process, and the intended use of the recovered water. A fundamental assumption of this report is that wastewater used to recharge the ground water must receive a sufficiently high degree of treatment prior to recharge so as to minimize the extent of any degradation of native ground water quality, as well as to minimize the need for and extent of additional treatment at the point of extraction.

After pretreatment, the water is ready for recharge, either through surface spreading and infiltration through the unsaturated zone or by direct injection into ground water. Recharge by infiltration takes advantage of the natural treatment processes, such as biodegradation of organic chemicals, that occur as water moves through soil. The quality of the water prior to recharge is of interest in assessing the possible risks associated with human exposures to chemical toxicants and pathogenic microorganisms that might be present in the source water. Although one can reasonably expect that such constituents will often be reduced during filtration through the soil, as well as subsequently in the aquifer, a conservative approach to risk assessment would assume that toxicants and microorganisms are not completely removed and some are affected only minimally prior to subsequent extraction and use. Thus when recharge water is withdrawn later for another purpose, it may require some degree of posttreatment, depending on its intended use.

Taking a systems perspective that encompasses all steps from pretreatment, through recharge, through transformation and transport, to extraction, this report assesses the issues and uncertainties associated with the artificial recharge of ground water using source waters of impaired quality. In particular, the report focuses on the methodologies and nature of the recharge systems and the subsequent impacts on the native ground water quality, especially as those impacts may affect public health following use of the recovered water. Economic, institutional, and regulatory questions are examined as well. First, this chapter presents a primer on artificial recharge of ground water to give the reader an introduction to the philosophy and techniques of the field.

A PRIMER ON ARTIFICIAL RECHARGE

Water continually evaporates from the oceans and other open water bodies, moves across the land as water vapor in clouds, falls back on the land as rain and snow, and then returns to the oceans through rivers and underground pathways to start the cycle—the hydrologic cycle—again. Part of the water that fails on the land evaporates from the soil or is transpired from plants back into the atmosphere. Another part flows overland to stream channels, lakes, or the sea. The remainder seeps downward through the soil under the influence of gravity to enter the ground water system. Once in the ground water system, the water moves slowly in response to ground water slopes or hydraulic gradients until it reenters the surface part of the cycle.

The term ground water applies to water of higher than atmospheric pressure



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