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442-901

Authors as Published

Introduction

In Virginia, groundwater is an important source of private and public water supplies. In fact, in 60 of Virginia's 95 counties, the majority of households obtain water from private wells and springs (see Figure 1). For 38 counties, groundwater is the sole source for public water supplies, and another 16 counties depend on groundwater to obtain more than 50 percent of their water for public supplies. Overall, more than one-third of Virginia's almost 6.4 million residents depend on groundwater. Agriculture, an important part of Virginia's economy, maintains its high productivity, partially by using groundwater. According to U.S. Geological Survey estimates for the year 1990, almost 22 percent of the 36 million gallons of fresh water source used per day for crop irrigation in Virginia was derived from groundwater.

I. Facts About Groundwater

A. Basics of Groundwater and Waterwells

The water that seeps down through the soil surface to the underlying soils and rocks is known as groundwater. Groundwater occasionally reappears above ground as springs and seeps, and is an important source of supply to surface waters. According to the U.S. Geological Survey, at least 30 percent of the annual average flow of streams in Virginia is derived from groundwater.

Groundwater occurs in the voids between soil and rock particles much as water fills the pores of a sponge. The formation in which groundwater occurs is referred to as an aquifer. Aquifers can be confined or unconfined (Figure 2). A confined aquifer is surrounded on all sides including the upper surface by confining beds or layers. Also known as an artesian aquifer, a confined aquifer contains water that is under pressure. A well that taps an artesian aquifer is an artesian well. A flowing artesian well taps an aquifer where the water is under enough pressure to rise to the land surface without pumping.

The upper surface of an unconfined aquifer (also called a water table aquifer) is not bound by a confining bed. The water table, defined as the height of water in the aquifer, rises and falls as the amount of water in the aquifer fluctuates. A well drilled into such an aquifer is called a water table well. The level of water in the water table well reflects the depth of the water table in the surrounding aquifer. The water table is not usually level; more often it follows the surface topography above it and has "hills" and "valleys" just as the land surface does. In general, the water table is at depths of zero to twenty feet in humid areas and can be hundreds of feet underground in desert areas.

The recharge area (the land surface area from which a given aquifer is replenished by precipitation) for an unconfined or water-table aquifer may include all the land surface above it; for a confined aquifer, the recharge area is typically less extensive. For both types of aquifers, it is important to protect recharge areas from land-use activities that might contaminate the aquifer.

The rate of groundwater movement is very slow compared to that of streams and rivers. The average rate of water movement through a coarse sand aquifer is 360 feet per year, whereas the average rate through a clay confining bed can be less than half an inch per year. The slow movement of groundwater means that groundwater contamination can have long-term and possibly delayed effects. Chemicals, after entering an aquifer, could remain in the aquifer for hundreds or thousands of years. Therefore, a spill occurring many years ago may ultimately reach a nearby well or spring at a much later date.

Karst formations have developed in carbonate and dolomite rocks through the dissolving action of surface water and groundwater. Karst areas include sinkholes, caves, sinking streams, and springs. In limestone areas, sizable surface streams disappear into underground channels and, conversely, some large springs emerge to become the headwaters for rivers. In karst terrain and limestone caverns, open lava tubes, or large rock fractures, the rate of groundwater movement resembles that of streams and rivers on the surface. The movement of contaminants through such formations is likewise very rapid.

B. Virginia's Five Physiographic Provinces

Virginia is a fascinating place because of its geologic diversity. The Commonwealth has five areas-called physiographic provinces-of distinct geologic structure and climate. From west to east, the provinces are known as the Cumberland Plateau, Valley and Ridge, Blue Ridge, Piedmont, and Coastal Plain (Figure 3). The geology of each province affects the quantity and quality of groundwater in it.

Cumberland Plateau

The Cumberland Plateau is underlain by sandstone, shale, and coal. Groundwater quality varies with depth. At 100 feet or less below the land surface, it is generally of poor quality, tending to be sulfurous and iron-rich. Between 150 and 300 feet in depth, better quality water generally occurs. Naturally saline waters occur at depths greater than 300 feet. The potential for groundwater pollution is moderate in the Cumberland Plateau.

Valley and Ridge

Limestone, dolomite, shale, and sandstone are the common rock types in the Valley and Ridge province. Where limestone dominates, groundwater yields may be as high as 3,000 gallons a minute. Some of the most productive aquifers in the state are located in lowlands, such as the Shenandoah Valley. Ridges and upland areas are often underlain by sandstone and shale, which yield only enough water for domestic use.

