Research paperMethodology for scenario-based assessments and demonstration of treatment effectiveness using the Leaching Environmental Assessment Framework (LEAF)
Graphical Abstract
Introduction
Leaching assessments estimate the release of constituents of potential concern (COPCs) from solid wastes and secondary materials into water resources and ecosystems that may result from contact with water. Typically, leaching assessments are based on single extraction test results, e.g., U.S. EPA’s Toxicity Characteristic Leaching Procedure (TCLP, Method 1311) or Synthetic Precipitation Leaching Procedure (SPLP, Method 1312) that assume a leaching condition with little relevance to the anticipated end use (Clavier et al., 2019, da Silva et al., 2017, Intrakamhaeng et al., 2019, U.S. EPA, 1991, U.S. EPA, 1999). The Leaching Environmental Assessment Framework (LEAF) was developed to provide a flexible basis for leaching assessments based on measured intrinsic leaching characteristics that can be used to develop more accurate leaching assessments for waste treatment, disposal, use, and remediation scenarios (Kosson et al., 2002, Garrabrants et al., 2010, Kosson et al., 2014, U.S. EPA, 2019a). LEAF test methods (U.S. EPA, 2019b) are designed to characterize for COPCs (i) the liquid-solid equilibrium as a function of pH and liquid-solid ratio (L/S in L/kg-dry), (ii) the elution during percolation under local equilibrium, and (iii) the mass-transfer (diffusion) rate controlling release from low permeability materials (see method summaries in SI-1).
Within LEAF, assessments are derived from material characteristics, leach testing results, and assumptions regarding disposal or use conditions (U.S. EPA, 2014a, U.S. EPA, 2014b) with the resultant leaching estimates directly compared to acceptance thresholds or used as input to fate and transport or exposure models supporting human health and ecosystem risk assessments. The required refinement of assessment, however, depends on the intended use of the leaching assessment and decision thresholds,1 while the accuracy of the estimate is constrained by the available input data, methodology, and underlying assumptions. For example, screening-level assessments (Garrabrants et al., 2020) are intended to be bounding, biasing toward over-estimating COPC leaching to allow management decisions to be protective of the environment while balancing simplifications and uncertainty in the assessment approach. Screening-level assessments may be helpful when preliminary estimates of leaching concentrations are desired or when the goal of the assessment is to rank or select relevant COPCs.
However, screening-level estimates lack the level of detail required (i) to evaluate potential leaching for site-specific scenarios, (ii) to provide comparisons between different leaching conditions (e.g., water percolating through granular material versus water flow around monolithic materials), or (iii) to account for material characteristic changes over time (Branch et al., 2017, Garrabrants et al., 2015, Kosson et al., 2002, Kosson et al., 2014). Scenario assessments refine the leaching assessment process using site- and scenario-specific information on application scale and environmental parameters to provide a more precise estimate, as well as a common basis, for comparison of leaching scenarios with dissimilar conceptual release models (e.g., percolation vs diffusion). The assessment scenario can be designed to reflect a range of anticipated environmental conditions with one or more sets of bounding conditions that account for the hydraulic, physical, and chemical nature of materials.
The Interstate Technology and Research Council (ITRC) recommended a scenario-based assessment approach using LEAF leaching tests for demonstrating treatment effectiveness of solidified/stabilized (S/S) materials (ITRC, 2011). However, the ITRC guidelines focus on release from S/S materials placed within groundwater, whereas literature examples are not widely available for S/S materials placed in the vadose zone.
Scenario-based assessments, extending recent LEAF technical guidance (U.S. EPA, 2019), are presented here using a hypothetical case study of S/S-treated soil from a copper and lead smelter site. These assessments compare estimated leaching from untreated soil and S/S treated material managed above the water table using the same materials, methods, and leaching results used to develop screening-level assessments (Garrabrants et al., 2020). Scenario-specific information, including the physical dimensions of the material, the mode of water contact based on the physical form of the material, and the water infiltration for selected wet and dry environments, was derived from regional information and is not intended to represent any particular application or location. This work illustrates (i) a methodology for developing scenario-based leaching assessments using LEAF testing results in conjunction with scenario-specific information; (ii) the effects of scenario parameters on scenario-based assessment results; and (iii) a pathway for evaluating the effectiveness of treatment.
Section snippets
Hypothetical case study
The soil surrounding a copper and lead smelter is contaminated with heavy metals and other COPCs, affecting a 400-m2 plan-view area (20 m by 20 m) from grade to a depth of 5 m (total volume of 2000 m3). The soil is a sandy loam with an assumed porosity of 0.4, dry density of 1600 kg/m3, and field capacity of 0.24 (Meyer and Gee, 1999). The water table does not directly contact, or otherwise affect, the source material. The field scenario was assumed oxic (chemically oxidizing or mildly
Methodology for scenario-based assessments
The general approach for scenario-based assessments is to calculate an estimated average leaching concentration over an assessment interval () based on percolation or diffusion scenarios (see flowcharts provided in SI-3). The estimated leaching concentration may be calculated for a single assessment time step (e.g., 1-year) or averaged over several time steps that comprise an assessment interval, I (e.g., yearly time steps over a 30-year assessment):
The assessment is
Leaching assessment results
The final step for each scenario assessment is to generate ARs based on threshold criteria and DAF values. The estimated leaching concentration and ARs for 1-year, 5-year, and 30-year assessment periods with DAF = 1 are calculated for CFS in a percolation scenario (Table 3) and for S/S-CFS in diffusion scenario (Table 4). ARs shown in bold italics (AR ≤1) indicate COPCs that are not expected to leach at concentrations greater than the threshold. The S/S treatment of smelter soil generally
Conclusions
This scenario assessment methodology provides a consistent, robust approach for estimating potential leaching concentrations by incorporating LEAF leaching tests results with conceptual release models and scenario-specific conditions. The methodology distinguishes percolation scenarios for granular materials (e.g., soils, sediments) and diffusion scenarios for low permeability or monolithic materials (e.g., cements, clays).
All leaching assessments make simplifying assumptions that add
CRediT authorship contribution statement
Andrew Garrabrants: Conceptualization, Methodology, Writing - original draft. David Kosson: Conceptualization, Methodology, Writing - original draft. Kevin Brown: Validation, Writing - review & editing. Daniel Fagnant: Writing - review & editing. Gregory Helms: Writing - review & editing. Susan Thorneloe: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments and disclaimer
The authors gratefully acknowledge the financial support and technical input of the U.S. EPA Offices of Research and Development (Research Triangle Park, NC) and Land and Environmental Management (Washington, DC). The research described here has been funded, in part, by the U.S. EPA [Contract EP-C-15-008] to Jacobs. It has been subjected to the Agency’s review process and approved for publication. Approval does not signify that the contents reflect the views of the Agency. This study is not
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