FEATURE
Predicting Chemical Risks with Multimedia Fate Models Regulators are turning to these simple desktop computer models to estimate pollutant fate and human exposure. REBECCA
η 1992 the Canadian government placed chlorobenzene on a list of "priority substances" of potential regulatory concern. Although not pro duced in Canada, roughly 63,000 kg are im ported annually for the production of pesti cides, rubber polymers, and textile dyes. As part of the chlorobenzene assessment, researchers at the University of Toronto's Institute for Environmental Studies used multimedia mass balance models to in vestigate the chemical's fate in the environment, working on the question of "where does it go" be fore considering what should be done to control it. Using only physicochemical data, emissions data, and a set of multimedia mass balance models that start simply and become more complex, the group discovered a surprising result: The chlorobenzene in southern Ontario came not from Canadian sources but from the United States. "We eventually figured out that this drift from the United States was about 300 times larger than local emissions," said Donald Mackay, a University of Toronto researcher in volved in the work and a pioneer in the use of mul timedia fate models in environmental applications. "Our results showed that this was a transboundary problem and that action in Canada would have been misguided." Since their introduction in the late 1970s, multi media mass balance models have gained growing ac ceptance as a powerful tool that provides a broad un derstanding of complex environmental processes—a critical consideration in crafting regulations and as sessing human exposure and risk from pollutants. In dustry giants such as Procter & Gamble, Monsanto, and 3M have recently begun using these models to predict the environmental fate of new chemicals. Sev eral countries and organizations have also been attracted to their power and relative simplicity. Can ada and the European Community are recommend ing these models to assess the safety of new chemi cals to humans and the environment (i). Government
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RENNER
researchers in the Netherlands have used multimedia models to determine whether their single-media en vironmental regulations have compatible objectives (2). In the United States, Minnesota has been using a mul timedia fate model to determine priorities for regula tory efforts since 1993. Just this fall, the California EPA introduced a multimedia fate model to estimate chem ical fate and human exposure near hazardous waste sites. "These models are best suited for chemicals chronically discharged into the environment: fuels, household products, and industrial chemicals," said Mackay. According to Bob Hazen, chief of the New Jersey State Bureau of Risk Assessment, this feature makes the models particularly applicable to cur rent regulatory needs: "Now that the major emit ters are under control, there are more small sources to account for. This means that multimedia models are the way to go." These models operate by simplifying environ mental media—air, water, soil, and sediment—into homogenous compartments or boxes and then track ing the flow and eventual fate of chemicals from box to box. Subcompartments allow for additional com plexity. Conservation of mass is the underlying prin ciple of this system. The models require two kinds of data: prevailing environmental conditions such as temperatures, flow rates, and accumulation rates; and chemical properties that influence partitioning and reaction tendencies. Mackay-type or fugacity models, the most widely accepted multimedia fate models, come in three types of increasing complexity. Level I models provide a first look at chemical fate by partitioning chemicals among compartments according to fugacity, a thermody namic quantity that determines how a substance will diffuse between compartments to reach equilib rium. The University of Toronto researchers used a Level I model to determine that most of the chloro benzene released into the atmosphere would re-
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main there. Level II models also account for transformations and reactions within a compartment using a rate constant, which can also be expressed as a residence time or half-life. A Level II model was used to characterize chlorobenzene's tendency to disperse widely in the atmosphere with a residence time of several days. Level III models allow for nonequilibrium distribution and require additional expressions for physical transfer between compartments. But despite widespread recognition of the value of multimedia fate models in risk assessment, some scientists and regulators have been concerned that different models applied by different users do not always give comparable results. These inconsistencies have raised doubts that the models, currently the tools of their developers and expert users, could be made sufficiently robust and user-friendly for routine regulatory applications. These issues were addressed in 1994 at two Society of Environmental Toxicology and Chemistry (SETAC) workshops held in Leuven, Belgium, and Denver, CO. Proceedings of these two meetings, including the results of a roundrobin comparison of four models, have just been published (i).
The power of simplicity Multimedia models provide a simplified, but comprehensive picture of what to expect from a chemical when it enters the environment. By dividing the environment into a few compartments and subcompartments and then using relatively simple relationships to track the chemical's movement, these models consider all of the primary processes likely to affect a chemical's fate and final destination.
Input: Chemical discharge into the environment
Output: Chemical fate by compartment
Movement to and from compartments and subcompartments is controlled by some or all of these mechanisms: fugacity, or thermodynamic equilibrium; transformations and reactions within compartments; and nonequilibrium physical transfer.
