Human virus and microbial indicator occurrence in public-supply groundwater systems: meta-analysis of 12 international studies

Authors: FOUT, SHAY - Us Environmental Protection Agency (EPA); Borchardt, Mark; KIEKE, BURNEY - Marshfield Clinic Research; MOHAMMAD, KARIM - City Of Santa Cruz Public Works Department

Submitted to: Hydrogeology Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2017
Publication Date: 4/26/2017
Publication URL: http://handle.nal.usda.gov/10113/5753396
Citation: Fout, S.G., Borchardt, M.A., Kieke, B., Mohammad, K.R. 2017. Human virus and microbial indicator occurrence in public-supply groundwater systems: meta-analysis of 12 international studies. Hydrogeology Journal. 25:903-919.

Interpretive Summary: Testing for pathogens in groundwater is complicated and expensive. Instead, public health officials have relied on more easily tested non-pathogenic microbes, known as the fecal indicator organisms (FIOs), to indicate the sanitary quality of water. When evaluating how well an FIO works, researchers often resort to characterizing its co-occurrence with one or more pathogens using simple tests of correlation. We combined data on human gastrointestinal viruses and FIOs in groundwater from 12 international studies, and we used this large dataset to demonstrate the full suite of statistical information that is necessary to evaluate virus and FIO co-occurrence. For six FIOs commonly used in many countries around the world, we report the probability of detecting a virus when an FIO is detected, and we calculate how often the FOIs are incorrect in indicating whether a virus is present or absent. Overall, the six FIOs are reliable for indicating clean water, but they do a poor job of indicating virus contaminated water. Finding the most effective FIO for pathogens in groundwater remains a major goal of public health microbiology. This study shows the most likely path for achieving this goal.

Technical Abstract: Groundwater quality is often evaluated using microbial indicators. This study examines data from 12 international groundwater studies (conducted 1992–2013). Sites were chosen from 718 public drinking-water systems with a range of hydrogeological conditions. 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. Escherichia coli (E. coli) and somatic coliphage are 7–9 times more likely to be associated with culturable virus-positive 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 virus-positive 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 subsets based on (1) presence of total coliform bacteria, (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 single indicators are E. coli and somatic coliphage, although single indicators may underestimate virus occurrence. Repeat sampling for indicators provides a superior estimate of the potential for viral contamination in a well.