Keynote Address: Flow and Transport Properties of Fractured Bedrock Aquifers in the Vertical Direction

Assessing the ability of an unconfined bedrock aquifer to transmit infiltrating water and contaminants vertically downwards has long been a difficulty for practicing hydrogeologists. Typically, the conduits for flow are vertical fractures or joints that may intersect the ground surface or subcrop beneath a thin veneer of glacial material or soil. Although this setting is widely recognized to pose a risk to the quality of water in the aquifer, we actually know very little about the fluid and transport processes across the bedrock surface.In this talk, the results of studies conducted recently at two field sites, one set in sedimentary rock and the other in a gneissic terrain, will be presented. The studies include the development of pumping and pulse interference methods for estimation of vertical K, the completion of tracer experiments from a surficial source to depth in a pumped well for the estimation of transport porosity, and the interpretation of unique infiltration experiments conducted adjacent to a small outcrop across the interface between the soil and bedrock. The results point out the significant importance of vertical fracture geometry, spacing and aperture in determining bulk vertical hydraulic conductivity and vertical transport porosity, and the difficulty we have in estimating these properties. The role of the materials covering the bedrock is also significant and even a thin veneer of soil can substantially impede rapid fluid and contaminant migration.