RECENT DEVELOPMENTS IN THE UNDERSTANDING OF KAROO AQUIFERS AND THE DEEPER UNDERLYING FORMATIONS

Peter Rosewarne1, Alan Woodford2, Millie Goes1, Siep Talma3, Richard O’Brien1, Gideon Tredoux4, Chris Esterhuyse1, Des Visser1 and Gerrit van Tonder5

1 SRK Consulting, Cape Town; prosewarne@srk.co.za; mgoes@srk.co.za; robrien@srk.co.za; cesterhuyse@srk.co.za; dvisser@srk.co.za

2 RPS Aquaterra, Perth, Australia; Alan.Woodford@rpsgroup.com.au

3 Consultant, Pretoria; siep.talma@gmail.com

4 Consultant, Cape Town; gideon.tredoux@gmail.com

5 Consultant, Bloemfontein; vtonderg@ufs.ac.za

ABSTRACT

The possibility of large reserves of shale gas underlying the Karoo and their exploitation has focused attention on the groundwater resources and aquifers of this region. Much is known about the relatively shallow aquifers (<300 m) which supply many local municipalities and farmers with water for domestic, stock and irrigation use (mostly from boreholes <150 m in depth). Conversely, little is known about the deeper formations (>500 m) and associated groundwater occurrences and their possible interconnection to the shallow aquifer systems. This paper covers a desk study of the southern Karoo, mostly above the Great Escarpment, carried out by a group of hydrogeologists/geochemists with a cumulative experience of about 250 years. The main consideration at this stage has been the collation and analysis of existing information, using GIS, conceptualisations, and identification of knowledge gaps. A Karoo Groundwater Atlas was published in 2012 and Volume 2 has been released at this conference, which present a summary of the main analyses. Borehole yield, recharge and quality in the shallow aquifer tend to improve, and water levels become shallower, from west to east, associated with higher rainfall and increased percentage of dolerite & sandstone. Aquifer yield, quality, lithology and presence of dolerites constitute 60 per cent of a groundwater attributes ratings classification and 54 percent of the study area has a high rating. Twenty four per cent of the study area has a high vulnerability rating. Knowledge of the characteristics of groundwater associated with the deeper formations is restricted to a few thermal springs and sparse data from some deep hydrocarbon exploration wells. Weak artesian flows were recorded from two such wells in the Dwyka Group below the Great Escarpment, with Total Dissolved Solids of up to 10 000 mg/L and temperature of up to 77oC, from depths of 2 347 to 3 100 m. Further work and cooperation with other researchers/institutions is ongoing and planned to fill in knowledge gaps and assess the risks to groundwater of shale gas exploration.

1. BACKGROUND

In September 2011 a group of groundwater experts was formed (initially known as the Water Expert Group or WEG) in a Shell-funded project to establish the current understanding of Karoo hydrogeology and its application to specific areas in the western, central and eastern Karoo. This was to assist with the assessment of risks and mitigating measures related to possible shale gas development activities. Figure 1 shows the location of the study area and other key regional features. The current group (Karoo Groundwater Expert Group or KGEG) is now comprised of the authors listed above. Other members will be co-opted as required. Between them this group represents a cumulative experience of about 250 years, much of it related to Karoo Aquifers.

The main objectives of this project are to:

• Identify and manage the potential risks to groundwater from shale gas exploration; • Use existing Karoo data beneficially; • Develop new approaches to data assessment and interpretation , e.g. through formation of

the KGEG; • Identify gaps in knowledge and plan and carry out future work to address such gaps.

A secondary objective of the project is to promote a broader participation and mentorship of students and recent graduates in research work, plus the dissemination of progress and results. This in order to inspire and assist with training of a new generation of hydrogeologists.

The motivations for the project are:

• Reliance of local communities on groundwater; • Presence of major and sole source aquifers; • In the light of the above, develop an understanding of aquifer systems to inform any

local use of groundwater; • Need to establish a comprehensive/versatile database; • Need for defensible strategy for gas exploration, regulatory compliance and management

of risks.

