Bacterial testing and what it means


If you follow any of the Riverkeepers of the Chesapeake Bay, you are probably familiar with their weekly Facebook posts about bacteria levels. Let’s talk about why they are posting this information, what it’s actually telling us, and what it means to you.



First, there are a variety of water quality monitoring programs that exist in and around the Chesapeake Bay. Some of these programs are run by nonprofits, like the various Riverkeepers, and some are run by individual states such as the testing performed by the Maryland Department of Natural Resources. Some of these programs are run year round while others run only during certain seasons. The reason for this is the purpose of the testing itself.



When we are talking about general water quality testing, this includes a variety of water quality parameters such as temperature, dissolved oxygen, nutrients like nitrogen and phosphorus, as well as bacteria and even harmful algal blooms. These tests are usually done year round and are intended to keep track of the health of the aquatic ecosystem.



Seasonal testing is often to address a particular parameter that is only a concern during a certain time of year. For example, during months when swimming is a common recreational activity, bacterial testing is performed as a way to make sure that people can swim and be in the water safely. Often, this bacterial monitoring does not include any additional tests and is only a test for fecal coliform bacteria - the type of bacteria associated with feces, both human and animal. 

Using a YSI to measure water quality.

Using a YSI to measure water quality.

When there are high levels of fecal bacteria in the water, it usually means there are other, more dangerous pathogens present. However, there are so many different bacteria that could be present that it would be time consuming and expensive to test for them all. Instead, the level of fecal bacteria is used as an indicator since measuring this requires a single, relatively quick test. By measuring for safe levels of this bacteria, folks like the Riverkeepers are trying to help people better understand the risk associated with swimming in a particular area. 



If you’ve ever seen the results of bacterial monitoring, you may wonder why some testing sites routinely fail or why some sites vary greatly in how much fecal bacteria is present from week to week. One of the biggest factors that contributes to the variability is rainfall. Most of the fecal bacteria in the water is coming from land in the form of animal waste (both wildlife and agriculture) and human waste (such as wastewater treatment plants and septic tanks). After a rain event, that waste is washed into nearby waterways. This is why the general rule of thumb is to avoid swimming in natural bodies of water (as opposed to a pool) for 24 to 72 hours after it rains. The variation in time depends on the type of water and how quickly the water moves; in areas where water is rapidly flushed out, a shorter amount of time is required for bacteria levels to drop after a rain event. In areas where there is less flow, it may take longer for the bacteria to be diluted or flushed out of the system so you should wait longer.



It’s also important to understand that these tests are only for fecal bacteria and do not test for other pathogens of concern such as Vibrio spp. bacteria. These bacteria, including Vibrio vulnificus, the flesh eating bacteria, prefer warmer water. They are normal residents of the Chesapeake Bay but can be found at elevated levels during warmer months of the year, which also happens to be when a greater number of people are coming in contact with the water. As permanent residents of the Chesapeake Bay, the amount of these bacteria in the water does not correlate to the level of fecal coliform bacteria.



While the thought of getting a flesh eating bacterial infection from swimming is a scary one, it's important to note that infections are relatively rare. If you are worried about Vibrio spp. bacteria, the National Oceanic and Atmospheric Administration has a predictive model to highlight when and where Vibrio spp. is likely to be present in the Chesapeake Bay. The Maryland Healthy Beaches program also notes that risk for contracting an infection from Vibrio spp. bacteria is highly variable depending upon the dose of bacteria a person is exposed to, the length of exposure, and an individual person’s vulnerability. Ultimately, the best way to protect yourself from Vibrio is to avoid contact with salt or brackish water when you have an open wound and to have antibiotic ointment or hand sanitizer on hand when you do come in contact with salt water.



Ultimately, if a site fails a fecal coliform bacteria test, no one is going to stop you from going in the water. These tests are really just to let you know how risky swimming at a particular site might be. If a site fails, it’s a good idea to find an alternative place to swim but that decision is up to you.



A fecal coliform test.

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