Tiered Approach to Corrective Action Objectives Fact Sheets

Tiered Approach to Corrective Action Objectives (TACO)
Fact Sheet 7: Tier 2

What is Tier 2?

Under Tier 2, site owners and operators can develop remediation objectives applying site-specific data to pre-established modeling equations provided in Appendix C (Tier 2 Tables).

When do I use Tier 2?

When your contaminant concentrations exceed Tier 1 objectives, Tier 2 may be used to develop remediation objectives that are based on the actual site conditions and are still protective of human health.

How do I obtain data for a Tier 2 equation?

Tier 2 allows the use of information on the physical and chemical properties of individual chemicals along with site-specific physical and groundwater properties. Any field data used must conform to standard practice in the geological and engineering industry. Many of the parameters in the equations can use either a default value (i.e., standardized values) or actual site-specific data. Appendix C, Tables B and D provide the default values (labeled "parameter values") and identify when site-specific data may be used. Methods for determining physical soil parameters are provided in Appendix C, Table F.

Where do I find a chemical's physical and toxicological properties?

Physical parameters can be found in Appendix C, Table E. If a contaminant's physical properties are not listed in Appendix C, Table E, properties can be proposed for Illinois EPA review and approval. Justification for the use of the values will need to be provided.

The U.S. EPA's Integrated Risk Information System (IRIS) should be consulted for toxicological parameters. Toxicological parameters may be requested from a BOL project manager, or values may be proposed for Illinois EPA review.

Are carcinogens and noncarcinogens calculated differently?

Yes. Fundamental to the use of Tier 2 equations is the determination of whether a contaminant is a carcinogen or a noncarcinogen. Appendix B, Tables A and B identify which chemicals are carcinogens (footnoted "e") and noncarcinogens (footnoted "b"). For carcinogenic effects, risk is associated with the probability of an individual developing cancer over a lifetime. For non-carcinogenic effects, risk is expressed as a hazard quotient (See Fact Sheet 2). It is important to note that a Hazard Quotient is not a statistical probability that can be related to a 10-6 risk. There are separate equations for carcinogens and for noncarcinogens.

Section 742.505(b)3 and 742.720 address chemicals with cumulative noncarcinogenic effects for a specific target organ(s). If a contaminant poses a risk of both cancer and noncarcinogenic effects, remediation objectives must be derived for the most health-sensitive effect.

What is a default value?

A default value is a standardized value that when factored into the equations produces a conservative, health protective remediation objective. For many parameters, site-specific values can be used in lieu of the default values. Not all parameters have default values, in which case a site-specific value must be obtained.

Should I use the SSL or RBCA equations?

Either the SSL or RBCA equations can be used. Both sets of equations are health protective. However, the RBCA and SSL equations themselves are different; their respective parameter values cannot be interchanged. Also, the Tier 2 equation models cannot be altered (except as provided in Tier 3).

Furthermore, RBCA models ingestion, inhalation, and dermal exposure pathways all together in one equation, while SSL models them separately. This means that you must calculate the ingestion and inhalation objectives for a particular contaminant using the same set of equations, either SSL or RBCA.

Either the SSL or RBCA approach can model the migration to groundwater portion of the groundwater ingestion route. Since both sets of equations are scientifically defensible, either approach may be used for the development of remediation objectives.

That is, if the SSL equation determines one migration to groundwater remediation objective and the RBCA equation determines a less restrictive migration to groundwater remediation objective, the less restrictive objective may be used.

Do the soil attenuation and soil saturation constraints factor into a Tier 2 evaluation?

Yes. When developing Tier 2 remediation objectives, the total organic concentrations of all the contaminants from a single sampling point (whether they exceed the Tier 1 objectives or not) must not exceed the soil attenuation capacity.

The soil saturation limit cannot be exceeded for individual organic chemicals. A pre-calculated soil saturation limit is provided in Appendix A, Table A, or a site-specific value can be calculated using equation S29.

Are there any limits to using Tier 2?

Yes. Both the SSL and RBCA equations presume that the assumptions on which the models are based are true at your specific site.

1. When modeling groundwater concentrations, there can be no layers confining the contaminated groundwater. That is, there can be no stratigraphic unit restricting the groundwater plume to a narrow seam and preventing vertical migration. This would result in a more concentrated flow occurring horizontally than the equations predict.

2. Sites in fractured bedrock or karst settings cannot be modeled because the dilution factor for groundwater does not adequately address such aquifer flow patterns.

3. The TACO equations do not model contaminant concentrations for indoor air. The U.S. EPA and Illinois EPA decided against modeling remediation objectives for indoor air due to the sensitivity of models to parameters which do not lend themselves to standardization on a statewide basis (i.e., building ventilation rates and the number and size of cracks in foundations or basement walls).

4. Toxicological and physical parameters may not be varied. A chemical's physical parameters may be obtained from Appendix C, Table E. Note: Some chemicals do not have default physical parameters. For instance, nitrate does not have a partition coefficient (k_oc) or degradation constant (l). Hence, Tier 2 equations will not be applicable for all chemicals.

Is the organic content of the soil important?

Yes, it is a measure of the capacity of the soil to adsorb organic contaminants. The organic carbon content (f_oc) is to be measured in the native soil where the organic contaminant is expected to migrate through to reach either the groundwater or the atmosphere.

For modeling migration to groundwater, the soil sample submitted for f_oc laboratory analysis is usually collected several feet below the ground surface. For modeling the inhalation exposure route, the soil sample submitted for f_oc laboratory analysis is usually collected within the top foot of the ground surface.

If sampled from fill areas with cinders, leaves, wood chips, slag, etc., the site-specific f_oc measurement cannot be used.

When calculating the Tier 1 migration to groundwater remediation objectives, the Illinois EPA used an f_oc value of 0.002. This default value is protective in soils containing little organic content where contaminants may migrate freely.

However, it has been the Illinois EPA's experience that the organic carbon content may commonly range from two to five percent (0.02 to 0.05). The laboratory test methods to determine a soil's organic carbon content are provided in Appendix C, Table F.

Can I use models or equations different than those presented in TACO to calculate remediation objectives?

Yes. This would be a Tier 3 evaluation. Tier 3 considers other fate and transport models and any modifications or updates to exposure and toxic criteria.

How can I use SSL mass-limiting equations S26, S27 and S28?

S26 and S27 are used to determine a volatilization objective and S28 is used to find the soil component of the groundwater ingestion exposure route. These equations may be useful when the soil contamination is less than seven feet deep. The primary parameters to obtain are f_oc and the depth of contamination; for the remaining equation parameters, the default values will suffice.