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TextMonitoring Methods
Countywide monitoring is conducted annually using a probabilistic design approach. Using this approach, statistically valid inferences may be made about the condition of the county's streams. Each year, all potential sampling sites are stratified by stream order and 40 sampling locations are selected randomly. These 40 locations are then evaluated for bacteria levels, benthic macroinvertebrates, fish community and physical habitat during that year. Water quality parameters (such as water temperature, dissolved oxygen, specific conductance, pH and levels of nitrogen and phosphorus) are also collected. The results of the yearly monitoring events are compiled into annual reports and are used to support the county's Environmental Quality Advisory Council's Annual Report on the Environment.
Bacteria and Water Chemistry
As recommended by the U.S. Environmental Protection Agency (EPA), the bacterium Escherichia coli (E. coli) is used as the water quality indicator for fecal contamination in surface water. E. coli is a species of bacteria present in the intestinal tracts and feces of warm-blooded animals. Although most strains of E. coli are generally not harmful, its presence indicates the possible existence of pathogenic (disease-causing) bacteria and viruses.
Grab samples of water are collected seasonally to determine the concentration of E. coli in our streams. At the time of collection, other water quality parameters are measured including pH, water temperature, dissolved oxygen and specific conductance. E. coli samples are processed at the Fairfax County Health Department laboratory, while chemical parameters are recorded by county ecologists at the site of collection using hand-held meters.
Based on the levels of E. coli found, the Fairfax County
Health Department discourages the recreational use of county
streams.
Benthic Macroinvertebrates
Benthic macroinvertebrates are animals that live on the bottom of a stream, river or lake (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 of nonpoint source pollutants such as toxins, excess 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. This method involves taking 20 separate collections from different habitat types, such as undercut banks, aquatic vegetation, cobble and snags from a 100-meter reach of stream.
Benthic macroinvertebrates are picked out of the vegetative debris
and identified to the genus taxonomic level in a county laboratory.
The Stream
Quality Index is based on the integrity of the benthic
macroinvertebrate communities found in the sampled stream
segment.
Fish Community
A 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. A backpack electrofisher unit is used to send electricity into the water. This momentarily stuns the fish and allows for easy collection with a net. Once collected, the fish are identified to the species taxonomic level and counted to track their respective populations within each 100-meter sample. The fish are then released back into the water.
More information including pictures, descriptions and species distribution can be found at the Fishes of Fairfax County website.
Habitat Quality
The quality of a stream is affected by its surrounding habitat. Habitat incorporates the physical, chemical and biological components that influence the structure and function of the aquatic community in a stream. Stream habitat is visually assessed and scored for ten features, such as how much of the stream bottom is affected by sediment, what percentage of the stream bank is covered by native vegetation, the frequency of riffles and bank stability. 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, cobbles and pools throughout the stream and vegetative debris in the stream.
The protocols for macroinvertebrate and fish monitoring and habitat assessment are based on the EPA's Rapid Bioassessment Protocol for Use in Wadeable Streams and Rivers. The modified protocols can be found in Section 2 of the 2005 Annual Report on Fairfax County's Streams.For more information on the Stream Quality Assessment Program, please email the Stormwater Planning Division or call 703-324-5500, TTY 711.
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