Environmental Engineering Science, In-press, 2002. 

An Integrated Framework for Evaluating Leaching in Waste 

Management and Utilization of Secondary Materials 

D.S. Kosson1*, H.A. van der Sloot2

, F. Sanchez1 and A.C. Garrabrants1

1

Department of Civil and Environmental Engineering 

Vanderbilt University 

Nashville, Tennessee, USA 

2

The Netherlands Energy Research Foundation 

Petten, the Netherlands

Abstract

A framework for the evaluation of inorganic constituent leaching from wastes and secondary materials is 

presented. The framework is based on the measurement of intrinsic leaching properties of the material in

conjunction with mathematical modeling to estimate release under field management scenarios. Site 

specific and default scenarios are considered, which may be selected based on the evaluation context. A 

tiered approach is provided to allow the end-user to balance between the specificity of the release estimate, 

the amount of testing knowledge required, a priori knowledge, and resources required to complete an

evaluation. Detailed test methodologies are provided for a suite of laboratory leaching tests. 

Key Words: leaching, metals, waste, soil, utilization, beneficial use, secondary materials, disposal, 

landfill, risk assessment, test methods 

 *

 to whom communications should be addressed. 

Box 1831 Station B, Nashville, TN 37235. Phone: (615) 322-1064, Email: David.Kosson@Vanderbilt.edu

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Environmental Engineering Science, In-press, 2002. 

INTRODUCTION 

Leaching tests are used as tools to estimate the release potential of constituents from waste 

materials over a range of possible waste management activities, including during recycling or reuse, for 

assessing the efficacy of waste treatment processes, and after disposal. They may also be used to develop 

endpoints for remediation of contaminated soils and the source term1

 for environmental risk 

characterization. The Resource Conservation and Recovery Act (RCRA) requires the USEPA to classify 

wastes as either hazardous or non-hazardous. In implementing this portion of RCRA, the USEPA asks, 

“Would this waste pose unacceptable environmental hazards if disposed under a plausible, regulatorily 

defined, mismanagement scenario?” This scenario typically represents “worst case” management (i.e., the 

estimated highest risk, plausible, legal management option) and wastes posing such unacceptable 

environmental hazards warrant classification and regulation as hazardous wastes. In developing the 

Toxicity Characteristic regulation (40 CFR 261.24), the USEPA defined the plausible, worst-case 

mismanagement scenario for evaluating industrial waste as co-disposal in a municipal solid waste (MSW) 

landfill. The assumption of this mismanagement scenario, in turn, resulted in the development of the 

Extraction Procedure Toxicity test and its successor, the Toxicity Characteristic Leaching Procedure

(TCLP; see 45 FR 33084, May 19, 1980, and 55 FR 11798, March 29, 1990), which attempts to replicate 

some key leaching factors typical of MSW landfills. 

The TCLP has come under criticism because of overbroad application of the test (and underlying 

assumption of MSW co-disposal) in evaluating and regulating wastes, and some technical specifications of

the methodology. The Science Advisory Board of USEPA reviewed the leaching evaluation framework 

being employed by the agency in 1991 and 1999 (USEPA, 1991; USEPA, 1999). In the 1999 review, the 

Science Advisory Board stated: 

The current state of the science supports, even encourages, the development and use of 

different leach tests for different applications. To be most scientifically supportable, a leaching 

protocol should be both accurate and reasonably related to conditions governing leachability under 

actual waste disposal conditions. 

And 

 1

 In this context, ‘source term’ refers to representation of constituent release from a waste or contaminated

soil that is used in subsequent fate and transport modeling for exposure evaluation in risk assessment. 

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Environmental Engineering Science, In-press, 2002. 

The multiple uses of TCLP may require the development of multiple leaching tests. The result 

may be a more flexible, case-specific, tiered testing scheme or a suite of related tests incorporating the 

most important parameters affecting leaching. Applying the improved procedure(s) to the worst-case

scenario likely to be encountered in the field could ameliorate many problems associated with current 

procedures. Although the Committee recognizes that these modifications may be more cumbersome to

implement, this type of protocol would better predict leachability. 

The Science Advisory Board also criticized the TCLP protocol on the basis of several technical 

considerations, including the test’s consideration of leaching kinetics, liquid-to-solid ratio, pH, potential for 

colloid formation, particle size reduction, aging, volatile losses, and co-mingling of the tested material with

other wastes (i.e., co-disposal). 

In response, this paper offers an alternative framework for evaluation of waste leaching potential 

that responds to many of the criticisms of the TCLP. It provides a tiered, flexible framework capable of 

incorporating a range of site conditions that affect waste leaching and so can estimate leaching potential 

under conditions more representative of actual waste management. The paper also addresses practical 

implementation of the framework in different applications, and an example application of this approach for 

evaluating alternative treatment processes for mercury contaminated soils is presented in a companion

paper (Sanchez et al., 2002). The leach testing protocols used in the framework also address technical 

concerns with the TCLP. The test protocols provided here are designed only for application to inorganic 

species; however, the concepts presented for the integrated framework are general, with application to both

inorganic and organic species. Applicable test methods for organic species are the subject of future 

development. Complete technical specifications for the protocols are provided in the Appendix. 

IS THE RIGHT QUESTION BEING ASKED?

In evaluating the leaching potential of wastes based on a single, plausible worst-case

mismanagement scenario via TCLP, the USEPA seeks to provide environmental protection for unregulated 

wastes. However, wastes are managed in many different settings, and under a range of conditions that affect 

waste leaching. The reliance of the USEPA on a single, plausible worst-case, management scenario for 

leach testing may be generally protective, but often at the cost of over regulation. It has also proven to be 

inadequately protective in some cases (see discussion of spent aluminum potliner regulation at 62 FR 

41005, July 31, 1997 and 62 FR 63458, December 1, 1997). While reliance on a single waste management

scenario as the basis for leach testing may simplify implementation of RCRA, many of the wastes evaluated 

using TCLP have little if any possibility of co-disposal with MSW; assessment of the release potential of

wastes as actually managed is needed to better understand the hazards posed by waste. Neither the TCLP, 

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nor any other test performed under a single set of conditions, can provide an accurate assessment of waste 

hazards for all waste. 

From an environmental protection perspective (and setting aside the particular requirements of

RCRA), the goal of leaching testing is to answer the question “What is the potential for toxic constituent 

release from this waste by leaching (and therefore the risk) under the selected management option?” For 

environmentally sound waste management, the following questions result from different perspectives: 

• From the waste generator’s prospective - Which waste management options are acceptable for a 

waste? 

• From the waste management facility’s perspective - Which wastes are suitable for disposal in a 

specific disposal facility? 

• From the potential end-user’s perspective - Is this secondary material acceptable for use in 

commerce (e.g., as a construction material)?

The framework for answering these questions should be consistent across many applications, ranging from

multiple waste disposal scenarios to determination of the environmental acceptability of materials that may 

be subject to leaching (e.g., construction materials). At the same time, the framework should be flexible 

enough to consider regional and facility-specific differences in factors affecting leaching (e.g., precipitation,

facility design). A methodology guideline (ENV 12920, 1996) developed under European standardization 

initiatives recommends that the management scenario be a central consideration in the testing and

evaluation of waste for disposal and beneficial use of secondary materials. This methodology is an

extension of the approach in the Building Materials Decree established in The Netherlands (Building 

Materials Decree, 1995). 

The answers to the questions posed above require several interrelated assessments including (i) the 

release rate and total amount over a defined time interval of potentially hazardous constituents from the 

waste, (ii) attenuation of the constituents of concern as they migrate from the waste, through groundwater, 

to the receptor being considered, (iii) exposure of the receptor, and (iv) the toxicity of each specific 

constituent. Considerable effort has resulted in accurate assessment techniques and data for evaluating

contaminant transport through the environment (and attenuation), and toxicity for a large number of

constituents. 

In contrast to the detailed research on constituent fate, transport and risk following release, 

estimation of constituent release by leaching most often assumes (i) the total content present is available for 

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release, or (ii) the contaminant concentration in the leachate will be equal to that measured during a single 

batch extraction and is constant with time

2

, or (iii) the fraction of the contaminant extracted during a batch

extraction is equal to the fraction that will leach (USEPA, 1986; Goumans et al., 1991). These approaches

frequently result in grossly inaccurate estimation of actual release (both over- and under-estimation). 

Inaccurate release estimation, in turn, forces disposal of materials that are suitable for beneficial use, 

mandates remediation of soils to levels beyond that necessary for environmental protection, unnecessarily

depletes disposal capacity, or results in groundwater contamination (if release is underestimated). In 

addition, treatment processes, that may be proven to reduce the extracted concentration for a regulatory test 

(TCLP), have resulted in increased release when compared to management scenarios without treatment

(Garrabrants, 1998). Thus, methodologies that result in a more accurate estimate of contaminant leaching 

may both improve environmental protection through more efficient use of resources and be economically

beneficial. 

