WATER RESEARCH COMMISSION
ECOLOGICAL AND ENVIRONMENTAL IMPACTS OF
LARGE VOLUME GROUNDWATER ABSTRACTION IN THE
TABLE MOUNTAIN GROUP (TMG) AQUIFER SYSTEM
DISCUSSION DOCUMENT FOR SCOPING PHASE
July 2003
WRC project K5/1327
WATER RESEARCH COMMISSION
ECOLOGICAL AND ENVIRONMENTAL IMPACTS OF
LARGE VOLUME GROUNDWATER ABSTRACTION IN THE
TABLE MOUNTAIN GROUP (TMG) AQUIFER SYSTEM
DISCUSSION DOCUMENT FOR SCOPING PHASE
July 2003
WRC Project K5/1327
Authors of this report:
Cate Brown – Southern Waters
Christine Colvin – CSIR
Chris Hartnady – Umvoto Africa
Rowena Hay – Umvoto Africa
David Le Maitre – CSIR
Kornelius Riemann – Umvoto
Africa
Project Manager:
Paul Lochner - CSIR
Report published by:
CSIR-Environmentek
P O Box 320
Stellenbosch 7599
South Africa
Tel: (021) 888 2400
Fax: (021) 888 2693
Southern Waters
P O Box 13280
Mowbray 7705
South Africa
Tel: (021) 685 4166
Fax: (021) 685 4630
Umvoto Africa (Pty) Ltd
P O Box 61
Muizenberg 7950
South Africa
Tel: (021) 788 8031
Fax: (021) 788 6742
This report is to be cited as:
Water Research Commission (WRC), 2003. Ecological and environmental impacts of large volume
groundwater abstraction in the Table Mountain Group (TMG) aquifer system: Discussion Document for
Scoping Phase. WRC Project K5/1327. Published by CSIR-Environmentek, Southern Waters and
Umvoto Africa. CSIR Report Number: ENV-S-C 2003-076, Stellenbosch.
CSIR Report Number: ENV-S-C 2003-076
CONTENTS
1.
PURPOSE OF THIS DOCUMENT __________________________________ 1
1.1
1.2
1.3
1.4
2.
The project _______________________________________________________________
The team ________________________________________________________________
The plan _________________________________________________________________
A one-day Workshop _______________________________________________________
1
2
3
3
AN INTRODUCTION TO GROUNDWATER ___________________________ 4
2.1 What is groundwater _______________________________________________________ 4
2.2 Types of aquifers __________________________________________________________ 5
2.3 Properties of fractured-rock aquifers ___________________________________________ 7
3.
GROUNDWATER INTERACTIONS IN THE ENVIRONMENT _____________ 8
3.1 Soil and Rock Interactions __________________________________________________ 10
3.2 Interactions with non-marine aquatic ecosystems ________________________________ 12
3.2.1
3.2.2
Surface water bodies ________________________________________________ 12
Subsurface water bodies ______________________________________________ 14
3.3 Interactions with terrestrial ecosystems ________________________________________ 14
3.4 Interactions with marine ecosystems __________________________________________ 16
3.5 The nature of groundwater dependency _______________________________________ 16
4.
TMG AQUIFERS _______________________________________________ 17
4.1
4.2
4.3
4.4
Table Mountain Group: Geological pattern and process __________________________
Structure and functioning of the deep fractured-rock TMG aquifer ___________________
Chemistry of TMG Aquifers _________________________________________________
Why the confined TMG aquifer is being explored for bulk water supply. _______________
4.5 Types of TMG aquifer settings _____________________________________________
4.6 Interactions TMG aquifer - ecosystems ________________________________________
5.
19
20
22
22
23
26
ASSESSING GROUNDWATER DEPENDENCE ______________________ 27
5.1 Possible evaluation methods ________________________________________________ 28
5.2 Assessing impact of long-term changes in hydrologic cycle ________________________ 30
6.
INPUT REQUIRED FROM SPECIALISTS ___________________________ 31
7.
REFERENCES ________________________________________________ 32
Appendix A Glossary of hydrogeological terms
TMG Ecosystems: Discussion Document for Scoping Phase
1.
PURPOSE OF THIS DOCUMENT
This document aims to assist specialists from a range of earth and life science disciplines in
developing an understanding of how abstraction of groundwater from the Table Mountain Group
(TMG) aquifer system, in the Western Cape, may affect water availability in the surface
environment. It does this by:
 giving a brief introduction to this project, its objectives and the team involved;
 providing an introduction to groundwater (see Section 2);
 providing an overview of the role of groundwater in the environment and its interactions
with ecosystems, in general (see Section 3);
 providing an overview of the character and functioning of the TMG aquifer in the Western
Cape (see Section 4); and
 summarising pertinent groundwater terminology to facilitate discussion (see Appendix A).
Isolating the potential impacts on the environment as a result of abstraction of water from the
TMG aquifer is likely to be a complex process, which cannot be addressed by one scientist or
discipline alone. Specialists will require a basic understanding of the above-mentioned issues,
and of each other’s disciplines, in order to promote cross-disciplinary interactions and facilitate
informed evaluation of the possible implications of abstraction on the ecosystems in which they
specialise.
The document represents the first in a series of reports and other structured activities aimed at
promoting the required interaction, including various one-on-one meetings between specialists
and core team members, and an inter-disciplinary workshop in March 2003.
1.1
The project
The project, “Ecological and Environmental Impacts of Large-scale Groundwater
Development in the Table Mountain Group (TMG) Aquifer Systems”, forms part of a
programme of research funded by the Water Research Commission (WRC) in partnership with
Department of Water Affairs and Forestry (DWAF) on “The hydrogeology of the TMG aquifers”.
The programme was created in the light of possible future large-scale groundwater development
of the TMG aquifer.
The full WRC programme has the following specific objectives:
1. Development of an understanding of the occurrence, attributes and dynamics of the TMG
aquifer systems.
2. Development of an understanding of the environmental impacts of exploitation from TMG
aquifer systems.
3. Integration of groundwater into the broader water management framework.
This project aims to initiate an assessment of the ecological role of groundwater in the TMG
aquifer systems as a first contribution towards the Programme Objective 2 as stated above.
Furthermore, the project aims to:
 use existing knowledge and develop existing study sites;
 promote capacity building to strengthen the human resources available to monitor and
manage possible water-resource developments of the TMG aquifer.
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The critical areas that will be addressed are:
 The development of predictive tools, innovative techniques and indicators to assess the
impact of groundwater abstraction on the environment.
 The improvement of our understanding of groundwater dependent ecosystems in the TMG
and sensitivity to groundwater level fluctuations.
 The development of an improved, conceptual understanding of the impact of changing low
flows on freshwater ecosystems.
The project is being run in close conjunction with the City of Cape Town’s (CCT) Pilot Study on
the feasibility of extracting groundwater from the TMG for municipal water supplies. The CCT
project also is in its public participation and scoping phase at present. The WRC project will
provide information that will help the CCT project in selecting the site for the wellfield and in
designing the monitoring component.
The proposed key deliverables of this project are:
 A spatial database of information on groundwater dependent ecosystems, related to the
TMG.
 A prototype design for the assessment of the groundwater dependent ecosystems and
monitoring results at field sites over one annual cycle.
 Establishment of an infrastructure for monitoring groundwater dependent ecosystems in
selected study areas.
 Analysis of field results to assess the extent of dependency of the identified ecosystems
on patterns of groundwater discharge at various spatial and temporal scales, and inferred
sensitivity to abstraction impacts.
 Recommendations for application of current knowledge and critical areas for future
research.
1.2
The team
The project team comprises a consortium of Western Cape scientists, which includes
hydrogeologists, terrestrial and aquatic ecologists, hydrologists and data base specialists.
Organisations involved in the study include:










Southern Waters Ecological Research and Consulting cc
Umvoto Africa
Environmentek – CSIR
Ninham Shand
University of Cape Town
University of Western Cape
Stellenbosch University
Geoss
Western Cape Nature Conservation Board
Department of Water Affairs and Forestry.
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1.3
The plan
The project has been organised in four phases:
Phase 1
Phase 2
Phase 3
Phase 4
Scoping: Detailed inception study and field study planning.
Field data collection.
Analysis of results.
Recommendations for the application of current knowledge and
identification of critical areas for future research.
The study commenced in June 2002, and is currently in Phase 1. The objective of Phase 1 is to
capture available knowledge that may be of use in assessing the dependency of ecosystems on
groundwater in the TMG. Phase 1 comprises three main activities:
 Collation of readily available date and information.
 Compilation of this Discussion Document and related aids to inform key specialists.
 One-on-one discussions and a workshop with key specialists.
1.4
A one-day Workshop
During the workshop, invited specialists will be asked to contribute:



Background to their area of expertise;
Their initial ideas on potential impact on groundwater dependent ecosystems;
Suggestions for impact and baseline monitoring.
We intend to:
 Scope the full range and types of potential ecological impacts;
 Identify geographical areas considered likely to be dependent on groundwater;
 Prioritise areas for future monitoring and research.
The criteria for prioritising future research and monitoring sites will be discussed during the
workshop. It is expected that they will include:





