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From Geological Complexity to Hydrogeological Understanding Using an Integrated 3D Conceptual Modelling Approach – Insights from the Cotswolds, UK

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From Geological Complexity to Hydrogeological Understanding Using an Integrated 3D Conceptual Modelling Approach – Insights from the Cotswolds, UK Chater 6 From geological complexity to hydrogeological understanding using an integrated 3D conceptual modelling approach – insights from the Cotswolds, UK S.H. Bricker, A.J.M. Barron, A.G. Hughes, C. Jackson & D. Peach British Geological Survey, Kingsley Dunham Centre, Nottingham, UK ABSTRACT Adequate hydrogeological conceptualisation of structurally complex fractured aquifers requires the support of detailed geological mapping and three dimensional understanding. With a geo- logical framework in place uncertainties in hydrological understanding and irregularities in hydraulic observations may be rationalised. Using the Cotswold of southern England, which are underlain by the ooidal limestone-dominated Middle Jurassic Inferior Oolite and Great Oolite groups, 3D modelling software GSI3D and Geographical Information Systems (GIS) have been used to integrate observed hydraulic behaviours with the 3D geological framework. In this way a conceptual model is developed to assist simulation of groundwater flow and the predicted response of groundwater levels and river flows to climatic extremes. The structural and lithological complexity of the bedrock results in sub-catchments which exhibit individual hydraulic responses and a hydrogeological setting dominated by shallow rapid fracture path- ways and copious spring discharge. 6.1 INTRODUCTION Geological maps have always been an expression of the geologist’s 3D understanding of the structure and composition of the earth. In recent years 3D geological ‘frame- work’ models describing the subsurface have begun to supersede the traditional 2D geological map. Initially these tended to be of shallow superficial deposits but are increasingly of complex faulted bedrock systems (Aldiss et al., 2012). The capabil- ity for 3D geological characterisation is being driven by multi-faceted applications e.g. for engineering (Merritt et al., 2007), land-use planning (Campbell et al., 2010) and hydrogeology (Robins et al., 2005, Royse et al., 2009) such that 3D models pro- vide the foundation for many integrated modelling approaches. Within the discipline of hydrogeology, 3D geological models have been applied to both hydrogeological and groundwater modelling e.g. for permeability mapping, as a framework for risk screening tools (Marchant et al., 2011), to delineate aquifer volumes and boundaries within groundwater flow models and to visualise model outputs (Wang et al., 2010). A detailed 3D geological model may also be applied where geology is structurally complex. The model supports hydrogeological conceptualisation where uncertainties CCH06.inddH06.indd 9999 22/13/2014/13/2014 55:56:53:56:53 PPMM 100 Fractured rock hydrogeology in hydrological understanding and irregularities in hydraulic observations may be rationalised (Royse et al., 2010). The Jurassic sequence of the Cotswolds, in which lie the headwaters of the River Thames, is complex both in terms of lithology, where units are thin and laterally variable, and structure. The strata are highly fractured, disrupted by steeply dipping normal faults and cambering and dissected by deep river valleys. By resolving the geo- logical configuration in 3D it is easier to visualise and understand the hydrogeology and subsurface flow processes. The limestones of the Middle Jurassic within the Cotswolds form a principal aqui- fer. Licensed groundwater abstraction nears 50 Ml/d much of which is used for public water supply serving 250 000 people. Non-consumptive abstractions for fish farming and mineral workings are also common within the region. Base flow derived from the Jurassic sequence of the Cotswolds is reputed to have sustained river flows further downstream within the Thames basin during recent droughts (e.g. 2004–2006) (pers. comm. Jones, 2011). Rivers draining the Cotswolds have high base flow indices (BFI) all year round; the BFI for the River Churn at Cirencester varies from 0.71–0.89, while the BFI for the River Coln at its downstream gauging station varies from 0.89– 0.98 seasonally (CENTRE FOR ECOLOGY AND HYDROLOGY, 2012). Base flow within the River Thames downstream of the Cotswolds remains above 50 Ml/d during a typical summer recession and exceeds 25 Ml/d during some of the more extreme drought periods (period of record 1992–2008) (CENTRE FOR ECOLOGY AND HYDROLOGY, 2012). However, the sustainable management of water resources in the Cotswold area is challenging particularly under low flow conditions where ground- water resources are already fully allocated (ENVIRONMENT AGENCY, 2007). Pub- lic water supply licences have been subject to review, both within