Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

Southern Waters Ecological Research and Consulting
Zeekoevlei / Rondevlei Rehabilitation Study
Executive Summary
Zeekoevlei and Rondevlei are adjacent, shallow lacustrine wetlands that, over a period of
many decades, have acted as receiving waters for runoff and pollution from a combined
catchment area of some 100 km 2 . The level of impact is such that restoration to natural
condition is not possible and, given the hydraulic modifications to each vlei, impractical.
However, rehabilitation to a vastly improved ecological condition in keeping with the
ecological importance of these vleis remains a very real option. While the rehabilitation of
these two vleis will be a costly and time-consuming process, the results are likely to be
significant and economically- and ecologically-sustainable in the long term. Both vleis are
eco-recreational resources of local and regional importance.
The recommendations made in this report range from catchment level assessments of the
efficacy of Best Management Practices, to gross rehabilitation activities required for
Zeekoevlei. Some of the recommendations are not new, but their consideration in holistic
terms has served to highlight their importance. It is apparent that, for Zeekoevlei, anything
less than large scale and concerted effort will not achieve the desired aims and objectives.
The report includes a detailed assessment of the possible management and rehabilitation
options, and condenses those deemed workable into an Action Plan. The recommended
actions are bolstered by a comprehensive assessment of monitoring and audit needs.
The most difficult step in the rehabilitation of Zeekoevlei, and to a lesser extent Rondevlei,
will be the implementation of the recommendations. These will require commitment from
both the metropolitan and local authorities, as well as buy-in from all involved stakeholders.
The present highly-impacted condition of these vleis is the product of years of abuse.
Progress along the road to recovery will be slow and costly, but is achievable. It is estimated
that implementation of the initial rehabilitation options will cost R17.5 million (at 2000 Rand
values) over a period of 5 years, and with the lion’s share of this expenditure required for the
gross rehabilitation of Zeekoevlei. It should be noted, however, that a major portion of the
total cost (c. R8 million) is for the acquisition of dredging plant that is likely to also be needed
at Princess Vlei, Zandvlei and several other vleis and rivers within the CMA. Accordingly,
the initial cost could be spread over several rehabilitation programmes, thus reducing the
burden on the Zeekoevlei project.
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Five Point Rehabilitation Action Plan Summary
1. Reduce total annual loading of phosphorus to Zeekoevlei by 60% through:
• dredging of 1.1 million m 3 of accumulated sediments;
• attenuating the seepage of phosphorus-rich water from the adjacent
wastewater treatment plant.
Core programme cost over five years: R 13 000 000.
(parallel reduction of pollution within the Zeekoevlei catchment is being
investigated and costed by a separate study)
2. Reduce total annual loading of phosphorus to Rondevlei by 45% through:
• rehabilitation of the Princess Vlei ecosystem and, in particular, internal
nutrient generation from the sediments and catchment management (30%
total reduction);
• implementation of Best Management Practices within the Rondevlei
catchment (10% total reduction);
• wetland treatment of low flows during the dry season (5 % total reduction).
Core programme cost over five years: R 3 500 000.
3. Implement landuse planning and management policies to reduce the
generation of nutrients at the catchment level, by:
• imposition of non-structural Best Management Practices including
legislation to reverse, modify or relocate identified polluting activities;
• adopt and/or develop Best Management Practices to reduce nutrient
generation by current landuse, or prevent same occurring from planned use;
• apply economic benefit and regulatory mechanisms to encourage catchment
and wetland management directed towards nutrient reduction.
Core programme cost over five years: R 500 000.
4. Monitor and audit water chemistry and biotic response of both vleis to the
implemented rehabilitation (years 3-5).
Core programme cost over five years: R 500 000.
5. Reassess ecosystem health and functioning in terms of:
• Post- vs pre-rehabilitation condition;
• Attainment of water quality targets
• Ecosystem structure, composition, functioning and direction of change,
and determine second phase (year 5+) management needs, inclusive of a
reassessment of the diversion option.
Core programme cost over five years: R 600 000
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Table of Contents
Executive Summary
i
Action Plan
1. Introduction 1
1.1 How to Use this Document 1
1.2 Contents of the Action Plan 1
2. Background to the Study
2.1 The Problems: History and Causes 3
2.2 Approach Adopted for this Study 4
2.3 Rehabilitation vs Restoration 5
3. The Zeekoevlei / Rondevlei Environment and Ecology
3.1 Geology and Climate 6
3.2 Vlei morphology 6
3.3 Ecology 7
3.4 Wetland importance 7
4. Issues, Options and Actions Required
4.1 Major Issues 9
4.1.2 The Key Role of Groundwater 9
4.1.3 Other key findings related to groundwater 10
4.2 Eutrophication and Phosphorus Loading 11
4.2.1 Sources of Phosphorus Loading in Zeekoevlei 13
4.2.2 Phosphorus Reduction Scenarios for Zeekoevlei 15
4.2.2.1 Sediment Removal 15
4.2.2.2 Seepage Curtailment 15
4.3 Sources of Phosphorus Loading in Rondevlei 17
4.3.1 Phosphorus Reduction Scenarios for Rondevlei 18
4.4 Water Level Regulation and Drawdowns
4.4.1 Response of Zeekoevlei to drawdowns 20
4.4.2 Why drawdowns benefit Zeekoevlei 20
4.4.3 Negative impacts of the drawdowns – midges 21
4.4.4 Need for deep water areas and sediment removal 21
4.4.5 Modification of drawdown rate and timing 22
4.4.6 Impact on bird populations 23
5. Secondary Issues
5.1 Issues pertaining to fish populations 24
5.2 Issues pertaining to midges 26
5.3 Issues pertaining to faecal pollution and water quality 26
Section A
1. Background and Terms of Reference
