collecting water samples at popular water recreation spots around our area. The goal of this program is to help people be aware of harmful bacteria levels in the water so they can make informed decisions about when, where, and how they want to enjoy the Chesapeake Bay.

The main thing to understand about this program is that when we use the general term “bacteria”, we are specifically testing for E. coli bacteria as part of this program. While there are other harmful bacteria and diseases out there, E. coli is used an indicator species. E. coli is a normal bacterial resident of human and animal intestines but in the environment it is typically seen in association with fecal matter, whether human or animal. Some strains of E. coli are harmless but others are pathogenic and can cause diarrhea or other illness. Ultimately, the level of E. coli bacteria in the water is used as a proxy for understanding certain types of pollution and run off.

To learn more about our local bacteria monitoring program, click the image above.

After a rain event, we tend to see higher levels of E. coli in the water. This is because the rainwater runs off the land and into the Chesapeake Bay and its tributaries. As the water runs off, it carries with it whatever contamination it picks up along the way, such as trash, dirt, and poop. We might also see stormwater/drain systems be overwhelmed during rain events, which can lead to raw sewage becoming runoff as well. This used to be a lot more common but with efforts to improve water quality, there’s been a push to help separate stormwater from sewage in wastewater treatment so that if a spillover event occurs, it’s less likely to contain untreated fecal matter.

The EPA sets a safe limit of E. coli bacteria in water at 104 CFU/mL (colony forming units per milliliter of water). As the amount of bacteria in the water climbs above this number, you are at greater risk of developing an infection from this bacteria. This is not to say you can’t get an E. coli infection from water containing below 104 CFU/mL, especially if you are immunocompromised, but the risk of infection is significantly lower. After a rain event, we tend to see levels above the recommended safe threshold so it’s a good idea to avoid the water for 24 to 48 hours, especially if you are immunocompromised or have an open wound or cut. Even when bacteria levels are low, it’s always a good idea to rinse off after spending time in the water.

The science behind collecting E. coli data is relatively simple. A water sample is collected into a sterile container with clean hands to reduce contamination. From there, there are a couple different ways to get a number for how many E. coli are present. One method uses growth media to grow colonies of bacteria from a known amount of the original water sample. The water is spread across the growth media (which contains a food source for the bacteria) and allowed to incubate for a specific amount of time. Once that time has passed, the plates are inspected and the number of colonies of bacteria are counted. Each individual colony is assumed to have come from a single E. coli cell, so we’re able to get a count of the total number of ‘colony forming units’ per millimeter of water. Other methods use a color change reaction to estimate the amount of bacteria in the water sample, whether on a test strip or in liquid. These methods are more rapid but give a range of E. coli amounts and not so much a specific number.

Over the next several months, we’ll be collecting a water sample from the Tred Avon on Thursday morning. By Friday, we’ll have the results so you can be in the know about bacterial levels. Thanks to SwimableShoreRivers, you can get this information every Friday for many popular recreation sites throughout the upper and mid-shore. This way, you’ll be able to make an informed decision about how you want to enjoy the Chesapeake Bay.

And even when you don’t have bacterial testing data readily available, you can better protect yourself if you remember that E. coli bacteria and other contaminants are likely to be present in the water at higher concentrations for 24 to 48 hours after a rain event.

Be safe out there and take some time to enjoy the Chesapeake Bay this summer!

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