RBCA

January 10, 2024

Calculation of an Alternative Soil Cleanup Target Level Based on Site-Specific Soil Properties

The contaminated site rehabilitation (cleanup) provisions promulgated in Florida are based on Risk-Based Corrective Action (RBCA) principles. This approach to rehabilitating sites combines traditional site assessment with risk assessment principles and site-specific conditions to develop risk management options (RMOs) that meet the client’s objectives and the mandated human-health risk levels. RBCA allows us to tailor our assessment and remediation strategies to the unique characteristics of each site and avoid costs associated with unnecessary environmental assessment or remediation.

The RBCA provisions offer a number of default RMOs to achieve site closure. The most commonly used RMOs include natural background evaluations, total recoverable petroleum hydrocarbon fraction analysis, and synthetic precipitation leaching procedure (SPLP) analysis. One of the lesser-used RMOs includes calculating alternative soil cleanup target levels (ASCTLs) using site-specific soil properties. A downside of this approach is that one of the input parameters, soil moisture content, requires one year of data collection; this is too long for most of our clients (particularly developers). Nevertheless, given the right conditions, this RMO can yield beneficial results by justifying a higher cleanup level without needing a year of data. An example of how SCS has used this approach is below.

Background – SCS calculated a leachability-based ASCTL using site-specific soil properties for a former agricultural site undergoing redevelopment for residential use. We considered this site to be a good candidate for this RMO due to the following site-specific conditions:

• The upper two feet of soil throughout the site was marl, which often exhibits a higher organic carbon content than the default organic carbon content.
• The contaminant of concern (COC) was dieldrin, which has a high organic carbon partition coefficient (Koc), meaning it strongly sorbs to organic carbon in the soil.
• Considering the soil type and target COC, we suspected that soil organic carbon content would be the driving factor in the ASCTL calculation or, in other words, that soil moisture content (the parameter that requires one year of data collection) would contribute very little to the ASCTL.

Data Collection – SCS collected discrete samples from the relevant intervals to obtain site-specific data for the required soil properties. The measured parameters include a fraction of organic carbon, dry soil bulk density, and soil moisture content. Using these measured parameters, SCS calculated the remaining site-specific soil parameters, specifically, the water-filled soil porosity, total soil porosity, and air-filled soil porosity.

ASCTL Calculation – We calculated the ASCTL using the site-specific soil properties and the equation below. The table following the equation presents the default and site-specific soil property input values and the method for deriving the site-specific values. Refer to the DERM Technical Report: Development of Cleanup Target Levels (CTLs) for Chapter 24, Miami-Dade County Code (September 2005) for information concerning the remaining input parameters.

Using the site-specific soil properties resulted in an ASCTL of 0.046 mg/kg, significantly higher than the default leachability SCTL of 0.002 mg/kg.

Sensitivity Analysis – As mentioned above, soil moisture content requires one year of data collection; generally, this parameter is an average of results obtained from four quarterly events (designed to represent variability in moisture content during wet and dry seasons). Since we speculated that organic carbon content is driving the ASCTL, and the client could not wait a year to get approval of the ASCTL, SCS performed a sensitivity analysis of the various soil parameters on the ASCTL calculation. The results indicated that soil moisture content does not impact the ASCTL calculation for dieldrin (at the site-specific organic carbon content), even using the unrealistic scenarios of 0% and 100% soil moisture content. With this finding, we could justify using a single data point for soil moisture content instead of averaging one year of soil moisture data.

Summary – ASCTLs using site-specific soil properties can be calculated for any COC. However, consider several criteria when evaluating the feasibility of this approach, some of which include:

• The target COC – Organic COCs are best suited for this RMO, as the ASCTL calculation is fairly straightforward. While the calculation is not impossible for inorganics (i.e., metals), it is more complex. The partitioning of metals is represented by the soil/water partition coefficient (Kd), as opposed to the Koc for organic COCs. Since various soil conditions significantly influence the Kd, deriving site-specific values involves a rather intricate process.
• Timing – If the project is time-sensitive, it will be important to eliminate the need to collect one year of soil moisture content. Similar to the example herein, under the right conditions, a sensitivity analysis may be used to support the calculation of the ASCTL based solely on organic carbon content. Organic COCs with low water solubility, high affinity for organic matter, and low volatility, coupled with highly organic soils, offer the best scenarios for this.
• Sample collection – The number and placement of soil samples must represent the soil characteristics from each applicable interval across the site.
• Size of impacted area – The default dilution attenuation factor (DF) of 20 represents a source area of 0.5 acres. Therefore, if the area of potentially leachable soil exceeds 0.5 acres, a site-specific DF may need to be calculated, requiring site-specific aquifer hydraulic conductivity, hydraulic gradient, mixing zone depth, infiltration rate, and source length parallel to groundwater flow.

We can also calculate the direct exposure ASCTLs based on site-specific soil properties. Use the soil properties in the volatilization factor portion of the direct exposure SCTL equation; therefore, COCs with a significant fraction of the risk associated with the inhalation route would benefit most from this RMO.

Ultimately, an environmental professional with expertise in environmental assessment, risk assessment, and soil science/geology can help assess whether calculating an ASCTL would be beneficial in meeting your client’s goals. Approval of a higher leachability-based ASCTL can result in significant savings by minimizing or eliminating the need for additional assessment, remediation, or groundwater monitoring.

Environmental Assessment or Remediation References

DERM Technical Report: Development of Cleanup Target Levels (CTLs) for Chapter 24, Miami-Dade County Code. September 2005, prepared by the Center for Environmental & Human Toxicology, University of Florida. Link

Hagan, D., F. Escobedo, G.Toor, H. Mayer, J. Klein, and C. Dobbs. 2010. Soil Bulk Density and Organic Matter in Urban Miami-Dade County, Florida. SL 327. Gainesville: University of Florida Institute of Food and Agricultural Sciences. Link

Nelson, D.W. and Sommer, L.E. (1982) Total Carbon, Organic Carbon and Organic Matter. Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties, 2nd Edition. ASA-SSSA, Madison, 595-579. Link

About the Authors:

Anabel Rodriguez-Garcia is an environmental scientist, with a decade of experience in sustainable management of soil, heavy metal contamination in soils and organic fertilizers, physical, chemical and biological characterization of soils, including sample collection and documentation. She serves SCS clients as a senior project professional, and is particularly valuable for environmental site assessments. She has worked on projects for government agencies such as Florida Department of Transportation, Miami-Dade County Regulatory and Economic Resources, public utilities, and for the private sector.

Lisa L. Smith has three decades of experience in a variety of roles in the field of environmental science. Lisa serves SCS clients as a senior technical advisor and expert in the field of risk based corrective action (RBCA). She has worked as an environmental regulator at the Miami-Dade County Department of Environmental Resources Management (DERM), a risk assessor at a national environmental consulting firm, and a research chemist at the University of Florida.

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