Technical Background

The first rule of ecology is that everything is related to everything else. This is especially important when determining the health of a stream, as all aspects of the surrounding environment must be considered, in addition to assessing its chemical and biological characteristics.

Bacteria and Water Chemistry

Bacteria Monitoring Fecal coliform, a bacteria group present in the intestinal tracts and feces of warm-blooded animals, is commonly used as an indicator of possible sewage contamination because it is found in human wastewater. Although fecal coliform is generally not harmful, it indicates the possible presence of pathogenic (disease-causing) bacteria and viruses.

Fairfax County conducts bacteria sampling throughout the county to determine the concentration of fecal coliform and E. coli (a particular type of fecal coliform) in streams.

Grab samples of stream water are collected four times a year to determine the concentration of fecal coliform. At the time of collection, other water quality parameters are also sampled for and measured including: nitrates, phosphates, pH, water temperature, dissolved oxygen, and specific conductance.

Fecal coliform, nitrates, and phosphate samples are processed at the Fairfax County Health Department laboratory, and the other chemical parameters are recorded by county ecologists at the site of collection.

Fairfax County Health Department Water Quality Statement - Recreational Use of County Streams

Benthic Macroinvertebrates

Benthic Macroinvertebrate Monitoring Benthic macroinvertebrates are aquatic insects or the larval form of many common terrestrial insects that live on the bottom of the streambed (benthic); are visible without the use of a microscope (macro); and do not have a backbone (invertebrate).

Benthic macroinvertebrates are diverse organisms with varying tolerances for pollution from toxins, nutrients, and sediment, making them well suited as indicators for determining stream health and water quality.

Samples are collected between mid-March and mid-April, using the "20-jab" multi-habitat sampling protocol. This method involves taking 20 separate "jabs" or collections from different habitat types, such as undercut banks, aquatic vegetation, riffles, and snags.

Benthic macroinvertebrates are picked out of the vegetative debris and identified to the genus taxonomic level in a county laboratory.

Fish Community

Fish Community MonitoringA healthy and diverse fish community is indicative of good stream health.

Fish are very sensitive to both natural and human-induced changes within a given stream system and surrounding watershed. The method for collection is based upon the Environmental Protection Agency's Rapid Bioassessment Protocols V.

A backpack elecrofisher unit is used to send electricity into the water, stunning the fish for a moment, allowing for easy collection with a net. Once collected, the fish are identified to the species taxonomic level and counted to keep track of their respective populations within each sampled reach.

Anything that appears strange or anomalous on the fish, such as fin and eye deformations, are also recorded. The fish are then released back into the water.

Habitat Quality

Habitat Assessment Stream quality and the types of aquatic organisms that live there are controlled by habitat factors, such as how well the stream is shaded by leaves (canopy cover), whether there are plants protecting the stream banks from erosion (ground cover), and how the land is being developed throughout the watershed (land use).

A stream with good habitat quality will have a wide, healthy forest growing along both stream banks; small amounts of erosion on the stream banks; riffles and pools; and vegetative debris in the stream.

Stream habitat is visually assessed and scored for 10 different features. Scores can range from zero at the very worst to 200 at the very best.


For the "20 jab" method, fish and habitat protocols please see the U.S. Environmental Protection Agency's Rapid Bioassessment Protocol for Use in Wadeable Streams and Rivers.

The modified protocols used for the collection of habitat information, benthic macroinvertebrates, and fish in the Fairfax County Stream Protection Strategy Baseline Study are included as appendices to the Stream Protection Strategy Baseline Report.

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Technical Questions: Web Administrator

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