Drilling to Augment Urban Water Supply – A Case Study of the Cape Flats and Table Mountain Group Aquifers
Umvoto Africa (Pty) Ltd (Umvoto) has 30 years’ experience in the groundwater industry and has undertaken numerous groundwater development projects across all aquifer types throughout South Africa and Africa, including fractured rock aquifers (TMGA), basement aquifers (Malmesbury Group, Cape Granite Suite), primary aquifers (Cape Flats Aquifer [CFA], Atlantis Aquifer), and karstic aquifers (Malmani Dolomites).
Umvoto has played an integral role in assisting the City of Cape Town (CoCT) to develop alternative bulk water supply, initiated with the Table Mountain Group Aquifer (TMGA) project as early as 2002 (see Figure 5). Prior to 2017, TMGA exploration phase work undertaken by Umvoto had already proven the feasibility of large-scale groundwater abstraction from the TMG aquifers. Through the feasibility study, select production drilling target zones (wellfields) with a clear geographical linkage to the existing surface water supply infrastructure for future groundwater development were identified and delineated.
The recent “Day-Zero” drought experienced throughout the Western Cape between 2015-2018, forced local government to seek alternative water resources to augment bulk water supply. These included
re-use, desalination and various groundwater resource development initiatives, including the TMGA and CFA, as part of the New Water Programme (NWP), which was initiated in 2017.
Historically, an abundance of literature about the potential of the CFA prompted the interest into exploring this resource for groundwater production. Umvoto undertook a Situation Assessment of the CFA in 2014 to evaluate the potential of the CFA for bulk water supply and provided recommendations on using the aquifer with an adaptive management approach and thereafter developed the Cape Flats Aquifer Management Strategy (2015) on behalf of the Department of Water and Sanitation which outlined measures to remediate and use the aquifer sustainably. Initial exploration of the proposed Cape Flats Aquifer Management Scheme (CFAMS) commenced through an airborne electromagnetic and magnetic geophysics campaign to define the lateral extent, thickness, and aquifer geometry over the delineated aquifer area. Accessible areas where higher groundwater potential zones were identified, were targeted for exploration drilling.
The CFA is a Quaternary unconsolidated, primary, coastal aquifer which covers an area of ~488 km2 from False Bay in the south to Tygerberg Hills and Milnerton in the northeast and northwest. Deposited by varying laterally extensive fluvial, marine and aeolian palaeodepositional processes, the vertical and lateral heterogeneity of the CFA is complex and not well understood. Target aquifer material generally comprises of palaeo-channel quartz-rich gravels with clay and peat of the Elandsfontyn Formation, fine to medium-grained sands with shells of the Varswater Formation and the well sorted and rounded, fine to medium-grained mature sands of the Springfontyn Formation. Collectively these target formations form part of the major CFA (refer to Table 1).
Table 1: Compilation and summary of the hydrostratigraphyand stratigraphy across the project study areas. Targeted aquifers (and associated units) highlighted in bold.
On the other hand, the TMGA study area is set within the structurally complex (Cape Fold Belt syntaxial zone) basal formations of the tripartite Cape Supergroup (see Figure 1), particularly focusing on the fractured aquifers of the mature quartz arenite dominated Table Mountain Group (TMG). Selected target zone areas aimed to intersect major structural feature(s) of hydrogeological interest (also referred to as hydrotects). The two main targeted TMG hosted aquifers included the locally unconfined to semi-confined Nardouw Aquifer and deeper confined Peninsula Aquifer. The interstitial Winterhoek Mega-aquitard acts as a confining unit for the underlying Peninsula Aquifer, with associated deep confining high pressure (>10 bar) artesian conditions.
Figure 1: An overview of the TMGA Steenbras Wellfield geology, associated target structures (faults) and Peninsula Aquifer target boreholes, with particular focus on the Peninsula wide-diameter production borehole (H8A5, H8A10, H8A4 and H8A9) site layouts during the drilling phase.
The diversity of the aquifer settings and targets related to each of the project areas required vast research into specific drilling techniques to meet the target design requirements for each setting (see Figure 2). For the TMGA, this entailed the drilling of comparatively shallow (~150-300 m; still considered deep in most of Africa for water supply) and deep (>800 m) wide-diameter Nardouw and Peninsula Aquifer production boreholes (telescopic 586-152 mm diameters), respectively. Prior and concurrent to wellfield development, exploration core drilling (123-60 mm diameters) informed potential production borehole design, hydrostratigraphic conditions and expected challenges. A total of 49 production and exploration boreholes were drilled. Additionally, the exploration boreholes were incorporated into the robust multi-technique monitoring network, allowing aquifer conditions to be monitored throughout the various phases of wellfield development.
Figure 2: A composite figure of the construction, geological, hydrogeological (waterstrikes and fractured zones[white]) and penetration rate logs, with corrresponding illustrations of the various drilling techniques and downhole assembly throughout the drilling process of the Peninsula Aquifer target borehole H8A9.
From the inception of the CFA project in early 2018, 285 exploration, monitoring, production, and Managed Aquifer Recharge (MAR) boreholes were drilled throughout the target areas identified in the Cape Flats (see Figure 3). Initially, exploration (203 mm diameter) and production borehole (457 mm diameter) drilling took place simultaneously. This approach ensured that a monitoring network was established through the repurposing of exploration boreholes. In addition to exploration and production boreholes, the CFAMS requires a MAR component comprising of production-diameter injection boreholes. A single test injection borehole with a diameter of 1016 mm was drilled to compare its efficiency, operation and clogging potential to the more standard 304.8 mm stainless steel casing.
Figure 3: Overview of Production and MAR wellfields situated across the CFA. Bishop Lavis East and West has been put on hold and possibly could be used in future should further supply be required.
