Groundwater Specialist Study - Cape Town International Airport Runway Re-alignment and Associated Infrastructure Project REPORT: GEOSS Report No: 2014/06-10 PREPARED FOR: Sharon Jones and Scott Mason SRK Consulting (South Africa) (Pty) Ltd. The Administrative Building Albion Spring 183 Main Road Rondebosch 7700 PREPARED BY: Julian Conrad GEOSS - Geohydrological and Spatial Solutions International (Pty) Ltd Unit 19, Technostell Building, 9 Quantum Street, TechnoPark Stellenbosch 7600 Tel: (021) 880-1079 Email: [email protected](www.geoss.co.za) 08 September 2014
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Groundwater Specialist Study - Cape Town International Airport Runway Re-alignment
and Associated Infrastructure Project
REPORT: GEOSS Report No: 2014/06-10
PREPARED FOR:
Sharon Jones and Scott Mason SRK Consulting (South Africa) (Pty) Ltd.
The Administrative Building Albion Spring 183 Main Road
Rondebosch 7700
PREPARED BY: Julian Conrad
GEOSS - Geohydrological and Spatial Solutions International (Pty) Ltd Unit 19, Technostell Building,
3. RELEVANT EXISTING DATA AND DATA AQUIRED FROM FIELD WORK .......................................................................................................................... 13
3.2 Field assessment ............................................................................................................... 18
4. IMPACT IDENTIFICATION AND ASSESSMENT ........................................ 23 4.1 Direct Impacts .................................................................................................................. 23
4.1.1 Removal of alien vegetation and associated impact on groundwater levels (construction phase). 23
4.1.2 Removal of alien vegetation and associated impact on groundwater quality (construction phase). 27
4.1.3 Impact on groundwater levels due to earthworks, ground surface levelling and runway construction (construction phase) ..................................................................................................... 28
4.1.4 Impact on groundwater quality due to earthworks, ground surface levelling and runway construction (construction phase) ..................................................................................................... 29
4.1.5 Impact of the presence of the runway and taxiways on groundwater levels (operational phase) 30
4.1.6 Impact of the presence of the runway and taxiways on groundwater quality (operational phase) 32
11. APPENDIX C: SITE PHOTOS .......................................................................... 48
LIST OF FIGURES AND MAPS Figure 1: Location of Cape Town International Airport and proposed project footprint
(SRK, 2014). ............................................................................................................................ 2 Figure 2: Proposed construction of realigned runway and taxiways (Naco-SSI, 2008a) ........ 3 Figure 3: Average monthly rainfall and evaporation (ET) for the study area (Schulze et al,
2008) ......................................................................................................................................... 7 Figure 4: Conceptual model for the current status at the CTIA ............................................. 10 Figure 5: Long term borehole water level graph – UWC (Bh02) ............................................. 14 Figure 6: Long term borehole water level graph – UWC (Bh06) ............................................. 15 Figure 7 A & B: A. Study site, recharge zones and pilot study monitoring borehole
locations. B. Groundwater levels in metres below ground level (mbgl) under 133 hectares cleared and un-vegetated ground subjected to wet conditions (CSIR, 2006). ................................................................................................................................................. 16
Figure 8: Piper diagram of the two groundwater samples collected from the piezometers (24th June 2014) ..................................................................................................................... 20
Figure 9: Stiff diagram of the two groundwater samples collected from the piezometers (24th June 2014) ..................................................................................................................... 20
Figure 10: Expanded Durov diagram of the two groundwater samples collected from the piezometers (24th June 2014) ............................................................................................... 21
Figure 11: Conceptual model post-construction phase ........................................................... 24 Figure 12: Conceptual model of the conditions during the operational phase ..................... 25
Map 1: Location of the Cape Town International Airport area within a regional setting .... 37 Map 2: The study site superimposed on a 1:50 000 topocadastral map, showing the
GEOSS auger sites and piezometer positions. ................................................................. 38 Map 3: The study site superimposed on an aerial photograph, showing the GEOSS auger
sites and piezometer positions. ........................................................................................... 39 Map 4: Geological setting of the study area and NGA boreholes (Council for Geoscience
map: 1:250 000 scale 3318 –Cape Town) .......................................................................... 40 Map 5: Aquifer type and yield within the study area (DWA 1:500 000 scale 3317 Cape
Town) ..................................................................................................................................... 41 Map 6: Groundwater level as metres below ground level (base data is SRTM, 2000) ......... 42 Map 7: Groundwater level as metres above mean sea level (base data is SRTM, 2000) ..... 43 Map 8: Aquifer quality within the study area (DWA 1:500 000 scale 3317 Cape Town) .... 44 Map 9: Groundwater vulnerability within the study area (DWA, 2005) ................................ 45
LIST OF TABLES Table 1: Geological formations within the study area ................................................................ 8 Table 2: National Groundwater Archive data within the study area ...................................... 13 Table 3: Groundwater levels, EC and pH, measured at the Airports Company South Africa
groundwater monitoring sites by GPT (2014). ................................................................ 18 Table 4: Auger and piezometer positions ................................................................................... 19 Table 5: Field descriptions of the piezometer and auger sites ................................................. 19 Table 6: Field chemistry of the two piezometer sites ............................................................... 20 Table 7: Water quality classification table ................................................................................... 21 Table 8: Details of borehole samples collected and DWA (1998) limits for domestic use. 22
Groundwater Specialist Study - Cape Town International Airport Runway Re-alignment and Associated Infrastructure Project
GEOSS Report No. 2014/06-10 08 September 2014 v
Table 9: Impact 1a – Alien vegetation clearing and groundwater levels ................................ 27 Table 10: Impact 1b – Alien vegetation clearing and groundwater quality ........................... 28 Table 11: Impact 2a – Impact of earth works and construction activities and groundwater
levels ....................................................................................................................................... 29 Table 12: Impact 2b – Impact of earth works and construction activities and groundwater
quality ..................................................................................................................................... 30 Table 13: Impact 3a – Runway and taxiways and groundwater levels ................................... 31 Table 14: Impact 3b – Runway and taxiways and groundwater quality ................................. 32
ABBREVIATIONS ACSA Airports Company South Africa CFA Cape Flats Aquifer ch collar height CSIR Council for Scientific and Industrial Research DWA Department of Water Affairs DWS Department of Water and Sanitation (formerly DWA) EC Electrical conductivity EIA Environmental Impact Assessment ET Evapotranspiration GCS Groundwater Consulting Services GIS Geographical Information System GPT Geo-Pollution Technologies ha hectare ℓ/s litres per second m metres m/d metres per day MAE Mean Annual Evaporation mamsl metres above mean sea level MAP Mean Annual Precipitation MAR Mean Annual Runoff mbch metres below collar height mbgl metres below ground level mg/ℓ milligrams per litre mm millimetres mm/a millimetres per annum mS/m milliSiemens per meter NGA National Groundwater Archive RP reduction potential RWL rest water level SANDF south African National Defence Force SANRAL South African National Roads Agency Ltd SRK SRK Consulting (South Africa) (Pty) Ltd SRTM Shuttle Radar Topography Mission TDS total dissolved solids ToR Terms of References UWC University of the Western Cape WGS84 Since the 1st January 1999, the official co-ordinate system for South Africa is based on the World Geodetic System 1984 ellipsoid, commonly known as WGS84.
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GLOSSARY OF TERMS Anisotrophy: Anisotropy is the property of being directionally dependent Aquifer: a geological formation, which has structures or textures that hold water or permit
appreciable water movement through them. Borehole: includes a well, excavation, or any other artificially constructed or improved
groundwater cavity which can be used for the purpose of intercepting, collecting or storing water from an aquifer; observing or collecting data and information on water in an aquifer; or recharging an aquifer.
