The Rural Crescent may have started with different intentions; but today the Rural Crescent is about water, groundwater and watershed preservation. I strongly support redevelopment of areas with preexisting infrastructure which would allow Prince William County to improve storm water management in the existing developed areas (and reduce nutrient contamination under the EPA mandated TMDL) as well as revitalize older areas of the county and preserve the Greenfields areas in general support of sustainable development and maintaining the Rural Crescent to preserve and protect our groundwater resources.
The Rural Crescent in Prince William aligns roughly with the Culpeper groundwater basin, one of the more important watersheds in Virginia. 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, but more importantly limits natural protection to the aquifer. These sedimentary rocks are productive aquifers and feed not only the groundwater wells that provide drinking water to Evergreen 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 creating the vast water resource our county enjoys. 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 storage tank, improper disposal, a leaking pipe, or surface 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, 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.
According to the U.S. Environmental Protection Agency, impervious cover levels of 10% can significantly impact watershed health increasing stormwater runoff. When runoff volume increases, runoff velocity increases, and peak storm flows causes flooding and erosion. Increased stormwater velocity increase soil erosion, increases nutrient contamination and reduces water infiltration into groundwater. The groundwater is essential as the base flow to the streams and rivers that feed the Occoquan Reservoir during the dry months. The groundwater stored in the watershed can supply adequate water to maintain river flow during droughts. Maintaining open areas provides areas of groundwater recharge and controls runoff. Decisions about the fate and management of the Rural Crescent will impact groundwater quantity and quality and in turn will impact water flows to the Occoquan Reservoir during dry periods. Flow to the Occoquan Reservoir is essential in managing the drinking water withdrawals from the Potomac River. The Interstate Commission on the Potomac River Basin, ICPRB, manages the Potomac River drinking water allocations for the entire region by “suggesting” the quantity that Fairfax Water draw from the Occoquan and Potomac daily. Prince William County’s decision on the fate of the Rural Crescent could impact drinking water supplies in Fairfax, Maryland, and DC as well as our own county.
Rural Crescent also provides a significant portion of our green infrastructure to our Northern Virginia community. Green infrastructure connects the still intact habitat areas through a network of corridors that provide for wildlife movement and trails as well as pathways for pollinators. Maintaining intact, connected natural landscapes is essential for basic ecosystem and watershed preservation to ensure that there will always be clean air and water in Northern Virginia. Maintaining a tree canopy and controlling runoff to prevent stream bank erosion and water quality impairments and maintaining adequate water flows through groundwater and surface recharge are vital to ensuring safe water supplies, water recreation and the ecological integrity of the region. The Northern Virginia Regional Commission (NVRC) has developed a Conservation Corridor Planning Project which is a regional effort to identify essential green infrastructure and help area governments to avoid the mistakes of the past and maintain the few remaining green corridors along the rivers and reservoirs that boarder Fairfax and integrate green infrastructure planning into the future development planning of Prince William and Loudoun counties.
According to NVRC there are three priority regional conservation corridors in Prince William County. Bull Run Mountain and Catoctin Mountain corridor is a north-south corridor connecting the foothills of the Blue Ridge Mountains in Northern Virginia. The corridor provides significant intact habitat for Northern Virginia wildlife. North of Leesburg, the corridor is the karst terrain of Loudoun underlain by limestone, and highly susceptible to pollution and sinkhole creation. South of Route 50 the Bull Run Mountain ridge is within the Rural Crescent and is the location of a significant area of recharge for the groundwater that ultimately maintains and feeds Bull Run and the Occoquan River. This area is fractured rock system with limited overburden and no natural attenuation. A polluted plume could be carried for miles without dilution.
The second priority conservation area is begins at the Bull Run Mountains and heads east across Route 15 to Manassas covering the land between Route 50 and 29 (the northwest portion of the Rural Crescent) to the confluence of the Occoquan River with Belmont Bay. This corridor is rich in water and environmental resources that ultimately deliver drinking water to over one million Northern Virginia residents. The Occoquan Reservoir, one of the country’s first water reclamation facilities where sewage treatment water is returned to provide water recreation. The western portion of the area is part of the Culpeper Basin Important Birding Area and the Culpeper Basin Groundwater Aquifer. Preventing water contamination and ensuring adequate groundwater recharge are vital to ensuring safe water supplies, recreation opportunities and the ecological integrity of the region.
The third priority conservation area is the Potomac Gorge and Quantico Corridor, the greenbelt that connects Prince William National Forest Park with Manassas Battlefield. This area includes large tracks of undeveloped private land. The Culpeper basin is part of a much larger Piedmont Geologic Province and has only begun to be studied thanks to the careful groundwater measurements taken by Loudoun County as excessive development of the western part of the county began to impact water supplies. Groundwater quantity and quality in our region impacts not only groundwater wells, but stream flow and recharge to the surface water. In short all the drinking water in Prince William County. Groundwater recharges at various rates from precipitation and other sources of infiltration. The recharge is not spread evenly across the land. Pave over the land, change surface flow and infiltration and groundwater recharge are reduced.
Important regional waterways, such as Goose Creek, Bull Run, the Potomac River and Occoquan Reservoir thrive because they are shaded by trees and vegetation that filter stormwater, prevent erosion, and facilitate ground water recharge and moderate temperatures. Green infrastructure maintenance ensures the forested buffers are maintained and enhanced over time, protecting public health and water quality. Maintaining and enhancing forested buffers near Northern Virginia’s waterways requires focus on how to maintain and protect these ecological resources. The EPA has identified nitrogen, phosphorus and sediment as the three primary pollutants that must be reduced to restore the health of the Chesapeake Bay and its tributaries. They have mandated to Virginia and the other Chesapeake Bay Watershed states and Washington DC an approximate 25% reduction in these pollutants. A wide range of approaches can address these impairments, including reducing runoff and restoring stream banks and buffer areas. The Rural Crescent is an extraordinary valuable resource that we cannot just throw away by building highways such as the Bi-County Parkway, roadways and buildings. Protecting our water supply infrastructure is more than a pipe that runs into your house, more than the Occoquan. If you pave and build over the landscape the water supply will be irreparably damaged. Without water there is no Prince William. On Saturday, December 7th 2013 there will be an open house and all day series or meetings at George Mason University, Prince William Campus in Manassas.