Investigation Approach

A series of workshops were held to debate current hydrogeological knowledge, agree on analytical methods for existing data and develop provisional approaches to inter alia a risk- based approach, monitoring and geochemical indicators. A Karoo Groundwater Atlas (KGA) (WEG, 2012) was produced and released for general circulation in early 2012 (WEG, 2012) and also via a web link (http://s03.static-shell.com/content/dam/shell/static/zaf/downloads/media/karoo-groundwateratlas.pdf). This atlas presented a hierarchy of features of importance in the southern Karoo Aquifers . This information has been visualized through a series of Geographical Information System (GIS)-based groundwater attributes maps depicting these features. Volume 2 of the KGA has been released at this conference and contains groundwater vulnerability maps, 2D and 3D conceptual models and some long-term hydrographs of Department of Water Affairs (DWA)-monitored boreholes.

Sources of Information

The main sources of information are:

• DWA National Groundwater Archive and monitoring borehole data; • DWA Groundwater Resource Assessment Phase 2 Project (DWA, 2005); • Geological logs of Soekor deep core exploration wells; • Water Research Commission (Woodford and Chevallier, 2002, Steyl et al, 2012), DWA,

Government task team (Department of Mineral Resources, 2012), consultants and Environmental Management Plan (Golder Associates, 2011) reports;

• Council for Geoscience 1:250 000 scale geological maps.

Some related topics are covered in more detail in sister papers at this conference by Goes et al (GIS applications), O’Brien et al (geochemistry) and Talma and Esterhuyse (methane gas occurrence).

2. GROUNDWATER MAPPING

Two types of groundwater maps have been developed, namely Attributes and Vulnerability. Attributes are an aquifer classification methodology developed by the KGEG while Vulnerability assessment uses the well-known DRASTIC (see explanation on next page) methodology (Aller et al, 1987). These maps will be used as tools to assist in siting exploration gas wells within the areas of interest. Where possible areas of relatively lower aquifer potential and low hydraulic conductivity will be targeted. Then significant buffer zones from known pathways and potential receptors will be added.

Attribute Maps Ten key attributes were agreed by the KGEG to be the most significant in contributing to groundwater/aquifer characteristics in the Karoo, as shown in Table 1.

Table 1. Key Groundwater Attributes and Ranking

1 Aquifer yield 6 Folded lithologies

2 Depth to water table 7 Lithology/Group

3 Quality 8 Depth to main water strike

4 Dolerite intrusions 9 Soil type

5 Faults 10 Depth of weathering

These were ranked and weighted and the first four criteria accounted for 60 per cent of the median weighting. A series of 10 maps has been produced plus a composite attributes maps, covering the study area shown in Figure 1. A ranking system of low, medium and high was applied. Structures, such as faults and fold axes, are not ranked highly in this paper because formations above the Great Escarpment are mostly horizontal to very gently dipping. Small-scale faulting has mostly been infilled by dolerite. The picture may be very different further to the south closer to the Cape Fold Belt and south of the ‘dolerite line’. Attributes ratings generally increase from west to east. This is as a consequence of dolerite intrusions becoming more common, water levels getting shallower, recharge increasing and groundwater quality improving in this direction (and from south to north in the eastern areas).

Vulnerability Maps Groundwater vulnerability is defined (Aller et al, 1987) as the tendency or likelihood for contaminants to reach a specified position in the groundwater system after introduction at some location above the uppermost aquifer. The general concept is based on the assumption that the physical environment can provide groundwater with some degree of protection against natural impacts, especially with regard to contaminants entering the subsurface environment. Differences in the physical environment from location to location therefore make some land areas more vulnerable to groundwater contamination than others. The DRASTIC model uses seven environmental parameters, namely Depth to water, net Recharge, Aquifer media, Soil media, Topography, Impact of vadose zone, and hydraulic Conductivity in order to characterise the hydrogeological setting and evaluate aquifer vulnerability.

The approximate percentage coverage of the different categories of attributes and vulnerability in the study area are shown in Table 3.