In general, leaching tests can be classified into the following categories (Environment Canada, 

1990): (i) tests designed to simulate contaminant release under a specific environmental scenario (e.g., 

synthetic acid rain leach test or TCLP), (ii) sequential chemical extraction tests, or (iii) tests which assess 

fundamental leaching parameters. 

Tests that are designed to simulate release under specific environmental scenarios are limited 

because they most often do not provide information on release under environmental scenarios different 

from the one being simulated. This type of limitation has led to widespread misuse and misinterpretation of

TCLP results. Reliance on simulation-based testing also results in treatment processes that are designed to 

“pass the test” rather than to improve waste characteristics or reduce leaching under actual use or disposal 

scenarios. For instance, it is common practice to include waste treatment additives to buffer the TCLP 

leachant at a pH resulting in minimum release of target constituents. However, when the buffered material 

is landfilled, the landfill leachate pH may be dominated either by the material buffering capacity (monofill 

scenarios) or by other sources (co-disposal scenarios). In either case, the release scenario may differ 

significantly from conditions simulated by the testing protocol and unpredicted leaching behavior may 

occur. 

Sequential chemical extraction tests evaluate release based on extraction of the waste with a series 

of increasingly more aggressive extractants. The sequential extraction approach, originally compiled by

 2

 This assumption is often referred to as the “infinite source” assumption. 

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Environmental Engineering Science, In-press, 2002. 

Tessier et al. (Tessier et al., 1979), has been adapted by others (Frazer and Lum, 1983). These adapted

approaches have limitations that require case-by-case evaluation (Khebohian and Bauer, 1987; Nirel and

Morel, 1990). In addition, the operationally defined nature of sequential extraction approaches make

generalized application in a waste management framework difficult. 

In addition, geochemical speciation modeling also can provide useful insights into leaching 

behavior, as it provides information on possible solubility controlling mineral phases (Meima et al., 1999; 

van der Sloot, 1999; Crannell et al., 2000), the role of sorption processes with Fe, Mn and Al phases 

(Meima and Comans, 1998; Meima and Comans, 1999), and complexation with dissolved organic mater 

(Keizer and van Riemsdijk, 1998; Kinniburgh et al., 1999; van der Sloot, 2002). However, geochemical 

modeling often requires detailed solid phase identification that is either impractical or not possible for 

complex materials, and needed solubility and adsorption parameters may be unavailable. Although the 

information it provides can be used effectively in waste management, geochemical modeling often only

provides qualitative or semi-quantitative results and is not a tool for regulatory control. 

The alternative framework described below was designed to assess intrinsic waste leaching 

parameters, thereby providing a sound basis for estimation of release potential in a range of different

potential waste management scenarios. It provides a basis either for choosing acceptable management or 

disposal from among several possible options or for judging whether a pre-selected management or disposal

option is in fact environmentally sound and appropriate. 

AN ALTERNATIVE FRAMEWORK FOR EVALUATION OF LEACHING 

Waste testing should provide information about potential contaminant release from a waste in the 

context of the anticipated disposal or utilization conditions. Thus, testing should reflect the range of

conditions (e.g., pH, water contact, etc.) that will be present in the waste and at its interface with its 

surroundings during the long-term, which may be significantly different than the properties of the material

immediately following production3

. 

 3 Examples where the material as produced has different constituent release behavior than that during

utilization are: (i) concrete pillars immersed in surface water where release reflects the neutral pH of 

surface water rather than the alkali pH of Portland cement concrete (van der Sloot, 2000), (ii) stabilized 

coal fly ash exposed to seawater showing surface sealing (Hockley and van der Sloot, 1991), (iii) MSWI 

bottom ash used in road base application being neutralized with a few years of field exposure (Schreurs et 

al., 2000), and (iv) use of steel slag in coastal protection applications where V and Cr leaching is reduced 

by the natural formation of ferric oxide coatings in the utilization environment (Comans et al., 1991). 

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The goals of a revised framework for evaluation of contaminant leaching should be to: (i) provide

conservative4

 but realistic estimates of contaminant leaching for a broad range of waste types, constituents 

of concern, environmental conditions, and management options, (ii) utilize testing strategies that can be

carried out using standard laboratory practices in reasonable time frames (e.g., several hours to several 

days, depending on requirements), (iii) provide for release estimates that consider site-specific conditions, 

(iv) encourage improvements in waste management practices, (v) provide flexibility to allow level of 

evaluation (and hence degree of over-conservatism5

) to be based on the user’s requirements, (vi) evolve in

response to new information and take advantage of prior information, and (vii) be cost effective. 

In concert with these goals, evaluation of constituent release can be approached by a series of 

steps: (i) define management scenarios and mechanisms occurring in the scenarios (e.g., rainfall infiltration) 

that control constituent release, (ii) measure intrinsic leaching parameters for the waste or material being

evaluated (over a range of leaching conditions), (iii) use release models incorporating measured leaching

parameters (corresponding to anticipated management conditions) to estimate release fluxes and long-term

cumulative release, and (iv) compare release estimates to acceptance criteria. Management scenarios can 

either be default scenarios that are designed to be conservative or incorporate site-specific information to

provide more-accurate estimates of release. In CEN TC 2926

, such a scenario-based approach has been

described as an experimental standard (ENV 12920, 1996). This standard describes steps very similar to

those identified above. 

The controlling release mechanisms most often can be described in terms of either equilibrium

controlled or mass-transfer rate controlled. Equilibrium controlled release occurs for slow percolation 

through porous or granular materials. Mass transfer rate controlled release occurs when flow is 

predominantly at the exterior boundary of monolithic materials or percolation is very rapid relative to mass 

transfer rate of constituent release to the percolating water. Intrinsic leaching parameters that are to be

measured using laboratory testing are: constituent availability, constituent partitioning at equilibrium

between aqueous and solid phases as a function of pH and liquid-to-solid (LS) ratio, acid and base 

 4

 In this paper, “conservative” estimates of release implies that the actual release will be less than or equal 

to the estimated release during the management scenario considered. 5

 For most cases, more detailed waste characterization results in more accurate estimates of actual 

contaminant release, providing safety margins by reducing the degree of over-estimated release. However, 

more detailed characterization requires additional testing cost and time which may not be justified because 

of either the limited amount of waste to be managed, time constraints, or other reasons. 6

 CEN/TC 292 is the European Standardization Organization (CEN) technical committee dealing with

characterization of waste (established in 1993). For additional information, see www.cenorm.be on the 

Internet. 

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neutralization capacities (ANC and BNC), and constituent mass transfer rates. Definition of management

scenarios and application of intrinsic parameters, release models and decision criteria are discussed in later 

sections of this paper. 

To achieve the desired framework goals and series of evaluation steps, a three-tiered testing

program is proposed (Figure 1). An analogous, tiered approach, developed with input from the authors of

this paper, has been recommended by Eighmy and Chesner for evaluation of secondary materials for use in

highway construction (Eighmy and Chesner, 2001). In the framework presented in this paper, each 

successive tier provides leaching data that is more specific to the material being tested and possible leaching 

conditions than the previous tier. Individual leaching tests are designed to provide data on intrinsic leaching 

parameters for a waste or secondary material. Results from multiple tests, used in combination with either 

default management scenario assumptions (more conservative, but with simpler implementation) or sitespecific information, provide more accurate release assessments. However, the results of a single test (e.g., 

the first tier availability test) can be used as the most conservative approach for management decisions 

when time or economic considerations do not justify more-detailed evaluations. 

Three tiers of assessment can be defined to efficiently address the above waste management

questions and criteria:

Tier 1 – Screening based assessment (availability). 

Tier 2 – Equilibrium based assessment (over a range of pH and LS conditions). 

Tier 3 – Mass transfer based assessment. 

Progressing from Tier 1 through Tier 3 provides increasingly more realistic and tailored, and less 

conservative, estimates of release, but also requires more extensive testing. 

Tier 1 is a screening test that provides an assessment of the maximum potential for release under 

the limits of anticipated environmental conditions, without consideration of the time frame for release to

occur. This concept of maximum potential release is often referred to as “availability.” In practical 

application, availability is operationally defined using a selected test method. Leaching potential is 

expressed on a mass basis (e.g., mg X leached/kg waste). The basis for this bounding analysis would be 

testing under extraction conditions that maximize release within practical considerations (see further 

discussion below). Tier 2 testing is based on defining liquid-solid equilibrium as a function of pH and LS 

(i.e., chemical retention in the matrix). Tier 3 testing uses information on liquid-solid equilibrium in

conjunction with mass transfer rate information (i.e., physical retention of constituents in addition to 

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chemical retention in the matrix). Both Tier 2 and Tier 3 testing may use either default or site-specific 

management assumptions (e.g., infiltration rates, fill depth) to estimate release as a function of time. For a 

scenario, leachate concentrations based on equilibrium will always be greater than or equal to those based

on mass transfer rate. Thus, equilibrium release estimates (Tier 2) may be a conservative approximation in

the absence of mass transfer rate information (Tier 3)7

. 

