Appendix B2
Progress report on groundwater field
work at WCNP
ENV-S-I 2004-006
INTERACTIONS BETWEEN
GROUNDWATER AND SURFACE WATER:
ASSESSMENT OF THE LANGEBAAN
LAGOON AS A STUDY AREA
I. Saayman, C. Colvin, J. Weaver, L. Fraser, J.
Zhang, S. Hughes, G. Tredoux, A. Hön, D. Le
Maitre, F. Rusinga, and S. Israel
Issued by:
Division of Water, Environment and Forestry Technology,
CSIR
P O Box 395, Pretoria 0001
March 2004
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Interactions between Groundwater & Surface Water:
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Table of Contents
1.
Motivation ......................................................................................................................... 1
2.
Background to Groundwater – Surface Water Interactions ....................................... 1
2.1
Climate ....................................................................................................................... 1
2.2
Geology ...................................................................................................................... 2
2.3
Cultural Influences ..................................................................................................... 2
3.
Methodology/Approach ................................................................................................... 2
4.
Study Site Description ...................................................................................................... 4
4.1
Bio-physical Environment.......................................................................................... 4
4.2
Regional Hydrology and Hydrogeology .................................................................... 5
4.3
Vegetation .................................................................................................................. 6
4.4
Conservation status .................................................................................................... 6
4.5
Salinity tolerances ...................................................................................................... 7
5.
Initial Conceptual Model ................................................................................................. 7
6.
Results of Initial Field Work and Discussion............................................................... 12
6.1
Study area ................................................................................................................. 12
6.2
Geophysics Results .................................................................................................. 12
6.2.1
6.2.2
6.2.3
6.2.4
7.
8.
Introduction ..................................................................................................................... 12
Field methods .................................................................................................................. 13
Results and preliminary interpretation ............................................................................ 14
Areas of uncertainty and lessons learned ........................................................................ 19
6.3
Remote Sensing ........................................................................................................ 20
6.4
Chemical Sampling .................................................................................................. 21
Data Inventory/Availability ........................................................................................... 23
7.1
Reports and papers: .................................................................................................. 23
7.2
Maps: (Hard copies) ................................................................................................. 24
7.3
GIS data: ................................................................................................................... 25
7.4
Berg River Monitoring data ..................................................................................... 25
7.5
Borehole data: .......................................................................................................... 25
References ....................................................................................................................... 26
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List of Figures
Figure 1: Geology of the Langebaan Lagoon area (Reaper, 1995) ............................................ 5
Figure 2: Conceptual model of Groundwater Discharge to the Langebaan Lagoon .................. 9
Figure 3: Scenarios to explain the distribution of fresh and saline water at Oosterval ............ 10
Figure 4: Relation between slope, geology and the fresh water/salt water interface ............... 11
Figure 5: Plotted resistivity data for the initial resistivity profile at Geelbek. The two data
points in red were the only bad data points that were removed. ...................................... 15
Figure 6: Resistivity profile 1 at Geelbek traversing from saline water vegetation to fresh
water vegetation. .............................................................................................................. 16
Figure 7: National Groundwater database listed boreholes in study area. ............................... 21
List of Tables
Table 1: Seasonal salinity ranges in selected parts of the Langebaan Lagoon (Flemming,
1977) ............................................................................................................................ 5
Table 2: Field measurements from reconnaissance borehole sampling at Langebaan. .......... 21
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1. Motivation
The South African National Water Act (no. 36 of 1998) prioritises the allocation of water to
meet basic human needs and the requirements of the environment, and recognises
groundwater as part of the water cycle. Water Resource managers are therefore required to
understand the contribution that groundwater makes to a surface water system and its role in
sustaining ecosystems, before an allocation may be made from that groundwater resource.
The scientific tools required to develop such an understanding, however, is largely lacking or
non-existent. A number of Water Research Commission (WRC) funded projects are underway
or have been recently completed to address the need for methodologies/approaches that
quantify the contribution of groundwater to rivers (Xu, et al., 2003), and to assess the degree
of ecosystem dependence on groundwater (Colvin, in progress).
This project is funded through the WRC Aquifer Dependent Ecosystem project (1337) CSIR’s
parliamentary grant, and aims to:
 Test methodologies to characterise the interactions between groundwater and surface
water systems and coastal ecosystems;
 Establish an integrated approach within the CSIR Groundwater Group when tackling
groundwater studies;
 Build capacity within the CSIR Groundwater Group, which can be applied to future
research studies.
2. Background to Groundwater – Surface Water Interactions
The interaction between groundwater and surface water takes place when and where
groundwater emerges at the surface or discharges to a surface water body, or where surface
water passes into groundwater. In many settings the flow of streams or rivers and water
logged conditions in wetlands and vleis during periods of little or no rainfall is maintained
through the discharge of groundwater. Surface water systems on its part represent important
zones of recharge to groundwater, especially in arid and semi-arid settings (Lerner, 1990).
Where groundwater and surface water occur in close association, it is the elevation of the
groundwater table relative to the surface water level that controls the direction and quantity of
flow in or out of the surface water system. The relation between these two systems is in part
influenced by climate (discharge volumes and patterns), geology, (aquifer properties and
changes in topography), and cultural activity (like landuse, groundwater abstraction and the
damming of rivers) (Lerner, 1996, Van Niekerk, et al., 1995, Xu, et al., 2002). Changes in
these factors along the reach of a river will influence the relative elevation of the water table,
and so dictate the type and extent of the groundwater/surface water interaction.
2.1
Climate
Different climatic zones are characterised by typical groundwater/surface water relations.
Humid and temperate regions for instance are characterised by high water tables and perennial
rivers. Seasonal changes in the water table may result in perennial rivers being seasonal in
their upper reaches. Ephemeral rivers on the other hand are more common in arid and semi-
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arid regions. In such regions rivers are typically perched, though such systems may switch
between being perched and connected as water table elevations change seasonally, or in
response to recharge events (Lerner, 1996). The greater depth of groundwater in arid and
semi-arid settings also results in saturated flow typically making a much smaller contribution
to stormflow discharge than in humid–temperate regions (22% in a semi-arid catchment vs.
40 to 80% in humid-temperate catchments – Sandström, 1996).
2.2
Geology
In all settings, the rate of exchange between groundwater and surface water will be dictated
by the hydraulic gradient between the two systems and the hydraulic conductivity of the
intervening soil, rock and bed sediments. Variations in hydraulic conductivity will also
influence the rate and location of groundwater discharge or recharge. Even in largely
homogonous systems, the hydraulic conductivity will dictate the location of the groundwatersurface water interaction. An example is the numerical model presented by the U. S. Army
Corps of Engineers (1999) of groundwater discharge to a seepage lake. The model shows that
the higher the hydraulic conductivity of the aquifer that underlie the lake, the more likely it is
that inflow through the lake bottom will be reversed to outflow.
