Scientific Investigations Report 2006–5290
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5290
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Testing was done for fecal-indicator bacteria and coliphages on samples from at least 15 wells in the 22 study units during Cycle I or Cycle II of the NAWQA program. Concentrations and detection frequencies of fecal-indicator bacteria and coliphage are summarized by study networks and by principal aquifers and their lithology. The occurrence of these microorganisms in water supplies and relations to selected well and water-quality characteristics are also discussed.
The 22 study units collected samples from 1,205 wells as part of the MAS, LUS, and SWQA sampling networks. Concentrations of fecal-indicator bacteria in 1,174 of these wells were used in the analyses described in this report. Samples collected from 450 wells in 11 of the 22 study units were analyzed for the presence of coliphage. Coliphage data from 423 of those wells were available for this analysis.
The study wells are completed in 16 principal aquifers and 7 types of lithology. Water-use types for these wells were originally coded by study-unit staff within 1 of 15 categories, including drinking-water supply (domestic or public), water used for power or aquaculture, and water use designated as “other” or ”unused.” For this report, several of the minor types of water use were combined—commercial, industrial, institutional, and recreational into one class; irrigation and stock watering into a second class; and a third class referred to simply as “other.” Water-use types described in this report include these three combined classes and the classes termed domestic, public, and unused.
Detection frequencies and concentrations of total-coliform bacteria were significantly different (p-value <0.05) among the study networks, principal aquifers, types of lithology, and water-use classes. These differences are further examined in the discussions of the microbial quality of ground water sampled for the NAWQA program.
Fecal-indicator bacteria were geographically widespread, with at least one well testing positive among all the wells sampled within each study unit and principal aquifer (fig. 3A and 3B). Coliphage, which occur less frequently than the bacteria, were present in well samples from 4 of the 11 study units that sampled and tested for coliphage—the Central Columbia Plateau–Yakima (CCYK), Georgia-Florida (GAFL), San Joaquin (SANJ), and Trinity (TRIN) study units, representing the Columbia Plateau, Floridan, Central Valley, and Coastal Lowlands aquifers, respectively.
Samples from 347 wells (29.6 percent of the 1,174 wells for which data were used in this analysis) tested positive for fecal-indicator bacteria (either total coliform, E. coli, or fecal coliform). Individual study-unit detection frequencies ranged from 1 out of 82 well samples (1 percent) in the USNK testing positive for fecal coliforms to 108 out of 155 samples (70 percent) in the LSUS testing positive for total coliforms (fig. 4). The greatest number of wells that tested positive for E. coli was in the TENN study unit; 32 out of 124 wells (26 percent) tested positive.
Concentrations of total coliforms in samples from the 1,174 wells ranged from <1 to 1,600 CFU/100mL and concentrations of E. coli ranged from <1 to 1,200 CFU/100mL. Median concentrations for both total coliforms and E. coli were at the reporting limit of <1 CFU/100mL. In addition, 67 percent of total coliform and 85 percent of E. coli concentrations were at the minimum reporting limit, indicating that most concentrations are low.
With 61 percent of all the wells sampled, the MAS is the largest of the three major study networks, followed by the LUS network and the SWQA network (table 4). For the MAS, total-coliform concentrations for 566 wells; E. coli concentrations for 592 wells; and coliphage results for 280 wells were used in this analysis (table 4). For the LUS network, concentrations of total coliforms and E. coli were available for 251 wells and 237 wells, respectively. For all three networks, the highest percentage of total coliform detections (50.2 percent) occurred in wells in the LUS network, and in the agricultural LUS network, total coliforms were detected in 67.2 percent of the wells (fig. 5A). In contrast with the LUS network, the lowest percentage of total-coliform detections (19.5 percent) occurred in wells in the SWQA network. Median concentrations of coliform bacteria were <1 CFU/100mL for the MAS and SWQA networks, 1 CFU/100mL for the LUS network, and 4 CFU/100mL for the agricultural LUS network. Excluding the high concentration of coliform bacteria in a sample from one well, total coliform concentrations were equal to or less than 420 CFU/100mL and E. coli concentrations were equal to or less than 440 CFU/100mL (fig. 5B).
Of the 16 principal aquifers studied by the NAWQA program during 1993–2004, the number of wells testing positive for coliform bacteria exceeded 50 percent in three aquifers—the Valley and Ridge, Floridan, and the Piedmont and Blue Ridge (fig. 6A and table 5). The lowest detection frequencies of coliform bacteria (less than 5 percent) among the 16 principal aquifers were in samples from wells in the Basin and Range and Snake River aquifers (fig. 6A, table 5). Detection frequencies of total coliform bacteria for the Valley and Ridge, Floridan, and Piedmont and Blue Ridge aquifers were significantly higher (p-value <0.0006) than detection frequencies for the Glacial Deposits, Columbia Plateau, High Plains, Stream and River Valley, Coastal Lowlands, Southeastern Coastal Plain, and Basin and Range aquifers (fig. 6B).
For the large MAS network, the frequency of wells testing positive for total coliform was 82 percent for the Central Valley aquifer (fig. 7A); however, this high frequency of detection might be a function of the low number of available samples. Detection frequencies of E. coli were highest for MAS wells in the Ordovician aquifer (30 percent), followed by detections in the Central Valley (25 percent) and the Mississippian-Pennsylvanian (19 percent) aquifers (fig. 7A).
