|
|
Underground Storage
Tanks |
In 1988, the EPA reported that an estimated 25% of all underground storage tanks (USTs) were leaking chemicals and petroleum products into the environment (EPA, 1988). The liquids from the USTs can migrate through the soil and contaminate ground water. Additionally, the liquids are often flammable and may cause fires or explosions. Finally, vapors from the spill may gather in a closed area, such as a basement or storage shed, until the concentrations reach dangerous levels. In response to the threat posed by leaking USTs to public health and the environment, Congress added Subtitle I to the Resource Conservation and Recovery Act (RCRA) in 1984 (EPA, 1988). Subtitle I required the EPA to develop underground storage tanks regulations for existing and future USTs. The EPA regulations have been enforced since 1988.
The regulations require that all USTs that are installed after December 1988 must meet the following requirements: 1) certification that the tank and piping are installed properly according to industry codes, 2) installation of devices that prevent spills and overfills, 3) corrosion protection of the tank and piping, and 4) installation of a leak detection system (EPA, 1988). Leak detection methods may include automatic tank gauging, monitoring for vapors in the soil, interstitial monitoring, monitoring for liquids in the ground water, and other approved methods (EPA, 1988).
All USTs that were installed prior to December 1988 must be retrofitted by December 1998 with corrosion protection, overfill/spill prevention devices, and leak detection systems. Leak detection systems may include monthly monitoring methods as listed above or monthly inventory control with tank tightness testing every five years (EPA, 1988).
If underground storage tanks do leak into the surrounding soil, a manager must inform the regulatory agency within 24 hours if the spill exceeds 25 gallons. Four main options may be pursued to mitigate the problem, including containment of the contamination in place, extraction of the contaminant from the site, in situ treatment of contaminants, and management alternatives that allow the contamination to remain, while minimizing risks to public health and the environment (Domenico and Schwartz, 1990). The following BMPs provide options for mitigating UST leaks. Sources of material for this section include Physical and Chemical Hydrogeology by Domenico and Schwartz (1990) and Musts for UST's published by USEPA (1988).
The containment option prevents contamination from spreading by the placement of physical or hydrodynamic barriers. The following are general examples of containment option that are currently used with success.
Slurry Walls: The placement of low-permeability barriers in trenched dug around the spill either contains the contamination itself or restricts water flow through the area by an upgradient barrier, preventing movement of the contamination. Low permeability barriers often consist of a cement/bentonite slurry (Domenico and Schwartz, 1990).
Grouting: A low-permeability grout wall is created by injecting fluids under high pressure into the ground. around the contaminated area. Grouting is especially successful in areas of fractured bedrock, where emplacement of other low-permeability barriers is not feasible. Grouting fluids are typically comprised of cement, bentonite, or specialty fluids such as silicate or lignochrome grout (Domenico and Schwartz, 1990).
Geomembranes: Installation of synthetic sheets in trenches dug around the contaminated area restricts the spread of contamination. Geomembranes are relatively new, and there are concerns about the long-term efficiency and compatibility of the synthetic fibers with organic solvents (Domenico and Schwartz, 1990).
Surface Seals: Installation of an impermeable cover above the contaminated area prevents infiltration of surface water and precipitation. Restriction of water flow through the area will slow the contaminant spread. Surface seals are commonly composed of compacted clay, mixtures of natural soils, and stabilizers such as cement, bitumen or fly ash; bentonite layers; sprayed bituminous layers; synthetic membranes, waste materials such as furnace slag, incinerator residues, fly ash, and clinkers (Domenico and Schwartz, 1990).
Surface Drainage: Alteration of the land surface to promote drainage of surface water and precipitation away from the contaminated area. Rapid removal of the water from the land surface will limit infiltration. Restriction of water flow through the area will slow the contaminant spread.
Hydrodynamic Control: The contaminant spread may be controlled by creating a cone of depression (lowering the water table) with pumping wells around the area. The lack of ground water will restrict movement of the plume. Additionally, a combination of pumping wells and injection wells around the area can create a potentiometric low that will trap the contamination in place. Vapor movement may be controlled with a pressure differential system or a vapor extraction system (Domenico and Schwartz, 1990).
Pumping: Pumping wells remove contaminated ground water. The contaminated water must be treated before disposal (Domenico and Schwartz, 1990).
Interceptor Systems: Drains, trenches, and lined trenches are used to collect contaminants that lie slightly above the water table. The contaminated water must be treated before disposal (Domenico and Schwartz, 1990).
Soil Venting: Volatile organic compounds (VOCs)are removed from the unsaturated zone by vacuum pumping air through a well. Air circulates rapidly across the spill, promoting volatilization and biodegradation. The VOCs should be removed from the airstream before being released to the atmosphere. Non-volatile compounds are not effectively removed by this method (Domenico and Schwartz, 1990).
Excavation: Soil is removed from the site. Excavation is often costly because of the labor and disposal costs (Domenico and Schwartz, 1990).
Biological Degradation: The addition of nutrients and oxygen encourages the microbial breakdown of complex organic molecules into simpler, more stable compounds such as carbon dioxide and water. Generally, existing populations of bacteria are encouraged to breakdown the contaminants, although some strains of bacteria have been genetically engineered to rapidly metabolize particular contaminants. The efficiency and predictability of this method remains uncertain (Domenico and Schwartz, 1990).
Chemical Degradation: The addition of appropriate chemicals or treatment agents through wells around the site can alter the composition or consistency of the contaminants. For example, alkalies or sulfides causes heavy metals to precipitate as insoluble compounds and oxygen causes the chemical alteration of cyanide to less hazardous chemicals. The reliability of this method in comparison to other methods remains uncertain (Domenico and Schwartz, 1990).
Management options are actions that will protect the public's health, while allowing the contaminants to remain in place. Management alternatives include 1) restricting or forbidding use of aquifer for water supply, 2) developing or purchasing an alternative water supply, 3) removing the source of the contamination to reduce or eliminate the problem, 4) monitoring the progress of the contamination, 5) issuing health advisories, 6) accepting increased risks, or the "no action" alternative (Domenico and Schwartz, 1990).
Domenico, P.A., and F.W. Schwartz, 1990. Physical and Chemical Hydrogeology. John Wiley & Sons, New York.
EPA, 1988. Musts for USTs. EPA/530/UST-88/008. U.S. Gov. Printing Office, Washington, D.C.
|
NCSU Water Quality Group |
|