For Tier 2 and Tier 3 assessments, three levels of testing (Levels A, B or C) are defined. Each of 

the three levels of testing may be used depending on the amount of previous knowledge (test data) of the 

waste, or the degree of site-specific tailoring desired. Level A (in either Tier 2 or 3) uses concise or

simplified tests. The basis for Tier 2A would be measurement of the leaching characteristics at conditions 

that bound the range of anticipated field scenarios for equilibrium (e.g., use of three extractions to define

release at acidic, neutral and alkali pH conditions with consideration of the material’s natural pH at LS=10 

mL/g). The basis for Tier 3A testing would be a coarse estimate of release rates (e.g., a four point, 5 day

monolithic leach test). The data from these tests would be used in conjunction with default management

scenario bounding conditions, and simplified release models, to provide a conservative assessment in the 

absence of more detailed knowledge. Example applications of Level A testing (in either Tier 2 or 3) include 

for routine disposal of wastes that may fail Tier 1 testing, simplified evaluations for disposal or utilization

that can be justified based on more conservative assumptions, and verification that a material being tested

exhibits characteristics similar to a class of materials that has previously been more extensively 

characterized (e.g., Level B, see below). 

Level B testing provides detailed characterization of the waste or secondary raw material. The

basis for Tier 2B testing would be definition of equilibrium over the full range of relevant pH and LS 

conditions (i.e., pH 2-13, and LS 0.5-10 mL/g). The maximum release observed under these conditions also

is functionally equivalent to the availability measured in Tier 1, although the specific values may differ 

based on the method of determination. The basis for Tier 3B testing would be a more complete definition of 

mass transfer rates (e.g., 10 data points over 60 days) and verification of material integrity (e.g., strength

after leaching). These more detailed data can be used in conjunction with either default or site-specific 

management scenario assumptions, and either simplified or advanced release models. For example, results

from Level B testing in conjunction with default scenarios and simplified release models can provide the 

basis for comparison of treatment processes. Results from Level B testing used in conjunction with site-

 7

 Extrapolation of laboratory mass transfer test results to field conditions requires careful consideration of 

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specific information and advanced models provide the most realistic and least conservative assessment. 

Level B testing would only be carried out initially for a material or class of materials generated in large 

quantities, and thereafter only if significant changes in material characteristics are indicated by periodic 

Level A testing. Level B testing provides insight into the critical components for a given material and thus 

providing the basis for selection of a reduced set of parameters for subsequent testing. After completion of 

Level B testing, Level A testing can be used to answer the question, “Does the material currently being 

tested have the same characteristics of the material that was previously characterized in more detail (Level 

B)?” The frequency of testing can be related to the degree of agreement with the level B testing. Good

performance is then rewarded by reduction in test frequency. A deviation then requires initially more 

frequent testing to verify the deviation, and if necessary a return to the level B testing to evaluate the cause. 

Additional examples of application of Level B testing include monofill disposal of special wastes and 

approvals for beneficial use of secondary materials. 

Level C provides the most simplified testing for quality control purposes, and relies on

measurement of a few key indicators of waste characteristics, as identified in the level B testing. An

example of Level C testing would consist of titration of a sample to a designated pH with measurement of 

the concentration of a limited number of constituents in the resulting single extract. Specific Level C testing 

requirements would be defined on a case specific basis. Level C should only be used after Level B testing

has initially been completed to provide a context for quality control. One application of Level C testing 

would be the routine (e.g., daily, weekly or monthly) evaluation of incinerator ash prior to disposal. 

A feedback loop is provided between Tier 2C and Tier 2A within the framework (Figure 1). This 

loop is provided to indicate that Tier 2A testing can be used on a random basis to provide further assurance 

of attainment of regulatory objectives when much more simplified testing is allowed on a routine basis (Tier 

2C). In this case, the Tier 2A testing is compared with the more complete Tier 2B characterization testing to

verify that the batch of material being tested has not deviated significantly from the material that was 

originally characterized and serves as the baseline assessment. A similar approach may be used when

quality control testing is based on mass transfer rate testing (Tier 3C) rather than equilibrium testing (Tier 

2C). 

While the above framework provides the specific basis only for evaluation of inorganic 

constituents, an analogous set of test conditions can be described for evaluation of organic constituents. 

 

the external surface area for water contact and the potential for external stresses. 

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Additional considerations for organic constituents would include (i) the potential for mobility of a nonaqueous phase liquid, (ii) the fact that pH dependence of aqueous partitioning is usually limited to the 

indirect (although important) effect of pH on dissolved organic carbon levels from humic or similar 

substances, and (iii) availability for many organic constituents is limited and may require a more complex

modeling approach. 

DECISION MAKING BASED ON THE EVALUATION FRAMEWORK 

Application of laboratory testing results to environmental decision-making requires linking the 

laboratory data to environmental endpoints of concern (protection of human health and environment). This

is done through data or models that represent environmental processes, including groundwater transport of

released constituents, exposure to humans or animals via drinking water, and the toxicity of the released 

constituents of concern.

This linkage was established for the TCLP based on assuming the test results yielded a leachate 

constituent concentration that reflected anticipated field leachate that would be produced during disposal in

the bounding scenario. This leachate constituent concentration in turn would be reduced through natural 

groundwater attenuation processes as it moved through the groundwater (e.g., dilution and adsorption) 

before reaching a drinking water well. This “concentration-based approach” implicitly assumes an infinite 

source of the constituents of concern and does not account for either the anticipated changes in release over 

time (including exhaustion of the source) or the potential for cumulative effects of release over time. 

Furthermore, this approach considers only the leaching behavior of the material; it does not consider the 

management context (e.g., disposal vs. utilization, design of the management scenario, geographic 

location). Thus, the concentration-based approach establishes a leachate concentration (as measured in the 

TCLP) below which no significant impact to drinking water is anticipated. This approach also can be 

misleading if the test conditions do not reasonably reflect the field conditions (e.g., with respect to pH and 

LS ratio). 

The proposed alternative is a performance or “impact-based approach.” This approach focuses on 

the release flux of potentially toxic constituents over a defined time interval. Thus, the management

scenario is evaluated based on a source term that incorporates consideration of system design, net

infiltration and the leaching characteristics of the material. Basing assessment and decisions on estimated

release allows consideration of the waste as containing a finite amount of the constituent of interest, the

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time course of release, and the ability to adapt testing results to a range of management scenarios. The 

measure of release would be the mass of constituent released per affected area over time (i.e., release flux). 

Knowledge of the release flux would allow more accurate assessment of impact to water resources (e.g., 

groundwater or surface water) by defining the mass input of constituent to the receiving body over time. 

Results of this impact-based approach can provide direct input into subsequent risk assessment for decision 

making, either based on site-specific analysis or using a generalized set of default assumptions. 

Management scenarios 

Waste management or utilization scenarios must be used to link laboratory assessment results to

impact assessment. Defining scenarios for this purpose requires the leaching mode controlling release 

(equilibrium or mass transfer), the site-specific LS ratio, the field pH, and a time frame for assessment. 

Values describing a specific waste management facility or a hypothetical default scenario could be used. 

Using these site conditions with laboratory measures of constituent solubility as a function of pH and LS 

ratio, a simple release model can be used to estimate the cumulative mass of the constituent released over 

the time frame for a percolation/equilibrium scenario. Including laboratory measurement mass transfer rates 

allows application of simple release models for mass transfer rate controlled management scenarios (e.g., 

monolithic materials). 

For a hypothetical default landfill disposal scenario, parameter values may be based on national 

data for different landfill types, or defined as a policy matter. Values for field pH and LS ratio may be either 

measured at an actual site or estimated for the site. Measuring field pH requires collecting landfill leachate 

or landfill pore water and measuring the pH before contact with the air begins to alter the pH. LS ratio

serves as the surrogate parameter for time. Good agreement has been obtained between laboratory test data 

and landfill leachate based on LS (van der Sloot, 2001). Measuring field LS ratio involves measuring the 

volume of leachate collected (annually) from the landfill, and comparing it with the estimated waste volume 

in the landfill, or the landfill design capacity. As an alternative to measuring the LS ratio, it may be

estimated, based on defining the geometry for the management scenario and local environmental

conditions. Parameters for defining the management scenario include fill geometry (relating waste mass to

impacted area), net infiltration rates (defining amount of water contact), and time frame. For example, a 

default disposal scenario may be a fill height of 10 m, 20 cm infiltration per year and 100 years 

(alternatively, the total mass of waste and footprint area may be specified). The selection of the default 

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management scenario is ultimately a consideration of typical waste management practices and of societal 

value judgments reflected in the regulatory development process. 

For discussion purposes, a 100-year interval is suggested as a hypothetical assessment period,

although other time frames could be used8

. For comparison of treated wastes, a cube one meter on edge is

assumed. Laboratory test results are presented primarily as release per unit mass of waste tested (e.g., mg

X/kg waste), but also are presented and used on a concentration basis for Tier 2 testing. 