Brecciated: breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, which can be either similar to or different from the composition of the fragments
Fractured aquifer: fissured and fractured bedrock resulting from decompression and/or tectonic action. Groundwater occurs predominantly within fissures and fractures.
Groundwater: water found in the subsurface in the saturated zone below the water level or piezometric surface i.e. the water level marks the upper surface of groundwater systems.
Hardness: hard water is water that has high mineral content. Hard water is not a health risk but is a nuisance because of mineral build up on plumbing fixtures and poor soap and or detergent performance.
Heterogeneity: a material that is distinctly non-uniform Hydraulic conductivity: measure of the ease with which water will pass through earth
material; defined as the rate of flow through a cross-section of one square metre under a unit hydraulic gradient at right angles to the direction of flow (in m/d)
Intergranular and fractured aquifers: Largely medium to coarse grained granite, weathered to varying thicknesses, with groundwater contained in intergranular interstices in the saturated zone, and in jointed and occasionally fractured bedrock.
Intergranular Aquifer: generally unconsolidated but occasionally semi-consolidated aquifers. Groundwater occurs within intergranular interstices in porous medium. Typically occur as alluvial deposits along river terraces.
Transmissivity / transmissive: the rate at which a volume of water is transmitted through a unit width of aquifer under a unit hydraulic head (m2/d); product of the thickness and average hydraulic conductivity of an aquifer.
Vadose zone: the unsaturated zone above the groundwater level and below the ground surface.
Suggested reference for this report: GEOSS (2014). Groundwater Specialist Study - Cape Town International Airport Runway
Re-alignment and Associated Infrastructure Project. GEOSS Report Number: 2014/06-10. GEOSS - Geohydrological & Spatial Solutions International (Pty) Ltd. Stellenbosch, South Africa.
Cover photo: On-site augering in order to assess groundwater conditions. GEOSS project number:
2014_04-1233
Groundwater Specialist Study - Cape Town International Airport Runway Re-alignment and Associated Infrastructure Project
GEOSS Report No. 2014/06-10 08 September 2014 1
1. INTRODUCTION
1.1 Background
The Cape Town International Airport comprises two active runways: the primary runway
and a secondary runway bisecting it. The Airports Company South Africa proposes to re-
align the existing primary runway. This will allow the runway to be lengthened (from
3 201 m to 3 500 m) and create space for future development. The existing secondary
runway will be decommissioned and be incorporated into a new integrated taxiway system,
which will entail the construction of parallel taxiways and rapid exit taxiways to increase the
capacity of the system to handle air traffic. SRK Consulting (South Africa) (Pty) Ltd
(“SRK”) has been appointed to undertake the Environmental Impact Assessment (EIA)
for the project.
Cape Town International Airport is located in the sandy Cape Flats region, immediately
north of the N2 highway, approximately 20 km east of Cape Town’s Central Business
District (Map 1, Appendix A). The surrounding area consists of mixed land uses
including residential, industrial and portions of undeveloped and derelict land. The current
Cape Town International Airport property is approximately 975 ha in extent, incorporating
the passenger terminal and related airport support infrastructure to the west of the
runways, undeveloped land to the east of the runways, and a portion of land to the south,
belonging to the South African National Roads Agency Ltd (SANRAL), but nominally
falling within the Cape Town International Airport property boundary. The Airports
Company South Africa is currently in the planning phase for the acquisition of additional
parcels of land for the realignment project. Airports Company South Africa proposes to
acquire a portion of land (to the east of the airport) which is owned by the State (i.e.
National Housing Board) as well as the portion of land to the south owned by SANRAL to
accommodate the project and future expansion of the airport. A separate facility, presently
used by the police and administered by the South African National Defence Force’s
(SANDF) 35 Squadron, is located to the immediate east of the runways within the Cape
Town International Airport property boundary.
Airport Industria is located to the west of the terminal, while Bishop Lavis and Belhar are
located to the north of Cape Town International Airport (including the M10 (Modderdam
Road) and M12 (Stellenbosch Drive) roads). Modderdam industrial area lies between these
areas. Immediately east of the Cape Town International Airport property is a large, portion
of undeveloped land. Small sand dunes, with isolated patches of indigenous vegetation on
the dune ridges and some small wetland(s) occur in this area. The area is used for illegal
dumping, harvesting of firewood, opportunistic grazing and, possibly, initiation rituals. To
the east of this vacant land and the M171 (Symphony Way), new affordable housing has
been built forming the residential areas of Delft and Delft South. Crossroads, consisting
mostly of informal shacks, is located to the south of Cape Town International Airport
beyond the N2. Philippi Industria is located to the south of Crossroads.
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Figure 1: Location of Cape Town International Airport and proposed project footprint
(SRK, 2014).
The current runway system at Cape Town International Airport comprises two active
runways, namely:
Primary runway – 3 201 m long and 60 m wide; and
Secondary runway – 1 700 m long and 46 m wide.
The proposed project comprises the re-alignment of the existing primary runway to an
“18L/36R“ configuration§. The new runway will be 3 500 m long and approximately 75 m
wide and will be built to international specifications to enable the airport to receive Code F
aircraft such as the A380. The existing primary runway will no longer be used as a runway
but will form part of the integrated taxiway system. The existing secondary runway will be
§ A runway's compass direction is indicated by a number written in a shorthand format. A runway with a
marking of "18" is actually close to (if not a direct heading of) 180 degrees. This is a south compass heading. A runway with a marking of "36" has a compass heading of 360 degrees, that is, a north direction. For parallel runways, 'L' and 'R' is added to the runway number.
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decommissioned in order to accommodate the new re-aligned runway (Figure 2). In
Figure 2 take careful note of the orientation of the drawing.
Figure 2: Proposed construction of realigned runway and taxiways (Naco-SSI, 2008a)
The proposed development will entail the construction of two parallel taxiways and Rapid
Exit Taxiways to accommodate increasing air traffic. Parallel 3 500 m long taxiways will be
aligned to the west of the primary runway. Taxiway access to the SANDF apron will be
maintained and accommodated within the new runway and associated taxiway system. The
total area occupied by the proposed new taxiway system will be approximately 34 ha.
In total, the new runway and taxiways will have a footprint of approximately 82.7 ha,
almost all located within the existing Cape Town International Airport perimeter fence,
though Cape Town International Airport will have to acquire some land to accommodate
the re-alignment. Additional key elements of the project include:
Eight connecting rapid exit taxiways;
One aircraft isolated parking position and compass calibration pad;
One dual lane taxiway;
Future taxiway tie-ins; and
Widening of existing taxiway.
Associated infrastructure required as part of the project includes stormwater pipelines and
control systems, internal roads, security facilities, etc. In addition, several services such as
Telkom cables, power cables and fibre optic cables will have to be relocated.
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Other areas where fill material will be sourced or placed will increase the disturbance
footprint. Approximately 350 ha will be required for cut operations and 170 ha will be
required for fill operations. It is anticipated that construction will be in six phases.
1.2 Terms of Reference (ToR)
1.2.1 Broader project - Terms of Reference
The principle objectives for this study are to:
Describe the existing baseline characteristics of the study area and place these in a
regional context;
Identify and assess potential impacts resulting from the project (including impacts
associated with the construction and operation phases);
Identify and describe potential cumulative impacts resulting from the proposed
development in relation to proposed and existing developments in the surrounding
area;
Recommend mitigation measures to minimise impacts and/or optimise benefits
associated with the proposed project;
Take into account comments and concerns raised by interested and affected parties
and authorities during the Scoping Phase of the project; and
Recommend a draft monitoring programme, if applicable.
The main deliverable from this specialist study is an Impact Assessment Report with
appropriate maps, drawings and figures. This specialist study must consist of the following
components:
Baseline Description: a description of the environment of the study area in its
current state, relevant to the specialist’s field of study (the specialist must provide a
sufficiently comprehensive description of the existing environment in the study
area to ensure that an adequate assessment of the potential impacts of the proposed
development can be made. The baseline should include data collected through a
thorough literature review as well as field surveys); and
Impact assessment: an assessment of how the proposed expansion project would
alter the status quo as described in the baseline description and recommended
measures to mitigate and monitor impacts.