1.1 Background 1
1.2 Terms of Reference 2
1.3 Other Relevant Studies and Guidelines 3
2. Approach and Study Team 7
3. Historical Review
3.1 Perspective of the Cape Flats vleis 13
3.2 Zeekoevlei 14
3.3 Rondevlei 24
3.4 Concluding Remarks 29
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4. Public Consultation Process
4.1 Introduction 33
4.2 Terms of Reference 33
4.3 EIA Regulations 33
4.4 Public Consultation Process 34
4.5 Key Issues Raised 39
Appendix A: Interested and Affected Parties 51
Appendix B: Background Information Document 55
Appendix C: Announcement of Study 61
Appendix D: Notes of Meeting held 22 March 2000 63
Appendix E: Notes of Meeting held 5 April 2000 73
Appendix F: Copies of IAAP Submissions 77
Section B: Specialist Reports
1. Morphology and Bathymetry 1
1.1 Zeekoevlei 2
1.2 Rondevlei 4
2. Geohydrology
2.1 Introduction 7
2.2 Background 11
2.3 Geohydrological Description 23
2.4 Vlei – Groundwater Interaction 39
2.5 Management Measures 50
2.6 Monitoring 57
2.7 Conclusions and Recommendations 61
Appendix A: Groundwater Fluxes through Unit Widths 71
Appendix B: Wellpoint Data 73
Appendix C: EC Depth Profiling Data 75
Appendix D: Chemical Data 77
Appendix E: Hydrocensus Data 79
Appendix F: Groundwater Level Data 81
3. Aquatic Ecosystem Assessments
3.1 Zeekoevlei
3.1.1 Biotic and abiotic characteristics 83
3.1.2 Management Issues 95
3.1.3 Zeekoevlei and Trophic State Concepts 95
3.1.4 Information Gaps 97
3.1.5 Assessment Terms of Reference and Approach 97
3.1.6 Phosphorus Load and Effect Modelling 98
3.1.7 Wetland Assessment 113
3.1.8 Summary and Recommendations 118
3.2 Rondevlei
3.2.1 Biotic and abiotic characteristics 125
3.2.2 Management Issues 128
3.2.3 Rondevlei and Trophic State Concepts 128
3.2.4 Information Gaps 130
3.2.5 Assessment Terms of Reference and Approach 130
3.2.6 Phosphorus Load and Effect Modelling 131
3.2.7 Wetland Assessment 144
3.2.8 Recommendations 155
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3.3 Sediments
3.3.1 Introduction 159
3.3.2 Sediment Characteristics 159
3.4 Issues pertaining to swarming insects (midges)
3.4.1 Characteristics of non-biting midges 161
3.4.2 Recent Incidence of midge problems 169
3.4.3 Previous Studies 169
3.4.4 Emergence Patterns and Control Mechanisms 170
3.4.5 Possible cause for increased swarms 171
3.4.6 Summary and Recommendations 171
3.5 Management of Fish Populations
3.5.1 Introduction 175
3.5.2 Approach and Methodology 176
3.5.3 Present Populations and Historic Changes 176
3.5.4 Barriers to Fish Movement 180
3.5.5 Facilitating Fish Movement 182
3.5.6 Artificial Fish Introductions 185
3.5.7 Effects of Drawdowns 186
3.6 Effect of Drawdowns on Bird Populations
3.6.1 Introduction 191
3.6.2 Response to Drawdowns at Rondevlei 192
3.6.3 Response to Drawdowns at Zeekoevlei 199
3.7 Zeekoevlei Bacteriology (Faecal indicator bacteria)
3.7.1 Introduction 203
3.7.2 Data utilization and Approach 203
3.7.3 Updated Assessment 204
3.7.4 Conclusions 206
3.8 Summarized Spatial Information
3.8.1 Introduction 207
3.8.2 Construction of Spatial Covers 207
3.8.3 Description of Information per Cover 208
Section C: Management Synthesis
1. Concepts of Restoration, Rehabilitation and Remediation 1
1.1 The Concept of Phosphorus as Limiting Nutrient 2
2. Identification of Management Needs for Zeekoevlei 3
2.1 The Case for Dredging 3
2.2 Management Options 4
2.3 Proposed Sequence of Rehabilitatory Options 5
Schedule of Management Options for Zeekoevlei 6
3. Identification of Management Needs for Rondevlei 10
3.1 Management Options 10
3.2` Proposed Timing of Rehabilitatory Options
Schedule of Management Options for Rondevlei 12
4. Matrices for watershed level BMP audit (Rondevlei) 15
5. Monitoring Needs
5.1 Introduction 23
5.2 Data Management 23
5.3 Surface Water Monitoring Requirements 24
5.4 Monitoring Frequency: Determination of Loads 28
5.5 Groundwater Monitoring Requirements 28
5.6 Recommended Monitoring Protocol 30
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Zeekoevlei / Rondevlei Rehabilitation Study
Action Plan
1. Introduction
1.1 How To Use This Document
This section of the report on the Zeekoevlei / Rondevlei Rehabilitation Study summarizes the
identified core ecological problems prevailing in Zeekoevlei and Rondevlei, the associated
key issues, and those options and actions deemed most likely to succeed in rehabilitating the
vlei ecosystems.
1.2 Contents of the Action Plan
The Zeekoevlei / Rondevlei Rehabilitation Study Report is comprised of four sections and
supportive appendices, namely:
• The Action Plan – this section should be read together with Section C;
• Section A: Background to the study and details of the stakeholder involvement
process;
• Section B: Reports of the Specialist Studies;
• Section C: Synthesis of rehabilitation options and monitoring requirements (for
an initial period of five years following completion of this
assessment).
The Action Plan integrates the key findings of the study, and suggests actions recommended
for the rehabilitation of the two vleis. The basis for the recommendations is provided in
Sections B and C.
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The Action Plan ranks those actions that are deemed necessary to:
• reduce the identified bulk sources of phosphorus reaching the vleis;
• address those problematical issues that stem from the eutrophication of either system;
• monitor the change in condition of the vleis in response to applied rehabilitatory
procedures;
• implement other ameliorative monitoring and management procedures that are
required to ensure the future dynamic functioning of these wetland systems.
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2. Background to the Zeekoevlei / Rondevlei Rehabilitation
Study
2.1 The Problems – History And Causes
Zeekoevlei and Rondevlei exist as two adjacent shallow vleis (lacustrine wetlands) that are
physically separate in terms of their surface characteristics, but linked via groundwater not
only to each other, but to the pattern of sub-surface water movement across the Cape Flats in
the general direction of False Bay. Despite their geographic proximity, the vleis are managed
separately for the purposes of recreation (Zeekoevlei) and nature conservation (Rondevlei).
