Monday, January 25, 2010

The Geological Regions of Virginia

Loudoun County’s proposed amendments to the zoning ordinances that would create a Limestone Overlay District (LOD) got me thinking of about the geology of the state. I did not grow up in Virginia, but not only is my husband a native son of the Commonwealth, but he comes from a long line of teachers and I have sweet memories of a road trip with his Aunt Louise amusing her great nephew with teachable moments as we drove through the Valley, to the Blue Ridge, to the Piedmont. It was not until years later that I realized that Aunt Louise had been teaching her five year old great nephew (and me) the geological regions of Virginia.

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. Historically, the geology of these regions has determined what the lands can be used for and by implication the nature of the communities that developed in these regions. Post World War II development of the suburbs had disconnected us from the direct ties to geology, but the density of the population and the demand for water, specifically groundwater is bringing us bank to understanding our geological destiny. The natural occurrence of groundwater depends on the geological conditions.

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. At its widest portion the Costal Plain is over 100 miles wide. Costal 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. There is some leakage from the upper to the lower aquifer, but that is relatively insignificant. 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 overbuilding 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 (like my neighborhood) along the base of the Blue Ridge Mountains. 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. The fractures and faults offer a route of transport for any contaminants so that the most water rich areas are the most susceptible to contamination.

The Blue Ridge province lies to the west of the Piedmont and is a narrow zone (4-25 miles wide) of mountains that runs from North Carolina to Maryland with the highest elevations in Virginia. The bedrock is near the surface and relatively impervious and contains limited amounts of water in joints, fractures and fault zones. Igneous and metamorphic rocks are most common on the eastern slope (and into the Piedmont) and sedimentary rocks are common on the western slope. Water yields are low and limited and typically very high in iron.

The Valley and Ridge region is to the west of the Blue Ridge Mountains and is underlain by consolidated sedimentary rocks of limestone, dolomite, shale and conglomerate. Limestone and dolomite occur beneath lowlands, such as the Shenandoah Valley (also within the lowlands between the Potomac and the Catoctin Mountains) these deposits consistently form productive aquifers. Karst features such as sinkholes, caves, and large springs are found in the Valley and Ridge province. The ridges in the upland area are typically underlain by sandstone and shale with limited groundwater yield. Limestone frequently contains underground channels that store and transmit groundwater. Rapid movement of water in the limestone area makes the pollution potential high. Aquifers are often recharged directly by streams crossing fault zones giving wells in these areas the highest yields. This direct surface water to groundwater recharge can create serious water quality problems. The groundwater in these zones bypasses any natural filtration the soil might have provided. The quality of the groundwater would reflect the quality of the seasonal streams and surface water.

The final and smallest geological region of Virginia is the Cumberland Plateau also called the Appalachian Plateau which includes the southwester tip of Virginia. This region is underlain by sedimentary rocks, primarily sandstone, shale and the coal. It is the presence of coal that has most determined the fate of this region. The gentle folding of these formations has created domes and basins and faulting has occurred. Groundwater quality is generally best in the bedrock above the stream level. The groundwater in the stream level contains high concentrations of sulfate, sulfite, nitrate, iron and carbon dioxide. The water improves at 150-300 feet below this area. Groundwater is generally used for small domestic purposes and processing coal. The shallow nature of the groundwater allows for relatively easy contamination.

Virginia is rich in water our actions will determine if we remain so. The process by which water from rainfall, snowmelt, streams and rivers flows into water bearing geologic formation is the groundwater recharge process. The climate change models (as limited and faulty as they may be) predict that Virginia will become a bit wetter and warmer (think North Carolina). A failure of the water supply in Virginia will be due to our own actions and decision. The land surface through which groundwater is recharged must remain open and uncontaminated to maintain the quality and quantity of groundwater of the Commonwealth of Virginia.

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