52-63 SuGWMR03

Introduction Hydrogeologic investigations are often hampered by insufficient information about the site-specific hydrostratigraphic features that control ground water flow and solute transport. Field and modeling studies in a wide variety of geologic settings have shown the importance of a detailed description of aquifer heterogeneity for applications ranging from prediction of contaminant transport (Sudicky and Huyakorn 1991) to design of effective remediation schemes (National Research Council 1994; Hyndman et al. 2000) to assessment of stream-aquifer interactions (Butler et al. 2001). Although numerous studies have demonstrated the critical role played by fine-scale hydrostratigraphic features, information about such features is usually quite limited. In this paper, we examine an approach that has the potential to characterize hydrostratigraphic features at a level of detail that has rarely been possible in routine field investigations. Geologic logs and wellbore-geophysical methods are common sources of detailed information about site hydrostratigraphy. These approaches, however, are limited in their ability to resolve fine-scale features. Although the quality of geologic logs varies greatly with drilling technology and logging personnel, small-scale features are difficult to detect without the collection of continuous cores. High-resolution wellbore geophysical methods (e.g., microresistivity logging) have been developed for use in consolidated materials. Most logging tools for unconsolidated formations, however, have large averaging volumes and thus are of limited effectiveness for the detection of fine-scale features. In addition, data from high-resolution wellbore logging methods are often biased by irregular borehole diameter and drilling fluids. Moreover, cost considerations typically result in a well spacing that is inadequate for the detailed characterization of heterogeneous systems. Surface-based geophysical methods overcome the restrictions imposed by well spacing, but are rarely capable of high-resolution characterization of heterogeneous sequences in a nonresearch context. Cone penetrometer technology (CPT) provides high-resolution records of geotechnical properties of unconsolidated materials that can be related to sediment type through empirical relationships (Lunne et al. 1997). Although CPT equipment has been augmented in recent years with a variety of sensors for contaminant transport investigations (Lieberman 2000; Shinn 2000; Kram et al. 2001), operating costs hinder its widespread use. Thus, as Huggenberger and Aigner (1999), among others, have pointed out, many features of hydrostratigraphic relevance continue to remain unrecognized in the vast majority of contaminant-transport investigations. Abstract Fine-scale hydrostratigraphic features often play a critical role in controlling ground water flow and contaminant transport. Unfortunately, many conventional drillingand geophysics-based approaches are rarely capable of describing these features at the level of detail needed for contaminant predictions and remediation designs. Previous work has shown that direct-push electrical conductivity (EC) logging can provide information about site hydrostratigraphy at a scale of relevance for contaminant transport investigations in many unconsolidated settings. In this study, we evaluate the resolution and quality of that information at a well-studied research site that is underlain by highly stratified alluvial sediments. Geologic and hydrologic data, conventional geophysical logs, and particle-size analyses are used to demonstrate the capability of direct-push EC logging for the delineation of fine-scale hydrostratigraphic features in saturated unconsolidated formations. When variations in pore-fluid chemistry are small, the electrical conductivity of saturated media is primarily a function of clay content, and hydrostratigraphic features can be described at a level of detail (<2.5 cm in thickness) that has not previously been possible in the absence of continuous cores. Series of direct-push EC logs can be used to map the lateral continuity of layers with non-negligible clay content and to develop important new insights into flow and transport at a site. However, in sand and gravel intervals with negligible clay, EC logging provides little information about hydrostratigraphic features. As with all electrical logging methods, some site-specific information about the relative importance of fluid and sediment contributions to electrical conductivity is needed. Ongoing research is directed at developing direct-push methods that allow EC logging, water sampling, and hydraulic testing to be done concurrently.