Spatial and temporal instabilities in water flow through variably saturated fractured basalt on a one-meter field scale

Traditional approaches for predicting water flow and migration of contaminants in fractured rock vadose zones are often based on volume-averaged behavior of fluids in homogeneous porous media. However, this approach may overestimate the travel time through the vadose zone to underlying aquifers. This paper presents empirical data for nonideal infiltration and discusses factors and processes of three-dimensional spatial and temporal water-flow dynamics in variably saturated fractured basalt at the one-meter scale. Three small-scale ponded infiltration tests were conducted in fractured basalt during the summer of 1998 at the Hell’s Half Acre field site in Idaho. A small reservoir (40 x 80 cm) was constructed on the surface exposure of a fracture at an overhanging basalt ledge. The tests were monitored with the purpose of determining factors that might generate nonideal flow behavior. Our major findings include: (1) infiltration and outflow were highly variable and apparently unrelated to the head in the reservoir; (2) the volume-averaged infiltration rate can be used to calculate an approximate range of the hydraulic conductivity, using Darcy’s law for variably saturated flow; (3) measurements using tensiometers do not provide adequate three-dimensional resolution of the fracture and matrix capillary pressure for determining the exchange of water between the fracture and the basalt matrix; (4) the complex interactions of multiple, inherently nonlinear processes of water flow affected by gravity and capillary forces in both the fracture and matrix, as well as the fracture-matrix interaction and the effect of entrapped air, create temporal and spatial instabilities; and (5) under certain conditions, water dripping from an open fracture can exhibit deterministic chaotic behavior.