Environmental Considerations 

Release estimates for most cases assume that conditions influencing release are controlled by the 

waste material and associated design conditions; however, properties of surrounding materials may 

dominate the release conditions in some scenarios. These external stresses (e.g., pH or redox gradients, 

carbonation, co-mingling effects) can lead to substantial deviation from material-driven leaching behavior. 

For instance, caution must be used if large pH or redox gradients exist between the waste and the 

surrounding environment or within the waste matrix. The solubility of many inorganic species may be

strongly a function of pH (e.g., Pb, Cd, Ba) or significantly altered by redox conditions (e.g., Cr, Se, As). 

Large gradients in pH or redox potential can result in precipitation or rapid dissolution phenomena for some

elements as concentration gradients within the material or at the material boundary redistribute over long 

time intervals (van der Sloot et al., 1994; Sanchez, 1996). The release of highly soluble species (e.g., Na, K,

Cl) is not considered a strong function of leachate conditions. 

Redox gradients and reducing conditions may result from material characteristics, biological 

activity or external inputs. Materials with inherent reducing properties include several types of industrial 

slag, fresh sediment and degrading organic matter. Testing of these materials under air-exposed conditions

may lead to unrepresentative answers for the situation to be evaluated. For an appropriate assessment of 

reducing materials, testing and release modeling that considers conditions imposed by external factors, 

rather than by the waste itself, will be necessary. This is still an underdeveloped area of research. 

For most alkaline wastes, the most prevalent interface reaction is absorption of carbon dioxide. 

Carbonation of waste materials results in the formation of carbonate species and neutralization of alkaline

buffering capacity. For Portland cement-based matrices, the conversion of calcium hydroxide to calcium

carbonate has been noted to reduce pore water pH towards 8 (Sanchez, 2001; Garrabrants, 2001). Thus, if

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pH-dependent species are a concern, carbonation of the matrix can play a significant role in predicting longterm release. 

Currently, the proposed approach does not consider the impact of co-mingling different types of

wastes during disposal other than the impact of resulting changes in pH. In cases where a pH gradient

appears to be the most significant factor, release estimates can be accomplished using advanced modeling

approaches in conjunction with characterization data interpolated from the concentration as a function of

pH as defined under Tier 2. Test methods and release models to assess the impact of material aging under 

carbonation and reducing conditions are under development (NVN 7438, 2000; Garrabrants, 2001; Sanchez

et al., 2001a). Experimental work is in progress to evaluate waste-waste interaction by quantifying

buffering of pH, dissolved organic carbon, and leaching from waste mixtures (van der Sloot et al., 2001a; 

van der Sloot et al., 2001b). 

TEST METHODS FOR USE IN THE FRAMEWORK 

CRITERIA FOR EQUILIBRIUM TEST METHODS 

Important considerations for the design of equilibrium test methods are (i) the relationships 

between particle size, sample size and contact time, (ii) definition of an appropriate LS ratio, (ii) selection

of the acid or alkali for pH modification, and (iii) practical mechanical limits. Experimental observations

with several wastes have indicated that use of a maximum particle size of 2 mm and contact time of 48

hours results in a reasonable measurement of equilibrium (Garrabrants, 1998). If diffusion is assumed to be

the rate controlling mechanism, the relationships between particle size and contact time required to 

approach equilibrium can be approximated as diffusion from a sphere into a finite bath (Crank, 1975). 

Critical parameters are the fraction of constituent released at equilibrium, observed diffusivity, particle 

diameter and contact time. The ratio between the fraction of constituent released at a given time and the 

fraction of the constituent released at equilibrium can be considered an index of the approach to 

equilibrium. Results of simulations using this modeling approach are consistent with approaching 

equilibrium after 48 hours for observed diffusivities less than 10-14 m2

/s (Garrabrants, 1998).

 8

 The authors have found 100 years to be a useful period for release estimates. This period is typically 

longer than a lifetime but short enough to be comprehendible. In addition, for many cases, a major fraction 

of the long-term release is anticipated to occur during a period less than this interval. 

 14 
Environmental Engineering Science, In-press, 2002. 

Equilibration times for different particle size systems, assuming all other properties remain

constant (e.g., observed diffusivity, liquid-solid ratio, fractional release at equilibrium), can be evaluated

using a dimensionless time parameter:

2 r

D t obs ⋅ τ = (Equation 1)

where τ is the dimensionless time parameter [-]; 

t is the contact time [s]; 

r is the particle radius [m]; and,

Dobs is the observed diffusivity [m2/s]. 

Based on this approach, achieving a condition equivalent to the 2 mm/48 hr case, a particle size of 5 mm

would require extraction for 12.5 days; for a particle size of 9 mm, 40.5 days would be required. However,

most materials undergoing testing would be sized reduced or naturally have a particle size distribution with

the maximum particle size specified. Thus, a maximum particle size of 2 mm with a 48 hr minimum contact

time is specified as a base case, with alternative conditions suggested considering both equivalent

approaches to equilibrium and practical limitations (Table 1). Demonstration of approximating equilibrium

conditions for the material being tested is recommended before using alternative contact times. 

Selection of sample sizes assumes testing of representative aliquots of the material being 

evaluated. For the base case with a maximum particle size of 2 mm, a sample size of 40 g (equivalent dry

weight) is recommended when carrying out an extraction at an LS ratio of 10 mL/g. Heterogeneous 

materials and materials with a larger particle size will require either testing of larger aliquots or 

homogenization and particle size reduction prior to sub-sampling for testing. A discussion and example of

sampling of heterogeneous materials and particle size reduction followed sub-sampling for leaching tests is 

provided elsewhere (IAWG, 1997). 

For many test methods, an LS ratio of 10 mL/g has been selected to provide adequate extract

volumes for subsequent filtration and analysis while using standard size extraction containers (i.e., 500 

mL). This liquid-to-solid ratio also provides for reasonable approach to equilibrium based on theoretical 

considerations. Typically, use of an LS ratio of 10 mL/g provides solubility-controlled equilibrium over the 

range of pH relevant for extrapolation to the field. The resulting solution concentration is generally only 

 15 
Environmental Engineering Science, In-press, 2002. 

weakly dependent on LS ratio between LS ratio of 10 and 2 mL/g. LS ratio dependence may be verified

using an extraction at lower LS (see methods below). 

In the experimental methods, pH adjustments are made using aliquots of nitric acid or potassium

hydroxide. Nitric acid was chosen to minimize the potential for precipitation (e.g., such as occurring with

sulfuric acid), complexation (e.g., with organic acids or hydrochloric acid) or analytical interferences. It is

also recognized that nitric acid is oxidizing which is a conservative selection due to the solubility behavior

of metal hydroxyl species (e.g., Pb(OH)3

-

, Cd(OH)3

-

) and the potential for oxidizing conditions during

management. However, oxyanions (e.g., chromate) exhibit maximum release at near neutral to slightly 

alkaline conditions that typically are achievable without significant acid additions. Testing for release under 

reducing conditions requires the development of additional test methods because consideration must be 

given to acid selection, sample handling and establishment of reproducible reducing conditions. Potassium

hydroxide was selected to avoid interference with the use of sodium ion as an inert tracer in some

applications; however, sodium hydroxide may be substituted for cases in which potassium characterization

is a concern. 

During extraction, complete mixing should be insured by end-over-end mixing. In all cases, it is

desired to test the material with the minimum amount of manipulation or modification needed prior to

extraction. Thus, it is preferable to avoid sample drying before testing, although this can be acceptable 

when non-volatile constituents are of primary interest and it is necessary to achieve particle size reduction. 

RECOMMENDED TEST METHODS 

The following test methods are recommended for use in the proposed tiered leaching framework.

The general purpose, approach and application of these test methods are shown in Table 2. Detailed 

protocols for these test methods are presented as Appendix A. 

Tier 1 – Screening Tests 

An ideal screening test would result in a conservative estimate of release over the broad range of 

anticipated environmental conditions. In addition, this screening test would require only a single extraction

that could be completed in less than 24 hours. However, this ideal scenario is impossible to achieve. Several 

approaches to measuring “availability” or maximum leaching potential have been developed or considered. 

One approach is a two step sequential extraction procedure with particle size <300 μm, LS=100 mL/g and

 16 
Environmental Engineering Science, In-press, 2002. 

control at pH 8 and 4 (NEN 7341, 1994). Another approach uses EDTA to chelate metals of interest in

solution at near neutral pH during a single extraction (Garrabrants and Kosson, 2000). Either of these 

approaches can be used as a screening test, but both approaches have practical limitations relative to

implementation. The NEN 73419

 requires a small particle size, two extractions and pH control. The 

approach of Garrabrants and Kosson (2000) requires a pre-titration and can have some difficulties in 

controlling the pH. This approach also has been criticized as providing a release estimate that may be too 

conservative. 

