Sediment filled fractures in the Permo-Triassic sandstones of the CheshireBasin: observations and implications for pollutant transport

Fracture mapping in a tunnel system and at nearby outcrop on the Runcorn Pe ninsula, UK, suggests the need for a review of the potential pathways for p ollutant transport in Permo-Triassic sandstone aquifers. Sediment infilling is pervasive in the largest sub-vertical multi-layer fractures in the stud y area, both at the surface and to a depth of about 40 m below ground level . Sediment infill is inferred to have formed in situ. The conventional mode ls of pollutant transport in fracture networks assume that they comprise op en fractures, with pollutant mobility depending on fracture connectivity (a function of density, length, orientation and intersection) and aperture. T he presence of extensive sediment fills in fractures will materially change their permeability, thereby reducing pollutant flux, and be of significanc e in the assessment of risks arising from chemical spillages. There has bee n little or no substantive evidence for such fills in Permo-Triassic sandst ones in the UK, apart from observations at outcrop and anecdotes of sand be ing pumped from boreholes. Here, we report surface and rare, but complement ary, subsurface observations of extensive fills in the Cheshire basin, and argue that they will only act as preferential pathways where they crosscut low-permeability horizons such as mudstones. (C)4 2001 Elsevier Science B.V . All rights reserved.