South Atlantic Water Science Center

Chattahoochee River BacteriAlert: Understanding BacteriAlert Data

E. coli data value

Picture of water sample being analyzed for E coli

The E. coli bacteria count value is reported as the number of colonies per 100 milliliters (ml) of water. As of mid-2002, only occasional water samples are being taken, as current turbidity values are used to predict E. coli counts. Information about E. coli bacteria is available.

To determine the E. coli concentration in a water sample, the water must be taken to a lab where it is separated into different dilutions. Chemicals are added to the water, which is then sealed in plastic containers and incubated in an oven for about 20 hours. This process allows the E. coli bacteria colonies to grow enough to fluoresce under ultra-violet light.

Turbidity

Picture of three glass beakers with water with turbidities of 10, 200, and 1500.

Picture of three glass beakers with water with turbidities of 10, 200, and 1500. Turbidity is the amount of particulate matter that is suspended in water. It is measured in nephelometric turbidity units (NTU). Turbidity makes the water cloudy or opaque and it is measured by shining a light through the water. At base flow, the Chattahoochee near Atlanta is usually a clear green color, and turbidities are low. During a rainstorm, particles from the surrounding land are washed into the river making the water a muddy brown color, indicating water that has higher turbidity values.

Using state-of-the-art equipment, we are now measuring turbidity instantaneously at the Medlock Bridge and Paces Ferry sites. The BacteriALERT project is using turbidity as an indicator of the presence of E.coli bacteria, since the data collected so far indicate a relation between E. coli counts and turbidity, for example, as turbidity values went up, so did E.coli bacteria counts. Thus, we are using the most recent turbidity value to ascertain the probability that today's E. coli bacteria levels present a high-health risk.

Find out more about turbidity.

USEPA E. coli criteria

The USEPA has determined that E. coli bacteria counts above 235 colonies per 100 mL indicate that more than 8 people out of 1,000 who come into contact with the water may become sick. But it is important to remember that as E. coli counts go up, it is the chance that someone will get sick that goes up - there are many other things that determine if a person will become sick:

U.S. EPA Criteria

Percent probability E. coli count is over 235
As an example, let's say the percent probability that the E. coli count was over 235 was 26 percent. This means that, according to the turbidity model, there is a 26-percent probability that the current E. coli bacteria count exceeds the USEPA criteria value of 235 colonies/100 ml of water.

Since the E. coli bacteria counts cannot be instantly determined, the probability percentage is determined using the most recent turbidity numbers. Historical turbidity and E. coli data were compared and a mathematical relation was found between the two parameters. This relation between turbidity and E. coli counts allows us to develop a mathematical formula that can predict the chance that the current E. coli bacteria count is higher than 235.

National Park Service health-risk determinations

Low risk: E. coli counts of less than 177 colonies per 100 mL of water
As determined by the U.S. Forest Service, a person in direct contact with the river water (i.e. swimming, diving, wading) has a LOW chance of getting sick. The water is in full compliance with recreation water-quality criteria.

High risk: E. coli counts above 235 colonies per 100 mL of water
Contact with the river is NOT recommended. A person in direct contact with river water has a increased risk of getting sick. The water quality does NOT meet federal recreation water-quality criteria.