Indiana Vapor Intrusion Evaluation - Screening, Sampling, and Response Under the RCG
How vapor intrusion is evaluated at Indiana cleanup sites. Published Level screening, indoor air and soil gas sampling, and response actions.
Overview
Vapor intrusion (VI) is the migration of volatile chemicals from contaminated soil or groundwater into overlying buildings through cracks, joints, and openings in foundations. In Indiana, VI evaluation is integrated into the Risk-based Closure Guide (WASTE-0046-R2) framework and applies to all IDEM cleanup programs.
Indiana’s approach to vapor intrusion is notably different from Ohio’s. Indiana publishes screening levels for indoor air and soil gas across 154 chemicals, three sample types (subslab, deep exterior, shallow exterior), and three land uses (residential, commercial/industrial, excavation). This means most VI evaluations in Indiana start with a direct comparison of sampling data to Published Levels rather than requiring the consultant to run fate-and-transport models or the EPA VISL calculator.
The VI evaluation is part of the R2 characterization phase (Tasks 1-3) and the risk evaluation phase (Tasks 4-7). Vapor is treated as another environmental medium alongside soil and groundwater - the same framework of characterization, delineation, risk comparison, and remedy selection applies.
When VI Evaluation Is Required
A vapor intrusion evaluation is required when volatile chemicals are present in soil or groundwater at concentrations that could result in indoor air exposures above health-based levels. The R2 requires VI evaluation when:
- Volatile organic compounds (VOCs) are detected in soil or groundwater above applicable Published Levels
- Buildings or occupied structures exist (or are planned) above or near known soil or groundwater contamination containing volatile chemicals
- Subsurface conditions (shallow groundwater, permeable soils, preferential pathways such as utility corridors) suggest a vapor migration pathway to occupied structures
The key question is whether there is a complete or potentially complete exposure pathway from the subsurface source to indoor air in an occupied building. If volatile contamination exists but no buildings are present and none are anticipated, the VI pathway may be incomplete - but this must be documented in the conceptual site model.
The R2 does not prescribe a fixed distance threshold for triggering VI evaluation. The determination is based on the conceptual site model - whether volatile chemicals are present in the subsurface at concentrations and locations where a vapor migration pathway to an occupied building could be complete.
Screening with Published Levels
Indiana publishes screening levels for three categories of vapor-related media:
Indoor Air Published Levels
Indoor air Published Levels are health-based concentrations for volatile chemicals measured in the breathing zone of occupied buildings. They are available for residential and commercial/industrial land use scenarios.
- Residential levels assume continuous exposure (24 hours/day, 350 days/year) for both adults and children
- Commercial/industrial levels assume worker exposure (8 hours/day, 250 days/year) for adults only
If indoor air sampling results are below the applicable Published Levels, the VI pathway does not pose unacceptable risk for that land use.
See the Indiana Indoor Air Published Levels - VOCs page for current values.
Soil Gas Published Levels
Indiana publishes soil gas screening levels for three sample types:
Subslab soil gas - collected from beneath the building foundation, representing the vapor concentrations immediately below the structure. This is the most direct measurement of what could enter the building.
Deep exterior soil gas - collected outside the building footprint at depths of 5 feet or more below ground surface. The 5-foot threshold is the standard dividing line between “deep” and “shallow” exterior soil gas - at shallower depths, ambient air dilution can affect results. Deep exterior probes represent vapor concentrations in the vadose zone below the zone of atmospheric influence.
Shallow exterior soil gas - collected outside the building footprint at depths less than 5 feet below ground surface. Shallow exterior probes are more susceptible to atmospheric influence but can be useful for evaluating vapor concentrations closer to the surface.
Each sample type has its own set of Published Levels because the attenuation factor between the sample point and indoor air differs. Subslab levels are the most conservative (highest attenuation factor - closest to the building), and exterior soil gas levels are less conservative (lower attenuation factor - more dilution expected between the sample point and indoor air).
See the Indiana Soil Gas Screening Levels page for current values.
No Default Migration-to-Indoor-Air Pathway for Groundwater
Unlike some states, Indiana does not publish default soil-gas-to-indoor-air or groundwater-to-indoor-air screening levels. The evaluation of whether groundwater contamination could cause a VI problem is handled site-specifically through the R2 framework. If volatile chemicals are present in groundwater, the consultant evaluates the VI pathway using site-specific conditions (depth to groundwater, soil type, building construction, etc.) rather than comparing to a lookup table.
