Detailed hydraulic assessment using a high-resolution piezocone

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Courtesy of US EPA - The Office of Solid Waste (OSW)

Department of Defense (DoD) ground water assessment and remediation projects require cost effective methods for determination of the direction and rate of ground water and contaminant flow. Monitoring wells have typically been used to estimate these parameters. However, detailed three-dimensional ground water and contaminant flow pathways cannot typically be delineated using monitoring well data. Understanding of three-dimensional flow pathways, gradients, and contaminant flux is essential to develop a remedial design, for risk determination, and to evaluate remediation effectiveness. DoD has hired contractors to install thousands of ground water monitoring wells for their site monitoring and remediation strategies. Since conventional wells are not adequate for determining ground water and contaminant flow pathways in three-dimensions, this can result in ineffective remediation, faulty monitoring strategies, poor model predictions and inaccurate risk assessments. Methods capable of providing the required level of resolution to evaluate site conditions in three-dimensions include non-conventional multilevel well installation networks, comprehensive soil sampling and laboratory analyses, or the use of tracer tests. These options can be cost-prohibitive, especially at sites where contamination may be aerially extensive or the site has complex hydrogeologic conditions. It is likely that decades and tens of billions of dollars will be required to cleanup DoD sites using standard hydrogeologic assessment methods.

This project employs the use of two innovative direct push sensor probes (the highresolution piezocone and GeoVIS) deployed with a standard cone penetrometer system for the purpose of determining direction and rate of ground water flow in three dimensions. The key to determining direction and rate of flow (or “seepage velocity”) is to understand the distribution of ground water head, gradient (i.e., change in head divided by the distance between measurement points along the direction of flow), soil porosity, and soil hydraulic conductivity. When coupled to the distribution of contaminant concentration, a contaminant flux estimate can be derived.

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