This study must devote considerable effort to the impact assessment and recommendations
for mitigation and not be overly focused on the baseline aspects. The study must also:
Provide an outline of the approach used in the study;
Clearly identify assumptions, limitations and sources of information;
Incorporate local knowledge if possible;
Include a short discussion of the appropriateness of the methods used in the
specialist study;
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Be based on accepted scientific techniques for the assessment of the data, where
possible, failing which the specialist is to make judgments based on professional
expertise and experience;
Provide a description of the affected environment, both at a site-specific level and
for the wider region, the latter to provide an appropriate context;
Address the uniqueness or “irreplaceability” of the site in the context of the
surrounding region, at a local, regional (and, if necessary, national) scale. This must
largely be based on a comparison to existing data sources, where available; and
Provide an indication of the sensitivity of the affected environment (Sensitivity, in
this instance, refers to the capacity of an environment to tolerate disturbance
(taking the environment’s natural capacity to recover from disturbance as well as
existing cumulative impacts into account).
The specialist must take care to obtain an understanding of the overall system of which
their specialist discipline is a part, in order to understand how changes to that system will
affect their subject.
Clear statements identifying the potential environmental impacts of the proposed project
must be presented. This includes potential impacts for the construction and operation
phases of the project. The specialist shall clearly identify the suite of potential direct,
indirect and cumulative environmental impacts in the study. Direct impacts require a
quantitative assessment which must follow the prescribed impact assessment methodology.
Indirect and cumulative impacts should be described qualitatively.
The specialist shall assess environmental impacts and also indicate any fatal flaws, i.e. very
significant adverse environmental impacts which cannot be mitigated and which will
jeopardise the project and/or activities in a particular area (if appropriate). Note that all
conclusions will need to be thoroughly backed up by scientific evidence.
Specialists must clearly state the impact to be assessed, followed by a brief description of
the impact and must then present the assessment of the impact, using the prescribed
impact rating system, in the format provided.
Specialists must recommend practicable mitigation measures or management actions that
effectively minimise or eliminate negative impacts, enhance beneficial impacts, and assist
project design. The significance of impacts must be assessed both without and with
assumed effective mitigation. If appropriate, specialists must differentiate between
essential mitigation measures which must be implemented (i.e. implicit in the “assuming
mitigation” rating) and optional mitigation measures which are recommended, i.e. “nice-to-
haves”, but which do not affect the impact rating. Unsubstantiated recommendations for
further studies should be avoided.
Specialists are also required to recommend appropriate monitoring and review programmes
to track the efficacy of mitigation measures (if appropriate). This should include where to
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monitor (locations), what (parameters), when (frequency and duration), how (methods) and
who.
1.2.2 Groundwater - specific Terms of Reference
With regard to this groundwater study, the following is required:
Describe and map the existing groundwater resources potentially affected by the
proposed project, including groundwater levels, groundwater quality, hydrological
linkages with other surface and groundwater resources and existing users of
groundwater resources in the area. The description of the affected environment
must be both at a site-specific level and for the wider region, the latter to provide
an appropriate context;
Define applicable legislative requirements regarding any permit applications
required;
Identify potential impacts of the proposed project on groundwater resources as
well as potential impacts of groundwater on the proposed development;
Assess the impacts of the proposed project on groundwater resources in the area
using the prescribed impact assessment methodology;
Identify and assess potential cumulative groundwater impacts resulting from the
proposed development in relation to proposed and existing developments in the
surrounding area;
Recommend practicable mitigation measures to avoid and/or minimise/reduce
impacts and enhance benefits. Assess the effectiveness of proposed mitigation
measures using the prescribed impact assessment methodology;
Recommend and draft a monitoring campaign to ensure the correct
implementation and adequacy of recommenced mitigation and management
measures, if applicable; and
Assist the EAP in addressing any relevant comments raised by stakeholders.
1.3 Study area
The Cape Town International Airport is located on the Cape Flats, near Cape Town
(Map 1, Appendix A).
1.3.1 General
The Cape Flats is characterised by an expansive, low-lying plain of white sand. The airport
is located on this flat plain. A poorly defined dune system is evident to the east of the
existing airport boundary. The dune system is stabilised by vegetation (mostly alien
vegetation with isolated patches of indigenous vegetation on the dune ridges) and with
wetlands in the dune slacks. Map 2 and Map 3 (Appendix A) show the study area
superimposed on a topocadastral map and aerial photograph respectively.
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1.3.2 Climate, Rainfall and Evaporation
The Western Cape has a semi-arid Mediterranean climate, which is strongly influenced by
the cold Benguela ocean current and coastal winds. The Cape Town area is characterised
by dry warm summer months (October to April) and wetter cool winter months (from May
to September).
Although rainfall occurs throughout the year, most rainfall occurs between May and
August. The long-term average monthly rainfall measured at the airport is shown in
Figure 3. The average annual rainfall is 569 mm/a (Schulze et al, 2008). The long-term
average monthly evaporation (ET) data, is also shown in Figure 3. The average annual
evaporation is 2 030 mm/a (Schulze et al, 2008).
Figure 3: Average monthly rainfall and evaporation (ET) for the study area (Schulze et al,
2008)
From the rainfall and evaporation data presented in Figure 3 it can be seen that the field
work (carried out on 24th June 2014) was completed during a high rainfall period. With this
monthly pattern of rainfall and evaporation groundwater recharge will be quite significant
as the rainfall occurs when evaporation is lowest. The risk of flooding will also be highest
during the months of June, July and August. Flooding can occur later in the year, especially
if an exceptionally heavy rainfall event occurs.
1.3.3 Regional Geology
The area is characterised by Quaternary-age sediments overlying basement rocks of the
Malmesbury Group. The Geological Survey of South Africa (now the Council for
Geoscience) has mapped the area at 1:250 000 scale (map sheet 3318 Cape Town). The
geological setting is shown in Map 4 (Appendix A). The main geology of the area is listed
in Table 1.
0
50
100
150
200
250
300
350
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mill
ime
ters
(m
m)
Month Mean ET Mean rainfall
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Table 1: Geological formations within the study area
Group Formation Lithology
Sandveld
Witzand Very fine to very coarse grained calcareous coastal dune sand
Springfontyn Fine to medium grained quartzitic sand
Malmesbury
Group Tygerberg
Grey to green phyllitic shale, siltstone and medium to fine
grained impure sandstone (greywacke)
Tertiary to Recent age sedimentary deposits underlie the area, comprising mainly calcareous
sands of the Sandveld Group, which are underlain by rocks of the Malmesbury Group.
The airport is predominantly underlain by deposits of the Witzand and the Springfontyn
Formations.
The Witzand Formation consists mainly of very fine to very coarse calcareous sand
and has abundant shells and shell fragments. These sands form an extensive
system of parabolic, vegetation-bound coastal dunes.
The Springfontyn Formation, which is aeolian in nature, comprises mainly fine to
medium grained quartzose sands. The grain size often increases with depth and
thin calcareous clay and peat lenses may locally be present. This Formation is
relatively uniform and free of inclusions. Vandoolaeghe (1989) suggest this
formation might be a decalcified facies of the Witzand Formation. The
decalcification is due to the relatively high surface and sub-surface permeability and
associated recharge rate of rainfall.