For several decades (almost 80 years for Zeekoevlei), both vleis have been subject to water
level controls and/or excessive loads of catchment-derived nutrient enrichment or deliberate
waste disposal, with the effects thereof being evident particularly in Zeekoevlei as sustained
blooms of blue-green algae and accelerated accumulation of organically-rich sediment. The
non-limiting availability of phosphorus, in particular, has for many years been recognized as
the cause of high levels of both plants and algae. The consequence of this sustained impact
has been a loss of biodiversity and reduced ecosystem health. Both vleis exhibit the effects of
eutrophication common in shallow, well-mixed lakes where natural cycles of flushing and
water level variation have been tampered with.
Eutrophication (from the Greek meaning well-nourished), refers to excessive enrichment of
a waterbody by the plant nutrients nitrogen and phosphorus, usually the consequence of
deliberate (“point source”) discharges, or the combined impact of polluted runoff (“diffuse
pollution”) emanating from the catchment(s) draining into a river, lake or wetland. The effect
may be sudden, i.e., the after effects of a single spill or discharge, or alternatively slow and
insidious, with the receiving water changing progressively through increasing levels of
enrichment from oligo- (poor) to hypereutophic (grossly enriched). The typical consequences
of eutrophication are imbalanced algal and/or plant growth, reduced water clarity, reduced
oxygen availability, blooms of toxic algae, fish kills and increased levels of (organic)
sedimentation. The process is often accompanied by an increasing level of resistance to
restoration or rehabilitation, i.e., the longer the impact is allowed to continue, the more
difficult it is to remedy. While eutrophication is relatively easy to diagnose, it is exceedingly
difficult to reverse.
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Phosphorus as a limiting nutrient. “Life can multiply until all the phosphorus has gone and
then there is an inexorable halt which nothing can prevent” (Isaac Asimov). Phosphorus is a
key component of the molecule commonly known as ATP (adenosine triphosphate) – the
molecule that provides the requisite energy for cell life. Accordingly, phosphorus is the
limiting factor to life, both in water and on land. Phosphorus does not exist in nature in its
elemental (P) form, but rather as one or other form of phosphate. Phosphorus cycles and
recycles through organic (living) and inorganic (non-living) matter. The deleterious impacts
of the disposal of phosphate-rich sewage effluent and/or detergent-rich grey water to lakes
and rivers first became evident during the 1950s, and is now recognized as a global threat to
water quality, ecosystem health and the recreational use of rivers and lakes. Phosphorus is
not a pollutant or a toxicant, but a naturally scarce and life-supporting element that, when
present in oversupply, frequently results in biological overgrowth. Its effective management
and control, especially in shallow lakes and vleis, can reverse or prevent this process.
2.2 Approach Adopted For This Study
A considerable and informative base of well-researched data exists for Zeekoevlei, and to a
lesser extent for Rondevlei. As its fundamental points of departure, the Zeekoevlei /
Rondevlei Rehabilitation Study encompassed the following key elements in its approach:
• to not undertake or initiate any new collections of primary data other than to determine
the role played by groundwater in the hydrological cycling and nutrient loading of these
two vleis (an understanding of the role of groundwater in the hydrological and ecological
functioning of the Cape Flats vleis has, for too long, been neglected;
• to model the cause and effect linkages between the identified phosphorus loads, and the
resultant in-lake condition;
• to develop rehabilitation scenarios based on phosphorus reduction which, in conjunction
with other management and/or biomanipulatory practices, would serve to reverse the
extant extreme levels of eutrophication in each vlei.
The primary focus of the study was to convert the existing knowledge and information into
guidelines for rehabilitation.
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2.3 The Concept Of Rehabilitation vs Restoration And Remediation
In terms of formulating management needs and recommendations for Zeekoevlei and
Rondevlei, it is important that the concepts of restoration, rehabilitation and remediation be
defined. The concept of restoration implies an inherent degree of naturalness, viz. that
whatever efforts are undertaken will strive to restore aspects such as vegetation, hydraulic
and/or hydrological patterns, water quality or faunal composition to their natural condition.
By contrast, rehabilitation recognizes that restoration may be neither pragmatic nor possible.
However, the enhancement of core aspects of ecological functioning and processes, in order
to resemble a semi-natural condition, is attainable. Remediation procedures are common for
systems that have been grossly abused (e.g. mine tailing wetlands), and any semblance of
original condition is no longer possible. Here the goal would be to improve the ecological
condition, to an endpoint that is unlikely to mirror any previous natural state.
The specialist investigations conducted for this study indicate that, for Zeekoevlei, any
management approach should encompass a blend of rehabilitation and remediation options.
In the case of Rondevlei, future management should be rehabilitation-focussed.
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3. The Zeekoevlei / Rondevlei Environment and Ecology
3.1 Geology and Climate
Zeekoevlei and Rondevlei are situated in the winter-rainfall region of South Africa. Annual
rainfalls average between 550 and 600 mm, with in excess of 80% of the total falling between
April and September. Air temperatures range from minima of 11°C during July to 21°C
during January and February, while water temperatures for these shallow vleis vary between
12 and 23°C. The mean annual day length is 8 hours, with 3000 hours of bright sunshine
measured during a typical year. The vleis experience a particularly windy climate, and with
an almost total absence of calm conditions – measured as windspeeds of less than 6 m s -1 .
Mean annual windspeeds exceed 6 m s -1 . The greater surface area of Zeekoevlei is
completely mixed on a daily basis. Annual evaporation totals 1.4 m, i.e., a nett loss of 0.85m.
