Risk-Based Waste Classification in California (1999)

In this chapter, the NRC committee identifies the task that was given to the committee for the review of the California Environmental Protection Agency (Cal/EPA) Department of Toxic Substances Control's (DTSC's) proposed risk-based classification system for wastes that are not subject to the federal Resource Conservation and Recovery Act (RCRA). The committee's approach to its task and the materials that were reviewed are discussed. The chapter then indicates those areas of waste management that the committee considered to be beyond the scope of its task. A description of the organization of the body of the report and the topics that are covered in the remaining chapters are then presented. The remainder of the chapter is an overview of DTSC's current and proposed waste classification systems.

The NRC Committee on Risk-based Criteria for Non-RCRA Hazardous Waste was given the following task:

DTSC provided the committee with a document entitled ''Risk Based Criteria for Non-RCRA Hazardous Waste: A Report to the National Research Council Introducing Proposed Changes to the Definition of Hazardous Waste in the California Code of Regulations'' (DTSC 1998a). That document contains the specifics of the DTSC approach for the development of new risk-based values for toxicity threshold limit concentrations (TTLCs) and soluble or extractable regulatory thresholds (SERTs), including the documentation for the CalTOX model and its modifications, the lead risk-assessment spreadsheet, the modified preliminary endangerment assessment model, the ecological risk assessments, and several sections on the use of the federal toxicity characteristic leaching procedure in place of the California waste extraction test. This 1,494-page document and the spreadsheets implementing the models were the bases for the committee's review of DTSC's proposed methodology. In addition, the committee made use of several publicly available documents on the DTSC's home page on the Internet (http://www.dtcs.ca.gov) that described the regulatory context in which the proposed waste classification was developed (DTSC 1998b). After a preliminary review of the DTSC report, the committee determined that additional clarification was required on specific issues, such as the scenarios that DTSC used to develop their upper and lower toxicity criteria. The committee prepared and submitted 86 questions to DTSC to clarify these issues. DTSC provided answers to each of the questions (DTSC, personal commun., October 9, 1998, see Appendix C, No. 28).

In addition to the DTSC report and ancillary materials, the committee held two public meetings (on September 10, 1998 and November 20, 1998) to gather information from DTSC and other interested parties. Before, during, and immediately after the public meetings, numerous comments were received from various industry representatives and waste generators, environmental and public interest groups, local waste handlers (e.g., sanitation facilities), and affected trade associations (e.g.,

scrap-recycling industry). DTSC also provided some supplemental materials to the committee (DTSC, personal commun., October 9, 1998, see Appendix C). A list of all materials reviewed by the committee is provided in Appendix C.

The regulatory, legal, and policy context within which DTSC is proposing to change its waste classification scheme is not considered to be within the scope of this report. Other issues not considered in this report include DTSC's authority under various California statutes; the list of chemicals for which DTSC has proposed TTLCs or SERTs; the economics of compliance with this classification method; ramifications for other California regulatory agencies; and the appropriateness of disposal requirements, predisposal management requirements, or regulation of discharges to sanitary sewers.

Instead, the focus of this report is on the scientific validity of DTSC's waste classification methodology. The management of a waste after it is classified as hazardous, nonhazardous, or special is outside the committee's task and was not included in the methodology provided by DTSC. When appropriate, however, the committee indicates where implementing the DTSC proposal might be difficult in terms of management of the various waste streams or where regulatory or management decisions that affect risk estimates might require comment. The committee notes that the terms hazardous waste and nonhazardous waste, while appropriate for a classification system based on toxicity, may not be technically appropriate for one based on risk, when the definition of the term "hazardous" refers to the toxicity of an agent and is independent of exposure.

This report is arranged to reflect the committee's analysis of the proposed DTSC risk-based method for classifying hazardous waste. In Chapter 2, the overall approach of the DTSC to identification of hazardous waste using a risk-based rather than a hazard-based process is discussed. Chapter 2 examines DTSC's proposed approach for protecting the California population and environment from risks due to exposure to hazardous substances, the use of technical data as support for policy decisions, and the use of a chemical-by-chemical approach for classifying wastes, with the uncertainties surrounding such an approach. In Chapter 3, the committee examines DTSC's method of selecting exposure scenarios and the application of the models to the selected scenarios. The

validity of the scenarios and models is assessed in terms of their protectiveness of the health of the California population and environment. The details of the component-model input parameters, assumptions made in running the exposure models, who the models are intended to protect, and other health-related values are explored in Chapter 4. Chapter 4 also compares the use of the two analytical extraction methods for leachates from solid waste (toxicity characteristic leaching procedure (TCLP) and waste extraction test (WET)) and DTSC's use of acute toxicity data for classifying wastes. Finally, in Chapter 5, the committee identifies some programmatic concerns with the proposed DTSC method and suggests modifications of the DTSC approach to enhance clarity and effectiveness.

