Multimedia pollutants concentrations

Lead acts as a multimedia pollutant in the human environment. Multimedia pollutants are complex to regulate because of some larger realities. For example, what are the major sources and pathways for the contaminant in terms of human exposures on either a national or case-specific basis How well are levels in the major media controlled relative to concentrations in the minor media and vice versa" ... 

Lead is a multimedia pollutant, i.e., it provides exposures through diverse environmental media. The specific characteristics of each environmental contributor, such as Pb concentrations and intake amounts of some environmental medium, help determine the extent to which different individuals and populations sustain actual exposures. This also means that while some Pb sources can be characterized on a national, international, or other macroscale as being significant, the actual sources and pathways in specific cases for substances such as lead or other elements require specific evaluation. Some segments of human populations may sustain exposures from Pb in more than one environmental medium, in which case one employs methods to sort out potential contributions or the relative total contributions to total exposures. 

Multimedia models can describe the distribution of a chemical between environmental compartments in a state of equilibrium. Equilibrium concentrations in different environmental compartments following the release of defined quantities of pollutant may be estimated by using distribution coefficients such as and H s (see Section 3.1). An alternative approach is to use fugacity (f) as a descriptor of chemical quantity (Mackay 1991). Fugacity has been defined as fhe fendency of a chemical to escape from one phase to another, and has the same units as pressure. When a chemical reaches equilibrium in a multimedia system, all phases should have the same fugacity. It is usually linearly related to concentration (C) as follows ... 

To address media-specific problems, single-media models for air, surface water, groundwater and soil pollution have been developed and used by different disciplines. Although these models generally provide detailed description of the pollutant distribution in space and time and incorporate mass transfer from other media as boundary conditions, they are not capable of characterizing the total environmental impact of a pollutant release. Multimedia models have been, therefore, developed to predict the concentration of chemicals in multiple environmental media simultaneously with consideration of chemical transport and transformation within and among media [1],... 

In this case-study, the issue of concern is the extensive exposure of human populations to PBLx, which is a persistent pollutant with multimedia and multipathway exposure potential. For PBLx, exposures occur primarily through food pathways. Here we focus on fish ingestion exposures from PBLx emissions to air that are deposited on ocean and fresh waters and accumulated in both ocean and freshwater fish. This situation focuses attention on three key pieces of information—concentration in water, biotransfer to fish and human consumption of fish. 

Of particular importance is the contamination of soil, because it receives pollutants from the atmosphere (e.g., sulfates and nitrates resulting from oxidation of nitrogen and sulfur oxides, and metals from smelters) and from the hydrosphere (e.g., sediments that concentrate heavy metals from aqueous bodies and mining operations). In multimedia mass-balance models, soil is an important sink as well as a conduit for mass transfer to vegetation and shallow groundwater. 

Multimedia models and field data indicate that this is a dominant process for the transfer of persistent organic pollutants to lakes (e.g., Bidleman McConnell, 1995 Mackay Wania, 1995). Fluxes of air-water gas exchange are typically calculated using the two-film resistance model (Schwartzenbach et al 1993). Fluxes can be in either direction and depend upon air and water concentrations, partition coefficients, and air and water resistivities. 

Pollution prevention offers industry an opportunity, but its exact cost, benefits, and risks are difficult to fully identify or quantify. Pollution prevention represents a significant change in the scope and methodology usually used in waste management. It is a multimedia approach that concentrates on preventing the production of waste in any form in all parts of the plant. 

As noted above, there are cases where we need more accurate representations of how chemical concentration varies with depth. For example, we may be interested in transfers of chemicals from air to shallow ground water or want to consider how long-term applications of pesticides to the soil surface can impact terrestrial ecosystems—including burrowing creatures. However, we also wish to maintain a simple mathematical mass-balance structure of the multimedia model. To illustrate how we can set up a multilayer model that accurately captures soil mass transport processes, we next derive a vertical compartment structure with an air and three soil compartments, but any number of environmental compartments and soil layers can be employed in this scheme. Figure 8.6 provides a schematic of three soil layers linked to an air compartment and carrying pollutants downward to a saturated zone. We represent the inventory in each vertical compartment i, as M, (mol), transformation rate constants as kt, and transfer factors as ky (d ). The latter account for the rate of transfer between each i and j compartment pair. 

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