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

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Volatilization is treated using the approach suggested by Jury et al. (1983). Three contributing D values are deduced. An air boundary layer D value, D E, is deduced as the product of area A, a mass transfer coefficient k V, and the Z value of air, i.e., A k V Z A. Jury has suggested that k V be calculated as the ratio of the chemical’s molecular diffusivity in air (0.43 m 2 /day or 0.018 m 2 /h being a typical value), and an air boundary layer thickness of 4.75 mm (0.00475 m); thus, k V is typically 3.77 m/h. Another k V value may be selected to reflect different micrometerological conditions. An air-in-soil diffusion D value characterizes the rate of transfer of chemical vapor through the soil in the interstitial air phase. The Millington–Quirk equation is used to deduce an effective diffusivity B EA from the air phase molecular diffusivity B A as outlined in Chapter 7, namely, ©2001 CRC Press LLC B EA = B A v A 10/3 /(vA + v W) 2 where v A is the volume fraction of air, and v W is the volume fraction of water. If v W is small, this reduces to a dependence on v A to the power 1.33. A diffusion path length Y must be specified, which is the vertical distance from the position of the chemical of interest to the soil surface; i.e., it is not the “tortuous” distance. The air diffusion D value D A is then B EA A Z A/Y A similar approach is used to calculate the D value for chemical diffusion in the water phase in the soil, except that the molecular diffusivity in water B W is used (a value of 4.3 ¥ 10 –5 m 2 /day being assumed), and the water volume fraction and Z value being used, namely, where D W = B EWAZ W/Y B EW = B Wv W 10/3 /(vA + v W) 2 Since the diffusion D values D A and D W apply in parallel, the total D value for chemical transfer from bulk soil to the soil surface is (D A + D W). The boundary layer D value then applies in series so that the overall volatilization D value, D V, is given as illustrated in Figure 8.2 as 1/D V = 1/D E + 1/(D A + D W) Selection of the diffusion path length Y involves an element of judgement. If, for example, chemical is equally distributed in the top 20 cm of soil, an average value of 10 cm for Y may be appropriate as a first estimate. This will greatly underestimate the volatilization rate of chemical at the surface. Since the rate is inversely proportional to Y, a more appropriate single value of Y as the average between two depths Y 1 and Y 2 is the log mean of Y 1 and Y 2, i.e., (Y 1 – Y 2)/ln(Y 1/Y 2). Unfortunately, a zero (surface) value of Y cannot be used when calculating the log
mean. For chemical between depths of 1 and 10 cm, a log mean depth of 3.9 cm is more appropriate than the arithmetic mean of 5.5 cm. It may be useful to consider layers of soil separately, e.g., 2 to 4 cm, 4 to 6 cm, etc., and calculate separate volatilization rates for each. Chemical present at greater depths will thus volatilize more slowly, leaving the remaining chemical more susceptible to other removal processes. It is acceptable to specify a mean Y of, say, 10 cm to examine the fate of chemical in the 2 cm depth region from 9 to 11 cm. This depth issue is irrelevant to reaction or leaching, but it must be appreciated that, if the soil is treated as separate layers, the leaching rate is applicable to the total soil, not to each layer independently. The total rate of chemical removal is then f D T, where the total D value is: ©2001 CRC Press LLC D T = D R + D L + D V the individual rates being f D R, f D L, and f D V. The overall rate constant k O is thus D T/V TZ T, where V TZ T is the sum of the V iZ i products, and the overall half-life t O is 0.693/k O hours. The half-life t i attributable to each process individually is 0.693V TZ T/D i, thus, 1/t O = 1/t R + 1/t L + 1/t V It is illuminating to calculate the rates of each process, the percentages, and the individual half-lives. Obviously, the shorter half-lives dominate. The situation being simulated is essentially the first-order decay of chemical in the soil by three simultaneous processes, thus the amount remaining from an initial amount M (mol) at any time t (h) will be M exp(–D Tt/V TZ T) = M exp(–k Ot) mols This relatively simple calculation can be used to assess the potential for volatilization or for groundwater contamination. Implicit in this calculation is the assumption that the chemical concentration in the air, and in the entering leaching water, is zero. If this is not the case, an appropriate correction must be included. In principle, it is possible to estimate atmospheric deposition rates as was done in the air-water example and couple these processes to the soil fate processes in a more comprehensive air-soil exchange model. It may prove desirable to segment the soil into multiple layers, especially if evaporation or input from the atmosphere is important. <strong>Models</strong> of this type have been reported by Cousins et al. (1999) for PCBs in soils. 8.4.3 Model The Soil model is available from the website in both Windows software and in the older DOS-based BASIC format, similarly to the AirWater model. The SoilFug model is also available and can be used to explore the effects of varying precipitation on soil runoff.
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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
Page 159 and 160: Figure 7.1 Illustration of nonequil
Page 161 and 162: 5. Diffusion within soils, and from
Page 163 and 164: sionally, the term flux rate is use
Page 165 and 166: ©2001 CRC Press LLC N = ABDC/ Dy m
Page 167 and 168: no currents or eddies. In practice,
Page 169 and 170: diffusivity, and some unknown layer
Page 171 and 172: where ©2001 CRC Press LLC X = y/ U
Page 173 and 174: fraction of the total volume that i
Page 175 and 176: Figure 7.6 Mass transfer at the int
Page 177 and 178: partition coefficient. The signific
Page 179 and 180: and the series retains a similar ra
Page 181 and 182: ments of resuspension rates are par
Page 183 and 184: Figure 7.8 Movement of air phase (k
Page 185 and 186: ates can thus be used to estimate m
Page 187 and 188: Figure 7.9 Combination of D values
Page 189 and 190: Figure 7.10 Four-compartment Level
Page 191 and 192: Table 7.2 Order of Magnitude Values
Page 193 and 194: Unlike the Level II calculation, it
Page 195 and 196: Figure 7.12 Sample Level III output
Page 197 and 198: McKay, Donald. "Applications of Fug
Page 199 and 200: applications. Citations are given t
Page 201 and 202: Another approach is to employ Monte
Page 203 and 204: LEVEL1B A six-compartment Level I p
Page 205 and 206: Figure 8.1 Air-water exchange proce
Page 207 and 208: The total rates of transfer are thu
Page 209: Figure 8.2 Chemical transport and t
Page 213 and 214: 8.5.2 Process Description The water
Page 215 and 216: learn the benefits of using fugacit
Page 217 and 218: where Figure 8.5 QWASI model: stead
Page 219 and 220: are similar to those described by M
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
Page 231 and 232: iomagnification is more significant
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 and 242: mated again in mg/day. Food, the ot
Page 243 and 244: models. Most interest is in LRT in
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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