Groundwater quality is often evaluated using microbial indicators. This study examines data from 12 international groundwater studies (conducted 1992?2013) of 718 public drinking-water systems located in a range of hydrogeological settings. Focus was on testing the value of indicator organisms for identifying virus-contaminated wells. One or more indicators and viruses were present in 37 and 15% of 2,273 samples and 44 and 27% of 746 wells, respectively. <
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Escherichia coli<
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(<
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E. coli<
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) and somatic coliphage are 7? 9 times more likely to be associated with culturable viruspositive samples when the indicator is present versus when it is absent, while F-specific and somatic coliphages are 8?9 times more likely to be associated with culturable viruspositive wells. However, single indicators are only marginally associated with viruses detected by molecular methods, and all microbial indicators have low sensitivity and positive predictive values for virus occurrence, whether by culturable or molecular assays, i.e., indicators are often absent when viruses are present and the indicators have a high false-positive rate. Wells were divided into three susceptibility subsets based on presence of (1) total coliform bacteria or (2) multiple indicators, or (3) location of wells in karst, fractured bedrock, or gravel/cobble settings. Better associations of some indicators with viruses were observed for (1) and (3). Findings indicate the best indicators are <
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E. coli<
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or somatic coliphage, although both indicators may underestimate virus occurrence. Repeat sampling for indicators improves evaluation of the potential for viral contamination in a well.