Tier 2 - Solubility and Release as a Function of pH

The objectives of this testing is to determine the acid/base titration buffering capacity of the tested 

material and the liquid-solid partitioning equilibrium of the “constituents of potential concern” (COPCs). 

For wastes with high levels of COPCs, the liquid-solid partitioning equilibrium is determined by aqueous 

solubility as a function of pH. For low levels of COPCs, equilibrium may be dominated by adsorption 

processes. However, the concurrent release of other constituents (e.g., dissolved organic carbon, other ions) 

will also impact the results by modifying the solution characteristics of the aqueous phase10. The two 

approaches that have been considered for achieving the objective of measuring solubility and release as a 

function of pH are (i) static (controlled) pH testing at multiple pH values through use of a pH controller at

desired set points (van der Sloot et al., 1997), and (ii) a series of parallel extractions of multiple sample 

aliquots using a range of additions of acid or alkali to achieve the desired range of endpoint pH values

(Environment Canada and Alberta Environmental Center, 1986; Kosson et al., 1996; Kosson and van der

Sloot, 1997; prEN14429, 2001). Both testing approaches have been shown to provide similar results (van 

der Sloot and Hoede, 1997), including determination of both the acid/base titration buffering capacities of 

the tested material and the characteristic behavior of the constituents of potential concern. The static pH

approach has the advantage of being able to achieve desired pH endpoints with a high degree of accuracy.

The parallel extraction approach has the advantage of mechanical simplicity. The range of pH examined

should include the extreme values of pH anticipated under field conditions and the pH when controlled by 

the tested material (i.e., “natural” or “own” pH). Thus, while the recommended method below provides a 

 9

 NEN is the national Dutch standardization organization, where a standardization committee has been

addressing the development of leaching tests for construction materials and waste materials since 1983. 

For additional information, see www.nen.nl on the Internet. 10 For example, the dissolution of organic carbon from a waste has been shown to increase the solubility of

copper in municipal solid waste incinerator (MSWI) bottom ash and several metals in matrices containing 

organic matter (van der Sloot, 2002). 

 17 
Environmental Engineering Science, In-press, 2002. 

full characteristic behavior curve (i.e., for Tier 2, level B testing), an abbreviated version based on three

analysis points may be used for simplified testing (i.e., for Tier 2A). The recommended method below is

also analogous to CEN TC 292 Characterization of Waste – Leaching Behavior Test – pH Dependence Test 

with Initial Acid/Base Addition (prEN14429, 2001). 

SR002.1 (Alkalinity, Solubility and Release as a Function of pH)

This protocol consists of 11 parallel extractions of particle size reduced material at a liquid-to-solid

ratio of 10 mL extractant/g dry sample. An acid or base addition schedule is formulated for eleven extracts 

with final solution pH values between 3 and 12, through addition of aliquots of HNO3 or KOH as needed.

The exact schedule is adjusted based on the nature of the material; however, the range of pH values 

includes the natural pH of the matrix that may extend the pH domain (e.g., for very alkaline or acidic 

materials). Using the schedule, the equivalents of acid or base are added to a combination of deionized (DI) 

water and the particle size reduced material. The final liquid-solid (LS) ratio is 10 mL extractant/g dry 

sample which includes DI water, the added acid or base, and the amount of moisture that is inherent to the 

waste matrix as determined by moisture content analysis. The eleven extractions are tumbled in an endover-end fashion at 28±2 rpm. Contact time is a function of the selected maximum particle size, with an

extraction period of 48 hr for the base case of 2 mm maximum particle size. Following gross separation of

the solid and liquid phases by centrifugation or settling, leachate pH measurements are taken and the phases 

are separated by vacuum filtration through 0.45-μm polypropylene filtration membranes. Analytical 

samples of the leachates are collected and preserved as appropriate for chemical analysis. The acid and base 

neutralization behavior of the materials is evaluated by plotting the pH of each extract as a function of 

equivalents of acid or base added per gram of dry solid. Equivalents of base are presented as opposite sign

of acid equivalents. Concentration of constituents of interest for each extract is plotted as a function of

extract final pH to provide liquid-solid partitioning equilibrium as a function of pH. Figure 2 (a-b) shows 

conceptual output from the recommended SR002.1 protocol with the recognition that a broad range of

behaviors is possible. In Figure 3a, the output data of the SR002.1 protocol for a cementitious synthetic 

waste matrix (Garrabrants, 2001) is compared to the total elemental content and constituent availability 

(Tier 1 value).

The abbreviated version of the SR002.1-A (Alkalinity, Solubility and Release as a Function of pH) 

protocol consists of three parallel extractions of particle size reduced material at a liquid-to-solid ratio of 10

mL extractant/g dry sample. The selection of the target pH values is dependent on the natural pH of the 

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Environmental Engineering Science, In-press, 2002. 

material. If the natural pH is <5, then natural pH, 7 and 9 are selected as the target pH values. If the natural 

pH ranges between 5 and 9, then 5, 7 and 9 are selected as the target pH values, and if the natural pH is >9, 

then 5, 7 and natural pH are selected as the target pH values. 

Tier 2 - Solubility and Release as a Function of LS Ratio 

The objective of this test is to determine the effect of low liquid-to-solid ratio on liquid-solid

partitioning equilibrium when the solution phase is controlled by the tested material. This is used to

approximate initial pore-water conditions and initial leachate compositions in many percolation scenarios 

(e.g., monofills). This objective is accomplished by a series of parallel extractions using multiple aliquots of 

the tested material at different LS ratio with deionized water to achieve the desired range of conditions.

When necessary, results can be extrapolated to lower LS ratio than readily achieved under typical

laboratory conditions. The range of LS ratio examined should include the condition used for solubility and 

release as a function of pH testing (i.e., LS=10 mL/g) and the lowest LS practically achievable that 

approaches typical pore water solutions (i.e., LS=0.5 mL/g). Thus, while the recommended method below

provides a full characteristic behavior curve (i.e., for Tier 2, level B testing), an abbreviated version based

on two analysis points may be used for simplified testing (i.e., for Tier 2A)11. 

For some materials, LS <2 mL/g may be difficult to achieve with sufficient quantity of eluate for 

analysis due to limitations of solid-liquid separation. In addition, the formation of leachate colloids can 

result in overestimation of release for some metals and organic contaminants. Use of a column test is an

alternative to use of batch testing for measuring release as function of LS. A column test (prEN14405, 

2001), similar to the Dutch standard column test (NEN 7343, 1995), has been developed within the 

European Standardization Organization CEN. 

SR003.1 (Solubility and Release as a Function of LS Ratio)

This protocol consists of five parallel batch extractions over a range of LS ratios (i.e., 10, 5, 2, 1,

and 0.5 mL/g dry material), using deionized (DI) water as the extractant with aliquots of material that has 

been particle size reduced. The mass of material used for the test varies with the particle size of the 

material. All extractions are conducted at room temperature (20±2o

C) in leak-proof vessels that are tumbled

in an end-over-end fashion at 28±2 rpm. Contact time is a function of the selected maximum particle size, 

with an extraction period of 48 hr for the base case of 2 mm maximum particle size. Following gross 

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Environmental Engineering Science, In-press, 2002. 

separation of the solid and liquid phases by centrifugation or settling, leachate pH and conductivity 

measurements are taken and the phases are separated by a combination of pressure and vacuum filtration

using 0.45-μm polypropylene filter membrane. The five leachates are collected, and preserved as 

appropriate for chemical analysis. Figure 2 (c-d) shows conceptual output from the recommended SR003.1

protocol with the recognition that a broad range of behaviors is possible. In Figure 3b, the output data of

equilibrium-based protocols (SR002.1 and SR003.1) are compared for a cementitious synthetic waste 

matrix (Garrabrants, 2001). 

The abbreviated version, SR003.1-A (Solubility and Release as a Function of LS Ratio) protocol

consists of two parallel extractions of particle size reduced material using DI water at liquid-to-solid ratio of 

10 and 0.5 mL extractant /g dry sample, respectively. The extraction at an LS ratio of 10 mL/g may be the 

same sample as used in SR002.1-A to reduce the required number of analyses. 

Tier 3 - Mass Transfer Rate (Monolithic and Compacted Granular Materials) 

The objective of mass transfer rate tests is to measure the rate of COPC release from a monolithic 

material (e.g., solidified waste form or concrete matrix) or a compacted granular material. Results of these 

tests are to estimate intrinsic mass transfer parameters (e.g., observed diffusivities for COPCs) that are then

used in conjunction with other testing results and assessment models to estimate release. Results of these 

tests reflect both physical and chemical interactions within the tested matrix, thus requiring additional test 

results for integrated assessment. While the recommended methods are derivatives of ANS 16.1 (ANS,

1986), a leachability index is not assumed nor used as a decision criterion. The recommended methods 

below are also analogous to NEN 7345 (NEN 7345, 1994) and methods under development by CEN/TC

292. 

