The Role of Multimedia Fate Models in Chemical Risk Assessment

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A L A N ESCHENROEDER Arthur D. Little, Inc., Cambridge, M A 02140

T h i s paper r e l a t e s mathematical models for chemicals moving through air, water, soil and b i o t a to methodologies f o r a s s e s s i n g h e a l t h r i s k s to individuals or ecosystems experiencing environmental exposures. The procedures f o r assessing risks are traced from sources to r e c e p t o r s , and the a p p l i c a t i o n of models to t h i s process is d e s c r i b e d . The paper sets out to answer questions of how to s e l e c t and l i n k models i n the context of r i s k assessment. The theory, s t r u c t u r e , verification and application of the models themselves is left to other papers i n this symposium. Acute r i s k s are d i s t i n g u i s h e d from chronic r i s k s i n the context of environmental r e g u l a t o r y requirements. A technique f o r s e l e c t i n g and assembling multimedia models based on r e l e a s e , environmental and receptor characteristics i s described. The content of the paper is designed t o unify other papers i n the framework and o r g a n i z a t i o n of t h i s symposium.

When chemicals are released i n the environment, their hazard p o t e n t i a l to human or e c o l o g i c a l receptors depends upon the extent of contact between the r e c e p t o r s and the chemical. This exposure l e v e l i s not only i n f l u e n c e d by where, when and how much of the chemical i s r e l e a s e d , but a l s o on i t s movement and changes i n a i r , water, s o i l o r b i o t a r e l a t i v e to the l o c a t i o n s of the receptors. Risk i s defined as the p r o b a b i l i t y of some adverse consequence i n the h e a l t h context, o r as the probability times the extent of the consequence i n the technology context. In t h i s paper we s h a l l examine and d i s c u s s how mathematical models are used to generate estimates of r i s k when more than one of the environmental media must be considered i n t r a c i n g pathways connecting sources with r e c e p t o r s . The principal objective here i s to place i n p e r s p e c t i v e the 0097-6156/83/0225-0089$06.00/0 © 1983 American Chemical Society

Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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s e l e c t i o n and a p p l i c a t i o n of f a t e models on the background of the needs perceived by government and i n d u s t r y f o r q u a n t i t a t i v e hazard or exposure a n a l y s i s . F i r s t , we i n v e s t i g a t e some of the r e g u l a t o r y motivations for chronic r i s k a n a l y s i s . Next, i t i s necessary to p o i n t up the s i m i l a r i t i e s and d i f f e r e n c e s between acute and chronic r i s k and d e l i n e a t e the steps i n e s t i m a t i n g h e a l t h r i s k s posed by environmental chemicals. Following some i l l u s t r a t i o n s of model s t r u c t u r e , we conclude by d i s c u s s i n g s p e c i f i c f a c t o r s i n f a t e a n a l y s i s that suggest choices of model components. Some Regulatory Background Environmental c o n t r o l s t a t u t e s and t h e i r administrative implementation through r e g u l a t i o n s have e i t h e r i m p l i c i t l y o r e x p l i c i t l y r e q u i r e d chronic r i s k assessment. T h i s has o f t e n been considered a y a r d s t i c k i n e v a l u a t i n g r e g u l a t o r y impact from a c o s t - e f f e c t i v e n e s s point o f view. Indeed, i n the c l o s i n g weeks of the 97th Congress 2nd Session H.R. 6159 passed the House by v o i c e vote and was pending i n the Senate Commerce Committee a t the time of t h i s w r i t i n g . The proposed l e g i s l a t i o n (Risk Analysis Research and Demonstration Act of 1982) e s t a b l i s h e s a program under the c o o r d i n a t i o n of the O f f i c e of Science and Technology P o l i c y (OSTP) f o r improving the use of risk analysis by those Federal agencies concerned with r e g u l a t o r y d e c i s i o n s r e l a t e d to the p r o t e c t i o n of human l i f e , h e a l t h and the environment. The b i l l would e s t a b l i s h research, demonstration and c o o r d i n a t i o n programs, among these agencies. It further requires the A d m i n i s t r a t o r o f OSTP to present Congress with a plan f o r implementing r i s k a n a l y s i s . The Clean A i r Act, the F e d e r a l Water P o l l u t i o n C o n t r o l A c t , the Safe D r i n k i n g Water Act, the F e d e r a l I n s e c t i c i d e , Fungicide and Rodenticide A c t , and the Toxic Substances C o n t r o l Act i m p l i c i t l y r e q u i r e r i s k a n a l y s i s i n s e t t i n g standards that the U.S. Environmental P r o t e c t i o n Agency imposes through the s t a t e and l o c a l c o n t r o l agencies. (Indeed, the "zero discharge" goal i n some s t a t u t e s bypasses a l l needs f o r r i s k a n a l y s i s . ) H i g h l y t o x i c a i r p o l l u t a n t s f a l l under S e c t i o n 112 o f the Clean A i r Act. U n l i k e c r i t e r i a p o l l u t a n t s , these hazardous a i r p o l l u t a n t s must be c o n t r o l l e d to p r o t e c t the p u b l i c h e a l t h with an "ample margin of s a f e t y . " Implied i n t h i s language i s the b e l i e f i n a d i s c r e t e t h r e s h o l d of exposure below which no e f f e c t s occur and from which a s a f e t y margin can be measured. Subsequent i n t e r p r e t a t i o n s , however, i n d i c a t e d c l e a r l y that Congress d i d not equate safeguarding the p u b l i c h e a l t h with complete r i s k e l i m i n a t i o n . The Federal Water P o l l u t i o n C o n t r o l Act (Clean Water A c t ) e s t a b l i s h e s n a t i o n a l l y a p p l i c a b l e e f f l u e n t l i m i t a t i o n s using c r i t e r i a based on d i f f e r e n t l e v e l s of c o n t r o l technology. For example, r i s k assessments were c a r r i e d out i n response to the settlement of l i t i g a t i o n a s s e r t i n g a f a i l u r e to set standards


