Representative Ground Water Monitoring in Fractured Porous Systems

Abstract

Ground water flow and transport processes in fractured porous rocks present special challenges for obtaining representative samples. Designing appropriate monitoring programs for these systems requires an assessment of spatial and temporal variability (1) to define representativeness, and (2) to determine the most efficient and cost-effective methods for obtaining representative samples. Gas tracer studies, borehole tests, and geochemical sampling conducted at a fractured rock site in east Tennessee indicate large spatial and temporal variations in transport processes. Ground water flow as indicated by helium tracers moves predominantly along strike, perpendicular to hydraulic gradients except during storm events, demonstrating temporal variability in transport directions. Distinct vertical changes in water chemistry encountered in multilevel wells are indicative of discrete transport pathways in a poorly mixed system. Comparison of samples obtained from standard screened wells and multilevel wells show that screened wells mask these vertical variations in ground water composition and miss important transport pathways. Experimental results demonstrate the need for adequate characterization of spatial and temporal variations for appropriate placement and construction of monitoring wells, timing of ground water monitoring, and evaluation of exposure risk and contaminant flux in support of remedial decision making.