Materials balance approach

A materials (or mass) balance approach for contaminants of public health and/or environmental concern is used to determine the presence, fate and transport of contaminants in the environment . The materials balance approach, a fundamental principle of science, engineering, and industrial research and risk analysis , is familiar to professionals trained in the physical or life sciences and engineering. The use of a materials balance approach provides a technique to describe the environment as it is today and as it might be under conditions resulting from remedial actions or from changes in the way society produces, uses, and disposes of chemicals of environmental concern. It also provides a rational and fundamental basis for asking specific questions and for obtaining specific information, which is necessary for determining fate and transport of contaminants, selecting and evaluating remedial treatment options, and monitoring treatment effectiveness.

The determination or construction of a materials balance is dependent on conservation of materials. Material is not created nor is it destroyed by ordinary processes, but it is transformed. When applied to chemicals of environmental concern, conservation of materials requires that a chemical entering a specific environment must be transformed, held in, or transported out of the environment. The amount of a chemical that leaves any process, environmental compartment, or area must be exactly balanced by the amount that enters minus net accumulation within the process or compartment boundaries. This materials balance can be stated as a simple equation:

Change in mass in a volume = Mass entering a volume - Mass leaving a volume

The control volume (compartment) for analysis, the shape of the volume, and the identification of input and output flows as well as the processes acting within the control volume all must be chosen carefully in order for the materials balance to provide useful information for the assessment of environmental impacts and the control of environmental effects.

The steps involved in determining a materials balance of contamination in an environment include:

• Where is the contamination, in what form(s) does it exist and in what concentrations is it present?
• Where is the contamination going under the influence of natural geochemical and geobiological processes (that is, what are key pathways of transport and fate; what are the processes that affect mobility and degradation of contaminants)?
• Based on the answers to questions 1 and 2, how can the contamination be contained, destroyed, or immobilized in the specific phases in which it is found? Through time, as natural and remedial processes act upon specific contaminants, additional determination of the materials balance is required to assess the fate and transport of the contaminants or their transformation products and to develop further treatment or containment methodologies.
• Based on the answers to questions 1 and 2, what environmental phases should be monitored through time to assess the fate and transport of the contamination and effectiveness of treatment or containment under both natural and remedial processes?

Data to answer questions 1 and 2 may be obtained from three categories:

• direct data from administrative records regarding the type and quantity of contamination, as well as where it entered the environment;
• direct data from chemical measurements;
• indirect data from modelling and simulations of natural processes.

Types of direct data may include an assessment of the presence of contaminants in all environmental compartments present in a specific environment, which may include the air phase, aqueous and non-aqueous liquid phases, and solid phases. Contaminants may be transformed partially or completely in the respiration process to obtain energy to synthesize new microbial cells, so an estimate of mineralization (for example, by measuring oxygen utilization or carbon dioxide production in aerobic systems ) may also provide important information.

A determination of the accuracy of a materials balance assessment is indicated by a good agreement between direct and indirect measurements, suggesting that the control volume was well-bounded and the processes acting within the volume were well-defined. The study of the materials balance approach involves both art and science, but if well-implemented, can provide an understanding of the environment of concern.

See also Biogeochemical cycles; Risk assessment; Waste management

[Judith Sims ]

RESOURCES

PERIODICALS

Sims, R. C. "Soil Remediation Techniques at Uncontrolled Hazardous Waste Sites: A Critical Review." Journal of the Air & Waste Management Association 40 (1990): 703–732.