The basement rocks of the area consist of the Tygerberg Formation of the Malmesbury
Group. This formation consists mainly of alternating layers of grey to green phyllitic shale,
siltstone and medium to fine grained impure sandstone (greywacke). The transition
between the sands and the Malmesbury rocks is characterised by a clay layer which is the
product of weathering of the shale. The degree and depth of weathering can change over
relatively short distances (SRK, 2011). The thickness of this weathered zone can vary
significantly, but can be up to 44 m thick (Wright and Conrad, 1995). The bedrock
gradient, in broad terms, is from + 80 metres above mean sea level (mamsl) near Bellville
to -40 mamsl at Zeekoevlei. At the study area the bedrock elevation is approximately
+20 mamsl and it slopes down to the south-west.
Potential mineral resources on the proposed site include building sand and calcrete
deposits. These mineral resources are used primarily in the building industry. Calcrete is
also used as a sub-base for roads. Formal sand mining takes place to the north-east of the
airport.
1.3.4 Regional Hydrogeology
The Sandveld Group deposits constitute what is known as the Cape Flats Aquifer (CFA).
The aquifer is regionally unconfined and internally is essentially free of lateral hydraulic or
geological boundaries which may influence regional behaviour. The aquifer is not
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hydrologically linked to any other aquifer except the talus / scree material along the foot of
the mountains in the west. The aquifer is extensive and pinches out against “impermeable”
boundaries in the east, west and north whilst the southern boundary is defined by the
coastline extending along False Bay, between Muizenberg and Macassar. The thickness of
the sand at the study area is approximately 25 m. The saturated thickness of the aquifer in
the study area is approximately 20 – 25 m.
Sands of the Witzand and Springfontyn Formations constitute the main target for
groundwater abstraction from the CFA. These sands range in grain size from very fine to
very coarse and are generally well-sorted and well-rounded. The aquifer transmissivity
within the study area is approximately 100 m2/d. These formations do, however, possess a
degree of heterogeneity and anisotropy due to vertical and lateral grain size gradation and
the occurrence of sandstone, clay or calcrete lenses. The vertical permeability can be
smaller by a factor of 10 to 20 (or even up to 100) when compared with the horizontal
permeability. The bedrock, which consists of weathered Malmesbury Group meta-
sediments, is generally regarded as an impervious basement. However in places the
Malmesbury Group contains brecciated and transmissive fault zones resulting in a fractured
aquifer with high yielding boreholes. The aquifer is recharged directly principally from
rainfall. The average annual rainfall, which occurs mainly in winter and early spring, ranges
from 500 to 800 mm across the Cape Flats. Vandoolaeghe (1989) confirmed that net
groundwater recharge through sandy soils of the primary aquifer varies between 15% and
37% of the annual precipitation. The evaporation rates from the aquifer have been
measured at the surface to be 2 x 10-8 m/s in summer. Evaporation is higher towards the
eastern portion of the CFA, where shallow calcrete cause perched water tables (Gerber,
1976). Groundwater flow in the Cape Flats is either west to Table Bay or south to False
Bay. In the regional study on the CFA (Wright and Conrad, 1995) indicate the flow
direction is mainly towards the south-west within the study area. According to the regional
1:500 000 scale groundwater map of Cape Town (3317), produced by the Department of
Water Affairs, the project area does host an intergranular aquifer with an average borehole
yield of 0.5 ℓ/s to 2.0 ℓ/s (Map 5, Appendix A).
The groundwater quality within the main part of the aquifer generally has a low salinity yet
a relatively high hardness. Due to the depositional history of the aquifer, zones of saline
water do occur at different stratigraphic levels. The high evaporation rates in summer also
result in high salinity areas. For the study area the regional groundwater quality, indicated
by Total Dissolved Solids is in the range from 500 mg/ℓ to 2 000 mg/ℓ. This equates to
an Electrical Conductivity range of 70 mS/m to 300 mS/m (DWAF, 2000) (Map 8,
Appendix A).
The regional groundwater vulnerability is shown in Map 9 (Appendix A) as determined by
DWA (2005). The groundwater in the entire area is classified as having a “medium to
high” vulnerability to surface based contamination.
Based on a conceptual understanding of the site geohydrology a schematic diagram was
developed of the status quo (Figure 4).
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Figure 4: Conceptual model for the current status at the CTIA
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1.4 Assumptions and Limitations
1.4.1 Assumptions
There is often a high degree of spatial variability in groundwater studies, relating to soil
conditions, geological setting and geohydrological parameters. It is difficult to glean a lot
of geohydrological information without drilling boreholes and sampling. Thus borehole
information and associated data (such as drill logs, water levels, and groundwater
chemistry) are useful for most geohydrological studies. It is assumed that the
geohydrological conditions are relatively consistent between sampling points, unless there
is clear evidence to the contrary. A lot of the work is based on averages, i.e. average annual
rainfall and average groundwater conditions. The absence of water level and rainfall data
make it difficult to make predictions regarding extreme events. For example: a very high
intensity rainfall event in winter, when the groundwater levels are at the shallowest (i.e.
closest to ground level), may result in flooding of certain portions of the study area. There
is insufficient data available to accurately predict the impact of the extreme conditions.
Assumptions will therefor need to be made in this regard.
1.4.2 Limitations
The limitations pertain to the lack of groundwater monitoring data and long-term records.
The study area is not entirely fenced and any monitoring sites established in the past have
been vandalized and rendered non-functional. Thus long-term groundwater level
responses cannot be determined. Additionally, the geographical distribution of existing
monitoring sites is not ideal, but is considered adequate for the purposes of this study.
There are more groundwater quality monitoring sites in the western portion of the study
area than in the east. This is because the airport infrastructure and fuel storage areas are
located in the western portion of the area. The borehole drilling in the eastern portion is
limited so geological detail in that area is sparse. For example, there is not accurate
information on the depth, thickness and extent of the calcrete layers in the eastern portion.
The areas prone to flooding are those where impermeable geological layers are shallow,
resulting in perched aquifers. Without detailed geological information it is difficult to
forecast exactly where flooding is likely to occur. However sufficient data does exist to
enable interpretation of a groundwater level map. For this reason no borehole drilling was
included in this geohydrological specialist study. Hand augering of piezometers to measure
groundwater levels and quality sufficed for this project. Deeper borehole drilling was not
considered necessary for this study.
2. METHODOLOGY
The methodology to achieve the objectives of the ToR is as follows:
Undertake a desktop study of existing information available from relevant
literature, the National Groundwater Archive (NGA), the Department of Water
and Sanitation (DWS) and previous groundwater monitoring undertaken by ACSA;
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Undertake a site visit to identify and verify the positions of boreholes at the Cape
Town International Airport and surrounding areas;
Undertake a hydrocensus targeting large industries, schools and institutions in the
area; and
If required, install a limited number of piezometers to establish groundwater levels
and quality.
The approach to the groundwater study was to obtain as much existing data and
information as possible. This included requesting borehole data from the Department of
Water Affair’s National Groundwater Archive (NGA), including borehole yields,
groundwater depths and groundwater quality. No borehole yield data was available and the
Electrical Conductivity data obtained was not usable.
Many relevant reports were obtained from SRK and the Airports Company South Africa.
The reports were reviewed and relevant information retrieved from the reports. This
information, if spatially referenced, was then captured in a Geographical Information
System (GIS) set up for the project by GEOSS. There was a lot of useful information
from the Geo-Pollution Technologies (GPT) (2014) report on groundwater levels and
quality in the western part of the study area.
A list of all the references is included in Chapter 8, but key sources of information
pertaining to this study and reviewed in detail include:
Arcus Gibb, 2004. Sub-soil drainage along the main runway of Cape Town
International Airport
CSIR, 2006. Impact of Alien Invasion Vegetation Report
Following on from the literature review discussions were held with those who have
knowledge of the aquifer in the vicinity of the Cape Town International Airport.
Discussions were held with SRK (Sheila Imrie); Airports Company South Africa (Sean
Bradshaw); University of the Western Cape (Dr Jaco Nel - GCS); and RoundOne Drilling
Solutions (Jason Greeff).