Both the TMGA and CFA project areas faced unique challenges, including a host of hydrostratigraphic, environmental and socio-economic factors, which had to be overcome during each drilling phase. The major factor contributing to challenging drilling conditions throughout each project area was the complex geological settings. In the TMGA this included extremely competent TMG quartz arenites, structural complexity, highly unstable fractured, brecciated and gouged fault zones, and large pseudokarstic cavities. The vertical and lateral lithological heterogeneity of the CFA and TMGA posed borehole design challenges that required adaptive approaches to the construction designs.
To overcome the geological conditions in the TMGA, various drilling methods were used to achieve target drilling depths, diameters and yields (see Figure 2). Exploratory diamond core drilling was undertaken with the primary focus on locating (depth and contacts) and hydrogeologically classifying the Nardouw and Peninsula Aquifers. Wide-diameter boreholes targeting the Nardouw Aquifer, relied completely on the traditional Down-The-Hole (DTH) rotary air percussion drilling technique. This is the most cost effective and rapid drilling technique for shallow targets in fractured aquifers. The reverse circulation (RC) air percussion method was used interchangeably with the DTH method to maintain target drilling diameters at depth and adhere to strict environmental protocols. Drilling advance and productivity in both surface pneumatic air percussion methods was largely limited by the up-hole velocities required to overcome the hydraulic head, which increased with depth and high-yielding water strikes. Boreholes targeting the Peninsula Aquifer, were initially drilled using the same air DTH and RC method up to ~250 m, whereafter the limit to up-hole velocity and drill operating pressures, required a shift to a non-air reliant technique. The rotary flooded RC technique was used for drilling between depths of ~250 m and 800 m and served to simplify environmental mitigations and advance through deep, unstable (fractured, faulted and/or heterogenous) lithologies. It is notable that the use of tri-cone bits and the flooded RC technique prior to the TMGA project deep drilling was novel for TMG boreholes. The water hammer technique was utilised in the last >800 m of drilling, where significant hydrostatic water column (>80 bar) and artesian pressures (up to >10 bar) were encountered. During use, the water hammer technique achieved extremely fast penetration rates of 3-6 min/m and minimal hole deviation.
In addition to site specific environmental method statements, on-site control officers and general mitigation measures, it was critical to completely isolate the target aquifers and seal off unwanted water strikes related to non-target TMG lithologies. This was achieved through the final installation sequence of annular grouted casing sets. In the deeply confined Peninsula Aquifer target boreholes this comprised of a tapered
256-203 mm set of 8 mm thick (1400 psi collapse strength) steel casing. Casing and sealing throughout the drilling process additionally served to ensure environmental compliance (no uncontrolled annular artesian discharge) and critical borehole sidewall integrity.
Due to the lithological heterogeneity of the CFA, a number of techniques were trialled and used for the drilling of the boreholes. Overburden Drilling EXcentric (ODEX) was attempted but failed due to the soft unconsolidated nature of the sediments. Majority of drilling was undertaken using the mud rotary drilling technique as this was the most cost effective and allowed rapid drilling of narrow and wide-diameter boreholes (see Figure 4). The lithological heterogeneity proved to be complex for the siting and planning of MAR boreholes. Discontinuous clay lenses needed to be accurately identified to ensure recharge would not cause any surface flooding. To overcome these and to ensure efficient planning, the air core drilling was tested, but due to the creation of possible cavities the method was rejected and therefore, sonic drilling was used and a total of 36 sonic boreholes were drilled. This technique was the most accurate for identifying and logging lithological changes, and proved to be cost effective, with the boreholes then been included into the monitoring network.
Figure 4: Typical borehole design for boreholes drilled during the CFAMS. A) Borehole design of exploration and monitoring boreholes. Asimilar design was used for sonic boreholes, but size of uPVC casing varied. B) Borehole design used for production boreholes. C) Borehole design for MAR boreholes which includes a monitoring borehole which will be used to control operation during injection.
The pristine setting of the TMGA within the Cape Floristic Kingdom (and corresponding protected areas and reserves) meant that all contractor activity had to be stringently planned, managed, and monitored to ensure limited impact to rare and endangered flora and fauna. Contrastingly, CFA mostly underlies densely populated residential and informal settlements, industrial – commercial and agricultural areas. Accompanying the densely populated surface area, an array of contamination hazards and complex social conditions was encountered throughout the project tenure. Additionally, numerous existing groundwater users, such as the farmers of the Philippi Horticultural Area (PHA), had to be taken into consideration during the design of the CFAMS. This was all undertaken whilst being accompanied by armed security, for the safety of consultants, contractors, and equipment alike.
Ultimately each drilling technique has its pros and cons, and is dependent on the type of geological material, accuracy of data required and target depth that needs to be achieved. Implementing multi-technique drilling technologies has allowed Umvoto to achieve >1000 m deep boreholes targeting the Peninsula Aquifer in the TMGA project. Identifying the need for accurate lithological detail in the CFA, has given Umvoto the opportunity to accurately site 25 injection boreholes for recharge, and plan for future MAR expansion, which is a step towards sustainable management.
The challenges were overcome by combined planning, continual adaptation and mitigations initiated between the hydrogeological sub-consultant (Umvoto), drilling contractors, consultant ECOs, client, and specialists. Umvoto’s extensive experience in the applications of various drilling techniques in a variety of aquifer settings played an integral role in this (see Figure 5). The truly innovative and integrated approaches resulted in a standard being set for large scale diverse aquifer-targets for South African municipal supply. Additionally, the TMGA boasts with producing the deepest wide-diameter municipal water supply boreholes in the Western Cape.
Figure 5: CFA and TMGA study areas with study highlights