Analysis of subsurface flow and formation anisotropy in a fractured aquitard using transient water level data

A new methodology is proposed to estimate the actual direction of water flow in a fractured aquitard. According to the conceptual model, the flow direction in a fractured aquitard is dictated by the azimuth of the fractures, since almost no flow occurs within the rock matrix, which possesses very low hydraulic conductivity. Thus, while groundwater in reality continues to flow along the fractures, calculations based on water level data may incorrectly indicate changes in hydraulic gradients and azimuth of flow. The present methodology is based on simultaneous analysis of hydraulic gradients and flow direction, as calculated from equipotential surfaces. The approach was tested in a fractured chalky aquitard, and the calculated direction of principal fractures was found to be in good agreement with subsurface fracture data obtained from a tunnel excavated in the vicinity of the study area.