Risk Assessment

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Arkansas has not implemented a single set of soil clean-up levels for general usage. Instead the state uses standards set in Regulation 23 for the management of remediation and related wastes, usually arriving at a site-specific standard for each clean-up.

The answer to "How clean is clean?" begins with one of two decisions: Will the waste be treated in situ and left in place, or will it be removed and treated somewhere else?

In what is typically referred to as the area of contamination (AOC) policy, ADEQ and EPA interpret the Resource Conservation and Recovery Act (RCRA) to allow certain discrete areas of generally dispersed contamination to be considered as RCRA units (usually landfills). Because an AOC is equated to a RCRA land-based unit, consolidation and in situ treatment of hazardous waste inside the boundaries of the AOC does not create a new point of hazardous waste generation.

This interpretation allows wastes to be consolidated or treated in situ within an AOC without triggering either the Land Disposal Restrictions (LDR) or minimum technology requirements (MTR) found in Regulation 23 Section 268. The AOC interpretation may be applied to any hazardous remediation waste (including non-media wastes) that is in or on the land. Note that the AOC policy only covers consolidation and other in situ waste management techniques carried out inside an AOC. Ex situ waste management or the transfer of wastes from one area of contamination to another triggers the land disposal restrictions, and the removed waste is subject to the appropriate treatment standards for all contaminants subject to treatment under Section 268.

The AOC policy is further explained in EPA's National Oil and Hazardous Substances Pollution Contingency Plan (NCP). See 53 FR 51444 for detailed discussion in EPA's proposed NCP preamble; and 55 FR 8758-8760, March 8, 1990, for final NCP preamble discussion. See also the March 13, 1996, EPA memo, "Use of the Area of Contamination Concept During RCRA Cleanups."

An extended application of the AOC policy is the Corrective Action Management Unit (CAMU) described in Regulation 23 Section 264 Subsection S. Implementation of a CAMU must be done under the provisions of a final RCRA permit or Remedial Action Plan (RAP).

Since waste treated in situ is left in place, the amount of contamination remaining must meet the criteria of "acceptable risk." ADEQ uses EPA Region VI's Human Health Media-Specific Screening Levels (HHMSSLs) as a point of departure to determine whether a potential health or environmental problem exists. If contaminant levels exceed the applicable HHMSSL, a formal risk assessment is performed to determine a site-specific clean-up level, or whether additional controls must be put in place.

Site-specific clean-up standards established through site specific, risk-based minimized threat variances should be within the range of values that ADEQ and EPA generally find acceptable for risk-based cleanup levels. That is, for carcinogens, these should ensure constituent concentrations that result in the total excess risk to an individual exposed over a lifetime generally falling within a range from 10-4 to 10-6, using 10-6 as a point of departure and with a preference for achieving the more protective end of the risk range.

For non-carcinogenic effects, standards should ensure constituent concentrations that an individual could be exposed to on a daily basis without appreciable risk of deleterious effect during a lifetime; in general, the hazard index should not exceed one (1). Constituent concentrations that achieve these levels should be calculated based on a reasonable maximum exposure scenario -- that is, based on an analysis of both the current and reasonable expected future land uses, with exposure parameters chosen based on a reasonable assessment of the maximum exposure that might occur; however, alternative LDR treatment standards (§268.44) may not be based on consideration of post-land disposal controls such as caps or other barriers.

Removal of waste or contaminated media outside the AOC for treatment or disposal triggers the applicable LDR treatment standard for all contaminants subject to treatment under Reg. 23 Section 268. With the exception of characteristic wastes that have been treated so as to eliminate the hazardous characteristic, wastes and contaminated media that have been treated to meet the LDR standards must still be placed in a permitted or interim status Subtitle C, hazardous waste landfill.

Contaminated environmental media, of itself, is not hazardous waste and, generally, is not subject to regulation under RCRA. Contaminated environmental media can become subject to regulation under RCRA if they "contain" hazardous waste. ADEQ and EPA generally consider contaminated environmental media to contain hazardous waste: (1) when they exhibit a characteristic of hazardous waste; or, (2) when they are contaminated with concentrations of hazardous constituents from listed hazardous waste that are above health-based levels.

If contaminated environmental media contain hazardous waste, they are subject to all applicable RCRA requirements until they no longer contain hazardous waste. Contaminated environmental media are considered to no longer contain hazardous waste:

  1. when they no longer exhibit a characteristic of hazardous waste; and
  2. when concentrations of hazardous constituents from listed hazardous wastes are below health-based levels.

Generally, contaminated environmental media that do not (or no longer) contain hazardous waste are not subject to any RCRA requirements; however, as discussed below, in some circumstances, contaminated environmental media that contained hazardous waste when first generated (i.e., first removed from the land, or area of contamination) remain subject to LDR treatment requirements even after they "no longer contain" hazardous waste.