Relevance of the impacted ecosystem in the wider environment;
Degree of ecosystem dependency on groundwater;
Potential ecosystem responses to changes in the hydrogeological regime;
Sensitivity of groundwater discharge zones to long and short term abstraction and climatic
variations;
Capacity to monitor and evaluate anticipated changes in the ecosystem.
Note: This workshop was held on 28 March 2003 at CSIR Stellenbosch. The
findings of the workshop are contained in a separate report entitled Scoping
Workshop Report prepared by CSIR, Umvoto and Southern Waters.
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2.
AN INTRODUCTION TO GROUNDWATER
Water is a scarce and unevenly distributed national resource which occurs in many different
forms, all of which are part of a unitary, interdependent cycle (National Water Act, 1998).
Groundwater is one part in the hydrologic cycle. In order to understand interactions between the
components of the hydrologic cycle it is required to develop an understanding of the processes
and the driving forces.
Figure 2.1: Groundwater in the hydrologic cycle (from Domenico and Schwarz, 1990)
2.1
What is groundwater
Groundwater is water that occurs underground and (‘daylights’) as springs, wetlands, seeps or as
baseflow into rivers or the sea. Hydrogeologists think of groundwater in terms of water that is
found in the saturated zone, as opposed to soil moisture, which is the water in the unsaturated or
vadose zone (see Figure 2.2). Potentially usable or abstractable groundwater is found in
aquifers.
An aquifer is formed from permeable earth material, such as porous sediments or fractured hard
rock, which is saturated with groundwater. An aquifer is considered to be a rock type (including
‘soft’ unconsolidated material), which has sufficient permeability to allow flow to a borehole or a
well. This implies that only utilisable groundwater constitutes an aquifer. Low permeability and
impermeable rock types are called aquitards and aquicludes, respectively (see Figure 2.4).
In addition to the permeability of an aquifer, the amount of groundwater stored is a key parameter
used in the assessment of aquifers. The water stored is normally described in terms of water
released per unit drop in water level. This is determined by the porosity of the aquifer, and the
compressibility of the aquifer material and water.
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Figure 2.2: Unsaturated and saturated zone, capillary fringe and water table
(Hölting 1984)
2.2
Types of aquifers
There are essentially two main categories of aquifers (see Figure 2.3):
Primary aquifers:
Aquifers in which the water moves through the spaces that were formed
at the same time as the geological formation was formed, for instance
intergranular porosity in sand (e.g. alluvial deposits).
Secondary aquifers:
Aquifers in which the water moves through spaces that were formed after
the geological formation was formed, such as fractures in hard rock.
a)
b)
c)
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Figure 2.3: Schematic block-diagramms of different aquifer types; a) primary
aquifer, b) secondary aquifer (red block not porous), c) double porosity aquifer
(yellow block porous) (van Tonder, 2001)
Within this broad characterisation, aquifers can be described as either unconfined or confined:
Unconfined aquifer: An aquifer where the water pressure is equal to atmospheric pressure. The
‘top’ of an unconfined aquifer, where the aquifer is fully saturated, is termed the ‘water
table’. If the water table reaches the surface, water flows from the ground as springs or
seepages, thus generating or augmenting the baseflow of rivers or wetlands. Where
water is drawn from an unconfined aquifer, the water table drops in a cone of depression.
There is a direct relationship between the volume of water in the aquifer and the height of
the water table.
Confined aquifer: An aquifer that is overlain by a geological layer of low permeability, under
which the water is confined at a pressure greater than atmospheric pressure. The
interaction between confined aquifers and the surface environment tends to be restricted
over most of its area by the low permeability layer. The water in these systems remains
trapped below the aquitard/aquiclude, and flows from the ground only in areas where the
impermeable layer is broken or weathered away at the surface. Drilling into a confined
aquifer will result in a release of the pressure and an expansion of the water into the
additional space. Thus, the water level in the borehole at rest after drilling (rest water
level) is higher than that intercepted during drilling (strike water level). If water is
abstracted from a confined aquifer, the pressure-head in the aquifer, represented by the
piezometric surface, falls in the vicinity of the borehole.
Confined and unconfined aquifers are the opposite end members of a continuum from
pressurised aquifers (confined) to ones that are in equilibrium with the atmosphere (unconfined)
(see Figure 2.4). Semi-confined (leaky) and semi-unconfined (delayed yield) aquifers are found in
between.
Figure 2.4: Differences between confined and unconfined aquifers (van Tonder, 2001)
Flow within aquifers, and therefore discharge to springs and baseflow, is controlled by the
permeability of the aquifer and the gradient of the water table or piezometetric surface.
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Groundwater flows through aquifers from areas of recharge to areas of discharge (see Figure
2.5). Recharge occurs where water from rainfall, surface water, neighbouring aquifers or even
from water supply systems, is added to the aquifer. Rainfall, which is not intercepted, transpired,
evaporated or does not run-off the land surface, percolates through the unsaturated zone. In the
unsaturated zone, water infiltrates downwards under gravity flow through pores and macro pores
such as cracks and fractures, until it reaches the water table. Once in the aquifer the flow is
controlled by hydraulic permeability and the gradient of the water level. Flow paths may be fairly
short in time and length (several 100m, weeks/months) or long (100s kms and thousands of
years).
Figure 2.5: Groundwater flow in unconfined and confined aquifers (after Driscoll,
1986)
2.3
Properties of fractured-rock aquifers
In fractured (secondary) aquifers, such as the TMG, most of the water flows along fractures. The
macro fractures are usually embedded in a porous matrix formed by blocks (e.g. sandstone) or
micro fissured blocks (e.g. quartzite). This matrix has lower permeability than the fractures, but is
capable of storing water in the innumerable pores or micro fractures (see Figure 2.3). In extreme
cases, the blocks between the fractures are of such low permeability (e.g. granite) that very little
water can be exchanged between the fracture network and the matrix, which is then called ‘inert’.
The most striking hydrogeologic feature of a fractured rock is the variability of aquifer parameters.
A parameter such as the hydraulic conductivity or permeability normally varies by several orders
of magnitude within the same rock unit, often within short distances, and over different scales.
Similarly, the storativtiy of an aquifer may vary at different scales. The flow of groundwater (in
both primary and secondary aquifers) may be anisotropic. In fractured aquifers, preferential flow
along fractures and fracture zones results in strongly anisotropic flow.
If fractures are densely interconnected, they conform to a ‘fracture network continuum’
characterised by a large storage capacity that contributes substantially to the volume extracted by
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a pumped well. A fracture network may be considered as continuum depending on the following
three properties:
 Representative elementary volume;
 Fracture connectivity;
 Conductivity contrast between fracture and matrix.
These properties of the fracture network control its hydraulic behaviour. Importantly anisotropic
flow occurs in restricted zones of high permeability where the water flows from recharge to
discharge areas.
According to the standard equations derived for primary aquifers, the radius – or distance – of
influence increases with increased permeability and decreased storativity. Since water bearing
fractures can be seen as conduits with very high permeability, the radius of influence can easily
reach several kilometers in the direction of the fracture set, while the radius perpendicular to the
fractures reaches only several meters. An example of the possible cone of depression during a
hydraulic test is shown in Figure 2.6.
Figure 2.6: Cone of depression (blue line) during hydraulic test at one borehole
(green dot), showing the anisotropy along the water bearing fault zones (red lines)
(Umvoto, 2002)
In most of the fractured-rock formations in RSA the flow of groundwater is controlled by two kinds
of fractures, namely: vertical and sub-vertical fractures (i.e. vertical faults and fault zones due to
tectonic stresses; contacts along dykes) and horizontal or bedding-plane fractures (i.e. fractures
formed by tension release, uplifting and weathered zones).
3.
GROUNDWATER INTERACTIONS IN THE ENVIRONMENT
Groundwater and surface water interact at many places throughout the landscape. These
interactions can be highly dynamic as they respond to the variations and changes in the hydraulic
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Baseflow, bank storage
Spring, wetland
“Cryptic” seasonal and
ephemeral rivers, pans
gradients which drive the flows
between them.
The potential
functions of aquifers in the broad
environment can be summarised as
provision of :
 sources or sinks for water;
 sources or sinks for nutrients
or trace elements;
 habitat for aquatic and semiaquatic biota.
impermeable layer
Cave ecosystems
Figure 3.1. Types of aquatic and
terrrestrial ecosystems, using
groundwater as water resource
(Le Maitre, in press).
Aquifers typically provide water to
non-marine aquatic ecosystems,
such as baseflow to rivers, wetland and spring ecosystems, cave and aquifer ecosystems, and
water to terrestrial plant communities (Figure 3.1). They also supply freshwater and nutrients to
marine and, in particular, coastal environments such as lagoons, estuaries and the near-zone
intertidal zone.
River ecosystems: In perennial and seasonal rivers, the character and composition of the
aquatic (in-stream) or riparian (near stream) ecosystem is often dependent on groundwater
discharge as base (dry season) flows. In many cases the flows are also critical for meeting
human needs both directly and by sustaining human enterprises.
Wetlands and spring systems: In terms of groundwater interaction, there is a continuum from
springs, which have definite discharge points, to wetlands, where discharge tends to be diffuse.
Groundwater dependent wetlands would include wetlands with a known or likely component of
groundwater discharge in their hydrological cycle; at least some endorheic pans and many of the
coastal wetlands are examples.
Aquifer and cave ecosystems: These include groundwater contributions to “hypogean life”
(subterranean), including those in the aquifer itself (Hatton and Evans 1998), and to associated
cave ecosystems. Areas with karst geology, such as dolomitic rock systems or limestones, are
examples.
Terrestrial ecosystems: Where terrestrial vegetation is directly dependent on groundwater (as
opposed to interflow) it is in areas where the groundwater body is within the rooting depth of the
plants and groundwater discharge occurs through the plant root systems. This is also called
“cryptic” discharge, and is most noticeable as oasis-type vegetation in arid environments.
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Marine ecosystems. Groundwater inflows to the near-shore marine ecosystems contribute
nutrients and silica and affect the productivity of these systems. Evidence indicates that
groundwater may also play a significant role in estuarine and salt marsh ecosystems, and that
freshwater seeps on the ocean floor could contribute to up-welling effects.
Each of these possible groundwater interactions is addressed in more detail in the sub-sections
that follow, with general descriptions and graphics based on examples and geological settings of
primary aquifers. This is for illustrative purposes only. It is not directly translatable into the
context of the TMG geology and topography. This aspect is dealt with more fully in section 4.
3.1
Soil and Rock Interactions
Groundwater also reacts with the soils,
weathered materials and rocks in which it
occurs and the longer the residence time, the
further these reactions progress.
These
reactions alter the biological and geochemical
