The primary polluter of the Occoquan reservoir, which supplies water for more than 600,000 residents in the city of Alexandria and Fairfax and Prince William Counties, is runoff from urbanized areas in the Occoquan watershed, according to a recent report by a study group of the Northern Virginia Planning District Commission.

Further urban development will increase the runoff pollution, the Occoquan study group concluded.

That conclusion reversed a previously held belief that runoff from sewage treatment plants in upstream areas of the watershed was primarily responsible for polluting the reservoir.

The Northern Virginia Planning District Commission staff, working with the Occoquan watershed monitoring laboratory of the Virginia Polytechnic Institute, discovered in a year-long study of pollutants contained in the runoff from commercial, residential and agricultural areas that total phosphorus and nitrogen from runoff was more than seven times greater than the amount of the same substances coming from the 11 major sewage treatment plants which discharge into the Occoquan. Nitrogen and phosphorus are important because they cause increases in algae which can reduce the amount of oxygen in the water and can reduce the useful life of the reservoir.

The study also showed that heavy metals such as lead and zinc, which accumulate on streets and parking lots and are washed out of the air during periods of poor air quality, flow into storm sewers and streams during rainstorms. Commercial high rise apartments produced more pollutants than farms and single family houses of the same land area. The wast majority of housing in the Occoquan watershed is made up of single family houses, and the runoff pollution from those areas is itself a significant problem, the group reported.

The study also described ways to significantly reduced the impact of runoff pollution on the water supply. Such controls as holding and sediment ponds were found to reduce runoff pollutant entry into the water supply.

As a result of the study group's findings, the Northern Virginia Planning District Commission approved, and sent to the Virginia State Water Control Board, a request that the Occoquan policy be amended to include guidelines to product the water supply from runoff pollution. In addition, the letter states that increased sewage treatment capacity, which will be provided the Upper Occoquan Sewage Authority Advanced Wastewater Treatment Plant, may not produce the overall cleanup of the reservoir anticipated by the control board when construction of the plant was approved.

The planning commission also urged the State Water Control Board to speed up initiation and completion of the Fairfax County Water Authority study on chloroform present in the treated Occoquan water and solutions to control the contamination of drinking water.