Tracer migration experiments in unsaturated fractured tuff

Abstract

Field tracer experiments were conducted at an underground tunnel—the Exploratory Studies Facility (ESF)—at Yucca Mountain, Nevada, to investigate the localized preferential flow and transport resulting from multiple episodic liquid-release tests in unsaturated fractured tuff. Liquid was released into an isolated test interval (0.30 m long) within a borehole drilled along ESF. Out of 60 liquid-release tests, 36 contained tracers (mostly food and fluorescent dyes) to help us to elucidate the flow paths in unsaturated tuff. After the tracer-migration test, an array of 12 boreholes was drilled into the test area to collect rock samples. These samples were then analyzed to delineate the extent of tracer migration. Complementary laboratory column transport studies using crushed rock showed that the dye tracers were—at most—very weakly sorbed to the tuff matrix. We found that the tracer-migration test, with a limited release volume of 1.5 L, resulted in a localized distribution of tracers, most likely confined to a 1.0 × 1.6-m area directly below the test interval (over a vertical experimental length of about 0.65 m). This conclusion was corroborated by detecting dyes that had been introduced during previous liquid-release seepage tests either before or after niche excavation; the spatial distribution of all dyes was near their individual release intervals. Limited lateral spreading of the dyes occurred, even when several non-dye-containing seepage tests were intermittently conducted following dye release. This work also demonstrates the utility of applying multiple tracers (including food and fluorescent dyes) to help delineate transport characteristics in unsaturated fractured rock.