Pat Quinn, Governor
Tiered Approach to Corrective Action Objectives (TACO)
|Tier 2, Example 1 - Application of the Tier 1 Tables in a Tier 2 Analysis|
In this example, the groundwater classification at the site is Class II. Using Appendix B, Table A (Tier 1, Residential) as a screening tool, here are the contaminant concentrations at the site and their respective Tier 1 residential remediation objectives:
Benzene: The level of contamination (3.0 mg/kg) is less than the ingestion remediation objective (22 mg/kg), but greater than the inhalation and migration to groundwater remediation objectives listed in Table A (0.8 mg/kg and 0.15 mg/kg, respectively). Therefore, a Tier 2 evaluation for benzene could be calculated for the migration to groundwater exposure route and the inhalation exposure route The most restrictive of these two objectives will be the soil remediation objective for benzene.
Ethylbenzene: The level of contamination (200 mg/kg) is less than the ingestion and inhalation remediation objectives, but greater than the migration to groundwater remediation objective; therefore, the only Tier 2 objective to be calculated is for the migration to groundwater route.
Toluene: The level of contamination (7 mg/kg) is less than the Tier 1 ingestion, inhalation, and migration to groundwater remediation objectives; therefore, no further remediation objectives for toluene are needed.
Chrysene: The level of contamination (250 mg/kg) is less than the migration to groundwater remediation objective, but greater than the ingestion remediation objective. Therefore, a Tier 2 remediation objective need only be evaluated for ingestion. Note: the inhalation objective has footnote "c" indicating no toxicity data is provided. In fact, for chrysene, the soil saturation limit is protective for the inhalation route.
|Tier 2, Example 2 - Calculating a Tier 2 Migration to Groundwater Soil Remediation Objective|
|This example uses the SSL Partition Equation for Migration to Groundwater for
protection of Class 1 groundwater from benzene contamination. S17 is the equation that
determines the remediation objective, but S18 and S19 are needed to determine inputs into
S17. In this example all default variables are used, except for the site-specific organic
content (foc). The site-specific foc
value is 0.05. The Illinois EPA has found that the two most sensitive variables in the
model are the GWobj and the foc
, and recommends that the default values be used for the remaining parameters.
Step 1: Determine the soil leachate concentration using Equation S18
Step 2: Determine the soil-water partition coefficient using Equation S19
Step 3: Use Equation S17 to determine the remediation objective
Note: 0.316 mg/kg reflects the site-specific conditions versus the Tier 1 remediation objective of 0.03 mg/kg.
Yes. Both the SSL and RBCA equations presume that the assumptions on which the models are based are true at your specific site.
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.
Sites in fractured bedrock or karst settings cannot be modeled because the dilution factor for groundwater does not adequately address such aquifer flow patterns.
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).
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 (koc) or degradation constant (l). Hence, Tier 2 equations will not be applicable for all chemicals.
Yes, it is a measure of the capacity of the soil to adsorb organic contaminants. The organic carbon content (foc) 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 foc laboratory analysis is usually collected several feet below the ground surface. For modeling the inhalation exposure route, the soil sample submitted for foc 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 foc measurement cannot be used.
When calculating the Tier 1 migration to groundwater remediation objectives, the Illinois EPA used an foc 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.
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.
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 foc and the depth of contamination; for the remaining equation parameters, the default values will suffice.
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