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

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.