The Audubon Naturalist Society conducted a study of the Occoquan Watershed. The Occoquan Reservoir and intake from the Potomac River supply the bulk of water for the Fairfax County Water Authority, which the Society considers a well run entity. Maryland controls the use of the Potomac River and so the water authority must receive permission from Maryland to take water from the Potomac River. The water authority uses the Occoquan Reservoir during periods of low water flow in the Potomac River to supply a larger percentage of its water. In addition, Fairfax County's soils contain aquifers that can support development, while neither Prince William or Loudoun counties soils can do the same. The FCWA sells some of its excess water capacity to both Prince William and Loudoun counties.

Development in Fairfax County and the further upstream counties will have an effect on the Occoquan Watershed. Other localities, such as New York, have decided it is cheaper to purchase the watershed area to prevent damage caused by development then to treat the runoff of developed land. Currently approximately 11% of the watershed area is covered by impervious surfaces (roofs, asphalt, etc.). When that percentage increases to over 15% noticeable decreases in water quality can result. The Society projects that by the year 2020 the percentage will be close to the 20% level.

Water quality also depends on the conditions of the streams that feed water into the reservoir. When checked in 1989, the Fairfax County portion of the Occoquan watershed contained 60% high-quality sensitive streams, with 33% of the streams showing signs of urban impact and 7% not capable of supporting aquatic life. Projections for 2005 show only 22% remaining high quality, with further degradation by the year 2020. While the degradation of the streams is an important consideration in its own right, their degradation can be expected to degrade reservoir water quality as well.

The Upper Occoquan Sewer Authority services Fairfax and Prince William Counties and produces potable water as its output. During periods of low rainfall, the reservoir water quality improves as a higher percentage of water in the reservoir comes from the sewer authority. The sewer authority is heavily involved in negotiations concerning the placement of a proposed chip manufacturing plant in the Manassas vicinity. The chip manufacturing process produces water heavily laced with chemicals, some of which are toxic. The sewer authority is in the midst of negotiating what it considers acceptable levels, if any, of these chemicals. This negotiation is taking longer that the chip plant proponents envisioned, but is an important part of the permitting process. As an example of other chip manufacturing facilities, some Japanese plants have closed systems that recover all toxic chemicals, or use manufacturing processes that rely on non-toxic chemicals.

The Society proposed various methods of maintaining the quality of the watershed. It recommends that more citizens sit on the water quality board, that better ?best management practices? designs be required and that county concentrate on managing stream valleys as well as the reservoir itself.

One of the points brought out by the Society's study was that 1/3 of the nitrogen deposited on the soil came from non-point sources, such as automobile exhaust. It doesn't matter if trucks and cars are kept from crossing the open reservoir, but instead diverted along major highways. Their exhaust will still settle in the watershed area and eventually be washed into the reservoir. Better ?best management practices? designs would require dry ponds with vegetative covers that can trap and use the excess nitrogen and phosphorus non-point pollution.

If Fairfax doesn't maintain its stream valleys, then too much water will erode the banks, carrying soil and nutrients into the reservoir. If the county maintains the stream banks, then the vegetation along the stream valley can capture the nutrients and soil and prevent them from entering the reservoir.