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for 129 potentially toxic materials called "priority pollutants." The agreement stemming from t h i s l i t i g a t i o n was e s s e n t i a l l y r a t i f i e d i n the 1977 amendments to the Act and 1984 is the deadline year f o r the establishment of p e r m i s s i b l e effluent levels. The r i s k methodology recommended f o r the water q u a l i t y c r i t e r i a i n v o l v e s e i t h e r q u a l i t a t i v e or q u a n t i t a t i v e estimates of concentrations of a p o l l u t a n t i n ambient waters which, when not exceeded, w i l l "ensure a water q u a l i t y s u f f i c i e n t to p r o t e c t a s p e c i f i e d water use." C r i t e r i a are intended f o r both the p r o t e c t i o n of human h e a l t h and of ecosystems; however, they do not c a r r y the a u t h o r i t y of law. The hazardous waste g u i d e l i n e s and r e g u l a t i o n s generated by the EPA i n response to the Resource Conservation and Recovery Act of 1976 (PL 94-580) propose to cover methods of d e f i n i n g and i d e n t i f y i n g hazardous waste, standards f o r keeping records of c o n t a i n i n g and t r a n s p o r t i n g these wastes, and standards f o r performance i n the management of hazardous waste f a c i l i t i e s , but do not e x p l i c i t l y r e q u i r e r i s k assessment. Most of the p r o v i s i o n s of the Toxic Substances C o n t r o l Act (TSCA) of 1976 (PL 94-469) r e l y i n some way on r i s k assessment of chemicals. Under the r e p o r t i n g requirements of the s t a t u t e , any manufacturer, processor, or d i s t r i b u t o r of a chemical f o r commercial purposes must inform the EPA immediately a f t e r discovering any information which "reasonably supports the c o n c l u s i o n " that a chemical substance or mixture "presents a s u b s t a n t i a l r i s k of i n j u r y to h e a l t h or to the environment" unless the EPA Administrator has been adequately informed already. EPA i s mandated to e s t a b l i s h r e g u l a t i o n s f o r t e s t i n g new or e x i s t i n g substances when i t i s determined that there i s not enough h e a l t h or environmental information, that t e s t i n g i s necessary to develop such information and that the chemical or mixture "may present an unreasonable r i s k of i n j u r y to h e a l t h or the environment." Representations of adequate c o n s i d e r a t i o n of chronic r i s k s are, t h e r e f o r e , necessary i n the planning of many schemes f o r manufacturing, transporting, s t o r i n g , use and d i s p o s a l of p o t e n t i a l l y t o x i c waste m a t e r i a l s . The combined e f f e c t s of the s t a t u t e s as described above have focused r e g u l a t o r y a t t e n t i o n on the multimedia ( a i r , water, s o i l and b i o t a ) aspects o f such activities. I t would appear as i f the trend i s toward acceptance of some r i s k r a t h e r than a guarantee (or hope) of complete s a f e t y of the p u b l i c . Chronic vs. Acute Risk A n a l y s i s Environmental chemical r e l e a s e s due to human a c t i v i t i e s may be a c c i d e n t a l ( u s u a l l y acute) or as an attendant consequence of some planned a c t i v i t y ( u s u a l l y c h r o n i c ) . Traditionally, spills have been separated from steady discharges because of s t a t u t o r y distinctions, but any i n t e g r a t e d p o l l u t a n t assessment must


