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Surface Water Quality
Loudoun County Water Quality  Map of Stream Monitors
Water quality in streams is important to the people and ecosystems in Loudoun as well as to those living downstream in other communities where “our” water eventually flows. All of Loudoun’s streams drain either directly or indirectly into the Potomac River and ultimately into the Chesapeake Bay. Surface water quality information is compiled or collected by the Department of Building & Development as part of the county’s Water Resources Monitoring Program.

Water Quality Monitoring
Stream water quality is routinely monitored by the Virginia Department of Environmental Quality (DEQ). Workers collect water samples from dozens of locations on a multi-year, rotating watershed basis. Samples are analyzed for temperature, chemistry, microbiology, benthic macroinvertebrate and stream habitat characteristics. Over 100,000 measurements have been collected since the early 1970s.

Citizen volunteers from several organizations monitor water quality for similar parameters. The USGS has conducted studies of trace elements in surface water as part of their National Water-Quality Assessment program. The county incorporates these data into its water resources database.

Monitoring Results
Every two years, DEQ assesses the water quality for various uses. Stream monitoring results are compared to numerical water quality standards to determine if the water quality "measures up.” If water quality falls below a certain level of cleanliness, DEQ identifies the location, the parameter of concern (such as high bacteria counts) and the likely sources (such as failing septic systems or feedlot runoff). The streams that do not pass muster are listed in a widely circulated pair of reports called the 305(b) and 303(d) reports.

Since 2004, DEQ has combined both the 305(b) Water Quality Assessment and the 303(d) Report on Impaired Waters into the Virginia Water Quality Assessment 305(b)/303(d) Integrated Report. Stream, impairments identified in the 2010 Water Quality Assessment include:
  • Bacteria for recreational use
  • Benthic for aquatic life use
  • PCB for fish consumption

Read more about benthic macroinvertebrate and stream habitat characteristics.

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