SHALLOW SUBSURFACE FRACTURE NETWORKS AND THEIR IMPLICATION ON SUBSURFACE FLOW PATHWAYS

The nature of fluid movement within the shallow subsurface is critical to the transmission of fluids and contaminates from the surface to the ground water. The Dounreay Nuclear Power Establishment is being decommissioned following a long and complex history of nuclear research and experimental power generation. As part of the decommissioning process the site must demonstrate an understanding of the subsurface and the processes within the subsurface.

This work presents a high resolution 3D geological incorporating bedrock and superficial deposits. Based on downhole geophysical data and surface outcrop, the model includes a discrete fracture network (DFN). The DFN was up–scaled and hydraulic conductivity estimated by the ‘cubic law’.

Three principal fracture sets were identified from DFN modelling. The prominent fracture set is orientated approximately parallel to the bedding. The second and third sets are at a high angle to the bedding and trend NW-SE and NE-SW. These fracture sets represent a systematic pair of approximately orthogonal fractures which is relatively consistent across the site and the region.

The primary flow within the bedrock geology is constrained by the fracture networks with the un‑fractured bedrock being relatively impermeable. From the up-scaled DFN it can be shown that the bedding­–parallel fractures are more hydraulically conductive than the high–angle fractures and approximately four orders of magnitude higher than the un-fractured bedrock.

The bedding parallel fractures intensity decreases exponentially with depth, approaching equilibrium at approximately 60 metres below ground level. This highly fractured zone will substantially increase flow within the shallow subsurface and restrict the contribution of surface water from the site entering the groundwater system. The majority of flow will enter the shallow subsurface and be rapidly moved via the bedding–parallel fractures towards the coast with minor flow to the deeper groundwater system through the high–angle fractures.

The integration of bedrock, superficial and discrete fracture network modelling is essential for the understanding of the shallow subsurface and the impact that the transition from the superficial deposits through a fractured weathered zone to the bedrock may have on the transport of contaminants.