Monday, November 14, 2011
Sustainability of Prince William County and the Rural Crescent
In the recent Prince William County local elections the candidates ran and were elected on schools, roads and jobs. Those are the immediate concerns of residents, but our elected officials need to look to the future and worry about the sustainability of Prince William County. We need to meet the ever tightening requirements under the Virginia Watershed Implementation Plan for the mandated Chesapeake Bay TMDL and we need to make sure that our water resources are not impaired. Preserving the Rural Crescent is essential to sustainable development of Prince William County. The first half of sustainable development is the redevelopment of Brownfields along the Route 1 corridor rather than Greenfield development in rural areas where there is no existing infrastructure. Redevelopment along Route 1 would help Prince William County to improve storm water management (and score nutrient reduction points under the Watershed Implementation Plan) as well as revitalize these older areas of the county. This redevelopment would take place without significantly increasing pavement and impervious surfaces. The second portion of sustainable development is to ensure adequate water for our county now and in the future.
Our geology and climate determines our water resources. The geological regions of Virginia are (from east to west) the Coastal Plain, the Piedmont, the Blue Ridge, the Valley and Ridge and the (Cumberland) Plateau. The Coastal 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, to a large extent along Route 95. At its widest portion the Coastal Plain is over 100 miles wide.
Coastal Plain deposits consist of alternating layers of unconsolidated sand, gravel, silt, shell strata and clay and slopes generally southeast. 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. In the 1990’s it was estimated that approximately half of Virginia’s groundwater use was in this region. The principal recharge area for these aquifers is the land around the fall zone where the aquifers outcrop, unfortunately that area was paved and covered along with the development of Route 95. The Costal Plain’s artesian aquifer has an enormous groundwater storage capacity and Virginia remains a relatively wet location, but pumping (possibly over pumping) has lowered the artesian pressure allowing some salt water intrusion near the coast and development and building in the recharge zone has impacted the availability of water. It is projected with little more population growth that during drought years Fairfax and the Norfolk-Virginia Beach area will have inadequate water.
The Piedmont is bordered by the “fall zone” on the east and the Blue Ridge Mountains on the west. The Piedmont is the largest geological region in Virginia and has a diverse geology largely dominated by igneous and metamorphic rocks, with some areas of sedimentary rocks. The area 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. 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. In other areas of the Piedmont, disintegration of the granite bedrock forms a zone of granular material with slow recharge and relatively high and annoying amounts of iron and sulfur. While providing very productive wells the fractures and faults offer a natural route of transport for any contaminant so that the most water rich areas that supply Bull Run and the Occoquan are the most susceptible to contamination.
Prince William traverses both the coastal plane and the Piedmont. The Rural Crescent in Prince William County is contained within the Piedmont region of the county within the water rich fracture and fault system and its waters feed the surface waters of the eastern portion of the county. The Rural Crescent should remain an urban growth boundary for the county not to preserve our agricultural heritage and sense of place, but to preserve our water. While I strongly support redevelopment of areas with preexisting infrastructure (Brownfield redevelopment) which would allow Prince William County to improve storm water management as well as revitalize older areas of the county and preserve the Greenfields areas in my general support of sustainable development; my strong support for preserving the Rural Crescent is about protecting the groundwater from depletion and contamination.
The Rural Crescent in Prince William County aligns roughly with the Mesozoic basin aquifer of 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 sandstone, siltstone, and conglomerate of Late Triassic age; with the fault and fracture system that produces water rich wells and the easy transport routes for contaminants into the groundwater supply.
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. Each groundwater system or basin is unique and must be understood and managed individually. 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 could be reduced.
There are limits to the amount of groundwater available for extraction from the aquifer. The amount of groundwater removed from an aquifer needs to be sustainable and should ideally match the recharge rate. Increasing the direct demand by pumping to supply water to commercial or industrial users or reducing the recharge rate by diverting surface flow and adding pavement and roads will result in changes in the local or regional hydraulic balance- a reduction in discharge to surface water at some other location, an increase in recharge from surface water, or a loss of storage in the aquifer by falling water table or some combination of these effects.
Our freshwater resources need to be managed as a whole. The utilization of groundwater resources in an unsustainable manner can result in impacts to the entire region, including the decrease in water level and aquifer storage, reductions in stream flow and lake levels, loss of wetland and riparian ecosystems, land subsidence, 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 by those who only see short term gain.