characteristics of the water through:
(1)
(2)
(3)
(4)
(5)
(6)
acid-base reactions,
precipitation and dissolution of minerals,
sorption and ion exchange,
oxidation-reduction reactions,
biodegradation, and
dissolution and exsolution of gases.
The net inflow of oxygen-rich surface water
into the subsurface environment creates an
environment
where
bacteria
and
geochemically active sediment coatings are
abundant. Aerobic (oxygen-using) micro
organisms are particularly active and may use
up all the dissolved oxygen so that the
“downstream” part of the localised subsurface flow system may be dominated by
anaerobic micro organisms. Anaerobic
bacteria can use nitrate, sulfate, or other
solutes in place of oxygen for metabolism.
The result of these processes is that many
solutes are highly reactive in shallow
groundwater in the vicinity of streambeds and
the return flow to the surface water may be
relatively nutrient-rich (see Figure 3.2 for an
example of the kinds of changes).
Figure 3.2: How concentrations of various
substances changed as the groundwater
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was drawn from a river to a borehole
(Winker et al 1999)
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3.2
Interactions with non-marine aquatic ecosystems
Aquatic systems are broadly defined as ones where water is the primary medium or substrate.
Although some saline water bodies are included in this section, the emphasis here is on
freshwater systems where groundwater plays a significant role. This includes both surface
aquatic systems, such as rivers, springs and wetlands, and sub-surface aquatic systems, such as
caves and aquifers. Lakes and vleis and endorheic systems, such as pans, sometimes also have
groundwater connections, but this is not always the case.
Estuaries and lagoons are dealt with in the section on the coast (Section 3.4).
3.2.1
Surface water bodies
Rivers
Many rivers originate as springs, where there is a localised or point discharge of groundwater.
These are most common where groundwater flows are confined to conduits, such as faults, or
concentrated in geological contact zones in the underlying rocks.
Thereafter the interactions between
groundwater and surface water bodies are
usually in the form of net fluxes (e.g.,
groundwater discharge from or recharge to
a river, Figures 3.3 and 3.4). Figures 3.3
and 3.4 represent distinct states along a
continuum, where the direction and rate of
flow between the river and groundwater
varies both spatially and temporally. As
the water level in a river rises during
rainfall events, so water may flow into the
groundwater (losing river), and as it drops
again following a dry period, so the
direction of flow may reverse (gaining
river). Some of the major rivers in the
Western Cape are gaining rivers in the
upper reaches but become losing rivers in
their lower reaches, particularly in summer.
Thus, the stability of riverine habitats is
enhanced
by
the
interaction
with
groundwater.
By receiving water that
sustains water levels during dry seasons and
droughts and replenishing this water during
periods of high flow, the extremes of high or
no-flow are moderated, and the risk of
complete loss of aquatic habitat is reduced.
Figure 3.3: A gaining stream, with a net
inflow of groundwater. (Winker et al
1999)
Figure 3.4: A losing stream, with a net
outflow of groundwater (Winker et al
1999)
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The temperature beneath the surface is also more constant with time so groundwater discharge
can help stabilise the temperature of the surface water body it interacts with. This is particularly
important in cold temperate climates but may also be important in the hotter, drier parts of South
Africa. For instance, in the Doring River groundwater-fed pools are used by the indigenous fish
during the dry season when surface flow ceases. During a recent survey in the summer
indigenous fish were only found in pools that had a freshwater source, while alien fish were found
in all pools.
A river may also become a losing stream when the transpiration of the riparian vegetation creates
a zone of depression, with the water table at a lower elevation than the water level in the river.
This can result in distinct diurnal cycles in the water levels in the river, which have been used to
provide rough estimates of transpiration by the riparian vegetation.
Wetlands
Similar situations to those described in Figures 3.3 and 3.4 apply in the case of wetlands. Most
wetlands are gaining systems, where there is a net discharge of groundwater into the wetland,
although there are some well-known examples of losing wetlands, such as the endorheic
Okavango Swamps. Many river floodplains in dry areas operate as gaining wetlands during
floods, and as losing wetlands, feeding back to the river, during the dry season. Rooted
vegetation in wetlands typically has a fibrous root mat which is highly permeable. Water
absorption through these roots drives a significant interchange between surface water and pore
water in the underlying sediments and can result in substantial nutrient fluxes. Nutrients from
groundwater may also play a significant role in the functioning of wetlands.
Hyporheic zones
Interaction between river water and groundwater typically occurs through the hyporheic zone,
which is a localised area or strip of permeable substrate beneath the river surface (Figure 3.5).
Extensive hyporheic zones are likely to be a feature of most of the streams and river systems in
and fed by the TMG. In Western Cape cobble-bed rivers, the hyporheic zone is recognised as
an important component of the habitat assemblage in a river, providing, inter alia, nursery areas
for larvae and juveniles, refuge areas for adults and supporting processes such as mineralisation
and organic matter decomposition.
The size and geometry of hyporheic zones surrounding streams vary greatly in time and space.
Mixing of ground and surface water in this zone may cause the chemical and biological character
of the hyporhoeic zone to differ markedly from adjacent river or groundwater. The upwelling
groundwater creates patches of high productivity in the hyporheic zones and in aquatic systems,
which support greater animal densities and diversity compared with non-up welling situations.
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Figure 3.5: Illustrations of the location of a hyporhoeic zone beneath a river and two
situations where there is flow or interchange between the stream water and the
groundwater driven by the hydraulic gradients (Winker et al. 1999)
Riparian vegetation
Groundwater may comprise a substantial proportion of the dry season flow that is essential for
maintaining the vegetation in riparian areas. Riparian zones, especially in semi-arid to arid areas,
are important for maintaining biodiversity, offering refugia and habitat for a variety of organisms
that are not able to survive solely in the adjacent terrestrial or aquatic environments. Riparian
zones create a buffer between the terrestrial and aquatic ecosystems, protect the river from the
effects of activities in the adjacent terrestrial environment, and stablise the river banks. They also
provide shade, lower temperatures and natural resources (e.g. reed, wood, fruit).
3.2.2
Subsurface water bodies
Subsurface water bodies are found mainly in cave systems and karst landscapes where the water
table reaches the bottom of the cave system. Where the rocks (e.g. limestone, dolomite) are
fairly easily dissolved, groundwater interactions can influence the formation of underground
caverns and caves, thus affecting the types and distribution of the different habitats, and the
organisms that live in them.
Little is known about these systems or their interaction with groundwater. However, work done
elsewhere, particularly in Eastern Europe and Australia, has indicated that they can exhibit
surprisingly diverse assemblages of biota, and that they should not be excluded from an
assessment of groundwater/surface water interactions.
3.3
Interactions with terrestrial ecosystems
Terrestrial ecosystems, using groundwater as the main water resource, are a significant feature
of semi-arid and arid environments but are also found in higher rainfall areas. They are
commonly found on seasonal and ephemeral river systems where there is no visible surface
water and are indicated by the presence of plant species not represented in the adjacent dryland
areas, for example, gallery forest or woodland. These types of terrestrial vegetation may also
occur in association with dykes, geological contact zones, faults and fractures and are often used
to site boreholes where there is no other evidence of groundwater near the surface.
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Afforestation (or conversion of short herbaceous vegetation to a dense,
vigorous vegetation) will increase evaporative losses and reduce
recharge (B vs A).
Pumping of groundwater (C) may reduce discharge
and so affect riparian or wetland vegetation (D). Bush clearing will
reduce evaporative losses and increase recharge (F vs E). Pumping
from rivers or alluvial aquifers (G) can affect riverine vegetation by
lowering groundwater levels. Large phreatophytes along rivers or
groundwater zones (H) can depress the groundwater locally causing,
for instance, diurnal dips in discharge hydrographs. Plantations of trees
with access to a near-surface water table, or exploitation of such
groundwater, can cause a draw-down and this may reduce the size of
adjacent wetlands and their plant composition (I and J).
Figure 3.6: Schematic illustrating some typical interactions between vegetation and
groundwater (Scott & Le Maitre, 1998; Le Maitre et al., 1999).
Groundwater usage of vegetation is highly likely in areas where the water table is <10 m deep but
may also occur where the water table is deeper, as many tree root systems can reach depths of
20 - 50 m (Scott and Le Maitre 1998; Le Maitre et al. 1999, Figure 3.7).
Trees
Shrubs
Grasses &
herbaceous plants
0
-10
Depth (m)
-20
Mean
Maximum
-30
-40
-50
-60
-70
Figure 3.7: Mean (+/- standard deviation) and maximum root depths for different
plant growth forms (After Canadell et al. 1996).
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3.4
Interactions with marine ecosystems
By volume, groundwater discharge into the marine environment is fairly small, but may play a
significant role in the ecological functioning of coastal, estuarine and salt marsh. Where
groundwater discharges into the marine environment, it not only moderates the salinity of the
receiving water, but also keeps saline water out of terrestrial ecosystems. Discharge can occur
either as broad seepage, often along the coastline, or as a point-discharge from a subsurface
spring at the shoreline or offshore (Figure 3.8). The discharge rate varies with time according to,
inter alia, variations in the sea level, increasing as sea levels decline.
In areas of groundwater seepage the groundwater flux through the seafloor influences the fauna
living in the sediment, providing the organisms with freshwater, dissolved silica and nutrients. For
instance, blooms of Anaulu, (a diatom) in the wave zone at the Alexandria Dunefield, Eastern
Cape have been linked to nutrient-rich groundwater entering the ocean just off the beach. A
similar phenomenon occurs on the False Bay coast off Zeekoeivlei, associated with silica-rich
groundwater (Engelbrecht, pers. comm.). The Anaulus is food for inter-tidal mussels and snails
that are eaten by shore-birds.
In brackish systems, such as salt marshes or estuaries, the fresh groundwater can provide a
refuge habitat for freshwater organisms by maintaining relatively low salinities.
Figure 3.8: Groundwater seepage into the surface water and groundwater discharge
through a sub-aqueous spring (Winker et al. 1999)
3.5
The nature of groundwater dependency
A groundwater dependent ecosystem, or component of an ecosystem, can be defined as: ‘An
ecosystem, or component of an ecosystem, that would be significantly altered by a change in the
volume and/or temporal distribution of its groundwater supply’.
A fundamental tenet of ecology is that ecosystems generally use a resource in proportion to its
availability, whether it is water, light, nitrogen or some other resource, and that the availability of
different resources will be a significant determinant of the structure, composition and dynamics of
an ecosystem (Tilman 1988). Thus, where groundwater is accessible, ecosystems will develop
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some degree of dependence on it, and that dependence is likely to increase with increasing
aridity of the associated environment. The following definitions of dependency may assist in this
evaluation:

