Eliminating the Rural Crescent

When the Prince William County Board of Supervisors vote last month to adopt the Comprehensive Plan, they voted to eliminate the Rural Crescent. The Board of Supervisors voted for sprawl, destruction of our watershed and the loss of habitat. The Rural Crescent is about water, groundwater, watershed and habitat preservation.

The Rural Crescent in Prince William roughly aligned with the Culpeper groundwater basin, one of the more important watersheds in Virginia. Much of what has been 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 limits natural protection to the aquifer. These sedimentary rocks until developed and paved over are productive aquifers and feed not only the groundwater wells that provide drinking water to Evergreen system and other communities, but also feeds the tributaries to Bull Run, the Occoquan 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.) 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 and region depends on.

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. Development of the Rural Crescent will introduce potential sources of contamination that could never (in our lifetimes) be remediated. In addition, development of the Rural Crescent threatens the hydrology of the area and the water supply itself.

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 and serve as the perennial flow during dry spells. 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. In a properly functioning watershed, much of the runoff 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. All of this is the hydrology of a watershed; stormwater control ponds cannot imitate this function.

According to the U.S. Environmental Protection Agency, impervious cover levels of 10% can significantly impact watershed health increasing stormwater runoff. As we continue to build in the watershed we chip away at the health of the watershed until it is overwhelmed. 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.

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. Conversion of natural land to residential, commercial and industrial development is the primary way habitat is permanently lost in Virginia. Without proper planning, this conversion can occur in decentralized and scattered patterns, consuming an excessive amount of land and causing unnecessary fragmentation of the landscape. Such has been the case in Virginia in recent decades and our own our Board of Supervisors has planned that as our future.

The consequences of this haphazard development include not only lost habitat and natural wildlife corridors but also the degradation of important ecosystem services that keep our air and water clean, assist in climate regulation, and reduce the impacts of natural disasters. The Northern Virginia Regional Commission (NVRC) in their Conservation Corridor Planning Project  identified essential green infrastructure to 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.

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  to the confluence of the Occoquan River with Belmont Bay. The is area includes the recently approved PW Digital Gateway. 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 not yet developed private land. These open lands protect groundwater quantity and quality in our region which impacts not only groundwater wells, but stream flow and recharge to the surface water.

All water in Prince William County is connected. 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 are fed by and recharge the groundwater. Green infrastructure maintenance ensures the forested buffers are maintained and protect public health and water quality.

The Rural Crescent is an extraordinary valuable resource that we cannot just throw away by building data centers, commercial and residential buildings and four lane roadways. Protecting our water supply infrastructure is more than a pipe that runs into your house, more than a storm water pond or a line of trees in an industrial development. If you pave and build over the landscape the water supply will be irreparably damaged. We are repeating and amplifying all the mistakes that have been made in the past.