used for potable purposes or other purposes that might result in human exposure?

Public health concern over the use of recovered water from ground water recharged with source waters of impaired quality centers on the difficulty in identifying and estimating human exposures to the potentially toxic chemicals and microorganisms that may be present. To some extent the assessment of possible health risks can rely on the vast body of knowledge that has been developed for water supplies using conventional source waters, such as ground water from relatively uncontaminated aquifers and surface waters. However, there is a substantial amount of uncertainty even for such waters, principally related to the presence of synthetic organic chemicals, inorganic chemicals disinfection by-products, and pathogenic organisms.

Studies have been made of the chemical and microbiological characteristics of recovered water, although they are limited in number and scope. Several studies have shown that the recovered water can meet drinking water standards, even when the recharge source is treated municipal wastewater. Such findings lead some experts to the conclusion that these extracted waters are as acceptable as water supplied from traditional sources. Other experts strongly disagree, saying that water originating from an impaired source is inherently more risky. For instance, disinfection of the recharge waters may develop a different mix of disinfection by-products (DBPs), often unidentified, from those found in conventional water supplies. Also, the characterizations of the organic material and the full range of microbiological constituents are incomplete. In addition, source waters of impaired quality and recharge water withdrawn from the aquifer at the point of use may contain some contaminants at higher concentrations than are likely to be present in conventional water supplies. And throughout the whole process there is increased reliance on technology and management, leaving open the door for errors. Thus, the question arises whether drinking water standards developed for conventional water supply systems are sufficiently protective of human health when ground water is recharged with waters of impaired quality.

The assessment of health risks associated with recharge using impaired sources is far from definitive because there are limited chemical and toxicological data and inherent limitations in the available toxicological and epidemiological methods. The limited data and extrapolation methodologies used in toxicological assessments provide a source of limitations and uncertainties in the overall risk characterization. Similarly, epidemiological studies suffer from the need for very long time periods required, because cancers have latency periods of 15 years or more. Also, such studies require large populations to uncover the generally low risks associated with low concentrations of toxicants. Past studies of the possible adverse health effects from reclaimed water have tended to be limited in terms of toxicological characterization and have focused on those chemicals for which drinking water standards exist.

The challenge in considering the health risks from recharge systems is to

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