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Title: Using multiple geophysical techniques for improved assessment of aquifer vulnerability
Author: Mejus, Lakam anak
Awarding Body: Lancaster University
Current Institution: Lancaster University
Date of Award: 2014
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There is increasing pressure to understand the pathways of potential contaminants In the subsurface environment. Such an understanding relies on the development of a reliable conceptual model of subsurface flow pathways. In this thesis, a combination of geophysical and conventional hydrological techniques was used to characterise the hydrogeology of the Pow sub-catchment, which is part of the River Eden catchment in Cumbria, United Kingdom. Field geophysical investigations of the subsurface are typically performed through the delineation/imaging of its physical properties. Information gathered from such geophysical methods can then be used to differentiate and characterise lithological units and, in some cases, related hydrological properties and processes. Despite some considerable advances in the use of geophysics for hydrogeological studies, hydrological quantification of the geophysical responses under field conditions has often remained challenging. The main focus of this study is on improving the hydrological conceptual model of the Pow sub-catchment, using multiple geophysical datasets as a means of assessing the vulnerability of the regional sandstone aquifer. Specifically, this thesis explores the potential of electrical properties to differentiate hydrological units at the site, with a specific focus on saturated hydraulic conductivity. This leads to the establishment of electrical-lithological and electrical-hydraulic conductivity relationships, which are used to improve the conceptual model of subsurface hydrology within the Pow sub-catchment. Such approaches could enable lithological and hydraulic characterisation of heterogeneous subsurface in a way that is currently prohibitively expensive across anything other than small spatial areas. The field geophysical investigations successfully revealed a three-layered model structure and.3 'channel' feature reflect to the lithological variations. The joint application of multiple geophysical datasets combined with borehole information allowed the development of an initial conceptual model of the site. The resulted conceptual model illustrates the potential importance of superficial deposits for controlling the hydrogeology of the Pow sub­catchment. Using synthetic studies to constrain the interpretation of the field datasets, limitations in the geoelectrical inversion were identified. The approach of using a priori information in an inversion routine significantly improved the inversion results. The link between electrical properties and hydraulic conductivity were investigated and evaluated using three induced polarisation-based electrical models and a surface conductivity model for hydraulic conductivity estimation. It was found that knowledge of the formation factor, cation exchange capacity and porosity was necessary to constrain electrical-hydraulic conductivity estimation models. Two approaches were used to produce a field-scale hydraulic conductivity estimation map: induced polarisation-based models for hydraulic conductivity estimation and an empirical relationship between the real part of complex conductivity and hydraulic conductivity. The mapping of low and high hydraulic conductivity zones helps to improve the conceptualisation of the Pow sub-catchment, which includes: (1) infiltration to the overburden unit through a network of zones comprising of sand and gravel-rich material, (2) possible flow pathways through a network of fractures, fissures and laminated sand features, and (3) preferential flow pathways through the paleo-channel and the occurrence of 'breaks' in the clayey till unit above the regional sandstone aquifer. This study shows the value of multiple geophysical datasets in providing comprehensive insights into the connection between the ground surface and the regional aquifer, and thus the pathways of potential contaminants from the surface to the aquifer in the sub­catchment. The ability to predict subsurface hydraulic properties from geophysics could provide important supporting information, help to reduce the cost of more conventional hydrogeological investigations and increases the confidence level in geophysical interpretations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available