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

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A key concept in this discussion that was introduced earlier, and is variously termed persistence, lifetime, residence time, or detention time of the chemical. In a steady-state system, as shown in Figure 6.1a, if chemical is introduced at a rate of E mol/h, then the rate of removal must also be E mol/h. Otherwise, net accumulation or depletion will occur. If the amount in the system is M mol, then, on average, the amount of time each molecule spends in the steady-state system will be M/E hours. This time, t, is a residence time and is also called a detention time or persistence. Clearly, if a chemical persists longer, there will be more of it in the system. The key equation is ©2001 CRC Press LLC t = M/E or M = t E This concept is routinely applied to retention time in lakes. If a lake has a volume of 100,000 m 3 , and if it receives an inflow of 1000 m 3 per day, then the retention time is 100,000/1000 or 100 days. A mean retention time of 100 days does not imply Figure 6.1 Diagram of a steady-state evaluative environment subject to (a) advective flow, (b) degrading reactions, (c) both, and (d) the time course to steady state.
that all water will spend 100 days in the lake. Some will bypass in only 10 days, and some will persist in backwaters for 1000 days but, on average, the residence time will be 100 days. The reason that this concept is so important is that chemicals exhibit variable lifetimes, ranging from hours to decades. As a result, the amount of chemical present in the environment, i.e., the inventory of chemical, varies greatly between chemicals. We tend to be most concerned about persistent and toxic chemicals, because relatively small emission rates (E) can result in large amounts (M) present in the environment. This translates into high concentrations and possibly severe adverse effects. A further consideration is that chemicals that survive for prolonged periods in the environment have the opportunity to make long and often tortuous journeys. If applied to soil, they may evaporate, migrate onto atmospheric particles, deposit on vegetation, be eaten by cows, be transferred to milk, and then consumed by humans. Chemicals may migrate up the food chain from water to plankton to fish to eagles, seals, and bears. Short-lived chemicals rarely survive long enough to undertake such adventures (or misadventures). This lengthy justification leads to the conclusion that, if we are going to discharge a chemical into the environment, it is prudent to know 1. how long the chemical will survive, i.e., t, and 2. what causes its removal or “death” This latter knowledge is useful, because it is likely that situations will occur in which a common removal mechanism does not apply. For example, a chemical may be potentially subject to rapid photolysis, but this is not of much relevance in long, dark arctic winters or in deep, murky sediments. In the process of quantifying this effect, we will introduce rate constants, advective flow rates and, ultimately, using the fugacity concept, quantities called D values, which prove to be immensely convenient. Indeed, armed with Z values and D values, the environmental scientist has a powerful set of tools for calculation and interpretation. It transpires that there are two primary mechanisms by which a chemical is removed from our environment: advection and reaction, which we discuss individually and then in combination. ©2001 CRC Press LLC 6.2 ADVECTION Strangely, “advection” is a word rarely found in dictionaries, so a definition is appropriate. It means the directed movement of chemical by virtue of its presence in a medium that happens to be flowing. A lazy canoeist is advected down a river. PCBs are advected from Chicago to Buffalo in a westerly wind. The rate of advection N (mol/h) is simply the product of the flowrate of the advecting medium, G (m3 /h), and the concentration of chemical in that medium, C (mol/m3), namely, N = GC mol/h
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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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Page 81 and 82: Figure 5.1 Some principles and conc
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Page 85 and 86: Another useful quantity is the rati
Page 87 and 88: experimentally, but not directly, u
Page 89 and 90: ©2001 CRC Press LLC 5.3 PROPERTIES
Page 91 and 92: 1.0. This, then, is the fugacity or
Page 93 and 94: where ©2001 CRC Press LLC pH is -l
Page 95 and 96: In terms of solubilities, S iA and
Page 97 and 98: The temperature coefficient is the
Page 99 and 100: (1982), Baum (1997) and Leo (2000).
Page 101 and 102: Although it may appear environmenta
Page 103 and 104: Figure 5.4 Illustration of quantita
Page 105 and 106: 5.5 ENVIRONMENTAL PARTITION COEFFIC
Page 107 and 108: C S = K PC W benzene = 1.1 ¥ 0.001
Page 109 and 110: Mackay (1986), in an attempt to sim
Page 111 and 112: Compartment Definition of Fugacity
Page 113 and 114: high concentration in the fish, but
Page 115 and 116: heat capacity of 0.38 J/g°C requir
Page 117 and 118: Therefore, ©2001 CRC Press LLC f =
Page 119 and 120: ©2001 CRC Press LLC Z T = S(V i/V
Page 121 and 122: ©2001 CRC Press LLC air Z = P S /R
Page 123 and 124: Table 5.1 Table 5.1 Summary of Defi
Page 125 and 126: Figure 5.7 Fugacity form 1 for dedu
Page 127 and 128: Calculate the following physical-ch
Page 129: ©2001 CRC Press LLC CHAPTER 6 Adve
Page 133 and 134: This enables us to conceive of, and
Page 135 and 136: ©2001 CRC Press LLC f = 15/0.5 = 3
Page 137 and 138: D AS dominates. A residence time of
Page 139 and 140: and Therefore, ©2001 CRC Press LLC
Page 141 and 142: eaction situation in which there is
Page 143 and 144: The rate constants in each case are
Page 145 and 146: ©2001 CRC Press LLC 1/t O = SD Ai/
Page 147 and 148: e achieved in 10 days. Detractors o
Page 149 and 150: sulfate to sulfide or by dechlorina
Page 151 and 152: to Zepp and Cline (1977) for the or
Page 153 and 154: ©2001 CRC Press LLC 6.7 LEVEL II C
Page 155 and 156: Figure 6.3 Fugacity Level II calcul
Page 157 and 158: McKay, Donald. "Intermedia Transpor
Page 159 and 160: Figure 7.1 Illustration of nonequil
Page 161 and 162: 5. Diffusion within soils, and from
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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
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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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Table 7.2 Order of Magnitude Values
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Unlike the Level II calculation, it
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Figure 7.12 Sample Level III output
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McKay, Donald. "Applications of Fug
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applications. Citations are given t
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Another approach is to employ Monte
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LEVEL1B A six-compartment Level I p
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Figure 8.1 Air-water exchange proce
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The total rates of transfer are thu
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Figure 8.2 Chemical transport and t
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mean. For chemical between depths o
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8.5.2 Process Description The water
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learn the benefits of using fugacit
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where Figure 8.5 QWASI model: stead
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are similar to those described by M
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C F ª Z Ff W ª 1.608 mol/m 3 ª 4
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y Mackay et al. (1983), and an appl
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accordingly, but the final solution
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Figure 8.10 Fish bioaccumulation pr
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The nature of the processes control
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iomagnification is more significant
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An alternative and more elegant met
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8.11.2 Model of a Chemical Evaporat
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Paterson et al. (1991), and Hung an
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8.14.1 Introduction ©2001 CRC Pres
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mated again in mg/day. Food, the ot
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models. Most interest is in LRT in
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available from the University of To
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McKay, Donald. "Appendix" Multimedi
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©2001 CRC Press LLC
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