Entirely dependent – Were groundwater fluxes to diminish or be modified only slightly the
ecosystem will collapse.

Highly dependent – Moderate changes to groundwater discharge or water tables would
lead to substantial decreases in either the extent or the condition of the ecosystem. One
form of response could be where the ecosystem has a critical threshold in its tolerance of
change. Its response would be abrupt but only evident once the threshold has been
exceeded.

Proportionally dependent – For a number of systems it is likely that a unit change in the
amounts of groundwater will result in a proportional change in the condition of the
ecosystem. For example, a 50% change in discharge would lead to the same change in
the ecosystem.

Facultative dependency – Changes in groundwater would have a minor effect on the
condition of the ecosystem.

No dependence – ecosystem independent of groundwater. It may be very difficult to restore
these ecosystems once they have been affected or altered because of changes in the
groundwater situation.
This section provided an overview of the types of interactions that generally can occur between
groundwater and other ecosystems. In reality there are few studies and even less data on the
nature of the interactions, or on the importance of those for maintaining the ecosystems. Often
the exact nature of a groundwater dependency may only be realised once an ecosystem has
been stressed beyond a critical threshold. Furthermore, it is worth remembering that
groundwater dependence is not limited to quantity considerations and could also include
dependence on the physical and chemical characteristics of the groundwater.
4.
TMG AQUIFERS
TMG formations underlie much of the Western Cape and extend into both the Eastern and
Northern Cape (Figure 4.1). The focus of this project is the confined secondary aquifer, where
water moves at depth through cracks and fractures in the Table Mountain Group quartzites.
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Figure 4.1a: Outcrop areas of TMG formations in the Western Cape, Eastern Cape and
Northern Cape with associated caves (Mlisa, 2003). Box indicates area of interest for
large scale abstraction for the City of Cape Town.
Figure 4.1b: Outcrops and subsurface extent of TMG formations with associated hot
springs and major fault zones (Hartnady and Hay, 2000). Yellow box indicates
‘Cage’ study area.
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4.1
Table Mountain Group: Geological pattern and process
The occurrence of groundwater in the deep, fractured rocks of the Table Mountain Group Aquifer
has an ancient geological history that can be traced to the protracted deposition of sediments of
the Cape Supergroup more than 400 million years ago.
The sediments, comprising quartz sands, clays and silts, were deposited in a shallow marine
environment. An estimated 5 – 8 km depth of sediment was deposited in the Cape Basin. Over
time the layers became buried, eventually forming rock under the increasing pressures and
temperatures. The Table Mountain Group, which is the focus of this study, is the lowest
component of the Cape Supergroup and forms the backbone of the Cape Fold Belt Mountains.
Continental movement caused the layers to be squeezed into folds. The pressure on these
layers of rock was immense, causing buckling and fracturing in the layers.
Two major events led to the current deformed state of the Table Mountain Group. They include a
mountain building period called the Cape Orogeny during which uplift and thickening occurred,
and the break up of Gondwana, a super-continent that existed before the formation of the Atlantic
Ocean. The Cape Fold Belt was formed around 250 million years ago. The results of these
events are fractures and faults in the brittle or competent layers e.g. the quartz sandstones, and
extensive folding in the more pliable shale layers.
Table 4.1 Lithostratigraphy1 (after De Beer 2002) and hydrostratigraphy2 (after Hartnady and
Hay 2002) of the TMG (thickness values mostly apply to southwestern outcrops)
Subgroup
Formation
Lithology (Rock type)
Nardouw
Rietvlei/Baviaanskloof
Feldspathic sandstone
Verlorenvalley
Skurweberg
Goudini
Shale
Quartz sandstone
Silty sandstone,
siltstone
Shale, siltstone
Diamictite shale
Quartz sandstone
Impure sandstone,
shale
Quartz sandstone,
conglomerate, shale
Peninsula
Cedarberg
Pakhuis
Peninsula
Graafwater
Piekenierskloof
1
2
Max.
Thickness (m)
Hydrostratigraphy
280
Aquifer
(limited)
Mini-aquitard
Aquifer
Mesoaquitard
Mesoaquitard
Aquifer
Mesoaquitard
Aquifer
(limited)
290
230
120
40
1800
420
900
The sequence in which rock types are layered
The sequence in which hydrological units (aquifer, aquitard) are layered
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4.2
Structure and functioning of the deep fractured-rock TMG aquifer
The fractured rock groundwater systems of the tectonically folded Table Mountain Group (TMG)
constitute a vast aquifer system extending from just north of Nieuwoudtville southwards to Cape
Agulhas and eastwards to Port Elizabeth. The full volume of the aquifer rocks in this whole
region comprises a staggering 100 000 km3.
The geological structure and geohydrological settings of the aquifers in the TMG primarily
considered for medium to large scale abstraction (i.e. the Skurweberg Aquifer in the Nardouw
Subgroup and the Peninsula Aquifer in the Peninsula Subgroup) differ in several respects. The
Piekenierskloof Aquifer has no regional relevance and is not considered further. The Rietvlei
aquifer, while having average thickness, is fine grained, has high feldspathic content and in
general a low storativity. It is therefore not considered suitable for large scale abstraction
schemes.
For further discussion of groundwater flow in the TMG and interactions with other ecosystems it is
necessary to distinguish between these two important aquifers. The main characteristics and
differences are listed in Table 4.2 below. For the occurrence and outcrops of these aquifers refer
to Figure 4.1a.
Table 4.2
Main characteristics and differences of the Skurweberg and Peninsula Aquifer
Characteristic
Skurweberg Aquifer
Peninsula Aquifer
Elevation of outcrops
Middle and lower ranges
High mountain ranges
Vegetation cover
Karroid shrublands, Renosterveld,
Fynbos
Fynbos, often bare rock
Weathering
Highly weathered, fractured
Resistant, highly fractured
Confined / unconfined
Often confined, overlaid by
Bokkeveld Shale
Mostly confined, overlaid by
Cedarberg Shale
Material
Sandstone, siltstone, shale layers
Sandstone, Quartzite
Bedding & jointing
Thin bedding, cross-bedding, small
vertical fractures; medium fracture
network
Thick bedding, dense vertical
fractures; good fracture network
Storage capacity
Medium storativity due to less
thickness and fracturing
High storativity due to thickness and
dense fracture network
Recharge
Less rainfall and less recharge
percentage
High rainfall and higher recharge
percentage
Discharge
Seep zones, seasonal low flow
springs
High elevation wetlands, perennial
springs
The groundwater intersections of pathways in the TMG aquifer are commonly at depths of greater
than 100 m below ground. The depth and capacity of the system is evidenced by the several
powerful hot-springs in the region with outflow temperatures of up to 64° C (e.g. Brandvlei, see
Figure 4.4). These temperatures are caused by the water being circulated to depths of at least
2000 m below ground level. All of the hot springs are situated in the Peninsula Aquifer and linked
to either the contact with the over- or underlying aquitard or an impermeable fault zone, or where
the Peninsula is hydraulically connected with the Nardouw due to faulting (see Figures 4.3 – 4.8).
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Most springs and other natural groundwater discharge areas in the TMG can be linked to highly
permeable fractures, which can have a large extent (called hydrotect). Usually the water flows in
the fracture network towards zones of high permeability. These fractures or fault zones act like
conduits, transporting the water over a long distance in a very narrow zone (see Figure 4.2).
Figure 4.2: Hydrotect near Voelvlei, connecting recharge and discharge area across a
synclinal structure (Umvoto 2001)
Due to the harder structure and texture the higher lying outcrop areas of the TMG consist mainly
of the quartzites from the Peninsula Formation, while the Nardouw forms the middle and lower
ranges in the mountainous regions. The valley infills consist of Bokkeveld rocks which overlie the
Nardouw except in upper regions and Alluvium cover. The river alluvium is generally derived from
the TMG and thus varies from coarse sand and boulders in upper regions to sand with increasing
silt content derived from Bokkeveld downvalley.
The recharge to the Table Mountain Group aquifer system is believed to be in the range of 7% to
23% of Mean Annual Precipitation (MAP). On the higher mountain peaks of the TMG in the
Western Cape the MAP exceeds 1000 mm and over the remaining TMG catchment it usually
exceeds 600 mm. High snowfalls in the vicinity of the Hex and Ceres Valley add considerably to
the total recharge. Recharge is dominant in the winter months. The average winter temperature
is significantly less than the average summer temperature. The EVT applicable to recharge
estimation is based on the winter month’s temperature average. The surfaces of the TMG
aquifers are also conducive to infiltration, comprising a spongelike jointed and blocky rock with
minimal soil cover. Taking the orographic rainfall pattern, the elevation of the outcrop areas of
the Peninsula Aquifer and its weathered and highly fractured surface into account, it is evident
that the Peninsula Aquifer receives the higher amount of recharge.
In the study area for the concurrent feasibility study for CMC, the potential groundwater recharge
for the Peninsula Aquifer only is of the order 100 million cubic metres or more a year (Umvoto
2001).
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4.3
Chemistry of TMG Aquifers
Table 4.3 provides a breakdown of the general water chemistry characteristics expected for water
abstracted from the Peninsula formation of the TMG aquifer. The water tends to be oligotrophic
(low in nutrients), acidic and low in salinity, which is characteristic of water flowing through or over
TMG formations.
Table 4.3 Typical water chemistry characteristics expected for water abstracted from the
confined TMG aquifer. All concentrations in mg/L unless otherwise indicated (Smith et
al. 2002).
EC
(mS/m)
pH
Na
Boreholes from Nardouw Subgroup
Mean
30.0
6.0 30.8
Minimum
9.2
3.1
7.2
Maximum
155.0
8.3 232.8
Boreholes from Peninsula Formation
Mean
10.4
6.2 11.1
Minimum
2.6
4.3
2.0
Maximum
26.3
7.6 21.2
Mg
5.8
1.5
43.1
1.8
0.9
3.2
Ca
Cl
SO4
10.2 56.2 27.6
1.3
6.1
3.2
73.4 395.2 220.5
3.3
0.4
30.4
18.0
4.5
34.1
5.2
1.0
14.0
Alkalinity as
CaCO3
Si
K
Fe
42.8 6.4 5.1 3.3
1.0 2.1 0.4 <0.1
147.3 18.1 16.2 15.4
14.5
3.5
77.9
3.8
1.4
9.4
0.8
0.2
2.3
0.2
0.1
0.2
D
(‰)
18O
(‰)
-45.2
-53.2
-27.7
-7.3
-7.9
-6.3
-42.8
-51.1
-35.5