In this section, the committee presents a synopsis of the DTSC's hazardous waste-classification system as presented to the committee (DTSC 1998a) and from other sources, such as "A Risk-Based Regulatory System: Revision of California Non-Resource Conservation and Recovery Act Waste Classification Regulation" (DTSC 1998b). This synopsis is intended to assist the reader in understanding the regulatory context in which DTSC has proposed changes to the hazardous waste classification system, and what those changes are. The committee has attempted to provide an accurate portrayal of DTSC's program based on the documentation provided by DTSC (DTSC 1998a), DTSC's written responses to questions from the committee (DTSC, personal commun., October 9, 1998, see Appendix C, No. 28), information gathered by the committee at two public meetings, and additional written materials from DTSC and the public; however, this synopsis is not intended to be a comprehensive or legal description of the current or proposed hazardous waste classification systems. For a further description of DTSC's proposed system, refer to the DTSC report.

In 1976, the U.S. Congress passed the Resource Conservation and Recovery Act (RCRA) (Public Law 94-580). RCRA gave the U.S. Environmental Protection Agency (EPA) the authority to control hazardous waste from

generation to disposal. Under Subtitle C of RCRA, EPA regulates the management, storage, treatment, and disposal of certain hazardous wastes including disposal in subtitle C landfills. In subtitle D, RCRA also set forth a framework for the management of nonhazardous wastes, including disposal in subtitle D landfills.

Under RCRA, states have primary responsibility for the implementation of hazardous-waste regulations. State programs, authorized and approved by the EPA, must be at least equivalent to the federal regulations; however, such state programs may be more stringent or broader in scope than the federal program. Under the Hazardous Waste Control Act of 1972 (California Health and Safety Code, §25100 et seq.), which precedes the federal RCRA statute, the State of California mandated that DTSC develop regulations for the identification, management, and disposal of hazardous wastes. DTSC, after complying with RCRA requirements, uses an additional regulatory process to supplement the federal statute. Under California's regulations, certain wastes not classified as hazardous under RCRA may be classified and managed as hazardous waste if DTSC determines that they might adversely affect human health or the environment when disposed of as unregulated wastes.

Like the federal RCRA, the DTSC waste-classification system identifies wastes as hazardous on the basis of their ignitability, corrosivity, reactivity, or toxicity. California uses the federal RCRA criteria for ignitability and reactivity. For corrosivity, it broadens the definition to include corrosive solids in addition to liquids. The California waste-classification system differs from the federal RCRA system in two primary ways. First, California has developed a list of 800 chemicals whose presence in a waste introduces a rebuttable presumption of hazard due to a potential characteristic of concern (corrosivity, reactivity, ignitability, and/or toxicity) annotated for each such chemical. The presumption can be rebutted by the testing the waste for the characteristics listed for the chemicals present in the waste. For toxicity, the relevant tests are acute oral, dermal, and inhalation toxicity, and the fish bioassay. These 800 chemicals are not discussed in the DTSC report (DTSC 1998a) and their designation and use were not evaluated by the committee.

Second, RCRA bases the toxicity of a waste on the results of a toxicity characteristic leaching procedure (TCLP) test, whereas California

uses eight criteria to determine the toxicity of a waste, only the first of which is equivalent to the federal statute. The California criteria are the following:

• Federal definition as a hazardous waste.
• Exceedence of TTLCs or soluble threshold limit concentrations (TTLCs) for specific chemicals.
• An acute oral toxicity less than 2,500 milligrams (mg) of waste per kilogram (kg) of body weight.
• An acute dermal toxicity less than 4,300 mg/kg of body weight.
• An acute inhalation toxicity less than 10,000 parts per million (ppm) in air.
• An acute aquatic toxicity, based on a fish bioassay, less than 500 mg/liter (L) of water.
• The presence of any one or more of 16 carcinogens in waste at a concentration exceeding 10 mg/kg.
• Shown by experience or testing to be hazardous to public health or the environment (new threats).

