Water quality impacts of urbanisation: Relating water quality to urban form

Background\ud Effective urban resource planning and management entails the mitigation of the impacts of urbanisation on the water environment. The significance stems from the fact that water environments are greatly valued in urban areas as environmental, aesthetic and recreational resources and hence are important community assets. Urbanisation has a profound influence on stormwater runoff quality. This is due to changes to the hydrology of the catchment and the introduction of pollutants resulting from various anthropogenic activities common to urban areas. Though the sources and causes of stormwater pollution are known, its control constitutes an intractable challenge in the drive towards sustainable human settlements.\ud \ud These difficulties can be ascribed to the fact that the current focus on urban water quality is of relatively recent origin. It is a paradigm shift from the sole focus in the past on quantity issues for flood mitigation. However the techniques and approaches adopted are strongly rooted in quantity research undertaken in the past. This applies not only to modelling philosophies, but also to the conducting of research and data analysis. There is undue reliance on physical processes and the neglect of important chemical processes in describing various stormwater associated phenomena.\ud \ud Therefore in the absence of appropriate guidance, current approaches to safeguard water quality focus primarily on ‘end-of-pipe’ solutions. Unfortunately, as a result of entrenched misconceptions, insufficient design knowledge, faulty value judgements or inadequate consideration of life-cycle costs, these approaches tend to be largely ineffective and even counter-productive in the long-term.\ud \ud The Project\ud The research project was located in Gold Coast, Queensland State, Australia. The primary focus of the project was to undertake an in-depth investigation of pollutant wash-off by analysing the hydrological and water quality data from six areas having different land uses in order to correlate urban form to water quality. The project entailed significant field work for water sample collection, extensive laboratory testing and data analysis. \ud \ud The study areas were selected so as to ensure that there was uniformity in the geological, topographical and climatic variables which could influence the water quality characteristics. The three main catchments selected were already established by Gold Coast City Council and are characterised by differing forms of land development and housing density; ranging from predominantly forested, to rural acreage-residential and forest to mixed urban development. For this project, three smaller subcatchments within the urban catchment were identified for more detailed investigations into effects of increasing urban density on water quality. \ud \ud Automatic monitoring stations were established at the outlet of each area to record rainfall, stream-flow and a number of water quality parameters. Each station was equipped with an automatic event sampler to augment grab samples taken during low flow conditions. Event samples collected and the grab samples taken during low flow conditions were analysed in the laboratory for a range of water quality parameters.\ud \ud The data derived were initially analysed using univariate statistical methods to obtain an insight into the trends and patterns of variations in water quality. Subsequently multivariate ‘chemometric’ techniques were applied to identify linkages between various parameters and their correlation with land use. The analytical techniques used included, Principal Component Analysis, Scores Plots and Partial Least Squares Regression.\ud \ud The Outcomes\ud The primary conclusions derived were:\ud • The mean values and standard deviations for the primary water quality parameters were generally found to increase with increasing urbanisation. The increase in standard deviations underlies the difficulties in developing predictive water quality models.\ud \ud • For all three main catchments, the particulate bound component of heavy metals was significantly higher than the dissolved component. This can be attributed to the relatively stable pH values.\ud \ud • The total concentrations of Al, Mn and Fe, which are sourced from the soil were found to increase with increasing urbanisation probably as a result of erosion.\ud \ud • There was no appreciable difference in the dissolved components of heavy metal concentrations between study areas which runs counter to the general trend of increasing concentrations with urbanisation. Secondly due to the fact that the dissolved fractions were below detection limit, it could be surmised overall that heavy metals are not a significant issue in these study areas. It is the dissolved component which is readily bioavailable.\ud \ud • The six study areas behaved quite differently to each other in terms of correlations between different parameters and strong correlationships were not common. This would make it difficult to develop stereotypical strategies for water quality management.\ud \ud • It was quite common for TOC to be in soluble form (as DOC) in a number of study areas. This parameter can exert a significant influence on urban water quality particularly in relation to the bioavailability of heavy metals and hydrocarbons.\ud \ud • Primary pollutants such as TN, TP and TOC were commonly in soluble form. This would mean that the effectiveness of structural pollutant abatement measures such as sediment traps is open to question as these measures are dependent on gravity settling.\ud \ud • The calibration models derived using partial least squares regression for predicting various parameter values were of questionable value generally resulting in large errors of prediction. This in effect means that strong correlationships between parameters were singularly lacking and underlies the reason for the large errors commonly encountered in water quality models.\ud \ud The outcomes from this study bring into question a number of fundamental concepts routinely accepted in stormwater quality management. The fact that the pollutant characteristics are not consistent across the study areas would mean that the urban form is the overriding factor influencing the water quality. This conclusion would mean that the effectiveness of structural measures would not be universal and stereotypical solutions will not always prove adequate. A significant fraction of the pollution is in dissolved form, it is more bio-available and is therefore more likely to cause pollution in receiving waters. It could well be that this condition is linked to the climatic and rainfall conditions experienced in the study region which significantly influences pollutant composition, build-up and wash-off. Therefore it is important that predictive water quality models developed have the versatility to take these characteristics into consideration.\ud \ud The above findings underline the need to move beyond the dependency on customary structural measures and end-of-pipe solutions and the key role that urban planning can play in safeguarding urban water environments. The univariate and multivariate statistical data analysis undertaken found that among the different urban forms, stormwater runoff from the area with detached housing in large suburban blocks exhibited the highest concentration and variability of pollutants. This is based on the concentration of various pollutants, their high variability and physico-chemical form. Rural residential on large blocks were only marginally better. It could be concluded that in terms of safeguarding water quality, high density residential development which results in a relatively smaller footprint should be the preferred option