Scientific Investigations Report 2006–5290
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5290
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In 1991, the U.S. Geological Survey (USGS) began the National Water-Quality Assessment (NAWQA) program to study the quality of the Nation’s surface- and ground-water resources in more than 50 major river basins and aquifers. During the first 10 years of study (Cycle I), the primary goals of the program were to assess the water-quality conditions of the Nation’s streams and ground water, the effects of human activities and natural features on the quality of water, and changes in the quality over time (Gilliom and others, 2001). In 2001, at the beginning of the program’s second 10-year cycle (Cycle II) of intensive studies, NAWQA investigators returned to 14 major river-basin and aquifer systems (Gilliom and others, 2001). The second set of Cycle II studies of 14 systems began in 2004, and studies of a third set of 14 systems will begin in 2007, bringing the total to 42 of the Nation’s most important river-basin and aquifer systems (Gilliom and others, 2001). Cycle II emphasizes building upon the initial assessments of water quality and studying in detail the long-term trends and the factors that affect water quality (Gilliom and others, 2001). NAWQA’s Cycle II ground-water studies focus on regional assessments to evaluate water-quality conditions and trends in 16 or more principal aquifers, most of which underlie several states (Lapham and others, 2005).
NAWQA study units generally are defined by the boundaries of major-river basins and aquifer systems. Within each study unit, well networks were established to study the ground-water resources. In some cases, study units extended data collection outside of original geographic delineations to meet particular study unit, local, or national objectives. A group of network wells typically is sampled once during each 10-year cycle; however, the sampling frequency can vary among the study units.
Fecal and sewage contamination of water can introduce pathogenic microorganisms, including bacteria and viruses, into a water resource (Geldreich, 1966). Data obtained from the collection of water samples from wells and analyzed for the presence of fecal-indicator microorganisms can be used in multiple ways. Perhaps most importantly, data indicating the presence or absence of fecal-indicator microorganisms in ground-water samples can help determine the suitability of a water resource for different purposes, particularly as a drinking-water resource. The collection of ground-water samples for the NAWQA program and the analyses for the occurrence of fecal-indicator bacteria and fecal-indicator viruses contributed to the discussion of the microbial quality of ground water in regions across the United States.
This report summarizes microbiological data collected from wells by the NAWQA program from 1993 through 2004. The report describes (1) the occurrence and distribution of fecal-indicator bacteria and viruses in principal aquifers, and (2) the quality of water used for drinking-water supplies in the United States. All microbiological data collected in 27 NAWQA study units during Cycle I and several Cycle II studies (fig. 1) were inventoried for this analysis. However, for describing the microbial quality of the ground-water resources of geographic regions and the principal aquifers, this assessment was limited to that of 22 Cycle I and Cycle II study units for which data sets consisted of information from at least 15 wells.
The wells sampled by these study units make up large networks designated as major-aquifer studies (MAS); land-use studies (LUS), including the focused land-use studies in agricultural and urban regions; and source-water quality assessments (SWQA) of ground water used for public supplies. A smaller network consists of reference wells, which are primarily for monitoring background water quality. Because only two reference-network wells in this data set were sampled for fecal-indicator bacteria, that network was not included in this analysis. The large networks included wells that represented 16 of the 62 principal aquifers and aquifer systems described by the USGS Office of Ground Water in The National Atlas of the United States of America (U.S. Geological Survey, 2003). Total withdrawals from the 16 principal aquifers used for irrigation, public supply, and industrial purposes amounted to about 53,000 Mgal/d (Maupin and Barber, 2005). Seventy-six percent of these withdrawals were for irrigation purposes and 19 percent for public supplies. The water withdrawn from the High Plains principal aquifer in 2000 amounted to 17,500 Mgal/d (Maupin and Barber, 2005), the greatest amount of the 16 principal aquifers (table 1). The Central Valley, Calif., principal aquifer supplied the second highest amount of water (9,800 Mgal/d) (Maupin and Barber, 2005), which was used mostly for irrigation.
Direct analysis of pathogens in water is difficult and impractical (IAWPRC Study Group, 1991; Havelaar and others, 1993), but certain readily cultured bacteria, particularly the coliform and fecal streptococcus groups and bacteriophages (viral pathogens of bacteria), can be used as indicators of contamination. The total-coliform group is a broad group with several members of fecal and nonfecal origin. Over time, microbiologists have developed methods for analysis and enumeration of bacteria (Geldreich, 1966), such as the coliform species Escherichia coli (E. coli) and enterococci, that are specific for fecal contamination of water because they are consistently and nearly exclusively found in feces of warm-blooded animals (Cabelli, 1978; Gerba, 1987). The fecal-coliform group is a heat-tolerant subset of the total-coliform group and is differentiated in part by incubation at body temperature (44.5°C) rather than at 35°C, the temperature used to culture total coliforms. The fecal-coliform group is intermediate to the total coliforms and E. coli with respect to functioning as a fecal indicator because the test recovers species of nonfecal origin as well as E. coli and other fecal-origin coliforms. However, because the major species of the fecal-coliform group is E. coli (U.S. Environmental Protection Agency, 1978), the group has been extensively used for indicating fecal contamination (Geldreich, 1966). Some NAWQA studies included analyses for the fecal-coliform group, either in conjunction with, or in place of, the total coliforms.