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consider both. Some m a t e r i a l s are so hazardous that any r o u t i n e emissions are p r a c t i c a l l y n i l ; t h e r e f o r e , inadvertent discharges may dominate. Space and time scales can be combined to draw the d i s t i n c t i o n s between the r i s k s due to these two types of release. Acute r i s k s are u s u a l l y a s s o c i a t e d with immediate e f f e c t s of a r e l e a s e o c c u r r i n g w i t h i n hours of the a c c i d e n t and confined to w i t h i n a few k i l o m e t e r s or l e s s of i t s l o c a t i o n . Examples of t h i s c l a s s of events are s p i l l s , f i r e s , e x p l o s i o n s and t h e i r e f f e c t s such as property damage, traumatic i n j u r y , or sudden death. Events that generate c h r o n i c r i s k s may be the same as those l e a d i n g to acute e f f e c t s or could be s u b t l e r e l e a s e s d i s t r i b u t e d over long periods of time. In e i t h e r case, the term " c h r o n i c " refers to longer term and potentially more widespread consequences than those p r e c i p i t a t e d by acute r i s k events as exemplified above. Whether caused by r a p i d or gradual r e l e a s e s , chronic r i s k s are occasioned by p o l l u t a n t exposures of r e c e p t o r s lasting days or even years. Some cases are d i f f i c u l t to c l a s s i f y such as the short-term exposure that leads to an e f f e c t which appears much l a t e r . Thus, the cause may be acute and the e f f e c t , chronic. T h e i r geographical ranges may extend over many k i l o m e t e r s around an i n c i n e r a t i o n s i t e or along a t r a n s p o r t a t i o n c o r r i d o r . The d i s t r i b u t e d use of p o t e n t i a l l y hazardous m a t e r i a l s such as p e s t i c i d e s generates chronic r i s k r e g a r d l e s s of geographical range. Any a n a l y s i s of r i s k should recognize these d i s t i n c t i o n s i n a l l of t h e i r e s s e n t i a l f e a t u r e s . A t y p i c a l approach to acute r i s k separates the s t o c h a s t i c nature of d i s c r e t e c a u s a l events from the d e t e r m i n i s t i c consequences which are t r e a t e d u s i n g engineering methods such as mathematical models. Another t o o l i f r i s k a n a l y s i s i s a r i s k p r o f i l e that graphs the p r o b a b i l i t y of occurrence versus the s e v e r i t y of the consequences (e.g., probability, of a f i s h dying or p r o b a b i l i t y of a person c o n t r a c t i n g l i v e r cancer; e i t h e r as a r e s u l t of exposure to a s p e c i f i e d environmental contaminant). In a way, t h i s p r o f i l e shows the f u n c t i o n a l r e l a t i o n s h i p between the p r o b a b i l i s t i c and the d e t e r m i n i s t i c p a r t s of the problem by showing p r o b a b i l i t y versus consequences. Let us now turn our a t t e n t i o n to the main steps of any procedure constructed to a n t i c i p a t e or respond to the r i s k a n a l y s i s requirements s e t f o r t h by the s t a t u t e s reviewed above or v o l u n t a r i l y e s t a b l i s h e d as product standards by i n d u s t r i e s . I t i s important to note that t h i s type of procedure i s a t e c h n i c a l means to a r r i v e at a q u a n t i t a t i v e estimate. The decisions regarding the a c c e p t a b i l i t y of the r e s u l t is s o c i o p o l i t i c a l and i s , t h e r e f o r e , beyond the scope of t h i s discussion.


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Components of a Chronic Risk Assessment Materials Balance Analysis* The f i r s t step i n our methodology i s the establishment of flows of hazardous p o l l u t a n t s and t h e i r distributions among the environmental compartments. The time phasing of r e l e a s e s must be considered; f o r example, some r e l e a s e s may be instantaneous at a frequency of once a month while others may be continuous with seasonal or d i u r n a l v a r i a t i o n s superimposed. Furthermore, the u l t i m a t e chronic r i s k w i l l a l s o depend upon the s p a t i a l d i s p o s i t i o n of r e l e a s e s ; f o r example, a moving e l e v a t e d p o i n t source w i l l give ambient concentration patterns d i f f e r e n t from those from a s t a t i o n a r y surface-based area source. Chemical s p e c i a t i o n a l s o must enter our m a t e r i a l s balance d e s c r i p t i o n i n some cases. A case i n p o i n t i s hexavalent chromium which has a higher order of c a r c i n o g e n i c i t y than t r i v a l e n t chromium, which i s a l s o found i n nature. F i n a l l y , p a r t i t i o n i n g among the media i s an e s s e n t i a l i n g r e d i e n t i n the c h a r a c t e r i z a t i o n of emissions or discharges i . e . , how much of a r e l e a s e enters the a i r , water, s o i l or biota? Or, put another way, i n t o what compartment i s an environmental release deposited initially? Answering this question sometimes i n v o l v e s s k i p p i n g ahead to a short-term chemical f a t e a n a l y s i s such as f o r a sudden s p i l l , depending on m a t e r i a l p r o p e r t i e s , f r a c t i o n s of the s p i l l e d m a t e r i a l may be found i n any or a l l of the four compartments ( a i r , water, s o i l , b i o t a ) or at t h e i r i n t e r f a c e s . T h i s i s an example of how the output of an acute r i s k a n a l y s i s can provide input to the chronic risk analysis by providing the instantaneous d i s t r i b u t i o n o f the r e l e a s e d substance among the compartments. Whether the r e l e a s e s to the environment are sudden or gradual, i t i s necessary to devise a systematic method to account f o r each component. One approach to t h i s problem i s based on a matrix having rows c o n s i s t i n g o f a c t i v i t i e s or sources (e.g., e x t r a c t i o n , p r o c e s s i n g , manufacturing, storage, t r a n s p o r t a t i o n , use, d i s p o s a l and reclamation) and columns r e p r e s e n t i n g the media, a i r , earth,, water and b i o t a . Each non-zero element of t h i s matrix array i s f i l l e d out with the s p a t i a l , temporal and chemical d e t a i l c a l l e d f o r above. The m a t e r i a l s balance thus d e r i v e d provides p o i n t s of entry i n t o the pathways of exposure that u l t i m a t e l y form the b a s i s of the chronic r i s k assessments. Brown and Bomberger have discussed the methodology f o r t h i s step e x t e n s i v e l y i n another paper i n t h i s symposium (J_). Environmental Fate. Having c h a r a c t e r i z e d the entry of m a t e r i a l s i n t o the environment, we move i n t o the second step of our procedure. The goal at t h i s stage of a n a l y s i s i s to d e f i n e ambient concentration of the m a t e r i a l or i t s products i n areas of concern f o r receptor (e.g., people, m a t e r i a l s or ecosystem components) exposure. A f a m i l y of computer s i m u l a t i o n models has been developed f o r c a l c u l a t i n g the ambient l e v e l s of a


