cusses potential impediments and drivers to implementing the framework. The chapter closes with a summary of its conclusions and recommendations.

Indicators for waterborne pathogens are used to achieve a variety of goals, fulfill various regulations, and meet differing applications under the Safe Drinking Water Act (SDWA) and Clean Water Act (CWA; see Chapters 1 and 4). Often they are used to provide an early warning of potential microbial contamination, an application for which a rapid, simple, broadly applicable technique is appropriate. They are used for health risk confirmation where resulting actions can be costly and time consuming. They are also used to identify the source of a microbial contamination problem which can have terrestrial origins. In both of these latter applications, the time frame and investment in indicators, indicator approaches, and methods must be greater.

Indicator applications also vary according to the media in which they are used. For example, warning systems for groundwater typically focus on the presence or absence of bacterial indicators of fecal contamination because high-quality groundwater does not normally contain fecal bacteria and is often used without disinfection. In contrast, quantitative tests for indicator bacteria are used in monitoring surface drinking water intakes because these waters often show some evidence of fecal contamination and are usually treated with filtration and disinfection. Interpretation of indicator data in recreational water applications is different again because the exposure can be more irregular and involves a more limited population at risk. Furthermore, all of the indicator applications discussed in this report are inextricably linked and to some extent must account for surrounding terrestrial ecosystems (e.g., through fecal loading from agricultural, wild, and domestic animals living in a flood plain) that can affect the microbiological quality of the water being assessed.

A single microbial water quality indicator or small set of indicators cannot meet this diversity of needs and applications. The complexity of issues surrounding microbial water quality assessment requires the use of a “tool box” in which the indicator(s) and method(s) are matched to the requirements of a particular application. Like health investigations, water quality studies may have to proceed through a series of phases with a different suite of tools needed for each phase.

The committee recommends use of a phased, three-level monitoring framework, as illustrated in Figure 6-1, for selecting indicators. The first phase of this framework is screening or routine monitoring (Level A). The objective of this phase is early warning of a health risk or of a change from background condition that could lead to a health risk. This is the most frequent type of monitoring and is routinely conducted throughout the country.

In general, the most important indicator attributes at this level are speed, low cost (logistical feasibility), broad applicability, and sensitivity. Speed is impor-