GROUNDWATER FLOW AND GROUNDWATER-STREAM INTERACTION IN FRACTURED AND DIPPING SEDIMENTARY ROCKS: INSIGHTS FROM NUMERICAL MODELS

Groundwater flow in unconfined aquifers is influenced by topography, but in fractured and dipping sedimentary rocks, it is also influenced by structure. Field evidence indicates that groundwater is older on the down-dip side of a stream (asymmetry) and that dip-aligned streams receive more baseflow than strike-aligned streams (anisotropy). We present detailed numerical models to evaluate the effects of various factors that influence groundwater flow pathways. The models simulate a small watershed drained by headwater streams and underlain by dipping strata. Groundwater flow can be characterized by three components: down the hydraulic gradient, down-dip, and along-strike. The degree of anisotropy and asymmetry depends on several factors: bedding anisotropy, efficiency of the weathered horizon, fracture depth, and bedding dip angle. Whereas anisotropy increases linearly with dip angle, asymmetry is greatest at a threshold angle. This threshold angle is related to the mean groundwater flow direction in an equivalent homogeneous and isotropic system.