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

Abstract

SUMMARY The structure and hydrogeological properties of subsurface faults in the Triassic Sherwood Sandstone Group and Ordovician Borrowdale Volcanic Group at Sellafield in west Cumbria were investigated in order to develop conceptual and numerical models for input to groundwater flow modelling at the possible site of an underground radioactive waste repository. Eighteen borehole intersections of faults were studied using core, borehole imagery and hydraulic test data (single and multi-well testing). The results were integrated with data and interpretations from seismic surveys and field studies in order to prepare conceptualizations of fault architecture and hydrogeology, suitable for taking forward into numerical modelling. Hydraulic test data indicate that fault zones in the Sherwood Sandstone Group are slightly more permeable than the host rock, and geological observations suggest that flow may be focused in the fault damage zones rather than in the faultrocks, which tend to act as flow baffles. Where present in sufficient numbers, granulation seams appear to be flow inhibitors. In the volcanic rock, hydraulic test data indicate that fault zones have little permeability contrast with the host rock. However, geological observations suggest that minor reactivation may have locally disrupted the baffling effect of faultrocks, thereby creating ‘leak points’. Overall, the study suggests that ground-water flow in faults at Sellafield is highly heterogeneous at the borehole scale, but at larger scales the hydraulic behaviour of faults is more homogeneous.