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m a t e r i a l subjected to the simultaneous i n f l u e n c e s of t r a n s p o r t , d i f f u s i o n and transformation i n a multimedia s e t t i n g . T h i s has been implemented by l i n k i n g s i n g l e medium models at the i n t e r f a c i a l boundaries (such as the l i n k i n g of an a i r model to a s o i l model by d e p o s i t i o n and v o l a t i l i z a t i o n processes). These c a p a b i l i t i e s have grown i n t e n s i v e l y over the past f i v e years l a r g e l y due to the sponsorship of government and i n d u s t r y . Examples of the need f o r multimedia models are found i n contemporary problem areas. Polynuclear aromatic hydrocarbons and metals are emitted i n t o the atmosphere as t r a c e i m p u r i t i e s with the products of c o a l combustion. The organics have low vapor pressure and p a r t i a l l y condense on emitted p a r t i c u l a t e s i n a stack plume. The p a r t i c u l a t e s are t r a n s f e r r e d to the s o i l by dry deposition, rainout or washout. The metals manifest themselves i n r e l a t i v e l y r e f r a c t o r y oxides formed s e l e c t i v e l y among the f i n e s i z e ranges of f l y a s h p a r t i c l e s . Both p a r t i c l e bound p o l l u t a n t s must be t r e a t e d i n chronic i n h a l a t i o n r i s k s t u d i e s by a sequence of a i r d i s p e r s i o n and surface d e p o s i t i o n processes, whereas the vapor f r a c t i o n of organics remains i n the air. Thus, the gas phase, a e r o s o l and s o i l components are t r e a t e d simultaneously i n multimedia model s t u d i e s . Another case of multimedia f a t e modeling may be e x e m p l i f i e d by human i n h a l a t i o n exposure estimates f o r PCB s p i l l s . The spill size i s estimated c o n s i d e r i n g both spread and soil infiltration. V o l a t i l i z a t i o n c a l c u l a t i o n s were c a r r i e d out to get t r a n s f e r r a t e s i n t o the a i r compartment. F i n a l l y , plume c a l c u l a t i o n s using l o c a l meteorological statistics produced ambient c o n c e n t r a t i o n patterns which can be subsequently f o l d e d together with population d i s t r i b u t i o n s to o b t a i n exposures. Numerous examples of f a t e models are reviewed i n other papers i n t h i s symposium. For example, s i n g l e media models a r e covered f o r a i r by Anderson ( 2 ) , f o r water by Burns ( 3 ) , and f o r s o i l and groundwater by Bonazountas ( 4 ) .

Receptor Exposure. Exposure modeling should produce a s t a t i s t i c a l l y r e p r e s e n t a t i v e p r o f i l e of p o l l u t a n t intake by a set of r e c e p t o r s . T h i s i s done by combining the space/time d i s t r i b u t i o n of p o l l u t a n t concentrations with that of receptor populations (whether they be people, f i s h , ducks or property made of some m a t e r i a l that i s v u l n e r a b l e to p o l l u t a n t damage). The accuracy and r e s o l u t i o n of the exposure estimates are chosen to be c o n s i s t e n t with the main purposes of d e c i s i o n making. These purposes i n c l u d e the f o l l o w i n g : o Screening of p o l l u t a n t s or sources to s e t p r i o r i t i e s ; o E v a l u a t i o n of l e g i s l a t i o n or rulemaking; o Comparison of a l t e r n a t e ambient standards; o Planning of f a c i l i t i e s at s p e c i f i c s i t e s ; o Support of f i e l d research programs; and o Design of r e a l - t i m e episode c o n t r o l systems.


