RESULTS OF RECENT TRACER EXPERIMENTS CONDUCTED AT INCREASING SCALE IN A FRACTURED DOLOMITIC AQUIFER

At most contaminated sites in fractured rock, the off-site migration of aqueous-phase contaminants is governed by the distribution of hydraulic head and the connection of discrete fracture pathways. In these settings it is usually possible to characterize the distribution of transmissivity and hydraulic head at individual well locations. In some cases, local fracture interconnections can be explored via inter-well testing. These data, however, provide little information on the potential transport pathways at increasing scale, as the nature of the fracture interconnections remains uncertain. To explore the potential for dominant pathways and develop methods to help us interpret transport at both the discrete fracture and bulk rock scale, a series of tracer experiments have been conducted in a flat-lying dolomite of Silurian age. The aquifer has undergone extensive hydraulic and geochemical characterization and several forced-gradient tracer experiments have been previously conducted in individual fractures. The present series of experiments was conducted over the entire thickness (10 m) of one lithological unit utilizing a novel injection process which provides a radial source 10-60 m in diameter. The arrival of a fluorescent tracer was detected at a series of open monitoring wells located in the down gradient direction. An in-situ fluorometer was used to detect the transport through discrete fractures, and the analysis of bulk samples was conducted using a fluorometer on surface. The transport distances ranged from approximately 15 m to more than 100 m. The results show that an increasingly fewer number of fractures contributed to the transport pathway over increasing distance. Dilution and mixing effects in the monitoring wells significantly impacted the interpretation of the results, although these effects were mitigated slightly with distance. The results of the experiments were also simulated using a discrete fracture numerical model (FRAC3DVS) to explore the nature of the fracture interconnections that gave rise to the observed distribution of transport pathways.