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Airborne geophysical mapping of aquifer water quality and structural controls in the Lower Okavango Delta, Botswana

2006; Geological Society of South Africa; Volume: 109; Issue: 4 Linguagem: Inglês

10.2113/gssajg.109.4.475

1996-8590

G. Campbell, Sean Johnson, T. Bakaya, Hrishikesh Kumar, J. Nsatsi,

Geological formations and processes

Research Article| December 01, 2006 Airborne geophysical mapping of aquifer water quality and structural controls in the Lower Okavango Delta, Botswana G. Campbell; G. Campbell GAP Geophysics (Pty) Limited, PO Box 2347, Saxonwold, 2132, South Africa. e-mail: gapgeo@icon.co.za Search for other works by this author on: GSW Google Scholar S. Johnson; S. Johnson GAP Geophysics (Pty) Limited, PO Box 2347, Saxonwold, 2132, South Africa. e-mail: gapgeo@icon.co.za Search for other works by this author on: GSW Google Scholar T. Bakaya; T. Bakaya Water Resources Consultants (Pty) Limited. P.O. Box 40459, Gaborone, Botswana e-mail: tej.bakaya@wrc.bw Search for other works by this author on: GSW Google Scholar H. Kumar; H. Kumar Water Resources Consultants (Pty) Limited. P.O. Box 40459, Gaborone, Botswana e-mail: tej.bakaya@wrc.bw Search for other works by this author on: GSW Google Scholar J. Nsatsi J. Nsatsi Department of Water Affairs Botswana, P/Bag 0018,Gaborone, Botswana e-mail: dwa@global.bw Search for other works by this author on: GSW Google Scholar Author and Article Information G. Campbell GAP Geophysics (Pty) Limited, PO Box 2347, Saxonwold, 2132, South Africa. e-mail: gapgeo@icon.co.za S. Johnson GAP Geophysics (Pty) Limited, PO Box 2347, Saxonwold, 2132, South Africa. e-mail: gapgeo@icon.co.za T. Bakaya Water Resources Consultants (Pty) Limited. P.O. Box 40459, Gaborone, Botswana e-mail: tej.bakaya@wrc.bw H. Kumar Water Resources Consultants (Pty) Limited. P.O. Box 40459, Gaborone, Botswana e-mail: tej.bakaya@wrc.bw J. Nsatsi Department of Water Affairs Botswana, P/Bag 0018,Gaborone, Botswana e-mail: dwa@global.bw Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2006 Geological Society of South Africa South African Journal of Geology (2006) 109 (4): 475–494. https://doi.org/10.2113/gssajg.109.4.475 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation G. Campbell, S. Johnson, T. Bakaya, H. Kumar, J. Nsatsi; Airborne geophysical mapping of aquifer water quality and structural controls in the Lower Okavango Delta, Botswana. South African Journal of Geology 2006;; 109 (4): 475–494. doi: https://doi.org/10.2113/gssajg.109.4.475 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Groundwater exploration in the Lower Okavango Delta to the west of Maun in Northern Botswana has focused on the search for shallow, fresh-water aquifers in Kalahari sediments. The target aquifers comprised thick packages of medium grained sands characterised by medium porosity and permeability, allowing for annual flushing and freshwater recharge during seasonal floods. Elsewhere, a combination of poor drainage, evaporation and evapotranspiration results in the widespread distribution of saline groundwater. The northeasterly-striking Tamalakane and Kunyere fault bounding the southern edge of the Okavango graben are still active, and in the recent past movements along these features has controlled the distribution of drainage systems across the Lower Delta. As a consequence, target freshwater palaeochannels may now be distant from present river channels, but still undergo recharge during annual Delta floods.The area is covered by Kalahari beds of variable thickness which overly Karoo sediments above a Damaran basement. Airborne geophysical mapping conducted by the Botswana Government at a scale akin to that of mineral exploration has played a major role in the groundwater exploration programme, as well as adding significantly to a structural understanding of the area. Airborne magnetic/DTM surveys have directly mapped the southern Thamalakane and Kunyere Fault zones as magnetic linears along fault-truncated magnetic lithologies of the Damaran basement, and as DTM scarp features at surface. Magnetic modelling of fault and dyke anomalies over the 65 km wide Okavango Dyke Swarm has quantified the expected step-wise northerly increase in thickness of the Kalahari sediments (from 10 m to 400 m). Basin or graben-like structures within the inter-fault area point to secondary fault linkages between the two major bounding fault zones.Airborne and follow-up time-domain EM (TDEM) surveys were used as hydrogeological mapping tools to locate and delineate potential freshwater-sand aquifers on the basis of their relatively high formation resistivities. Such aquifers appear as sinuous zones of moderate conductivity in an otherwise high-conductivity background environment reflecting the widespread distribution of saline groundwater away from permanent river channels and, to a possible lesser extent, clay-rich units interbedded with the sands.Airborne survey data was used to set the surface lateral limits of potential freshwater aquifers, and follow-up ground EM sounding surveys to determine the maximum thickness of such systems from mapping of the fresh-saline water interface. Over the better-developed aquifers this interface is found at depths of 60 m to 80 m. Elsewhere it may be within 10m of surface. Findings are exemplified by reference to follow-up work and borehole results over the Upper Thamalakane Aquifer which is proximal to the permanent river channel, and for which an isopach model was developed from extensive EM sounding work. 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