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It i s clear that these goals place widely differing requirements on both the r e s o l u t i o n and the accuracy of exposure estimates; thus, the approach s e l e c t e d should be optimized to f i t the requirements. The exposure models are designed to be compatible w i t h f a t e a n a l y s i s outputs whether they be f o r a i r , water or s o i l . If there are s i g n i f i c a n t l y d i f f e r e n t exposure or dose/response c h a r a c t e r i s t i c s among v a r i o u s subpopulations, we form cohort groups d i f f e r e n t i a t e d by age, sex, occupation, l e v e l of a c t i v i t y or geographical h a b i t s . L i m i t a t i o n s of a v a i l a b l e data d e r i v e d from c l i n i c a l , e p i d e m i o l o g i c a l and t o x i c o l o g i c a l s t u d i e s u s u a l l y preclude d i s t i n c t i o n of dose/response curves among the cohorts; however, there o f t e n are s u f f i c i e n t data on indoor v s . outdoor l e v e l s , geographical v a r i a t i o n , and o c c u p a t i o n a l surroundings to allow some d i s t i n c t i o n s to be drawn among cohorts. Thus, p e c u l i a r i t y o f microenvironments l e a d to d i f f e r i n g exposure levels. Because the s i g n i f i c a n c e of exposure has only been considered over the past few years, there i s not as wide a s e l e c t i o n of exposure models a v a i l a b l e as that f o r f a t e models. The latter have been a p p l i e d f o r s e v e r a l decades to the calculation of ambient exposure l e v e l s compared with some standard values. Papers illustrative of human exposure assessments i n t h i s symposium i n c l u d e one on airborne p o l l u t a n t exposure assessments by Anderson ( 2 ) , a g e n e r i c approach to e s t i m a t i n g exposure i n r i s k s t u d i e s by F i k s e l (5)> and a d e r i v a t i o n of p o l l u t a n t l i m i t values i n s o i l or water based on acceptable doses to humans by Rosenblatt, Small and Kainz ( 6 ) . Risk E s t i m a t i o n . As mentioned above, c h r o n i c r i s k i s expressed as a p r o b a b i l i t y of occurrence per year or per l i f e t i m e of some adverse consequence caused by exposure t o the pollutant. S t a t u t o r y mandates have focused on human h e a l t h e f f e c t s as the primary expression o f c h r o n i c r i s k s . The b a s i s of the r i s k c a l c u l a t i o n i s the dose/response curve that r e l a t e s the adverse e f f e c t to the amount o r r a t e of a chemical taken i n to the s u b j e c t . Because of r e g u l a t o r y emphasis of cancer, most of the work devoted t o the d e v i a t i o n o f dose/response curves has been concerned with the p r o b a b i l i t y of appearance of a tumor as the adverse e f f e c t . The risk e s t i m a t i o n procedure may be thought of as performing three d i s t i n c t f u n c t i o n s : 1. Conversion of experimental dose/response data i n t o a form s u i t a b l e f o r e x t r a p o l a t i o n o f human r i s k u s i n g l e a s t squares o r , more u s u a l l y , maximum likelihood curve f i t s . 2. Generation of a l t e r n a t i v e dose/response models f o r r i s k estimation to emphasize the range o f r e s u l t s generated by widely d i f f e r i n g assumptions. 3. D i s p l a y o f r i s k l e v e l s f o r v a r i o u s subpopulations under v a r i o u s a p p l i c a t i o n s of t e c h n o l o g i c a l or r e g u l a t o r y c o n t r o l of r e l e a s e s i n t o the environment i n order to r e l a t e s o c i a l c o s t s to r i s k r e d u c t i o n .


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The dose/response models are intended to e x t r a p o l a t e both from t e s t animals to humans and from high doses to low. The user should be concerned with assumptions u n d e r l y i n g these models. A range of assumptions f o r r i s k e v a l u a t i o n might be obtained by making three choices of e x t r a p o l a t i o n formulas: the one-hit l i n e a r model, the multistage model and the l o g p r o b i t model. The one-hit formulation i s based on a p o s t u l a t e that the i n v a s i o n of a c e l l by a s i n g l e p o l l u t a n t molecule can i n i t i a t e a tumor. This g i v e s a s t r a i g h t - l i n e r e l a t i o n s h i p between dose and response. The m u l t i s t a g e models depend on a mechanism i n v o l v i n g m u l t i p l e processes a t v a r i o u s stages of c e l l d i v i s i o n to cause a tumor. Going from high doses to low doses, the m u l t i s t a g e r i s k drops o f f more sharply than the one-hit r i s k as dose i s decreased. At intermediate to low doses, however, multistage a s y m p t o t i c a l l y approaches l i n e a r i t y at some constant factor p l a c i n g i t somewhat below one-hit. Log p r o b i t i s a model based e m p i r i c a l l y on a sigmoid-shape assumption f o r a l l dose/response curves. T h i s shape approximates the n o t i o n of a t h r e s h o l d ; i.e., a dose below which defense mechanisms, metabolism or e l i m i n a t i o n processes intervene to prevent tumor formation. A l l of the e x t r a p o l a t i o n models are p r e d i c a t e d on the s u p p o s i t i o n that there are no i n t e r s p e c i e s d i f f e r e n c e s . None assumes any synergism or antagonism with other p o l l u t a n t s , and a l l of them s c a l e e f f e c t s by surface area i n order to consider the s i z e of the receptor organism. No d i s t r i b u t i o n i s made among the v a r i o u s entry routes into the body s i n c e the pharmacokinetics, which d e s c r i b e the chemical's f a t e i n the organism, are not d i f f e r e n t i a t e d . Despite these l i m i t a t i o n s , r e g u l a t o r y agencies use dose/response e x t r a p o l a t i o n f o r d e c i s i o n making; t h e r e f o r e , the a n a l y s t must be mindful of the wide range of values y i e l d e d by the v a r i o u s models at low dose and be aware of the u n c e r t a i n t y of the r i s k r e s u l t s . Because o f these difficulties, i t i s o f t e n u s e f u l to stop at the exposure c a l c u l a t i o n s and compare exposure s t a t i s t i c s with ranges of values accepted and experienced i n everyday l i f e . In t h i s symposium a comprehensive overview of the r i s k e s t i m a t i o n step and i t s r e l a t i o n s h i p to the output of multimedia f a t e models i s given i n the paper by F i k s e l ( 5 ) . Examples of the a p p l i c a t i o n of and l i n k a g e among the v a r i o u s techniques are a l s o presented i n that paper. Multimedia

