The Rural Crescent in Prince William County is an urban growth boundary for the county that is intended to preserve our agricultural heritage and sense of place as well as achieve the goals of favoring redevelopment along the Route 1 corridor rather than Greenfield development in rural areas where there is no development. However, while I strongly support redevelopment of areas with preexisting infrastructure which would allow Prince William County to improve storm water management (and score nutrient points for the EPA mandated TMDL) as well as revitalize older areas of the county and preserve the greenfields areas in my general support of sustainable development; the Rural Crescent is about water, specifically groundwater.
The Rural Crescent in Prince William aligns roughly with the Culpeper groundwater basin, one of the more important watersheds in Virginia. My home and much of the Prince William County Rural Crescent is located within the northeast quadrant and eastern quadrant of the Culpeper basin and consists of an interbedded sequence of sedimentary and basaltic rocks formed about 200 million years ago. These volcanic rocks are intersected by diabase intrusives and thermally metamorphosed rocks. The rocks of the Culpeper basin are highly fractured and overlain by a thin cover of overburden. The lack of overburden is a challenge to gardeners and limits natural protection to the aquifer. These sedimentary rocks are productive aquifers and feed not only the groundwater wells that provide drinking water to Manassas and other communities, but also feeds the tributaries to Bull Run and the Potomac.
Ground water flows under ambient pressure from Bull Run Mountain towards Bull Run generally west to east with a slight southern slant in the northeast quadrant. The soils in this area are described by the USGS as Balls Bluff Siltstone with a gravel, sand and clay type bedding plane. (That would be those flat plane, edged orange red rocks that are everywhere you put a shovel.) In the siltstone bedding plane, the fractures within the rock run predominately north south. Thus while ground water flows generally speaking west to east, water or a contaminant that catches a fracture will carry the contaminant to depth in a north south pattern. Contaminants can enter the groundwater at these fractures and zigzag through the aquifer, but these fractures also serve as recharge areas. Groundwater is usually cleaner than surface water and is typically protected against contamination from the surface by the soils and rock layers covering the aquifer, but there is inadequate overburden in much of the Rural Crescent. Once contaminated, groundwater is very difficult to clean and often after removal of contaminated plumes only long term abandonment of use to allow for natural attenuation is the only possible course of action.
The fractured rock system that is so rich in water is also our weakness, there is no natural attenuation in a fractured system so that the groundwater as a drinking water resource can be easily destroyed without any real ability to recover. Any malfunctioning septic system, underground fuel storge tank, improper disposal, or hazardous spill on any property within this area has the potential to impact the drinking water wells to the south, southeast or east. Development of the Rural Crescent would introduce potential sources of contamination that could never (in our lifetimes) be remediated. In addition, development of the Rural Crescent threatens the water supply itself.
Generally, groundwater in the Culpeper Basin is renewed each year through precipitation. The water stored in the watershed can supply adequate water in wet years and droughts provided that there is adequate replenishment, the withdrawal of water is within the average recharge rate and that the source is protected from pollution. Properly managed and protected groundwater can be abstracted indefinitely. Groundwater recharge through precipitation requires adequate area for infiltration, control of sheet flow created by roads and paved areas, as well as protecting the most geologically favorable infiltration points. Precipitation flows over the ground as surface runoff. Not all runoff flows into rivers, much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers (saturated subsurface rock), which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into rivers, creeks, and ponds (and the ocean) as ground-water discharge, and some ground water finds openings in the land surface and emerges as freshwater springs.
Prince William has more than adequate water to supply the County even with significant growth in population and industry without rationing, but conservation and water management are necessary and protection of the groundwater recharge areas are essential to maintain the water supply. Most of Prince William County is within the Piedmont geologic region, the largest geological region in Virginia. The Piedmont is bordered by the “fall zone” on the east and the Blue Ridge Mountains on the west. Our little corner of the Piedmont has limited overburden and the fractures and fault lines formed in the rocks store and transmit groundwater. The size and number of water bearing fractures decrease with depth so significant supplies of water are generally located in the first few hundred feet and recharged by rainfall through near surface. There is a wide variation in groundwater quality and yield ranging from under 1 gallon to over 50 gallons a minute. The largest yields are obtained where fracture and fault system are extensive along the base of the Blue Ridge Mountains and in the Rural Crescent area near Bull Run Mountain.
A small portion of Prince William County is located within the Costal Plain and should serve as a warning to us of what happens if you do not protect your watershed and its recharge. The Costal Plain of Virginia is composed mostly of unconsolidated geologic deposits and extends from the Atlantic coast to the “fall zone” a geological line that runs north-south through Fairfax, Fredericksburg, Richmond, and Petersburg along Route 95. There are two groundwater systems, an unconfined aquifer and a lower artesian aquifer both flow in the general direction of the topography slope towards the ocean. The fall zone was the area of recharge for the artesian aquifer, it was the geologic area where the earth folded and the lower mostly isolated artesian aquifer reached the surface and could be recharged with rainfall. By building Route 95 along the fall zone and developing the adjacent areas Virginia essentially paved over a significant portion of the recharge zone for the artesian groundwater aquifer.
The groundwater withdrawals of the Coastal Plain now total more than 130 million gallons a day. USGS monitoring wells in the region indicate that artesian water levels are falling at rates of roughly1.0 to 3.0 feet per year. The water level decline is the result of a decrease in the hydraulic pressure of the artesian aquifer. Hydraulic pressure falls because groundwater is lost from aquifer storage by withdrawals (pumping) and reduced recharge caused by development of the recharge zones. It is predicted by Frank W. Fletcher, Ph.D., P.G. that Fairfax and the Eastern Shore areas could run out of groundwater to meet the demand if appropriate groundwater management does not take place. Fortunately, to a large extent the Coastal Plain areas of Prince William County are provided drinking water from public supply that originates within the Culpeper Basin, but we need to protect our groundwater and the only way to ensure an adequate clean supply of water for Prince William County is to preserve the Rural Crescent from any further development. The land must remain open and unpaved to allow for adequate rainfall infiltration to recharge the groundwater and the area must be protected from potential sources of contamination. Protect the Rural Crescent to protect our future.