This was followed up by field work carried out on 24 June 2014, to try and locate previous
relevant field installations as well as to obtain new data. Note the western portion of the
study area (within the existing airport boundaries) was not visited, however data was
obtained from previous reports. In the eastern portion of the study area all previously
referred to boreholes/piezometers could not be located (however the Naco-SSI (2010) data
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was still used in this project). The area is unfenced, thus open to the public and prone to
vandalism (and illegal dumping).
Hand augering of 70 mm diameter holes was completed along the proposed route of the
re-aligned runway in order to establish groundwater depth and the presence of
impenetrable layers. Piezometers were installed at two of the auger sites, where
groundwater was intersected. Samples were collected at these two sites and submitted to
the CSIR for analysis.
This approach is appropriate for the assessment of shallow groundwater and vadose zone
conditions. This approach, however, does not provide much information about the deeper
CFA. In order to investigate the CFA more thoroughly within the study area it will be
necessary to drill boreholes through the hard layers down to the bedrock. It was decided
that such drilling was not required to meet the Terms of Reference for this project. In
addition the primary purpose of the groundwater study is to assess the likelihood of
flooding occurring and the drilling of the total depth of the CFA will not contribute
significantly to the prime purpose of this study.
The combined approach of a literature survey and field work enabled characterisation of
the area.
3. RELEVANT EXISTING DATA AND DATA AQUIRED FROM FIELD WORK
3.1 Desktop investigation
3.1.1 National Groundwater Archive (NGA)
A search was completed of the Department of Water Affairs groundwater database,
specifically the National Groundwater Archive (NGA). The results obtained for the study
area are presented in Map 2 and Map 3 (Appendix A), and are listed in Table 2. A total
of 16 NGA sites were obtained. The data associated with these sites is limited; however
some groundwater level and groundwater quality does exist (presented in Table 2).
Table 2: National Groundwater Archive data within the study area
ID Map ID Latitude Longitude Water Level
(mbgl) pH
3318DC00263 NGA01 -33.98023 18.60093 1.28 7.3
3318DC00262 NGA02 -33.97828 18.59787 1.34 7.6
3318DC00261 NGA03 -33.97773 18.59926 1.55 -
3318DC00264 NGA04 -33.97495 18.59732 2.22 7.5
3318DC00265 NGA05 -33.97495 18.59759 2.37 7.3
3318DC00270 NGA06 -33.97467 18.61287 3.79 7.6
3318DC00267 NGA07 -33.97384 18.59454 1.70 7.4
3318DC00274 NGA08 -33.97273 18.59870 2.19 -
3318DC00271 NGA09 -33.97217 18.59426 0.72 7.2
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3318DC00273 NGA10 -33.97134 18.59370 1.03 7.5
3318DC00272 NGA11 -33.97134 18.59398 0.89 -
3318DC00275 NGA12 -33.96773 18.59732 2.4 7.2
3318DC00269 NGA13 -33.96551 18.60870 0.87 7.6
3318DC00266 NGA14 -33.95912 18.59370 0.56 7.3
3318DC00268 NGA15 -33.95912 18.60870 1.45 7.7
3318DC00001 NGA16 -33.95761 18.59883 2.66 -
3.1.2 University of the Western Cape (UWC)
There is very little continuous monitoring of groundwater levels within the Cape Flats
Aquifer. However the University of the Western Cape (UWC) did drill a number of
research boreholes which were equipped with borehole water level sensors and data
loggers. It was not possible to track down any reports on the UWC boreholes; however it
was possible to obtain some groundwater level data. The geological setting, as revealed by
these boreholes, consists of approximately 30 m of unconsolidated sands, with peat layers,
overlying Malmesbury Group bedrock. The sands increased in grain size with depth. The
boreholes were drilled to approximately 130 m and fractures were intersected in the
bedrock at approximately 100 m. The primary aquifer had a yield of approximately 2 ℓ/s
and the fractures in the bedrock were higher yielding at approximately 5 ℓ/s (pers. comm.,
Dr J. Nel, 2014).
The groundwater level data was obtained for two of the UWC boreholes and the
groundwater levels are plotted in Figure 5 and Figure 6. The UWC boreholes are
approximately 2.8 km north-east of the Cape Town International Airport.
Figure 5: Long term borehole water level graph – UWC (Bh02)
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Figure 6: Long term borehole water level graph – UWC (Bh06)
From Figure 5 the range in the natural seasonal borehole water level (UWC_02)
fluctuation is approximately 0.68 m, with the water level being approximately 4.87 mbgl at
its shallowest. From Figure 6 it can be seen the natural seasonal borehole water level
(UWC_06) range is approximately 0.91 m, with the shallowest groundwater level being
approximately 5.19 mbgl.
3.1.3 Proposed Belhar Housing Development
From a study completed by GEOSS at a proposed development in Belhar (GEOSS, 2013),
six piezometers were installed on site and the groundwater levels were measured from
summer to winter. The range in water levels was greater than measured at UWC, with the
average range of the six sites being 1.590 m. The smallest fluctuation measured was
1.497 m and the largest was 1.691 m. The Belhar study site is 3 km north-east from the
Cape Town International Airport.
3.1.4 CSIR (2006)
The CSIR completed a study in 2006 to address the impact of alien vegetation removal on
groundwater levels (CSIR, 2006). The extent of the study area and results obtained by the
CSIR (2006) study area are shown in Figure 7A and Figure 7B respectively. The CSIR
study area focussed more on the current runway area. The current study area is just to the
east of the CSIR (2006) study area. The results of the CSIR study indicated:
The alien acacia vegetation is not measurably discharging groundwater by
evapotranspiration;
For the vegetated areas the groundwater recharge was 7%;
For the areas that are grassed and cleared of vegetation, the recharge rate increased
to 24% of the rainfall;
Therefore groundwater levels and risk of flooding are higher in cleared areas.
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For areas cleared of vegetation the following is concluded:
short term: high risk of flooding and very high risk to runway integrity
long term: moderate risk of flooding and high risk to runway integrity if an area of
133 ha is cleared simultaneously.
Figure 7 A & B: A. Study site, recharge zones and pilot study monitoring borehole locations. B. Groundwater levels in metres below ground level (mbgl) under 133 hectares
cleared and un-vegetated ground subjected to wet conditions (CSIR, 2006).
The CSIR (2006) stated the risks could be mitigated to a very low risk if the following
measures are taken:
The clearance and re-vegetation is conducted in a phased manner.
Groundwater is abstracted from the eastern part of the site.
However please note the new runway will be built substantially higher than the existing
ground levels (SSI, 2010) (see Section 3.1.5) thus the CSIR (2006) mitigation measures are
considered unnecessary.
3.1.5 Naco-SSI (2010)
A hydrological investigation was completed by Naco-SSI in February 2010 (Naco-SSI,
2010) for the new aligned runway RWY 18-36 and associated taxiways. The purpose of
this hydrological study was to investigate and record the existence and fluctuation of the
groundwater level in the study area. This information was required to determine the
optimum fixed construction ground level posing the minimum risk of ground water
influence on the detail design. In the report it is stated that a common feature of the Cape
Flats area is the general occurrence of a shallow groundwater level, during the winter
rainfall season. Perched groundwater levels have also been observed at several locations
towards the south-eastern end of the proposed runway area. It is apparent that these
groundwater levels are elevated in areas of hard impermeable calcrete where the sand
overburden is of limited thickness.
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To accurately determine the actual levels of the groundwater as well as the magnitude of
the seasonal fluctuation in these levels, a set of 15 piezometers were installed across the
future construction site. During November 2008 these piezometers were installed by
Fairbrother Geotechnical Engineering, mostly by means of water jetting, to a depth of
5.0 m below ground level. Several of the holes had to be core drilled due to the presence
of shallow hard calcrete lenses. The installation of the piezometers was completed in
January 2009 with the first readings taken at the end of that month.
