Determining Level of Contamination
Once targets are determined to be actually contaminated, they are further evaluated to determine whether they are subject to either Level I or Level II concentrations. The HRS does not distinguish between levels of contamination in evaluating air pathway sensitive environment target factors.
Level I concentrations can only be established in those cases where the observed release has been established by chemical analysis. Level I concentrations are established when the concentrations at the observed release sampling location meet or exceed media-specific benchmarks. The HRS specifically defines these benchmarks for each pathway.
Level II concentrations are established whenever actual contamination is present, but Level I concentration requirements have not been met, i.e.:
- targets are subject to actual contamination established by direct observations, or
- targets are subject to actual contamination established by chemical analysis but the concentrations at the observed release sampling locations(s) are less than all of the applicable, media-specific benchmarks.
The HRS lists the nine types of media-specific benchmarks to which samples are compared in establishing Level I contamination:
- Non-zero Maximum Contaminant Level Goals (MCLGs)
- Maximum Contaminant Levels (MCLs);
- Food and Drug Administration Action Levels (FDAAL) for Fish or Shellfish;
- EPA Ambient Water Quality Criteria (AWQC) for Protection of Aquatic Life;
- EPA Ambient Aquatic Life Advisory Concentrations (AALAC);
- National Ambient Air Quality Standards (NAAQS);
- National Emission Standards for Hazardous Air Pollutants (NESHAPs);
- 10-6 Cancer Risk Screening Concentrations; and
- Reference Dose Screening Concentrations.
As explained in HRS Rule, Section 2.5.2, Page 51593, many of these benchmark types are applicable to evaluations in only some of the pathways and threats. For example, MCLs apply only in evaluating the ground water migration pathway and the drinking water threat in the surface water migration pathway. Similarly, the NAAQS apply only to the air pathway. The cancer risk screening concentrations and the reference dose screening concentrations apply in all pathways (except the surface water pathway, environmental threat). The values for these benchmarks vary between pathways based on the type of exposure implicitly assumed in the pathway.
In addition, not all hazardous substances have benchmark values, although the most common substances do have values. SCDM contains the applicable benchmarks values for most hazardous substances. In general, if a substance is included in SCDM, then only the benchmarks for which values are presented in SCDM are applicable. EPA should be consulted whenever benchmark values are needed for substances not appearing in SCDM.
Many substances may have more than one media-specific benchmark as a result of overlap among the types of benchmarks. If more than one media-specific benchmark is available for a hazardous substance in a pathway, the lowest benchmark value is used in the evaluation. For example, three benchmark values are provided in SCDM for evaluating the level of contamination for drinking water targets in the ground water pathway:
Non-zero MCLG: 50 ug/l,
10-6 Cancer Risk Screening Concentration: 0.02 ug/l, and
- Reference Dose Screening Concentration: 10 ug/l.
The lowest of the three concentrations (0.02 ug/l) is the benchmark concentration used in assessing the level of contamination associated with drinking water targets in the ground water and surface water pathways.
The graphic above illustrates the determination of actual contamination and the level of contamination associated with three soil samples attributable to a site. The background level for the hypothetical substance is established at 25 mg/kg, which is above the detection limit. Therefore, any sample with a concentration exceeding 75 mg/kg is considered as demonstrating an observed release. The benchmark concentrations is 150 mg/kg. This leads to the following classification:
- Level I Concentrations: > = 150 mg/kg,
- Level II Concentrations: 75 to < 150 mg/kg, and
- Potential Contamination: < 75 mg/kg.
Accordingly, Sample A is subject to potential contamination; Sample B is subject to actual contamination, Level II concentrations; and Sample C is subject to actual contamination, Level I concentrations.
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