SEEKING DOMINANT FLOW PATHWAYS IN FRACTURED CARBONATE ROCK USING TRACER EXPERIMENTS CONDUCTED WITH IN-SITU DETECTION

Groundwater flow in carbonate aquifers is typically governed by horizontal fractures formed from bedding planes which are weakly interconnected by vertical fractures. In flat lying sedimentary basins, flow tends to be horizontal, and the relative role played by individual bedding plane features in the bulk transport of mass, is uncertain. Some recent large-scale studies conducted using detailed measurements of hydraulic properties and time-series measurement of hydraulic head have suggested that dominant fracture features may strongly influence the transport pathways in these interconnected fracture networks. To explore this, we have conducted a tracer experiment in a Silurian-aged dolomite using in-situ detection of tracer arrival. The experiment was conducted over the entire thickness (10 m) of one lithological unit utilizing a radial divergent flow field. The unit has 3-4 horizontal bedding plane fractures of 0.5 to 1.2 mm aperture that were identified in the source and observation wells using hydraulic testing methods. An in-situ fluorometer was used to detect the transport of the fluorescent tracer through the discrete features. The transport distances ranged from approximately 25 m to more than 125 m. The results show that dilution and mixing effects in the monitoring wells significantly impacted the interpretation of the results, although these effects were mitigated slightly with distance. Using the timing and location of first arrivals in each of the wells, a conceptual model was developed and several numerical simulations conducted to explore the dominant pathways. The simulations were conducted using HydroGeoSphere, a discrete fracture model which accounts for advection-dispersion in each fracture and diffusion into the adjacent matrix. The analysis shows that although dominant pathways appear to develop, the ability to simulate these is confounded by uncertainties in the geological features between observation wells, and by the dominating effect of matrix diffusion (or possibly tortuosity in the hydraulic gradients).