A 408 VOLUME 110 | NUMBER 7 | July 2002 • Environmental Health Perspectives

Innovations

T he continuing existence in recent years of signifi-cant toxaphene levels in the Great Lakes has been

somewhat of a mystery, especially since the chemicalhad never been used very much in that watershed. Firstintroduced in the 1940s, toxaphene was used primarilyin Southeast cotton fields as an insecticide and herbi-cide. It was banned for use in the United States in1986, but due to its persistent nature, detectable levelsstill exist in the environment today—and to an unusualdegree they affect the Great Lakes, a thousand milesfrom the chemical’s heaviest use.

Scientists have always suspected that the toxaphenewas transported out of the southeastern United States,but it’s been the stubbornness of the chemical’s clear-ance from the Great Lakes watershed in the last 10years that has been perplexing. Now, however, a newmodel developed by scientists in Canada and theUnited States purports to explain why. A joint productof Lawrence Berkeley National Laboratory in Berkeley,California, and the Canadian EnvironmentalModelling Centre at Trent University in Peterborough,Ontario, this so-called “mass balance contaminantfate model” may describe how chemicals move overlong distances.

The model, known as BETR North America (for thetwo institutions involved), incorporates an enormous

variety of meteorologic and toxic-release data, but it alsofactors in how a region’s soil, water, and vegetationabsorb and release pollutants; that is, it incorporates massbalance equations. While there are other continentalmodels for evaluating the movement of contaminants,they focus on air transport patterns and don’t addresshow pollutants interact with vegetation, soil, and water.As explained by codeveloper Tom McKone, a senior sci-entist at the Berkeley lab, the multimedia BETR NorthAmerica model measures absorption traits of a region’ssoil, water, and vegetation against the solubility of specif-ic contaminants. These differences in a region’s ability toabsorb contaminants determine the degree to which theyget passed on.

“There’s this sort of exchange that goes on,”McKone says. “[Pollutants] partition into the soil andthe vegetation, and then the wind changes, they goout, and they come back in. If you take a pollutant liketoxaphene or dioxin and look at a cross-section of theirpartitioning, ninety-nine point nine percent of it is inthe soil and vegetation. But it’s the small fractionthat’s in the air that moves. It turns out that becausethere’s enough vapor pressure to get some of it into theair, it’ll redistribute itself—but it’s going to redistrib-ute itself with this real drag caused by the fact that it’strying to partition into the soil and vegetation. This is

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the process that has not been captured previ-ously, this coupling between air, surface soil,and vegetation.”

This attention to absorption rates is onecharacteristic that sets BETR North Americaapart from other models, McKone says.Another is its use of broad natural regions formeasurement instead of the grids that are typ-ical of other modeling methods. BETR NorthAmerica divides Canada, the United States,and Mexico into 24 regions, largely based onwatersheds and soil types. Each region is fur-ther divided into 7 compartments: upperatmosphere, lower atmosphere, freshwater,freshwater sediment, soil, coastal water, andvegetation. This configuration creates the 168mass balance equations that define the model.

To date, the Berkeley–Trent scientistshave applied the model to several chemicals,but the toxaphene examination has been themost detailed. Matt MacLeod, a graduate stu-dent at Trent University and the model’sprincipal developer, says that study verifiedBETR North America’s accuracy because itsresults agreed with previous measurements.That is, he says, the model correctly predictedthat atmospheric conditions and regional pol-lution absorption rates would result in migra-tion of toxaphene from the Mississippi Deltato the Great Lakes and a relative plateauing ofits levels there over the last 10 or 15 years.

BETR North America’s GenesisAccording to MacLeod, BETR NorthAmerica evolved from previous models. Onenotable predecessor is ChemCAN, to which it

bears a close resemblance. The creation ofDon Mackay, a Trent professor of environ-mental and resource studies who also workedon BETR North America, ChemCANdivides Canada into 24 regions with the capa-bility of describing how those regions individ-ually absorb pollutants into their water, soil,and vegetation. But with ChemCAN, thefocus was on chemical properties and howthey would interact with generic environmen-tal conditions, MacLeod says. “There wasn’ta focus on where the chemical was moving; itwas just on how it partitioned between airand water or air and soil,” he says. “What wasmissing was tracking where the chemicalwent when it left the region that you wereworking on.”

With funding under the Canadian gov-ernment’s Toxic Substances ResearchInitiative, a research program managed byHealth Canada and Environment Canada,the Canadian scientists began developinginterregional mapping that described themovement of pollutants. Because theCanadians had already formed a collaborativelink with the Berkeley lab, which was interest-ed in working on the new model, the interna-tional effort to apply the method to all ofNorth America was born.

The prospect of creating a continentalmodel was enticing because, as McKone says,scientists still have much to learn about long-distance movement of persistent pollutantssuch as dioxin, mercury, and DDT. To date,the bulk of the work on BETR NorthAmerica has been to develop data on the mass

balance of chemical substances—that is,where and in what amounts they will tend toend up following release. But the next phase,according to McKone, will be greater exami-nation of human exposure calculations.

“What we’re investing our time in now isfood distribution,” he says. “We tend to thinkof exposure and risk assessment as being doneon a fairly local scale. But with a model likethis, we now can do the kind of things thatwere done with [nuclear] fallout studies in thefifties, where they would track fallout acrossthe country and realize that, even though anuclear test was done in Nevada, it impactedthe milk in New York State.”