-7.3
-7.7
-7.06
Thus, unlike the Nardouw Aquifer in the Western Cape, which tends to be high in iron and salts,
the water from the Peninsula formation can be expected to be relatively pure. In contradiction,
results from a study in the Hermanus area show high iron content and higher EC values for water
samples from the Peninsula Aquifer due to the position of the water strikes close to the
Cedarberg / Pakhuis contact (Umvoto 2002).
4.4
Why the confined TMG aquifer is being explored for bulk water supply.
As with a surface water resource being considered for use as supply, the main concerns when
evaluating a possible groundwater source are the quality, abstractability, economic (i.e. the
higher the yield the lower the cost in R/m3), storage capacity of the aquifer and environmental and
social considerations.
The great thickness of the TMG, its high permeability in faulted areas, the generally good water
quality (especially in the Peninsula Formation) and its exposure in high rainfall areas means that
this aquifer may be able to provide significant volumes of useable water on a sustainable basis.
In several areas, farmers are already abstracting significant volumes of groundwater from the
Nardouw Aquifer, associated alluvium and alluvial fans, and Bokkeveld sandstones, e.g. Hex
River (20 mill m3/yr Rosewarne 2002), Olifants – Doring (~12 mill. m3/yr, Hartnady and Hay
2000). There is currently only little abstraction from the Peninsula Aquifer for bulk water supply,
e.g. Kammanassie area (1 mill. m3/yr), Hermanus (planned 1 mill. m3/yr). The CCT has started a
project to gain a better understanding of the potential yield and impact of the TMG within 200 km
of Cape Town. This project aims to establish a pilot well field delivering 5 mill. m 3/yr within three
years.
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4.5
Types of TMG aquifer settings
The geological setting and geometry of the TMG aquifer requires a three-dimensional conceptual
understanding of groundwater flow when evaluating flow paths and groundwater discharge into
surface water bodies. The types of interaction with aquatic and terrestrial ecosystems can differ
from the examples given in section 3 above. Groundwater discharge is mostly locally restricted
and directly linked to lineaments such as fractures and faults.
In this context it is also important to distinguish between the two main TMG aquifers, namely the
Skurweberg (in the Nardouw sub-group) and the Peninsula (Table 4.1). For instance, the
Skurweberg aquifer contributes more directly to river baseflow both via the river bottom and via
springs at the Nardouw – Cedarberg contact, while the Peninsula contributes to river flow mainly
as surface run-off. Springs related to the Skurweberg/ Nardouw are often low volume springs and
seasonal (depending on the short term rainfall patterns), while Peninsula springs mostly flow all
year around and respond to loner term climatic patterns.
The following diagrams illustrate some of the main types of situations – type-settings – in the
TMG aquifer showing the groundwater flow paths and the kinds of springs that are associated
with these systems. Note that these diagrams are two-dimensional cross-sections. In some cases
additional flow at an angle to the cross-section can occur in both the Peninsula in the overlying
confined Skurweberg/ Nardouw aquifers.
1. FOLD & LITHOLOGY CONTROLLED [‘CAGE’]
Recharge
Recharge
Cold
Springs
‘Hot’ Springs
Groundwater
Head (Peninsula)
Cold
Springs
Flow
Paths
Bokkeveld (shale)
Nardouw (quartzite)
TMG
Deep Flow
Cedarberg (shale)
Peninsula (quartzite)
Pre TMG Basement (granite)
Figure 4.3: Cross-section of TMG-flow –fold & lithology controlled [‘CAGE’-Type]
Recharge to exposed Peninsula either side of a folded syncline. Cold spring discharge from short flow
paths. Hot springs discharge from long, deep flow paths. Flow follows the folded formation and is
focussed by faulting (parallel to cross-section).
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2. FAULT-LED [BRANDVLEI]
Recharge
Recharge
F
Groundwater
Head
Fa
u
lt
Hot
Spring
Post-TMG rocks
Nardouw (quartzite)
Cedarberg (shale)
TMG
Flow
Path
Peninsula (quartzite)
Pre TMG Basement (granite)
F’
Figure 4.4: Cross-section of TMG-flow – ‘Brandvlei’-Type, fault led
Fault itself is ‘sealed’ (impermeable) but fault zone has high permeability. Recharge in high elevation
area of the Peninsula and flow controlled by lithology. Long, deep flow paths as shown by hot springs
controlled faul tzone creating hydraulic contact between Peninsula and Nardouw.
3. FAULT-BOUNDED [WEMMERSHOEK]
Recharge
Cold
Springs
Cold
Springs
F
Flow
Paths
Berg River
Alluvium
ult
Fa
F’
Peninsula (quartzite)
TMG
Nardouw (quartzite)
Cedarberg (shale)
Granite basement
Figure 4.5: Cross-section of TMG-flow – ‘Wemmershoek’-Type, fault bounded
Faulting throws up impermeable basement. Short local flow paths. Discharge to springs against
impermeable basement and aquitard.
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4. SYNCLINAL AQUIFER & FAULT-LED PATHWAYS
Recharge
Cold
Springs
Groundwater
Head (Peninsula)
Cold
Springs
F
Fault
Recharge
Cold
Springs
Sea level
Bokkeveld (shale)
Nardouw (quartzite)
TMG
Submarine
Hot Springs
Cedarberg (shale)
Peninsula (quartzite)
Pre TMG basement (Shale)
F’
Figure 4.6: Cross-section of TMG-flow – Synclinal aquifer & fault led
Complex interaction of faults and folds. Short and long pathways to hot and cold springs, including
coastal and sub-marine springs.
5. SYNCLINAL AQUIFER & ALLUVIUM [VÖELVLEI]
Recharge
Cold
Spring
Vöelvlei
Recharge
Cold
Spring
Recharge
Groundwater
Head (Peninsula)
Cold Spring
Alluvium
Cedarberg (shale)
TMG
Nardouw (quartzite)
Peninsula (quartzite)
Pre TMG basement (Shale)
Figure 4.7: Cross-section of TMG-flow – ‘Voelvlei’-Type, synclinal aquifer & alluvium
Precipitation and topographic gradients drive longer flow paths in the Peninsula. Discharge to springs
and alluvium. Flow paths in Nardouw are perpendicular to the cross-section
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4.6
Interactions TMG aquifer - ecosystems
Using the same grouping of ecosystems as in section 3, the TMG specific interactions between
groundwater and ecosystems are described below, as far as they are known:
River ecosystems
The groundwater fed river low flow or baseflow is mainly contributed by springs in the upper parts
of the river system. Streams, rivers and estuaries are not directly connected to the TMG aquifers.
However, the alluvium aquifer which might feed into the river or estuarine system might receive
some contribution from higher lying areas in the TMG aquifer via springs and seep zones.
A WRC funded study has been carried out in the Kammanassie Mountains on TMG discharge
and the impacts of abstraction. Groundwater in the area is thought to play a key role in providing
baseflow to rivers and riparian vegetation. The environmental importance of groundwater in the
area in and around the Kammanassie Nature Reserve is high.
Wetlands and spring ecosystems
In the above-mentioned study area (Kammanassie Nature Reserve), three main types of springs
were identified (see Figure 4.8):
Type 1:
Type 2:
Type 3:
Shallow seasonal springs and seeps emanating at perched water tables, which
can be interpreted as interflow or rejected recharge
Lithologically controlled springs, due to the presence of inter-bedded aquitards,
located mainly at the Peninsula - Cedarberg, Goudini – Skurweberg and Nardouw
– Bokkeveld contacts
Fault controlled Springs (FCS)
LEGEND
Study area
+
Cold
"perched" spring
Cold
"perched" spring
Cold
spring
(watertable)
Leakage
Cold
spring
f
+
+
+
+
+
Figure 4-2
production boreholes
spring
valley floor
piezometric surface
f
Hot spring
(artesianC
) old
spring
f
Alluvium/Colluvium
Bokkeveld (shale)
Nardouw(Quartzite)
TMG
Cedarberg (shale)
Peninsula (Quartzite)
Kaaimans/Cango (Metased)
+
+
+
f+
Typical spring localities inthe Kamm
anassie area.
Figure 4.8: Typical spring localities in the Kammanassie area (Cleaver et al., in prep)
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Type 1 springs occur across the Peninsula and Nardouw aquifers, and are not connected to the
greater groundwater flow system (Kotze, 2001). The springs seep from a network of fractures
within the TMG aquifers directly above localised aquitards and are highly seasonal. According to
Kotze (2001) groundwater abstraction from any part of the TMG aquifers will not impact Type 1
springs.
Types 2 & 3 are the most significant with regard to the regional water balance (Kotze, 2001).
Type 2 refers to those emanating from contacts between Cedarberg shale aquitard and the
Peninsula Aquifer, the different strata in the Nardouw group and the Nardouw and Bokkeveld
shales, as well as unconformities. The Type 2 springs provide an important portion of the stream
run-off in the form of groundwater fed baseflow. Type 3 is represented by the hot springs present
in the area. Both types can be impacted by groundwater abstraction provided there is
interconnection and the spring occurs within the radius of influence of abstraction.
In the high elevation areas of the TMG outcrops a number of high lying wetlands occur, which
depend on either the regional water table or perched water.
Aquifer and cave ecosystems
Subsurface water bodies and caves occur in certain formations within the Cape Supergroup
(Mlisa 2003, see Figure 4.1a). These caves are typical of those more usually found in limestone
formations and thus the Table Mountain Group is said to have a pseudokarstic character. They
are likely to be important in TMG aquifer recharge and discharge patterns and in particular in the
distribution of highland seeps and localised occurrence of apparent groundwater dependent
ecosystems.
Terrestrial ecosystems
The typical vegetation in the high elevation areas, underlain by TMG formations, utilise rain, air
moisture and soil moisture as their main water sources. Vegetation depending on groundwater is
fairly localised and can be linked to springs, seep zones and wetlands.
Marine ecosystems
Due to the physical character of the fractured TMG aquifers discharge of groundwater direct into
the sea occurs mostly at distinct points, which could provide small but important ecosystems, as
described in section 3.4. There is no direct groundwater discharge from the TMG into lagoons
and estuarines.
5.
ASSESSING GROUNDWATER DEPENDENCE
Evaluation of the potential impacts of water abstraction from TMG aquifer systems requires an
understanding of the nature and extent of dependency of the ecosystems which use groundwater
from this system.
As described in section 3.5 a groundwater dependent ecosystem, or component of an
ecosystem, can be defined as: ‘An ecosystem, or component of an ecosystem, that would be
significantly altered by a change in the volume and/or temporal distribution of its groundwater
supply’.
From this definition, and using the information provided in Sections 3 and 4, it follows that:
 demonstration of groundwater use does not necessarily equate to groundwater
dependence;
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

