A comprehensive approach to plume stability

This article defines and presents a systematic approach to groundwater plume-stability assessment. Qualitative and quantitative methods that have been used to assess plume stability at National Priority List sites undergoing optimization are reviewed. Example case studies are included to illustrate the advantages of combining multiple analysis methods. Relevant statistical methods include identifying normal data distributions, detection frequencies, coefficients of variation, individual well trends, and rates of change at individual monitoring locations. Trend estimates for total plume mass and center of mass provide a broader picture of plumewide processes. Deterministic methods, such as quantitative mass-balance approaches, may be useful for larger plumes. Qualitative assessments include evaluations of the conceptual site model, source strength, attenuation mechanisms, and hydrogeology. Because groundwater plumes are always dynamic, the determination of plume stability has to include qualitative steps relating the rate and magnitude of change to the goals and objectives of the program and the time frame over which critical management decisions are to be made. The assessment of plume stability is, therefore, presented as a process that should involve both qualitative and quantitative steps for determining the acceptability of variability in groundwater contaminant concentrations.