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Containment Features

This section provides definitions for many of the terms used in the ground water containment descriptions and explains how to score the containment factor. If an observed release to an aquifer cannot be established, then that aquifer is evaluated based on potential to release. Four factors are used to evaluate the potential to release factor: containment, net precipitation, depth to aquifer, and travel time. The containment factor is a measure of the methods (either natural or engineered) that have been used to restrict the release of hazardous substances from a source to the subsurface or to prevent released substances from entering ground water.

Containment criteria have been compiled for several types of sources on a numerical scale selected to provide a relative degree of discrimination among different levels of containment. HRS Table 3-2 includes containment factor rating descriptions for the following specific categories of hazardous waste sources: surface impoundments, land treatment facilities, containers, and tanks. The table also provides containment factor rating descriptions that apply to all other hazardous waste sources, including landfills, piles, and contaminated soil.

The containment factor is evaluated for each source for the aquifer being evaluated, and the highest containment factor value for any source that meets the minimum size requirement is assigned as the containment factor value. If none of the sources meets the minimum size requirement, the highest containment factor value of any source is assigned.


Section   Containment


The following definitions elaborate on terms used in the containment descriptions in HRS Table 3-2.

Above-ground Tank: Any tank that does not meet the definition of a below-ground tank (including any tank that is only partially below the surface).

Associated Containment Structures: As used in HRS Table 3-2, constructed barriers (e.g., liners, dikes, berms) that may have been placed under, over, or around a source (e.g., a landfill or a waste pile) to prevent the release of hazardous substances to the environment.

Below-ground Tank: A tank with its entire surface area below the surface and not visible; however, a fraction of its associated piping may be above the surface.

Bulk Liquids: Noncontainerized liquids deposited directly into a source by pipe, tanker truck, or other means of transport.

Essentially Impervious Base: A base underlying containers that is free from cracks and gaps and prevents penetration of leaks, spills, or precipitation.

Evidence of Hazardous Substance Migration: Chemical analyses and/or visual evidence that demonstrate hazardous substances attributable to a source have migrated away from that source into the surrounding soil, ground water, surface water, or air (e.g., leachate containing hazardous substances coming out of the source; stained or contaminated soil that can be attributed to migration from the source; evidence of the overflow from a surface impoundment containing hazardous substances).

Free Liquids: Liquids that readily separate from the solid portion of a substance under ambient temperature and pressure.

Freeboard: Vertical distance between the top of a tank or surface impoundment dike and the surface of the hazardous substance contained therein. Freeboard is intended to prevent overtopping resulting from normal or abnormal operations, wind and wave action, rainfall, and/or run-on.

Functioning Ground Water Monitoring System: A system of test wells installed around a source to detect migration of hazardous substances. In evaluating the containment factor in the ground water pathway, wells should be sampled and maintained to constitute a functioning ground water monitoring system.

Land Treatment Zone: Soil layer in the unsaturated zone of a land treatment unit within which hazardous substances are intended to be degraded, transformed, or immobilized.

Liner: A continuous barrier that covers all the earth likely to be in contact with a source so that hazardous substances or leachate containing hazardous substances would not migrate to the surrounding earth. The barrier may be synthetic material (e.g., a thick, continuous, polyethylene membrane) or engineered, compacted, natural material (e.g., re-worked and low permeability clay). An in-situ clay layer that has not been re-engineered by compaction or other methods is not considered a liner.

Maintained Engineered Cover: Vegetated cover, usually made of compacted clean soil. It is generally placed over a source at its closure and is designed and constructed to minimize the migration of liquids through the closed source, function with minimum maintenance, and accommodate settling and subsidence. Maintenance of the integrity and effectiveness of the final cover may include repairing it as necessary to correct the effects of settling, subsidence, erosion, and other events.

Secondary Containment: As used in HRS Table 3-2, secondary containment is applicable to the evaluation of the containment factor for tanks. Methods of secondary containment include a liner external to the tank, a vault, a double-walled tank, or an equivalent device.

Tank and Ancillary Equipment: Tanks and associated pipes, pumps, sumps, manifolds, fittings, flanges, and valves used to distribute, meter, or control flow of hazardous substances to or from the tank.


  1. Identify the sources at the site. HRS section 1.1 defines a source as "any area where a hazardous substance has been deposited, stored, disposed, or placed, plus those soils that have become contaminated from migration of a hazardous substance." The HRS divides sources into five categories for evaluating ground water containment: surface impoundments, land treatment, containers, tanks, and all other sources. Each category has a separate list of criteria used to assign containment values.