2.3
Cultural Influences
Human activities commonly affect the distribution, quantity, and chemical quality of water
resources. Where groundwater resources are exploited, groundwater pumping can result in a
change in the rate and direction of groundwater flow. This has the ability to decrease flow in
streams that are hydraulically connected to aquifers, either by reducing leakage of
groundwater to the stream or by inducing leakage from the stream into the subsurface (U. S.
Army Corps of Engineers, 1999). Controlled or altered discharge to streams and rivers also
has the ability to induce surface water recharge to aquifers or to limit groundwater discharge
to streams.
In most settings agriculture is the biggest groundwater user, but also impact water resources
through the modification of landscapes. This in turn results in a modification of the
infiltration and runoff characteristics of the land, which in turn affects recharge to
groundwater, the delivery of water and sediment to surface water bodies, and
evapotranspiration (Winter, 1998). The application of fertilizers, herbicides and pesticides to
agricultural land also results in the contamination of water resources. Other sources of water
resources contamination include waste disposal sites, cemeteries, and discharges from waste
water treatment plants, industrial facilities and stormwater drains.
3. Methodology/Approach
The interrelated nature of systems where interactions between surface water and groundwater
occur, requires the input of a multi disciplinary team, which would typically include
hydrogeologists, geochemists, geophysicists, hydrological/hydrogeological modellers,
botanists, and remote sensing specialists. The CSIR is in the fortunate position of having
specialists in all of theses fields. The project leadership identified persons within the CSIR
(and in particular the Groundwater Group), which had these skills, and invited them to project
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planning meeting. During this meeting the approach to be followed was discussed, and
possible study sites identified.
The study sites that were identified during the planning meeting (nine sites were identified),
and the criteria which the study team viewed as significant for the selection of a research site
was passed on to a MBA study group at the University of Stellenbosch, who had agreed to use
an ‘Analytical Hierarchy Process” to rank the sites in terms of their suitability, relevance and
practicality. Their analysis found that the Zeekoevlei and Langebaan Lagoon sites scored
highest when evaluated in terms of the stated criteria. Following review of the results of the
analytical hierarchy exercise, the study team decide on the Langebaan Lagoon as their
preferred study site, as:
1. The Langebaan Lagoon is recognised as an area of high ecological significance, which
is threatened by human activity;
2. The west coast is semi-arid, with very little surface water run-off and a high reliance
on groundwater for water supply;
3. Use of the Langebaan Lagoon as a study site makes possible cooperation and research
partnerships with the WRC funded “Aquifer Dependent Ecosystems” project, the
National Parks Board, and the University of Stellenbosch (Botany Department).
Once the Langebaan Lagoon was selected as the research study site, permission was obtained
from the National Parks Board to do research within the West Coast National Park (Appendix
2), the area visited on the 12th and 13th of January 2004 to familiarise team members with the
area. A workshop was held subsequent to the study visit during which the study team
formulated their conceptual understanding of hydrogeological system and developed a
number of tasks to test its validity. The outputs of the hydrological system conceptualisation
(sketched cross sections and plans) is presented in section 5.
The work-plan to test the validity of the conceptual model comprised the following:
 Analysis of remote sensing images so that the distribution of freshwater vegetation on
the edge of the lagoon may be delineated;
 Geophysics using multi-electrode resistivity imaging to delineate the contact between
fresh groundwater and saline/brackish lagoon water;
 A drive-peizometer survey, along the edge of the lagoon where the remote sensing
analysis and geophysics indicated the discharge of freshwater, so that groundwater
samples may be obtained for lab analysis;
 A survey of existing boreholes and an assessment of their suitability for groundwater
sampling, so that the chemical/isotopic character of the Langebaan Lagoon may be
defined.
 An inventory of existing data of the study area, and an evaluation of its suitability for
the construction of a numerical groundwater flow/discharge model.
The results of and progress made on each of these tasks are presented in Section 6.
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4. Study Site Description
4.1
Bio-physical Environment
The Langebaan Lagoon covers an area of approximately 57 km2 and lies within the 187 km2
West Coast National Park, which was proclaimed in 1985. The Langebaan Lagoon has been
classified as a wetland of international importance in terms of the Ramsar Convention (1975),
mainly because it supports more birdlife than any other wetland in South Africa (Reaper,
1995).
The high conservation status of the West Coast National Park results in part from the large
areas of the highly threatened Lowland fynbos (or Strandveld) that it protects, while the
lagoon plays host to a wide variety and large number of bird species. Three veld types can be
discerned in the Langebaan area; West Coast Strandveld, Coastal Renosterveld and Cape
Macchia (Reaper, 1995). Halophytic vegetation is found along the edge of the Langebaan
Lagoon.
The area is essentially semi-arid with an average annual rainfall of about 260 mm, half of
which occurs during the months May to July. The low rainfall and the highly permeable and
unconsolidated nature of the geology limit surface runoff. Annual temperatures range from
an average winter low of 5°C to an average summer high of 34°C.
The Saldanha Bay and the Langebaan Lagoon formed as a result of dramatic sea-level
changes during the Cenozoic era, which resulted in the development of barrier sand dunes
along the coast. These dunes were later (during the Tertiary - ±9000 years ago) submerged
and washed away. The Donkergat Peninsula remains as a remnant of the barrier dune system.
It has remained intact thanks to the outcrop of resistant Cape Granite at its headlands.
The geology of the area consists of Cape Granite (here known as the Darling Granites), which
forms the areas’ geological basement, The Granite is covered by unconsolidated marine sand,
deposited by numerous marine transgressions and regressions during the Tertiary and
Quaternary periods. Extensive calcrete sheets are found in and around the lagoon. These are
believed to be the fossilised remains of oyster beds. Outcrops of these sheets have been found
under all the salt marshes at Langebaan Lagoon (Reaper, 1995).
The Langebaan Lagoon forms the southern extension of Saldanha Bay, from which it is
separated by Skaapeiland. From Skaapeiland to Geelbek (on the southern edge of the lagoon),
the lagoon measures 14.5 km. At its widest point it is approximately 4.0 km, and has an
average depth of 1 – 2 metres.