3.2 Vlei morphology
Morphological details for the two vleis are summarized in the following table:
Summarized morphological and hydrological data for Zeekoevlei and
Rondevlei
Hydrologic /
morphometric variable
Zeekoevlei
Rondevlei
Catchment drainage area 8.01 x 10 7 m 2 1.23 x 10 7 m 2
Annual runoff volume (estimated) 2.01 x 10 7 m 3 4.26 x 10 6 m 3
Lake surface area 2.56x 10 6 m 2 4.49 x 10 5 m 2
Lake volume 5.0 x 10 6 m 3 6.8 x 10 5 m 3
Lake mean depth 1.91 m 1.43 m
Precipitation – evaporation -0.85 m -0.85 m
Hydraulic loading 1.79 x 10 7 m 3 y -1 3.87 x 10 6 m 3 y -1
Areal water load 7.0 m y -1 8.62 m y -1
Lake flushing rate 0.28 y 0.18 y
Sediment volume 1.1 x 10 6 m3 Not determined
Area covered by sediment 60 ha Not determined
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3.3 Ecology
The hydraulic regimes of both vleis are attenuated by weirs that were constructed during the
1940s. Both vleis are hypertrophic (see below), but with the degree of anthropogenic nutrient
enrichment in Zeekoevlei equivalent to 3-fold that for Rondevlei (see below). Both vleis
exhibit the microphyte (phytoplankton) dominance typical of shallow, regulated lakes, but
with this dominance considerably more sustained and ingrained in Zeekoevlei. Prior to the
recent implementation of water level drawdowns, Zeekoevlei was characterized by nearpermanent
algal blooms, an impoverished population of small-bodied cladoceran
zooplankton, and a large population of coarse fish. The drawdowns served to release the topdown
pressure on the zooplankton community, resulting in a considerable reduction in the
algal population. Algal dominance in Rondevlei followed a more distinct periodicity prior to
drawdowns being applied. Given a preceding greater level of ecological balance and stability
in this smaller vlei, the benefits that may have accrued from the drawdown procedure have
not been visibly apparent, and have not been subject to scientific investigation.
Macro-hydrophyte dominance in both vleis is limited to dense stands of emergent reeds. In
Zeekoevlei Typha capensis predominates, with a lesser contribution by Scirpus nodosus in the
deeper water. Rondevlei reflects an apposite situation with S. nododus as the dominant genus,
but with Typha encroachment. Rondevlei supports some 18 species of endangered or
vulnerable flora, and with one endemic species.
Avifaunal composition is much more diverse in Rondevlei than in Zeekoevlei, but the recent
drawdown-facilitated improvement in water clarity has allowed a return of greater numbers of
piscivorous birds to Zeekoevlei. The low human disturbance levels of the Rondevlei Nature
Reserve environment, together with a greater seasonal availability of shallow wading habitat
(that Zeekoevlei lacks), makes the smaller vlei more attractive to bird life. Rondevlei
provides habitat for 10 species of birds that are of rare or indeterminate importance.
3.4 Wetland importance
Both vleis have been provisionally assessed in terms of their present condition, and their
ecological importance and sensitivity. Ecological importance implies the value of a wetland
environment to maintain ecological diversity and functioning on local and wider scales, while
sensitivity relates to the capacity of the wetland to resist disturbance, and its ability to recover
once an impact has been endured (resilience). The level of variation between the present
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condition, and the predicted ecological importance and sensitivity provides an indication of
the scope, if any, and need for rehabilitation.
Zeekoevlei: The present condition of Zeekoevlei is considered to be seriously impaired
(modified), and found to be a Class E condition, which is regarded as being unacceptable.
However, the vlei rates a Class C in terms of its importance and sensitivity. This implies that,
at the very least, efforts should be made to raise the present condition to a Class C – i.e.,
moderately modified.
Rondevlei: The present condition of Rondevlei was deemed to be moderately modified
(Class C). This was expected given the protected nature of the vlei, and the considerably
lower overall level of impact. The ecological importance was rated two classes higher, i.e.
Class A, again indicating scope and need for rehabilitation in order to improve the present
state.
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4. Issues, Options and Actions Required
4.1 Major Issues
Three major rehabilitation issues were identified, namely:
• the role played by groundwater, and particularly regarding the nutrient enrichment of
Zeekoevlei as a consequence of polluted seepages;
• phosphorus enrichment of the vleis, and the sources of said enrichment, including that
from the aforementioned seepage;
• the ecosystem processes that derive benefit from the drawdown process.
Several other issues, classified here as secondary in that they are components of greater
rehabilitation needs, were also identified and are dealt with later in this Action Plan (see
Section 5).
Together, all of the identified issues encompass and address those ecological aspects
identified by the Public Participation component of the Zeekoevlei / Rondevlei Rehabilitation
Study.
4.1.2 The Key Role of Groundwater
The geohydrological study conducted as part of this investigation revealed that groundwater
intrusion to both Zeekoevlei and Rondevlei forms a crucial component of the annual water
balance of these wetlands. This finding provided the last pieces of the puzzle of “ghost”
sources of phosphorus identified using mass balance modelling. The geohydrological
investigation encompassed the creation of a field of some 40 wellpoints around Zeekoevlei
and Rondevlei. This provides an established physical infrastructure for the continued
monitoring of groundwater patterns and quality in the groundwater control zone of these vleis.
The geohydrological investigation clearly illustrated the role of groundwater in the
functioning of the vleis, as well as inter-vlei sub-surface linkages across the Cape Flats. It
was determined that groundwater inflows contribute approximately 15% of the total annual
inflow into the two vleis, and play a critical role during summer when groundwater becomes
the sole source of supply. A key finding was that the inflow from groundwater approximates
between 4 and 5 mm per day, throughout the year. The study highlighted the importance of
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the primary aquifer in the vicinity of Zeekoevlei and Rondevlei, and recommended that it be
afforded protection as a special aquifer system.
The investigation also showed that Rondevlei and Zeekoevlei function as a single
hydrological unit. In terms of linkage to the groundwater regime of the greater Cape Flats
area, the vleis play an important role as groundwater flows towards them discharge through
them as evaporation.
A most significant finding was that seepage (= leakage) from the wastewater treatment plant
to the south of Zeekoevlei accounted for 34% of the total annual load of phosphorus in the
vlei. Decades of impact of sewage management in the immediate area had resulted in the
collapse of the phosphorus retention barriers typical of coastal plain soils. A further finding
that dispelled earlier speculation was that the groundwater to the east of Zeekoevlei was not
contaminated by agricultural practices.
