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Using Compound Specific Isotope Analysis (CSIA) in Groundwater Assessments

Dennis G. Shin, PE

The U.S. Environmental Protection Agency (USEPA) is committed to protecting the Nation’s land, air, and water resources. Under a mandate of national environmental laws, the Agency strives to formulate and implement actions leading to a compatible balance between human activities and the ability of natural systems to support and nurture life. To meet this mandate, the USEPA’s research program is providing data and technical support for solving environmental problems today and building a science knowledge base necessary to manage our ecological resources wisely, understand how pollutants affect our health, and prevent or reduce environmental risks in the future.

Managing the risk associated with hazardous organic compounds in ground water at hazardous waste sites often requires detailed knowledge of the extent of degradation of the organic contaminants at the site. An evaluation of the contribution of natural biodegradation or abiotic transformation processes in ground water is usually crucial to the selection of Monitored Natural Attenuation (MNA) as a remedy for a site. Documentation that the organic contaminant is actually being degraded is important for performance monitoring of MNA, performance monitoring of active in situ bioremediation, and performance monitoring of many other active remedial technologies.

The traditional approach of monitoring a reduction in the concentrations of contaminants at sites often does not offer compelling documentation that the contaminants are actually being degraded. When data on concentrations are the only data available, it is difficult or impossible to exclude the possibility that the reduction in contaminant concentrations are caused by some other process such as dilution or dispersion, or that the monitoring wells failed to adequately sample the plume of contaminated ground water. Stable isotope analyses can provide unequivocal documentation that biodegradation or abiotic transformation processes actually destroyed the contaminant.

When organic contaminants are degraded in the environment, the ratio of stable isotopes will often change, and the extent of degradation can be recognized and predicted from the change in the ratio of stable isotopes. Recent advances in analytical chemistry make it possible to perform Compound Specific Isotope Analysis (CSIA) on dissolved organic contaminants such as chlorinated solvents, aromatic petroleum hydrocarbons, and fuel oxygenates, at concentrations in water that are near their regulatory standards.

At many hazardous waste sites, progress toward cleanup of contamination in ground water depends on successful identification of the true source of the contamination. Often, the ratio of stable isotopes in materials in commerce will vary, depending on the isotope ratio in the feed stock used for synthesis of the material, and on the particular chemical process used to manufacture the material. Different spills of the same material may have different isotopic “signatures” that can be used to associate a plume of contamination in ground water with a particular spill. Because CSIA is a new approach, there are no widely accepted standards for accuracy, precision and sensitivity, and no established approaches to document accuracy, precision, sensitivity and representativeness. This course reviews general recommendations on good practice for sampling ground water for CSIA, and quality assurance recommendations for measurement of isotope ratios. The course also reviews recommendations for data evaluation and interpretation to use CSIA to document degradation of organic contaminants, or to associate plumes of contaminants in ground water with their sources.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

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