Tracer Penetration into Welded Tuff Matrix from Flowing Fractures

Abstract

Field and laboratory tracer experiments were conducted to investigate the extent of tracer imbibition and penetration into unsaturated, fractured rock matrix at Yucca Mountain, Nevada. Field experiments were carried out in the Exploratory Studies Facility (ESF), an underground tunnel at Yucca Mountain. Water containing dye was released into horizontal boreholes drilled into the wall of the ESF main drift. The region was then mined to observe the flow pathways and to collect dye-stained rock samples for subsequent laboratory quantification. Dye concentration profiles in the rock, measured using a newly developed sampling technique, showed that liquid flowing through the fractures penetrated into the matrix to a depth of several millimeters. Laboratory studies of tracer penetration into the rock matrix were conducted using tracer-free rock samples, collected from the same hydrogeologic unit and machined into cylindrical cores. Tracer-imbibition tests were performed on cores at two different initial water saturations with both sorbing (dyes) and nonsorbing tracers. The travel distance for sorbing dyes was a few millimeters after ∼16 to 20 h, similar to the extent measured in samples from the field test. The nonsorbing bromide front coincided with the wetting front in the rock core at the initial water saturation of 12%, and the imbibition depth agreed very well with the prediction, using independently measured properties. At the high initial water saturation of 76%, the bromide front lagged significantly behind the wetting front. Sorption coefficients for the dyes in the partially saturated core samples were calculated using two independent approaches, based on tracer travel-distance and mass-distribution calculations, and were found to yield comparable results. Utilization of nonsorbing tracers with different molecular sizes helped to identify the effects of pore-size restriction on tracer transport during imbibition. The results from this work have a direct application to radionuclide transport at Yucca Mountain, and the methods presented are broadly applicable to the investigation of water and solute transport in unsaturated rock.