Nearly 80 percent of communities rely on ground water as their primary drinking water source. Wells extract ground water for use in homes and businesses. In addition, about 42 million Americans use private wells for drinking water. In light of this information, preventing ground water contamination is of utmost importance, especially since a number of factors can contribute to ground water contamination.
To prevent ground water contamination, the first step is to locate the well so that surface water and contaminants cannot flow into it. Site engineers try to install the well uphill from any potential contamination source. This means avoiding potential pollution sources, such as industrial plants, home septic systems, landfills and underground storage tanks. Hiring a qualified hydrogeologist to investigate potential contaminant sources and likely subsurface conditions makes locating a well easier.
For most private wells, the primary contaminant source is the owner’s septic system. The best protection practice is to locate the well above the area where contaminants can enter it, usually about 50 feet to 100 feet away. In addition, install a surface seal into a fine-grained layer or non-fractured zone above the aquifer. To prevent water from collecting near the casing, the ground surrounding the well should slope away from the wellhead on all sides. In addition, most states regulate how far a well must be located from potential contamination sources. For instance, most states require that wells be a minimum of 50 feet away from a septic system.
The Well’s Design
Proper selection of well casings, seals, screens, filter packs and pump chamber casings are important factors that determine the efficiency of the well, and prevent contamination. The illustration (right) shows the components of a well that prevent pollutants from entering it. Most states have well construction standards and permitting processes that must be followed. The American Water Works Association has a standard A100-90 that deals with construction design.
Casing the Well
A casing is a pipe that usually is made of steel or plastic. It lines the borehole in the earth, keeps the well from caving in, and prevents runoff and other material from getting into the well. When contactors select casing, they must take into account the forces that are exerted while installing. In addition, the surrounding materials – such as soil and rocks – tend to collapse into the hole. If possible, the driller should use a temporary casing for the borehole. The temporary casing diameter must be at least four inches larger than the permanent casing to provide sufficient space for a good well seal.
The American Society for Testing and Materials, the American Petroleum Institute, and the American Iron and Steel Institute all have specifications for casings. Most state standards require steel casing of a specified wall thickness for wells, whether for a community or private individual. The diameter of the casing must leave enough room to install the submersible pump, and still have space for maintenance. The size of the pump depends upon the desired well yield. Casing depth also helps prevent well contamination. Logs of any other nearby wells, and the local geology can help determine how deep the casing should go. The casing should extend at least 12 inches above the ground for sanitary protection. Reducing the casing’s diameter requires a minimum of eight feet of casing overlap. A watertight well cap should be placed on top of the casing. The Water Systems Council (WSC) has standards for well caps and other well components.
The Well Screen
A well screen is a cylindrical, sieve-like structure that serves as the intake portion of the well. It is a metallic pipe that has holes or perforated or slotted sections that are placed on the water-carrying zones of the aquifer. Proper selection, design, placement and development of the screened section are very important, and determine the well’s efficiency and yield. Because certain sections of the ground are more porous than others and, hence, carry more water, placing the screens in these sections will yield the highest flow rates. By looking at the data collected during drilling, a good well driller can locate and place the screen in the proper zones.
To better understand conditions at the site, use borehole geophysical logs to grasp the subsurface conditions. In addition, visual inspection of the cuttings or samples can show if the layers of earth are sandy, coarse or clay-like. And to help determine well yield, use sieve analysis and hydraulic conductivity tests.
The Filter Pack
A filter pack typically is made up of sand or gravel that is smooth, uniform, clean and well-rounded. It is placed in the area between the borehole wall and the well screen to prevent formation material from entering the screen. To enhance the permeability of the zone surrounding the screen, place a filter pack around it. A good filter pack keeps sediment out and decreases friction losses around the screen, and is especially important if the aquifer consists of uniform fine sand. A filter pack allows for larger openings in the screen, and improves well yield. To install a filter pack, start from the bottom of the screen, filling in to at least three feet above the top of the screen. Domestic wells do not require a filter pack.
The Well Seals
The most important components that prevent contaminants from entering the well are well seals. A seal is a cylindrical layer of material, usually cement, bentonite or clay, which surrounds the casing up to a certain well depth. It prevents runoff or other contaminants from entering the well, and serves to further protect the casing. The drilled hole must be four inches larger in diameter than the outer diameter of the casing so that the seal can be placed in the space between the casing and the hole.
Well construction standards specify the material that well installers must use to seal the well, as well as the depth to which the well is grouted. Typically, public water supply wells are grouted to a depth of 50 feet. A cement slurry is pumped in the ring-shaped space between casing and hole, and the well is sealed from the bottom up. Grout is placed using a tremie. A layer of two-foot-thick bentonite should be placed on top of the filter pack.
Pitless adapters and pitless units are devices that attach to the well casing below the frost line, and provide sanitary connections. They prevent entry of contaminants into the well near the surface. These devices provide access to the well for servicing. The adapter connects the casing with a horizontal line that supplies water through a removable seal joint. This connection allows the drop pipe and pumping equipment in the well to be easily removed for repair or maintenance work without digging the ground around the well. WSC has performance standards for pitless adapters, pitless units and watertight well caps. A list of manufacturers that meet those standards can be obtained from the WSC.
Well installers must disinfect all equipment and tools using a chlorine solution before any drilling operation to prevent bacterial contamination. The well must be disinfected after it’s completed. Some types of bacteria, such as E. coli, are found in soils, and can contaminate the well. By dissolving calcium hypochlorite or sodium hypochlorite, installers can make a chlorinated water solution. ND
NESC Grows in both Scope and StatureWell into its fourth decade, the National Environmental Services Center (NESC), located at West Virginia University, has been providing environmental services to small communities. NESC traces its roots back to 1979 with the establishment of the National Small Flows Clearinghouse, a non-profit organization devoted to protecting the public health and environment of the nation’s small communities by providing wastewater information and assistance. Since that time, NESC has grown to become a national expert – not just in wastewater, but also in drinking water, environmental training, infrastructure resilience, and utility management.
Part of NESC’s success and longevity can be attributed to its former director, John Mori, who said, “We are valued because we don’t have an agenda. We are valued because we’re only interested in what is going to work in the community, and that’s a rare commodity.”
And the group has no intention of resting on its laurels. The group’s current director, Gerald Iwan, says, “The next few years are going to be exciting, considering population growth, public health, climate, economy, security, and social restructuring. The need for reliable infrastructure, safe, sustainable drinking water resources, and sound waste and wastewater management systems and practices will become even more apparent as we wrestle with these and other emerging issues.”
Learn more at www.nesc.wvu.edu.