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Areas of Observed Contamination

  • The areas of observed contamination are established by a three-step process:
    1. Establish points of observed contamination by sampling.
    2. Establish areas associated with these points.
    3. Delete sub-areas as required.

1.    Sampling locations document points of observed contamination:

HRS rule, page 51646 section 5.0.1, first bullet, left column

  • The sampling points will be associated with HRS sources (landfill, impoundment, contaminated soil...) but document a point of observed contamination only if hazardous substances are found at observed release criteria within the top two feet, if not at the surface.
  • When soil is sampled, care must be taken to assure an adequate and appropriate background level.

    Guidance for Performing Site Inspections Under CERCLA (540-R-92-021), page 76

  • The HRS Guidance Manual suggests what might constitute adequate background samples or level for comparison.

    Targets Matrix

  • Points of observed contamination cannot be documented without sampling. No samples, no soil exposure pathway.
  • The "covered by 2 feet or less of cover material" is meant to be protective of normal homeowner activities.
  • Do not composite samples over two feet in depth for this pathway. Because the areas of observed contamination will be overlain by property boundaries, it is also preferable not to composite samples across location.

2a.    If a sample that documents observed contamination is taken from a source other than contaminated soil, the entire source is considered to be an area of observed contamination.

HRS rule, page 51646, second bullet, first item under bullet

  • Sample, for instance, at a point in a landfill where cover is inadequate or lacking.
  • Only one "hit" is required to document the entire source as an area of observed contamination.

    Targets Matrix

2b.    If samples that document observed contamination were taken from contaminated soil, consider the areas lying between the sampling points to be an area of observed contamination "unless available information indicates otherwise." Examples:

  • If the deposition was from stack emissions or wind dispersion, treat the entire area between the outermost samples as an area of observed contamination, even if some samples within that area failed to meet the observed release criteria.

    Targets Matrix

  • If the deposition was from runoff or flooding, consider topography in establishing the area of observed contamination.
  • If clean fill has been brought in such that a portion of the area under consideration is now covered by more than 2 feet of clean cover, do not consider that area.
  • If deposition involved different mechanisms or events, be wary of connecting the areas.

    Targets Matrix

For an exercise, press here Graphic A

QUESTION:    Can surface water sediment samples be considered soil samples in the soil exposure pathway? ANSWER

3.    After the area of observed contamination has been established, delete sub-areas that are "covered by a permanent, or otherwise maintained, essentially impenetrable material (for example, asphalt) that is not more than 2 feet thick."

  • What observations should be recorded at the site inspection? ANSWER
  • What if the asphalt cover is cracked and pitted? ANSWER
  • Can a tank which is shown by sampling to contain a hazardous substance be considered to be an area of observed contamination? ANSWER

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