There are two critical factors to consider when establishing a ground level for future
construction purposes. The one is the risk of a shallow groundwater level below the
runway and the other is the cost involved in site clearing and preparing of the site for
construction work (mass earthworks). Naco-SSI (2010) stated that a geotechnical
investigation revealed the presence of scattered, hard, calcrete lenses to the south-east of
the site resulting in several perched groundwater levels. It is also important to note that
removal of vegetation during site preparation could have an influence on the level of the
groundwater level as well. Due to the vast surface area involved the slightest raising or
lowering of the construction ground level can lead to a very large increase or saving in
construction cost (Naco-SSI, 2010).
Naco-SSI (2010) stated the results obtained indicated:
The shallowest water level recorded in relation to existing ground level was 410 mm
below existing ground level (0.410 mbgl).
The deepest water level recorded in relation to existing ground level was (4 590 mm
below existing ground level (4.590 mbgl).
The smallest seasonal fluctuation recorded was 150 mm (piezometer No. 10) and
the highest fluctuation was 1 330 mm.
The average fluctuation between the shallowest and deepest recording was
616 mm.
3.1.6 Geo Pollution Technologies (GPT, 2014)
Geo Pollution Technologies (GPT) was appointed by the Airports Company of South
Africa to conduct groundwater sampling at the Cape Town International Airport. Water
was collected from 32 on-site monitoring wells. All the monitoring wells are to the west of
the existing runway. Samples were collected and analysed for organic parameters. The
depth to groundwater was measured to lie between 1.585 mbgl and 3.070 mbgl. The field
work was completed on 13 and 14 November 2013 (a time when the groundwater levels
are expected to be quite shallow following the winter rains). Being a primary aquifer, the
water levels respond quickly to rainfall events and are recharged rapidly during the wet
winter months. The existing monitoring well network can be used for on-going
monitoring. The groundwater levels and basic water chemistry is listed in Table 3. Please
note with regard to the values in Table 3, the groundwater levels and pH were measured
on 13 and 14 November 2013 and the EC readings are from sampling carries out in 2012.
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Table 3: Groundwater levels, EC and pH, measured at the Airports Company South Africa groundwater monitoring sites by GPT (2014).
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4. IMPACT IDENTIFICATION AND ASSESSMENT
Each major potential geohydrological impact (whether positive or negative) associated with
the proposed project has been outlined below and rated in terms of its significance using an
impact rating methodology. The degree of confidence of each assessment is also stated.
4.1 Direct Impacts
With regard to the groundwater setting of the area and the proposed runway and associated
taxiways development, the activities that may impact on the groundwater of the study area
are considered to be:
1. Removal of alien vegetation and associated impact on groundwater levels and
groundwater quality (construction phase).
2. Earthworks required to level the area (i.e. cut and fill) and removal of existing
wetlands on groundwater levels and quality (construction phase).
3. Presence of increased impermeable surfaces (i.e. the new runway and taxiways)
on groundwater levels and quality (operational phase).
These are discussed individually, but are also considered collectively as it is anticipated that
all three will take place. The impacts during the construction phase are presented in
Figure 11. The impacts during the operational phase are shown in Figure 12.
4.1.1 Removal of alien vegetation and associated impact on groundwater levels (construction phase).
4.1.1.1 Description
Prior to the development of the runway and taxiways it will be necessary to remove the
alien vegetation on the portion of the site to the east of the existing airport infrastructure.
It is most probable that all the alien vegetation will have to be removed with regular follow
up to ensure there is no re-growth. In all likelihood the alien vegetation will be replaced
with a grass after the earthworks have been completed. The CSIR (2006) reported the
groundwater recharge is 7% of the rainfall in vegetated areas. Due to the high permeability
of the soils and sands this relatively low recharge rate is attributable to the vegetation
intersecting and abstracting the soil moisture as it passes the root zone. The CSIR (2006)
report stated that in grassed and cleared areas the recharge will be 24%. It is expected that
520 ha of land will be cut and filled (see Section 1.1) for the proposed new upgrades.
Based on a mean annual rainfall of 569 mm/a, the recharge volumes will be:
207 116 m3/a for the lower recharge scenario (7% of mean annual rainfall); and
710 112 m3/a for the higher recharge scenario (24% of mean annual rainfall).
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Figure 11: Conceptual model post-construction phase
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Figure 12: Conceptual model of the conditions during the operational phase
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This is a concern for the areas where groundwater is already relatively shallow, as presented
in Map 6 (Appendix A). The Naco-SSI (2010) report stated the shallowest groundwater
level measured to be 0.410 mbgl. Over an area of 520 ha this increased recharge amounts
to 0.0967 m (say 100 mm) (for cleared land – i.e. high recharge scenario), so the
groundwater level could be as shallow as 0.310 mbgl. In addition to this Arcus Gibb
(2004) warns that isolated large rainfall events could significantly raise the groundwater
level for short periods of time.
These highlight the potential for flooding if the land surface is not sufficiently elevated, as
well as for ponding in lower lying areas where the groundwater surface exceeds the land
surface. Ponding would also be expected in areas where the ground surface is lowered
during the construction activities. Associated to ponding on the site would be an increased
number of wetland birds and animals.
4.1.1.2 Proposed Mitigation Measures
CSIR (2006) reported that groundwater levels will rise more rapidly in cleared areas and the
risk of flooding is higher in these areas. The integrity of the runway is considered to be at
high risk if the groundwater level comes within 0.8 m of the ground surface. For the
reduction in the rapid increase in the groundwater level in the wet winter months, the type
of grass planted needs to be a grass type that has a deep root system and is able to absorb
and transpire moisture at a relatively high rate. This will have limited efficacy as the rainfall
and recharge mainly occurs when plants and vegetation are relatively dormant. It is outside
the specialist’s area of expertise however it is assumed grass is best for the vegetative cover
in the runway environs.
There are two concerns with regarding to flooding. Firstly during high rainfall events the
runway is to have no standing water on it and this is being addressed by having a
“thickened keel” section (Naco-SSI, 2008a). The “thickened keel” means the central
portion (midline) of the runway will be slightly elevated and rainwater will drain off the
runway away from this central “keel”. Secondly the runway and taxiways must be elevated
above the ambient groundwater level. The re-aligned runway will be built substantially
higher than the existing ground levels (Naco-SSI, 2010). If the shallowest groundwater
level can be 0.237 mbgl and the CSIR (2010) consider 0.8 m to be the minimum distance
from the groundwater level to the runway, then it needs to be elevated at least 0.527 metres
above ground level. The second issue is to ensure the runway environs is not flooded as
this will increase the favourability of the habitat for birds and wildlife. This latter issue is
addressed in the two paragraphs above.
4.1.1.3 Assessment of impact
The study area comprises only a small portion of the Cape Flats Aquifer and removal of
alien vegetation from the footprint of the proposed development will result in a seasonal
increase of groundwater levels across the area where the alien vegetation has been cleared,
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however it will not extend much beyond the cleared areas. Thus the extent of the impact is
considered to be “local”. It is unlikely that groundwater levels will rise an additional height
(i.e. greater than the norm) beneath adjacent residential areas. This impact is considered to
be of medium intensity and be of long term duration). The consequence rating is thus
“very low” (i.e. a score of 4). The probability is “probable” (i.e. > 70% - 90% chance of
occurring). The overall significance is thus “very low”. The status is considered to be
negative. The level of confidence associated with the assessment is “high”. With the
implementation of mitigation measures the intensity reduces from “medium” to “low” and
the consequence remains “very low”; the probability remains “possible” and the
significance remains “insignificant”. The status is still “negative” and the confidence of the
assessment remains “high”. Table 9 presents the results of the assessment of Impact 1. .
