A limnological investigation of Lake Manassas, Virginia

Lake Manassas is a man-made impoundment in the Northern Virginia suburbs of Washington, D.C. The lake currently supplies drinking water at a rate of 6.7 million gallons per day to the City of Manassas, Virginia. The lake discharges, via the stream Broad Run, to the Occoquan Reservoir. The Occoquan Reservoir supplies potable water to over 750,000 people in the Northern Virginia area.

As the population of Washington, D.C., continues to increase, the development of the surrounding suburbs changes the quality of surface runoff water into existing reservoirs. These reservoirs can become enriched with both toxic and biomass inducing nutrient pollutants. The result can be less desirable and less dependable supplies of drinking water.

A State of Virginia mandated Environmental Monitoring Program is in force in this area to ensure the Occoquan Watershed remains a dependable supply of potable water. A computerized database, containing the results of the environmental monitoring program, allows for a quantitative estimate of the overall water quality of the reservoirs to be made.

This thesis presents the results of a limnological analysis of Lake Manassas. The analysis techniques used are established limnological techniques to arrive at a profile which can be compared to accepted scales of ranking.

One conclusion from the analysis is that Lake Manassas is eutrophic, which means that the production of biomass in the lake is at a higher than desired rate. The result of this eutrophic condition is that the water quality of the lake will decline rather rapidly. Another conclusion is that Broad Run is the major supplier of nutrients into Lake Manassas, but that conditions are also affected by a point source discharge from a sewage treatment plant. These conclusions are consistent with previous studies done on Lake Manassas.

In summary, Lake Manassas is an important water resource in the Northern Virginia area, and it is important to continue to closely monitor and manage runoff practices in the watershed to ensure the lake does not degrade to unacceptable conditions.