Under the current one-threshold system, DTSC classifies wastes as either hazardous or nonhazardous based on whether the toxicity of the waste or its constituents exceed one or more of the toxicity criteria; there is also a small category of hazardous wastes called "special" wastes for which DTSC has granted variances for disposal to specific nonhazardous landfills. Hazardous wastes must be disposed to landfills that have double composite liners and leachate collection systems (class I).¹ Nonhazardous wastes may be disposed to municipal solid-waste (class III) landfills and are not further regulated by DTSC. Special wastes may

go to class I or non-class I landfills, such as class II, but must meet all other hazardous waste regulations before disposal.

DTSC distinguishes between wastes that are likely to leach into groundwater and those wastes for which exposure is from pathways other than groundwater. To determine whether a waste may be classified as hazardous due to the groundwater pathway, the waste is extracted using a California-specific test called the Waste Extraction Test (WET). The concentrations of specific chemicals in the extract are then compared with the STLCs to determine whether the waste is hazardous. STLCs are derived from the California drinking-water maximum contaminant levels (MCLs) or from fish toxicity values as designated in the EPA's ambient water quality criteria. STLCs have been established for 36 chemicals: the values of 13 are identical to the RCRA toxicity characteristic values, 4 are higher than the RCRA toxicity characteristic values, and 1 is lower; although it must be borne in mind that different tests are used (WET versus TCLP) so that the values are not directly comparable. The other 18 chemicals with STLCs are not on the RCRA list of chemicals with TCLP regulatory concentrations. If the concentration of any of the 36 chemicals in the waste extraction test is higher than the STLC threshold for that chemical, the waste is considered to be hazardous.

In addition to STLCs, which address leachable contaminants, the waste is analyzed to determine whether the concentration of any chemical exceeds the total threshold limit concentrations (TTLC). TTLCs are used to account for exposure pathways other than groundwater (e.g., air, surface water, and soil). For any particular chemical, the numeric value of the TTLC, expressed in milligrams per kilogram, is generally 100 times the numeric value of the STLC, expressed in milligrams per liter. TTLCs have been derived for 38 chemicals. If the concentration of any of these chemicals in a waste exceeds the TTLC for that chemical, the waste is considered to be hazardous and is subject to disposal in a class I landfill.

The current system has only one set of toxicity thresholds for STLCs or TTLCs. If a waste does not contain chemicals for which STLCs or TTLCs have been established, it may still be classified as hazardous if it is determined to be acutely toxic to mammals or fish² by direct testing or

by a calculation that factors in the acute toxicity and weight proportion of its constituents. Wastes are considered to be hazardous if they have an acute oral LD₅₀ of less than 2,500 mg/kg of body weight, an acute dermal LD₅₀ of less than 4,300 mg/kg, an acute inhalation LC₅₀ of less than 10,000 parts per million (ppm), or an aquatic LC ₅₀ (based on a fish bioassay) of less than 500 mg/L. An acute oral toxicity threshold of 2,500 mg of a chemical per kilogram of body weight was established by California statute on January 1, 1997 (DTSC 1998b). Acute dermal and inhalation (vapor) toxicity limits were established by multiplying the "highly hazardous" level (LD₅₀ or LC₅₀ values) recommended by the National Institute for Occupational Safety and Health (NIOSH 1974) by a safety factor of 100. Aquatic toxicity levels are based on the list of hazardous substances at 40 CFR 116.

A waste may also be hazardous if it contains any of 16 carcinogens at a mass fraction greater than 10 mg/kg or if data are available to suggest that a new chemical, or a new route of exposure may pose a threat to human health or the environment. The 16 carcinogens are those identified in the California Occupational Carcinogens Control Act of 1976 (California Labor Code § 9004).

Under the existing waste-classification system, DTSC regulates hazardous wastes that pose a low risk to human health or the environment in the same way it regulates wastes that pose a high risk; mitigating information on potential exposures is not factored into the classification of waste containing the chemicals. The small group of "special" wastes consists of wastes that contain only metals (on the list of 800 chemicals) that are in a non-leachable form. Again, the risks they might pose to human health or the environment are not known. Wastes that do not satisfy any of the criteria for a hazardous or special waste are not further regulated by DTSC, but may be subject to regulation by other California waste management agencies.