Table 9: Impact 1a – Alien vegetation clearing and groundwater levels Extent Intensity Duration Consequence Probability Significance Status Confidence
Without mitigation
Local Medium Long term Medium Probable MEDIUM - ve High
(1) (2) (3) (6)
Essential Mitigation Measures:
Ensure the vegetation / plant material replacing the alien vegetation has a high evapotranspiration rate.
Correct design of the runway and taxiways and elevation of the runway and taxiways.
With mitigation
Local
(1)
Low
(1)
Long term
(3)
Low
(5) Possible VERY LOW - ve High
4.1.1.4 Statement of acceptability
The impact of the removal of the alien invasive vegetation, from the perspective of
groundwater levels, is acceptable, and is not a fatal flaw to the project.
4.1.2 Removal of alien vegetation and associated impact on groundwater quality (construction phase).
4.1.2.1 Description
The removal of alien vegetation is not considered to have a tangible negative impact on
groundwater quality. The removal of alien vegetation will result in increased groundwater
recharge and a possible improvement in groundwater quality (assuming no surface based
contamination). For this reason there are no mitigation measures applicable to the issue of
groundwater quality.
4.1.2.2 Proposed Mitigation Measures
No mitigation measures are required.
4.1.2.3 Assessment of impact
This impact is considered to be of low intensity and have long-term duration. The
consequence rating is thus “very low” (i.e. a score of 3). The probability is “probable” (i.e.
> 70% - 90% chance of occurring). The overall significance is thus “very low”. The status
is considered to be positive. The level of confidence associated with the assessment is
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“high”. No mitigation measures need to be implemented. Table 10 presents the results of
the assessment of Impact 1 with regard to groundwater quality.
Table 10: Impact 1b – Alien vegetation clearing and groundwater quality Extent Intensity Duration Consequence Probability Significance Status Confidence
Without mitigation
Local Low Long term Low Probable LOW + ve High
(1) (1) (3) (5)
Essential Mitigation Measures:
None
With mitigation
Local
(1)
Low
(1)
Long term
(3)
Low
(5) Probable LOW + ve High
4.1.2.4 Statement of acceptability
The impact of the removal of the alien invasive vegetation, from a groundwater quality
perspective, is acceptable, and is not a fatal flaw to the project.
4.1.3 Impact on groundwater levels due to earthworks, ground surface levelling and runway construction (construction phase)
4.1.3.1 Description
Prior to the construction of the runway, earthworks will be required on the site which will
be post- alien vegetation removal (or possibly the two activities will run in parallel). The
earthworks will require extensive “cut and fill” to level the area. These levels have however
not yet been defined and will depend on the final groundwater levels and the cost of fill
material. The engineers have indicated that the final levels on the eastern side of the site
have not been determined and discussion with the project geohydrologist is necessary for
finalisation of the design. The airport runways and taxiways will also be constructed during
this phase. With the levelling process it is recommended the elevation of the ground
surface is kept as high as possible. In other words as shallow groundwater occurs in many
parts of the area in winter, the land surface must be as elevated as economically feasible in
order to avoid ponding. In addition, this is also applicable to the runway and taxiways as
well.
4.1.3.2 Proposed mitigation measures
There are no relevant mitigation measures.
4.1.3.3 Assessment of impact
With regard to this impact, the extent is “local”; the intensity is “local”; and the duration is
considered “medium term”. The consequence rating is thus “very low”. The probability
of this impact occurring is considered to be “possible” (i.e. 40 % – 70% chance of
occurring). The significance rating is thus “insignificant”. The impact will be negative if it
occurs and the assessment is completed with a “high” level of confidence.
As no mitigation measures are applicable the significance rating thus remains
“insignificant”. The impact will be negative if it occurs and the assessment is completed
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with a “high” level of confidence. Table 11 lists the impact and associated rating, as well as
the rating with mitigation measures in place.
Table 11: Impact 2a – Impact of earth works and construction activities and groundwater levels
Extent Intensity Duration Consequence Probability Significance Status Confidence
Without mitigation
Local Local Medium
term Very low Possible
(40-70%) INSIGNIFICANT - ve High
(1) (1) (2) (4)
Essential Mitigation Measures:
None
With mitigation
Local
(1)
Low
(1)
Short term
(1)
Very low
(3)
Improbable
(<40%) INSIGNIFICANT - ve High
4.1.3.4 Statement of acceptability
The extensive earthworks planned for the site are not considered a fatal flaw to the project
and they are acceptable from a geohydrological point of view.
4.1.4 Impact on groundwater quality due to earthworks, ground surface levelling and runway construction (construction phase)
4.1.4.1 Description
During these earthwork activities the threat does exist of machinery leaking fuel and
lubricants onto the ground which may then contaminate groundwater. Even though the
groundwater is not planned to be used for domestic consumption, all measures should be
taken to prevent the contamination of the groundwater.
4.1.4.2 Proposed mitigation measures
During the earthworks, it is imperative the equipment is well maintained so that there are
no leakages of fuel or lubricants. Where the vehicles are stored overnight they must be
parked on a hardened surface, otherwise drip trays and / or sand trays must be positioned
under the equipment. The filling of the vehicles with fuel must be done on a hardened
impermeable surface, preferably on concrete surface, so ensure no spillages enter the
ground. Service bays also need to be hardened, impermeable surfaces, also a concrete
surface is best. If any spillage occurs the impacted material must be removed and taken in
the proper manner to a hazardous landfill (Vissershok). One must be particularly vigilant
when mobile fuel trucks are used for filling heavy equipment. Operators must also be
trained and informed about the procedures should a spillage occur.
4.1.4.3 Assessment of impact
The shallow groundwater within the development footprint is saline and not suitable for
domestic use (this may well apply to the deeper aquifer as well) and high levels of localised
contamination are present in the CFA. It is still essential that the quality of the CFA (both
the perched and deeper aquifer) must be protected and further contamination prevented.
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With regard to this impact, the extent is “local”; the intensity is “local”; and the duration is
considered “medium term”. The consequence rating is thus “very low”. The probability
of this impact occurring is considered to be “possible” (i.e. 40 % – 70% chance of
occurring). The significance rating is thus “insignificant”. The impact will be negative if it
occurs and the assessment is completed with a “medium” level of confidence.
With regard to the impact rating, taking into account mitigation measures, the large
construction companies do operate with high levels of awareness about contamination
prevention and the impacts are re-assessed below. The duration factor is reduced to “short
term” and the probability rating reduces to “improbable” (i.e. < 40 % chance of occurring).
The significance rating thus remains “insignificant”. The impact will be negative if it
occurs and the assessment is completed with a “high” level of confidence. Table 12 lists
the impact and associated rating, as well as the rating with mitigation measures in place.
Table 12: Impact 2b – Impact of earth works and construction activities and groundwater quality
Extent Intensity Duration Consequence Probability Significance Status Confidence
Without mitigation
Local Local Medium
term Very low Possible
(40-70%) INSIGNIFICANT - ve High
(1) (1) (2) (4)
Essential Mitigation Measures:
Service the vehicles on an impermeable surface
Drip trays / sand trays must be placed under engines when stored overnight or for longer periods.
Filling of vehicles must be done on an impermeable surface.
Vehicles filled on site by a tanker must have a drip tray / sand tray placed beneath the filling nozzle.
All relevant staff must be made aware of trying to prevent spillages and leakages and then trained in the procedures to follow
up should a leak or spillage occur.
With mitigation
Local
(1)
Low
(1)
Short term
(1)
Very low
(3)
Improbable
(<40%) INSIGNIFICANT - ve High
4.1.4.4 Statement of acceptability
The extensive earthworks planned for the site are not considered a fatal flaw to the project
and they are acceptable from a geohydrological point of view.