MT001.0 (Mass Transfer Rates in Monolithic Materials)

This protocol consists of tank leaching of continuously water-saturated monolithic material with 

periodic renewal of the leaching solution. The vessel and sample dimensions are chosen so that the sample

is fully immersed in the leaching solution. Cylinders of 2 cm minimum diameter and 4-cm minimum height

or 4-cm minimum cubes are contacted with DI water using a liquid to surface area ratio of 10 mL of DI 

water for every cm2

 of exposed solid surface area. Larger cylinder sizes are recommended for treated 

materials that have a particle size greater than 2 mm prior to solidification. Typically, the cylinder diameter 

 11 The abbreviated methods for testing solubility as a function of pH (three points) and solubility as a 

function of LS (two points) include one common point in both tests. Thus, for integrated testing under Tier 

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Environmental Engineering Science, In-press, 2002. 

and height or cube dimension should be at least ten times the maximum particle size of the material

contained therein. Leaching solution is exchanged with fresh DI water at pre-determined cumulative times 

of 2, 5 and 8 hours, 1, 2, 4 and 8 days

12. This schedule results in seven leachates with leaching intervals of

2, 3, 3, 16 hours, 1, 2 and 4 days. At the completion of each contact period, the mass of the monolithic 

sample after being freely drained is recorded to monitor the amount of leachant absorbed into the solid

matrix. The solution pH and conductivity for each leachate is measured for each time interval. A leachate 

sample is prepared for chemical analysis by vacuum filtration through a 0.45-μm pore size polypropylene 

filtration membrane and preservation as appropriate. Leachate concentrations are plotted as a function of 

time along with the analytical detection limit and the equilibrium concentration determined from SR002.1 

at the extract pH for quality control to insure that release was not limited by saturation of the leachate. 

Cumulative release and flux as a function of time for each constituent of interest are plotted and used to

estimate mass transfer parameters (i.e., observed diffusivity). Figure 4 shows sample output data from the 

MT001.1 test for a solidified waste matrix (van der Sloot, 1999). The solubility data shown in the figure 

corresponds to data derived from SR002.1. 

MT002.0 (Mass Transfer Rates in Compacted Granular Materials)

This protocol consists of tank leaching of continuously water-saturated compacted granular 

material with intermittent renewal of the leaching solution. This test is used when a granular material is

expected to behave as a monolith because of compaction during field placement. An unconsolidated or

granular material is compacted into molds at optimum moisture content using a modified Proctor 

compactive effort (NEN 7347, 1997). A 10-cm diameter cylindrical mold is used and the sample is packed

to a depth of 7 cm. The mold and sample are immersed in deionized water such that only the surface area of 

the top face of the sample contacted the leaching medium, without mixing. The leachant is refreshed with 

an equal volume of deionized water using a liquid to surface area ratio of 10 mL/cm2

 (i.e., LS ratio of 10 

cm) at cumulative times of 2, 5 and 8 hours, 1, 2, 4 and 8 days (see footnote 12). This schedule results in

seven leachates with leaching intervals of 2, 3, 3, 16 hours, 1, 2 and 4 days. The solution pH and 

conductivity for each leachate is measured for each time interval. A leachate sample is prepared for 

chemical analysis by vacuum filtration through a 0.45-μm pore size polypropylene filtration membrane and 

 

2, four analysis points are recommended. 12 This schedule may be extended for additional extractions to provide more information about longer-term

release. The recommended schedule extension would be additional cumulative times 14 days, 21 days, 28

days, and every four weeks thereafter as desired. Alternately, the duration of the test may be shortened 

(e.g., cumulative time of 4 days) for compliance testing.

 21 
Environmental Engineering Science, In-press, 2002. 

preservation as appropriate. Leachate concentrations are plotted as a function of time along with the

analytical detection limit and the equilibrium concentration determined from SR002.1 at the extract pH for 

quality control. Cumulative release and flux as a function of time for each constituent of interest are plotted

and used to estimate mass transfer parameters (i.e., observed diffusivity). 

RELEASE ASSESSMENT ESTIMATES 

Release estimates may be obtained for site-specific and management scenario-specific cases when

appropriate environmental data (e.g., precipitation frequency and amounts) and design information (e.g.,

placement geometry, infiltration rates) are available. For many situations, site-specific information either 

may not be readily available or may not be necessary (e.g., as in the case when the intent of testing is only

to provide uniform side-by-side comparisons of treatment processes). For these situations, default scenarios 

may be defined; an application of this approach is provided in the companion paper (Sanchez et al., 2002). 

These default scenarios are for illustrative purposes only, and other parameter values may be more 

appropriate for different management scenarios and geographic locations. 

Percolation-controlled scenario 

Percolation-controlled release occurs when water flows through a permeable fill with low 

infiltration rate and low liquid-to-solid ratio (Figure 5). In this case, local equilibrium at field pH is assumed

to be limiting release. The information required to estimate constituent release during this scenario is the (i) 

field geometry, (ii) field density, (iii) anticipated infiltration rate, (iv) anticipated field pH, (v) anticipated

site-specific liquid-to-solid ratio, and (vi) constituent solubility at the anticipated field pH. The anticipated

site-specific liquid-to-solid (LSsite) ratio represents the cumulative liquid-to-solid ratio that can be expected

to contact the fill over the estimated time period. It is based on the infiltration rate, the contact time, the fill 

density and the fill geometry and can be determined according to (Hjelmar, 1990; Kosson et al., 1996): 

H

inf t

LS 10 

fill

year

site ⋅

⋅ = ρ

 (Equation 2)

where, LSsite is the anticipated site-specific liquid-to-solid ratio [L/kg]; 

inf is the anticipated infiltration rate [cm/year]; 

tyear the estimated time period [year]; 

 ρ is the fill density [kg/m3]; 

 22 
Environmental Engineering Science, In-press, 2002. 

 Hfill is the fill depth [m]; and, 

10 is a conversion factor [10 L/cm-m2

].

Over an interval of 100 years or longer, LSsite values greater than 10 mL/g may be obtained for cases that

have relatively high rates of infiltration or limited placement depth (Kosson et al., 1996; Schreurs et al., 

2000). However, for many disposal scenarios, the observed LSsite has been less than 2 L/kg over a period of

ca. 10 years and for an isolated landfill site with reduced infiltration, it may take 1000 years to reach LSsite

of 1 L/kg (Johnson et al., 1998; Johnson et al., 1999; Hjelmar et al., 2001). 

An estimate of the cumulative mass release per unit mass of material can then be obtained using

the anticipated site-specific LS ratio and the constituent solubility at the anticipated field pH (Sfield pH) 

according to: 

( ) site field pH

t

mass M LS )(S year = (Equation 3)

where, is the cumulative mass of the constituent released (mass basis) at time t year t M mass year [mg/kg]; and,

 Sfield pH is the constituent solubility [mg/L] at the pH value corresponding to field pH. 

Mass transfer-controlled scenario 

Mass transfer-controlled scenario occurs when infiltrating water is diverted around a low 

permeability fill or prevented from percolating through the fill due to an impermeable overlay (Figure 6) or 

adjacent high permeability channels. In this case, mass transport within the solid matrix is rate limiting. The 

information required to estimate constituent release during such scenario are the (i) field geometry, (ii) field

density, (iii) initial leachable content and (iv) observed diffusivity of the species of concern. 

The mechanisms of release under mass transfer control can be quite complex and constituentspecific. The rate of COPC diffusion through the material can be retarded by surface reactions or 

precipitation of insoluble compounds. Alternately, mass transport may be enhanced by species 

complexation or mineral phase dissolution. Numerical techniques often are required to fully describe 

release under complex mechanistic conditions. Sophisticated models have been developed, or are under 

development, to dissolution/precipitation phenomena (Batchelor, 1990; Cheng and Bishop, 1990; Batchelor, 

1992; Hinsenveld, 1992; Batchelor and Wu, 1993; Hinsenveld and Bishop, 1996; Moszkowicz et al., 1996;

Sanchez, 1996; Baker and Bishop, 1997; Moszkowicz et al., 1997; Batchelor, 1998; Moszkowicz et al., 

1998), sorption/desorption phenomena, and material heterogeneity (Sanchez et al., 2001b). 

 23 
Environmental Engineering Science, In-press, 2002. 

Fickian Diffusion model

The Fickian diffusion model, based on Fick’s second law, assumes that the species of interest is 

initially present throughout the homogeneous porous medium at uniform concentration and considers that 

mass transfer takes place in response to concentration gradients in the pore water solution of the porous

medium. The assumptions and release estimation approach shown here is most appropriate for release 

scenarios for which only highly soluble species are a concern or for which external stresses (e.g., pH

gradients, carbonation, redox changes) are not significant. 

