Mass balance, environmental fate

Best EPH et al., Environmental behavior of explosives in groundwater from the Milan army ammunition plant in aquatic and wetland plant treatments. Removal, mass balances and fate in groundwater of TNT and RDX, Chemosphere, 38, 3383, 1999. 

MASS BALANCE MODELS OF CHEMICAL FATE 1.5.1 Evaluative Environmental Calculations... 

The discussion above provides a brief qualitative introduction to the transport and fate of chemicals in the environment. The goal of most fate chemists and engineers is to translate this qualitative picture into a conceptual model and ultimately into a quantitative description that can be used to predict or reconstruct the fate of a chemical in the environment (Figure 27.1). This quantitative description usually takes the form of a mass balance model. The idea is to compartmentalize the environment into defined units (control volumes) and to write a mathematical expression for the mass balance within the compartment. As with pharmacokinetic models, transfer between compartments can be included as the complexity of the model increases. There is a great deal of subjectivity to assembling a mass balance model. However, each decision to include or exclude a process or compartment is based on one or more assumptions—most of which can be tested at some level. Over time the applicability of various assumptions for particular chemicals and environmental conditions become known and model standardization becomes possible. 

All pesticides that can come into contact with the environment are subject to a risk assessment. The basis for this risk assessment is provided by data from environmental fate and environmental toxicity studies, which are carried out in the laboratory or under field conditions. The fate (adsorption, degradation, and mobility) of the active substance must be studied in soil, air, water, and sediments. The laboratory studies are frequently performed with C-labeled substances to make the mass balance easier. It is important to know how a substance degrades in the environment, because sometimes the degradation products are more persistent than the parent substance. DDT, for instance, is converted to metabolites by stepwise dechlorination (Eq. 11.9). The metabolites (e.g., DDD or DDA) can be found in soil for many years after the DDT itself is degraded. 

Mass-balance considerations, in particular the observed consumption of contaminants, were useful in showing the importance of biodegradation processes for limiting the mobility of petroleum hydrocarbons in groundwater systems. The mass-balance approach also contributed to our understanding of the environmental fate of chlorinated solvents in groundwater systems. In the 1980s, the observed behavior of chlorinated... 

Three possible outcomes exist for a chemical present at a specific location in the environment at a particular time the chemical can remain in that location, can be carried elsewhere by a transport process, or can be eliminated through transformation into another chemical. This very simple observation is known as mass balance or mass conservation. Mass balance is a concept around which an analysis of the fate and transport of any environmental chemical can be... 

The fundamentals of mathematical models lie in the mass balance of the chemical in the environment, which is quantitatively expressed in terms of equilibrium and rate constants of the environmental fate processes. Incorporating these constants into a set of the mass balance equations, and solving these equations are complicated, so that computers are used frequently to reduce the time and cost. 

In multimedia box models, the environmental fate of a chemical is described by a set of coupled mass-balance equations for all boxes of the model. These equations include terms for degradation, inter-media exchange such as settling and resuspension of particles, and transport with air and water flows [19,20]. Equations for different boxes are coupled by inter-media exchange terms (linking different environmental media) and terms for trans-... 

A key concept in describing environmental chemical fate is the principle of mass preservation. A chemical in a par-ticirlar location at a specific time can remain at that location, can be transported elsewhere, or can be transformed into another chemical. A mass balance can be formulated for a specific subsection of the environment, called the system or control volume. Defining a system boundary involves a decision of what is considered a part of the system and what is part of its srrnoundings. The mass balance then accormts for how much chemical crosses the system botmdary and how much chemical is generated and lost within the system dtuing a particular time interval (Fig. 5) ... 

Multimedia environmental models often incorporate the concept of fugac-ity into mass balance calculations. As pioneered by Dr. Donald Mackay and described in Table 2.3, fugacity models can reflect four levels of sophistication [64]. Level III fugacity models are commonly used to describe the fate and transport of a chemical released to the environment that undergoes degradation and advective transport between compartments. One such model is described below. 

Every mass balance on the fate and transport of a chemical in the environment requires numerous assumptions the results should not be used blindly or assigned too much weight as absolute predictors of environmental concentrations. 

Although LCA contains elements of the tools discussed earlier in this chapter—mass balance, multimedia modeling of environmental fate and transport, and risk characterization— it is a distinct discipline with its own jargon, precepts, and limitations. 

This section describes how our understanding of the behavior of 1,4-DCB in the environment has evolved over the 40-year history described in this case study. After an overview of fhe fate and transport of 1,4-DCB from a contemporary perspective follows a summary of early environmenfal mass balances that predated the practice of life cycle analysis, and a view of environmental fate and transport from fhe perspective of the EU restriction under REACH. 

In a forerunner of fhe current practice of life cycle analysis, the US EPA commissioned a mass balance on chlorobenzenes in 1979. The study was intended "to determine, within the constraints of time and information availability, the quantities of chlorobenzenes annually released to the environment and the sources of those releases" [54, p. 1-1]. At that time, regulators knew relatively little about the environmental fate of 1,4-DCB, simply that it "[u]ndergoes degradation at a moderate to rapid rate. It is degradable... 

The conservation of mass, performed by applying the Lavosier species mass balance to chemicals in the natural media, is the basic concept underlying environmental chemodynamics. The species CE is the result and is a good context in which to present the various types of chemical mass transport processes needed for environmental chemical modeling and chemical fate analysis. For constant physical properties first-order reaction, and dilute solutions in any media the CE in vector notation is... 

The calculation of characteristic times aids in the interpretation of multimedia mass balance for environmental compartments by conveying how long it will take for the compartment or phase to adjust as a result of each rate process. Obviously, the shorter characteristic time processes are the most important with respect to chemical fate. The following simple example demonstrates the use of fugacity, Z values, D values, and characteristic times when interpreting the results of mass balances. 

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