Model C h a r a c t e r i s t i c s

Model Types. I f i t i s determined that exposure pathways of i n t e r e s t i n t e r s e c t more than one o f the media, the a n a l y s t i s faced with the need to l i n k together s i n g l e media models (or to apply e x i s t i n g multimedia models). Despite claims t o the contrary, there i s probably no s i n g l e model that i s appropriate to a l l problems. Thus, a h y b r i d combination o f boundary


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c o n d i t i o n s , algorithms and output d i s p l a y s i s assembled to respond to s p e c i f i c problem needs. The requirements a r e expressed i n terms of f a c t o r s such as the f o l l o w i n g : o accuracy as evidenced by v a l i d a t i o n t e s t s o time/space r e s o l u t i o n o o v e r a l l i n t e r v a l or s p a t i a l scale o resource a v a i l a b i l i t y (e.g., data p r o c e s s i n g system capability, user personnel level, budget and turnaround time) These requirements are d r i v e n by the a p p l i c a t i o n whether i t be product design, r e g u l a t o r y mandates, r e g i o n a l planning, standard setting, legislative drafting or control strategy design. The techniques a v a i l a b l e f o r multimedia modeling up to around 1978 were reviewed i n a previous paper (7); this symposium i s intended to provide the fundamentals and a p p l i c a t i o n s r e f l e c t i v e o f development e f f o r t s as w e l l as the current s t a t e - o f - t h e - a r t . Much i n the way of u s e f u l background m a t e r i a l i s summarized i n the proceedings of a workshop convened by the U.S. Environmental P r o t e c t i o n Agency (8). Beyond the stage of development described i n these documents, multimedia model designs can be roughly c a t e g o r i z e d as e i t h e r well-mixed compartment types o r t r a n s p o r t types. E i t h e r type may or may not handle chemical transformations. The p r i n c i p a l d i s t i n g u i s h i n g c h a r a c t e r i s t i c of a multimedia model i s i t s c a p a b i l i t y to c a l c u l a t e flows across media boundaries. The content of the well-mixed compartment model i s mostly concerned with boundary processes s i n c e s p a t i a l u n i f o r m i t y i s assumed i n each medium or phase. The p a r a l l e l developments of Mackay s approach ( 9 ) and that reported by Neely and Blau ( 1 0 ) are examples of well-mixed compartment models. The most rudimentary form of Mackay s approach uses the thermodynamic e q u i l i b r i u m scenario by d e f i n i n g a set of f u g a c i t i e s whose e v a l u a t i o n determines the p a r t i t i o n i n g of a chemical among the media. Higher l e v e l s allow f o r steady flow and unsteady flow behavior i n the compartments, but the key element i n a p p l y i n g any of the well-mixed compartment models i s e s t i m a t i o n of compartment volume. This step inherently presumes some estimate of transport. The approach of Neely i s laboratory-based and i n v o l v e s the use of a d e c i s i o n tree to s e l e c t calculation algorithms. The two methods were comparatively analyzed by Lyman (11) who concluded that f o r s i n g l e component organic chemicals both models are easy to use with a minimum of data and can be executed on a hand-held calculator. Considering the severe limitations on these models, they are u s e f u l f o r screening approximations. The t r a n s p o r t type of model becomes necessary where s i t e - s p e c i f i c p r e d i c t i v e c a b i l i t y i s needed. Mathematically t h i s type i s d i s t i n g u i s h e d from the well-mixed compartment by i t s dependence upon p a r t i a l d i f f e r e n t i a l equations generated by 1

T


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the space/time equations governing chemical composition.* In general, these equations have v a r i a b l e c o e f f i c i e n t s and can be nonlinear. T h i s almost c e r t a i n l y r e q u i r e s the use of at l e a s t a minicomputer i f not a main frame system. The UTM model f a m i l y (8) e x e m p l i f i e s the transport type approach by l i n k i n g a s e r i e s of s i n g l e media s i m u l a t i o n s . One weakness of some multimedia models that must be considered by the user i s i n c o n s i s t e n c y of time s c a l e s . For example, i f we employ monthly averaged a i r concentrations to get rainout values on f i f t e e n - m i n u t e i n t e r v a l inputs to a watershed model, l a r g e e r r o r s can o b v i o u s l y occur. The a i r - l a n d - w a t e r simulation (ALWAS) developed by Tucker and co-workers (12) overcomes t h i s l i m i t a t i o n by a l l o w i n g f o r s e q u e n t i a l a i r q u a l i t y outputs to provide d e p o s i t i o n data to d r i v e a s o i l model. This i n t u r n i s coupled to a surface water model. Current Examples. In t h i s symposium the c h a r a c t e r i s t i c s , types and a p p l i c a t i o n s of multimedia models are exemplified. The compartment type i s reviewed i n a paper by Mackay and Paterson (13). The fugacity approach i s discussed and a p p l i c a t i o n s are described f o r p o l y c h l o r i n a t e d biphenyls i n the Great Lakes r e g i o n . A p p l i c a t i o n s of compartment modeling to organic chemicals are covered by M c C a l l , Swann and Laskowski (14). The implementation of t h i s type of approach using l a b o r a t o r y data based p r o p e r t i e s estimates i s i l l u s t r a t e d . The key r o l e of i n t e r f a c i a l t r a n s p o r t i n compartment or t r a n s p o r t models i s the focus of a paper by Bomberger and co-workers (15). The combined influences of chemistry, phase change and biotransformation are processes modeled at the terrestrial-atmospheric interface. Another compartmental partitioning issue of major consequence f o r p e s t i c i d e s i s the d i s s o l v e d versus adsorbed f r a c t i o n i n an aqueous environment. C a r t e r and S u f f e t (16) present measurements o f b i n d i n g of p e s t i c i d e s to d i s s o l v e d f u l v i c a c i d s that • w i l l provide inputs to compartment models. Data from l a b o r a t o r y measurements used i n compartment models can o f t e n bypass c o s t l y f i e l d experiments i n the screening stage. Thomas, S p i l l n e r and Takahashi (_17) have r e l a t e d the s o i l m o b i l i t y of a l a c h l o r , b u t y l a t e and metachlor to physicochemical p r o p e r t i e s of these compounds. In the area of transport-type models, s o i l / w a t e r systems have been a primary area of development. The Hydrologic Simulation Program (18) described i n the paper by Johanson simulates chemical movement and transformation i n runoff, groundwater and surface water i n contact with s o i l or sediments. * S t r i c t l y speaking, f i n i t e d i f f e r e n c e or f i n i t e element s o l u t i o n s to d i f f e r e n t i a l equations are simply m u l t i p l y i n g the number of comparments many times, but the mathematical r u l e s f o r l i n k i n g c e l l s i n d i f f e r e n c e c a l c u l a t i o n s are r i g o r o u s l y s e t by the form of the equations.