In the classical representation of the diffusion model, two coupled parameters characterize the 

magnitude and rate of the release: C0, the initial leachable content (e.g., available release potential, total 

elemental content)13 and Dobs, the observed diffusivity of the species in the porous medium. When the 

species of concern is not depleted over the time period of interest, the cumulative mass release can be 

described by a one-dimensional semi-infinite geometry. Depletion is considered to occur when more than

20% of the total leachable content has been released (de Groot, 1993). 

For a one-dimensional geometry, an analytical solution for Fickian diffusion is provided by Crank 

(1975) with the simplifying assumption of zero concentration at the solid-liquid interface (i.e., case of a 

sufficient water renewal; infinite bath assumption): 

1 2















 ⋅ = ⋅ ⋅ π

ρ

D t M 2 C

obs

0

t

area (Equation 4)

where is the cumulative mass of the constituent released (surface area basis) at time t [mg/m t M area 2

]; 

C0 is the initial leachable content (i.e., available or total elemental content) [mg/kg]; 

ρ is the sample density [kg/m3]; 

t is the time interval [s]; and,

Dobs is the observed diffusivity of the species of concern [m2

/s]. 

The test conditions for the MT series protocols (i.e., MT001.1 and MT002.1) are designed to ensure a nondepleting matrix and approximate the zero-concentration boundary, although field conditions may not

satisfy these simplifications for many cases and the resulting release estimate may overestimate release. 

Therefore, other modeling approaches may be required to more accurately extrapolate to field conditions. 

 24 
Environmental Engineering Science, In-press, 2002. 

In release scenarios for which COPC depletion does not occur and Fickian diffusion is considered

the dominant release mechanism, the mass release is proportional to release time by a t1/2 relationship. After

a log transform, Equation 4 becomes:

t

D log M log 2 C

obs

o

t

area log

2

1

1 2

+































 = ⋅ ⋅ π

ρ (Equation 5)

Thus, the logarithm of the cumulative release plotted versus the logarithm of time is expected to be a 

straight line with a slope of 0.5. Often, initial release as observed from laboratory testing reflects wash off 

or dissolution of surface-associated constituents. The apparent constituent release then may be followed by

diffusion-controlled release. Mass release over this initial time when surface phenomena are observed

would result in a line with a slope greater than 0.5. In these cases, only the data points reflecting diffusioncontrolled release are used to estimate observed diffusivity. The initial release should be verified to be

insignificant in relation to the long-term field estimate of release (see Sanchez et al, 2002 for an illustration

of this phenomena). 

Estimation of observed diffusivity 

Under the assumptions of the Fickian diffusion model, an observed diffusivity can be determined 

for each leaching interval where the slope is 0.5±0.15 by (de Groot and van der Sloot, 1992): 

2

















⋅ ⋅ = 2 C ( t - t )

M

D 

0 i i-1

t

obs area i

i

ρ

π (Equation 6)

where is the observed diffusivity of the species of concern for leaching interval i [m obs Di 2

/s]; 

i is the mass released (surface area basis) during leaching interval i [mg/m t M area 2

]; 

ti is the contact time after leaching interval i [s]; and, 

ti-1 is the contact time after leaching interval i-1 [s].

The overall observed diffusivity is then determined by taking the average of the interval observed 

diffusivities. 

Release estimates 

 13 The value used for the initial leachable content and the determined observed diffusivity are coupled 

parameters such that the same set of parameters obtained from experimental data must be used in

determining long-term release estimates. 

 25 
Environmental Engineering Science, In-press, 2002. 

An estimate of the cumulative mass release for the management scenario can then be obtained 

using the analytical solution (Equation 4) over the anticipated assessment interval. When COPC release per 

unit mass of material is desired, conversion based on material field geometry can be applied to Equation 4. 

1 2















 ⋅ = ⋅ ⋅ ⋅ π

D t

V

S M 2 C

obs

0

t

mass (Equation 7)

where, is the cumulative mass of the constituent released (mass basis) at time t [mg/kg]; t M mass

S is the fill surface area [m2

]; and,

V is the fill volume [m3

].

In the case where initial surface wash-off is considered to provide significant contribution to the release 

prediction (i.e., >5% of cumulative release), release from initial surface wash-off is added to release 

estimate from diffusion-controlled phenomena. An estimate of the cumulative mass release can then be

obtained using:

1 2















 ⋅ = ⋅ + ⋅ ⋅ ⋅ −

π

D t

V

S M M S 2 C

obs

0

wash off

area

t

mass (Equation 8)

where, is the mass of constituent released (surface area basis) from surface wash-off [mg/m wash off M area − 2

].

When depletion of the COPC is anticipated to occur over the release interval, three-dimensional analysis 

using finite body models may be required to estimate cumulative release. Analytical solutions may be found

for different geometries in mass transport literature (Crank, 1975) or simplifying assumptions may be

applied to validate the above 1-D approach (Kosson et al., 1996). Alternately, numerical methods may be 

used to solve the Fickian diffusion equation in three dimensions (Barna, 1994). 

The above estimates represent a conservative approach for most mass transfer-controlled release 

scenarios where significant external stresses are not present. A zero surface concentration assumes a 

maximum gradient, or driving force, for mass transport (infinite bath assumption). In the case of slow water 

flow past the surface or small liquid-to-surface area ratios, accumulation of the COPC concentration in the 

leachate reduces the concentration gradient and limits leachate concentration to the mass of COPC in 

equilibrium with the solid phase. Thus, the upper bound (or maximum concentration) for mass transfercontrolled release should be estimated using release estimates obtained from equilibrium assumptions (e.g., 

Tier 2 testing in conjunction with percolation controlled release). 

 26 
Environmental Engineering Science, In-press, 2002. 

Other modeling considerations 

Mass transport modeling approaches (Garrabrants, 2001; Garrabrants et al., 2001; Sanchez et al., 

2001a; Tiruta-Barna et al., 2001) are under development to address environmental conditions that are more

likely to be encountered in the field such as intermittent wetting under varied environmental conditions (i.e., 

relative humidity and CO2 content). Additional modeling also has been done to relate column test results to

field leaching through application of geochemical speciation (Dijkstra et al., 2001). These models can

provide more accurate release estimates, but typically require additional information (experimental and

field) and greater expertise for use. The simple modeling approach provided here is intended to be a 

conservative, first-order approximation that will result in overestimation of actual release for most cases. 

EXAMPLE APPLICATIONS OF THE FRAMEWORK 

Important potential applications of the leaching framework defined here include (i) the 

comparative assessment of waste treatment processes, such as for determinations of equivalent treatment 

under RCRA, (ii) estimating environmental impacts from utilization of secondary materials in construction

applications, or (iii) estimating releases from large scale waste monofills. For these cases, Tier 2B and Tier 

3B testing is recommended for initial evaluation. An example of this application is provided in the 

accompanying paper (Sanchez et al., 2002). Subsequently, Tier 2A testing can be used to establish 

consistency between the materials initially tested and other similar materials.

ECONOMIC CONSIDERATIONS 

The more extensive testing recommended in the proposed framework will obviously increase 

initial testing costs. However, these initial costs should be offset by several factors. First, detailed

characterization of a material is only necessary initially to define its characteristic leaching properties, and 

only for materials that are produced in relatively large quantities. Subsequently, much less testing is needed 

to verify that new samples conform to the previously established properties. Second, cost savings should be

realized through the framework by enabling alternative management strategies that are not possible under 

the current rigid system. Treatment processes evaluated under this system will be better targeted to reducing

leaching under field scenarios. Reduced treatment costs may be achieved in many cases (however, 

 27 
Environmental Engineering Science, In-press, 2002. 

treatment costs may increase in cases where treatment processes were only effective at meeting TCLP, but 

were ineffective at reducing leaching in the field to levels consistent with risk-based endpoints). In addition, 

the potential for environmental damage and future liability will be reduced because of the closer 

relationship between testing and field performance. Costs for Tier 1 and Tier 2A testing should be of the

same order-of-magnitude as current TCLP testing. Reductions in costs are anticipated as the methods

become commercialized and data interpretation is automated.

CONCLUSIONS 

The proposed framework presents an approach to evaluate the leaching potential of wastes over a 

range of values for parameters that have a significant impact on constituent leaching (e.g., pH, LS, and 

waste form) and considering the management scenario. This approach presents the potential to estimate 

leaching much more accurately (than many currently used leach tests), relative to field leaching, when 

conditions for leach test data are matched with field conditions. The greater accuracy of the proposed 

approach makes it a useful tool for examining waste and assessing the environmental soundness of a range 

of waste management options as well as for assessing the effectiveness of proposed waste treatment

methods. In addition, the proposed framework provides flexibility to the end user to select the extent of 

testing based on the level of information needed and readily permits the incorporation of new testing

methods and release models as they are developed for specific applications. Appropriately used in waste 

regulatory programs, this approach could make those programs substantially more cost-effective and 

protective of the environment. The flexibility of the proposed approach allows for development of the 

framework to provide a greater degree of tailoring to site conditions, to account for the effects of other 

waste leaching parameters critical to a particular site. Reliance on a tiered approach to testing can also make 

this approach more economical for smaller waste volumes and therefore more broadly feasible. 

