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IDEM Issues New RISC Guidance on Non-Default Closures


In September 2007 the IDEM issued new guidance regarding a Risk Integrated System of Closure (”RISC”) method for calculating a site specific or nondefault migration to groundwater closure level. The new guidance explains the use of site-specific fraction of organic Carbon (“foc”) in the soil-to-ground water partitioning model (“Foc Guidance”). The Foc Guidance outlines sample handling and sample pretreatment requirements, acceptable Foc analytical methods, and documentation requirements.

The soil-to-ground water partitioning model estimates the potential for contaminants to migrate from impacted soils into ground water. Such migration prompts many cleanups at facilities with soils impacted by organic contaminants. Typically, the soil-to-ground water migration rate is determined using default values. However, under the IDEM’s RISC Guidance, site specific data may be collected and utilized for several variables, including Foc, required to calculate the soil-to-ground water migration rate. See IDEM RISC Technical Guide, Section 7.1.

Foc represents the portion of total soil organic matter comprised of simple organic compounds. These simple organic compounds provide adsorption sites to organic contaminants. The higher the organic carbon content of soil, the more organic chemicals may be adsorbed to the soil thereby impacting the migration rate of contaminants. The practical effect of greater soil adsorption of contaminants is that less contaminant is available to migrate to ground water.

The Foc Guidance establishes minimum sampling requirements including that soil samples must be collected from four borings in an area of the facility not impacted by contaminants. Sample handling and pretreatment requirements indicate that soil samples collected for Foc analysis should be free of extraneous organic matter (e.g., roots, sticks, wood, paper, etc.) and must be air-dried and screened. For most Indiana soils, the samples must be pretreated to remove inorganic carbon prior to analysis. In addition, the Foc Guidance describes acceptable analytical methods for determining Foc content and outlines the documentation requirements for the submission of a site-specific Foc calculation.

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