caused by declining ground water levels, maintain base flow in some streams, and raise ground water levels to reduce the cost of ground water pumping.

As the benefits of artificial recharge of ground water have become evident, water planners have sought alternative sources of water for recharge projects. The National Research Council's Committee on Ground Water Recharge was established to study issues associated with the recharge of ground water using source waters of impaired quality, specifically treated municipal wastewater, stormwater runoff, and irrigation return flow, and issues associated with the use of recovered recharge water for potable as well as nonpotable purposes. (This report does not address industrial wastewater, which can contain too wide and too different an array of possible constituents to be dealt with in this same volume.) The committee was asked to address a range of topics such as source water characteristics, pretreatment and recharge technologies, public health, and the nature of the physical, chemical, and biological processes and transformations that occur during transport through the subsurface. Economic, institutional, and regulatory issues were also to be considered.

The committee is aware that much work has been done in the past to understand the opportunities and potential problems related to artificial recharge of ground water, and we have been careful to try to build on this strong foundation. The committee also recognizes that artificial recharge using waters of impaired quality is one of many strategies that can be used, alone or in conjunction with other strategies, to augment water supplies, such as reducing water consumption or creating secondary water systems that deliver certain wastewaters directly to nonpotable uses (e.g., the use of gray water for landscape irrigation). This report summarizes the state of our knowledge about artificial recharge using source waters of impaired quality and its usefulness and makes recommendations to help the nation use this water management tool more effectively.

SOURCE WATERS AND THEIR TREATMENT

The quality of the source waters used to recharge ground water has a direct bearing on operational aspects of the recharge facilities and also on the ultimate use to be made of the recovered water. In general, the source water characteristics that affect the operational aspects of recharge facilities include suspended solids, dissolved gases, nutrients, biochemical oxygen demand, microorganisms, and the sodium adsorption ratio (which affects soil permeability). The constituents that have the greatest potential effects when potable reuse is being considered include organic and inorganic toxicants, nitrogen compounds, and pathogens.

Of the three types of impaired quality water considered in this report—treated municipal wastewater, stormwater runoff, and irrigation return flow—municipal wastewater is by far the most consistent spatially and temporally and in terms of both quantity and quality. Exceptions to this generalization are



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