McKone says this investigation will meantaking food distribution patterns and layingthem over the model’s pollution movementpatterns. “It may be that you’re exportingyour pollutant but bringing it back in withyour food supply,” he says. For example, heexplains, San Francisco enjoys clean airbecause pollution is usually blown inland toagricultural areas—which sell produce thatmight contain the same pollutants back toSan Franciscans. On a broader, continentalscale, McKone says that BETR NorthAmerica may be able to identify where a pol-lutant that’s showing up in Washington Stateapples—which could be eaten in, say, NewEngland—is coming from.

“What we’ll be able to do once this is upand running,” MacLeod says, “is, for example,take data from the Toxics Release Inventory [aU.S. Environmental Protection Agency–oper-ated public database of information on toxic

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chemical releases and otherwaste management activityreported by industry] andestimate the resulting humanexposure. Then we can lookat individuals who live in dif-ferent parts of the continentand where the food that theyeat comes from and extrapo-late further to a dose, or rateof intake, of the chemicalthat can be expected. Thetack we’re taking is todescribe the intake rate interms of the fraction of thetotal release that leaves theend of the pipe that’s inven-toried in the Toxics ReleaseInventory that will eventuallymake its way into somemember of the population.”

Other PossibleApplicationsMcKone and MacLeod pre-dict that BETR NorthAmerica may be an effectivetool in assisting states,regions, or even nations infostering greater understand-ing of pollution conse-quences and perhaps moreeffective regulatory activity.“[The model] really does allow you to lookat the reach of pollutants,” McKone says. “Ithink that will have ramifications for thingslike transboundary pollution, which hasalways been an issue even state to state. [Thetransboundary pollution issue] came upwith acid precipitation, but it hasn’t comeup with persistent pollutants.”

MacLeod points to the toxaphene studyas a good example of how the BETR NorthAmerica model can enlighten policy makers.“One of the incentives for looking attoxaphene was that people were perplexed bywhat was apparently a ‘stickiness’ to theseconcentrations—you institute severe restric-tions on a chemical that you are concernedabout, and you don’t see continued improve-ment in environmental quality,” he says. “But

what the model says is that the improve-ments are happening; it’s just that once youeliminate the local sources, the concentra-tions in the whole environment start tobecome influenced by these larger-scaleprocesses that are going on.” MacLeodthinks that in terms of encouraging govern-ments and individuals to support programsto regulate chemicals, the model will be veryvaluable. “It will show you in a tangible waywhat kinds of benefits you’re going to getfrom taking this action right now,” he says.“And it encourages the idea that you have tocooperate internationally.”

Tim Watkins, an assistant laboratorydirector at the U.S. EnvironmentalProtection Agency’s National ExposureResearch Laboratory in Research Triangle

Park, North Carolina, hasstudied and discussed BETRNorth America at length andconcludes that it has poten-tial use in many differentapplications in one of theareas he works in: develop-ing a strategy for trackingtrends of persistent toxics inthe environment. Models arenothing new at environmen-tal agencies. But accordingto Watkins, the model’smultimedia framework issomething new that offerscertain advantages over othermodel types. “Any attemptto draw connectionsbetween air and water andbetween water and soil are ofpotentially great benefit, Ithink, because linking thosedifferent compartments is atough nut to crack,” he says.

Does BETR NorthAmerica hold promise as aglobal tool? Perhaps. Mac-Leod says that the Berk-eley–Trent team has alreadybegun work on a globalapplication for the model.But the task is a big onebecause political realities as

well as differences in data collection andmonitoring in different parts of the worldpose serious problems. “For North America,it’s not that difficult because Canada and theUnited States have existing databases for theenvironmental parameters that we need—things like rate of rainfall and river flow ratesand weather data,” he says. “But when youextend the model to Africa and China andthe former Soviet Union, the data quality andquantity aren’t as good, and gathering thisdata becomes a much bigger task.”

But as McKone points out, at a timewhen some people believe that the biggestthreat to the quality of California’s air maysoon be China’s emissions of persistent pollu-tants, global air pollution is an increasinglyserious issue. BETR North America, he says,can be an effective tool in helping policymakers understand the problem.

“It does provide an opportunity to dosome scenario development,” he says, “like,what happens with rapid industrialization inMexico along the border if there are lots ofnew emissions of hundreds of organic chemi-cals? How will that affect the United States? Ithink it has a certain benefit as a tool for poli-cy makers to look at those sorts of problems.”

Richard Dahl

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Sugge s t ed Read ingBetts KS. Modeling continent-wide contamination. 2001. Environ Sci Technol

35(23):481A.

MacLeod M, Woodfine D, Mackay D, McKone T, Bennett D, Maddalena R. 2001.BETR North America: a regionally segmented multimedia contaminant fate modelfor North America. Environ Sci Pollut Res 8(3):156–163.

Woodfine DG, MacLeod M, Mackay D, Brimacombe JR. 2001. Development of conti-nental scale multimedia contaminant fate models: integrating GIS. Environ SciPollut Res 8(3):164–172.

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Traveling toxaphene. This insecticide, used primarily in the southeastern UnitedStates, is transported across North America and deposited from the atmosphereto the Great Lakes. The darker the region on the map, the higher its contributionof toxaphene to the Great Lakes. The number in each region is the percentage oftotal atmospheric loading that is due to toxaphene usage in that region.