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An a p p l i c a t i o n of t r a n s p o r t and compartment-type models to hazard a n a l y s i s i s described i n the paper by Honeycutt and Ballantine ( 1 9 ) . The compound CGA-72662 running o f f from a g r i c u l t u r a l areas i n t o surface waters was modeled i n order t o set safe a p p l i c a t i o n procedures c o n s i s t e n t with the p r o t e c t i o n of aquatic environments. Patterson, e t a l (20) have adapted the UTM model to a software package that i s g e n e r a l l y a p p l i c a b l e to f a t e assessments of t o x i c substances i n a i r , water, s o i l and biota. T h e i r work, now i n working d r a f t form, i s being used by Dr. W i l l i a m Wood and Dr. Joan L e f l e r i n the O f f i c e o f Toxic Substances of the U.S. Environmental P r o t e c t i o n Agency. Despite the i n t e n s i v e e f f o r t s devoted t o making new multimedia models, i t seems as i f r e l a t i v e l y l i t t l e a t t e n t i o n i s given to t h e i r verification through field or l a b o r a t o r y measurements. (One notable exception i s US EPA's p e s t i c i d e e v a l u a t i o n p r o j e c t sponsored by i t s Athens, GA l a b o r a t o r y . ) The philosophy of model v a l i d a t i o n and the conclusions from t e s t i n g programs are reviewed by Donigian (21) i n h i s paper. Although the work described i s mainly concerned with a q u a t i c s i m u l a t i o n s , the need f o r c a r e f u l l y designed e v a l u a t i o n s t u d i e s w i l l continue to grow f o r multimedia models proposed f o r use i n r i s k assessments. S e l e c t i o n and A p p l i c a t i o n o f Model Components Influence o f Entry Modes of Pollutants i n t o the Environment. In s e l e c t i n g an appropriate multimedia model, the user must begin by identifying several features that c h a r a c t e r i z e the emission, discharge or r e l e a s e of the p o l l u t a n t of interest into the environment. Following this identification, quantitative estimates of rates and d i s t r i b u t i o n s must be developed because the u l t i m a t e use of a set of f a t e models i s to c a l c u l a t e ambient l e v e l s i n terms of r e l e a s e r a t e s , p o l l u t a n t p r o p e r t i e s and environmental s c e n a r i o s . Since the designation o f pathways i s the primary step i n establishing the f a c t o r s determining model s e l e c t i o n , the process begins with a s e t of i n i t i a l p o i n t s f o r the candidate pathways. Choice of model s t r u c t u r e depends on release p o i n t s and on the main aspects o f f a t e processes i n f l u e n c i n g the movement along each pathway. A formal d e s c r i p t i o n of t h i s approach i s i n p r e p a r a t i o n by Bonazountas and F i k s e l (22). T h e i r handbook/catalogue w i l l provide users with a d i r e c t and simple way t o s e l e c t appropriate models. It will supply background of the p h y s i c a l , chemical and b i o l o g i c a l i s s u e s that must be considered i n f i t t i n g model c h a r a c t e r i s t i c s to problem needs. Releases into the environment may be natural or anthropogenic. B a c t e r i a l o r mineral a c t i o n may c o n s t i t u t e worldwide generation sources that f u n c t i o n independently o f any human a c t i v i t y . I f p o l l u t a n t impacts are to be evaluated, both these sources and n a t u r a l s i n k s , such as the oceans, s o i l


