What is an engineered barrier?
An engineered barrier as defined in TACO limits exposure and/or controls migration of contaminants. A barrier may be natural or human-made, but its effectiveness must be verified by engineering practices.
For an exposure (and therefore, a risk) to occur, three factors must be present:
- An exposure route, and
- A receptor.
The purpose of an engineered barrier is to limit exposure by "cutting off" the route. The use of an engineered barrier is an option in situations where contaminant concentrations exceed the applicable Tier 1 or Tier 2 remediation objectives.
The type of barrier used is based on the exposure route being intercepted and the barrier's effectiveness in doing so.
If an engineered barrier is used, it must be accompanied by an institutional control (
Fact Sheet 4) which assures the proper maintenance of the barrier. This institutional control is transferrable with the property and must provide procedures to be followed if intrusive work (breaching of the barrier) is necessary.
An engineered barrier will only be approved by the Bureau of Land (BOL) if the barrier, as part of the final corrective action, is intended to be permanent. That is, barriers will not be approved as part of the final corrective action if they are only intended for temporary use.
Examples of unacceptable engineered barriers include natural attenuation, fencing, and point of use water treatment.
When "cutting off" an exposure route, the soil must
not exceed the soil attenuation capacity (742.215) and the soil saturation limit (742.220); exhibit reactivity (742.305(c)); exhibit a pH less than or equal to 2.0 or greater than or equal to 12.5 (742.305(d)); or, exhibit toxicity for inorganic chemicals or their salts (742.305(e)).
Migration to Groundwater Barriers
For the migration to groundwater route, the goal of a barrier is to prevent the leaching of contaminants out of the soil and into the groundwater by reducing the infiltration rate (742.905). The two types of barriers discussed in TACO are caps and permanent structures. Using engineered barriers to impede contaminant migration to groundwater may be more feasible under a Tier 3 exposure route review (742.925).
A cap is a horizontal barrier that covers the entire area of contamination to prevent infiltration of water. A cap must be constructed of compacted clay, asphalt, concrete, or other materials capable of producing similar results.
Permanent structures are acceptable barriers due to their capping effects. A roadway, for example, may represent an adequate cap, as could a building.
Soil Ingestion and Inhalation Barriers
For both the soil ingestion and inhalation exposure routes, barriers can prevent human exposure to contaminated media.
The two types of barriers acceptable for both of these routes are caps and permanent structures. A clean soil cover is also acceptable for the soil ingestion route.
Caps used to prevent soil ingestion and/or inhalation are similar to those required for the migration to groundwater pathway, and may be constructed with the same materials. Caps for this use, however, are intended to prevent the upward migration of soil and vapors instead of the downward infiltration of water.
Permanent structures may provide adequate protection from contamination in instances where the contaminants have migrated beneath the structure or when a structure is built above the contamination.
A clean soil cover may be used to prevent the ingestion of contaminated soil provided that the clean cover is at least three feet thick. Clean cover consists of materials that have contaminant levels not exceeding the applicable Tier 1 residential remediation objectives.
Ingestion of Groundwater Barriers
For the ingestion of groundwater route, two barriers accepted under TACO are slurry walls and hydraulic control of groundwater.
A slurry wall is a vertical barrier constructed of a material that will prevent or impede the horizontal movement of soil or groundwater contamination. A slurry wall may be used in conjunction with a cap to prevent the migration of the contaminated groundwater.
Hydraulic groundwater control is used to:
- prevent groundwater from coming in contact with contaminated soil, and
- prevent contaminated groundwater from migrating.
The engineered barriers discussed here are all acceptable options. TACO, however, also allows the proposal of other types of barriers. Other barriers will be considered by the BOL if it can be demonstrated that the proposed barrier is as effective as those described here.
Point of Human Exposure
For both carcinogenic and non-carcinogenic contaminants, it is important to identify the location of the risk on a site. Essentially the risk is at the point of human exposure, because without exposure there is no risk. In the TACO procedure, it is assumed that the point of human exposure, i.e., the risk, is at the contaminant source. If, however, an institutional control or an engineered barrier is in place, the point of human exposure is moved to the edge of such controls.