A critical need for risk assessment An EPA Science Advisory Board (SAB) report on human exposure assessment, which was published this year, identifies the current lack of reliable multimedia models as a significant barrier to improved risk assessment. "The current fragmentation and lack of coherence of available models for different media, pathways, and chemicals hampers current exposure assessment capabilities," according to the report, which also identifies the regulatory use of multimedia fate models as a goal for improving exposure assessment (2). "A comprehensive suite of models that could credibly predict movement, degradation, and accumulation of chemicals in the environment could serve as the common metric for regulatory decisions in the areas of standard development, permitting, site-specific risk assessments, site ranking, and cleanup." However, the report cautions that as yet no validated model meets these goals. The results of the SETAC workshops have helped to improve this situation. In Belgium, the first results of the round-robin comparisons turned up some unexpectedly large discrepancies, often of more than an order of magnitude. The largest of these was caused by differences in data interpretation and the use of different units, such as wet- versus dryweight concentrations. At the second meeting in Denver, another comparison run of corrected models produced a more encouraging outcome: The four models gave essen-
tially the same results, but only if all relevant mechanisms and parameters were set to the same value. To fortify the models, it was agreed that they include a list of values for mass transfer coefficients to eliminate errors introduced by operator input of these values. As a further quality control for regulatory applications, workshop participants agreed that models should meet performance criteria such as the ability to generate the same results for specified chemicals, scenarios, and defined environments.
Comparing air pollution hazards Minnesota is one of the first states in the United States to use multimedia models in its regulatory operations. Minnesota's indexing system uses a Level III multimedia fate model to compare and evaluate different sources of atmospheric pollution. Using geographical data for the state and a given chemical's physical and chemical characteristics, the model predicts the proportion and concentration of the chemical in each media compartment (3). This fate information combined with exposure scenarios and toxicity data yields a total hazard potential for pollution sources and allows the state to establish priorities, according to Gregory Pratt, environmental research scientist at the Air Quality Division Program Development Unit. Sensitivity analysis suggests that
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TABLE 1
Leading multimedia fate models These four models were the subject of recent round-robin evaluations to address concerns of researchers and regulators that different models did not consistently produce comparable results. The models can be used to evaluate fates of chemicals that partition into all media and those soluble in water, but not metals, minerals, polymers, or speciating chemicals. Model name
Description
Developer
CalTOX
Fugacity model combining a soil-layer model with a four-compartment Level III model; includes terrestrial vegetation compartment but not aquatic biota. Numerical solution method: Excel. Default environments: residential, commercial, and industrial landscapes in California.
Tom McKone, Lawrence Livermore National Laboratory, University of California-Davis
ChemCAN
Level III model with four main compartments and numerous subcompartments; vegetation included in add-on indirect exposure model. Numerical solution method: analytical solution. Default environments: 24 Canadian regions plus generic areas.
Donald Mackay, University of Toronto-Ontario
HAZCHEM
Level III model with four compartments; vegetation included in add-on indirect exposure model. Numerical solution method: matrix inversion routine. Default environments: 10 European regions and generic European.
European Center for Ecotoxicology and Toxicology of Chemicals, Brussels, Belgium
SimpleBOX
Level III model with six compartments (including aquatic biota, suspended solids as separate compartments); vegetation included in add-on indirect exposure model. Numerical solution method: compiled Lotus 1-2-3 spreadsheet.
Dik van de Meent, National Institute of Public Health and Environmental Protection, Bilthoven, the Netherlands
Source: Reference i.
the results are robust to the order-of-magnitude level. Initially the department considered developing a more specific process-oriented simulation, Pratt says, but "for most of the substances, we don't have enough information available to run detailed simulations. The fugacity model is great because it tells us that given these properties the chemical will partition out this way." A project is on the drawing board for calculating emissions fees based on environmental hazard. The scheme would use the index system to calculate a standard total hazard potential on which emissions fees could be based. The state is already holding discussions with industry about the new project, which would require legislative approval to be implemented, and has received an EPA grant to continue work on incentive-based fees.