  1. Use HRS Table 3-2 to assign a containment value to each source. Use the definitions provided above to interpret the criteria in Table 3-2. Highlight 7-22 summarizes the types of information that generally should be collected during the SI for the purposes of evaluating the containment factor.

  1. For each source for the aquifer being evaluated, determine whether the source hazardous waste quantity value is 0.5 or greater. Only sources with a source hazardous waste quantity value of 0.5 or greater can be used to assign the containment value, unless no source for the aquifer being evaluated has a source hazardous waste quantity value of 0.5 or greater. This limitation is referred to as the "minimum size requirement". Highlight 7-23 summarizes the minimum measurements of sources that will give a source hazardous waste quantity value of 0.5. Any of the hazardous waste quantity tiers can be used to determine whether a source meets the minimum size requirement. Detailed guidance on determining hazardous waste quantity values is provided in Chapter 6.
  2. Assign a pathway containment factor value for the aquifer being evaluated.

:    Assign the highest containment value for those sources with hazardous waste quantity values greater than or equal to 0.5 as the containment factor value for the ground water pathway.

:    If none of the sources at the site for the aquifer being evaluated has a source hazardous waste quantity value of greater than or equal to 0.5, assign the highest containment value among all sources as the containment factor value for the ground water pathway.


The following types of information are helpful for evaluating the containment factor:

The physical location of the hazardous substance(s) (e.g., buried, in a below-ground tank).

Evidence of hazardous substance migration (e.g., overflow from surface impoundments).

Evidence, or lack thereof, of diking, berms, or other engineered physical barriers that completely surround the source area.

The presence of bulk and/or free liquids.

Evidence of liners that are continuous and that would prevent the source hazardous substance(s) from coming in contact with the earth beneath (or around) the source. In the case of liners, the site investigator may assume that there is not a liner unless evidence indicates otherwise.

Evidence, or lack thereof, of leachate collection systems (functioning or not), and ground water monitoring systems.

Evidence of the existence and condition of physical structures that provide protection from precipitation, and/or run-on and runoff control.

The above list is illustrative. It is meant neither to be all inclusive of the types of information that can be used to characterize the containment of any particular hazardous substance source nor to establish minimum requirements.


Tier Measure or Source Type Minimum Measurements for Hazardous Waste Quantity Value of 0.5
A Hazardous constituent quantity 0.5 pounds
B Hazardous wastestream quantity 2,500 pounds
C Volume Landfill 1,250 cubic yards
  Surface impoundment 1.25 cubic yards
  Surface impoundment (buried/backfilled) 1.25 cubic yards
  Drums 250 gallons
  Tanks and containers other than drums 1.25 cubic yards
  Contaminated soil 1,250 cubic yards
  Pile 1.25 cubic yards
  Other 1.25 cubic yards
D Area Landfill 1,700 square feet
  Surface impoundment 6.50 square feet
  Surface impoundment (buried/backfilled) 6.50 square feet
  Land treatment 135 square feet
  Pile 6.50 square feet
  Contaminated soil 17,000 square feet


Regardless of source type, if there is evidence of hazardous substance migration from the source, a containment factor value of 10 applies. Note that evidence of migration from a source does not have to meet the criteria for observed release.

Every source may not be evaluated for every aquifer, depending on the location of the source and the hydrogeology in the area of the site. Only sources for the aquifer being evaluated are used in assigning the containment factor value for that aquifer.

Only those sources that have a non-zero containment factor value for ground water should be evaluated.

The presence of a liner that extends under the entire source area is considered when evaluating containment; if the liner does not extend under the entire source area (i.e., a partial liner), the source should be evaluated as if no liner were present. The condition of the liner (e.g., damaged, torn, or leaking) would typically not be discernible during the SI.

A site may be considered to have a "natural" liner only if the clay underlying the site has been reworked to provide an engineered barrier. The mere existence of a natural clay layer or a confining layer is not sufficient. However, such a layer would be accounted for when evaluating the travel time factor.

Assign a containment factor value for only those sources with a source hazardous waste quantity value of 0.5 or more. If no source meets this minimum size requirement, select the highest containment factor value among all sources for the aquifer being evaluated as the containment factor value.

Any hazardous waste quantity tier (A, B, C, or D) can be used to determine if a source meets the minimum size requirement.


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