Groundwater Protection

Introduction: The atoms of a particular element must have the same number of protons and electrons, but they can have different numbers of neutrons. When atoms differ only in the number of neutrons, they are referred to as isotopes of each other. If a particular isotope is not radioactive, it is called a stable isotope. Because they differ in the number of neutrons, isotopes differ in mass, and they can be separated using a mass spectrometer. In recent years mass spectrometers have been joined to gas chromatographs to allow separation of individual organic compounds in a mixture, followed by combustion of each separate organic compound to carbon dioxide, and then determination of the ratio of isotopes in the carbon dioxide with a mass spectrometer. Even more recently, new techniques of sample preparation, such as purge and trap or solid phase micro-extraction, have made it possible to obtain adequate material for analyses from water with low concentrations of organic contaminants. For the first time, it is 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. Biodegradation can come about through natural biological processes, or through active in situ bioremediation. 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; CSIA has great promise to improve our understanding of the behavior of organic contaminants at hazardous waste sites. Better understanding can lead to better decisions on the remedies that are selected. CSIA can also be used to monitor the progress of natural attenuation or active biological remediation, and identify remedies that are not performing as expected. The U.S. Environmental Protection Agency requires that data quality objectives be developed for the methods and procedures that are used to characterize hazardous waste sites. The U.S. EPA also requires that the data that are used to make decisions must meet predetermined goals for data quality, including the accuracy, precision, and sensitivity of the measurement, and the extent to which the sample submitted for analysis are representative of the environmental medium being sampled. Other regulatory agencies world-wide have similar expectations. Because CSIA is a new approach in environmental investigations, there are no widely accepted standards for accuracy, precision and sensitivity, and no established approaches to document accuracy, precision, sensitivity and representativeness. This Guide is intended for managers of hazardous waste sites who must design sampling plans that will include CSIA and specify data quality objectives for CSIA analyses, for analytical chemists who must carry out the analyses, and for staff of regulatory agencies who must review and approve the sampling plans and data quality objectives, and who must review the data provided from the analyses. This Guide provides recommendations and suggestions to site managers, chemists and regulators. The recommendations and suggestions in this Guide are not legal guidance, and the site managers, chemists, and regulators may negotiate among themselves to develop objectives and approaches that are most appropriate for their site.