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

Chapter 4 - From the Field to the Lab

How to Measure Fecal Coliform Bacteria

Field Sampling Considerations

Bacteria samples, more than any of the other water quality parameters, are the easiest to contaminate. The sample bottles and their caps must be thoroughly cleaned and sterilized in an autoclave before each use. The caps must remain tightly on the bottles until just before the sample is collected. Care must be taken when unscrewing the cap and collecting the sample to ensure nothing touches the inside of the cap or bottle – including your hands or fingers.

Because bacteria attach themselves to small particulate matter, there are two sampling precautions that bear emphasis. First, always collect the sample upstream of any area that may have been disrupted by entering the stream – take a few steps forward, lean into the current, and collect the sample at arm’s reach. Second, the top film of water will have an excess accumulation of bacteria, so it is not representative of stream or lake conditions. For the sample to be a good representation of water conditions, only a small portion of this surface film should be sampled.

A standard technique has been developed for collecting bacteria samples to account for these differences. To avoid the surface layer, the bottle is "plunged" through the surface film by holding the bottle directly upside down and quickly submerging it. An inch or two below the surface, tilt the bottle toward the current and slide it in an arc toward the surface, removing the bottle in a vertical position. Leave about one-half inch of air space at the top. Immediately cap the bottle and put the sample on ice in a dark place (an ice chest).

The official procedure for analyzing bacteria requires analysis within 6 hours of collection. This is rarely possible. However, they must be run within 30 hours or the results should be discarded.

Measurement Methods

There are two common laboratory procedures for analyzing fecal coliform bacteria: membrane filtration (MF) and most probable number (MPN). The two are not directly comparable, so pick one method and stay with it. MF is the recommended procedure for freshwater monitoring and is more commonly used at this time. However, it is not as reliable a method as MPN in very turbid samples.

Field kits also are available for measuring bacteria. They still require access to some special equipment – such as petri dishes and a small oven or incubator – but if your group has access to a small lab and some equipment, field kits can be used successfully for educational purposes.

QA/QC Considerations

Bacteria often exhibit a large amount of field variability, so collection of field replicates (5 to 10 percent of the samples) can be important for this parameter. Lab replicates for 5 to 10 percent of the samples can be collected, but some error is introduced when pouring from one sample container to the other. Lab replicates also should be analyzed as part of the lab’s standard QA/QC policy.

The next chapter - Getting a Handle on Hydrology - discusses measuring stream flow.

Measuring Temperature | Measuring Dissolved Oxygen | Measuring pH | Measuring Secchi Disk Depth | Measuring Nutrient Concentrations | Measuring TSS | Measuring Chlorophyll a | Measuring Fecal Bacteria

Return to Table of Contents