Deep Artesian Groundwater for Oudtshoorn Municipal Supply - Phase D - Target Generation & Borehole/Wellfield Siting usin Structural Geology and Geophysical Methods

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:

PHASE A — Inception Review and Data Scoping
PHASE B — Regional Hydrogeological Survey
PHASE C — Financial and Business Planning
PHASE D — Target Generation and BoreholelWellfield siting
PHASE E — Exploration Drilling and Resource Assessment
PHASE F — WelIfield Establishment and Licensing

Among the overall DAGEOS key deliverables are:

Location of exploration borehole targets and well-fields;
Quantification of potential groundwater yield;
Determination of access and distribution costs of groundwater;
Assessment of possible environmental impacts;
Assessment of legal implications;
Assessment of cost/benefit and risks;
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 [Mm³ 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:

Scientific pump-testing of an experimental deep well to determine aquifer parameters and prove a sustainable and environmentally acceptable resource
Development and implementation of modern methods of technical and financial risk assessment for a model deep groundwater project (Task C5 of the overall DAGEOS study)

A number of reports are available from this study and document the progress of this study. These are:

The Inception Report (Phase A) of the DAGEOS Project (Umvoto, 2000) developed the hydrogeological understanding and phased approach that forms the basis of the present work
The Regional Hydrogeological Reconnaissance Report (Phase B) of the DAGEOS Project (Umvoto, 2003) described the methodology and scientific approach for groundwater exploration in the regional context.
Phase C of the study involved ongoing liaison with key stakeholders in the area and negotiation with Provincial Government and municipal officials as regards budget planning and management in response to changes in regulatory processes, approaches and input. The record is outlined in the project correspondence, project minutes, summary progress memos as well as in this report

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:

Digital GIS compilations of diverse data sets, and their processed derivatives provide a sound basis for quantitative hydrogeological mapping and groundwater exploration in the fractured rock terrain of this arid to semi-arid region.
The fractured-rock aquifers of the TMG have a definite hydrostratigraphy, consisting of a major Peninsula Aquifer overlain by a subordinate Nardouw Aquifer. The latter is in turn divided into two sub-aquifers, namely the lower and more substantial Skurweberg Subaquifer and the top, relatively thin Kareedouw Subaquifer
within the four target zones (A, B, C, D) previously defined during Phase B of the DAGEOS Project, sixteen (16) potential exploration target sites were generated by remote-sensing analysis and reconnaissance fieldwork.
Target Zones C and D to the south of Oudtshoorn have three of the four highest priority sites because the Outeniqua compartments of the TMG aquifers are most extensive in the subsurface around these zones and have the best potential for sustainable recharge from the higher rainfall areas along the Outeniqua mountain range.
Detailed structural and borehole-siting studies within the C Zone are complemented by exploration drilling, in order to obtain a firm data on aquifer/aquitard thicknesses and aquifer structural properties at depth immediately beneath the target site. This initial exploratory drilling was undertaken by the Department of Water Affairs, with a view to maintaining the exploration boreholes as permanent monitoring sites for both the deep confined aquifers and the shallow stream-related groundwaters.
At an analysis scale of 1:50 000, the WNW/ESE joint direction appears as the most prominently developed set. There is km-scale length (and by inference also depth) persistence of individual master-joint arrays in this set, particularly along a trend from the summit of the Outeniqua Range through the Mount Hope region and further west.

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;

Preliminary recharge estimations, concentrated only on the exposed outcrop areas of the Peninsula and Nardouw aquifers, indicate that recharge to the Peninsula Aquifer over the whole DAGEOS area ranges between 68 million cubic metres per year (in a “worst case”) and 123 Mm³/yr in a “best case” scenario. Recharge to the Skurweberg Aquifer ranges between 41 and 52 Mm³/yr.

Monitoring:

A gravity-based method for groundwater resource assessment monitoring has the advantage that precisely levelled (±1 cm) benchmarks for regular, repeat gravity observation (± 5 microGal) substitute for many expensive boreholes. The gravity monitoring technique is therefore relatively non-invasive, which is important from an environmental-impact perspective. The method also provides for better and more flexible geographic coverage of the monitoring system in heterogeneous, anisotropic fractured-rock aquifers. Fewer calibration wells record water-level at gravity benchmarks, which can be tied to a gravity reference station for continuous monitoring (30 mm interval), where water level(s), soil moisture and weather (temperature, pressure, rainfall, etc.), are also recorded;

Risk Management:

The technical, financial and environmental risks associated with bulk abstraction of groundwater for projects such as DAGEOS were identified by means of a risk register. Specifically, the preliminary risk ranking exercise has identified the following as the issues or threats of highest risk:
- The aquifer recharge and flow rates are found to be lower than predicted, due to differing hydraulic & hydrologic pro panties.
- Deliberate sabotage of abstraction and/or supply infrastructure may lead to temporary disruption of water supplies, and necessitate expensive alternative emergency supply sources and repair of damaged eguipment and/or infrastructure. This is a generic issue that does not specifically pertain to groundwater development.
- Future water demand may be significantly higher than current forecasts. This may imply future water shortages, and/or concomitant implementation of other (higher cost) water supply options. This is a generic issue that does not specifically pertain to groundwater development.
- Possible contamination or pollution of water in aguifer, making the water temporarily or permanently unfit for human use and possibly resulting in environmental damage.

