Tracer Penetration into Welded Tuff Matrix from Flowing Fractures

based on volume-averaging concepts, liquid migration along subvertical fractures typically is predicted to be Field and laboratory tracer experiments were conducted to investisubject to strong matrix imbibition into the partially satugate the extent of tracer imbibition and penetration into unsaturated, fractured rock matrix at Yucca Mountain, Nevada. Field experiments rated rock (e.g., Wang and Narasimhan, 1993). Mounting were carried out in the Exploratory Studies Facility (ESF), an underevidence, however, points to the occurrence of fast and ground tunnel at Yucca Mountain. Water containing dye was released preferential flow, which suggests limited interaction of into horizontal boreholes drilled into the wall of the ESF main drift. fractures with the surrounding matrix (Nativ et al., 1995; The region was then mined to observe the flow pathways and to Glass et al., 1995; Yang et al., 1995; Tokunaga and Wan, collect dye-stained rock samples for subsequent laboratory quantifica1997; Hu et al., 2001). tion. Dye concentration profiles in the rock, measured using a newly To explain the fast and preferential flow in unsatudeveloped sampling technique, showed that liquid flowing through the rated fractured rock, investigators have proposed modfractures penetrated into the matrix to a depth of several millimeters. els with different fracture–matrix interactions. Liu et al. Laboratory studies of tracer penetration into the rock matrix were (1998) presented an “active fracture model” in which conducted using tracer-free rock samples, collected from the same hydrogeologic unit and machined into cylindrical cores. Tracer-imbibionly a portion of the connected fractures are active in tion tests were performed on cores at two different initial water saturaconducting fluid in a steady-state flow field. Pruess tions with both sorbing (dyes) and nonsorbing tracers. The travel (1999) proposed a mechanistic model that allowed water distance for sorbing dyes was a few millimeters after 16 to 20 h, to flow relatively freely in networks of interconnected similar to the extent measured in samples from the field test. The fractures despite strong suction from an unsaturated nonsorbing bromide front coincided with the wetting front in the rock rock matrix; this model conceptualizes fracture flow as core at the initial water saturation of 12%, and the imbibition depth a spatially localized (within a fracture network) and agreed very well with the prediction, using independently measured transient (intermittent) process. However, Pruess noted properties. At the high initial water saturation of 76%, the bromide that direct field observations remain incomplete. Field front lagged significantly behind the wetting front. Sorption coeffistudies are required to directly estimate fracture–matrix cients for the dyes in the partially saturated core samples were calculated using two independent approaches, based on tracer travel-disinteraction to calibrate or verify these conceptual models. tance and mass-distribution calculations, and were found to yield While fast and localized fracture flow has recently comparable results. Utilization of nonsorbing tracers with different received broad acceptance, the importance of matrix molecular sizes helped to identify the effects of pore-size restriction imbibition and matrix flow in retarding contaminant on tracer transport during imbibition. The results from this work have migration should not be overlooked. The objective of a direct application to radionuclide transport at Yucca Mountain, and this study was to investigate, through a combination of the methods presented are broadly applicable to the investigation of field and laboratory experiments, the spatial extent of water and solute transport in unsaturated rock. matrix penetration by nonsorbing and sorbing tracers into fractured, unsaturated tuff. The purpose of the field tests was to observe the flow pathways; the purpose of I low-permeability unsaturated fractured rock, the laboratory tests was to more completely quantify water flows predominantly through the interconand understand tracer penetration into the tuff. nected fracture network, with some water imbibing into the neighboring matrix rock. Imbibition (from the capilTRACER STUDIES IN FIELD FRACTURES lary pressure gradient) and diffusion (from the concenField tests were conducted in two specially mined openings, tration gradient) drive mass transfer of solutes between called niches, located in the ESF. The niches are approxithe fractures and rock matrix. Imbibition and diffusion mately 4 m wide, 3 m high, and 9 m deep within the Topopah into the rock matrix can move contaminants away from Spring welded (TSw) volcanic tuff, the potential repository the flowing fracture, thereby contributing to their retargeologic unit. The intrinsic permeability for the rock matrix dation. Water flow in rock fractures and imbibition into of the TSw is on the order of 10 18 m (Flint, 1998). Figure 1 the surrounding matrix in the unsaturated zone are key shows the location of seven horizontal boreholes installed at processes in evaluating the potential underground highNiche 3650 (at the construction station 3650 m from the ESF level radioactive waste repository at Yucca Mountain, North Portal) before the niche was excavated. The boreholes Nevada. (0.076 m in diameter and 10 m in length) were dry coredrilled. Three upper boreholes (UL, UM, and UR) were about A variety of conceptual models have been developed 0.65 m above the top of the unmined niche. The remaining to describe flow and transport behavior in unsaturated boreholes were drilled within the limits of the niche and were fractured rock. In conventional continuum approaches intentionally mined out when the niche was constructed. The configuration was similar for Niche 4788. Q. Hu, T.J. Kneafsey, R.C. Trautz, and J.S.Y. Wang, 1 Cyclotron Road, Liquid-release tests were conducted prior to the excavation MS 90-1116, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. Received 15 Nov. 2001. *CorrespondAbbreviations: ESF, Exploratory Studies Facility; HPCD, hydroxying author ([email protected]). propyl-cyclodextrin; PFBA, pentafluorobenzoic acid; TSw, Topopah Spring welded [tuff]. Published in Vadose Zone Journal 1:102–112 (2002).