Towards prediction of saturated-zone pollutant movement in groundwaters in fractured permeable-matrix aquifers: the case of the UK Permo-Triassic sandstones

Abstract

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 rockmay have a dominating effect. Where present, low-permeability matrix (e.g. mudstones) will significantly affect vertical flow, but will rarely prevent eventual breakthrough. Quantitative 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 uncertainty 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) hydrogeological 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.

Comparison of rates of ureolysis between Sporosarcina pasteurii and an indigenous groundwater community under conditions required to precipitate large volumes of calcite

“…This aquifer is representative of Permo-Triassic Sandstone aquifers that underlie several other major cities, including Nottingham, Liverpool, and Manchester (detailed in Tellam and Barker, 2006). Sampling was conducted in December 2009 and January 2010 at Birmingham University from Borehole 1 (BH1).…”

Characterization of a fluvial aquifer at a range of depths and scales: the Triassic St Bees Sandstone Formation, Cumbria, UK

“…The Sherwood Sandstone Group (Lower Triassic) is a continental succession that has long been ascribed to a mixed fluvial and aeolian origin (e.g., Mountney and Thompson 2002;Tellam and Barker 2006;Thompson 1970;Turner 1981). This sandstone-dominated succession represents the UK's second most important aquifer, covering 25% of the total licensed volume of groundwater abstractions in the country (Abesser and Lewis 2015;Binley et al 2002;Bricker et al 2012).…”

“…During the Quaternary, intense weathering developed up to~10 m below the overlying glacial till; this was recognized in the St Bees Sandstone aquifer by McMillan et al (2000) and Hough et al (2006). Additionally, acidic conditions (pH = 5) capable of dissolving calcite are common in the unsaturated zone (such low pH can be generated by high levels of dissolved CO 2 produced by bacterial respiration in the soil zone) of the UK Sherwood Sandstone aquifer (Moss and Edmunds 1992;Tellam and Barker 2006). This work shows how samples collected~5-10 m below the glacial till (see Fig.…”

“…In some basins, these basement rocks are used to host nuclear waste repositories (Berglund et al 2009;Michie 1996;Waugh 1973). Furthermore, fluvial deposits represent porous media, which can serve as important hosts for hydrocarbon and geothermal resources (McKie and Williams 2009;Ruggeri and Gianelli 1999); additionally, they serve as important groundwater aquifers (Tellam and Barker 2006;Tellam 2004). …”

“…Assessment of groundwater behavior at such depths is important in geoscience since depth of well screens for water abstraction is typically < 100 m below ground level (BGL) in the major European and American industrial cities (De Simone 2008;Tellam and Barker 2006;Toccalino et al 2010;Worthington et al 2016), whereas depth intervals associated with nuclear waste repositories are typically 200-1,100 m BGL (Martin and Christiansson 2009;Min et al 2004;Tsang et al 2012;Voss and Andersson 1993). Thus, the proposed characterization allows investigation of the role of a range of geological discontinuities (e.g., stratabound joints and faults) which potentially establish hydraulic connection between a nuclear waste repository and aquifers exploited for civil use (Tsang et al 2016).…”