Table 3. Approximate Percentage Coverage of Categories of Attributes and Vulnerability

Category Attributes (%) Vulnerability (%) Very Low/Low 39 55 Medium 7 21 High/Very High 54 24

2. KAROO AQUIFER SYSTEM

The upper shallow aquifer system (c.300 m) is relatively well understood based on decades of detailed research and groundwater exploration, much of it by members of the KGEG, related to municipal and other use and monitoring. Some broad trends identified from this research and the KGA work are summarized below.

General geology: Geological strata in the Karoo are generally horizontal to very gently dipping, except adjacent and sub-adjacent to the Cape Fold Belt. They comprise of a thick sequence of multiple layers of shale and mudstone, with subordinate sandstones, and dolerite sills of variable thickness (usually tens of metres but up to 100-200 m).

Thick dykes and their contact zones extend for up to tens, even hundreds of kilometres (E-W) on surface but their vertical continuation is uncertain. It remains to be proven to what depths single dykes and their contact zones continue uninterrupted. Regional east-west trending ‘shear’ dykes as well as a number of regional north-west trending ‘feeder’ dykes, e.g. the Middelburg Dyke, probably extend to the base of the Karoo sediments. The thickest sills are associated with the Ecca Group, while nested, saucer-shaped ring structures occur in the Beaufort Group, with individual sill thicknesses of c.100 m (Svensen et al, 2012)

Transmissivity: This will vary along dyke contact zones, with depth and along strike, dependent on host rock, stress regime, mode of intrusion and subsequent weathering. It is unlikely that transmissivity will be constant along such zones given the multiple layers of alternating mudstone, siltstone, shale and sandstone that make up the Karoo sedimentary sequence. The absence of thermal springs in the study area with temperatures >25oC suggests that interconnecting transmissive zones from depths of >1 000 m to surface are rare. However, under sub-artesian conditions, groundwater would not reach the surface and pressure could also dissipate within the shallow aquifer.

Hydrogeochemistry: Baseline information on groundwater levels and hydrochemistry has been obtained from the numerous DWA monitoring boreholes, the locations of which are shown on Figure 1. Monitoring records for these boreholes go back to 1994, with biannual chemical analyses covering macro-chemistry, trace elements and isotopes. Statistical analysis of these data is being carried out to establish any spatial, seasonal and/or long-term trends. This will assist in determining baseline conditions and assessing any future trends if shale gas exploration takes place.

Gas: The occurrence of methane gas in existing supply boreholes and thermal springs in the Karoo is a relatively common phenomenon. Available isotope data indicate a thermogenic (deep) as well as biogenic origin for the methane (Talma and Esterhuyse, 2013).

Consideration Zones: Some point or linear features, such as boreholes and dykes, respectively, have potential ‘capture zones’ (boreholes) or preferential pathway zones (dykes) for groundwater flow. The application of a surrounding ‘consideration zone’ to take into account the areas of influence beyond mapped coordinates or boundaries of the feature is therefore proposed. Within these consideration zones, some particularly sensitive areas may be identified as exclusion zones where no gas wells should be sited. As gas exploration progresses and additional geological and hydrogeological data are gathered, understanding of areas of influence will improve and consideration/exclusion zones may change.

Preliminary features and associated consideration zones (3D) currently proposed for initial exploration drilling are shown in Table 2.

Table 2. Proposed Consideration Zones

Dolerite intrusions 250 m Thermal springs 1 000 m

Faults/fold axes 250 m Hydrothermal plugs 100 m

Production boreholes 1 000 m Kimberlites 100 m

3. DEEPER FORMATIONS

Some conceptualisations on the deeper formations are shown in Figure 2, a schematic N-S cross- section through the study area, and Figure 3, a preliminary 3D conceptualisation with attributes and vulnerability overlain. Some aspects are summarised below while others are annotated on the figures.

