Washington Administrative Code › Title 173 - Ecology, Department of (See also Titles 197, 317, 372, and 508) › Chapter 173-340 - Model Toxics Control Act-Cleanup › Part 3 - SITE REPORTS AND CLEANUP DECISIONS › Section 173-340-357 - Quantitative risk assessment of cleanup action alternatives

Current through Register Vol. 24-14, July 15, 2024
(1)
Purpose.
A cleanup action must protect human health and the
				environment, including likely vulnerable populations and overburdened
				communities (see WAC
				173-340-360(3)(a)(i))
				. A quantitative site-specific risk assessment may be conducted to help
				determine whether cleanup action alternatives, including those relying on
				engineered or institutional controls to limit exposure to contamination
				remaining at a site, protect human health and the environment. Other methods
				may be used in addition to, or instead of, a quantitative site-specific risk
				assessment to determine whether a cleanup action alternative is
				protective.
(2)
Human health
				risk assessment.
A quantitative site-specific human health risk
				assessment may be conducted to help determine whether cleanup action
				alternatives, including those relying on engineered or institutional controls
				to limit exposure, protect human health. This subsection defines the framework
				for assessing cleanup action alternatives relying on engineered or
				institutional controls to limit exposure. References to Method C in this
				subsection apply to an environmental medium only if the medium for which a
				remediation level is being established qualifies for a Method C cleanup level
				under WAC
				173-340-706.
(a)
Reasonable maximum exposure.
Standard reasonable maximum exposures and corresponding Method B and C
				equations in WAC
				173-340-720 through
				173-340-750 may be modified as
				provided under WAC
				173-340-708(3)(d).
				For example, land uses other than residential and industrial may be used as the
				basis for an alternative reasonable maximum exposure scenario for the purpose
				of assessing the protectiveness of a cleanup action alternative that relies on
				engineered or institutional controls (such as containment) to limit exposure to
				contaminated soil.
(b)
Exposure parameters.
Exposure parameters for the standard Method B
				and C equations in WAC
				173-340-720 through
				173-340-750 may be modified as
				provided in WAC
				173-340-708(10).
(c)
Acceptable risk level.
The
				acceptable risk level used to establish a remediation level for a hazardous
				substance must be the same as that used to establish the cleanup level for the
				substance.
(d)
Soil to
				groundwater pathway.
The methods specified in WAC
				173-340-747 to develop soil
				concentrations that are protective of groundwater beneficial uses may also be
				used to help assess whether a cleanup action alternative that relies on
				engineered or institutional controls (such as containment) will protect
				groundwater.
(e)
Burden of
				proof, new science, and quality of information.
Any modification of the
				default assumptions in the standard Method B and C equations, including
				modification of the standard reasonable maximum exposures and exposure
				parameters, or any modification of default assumptions or methods specified in
				WAC 173-340-747 requires compliance
				with WAC 173-340-702(14), (15) and
				(16).
(f)
Commercial gas station
				scenario.
At active commercial gas stations, where there are retail
				sales of gasoline or diesel, one of the following may be done to demonstrate
				when a cap is protective of the soil ingestion and dermal pathways:
(i) Equations 740-3 and 740-5 may be modified
				by reducing the exposure frequency to 0.25. This exposure frequency is intended
				to be a conservative estimate of a child trespasser scenario at a commercial
				gas station where contaminated soil has been excavated and stockpiled or soil
				is otherwise accessible. To rely on this exposure frequency:
(A) The cleanup action must include
				institutional controls that prevent uses that could result in a higher level of
				exposure; and
(B) Other exposure
				pathways (e.g., soil vapors and soil to groundwater) must be assessed to
				determine whether they are protective; or
(ii) Equations 740-3 and 740-5 may be
				modified on a site-specific basis as described in WAC
				173-340-740(3)(c).