Thursday, November 4, 2010

Groundwater in Virginia

According to George Harlow at the US Geological Survey (USGS) in Richmond, VA about 34% of all drinking water in Virginia is supplied by groundwater and there are 1.7 million Virginians whose drinking water is sourced from groundwater and supplied by their own private wells. The information below is from a talk Mr. Harlow gave and the Private Water Supply Handbook.

The geology-the underlying types of soil and rocks of an area determines the characteristic and availability of groundwater. To survive over time, a population must live within the carrying capacity of its ecosystem, the most important element of the ecosystem is potable water. Without water there can be no life. Water is needed for drinking, bathing, to support irrigated agriculture and industry. In Virginia, our rainfall is usually adequate and there is limited need to irrigate. Precipitation and soil type determines how much the shallower groundwater is recharged annually. However the volume of water that can be stored is controlled by the reservoir characteristics of the subsurface rocks. Groundwater may be present today that was precipitation months, years or eons ago. Using more groundwater than is recharged through precipitation is unsustainable over the long run.

The nature of the soils and rocks varies across Virginia by physiographic province. 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. There is also a limited areas of Mesozoic Lowlands within the Piedmont that is not a geographic region but is a physiographic province and is groundwater rich. I happen to live within the Mesozoic Lowlands. The natural occurrence and availability 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 unconsolidated sediments well casings must reach to the water table and the well must be screened in the saturated zone, but just about anywhere you drill a well, you will find groundwater. Water tends to be of good quality for the most part, but there are areas where over pumping has resulted in salt water intrusion and areas where iron and hydrogen sulfide occur. It is very possible 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 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. To be productive a well must be located within a fracture. Water tends to be hard and in many areas contains high levels of iron, sulfur, and can be acidic.

The Mesozoic Lowlands are within the Piedmont region. These areas consists of an interbedded sequence of sedimentary and basaltic rocks. The rocks of the lowlands are highly fractured and overlain by a thin cover of overburden. The lack of overburden limits natural protection to the aquifer. The sedimentary rocks are productive aquifers. The soils are described by the USGS as Balls Bluff Siltstone with a gravel, sand and clay type bedding plane. 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 spread easily. Groundwater is easy to locate and tends to be hard.

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. Water containing fractures can be few and far between and it is very possible not to find water on a home site or to have a well run dry regularly.

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 and tends to be acidic.

The 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 groundwater travels in the coal veins. 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.

The quality and minerals in the groundwater are determined to a large extent by the local geology. 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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