Dolerite: The exact configuration of dykes and sills with depth is unknown at this stage. Generalisations based on the literature (e.g. Galerne et al, 2011 and Svensen et al, 2012) and maps have therefore been used to adjust thicknesses and continuity with depth, lateral extent and geological formation. In general, dolerite content decreases to the west and south and the contact zones of the Dwyka-Ecca groups, Prince Albert-Whitehill formations and Upper Ecca-Lower Beaufort groups are preferential zones for sill emplacement. It is postulated (Woodford and Chevallier, 2002) that intrusion of the dolerite magma was a single event in geological terms (c.3 to 4 million years), with a source area to the east. It is therefore unlikely that cross-cutting contact fracture zones of dykes through sills are common but they do occur.

The ratio of dolerite to sediment decreases to the west and south, away from the main postulated source of magma to the east. Some sills are up to c.200 m thick but their continuity is difficult to predict. They quite often truncate against/or step up or down along regional dykes and ring structures.

Groundwater occurrence: Groundwater is unlikely to occur in a continuous aquiferous zone from the <300 m shallow aquifer zone to the deep formations where thermal (defined as >25oC), brackish to saline, very old water occurs under pressure. The shallow aquifer may be separated from this deeper groundwater (Dwyka Group and below) by zones of effectively impermeable rocks, possibly hundreds to thousands of metres thick, varying from area to area. The existence of hitherto undetected deep (>1 000 m) aquifers is an intriguing possibility that may be resolved by exploration drilling. A more likely scenario is the presence of aquifers in the largely unexplored >300-<1 000 m zone, where active groundwater circulation may still be taking place.

A key unknown is hydrostatic pressure heads in the Ecca Group, with numerous dykes and sills likely forming compartments and increasing the complexity of the system.

Deep (up to 4 692 m) exploration drilling in and around the study area by Soekor in the 1960s/’70s indicated isolated occurrences of deep, saline groundwater in the Karoo formations and fresher groundwater from the underlying Witteberg Group. Artesian flow was only noted from some wells below the Great Escarpment, after penetrating the Dwyka Group rocks (Woodford and Chevallier, 2002). Data on water ingress, drilling mud losses and fractures are only recorded in some wells, presumably because these are the only wells where they occurred.

It is not known with any certainty what the organic shale layer in the Ecca Group might yield. However, due to the potentially low hydraulic conductivity and age of this aquitard it is probable that the water is saline.

Hydrogeochemistry: Groundwater salinity generally increases with depth due to a combination of gravity, longer residence time due to longer flow paths, lower T (both of which promote interaction with and dissolution of minerals in the host rocks), and decreasing contributions from rainfall recharge. There are only two known hot springs in the Karoo Basin which are estimated to originate >1 000 m below surface (based on their water temperature). The water from these springs is not saline, the Aliwal North spring having Total Dissolved Solids (TDS) of c.1 200 mg/L and the Fort Beaufort spring a TDS of 520 mg/L (Kent, 1949).

A few TDS measurements were taken from weakly artesian flow emanating from water strikes at various depths (from the Dwyka Group, below the Great Escarpment) in two Soekor wells (Woodford and Chevallier, 2002) giving the following results:

• 1 006 m (KL1/65): TDS 1 390 mg/L;

• 2 975 m (SA1/66): TDS 6 460 mg/L (65.5oC);

• 2 347+3 184 m (KL1/65): TDS 10 010 mg/L (76.7oC).

The apparently layered and progressive increase in TDS could be an indication that mixing of deep and shallow groundwater is not occurring rapidly or on a significant scale, but there are too few data at this stage to draw conclusions. It could also be that this situation only occurs below the Great Escarpment where relatively more dynamic gravity-driven deep groundwater circulation is taking place. To the north of the escarpment, groundwater systems may be more ‘stagnant’ and hence more saline, i.e. ‘flushing’ not taking place and isolated from the active circulation zone of the upper c.1 000 m, as indicated by thermal springs. The possibility of upward movement of deep groundwater into the shallow aquifer is being investigated by a WRC-sponsored research team.

5. CONCLUSIONS

Significant insights into Karoo hydrogeology have been gained through new analysis of existing public-domain information. These insights have helped identify data/knowledge gaps and develop generic target areas for further consideration prior to progressing to shale gas exploration.