This is a significant practical difference from Ohio, which publishes indoor air standards through CIDARS that can be compared directly to groundwater data using vapor intrusion attenuation factors.
Building Assessment
Before collecting indoor air or soil gas samples, assess the building conditions that affect vapor intrusion potential:
Foundation type and condition. Slab-on-grade foundations have different VI characteristics than buildings with basements or crawl spaces. Cracks, joints, pipe penetrations, and sump pits are preferential pathways for vapor entry. Document the foundation type and any observed openings.
HVAC systems. Building ventilation affects indoor air concentrations. Forced-air systems can create negative pressure that draws vapors into the building. Document the HVAC type, operation schedule, and any sub-slab ventilation or depressurization systems already in place.
Current and historical building use. Indoor sources of VOCs (cleaning solvents, stored chemicals, recent renovation materials, attached garages) can confound indoor air sampling results. Document potential indoor sources to distinguish them from subsurface VI contributions.
Utility penetrations. Sewer lines, water lines, and other utilities that penetrate the foundation can serve as preferential pathways for vapor migration. Document utility entry points and their condition.
IDEM provides household chemical product lists for common VOCs (benzene, TCE, PCE, and others) on the Technical Guidance for Cleanups page. IDEM recommends sharing these lists with building occupants before indoor air sampling to help identify potential indoor sources. These lists can be incorporated into the investigator’s indoor air building survey checklist.
Sampling Methods
Indoor Air Sampling
Indoor air samples are collected using evacuated canisters (typically Summa canisters or equivalent) with calibrated flow controllers typically set for a 24-hour sampling period per standard EPA guidance. Samples are analyzed by EPA Method TO-15 or TO-15 SIM for target VOCs.
Key considerations for indoor air sampling in Indiana:
- Collect paired outdoor air samples to distinguish ambient contributions from VI
- Document potential indoor VOC sources during sampling
- Sample during building operating conditions representative of typical occupancy
- Heating season sampling is preferred per EPA guidance because closed-building conditions and HVAC operation create worst-case conditions for vapor entry. Heavy precipitation (one-half inch or more) in the 24 to 48 hours before sampling can reduce volatile concentrations in soil gas - postpone sampling at least 24 hours after a major rain event
- Close windows and doors during the sampling period per EPA 2015 VI Technical Guide building preparation requirements
Soil Gas Sampling
Soil gas samples are collected using temporary or permanent soil gas probes installed at appropriate depths. Samples are collected in Summa canisters or equivalent and analyzed by EPA Method TO-15.
Subslab sampling requires coring through the building foundation and installing a sampling port with an airtight seal. Deep and shallow exterior probes are installed outside the building footprint using direct-push or hand-driven methods.
Key considerations:
- Purge the sample tubing and probe before collecting the sample to remove ambient air
- Conduct leak testing (helium shroud or similar) to verify the sample is representative of subsurface vapors and not diluted by ambient air leakage
- Allow installed probes to equilibrate before sampling. Cemented subslab and exterior soil gas points generally require at least overnight equilibration before sampling
- Collect samples at locations and depths specified by the soil gas Published Level categories (subslab, deep exterior, shallow exterior)
Passive Soil Gas Sampling
Passive soil gas samplers (such as Waterloo Membrane Samplers or GORE modules) are used at some sites as a screening tool before committing to active soil gas sampling. Passive samplers provide mass flux estimates rather than concentration data, so they are not directly comparable to Published Levels.
Passive samplers provide mass flux estimates rather than concentration data, so results are not directly comparable to Indiana’s Published Levels. Confirm with the IDEM project manager whether passive results will be accepted as a screening tool for a specific site before relying on them for regulatory decisions.
Data Evaluation
Comparison to Published Levels
The primary evaluation step is comparing analytical results to the applicable Published Levels for the relevant media, sample type, and land use:
- Compare indoor air results to indoor air Published Levels (residential or commercial/industrial)
- Compare subslab soil gas results to subslab Published Levels
- Compare exterior soil gas results to the appropriate deep or shallow Published Levels
If all results are below the applicable Published Levels, the VI pathway does not pose unacceptable risk and no further VI action is needed. Document this conclusion in the risk evaluation report.
If any results exceed Published Levels, additional evaluation is needed - this could include additional sampling, source investigation, or proceeding to remedy selection.
