: Experiments were performed to determine the extent of penetration of drill water into Grouse Canyon densely welded t ff during use of normal drilling practices. Core samples were examin d from a borehole cored in a rib of the Rock Mechanics drift in G Tunnel at the Nevada Test Site, Nye County, Nevada. Methylene blu dye was added to the drill water to act as a tracer which stained the rock blue on contact. We found the rock stained blue only in a thin layer about 0.5 mm thick at the surface of the core. However we were concerned about the uniformity of penetration depth observed in the core and this prompted a simple experiment to test the ability of methylene blue to penetrate the matrix of densely welded tuff. We found that in the imbibition process, the dye and water separated such that the water penetrated the matrix to a much greater depth. This result meant that any interpretation of drill water imbibition in borehole core based on this dye as a tracer is unreliable.More important, however, is the conclusion that the presence of methylene blue dye on the rock indicates the presence of tracer water flow, but the absence of the dye does not rule out the presence of water flow.