Keywords

1 Introduction

The City of Cape Town (CCT; within the Western Cape Province of South Africa) is the most-populated coastal city, and second most-populated city within South Africa (~4 million people). The city is one of the most multi-cultural in the world, and is a major economic, transport, tourist, design, and agricultural (in association with surrounding farm regions) hub in South Africa and Africa. The Cape Floristic Kingdom (CFK) also occurs within the city and surrounding mountain catchments, and despite it being the smallest of the six floral kingdoms of the world, is a biodiversity hotspot with high economic and ecological value. The CCT is currently supplied by surface water stored in the six major dams of the Western Cape Water Supply System (WCWSS), which has a total storage of ~900 million m3, and an original 98% assurance of supply of 600 million m3 per annum (~50–60% of which is used by the CCT, and the remaining ~40–50% used by surrounding agriculture/other urban users).

The Western Cape is a relatively water-scarce area as a result of the Mediterranean climate experienced, and this scarcity is likely to be exacerbated as a result of climate change. An extreme, extended 1:590 year drought (CCT 2019) in the Western Cape from 2015 to 2017 (although continuing through to present in certain areas) has put severe strain on the WCWSS to such an extent that the supply system came close to failing in early 2018 (total dam storage volumes of <20%), even with severe water restrictions and water conservation/demand measures in place (reducing the city’s water demand from ~1000–1200 Ml/day to ~500–600 Ml/day) (CCT 2019). Consequently, the CCT was at risk of becoming the first modern day city on Earth to run out of water (as extensively reported nationally and internationally as “Day Zero”), which would have had enormous societal and economic impacts.

The CCT initiated its “New Water Programme” (NWP) in earnest to diversify its water supply to improve long-term water security and resilience against future droughts by implementing alternative bulk water supply options (CCT 2019). The NWP also aims to meet the demand by an ever growing urban population, improving the standard of living of approximately half of the city’s population through meeting Sustainable Development Goals (SDGs) 6 and 11. Identified alternative bulk water supply options include desalination, water re-use, and the abstraction of groundwater from three major aquifer systems that the city has access to, namely:

  • the Atlantis Aquifer and Cape Flats Aquifer, both primary sand aquifers within the city’s municipal boundaries; and

  • the Table Mountain Group (TMG) aquifers (largest of the three major aquifer systems; sub-divided into the fractured quartzitic sandstone Peninsula and Nardouw Aquifers), which occurs within the mountain catchments that surround the city and incorporate the dams of the WCWSS.

2 Project Implementation

The vast potential of the globally unique, deep (up to 1–2 km thick), evaporation free, extensive (~11,000 km2), voluminous (saturated aquifer volume of ~400,000 km3) and high yielding (individual borehole yields of ~10–70 l/s, potential total yield of ~140–400 Ml/day or ~50–150 million m3/a) Palaeozoic TMG aquifers was identified through pioneering groundwater exploration and supply projects by Umvoto Africa and others since the 1990s (Hartnady and Hay 2002, 2008; Hartnady et al. 2005, 2009; Blake et al. 2010).

The CCT has been investigating the feasibility of groundwater abstraction from the TMG aquifers since the early 2000s (Hartnady et al. 2004). Current exploration (including additional exploration via detailed geological mapping and heliborne geophysics) and wellfield development is occurring in the vicinity of the WCWSS Steenbras Dam east of the CCT, in the form an ~15–20 Ml/day wellfield scheme (expected to be operational in early-mid 2020) (Blake et al. 2019) (see Fig. 1).

Fig. 1
figure 1

Steenbras Wellfield lithology, structure, borehole positions, and wetlands/GDEs identified using Sentinel 2 imagery. Skurweberg and Rietvlei Formations of the TMG form the Nardouw Aquifer, whereas the Peninsula Formation (also TMG) forms the Peninsula Aquifer

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The planned “Steenbras Wellfield” targets both TMG aquifers along a high-yielding major crustal strike-slip structure/hydrotect termed the “Steenbras-Brandvlei Megafault Zone” (see Fig. 2). Current drilling activities have included shallow to deep (~100–400 m, Nardouw Aquifer) and ultra-deep (up to 1000 m, Peninsula Aquifer), wide diameter abstraction and core exploratory boreholes, with tested abstraction borehole yields ranging between 10–70 l/s. Further CCT TMG groundwater exploration and wellfield scheme development (potential total combined supply of ~50–150 million m3/a or ~140–400 Ml/day) is planned along major TMG structures within the Grabouw-Eikenhof and Theewaterskloof basins, Wemmershoek, Voelvlei, Berg River and the CCT South Peninsula region. These areas are in vicinity of the major WCWSS dams (which are fortuitously near large fault systems and hydrotects), as the infrastructure costs of transporting large volumes of groundwater from extremely rugged, ecologically sensitive mountain terranes can be excessive.