For the past 3 years, California has been engaged in a regulatory structure update (RSU). The aim of the RSU is to reduce regulatory burdens while retaining requirements needed to protect the citizens and environment of California. The approach of the RSU is to review and update California's hazardous-waste regulations using the most current scientific

data on health and environmental effects of chemicals. The RSU is based on the four guiding principles (DTSC 1997):

• Protect public health and the environment without unnecessarily hindering sustainable growth and development.
• Eliminate or modify regulations that are duplicative, ineffective, or do not provide needed protection or information.
• Foster voluntary compliance through regulatory flexibility, simplicity, and facilitating self responsibility.
• Ensure that California's hazardous-waste regulatory program reflects the current roles of other federal, state, and local regulatory programs.

To comply with the goals of the RSU, DTSC reviewed its waste-classification system, which has been in use for 12 years, and determined that changes are required. DTSC indicated that revisions to the current classification system are needed because the present TTLCs are inflexible, in that they (DTSC 1998a, p. 7):

• Require all wastes to be either rigidly regulated or completely unregulated by DTSC.
• Cannot incorporate advances in technical information.
• Provide no guidance for decisions on specific waste streams (variances or reclassification).
• Do not provide a defined mechanism for regulating additional chemicals.

The DTSC has proposed changes to its current approach to incorporate recent advances in exposure assessment, toxicology, and risk assessment. DTSC recognizes that there is new information available on many waste constituents indicating they are more toxic or less toxic than previously thought. Moreover, new technologies have permitted the separation, reduction, and recycling of many compounds, affecting the composition of some waste and the way in which it is disposed.

According to the proposed plan, the waste-classification criteria would use a risk-based approach to create a mid-level classification for waste that is not risky enough to be classified as hazardous but too risky to be classified as nonhazardous. Thus, waste not classified as RCRA

hazardous waste would be classified into the following three categories: (1) fully regulated hazardous wastes, (2) special wastes with reduced regulatory requirements, and (3) nonhazardous wastes with no regulatory requirements imposed by DTSC. Figure 1-1 compares the current and the proposed systems.

Under the RSU, DTSC's new regulatory approach has several goals:

• Incorporate risk into the classification of hazardous waste.
• Replace WET with the RCRA TCLP.
• Provide a step-by-step process for classifying wastes.
• Revise the criteria for special classification of waste so that they are risk-based.
• Classify waste on the basis of a constant, incremental risk threshold.

A schematic of the proposed DTSC non-RCRA waste-classification

* Under the current system, a very small number of hazardous wastes receive variances for disposal as special wastes.

Figure 1-1

DTSC's waste-classification systems.

system is shown in Figure 1-2. Under the proposed two-threshold classification system, DTSC would establish two risk thresholds by which wastes would be classified as hazardous, nonhazardous, or special. If the

Figure 1-2

Proposed Non-RCRA Waste Classification System. Source: DTSC (1998a, p 36). A slightly amended version with a corrected value for the upper threshold inhalation LC₅₀ was submitted to the committee by Dr. James Carlisle (personal commun., January 28, 1999). The figure omits the comparisons for toxicity of vapors and does not appear to exactly match the discussion in DTSC 1998a. Also omitted are any considerations of ignitability, reactivity, carcinogenicity, or new threats; and "CA Exempt" does not appear to be explained anywhere in DTSC, 1998a).

concentration in a waste of each regulated substance is lower than a certain threshold (an ''exit'' threshold), the waste would be considered nonhazardous and could be disposed to landfills other than class I landfills. If any regulated substance is present at a concentration at or above the upper threshold established for that substance, the waste would be considered hazardous and would need to be disposed to a class I landfill with a double composite liner and leachate collection system. If the concentration in a waste of all regulated substances is below the upper threshold, but the concentration of one or more of such substances exceeds the lower threshold, the waste would be classified as special. Such waste could be disposed to a landfill with a single composite liner and leachate collection system provided the waste is not determined to be hazardous by the other criteria of reactivity, corrosivity, or ignitability. Special wastes would also be subject to modified reporting and handling requirements. Risk-based thresholds have been proposed by DTSC (1998a) for the chemicals (exceptions noted) in the list shown below.

