Deep Artesian Groundwater for Oudtshoorn Municipal Supply - Phase D - Target Generation & Borehole/Wellfield Siting usin Structural Geology and Geophysical Methods
Report No. 1254/1/05
September 2005

EXECUTIVE SUMMARY

BACKGROUND AND SCOPE OF WORK

The general background, concept, scope, project structure, and purposes of the Deep Artesian Groundwater Exploration for Oudtshoorn Supply (DAGEOS) Project have been given in an earlier report (Umvoto, 2000). (See Frontispiece and Figure 3.1 for Locality map of Study domain).

The DAGEOS Project follows on from previous investigations of deep groundwater potential in the Oudtshoorn area by the Municipality, The Department of Water Affairs and Forestry (DWAF) and the Council for Scientific and Industrial Research (CSIR) in the greater Oudtshoorn area.

The purpose of the DAGEOS study itself is to establish the feasibility of augmenting the water supply to the town of Oudtshoorn situated in the Gouritz Water Management Area (WMA) in the Western Cape South Africa and or to contribute to a conjunctive surface- and groundwater augmentation scheme.

The DAGEOS Project is divided into six phases, some of which are overlapping in time:
Among the overall DAGEOS key deliverables are:
  1. Location of exploration borehole targets and well-fields;
  2. Quantification of potential groundwater yield;
  3. Determination of access and distribution costs of groundwater;
  4. Assessment of possible environmental impacts;
  5. Assessment of legal implications;
  6. Assessment of cost/benefit and risks;
  7. Implementation of supply scheme(s). 
The Water Research Commission (WRC) funded project K5/1 254 is Phase D of the overall DAGEOS study and focuses mainly on the target generation and borehole I wellfield siting. Within this broader context the objectives of Phase D are defined in the Terms of Reference as:

“To explore and quantify the deep artesian groundwater resource potential in the confined fractured-rock aquifers of the Table Mountain Group (TMG) within a water-stressed catchment through the integrated application and further enhancement of structural-geological and remote-sensing/geophysical methods”;

“To develop the technical capacity for drilling deep (>300 m) wells at selected sites of potentially high water resource yield (>35 I/s or > 1 million cubic metres [Mm│ per year)”;
To complete and pump-test at optimum yield an experimental deep groundwater well at one or more target sites for the purpose of constraining the aquifer parameters (e.g., permeability, storativity) and proving a sustainable and environmentally acceptable augmentation of the Oudtshoorn municipal water supply”.

Phase D is a combination of desk-top study and reconnaissance, with anticipated exploration fieldwork. It aims at a regional integration and, where required, reinterpretation of the existing database into a comprehensive groundwater systems analysis. In addition to the objectives outlined above, the additional aims of the WRC project are defined as:
A number of reports are available from this study and document the progress of this study. These are:
This report summarises the results of the WRC funded Phase D of the DAGEOS Project and makes recommendations applicable to Phases E and F of the project as well as to the current WRC, DWAF and municipal funded initiatives in TMG and other fractured rock terrain.

WORK UNDERTAKEN

The results of Phase 0 of the Dageos Study are dependent upon the data acquisition, processing and analysis undertaken in Phase B of the project. The raw datasets are owned by the Municipality of Oudtshoorn. A summary of the data processing and interpretation undertaken in Phase B is presented in Chapter 2. The relevant data sets are: topography, geology, remote-sensing (aerial photography and satellite imagery) and hydroclimatic data (mean annual precipitation) and processed derivatives thereof.

Chapter 3 reviews the geological data from a hydrostratigraphic and structural perspective, and develops the basis for a tectonic and hydromechanical approach to groundwater exploration and development. To define the target sites for deep drilling, the structural geology and geometry of the large-scale fracturing is analyzed from satellite images and aerial photographs at different scales, so as to determine its dependence on scale of observation and rock type.

Quantitative data gathered through the 20 GIS-based mapping is augmented by outcrop-based studies on the 3D orientation, properties (aperture, mineral fill) of fracture sets, to determine the relative importance of particular sets for deep groundwater flow. The network properties of the fracture systems, particularly density and connectivity, are a focus of the effort to generate a set of deep groundwater targets for subsequent exploration drilling. Within two Target Zones (C and D) it summarizes the results of this approach at selected target sites, and makes recommendations for further development and application of the method.

