Soil Attenuation Model for Derivation of Risk-Based Soil Remediation Standards

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Overview of Soil Attenuation Model (SAM)

Under a risk-based corrective action (RBCA) process, soils and groundwater impacted by a chemical release are to be remediated or controlled to concentration levels such that further migration will not expose human or environmental receptors to unsafe levels of hazardous constituents. For this purpose, site-specific target levels (SSTLs) must be established for the affected soil mass such that subsequent soil leachate migration to an underlying water-bearing unit does not cause exceedance of applicable exposure limits for groundwater. To derive such groundwater protection standards, a new Soil Attenuation Model (SAM) has been developed to provide a conservative estimate of soil-to-groundwater contaminant release based on readily available information regarding annual rainfall, soil type, depth to groundwater, and the hydrogeologic properties of the underlying water-bearing unit. Using either site-specific or generic site properties, this analytical model can be used either to i) predict upperbound constituent concentrations in groundwater, based on an observed soil concentration, or ii) backcalculate a lower-bound soil SSTL value, based on the applicable risk-based screening level (RBSL) at the groundwater point of exposure (POE). The model is applicable to analysis of porous media soils impacted by either organic and inorganic constituents, in the absence of mobile non-aqueous phase liquids (NAPLs).

The SAM represents a modification to the soil-leachate equations presented in Appendix X.2 of ASTM E-1739 Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites (ASTM, 1995). These modifications are directed toward providing a more rigorous characterization of the soil-to-groundwater leachate process and assisting the user in estimation of critical input parameters. However, to accommodate use in Tiers 1 and 2 of the risk-based site evaluation effort, the SAM retains the format of a simple, screening-level analytical expression requiring limited site-specific data input. The model includes empirical relationships to assist the user in characterization of net infiltration and equilibrium soil moisture content parameters based on annual site rainfall and the predominant soil type overlying the groundwater unit. The soil-togroundwater leachate process is characterized as a three-step procedure, beginning with i) equilibrium partitioning of soil contaminants from a finite source mass to infiltrating rainwater, followed by ii) sorptive redistribution of contaminants from the leachate onto underlying clean soils, and iii) subsequent leachate dilution within the receiving groundwater flow system. In the base version of this model, contaminant loss through the processes of volatilization and biodegradation are neglected for purpose of conservatism and simplicity.

Soil Attenuation Model for Derivation of Sponsored by Partners in Risk-Based Soil Remediation Standards 1 RBCA Implementation (PIRI) July 1997
This paper reviews the principal components of the Soil Attenuation Model (SAM) and presents detailed background information regarding model derivation and key assumptions. The SAM model has recently been specified for incorporation in the proposed Texas Risk Reduction Program (TRRP) (see TNRCC 1996). Sample default values to be used in Tier 1 and Tier 2 of the Texas process are also discussed herein. Summary guidelines regarding calculation procedures and input parameter selection are provided on Figure 2 and Table 4, respectively.

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