A Borehole Methodology for Hydrogeochemical Investigations in Fractured Rock

Research into the development of borehole techniques for hydrogeochemical investigations in fractured rock has resulted in the construction of a geochemical probe controlled and operated with an umbilical cable system. This probe has been field tested to a depth of 600 m in a multilevel ported casing at the Chalk River Nuclear Laboratories (CRNL). The probe provides in situ measurements of pH, redox potential, dissolved oxygen, temperature, conductivity, and pressure while allowing groundwater sample collection both at surface and downhole. The multilevel ported casing preserves the subsurface hydrogeochemical zonation and facilitates sampling at various depths. In the CRNL borehole, dilute Na/HCO3 and Na/Cl waters ranging in age from 500 to 12,000 years have been observed in gneissic rock at depths of 85 to 350 m. However, accurate hydrogeochemical characterization of the low‐permeability metagabbro present from 400 to 600 m has been frustrated by contamination from casing installation water.