Distribution Of Contaminants
In The Environment
We have covered some of the environmental legislation in force in the United States. We have also talked about the need for this legislation and the history behind some of the most comprehensive measures. We have learned that when one talks about chemical contaminants in the environment one is generally thinking about synthetic chemicals. The boom in synthetic chemistry started in the early 1900s and has continued to the present time.
The improvement in quality of life brought about by the chemical industry has been astounding. One cannot think of agriculture without fertilizers and pesticides, travel without automobiles or airplanes. It is difficult to think of life without modern pharmaceuticals and plastics. But we have and, to a certain extent, will always pay a price for this improved quality of life. One price has been in terms of adverse effects on the ecosystem and on human health. Because of some real fears and some exaggerated ones, the general public is increasingly aware of environmental issues. The most substantive questions are:
- €What are the harmful effects of a chemical on the environment in general and on human health in particular?
- €What is the likelihood of the perceived harmful effects?
The answer to the first question is derived from toxicological evaluation or epidemiological studies. The idea here is to establish a dose/response relationship. This is a vast and growing field; however, it is beyond the scope of this course. Instead, we will dwell on part of the second question, i.e., What is the likelihood of exposure and at what level or concentration? One can rephrase the question as, What is the concentration of pollutant(s) likely to affect human health? and What is the time trend associated with it?
A simplified schematic representation of this, in the case of a synthetic chemical, is presented in Figure 2-1.
It is obvious from the schematic that, in order to estimate exposure of a chemical, it is necessary to understand how materials are introduced, transported and transformed as they move from the point of entry to the final sink, if there is one. The routes of exposure can be broken down into segments. The substance's intersegment and intrasegment movement can then be considered.
The ocean can be considered as one segment and the troposphere another. The food chain can be considered a segment, albeit a rather complex one. The relationship between some of the segments can be expressed simply in terms of the first order rate constant.
When we look at exposure and transport of a chemical in the ecosystem, we need to consider four factors:
- €Input
- €Kinetic processes
- €Transport processes
- €Output
FIGURE 2-1. A schematic representation of the flow of a chemical product from production to the environment. (From Neely, W.B., Chemicals in the Environment, Marcel Dekker, N.Y., 184, 1980.)
Input
Every chemical pollutant in the ecosystem has a source. It could be discharge from an industrial plant, pesticide application in the field, or a waste dump site. In every case, an estimate of the concentration needs to be made to answer the question of how much pollutant is in the ecosystem.
It goes without saying that, in order to determine concentration, one must know or characterize the contaminant. This is the first task for environmental monitoring.
How can we know what the chemical is?
- €Prior (background) Information; e.g., information on plant operation, waste dump history, reagent and product lists and byproducts of the process.
- €Chemical Determination
- Targeted
- Comprehensive
- €Biological Monitoring and Toxicity Monitoring
Transport Process
The effect of chemicals on human health and the ecosystem is also a function of transport processes. Transport of chemicals in the ecosystem is primarily controlled by the physicochemical properties of individual chemicals on bulk contaminant mixture.
Kinetic Process
The ultimate fate of a chemical and its rate of disappearance from the ecosystem is determined by specific reactions and their rates. Most pertinent reactions depend on the chemical and the matrix. For instance, the most important reaction for chlorinated organics in the troposphere and stratosphere is photodegradation, whereas the most important reaction in or near the subsurface environment is microbial degradation.
Output
The chemical material balance, and consequently the exposure, is also dependent on the ultimate (or close to one) sink or final resting place of the chemical. Warren J. Lyman of Arthur D. Little Environmental Consultants has coined a first law of environmental pollution. It states: "Everything has to go somewhere."
We can restate our discussion as, What is the expected environmental concentration-time profile of a chemical at specific locations in various media during manufacture, use and dissipation of products and byproducts?
Systematic answers to these questions can be obtained, to a degree, from the properties of the chemical. Which leads into the question, What chemical properties are needed to assess environmental transport and fate? As many as 40 chemical specific properties have been listed on various wish lists. Some of these are listed in Table 2-1.
There are more than 40,000 chemicals listed on the EPA Toxic Substances Inventory List (representing an annual production of 1.8 x 109 tons). Of these, 3800 materials are produced at a rate greater than one million lb/year. However, well-defined structures constitute 34%, or 1300, of this group. As a result, all desired properties are not known and other approaches to assess environmental behavior have been established. Ultimately, however, the exposure must be monitored experimentally.
Estimation Methods
Most methods of estimation are based upon one of the following:
- €Theoretical equation, generally with experimentally or empirically derived parameters
- €Group or atomic fragment constants, sometimes with structural factors, derived from regression analysis of data sets
- €Correlation, usually in the form of linear regression analysis between two or three properties
- €Rules for topological calculations and subsequent use of correlation equations between the index and the property of interest
Table 2-1. Important Chemical-Specific Properties
Bulk (condensed phase) properties affecting mobilitya
Physical state (solid, liquid, gas) of waste
Chemical composition of waste
Density (liquid)
Viscosity (liquid)
Interfacial tension (with water and minerals) (liquid)
Properties to assess mobility of low concentrationsb
Henry's law constant (or vapor pressure and water solubility)
Bioconcentration factor
Soil adsorption coefficient
Diffusion coefficient (in air and water)
Acid dissociation constant
Related properties
Octanol-water partition coefficient
Activity coefficient
Mass transfer coefficients (and/or rate constants) for each interphase transfer
Boiling point
Melting point
Properties to assess persistencec
Rate of biodegradation (aerobic and anaerobic)
Rate of hydrolysis
Rate of oxidation or reduction
Rate of photolysis (in air and water)
- aThese properties will be important when it is known or suspected that a separate organic phase exists in the environment, e.g., due to a large spill.
- bThese properties are important in assessments of the mobility of chemicals present in low concentrations (i.e., not as a separate phase) in the environment.
- cFor these properties it is generally important to know: (1) the effects of key parameters on the rate constants (e.g., temperature, concentration, pH) and (2) the identity of the reaction products.