During tidal fluctuations partial exchange of water occurs with Saldanha Bay (half the lagoon
water passes into the bay), with little direct exchange of coastal water from the Atlantic
(Shannon and Stander, 1977). The water clarity and shallow nature of the lagoon result in
relatively high water temperatures (14°C in winter to 25°C in summer). This results in higher
evaporation rates and consequently higher salinities than in Saldanha Bay and the coastal
waters. Table 1 presents a comparison of winter and summer salinity at various parts of the
Lagoon (Flemming, 1977)
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Table 1: Seasonal salinity ranges in selected parts of the Langebaan Lagoon (Flemming, 1977)
Station
Mount
Kraalbaai
Bottelary
Churchhaven
Geelbeck
Salt Marsh Creek (Geelbek)
4.2
Winter
33
32
30
32
33
22
Summer
34
34
34.5
35
36.5
48
Regional Hydrology and Hydrogeology
Most of the hydrogeological investigations have focussed on understanding and developing
the Langebaan Road aquifer (Timmerman, et al., 1986, Weaver, et al., 1997, etc.). The
Langebaan Road aquifer lies in the Cenozoic deposits in the area between the Berg River, the
Langebaan Lagoon, Darling and Hopefield. The aquifer is bound by the Berg and Sout rivers
to the east and north, a zero flow boundary between Langebaan and Hopefield in the south,
the Vredenburg headland in the north west and Saldanha Bay in the west. The major aquifer
types that occur in the Langebaan Road aquifer are:
 The Langebaan Limestone of the Bredasdorp Formation, which is typically
unconfined, and varies in thickness from 10 to 20 m.
 The Sand and Gravel deposits of the Elandsfontein Formation, which is between 40
and 60 m thick.
The Elandsfontein coarse sand
and gravel aquifer is in places
confined by the Elandsfontein
clays and peats, which may be
up to 20 metres thick.
Figure 1: Geology of the
Langebaan Lagoon area (Reaper,
1995)
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Generally groundwater flows in a westerly direction from Hopefield. From Langebaan Road a
portion of the water flows in a northerly direction towards the Berg river, while the remaining
portion continues its westward migration to Saldanha Bay. Artesian boreholes occur in the
vicinity of Langebaan Road. Groundwater quality in the aquifer is variable. (Timmerman,
1985a & b, and Weaver, et al., 1997)
A few small, seasonal streams enter the lagoon but there is no other surface runoff. The
hydrogeology of the area is quite complex with different formations having different
properties (Weaver and Wright 1994). The Elandsfontein primary aquifer unit probably
discharges into the lagoon and the overlying dune areas are also likely to store groundwater
which could discharge into the southern end of the lagoon (Weaver and Wright 1994).
4.3
Vegetation
The terrestrial vegetation is a mixture of west coast dune thicket and sand-plain fynbos whose
composition is strongly controlled by the different soil sources: unconsolidated and
consolidated aeolian sands, limestones and granites (Boucher and Jarman 1977). The granite
and limestone communities are particularly rich in rare plant species. These communities
grade into freshwater marshes dominated by Phragmites and Typha and then into salt marshes
characterised by varying abundances of Arthrocnemum (Salicornia), Juncus kraussii,
Chenolea, Limonium, Spartina and a mixture of other sedges and shrubs (O’Callaghan 1994).
The marsh supports a very rich fauna of algae, molluscs and crustaceans and is a nursery
ground for many fish species (Barnes 1998).
The freshwater wetlands are characterised by a high water table (Boucher and Jarman 1977).
There do not seem to have been any studies which have examined the role of groundwater in
the salt marsh but groundwater discharge sustains the freshwater marshes (Boucher and
Jarman 1977; Christie 1981; O’Callaghan 1994) which probably would not be sustained by
rainfall alone. The groundwater is very likely to have some influence on the salt marsh as
well, for example the distribution of Juncus kraussii (O’Callaghan 1994; Reaper 1995). The
potential role of groundwater in this ecosystem will be investigated in the near future as part
of a research project being conducted by the authors. Although the lagoon is not a traditional
form of estuary, its mixture of fresh and salt water habitats qualifies suggests that it is
functioning as an estuary and that groundwater is an important component of the hydrology.
This hypothesis is supported by the estuarine character of the zooplankton at the southern end
of the lagoon (Grindley 1977) and similar north-south increases in dissolved silica (and
phosphorus) although nitrogen concentrations decrease (Christie 1981).
4.4
Conservation status
Langebaan lagoon was proclaimed a Ramsar wetland in 1998 and is recognised as an
internationally important bird area (IBA No 105, Barnes 1998). It forms part of the West
Coast National Park. More than 250 bird species have been recorded in the park and the
highly productive salt marsh regularly supports to more than 35 000 birds of a range of
species, particularly 15 species of Palearctic wader and other waterbirds. Many of these
species are classified as globally or nationally threatened. Other species make use of the
freshwater marsh and adjacent terrestrial vegetation, including Marsh Harrier (Circus
ranivorus), Black Harrier (C. maurus), Red-chested Flufftail (Sarothrura rufa) and African
Rail (Rallus caerulescens) and various reptiles and amphibians.
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4.5
March 2004
Salinity tolerances
There have been few studies of the effects of salinity on South African salt marsh and
freshwater marsh species. Studies by Adams (UPE) found that Phragmites is tolerant of
salinities of 3 ppt (‰) for up to 3 months.
Studies of Phragmites australis from mediterranean France found that seed germination was
reduced by salinity levels above 10 ‰ from 90-100% in freshwater to 5-60 % at 25 ‰
(Mauchamp and Mesleard 2001). Seedling growth decreased with increasing salinity: 50%
decrease at 7.5 ‰ compared with freshwater and 7-100 % mortality occurred at 15 and 20 ‰.
Salt tolerance in P. australis has been shown to involve an energy dependent sodium
exclusion mechanism (Lissner et al. nd). Plants from warmer environments salt tolerance had
growth rates which were 20-100% higher, especially at salinities ranging from 0.5-2.0%,
largely because of an improved energy balance and better osmoregulation. Most emergent
plants do not survive salinities of more than 5 g/l, an exception is Phragmites which can
survive up to 12 g/l (Anon nd). Other relatively salt tolerant aquatic or marsh plants such as
Triloglochin can tolerate up to 10 g/l.
Typha's maximum salinity tolerance is apparently lower than that of Phragmites with an
absolute max 17g/l versus 20g/l under cool climatic conditions (find reference#). The general
opinion is that differences in distribution of Phragmites and Typha species are due more to
their different tolerances of inundation rather than salinity. Based on the literature and the
healthy looking reeds (Phragmites) and Typha nearby the site, salinities probably do not
exceed 5 g/l for long periods, and higher salinities only occur for short periods. Apparently
Phragmites can survive with only 0.3 m of freshwater above much more saline water. As the
salinity of the lagoon water are high, at least seawater, the vegetation indicates that there must
be substantial freshwater inflows. This would also accord with observations by Reaper
(1995), O’Callaghan (1993, 1994), and Boucher and Jarman (1977). Coral Birrs (Gauteng
Environment) told us that there is a fresh water spring on Jutten Island (near Saldanha) and
springs and freshwater pools in the saltmarshes. The WCNP staff know where these are
located.
