Material- and Site-Specific Partition Coefficients for Beneficial Use

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Environmental Modeling

Material and Site Specific Partition Coefficients for Beneficial Use Assessments Nawaf I. Blaisi, Kyle Clavier, Justin Roessler, Jaeshik Chung, Timothy G. Townsend, Souhail R Al-Abed, and Jean-Claude J Bonzongo Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01756 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on August 7, 2019

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Environmental Science & Technology

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Material and Site Specific Partition Coefficients for Beneficial Use Assessments

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Nawaf I. Blaisia, Kyle A. Claviera, Justin G. Roesslera, Jaeshik Chunga,b, Timothy G.

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Townsenda*, Souhail R. Al-Abedc , Jean-Claude J. Bonzongoa

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a Department

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PO Box 116450 Gainesville, FL 32611 – 6450, USA

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b Center

for Water Resource Cycle, Korea Institute of Science and Technology, Seoul 136791, Republic of Korea

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of Environmental Engineering Sciences, University of Florida,

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National Risk Management Research Laboratory, U.S. Environmental Protection Agency,

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26 West Martin Luther King Drive, Cincinnati, OH 45268, USA

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Submitted to:

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Environmental Science and Technology

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* Corresponding

author: Phone: +1 352-392-0846, Fax: 352-392-3076, email [email protected]

ACS Paragon Plus Environment


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TOC ART

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Abstract

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Partition coefficients (Kd) available in the literature are often used in fate and transport

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modeling conducted as part of beneficial use risk assessments for industrial byproducts. Since

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element partitioning depends on soil properties as well as characteristics of the byproduct

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leachate, site-specific Kd may lead to more accurate risk assessment. In this study,

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contamination risk to groundwater of beneficially reused byproducts was assessed using batch

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leaching tests on waste to energy bottom ash (WTE BA) and coal combustion fly ash (CFA).

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Leachates were equilibrated with 8 different soils to obtain the waste-soil-specific Kd,exp for the

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metals of interest. The Kd,exp values were used as inputs in the Industrial Waste Management

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Evaluation Model (IWEM) to demonstrate the degree to which Kd estimates affect risk

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assessment outcome. Measured Kd,exp for the most part fell within the large range of Kd reported

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in the literature, but IWEM results using default Kd for some types of soils resulted in

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overestimated risk compared to those derived from Kd,exp. Modeled concentration at the

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receptor location was much lower for some elements for those soils with high concentrations

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of iron and aluminum.

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Keywords: coal combustion fly ash, dilution attenuation factor, leachate, partitioning

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coefficient, waste to energy bottom ash



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Introduction

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Recycling of industrial byproducts through beneficial use applications such as aggregate

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replacement, fill material, and soil amendment plays an important role in sustainable materials

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management.

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resource consumption and reduced land disposal requirements, must be weighed against the

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potential risks posed by chemicals present in these byproducts [1, 2]. In addition to assessing

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potential for direct human exposure, a reused byproduct’s environmental risk is assessed by

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evaluating its potential to leach contaminants into groundwater or surface water [3–5]. This

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type of risk assessment involves estimation of concentrations of contaminants of potential

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concern (COPCs) using leaching tests and relies on fate and transport modeling to predict

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COPC concentrations at target receptor location or compliance point [6,7]. Fate and transport

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models for contamination of groundwater typically require inputs that quantify subsurface

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environment characteristics such as: infiltration rate, aquifer and vadose zone thickness, aquifer

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pH, soil partitioning coefficient (Kd), receptor distance, and COPC concentration, among others

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[7–10]. Each of these inputs influence modeled COPC concentrations at receptor sites and most

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have been the subject of considerable research [11–14].

Benefits of these materials management strategies, such as reduced natural

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The solid to liquid partitioning coefficient (Kd) between a waste leachate and soil interface,

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a measure of the ratio of a contaminant in the solid and liquid phase at equilibrium, is one input

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parameter used in fate and transport modeling that is extensively studied in the literature

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[6,15,16]. While most beneficial use evaluations employ leaching tests to determine waste-

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specific COPC concentrations, determinations of site-specific Kd’s are less frequent. Most risk

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assessment protocols utilize reference Kd values selected from the literature or Kd values

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determined from programs such as MINTEQA2 [8]. Relying on reference Kd that are not

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specific to site and waste characteristics when assessing beneficial reuse risk may result in less


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accurate contamination risk profiles, especially considering the broad range of Kd reported in

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the literature and their dependence on material-specific properties [17]. Consider, for instance,