Groundwater quality is affected by the chemical composition of rock formations. Limestone or carbonate, for example, contributes to the "hardness" of water in this province. Because of the features of karst terrain, the pollution potential in the Valley and Ridge is very high. Streams and surface runoff entering sinkholes or caves contribute to aquifer recharge, providing direct conduits for surface watercarrying contaminants and bypassing filtration through the soil, which can remove some contaminant. In karst terrain, groundwater can travel much faster through underground networks of pathways - up to several miles a day - and contaminants can quickly be transmitted to wells and springs nearby or miles away.

Blue Ridge

The Blue Ridge province is a relatively narrow zone of mountains with the highest elevations in the state. Only between 4 to 25 miles in width, the rocks underlying the area are granite, gneiss, and marble. Steep terrain, thin soil, and weathered rock covering bedrock result in rapid surface runoff and low groundwater recharge. Bedrock is somewhat impervious, so water is held in joints, fractures, and faults. In the eastern portion of the Blue Ridge, igneous and metamorphic rocks are most common, and in the west, sedimentary rocks predominate. Groundwater use is limited to domestic rather than public wells, because of limited industrial and residential development in the province. Along the lower slopes, which favor groundwater accumulation, springs are common and serve as private water supplies. Water quality is generally good because the rocks are relatively insoluble, but the iron content may be high in some locations.

Piedmont

The Piedmont extends from the fall line (an imaginary line passing through the cities of Emporia, Petersburg, Richmond, Fredericksburg, and Washington, D.C.) westward to the Blue Ridge Mountains. This is the largest province with some sedimentary rocks, but mainly dominated by hard, crystalline igneous and metamorphic formations or bedrock overlain by saprolite. Most of the groundwater is found within a few hundred feet of the surface because the occurrence of fractures and faults which store water in bedrock decreases with depth.

The subsurface geology of the Piedmont province is diverse, resulting in wide variations in groundwater quality and well yields. Where fractures and faults are extensive, the greatest yields occur, such as in the western Piedmont along the base of the Blue Ridge Mountains. Groundwater is generally of good quality; in a few areas, high iron concentrations and acidity cause problems.

Coastal Plain

Extending from the coast inland about 110 miles to the fall line, the Coastal Plain is composed primarily of sand, gravel, clay, shell rock, and other unconsolidated deposits. This province stores more groundwater than any other in the state, and about half the state's ground water use occurs in the Coastal Plain.

Two major groundwater systems, shallow and deep, respectively, are located in the Coastal Plain. The deep system of confined aquifers is the major source of groundwater withdrawals in the province that serves municipal and industrial users. Some large production wells yield 2,000 to 3,000 gallons a minute.

A shallow unconfined (water table) aquifer system overlies impermeable clay beds in many areas and serves as the source of water for domestic and low capacity wells of about 10 to 50 gallons a minute. The pollution potential in the upper unconfined aquifer is high because of the rapid infiltration rates, high population densities, and agricultural activities. Natural water quality is good, except in a few areas where saltwater, iron, and hydrogen sulfide occur.

On the Eastern Shore, high salt concentrations in water below a depth of 300 feet render it unpotable. Where saltwater interfaces fresh, brackish water may migrate inland as aquifers are pumped. This has resulted in water from deep aquifers, as well as some shallow aquifers, on much of the lower York-James Peninsula and in the Norfolk-Virginia Beach area being too salty for domestic use.

C. Threats to Virginia's Groundwater

The Virginia Department of Environmental Quality's Pollution Response Office, which cleans up groundwater pollution and takes action against individuals or firms responsible for contamination, began recording citizen complaints about groundwater problems in 1978. The number of complaints increased sharply over the next several years. In 1988 the Pollution Response Office had received 1,100 complaints and in 1993 the total number of complaints was more than double the 1988 levels. In spite of this increase in complaints, the Virginia Department of Environmental Quality has determined that a very low percentage of the state's aquifers are seriously contaminated.

Discussed below are the major threats to Virginia's groundwater in farmstead environments. The risks to farmstead water supplies may be greater when such pollution sources/activities are coupled with improperly constructed and maintained wells and springs (Fact Sheet No. 2).

Septic Systems:

As the leading contributor to the total volume of waste discharged directly into the ground, septic systems are a significant source of groundwater contamination. Of particular concern are nitrate and disease-causing organisms such as pathogenic bacteria and viruses. Septic systems rely on soil to filter and treat domestic sewage; they are not designed to handle industrial wastes, petroleum by-products, or farm and household hazardous wastes, such as pesticides, varnishes, and cleaning products. For details, see Fact Sheet No. 3.

Hazardous Wastes:

Products used around home-sites and farmsteads such as motor oil, antifreeze, wood preservatives, paints, batteries and some household cleaners are potentially hazardous wastes. If improper disposal practices are used, hazardous waste may contaminate groundwater and can be harmful to humans, animals and the environment. For details see Fact Sheet No. 4.