5. Initial Conceptual Model
A workshop was held on the 15th of January, during which team members formulated their
conceptual understanding of the Langebaan Lagoon/Groundwater system based on their
observations during the field visit and research presented in the literature. The initial
conceptual model that was developed during the workshop formed the basis of the data
acquisition phase of the project.
The discussions and relevant issues were captured in the form of sketches. These largely
capture the consensus thinking within the group, and highlight areas of uncertainty.
Sketches 1 and 2 (Figure 2): Regional Model. The area is underlain by two aquifer systems,
which are separated by a largely impervious clay layer. 1. Groundwater from the unconfined
aquifer discharges to the lagoon. There is little or no interaction between the lagoon and the
deeper confined aquifer. 2. Groundwater from both the shallow and deeper aquifer units
discharges to the Langebaan lagoon.
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Areas of uncertainty:
 Does the clay layer occur continuously across the whole area? Experiences at the
Langebaan Road Aquifer system suggest that there may be “windows” in the clay
layer (Weaver, et al., 1997). Sketch 2 (Figure 2) shows a scenario in which the clay
layer is absent in parts of the lagoon, so that water from the deeper aquifer discharges
to the lagoon. Understanding the connectivity between the aquifers and the extent to
which each contributes discharge to the lagoon will provide valuable input to design
of groundwater abstraction models and assessments of environmental impact.
 Does groundwater in the deeper aquifer discharge to the sea, or is the aquifer pinched
out between the clay layer and bedrock?
 What is the quality of water beneath the lagoon? and if it is salty,
 Where will the fresh water – salt water interface occur in the shallow and deep
aquifers?
Sketch 3 (Figure 2): The distribution of freshwater vegetation is controlled by shallow
calcrete lenses that forces shallow circulating groundwater to the surface.
Areas of uncertainty:
 The relationship between the calcrete layers and groundwater discharge is unclear.
The shallow nature of the calcrete layers makes it unlikely that they will be the major
factor controlling groundwater discharge. Even if a relationship is found between the
two, whether this is a major factor controlling groundwater discharge will have to be
tested. Calcrete may simply be controlling topography in the tidal zone.
Sketches 4 to 6 (Figure 3): Plan layout of the Oosterval area, with cross-sections of two
models that explain the local hydrology, based on the vegetation distribution.
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REGIONAL MODEL
E
March 2004
1
W
RAIN
1 (a) Where is the interface?
1 (b) Is there an interface?
T T
T S
S
1a
1b
CLAY
L
LAGOON
ATLANTIC
OCEAN
?SALTY?
?
2
Clay
3
typha
Standing fresh
water
calcret
e
Clay
Figure 2: Conceptual model of Groundwater Discharge to the Langebaan Lagoon
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Interactions between Groundwater & Surface Water:
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T
N
T
M
M
M
M M
M M
M
M
M
M
M
T
T
T
T
T
T T TT T
T
T
T
T
T
T
T
M
T
T
T
T
March 2004
T
T
T T
T
T T
ROAD
S
T
T
F
F
F
Start of
Boardwalk F
F
F
T
F
F
M
F F
F- Fynbos
T- Typha
S- Salt Marsh
M- Mud Flats
F
F
F
F F
F
F
F
S
N
Evaporite
1
T
T
T
calcrete
N
2
S
Layer of
fresh water
Evaporation
Figure 3: Scenarios to explain the distribution of fresh and saline water at Oosterval
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Fresh
Saline
calcrete
Evaporation
Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
Saline
Windflat
Lagoon
Windflat
N
S
Granite
Hills
Symbols
Granite
Calcrete
Evaporite
E
Water Bodies
i.e. Lagoon and Ocean
Inter face
Granite
SAND
Sand
Clay lenses
Figure 4: Relation between slope, geology and the fresh water/salt water interface
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6. Results of Initial Field Work and Discussion
6.1
Study area
The study area is located about ½ km north-east of the Geelbek farm complex, near the bird
hide and the walkway which connects it to the shoreline. The nearshore environment
comprises low dunes which grade into a gently sloping terrace with a shallow layer of saline
sand. This environment supports a salt marsh habitat described as the Limonium-Disphysma
(salt-bush) form by Boucher and Jarman (1977). This community indicates highly saline
conditions. There appears to be little leaching of the salts by the rainfall. This terrace may be
inundated by extreme tides (spring highs?) and probably gets some marine water inundation
from overtopping waves when there are strong winds from the north-west.
The bird hide itself is located in a belt of the Spartina-Triloglochin (salt marsh sedge) form
which fringes the shoreline of the lagoon. The shoreline itself is marked by a narrow strip of
the invasive European sedge species Schoenoplectus (Scirpus) triqueter which extends to
below the high tide line. There are also localised patches of the Juncus kraussii-Nidorella and
Typha capensis (bullrush) vegetation types in the area below the step.
Just to east of the walkway, and crossing it near the shoreline, is a clear boundary in the
vegetation associated with a step or drop of about 0.15 m in the ground level. The vegetation
on top of the step is the salt marsh and below the step it is the Spartina-Triloglochin and
Typha, clear evidence of an abrupt reduction in salinity below the step. To the east of the
walkway the sedge and bulrush vegetation is replaced by Phragmites australis (fluitjiesriet)
marsh. Further east the Phragmites extends right to the shoreline of the lagoon with just a
narrow strip of Schoenoplectus between it and the saline lagoon water.
6.2
Geophysics Results
6.2.1 Introduction
Resistivity is a non-invasive geophysical tool that can provide cost-effective answers to
geological problems. The Lund imaging system is a completely automated resistivity data
acquisition system. A pseudo-section of apparent resistivity data is acquired that shows both
lateral and vertical changes in resistivity along the profile. The inversion software
RES2DINV has revolutionised the inversion of such a complex data set and the interpretation
of the resistivity data has been optimised. The software output is a true resistivity pseudosection that can be directly related to geological changes.
The bulk resistivity of different geological units varies mostly because of changes in either
salinity of the pore fluid or changes in porosity. The resistivity method can thus be applied
very effectively to delineate changes in water quality, for e.g. to delineate the interface
between the fresh water and the saline water in the coastal zone/lagoonal areas. It can also be
applied to distinguish between different lithologies, if there is a variation in either porosity or
salinity of the pore fluid.
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
The resistivity technique was proposed as a tool for providing information for research
questions relating to the interaction between aquifer water from the Elandsfontein aquifer and
the lagoon at Langebaan. The vegetation around the lagoon varies from terrestrial to fresh
water to salt marsh vegetation. These changes in vegetation can probably at least in part be
attributed to groundwater flow regimes. Some of the potential hypotheses for the differing
vegetation around the lagoon will be tested using the Lund imaging system and includes the
following:
 The saline vegetation is underlain by calcrete and the fresh water discharge from the
aquifer does not reach the rootzone of the vegetation.
