A Field Evaluation of In‐Situ Biodegradation of Chlorinated Ethenes: Part I, Methodology and Field Site Characterization

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Abstract
Careful site characterization and implementation of quantitative monitoring methods are prerequisites for a convincing evaluation of enhanced biostimulation for aquifer restoration. This paper describes the characterization of a site at Moffett Naval Air Station, Mountain View, California, and the implementation of a data acquisition system suitable for real‐time monitoring of subsequent aquifer restoration experiments. A shallow, confined aquifer was chosen for the enhanced biodegradation demonstration, and was shown to have suitable hydraulic and geochemical characteristics. Injection and extraction wells were installed at a distance of 6 m, with intermediate monitoring wells at distances of 1, 2.2, and 4 meters from the injection well. Bromide tracer tests revealed travel times of S to 27 hours from the injection well to the various monitoring wells, and 20 to 42 hours from the injection well to the extraction well. Complete breakthrough of the tracer at the monitoring wells was facilitated by choosing a line of wells aligned with the regional flow, and selecting injection and extraction flow rates of approximately 1.5 and 10 liters/min. Transport studies were conducted with selected halogenated organic compounds. The retardation factors were found to range from approximately 2 to 12. The breakthrough responses for the more strongly sorbing compounds, e.g. TCE, exhibited pronounced tailing, such that a minimum period of several weeks was required to achieve complete saturation of the aquifer. The microcomputer‐driven sampling, analysis and data management system provided automated data acquisition at sample intervals of 40 minutes, with coefficients of variation smaller than 20%, and allowed for real‐time surveillance of the dynamic responses. Overall, the conditions were favorable for a quantitative evaluation of in‐situ aquifer restoration by enhanced biodegradation.