RECOMMENDATIONS

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

Hydrogeological Study:

A hydrocensus survey that includes measurement of flow rates and hydrochemical sampling should be undertaken in order to establish recharge/discharge patterns of the Peninsula and Skurweberg aquifers in particular target areas, especially around in parts of Target Zone D (Waboomskraal and Herold) where groundwater exploitation constitutes a larger proportion of water usage than previously estimated;
A basin scale groundwater recharge and discharge study that integrates the surface groundwater interaction and measurement of the time lag between the two as it varies spatially and temporally should be initiated.
It is strongly suggested that the opportunity presented by the ongoing drilling be utilised to the fullest to obtain the following data to address the remaining uncertainties:
- Porosity and compressibility from core samples, taken from different horizons within the targeted aquifer to update the storage model;
- Long-term water level and climate data from the Peninsula Aquifer as data input for the SVF I CRD model;
- Isotope analysis of samples taken from the Peninsula Aquifer, selected springs and rainfall collectors to be installed, to calibrate the recharge model
- Macro and trace element analysis from water samples taken from the Peninsula Aquifer during or after drilling completed.

Monitoring

A GIS based water resource and planning database, based on the DAGEOS inputs, should be established by the CMA in order to support Integrated Water Resource Management (IWRM) in the wider Klein Karoo area. Without such a systematic data base any monitoring results would be of limited use;
The requirement for a regional groundwater monitoring network, supported by a formal monitoring protocol, remains urgent, as it will significantly impact on the licensing procedure, promote support for exploration and testing among the stakeholder communities, and possibly shorten the time required to gain acceptance of future groundwater exploration, development and management;
The area of most immediate concern to the monitoring proposal is the upgradient area around Waboomskraal, which is close to or within the recharge zone of the Peninsula Aquifer. Potential effects could be observed through conventional water-level monitoring in shallow boreholes, provided that corrections for natural recharge and for local groundwater abstraction can be applied to the data, which in turn presupposes a substantial period of baseline monitoring during which the necessary correction factors can be derived.
The baseline monitoring network in the D Target Zone must be extended throughout the 0 Target Zone (Waboomskraal area). The effects of groundwater recharge originating in the Outeniqua Mountains, and the effects of groundwater abstraction in the DAGEOS area, will have more rapid and greater impacts on the monitoring record in the D Target Zone because of its higher altitude and closer proximity to the recharge zones, than monitoring stations downgradient in the system;

Because of potential concern about long-term impacts of sustained groundwater abstraction from the Peninsula Aquifer on the deep, down-gradient flowpaths to discrete discharge sites, such as the Calilzdorp hot spring, extension of the detailed fracture mapping to the area between Mount Hope and the Gamka mountain range is advisable. There is a small probability that future abstraction in the C Target Zone may reverse the hydraulic gradient between the Outeniqua range and the Calitzdorp spring site along the Mount Hope neotectonic fracture array, and thus affect rates of outflow at the spring. The potential fracture connections between the north-western end of the Mount Hope array and C Zone monitoring sites therefore require more detailed mapping.

Provided that the groundwater exploration of the Peninsula Aquifer in the C Target Zone is successful, the drilling and equipping of one deep borehole, to access and monitor both the Skurweberg Subaquifer and the Peninsula Aquifer, is recommended;
There is a definite future role for modern methods of microgravity monitoring and centimetre-precision GPS levelling, in order to detect the development and lateral propagation of small density changes and associated land-surface subsidence associated with pressure (potentiometric surface) drawdown in the deeper confined aquifers. The incremental development of a gravity-GPS web for groundwater resource assessment and monitoring in deep fractured-rock aquifers could capitalize effectively on the concurrent progress of the GRACE mission during its 2002-2007 duration, on the associated SAGOS SG facility that forms a crucial validation pillar for the GRACE mission.
Vegetation change vector analysis (CVA) should be undertaken within the framework of the geohydrological regional flow model in order to assess the preliminary potential aquatic and terrestrial ecological impacts of groundwater abstraction from either the Peninsula or the Skurweburg aquifers. Groundtruthing of the findings, calibration of the model and a reconnaissance would be required.

Exploration, Development and Management

A formal and detailed risk management plan needs to be developed for the exploration and potential development and management of the TMG Aquifers in this area. Such a plan should formalise:

Iterative scenario analysis using the probabilistic risk assessment model
Regular and structured updating of the risk register
Allocation of risk and resource management and mitigation responsibilities.
The monitoring, reporting and review cycle.

DWAF should initiate the development of a regional Aquifer Protection Strategy as soon as possible.

Education and Training

A Groundwater Information and Education Programme be closely integrated with the development of Water User Associations in the area;
The present study domain is appropriate to target selection for the Dageos Project but must be expanded in context of legal and institutional developments. It is therefore recommended that the study domain be broadened to include the newly demarcated regional council.