The potential of urban runoff as a water resource

Abstract

With the population of urban areas rapidly increasing, a much greater demand is being placed on existing water supplies. The arid southwestern region of the United States, in particular, is experiencing large population increases while possessing limited water resources. Tucson is a representative city in the region facing problems of providing an adequate water supply to the public in the future. Presently, Tucson is being supplied entirely with groundwater. Increases in population and industrial activities, however, have caused a steady decline of the groundwater table in the Tucson Basin. The reclamation of wastewater and the importation of water have been studied as alternatives in alleviating the annual decline of the groundwater table. Problems still exist, however, preventing the immediate use of both aforementioned supplies of water. In developing the water resources of an area, every possible source of water must be evaluated. A source which has not received much attention, but which merits much attention, is the water occurring as urban runoff following intense storms. In order to evaluate the potential of urban runoff as a water supply, the study includes investigations of water quality, water treatment through storage and coagulation, and problems involved with the utilization of storm water. Samples of runoff from three diversified urban watersheds in the Tucson area were analyzed for bacterial, mineral, pesticide, solids, and chemical oxygen demand concentrations. The watersheds were characterized according to the percentage of the total area devoted to a particular land use. In addition, the hydrologic characteristics of each storm were tabulated. Correlation coefficients were determined between the quality parameters and the watershed and hydrological characteristics. Development of regression equations equating quality parameters as a function of both watershed and hydrological characteristics was also undertaken. The final analysis of the quality study involved the determination of relationships between quality parameters of chemical oxygen demand, total coliforms and suspended solids and the point of time on the hydrograph at which runoff was sampled. Prior to beneficial use of the urban runoff, treatment to varying degrees will be required. In the second phase of this study, the efficiency of treatment by the simple methods of storage and alum coagulations was studied. Five gallon samples were collected from randomly selected storms and used either in the storage or coagulation study. Changes in chemical oxygen demand, solids and bacterial concentrations were evaluated at selected intervals during storage for a period of a week. Jar test studies utilizing varying doses of alum were undertaken on water collected from each of the watersheds, determining the efficiency of chemical oxygen demand, turbidity, and total coliform removals. The final phase of the study involved discussion of the problems attendant with the planning and design of treatment facilities. Included in this phase were sections involving water quality standards and the related treatment processes, waste sludge production and treatment methods, and costs pertaining to treatment. Legal aspects of appropriating the urban runoff were considered and the possible conflicts between upstream and downstream interests noted. The study concluded with a demonstration of the application of dynamic programming for optimally planning the location and capacity of storage treatment facilities at urban sites.