Fluid flow in faults: a study of fault hydrogeology in Triassic sandstone and Ordovician volcaniclastic rocks at Sellafield, north-west England

The structure and hydrogeological properties of subsurface faults in the Tr iassic Sherwood Sandstone Group and Ordovician Borrowdale Volcanic Group at Sellafield in west Cumbria were investigated in order to develop conceptua l and numerical models for input to groundwater flow modelling at the possi ble site of an underground radioactive waste repository. Eighteen borehole intersections of faults were studied using core, borehole imagery and hydra ulic test data (single and multi-well testing). The results were integrated with data and interpretations from seismic surveys and field studies in or der to prepare conceptualizations of fault architecture and hydrogeology, s uitable for taking forward into numerical modelling. Hydraulic test data in dicate that fault zones in the Sherwood Sandstone Group are slightly more p ermeable than the host rock, and geological observations suggest that flow may be focused in the fault damage zones rather than in the faultrocks, whi ch tend to act as how baffles. Where present in sufficient numbers, granula tion seams appear to be flow inhibitors. In the volcanic rock, hydraulic te st data indicate that fault zones have little permeability contrast with th e host rock. However, geological observations suggest that minor reactivati on may have locally disrupted the baffling effect of faultrocks, thereby cr eating 'leak points'. Overall, the study suggests that groundwater flow in faults at Sellafield is highly heterogeneous at the borehole scale, but at larger scales the hydraulic behaviour of faults is more homogeneous.