A Practical Approach to Evaluating Natural Attenuation of Contaminants in Ground Water |
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| Authors: | PM McAllister CY Chiang |
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| Institution: | Paul McAllister received a Ph.D. in chemical engineering from the University of Notre Dame in 1991. He is an associate research engineer in environmental RD&T at Shell Development Co.'s Westhollow Research Center in Houston, Texas. Since joining Shell, he has provided technical support to locations on ground water modeling and remediation issues. His research interests include in situ bioremediation of ground water and residual hydrocarbons, fate and transport phenomena in the subsurface, and natural attenuation processes.;Chen Y. Chiang received his Ph.D. in environmental engineering from Rice University. He is a senior research engineer in the environmental RD&T at Shell Development Co. 's Westhollow Research Center. He serves on the Hydrologic Science Panel of the National Science Foundation and on the Ground Water/Soil Technical Task Force of the American Petroleum Institute. His research interests include in situ bioremediation processes, ground water/soil sampling and monitoring, multiphase fluid flow phenomenon, and risk/exposure assessment. |
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| Abstract: | The extent of natural attenuation is an important consideration in determining the most appropriate corrective action at sites where ground water quality has been impacted by releases of petroleum hydrocarbons or other chemicals. The objective of this study was to develop a practical approach that would evaluate natural attenuation based on easily obtained field data and field tested indicators of natural attenuation. The primary indicators that can he used to evaluate natural attenuation include plume characteristics and dissolved oxygen levels in ground water. Case studies of actual field sites show that plumes migrate more slowly than expected, reach a steady state, and decrease in extent and concentration when natural attenuation is occurring. Background dissolved oxygen levels greater than 1 to 2 mg/L and an inverse correlation between dissolved oxygen and contaminant levels have been identified through laboratory and field studies as key indicators of aerobic biodegradation. an important attenuation mechanism. Secondary indicators such as geochemical data, and more intensive methods such as contaminant mass balances, laboratory microcosm studies, and detailed ground water modeling can demonstrate natural attenuation as well. The recommended approach for evaluating natural attenuation is to design site assessment activities so that required data such as dissolved oxygen levels and historical plume flow path concentrations are obtained. With the necessary data, the primary indicators should be applied to evaluate natural attenuation. II the initial evaluation suggests that natural attenuation is a viable corrective action alternative, then a monitoring plan should be implemented to verify the extent of natural attenuation. |
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