In the case of Rondevlei, groundwater was estimated to contribute less than 2% of the total
annual phosphorus load, a background and insignificant portion of the total.
4.1.3 Other key findings relating to groundwater
• Monitoring of water levels around both Zeekoevlei and Rondevlei showed that the
drawdown procedure exerted a limited impact on the geohydrological regime. The
impact of the drawdown procedure was highly localized, with a 0.2 m drop in water
levels, concurrent with the drawdown, measured 500 m from the edge of the vlei.
• The shallow water table in the area was found to have resulted in some minimal
interference with the operation of septic tanks systems, but no abnormal contamination of
the groundwater from this source was detected.
Recommendation 1:
1.1 Strengthen the conceptual understanding of the role of groundwater in
the ecological functioning of Zeekoevlei and Rondevlei through the
continued monitoring of the established wellpoint field, and associated
surface water characteristics (see Monitoring Protocol).
1.2 Make application for protection of the primary aquifer system in terms
of the Water Act of 1998.
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4.2 Eutrophication and Phosphorus Loading
Nutrient concentrations in waterbodies can range from very low to very high. Such
concentrations are normally characterized in terms of the trophic state or condition of the
system, and ranked as oligotrophic (= poorly nutrient enriched) through meso- (= moderately
enriched) to eutrophic or even hypertrophic (highly to grossly-high in-lake levels of
phosphorus availability). For both Zeekoevlei and Rondevlei, restriction of the natural cycles
of filling, flushing and drying (through the construction of weirs), and their use as detention
ponds for effluent and urban runoff, has resulted in their being classified as highly eutrophic,
with impaired ecosystem functioning and biodiversity. While some relief of water level
regulation has been possible through the implementation of drawdowns, the greater problem
of excessive nutrient enrichment continues unabated.
Total phosphorus
(mg/l)
1
0.8
0.6
0.4
0.2
0
400
350
300
250
200
150
100
50
0
90/91
91/92
92/93
93/94
Chlorophyll-a (ug/l)
94/95
95/96
96/97
97/98
98/99
Growing season (GS)
Total phosphorus
Chlorophyll-a
Graph showing the change in growing season phosphorus concentration and chlorophyll-a in
Zeekoevlei during the 1990s. Note, despite the continuing year-to-year increase in the
ambient concentration of phosphorus, the marked decrease in chlorophyll-a as a consequence
of the applied drawdowns.
Appropriate management philosophy for eutrophication encompasses two broad options,
either: (a) minimizing the causes or (b) learning to live with the symptoms. The process, once
ingrained in an aquatic ecosystem, often adopts a high level of resilience to rehabilitation, and
the “live with it” option is a common choice, unwillingly made by many, in the face of the
extremely high costs associated with reversing the process.
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The findings of this report clearly indicate that, despite the high levels of eutrophication
present in both vleis (see Table below), meaningful rehabilitation is possible, although at a
relatively high initial cost. The cost implications should, however, be weighed against the
period of time that the vleis have been abused. Also, the eco-recreational value of having the
wetlands in a significantly better condition than at present, is likely to render the cost of
rehabilitation relatively small when viewed long-term.
Comparison Of Zeekoevlei And Rondevlei With The Trophic State Classification System For
African Lakes And Lacustrine Wetlands
Indicator
Upper limit of
Zeekoevlei
Rondevlei
mesotrophy
(median)
(median)
Phosphorus
concentration
0.05-0.06 0.70 0.24
mg l -1
Chlorophyll-a
concentration
10-15 200 125
mg m -3
Nitrogen concentration
mg l -1 0.2-1.0 2.0 2.8
Primary production
level
g C m -2 d -1 2-3 1.2 – 4.4 Not determined
Dominant algal family Cyanophyta Cyanophytes
Cyanophytes
Chlorophytes
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4.2.1 Sources of phosphorus loading in Zeekoevlei
On average an estimated 34 000 kg of phosphorus enters Zeekoevlei per year. This exceeds
the guideline annual loading for a vlei of this area and depth by two orders of magnitude
(100x). The largest single contribution, 35%, originates from seepage from the adjacent
WWTP; the second-largest contribution, from the catchment of the Big Lotus River (28%);
and the third-largest contribution as internal loading generated from the considerable volume
of organically-rich sediments (25%).
The various phosphorus loadings for Zeekoevlei are summarized in the following diagram:
Groundwater
3.8%
Lake surface
0.1%
Septic tanks
1.5%
Big Lotus catchment
28.4%
Seepage
34.6%
Little Lotus catchment
4.7%
Reed beds
1.1%
Internal loading
25.1%
Local catchment
0.6%
Diagrammatic representation of the identified annual phosphorus loads, by source and
percentage of the total, for Zeekoevlei.
This study concluded that reducing the total phosphorus loading to an in-lake growing season
median concentration of between 0.1 and 0.2 mg l -1 , in conjunction with continued
drawdowns and other management options (see elsewhere in this summary) would:
• reduce algal growth and sedimentation;
• restore clear water lake conditions;
• allow sufficient light penetration to support the re-establishment of desired rooted
aquatic plants;
• lead to increased levels of biodiversity;
• reduce the rate of reed encroachment.
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Attaining the desired in-lake phosphorus level would require a load reduction in excess of
70% of the total (see diagram below). However, attenuation of the two well-defined sources
of phosphorus, namely seepage from the WWTP, and the sediments accumulated in the vlei
(combined 60% reduction), together with continued drawdowns (see Section 4.4), is
anticipated to result in the above-mentioned improvements in lake water quality and
attributes.
TP or chlorophyll-a (ug/l)
800
700
600
500
400
300
200
100
0
691
634
576
516
455
392
326
258
225 208 189 170 185
151
131 109
87 105
63
37
0 10 20 30 40 50 60 70 80 90
% reduction in P load
In-lake total P
In-lake chlorophyll-a
Graph showing the predicted change (reduction) in the concentration of phosphorus during
the algal growth season in response to reduction of the gross annual load of this nutrient to
Zeekoevlei.