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a beneficial use scenario in which the default Kd value for a given element used in a fate and

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transport model is higher than the actual Kd of the soil on site. The model would predict greater

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attenuation of that element in the soil and thus lower concentrations at the receptor site location;

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this would result in an under-estimation of leaching risk associated with the beneficial use

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application and potential harm to human health or the environment. Conversely, a low default

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Kd value would overestimate leaching risk and might inappropriately disqualify a candidate

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beneficial use material based on perceived risk. An ideal scenario involves using a Kd value

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that is specific to a given application and will allow for more accurate quantification of risk. It

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is highly unlikely that default partitioning coefficients account for all of the complex

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interactions between waste and soil with leachate, such as organic matter, trace element

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content, and soil pH. Thus the default Kd values may be inaccurate for specific scenarios and

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lead to one of the above-mentioned scenarios.

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Here we demonstrate a method to determine waste- and soil-specific Kd values (herein

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referred to as Kd,exp) and use them in fate and transport models to provide a more representative

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prediction of risks associated with beneficial reuse projects utilizing common industrial waste

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byproducts.

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Leaching concentrations measured from waste to energy (WTE) bottom ash (BA) and coal

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combustion fly ash (CFA) were input into US EPA’s Industrial Waste Management Evaluation

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Model (IWEM) along with waste- and soil- specific Kd,exp. The Kd,exp values were determined

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by experimentally measuring partition coefficients for eight different soil sources contacted

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with WTE BA and CFA leachate; the different soil sources help highlight the importance of

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major soil properties (e.g. iron and aluminum content) on the outcome of beneficial use

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determinations.



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Materials and Methods

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Soil and Waste Sample Collection. Soil samples were collected at a depth of 5-25 cm below

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the ground surface from eight different locations in Florida, US. Soil sampling locations were

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selected to provide a wide range of soils which would differ in both chemical composition and

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physical characteristics, so that the impact of these parameters on the Kd,exp could be evaluated.

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Each soil sample was air dried at room temperature (~ 24±2 oC) for three days, passed through

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a 2-mm sieve, and refrigerated at 4 oC until used in laboratory experiments. Two grab samples

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of industrial waste byproducts were collected from facilities in Florida, US. CFA samples were

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collected from a coal-fired power generation unit. The samples were collected from the

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discharge of the facility’s fabric filters and did not contain the scrubber residues associated

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with acid gas removal. Samples of WTE BA were collected from a mass burn WTE facility.

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This facility subjects its bottom ash to ferrous (magnet) and non-ferrous (eddy current

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separation) metals recovery following combustion. As commonly performed prior to beneficial

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use applications, the material was aged for three months before collection.

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Soil

Classification

and

Elemental

Content.

The

operationally

defined

total

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environmentally available concentrations of metals in soil samples were determined after

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subjecting six replicates of each soil sample to acid digestion following EPA Method 3050b.

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This strong acid digestion dissolves almost all elements, except for those in forms that are

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generally not considered “environmentally available,” such as those bound by silicate

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structures [18]. The percentages of sand, silt and clay in each soil sample were determined

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based on grain size distribution in accordance with the procedures outlined in ASTM D422

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[19]. The pH of the soil samples was determined following EPA Method 9045D [20]. Soil

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moisture content was measured by determining the initial and oven-dried mass in accordance

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with ASTM D2216 [21]. The soil organic matter content was estimated through loss on ignition

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(LOI) after four hours of heating at 550 °C as described by Santisteban et al. (2004) [22].


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Batch Leaching and Soil-Liquid Partitioning Tests. Samples were first size-reduced to

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pass a 4.75 mm sieve and leachates were generated using a modified EPA method 1316 test at

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a set liquid to solid ratio (L/S) of 10 mL-reagent water/g-waste [23]. Each waste was leached

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in eight replicates; a 200-g dry mass of waste was used to generate a sufficient quantity of

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leachate for the soil sorption tests.

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To determine the Kd,exp , the leachate generated from the 1316 test was contacted with each

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of the eight soils (each soil sorption test was conducted in triplicate). The soil sorption testing

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was performed following ASTM D4646, a procedure designed to measure a soils affinity for

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select constituents in leachates. This test involves equilibrating a given mass of soil sample

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with a waste solute, such as a laboratory extraction leachate, of known trace element

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composition in a 1:20 soil-to-solution ratio. Adsorbed solute can be calculated by measuring

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remaining solute concentration after the sorption test, adjusted for extract volume and soil mass

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[24]. For the purposes of this study, soil sorption tests assume trace element contribution from

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the clean soil is negligible. It is also assumed that sorption and desorption are reversible and

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that the leachate and soil have equilibrated during the contact time prescribed by ASTM D4646.