ACKNOWLEDGEMENT AND DISCLAIMER 

Primary support for this research was provided by the USEPA Northeast Hazardous Substances Research 

Center and the USEPA Office of Solid Waste. Limited support also was provided by (i) The Consortium for

Risk Evaluation with Stakeholder Involvement (CRESP) through U.S. Department of Energy grants DEFG26-00NT 40938 and DE-FG02-00ER63022.A000 and (ii) EU DG Research funded projects. The authors 

 28 
Environmental Engineering Science, In-press, 2002. 

gratefully acknowledge the thoughtful feedback from Mr. Greg Helms (USEPA), and Dr. Charles W.

Powers (Institute for Responsible Management) during the development of this manuscript and the 

technical support of Ms. Teresa Kosson during the development of the test methods. The authors also

gratefully acknowledge the thoughtful comments and feedback of the anonymous reviewers and the 

assistance of the Editor, Dr. D. Grasso, lead author of the USEPA Science Advisory Board Review (1999) 

for which this paper is primarily in response. The viewpoints expressed in this paper are solely the 

responsibility of the authors and do not necessarily reflect the view or endorsement of the USEPA. 

 29 
Environmental Engineering Science, In-press, 2002. 

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Table 1. Specifications for the base case and suggested alternative conditions for equilibrium extractions. 

Maximum Particle Size [mm]

Base case Suggested alternates 

2 0.3 5

Minimum sample size [g] 40 20 80 

Minimum contact time [hr] 48 18 168 (7 days) 

Container size [mL] 250 500 1000

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Environmental Engineering Science, In-press, 2002. 

Table 2. Comparison of recommended leaching protocols and applications. Tier Test Name Purpose Methodology Output Application

1 AV001.1 To determine the

potentially extractable content of constituents under environmentalconditions. 

Parallel extractions at pH 4and 8 in DI water; Liquid-tosolid (LS) ratio of 100 mL/g; contact time dependent onparticle size. 

Availability at pH 4Availability at pH 8

Screening: conservative release estimate. Characterization: realistic source term for 

modeling mass transport-controlledrelease. 

1 AV002.1 To determine the

potentially extractable content of constituents under environmentalconditions. 

Single extraction using 50mM EDTA; LS ratio of 100mL/g; contact timedependent on particle size 

Availability in EDTA Screening: conservative release estimate. 

Characterization: realistic source term for 

modeling mass transport-controlledrelease. 

2 SR002.1 To obtain solubility and

release data as a function of leachate pH

Multiple parallel extractionsusing DI water and HNO3 or

KOH; LS ratio of 10 mL/g; contact time dependent onparticle size. 

Material-specific 

acid/base titration curve. 

Solubility and release as 

a function of pH

Characterization: detailed behavior of 

COPC as a function of pH. 

Compliance: abbreviated protocol to

indicate consistency with previouscharacterization. 

2 SR003.1 To estimate pore water 

conditions by obtainingsolubility and release data as a function of LSratio. 

Multiple parallel extractionsusing DI water; LS ratios of0.5 to 10 mL/g; contact time dependent on particle size. 

Solubility and release as 

a function of LS ratio. 

Characterization: detailed behavior of 

COPC as a function of LS ratio. 

Compliance: abbreviated protocol to

indicate consistency with previouscharacterization. 

3 MT001.1 To determine mass

transfer parameters. 

To estimate rate of 

release under continuously saturated conditions. 

Semi-dynamic tank leachingof monolithic material; Liquid-to-surface-area ratio of 10 [mL/cm2] 

Observed constituent 

diffusivity. 

Rate and cumulative 

release of constituent release under continuously saturated conditions. 

Characterization: detailed leaching 

mechanisms and rate of release under mass-controlled leaching scenario. 

Compliance: abbreviated to indicate 

consistency with previouscharacterization. 

3 MT002.1 To determine mass

transfer parameters. 

To estimate rate of 

release under continuously saturated conditions. 

Semi-dynamic tank leachingof compacted granular material; Liquid-to-surfacearea ratio of 10 [mL/cm2] 

Observed constituent 

diffusivity. 

Rate and cumulative 

release of constituent release under continuously saturated conditions. 

Characterization: detailed leaching 

mechanisms and rate of release under mass-controlled leaching scenario. 

Compliance: abbreviated to indicate 

consistency with previouscharacterization. 

36 
Environmental Engineering Science, In-press, 2002. 

MATERIAL

WASTE, SOIL OR PRODUCT

MANAGEMENT SCENARIO

y Specific disposal or

utilization scenario

y Default cases

SCREENING

TIER 1 Tier 1

TIER 2

TIER 3

MASS

TRANSFER RATE

Compliance

Tier 3A

LEVEL A LEVEL B LEVEL C

EQUILIBRIUM

Compliance

Tier 2A

EQUILIBRIUM

Characterization

Tier 2B

MASS

TRANSFER RATE

Characterization

Tier 3B

EQUILIBRIUM

Quality Control

(Material specific)

Tier 2C

MASS

TRANSFER RATE

Quality Control

(Material specific)

Tier 3C

Random compliance testing

Default Scenario

Specific or

Default (see text)

Scenario

Random compliance testing

Figure 1. Alternative Framework for evaluation of leaching 

 37 
Environmental Engineering Science, In-press, 2002. 

2

4

6

8

10

12

14

-2 0 246 8 10

Acid Added [meq/g dry]

pH

Matrix A

Matrix B

a)

KOH HNO3

0.00001

0.0001

0.001

0.01

0.1

1

10

100

1000

4 6 8 10 12

Leachate pH

Conc [mg/L]

Cationic Amphoteric Oxyanionic Highly Soluble

b)

10

11

12

13

14

0 2 468 10

LS Ratio [mL/g dry]

pH

Matrix A

Matrix B

c)

0

200

400

600

800

1,000

0 2 468 10

LS Ratio [mL/g dry]

Conc [mg/L]

Constituent 1

Constituent 2

d)

Figure 2. Conceptual data obtained using equilibrium-based testing protocols: a) titration curve (SR002.1), 

b) constituent release as a function of pH (SR002.1), c) pH as a function of LS ratio (SR003.1), and d)

constituent concentration as a function of LS ratio (SR003.1). 

DRAFT – do not cite, quote or distribute

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0.0001

0.01

1

100

10000

2 4 6 8 10 12 14

Leachate pH

Lead Release [mg/kg]

Release f(pH)

Avail (pH4)

Total

R l fi

a)

0.001

0.01

0.1

1

10

100

8 10 12

Leachate pH

Lead Conc [mg/L]

14

SR002.1 Conc f(pH)

SR003.1 Conc f(LS)

b)

Figure 3. Actual data obtained using equilibrium-based testing protocols from a cementitious synthetic 

waste: a) lead release as a function of pH compared to lead availability and total lead content and b)

comparison of SR002.1 and SR003.1 concentration data. 

DRAFT – do not cite, quote or distribute

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Environmental Engineering Science, In-press, 2002. 

6.5

7

7.5

8

0 20 40 60 8

Time (days)

Leachate pH

a)

0

0

200

400

600

800

1000

0 20 40 60 8

Time (days)

Ba Concentration 

μg/L

0

MT001.1 data

SR002.1 data

b)

0

40

80

120

160

0 20 40 60 8

Time (days)

Cumulative Ba Release (mg/m2

) c)

0

0.01

0.1

1

0.1 1 10 100

Time (days)

Ba Flux (mg/m2 s)

d)

slope=-0.5

Figure 4. Actual data obtained using MT001.1 protocol from a stabilized waste (van der Sloot, 1999): a) 

leachate pH as a function of cumulative time, b) comparison of leachate barium concentration (MT001.1) 

and barium solubility as a function of pH (SR002.1), c) cumulative release of barium as a function of

cumulative time, and d) barium flux as a function of mean cumulative time. 

DRAFT – do not cite, quote or distribute

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Release scenario: Percolation

Seepage Basins

V

Sfield pH

@ Local equilibrium at field pH is rate limiting

Scenario characteristics

- Granular or highly permeable material

- Low infiltration rate 

- Low liquid-solid ratios [mL/g]

Site information

- Infiltration rate inf

- Fill density ρ

- Fill geometry H fill

- Field pH

Figure 5. Release scenario: percolation. 

DRAFT – do not cite, quote or distribute

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Environmental Engineering Science, In-press, 2002. 

Release scenario: Diffusion-controlled 

scenario

Csat

Cs≈0

Roadbase material

@ Mass transport within solid matrix is rate limiting

Scenario characteristics

- Low permeability material

- High infiltration rate 

- High liquid-surface area ratios

Site information

- Fill density

- Fill geometry S, V

- Fill porosity

Figure 6. Release scenario: diffusion-controlled scenario. 

DRAFT – do not cite, quote or distribute

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