The presence, or absence, of the total-coliform group has long been used for assessing drinking-water quality and has been written into water-quality standards promulgated as early as 1914 by the Public Health Service (McKee and Wolf, 1963). The coliform group, as defined by the American Public Health Association (APHA), is the principal indicator of the suitability of a water resource for domestic household or other uses. Although differentiation of the members of the coliform group, such as the fecal coliforms or E. coli, can be useful for evaluating a water resource for special study purposes, such as possible sources and types of contamination or how recent the contamination, the APHA considers the presence of any coliform bacteria in drinking water as unsatisfactory (American Public Health Association, 1998).
The U.S. Environmental Protection Agency (USEPA) regulates public drinking-water supplies through the Safe Drinking Water Act of 1974 (U.S. Environmental Protection Agency, 2003). USEPA’s Total Coliform Rule of 1989 established maximum contaminant levels for total coliform bacteria, including fecal coliforms and E. coli, in public-supply distribution systems and specified routine sampling requirements (U.S. Environmental Protection Agency, 2001a). The Total Coliform Rule also requires that if a sample of the finished (treated) water tests positive for total coliforms, the positive sample is tested for the presence of fecal coliforms or E. coli. The Ground Water Rule (U.S. Environmental Protection Agency, 2006) requires among other activities, disinfection of ground water “as necessary,” sanitary surveys, and source-water monitoring.
The USEPA does not regulate the quality of water from privately owned, domestic wells used for drinking water. However, the agency provides a web site (http://www.epa.gov/safewater/privatewells/index2.html) at which private well owners can access an array of basic information and guidance on protecting their well water from contamination. USEPA also participates in a partnership of a number of other organizations, including Farm*A*Syst/Home*A*Syst, American Ground Water Trust, and the National Ground Water Association, that provide technical assistance and information for well owners.
Clostridium perfringens (C. perfringens) is a noncoliform species of bacteria that forms resistant spores that enhance survival of the organism outside of its normal habitat. The spores are resistant to sewage treatment processes and disinfection that readily kill members of the coliform group. C. perfringens is a useful fecal indicator in tropical climates and for tracking point sources of wastewater. The organism has been proposed as a surrogate for other spore-forming microorganisms such as Cryptosporidium that the traditional coliform groups might not adequately represent (Bisson and Cabelli, 1980)
Microorganisms of fecal origin are affected differently on entering the aquatic environment (Bisson and Cabelli, 1980; Havelaar and others, 1993), and no single indicator can address all concerns about the sanitary quality of the water (Cabelli, 1978). For ground water, enteric viruses are of particular concern because they are thought to be responsible for many infectious gastrointestinal illnesses related to the consumption of ground water (Borchardt and others, 2004). The U.S. Centers for Disease Control reported that during 2001–02, 23 of 25 non-Legionella outbreaks (92 percent) of drinking-water illnesses were linked to ground-water sources, and 5 of the 25 outbreaks (20 percent) were attributed to viruses (norovirus) (Blackburn and others, 2004). Bacteria, partly because of different survival rates, might be inappropriate for indicating the presence of viruses in water (Metcalf, 1978; Palmateer and others, 1991; Havelaar and others, 1993). Therefore, some researchers have suggested using the coliphage viruses (referred to as coliphage), particularly the male-specific coliphage (Gerba, 1987; Havelaar and others, 1993), and other bacteriophages as indicators of the potential presence of enteric viruses (DeBartolomeis and Cabelli, 1991; Havelaar and others, 1993; Sobsey and others, 1995).
The specific interest in the male-specific coliphage is their resemblance to human enteric viruses in morphology, genetic material, and similar resistance to environmental stresses and disinfection processes (Bitton, 1980; Sobsey and others, 1995). Coliphage infect cells of E. coli by attaching directly to the outer cell membrane (somatic coliphages) or to the F pilus of E. coli cells that contain the F plasmid (male-specific or F-specific coliphages) (International Association on Water Pollution Research and Control Study Group, 1991). Some organisms, such as the coliphages, appear to be specific indicators for sewage because they are consistently isolated in large, but variable numbers from sewage, and infrequently isolated or isolated only in low numbers from the feces of healthy individuals (Cabelli, 1978; Gerba, 1987; International Association on Water Pollution Research and Control Study Group, 1991). Studies conducted to show relations between the presence of enteric viruses with the presence of coliphage or the fecal-indicator bacteria in ground water supplies have reported mixed results (Francy and others, 2004). Some studies have shown a poor relation between the occurrence of enteric viruses and coliphage or with the fecal-indicator bacteria (Borchardt and others, 2003; Borchardt and others, 2004; Francy and others, 2004), and others have shown significant positive correlation between infectious total virus and the coliphage or total coliforms, E. coli, or enterococcus (Francy and others, 2004). Thus, the detection of coliphages in the NAWQA well samples does not indicate that pathogenic viruses also will be detected in the water; rather, the coliphage detection is more an indication of the potential for the transport of other viruses into the subsurface.
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