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Towards Prediction of Saturated-Zone Pollutant Movement in Groundwaters in Fractured Permeable-Matrix Aquifers: the Case of the UK Permo-Triassic Sandstones

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Towards Prediction of Saturated-Zone Pollutant Movement in Groundwaters in Fractured Permeable-Matrix Aquifers: the Case of the UK Permo-Triassic Sandstones Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 Towards prediction of saturated-zone pollutant movement in groundwaters in fractured permeable-matrix aquifers: the case of the UK Permo-Triassic sandstones JOHN H. TELLAM & RONALD D. BARKER Hydrogeology Research Group, Earth Sciences, School of Geography, Earth and Environmental Sciences, Birmingham University, Birmingham B15 2TT, UK (e-maik J.H. [email protected]. uk; R.D. Barker@bham. ac. uk) Abstract: The UK on-shore Permo-Triassic sandstones are fluvial and aeolian red beds showing a nested cyclic architecture on scales from millimetres to 100s of metres. They are typical of many continental sandstone sequences throughout the world. Groundwater flows through both matrix and fractures, with natural flow rates generally of less than 200 m year-1. At less than 30 m horizontal distances, below important minimum representative volumes for both matrix and fracture network permeability, breakthroughs are likely to be multimodal, especially close to wells, with proportionately large apparent dispersivities. 'Antifractures' - discontinuities with permeability much less than that of the host rock - may have a dominating effect. Where present, low-permeability matrix (e.g. mudstones) will significantly affect vertical flow, but will rarely prevent eventual breakthrough. Quan- titative prediction of breakthrough is associated with large uncertainty. At scales of 30 to a few 100s of metres, multimodal breakthroughs from a single source become less common, although very rapid fracture flow has been recorded. At distances of hundreds of metres to a few kilometres, there is evidence that breakthroughs are unimodal, and may be more immediately amenable to quantitative prediction, even in some cases for reacting solutes. At this and greater scales, regional fault structures (both slip surfaces and granulation seams) can have major effects on sub-horizontal solute movement, and mudstones and cemented units will discourage vertical penetration. The aquifer has limited oxidizing capacity despite the almost ubiquitous presence of oxides, limited reductive capacity and limited organic sorption capacity. It has a moderate cation-exchange capacity, and frequently contains carbonate. Mn oxides are important for sorption and oxidation, but are present in limited quantity. Relationships between hydraulic and chemical properties are largely unknown. 'Hard' evidence for the solute transport conceptual model presented above is relatively limited. To be able to predict to a reasonably estimated degree of un- certainty requires knowledge of: the geological, and thence the hydraulic and geo-chemical, structure of the complex sandstone architecture (including the correlations between these properties); the development of suitable investigation techniques (especially geophysical) for mapping the structures; and the development of modelling tools incorporating matrix, fractures, 'antimatrix' and antifracture elements, each with associated hydraulic and possibly geochemical properties. In common with solute movement studies in most aquifer types, much more geological characterization needs to be undertaken. Although new investigation and modelling tools are being developed specifically for (shallow) hydrogeo- logical applications with some considerable success, much greater advantage could be taken of importing techniques from other disciplines, and in particular from oil exploration and development. The development of a quantitative understand- continental USA. They are also well known in ing of groundwater solute movement is an Gondwanaland, especially South America, important goal for all aquifer types. This paper Africa, Antarctica and Australia. Although the is concerned with evaluating the progress character of the sandstones varies with location, towards this goal in an example fractured in broad terms the UK sandstones are typical permeable matrix aquifer - the UK Permo- fractured continental red-bed sequences with Triassic fluvial-aeolian sandstone sequence. well-marked sedimentary structures. After the Permian and Triassic continental sandstones (Cretaceous) chalk, they constitute the most were widely distributed prior to the break-up of heavily used set of aquifers in the UK; they are Pangaea. They now occur in NW Europe, East also extensively used elsewhere for water Greenland and in many locations across the supply, and form hydrocarbon reservoirs From: BARKER,R. D. & TELLAM,J. H. (eds) 2006. Fluid Flow and Solute Movement in Sandstones: The Onshore UK Permo-Triassic Red Bed Sequence. Geological Society, London, Special Publications, 263,1-48. 