Fig. 2
figure 2

Example of likely groundwater-related wetland/seep/channel ecosystems in the Steenbras Wellfield. SBMZ (red dashed line)—Steenbras-Brandvlei Megafault Zone strand, Ss— Skurweberg Fm., Dr—Rietvlei Fm. (both Nardouw Aquifer, contact line in solid blue)

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3 Geoethical Considerations

The “Cape Town Statement on Geoethics” (Di Capua et al. 2017), prepared during the 35th International Geological Congress in Cape Town (2016), describes the purpose of geoethical thinking as: “to improve both the quality of professional work and the credibility of geoscientists, to foster excellence in geosciences, to assure sustainable benefits for communities, as well as to protect local and global environments; all with the aim of creating and maintaining the conditions for the healthy and prosperous development of future generations”.

The TMG aquifers are essential in sustaining groundwater-dependent ecosystems (GDEs; both floral and faunal components) associated with the CFK (see Fig. 2), through spring/seep discharge of high quality, low nutrient, acidic groundwater along geological (specifically aquifer-aquitard) contacts and structural features (e.g., faults, fractures and dolerite dykes), and the provision of mountain stream baseflow (Colvin et al. 2009). The understanding of any potential changes in groundwater quality and quantity within the TMG aquifers as a result of bulk groundwater abstraction, through monitoring of GDEs and monitoring-modeling-management of surface–groundwater interactions, is a critical geoethical approach in determining the long-term sustainable viability of the groundwater resource (while still supplying water to the city).

Planned bulk TMG groundwater abstraction has raised both valid concerns and perceived claims by numerous botanists, ecologists, and environmentalists, especially in the Western Cape where there is a strong (and sometimes over-zealous) ecological conservation culture and green environmental lobby. There is valid concern in protecting the CFK, which is a global biodiversity hotspot with exceptional endemic diversity (~3% of all species on Earth, covering only 0.06% of Earth’s land surface) (Myers et al. 2000), but also a global extinction hotspot—37 seed-bearing plants (13 within CCT municipal boundaries; Rebelo et al. 2011) in the CFK have gone extinct since 1900 (2nd most to only Hawaii with 79 extinctions) (Humphreys et al. 2019), with ~13% of all threatened plant species on the planet occurring within the CFK (with an additional ~1500 species considered rare or critically rare). Main ecological concerns identified by botanists/ecologists include:

  • direct habitat and biodiversity loss due to drilling/wellfield construction;

  • indirect impacts such as changes in hydrology and fire regimes due to wellfield infrastructure, which can potentially lead to increased alien species invasion potential, and soil erosion (depending on the terrane);

  • impacts of bulk abstraction on ecosystems (incorporating both fauna and flora) that are directly (GDEs) and indirectly dependent on groundwater.

The CCT has taken a “no-regrets” (and in essence geoethical) approach in the development of TMG wellfield schemes to reduce the risk of any negative ecological and environmental impacts (sometimes at additional excessive costs to reduce perceived ecological fears), while still enhancing the drought resilience of the city, providing water for future urban growth, and meeting SDGs 6 and 11:

  • restricting drilling target sites and wellfield infrastructure to existing access roads/tracks/firebreaks to reduce direct habitat and biodiversity loss;

  • if natural area clearing is required for drill rig setup or other wellfield infrastructure, then search-and-rescue of endangered flora species (stored in a temporary on-site botanical nursery) and topsoil removal/storage is undertaken for subsequent site rehabilitation to the prior natural state;

  • containment of all groundwater expelled during the drilling process (which can be very large volumes, with some boreholes having blowyields of ~100–300 l/s), as a sometimes over-cautious approach to ensure no groundwater and biodegradable drilling foam enters the receiving environment (even in plantation forested areas earmarked for rehabilitation);

  • transfer of all groundwater expelled during test-pumping to Steenbras Dam via temporary PVC pipelines, to ensure the characteristic of streams and drainage lines are not altered;

  • field and remotely sensed mapping/identification and monitoring (see Figs. 1 and 2) of GDEs relating to the TMG aquifers;

  • detailed geological/hydrogeological mapping (especially aquifer-aquitard contacts and structural features associated with GDES) and numerical groundwater modeling, to determine potential groundwater abstraction-related impacts to at-risk GDEs;

  • development of detailed hydrogeological, hydrological and ecological monitoring protocols for wellfield operation, in line with (and in addition to) the water use license requirements detailed by government regulators;

  • establishment of an aquifer monitoring committee and ecological focus groups with relevant stakeholders, in order to present monitoring results, educate on earth/groundwater science principles, capture/develop relevant hydrogeological-ecological concerns/solutions, and holistically manage the various planned TMG wellfield schemes.

This “no-regrets” geoethical approach should be applied (within reason) across future TMG wellfield scheme developments in the Western and Eastern Cape of South Africa.