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In the wake of the Surface Water Treatment Rule, primacy agencies are required to submit policies on handling source water determinations without the benefit of a national consensus method for evaluation or interpretation. Despite guidance offered by the US Environmental Protection Agency and other work done in this area, there are some unresolved problems in the classification methods for groundwater that may be under the influence of surface water. This paper shares Montana's experiences with the recommended procedures for classifying such source waters and provides information on complicating factors. For investigations in Montana to date, the most useful tools have been hydrogeological information, construction details, site observations, and particulate analyses. Montana believes that there is no one indicator that will give a clear-cut answer as to whether a system is at risk for Giardia. Particulate analyses should not be used as the sole determinant, and results from laboratories should be evaluated carefully to determine what they really indicate. Further complicating the issue, it is possible that the protocols developed for Surface Water Treatment Rule implementation will end up in courts as a water rights issue, too. A flow chart shows the steps used in the Montana process for classifying source water. See more