NUMERICAL-ANALYSIS OF SOLUTE MIGRATION THROUGH FRACTURED CLAYEY DEPOSITS INTO UNDERLYING AQUIFERS

This study examines the degree to which vertically fractured clayey aquitards protect an underlying aquifer from near-surface sources of contamination. Evidence from field studies indicates that many clayey aquitards previously assumed to be unfractured at depth have vertical fractures that actively transmit groundwater. Calculated groundwater velocities within these fractures can exceed several meters per day for apertures in the range of those reported to occur in clays. If this is the case, significant quantities of dissolved contaminants originating in near-surface zones may move rapidly downward through the fracture network into underlying aquifers. A two-dimensional Laplace transform Galerkin finite element model (Sudicky and McLaren, this issue) was used to assess the importance of idealized planar vertical fractures on flow and solute transport through a 15-m-thick aquitard and subsequent plume evolution within an underlying aquifer in which groundwater flows horizontally. Sensitivity analyses indicate that solute transport through the aquitard is strongly affected by downward flow in widely spaced deep fractures with apertures as small as 10-mu-m and that a plume of large lateral extent and significant concentration forms within the underlying aquifer in a few tens of years for apertures of the order of 20-mu-m or much sooner if the apertures are 50-mu-m. This finding suggests that small hydraulically active fractures that are very difficult to detect or identify within an otherwise protective clayey barrier may cause rapid and large-scale contamination of groundwater beneath the barrier.