 The fresh vegetation is underlain by calcrete, preventing fresh rainwater from
infiltrating, thus keeping a freshwater lens on top.
 A combination of above – where groundwater flow can be delineated.
6.2.2 Field methods
The Lund imaging system, consisting of the SAS1000 resistivity unit, the ES464 switching
unit, multi-core cables and electrodes, was used for the acquisition of resistivity data in the
study area. The Lund imaging system is completely automated and pre-designed protocols
are used to optimise data coverage for specific profiles for e.g., for groundwater flow regimes,
data quality and resolution/coverage must be excellent. Multi-core cables with a two metre
electrode spacing were used to improve data coverage and quality at shallow depths.
The Wenner array was used for all profiles. The field procedure involved rolling out the
multi-core electrical cables in the pre-selected areas. The electrodes are then inserted next to
the electrode take-outs on the cables. Where possible the electrodes were pushed into the
ground to minimise disturbance of soil around the electrode and thus maximising electrode
contact. The electrodes were then connected to the electrode take-outs on the multi-core
cables using the earthing cables. This is also the opportunity to ensure that electrode
grounding is optimum, ensuring good quality data. The cables are then plugged into the
ES464, which is connected to the SAS1000. The SAS1000 is then connected to the battery
and the system started.
Quality control during the acquisition phase starts with an “electrode test”, which checks the
grounding of all the active electrodes. Electrodes that are not properly grounded can then be
investigated and re-checked. Generally, more than 90 % of the electrodes must be active –
i.e. properly grounded. In the saline conditions next to the coast a pass rate of 100% is
expected and were achieved.
A further quality control measure associated with the Lund imaging system is the leastsquares error. The resistivity data are acquired using at least two stacks (maximum of six
stacks to optimise data quality, total acquisition time and battery power). The least squares
error in the measured resistivity reading is displayed and saved as a standard deviation. This
error is saved in the data file and can be seen by converting the field data to the ABEM Multipurpose Protocol (.AMP) file format. The data quality can be assessed by looking at the
general error percentage. The number of data points with an error in excess of 5% should be
minimal. These erroneous data points can be removed from the data set. Unfortunately for
the initial trial survey, the standard deviation error of more than 60 % of the data was above
5%. This was attributed to an equipment error and the resistivity unit had to be repaired,
causing further delay.
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
The data were inverted using the RES2DINV software. The resistivity pseudo-section is
automatically plotted and the only pre-processing done was to erase obviously erratic data
points. The data was then automatically inverted using the finite element inversion technique.
The apparent resistivity pseudo-section (field data) is displayed with the inversion starting
model and the inverted true resistivity sections. The true resistivity pseudo-sections are used
for the interpretation of the resistivity data.
Quality control for the inversion process occurs through the least-squares error that is
calculated for the inverted true resistivity pseudo-section. This RMS (Root-Mean-Square)
error should ideally be below 10%. The RMS error for the Geelbek profile is ~ 8%.
6.2.3 Results and preliminary interpretation
Plans for detailed geophysical surveying at the Langebaan Lagoon study site had to be reassessed when the electrical imaging equipment that were to be used during the study became
unavailable due to a breakdown. The equipment was in use in Mozambique until December
2003 after which it had to be sent to Sweden for repairs. Some geophysical surveying could
however be attempted in February and March 2004. The rest of the resistivity data were
acquired in April 2004.
The first data set was acquired near the Geelbek bird hide, traversing from salt marsh
vegetation to fresh water vegetation. The salt marsh area is flat and then “steps down” into the
fresh water vegetation. There is a small drop (~10 – 15 cm) in ground elevation from the salt
“flats” into the fresh water vegetation. The fresh water vegetation stands in lagoon water all
the time because of this change in elevation. The tide was coming in during the actual
acquisition phase of the profile and the general change in water level could be closely
observed. The water level increased by about 20 cm in the fresh water vegetation, with the
end of the profile covered in about 0.5 m of water during peak tide. The zone just above the
root zone of the saline vegetation was washed – the saline marsh was water logged, but not
flushed. The salt marsh vegetation most probably gets flushed at spring tides only.
The repair of the resistivity unit did wonders for the data quality. The data are of excellent
quality with less than 1% of the data with an error of above 5%. The raw data are shown in
Figure 5 with the bad data points highlighted.
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
Figure 5: Plotted resistivity data for the initial resistivity profile at Geelbek. The two data points in red
were the only bad data points that were removed.
The data were initially inverted using a vertical/horizontal ratio of 1:1, i.e. the elongation of
structures in the horizontal or vertical direction was ignored. The data was then inverted
using a vertical/horizontal ration of 2:1, assuming strong horizontal layering. This was done
to attempt to delineate any thin horizontal lenses for e.g. calcrete lenses under the salt marsh
vegetation. However, the latter data inversion did not significantly re-align structures to
warrant the use of the increased horizontal ratio for the inversion – the inversion was not more
meaningful using a greater horizontal flatness. The inverted data are shown in Figure 6, with
the vegetation types indicated above the profile. Also note that all resistivity profiles acquired
in this area have been rescaled to the same resistivity, for comparison.
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
Salicornia (saline)
Juncus (fresh)
Typha (fresh)
March 2004
Phragmites (fresh)
18/03/04 13:25
Barrier zone
Top sandy/silty lagoon sediments
Aquifer sand
Figure 6: Resistivity profile 1 at Geelbek traversing from saline water vegetation to fresh water
vegetation.
The data shows a low resistivity layer at the top underlain by a high resistivity zone. The low
resistivity top layer varies slightly from very low resistivity under the salt marsh vegetation to
higher under the fresh vegetation. This correlates with what is observed in the field. The
saline vegetation is surrounded by pools of hypersaline water (very low resistivity) and the
fresh water vegetation stands in fresher (brackish) water (low resistivity). The water salinity
also seems to decrease the further you progress into the fresh water vegetation – the further
you are away from the lagoon. This is probably because the lagoon water reaches this far into
the flats at spring tide.
