Examining the Role of Topological Factors in Controlling the Hydraulic Conductivity of Granular Deposits Through the Analysis of Geophysical Well Logs: Results From the USGS Toxic Substances Hydrology Site

Long-term spatial and temporal monitoring of a treated-wastewater plume moving through a sand-and-gravel aquifer has been ongoing at the USGS Toxic Substances Hydrology research site on Cape Cod, Massachusetts, for three decades. The site offers access to numerous wells that penetrate the glacial outwash-plain deposits, and a variety of field experiments has been designed and implemented to investigate aquifer heterogeneity and transport processes. As part of this effort, geophysical logging programs have delineated the spatial variability of hydraulic conductivity, K, by means of a comprehensive series of flowmeter/pumping tests, and correlations among K and other log-derived properties such as porosity, electrical conductivity, and natural gamma activity have been examined statistically to extract information regarding the nature of fluid flow through these materials. Results revealed only a weak or inconclusive dependence of K on porosity from which was inferred that hydraulic conductivity may be influenced more by the topology, or geometric configuration, of the granular mixtures than by the volume of the fluid phase. To investigate the role of pore geometry further, a field experiment was recently conducted in which a sequence of electromagnetic induction logs was recorded in a well to monitor changes in the electrical properties of the surrounding sediments during injection of a saline tracer. The changes resulted from marked contrasts in the specific conductance between the ambient and injected fluids. Analytical mixing models, as well as empirical relations such as Archie's law, have been developed to characterize the electrical properties of saturated rocks and sediments. By applying these relations to the electrical conductivity data combined with the porosity log, topological measures of phase connectivity and surface area along grain boundaries can be derived from downhole measurements. Petrophysical parameters such as formation factor, F, and cementation factor, m, are computed and interpreted with respect to the K distribution. Results show that hydraulic conductivity is controlled by the degree of connectivity of the fluid phase and the grain-size distribution, both of which are established by the particular packing arrangement of the granular deposits. The porosity plays only a minor role in affecting the magnitude of K. These conclusions have spurred the design of future field experiments at this research site that will examine the role of interstitial fluid velocities and tortuosity on ground-water transport and dispersion.