Public Works and Environmental Services

Fairfax County, Virginia

CONTACT INFORMATION: Our administrative offices are open 8 a.m. – 4:30 p.m., Mon - Fri

703-324-5500
TTY 711

12000 Government Center Parkway
Suite 449, Fairfax, Va 22035

Craig Carinci,
Director, Stormwater Planning

Stream Quality Assessment Program

A stream is a combination of all of its physical, chemical and biological characteristics. Human activities shape and alter many of these characteristics. The health of our rivers and streams is closely linked to their surrounding watersheds. Ecologists measure the extent to which human activities affect watershed health by gathering and recording information on physical conditions, water quality and living communities in streams and in the surrounding environment. The long-term stream monitoring program meets the requirements of state and federal regulations and supports the Board of Supervisors' environmental improvement program by providing an ongoing evaluation of the streams. The program maintains a substantial database, which over time will be used to determine the overall rate of change or trends in the conditions of our waterways.

Freshwater ecologists monitor Fairfax County’s streams and lakes to assess water and habitat quality and gauge the overall health of waterways. The long-term stream monitoring program meets the requirements of state and federal regulations and supports the Board of Supervisors' environmental improvement program.

Stream Conditions

Fairfax County collects biological data at 40 randomly selected locations each year. The Stream Quality Index (SQI) is based on annual data collected on benthic macroinvertebrates (animals without a backbone that live on the bottom of a stream and are visible to the naked eye). The SQI uses benthic macroinvertebrates because they are good indicators of water quality.

Each of the 40 locations receives a score (excellent, good, fair, poor, or very poor) based on the benthic macroinvertebrates found in that stream. The higher SQIs indicate better stream quality. In many years, greater than 75 percent of Fairfax County's streams are in fair, poor, or very poor biological condition. This indicates that most of our streams are impaired and lack biological diversity. Annual results can be found in the table below.

 

Monitoring 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. 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 the Standard Operating Procedures Manual for the Fairfax County Biological Stream Monitoring Program.

Bacteria and Water Chemistry

Bacteria and Water ChemistryThe bacterium Escherichia coli (E. coli) is used as the water quality indicator for fecal contamination in surface water. Grab samples of water are collected seasonally to determine the concentration of E. coli in our streams. Based on the levels of E. coli found, we discourage swimming in natural waters. See Health Department Advisory.

Benthic Macroinvertebrates

Benthic MacroinvertebratesThese diverse organisms have 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. They are picked out of the vegetative debris and identified to the genus taxonomic level.

Fish Community

Fish CommunityA healthy and diverse fish community is indicative of good stream health. A backpack electrofisher unit is used to send electricity into the water to momentarily stuns the fish and allow for easy collection. Once collected, the fish are identified to the species taxonomic level and counted to track their populations. The fish are then released back into the water.

Habitat Quality

Habitat CommunityStream habitat incorporates the physical, chemical and biological components that influence the structure and function of the aquatic community. It is visually assessed and scored for 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.

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