Pesticides:

Improper storage, mixing, and application (rate and timing) of pesticides may contribute to groundwater contamination. The best way to reduce the threat of groundwater contamination by pesticides from agriculture, forestry, and lawn and garden care is to use proper pesticide management and less toxic, rapidly degradable pesticides with low leaching potentials. For details, see Fact Sheet No. 7.

Fertilizers/Animal Waste:

Nitrates from livestock production, feedlots, fertilizers applied to farmlands, gardens, and lawns, and other sources can leach through soil into groundwater. Artificial drainage, used to dispose of excess water from croplands during wet seasons, provides a direct route for fertilizers to contaminate water supplies, as do improperly constructed or abandoned domestic wells. For details, see Fact Sheet No. 3 , Fact Sheet No. 6, Fact Sheet No. 8 , Fact Sheet No. 9 , Fact Sheet No. 10 , Fact Sheet No. 11 , Fact Sheet No. 12 .

Underground Storage Tanks:

Buried steel tanks used to store gasoline, diesel fuel, heating oil, and other petroleum products have a lifespan of about 15 years. Rusted and corroded tanks and lines can leak petroleum into groundwater and contaminate large amounts of drinking water. For details, see Fact Sheet No. 5.

Abandoned Wells:

Improperly constructed or abandoned wells can be a significant contributor to groundwater contamination in Virginia. If wells are not properly capped and sealed, they provide direct conduits for surface contaminants to enter groundwater. Abandoned wells should not be used for the disposal of any waste materials. For details, see Fact Sheet No. 2 .

Other:

Overpumping groundwater in areas where naturally occurring brackish aquifers or tidal waters are adjacent to freshwater aquifers can result in the movement of brackish water into fresh waters. Once an area has lost its groundwater to saltwater intrusion, developing new water supplies can be extremely expensive. Accidents, such as spills of petroleum and chemicals, can occur. Government agencies will respond immediately to contain and lessen the effects of a chemical spill on land, in the air, or in water.

Spills of oil or chemicals should be reported immediately to the National Response Center or Virginia Emergency Response Council. See Resource Directory for appropriate contacts.

 

II. Site Evaluation

How farmstead operations such as pesticide handling or manure management affect groundwater depends in part on the physical characteristics of the farmstead: soil type, bedrock characteristics and depth to groundwater. Therefore, evaluating the site properties is an important step in protecting groundwater and the quality of water supplies. Keep in mind that, even in areas that have soil types and geologic conditions that are less likely to contribute to groundwater contamination, poor management practices can lead to serious groundwater/drinking water contamination.

A. Soil Type

Soil characteristics are very important in determining whether a contaminant breaks down to harmless compounds or pollutes the groundwater. Because most breakdown occurs in the soil, there is a greater potential for groundwater contamination in areas where contaminants can move quickly through the soil.
  • Sandy soils have large pore spaces between individual particles. Large amounts of rainfall can percolate through these soils and dissolved contaminants can move rapidly down through the soil and into groundwater.
  • Clayey soils, on the other hand, are made up of extremely small particles that slow the movement of water and dissolved contaminants through the soil. Some contaminants, such as phosphorus, also may adhere or stick to clay surfaces and not reach the groundwater table.

Soil organic matter content is important in holding contaminants. Soils high in organic matter provide an excellent environment for chemical and biological breakdown of many contaminants before they reach groundwater. Most chemical and biological breakdown takes place in the loose, cultivated surface layers, where the soil tends to be warm, moist, higher in organic matter and well aerated. However, the natural purification capability of the soil is limited. Certain conditions, such as heavy rainfall or chemical spills, may exceed the soil's purification capacity, allowing leaching to occur. In such cases, the subsurface geologic material and the depth to groundwater (the distance from the ground surface to the water table level) are important factors in determining whether a contaminant actually reaches the groundwater.

B. Subsurface Conditions

Depth to groundwater is important primarily because it determines the depth of material through which a contaminant must travel before reaching an aquifer, and the time during which a contaminant is in contact with the soil layer before it reaches the groundwater. As a result, where soil and subsurface deposits are fairly deep, contaminants are less likely to reach groundwater.

Subsurface geology influences groundwater movement and contamination. For example, sedimentary rocks have a wide range of permeability-from highly permeable fractured limestone to nearly impermeable shales and crystalline formations. Movement of pollutants in fractured limestone or dolomite is unpredictable and pollutants can readily spread over large areas. Where bedrock material contains significant cracks and fractures, the depth and characteristics of soil and subsurface geologic deposits largely determine the potential for groundwater contamination.

 

Contacts and References

For additional information and contacts, see the Virginia Farm*A*Syst Resource Directory.

Acknowledgements

View a list of the Virginia Farmstead Assessment System publications

 

Reviewed by Brian Benham, Extension Specialist, Biological Systems Engineering


Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.

Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.

Publication Date

May 1, 2009