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IDEM program areas make this determination, using project milestones described on pages 10 and 11 of the Remediation Program Guide [PDF]. For projects that have not met program-specific milestones, IDEM will use the most current guidance when evaluating submittals. Where applicable, IDEM will honor its pre-existing commitments regarding use of particular guidance or remediation objectives.
When IDEM updates its guidance (including screening level tables), it will provide advance notice of the revisions or updates, and when appropriate, define a transition period of 6 months that will begin on the effective date of the new guidance. IDEM will consider both the guidance that is being replaced and the new guidance to be in effect during the transition period.
Parties may choose to update to newer guidance at any time. However, if a party has an approved work plan and proposes new site-specific remediation objectives, the scientific methodology and any inputs used to evaluate risk or to establish those new remediation objectives should be current at the time they are presented to IDEM for approval.
If a party does not complete a remediation project consistent with the terms and conditions of the Remediation Work Plan (or an existing agreement), or if a hazardous substance or petroleum contaminant becomes an imminent and substantial threat to human health or the environment, then IDEM may withdraw its prior approval of the work plan.
IDEM anticipates updating the screening levels table as early as possible every calendar year. Screening level changes made by the EPA throughout the year can be proposed to IDEM on a site-specific basis, but won’t be incorporated into the IDEM table until the January update.
Demonstrating that chemical concentrations meet numeric remediation objectives is one option for sites in IDEM’s remediation programs. It is also acceptable to demonstrate that there are no complete exposure pathways, even for chemicals that exceed numeric remediation objectives. Acceptable implementation of the latter approach will generally involve engineering or institutional controls, or both.
Section 1.3 of the Remediation Closure Guide provides more information about remediation objectives.
Effective with the March, 2012 implementation of the Remediation Closure Guide (RCG), IDEM no longer publishes closure or screening levels for TPH. Important reasons for this include the lack of a petroleum-specific analytical method, the lack of toxicological data for various petroleum mixtures, and the highly variable nature of petroleum products both when sold and as they degrade over time. Instead, IDEM evaluates risk arising from individual chemicals that occur in petroleum products. More information on IDEM’s position is available in a Status Update memorandum dated December 9, 2013.
Remediation sites following older guidance may still choose to include TPH as an investigative parameter. In such cases, IDEM will follow its transition policy to determine the appropriate guidance for evaluating the site.
Yes, in some circumstances. It is important that the sample distribution reasonably represent the entire exposure area. Note that because sampling from a known or suspected source area is a common means of site characterization, it is likely that such judgmentally collected samples are from areas of relatively high concentration, and may overestimate exposure risk.
How do I determine an appropriate exposure area for calculating an upper confidence limit of the mean (UCL) for soil exposure? When calculating a UCL for soil direct contact exposure, it may be appropriate to assume that receptors move randomly around a site and undergo equivalent exposure to soils all over the site. If it is not reasonable to assume random movement, or equal exposure to all areas of a site, divide the site into separate exposure areas and calculate UCLs for each subdivision. For example, children at a city park may spend more time in the playground area than in an adjacent open field. In that case, it would be appropriate to calculate a separate UCL for each area, and not combine the data. Evaluate each area separately to reflect the different exposures.
IDEM’s screening level table includes soil direct contact values for several hundred chemicals, including VOCs. However, several forces (volatilization, leaching, biodegradation, etc.) act quickly to attenuate VOC concentrations in soils at or very near the ground surface. This is why IDEM guidance calls for collecting and preserving VOC soil samples within minutes of bringing soil boring cores to the surface. Given that IDEM’s soil direct contact screening levels assume twenty-five or thirty years of exposure, the screening levels in our tables greatly overestimate risk from soil direct contact to VOCs, and IDEM risk evaluation staff will take this into consideration when evaluating risk from exposure to VOCs in soils.
Most IDEM remediation programs specify unfiltered ground water samples, unless turbidity cannot be reduced to less than ten nephelometric turbidity units (NTUs). Methods for reducing turbidity include proper well construction, well development, and sampling technique (e.g., low-flow sampling). If IDEM Science Services staff agree that turbidity remains above ten NTUs despite implementation of the measures above, the samples may be filtered, and the results compared to screening levels or used in a risk assessment. Responsible parties may also use filtered samples in conjunction with unfiltered samples as a line of evidence that the unfiltered sample data overestimates the chemical concentration that would be present in a properly constructed drinking water well. Note that the RCRA program generally specifies filtering of ground water samples before analysis.
A municipal water supply and private water wells may both occur in the same area. Some residents or other users may keep pre-existing wells, or choose to install wells for various purposes, even when a municipal supply exists. Therefore, it is important to confirm the presence or absence of private wells in areas over which a plume extends or is likely to extend, whether or not a municipal supply exists.
An adequate demonstration will likely involve several approaches. The DNR Water Well Record Database, while useful, is incomplete and therefore not by itself sufficient to show that exposure is not occurring. Local government officials and the water utility may be able to provide additional information about the existence of private wells. In some cases, a door-to-door survey of residents and other water users may be necessary.
Contamination within a WHPA increases the probability of human exposure. Remedy selection for ground water contamination within a WHPA will be a site-specific decision that should involve IDEM and local stakeholders, such as drinking water providers. Appropriate remedies will address risk to receptors and risks to the ground water resource.
Yes, given at least eight quarters of data from each relevant monitoring well. A UCL can show that ground water chemical concentrations are less than remediation objectives throughout the plume. Section 9.6 of the Remediation Closure Guide provides more information about evaluating ground water exposure risk.
They can be. Migration-to-ground water (MTGW) screening levels are modeled soil concentrations that, if exceeded, indicate a potential for soil to leach chemicals to ground water, resulting in ground water concentrations that exceed screening levels. IDEM strongly prefers to make decisions based on observed concentrations rather than modeled concentrations. Therefore, in most cases, IDEM will expect direct measurement of ground water concentrations as part of a site investigation. In some situations, such as a very new release, or a soil source under a building or parking lot where infiltration may be limited, current ground water conditions may not reflect potential increases in ground water concentrations due to leaching after removal of the building or parking lot. In such cases, MTGW screening levels can provide insight into leaching potential or serve as remediation objectives. The synthetic precipitation leaching procedure is an alternative approach to evaluating the leaching potential of chemicals in soils.
No. VIGWSLs are trigger levels that, if exceeded, indicate a need to evaluate vapor intrusion (VI) at structures or properties within specified distances of the exceedance. Section 5.4.2 (chlorinated chemicals) and 5.4.3 (petroleum chemicals) of the Remediation Closure Guide describe procedures for evaluating VI based observed ground water concentrations relative to VIGWSLs and distance to potential receptors. Typically, if additional VI evaluation in accordance with IDEM guidance shows that no further action is warranted, VIGWSLs become irrelevant.
Congress established OSHA to protect workers from hazards originating in the workplace. OSHA Permissible Exposure Limits apply to chemicals routinely used in the workplace. IDEM’s vapor-related screening levels apply to chemicals that originate from a subsurface source and are not routinely used in the workplace.
Benzene is usually the principal risk driver at gasoline stations, and OSHA exempts most active gasoline stations from meeting benzene standards. In addition, ambient benzene concentrations in air at active gasoline stations usually dwarf any concentrations that may move into station buildings from the subsurface. For these reasons, IDEM does not anticipate routinely requesting vapor intrusion investigations in structures on active gasoline station sites. However, we will still generally expect vapor intrusion investigations at potentially affected off-site structures or, where appropriate, at structures on former gasoline station sites.