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4.2.2 Phosphorus reduction scenarios for Zeekoevlei
Note: The brief for this study did not encompass close examination of, or derivation of Best
Management Practices for, the Zeekoevlei catchment. This aspect is receiving attention
through a study funded by the Water Research Commission. Diversion of riverine flows
around Zeekoevlei is not recommended until all other possible watershed-level management
control options have been implemented.
4.2.2.1 Sediment removal
The removal of the accumulated sediments in Zeekoevlei will:
• reduce the annual phosphorus load by 25% of the present total value;
• restore presently inaccessible deep water habitat (21% of the volume of the vlei, and some
40% of the surface area).
These benefits would be best achieved through removal by dredging. Sediment analyses have
shown that the composition of the sediments does not preclude their reuse as admixture for
fertilizers or soil stabilizers.
As part of this project, a specialist dredging engineer from Australia visited Zeekoevlei, and
advised on appropriate procedures and costs. It was apparent from this investigation that the
commissioning of this level of expertise could result in cost savings of as much as 3x over
alternative commercial offers to dredge the system. Scenario case studies support the value
and potential success of dredging as a rehabilitatory option for Zeekoevlei.
4.2.2.2 Seepage curtailment
Curtailment of the seepage flows will:
• Reduce the annual phosphorus load by 35% of the present total value.
A suite of options for attenuating the seepage flows, ranging from the unlikely and
impractical cessation of WWTP operations and closure of the works, to the creation of
physical and/or hydraulic barriers between the WWTP and the vlei were identified.
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This report considers the removal of sediments from Zeekoevlei, and curtailment of the
seepages to be achievable short term objectives. It is predicted that the successful mitigation
of nutrient loading from these two sources will result in a reduction in algal growth of
between 2.5 and 5x the present level (as measured using chlorophyll-a). Any further
reductions in nutrient attenuation achieved in the catchment (a medium to long term process),
will further enhance the level of water quality improvement.
Recommendation 2:
2.1 Remove the estimated 1.1 million m 3 of sediment from Zeekoevlei by
dredging. This process will amount to an annual load reduction of
25% of the present total. The estimated cost of the dredging
procedure is 1 million US dollars, plus a further 0.5 million USD for
the processing of the dredge spoils (the cost of the latter operation
may be offset against resale of the product). Between nine and fifteen
months will be required to complete the operation.
2.2 Concurrent with the dredging operation, implement those measures
deemed most appropriate for the attenuation of the phosphorus-rich
seepage flows from the Strandfontein WWTP to Zeekoevlei. This
will reduce the annual phosphorus loading of the vlei by up to 35%.
The cost of this mitigation is estimated at between ZAR 2.5 and 5
million (2000 prices).
2.3 That watershed-directed Best Management Practices be identified and
implemented as part of the envisaged Catchment Management Plans
for the Big and Little Lotus River catchments.
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4.3 Sources of phosphorus loading in Rondevlei
Phosphorus loading to Rondevlei stems from diffuse catchment sources, and from sediment
loading of the upstream Princess Vlei (see diagram). In contrast to Zeekoevlei, internal
sediment loading in Rondevlei was not deemed to be a significant (13%) source of nutrients.
Reed beds
5.4%
Internal loading
13.2%
Local catchment
1.1%
Groundwater
0.7%
Lake surface
0.3%
Rondevlei
catchment
23.0%
Princess Vlei
catchment
56.3%
Diagrammatic representation of the identified annual phosphorus loads, by source and
percentage of the total, for Rondevlei.
A total annual load of some 2300 kg of phosphorus per annum was estimated for Rondevlei.
The bulk (56%) of this loading stems from the upstream Princess Vlei (a vlei for which a
separate rehabilitation study is currently underway), with the urban catchment draining
directly to Rondevlei contributing a further 23% of the estimated total. Given the surface area
to volume proportions of Rondevlei, this amounts to a loading of approximately 40x the
guideline value for a shallow lake.
Notwithstanding the level of phosphorus loading, the ecological functioning of Rondevlei is
impaired to a much lesser degree compared with Zeekoevlei. This is probably because the
vlei has been impacted at a lower level and for a shorter period of time; has a lower
dominance by bloom-forming algae; a greater degree of natural depth and water level
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variability and possesses a greater flushing rate (0.18 vs 0.28 year). Accordingly, relatively
less (compared with Zeekoevlei) rehabilitation effort is anticipated for Rondevlei.
TP or chlorophyll-a
(ug/l))
250
200
150
100
50
0
220
200
182
163
144
124
103
82
130 120 59
109 98 34
87
76
63
51
37
22
0 10 20 30 40 50 60 70 80 90
% reduction in P-load
In-lake total P
In-lake chlorophyll-a
Graph showing the predicted change (reduction) in the concentration of phosphorus during
the algal growth season in response to reduction of the gross annual load of this nutrient to
Rondevlei.
4.3.1 Nutrient enrichment attenuation options for Rondevlei.
The following options were identified, by this and previous studies, for reducing nutrient
pollution of Rondevlei:
• rehabilitation of Princess Vlei, including implementation of watershed-directed measures
within the Princess Vlei catchment;
• watershed-directed Best Management Practices;
• treatment wetlands – this option has already been implemented for low flows discharging
to Rondevlei;
• drawdowns (discussed later in this section);
• diversion of flows around Rondevlei.
Outflow from Princess Vlei discharges directly to Rondevlei, thus it is logical for effort to be
concentrated on improving the water quality of the former as close to source as is possible,
and thereafter implementing such measures as may be feasible and possible to deal with the
balance. The situation in Princess Vlei is somewhat similar to that in Zeekoevlei in that the
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vlei contains a large accumulation of nutrient rich sediments spread over much of the floor of
the vlei. Unless this nutrient source is effectively managed, Princess Vlei will continue to
generate nutrients that flow to Rondevlei. Given the linkage of the Princess Vlei catchment to
Rondevlei, it would be beneficial to link the catchment management resources for these two
vleis.
In addition, an assessment needs to be made as to whether catchment-based Best Management
Practices within the Rondevlei catchment area are likely to be effective, or indeed
implementable, given space and existing infrastructure limitations. Nutrient loads generated
by this catchment may be at “background” levels, i.e. levels that are high enough to generate
problematical loads for small waterbodies, but so low as to preclude effective mitigation using
conventional BMPs.
