A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 2 - Lakes

Fecal Coliform Bacteria Concentrations in Lakes

Why Is It Important?

Fecal coliform bacteria are microscopic organisms that live in the intestines of warm-blooded animals. They also live in the waste material or feces excreted from the intestinal tract. When fecal coliform bacteria are present in high numbers in a water sample, it means that the water may have received fecal matter from one source or another. Although not necessarily agents of disease, fecal coliform bacteria may indicate the potential presence of disease-carrying organisms, which live in the same environment as the fecal coliform bacteria.

Reasons for Natural Variation

Unlike the other conventional water quality parameters, fecal coliform bacteria are living organisms. They multiply quickly when conditions are favorable for growth and die in large numbers when they are not. Because bacterial concentrations are dependent upon specific conditions for growth and these conditions change quickly, fecal coliform bacteria counts are not easy to predict. For example, although winter rains may wash more fecal matter from urban areas into a lake, cool water temperatures may cause many of the organisms to die. Direct exposure to sunlight is also lethal to bacteria, so dieoff may be high even in the warmer water of summertime.

Expected Impact of Pollution

A lake heavily polluted by nutrients may have very low concentrations of fecal coliform bacteria. It depends on the source of pollution. Urbanization of watersheds may generate new sources of fecal coliform bacteria, even as "old" sources disappear – for example, when agricultural land fertilized by cow manure is converted into residential developments. In this case, pet wastes, failing septic systems, and interconnections with leaking sanitary sewers may replace cow manure as a fecal coliform source. Stormwater runoff in urbanized areas has been found to be surprisingly high in fecal coliform bacteria concentrations. The presence of disintegrating storm and sanitary sewers, misplaced sewer pipes, and good breeding conditions are common explanations for the high levels measured.

Most states have strict standards for fecal coliform bacteria concentrations, primarily for reasons of public health. The abundance of fecal coliform bacteria is measured as the number of "colonies" in 100 mL of water - #/100mL. The Washington State standard for lakes reads "fecal coliform organisms shall not exceed a geometric mean value of 50 organisms/100 mL, with not more than 10 percent of the samples exceeding 100 organisms/100mL."

If the lake is used for a drinking water supply, more stringent standards apply. The standards differ depending upon the method used and the number of samples collected. Suffice to say that for drinking water coliform numbers should be one or less.

Fecal coliform concentrations for three Western Washington lakes are shown here to provide examples of the range you may expect to measure. This group of lakes displays a fairly narrow range in measured bacteria concentrations. Large variations (tenfold or more) within a lake are not unusual, given the rate at which bacteria multiply and die.

The next section provides an example of a typical lake monitoring program.

Chapter Four provides information about how to measure fecal coliform bacterial levels in lakes.

Temperature | Oxygen | pH | Secchi | Nutrients | Turbidity | Chlorophyll | Fecal Coliforms

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Last updated on April 01, 2008