The Environmental Protection Agency (EPA) has released and is seeking comments on draft general guidance on methods of addressing indoor vapor intrusion (VI) from contaminated soil and groundwater plumes (Draft VI Guidance) (http://www.epa.gov/oswer/vaporintrusion/documents/vaporIntrusion-final-guidance-20130411-reviewdraft.pdf) and a companion draft guidance focused particularly on petroleum hydrocarbon contaminants, such as gasoline, diesel and jet fuel (http://www.epa.gov/oust/cat/pvi/petroleum-vapor-intrusion-review-draft-04092013.pdf). The Draft VI Guidance updates the EPA draft vapor intrusion guidance issued in 2002, which never was finalized.
The Draft VI Guidance is long and complex, presenting a step-by-step assessment process beginning with assembling and evaluating the data necessary to adopt an initial conceptual site model, through the gathering of additional data and evaluating the multiple lines of evidence culminating in a risk assessment. Several key elements of the Draft VI Guidance may need to be clarified and the cost to complete the process, as proposed, is likely to be high. The process described in the Guidance involves the exercise of considerable discretion by the regulators and the outcome of applying the Guidance to a particular site is uncertain. For example, one of the key touchstones throughout the process is the determination that sufficient data of appropriate quality exists to support decisions based on current and future site conditions. This gives a lot of leeway to EPA and other regulators who may use this guidance.
The risk management goal for determining that no remedial action is warranted remains a finding that the lifetime cancer risk (based on a reasonable maximum exposure condition) is 1 in 10,000 or less, and the non-cancer risk presents a hazard index of less than 1, although Draft VI Guidance states that a risk manager may decide that a risk level of less than 1 in 10,000 is unacceptable and that response is warranted without further explanation concerning how the traditional remedy selection criteria apply in this situation. Screening levels (i.e., levels that allow for early determination of a remedy) are set at a 1 in 1 million risk level. EPA reserves the right to consider taking action based on short-term and acute exposures.
However, the guidance does not define the most controversial issue in setting short-term limits, the time over which one calculates the average of the indoor air concentrations. Some EPA regional offices have adopted differing approaches to setting the period over which the indoor vapor air concentration should be averaged (e.g., hours, days, weeks or longer) and discussions at recent scientific symposia indicate that there is no scientifically accepted methodology governing how to apply these short-term noncancer limits.
The Draft VI Guidance specifies default medium-specific attenuation factors for residential buildings of 0.001 for the generic vadose zone, 0.005 for the grained vadose zone, 0.03 for the sub-slab soil gas, 0.03 for the near-source exterior soil gas, and 1.0 for the crawl space, based on the 95th percentile attenuation factors determined in EPA site-specific studies (except for the near source exterior soil gas attenuation factor). These values are very health protective, i.e., according to EPA, they have a less than 2 percent probability of resulting in a false negative.
The Draft VI Guidance encourages preemptive mitigation (or early action), i.e., where a party wishes to implement mitigation or control measures for vapor intrusion, even though only limited lines of evidence or measurements may be available to characterize the overall vapor intrusion pathway. For example, when vapor intrusion has been documented at neighboring structures, a party may find a reasonable basis to take action, even though each building presents a fact-specific situation that calls for its own individual judgment. Similarly, a preemptive approach may be taken because it is more cost-effective to design, install, operate and monitor engineered exposure controls to mitigate vapor intrusion in newly constructed buildings, or in buildings to be constructed in the future, that are in areas of vapor-forming subsurface contamination, rather than potentially allow vapor intrusion to occur later and assess it after the fact. Again, details on whether and how the adoption of the preemptive approach is determined through the application of the traditional remedy selection criteria is absent. The Draft VI Guidance states that for residential buildings, EPA may consider reviewing these decisions if the land use changes, or new information becomes available that suggests circumstances supporting past risk management decisions have changed, prompting the need to revisit those decisions.
The Draft VI Guidance contains extensive direction concerning obtaining public comment and implementing institutional controls at individual sites. The Draft recommends that regulators be notified of significant changes in building ownership, uses, access by the general public, or building construction (e.g., renovations), which may affect its risk management decisions pertaining to potential vapor intrusion assessment and mitigation, subsurface remediation, or institutional controls. The Draft Guidance also recommends that remediation options be fairly presented to occupants of buildings and that the occupant’s preference be addressed in an agreement between occupants and the property owner. However, it has long been recognized that the environmental statutes do not provide a building occupant any legally enforceable right to veto a remedy agreed upon by the potentially responsible party and the regulator (see preamble to the National Contingency Plan). Nothing in this Draft VI Guidance describes how to resolve disputes between an occupant’s preference and the property owner’s right to decide how best to implement the remedy.
The Draft VI Guidance includes many more details. Overall, EPA has proposed a relatively wide-ranging and aggressive approach and is keeping its options open on some key issues. Comments on the guidance will be accepted through May 24 at http://www.regulations.gov under Docket ID No. EPA-HQ-RCRA-2002-0033-0090. State (and, as appropriate, other federal agency) VI guidance also should continue to be consulted.