Lake County Water Atlas


Home Learn More

Not Available

Bacteria

What does this mean?

Bacteria are everywhere, including in our surface waters. Most are harmless. Water resource managers test for the presence of certain bacteria in waterways to protect human health. Some kinds of bacteria may cause illness if people are exposed to them, and others are harmless but their presence may indicate contamination from sources that may contain harmful pathogens. The presence of bacteria can affect our ability to use surface water for drinking, swimming, and shellfish production and harvesting.

Bacteria come from a variety of sources, but those of most concern for human health come from the fecal waste of animals and people. Sources of fecal bacteria include:

Water quality regulations identify the specific type(s) of bacteria used to indicate contamination from sources that may contain harmful pathogens.

The maximum allowed number of these specific type(s) of bacteria in a waterway depend on the “designated use” of the waterway. Each waterway is assigned a “Class” which indicates how it is used by people.

The state water quality standards establish bacteria limits for each class.

*Class III-Limited waters are similar to Class III, but have special water quality rule(s) applied to them because of exceptional environmental circumstances. These “Site Specific Alternative Criteria” are defined in Rule 62-302.800 of the Florida Administrative Code).

In addition to performing bacteriological testing required by the water quality standards, some water resource managers may test for other bacteria as well. These tests, and tests of other water quality characteristics, help in identifying possible source(s) of contamination.


How are the data collected? (Methods)

Samples are collected in sterile 100-milliliter (ml) containers and transported to a certified laboratory. The samples must be kept cool during transit. During sample collection, environmental conditions are also observed and recorded, along with recent rainfall amounts.

Historically, sample analysis involved filtering water samples to isolate targeted bacteria types, incubating the result, and counting bacteria colonies. Newer methods use chemical analysis to detect metabolized enzymes produced by targeted bacteria to detect their presence and estimate their abundance. These methods are more economical, take less time, and are less subjective than the earlier methods.

Analysis results are reported as “Colony Forming Units (CFU)”, as “Most Probable Number (MPN)” of colonies, or as “Membrane Filter (MF)”, which is equivalent to CFU.


Calculations

Below is an excerpt from Rule 62-302.530 of the Florida Administrative Code, effective 2/17/2016, which defines water quality standards related to the maximum allowable level of each type of bacteria for each waterway class.

A geometric mean is the nth root of the product of n values:

geometric mean formula

A geometric mean minimizes the effect of very high or very low values, which would bias the result if a straight average (arithmetic mean) were used.

Class

Water Quality Standard

I
(Potable water)

Escherichia coli

Counts shall not exceed a monthly geometric mean of 126 nor exceed the Ten Percent Threshold Value (TPTV) of 410 in 10% or more of the samples during any 30-day period. Monthly geometric means shall be based on a minimum of 5 samples taken over a 30-day period.

II
(Shellfish production and harvesting)

Fecal Coliform

Counts shall not exceed a median value of 14 with not more than 10% of the samples exceeding the Ten Percent Threshold Value (TPTV) of 43 (for MPN) or 31 (for MF), nor exceed 800 on any one day. To determine the percentage of samples exceeding the criteria when there are both MPN and MF samples for a waterbody, the percent shall be calculated as 100*(nmpn+nmf)/N, where nmpn is the number of MPN samples greater than 43, nmf  is the number of MF samples greater than 31, and N is the total number of MPN and MF samples.

III and III-Limited, Freshwater

(Recreation)

Escherichia coli

MPN or MF counts shall not exceed a monthly geometric mean of 126 nor exceed the Ten Percent Threshold Value (TPTV) of 410 in 10% or more of the samples during any 30-day period. Monthly geometric means shall be based on a minimum of 10 samples taken over a 30-day period.

III and III-Limited, Marine

(Recreation)

Enterococci

MPN or MF counts shall not exceed a monthly geometric mean of 35 nor exceed the Ten Percent Threshold Value (TPTV) of 130 in 10% or more of the samples during any 30-day period.  Monthly geometric means shall be based on a minimum of 10 samples taken over a 30-day period.


Caveats and Limitations

Environmental contamination can be very localized or patchy, especially if the source of contamination is wildlife. Bacteria may persist in a waterbody for a period after its source is eliminated. Poor sample results may reflect the immediate area where sampling occurred, but not necessarily be representative of water quality in the entire water body. Conversely, while waters with indicators exceeding the levels in water quality standards may be considered a potential health risk, that does not mean that levels within acceptable ranges are entirely free of risk.


Additional Information


Mapping
Analysis
Learn
Participate
An Edition of wateratlas.org
Connect With Us

The USF Water Institute is committed to ensuring that our websites conform with Accessibility Support guidelines for people who need to use assistive technologies. We are continually improving the user experience for everyone, and applying the relevant accessibility standards. View our Accessibility Statement for more information.