demonstration of groundwater use in general does not necessarily equate to use of TMG
groundwater in particular;
abstraction of water from the TMG aquifer will not necessarily affect the supply of TMG
groundwater to a TMG-dependent ecosystem.
Section 3.5 provided an overview of the types of interactions that can occur between groundwater
and other ecosystems. In reality there are few studies and even fewer data on the nature of the
interactions, or on the importance of those for maintaining ecosystems. Often the exact nature of
a groundwater dependency may only be realised once an ecosystem has been stressed beyond
its range of tolerance of change. Groundwater dependence is not limited to the quantity or flows
it can also include dependence on the physical and chemical characteristics of the groundwater.
Thus, different parameters may be important in assessing dependency in different ecosystems.
For instance, the depth to the water table is likely to be an important hydrogeological parameter
controlling the availability of groundwater to the terrestrial plant communities, but salinity
gradients and distributions are an important parameter in estuaries.
5.1
Possible evaluation methods
There are generally two ways of evaluating the groundwater dependency of a specific ecosystem,
namely (1) assessing the relative groundwater contribution to the ecosystem as a whole and (2)
assessing the extent to which key species are dependent on groundwater and their ability to
tolerate water stress. While the possible methods for (1) vary widely from field measurements to
remote sensing approaches, the methods for (2) depend upon field and laboratory experiments.
The methods to identify and assess the groundwater contribution to the ecosystem include:

Identifying groundwater discharge regimes and related water availability to terrestrial
ecosystems, using either direct measurements in the field or remote sensing approaches;

Identifying the spatial relationship of ecosystems to groundwater flow regimes, particularly
with respect to the recharge-discharge patterns of TMG aquifers.

Detailed geohydrological mapping and interpretation of the flow systems on, and affecting,
the ecosystem;

Quantifying the groundwater contribution to baseflow in TMG rivers and streams, using
hydrograph separation methods.
Methods to assess groundwater dependency of terrestrial ecosystems in southern Africa have
been summarised in Colvin et al., 2002. The figure below summarises some of the tools
available for use at different scales.
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Satellite
SPOT Veg
Landsat MSS
Landsat TM
SPOT HR
Ikonos
Airborne
Scanner
Video
Photography
Resistivity
Evaporation modelling
Community transpiration
Plant transpiration
Leaf transpiration
Xylem analysis
Plant moisture stress
Isotopes/chemistry
Tracers
Experimental abstraction
mm2
m2
100m2
1ha
1km2
100km2 10000km2
Area
Figure 5.1: Tools to assess groundwater use by plants according to spatial scale
(Colvin et al., 2002)
One technique using remote sensing is the change vector analysis technique (Hay, 2002). It
identifies vegetation change anomalies that arise from seasonal or circumstantial growth patterns.
Relationships between groundwater, surface water, terrestrial and aquatic ecology are a function
of rainfall, topography, stratigraphy and geological structure. In order to facilitate the objective
assessment of the probable combination of causes for vegetation change, a geospatial model is
required. Combined with regional groundwater flow path modeling, the tool has potential to
assess the impact of groundwater abstraction on the terrestrial and aquatic ecology. Geospatial
modeling techniques allow for semi quantitative evaluation of precedent or event response
causes.
The methods to assess the groundwater dependency of specific plant species involve at least the
following:





