REPRESENTATIVE GROUND-WATER MONITORING IN FRACTURED POROUS SYSTEMS

Ground water flow and transport processes in fractured porous rocks pr esent special challenges for obtaining representative samples. Designi ng appropriate monitoring programs for these systems requires an asses sment of spatial and temporal variability (1) to define representative ness, and (2) to determine the most efficient and cost-effective metho ds 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 trans port processes. Ground water flow as indicated by helium tracers moves predominantly along strike, perpendicular to hydraulic gradients exce pt during storm events, demonstrating temporal variability in transpor t 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 sc reened wells and multilevel wells show that screened wells mask these vertical variations in ground water composition and miss important tra nsport pathways. Experimental results demonstrate the need for adequat e 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 s upport of remedial decision making.