The resistivity of the layer underlying the fresh water vegetation is very high and this changes
laterally into a lower resistivity zone under the salt marsh vegetation. This seems almost to
form a vertical barrier zone in the subsurface between the fresh water vegetation and the salt
marsh vegetation. The data are not conclusive and the controls on the vegetation in this area
cannot be defined from the resistivity data. However, it does seem that it is most probably a
significant change in the properties of the underlying (aquifer) sand. This change could be a
change in porosity – a lower porosity sand underlying the salt marsh – or even a change in
lithology, i.e. a facies change – for e.g. clay in the sediments underlying the salt marshes. The
control on the vegetation could then be: the fresh water vegetation in underlain by sandy
material with a higher hydraulic conductivity that acts as an aquifer discharging groundwater
into the fresh water vegetation area. This continual fresh groundwater discharge allows
mixing with the higher salinity lagoon water and thus lowers the electrical conductivity of the
water enough to allow fresh water vegetation to grow. The lateral change in properties of the
subsurface sediments forms a barrier (dam) wall for this groundwater, pushing the
groundwater up in the fresh water vegetation zone, but preventing it from discharging in the
saline marsh area. The elevation changes can also account for part of the control on the
vegetation. The lower elevation around the fresh water vegetation allows wet conditions for
the fresh water vegetation. The saline vegetation might be just above mean sea level
preventing groundwater discharge on the flats. Some geological control should be acquired
for the resistivity data in the form of auger holes. Changes in sand coarseness or lithology
should be closely observed. These should be located to confirm lateral changes in the soil
properties.
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
The second profile was acquired closer to the Geelbek visitors centre and traversed from
terrestrial vegetation to salt marsh vegetation. The profile line started at the line of bluegum
trees lining the road to Geelbek. The elevation decreases slowly from the start of the line to
the salt marshes where the elevation flattens out. This part of the salt marsh is further away
from the lagoon – the marsh vegetation will probably only be inundated during spring tides.
No tidal variation was observed here while doing the survey. Tidal data will be correlated
with the data.
The inverted resistivity data are shown in Figure 7. The shallow resistivity below the salt
marsh (and a bit onto the terrestrial side) is low. This grades laterally into a higher resistivity.
This correlates well with observed conditions. The saline conditions (salt marsh) will result
in a low resistivity that will increase laterally away from the flats due to less salt. The shallow
low resistivity zone on the terrestrial vegetation is either tidal variation or more likely saline
patches on the surface. The discontinuous shallow low resistivity layer is underlain by a layer
of higher resistivity. Similar to the previous profile, this is probably the sandy aquifer
material with fresher water. The resistivity changes across this profile are not significant,
with an average resistivity between 7 and 50 Ω.m. There is a lower resistivity barrier, similar
to the first profile’s, where the vegetation changes from terrestrial to saline. However, the
change in resistivity is not that significant, mostly because the variation in resistivity is
smaller. This is thus not necessarily expected to be a real feature controlling the vegetation.
It should however be further investigated. It is important to note that the shallow lower
higher resistivity zone at the start of the profile might be due to the drying out of the soil due
to the abstraction of groundwater by the bluegums.
Saline patch
Terrestrial
Saline
reeds with
saline veg
in between
Saline
Aquifer sand
20/04/04 10:28
Barrier zone?
Top sandy/silty lagoon sediments
Bluegum
abstraction?
Figure 7: Resistivity profile 2 at Geelbek traversing from terrestrial vegetation to saline water vegetation.
The possible control on vegetation along this profile is expected to be elevation changes. The
terrestrial vegetation grows on the higher elevation sandy area above the neap high tide mark.
The saline vegetation then grows on the salt flats. The general lower resistivity as opposed to
that of the first profile could be an indication of lower porosity. This in turn can indicate less
groundwater discharge, preventing fresh water vegetation. Alternatively, the fresh water
vegetation might not grow here because the elevation does not drop enough to form flooded
conditions.
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
The third resistivity profile was acquired closer to the road – on the far side of the Geelbek
bird hide. The profile started on the terrestrial vegetation and then crossed into fresh water
vegetation. The elevation decreases from the start of the profile, and then flattens out at
around -26 m, then it drops of again slightly after +5 m along the profile. The fresh water
vegetation was in water all the time. The tidal variation was not obvious, but it seemed high
during the data acquisition phase. Tidal data will be acquired to correlate. This profile line
was further away from the lagoon and the tidal influence here is probably minimal, in terms of
both water level and salinity.
The inverted data are shown in Figure 8. The data correlates well with the observed
conditions. The shallow low resistivity data layer is indicative of the wetter and more saline
conditions in the fresh water vegetation. Note that the general resistivity of this top layer is
higher than on the other resistivity profiles. This is probably because this profile was
acquired further away from the lagoon and the tidal influence in minimal. The general
salinity of the water is probably lower than the fresh water vegetation on the first profile, but
this should be confirmed. The high resistivity patch at about –50 m along the profile is a
small sand dune – the high resistivity indicative of the dry sand. The resistivity of the lower
layer is approximately the same as that of the previous profile. There are also no significant
changes in the resistivity of this layer. The control on the vegetation does not seem to
originate from the subsurface. The control on the vegetation is most probably elevation again.
The terrestrial vegetation grows on the higher sandy areas above the neap high tide. The fresh
water vegetation grows where the elevation has dropped below mean sea level, where the
vegetation is constantly flooded. The tidal influence is minimal and some fresh water
discharge occurs; the salinity of the water is low enough for the fresh water vegetation to
grow.
Terrestrial
Juncus with
salicornia in
between
Typha (fresh water)
Top sandy/silty lagoon sediments
Aquifer sand
22/04/04 10:28
Figure 8: Resistivity profile 3 at Geelbek traversing from terrestrial vegetation to fresh water vegetation.
The data for the fourth resistivity profile were acquired at the northern end of the lagoon,
traversing from saline vegetation to terrestrial vegetation. The saline vegetation grows on a
“salt flat” and then the elevation increases gradually from about –30 m. From about +7 m
along the profile the elevation increases more rapidly. The tidal variation was not observed
during data acquisition, but tidal data will be acquired for future correlation.
The interpreted resistivity data can be seen in Figure 9. The general resistivity variation
correlates well with observed surface conditions. The low resistivity at the start of the profile
relates to the salt flats with associated saline vegetation. The resistivity increases laterally as
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
the terrain changes from salt flats to higher elevation sandy dune area. The high resistivity
after the centre of the line is typical of the saline sand. It is important to note that this profile
is the only profile where some sort of a salt water/fresh water interface can be seen (this has
been indicated on Figure 8). The saline interface is more pronounced here than in the other
profiles most probably because the hydraulic conductivity of the subsurface material is not
that high. The groundwater flow related to the higher hydraulic conductivity normally pushes
the saline interface further out to sea, and it cannot be observed on the resistivity sections.
Also note that the low resistivity zone extends into the terrestrial area; this is similar to the
second profile and indicates either neap high tide influences or saline soil that extends this far.
The vegetation control in this area seems to be elevation changes again. The saline vegetation
grows on the salt flats and the terrestrial vegetation grows above the neap high tide mark.
Salicornia + sparabolis (saline grass?)
Terrestrial
Top dry sand
Top sandy/silty lagoon sediments
Saline interface?
