Comparing The Influence Of Aquifer Heterogeneity On Borehole Design, Test Pump Results And Interpretations Between The Primary Cape Flats And Atlantis Aquifers Of The Western Cape, South Africa.

The recent Western Cape drought initiated large scale development of the Cape Flats Aquifer (CFA) and refurbishment of the Atlantis Water Resource Management Scheme (AWRMS). Both aquifers are comprised of primary sediments of the Sandveld Group. Lithologies and depositional environments of the two aquifers are often directly compared and linked, but recent borehole drilling in these two aquifers is highlighting their inherent differences. The use of conventional mud rotary drilling techniques in these aquifers and changing nomenclature over time, has created uncertainty in their lithological character, leading to complications in borehole design and interpretation of test pumping results. Sonic drilling - Atlantis (20) and CFA (25) - was undertaken and incorporated with approximately 200 mud rotary borehole drill logs and geophysical survey results to investigate aquifer geometry and hydro-lithological characteristics. Results to date indicate the CFA is more heterogenous and has greater lateral variation compared to the Atlantis Aquifer. The CFA is interspersed with clay lenses, organic rich layers, calcrete and thick basal shell units. Whilst the Atlantis Aquifer displays a more homogeneous character with limited clay lenses, minor organic layers, interspersed calcrete and a near non-existent basal shell layer. Results of the sonic drilling have led to increased confidence in boreholes design, test pump analyses and numerical model results. The influence of CFA's heterogeneity on test pumping interpretation is displayed in the results through a variety of unconfined, confined, semi-confined and leaky type curves. Atlantis however, typically displays unconfined Neuman-type curves. Delayed gravity drainage signals, test pump duration, varying hydraulic conductivities of different lithological units and other boundary effects not only have an economic impact on test pump design, but can lead to the misinterpretation of test pump data which greatly influences planning for the aquifers' response to large scale abstraction and Managed Aquifer Recharge (MAR) alike.

Presenter Name
Luke
Presenter Surname
Towers
Area
Western Cape
Conference year
2019