McKay, Donald. "Front matter" Multimedia Environmental Models ...

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Exposure routes vary greatly in magnitude from chemical to chemical, depending on the substance’s physical chemical properties such as K AW and K OW, and it is not usually obvious which routes are most important. Data from the environmental fate models can provide a sound basis for estimating risk when used to assess quotients and to determine dominant exposure routes. If such information can be presented to the public, the individuals will be, at least in principle, able to choose or modify their lifestyles to minimize exposure and presumably risk. Individuals then have the freedom and information to judge and respond to acceptability of risk from exposure to this chemical compared to the other voluntary and involuntary risks to which they are subject. ©2001 CRC Press LLC 8.15 THE PBT–LRT ATTRIBUTES A regulatory issue in which evaluative mass balance models are playing an increasingly important role is in assessing the persistence, bioaccumulation, toxicity and long-range transport (PBT-LRT) attributes of chemicals. If chemicals that display these undesirable attributes can be identified, they can be considered for regulation, as has been done by UNEP for the “dirty dozen” high-priority substances discussed earlier. Monitoring data are usually too variable to enable them to be used directly in this priority setting task, and monitoring is impossible for chemicals not yet in use. Since there are many thousands of chemicals of commerce that require assessment, and (it is hoped) most are innocuous, there is an incentive to develop a tiered system in which there are minimal data demands initially and perhaps 90% of chemicals evaluated are rejected as of no concern. The remaining 10% of potential concern can be more fully evaluated in a second tier with a similar rejection ratio. A third tier may be needed to select the (perhaps) 100 top priority chemicals from a universe of 100,000 chemicals in a three-tier system. The challenge is to devise models or evaluation systems that will accomplish this task efficiently. Webster et al. (1998) have suggested using a Level III model similar to EQC, but with advection shut off, to evaluate persistence. Gouin et al. (2000) have described an even simpler Level II approach. This has the advantage that no “modeof-entry” information is required. Regardless of which model is used, it seems inevitable that models will play a key role in assessing persistence or residence time, since these quantities cannot be measured directly in the environment. Bioaccumulation can be evaluated most simply by calculating the equilibrium bioconcentration factors as the product of lipid content and K OW. For more detailed evaluation involving considerations of bioavailability, metabolism, and possible biomagnification from food uptake, the Fish model or a variant of it can be used. In some cases, a food web model may be required to determine if significant biomagnification occurs. Foodweb can be used for this purpose. Mackay and Fraser (2000) have suggested such a three-tiered approach for assessing the bioaccumulation potential of chemicals. Toxicity is not within our scope here. Long-range transport (LRT) presents an interesting challenge because, like persistence, LRT cannot be measured in the environment. It can only be estimated using
models. Most interest is in LRT in the atmosphere but, in some cases, oceans and rivers can play a significant role. Even migrating biota can contribute to LRT. The most promising approach is to consider the fate of chemical in a parcel of Lagrangian air passing over soil or water and subject to degradation, deposition, and reevaporation. Such systems have been suggested by Van Pul (1998), Bennett et al. (1999), and Beyer et al. (2000). Beyer et al. (2000) showed that a LRT distance in air can be deduced from a simple Level III model as the product of the wind velocity, the overall persistence or residence time of the chemical, and the fraction of the chemical in the atmosphere. A model, TaPL3 (Transport and Pesistence Level 3), can be used for a Level III evalution of persistence and LRT. It is available on the website. It is expected that new models will be developed to assist in the evaluation of these attributes, especially in situations where there is no easy method of obtaining the required information from environmental monitoring data. ©2001 CRC Press LLC 8.16 GLOBAL MODELS The ultimate mass balance model of chemical fate is one that describes the dynamic behavior of the substance in the entire global environment. At present, only relatively simplistic treatments of chemical fate at this scale have been accomplished, but it is likely that more complex and accurate models will be produced in the future. Meteorologists can now describe the dynamic behavior of the atmosphere in some detail. Oceanographers are able to describe ocean currents. Ultimately, there may be linked meteorological/oceanographic/terrestrial models in which the ultimate fate of 100 kg of DDT applied in Mexico can be predicted over the decades in which it migrates globally. The obvious ethical implication is that a nation should not use a substance in such a way that other nations suffer significant exposure and adverse effects. These situations have already occurred with acid rain and Arctic and Antarctic contamination by persistent organic substances. The construction of global-scale models opens up many new and interesting prospects. It appears that there is a global fractionation phenomenon as a result of chemicals migrating at different rates and tending to condense at lower temperatures. Chemicals that do reach cold regions may be better preserved there because of the reduced degradation rates. Chemicals appear to be subject to “grasshopping” (or “kangarooing” in the Southern Hemisphere) as they journey, deposit, evaporate, and continue hopping from place to place until they are ultimately degraded as shown in Figure 8.13. Accounts of these phenomena, and models that attempt to quantify them, are given in a series of papers by Wania and Mackay (1993, 1996, 1999) and Wania et al. (1996). The most successful modeling to date has been of a-HCH, which was produced as an impurity in the insecticide, technical lindane (Wania and Mackay, 1999) but is no longer produced. An interesting insight from that study is an assertion that, despite a-HCH never having been used in the Arctic, about half the remaining mass on this planet now resides in the arctic oceans. This GloboPOP model is
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McKay, Donald. "Front matter" Multi
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Multimedia Environmental Models The
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hensive, and reliable, and they hav
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Chapter 1 Introduction Chapter 2 So
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McKay, Donald. "Introduction" Multi
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It is now evident that our task is
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McKay, Donald. "Some Basic Concepts
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give the derived units directly wit
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Energy (joule, J) The joule, which
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particles (aerosols), or biota in w
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Worked Example 2.1 A three-phase sy
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(i) What is the concentration (C) i
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Answer ©2001 CRC Press LLC 5.1 kg,
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or or When t is zero, C is zero, th
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What is the absolute minimum piscic
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©2001 CRC Press LLC C 1 = C O/(1 +
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containing nonequilibrium quantitie
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1. Fallout of chemical from air to
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validated using real environments.
