Tiered Approach to Corrective Action Objectives Fact Sheets

Tiered Approach to Corrective Action Objectives (TACO)
Fact Sheet 8: Pathway Exclusion

Introduction

Pathway exclusion is optional under TACO.

Pathway exclusion allows for the exclusion of an exposure route based on incompleteness of the route. Incompleteness means that the pathway is unable to transport contaminants to potential receptors. When the pathway exclusion requirements are met, you no longer need remediation objectives for that particular pathway. Exclusion of any exposure route requires an institutional control (See Fact Sheet 4) and may also require an engineered barrier (See Fact Sheet 5).

Minimum Requirements

The following two requirements, if applicable, must be met before pathway exclusion can be considered at a site:

1. The attenuation capacity of the soil and the soil saturation limit cannot be exceeded. This prevents free product and potentially unacceptable risks from either single or multiple contaminants from remaining in the soil. The attenuation determination is made by comparing the sum of all remaining organic contaminants from a single sample to the site-specific or default values for naturally occurring organic carbon of the soil (f_oc). The soil saturation limit is determined by using either Appendix A, Table A or by calculating a specific value from equation S29.

2. To prevent leaving behind materials with the potential impact of hazardous waste, soils cannot exhibit characteristics of reactivity or have a pH equal to or less than 2.0 or a pH greater than or equal to 12.5. Also, soil containing the following inorganics or their salts must not exhibit characteristics of toxicity: arsenic, barium, cadmium, chromium, lead, mercury, selenium, or silver.

 Pathway Exclusion

Subpart C sets forth the requirements for demonstrating that an exposure route is incomplete.

The three routes addressed in this Subpart are inhalation, soil ingestion and groundwater ingestion (which includes migration to groundwater). The "pathway exclusion" option in TACO can take into account natural conditions and/or engineered barriers and institutional controls.

Although Subpart C presents prescriptive requirements to pathway exclusion, Tier 3 is also an option for pathway exclusion under site-specific conditions not addressed in Subpart C.

Before any pathways can be excluded, the extent and concentrations of the contaminants of concern above the Tier 1 residential objectives must be known. The requirements for this characterization may vary by program within the BOL.

Excluding the Inhalation Pathway

To exclude the inhalation pathway, the contaminants of concern must not exceed the Tier 1 inhalation objectives within ten feet of the land surface or within ten feet of any man-made migration pathways (e.g., utility conduit). The ten foot requirement can, however, be modified by the BOL if an adequate engineered barrier is utilized.

If the inhalation pathway is to be excluded, an institutional control must be in place which satisfies all of the following:

• Soils exceeding Tier 1 levels will not be located or relocated to positions within ten feet of the surface or within ten feet of any man-made pathway,

• Safety precautions will be taken for future construction worker populations if the remaining levels exceed the construction worker scenario objectives for inhalation, and

• The engineered barrier will be maintained when it is relied upon for the objectives used.

Excluding the Ingestion Pathway

To exclude the ingestion pathway, the contaminants of concern must not exceed the Tier 1 ingestion objectives within three feet of the land surface. This three foot requirement can, however, be modified by the BOL if an adequate engineered barrier is utilized.

If the ingestion pathway is to be excluded, an institutional control must be in place which satisfies all of the following:

• Soils exceeding Tier 1 levels will not be located or relocated to positions within three feet of the land surface,

• Safety precautions will be taken for future construction worker populations if the remaining levels exceed the construction worker scenario objectives for ingestion, and

• The engineered barrier will be maintained when it is relied upon for the objectives used.

Excluding the Ingestion of Groundwater Pathway

To exclude the groundwater ingestion pathway, you must demonstrate that the groundwater in the area of the release will not be consumed as drinking water, and that contamination will not migrate to a location where it could be consumed. This demonstration must show the following:

• Free product has been removed to the extent practicable,

• The source of the release is not within a setback zone or a regulated recharge area of a potable water supply well,

• All areas within 2,500 feet of the source of the release are governed by an ordinance adopted by a unit of local government that prohibits the use of groundwater as a potable water supply,

• Using equation R26 in Appendix C, Table C, all contaminants will meet the Tier 1 objective at the nearest setback zone, and

• Using equation R26, any contaminated groundwater discharging to a surface water body will meet the surface water quality standards under 35 IAC 302.

Can I exclude the migration to groundwater pathway?

Yes. Under Tier 3, if you can't exclude the ingestion of groundwater pathway as discussed above, you can demonstrate that there is no actual or potential impact of contaminants to receptors from the migration to groundwater route. Or, you can propose to exclude the migration to groundwater route. Such a proposal must contain the following information:

• Description of the route,

• Descriptions of the chemical and physical properties of the contaminants,

• Contaminant migration properties,

• Descriptions of the site and its physical characteristics,

• Discussion of why the route is unlikely to become active in the future, and

• Any modeling or engineered barriers, if applicable.

Can pathway exclusion be used in the LUST Program?

Yes. Subpart C of TACO may be applied in two ways under the LUST Program:

1. You may use the exposure pathway exclusion under 732.312 as site classification Method Three, or

2. If your site is classified as high priority under Method One or Method Two, you may use Subpart C of TACO in developing a corrective action approach.