Under the proposed DTSC scheme, the risk-based approach to TTLCs requires the identification of possible waste-management situations and the potential exposure scenarios that could result. For each exposure scenario, the possible pathways by which humans and other organisms could be exposed to chemicals in the wastes are identified. The exposure-pathway information provides input into a mathematical model that estimates the risk associated with each scenario (see Table 1-1). For the upper TTLCs (i.e., separating special and hazardous wastes), DTSC decided that an appropriate exposure scenario is disposal in a municipal solid-waste landfill comparable to a RCRA Subtitle D landfill. The lower TTLC (i.e., separating special and nonhazardous wastes) is derived to be protective of humans and ecological receptors if the waste is used as a soil amendment (i.e., the land conversion scenario).

For the upper TTLC, DTSC determined that risks are likely to be greatest for workers at the landfill and for residents living adjacent to the landfill. Risk estimates for the workers are generated by a modified preliminary endangerment assessment (PEA) model that is used to estimate exposures to both organic and inorganic waste constituents. Risks

to residents living near the landfill are calculated using the CalTOX landfill model for organic waste constituents or the PEA model for inorganic constituents (DTSC 1998a, p. 57). The risk of chronic lead exposure, excluding carcinogenesis, for on-site workers is estimated using the DTSC lead risk-assessment spreadsheet (LeadSpread). These

exposure scenarios and the models used to estimate risks are shown in Table 1-1.

For lower TTLCs, an exposure scenario is used that corresponds to residents living on land to which waste has been applied at a rate of 7,000 kilograms per hectare annually for 20 years. In this scenario, DTSC assumes that the waste has been thoroughly mixed with the upper 15 centimeters of soil. PEA, CalTOX, and the lead risk-assessment spreadsheet are used to estimate risks for this scenario.

The management of special and hazardous wastes was assumed to be adequate to protect nonhuman biota, so that only lower TTLCs needed an ecological assessment. That assessment is performed using a two-step approach. The first step consists of a comparison of the human toxicity exit concentration designated in the proposed EPA Hazardous Waste Identification Rule (HWIR) (61 Fed. Register 18780, Apr. 29, 1996) with the ecological toxicity exit concentration in that same proposal. If, for a particular chemical, the human toxicity exit concentration was lower than the ecological toxicity exit concentration in the HWIR proposal, DTSC considered that the criterion derived in the lower TTLC exposure scenario, based on human toxicity, would also protect the environment. Otherwise, a second step was used. If the SERT values for the chemicals were based on the Ambient Water Quality Criteria (AWQC) for the protection of aquatic life, then DTSC assumed again that the human-toxicity-based lower TTLC would also be protective of the environment. Otherwise, the lower TTLC was determined on a chemical-by-chemical basis.

Once the upper or lower risk-based TTLCs are derived for any chemical using the procedures described so far, they are compared with twice the estimated quantitation level to determine if they can be adequately measured. The TTLCs are also compared with the background soil mass fractions found in California. (The background soil mass fraction is the highest mass fraction of the element found in 50 samples of native California soil (DTSC 1998a, p 51); other background values are also given in the documentation for specific chemicals (DTSC 1998a, p. 32, 52, 859).) The highest of the risk-based value, twice the quantitation limit, or the ambient background value is then considered to be the upper or lower TTLC, depending on the exposure scenario chosen.

DTSC is proposing to replace the STLCs with upper and lower soluble or extractable regulatory thresholds (SERTs) to protect groundwater. Wastes with an extractable concentration of any one or more of the designated chemicals greater than the upper SERT would be considered hazardous wastes. If the extractable concentration of each of the designated chemicals is below the lower SERT, the waste would be considered nonhazardous, providing it is not classified as hazardous by any other applicable criterion.

SERTs are based on the lowest value of the following (DTSC 1998a, p. 43):

• California maximum concentration limits (MCLs).
• EPA AWQC for the protection of aquatic life at 100 ppm total hardness and a pH of 6.5 (as applicable).
• Human-health-based levels calculated by DTSC and based only on exposure to humans from drinking groundwater.

