A FIELD ASSESSMENT OF HYDRAULIC TOMOGRAPHY: A NEW APPROACH FOR CHARACTERIZING SPATIAL VARIATIONS IN HYDRAULIC CONDUCTIVITY

Obtaining information about spatial variations in hydraulic conductivity (K) on a scale of relevance for transport investigations has proven to be a considerable challenge. Conventional pumping tests yield an average hydraulic conductivity for the aquifer, but provide little information about the spatial variability of K. Slug tests and related single-well methods can identify zones of differing K but provide little information about the continuity of those zones. Theoretical investigations have demonstrated that hydraulic tomography has the potential to provide detailed information about the characteristics and continuity of such features. In order to assess the practical feasibility of the approach, a series of field experiments was conducted at the Geohydrologic Experimental and Monitoring Site (GEMS) located in the Kansas River floodplain. Previous work at the site, which has involved conventional well tests, direct-push electrical conductivity logging, and direct-push hydraulic profiling, has indicated the existence of several lenses of low K material amongst the generally high conductivity materials of the alluvial aquifer. A major goal of the tomography experiments was to assess the continuity of those lenses. The experiments consisted of a series of short (15 minute) pumping tests performed in a tomographic fashion. Each of the tests involved pumping from a two-foot interval isolated with straddle packers. The pumped interval was moved between tests so that the entire thickness of the aquifer was covered. Recently developed extruded PVC multilevel sampling wells were used in conjunction with small diameter (1 cm) pressure transducers to measure drawdown. Integrated pressure sensor/data logger units, which do not require cables to the surface, were used to monitor the impact of a nearby pumping well and to record drawdown above, below, and within the isolated pumping interval. Drawdown data were analyzed using a new steady-shape-based theory developed for this application. Results of the tomography experiments will be assessed through a comparison with the K distribution obtained from a detailed set of direct-push hydraulic profiles obtained at nearby locations.