Scientific Investigations Report 2006–5290
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5290
Three objectives governed this analysis of the microbiological quality of the Nation’s ground water. First, to describe the spatial distribution and occurrence of selected fecal-indicator microorganisms in ground water on the basis of data collected for the NAWQA program from 1993 to early 2004. In addition, to determine how well the 19 principal aquifers that are the focus of the NAWQA Cycle II program are represented by the data and determine whether gaps are in the data on a regional basis. Second, to describe how fecal-indicator bacteria and coliphage are distributed within the aquifers by evaluating how microbial concentrations or detection frequencies relate to well depths, to types of aquifer materials, or to selected water-quality constituents or properties. Third, to identify what types and at what frequencies the fecal-indicator bacteria and coliphage are detected in water of the principal aquifers or in water used for drinking-water supplies.
The set of microbiological data collected for the NAWQA program provided a relatively large number of observations with which to describe, in a general way, the geographic distribution and occurrence of fecal-indicator bacteria and coliphage in ground-water resources across the Nation. The data showed that wells testing positive for coliform bacteria exist within each study unit in which microbiological data were collected, with the highest frequencies of detections in wells in the northeastern and southeastern regions of the country and the fewest in the Idaho Upper Snake River and Nevada Basin and Range study areas. However, at the time of this analysis, fecal-indicator bacteria data had been collected in only 22 of the 50 NAWQA study units, and these data represented 12 of the 19 principal aquifers targeted by NAWQA in Cycle II. Additionally, data on the presence or absence of the coliphage viruses were collected in 11 of the 22 study units. Gaps in the data, therefore, included more than 20 study units and 7 principal aquifers for which fecal-indicator bacteria data were unavailable and more than 30 study units for which coliphage data were unavailable for this analysis.
The Glacial Deposits, the Valley and Ridge, and the Piedmont and Blue Ridge aquifers had the greatest representation in the data, with a total of more than 420 analytical results for the fecal-indicator bacteria; no data, however, were available for the New England, Edwards-Trinity, California Coastal Basins, Biscayne, and Cambrian-Ordovician aquifers, or the Surficial aquifer systems of the Southeast. Aquifers that might be geographically underrepresented by this data set include the High Plains, except for the northernmost part; that part of the Basin and Range aquifer in Arizona and southern California; and the Denver Basin. Also geographically underrepresented are the Midwest regions and the greater Mississippi River Basin.
Another type of geographic underrepresentation would apply to the Glacial Deposits, North Atlantic Coastal Plain, Piedmont and Blue Ridge, and Coastal Lowlands principal aquifers. Although the data set contains relatively large numbers of results for these aquifers, the sampled wells are clustered in small, isolated areas and leave extensive areas of the aquifer without representation. The clustering is likely due to geographic boundaries of study units and to the types of well networks established for the NAWQA program, which dictated certain locations with suitable wells for meeting network study objectives.
Eleven of the 42 study units scheduled for inclusion in Cycle II are represented in this analysis. The advantages of microbiological data collected as part of Cycle II is a data set consisting of common analytical methods, target organisms, and quality assurance-quality control procedures. Forthcoming Cycle II data will help to fill some of the spatial gaps and greatly enhance the information on the occurrence of the coliphage viruses, for which testing only recently became part of regularly scheduled analyses for the well networks. These additional data should prove valuable for a more comprehensive understanding of the quality of source waters for drinking-water supplies than described in this report.
The data showed general tendencies for detection frequencies and concentrations of total coliform bacteria to be higher (1) in principal aquifers with median well depths less than 200 feet than in principal aquifers with median well depths greater than 200 feet; (2) in domestic wells than in public-supply wells; and (3) in wells completed in fractured rocks or porous carbonate rocks than in sand, gravel, semiconsolidated sand, and volcanic rocks. Evaluations of the additional water-quality data (analyses for major chemical constituents) also showed tendencies for higher rates of total coliform detections with increasing concentrations of nitrite-plus-nitrate nitrogen and dissolved oxygen. With no strong correlations, however, the presence or absence of coliform bacteria cannot be predicted on the basis of well depth, lithology, or water chemistry, a situation that has been historically documented (Pipes, 1978, p. 106) and shown more recently in NAWQA studies by Bickford and others (1996) and Francy and others (2000). The data collected for the NAWQA program during 1993-2004 also showed that coliform bacteria tend to occur relatively frequently in ground water, with nearly 30 percent of the wells testing positive for these bacteria. Even though some of the members of the total coliform group originate from nonfecal sources, they can still function as conservative indicators of the potential for other microorganisms to enter the ground water. Conversely, E. coli and coliphage viruses indicate contamination from fecal and sewage sources, which could be a more serious situation with respect to potential presence of pathogenic microorganisms and human health. Fortunately, E. coli and coliphage were detected in ground water at much lower frequencies (11 percent and 3 percent, respectively) than the total-coliform bacteria.
This analysis provided insight into the quality of the ground water used for drinking-water supplies by revealing the large numbers of domestic wells affected by the presence of coliform bacteria. Total-coliform bacteria were detected in samples from 33 percent of 405 domestic wells and 16 percent of 227 public supply wells. No USEPA criteria or standards govern the acceptability of private-well water, and no routine monitoring is required for domestic supplies as for public supplies (U.S. Environmental Protection Agency, 2005). A study of ground water in California determined that about one-fourth of the privately owned wells used for domestic supply in a two-county study area had some level of contamination by fecal-indicator bacteria (Bowman, 2005). In response to that determination, the California State Water Resources Control Board suggested that the detections of bacteria and other contaminants might warrant a program of testing by homeowners at a proposed frequency of every 5 years (Bowman, 2005). The USEPA, recognizing that about 15 percent of the population relies on private drinking-water supplies, provides Internet-based educational materials and also recommends that these households take precautions to protect and maintain their systems in addition to obtaining periodic testing by a certified laboratory (U.S. Environmental Protection Agency, 2005).
The data collected for the NAWQA program and the summary of the microbial quality of ground water in this report provide a basic understanding and current description of occurrence and relatively widespread distribution of coliform bacteria. The results presented here can serve as a starting point for continued assessments of the microbial quality of water in the principal aquifers of the United States as the NAWQA data set expands and perhaps also serve as a basis for water and public-health managers concerned with ground-water resources at the local level, particularly those used for domestic purposes.
U.S.
Department of the Interior | U.S. Geological
Survey
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