For carcinogens, California cancer potency factors are used to derive the human-health-based levels of a chemical corresponding to a risk of 10^-5 for an exposure scenario that appears to correspond to drinking the groundwater for the typical time that a person stays in a single residence. That risk level is the same as the risk level required by the California Safe

Drinking Water Act and Toxic Enforcement Act of 1986.³ When California cancer potency factors are not available, the EPA cancer potency factors are used. For noncarcinogens, EPA oral reference doses or data from EPA Region 9 preliminary remediation goals are used to develop the health-effects levels (no reference for these values were provided in DTSC 1998a, p. 43). Various uncertainties, such as those involved in extrapolation from animal data to humans and the uncertainty of cancer potency factors in animals, are factored into the health-based values by using point estimates that are expected to overestimate risks (a surface-area-based interspecies extrapolation, and the 95^th percentile of the experimental potency in animals).

Once the lowest value is chosen for calculation of the SERT, that number is multiplied by a dilution-attenuation factor of 100 to account for movement of the waste leachate through the unsaturated zone of soil and mixing with and dispersion in groundwater (DTSC 1998a, p. 44). The adjusted value is compared with a number that is twice the estimated quantitation limit, and the higher value becomes the exit-level SERT. In the case of arsenic, the resultant value is also compared with the 90^th percentile of arsenic concentrations in drinking water supplies (which is used as a surrogate for background concentrations) and the higher value selected. For upper SERTs, the lowest value of the MCL, AWQC, or health-based level is multiplied by a dilution-attenuation factor of 100 and by a liner protection factor (to account for the reduced percolation from a landfill with a liner—the DAF of 100 was based on a landfill with no liner). Again, the resulting value is compared with twice the estimated quantitation limit, and the higher value becomes the upper SERT.

For the derivation of the SERT values, DTSC is proposing to rescind the use of the California WET for determining the extractable constituents of a waste. In lieu of the WET, DTSC is proposing the use of the TCLP that is currently used by the EPA to identify soluble toxic constituents of RCRA hazardous wastes.

A two-threshold system is also proposed for evaluating acute toxicity of wastes, for human exposure by oral, dermal, or inhalation routes or for aquatic toxicity. For acute oral toxicity, wastes with an LD₅₀ of greater than 500 mg/kg of body weight would be nonhazardous and wastes with an LD₅₀ of less than 30 mg/kg would be hazardous; those with intermediate LD₅₀ values would be special wastes. The upper threshold is based on an adult exposure scenario assuming ingestion of waste at 0.31 mg/kg of body weight. The lower threshold is based on a child's exposure scenario involving ingestion of waste at 5 mg/kg of body weight. These ingestion rates are based on the ingestion rate parameters used in the CalTOX model, with fixed uncertainty factors of 100 applied to account for extrapolation from animal data to human (factor of 10) and from lethal concentrations to minimal effect concentrations (factor of 10).

For acute dermal toxicity, the upper (hazardous) and lower (nonhazardous) thresholds are 5,500 mg/kg and 7,400 mg/kg of body weight, respectively, based on dermal LD₅₀ values. As for acute oral toxicity, the upper threshold is based on adult contact with waste and the lower threshold is based on a child's exposure, with dermal contact rates derived from the dermal contact rate parameters used in the CalTOX model, and uncertainty factors of 100 applied to acute toxicity values.

For both oral and dermal toxicity classification, the LD₅₀ values are either directly measured for the whole wastes, or they may be calculated from constituents of the waste.

Under the current classification system, only vapor concentrations are considered for the acute inhalation toxicity of a waste. The proposed system will account for both vapor and particulate concentrations of all chemical constituents of a waste. For volatile chemicals, the hazard classification is based on a ratio of the chemical's vapor pressure to its inhalation LC₅₀. The ratios are summed for each volatile chemical in the waste. If the sum of the ratios is less than 0.1 the waste is nonhazardous, and if the sum of ratios exceeds 1, the waste is hazardous; special wastes have sums of ratios between 0.1 and 1.

Classification of particles is more complex. DTSC proposes to classify a waste as hazardous if the respirable fraction of the waste (fraction with particle size less than 10 microns) times the sum of the

ratios of concentrations of the individual chemicals in the respirable fraction (in milligrams per kilogram) divided by its inhalation LC₅₀ value (in milligrams per cubic meter) exceeds 10⁵. [Note: in the DTSC documentation (DTSC 1998a, p. 73) the upper acute inhalation value is given as 2 × 10⁶, this discrepancy was not clarified by DTSC]. If the sum is less than 10⁵, the waste will be classified as nonhazardous; no special waste category is designated for particles.

