A Method to Evaluate the Vertical Distribution of VOCs in Ground Water in a Single Borehole |
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| Authors: | Fredric Hoffman Michael D Dresen |
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| Institution: | Fredric Hoffman is a hydrogeologist with the University of California at Lawrence Livermore National Laboratory (L-528, P. O. Box 808, Livermore, CA 94551) where he is currently the division leader of the Environmental Restoration Division in the Environmental Protection Department. He earned an A.B. degree in geology from Dartmouth College in 1966 and an M.S. degree in geology from the University of Nevada at Reno in 1969, and is a registered geologist in California. Prior to joining LLNL in 1984, he was a hydrogeologist with the Environmental Protection Agency in San Francisco.;Michael Dresen is a registered geologist and certified engineering geologist in California, and a senior project hydrogeologist with Weiss Associates (5500 Shellmound St., Emeryville, CA 94608). He received his B.S. in geology and M.S. in environmental geology from California State University, Hayward, in 1975 and 1979. Since 1982, he has conducted and managed soil and ground water contamination investigations at sites in the San Francisco Bay Area. Prior to 1982, he worked for three years as a consultant in engineering/environmental geology and was a geology instructor from 1975 to 1987. |
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| Abstract: | Volatile organic compounds (VOCs) are present in multiple water-bearing zones beneath and downgradient of Lawrence Livermore National Laboratory. This area is composed of interfingering unconsolidated alluvial sediments with hydraulic conductivities ranging over four orders of magnitude. The more permeable sediments exhibit moderate hydraulic interconnection horizontally and less interconnection vertically, and appear to consist largely of interconnected stream channel deposits. To optimize selection of monitoring well screened intervals in this complex environment, a technique that enables collection of saturated formation samples from each water-bearing zone without contamination from other VOC-containing zones was developed, tested, and implemented. The technique utilizes a wireline punch-coring system that allows the drill bit to be replaced with a core barrel without removing the drill rod from the borehole. To help ensure that a sample from one water-bearing zone is not contaminated by VOCs from another zone, the drilling fluid is replaced with new fluid before each sampling run. Overnight chemical analysis by gas chromatography enables field personnel to know the vertical distribution of VOCs as drilling proceeds. Since its first use in 1985, the technique has successfully characterized the presence or absence of VOCs in ground water in 123 of 140 wells, many with concentrations in ground water in the low parts-per-billion range. Our sampling technique is a cost-effective and rapid method of evaluating the vertical distribution of VOCs in ground water in a complex hydrogeologic environment. |
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