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Acid Digestion and Elemental Analysis. All leachate samples were digested prior to

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analysis using an automated hot block digestion system following EPA Method 3010A [25].

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The acid digestates were evaluated for their major and trace element content using Inductively

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Coupled Plasma – Atomic Emission Spectrometry (ICP-AES), in accordance with EPA

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Method 6010B [26]. Cl- and SO42- in leachates were measured using ion chromatography (IC)

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following EPA Method 9056A [27].

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Waste- and Soil-specific Partitioning Coefficient Determinations. Kd,exp was calculated

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as the difference between constituent waste leachate concentration (Cinitial) from method 1316

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and the equilibrium aqueous concentration following the soil sorption test (Cfinal) divided by

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Cfinal, adjusted for soil mass (Msoil) and extract volume (Vsolution), assuming the linear region in



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the sorption isotherm. The Kd,exp’s reported in this study are all expressed in units of L/kg (see

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equation 1).

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Equation 1. Kd,exp calculation using data generated from L/S 10 1316 extractions and the soil

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sorption test outlined in ASTM D4646. Units for Cinitial and Cfinal are mg/L, Vsolution is in L, and

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Msoil is kg. Final units for Kd,exp are therefore L/kg.

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𝐾

𝑑,exp =

(𝐶𝑖𝑛𝑖𝑡𝑖𝑎𝑙 ― 𝐶𝑓𝑖𝑛𝑎𝑙)(𝑉𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛) (𝐶𝑓𝑖𝑛𝑎𝑙)(𝑀𝑠𝑜𝑖𝑙)

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Example of Beneficial Use Assessment. To determine the scale at which COPCs were

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present in the WTE BA and CFA, leachate concentrations were compared to Florida, US

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Groundwater Cleanup Target Levels (GCTLs). Risk thresholds such as GCTL are commonly

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used in beneficial use assessments; as a screening step, leach test results are compared to

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GCTL, but a complete risk assessment requires using leach test results as an input to a fate and

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transport model to assess whether GCTL are expected to be exceeded at receptor sites or

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compliance points [28]. Critical components of this assessment technique include the Kd, waste

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leachate concentrations, soil and hydrogeologic conditions, and the risk-based thresholds. To

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illustrate the impact that the Kd,exp can have on fate and transport modeling outcomes, two

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beneficial use scenarios were evaluated using IWEM. IWEM has been used as a modeling tool

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in the US at the state and federal level for beneficial use assessments, and was recently

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employed as a part of the decision-making efforts related to the regulatory status of coal

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combustion residuals [8]. A detailed description of the program can be found in the Supporting

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Information (SI). The two scenarios evaluated were the use of WTE BA as a roadway sub-base

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(Table S2) and the use of CFA as a structural fill material (Table S1). Many of the input


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parameters (infiltration rate, fill volume, roadway thickness, subsurface hydrogeology) for

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these models were set using values developed for two previous assessments conducted by

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Benson and Edil, and the Electric Power Research Institute [29-30].

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Input concentrations were set at the COPC concentrations measured in the Method 1316

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leachates for both the CFA (structural fill) and WTE BA (road base). Each of the modeling

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evaluations were performed at three different Kd: the default Kd for each element (from IWEM)

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and values representing the lowest and the highest calculated Kd,exp for each element from the

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soil sorption testing (on 8 different soils). The default Kd values in IWEM employ multiple sets

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of sorption isotherms derived from the geochemical speciation model MINTEQA2. Additional

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information on IWEM can be found in the IWEM Model 3.1 Technical Background Document

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[8].

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Results and Discussion

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Soil Characterization. Soil characterization results of the 8 samples tested, including soil

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pH, conductivity, percentage of organic matter, total metal (Al, Fe, and Mn) content, and soil

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classification conducted in accordance with the methodology outlined by the US Department

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of Agriculture (USDA) [31] are presented in Table 1. Total concentrations of Al, Fe and Mn

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are especially pertinent because (hydr)oxide forms of those metals on the sorbent surface are

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known to be efficient scavengers of some trace elements [32–36]. Iron concentrations in soil

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samples ranged from 167 to 5,800 mg-Fe/kg-soil. Soils 2, 4, 5 and 7 had relatively high Fe

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contents (>1,900 mg-Fe/kg-soil), while soils 1, 3, 6, and 8 had lower Fe contents (

Material- and Site-Specific Partition Coefficients for Beneficial Use

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