Botanical and ecological surveys of wetlands to develop a classification of groundwater
dependence and to identify indicators and key species.
Measurement of evaporation and plant water use in representative wetlands.
Sampling of selected plant species to assess their groundwater use and moisture stress
tolerances.
Measurement of impacts of water abstraction on surface water flows and of the responses
of the stream(s) and /or springs to rainfall inputs.
Measurement of ground and surface water quality.
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TMG Ecosystems: Discussion Document for Scoping Phase
Guidance on identifying groundwater dependent terrestrial ecosystems in order to set Resource
Quality Objectives (RQOs) as a protective measure is given in Colvin et al..
5.2
Assessing impact of long-term changes in hydrologic cycle
The next step after evaluating groundwater dependency of an ecosystem is to assess the impact
of long-term changes of the water flow pattern and water quality can have on the ecosystems.
The first step for the assessment is to quantify the changes related to different causes.
Long-term changes of the flow pattern are here defined as relevant changes, which are observed
over a cycle of several years and trends against the mean annual behaviour. They can be caused
by several factors:

change in climatic conditions, such as global warming, and therefore increase or decrease
of available water in the local system of the hydrologic cycle;

change in preferential flow paths due to reduced permeability of fracture zones, e.g.
caused by earthquakes or chemical precipitation;

change in flow paths due to constructions, which intersect the natural flow path, such as
tunnels, roads, dams;

decrease of actual recharge due to change of landuse in recharge area;

decrease of water table due to over abstraction.
It is often difficult, or even impossible, to distinguish between different potential causes, as the
impact cannot be related to one cause separately. Furthermore, the significance of the impact of
any one variable depends on the contribution this variable may have to the total change.
It is therefore of utmost importance for the assessment to establish a properly designed
monitoring system, in order to quantify the changes and related impacts. Without proper baseline
measurements and ongoing monitoring, it is impossible to quantify changes or predict the
associated impacts.
Data from the monitoring system and the assessment of impacts can be used to defined and
quantify the related risks.. Based on this approach a risk management plan can be derived for an
adaptive risk management and mitigation strategy.
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TMG Ecosystems: Discussion Document for Scoping Phase
6.
INPUT REQUIRED FROM SPECIALISTS
Note: This section was provided in the February 2003 version of this Discussion
Document in order to assist specialists in preparing for the scoping workshop
held on 28 March 2003.
In the Scoping Phase of the project, the intention is to obtain an indication from a range of
specialists of the type, nature and degree of interaction between their area of ecological
experience and groundwater. It is planned to use this information as a basis from which to:

Evaluate the potential links between ecosystems and the TMG aquifer, as opposed to
those from other aquifer systems in the Western Cape;

Identify key areas of information deficiency.
It is planned that the information, meta data, hard data, description, and opinion required from
specialists, where available, will include an assessment of:

The temporal degree of utilisation of groundwater, i.e., seasonal or year-round.

The significance of groundwater for the ecosystem and the likely consequences of
changes in groundwater supply, e.g., if groundwater was no longer available would this
result in the demise of the ecosystem? The definitions of dependency provided in section
3.5 may assist in this evaluation.

The spatial extent of the dependence, e.g. widespread or localised. This includes
consideration of both the nature and conservation importance of the ecosystem. It should
be noted that even a small aquifer may support a keystone ecosystem, which has an
importance greater than its geographical extent.