Recommendation 3:
3.1 Integrate the management needs of Rondevlei with those for Princess
Vlei, and manage these vleis as interlinked components of the same
system;
3.2 Implement whatever identified catchment level BMPs are deemed likely
to reduce pollution with the Rondevlei catchment;
3.3 Should the rehabilitation of Princess Vlei not achieve the desired
improvement in the quality of water draining to Rondevlei, investigate
options for the introduction of a low-flow bypass system to the Zeekoe
Canal, and the creation of a non-hydraulically-occlusive treatment
wetland in the latter.
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4.4 Water level regulation and drawdowns
4.4.1 Response of Zeekoevlei to the drawdown procedure
The use of drawdowns as a management technique was instituted at Zeekoevlei during 1997,
and at Rondevlei the following year. The original purpose of lowering the water levels was to
enable physical access to the shoreline areas of the vlei for the purposes of litter and reed
removal. The 1997 event was subjected to detailed scientific evaluation of changes to the
biota of the vlei, and in particular the phyto- and zooplankton populations. The findings were
profound. The receding water levels isolating the reed beds, and the absence of deep water
refugia, resulted in a high level of fish predation by piscivorous birds. This reduction in
pressure on the zooplankton enhanced phytoplankton grazing pressure, and with a sustained
clear-water phase being the result. This characteristic was reconfirmed during the second
(1998) drawdown. The improved water clarity also resulted in the reappearance of pondweed
(Potamogeton pectinatus) in the shallow northern and north-western bays of the vlei.
Subsequent drawdowns (post-1998) have not been subject to detailed investigation, and no
comparative studies have been conducted for Rondevlei. Notwithstanding this, aggregate
measurement of the level of algal pigments (chlorophyll-a) in Zeekoevlei has revealed that,
despite no change in the ambient level of nutrient availability, phytoplankton populations
have remained at approximately 50% of the level measured prior to implementation of the
drawdown procedure.
4.4.2 Why the drawdowns benefit Zeekoevlei
The drawdowns have resulted in improved ecological functioning for the following reasons:
1. At a gross level, the procedure served to relieve top-down pressure on the vlei food
chain by reducing grazing by fish on the organisms that feed on algae. This cause
and effect response is both predictable, and relatively easy to measure.
2. At a more subtle but no less important level, the procedure has released pressure on a
mechanism common to shallow lakes, viz. that water level regulation forces the
system towards dominance by phytoplankton rather than by more desirable rooted
plant species such as pondweeds – i.e. plant communities that support a wide range of
valued ecosystem services. Shallow lakes are known to exist in one of two
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predominant conditions, namely a clear water condition dominated by rooted plants,
or a turbid state in which algae are the major form of aquatic plant life. The
implication here is that the response will be directly proportional to the release on
water level regulation. This benefit will, however, be limited by other prevailing
factors such as the presence of sediment in the vlei (see below).
4.4.3 Negative impacts of the drawdowns - midges
A negative consequence of the drawdown procedure is a disturbance of the fish – aquatic
insect (midge) dynamic. The absence of deep water refugia during the drawdowns resulted in
a high level of fish predation by birds, as the fish had no means of escaping this impact.
Annual re-implementation of the drawdowns under the same conditions has allowed the
impact to be sustained.
The predominant fish species in Zeekoevlei is the common carp (Cyprinus carpio), a species
known to be an excellent biological control for the development of chironomid midge larvae
in shallow lakes. Immediately following the 1997 winter drawdown of Zeekoevlei, as well as
during the subsequent years, midge emergence reached plague proportions. This is now
considered to be a direct consequence of the reduction in the fish population, and the
consequent reduction in control on the midge population. However, as midge emergence on
the Cape Flats varies widely on inter-annual basis, and no emergence trapping trials have
been conducted after the drawdowns, other natural forcing functions may also be in effect.
4.4.4 The need for deep water areas and sediment removal
Apart from generating an enormous internal nutrient load (see above), the sediments
accumulated in Zeekoevlei occupy in toto the deep water areas (> 1.9 m) of the vlei. During
the drawdowns, average water levels are reduced to 0.5 and < 1m, a time when, should the
sediment basins that cover an area of some 60 ha (= 40% of the vlei area), be open water, they
would provide an additional water depth of up to 3 m.
The sediments currently present in the vlei have accumulated over a period of approximately
70 years, i.e., from the time of establishment of the first wastewater treatment facility on the
south-eastern shoreline. Rehabilitation of the vlei, as is envisaged and recommended in this
report, would reduce algal sedimentation rates to very low levels. Thus the benefits for the
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future aquatic health of the vlei would not only be immediately profound, but likely to be
sustainable for a very long period of time.
4.4.5 Options for modifying the timing and rate of the drawdown
procedure
The present timing of the drawdowns at Zeekoevlei corresponds with the time of year when
the water levels would naturally have receded. However, the current drawdown procedure
suddenly exaggerates this cycle, but at a point towards the natural end of the low water
period. This is not an ideal ecological scenario.
Zeekoevlei, and to a lesser degree Rondevlei, provide very little in the way of shallow,
exposed shorelines for migrant waders. By the time that the drawdowns are implemented,
usually late-April, it is too late for wading birds to benefit. Furthermore, the shorelines of
Zeekoevlei are rather abrupt, either bordered by reed beds extending into relatively deep
water, or by houses and lawns extending to the water’s edge. These features also reduce the
availability of suitable habitat for wading birds. A benefit of increasing wading bird
populations would be additional predation on midge larvae and other aquatic invertebrates, a
dynamic that is currently precluded by the present policy of water level management.
The earlier implementation of a gradual drawdown process, i.e. by progressively reducing
water levels by increments of approx 100 mm between November and April, such that the vlei
is fully drawn down (1 m ) by the end of the process, would be a more representative
simulation of natural events, and provide a greater range of habitat for birds. This process
would further benefit from compositional restructuring of the certain of the reed bed areas to
provide exposed or sparsely-reeded areas away from high recreational usage zones. In the
case of Rondevlei, drawdowns should be timed so as to not connect islands to the mainland
too early in the breeding cycles of birds.
