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Sediment-filled fractures in Triassic sandstones : pathways or barriers to contaminant migration?

By J.M. Pearce, E. Hough, G.M. Williams, G.P. Wealthall, J.H. Tellam and A. Herbert

Abstract

Observations of sediments infilling fractures in Triassic Sandstones, an important aquifer in the\ud United Kingdom may explain sand production in some water supply boreholes in the UK and why the\ud aquifer transmissivity increases with time, which would occur if sediments were being washed from the\ud fractures. The fracture infills are variable from sands to complex interlaminated sand, silt and clay. It is\ud clear that, depending on the fill material, fractures may form either pathways or barriers to contaminant\ud migration, questioning current concepts for flow in fractured sandstones, and throwing doubt on our\ud ability to predict or remediate contaminant migration in this nationally important aquifer.\ud Mineralogical differences between the fracture fill material and the host sandstone may also\ud affect sorption and precipitation reactions, which, if not considered, may significantly alter the\ud prediction of contaminant migration through either the fracture fill or host sandstone matrix. At present,\ud there is little knowledge of the extent of fracture fills, how they form, whether they occur above and\ud below the water table and whether in vertical joints or bedding plane fractures. There are few\ud measurements of their hydraulic properties to be able to assess their hydrogeological significance and\ud how they should be represented in contaminant transport models. For instance, if the fill has a similar\ud permeability to the rock matrix, a porous medium model may suffice. If the fractures are filled with clay\ud with lower permeability, or if preferential flow along fractures exists, a different modelling approach\ud involving a porous medium with planar barriers may be required.\ud It is hoped the improved representation of fractures in solute or NAPL migration models will\ud lead to better assessment of contaminant migration and risk, focused remediation techniques, and more\ud precise evaluation and management of groundwater resources.\ud This ongoing study aims to understand the nature and geological controls on how fracture fills\ud develop and to improve the representation of fractures in solute or NAPL migration models. The first\ud stage in this is to establish the distribution and occurrence of these features at outcrop, to establish\ud formation mechanisms and provide data such as textural information and permeability measurements\ud that will contribute to a better understanding of their influence on hydrogeology. This will lead to a\ud better assessment of contaminant migration and risk, more focused remediation techniques, and more\ud precise evaluation and management of groundwater resources. A key problem is that sediment infills\ud may not be preserved at outcrop and may also be destroyed during standard borehole sampling\ud procedures. This requires a careful approach to sampling, both at outcrop and in the borehole

Topics: Earth Sciences
Year: 2001
OAI identifier: oai:nora.nerc.ac.uk:20046

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