FEATURE

Predicting Chemical Risks with Multimedia Fate Models

Regulators are turning to these simple desktop computer models to estimate pollutant fate and human exposure.

R E B E C C A R E N N E R

η 1992 the Canadian government placed chlo-robenzene on a list of "priority substances" of potential regulatory concern. Although not pro­duced in Canada, roughly 63,000 kg are im­ported annually for the production of pesti­cides, 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 in­vestigate the chemical's fate in the environment, working on the question of "where does it go" be­fore 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 in­volved in the work and a pioneer in the use of mul­timedia 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, multi­media mass balance models have gained growing ac­ceptance as a powerful tool that provides a broad un­derstanding of complex environmental processes—a critical consideration in crafting regulations and as­sessing human exposure and risk from pollutants. In­dustry giants such as Procter & Gamble, Monsanto, and 3M have recently begun using these models to predict the environmental fate of new chemicals. Sev­eral countries and organizations have also been attracted to their power and relative simplicity. Can­ada and the European Community are recommend­ing these models to assess the safety of new chemi­cals to humans and the environment (i). Government

researchers in the Netherlands have used multimedia models to determine whether their single-media en­vironmental regulations have compatible objectives (2). In the United States, Minnesota has been using a mul­timedia fate model to determine priorities for regula­tory efforts since 1993. Just this fall, the California EPA introduced a multimedia fate model to estimate chem­ical 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 cur­rent regulatory needs: "Now that the major emit­ters 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 environ­mental media—air, water, soil, and sediment—into homogenous compartments or boxes and then track­ing the flow and eventual fate of chemicals from box to box. Subcompartments allow for additional com­plexity. Conservation of mass is the underlying prin­ciple 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 thermody­namic quantity that determines how a substance will diffuse between compartments to reach equilib­rium. The University of Toronto researchers used a Level I model to determine that most of the chloro­benzene 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 nonequi-librium 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 round-robin comparison of four models, have just been pub­lished (i).

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 dry-weight concentrations.

At the second meeting in Denver, another com­parison run of corrected models produced a more encouraging outcome: The four models gave essen-

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.

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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Input: Chemical discharge into the environment

Output: Chemical fate by compartment

Movement to and from com­partments and subcompart-ments is controlled by some or all of these mechanisms: fugacity, or thermodynamic equilibrium; transformations and reactions within com­partments; and nonequilib-rium physical transfer.

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.

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.

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.

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.

Source: Reference i.

Tom McKone, Lawrence Livermore National Laboratory, University of California-Davis

Donald Mackay, University of Toronto-Ontario

European Center for Ecotoxicology and Toxicology of Chemicals, Brussels, Belgium

Dik van de Meent, National Institute of Public Health and Environmental Protection, Bilthoven, the Netherlands

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 Af­

fairs. The state decided to come up with a way to re­place the default point estimates used in the Super-fund 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 four-compartment 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

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TABLE 1

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 (2,3,7,8-tetrachlorodibenzo-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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