Beyond the purposes of borehole siting, the fracture-network data will later serve as input to models of aquifer hydraulic behaviour or hydromechanics in the target-site areas, and for the understanding of deep groundwater movement along preferred flow paths (‘hydrotects”) on the routes between recharge and discharge zones.

The TMG aquifers represent a challenging case because of their fractured-rock nature and large spatial scale. The need for a quantitative hydromechanical approach to aquifer characterization is imperative as the foundation of TMG groundwater resource assessment.

Chapter 4 presents the final hydrogeological analysis dealing with aspects of storage, recharge and hydrochemistry. Relevant aquifer parameters for the fractured rock aquifer are based on both documented data as well as geological inference. Several GIS based methods for aquifer volume calculation and recharge estimation were developed. The recharge model is based on existing maps of rainfall distribution, lithology and catchment boundaries. To distinguish the recharge per aquifer unit, the exposed outcrop areas of the different formations were calculated from a common GIS overlay of the digital geological map and digital map of quaternary sub-catchments with area polygons of 1MG units differentiated for each sub-catchment.

The results of the models are presented in the context of Adaptive Management outlining the necessity for ongoing management and monitoring in order to refine resource evaluation figures and aquifer management strategy. An adaptive approach to groundwater management necessarily requires appropriate analytical tools or models and a comprehensive monitoring programme. What is most notable about adaptive management is its emphasis on monitoring programs and the assimilation of monitoring results into analytical models for testing a working hypothesis, or preferably multiple working hypotheses, about aquifer responses.

Chapter 5 expands on the monitoring aspect and develop a strategy for the design of land-and space-based systems for the monitoring of changes in continental water storage (surface and subsurface) and the remote-sensing of the hydromechanical structure and properties of the deep confined fractured-rock aquifer systems of the Western Cape province, with particular reference to the application of new Global Earth Observation (GEO) technologies. Its focus is the design of an experimental system for (i) monitoring deep-aquifer storage changes around Target Zones C and D, and (H) determining fundamental hydromechanical properties of the aquifer such as its bulk compressibility, by using a combination of land-based microgravity and GPS observations, complemented by satellite gravity and satellite radar

Chapter 6 describes the approach and methodology for a technical and financial risk assessment, and presents a preliminary DAGEOS Risk Register. The approach is based on international best practice, as described, inter alia, by the International Strategy for Disaster Reduction of the United Nations (UN-ISDR). Several potential risks are identified and assessed in the project Risk Register, resulting in recommendations for further emphasis in subsequent phases of the study. In order to deal with these risks a risk management framework and a financial risk model are developed.

CONCLUSION S

The main conclusions to be drawn from this study are the following:

Target Generation:
The Mount Hope neotectonic fracture array was probably the seismic source zone for the minor earthquake on 28th October 2001, and may be hydraulically connected to the source region for the Calitzdorp hot spring. Although this hot spring is located close to the TMG-Bokkeveld contact, the actual source of the deep groundwater (Peninsula or Skurweberg aquifer), or relative contributions, if it is from a mixture of aquifer sources, remains to be determined.

Regional Hydrogeology:

Preliminary calculations for the confined Peninsula Aquifers of the southern target zones indicate that, for regionally averaged drawdowns of 1 m the potential groundwater abstraction ranges between 2.9 million cubic metres for pessimistic and 16 million cubic metres for optimistic estimates of key aquifer properties. The total amount of groundwater in storage in the same area is estimated at between 1 and 23 billion (109) cubic metres;
Monitoring:
Risk Management:
RECOMMENDATIONS

In the light of the above key conclusions, the foflowing recommendations are made:

Hydrogeological Study:

Monitoring
Exploration, Development and Management
  1. Iterative scenario analysis using the probabilistic risk assessment model
  2. Regular and structured updating of the risk register
  3. Allocation of risk and resource management and mitigation responsibilities.
  4. The monitoring, reporting and review cycle.
Education and Training