A Method to Evaluate the Vertical Distribution of VOCs in Ground Water in a Single Borehole

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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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