the Cotswolds and further downstream in the Thames basin, in an effort to reduce the impacts of ground- water abstraction on river flows. Meanwhile climate change predictions (UK Climate Impacts Programme) suggest reductions in summer river flows typically between 20–50% by 2050 within the Thames Basin (ENVIRONMENT AGENCY, 2012). This chapter describes the integration of hydrogeological data within a geological framework model as a means to further our understanding of the catchment hydrogeol- ogy and controls on groundwater flow. The mid-Jurassic series of the Cotswolds is used as a case study. Using the 3D modelling software and Geographical Information Systems (GIS) the influence of lithology and structure on groundwater flow and discharge proc- esses is examined along with the importance of perched water tables and shallow flow paths for river base flow contribution. The relationship between the Middle Jurassic Great and Inferior Oolite limestone successions controls the potential for hydraulic inter- connectivity between the Great Oolite and Inferior Oolite aquifers. The refined concep- tual model provides a basis for onward numerical simulation of groundwater flow to assess the response of hydrological system to climate change and extreme events. 6.2 3D GEOLOGICAL MODELLING A 3D platform for geological mapping using the Geological Surveying and Investiga- tion in three dimensions tool (GSI3D, ©Insight GmbH) has been available to geolo- gists since the late 1990s. Its successful uptake since then is largely due to the intuitive CCH06.inddH06.indd 110000 22/13/2014/13/2014 55:56:53:56:53 PPMM Hydrogeological understanding using an integrated 3D conceptual modell 101 220000 River Coin Legend River Chum GSI3D model area River leach Rivers Fault Confining strata Great Oolite Group Fuller’s Earth Fm Inferior Oolite Group STROUD Ampney Bk Lias Group CIRENCESTER Thames headwaters 200000 CRICKLADE River Thames catchment 380000 400000 0 5 10 20 kilometers Figure 6.1 Location of the study area showing extent of the Cotswolds 3D geological model and 250k mapped geology with location of faults and drainage network. The position of the River Thames catchment (grey) and study area (green) is indicated on the smaller location map. Contains Ordnance Survey data © Crown Copyright. (See colour plate section, plate 17). geologically-based methodology that allows the incorporation of a geologist’s expert knowledge (Kessler et al., 2009). This approach is similar to 3D geology methodolo- gies adopted elsewhere (e.g. Kaufmann & Martin, 2008). Such 3D models are able to encapsulate the implicit and tacit knowledge from the mapping geologist and convey the degree of uncertainty in the depth profile which is otherwise absent in the inter- pretation of 2D maps and cross-sections by the non-expert viewer (Howard et al., 2009). The end product from such models may, therefore, be viewed as a geological interpretation as opposed to a modelled output. This more qualitative knowledge- driven approach has advantages over interpolation-based software packages, where geological data are unevenly distributed and the complexity of the geological struc- ture requires significant extrapolation (Kessler et al., 2009). The confidence in GSI3D and other similar modelling approaches is manifest in the wide range of applications and in the variety of end-users which includes geologi- cal surveys, regulators, the water industry and local government (Kessler et al., 2009). For the Cotswolds the environmental regulator provided the original impetus for the development of a 3D geological framework model to assess groundwater catchment divides for better groundwater resource management (Maurice et al., 2008). Recog- nising the complexity of the geological setting, and the implications for successful resource management the original 3D framework model was refined to provide the basis for an enhanced hydrogeological conceptual model. The 3D geological model developed in GSI3D covers some 600 km2 of the Cots- wolds between Cricklade in the south-east and Gloucester in the north-west (Figure 6.1). CCH06.inddH06.indd 110101 22/13/2014/13/2014 55:56:53:56:53 PPMM 102 Fractured rock hydrogeology While the model does not provide complete coverage of the Cotswolds the lithological sequence and geological features present within the study area are representative of the wider Cotswolds area. Inferences made with respect to the integration of the 3D geology with hydrogeological characteristics are, therefore, more widely applicable. The 3D geology was modelled by the regional geologist using 139 borehole records within 42 cross-sections with a combined length of nearly 600 km in conjunction with digital surface l bedrock maps at the 1:50 000 scale. 35 bedrock units from the Whitby Mudstone Formation at its base through to the Oxford Clay Formation are incorporated into the 3D model with strata mapped at the formation or member scale where
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