The nature of the dependency. This is possibly the most difficult to define given current
knowledge on groundwater dependence and, as already mentioned, may only be realised
once an ecosystem has been stressed beyond a critical threshold. Section 4 provided
some clarification using examples of groundwater interactions in the Western Cape, where
possible, and specialists may find these useful in their assessments.
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TMG Ecosystems: Discussion Document for Scoping Phase
7.
REFERENCES
Bear, J. 1972. Dynamics in fluids of porous media. American Elsevier, 764 pp.
Canadell, J, Jackson, RB, Ehleringer, JR, Mooney, HA, Sala, OE & Schulze, E-D 1996.
Maximum rooting depth of vegetation types at the global scale. Oecologia 108: 583-595.
Cleaver, G, Brown, LR, Bredenkamp, GJ and Smart, M in prep. Assessment of environmental
impacts of groundwater abstraction from Table Mountain Group (TMG) aquifers on
ecosystems in the Kammanassie Nature Reserve and environs. Draft report to the Water
Research Commission (WRC)
Colvin, CA, Le Maitre, DC, Hughes, S. 2002. Assessing terrestrial ecosystem dependence on
groundwater. Water Research Commission.
De Beer, CH 2002. The stratigraphy, lithology and structure of the Table Mountain Group. In: A
synthesis of the hydrogeology of the Table Mountain Group – formation of a research
strategy (eds K. Pietersen and R Parsons), pp 8-18. Report No. TT 158/01, Water
Research Commission, Pretoria.
Driscoll, F.G. 1986. Groundwater and wells. Second edition. Published by Johnson Filtration
Systems Inc., St Paul, Minnesota.1089 p.
Domenico, PA, and Schwartz, FW. 1990. Physical and chemical hydrogeology. John Wiley and
sons, New York.
Hartnady, CJH and Hay, ER 2000. Reconnaissance Investigation into the Development and
Utilisation of Table Mountain Group artesian Groundwater using the E10 Catchment as a
Pilot Study Area [CAGE Study]. Report by Umvoto Africa cc to the Department of Water
Affairs and Forestry
Hartnady, CJH and Hay, ER 2002. Fracture system and attribute studies in Table Mountain
Group groundwater target generation. In: A synthesis of the hydrogeology of the Table
Mountain Group – formation of a research strategy (eds K. Pietersen and R Parsons), pp
89-96. Report No. TT 158/01, Water Research Commission, Pretoria.
Hatton, T and Evans, R 1998. Dependence of ecosystems on groundwater and its significance to
Australia. Occasional Paper No 12/98, Land and Water Resources Research and
Development Corporation, CSIRO Australia.
Hay, ER, Tyson, C and Mlisa, A 2002. Vegetation Change Vector Analysis for evaluating the
impact of groundwater abstraction on terrestrial and riparian vegetation. AAWG
Hölting, B 1984. Hydrogeologie - Einführung in die Allgemeine und Angewandte Hydrogeologie
(Geohydrology – Introduction into general and applied geohydrology). Ferdinand Enke
Verlag Stuttgart, Germany
Kelbe, B 2001. The importance of groundwater in the hydrological cycle and the relationship to
surface water bodies. Progress Report on the WRC-Research Project K5/1168 to the
WRC
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TMG Ecosystems: Discussion Document for Scoping Phase
Khan, R, Hay, ER, Hartnady, CJH, Shand, M, Görgens, A, Brown, C, Boucher, C and
MacDevette, M 2000. Groundwater – Enhancing the water supply to the Western Cape.
Presentation to the Minister of Water Affairs 4 September 2000
Kotze, JC 2001. Modelling of groundwater flow in the Table Mountain fractured sandstone
aquifer. PhD Thesis, University of the Free State.
Le Maitre, DC, Scott, DF and Colvin C 1999. A review of information on interactions between
vegetation and groundwater. Water SA 25: 137-152.
Le Maitre, DC, Colvin C and Scott, 2002. Groundwater dependent ecosystems in the fynbos
biome and their vulnerability to groundwater abstraction. In ed Peitersen, K, and
Parsons, R, A synthesis of the hydrogeology of the Table Mountain Group – Formulation
of a Research Strategy. Report TT 158/01 Water Research Commission.
Luger, M and Hay, ER 2002. Are Environmental Impact Assessments meeting the challenges of
groundwater utilisation? Motivation for an alternative risk management approach. In
Proceedings Groundwater – The hidden treasure, Conference of the Groundwater
Division Branch Western Cape September 2002
Mlisa, A 2003. GIS and Remote Sensing Application for Determining the Relationship between
Geological Structures and Cave Distribution. B.Sc. (Hons.) Thesis at University of Fort
Hare, Department of GIS
Pietersen, K. and Parsons, R. 2002. A synthesis of the hydrogeology of the Table Mountain
Group – formulation of a research strategy. Report No. TT 158/01, Water Research
Commission, Pretoria.
Riemann, K and Hay, ER 2002. Risk management policy for sustainable water supply and
management of economical and ecological impact. In Proceedings Groundwater – The
hidden treasure, Conference of the Groundwater Division Branch Western Cape
September 2002
Rosewarne, P 2002. Case study: Hex River valley. In: A synthesis of the hydrogeology of the
Table Mountain Group – formation of a research strategy (eds K. Pietersen and R
Parsons), pp 178-182. Report No. TT 158/01, Water Research Commission, Pretoria.
Scott, DF and Le Maitre, DC 1998. The interaction between vegetation and groundwater:
research priorities for South Africa. Report No. 730/1/98, Water Research Commission,
Pretoria.
Smart, M in press. Kammanassie
Smith, ME, Clarke, S and Cavé, LC 2002. Chemical evolution of Table Mountain Group
groundwater and the source of iron. Groundwater Division, Western Cape Conference:
Tales of a hidden treasure; Somerset West, 16 Sept. 2002.
Tilman, D 1988. Plant strategies and the dynamics and structure of plant communities.
Monographs in Population Biology. Princeton University Press, Princeton, New Jersey.
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TMG Ecosystems: Discussion Document for Scoping Phase
Umvoto 2001. CMA Bulk Water Supply Study – Table Mountain Sandstone Deep Well Drilling
Umvoto 2002. Water Source Development and Management Plan for the Greater Hermanus
Area, Overstrand Municipality – Interim Report on Drilling and Pump testing of Exploration
Boreholes
Van Tonder, GJ 2001. Aquifer Mechanics. Lecture notes of B.Sc. hons. And M.Sc. Course at the
Institute for Groundwater Studies (IGS), University of the Free State, Bloemfontein
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TMG Ecosystems: Discussion Document for Scoping Phase
APPENDIX A - GLOSSARY OF
HYDROGEOLOGICAL TERMS
Acidic – water with a low pH.
Alluvial – recent unconsolidated sediments, resulting from the operations of modern rivers, thus
including the sediments laid down in the river beds, flood plains, lakes, fans at the foot of
mountain slopes, and estuaries.
Anisotropic – having physical properties that vary in different directions.
Aquiclude – An impermeable geological unit that cannot transmit water at all. (Very few natural
geological materials are considered aquicludes.)
Aquifer - A saturated permeable geological unit that can transmit significant (economically useful)
quantities of water under ordinary hydraulic gradients. Specific geologic materials are not
innately defined as aquifers and aquitards, but within the context of the stratigraphic
sequence in the subsurface area of interest.
Aquitard - A saturated geological unit of relatively lower permeability within a stratigraphic
sequence relative to the aquifer of interest. Its permeability is not sufficient to justify
production wells being placed in it. (This terminology is used much more frequently in
practice than aquiclude, in recognition of the rarity of natural aquicludes.)
Capillary fringe - (a.k.a. tension-saturated zone) - The subsurface zone directly above the water
table in which pores within the geologic matrix are saturated with water, but the fluid
pressure is less than atmospheric. Water is held within pores of the geologic matrix by
capillary forces.
Cone of depression - The cone-shaped area around a well where the groundwater level is
lowered by pumping. The shape of the cone is influenced by both the permeability and
storativity of the aquifer, and the abstraction rate and time. In fractured-rock aquifers the
geometry of the fracture network and the contrast between fractures and matrix have the
main impact on the shape.
Confined aquifer - An aquifer that is physically located between two aquicludes, where the
piezometric water level is above the upper boundary of the aquifer. The water level in a
well tapping a confined aquifer usually rises above the level of the aquifer. If the water
rises above ground level, the aquifer is called artesian.
Contact zone – the join or interface between different geological units. Where a permeability
contrast occurs, springs may form.
Discharge area - The area or zone where ground water emerges from the aquifer naturally or
artificially. Natural outflow may be into a stream, lake, spring, wetland, etc. Artificial
outflow may occur via pump wells.
Down gradient - Direction toward lesser hydraulic head than point of origin, or point of interest
Endorheic – a chatchement area with a closed drainage system i.e. one with no outlets. Often
with a perennial or seasonal water body (pan) at its lowest point.
Fracture – breaks in rocks such as joints, due to intense folding or faulting
Fracture connectivity – a measure of how well the individual fractures or fracture systems, are
connected to each other and thus of the potential flows.
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Groundwater - Water in the subsurface, which is beneath the water table, and thus present within
the saturated zone. In contrast, to water present in the unsaturated or vadose zone which
is referred to as soil moisture.
Heterogeneous - A characteristic of the geologic matrix of interest in which hydraulic conductivity
is dependent upon position and or direction.
Homogeneous - A characteristic of the geologic matrix of interest in which hydraulic conductivity
is independent of position and or direction
Hydraulic conductivity - The constant of proportionality in Darcy’s law. It is defined as the volume
of water that will move through a porous medium in unit time under a unit hydraulic
gradient through a unit area measured at right angles to the direction of flow. Hydraulic
conductivity is a function of both the porous medium and the fluid flowing through the
porous medium.
Hydraulic gradient - The difference in hydraulic head between two measuring points within an
aquifer located to each other in the direction of flow, divided by the distance between the
two points.
Hydraulic head - The fluid potential for flow through porous media largely comprised of pressure
head and elevation head. This satisfies the definition of potential in that it is a physical
quantity capable of measurement (such as with manometers, piezometers, or wells
tapping the porous medium), where flow always occurs from regions of higher values to
regions of lower values.
Hydrotect – high permeable fracture or fault zone that extends over a long distance
Hyporheic zone - The saturated and biologically active zone in the unconsolidated material
underlying and next to a water-course. The hyporhoeic zone is important in river system
nutrient budgets as it acts as a nutrient storage system. It also provides a habitat and
refuge for aquatic organisms, thus also serving a buffering function which promotes rapid
recovery of aquatic ecosystems after floods or droughts.
Interflow – the lateral movement of water through upper soil horizons, normally during or following
significant precipitation events.
Lithology – the description of the macroscale features of a rock, eg texture, grain type etc.
Oligotrophic – refers to lakes with considerable oxygen in the bottom waters and with limited
nutrient matter
Orographic – related to elevation; orographic rainfall pattern means a strong relationship between
amount of precipitation and elevation of the area
Perched water - Unconfined groundwater held above the water table by a layer of impermeable
rock or sediment
Percolate - The downward flow of water through the pores or spaces of unsaturated rock or soil
Permeability - The capacity of rock or soil to transmit water. It is the portion of the hydraulic
conductivity, which is a function of the porous medium alone. In primary aquifers
permeability is an intrinsic property, which is a function of mean grain diameter, grain size
distribution, sphericity and roundness of grains and the nature of grain packing.
Porosity - The degree to which the total volume of soil or rock is permeated with spaces or
cavities through which water or air can move.
Potable water - Water, which is free from impurities that may cause disease or harmful
physiological effects, such that the water is safe for human consumption
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Potentiometric or piezometric surface - An imaginary surface formed by measuring the level to
which water will rise in wells of a particular aquifer. For an unconfined aquifer the
potentiometric surface is the water table; for a confined aquifer it is the static level of
water in the wells. (Also known as the piezometric surface.)
Primary aquifer - Aquifers in which the water moves through the spaces that were formed at the
same time as the geological formation was formed, for instance intergranular porosity in
sand (e.g., alluvial deposits).
Recharge areas - Areas of land that allow groundwater to be replenished through infiltration or
seepage from precipitation or surface runoff.
Representative elementary volume – a volume of sufficient size where there are no longer any
significant statistical variations in the value of a particular property with the increasing size
of the element (Bear, 1972).
Rejected recharge – groundwater discharge occurring relatively close to the recharge area of a
regional aquifer system. The groundwater discharge has followed a flow path that is short
in relation to the rest of the flow system.
Salinity - The concentration of dissolved salts in water. The most desirable drinking water
contains 500 ppm or less of dissolved minerals.
Saturated zone - The subsurface zone below the water table where pores within the geologic
matrix are filled with water and fluid pressure is greater than atmospheric. Aquifers are
located in this zone.
Secondary aquifer – (a.k.a. as fractured-rock aquifer) - Aquifers in which the water moves through
spaces that were formed after the geological formation was formed, such as fractures in
hard rock.
Semi-confined aquifer – (a.k.a leaky aquifer) - An aquifer that is physically located between two
aquitardes, and where the piezometric water level is above the upper boundary of the
aquifer.
Storativity – capacity of the aquifer to store water in its pores, voids, fissures and fractures. It is
given as the volume of water released from storage per unit surface area of the aquifer
per unit decline in the hydraulic head (typically m3/m2/m, ie dimensionless).
Surface water - Bodies of water, snow, or ice on the surface of the earth (such as lakes, streams,
ponds, wetlands, etc.).
Syncline – a fold in rocks in which the strata dip inward from both sides toward the axis, often
forming a valley
Tectonic – designating the rock structure and external forms resulting from the deformation of the
earth’s crust
Unconsolidated – where the matrix of the aquifer is formed from uncemented materials such as
sand, gravel pebbles or mixtures of these.
Unconfined aquifer - (a.k.a. water table aquifer) - An aquifer which is not restricted by any
confining layer above it. Its upper boundary is the water table, which is free to rise and
fall. The water level in a well tapping an unconfined aquifer is at atmospheric pressure
and does not rise above the level of the water table within the aquifer. An unconfined
aquifer is often near to the earth's surface and not protected by low permeable layers,
causing it to be easily recharged as well as contaminated.
Unsaturated zone - An area, usually between the land surface and the water table, where the
openings or pores in the soil contain both air and water. (See also “vadose zone”)
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Up gradient - Direction toward greater hydraulic head than point of origin, or point of interest.
Vadose zone - (a.k.a. unsaturated zone) - The subsurface zone above the water table and the
capillary fringe in which pores within the geologic matrix are partially filled with air and
partially filled with water, and fluid pressure is less than atmospheric.
Water table - The top of an unconfined aquifer where water pressure is equal to atmospheric
pressure. The water table depth fluctuates with climate conditions on the land surface
above and is usually gently curved and follows a subdued version of the land surface
topography.
Watershed - All land and water within a drainage area, defined by topographic high points.
Well - An opening in the surface of the earth for the purpose of removing fresh water.
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