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Water: Drinking Water Standards

GROUND WATER RULE: Ground Water Microbial Occurrence Studies


EPA has the responsibility to develop a ground water rule which not only specifies the appropriate use of disinfection but, just as important, addresses other components of ground water systems to assure public health protection. Section 1412(b)(1)(A) of the Safe Drinking Water Act (SDWA) requires EPA to establish National Primary Drinking Water Regulations for contaminants that may have an adverse public health effect and that present a meaningful opportunity for health risk reduction. This general provision is supplemented with an additional requirement under Section 1412(b)(8) that EPA also develop regulations specifying the use of disinfectants for ground water systems as necessary. To meet these requirements EPA is working with stakeholders to develop a Ground Water Rule (GWR) proposal by March 1999, and a final rule by November 2000.


The overall goal of the Safe Drinking Water Act (SDWA) is to establish standards that will protect public health. The Ground Water Rule is designed to protect against pathogenic bacteria and viruses in source water, against growth of opportunistic pathogenic bacteria in ground water distribution systems, and to mitigate against any failure in the engineered systems, such as cross-connections or sewage infiltration into distribution systems.

Microbial pathogen and indicator occurrence data are important to GWR development for several reasons: 1) pathogen occurrence in a well represents an unequivocal message that disinfection is necessary; 2) national occurrence estimates of pathogens in wells, when combined with dose-response data for those pathogens, yield a national estimate of public health risk to be addressed in a GWR; 3) vulnerability assessments, designed to meet the SDWA mandate to disinfect "as necessary," can be evaluated using pathogen and indicator organism occurrence; and 4) indicator occurrence can be used, in an appropriate monitoring framework, so as to identify wells where further corrective action is necessary.


Recognition of pathogenic human enterovirus occurrence in drinking water wells dates back at least to the recovery of poliovirus from a Michigan well in 1972 (Mack et al., 1972). As part of the current GWR proposal development process, EPA is analyzing 16 recent studies that sampled wells for viral pathogens and fecal indicator organisms such as coliphage (viruses that infect coliform bacteria). Twelve studies evaluated the occurrence of pathogens, such as human enteroviruses in the source ground water (prior to disinfection) of drinking water wells (either PWS wells or private wells or both). These twelve studies also sampled for pathogen indicator organisms such as coliform bacteria and coliphage. Three additional studies sampled only for coliphage and bacteria. One study sampled only for bacteria.

These sixteen studies were designed and conducted to meet a range of objectives. All studies were conducted to learn more about the occurrence of pathogens and/or their indicators in well water. Other objectives included: 1) pathogen and/or indicator occurrence in Public Water Supply (PWS) wells from a variety of hydrogeologic settings; 2) pathogen and indicator occurrence in PWS wells that had a history of indicator organism occurrence or had been flooded; 3) pathogen and/or indicator occurrence in PWS or private wells sited within a single hydrogeologic setting, such as cavernous karst; 4) pathogen and indicator organism occurrence to be used to develop and evaluate methods for analysis of ground water samples; 5) pathogen and indicator organism occurrence to evaluate hydrogeologic vulnerability assessments; 6) pathogen and indictor occurrence in wells with proximity to treated sewage injection or percolation recharge sources; 7) indicator occurrence in private wells (with a few PWS wells) in a variety of hydrogeologic settings. All but one study sampled each well once or no more than twice; one study sampled each well monthly for a year.

In all studies, wells were selected from owner/operators that volunteered to be included. Typically, private wells are not disinfected and may be located in closer proximity to potential contamination sources than PWS wells. PWS wells sampled in all of the occurrence studies discussed herein are almost universally disinfected.

