CHALK FRACTURE SYSTEM CHARACTERISTICS - IMPLICATIONS FOR FLOW AND SOLUTE TRANSPORT

Bulk groundwater flow and solute advection occur in the fractures of the UK Chalk, and a knowledge of the frequency and aperture of these fractures is crucial to understanding these two key processes in the Chalk. Fracture frequencies in the Chalk of southeast England have been assessed regionally by means of outcrop scanline measurements. Measurements of radon dissolved in Chalk groundwaters allow the estimation of fracture apertures in the saturated zone, These data provide valuable controls on the conceptualization and estimation of hydraulic properties for the Chalk aquifer. In particular, standard hydraulic models of fractured rock permeability applied on the basis of these data are shown to be insufficient to explain the higher permeabilities measured in the unconfined Chalk beneath valley axes. In these settings, conduit flow mechanisms must be invoked; this in turn implies that modifications to standard matrix diffusion models are required to describe solute transport accurately.