Protecting human health and the environment is the department’s main goal at facilities where hazardous wastes or hazardous waste constituents have been released to the environment. These releases can contaminate several different media at the site, such as soil, groundwater and surface water. Soil and groundwater at sites are often affected where the contamination has been released over a long period. However, surface water can also be affected if rain or other surface water flows over the contaminated soil and carries the contamination to streams, rivers and lakes or contaminated groundwater is received by a stream, river or lake through a pathway such as a spring.
In addition to performing site remediation, also known as corrective action or cleanup activities, the department works to prevent humans, animals, habitats and other “receptors” from being exposed to the contamination at levels that may threaten human health or the environment or injure natural resources. The site owner and the department work together to determine what type of contaminants were released at the site and where the contamination is located. The department uses that information to determine possible pathways, such as air, soil, water and food, that humans and the environment could be exposed to the contamination. In addition to site remediation, engineered controls are often used to contain or otherwise limit the contamination from spreading.
There is no universal definition of engineered controls, and specific definitions may vary from one environmental agency to another. In general, engineered controls include any actions that physically change the site in order to contain or otherwise limit the contamination from moving in the environment. An engineered control could be a physical barrier between the contamination and receptor or a system or device use to reduce the contamination levels. The engineered controls discussed in this fact sheet are examples of controls used at contaminated sites in Missouri.
Engineered Controls for Contaminated Soil
A cap is a horizontal physical barrier built over the contaminated soil to prevent receptors from being exposed to the hazardous wastes and hazardous waste constituents. Caps are also used to prevent rain from soaking into the contaminated soil and leaching, or carrying, the hazardous wastes and hazardous waste constituents to the groundwater. Caps at Missouri hazardous waste sites are built according to certain technical standards specified by the department, such as using a water-resistant clay layer or flexible synthetic membrane covered with vegetated topsoil, asphalt or concrete. The cap’s design is influenced by many factors, including the potential for the cap and any fill beneath to settle, water drainage above and below the cap and managing any gases produced under the cap.
Vertical Engineered Barriers
Vertical engineered barriers, also called subsurface engineered barriers, are physical structures built around contaminated soil to prevent receptors from potential exposure to the hazardous wastes and hazardous waste constituents. They are also used to prevent hazardous wastes and hazardous waste constituents from spreading by treating contaminated groundwater or keeping the groundwater from coming into contact with the contaminated soil. Examples of subsurface engineered barriers are slurry trenches, sheet-pile walls and permeable reactive barriers.
Stabilization and Solidification (Immobilization)
The stabilization process chemically changes the hazardous wastes and hazardous waste constituents into less harmful or less mobile compounds; however, other physical or chemical characteristics may or may not change. For example, applying cement or lime to soil contaminated by metals changes the metals into compounds that do not move as easily through the soil.
Solidification is the process of encapsulating the hazardous wastes and hazardous waste constituents in a solid material, preventing the chemicals from moving. Chemical bonding does not necessarily occur between the encapsulated compounds and the additives causing the solidification; however, the solid can be left in place or removed to another location. For example, cement mixed with contaminated soil results in a solid that can be transported after it dries.
Subsurface Venting Systems
Soil vapor extraction, also known as “soil venting” or “vacuum extraction,” removes vapors released from contaminated soils or groundwater in the subsurface. These vapors can move through the soil and into a home or building through cracks or openings in the foundation slab or basement. In this process, either a vacuum can be applied to the contaminated soil through extraction wells or the vapors can be vented from the soil through a series of pipes. The vapors are “sucked” through the soil pores toward the wells or piping designed to capture and remove them. After the vapors are captured, it may be treated and released to the atmosphere or simply vented to the atmosphere without treatment if allowed by local and state laws and regulations.
Engineered Controls for Contaminated Groundwater
Groundwater Migration Barriers
Groundwater migration barriers are subsurface vertical engineered barriers built around the contaminated groundwater to prevent receptors from being exposed to the hazardous wastes and hazardous waste constituents. Barriers are designed to prevent the contaminated groundwater from moving off the contaminated property, allowing the facility to contain or treat it in place. An example of a groundwater migration barrier is a permeable reactive barrier. These barriers are a little different in that instead of preventing the groundwater from moving, they are designed to allow the contaminated groundwater to flow through the barrier, treating the contaminants with reactive materials as they pass through with the water and exit on the other side.
Groundwater Extraction Systems
A groundwater extraction system controls contaminant movement in the groundwater by removing the groundwater and contaminants from the soil through wells, trenches or drains. As the groundwater is removed from the well, trench or drain, more groundwater and contaminants flow toward the well, trench or drain. After the groundwater is removed, it will often require treatment before being disposed. Groundwater extraction and treatment not only keeps contaminated groundwater from moving to and affecting neighboring properties, but also reduces the contaminant levels in the groundwater.
In-situ Groundwater Treatment Systems
In-situ, or in place, groundwater treatment involves treating the contaminants without removing the groundwater from the soil. Organic contamination and occasionally metals and other inorganic contaminants can be destroyed or reduced through this type of treatment. There are several types of in-situ groundwater treatment processes, such as electrochemical methods or injecting chemicals into the contaminated groundwater at one location and removing it at another location. There are also biological methods, such as injecting bacteria into the contaminated groundwater or phytoremediation, which uses certain plants to remove contaminants from shallow contaminated groundwater. This type of treatment is typically very specific to the type of contaminants and their location.