0305-8719/06/$15 9The Geological Society of London 2006. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 2 J.H. TELLAM & R. D. BARKER particularly in the shelf areas surrounding the of overlying deposits will not be directly UK. considered: although these have a major effect The development of the attributes that affect on flow patterns and water chemistry, they are reacting solute movement in present-day not intrinsic to the phenomenon of solute groundwater systems is determined by the inter- movement in the sandstone. In addition, the action of three main geological factors: deposi- unsaturated zone and non-aqueous phase liquid tional environments; palaeo-groundwater-rock movement will not be covered, but papers on interactions; and stress (Fig. 1). In most ground- these topics are included within the volume water systems, solute movement, as observed, (Binley et al. 2006; Gooddy & Bloomfield 2006; depends strongly on the scale(s) of heterogen- Privett 2006; Rees 2006; Taylor & Barker 2006). eity of the sequence, the scale of measurement Evidence is drawn from the very considerable and the scale of interest (e.g. Dagan 1989), and literature, 'white', 'grey' and unpublished: it is well recognized that this needs to be because of the difficulty of obtaining much of considered when evaluating evidence. Accord- the latter two, the selection is unavoidably ingly, the structure of this paper (and this biased. Wherever relevant, an attempt has been Special Publication) reflects these issues: first, made to include reference to the very signifi- the geological setting is briefly described, then cant petroleum geology literature, a resource flow and, finally, solute movement. In the latter that often is not fully exploited in hydrogeology: two sections, the general approach is to consider it is, however, important to note that observa- evidence in order of increasing scale of investi- tions from deep systems with a different geo- gation, and in the final discussion scale is a major logical history are not necessarily directly consideration. Aquifer geometry and presence transferable. :De~iti~a~ I Geological Environments I ff [Gw Interactions]dxdt ! Development i I / ' gu~fi~ltx % 1< > ' + Uipli~tit!x) ~ fi [Stress]dxd! I Lithology(t, x) I ~ _ N) J~ : ' ~ ctures(t,x) ~' -- --~ I-Fig -- II | Km/4,/ mll I,:/+/.,(t.x) z5 Flow l .l tx, II Reactivity(t,x~~ K /d~ /'r (t,x) ~ I Dispersivity(t'x) I Breakthrough SoluteMovement dislributions(t,x) Fig. 1. Geologicalfactors and their relationships with solute movement (ff[]dxdt = integrated effect over space and time; K is hydraulic conductivity; 0 is porosity; x is tortuosity; subscripts f, m, and rm are fracture, matrix and rock mass; double line boxes indicate properties that may be amenable to geophysical measurement). Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 SOLUTE TRANSPORT 3 Geological considerations example, Wills (1970) recognized 'microcycles' at the scale of a few beds, 'miocycles' compris- Introduction ing groups of microcycles up to approximately 100 m thick at most and 'magnacycles' compris- An outline stratigraphy is given in Table 1, and ing groups of miocycles (usually equivalent to the distribution of outcrop is indicated in Figure formations). Each microcycle represents a wet 2 (for comprehensive details see, for example, to dry transition. Thus, an ideal microcycle Warrington et al. 1980 and Benton et al. 2002). sequence according to Thompson (1970a) and In general, the sequence, up to over 1 km in Wills (1970) might be: thickness at its maximum development, is underlain by deposits of low permeability, top Aeolian sandstones sometimes Permian in age but often Carbonif- Mudstone, with desiccation cracks erous or older. Usually it is immediately Finer-grained plane-laminated sandstone overlain by either the Triassic Mercia Mudstone Medium-grained cross-laminated sand- Group (mudstones and evaporates with stone occasional thin sandstones) or by Quaternary Coarse (pebbly) channel deposit deposits (tills to outwash gravels). The Permian base Erosion surface. and Triassic sandstone sequences are broadly Miocycles were defined by Wills (1970), similar in origin and lithology, and, for the rather subjectively, using indicators of maximum purposes of this review, will be considered water velocity with high-energy conditions together. passing upwards into lower energy conditions. At the largest scale (magnacycles), the main Depositional environments formations in any one locality may be paired into a cycle: for example, in the Triassic During the Permo-Triassic, the area that is now Sherwood Sandstone Group of the Midlands the UK migrated from approximately 10 ~ to (Table 1) the first magnacycle would comprise 30 ~ N of the equator. The climate was semi-arid the Kidderminster Formation (coarser, pebbly) to arid, with
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