Underground storage tanks (USTs) are containers that hold liquids or gases, with at least 10% of their volume, including piping, located beneath the ground. These tanks serve various industries by storing substances like fuels or hazardous chemicals. Despite their utility, their underground placement and contents present a potential for environmental harm.
Understanding Underground Storage Tanks
Underground storage tanks are constructed from materials such as steel, fiberglass-reinforced plastic, or a combination of these, and can range in size from a few hundred to several million gallons. They primarily store petroleum products, including gasoline, diesel fuel, jet fuel, and hydraulic fluid, accounting for approximately 95% of all USTs. Beyond petroleum, these tanks also store industrial chemicals, such as perchloroethylene used in dry cleaning, heating oil, pesticides, and food products.
USTs are placed underground for several practical reasons. This placement conserves surface space, allowing for more productive land use, especially in urban or commercial areas. Their subterranean location also offers aesthetic benefits by keeping industrial containers out of sight. Burying these tanks reduces the risk of fire or explosion by shielding flammable contents from ignition sources and external impacts. The earth also provides natural insulation, helping to maintain a stable temperature for the stored substances.
How Releases Occur
Substances stored within underground storage tanks can be released into the environment through several mechanisms. One common pathway is corrosion, which involves the degradation of the tank’s metallic structure or piping. Older tanks, particularly those made of bare steel before the mid-1980s, are susceptible to rust when exposed to water or certain soil conditions. This can compromise the tank’s integrity, leading to leaks.
Faulty installation is another source of releases. If tanks or their connecting pipes are not properly installed, sealed, or joined, leaks can occur early in their operation. Improper backfilling around the tank or inadequate pipe connections can create stress points that fail. Physical damage can also cause releases; external forces such as excavation activities, ground movement from seismic events, or vehicle impacts can compromise the structural integrity of the tank or its associated piping.
Operational failures are another cause of releases. Overfilling a tank during delivery can lead to spills that seep into the ground. Spills can also occur during content transfer due to equipment malfunction or operator error. Equipment failures involving pumps, valves, or other components of the UST system can also result in releases.
Environmental Impact of Contamination
When a release from an underground storage tank occurs, the environmental consequences can be serious. Leaked substances infiltrate the surrounding soil, leading to soil contamination. These chemicals can saturate the soil, altering its physical and chemical composition, which may render it infertile and toxic to plants and microorganisms. Contaminants in the soil can disrupt natural ecosystems and limit future land use.
UST leaks often lead to groundwater contamination, as groundwater serves as a major source of drinking water for many communities. Contaminants can migrate downward through soil layers and reach underground aquifers. Once in the groundwater, these substances can spread over large areas, polluting wells and affecting aquatic ecosystems if the contaminated groundwater discharges into nearby streams, rivers, or lakes. Even a small release, such as three gallons of gasoline, can contaminate drinking water for an entire town.
Volatile organic compounds (VOCs) can vaporize and migrate through subsurface pathways. This process, known as vapor intrusion, allows these vapors to travel through the soil and enter basements or nearby indoor spaces. Once inside, these vapors can degrade indoor air quality, posing respiratory and other health risks to occupants. Such vapors can also create an explosion hazard in enclosed spaces.
Why Detection and Cleanup Are Difficult
The underground placement of storage tanks makes detecting leaks particularly challenging, often leading to significant environmental damage before discovery. Unlike aboveground tanks where spills are immediately visible, UST leaks are hidden from view. This invisibility means that contaminants can accumulate in the soil and groundwater for extended periods before detection.
Once a leak is identified, tracking the contamination becomes complex due to unpredictable contaminant pathways. Substances released underground do not always move in a straight line; their migration is influenced by soil type, geology, groundwater flow, and the contaminant’s physical and chemical properties. This variability makes it difficult to define the boundaries of the contaminated area, complicating efforts to contain and remediate the pollution.
Remediation of contaminated sites is often a complex and costly endeavor. Excavating large volumes of contaminated soil is a labor-intensive and expensive process, requiring specialized equipment and proper disposal of hazardous waste. Treating polluted groundwater involves advanced techniques, such as pump-and-treat systems or in-situ bioremediation, which can take years or even decades to achieve desired cleanup levels. These challenges underscore the long-term nature and substantial investment required to address the impact of UST releases.