Aquifer sand
23/04/04 09:40
Figure 9: Resistivity profile 4 at the northern end of the park traversing from saline water vegetation to
terrestrial vegetation.
6.2.4 Areas of uncertainty and lessons learned
The controls on the vegetation around the lagoon cannot be conclusively derived from the
resistivity data. It does however clarify some areas of uncertainty and then it also created
some other areas of uncertainty:
There is no evidence of any sort of horizontal calcrete or clay layers controlling the saline
vegetation and/or terrestrial vegetation.
The vegetation seems to be controlled by elevation changes: the terrestrial vegetation grows
above the average high tide mark and or neap high tide mark. One of the uncertainties arising
from this is that the position of the neap high tide mark should be investigated to see if some
of the terrestrial vegetation gets flooded during neap high tide. The saline vegetation grows
on the flats where the vegetation is mostly in wet conditions but not covered by water all the
time – gets inundated by the tides every now and again. The uncertainties arising from this
are: firstly, how did the flats form – some geologic process (stream weathering, etc.) or some
lithologic process (i.e. calcrete lenses preventing rapid weathering). Secondly, the vegetation
control should have something to do with the physiology of the plants (apart from obvious
specialisations to adapt to saline conditions) – does the saline vegetation need to sometimes
dry out a bit (considering it does not grow at elevations where it can be submerged for long
periods of time). The fresh water vegetation grows in the areas lower than the salt flats where
the root zone in permanently submerged. There should also be fresh groundwater discharge
to allow mixing with the saline lagoon water to decrease salinity to acceptable levels for the
fresh water species.
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
In terms of general geophysics lessons learned: the initial geophysics fieldtrip produced data
with a high error. A subsequent field trip was undertaken on 5 February to ascertain whether
this was the result equipment failure or human error. During the field trip the following
became apparent:
 Always check background voltage using a multi-meter. This background voltage
should be corrected for by the LUND system itself. However, significant voltage
variation can result in large errors and negative readings. When the voltage varies
significantly it is important to increase the input current significantly to improve signal
to noise ratio. In the Langebaan case, the background voltage was stable at around 47
mV, but spikes did occur occasionally. One spike for e.g. caused a negative reading,
which was ratified by re-measuring.
 In the coastal zone, the natural background self-potential drops off towards the coast.
This causes fairly significant potential lines parallel to the coast, which will influence
resistivity data when acquired perpendicular to the coast. Once again the LUND
system should be able to correct for this error, but the current should be increased to
improve signal to noise ratio.
 Also to overcome significant voltage and self-potential (which can be classified as
sources of noise), the data acquisition time and delay time between cycles should be
increased to ensure that true direct current is used for surveying.
 In general, the LUND manual error codes should be kept handy to prevent
unnecessary data loss due to errors that could be corrected for in the field.
 In the coastal zone, input current should be at least 500 mA – this is to improve signal
to noise ratio. You will need two 12V batteries since one can supply a maximum of
200 mA.
 Always check the cables using the cabletest option on the LUND system before even
going to the field. This test takes a while, but it is worthwhile.
 Check the switching unit as well, using the ES464 test on the LUND system.
 Resistivity interpretation is ambiguous and groundtruth is necessary for control.
Hypothesis should always be tested and background information needs to be applied
where necessary.
6.3
Remote Sensing
Remote sensing satellite and aerial photographic images was to be used to generate map high
resolution vegetation maps of the study area. This was meant to generate a detailed georeferenced map of the study area on which areas of interest could be delineated. However, due
to time constraints the detailed remote sensing analysis could not be performed.
The GIS component was able to import data from the National Groundwater Database
(NGDB) of the Department of Water Affairs and Forestry. Boreholes and wells in the
immediate area of the Langebaan Lagoon listed in the NGDB is shown in Figure 7.
Page - 20 -
Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
Figure 7: National Groundwater database listed boreholes in study area.
6.4
Chemical Sampling
Limited hydrogeochemical sampling has been carried out in areas of interest around the
Lagoon. The sites sampled are listed in the table below and shown in the map above. Initial
results show predominantly of Na-Cl type water groundwater with a highly variable salinity
in the vicinity of the lagoon. There is a poor correlation in borehole position and data with
existing information from the National Groundwater Database, making these data of
questionable value.
Table 2: Field measurements from reconnaissance borehole sampling at Langebaan.
Sampling of Wells in the West Coast National Park
Field Parameters
Sample ID
W/Level (M) Depth (M) Electrical Conductivity
pH
Temp
(mS/m)
deg C
Geelbek Well 1
-1.26
4.28
260
7.48
21
Geelbek Well 2
1.30
4.64
205
Geelbek Well 3
-1.64
4.64
416
7.78
21
Geelbek Well 4
-1.91
3.73
Point 178
-1.41
3.49
Oesterwal Borehole
174
6.58
27
Oesterwal Well
-2.00
3.00
1659
7.95
20
Oesterwal Well 3
-1.46
2.04
665
20
Andrag Well
1.00
9.00
199
7.94
22
Roadworks Borehole
85
7.64
23
Churchhaven Well
-2.20
-2.50
503
8.01
22
Page - 21 -
Colour
Clear
clear
Clear
Odour
None
None
None
Slight Yellow
H2S
Slight Yellow
None
clear
none
Clear
none
clear
none
clear
none
Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
Langebaan Water Chemistry
80
80
60
Sampling Site
60
40
Andrag Well
Churchhaven
Geelbek well
Geelbek Well
Oesterwal Borehole
Oesterwal Well
Roadworks borehole
40
20
20
Mg
SO4
80
80
60
60
40
40
20
20
80
Ca
60
40
20
20
Na
40
60
80
HCO3
Figure 8: Piper Trilinear Plot of Groundwater Quality at selected boreholes at Langebaan Lagoon.
Page - 22 -
Cl
Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
7. Data Inventory/Availability
7.1
Reports and papers:
Geology
Coles, S. K. P., and Van Den Bossche, J. L., 2003, Brief
Operations/Cruise Report for the Langebaan Marine
Magnetometer Survey 9Ignimbrite Project) Conducted on
behalf of the University of Stellenbosch Geology
Department 9Surveyed 6 June 2003), Report No. 2003-0158,
Marine Geoscience Unit, Council for Geoscience.
Scheepers R. and Poujol M., 2002, U-Pb Zircon age of Cape
Granite Suite ignimbrites: characteristics of the phase of the
Saldanian magmatism, South African Journal of Geology,
volume 105, p 163-178
Leygonie, F. E., 1975, The granites of Langebaan, Cape
province (in Afrikaans), M.Sc. Thesis (unpubl.), University
of Stellenbosch, 97 p.
Van der Walt G. N., 2003, Geophysical Investigation of the
Postberg Ignimbrite Succession and Surrounding Rocks.