For aquatic toxicity, a lower threshold for nonhazardous wastes would be an LC₅₀ greater than 500 mg/L. Hazardous wastes would be those with an aquatic LC₅₀ of less than 30 mg/L. This latter value is the current aquatic toxicity threshold of 500 mg/L divided by the tenth percentile estimate of the liner protection factor (18) and rounded to one significant digit.

The current one-threshold system (at 10 mg/kg) for carcinogens would remain the same, except that vinyl chloride would be removed from the list of carcinogens and a SERT and TTLC would be developed for it.

Several exposure models are used to develop the risk-based concentrations for the exposure scenarios: residents near a landfill and workers at a landfill for the upper TTLC, and residents living on converted land and ecological effects for the lower TTLCs. Descriptions of each of the computational models is given below (DTSC 1997).

The CalTOX model (DTSC 1998a, p. 105) is a multiple pathway, multi-media approach for determining the risk associated with the concentration of an organic chemical in soil. The model builds on and extends EPA's risk-assessment approaches as defined in the "Risk Assessment Guidance for Superfund" (RAGS) (EPA, 1989). The advantages of CalTOX include: (1) explicit treatment of mass conservation and chemical equilibrium; (2) calculations of gains and losses in multiple environmental media (e.g., air, soil, or groundwater) over time; (3) consideration of both transportation of the chemical between environmental

media and transformation of the chemical within a medium (e.g., photolysis in air or hydrolysis in water); and (4) the capability for quantitative treatment of uncertainty and variability.

The model examines the distribution of organic chemicals in the environment and the resulting exposures of human populations via all routes and through multiple exposure pathways (including air, drinking water, food, and soil (ingestion or dermal exposure)). The CalTOX model version 2.3 is designed to model risks to humans living on or near soil with a fixed concentration of a chemical. However, this version of the CalTOX model was not considered to be appropriate for modeling risks to people living near a landfill or on residential land converted from land where there had been continuous soil amendments with waste (DTSC 1998a, p. 59).

Therefore, two modifications were made to the CalTOX model by DTSC to account for these two exposure scenarios for organic chemicals. The CalTOX landfill model relates the landfill compartment concentration to the waste concentration by allowing the use of a waste dilution factor. The model also adds an estimate of transport of the chemical in gases produced in the landfill from the landfill compartment to the air. The modifications also incorporate the differences between landfill contents and soil and the differences in areas and depths of landfills compared with residential yards. The CalTOX land-conversion model includes estimation of the root soil concentrations of a chemical from an application rate, mixing depth, application duration, and waste concentration rather than using a specific initial root soil concentration.

DTSC stated that the CalTOX model cannot be used for inorganic chemicals as it requires estimates of the soil/water partition coefficient. The model used to evaluate inorganic chemicals and to address the landfill-worker scenario is based on DTSC's PEA model (DTSC 1998a, p. 784). Like the CalTOX model, the PEA model is an adaptation of U.S. EPA's Risk Assessment for Superfund (RAGS) methods, and therefore, the treatment of toxicity and the intake equations for the routes of exposure are nearly identical in both models. However, the algorithms used to predict the environmental fate and transport of chemicals are quite different. Whereas, CalTOX considers many exposure pathways includ-

ing food, drinking water, and air, the PEA model considers only the major pathways of inhalation of vapors and dust, dermal contact with soil, and ingestion of soil. The PEA model is used to evaluate the exposure of landfill workers to both organic and inorganic chemicals, because the pathways by which a worker can be exposed to a chemical in waste are more limited and more direct than those for residents. The PEA model is also considered to be appropriate for evaluating exposure to inorganic chemicals for residents living near landfills and those living on converted land.

