Kalahari aquifers in the Gam area of north-eastern Namibia
Groundwater exploration in the Gam area of eastern Namibia has largely been aimed at fractured bedrock aquifers. An extensive cover of thick Kalahari Sequence sediments makes the location of bedrock structures notoriously difficult. In a number of localities boreholes drilled by air percussion indicate a piezometric surface situated above the Kalahari-bedrock contact although water strikes were only recorded below the contact, suggesting confinement. The absence of a clear, semi-regional aquitard in the Kalahari Sequence tends to discount such a possibility. To the south of Gam, the existence of the 'Eiseb Graben' aquifer, hosted in Kalahari Sequence strata, has been proved by drilling. During exploration of this aquifer it was demonstrated that conventional rotary air percussion drilling, as was used elsewhere in the Gam area, commonly fails to detect low-yield water strikes in Kalahari strata. This is concluded to be the result of the sealing off of the low-permeability strata by the invasive nature of this drilling technique. Development of boreholes by application of passive plunger methods and installation of appropriately designed casing results in low but dependable yields in many areas previously thought to be 'dry'. Re-interpretation of the 'piezometric' surface within Kalahari strata leads to the conclusion that in parts of the Gam area this is in fact a phreatic surface which was not detected during air percussion drilling. For groundwater exploration to be more successful in these areas of saturated Kalahari cover, a revision of the approach to groundwater exploration and the use of appropriate drilling and borehole completion techniques is required.
Communications of the Geological Survey of Namibia
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