Cone Penetrometer Testing And Discrete-Depth Groundwater Sampling Techniques: A Cost-Effective Method Of Site Characterization In A Multiple-Aquifer Setting

Cone penetrometer testing (CPT), combined with discrete-depth groundwater sampling methods, can<br>reduce significantly the time and expense required to characterize large sites that have multiple aquifers,<br>especially compared to the more common practice of installing monitoring wells. Results from the<br>screening site characterization then can be used to design and install a cost-effective monitoring well<br>network. At a site in northern California, it was necessary to characterize the stratigraphy and the<br>distribution of volatile organic compounds (VOCs) to a depth of 80 feet within a %-mile-by-%-mile<br>residential and commercial area in a complex alluvial fan setting. To expedite characterization, a tiveweek<br>field screening program was implemented that consisted of a shallow groundwater survey, CPT<br>soundings and pore-pressure measurements, and discrete-depth groundwater sampling. Based on<br>continuous lithologic information provided by the CPT soundings, four predominantly coarse-grained,<br>water-yielding stratigraphic packages were identified. Individual coarse-grained units within each<br>package are discontinuous, as they coalesce and pinch out in longitudinal and transverse directions.<br>Seventy-nine discrete-depth groundwater samples were collected using either shallow groundwater<br>survey techniques, the BAT Enviroprobe, or the QED HydroPunch I, depending on subsurface<br>conditions.<br>Using results from these efforts, a 20-well monitoring network was designed and installed to monitor<br>critical points within each stratigraphic package. Good correlation was found for hydraulic head and<br>chemical results between discrete-depth screening data and monitoring well data. Understanding the<br>vertical VOC distribution and concentrations produced substantial time and cost savings by minimizing<br>the number of permanent monitoring wells and reducing the number of costly conductor casings that had<br>to be installed. Additionally, significant long-term cost savings will result from reduced sampling costs,<br>because fewer wells comprise the monitoring network. We estimate these savings to be 50 percent for site<br>characterization costs, 65 percent for site characterization time, and 60 percent for long-term monitoring<br>costs.