Hazardous waste site evaluations California, also in the vanguard of using fate models in regulatory decisions, is enthusiastic about the models' benefits, but the state's initial experience has pointed out some of the difficulties in introducing a new methodology. About five years ago, the state EPA's Department of Toxic Substance Control, which regulates hazardous waste, began working to improve the exposure component of the U.S. EPA's recommended risk assessment procedure. "We felt that the use of point averages for exposure was weak," says Jeff Wong, head of the agency's Office of Science Af5 5 8 A • VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
fairs. The state decided to come up with a way to replace the default point estimates used in the Superfund Risk Assessment Guidelines with distributions that added fate and transport information. "We thought that risk assessment should be totally transparent and auditable," says Wong, "so we wanted a powerful but relatively simple model." To achieve this goal, the state turned to Tom McKone, a multimedia model developer at Lawrence Livermore National Laboratory, Livermore, CA. McKone developed CalTOX, a fugacity-based model that combines a separate soil layer model with a fourcompartment Level III model. CalTOX also includes indirect exposure algorithms and tools to quantify and assess uncertainties using Monte Carlo methods (4). The CalTOX fate model attempts to provide realistic ranges of contaminant concentrations available in environmental media at contaminated sites. This information then links to an exposure component of the model that produces a comprehensive list of possible exposure routes. "We asked the question, 'If soil is contaminated, how does it contaminate the day-to-day environment?'" McKone explains. The 23 exposure paths include soil and dust in the house, household water, water from showers and dishwashers, and home-grown vegetables. Described by the SAB report as "potentially the most advanced of all of the models reviewed" (2), CalTOX is a relatively simple and accessible model that runs on
a PC or Macintosh computer using Microsoft Excel spreadsheet software. The model was subjected to a technical peer review followed by a public review. For the past year, the model has been out for beta testing with consultants, industry, and environmental groups. However, the state's effort over the past year to find volunteer potentially responsible parties to test the CalTOX process on an actual site has been unsuccessful—not a single volunteer has come forward. Ned Butler, lead scientist in the state's risk policy group, believes that the private sector's reluctance to participate in the new method reflects the magnitude of the change that this new risk assessment method proposes. "Historically, regulations have been clear-cut and highly prescriptive," he says. "Regulatory agencies generate a specific value and specify what data to collect. We are shifting this so that environmental control decisions are no longer black and white, but instead have some flexibility." Some consultants familiar with the model believe that although CalTOX may be very useful for deciding on controls for wastes currently being produced, using the model for waste in existing sites is more difficult. "Imagine the typical former municipal landfill," said one consultant. "Who knows what chemicals are in that site? The model needs to know the mass of contaminants, but working that out requires extensive, expensive sampling." Butler disagrees. "There is a tremendous problem with the current description of risk in regulations. CalTOX involves an iterative, cooperative approach to modifying this methodology so that we can take advantage of all of the information available and come up with realistic risk assessments and not the overly conservative assessments of today's methodology." Although there does not yet appear to be a concerted effort to use multimedia fate models at the national level, EPA has not been immune to these issues. The Soil Screening Level Guidance released in November aims to account for contaminant volatility, migration to groundwater, and dust transport, according to Janine Dinan, an environmental health scientist in the Superfund Hazardous Site Evaluation Division. In California, potentially responsible parties currently have the option of using CalTOX as part of the risk assessment for their sites. A state task force looking at risk assessment is likely to consider adopting CalTOX into state regulations. Meanwhile the model verification work that started with the SETAC workshops is continuing, and Donald Mackay, Miriam Diamond, and their University of Toronto colleagues are working to expand the range of chemicals to which these models apply. Both Jeff Wong in California and Bob Hazen in New Jersey believe that the recent political emphasis on cost-effective regulations makes this a good time to look at more efficient risk analysis systems based on good science. But the ongoing efforts to improve these models and the reluctance of the California regulated community to adopt the new methodology suggest that widespread acceptance of multimedia fate models still faces many hurdles.
From concentrations to human exposures The CalTOX fate model produces realistic ranges of chemical concentrations at contaminated sites and goes one step further to produce a comprehensive list of possible exposure routes to humans. Average daily intakes are calculated in the exposed population over a defined exposure period. These sample calculations for PCE (tetrachloroethylene) and TCOD (2,3,7,8tetrachlorodibenzo-p-dioxin) demonstrate that both environmental dispersion and potential human exposure pathways are dependent on chemical properties. Both scenarios calculate exposures for a one-year exposure duration that begins one year after an initial uniform incorporation of 1 ppb contaminant in the first 6 m of soil. Exposure histogram for PCE
Exposure histogram for dioxin (TCDD)
* Micrograms of contaminant ingested per kilogram body weight per day. Source: Tom McKone, Lawrence Livermore National Laboratory.
References (1) Cowan, C. E. et al. The Multimedia Fate Model: A Vital Tool for Predicting the Fate of Chemicals; SETAC Press: Pensacola, FL, 1995. (2) Human Exposure Assessment: A Guide to Risk Ranking, Risk Reduction and Research Planning, Science Advisory Board. U.S. Environmental Protection Agency: Washington, DC, March 1995; EPA/SAB/1AQC-95/005. (3) Pratt, G. C. et al. Chemosphere 1993, 27, 1359-79. (4) McKone, T. E. "CalTOX, A Multimedia Total-Exposure Model for Hazardous Wastes Sites, Part II: The Dynamic Multimedia Transport and Transformation Model"; report prepared for the State of California, Department Toxic Substances Control. Lawrence Livermore National Laboratory: Livermore, CA, 1993; UCRL-CR-111456 Pt II. Rebecca Renner is a contributing
editor to ES&T.
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