GIS-based risk mapping of illicit discharges into stormwater drainage systems in Cape Town

Abstract

Migration into urban areas is driven by a diverse range of factors, such as opportunities to find employment and operate businesses, access to healthcare and educational facilities, and peace and stability. Although the effects have been felt throughout South Africa, Cape Town is a particularly popular destination. The phenomenon has increased pressure on available resources and infrastructure that was not designed to cope with dramatic increases in the size of the population. A myriad of problems have resulted as a direct consequence, including illicit discharges into stormwater drainage systems from all land uses. As local governments are faced with severe budgetary constraints, cost-effective methods need to be developed to mitigate pollution, which necessitates the conducting of appropriate research to understand the spatial distribution and extent of the problem, in order to identify and prioritise specific concerns for the deployment of limited resources and to provide solutions for the containment of pollution. Consequently, it was necessary to conduct a cost-effective desktop assessment study to map areas that are at high risk of illicit discharges in the city. The aim of this research is to develop a reliable geographic information systems (GIS) desktop methodology for mapping areas in the urban catchments that are at risk of illicit discharges into stormwater systems. Among the specific objectives is to perform a spatial analysis of the hydraulic, hydrological, and land use datasets to classify and identify areas with high illicit discharge potential and to develop effective procedures for risk mapping illicit discharge potential (IDP). The methodology entailed the use of datasets pertaining to twelve risk factors, namely, population density, development age, outfall density, ageing sanitary infrastructure, drainage density, generating site density, infrastructure access density, on-site sanitation, percentage impervious surface, and commercial, residential, and industrial land uses as indicators of the likelihood of illicit discharges into stormwater drainage systems. The study commenced with the collecting and compiling of data to transform datasets for individual risk factors into spatial data. Risk factors were then scored to determine thresholds in order to evaluate catchments on the basis of their IDP and to classify areas as being at low, medium, or high risk in relation to individual risk factors. A spatial overlay and analysis of past discharge records were performed to rank the influence of risk factors in relation to their ability to predict locations of illicit discharges. The past discharge records were categorised as pertaining to the water, wastewater, greywater, solid waste, and all/combined pathway groups. The five groups were then overlaid and analysed separately. A multi-criteria evaluation was performed to quantify risk factors as weights. The final step entailed the generating of composite maps to prioritise catchment areas as having low, medium, or high IDP.

The findings of the research study revealed that many land use, hydraulic, and hydrologic characteristics influenced the locations of illicit discharges. It also emerged that two principal risk factors, namely, residential land use and percentage impervious surface, were most crucial to predicting the locations of illicit discharges. The percentages of high-risk areas in the study area were found to vary from 25% and 32% for different discharge pathway groups. It is abundantly evident from the findings of this study that municipalities should avail themselves of risk mapping as a highly effective means of facilitating the structuring of the implementation of illicit discharge detection and elimination (IDDE) programmes. Risk mapping makes it possible for municipalities to follow a structured approach to formulate appropriate priorities for inspections of facilities, public education, investigations of outfalls and drainage areas, and projects to improve infrastructure. Although the findings of the study are promising, further study is still recommended in the study area, in order to develop a truly comprehensive appraisal of the risk of illicit discharges. The inclusion of agricultural land use as a risk factor is recommended, as there is a great deal of agricultural activity in the area. It is also recommended that the most up-to-date data should be used at all times, in order to maximise the accuracy of findings.