4.4.6 Impact of the drawdowns on bird populations
The drawdowns have invoked certain predictable responses in terms of the effect on bird
populations. These have largely centred on increases in counts due to habitat provision
(flamingoes) and food availability (pelicans). Other changes or influences have not been
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discernible as a consequence of the absence of count data (Zeekoevlei), or the short study
period (Rondevlei). It is, however, deemed likely that, should the proposed phased
drawdowns be implemented, positive benefits will accrue to both vleis insofar as their bird
populations are concerned.
Recommendation 4:
4.1 Create or re-create deep water habitat areas in Zeekoevlei (existing
basins) and Rondevlei (as desired) – see also Recommendation 2.1.
4.2 Continue to implement the annual drawdowns in a gradual and phased
manner over a period of six months, and at an incremental rate of 100
mm per interval of 10% of the total drawdown period.
4.3 In conjunction with 4.2, restructure the vegetation of reeded areas of
shoreline so as to enhance the quality of habitat for migrant wading
birds.
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5. Secondary Issues
The following secondary issues were identified as part of this study. Please note that use of
the term secondary does not imply lesser overall importance of the topics, but rather a lower
tier of linkage to one or more of the above major issues.
5.1 Issues Pertaining to Fish Populations
The following aspects pertaining to the management of fish populations were identified by
this study.
• The merits of trying to facilitate the movement of marine fish into the vleis;
• The merits of installing fish ladders;
• Stocking of the vleis with additional fish species;
• The impact of drawdowns on fish populations and midge control (addressed above).
The specialist fish study drew the following conclusions:
• The fish fauna of both vleis comprised only exotic species introduced for angling, or
possibly for midge control. Of those species identified, common carp and banded tilapia
were dominant.
• In addition to the exotics identified for the vleis, the lower (seaward) 500 m of the Zeekoe
Canal also contained mullet.
• Several major barriers preclude fish movement between the lower Zeekoe Canal and the
vleis, these barriers being:
• the gabion weir and bridge supporting the Muizenberg sewage rising main (no
fish other than possibly eel, would be able to negotiate this obstruction);
• the outlet weirs of each vlei;
• the outlet sluices. No fish would be able to move into the vleis against the force
of flow during the drawdowns, or thereafter;
• the Rondevlei fish ladder. The fish ladder that has been constructed at Rondevlei,
while suitable for large strong swimmers such as trout and salmon, is wholly
unsuitable for the Rondevlei flow regime and the type of fish that would be in a
position to attempt use of it.
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• aquatic plant growth, including water hyacinth, that predominates in the Zeekoe
Canal from time to time, reduces oxygen levels, rendering the canal unsuitable
for the movement of fish.
• It is doubtful whether the benefits, if any, of facilitating the movement of marine fish
species into Zeekoevlei and Rondevlei would outweigh the considerable cost and effort
that would be required for this purpose.
• If marine fish did attempt to move up the Zeekoe Canal, only southern mullet Liza
richardsonii and flathead mullet Mugil cephalus are likely to do this. Both species could
be artificially introduced into the vleis quite cheaply. As they would not breed in the
vleis, reintroductions would be required from time to time.
• Both vleis and the canal essentially contain the same assemblage species, and there exists
little motivation exists for undertaking a costly interconnection of the two systems.
The fish study concurred with the concomitant specialist findings regarding the dynamic
relationship between birds, fish and midges, and the need to create deepwater habitat to offset
attrition of the fish population during the drawdown period.
This study has recommended (see above) that Rondevlei and Princess Vlei be managed as
linked components of the same system. In this regard, the fish population of Princess Vlei is
important as it is known to contain a population of introduced barbel Clarias gariepinus. This
fish, in small enclosed environments, can be utterly devasting for not only other species of
fish, but for populations of aquatic organisms, including birds, in general. Accordingly, the
population characteristics of barbel in relation to other fish and aquatic organisms, requires
close monitoring.
It is unlikely that a natural fishway, other than an intermittent connection via flooded lagoons
and pans, ever existed between the Zeekoevlei / Rondevlei complex and the sea. The
deliberate creation of such a fishway has no obvious merit, and would necessitate costly
financial outlay for little or no benefit. Augmentation of the fish communities of these vleis
through re-stocking is deemed the most sensible option, provided that this is undertaken after
the vleis have been rehabilitated to an improved ecological condition. Given the
demonstrated effect of the top-down control exerted by fish on the zooplankton, and hence
phytoplankton populations of Zeekoevlei, considerable benefit is likely to be derived from the
deliberate management of the fish community of this vlei.
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Actions required
• Annual fish population surveys of Zeekoevlei and Rondevlei, as well as Princess Vlei and
the upper and lower reaches of the Zeekoe Canal, and integration of the findings with the
results of catch-and-return angling activities.
Recommendation 5:
5.1 Artificial stocking of Rondevlei with mullet and/or other desirable
fish species may be undertaken.
5.2 Introductions of a wider spectrum of fish species to Zeekoevlei
should be undertaken after implementation of the rehabilitation
recommended above.
5.3 Catch-and-return fishing activities should be managed through
compulsory fish tagging.
5.2 Issues Pertaining to Midges
This aspect has been dealt with under the section dealing with drawdowns (Section 4.4.3).
5.3 Issues Pertaining to Faecal Pollution and Bacteriological Water
Quality
Although the bacteriological water quality, as measured using faecal indicator bacteria, of the
rivers draining to Zeekoevlei is exceedingly poor, rapid attrition of the high counts occurs in
the vlei itself. This results in the vlei complying, on a seasonal basis, with the guidelines for
contact recreation. No appreciable change in the trend of faecal pollution discernible for the
past 20 years.
Notwithstanding the above, the high counts present in the rivers and the indirect use of
Zeekoevlei as a treatment pond for this pollution, is undesirable. Every effort should be made
to contain faecal pollution within the catchment to acceptable background levels.
Actions required
• Catchment level identification and control of all sources of faecal pollution over and
above the reasonable background levels deemed acceptable for the landuse type.
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