The key attributes contributing to shallow aquifer characteristics are considered to be yield, depth to water table, quality and dolerite intrusions. Attributes ratings increase from west to east due to increasing rainfall and increasing occurrence of sandstone and dolerite. Approximately 55 per cent of the study area is covered by areas with a high attributes rating and 24 per cent with a high vulnerability rating These attributes/ratings and consideration zones around key features, such as boreholes, dolerite intrusions and faults, are being taken into account during preliminary gas well siting (desk top).

Initial indications are that hot, saline artesian groundwater can occur from the Dwyka Group below the Great Escarpment. Groundwater seems to show a progressive increase in TDS with depth, up to c.10 000 mg/L at c.3 000 m.

Key knowledge gaps are considered to be:

• Hydraulic parameters and hydrogeochemistry of the deep formations;

• The presence or otherwise of deep aquifers;

• Hydrostatic pressures in the Ecca Group rocks;

• The compartmentalizing effects of sills and dykes at depth;

• Hydraulic interconnectivity between the shallow aquifer and the deeper formations and whether there is existing upward migration of deep groundwater.

Appropriate investigation of suitable Soekor wells could shed light on many of the above issues.

The information obtained and analysed so far will be used to inform follow-up desk study and field investigations to provide guidance to identify and manage the risks to groundwater resources from shale gas exploration. Cooperation and sharing of results with other researchers and liaison with organisations such as the WRC, and the regulator(s), will form part of this ongoing work. The participation and mentorship of students and recent graduates in research work will be encouraged in order to inspire and assist with the training of a new generation of hydrogeologists.

5 REFERENCES

Aller, L. Bennet, T, Lehr, J.H. Petty, R.J. and Hackett, G. (1987), “DRASTIC: A Standardized System for Evaluating Ground Water Pollution Potential Using Hydrogeologic Settings.” U.S. Environmental Protection Agency, Ada, OK, EPA/600/2-87-036, 455P.

Department of Mineral Resources. (2012), “Report on Investigation of Hydraulic Fracturing in the Karoo Basin of South Africa.” Pretoria.

Department of Water Affairs and Forestry. (2005), “Groundwater Resource Assessment Phase 2.” Pretoria.

Galerne, C.Y. Galland, O. Neumann, E-R. and Planke, S. (2011), “3D Relationships Between Sills and Their Feeders: Evidence from the Golden Valley Sill Complex (Karoo Basin) and Experimental Modelling.” Journal of Volcanology and Geothermal Research 202 (2011) 189-199. Elsevier.

Golder Associates. (2011), “Environmental Management Plan. South Western Karoo Basin Exploration Application.” Final Report. Johannesburg.

Karoo Groundwater Expert Group (2013), “Karoo Groundwater Atlas.” Vol. 2. Cape Town.

Kent, L.E. (1949), “The Thermal Waters of the Union of South Africa and South West Africa.” Trans. Geol. Soc. S. Afr., 51.

Steyl, G. van Tonder, G. and Chevallier, L. (2012), “State of the Art: Fracking in South-Africa and the Impact on Water Resources.” WRC Report No. KV 294/11. Pretoria.

Svensen, H. Corfu, F. Polteau, S. Hammer, Ø. and Planke, S. (2012), “Rapid Magma Emplacement in the Karoo Large Igneous Province.” Earth and Planetary Science Letters 325-326 (2012) 1-9. Elsevier.

Water Expert Group (2012), “Karoo Groundwater Atlas.” Cape Town. http://s05.static-shell.com/content/dam/shell/static/zaf/downloads/media/karoo-groundwateratlas.pdf

Woodford, A.C. and Chevallier, L. (Editors) (2002), “Hydrogeology of the Main Karoo Basin: Current Knowledge and Future Research Needs.” WRC Report No. TT 179/02. Pretoria

ACKNOWLEDGEMENTS

Financial support from Shell for the project is gratefully acknowledged as is the use of archive/published information from the DWA, WRC and CGS.