[en] The effects of invasion of bentonite-base drilling fluid on hydraulic characterization of low permeability basalt horizons were examined in a designed field evaluation at the Hanford Site, in south-central Washington State. The test formation selected for the field evaluation was a 24.7-m section of basalt flow interior that had been previously core-drilled utilizing only fresh water. The test design of the field evaluation included: an initial phase (Phase I) of hydrologic testing followed by the injection of bentonite-base drilling fluid into the test section, removal of the drilling fluid from the borehole, and a final phase (Phase II) of hydrologic testing. Hydrologic tests conducted during each phase included over-pressure pulse tests (pressurized slug tests) and multistep, constant-head injection test. Results of the field evaluation indicate that no discernible impact on hydraulic property estimates or test response could be attributed to drilling fluid invasion. The best estimate of equivalent hydraulic conductivity for Phase I and II testing was 1.4 x 10^-11 and 1.5 x 10^-11 m/s, respectively