Level I Fugacity Calculations

The atmospheric height is selected as an arbitrary 1000 m reflecting that region of the troposphere which is most affected by local air emissions. A water surface area of 10% or 10,000 km2 is used, with a water depth of 20 m. The water volume is thus 2 x 10n m3. The soil is viewed as being well mixed to a depth of 10 cm and is considered to be 2% organic carbon. It has a volume of 9 x 109 m3. The bottom sediment has the same area as the water, a depth of 1 cm and an organic carbon content of 4%. It thus has a volume of 10s m3. 

For the Level I calculation both the soil and sediment are treated as simple solid phases with the above volumes, i.e., the presence of air or water in the pores of these phases is ignored. 

Two other phases are included for interest. Suspended matter in water is often an important medium when compared in sorbing capacity to that of water. It is treated as having 20% organic carbon and being present at a volume fraction in the water of 5 x 10 6, i.e., it is about 5 to 10 mg/L. The volume is thus 106 m3. Fish is also included at an entirely arbitrary volume fraction of 10 s and are assumed to contain 5% lipid, equivalent in sorbing capacity to octanol. The volume is thus 2 x 10s m3. These two phases are small in volume and rarely contain an appreciable fraction of the chemical present, but it is in these phases that the highest concentration of chemical often exists. 

Another phase which is introduced later in the Level III model is aerosol particles with a volume fraction in air of 2 x lO11, i.e., approximately 30 pg/m3. Although negligible in volume, an appreciable fraction of the chemical present in the air phase may be associated with aerosols. Aerosols are not treated in Level I or II calculations because their capacity for the chemical at equilibrium is usually negligible when compared with soil. 

These dimensions and properties are summarized in Tables 1.5.1 and 1.5.2. The user is encouraged to modify these dimensions to reflect conditions in a specific area of interest. 

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FIGURE 1.7.6 Level I fugacity calculations for benzene in a generic environment. 

FIGURE 1.7.15 Level I fugacity calculations for PCP at data determination pH of 5.1. ... 

Figures 1.7.15 to 1.7.18 show the mass distributions obtained in Level I calculations and the removal distribution from Level II fugacity calculation of pentachlorophenol (PCP) at two different environmental pHs for the generic...

A fugacity level I calculation may be 6 compartment equilibrium, no reaction, no advection, steady state a level II may be equilibrium, with reaction and advection, steady state level III may be non equilibrium, with reaction and advection, steady state,and level IV and EXAMS are non equilibium, with reaction and advection, unsteady state. 

Paterson and Mackay (1985), Mackay and Paterson (1990, 1991), and a recent text (Mackay 2001). Only the salient features are presented here. Three evaluations are completed for each chemical, namely the Level I, II and III fugacity calculations. These calculations can also be done in concentration format instead of fugacity, but for this type of evaluation the fugacity approach is simpler and more instructive. The mass balance models of the types described below can be downloaded for the web site www.trentu.ca/cemc... 

The Level I calculation proceeds by deducing the fugacity capacities or Z values for each medium (see Table 1.5.3), following the procedures described by Mackay (2001). These working equations show the necessity of having data on molecular mass, water solubility, vapor pressure, and octanol-water partition coefficient. The fugacity f (Pa) common to all media is deduced as... 

The Level I calculation suggests that if 100,000 kg (100 tonnes) of benzene are introduced into the 100,000 km2 environment, 99% will partition into air at a concentration of 9.9 x 10-7 g/m3 or about 1 pg/rn3. The water will contain nearly 1% at a low concentration of 4 pg/rn3 or equivalently 4 ng/L. Soils would contain 5 x 10-6 pg/g and sediments about 9.7 x 10 6 pg/g. These values would normally be undetectable as a result of the very low tendency of benzene to sorb to organic matter in these media. The fugacity is calculated to be 3.14 x 10-5 Pa. The dimensionless soil-water and sediment-water partition coefficients or ratios of Z values are 2.6 and 5.3 as a result of a Koc of about 55 and a few percent organic carbon in these media. There is little evidence of bioconcentration with a very low fish concentration of 3.0 x FT5 pg/g. The pie chart in Figure 1.7.6 clearly shows that air is the primary medium of accumulation. 

The Level II calculation includes the half-lives of 17 h in air, 170 h in water, 550 h in soil and 1700 h in sediment. No reaction is included for suspended sediment or fish. The input of 1000 kg/h results in an overall fugacity of 6 x 10 6 Pa, which is about 20% of the Level I value. The concentrations and amounts in each medium are thus about 20% of the Level I values. The relative mass distribution is identical to Level I. The primary loss mechanism is reaction in air, which accounts for 802 kg/h or 80.2% of the input. Most of the remainder is lost by advective outflow. The water, soil and sediment loss processes are unimportant largely because so little of the benzene is present in these media, but also... 

Fugacity Level I calculations (six-compartment model) at environmental pH of 7... 

This analysis would thus predict that the chlorobenzene would distribute primarily into the air. Higher concentrations are associated with higher values of the fugacity capacity factor, Z. It is understood, however, that this Level I model is hypothetical and at best would represent how a persistent chemical could distribute given sufficient time to approach an equilibrium situation. On the other hand, the model does not require a lot of data and is simple to calculate. 

Several models of varying complexity have been developed to calculate and predict the distribution of chemicals in the environment (OECD, 1989a, 1993c). Most of them are derived from the Mackay model (Mackay 1979 Mackay and Paterson, 1981, 1982, 1990 Mackay, Paterson and Shiu, 1992) to estimate the environmental compartment (air, soil, water and biota) in which the chemical is most likely to be found. Based on the concept of fugac-ity, models were derived for four levels of increasing complexity. Level I... 

The Level IV calculation (Fig. 6) shows the buildup in concentrations and fugacity to the steady State (level III values) then the subsequent decay. Clearly, sediments are slower to respond to buildup and decay, i.e. they have a longer "time constant." A tenfold drop in sediment concentration would require 15 years. 

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