TRACER TECHNIQUES TO DETERMINE EFFECTIVE FIELD SITE SCALE PARAMETERS FOR REMEDIATION DESIGN
|G. Barth1, T.H. Illangasekare, M. Hill, and H. Rajaram, Department of Civil and Environmental Engineering, University of Colorado at Boulder, Boulder, CO 80309; United States Geological Survey, Denver Federal Center, P.O. Box 25046, Mail Stop 413, Lakewood, CO 80225||
Non-aqueous phase liquids (NAPLs) entrapped in the soil have the potential to act as long-term sources of ground water contamination. Efficient remediation design requires determination of the distribution and degree of NAPL saturation as well as the effective field site scale parameters.
Contaminant entrapment is primarily controlled by the interfaces of heterogeneous formations. Laboratory experiments have demonstrated that random heterogeneous formations result in a random distribution of NAPL.
Typical characterization methods such as soil coring cannot determine the distribution of entrapment and produce parameter estimates which are at a scale different than that required for simulations. This study investigates the feasibility of using a conservative tracer to determine the parameters of the saturation distribution. It is our hypothesis that a tracer breakthrough curve (BTC) contains information on the random distribution of entrapment and the effective field site scale parameters.
To test the hypothesis a series of three tracer experiments was undertaken. Experiments one and two provided insight for the design and implementation of the third experiment. The first experiment consisted of a qualitative evaluation using a complex random distribution of materials in a large two dimensional tank. For the second experiment a simpler, heterogeneous packing was used under more stringently controlled and monitored conditions.
The third experiment combined the complex, randomly generated packing of the first with the precisely controlled and monitored conditions of the second experiment. Tracer tests were performed at a variety of plume scales, sampled at two different scales, and repeated over a range of NAPL entrapment from "clean" to highly contaminated conditions. By thoroughly characterizing the system, the sensitivity of the tracer BTC with regard to a variety of issues could be evaluated.
Foremost was the issue of sensitivity to the amount of entrapped NAPL. The effect of sampling scale and plume scale with respect to the correlation scale of the heterogeneous system was also investigated. Finally, the impact of the temporal and spatial dependence of dispersivity on the quantification of entrapped NAPL was studied. These relations will dictate the implementation of field site scale evaluations, serving as guide-lines for protocols governing the injection, sampling, and duration of tracer tests used at an actual spill site.
Key words: tracers, effective parameters, site characterization, NAPL entrapment.
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