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©2001 CRC Press LLC CHAPTER 3 Envi
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of other, somewhat similar or homol
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Table 3.2 List of Chemicals Commonl
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elatively high concentrations. This
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exists between toxicologists and ch
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determine priority. This is a subje
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Another important classification of
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Figure 3.1 (continued) ©2001 CRC P
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Table 3.5 (continued) dichlorometha
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Table 3.5 (continued) total PCB 326
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Figure 3.2 Plot of log K AW vs. log
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chlorofluoro compounds are very sta
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3.3.2.7 Arsenic Compounds Arsenic,
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McKay, Donald. "The Nature of Envir
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Figure 4.1 Evaluative environments.
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are subject to two important deposi
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metals and organic chemicals from w
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carbon figure for deeper sediments
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flows. The quality of this groundwa
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Table 4.2 Evaluative Environments A
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McKay, Donald. "Phase Equilibrium"
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Figure 5.1 Some principles and conc
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likely using an equilibrium criteri
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Another useful quantity is the rati
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experimentally, but not directly, u
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©2001 CRC Press LLC 5.3 PROPERTIES
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1.0. This, then, is the fugacity or
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where ©2001 CRC Press LLC pH is -l
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In terms of solubilities, S iA and
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The temperature coefficient is the
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(1982), Baum (1997) and Leo (2000).
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Although it may appear environmenta
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Figure 5.4 Illustration of quantita
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5.5 ENVIRONMENTAL PARTITION COEFFIC
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C S = K PC W benzene = 1.1 ¥ 0.001
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Mackay (1986), in an attempt to sim
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Compartment Definition of Fugacity
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high concentration in the fish, but
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heat capacity of 0.38 J/g°C requir
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Therefore, ©2001 CRC Press LLC f =
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©2001 CRC Press LLC Z T = S(V i/V
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©2001 CRC Press LLC air Z = P S /R
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Table 5.1 Table 5.1 Summary of Defi
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Figure 5.7 Fugacity form 1 for dedu
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Calculate the following physical-ch
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©2001 CRC Press LLC CHAPTER 6 Adve
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that all water will spend 100 days
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This enables us to conceive of, and
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©2001 CRC Press LLC f = 15/0.5 = 3
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D AS dominates. A residence time of
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and Therefore, ©2001 CRC Press LLC
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eaction situation in which there is
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The rate constants in each case are
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©2001 CRC Press LLC 1/t O = SD Ai/
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e achieved in 10 days. Detractors o
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sulfate to sulfide or by dechlorina
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to Zepp and Cline (1977) for the or
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©2001 CRC Press LLC 6.7 LEVEL II C
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Figure 6.3 Fugacity Level II calcul
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McKay, Donald. "Intermedia Transpor
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Figure 7.1 Illustration of nonequil
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5. Diffusion within soils, and from
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sionally, the term flux rate is use
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©2001 CRC Press LLC N = ABDC/ Dy m
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no currents or eddies. In practice,
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diffusivity, and some unknown layer
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where ©2001 CRC Press LLC X = y/ U
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fraction of the total volume that i
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Figure 7.6 Mass transfer at the int
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partition coefficient. The signific
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and the series retains a similar ra
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ments of resuspension rates are par
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Figure 7.8 Movement of air phase (k
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ates can thus be used to estimate m
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Figure 7.9 Combination of D values
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Figure 7.10 Four-compartment Level
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Page 195 and 196: Figure 7.12 Sample Level III output
Page 197 and 198: McKay, Donald. "Applications of Fug
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Page 203 and 204: LEVEL1B A six-compartment Level I p
Page 205 and 206: Figure 8.1 Air-water exchange proce
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Page 209 and 210: Figure 8.2 Chemical transport and t
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Page 213 and 214: 8.5.2 Process Description The water
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Page 221 and 222: C F ª Z Ff W ª 1.608 mol/m 3 ª 4
Page 223 and 224: y Mackay et al. (1983), and an appl
Page 225 and 226: accordingly, but the final solution
Page 227 and 228: Figure 8.10 Fish bioaccumulation pr
Page 229 and 230: The nature of the processes control
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Page 233 and 234: An alternative and more elegant met
Page 235 and 236: 8.11.2 Model of a Chemical Evaporat
Page 237 and 238: Paterson et al. (1991), and Hung an
Page 239 and 240: 8.14.1 Introduction ©2001 CRC Pres
Page 241: mated again in mg/day. Food, the ot
Page 245 and 246: available from the University of To
Page 247 and 248: McKay, Donald. "Appendix" Multimedi
Page 249 and 250: ©2001 CRC Press LLC
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