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

Chapter 3- Streams

Fecal Coliform Bacteria in Streams

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 has received fecal matter from one source or another. Although not necessarily agents of disease, fecal coliform bacteria may indicate the 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 do not simply mix with the water and float straight downstream. Instead they multiply quickly when conditions are favorable for growth, or die in large numbers when conditions are not. Because bacterial concentrations are dependent on 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 stream, cool water temperatures may cause a major dieoff. Exposure to sunlight (with its ultraviolet disinfection properties) may have the same effect, even in the warmer water of summertime.

Expected Impact of Pollution

The primary sources of fecal coliform bacteria to fresh water are wastewater treatment plant discharges, failing septic systems, and animal waste. Bacteria levels do not necessarily decrease as a watershed develops from rural to urban. Instead, urbanization usually generates new sources of bacteria. Farm animal manure and septic systems are replaced by domestic pets and leaking sanitary sewers. In fact, stormwater runoff in urbanized areas has been found to be surprisingly high in fecal coliform bacteria concentrations. The presence of old, disintegrating storm and sanitary sewers, misplaced sewer pipes, and good breeding conditions are common explanations for the high levels measured.

Fecal coliform concentrations are reported in units of the number of bacterial colonies per 100 mL of sample water (#/100 mL). The Washington State standards for fecal coliform bacteria vary according to stream classification. For Class AA streams, the geometric mean can not exceed a value of 50 organisms per 100 mL, and fewer than 10 percent of the samples can be greater than 100/100 mL, and fewer than 10 percent can be greater than 200/100 mL, and fewer than 10 percent of the samples can be greater than 400/100 mL.

The table below provides comparison values from three Western Washington streams.

The next section provides information about stream pollution concentrations versus pollutant loading.

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

Stream Temperature | Stream Disolved Oxygen | Stream pH | Stream Nutrients | Stream TSS and Tubidity | Stream Fecal Coliforms | Return to Table of Contents

Last updated on April 01, 2008