100


surface or atmospheric p h o t o l y s i s must be considered along with the anthropogenic m a t e r i a l s balance. Owing to t h e i r r e l a t i v e l y good l e v e l of p r e d i c t a b i l i t y , chronic emissions or discharges can be classified and q u a n t i t a t i v e l y c h a r a c t e r i z e d i n a u n i f i e d manner u s i n g a matrix or t a b u l a r form as d e s c r i b e d p r e v i o u s l y . For a geographic study, s p a t i a l d i f f e r e n t i a t i o n can be introduced, f o r example, by s u b d i v i d i n g water i n t o p a r t i c u l a r stream reaches, ponds, aquifers, etc. A broad c l a s s i f i c a t i o n of a c t i v i t y c a t e g o r i e s should transcend the u s u a l inventory of stacks or discharge pipes; i t should i n c l u d e e x t r a c t i o n , p r o c e s s i n g , manufacturing, storage, t r a n s p o r t a t i o n , use, d i s p o s a l and reclamation. Again, the c a t e g o r i e s can be r e f i n e d by storage at manufacturing s i t e , storage at forwarding t e r m i n a l , storage at d i s t r i b u t i o n depot and storage by users, to c i t e one example. For acute r e l e a s e s , the f a u l t tree a n a l y s i s i s a convenient t o o l f o r o r g a n i z i n g the q u a n t i t a t i v e data needed f o r model s e l e c t i o n and implementation. The fault tree represents a h e i r a r c h y of events that precede the r e l e a s e of concern. This h e i r a r c h y grows l i k e the branches of a tree as we t r a c k back through one cause b u i l t upon another (hence the name, " f a u l t tree"). Each l e v e l of the tree i d e n t i f i e s each antecedent event, and the branches are c h a r a c t e r i z e d by probabilities attached to each c a u s a l l i n k i n the sequence. The model applications are needed to describe the environmental consequences of each type of impulsive r e l e a s e of p o l l u t a n t s . Thus, combining the probability of each event with i t s q u a n t i t a t i v e consequences s u p p l i e d by the model, one i s l e d to the expected value of ambient concentrations i n the environment. T h i s d i s t r i b u t i o n , i n t u r n , can be used to generate a p r o f i l e of exposure and r i s k . I f r e q u i r e d by the model(s) to be used, back-up data f o r each entry i n the matrix or t a b l e may be s u p p l i e d to r e s o l v e the t o t a l mass flow i n t o s p a t i a l c e l l s (UTM c o o r d i n a t e s , depth or h e i g h t ) , temporal c e l l s (hourly frequency d i s t r i b u t i o n s , d i u r n a l cycles, seasonal s u b d i v i s i o n s or s e c u l a r trends on annual i n t e r v a l s ) or s p e c i a t i o n c e l l s (by valency s t a t e of anions or by hydrocarbon s t r u c t u r e , f o r example). The l e v e l of d i f f i c u l t y encountered by the user i n supplying these data may i n f l u e n c e the choice of model(s). Dynamics of Chemicals i n the Environment. In i d e n t i f y i n g pathways and, hence, models, the user must a l s o consider what becomes of the p o l l u t a n t as i t enters the environment. The dominance of v a r i o u s f a c t o r s over others w i l l determine both pathway s e l e c t i o n and model s e l e c t i o n i n an i n t e g r a t e d p o l l u t a n t assessment. Within any medium of the environment, three types of process (defined here as intramedia processes) govern the p o l l u t a n t c o n c e n t r a t i o n at each point at each time:


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o

Advection - mass movement of the medium c a r r y i n g the m a t e r i a l along, o D i f f u s i o n - movement or spread of the p o l l u t a n t as r e l a t i v e to the mass of the medium as d r i v e n by molecular or turbulence s c a l e dynamics, o Transformation - production or consumption of the p o l l u t a n t u s u a l l y d r i v e n by chemical r e a c t i o n s . Superimposed on these mechanisms of change operating i n the bulk volume of each medium are processes that t r a n s f e r the p o l l u t a n t from one medium to another. Some conceptual model frameworks lump intermedia t r a n s f e r s together with embedded transformation processes* causing unnecessary mathematical confusion of boundary value s p e c i f i c a t i o n s w i t h source term formalisms. Examples of intermedia p o l l u t a n t t r a n s f e r s are as follows: o Surface d e p o s i t i o n ( r a i n o u t , washout, f a l l o u t and dry) o Evaporation ( c o d i s t i l l a t i o n or v o l a t i l i z a t i o n ) o Adsorption - desorption In choosing a model, the user can optimize f a t e assessment e f f o r t s by d e l i n e a t i n g f i r s t , the source r e l e a s e p a t t e r n s and second, the dominant dynamical processes. Taking the intramedia processes f i r s t , one can address model c r i t e r i a by c o n s i d e r i n g the r a t i o of c h a r a c t e r i s t i c times. The advection time i s the p r i n c i p a l length s c a l e of the domain L d i v i d e d by the average flow speed u; i . e . T * L/u d

T y p i c a l l y , L may be stream reach d i s t a n c e and u, flow v e l o c i t y . The d i f f u s i o n time i s approximated by the random walk hypothesis and i s approximated by: 2

T j ^ A /2D a

where A i s the c h a r a c t e r i s t i c transverse d i r e c t i o n (e.g. stream depth) and D, the transverse d i f f u s i v i t y , be i t t u r b u l e n t or molecular. F i n a l l y , the transformation time i s approximated by

T

t

* C /C

t

*Source terms, f o r example, are sometimes w r i t t e n i n the equation s e p a r a t e l y f o r chemical production sources and f o r emission sources.


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where C i s the average r a t e of concentration change due only to transformation ( t y p i c a l l y a chemical r e a c t i o n r a t e ) and C, the average concentration i n the domain. Let us examine three examples of how these times are used i n model s e l e c t i o n . If T and

The Role of Multimedia Fate Models in Chemical Risk Assessment

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