The determination that any given volume of contaminated media does not contain hazardous waste is called a "contained-in determination". In the case of media that exhibit a characteristic of hazardous waste, the media are considered to "contain" hazardous waste for as long as they exhibit a characteristic. Once the characteristic is eliminated (e.g., through treatment), the media are no longer considered to "contain" hazardous waste. Since this determination can be made through relatively straightforward analytical testing, no formal "contained-in" determination by the Department is required. Just like determinations about whether waste has been adequately decharacterized, generators of contaminated media may make independent determinations as to whether the media exhibit a characteristic of hazardous waste. In the case of media that are contaminated by listed hazardous waste, current EPA guidance recommends that contained-in determinations be made based on direct exposure using a reasonable maximum exposure scenario and that conservative, health-based, standards be used to develop the site-specific health-based levels of hazardous constituents below which contaminated environmental media would be considered to no longer contain hazardous waste. Since this determination involves development of site-specific health-based levels, the approval of the Department is required.

In certain circumstances the, RCRA land disposal restrictions will continue to apply to contaminated media that has been determined not to contain hazardous waste. This is the case when contaminated media contain hazardous waste when they are first generated (i.e., removed from the land, or area of contamination) and are subsequently determined to no longer contain hazardous waste (e.g., after treatment), but still contain hazardous constituents at concentrations above land disposal restriction treatment standards. It is also the case when media are contaminated as a result of disposal of untreated (or insufficiently treated) listed hazardous waste after the effective date of an applicable LDR treatment requirement. Of course, if no land disposal will occur (e.g., the media will be legitimately recycled) the LDR treatment standards do not apply. In addition, contaminated environmental media determined not to contain any waste (i.e., it is just media, it does not contain solid or hazardous waste) would not be subject to any RCRA Subtitle C requirements, including the LDRs, regardless of the time of the "contained-in" determination.

The contained-in policy was first articulated in a November 13, 1986 EPA memorandum, "RCRA Regulatory Status of Contaminated Groundwater". It has been updated many times in Federal Register preambles, EPA memos and correspondence, see, e.g., 53 FR 31138, 31142, 31148 (Aug. 17, 1988), 57 FR 21450, 21453 (May 20, 1992), and detailed discussion in HWIR-Media proposal preamble, 61 FR 18795 (April 29, 1996). A detailed discussion of the continuing requirement that some soils which have been determined to no longer contain hazardous waste (but still contain solid waste) comply with land disposal treatment standards can be found in the HWIR-Media proposal preamble, 61 FR 18804; the September 15, 1996 letter from Michael Shapiro (EPA OSW Director) to Peter C. Wright (Monsanto Company); and the preamble to the LDR Phase IV rule, 63 FR 28617 (May 26, 1998).

Note that the contained-in policy applies only to environmental media (soil, ground water, surface water and sediments) and debris. The contained-in policy for environmental media has not been codified, however the contained-in policy for hazardous debris was codified in 1992 (Reg. 23§ 268.45).

Arkansas has adopted and implemented the new federal land disposal restriction treatment standards specific to contaminated soils. These treatment standards require that contaminated soils which will be land disposed be treated to reduce concentrations of hazardous constituents by 90 percent, or meet hazardous constituent concentrations that are ten times the universal treatment standards (UTS), whichever is greater. (This is typically referred to as "90% capped by 10xUTS.") For contaminated soil that exhibits a characteristic of ignitable, reactive or corrosive hazardous waste, treatment must also eliminate the hazardous characteristic.

The soil treatment standards apply to all underlying hazardous constituents reasonably expected to be present in any given volume of contaminated soil when such constituents are found at initial concentrations greater than ten times the UTS. For soil that exhibits a characteristic of toxic, ignitable, reactive or corrosive hazardous waste, treatment is also required for: (1) in the case of the toxicity characteristic, the characteristic constituent; and, (2) in the case of ignitability, reactivity or corrosivity, the characteristic property. Although treatment is required for each underlying hazardous constituent, it is not necessary to monitor soil for the entire list of underlying hazardous constituents. Generators of contaminated soil can reasonably apply knowledge of the likely contaminants present and use that knowledge to select appropriate underlying hazardous constituents, or classes of constituents, for monitoring. As with the LDR treatment standards for hazardous debris (discussed below), generators of contaminated soil may use either the applicable universal treatment standards for the contaminating hazardous waste or the soil treatment standards. (Reg. 23 §268.49).

As with treatment in an AOC, any level of residual contamination left in place after the bulk of the waste has been removed should be within the range of values that ADEQ and EPA generally find acceptable for risk-based cleanup levels.

A Screening Risk Assessment utilizes predetermined human health and/or ecological screening values to determine if a more detailed-site risk assessment is needed before adequate risk management decisions can be made.

A Baseline risk assessment describes the source of contamination, how the contamination reaches people and the environment, the amount of contamination to which people or the ecological environment may be exposed, and the health or ecological effects that could result from exposure.

If no remedial action is planned and contaminant levels are above the applicable screening levels or at levels that may pose a risk to human health or the environment a site-specific Human Health and Ecological Risk Assessment should be conducted.

The most current EPA Regions 3, 6, and 9 Regional Screening Levels.

Note: These screening levels are available for viewing or downloading under the EPA Regions 3,6, and 9 Regional Screening Levels for Chemical Contaminants at Superfund Sites.

Risk Assessment Guidance for Superfund (RAGS): Vol. I B Human Health Evaluation Manual, RAGS Vol. I, Part A, Part B, Part C, and Part D can be viewed or downloaded.