Scientific Investigations Report 2006–5290

Scientific Investigations Report 2006–5290

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Microbiological data were collected from 1,205 wells in 22 study units of the National Water-Quality Assessment (NAWQA) program during 1993 to 2004, a period spanning the first decade (Cycle I) and the early part of the second decade (Cycle II) of the program. The samples of untreated ground water were analyzed primarily for concentrations of total-coliform bacteria, fecal-coliform bacteria, and Escherichia coli (E. coli), and for the presence of coliphage viruses. Water temperature, pH, specific conductance, and dissolved-oxygen concentrations were measured at the time of sampling for microorganisms, and water samples were collected for analysis of a broad suite of chemical constituents, including nutrients.

The wells sampled for this analysis were part of NAWQA’s specific study networks: those for the major-aquifer studies (MAS) and land-use (LUS) studies, and the wells sampled for the source-water-quality assessments (SWQA) of ground water used for public supplies. The networks were situated within 16 principal aquifers.

For the purpose of this analysis, wells were assigned to one of seven types of principal-aquifer lithology (consolidated carbonate rocks; crystalline rocks; sandstone-shale rocks; semiconsolidated sand; unconsolidated sands, gravels, or alluvium; and volcanic rocks) on the basis of well completion information. Wells were also assigned to one of six classes of water use.

Nearly 30 percent of the 1,174 wells used in the study tested positive for coliform bacteria. With at least one well in each study unit or principal aquifer testing positive, fecal-indicator bacteria were geographically widespread. Detection frequencies for bacteria in wells within individual study units ranged from 1 percent of sampled wells in the Upper Snake to 70 percent of wells in the Lower Susquehanna. Concentrations of total coliforms ranged from less than (<)1 to 1,600 colony-forming units per 100 milliliters (CFU/100mL) and concentrations of E. coli ranged from <1 to 1,200 CFU/100mL. Median concentrations for both total coliforms and E. coli, however, were at the reporting limit of <1 CFU/100mL. Sixty‑seven percent of total-coliform and 85 percent of E. coli concentrations were <100 CFU/100mL, indicating that overall, concentrations tend to be low.

In 11 of the 22 study units, samples were collected from 423 wells to test for the presence of coliphage viruses. Coliphage were present in samples from 4 of the 11 study units—the Central Columbia Plateau-Yakima , Georgia-Florida, San Joaquin, and Trinity, representing the Columbia Plateau, Floridan, Central Valley, and Coastal Lowlands aquifers, respectively.

Wells used for domestic supply made up the largest class of water use, with total-coliform concentrations for 405 wells and E. coli concentrations for 397 wells, followed by public-supply wells and unused wells with 227 and 37 concentrations of total-coliform bacteria, respectively. Total coliforms were detected in untreated water from 33 percent of domestic wells and 16 percent of public-supply wells; E. coli were detected in 8 and 3 percent of domestic and public-supply wells, respectively. Although median concentrations were <1 CFU/100mL for all classes of water use as defined in this report, the overall distribution of total–coliform concentrations was significantly higher in domestic wells than in public-supply wells. Coliphage were present in less than 4 percent of domestic and public wells used for drinking-water supply.

More than 50 percent of the wells sampled in the Valley and Ridge, the Floridan, and the Piedmont and Blue Ridge principal aquifers tested positive for coliform bacteria. Detection rates for the Floridan and the Piedmont and Blue Ridge aquifers were significantly higher than those for the Glacial Deposits, Stream and River Valley, Columbia Plateau, Basin and Range, High Plains, Southeastern Coastal Plain, and Coastal Lowlands aquifers. In 9 of the 16 aquifers, more than 75 percent of the concentrations of total coliform and E. coli in samples were <1 CFU/100mL. The maximum concentrations of total coliform bacteria and of E. coli detected in all samples analyzed were from a domestic well completed in dolomite of the Valley and Ridge principal aquifer. The highest overall concentrations of total coliform bacteria were detected in this aquifer with a median of 2 CFU/100mL. High concentrations of coliform bacteria (greater than [>] 300 CFU/100mL) also were detected in samples from the Mississippian-Pennsylvanian aquifer and the Ordovician aquifer in the lower Tennessee region.

Generally, coliform bacteria were detected more frequently and in higher concentrations in wells completed in sandstone or shale, and in sedimentary, carbonate, and crystalline rocks than for wells in unconsolidated materials, in semiconsolidated sand, or in volcanic rocks. More than 50 percent of sampled wells completed in carbonate rocks (limestone, dolomite) or in crystalline rocks (schist, granite) tested positive for coliform bacteria. The Floridan, Piedmont and Blue Ridge, Ordovician, and Valley and Ridge aquifers, all of which had high detection rates or concentrations of coliform bacteria, are composed of these fractured and porous rocks. The lowest rates of detections (less than 5 percent) were for wells in the Basin and Range and Snake River aquifers. Materials in these aquifers are primarily unconsolidated sand, gravel, and clay, or basalt with interbeds of sand, gravel, or clay.

The depths of public-supply wells (median of 427 feet below land surface) and of the wells in the Basin and Range aquifer (median depth of 400 feet) might explain in part, the relatively low detection frequencies of the coliform bacteria observed in these samples. A thick unsaturated zone increases the potential for natural attenuation of microorganisms, preventing the bacteria being transported into the ground water. Fifty-percent of wells in principal aquifers with median depths of sampled wells ranging from 100 to 200 feet below land surface tested positive for total coliform bacteria, whereas only 9 percent of wells in principal aquifers with median depths of sampled wells greater than 200 feet tested positive. However, there were no strong correlations between well depth and coliform bacteria detections or concentrations that would predict the presence of coliform bacteria for any given well.

The data collected for the NAWQA program and the summary of the microbial quality of ground water in this report provide a current description of the occurrence and relatively widespread distribution of coliform bacteria in the Nation’s ground-water resources. The results presented here will serve as a starting point for continued assessments of the microbial quality of water in the principal aquifers.

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