PRIORITIZATION OF RIVER BASINS IN THE TSHWANE AREA WITH REFERENCE TO FAECAL COLIFORM BACTERIA FOR THE PURPOSE OF THE IDENTIFICATION OF CANDIDATE WETLANDS FOR REHABILITATION Adri Venter A research report submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg in partial fulfilment of the requirements for the degree of Master of Science in Environmental Studies. Johannesburg, 2007 ABSTRACT Wetlands are considered a last line of defence against poor water quality. Despite the natural capabilities of wetlands to remove a variety of contaminants from surface water, the track record for wetland conservation leaves much to be desired. In the northern parts of the City of Tshwane, 84% of wetlands have been degraded. When viewed against the poor bacteriological quality of river water in the study area, the lack of wetland conservation efforts is of particular concern. Given the large number of wetlands in the Tshwane area in need of rehabilitation, this study aimed to devise a methodology to prioritise these wetlands for rehabilitation. No blueprint for such a prioritisation process exists, as studies are adapted to take into account the availability of data and the unique requirements of the study area. The methodology for this study is based on the prioritisation of a specific river basin, based on expected maximum faecal bacterial load originating from various sources of pollution. Four river basins were compared with each other in a series of screening processes. Screening was done on a landscape level using a Geographic Information System (GIS) to generate various composite layers as part of the screening process. The screening processes relied on the application of several weighted criteria. Weights for criteria are based on scientific literature. Weights are also allocated in line with the “worst case scenario”, as the study is in essence an assessment of the various pollution sources and their maximum possible contribution to deteriorating surface water quality. A Simple Additive Weighting technique was used to assess the total pollution loads and total numbers of users at risk from contaminated surface water in each of the river basins. It is important to note that the objective is to only rate the pollution sources, whilst exact pollution loads were not calculated. Diffuse, areal and point sources of pollution were rated using the estimated contributions to faecal coliform loads. The river basin with the highest score II was selected for the selection of candidate wetlands for rehabilitation purposes. The Apies River Basin scored highest for most of the criteria, with the exception of the number of households at risk from contaminated surface water. Despite the 0.60 weight allocated to households at risk, the extent of pollution sources in this river basin allowed it to be singled out as the basin in which a wetland for rehabilitation is most urgent in order to attenuate bacterial load. Two wetlands were short listed, based on their high need for rehabilitation, their hydrogeomorphic location (valley bottom with a channel), and given that they are larger than 1ha in size and within a minimum distance from the households at risk. Site level assessments are required for a final selection between the two, taking into account the nature of the current disturbances, the possibility of risk due to back-flooding, the projected costs associated with rehabilitation, the nature of the vegetation associated with the wetlands and the general conservation value of each of the wetlands. III DECLARATION I declare that this research report, apart from the contributions mentioned in the acknowledgements, is my own, unaided work. It is being submitted for the Degree of Master of Science at the University of the Witwatersrand, Johannesburg. It has not been submitted before for any degree or examination at any other university. ___________________________ (Signature of candidate) _______day of________________2007 IV ACKNOWLEDGEMENTS I wish to express my appreciation and gratitude to the following persons for their contributions to the successful completion of this study: • My study leader, Prof. S. Grab, who provided technical guidance and encouragement. • Jan van den Bergh, my colleague, who provided assistance with the compilation of composite maps on ARCView, acted as a sounding board and provided much encouragement. V CONTENTS DECLARATION............................................................................................. IV LIST OF TABLES.......................................................................................... IX LIST OF FIGURES......................................................................................... XI CHAPTER 1.....................................................................................................1 INTRODUCTION .............................................................................................1 1.1 OVERVIEW OF IMPORTANT CONCEPTS .......................................1 1.2 THE STUDY AREA............................................................................3 1.3 BACKGROUND.................................................................................9 1.4 PROBLEM STATEMENT.................................................................11 1.5 PURPOSE OF THE STUDY ............................................................13 1.6 LIMITATIONS AND DELIMITATION................................................14 CHAPTER 2...................................................................................................15 LITERATURE REVIEW .................................................................................15 2.1 INTRODUCTION.............................................................................15 2.2 AN OVERVIEW OF WETLANDS .....................................................15 2.3 MICROBIAL WATER QUALITY .......................................................20 2.4 POINT AND NON-POINT SOURCE POLLUTION ...........................24 2.5 WETLAND CLASSIFICATION AND WETLAND PRIORITISATION RELATED TO FUNCTION...................................................................29 2.6 WATER QUALITY MODELLING......................................................34 2.7 ASSIGNMENT OF WEIGHTS TO WATER QUALITY VARIABLES..36 2.8 SUMMARY AND CONCLUSION .....................................................40 CHAPTER 3...................................................................................................43 METHODODOLOGY .....................................................................................43 3.1 SELECTION OF METHODOLOGY..................................................43 3.2 DATA AND SPATIAL DATA ANALYSIS...........................................45 3.3 SAMPLING......................................................................................46 3.4 PRIORITISATION OF RIVER BASINS ............................................48 VI 3.4.1 Compilation of scores for diffuse pollution....................................52 3.4.2 Compilation of scores for non-point and point source pollution....53 3.4.3 Number of households at risk from contaminated surface water .58 3.4.4 Final river basin ranking...............................................................59 3.5 SELECTION OF CANDIDATE WETLANDS IN THE PRIORITY RIVER BASIN .................................................................................................59 3.5.1 Selection of wetlands with a high need for rehabilitation..............60 3.5.2 Selection of wetlands according to hydrogeomorphic location.....60 3.5.3 Selection of wetlands according to minimum size........................60 3.5.4 Selection of wetlands according to location .................................61 3.5.5 Proximity to receptors ..................................................................61 3.5.6 Storm water outlets ......................................................................61 3.6 SUMMARY ......................................................................................61 CHAPTER 4...................................................................................................63 RESULTS ......................................................................................................63 4.1 INTRODUCTION.............................................................................63 4.2 IDENTIFICATION OF RIVER BASINS.............................................64 4.3 PRIORITIZATION OF RIVER BASINS.............................................64 4.3.1 The diffuse pollution loads in the river basins ..............................66 4.3.2 Analysis of pollution point and area sources per river basin ........67 4.3.3 Households at risk from contaminated surface water...................70 4.3.4 The Simple Additive Weighting technique....................................71 4.4 SELECTION OF CANDIDATE WETLANDS IN THE PRIORITY RIVER BASIN .................................................................................................75 4.5 SUMMARY ......................................................................................78 CHAPTER 5...................................................................................................79 CONCLUSION AND RECOMMENDATIONS................................................79 5.1 INTRODUCTION.............................................................................79 5.2 DISCUSSION ..................................................................................80 5.3 RECOMMENDATIONS ...................................................................82