The studies were conducted using a variety of microbial analytical methods, including cell culture methods for both pathogens and indicators, and polymerase chain reaction (PCR) methods for pathogens. Samples were analyzed using one or more of the following methods: virus cell culture, bacteriophage cell culture, and PCR assay. Cell culture assays were performed to determine the presence of one or more of the following: enteroviruses, total coliform, enterococci, fecal coliform, fecal streptococci, E. coli, Legionella sp., Legionella pneumophila, Clostridium perfringens, Pseudomonas aeruginosa and Aeromonas hydrophila. Bacteriophage cell culture assays were performed using a variety of bacterial host media, including: S. typhimurium WG 49 for male-specific coliphage, E. coli C3000, E. coli 3000 for somatic coliphage, and Bacteroides fragilis HSP-40. PCR analyses were conducted using methods specific to identifying enteroviruses, rotavirus, Norwalk virus and Hepatitis A virus. A commercial test (Colilert) was also used to measure total coliform presence or absence.


The following is a discussion of the data available in the 16 studies analyzed. Some of these studies are still underway and will provide more data which will supplement the following analysis and possibly change any conclusions which could be drawn from the data summarized here.

Four studies include data indicating the presence of pathogenic human enterovirus in well samples. These data are available from more than 600 samples representing 460 wells. Thirty-six wells were positive for enteroviruses using cell culture methods, representing 8% of the wells.

Seven (23%) enterovirus cell culture positive wells were identified in a study that sampled 30 PWS wells monthly. Twelve (15%) enterovirus cell culture positive wells were identified in a study that sampled 81 PWS wells along the Mississippi River, some of which had been flooded. Sixteen (7%) enterovirus cell culture positive wells were identified in a study of 246 wells designed to develop methods for PCR analysis of ground water. One (4%) enterovirus cell culture positive well was identified in a study of 24 private wells in a karst hydrogeologic setting.

Opportunistic bacterial pathogens were evaluated in 47 private wells in one study of the Rio Grande River alluvial aquifer. Pseudomonas aeruginosa was found in 14 (30%) and Aeromonas hydrophila were found in 4 (8%) wells.

Total coliform bacteria were found in 93 of 446 (21%) and fecal coliform bacteria or E. coli were found in 25 of 175 (15%) of wells so tested. Enterococci were found in 59 of 337 (18%) wells tested. Male-specific coliphage were found in 46 of 396 (12%) and somatic coliphage in 37 of 463 (8%) of wells tested.

One study of Midwestern private wells by the Centers for Disease Control (CDC) found Total coliforms in 41.3% and E. coli in 11.2% of 5,520 wells.

In one study of Public Water Supply wells, 38% of 246 wells were positive by PCR for either enterovirus, rotavirus, or Hepatitis A virus. Currently, the PCR test can only be considered, at most, as a test for pathogen indicators. This method does not differentiate viable from non-viable organisms. PCR data are subject to considerable interpretation about the number of false positive results because of the lack of specificity associated with analysis of environmental (water) samples.

The differing studies were aggregated so as to combine the enterovirus cell culture data from 491 wells. Wells were combined based on whether they were "Potentially at risk" or "clean." "Potentially at risk" wells have historical total coliform occurrence (25 wells had positive TC or other fecal indicators in source water) or documented evidence of sampling during flooding. Grouping well samples into these two categories, 12.3% of known "Potentially at risk" wells and 5.5% of "clean" wells were positive for pathogenic enteroviruses. Twelve percent of "Potentially at risk" wells represents less of a potential health concern because most wells with repeated total coliform occurrence or that have been flooded presumably already disinfect. However, a virus occurrence of five percent in "clean" wells represents a large potential health concern, because many wells without a history of flooding or TC occurrence remain undisinfected.


1. How do the results from these studies relate to a national understanding of the present risk from pathogen-contaminated ground water?

2. What is the national estimate of microbial pathogen and indicator organism occurrence and what is the most appropriate method for determining such an estimate using the available data?

3. What is the proper role for and usage of indicator organism occurrence data in this rule-making, and which organisms are most useful as indicators of human pathogens?

4. What is the proper role for PCR occurrence data in this rule-making and what is the appropriate method for PCR analysis?

5. How representative are the available data in characterizing the variety of hydrogeologic settings and PWS systems in the US?


EPA will continue to evaluate occurrence data as it becomes available, and work with stakeholders to develop an estimate of national occurrence and risk.

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