Honours Thesis, University of Stellenbosch
Hydrogeology
Bertram W E., 1985 Hidrosensus in die weskusvlakte tussen
velddrif, yzerfontein, Atlantis, Darling en Hopefield.
Tegniese verslag No. Gh3371. Direktoraat: Geohidrologie,
Kaapstad
Du Toit I. M. and Weaver J., 1995, Saldanha steel project:
groundwater investigations, CSIR report no. 27/95,
Stellenbosch
Timmerman K M G, 1986, Hydrocensus in the area of
between Darling, Malmesbuey and Mamre, Directorate of
Geohydrology, Cape Town
Timmerman L.R.A., 1985a, Preliminary report on the
geohydrology of the Langebaan and Elandsfontein aquifer
units in the lower Berg River region, Report GH3373.
DWAF, Cape Town
Timmerman L.R.A., 1985b, Possibilities for the
development of groundwater from Cenozoic sediments of
lower Berg river region. Report Gh3374. DWAF, Cape
Town
Timmerman L.R.A., 1987a, PhD Thesis: Regional
hydrogeological study of the lower Berg river area, Cape
Province, South Africa, Volume I, State University Ghent
Timmerman L.R.A., 1987b, PhD Thesis: Regional
hydrogeological study of the lower Berg river area, Cape
Province, South Africa, Volume II, State University Ghent
Page - 23 -
The work of Timmerman
(1985, 1986, and 1987) gives
the most comprehensive
overview of the area’s
geology and hydrogeology.
Building on the work of
Timmerman, Weaver, et al.
(1997) provides a further
assessment of the area’s
hydrogeology. These
assessments are however,
largely focussed on describing
and assessing the state of the
Langebaan Road Aquifer
Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
Geology
Timmerman L.R.A., 1987c, PhD Thesis: Regional
hydrogeological study of the lower Berg river area, Cape
Province, South Africa, Volume III, State University Ghent
Timmerman L.R.A., Timmerman K M G and Candoolaeghe
M A C, 1986, Proposal for the development of a
departmental production wellfield in the Langabaan road
primary aquifer unit, Directorate Geohydrology, Cape Town
Weaver J. and Talma S., 2000, Langebaan road wellfield
isotope data from the new boreholes, Report no. ENV/S-C
2000-082, Stellenbosch, South Africa
Weaver J. and Fraser L., 1998, Langebaan road aquifer:
Drilling and testing of new wellfield, Report no. ENV/SC98042, Stellenbosch, South Africa
Weaver J., Van der Voort I., Hön A., Conrad J. 1997,
Langebaan road aquifer unit –water supply project, Report
no. ENV/S-C97015, Stellenbosch, South Africa
Woodford A C., 1999, South African National Parks:
Langebaan road aquifer system: review of reports outlining
the hydrogeological conditions, exploitation potential and
proposed development f the Langebaan road aquifer system.
T&P report no. 990195. Toens & Partners CC.: Geological,
Hydrogeological, civil Engineering &Environmental
Consultants. , 8 Lester Road Wynberg, 7800
Botany
Reaper, M. B. S., 1995, A study of the relationship between
environmental conditions and saltmarsh zonation at
Langebaan Lagoon, M.A. Thesis (Unpubl.), University of
Cape Town.
March 2004
system.
Zoology
7.2
Maps: (Hard copies)
Maps of the Langebaan area can be found in Timmerman (1985a, b, 1986, 1987a, b, c, and
Timmerman, et al., 1987). These illustrate the geology, thickness contour of geological
formation, hydrochemistry (water type, water class, hardness, hydrochemical subareas),
hydraulic conductivity, recharge area, and modelled water level contour, vegetation map.
Also available are:
 Langebaan road aquifer: monitoring network, DWAF
 Langebaan road aquifer: clay thickness, CSIR
 Langebaan road aquifer: aquifer thickness, CSIR
 Langebaan road aquifer: EC, CSIR
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
7.3
March 2004
GIS data:



DEM data for the lower Berg River and digital aerial photography of the Langebaan
Lagoon (1999) is available on CD from the Western Cape Nature Conservation Board.
Topography information in CGM file format is available from DWAF.
The CSIR has elevation contours and the ability to build a DEM and to do other
topographic analysis.
Coverages are also available for:
 Coastline
- 1: 50 000
 contours
- 1: 50 000
 roads
- 1: 50 000
 rivers
- 1: 50 000
 waterbodies - 1: 50 000
 wetlands
- 1:250 000
 Landcover
- 1:250 000
Satellite Imagery
 Landsat ETM+ satellite data November 1999
 Landsat 4/5 Satellite Mosaic circa 1990
- 28.6m Pixel/Ground Resolution
- 28.6m Pixel/Ground Resolution
Also available in digital form are scanned 1:50 000 topo-sheets.
7.4
Berg River Monitoring data
GIS data which covers sub-catchment G10 (low Berg River) is available. Time series data is
available for rainfall, river flow, dam levels, and some groundwater abstraction.
Tides
Tides: we have tidal constituents at Saldanha Port -> can be used to generate tidal levels for
any time period in Saldanha Bay. The tides in the lagoon are howver somewhat different. The
Delft3D computer model can be used to simlate the corresponding tidal levels at any location
in the lagoon. Model is already set up, but would need to be run for a particular time and
location (1 days work).
Air temperature, pressure and wind: 1994 -2003 measured at Port Control in Saldanha Bay.
Need permission to use from NPA. NB: wind has a significant impact on water levels in
lagoon - this can also be simulated with Delft3D model.
An example of Delft3D model of Saldanha Bay/Langebaan Lagoon attached (animated gif
format - play with e.g. internet explorer)
7.5
Borehole data:
The National Groundwater Database contains records of 486 boreholes data in the 1:50 000
topographic map sheets 3318AA and 3318AB, with information on: borehole locations,
altitude, water level, borehole yield, lithology, recharge, and land use etc.
Timmerman’s documents give some borehole log, and pumping test graphs
Page - 25 -
Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
March 2004
8. References
Anon nd. The nature of salt-affected natural ecosystems. Information Step B3. Tools for
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Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
Plates
Plate 1: Salt water vegetation near the edge of the lagoon, with tall trees that
must be accessing freshwater at depth.
Plate 2: Freshwater vegetation (Typha) alongside saline vegetation. The coast
(lagoon) is about 1 km away.
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March 2004
Interactions between Groundwater & Surface Water:
Assessment of the Langebaan Lagoon as a Study Site.
Plate 4: Close-up view of freshwater vegetation
(Typha) alongside saline vegetation.
Plate 3: View of Langebaan Lagoon from a granite hill.
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March 2004
Appendix 1: ‘Analytical Hierarchy Process’ – Decision Support Systems
Page – A-1 -