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The use of electrical resistivity tomography in deriving local-scale models of recharge through superficial deposits

Cuthbert, Mark, Mackay, R., Tellam, J. H. and Barker, R. D. 2009. The use of electrical resistivity tomography in deriving local-scale models of recharge through superficial deposits. Quarterly Journal of Engineering Geology 42 (2) , pp. 199-209. 10.1144/1470-9236/08-023

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The way in which superficial deposits affect groundwater recharge is often a significant source of uncertainty in groundwater resources and vulnerability assessments. A study of a small catchment in Shropshire, UK, shows how electrical resistivity tomography (ERT), with a degree of borehole control, can be an effective tool for defining the geometry of superficial deposits for purposes of inferring the hydraulic processes controlling groundwater recharge. Major lithological units were mapped to within c. 0.5 m vertically and 5 m horizontally using ERT surveys with a minimum electrode spacing of 2 m. Interpretation was aided by the strong contrast in resistivity between till and glaciolacustrine deposits (20–40 Ω m) and glaciofluvial deposits (generally >100 Ω m) that overlie the Permo-Triassic sandstone aquifer (saturated resistivity 60–145 Ω m) in the study area. A range of local-scale (tens to hundreds of metres) recharge models are presented, based on the findings of the field surveys, and it is shown how existing mapping misses key features of the superficial geology that may be very significant in enhancing or restricting aquifer recharge. Estimates of groundwater recharge are critical for developing robust models of water resource availability and aquifer vulnerability (Robins 1998). Understanding the complex set of hydraulic processes that control subsurface flows is fundamental to such estimation of recharge. However, difficulties in recharge estimation are often noted because of uncertainties in measured climatological variables, the lack of data to understand soil zone processes and the unknown influence of superficial deposits amongst other factors (Lloyd 1980; Lerner et al. 1990). The influence of superficial deposits on groundwater recharge in the catchment of the River Tern, Shropshire, UK, is the subject of this paper. Compared with many other catchments in the UK, data availability and quality are very good for the Tern catchment, largely as a result of the presence of the Shropshire Groundwater Scheme and, more recently, the Lowland Catchment Research (LOCAR) monitoring installations. Thus, approaches to quantifying recharge such as that taken recently by the UK Environment Agency, which make good use of available data, are as accurate as is possible at present for a regional water resources study (Streetly & Shepley 2005). However, given all this, major uncertainty still remains at the scale of the subcatchment with regard to the distribution and magnitude of groundwater recharge in areas covered by superficial deposits of variable lithology and thickness, and the generation of intermediate timescale stream flows (Cuthbert 2006). In part this is due to uncertainty in the detail of the thickness, permeability, extent and geometrical relationships between different superficial deposits in the catchment. The best available data are given by widely spaced geological logs and the existing 1:50 000 superficial geology map. However, the latter has no information about geological layering and makes assumptions about dominant lithologies; for example, it ignores very thin, but potentially hydrogeologically significant, superficial sands where they overlie till. In addition, the hydraulic processes involved in recharge at the site (<10 m) and local scale (tens to hundreds of metres) are not well understood and hence upscaled parameters for confident modelling of recharge at larger scales are not available. A more accurate knowledge of the distribution and geometry of superficial deposits within the catchment is vital as a first step to a better understanding of its effects on aquifer recharge. This paper presents the results of a geophysical investigation of the Potford Brook catchment, a subcatchment of the Tern catchment, designed to reveal the geometry and relationships between superficial deposits so as to infer hydraulic processes contributing to recharge.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Publisher: Geological Society
ISSN: 1470-9236
Last Modified: 01 Nov 2017 14:50

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