New understanding of deep unsaturated zone controls on recharge in the Chalk: a case study near Patcham, SE England

This paper describes data obtained from tensiometers installed through 60 m of the unsaturated zone of a site situated on the Chalk aquifer of southern England. The data are interpreted together with consideration of previous work on unsaturated zone processes. The data show that, at the North Heath Barn site near Patcham (West Sussex, UK), matric potentials at many depths remain high throughout the year, and therefore the potential for fracture flow to occur remains high throughout the year. The high matric potentials support earlier closed-circuit TV surveys that indicated that wet borehole walls occurred throughout the year at some horizons. Rapid bypass flow (taken to mean significant fracture flow that is sufficient to cause a water level rise) occurs relatively frequently through winter, and there is a characteristic sequence of events leading to a water level rise. Although, during the summer, effective precipitation ceased (i.e. precipitation was exceeded by evapotranspiration) at the surface, beyond the root zone, downward drainage of water through the unsaturated zone occurred throughout the year, with recharge occurring at the water table. The recession of water levels in the spring reflected the fact that recharge at the water table was less than discharge through saturated zone flow, rather than a cessation in drainage altogether. This is the first time continuing drainage through the deep unsaturated zone has been directly observed, although it has been inferred for many years.