Background Indoor Air Evaluation
Indoor VOC concentrations may reflect indoor sources rather than subsurface VI. Comparing indoor air results to outdoor air samples helps identify ambient contributions. If indoor concentrations are comparable to outdoor concentrations, subsurface VI is unlikely to be the source.
Common indoor VOC sources that can confound VI sampling include dry-cleaned clothing, paints and solvents, gasoline storage in attached garages, cleaning products, and new building materials.
The EPA Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway (2015) provides the general framework for evaluating indoor versus outdoor air comparisons. IDEM supplements this with its household chemical product lists to help investigators identify common indoor sources of VOCs.
Response Actions
If the VI evaluation confirms unacceptable indoor air exposures from subsurface sources, response actions are required. Options include:
Source Removal or Treatment
Addressing the subsurface source (excavation of contaminated soil, groundwater treatment, in-situ chemical oxidation) reduces or eliminates the vapor source. Source removal is the most permanent solution but may not be feasible at all sites.
Sub-Slab Depressurization (SSD)
SSD systems are the most common engineering control for vapor intrusion. The system maintains negative pressure beneath the building foundation, preventing vapors from entering the structure. SSD systems require ongoing operation, monitoring, and maintenance.
If an SSD system is part of a conditional closure, an Environmental Restrictive Covenant is required, and the property owner has ongoing operation and maintenance obligations.
Building Ventilation Modifications
Increasing building ventilation can reduce indoor air concentrations by diluting vapors that enter the structure. This is generally considered a supplemental measure rather than a standalone remedy.
Vapor Barriers
Vapor barriers installed during new construction or major renovation can prevent vapor entry through the foundation. Barriers are most effective when combined with a passive or active sub-slab venting system.
Institutional Controls
If VI risk is managed through engineering controls, an Environmental Restrictive Covenant is required to ensure the controls are maintained over time. The ERC must describe the vapor mitigation system and the property owner’s obligations for operation, maintenance, and reporting.
Comparison to Ohio
| Feature | Indiana | Ohio |
|---|---|---|
| Screening values | Published Levels for indoor air and soil gas (154 chemicals, 3 sample types, 3 land uses) | CIDARS includes indoor air generic standards; no published soil gas values |
| VI guidance document | Integrated into R2 framework | Standalone VI Guidance (March 2020) |
| Soil gas screening | Subslab, deep exterior, and shallow exterior Published Levels | No default soil gas screening levels - uses EPA VISL calculator |
| Groundwater-to-indoor-air | Evaluated site-specifically | Attenuation factors in CIDARS for GW-to-indoor-air screening |
| Response action threshold | Indoor air Published Levels | Case-by-case evaluation against CIDARS and EPA guidance |
| Building preparation | Follows EPA 2015 VI Technical Guide; IDEM provides household chemical product lists | Follows EPA 2015 VI Technical Guide |
The practical difference: Indiana’s Published Level approach is more prescriptive but simpler - compare your sample result to the table and determine whether you exceed the screening level. Ohio’s approach requires more modeling and professional judgment but provides more flexibility in how VI risk is evaluated.
Practical Tips
Check the Published Levels before planning your sampling. Know what the screening levels are for your chemicals of concern before you design the investigation. This determines which sample types you need (indoor air, subslab, exterior soil gas) and what detection limits your laboratory must achieve.
Detection limits matter. Some Indiana indoor air Published Levels are in the sub-part-per-billion range. Confirm that your analytical laboratory can achieve reporting limits below the applicable Published Levels before collecting samples. If the reporting limit is above the screening level, the data cannot be used to demonstrate compliance.
Sample during representative conditions. Indoor air concentrations vary with building operation (HVAC mode, occupancy, weather). Sample during conditions that represent typical exposure - not during renovation, not with all windows open, not during extreme weather events.
Document indoor sources thoroughly. The most common complication in VI evaluation is indoor VOC sources masking or mimicking subsurface contributions. Walk the building before sampling and document every potential indoor VOC source. Photograph stored chemicals, note recent renovations, and identify attached garages with fuel storage.
Subslab is the most diagnostic sample type. If you can only collect one type of soil gas sample, subslab is the most informative for evaluating the VI pathway to an existing building. It represents the vapor concentration at the point of entry and is less affected by atmospheric dilution than exterior probes.
Source
IDEM Nonrule Policy Document WASTE-0046-R2: Risk-based Closure Guide, July 8, 2022 (vapor intrusion provisions). IDEM Published Level Tables (indoor air and soil gas). EPA Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway (June 2015).