Lead is evaluated differently from the other inorganic chemicals. For the derivation of SERTs, DTSC indicated that it assessed carcinogenicity using the Cal/EPA cancer potency factor, as is done for other carcinogens (DTSC 1998a, p. 775). There is no reference dose for lead, and evaluation of its chronic toxicity is based on blood lead concentrations resulting from exposure to lead in the environment. The principal difference between the lead risk-assessment spreadsheet and the PEA model is that the spreadsheet uses empirical ratios between exposure media concentrations and blood lead concentrations and adds the incremental contribution from each of the five exposure pathway to arrive at a ninetieth percentile estimate of total blood lead concentration. The latter value is combined with an estimate of a safe blood lead concentration of 10 micrograms/deciliter to back-calculate a tenth percentile soil/waste concentration, which is used for the proposed upper and lower TTLCs for lead. Adult-worker and nearby child-resident scenarios are assumed for calculating the upper TTLC, and a child-resident scenario is assumed for calculating the lower TTLC.

DTSC has indicated that some of the changes it proposes would result in the inclusion of more wastes as hazardous, and other changes would result in the exclusion of some wastes from the hazardous category. Possible changes in the volumes of hazardous and nonhazardous waste will likely affect a broad range of regulated parties and state government

agencies that deal with the regulation and management of various wastes. Municipal and county agencies might be particularly affected because the type and volume of waste disposed to some municipal solid-waste landfills might change under this proposal. DTSC has indicated that there likely will be concern about placing wastes formerly classified as hazardous in those landfills. In addition, some landfill operators might refuse to accept certain wastes whose classification changes as a result of the proposed regulations.

Changes to the method used by DTSC to classify wastes will have ramifications for other California regulatory agencies that are part of the Hazardous Waste Management Program (DTSC 1997). These agencies include the State Water Resources Control Board (SWRCB), the Integrated Waste Management Board (IWMB), and the Certified Unified Program Agencies (CUPA). SWRCB is responsible for protecting the quality of water in California by regulating the siting, operation, and closure of waste disposal sites. SWRCB oversees the discharge of wastes to land, such as application of solid waste to agricultural land. SWRCB protects water quality by classifying wastes as to their risk of contaminating the water, classifying waste-disposal sites according to their protection of receiving waters, and by adopting standards and regulations for the waste sites. The proposed DTSC method might result in some waste previously classified as non-RCRA hazardous requiring further consideration by the waste generator to determine whether the waste meets the SWRCB definition of designated waste, thus requiring disposal in class II disposal facilities.

IWMB provides for solid-waste planning and implementation and oversees local agencies that manage solid waste. Solid waste is waste not classified as hazardous. Under the proposed DTSC classification system, some wastes previously classified as non-RCRA hazardous might be reclassified as nonhazardous and, therefore, might be disposed of as solid waste to a class III landfill (DTSC 1997).

Certified Unified Program Agencies (CUPA) was initiated by California in 1996 to consolidate and coordinate the regulation and management of hazardous waste, currently mandated by six regulatory programs. CUPA is implemented by local agencies with state oversight. The six programs in the CUPA are

• Hazardous Waste Generators and Hazardous Waste On-site Treatment Program

• Underground Storage Tanks Program
• Above Ground Tanks Program (Spill Prevention Countermeasure Control Plan)
• Hazardous Material Release Response Plans and Inventories
• Risk Management and Prevention Program
• Uniform Fire Code Hazardous Materials Management Plans and Inventories

The proposed DTSC waste-classification system will affect several of the CUPA programs and require adjustments not only by the local agencies but also by the waste generators themselves, particularly if their wastes are reclassified and subject to different management and disposal requirements.

EPA also developed a risk-based approach for the national hazardous-waste program under RCRA to identify wastes that no longer warrant being placed in the hazardous category. In 1996, EPA proposed this approach as part of its Hazardous Waste Identification Rule (HWIR) (61 Fed. Register 18780, Apr. 29, 1996). The proposed rule would have set risk-based exit levels for toxic constituents in RCRA-listed wastes generated at a facility for 192 chemicals for humans and approximately 50 chemicals for ecological receptors. The risk methodology was based on consideration of five types of waste-management units (sources); numerous release, transport, and exposure pathways; and biological-effects information. The approach to establish the exit levels was generally similar to DTSC's approach but was faulted by EPA's Science Advisory Board for several reasons, including inadequate consideration of all environmental media and setting exit levels based only on the single exposure pathway considered to be most sensitive. EPA is revising its approach, and its Office of Solid Waste has expressed interest in DTSC's approach.