The Rural Crescent, the Occoquan and the Water Supply

The Board of Supervisors of Prince William County have allocated $60,000 to engage a study to examine if the goals desired from the creation of the Rural Area of the county have been met through the implementation and management of the Rural Crescent. In addition, the study is to identify other rural preservation tools that would allow the elimination of the Rural Crescent restrictions on development density. This is a direct result of a motion by Supervisor Martin Nohe at the regular March meeting of the Prince William County Board of Supervisors.

The Rural Crescent was created in 1998 and originally intended as an urban growth boundary for the county designed to preserve the agricultural heritage and force redevelopment along the Route 1 corridor rather than development in the remaining rural areas. This was to be accomplished by limiting development to one home per 10 acres with no access to public sewers. The Rural Crescent has been chipped away at for years, but still contains 80,000-acres; however, active farming in Prince William continues to decrease.  To adequately judge the utility of the Rural Crescent the study must consider its impact on water resources and water ecology. While the Rural Crescent may have been the wrong policy to preserve our agricultural heritage, it has been a success at preserving water resources, protecting our groundwater and supporting the ecosystem of our estuary. In addition, continued redevelopment of areas with preexisting infrastructure will allow Prince William County to improve storm water management in those areas and score nutrient points for the EPA mandated TMDL as well as revitalize older areas of the county and 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 and essential to the health of the Occoquan Reservoir which in turn is an essential element in the drinking water supply of Fairfax Water - Prince William Service Authority, PWSA, obtains most of the drinking water they distribute in the county wholesale from Fairfax Water. Besides purchased water from Fairfax Water, PWSA operates the Evergreen water wells that draw water directly from the Culpeper Basin and thousands of home owners have private wells that also draw from the aquifer. The Virginia-American Water Company also distributes water purchased from Fairfax Water. Any changes in land use have the potential to negatively impact groundwater, the watershed and the Occoquan Reservoir and should be considered in the study of “other preservation tools.”  

The 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 that is highly fractured and overlain by a thin cover of overburden. While ground water flows generally speaking west to east, the fractures within the rock run predominately north south. Contaminants can enter the groundwater at these fractures and zigzag through the aquifer, but these fractures also serve as recharge areas. Groundwater 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 in a fractured rock system there is limited if any natural attenuation and the aquifer could be polluted beyond our ability to remediate.

Generally, groundwater in the Culpeper Basin is replenished each year through precipitation. Groundwater recharge through precipitation requires adequate area for infiltration of rainwater, control of sheet flow created by roads and paved areas, as well as protecting the most geologically favorable infiltration points. Precipitation and snow melt 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 the saturated subsurface rock of the aquifer, 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 as base flow for the rivers, and some ground water finds openings in the land surface and emerges as fresh water 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.

 

The “rural preservation tools” to be investigated as part of the study are sustainable community concepts, high density communities utilizing the strategies of Low Impact Development, LID, which include dedicated open space. While high density communities built adjacent to dedicated open space of cute community farms as is being done in Loudoun might preserve our agricultural heritage, it will not guarantee the preservation of our ecosystem and water. When development disturbs more than 10% of the natural land by covering surfaces with roads, driveways, walkways, patios, and homes the natural hydrology of the land is disturbed, irreparably disturbed. These developments while much better than traditional developments still disturb more than half the land area by significantly increasing building density. 

The lack of overburden limits natural protection to the aquifer, but has allowed easy infiltration. The sedimentary rocks of the Rural Crescent 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 assuring the base flow to the rivers and streams that feed the Occoquan. Our freshwater resources need to be managed as a whole. Development that will impair the recharge of the aquifer can result in impacts to the entire region, including the decrease in water level and aquifer storage, reductions in stream base flow and lake levels, loss of wetland and riparian ecosystems, saltwater intrusion and changes in groundwater quality. Our future and our children’s future is our water. We can’t allow it to be destroyed  in paving roads and building houses for short term gain.