Occoquan Overlay

To protect the Occoquan Reservoir that provides drinking water to the eastern portion of Prince William County, the county has created an overlay district in the lower Occoquan Watershed. Now that the overlay district is being added to the Comprehensive Plan, there are attempts to exempt the largest undeveloped parcels from the protections. The whole point of the ORPA is to prevent more intense development and protect the drinking water supply for eastern Prince William.

Protecting the recharge of groundwater within the Occoquan watershed serves to protect all of the water resources in the watershed. Today, the Occoquan watershed is often described as the most urbanized watershed in the nation. Certainly, there are far more urbanized areas in the United States, but they do no longer have functioning watersheds. We need to effectively protect ours and our source of drinking water.

from NVRC

As the US Geological Survey points out: “Nearly all surface-water features (streams, lakes, reservoirs, wetlands, and estuaries) interact with ground water. These interactions take many forms. In many situations, surface-water bodies gain water and solutes from ground-water systems and in others the surface-water body is a source of ground-water recharge and causes changes in ground-water quality. As a result, withdrawal of water from streams can deplete ground water or conversely, pumpage of ground water can deplete water in streams, lakes, or wetlands.”

Ground water flow and storage, often viewed as static reservoirs, are dynamic and continually changing in response to human and climatic stress [Alley et al., 2002; Gleeson et al., 2010]. Increase or decrease in precipitation patterns impacts available surface and groundwater. Man’s hand in changing the land surface also impacts water resources.

Land use changes that increase impervious cover more than 5-10% from roads, pavement and buildings does two things. It reduces the open area for rain and snow to seep into the ground and percolate into the groundwater and the impervious surfaces cause stormwater velocity to increase preventing water from having enough time to percolate into the earth, increasing storm flooding and preventing recharge of groundwater from occurring. 

Slowly, this can reduce water supply over time. As groundwater levels fall, perennial steams that feed the rivers become ephemeral. The groundwater becomes disconnected from the surface water network. Once the hydrology is destroyed by development, it cannot be easily restored, if at all. Though there have been a few attempts we have not succeeded in restoring a watershed. According to the NVRC in 2015 impervious surfaces in this lower Occoquan area was already at 11%.

from NVRC

Protecting the Occoquan Reservoir requires protecting all the water resource in a region because all water in the watershed is connected. Precipitation moves into the water table (the hyporheic zone) down to groundwater or into rivers and streams. Disrupting the balance of water flow can have dire consequences. The available supply of fresh water is continually renewed by the hydrologic cycle and in the case of the Occoquan Reservoir the actions of mankind. The need for water is constant and grows with population, wealth and business activity. There is also a seasonality to water- we use more in summer.

There has been a six-fold increase in population from when the statistics were first collected in the early 1970’s. Ironically, the defining study of water quality issues within the Occoquan Reservoir (Metcalf and Eddy, Inc., 1970) which led to the development of a management plan for the Occoquan included one recommendation to “provide highest treatment technically achievable; discharge reclaimed water to the Occoquan Watershed; and limit basin population to 100,000.”

The quantity and quality of ground water in Prince William County varies across the county depending on the geologic and hydrogeologic group you are in. Generally speaking, the groundwater in the county is recharged in elevated areas and discharges to streams and estuaries. However, the paths and duration of groundwater flow are different between consolidated rocks and unconsolidated material. 

In the area of the proposed ORPA is beyond the Culpeper Basin in the Piedmont region. This area of the ORPA is primarily hydrogeologic group D composed of igneous rock formations with areas of hydrogeologic group E that transition at the bounds of the ORPA to group E and then to the Coastal Plain.

Hydrogeologic group D is located within the Piedmont formation and consists of three igneous plutons in the eastern part of Prince William County: the Goldvein, Lake Jackson, and Occoquan Plutons. Rocks within hydrogeologic group D tend to have moderate water-bearing potential and ground-water storage is predominantly in the overburden, which is the soils above the bedrock. Wells in this area are most susceptible to drought and tend to be slightly acidic. The igneous rocks have subhorizontal sheeting and near vertical joints overlain by thick overburden. Groundwater wells in the area tend to have yields range from 1.2 to 100 gal/min which has resulted in the development of homes with wells in the area due to the thickness of the water storing overburden.

Hydrogeologic group E is also in the Piedmont formation in the eastern part of the county, and consists of metasedimentary, metavolcanic, and other metamorphic rocks. Rocks within hydrogeologic group E tend to have poor water-bearing potential, and thin- to thick cover of overburden. Similar to the rocks of hydrogeologic group D, ground-water storage tends to be predominantly in the overburden. Some of the poorest yielding wells in Prince William County are located in this hydrogeologic group and can be as low a 0.25 gallons per minute, but can also reach upto 70 gallons per minute-, but tending towards the low end because of the thinness of the overburden beyond the limits of what is the proposed ORPA. Homes and businesses in this area have depended on public water supply due to the limitations on well development and that water comes from the Occoquan Reservoir.