Urban wastewater as groundwater recharge : evaluating and managing the risks and benefits

Abstract

The note reviews how urban wastewater relate to groundwater, to what extent is wastewater a groundwater pollution hazard, and what types of measures are available for reducing risks, and increasing benefits. It further analyzes how can wastewater, and groundwater use, be integrated into urban planning. The expansion of waterborne sewerage in developing cities, has gone on intermittently over many decades, with the earliest small systems having been introduced during the first half of the 20th century. Although sewerage provision lags behind water supply, rapid growth in urban water demand is resulting in steadily-increasing wastewater generation by most developing cites, and this will be further stimulated by the United Nations (UN) Millennium Goals for sanitation. Many sewerage systems discharge directly to surface watercourses, with minimal treatment, and little dilution in the dry season, and thus the 'wastewater' available for irrigation reuse can often be (in effect) largely sewage effluent. It has also become apparent that common wastewater handling, and reuse practices in developing nations (which are frequently unplanned and uncontrolled) generate high rates of infiltration to underlying aquifers in the more arid climates. This incidental infiltration is often volumetrically the most significant local 'reuse' of urban wastewater, but one which is rarely planned, and may not even be recognized. It both improves wastewater quality, and stores it for future use, but can also pollute aquifers used for potable water supply. The topic has major implications in terms of future approaches to groundwater, and wastewater management in many rapidly-developing urban centers. The groundwater dimension is still often one of the 'missing links'. Major incidental recharge of aquifers through wastewater handling, and reuse is so widespread that it should always be contemplated as an integral part of wastewater management, and thus planned for accordingly. Those responsible for wastewater, need to be made aware of the benefits and hazards of wastewater recharge to aquifers, and how hydro-geological environments vary with regard to pollution vulnerability, and thus to wastewater safe loading rates, and patterns. A stronger element of municipal planning will be needed for the worst (and least sustainable) of past practices to be avoided in future.