Tracer dilution sampling technique to determine hydraulic conductivity of fractured rock

Groundwater in foothill and mountain watershed areas commonly occurs in fractured rock. The small well diameters and low apparent groundwater velocities in fractured rock require modification of normal techniques for the investigation of unconfined groundwater movement. The determination of hydraulic conductivity by the tracer dilution method normally employs injected radioisotope tracers. The dilution is determined by monitoring the isotope activity in the well with a scintillation probe. A modification of this method, using fluorescent dye tracers and physical sampling and analysis to determine dilution, has been applied in small wells with consistent results. Hydraulic conductivities of 0.02 to 0.5 ft/day have been determined in sixteen wells. Where comparison is possible, the values agree favorably with hydraulic conductivities determined by pumping tests. (Key words: Groundwater; permeability; tracers; wells)