Liquid phase structure within an unsaturated fracture network beneath a surface infiltration event: Field experiment

We conducted a simple field experiment to elucidate structure (i.e., geometry) of the liquid phase (water) resulting from ponded infiltration into a pervasive fracture network that dissected a nearly impermeable rock matrix. Over a 46 min period, dyed water was infiltrated from a surface pond while electrical resistance tomography (ERT) was employed to monitor the rapid invasion of the initially dry fracture network and subsequent drainage. We then excavated the rock mass to a depth of ∼5 m, mapping the fracture network and extent of dye staining over a series of horizontal pavements located directly beneath the pond. Near the infiltration surface, flow was dominated by viscous forces, and the fracture network was fully stained. With increasing depth, flow transitioned to unsaturated conditions, and the phase structure became complicated, exhibiting evidence of fragmentation, preferential flow, fingers, irregular wetting patterns, and varied behavior at fracture intersections. ERT images demonstrate that water spanned the instrumented network rapidly on ponding and also rapidly drained after ponding was terminated. Estimates suggest that our excavation captured from ∼15 to 1% or less of the rock volume interrogated by our infiltration slug, and thus the penetration depth from our short ponding event could have been quite large.