4.1.5 Impact of the presence of the runway and taxiways on groundwater levels (operational phase)
4.1.5.1 Description
The runway and taxiways are impermeable surfaces and this will reduce direct recharge to
groundwater beneath these surfaces. The recharge of the CFA taking place at the airport is
very small in comparison with the total aquifer recharge area of the CFA and will have an
extremely small impact. However the run-off from the runway and taxiway will be
concentrated along the edges of these surfaces. Where these “edges” occur above shallow
impermeable geological layers, the possibility for ponding to occur will increase. For the
airport this is a negative issue as it could lead to flooding of the runway and taxiways and
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GEOSS Report No. 2014/06-10 08 September 2014 31
attract birds to the area. The planning of storm water run-off must be addressed by the
design engineers to adequately manage run-off from these areas.
The run-off water may also have a chemistry that is slightly different to natural rainwater
due to accumulation of tyre compounds on the surface from landing planes. This could
potentially impact groundwater quality.
4.1.5.2 Proposed mitigation measures
The measures proposed by Naco-SSI (2008b) with regard to managing the edges of the
impermeable surfaces and avoidance of ponding are considered acceptable. These include:
Designing slopes of pavement and adjacent areas to create rainfall runoff flowing as
quickly as possible towards drainage inlet structures, without exceeding the
maximum allowable slopes for safe ground traffic and aircraft movement area, as
prescribed in the relevant design standards.
Designing structures for inlet, collection and storage of surface drainage runoff:
o Having sufficient capacity to collect, transport and store the resulting
volumes of water according to the agreed rainfall intensity-duration limits.
o Being structurally sufficiently strong to withstand static and dynamic loads
generated by critical aircraft, vehicles and equipment for the design life.
o Being installed so that all expected traffic will encounter no operational
obstruction or safety hazard.
4.1.5.3 Assessment of impact
The assessment is only applicable where runways, taxiways and impermeable surfaces
occur. The extent of the impact is local, of short intensity and of short duration. Table 13
summarizes the impacts for this factor.
Table 13: Impact 3a – Runway and taxiways and groundwater levels Extent Intensity Duration Consequence Probability Significance Status Confidence
Without mitigation
Local Local Short term
Very low Definite
(>90%) Very Low - ve High
(1) (1) (1) (3)
Essential Mitigation Measures:
Include sub-surface drainage and storm water removal at the edges of impermeable surfaces where impermeable geological
layers are shallow.
With mitigation
Local
(1)
Low
(1)
Short term
(1)
Very low
(3)
Possible
(<40%) Very Low - ve High
4.1.5.4 Statement of acceptability
The impact of the presence of the runway and taxiways on groundwater levels are
considered acceptable.
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GEOSS Report No. 2014/06-10 08 September 2014 32
4.1.6 Impact of the presence of the runway and taxiways on groundwater quality (operational phase)
4.1.6.1 Description
The runway and taxiways are impermeable surfaces and thus will accumulate mainly rubber
compound from the wheels of the aircraft. This material and any others that may
accumulate will be washed off these surfaces during a rain event. The run-off from the
runway and taxiway will be concentrated along the edges of these surfaces and may result
in areas of localised groundwater recharge.
4.1.6.2 Assessment of impact
The assessment is only applicable where runways, taxiways and impermeable surfaces
occur. The extent of the impact is local, of short intensity and of short duration. Table 14
summarizes the impacts for this factor.
Table 14: Impact 3b – Runway and taxiways and groundwater quality Extent Intensity Duration Consequence Probability Significance Status Confidence
Without mitigation
Local Local Short term
Very low Definite
(>90%) Very Low - ve High
(1) (1) (1) (3)
Essential Mitigation Measures:
None.
With mitigation
Local
(1)
Low
(1)
Short term
(1)
Very low
(3)
Definite
(>90%) Very Low - ve High
4.1.6.3 Statement of acceptability
The impact of the presence of the runway and taxiways on groundwater quality are
considered acceptable.
4.2 Cumulative Impacts
It is expected that there will be no cumulative impacts from the presence of the runway
and taxiways on groundwater at the local, regional or national scale. The increased tarred
surface will result in localised increased storm run-off which must be managed, and the
removal of vegetation may result in a small increase in water levels. These are not
considered significant.
4.3 Indirect Impacts
There are no anticipated indirect impacts of the development on the groundwater of the
area. In addition it is assumed that existing fuel storage tanks will not be moved and the
current fuel storage facilities are not being changed, increased in capacity or moved.
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GEOSS Report No. 2014/06-10 08 September 2014 33
5. CONCLUSION
It is concluded (and confirmed) that groundwater is very shallow beneath the study area
and the design engineers are well aware of this. The shallow, hard, laterally discontinuous
and impermeable calcrete (and sandstone) layers result in perched groundwater levels. This
is especially the situation in the wet winter months. The groundwater levels are particularly
shallow at the south-eastern and north-western ends of the proposed runway, and increase
towards the east as presented in Map 6 (Appendix A). These geological layers will result
in differential compaction of the site and these are the areas where flooding will occur if
the antecedent conditions are favourable in combination with a very heavy rainfall event.
The delineation of the calcrete layers (and causes for the perched aquifer conditions) is
considered important so that mitigation measures can be taken to avoid flooding of these
areas.
This specialist study has also addressed the concerns raised by Interested and Affected
Parties during the Scoping Phase of the project.
As there is no planned abstraction of groundwater, Section 21a of the National Water Act
is not applicable and it is likely that a Section 21a Water Use License Application is not
required from a groundwater perspective. This will be addressed and reviewed by the
Department of Water and Sanitation.
6. RECOMMENDATIONS
It is recommended that the levelling of the site be carried out with the objective to keep the
land surface as high above mean sea level as possible. In addition the runway and taxiways
need to be elevated above this level. Liaison with the project geohydrologist and design
engineers is necessary so that the optimal elevations can be decided upon.
It is important to ensure the groundwater is not contaminated during the construction or
operational phase.
Whether a long-term groundwater monitoring programme needs to be established or not
will be decided upon by the relevant authorities and should be in line with a risk based
approach.
7. ACKNOWLEDGEMENTS
The following are thanked for input to the project:
Sean Bradshaw (Airports Company South Africa) for sharing literature and his
knowledge of the area.
Sheila Imrie, Sharon Jones and Scott Mason (SRK) for providing a lot of literature
and insight into the project.
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GEOSS Report No. 2014/06-10 08 September 2014 34
Simo Constantatos and Zaid Gangraker (GPT) for providing the groundwater level
and water quality (EC) data for the Airports Company South Africa monitoring
boreholes.
Dr Jaco Nel (GCS) for imparting his knowledge on the UWC boreholes.
Henry de Haast (DWA) for supplying the UWC logger data.
Jason Greeff (RoundOne Drilling Solutions) for sharing his geological knowledge
of the area.
8. REFERENCES
Arcus Gibb (2004). Geotechnical Site Investigation Report. Sub-soil drainage along the main runway
at Cape Town International Airport. Arcus Gibb Report J24245A. November 2004.
CSIR (2006). Report on Pilot Removal of Alien Invasive Vegetation at Cape Town International Airport and Monitoring and Modelling of Groundwater Levels. Authors: C. Colvin, D le Maitre, F. Rusinga and R. Campbell. Environmentek, CSIR, Stellenbosch.
DWAF (2000). 1:500 000 Hydrogeological Map Series, 3317 Cape Town, DWAF (2005). Groundwater Resource Assessment – Phase II (GRAII). Department of
Water Affairs and Forestry. Pretoria. GEOSS (2013). Geohydrological assessment of a proposed Belhar residential development. GEOSS
Report Number: 2013/07-18. GEOSS - Geohydrological and Spatial Solutions International (Pty) Ltd. Stellenbosch, South Africa.
Gerber (1976). An investigation into the hydraulic characteristics of the groundwater source in the Cape
Flats. MSc thesis, University of the Orange Free State.
GPT (2014). Groundwater sampling report for Cape Town International Airport. Geo Pollution