Fugacity soil water

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. 

Yaron, B., Heuer, B., Birk, Y. (1974) Kinetics of azinphosmethyl losses in the soil environment. J. Agric. Food Chem. 22(3), 439 -41. Yin, C., Hassett, J.P. (1986) Gas partition approach for laboratory and field studies of mirex fugacity in water. Environ. Sci. Technol. 20, 1213-1217. 

Although activity and fugacity are closely related, they have quite different characteristics in regard to phase equilibria. Consider, for example, the equilibrium between liquid water and water vapor in the interstices of an unsaturated soil. At a given temperature and pressure, the principles of thermodynamic equilibrium demand that the chemical potentials and fugacities of water in the two phases be equal. However, the activities of water in the two phases will not be the same because the Standard State for the two phases is not j the same. Indeed, f° = 1 atm for the water vapor, so its activity is numerically ]... 

So at equilibrium, the fugacity of any species must be equal in phases a and p. This result is general, in that the fugacity of any species must be equal in all phases at equilibrium. For example, if benzene (B) is distributed between soil, water, and air at equilibrium, then the fugacity of benzene in each of these phases is identical. 

Fugacity can be regarded as the escaping tendency of a chemical substance from a phase, usually expressed in units of pressure. Five environmental compartments are considered as phases atmosphere, soil, water, sediment, and aquatic biota. 

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 III model includes all the important fate and transport processes in a real environment and is one step more complex than Level n. As in the Level II model, the chemical is discharged at a constant rate into the environment to reach a steady state (at which input equals output). Unlike Level II, equilibrium between different media is not assumed and rates of chemical transfer by intermedia transport processes are defined. The individual discharges to all environmental media must be specified because fhe disfribufion of the chemical between media now depends on how the chemical enters the system. Depending on the properties of a chemical, the mode of entry can also significantly alter chemical persistence or residence time in the environment to viues that are quite different from Level II results. A series of 12 transport velocities control chemical transfer between the four primary environmental media (air, water, soil, and sediment). Equilibrium is assumed, however, within each medium. For example, suspended matter and fish are assumed to be at the same fugacity as water. 

In this case the D values, which are conductivities, add reciprocally since the 1/D quantities are effectively resistances. The relative resistances become immediately apparent. In more complex situations, there may be several series and parallel resistances, an example being volatilization from soil in which there is diffusion in both soil air (pore air) and soil water (pore water) followed by an air boundary layer resistance. Later an example is given involving multiple transport processes and illustrates the simplicity and transparency of the fugacity mass balance equations. 

An advantage of the fugacity capacity approach is that for N compartments N values of Z are defined while there may be N(N-l)/2 partition coefficients. Using Z values the partitioning properties between two phases are attributed independently to each phase. It is possible to assign (accidentally) three inconsistent partition coefficients between air, soil and water but the three Z values are inherently consistent. 

Air-soil diffusion thus appears to be much slower than air-water diffusion because of the slow migration in the soil matrix. In practice, the result will be a nonuniform composition in the soil with the surface soil (which is much more accessible to the air than the deeper soil) being closer in fugacity to the atmosphere. 

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... 

Finally, it is interesting to note that the fugacity in this final case (in units of mPa) are for the four media 5.0 x HP3, 1.4, 1.6 and 1.1. The soil, sediment and water are fairly close to equilibrium, with the air notably under-saturated by a factor of about 200. This is the result of the rapid loss processes from air. 

Rainwater recharges the top of the profile and reacted water drains from the bottom. We take discharge through the soil to be 4 m yr-1 and assume the dispersivity or (see Chapter 20) is 1 cm. The rainwater is dilute and in equilibrium with the CO2 fugacity of the atmosphere, 10-3 5. Within the soil, however, the soil gas is taken to contain additional CO2 as a result of the decay of organic matter, and root respiration. The pore fluid is assumed to maintain equilibrium with the soil gas and CO2 fugacity within the soil is held constant over the simulation, at 10-2. 

Possible fate in the environment. An industrial chemical that has been released into the environment will exist in differing concentrations in the various environmental compartments. The concentrations of a substance in air, water, soil and other media following release can be modelled using the concept of fugacity.2 At its simplest, this involves only the use of standard physico-chemical data to estimate the partitioning between the various media. 

In this section solids represent a partitioning or adsorption phase such as soil, asphalt pavement, or granular activated carbon. In contrast to air, water, and octanol, solid phases are typically very complex and poorly characterized. For example, many studies have shown that soils and sediments are characterized by many different types and amounts of organic matter and minerals, and that these different environments have various affinities for an organic chemical. The fugacity for the solid or sorbed phase is expressed as follows ... 

The advantage of using fugacity to calculate the equilibrium distribution coefficients becomes apparent when one compares the fugacity capacities of a HOP for several different phases. For example, consider a region of the unsaturated zone just below the ground surface where naphthalene is distributed between air, water, pure phase octanol, and soil at equilibrium. The fugacity capacities for these phases are repeated below in Eqs. (46)-(49) ... 

The multimedia urban model (MUM) is a fugacity-based mass balance model that treats the movement of POPs in an urban environment and links emissions to ambient chemical concentrations, and thus outdoor exposure (Diamond et al., 2001). MUM considers longterm, average conditions of chemical transport and transformation among six environmental compartments in urban areas (air, soil, surface water, sediment, vegetation and surface film see Figure 6.1) shows a concepmal version of the model). The model does not estimate event-specihc processes as do meteorological-based air or stormwater models. 

PCB concentrations in air and water bodies have declined since efforts to reduce their escape into the environment were enacted in the 1970s. With falling PCB fugacities in the air, soils, and sediments may release previously deposited PCBs to water bodies and to the atmosphere [134]. This aspect needs further consideration in a congener-specific manner. 

In this section the environmental distribution of PCAs will be estimated using Mackay s Equilibrium Criterion (EQC) level III fugacity model [79]. Level III refers to a steady state, nonequilibrium system among soil, air, and water compartments, with the chemical undergoing reactions or inputs and removal processes (advection, volatilization, deposition, photolysis, hydrolysis, and biodegradation). 

Figure 3 Distributions of atrazine, chlorpyrifos, and chloropicrin among air, surface water, soils, and aqueous sediments, based on fugacity calculations. Percentages sum to less than 100% because partitioning into fish and suspended sediment was not accounted for (Mackay et al. (1997) reproduced hy permission of CRC Press, Lewis Publishers from Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic...

The fate of chemicals in the environment depends not only on processes taking place within compartments, but also by chemical partitioning between compartments. For example, there may be exchange of chemicals between air and water or soil. Movement from the water or soil into the air is accomplished by volatilization and evaporation of volatile or semivolatile compounds. Movement of chemicals from the air to water or soil is accomplished by deposition or diffusion into the water. Chemicals can also move from water to soil or sediment and vice versa. If a solid chemical in the soil or sediment dissolves into the water, this is called dissolution , while the opposite is called precipitation . If a chemical dissolved in water attaches to a soil or sediment particle, this is called adsorption , while the opposite is called desorption . The fugacity of a chemical, that is, its tendency to remain within a compartment, is affected by the properties of that chemical, as well as the chemical and physical properties of the environments such as temperature, pFF, and amount of oxygen in water and soil. Wind or water currents, wave action, water turbulence, or disturbance of soil or sediment (through the action of air or water currents, animals, or human activities) may also affect partitioning of chemicals. 

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