The Piedmont and Blue Ridge, Floridan, Coastal Lowlands, Columbia Plateau, Glacial Deposits, Basin and Range, and Central Valley aquifers, or just less than one-half the 16 aquifers studied since 1993, were the first principal aquifers to be sampled as part of the new SWQA network of NAWQA Cycle II. Samples with the highest detection frequencies of total coliforms were collected from Piedmont and Blue Ridge wells (greater than 50 percent) followed by detections in samples from wells completed in the Floridan aquifer (30 percent). Detection frequencies of E. coli were low, however, with nondetections reported for all wells in 4 of the 7 aquifers and only one detection in each of the others (fig. 7B).
Distribution of coliform bacteria concentrations was highly influenced by many samples that tested negative; and, of the samples that tested positive, many concentrations were near the minimum reporting limit (fig. 8). More than 75 percent of total coliform and E. coli concentrations in samples from 9 of the 16 aquifers were <1 CFU/100mL. Concentrations of total coliforms tended to be distributed somewhat higher in samples from four aquifers: 25 percent of the concentrations ranged from 1 to 7 CFU/100 mL in samples from the Floridan aquifer; from 1 to 4 CFU/100 mL in samples from the Piedmont and Blue Ridge aquifer; from <1 to 4 CFU/100 mL in samples from the North Atlantic Coastal Plain aquifer; and from <1 to 2 CFU/100 mL in samples from the Central Valley aquifer.
Maximum concentrations of total coliforms of 1,600 CFU/100mL and of E. coli of 1,200 CFU/100mL were detected in a sample from a well completed in carbonate rocks (dolomite) of the Valley and Ridge aquifer. This aquifer also had the highest overall concentrations of total coliform bacteria, with a median of 2 CFU/100mL (table 5) and 25 percent of the concentrations ranging from 2 to 34 CFU/100mL (fig. 8). High concentrations of coliform bacteria (greater than 300 CFU/100mL) also were detected in samples from the Mississippian-Pennsylvanian and Ordovician aquifers. For wells in the Floridan, Piedmont and Blue Ridge, and North Atlantic Coastal Plain aquifers, high concentrations were reported only to the upper limit of the analytical method (80 CFU/100mL) for an undiluted sample, so the maximum concentrations are unknown. For the MAS, however, the aquifers with the highest overall concentrations of total coliform bacteria were the Floridan, with a median of 4 CFU/100mL, and the Central Valley, with a median of 3 CFU/100mL.
Variations in the detection frequencies of coliform bacteria and in the distribution of their concentrations among principal aquifers are explained in part by types of lithology. Detection frequencies were greater for wells in carbonate rocks (primarily limestone and dolomite), crystalline rocks (primarily undifferentiated metamorphic rocks and schist) or sandstone, shale, undifferentiated sedimentary rocks, than for wells in unconsolidated materials (different mixtures of sand, gravel, and clay, or alluvium), semiconsolidated sand, or volcanic rocks (basalt, with sand and gravel interbeds) (fig. 9A). More than 50 percent of all sampled wells completed in carbonate rocks, such as those that make up part of the of the Valley and Ridge, the Floridan, and the Piedmont and Blue Ridge aquifers (table 2), or in crystalline rocks of the Piedmont and Blue Ridge aquifers tested positive for coliform bacteria (fig. 9A, table 6). The frequency of detection of coliform bacteria in wells completed in unconsolidated materials tends to be relatively low (less than about 20 percent), and is significantly lower (p-value <0.002) than detections for wells completed in consolidated rocks (fig. 9B). Principal aquifers composed primarily of these unconsolidated materials and with low detection frequencies include the Basin and Range, Coastal Lowlands, Southeastern Coastal Plain, Stream and River Valley, and High Plains.
Relatively high concentrations of fecal-indicator bacteria were detected in wells completed in carbonate rocks, with 25 percent of the total-coliform values ranging from 1 to 24 CFU/100mL (fig. 10). Carbonate-rock aquifers with high concentrations of total coliforms or E. coli include the Floridan, the Ordovician in the lower Tennessee region, the Valley and Ridge, the Piedmont and Blue Ridge, and the Mississippian-Pennsylvanian. Concentrations of total coliforms in unconsolidated materials were generally low, with medians of <1 CFU/100mL.
The MAS and SWQA study networks focused on the quality of ground water used for drinking-water supplies through the sampling of about 630 wells used for domestic and public supplies. Total coliforms were detected in 27 percent and E. coli in 6 percent of these wells. Most (71 percent) of the MAS wells were domestic wells; however, an additional 15 percent of MAS wells were used for public supplies (fig. 11). Conversely, and by study design, nearly all of the SWQA wells were used for public supplies. A few SWQA wells, however, were described as unused at the time of sampling and one was described as a domestic well (fig. 11).
Of the 6 classes of water use considered in this analysis, wells used for domestic purposes made up the largest class, with data on total-coliform concentrations available for 405 wells and on E. coli concentrations for 397 wells. The domestic-use class was followed by public supply and unused classes with total-coliform concentrations for 227 and 37 wells, respectively (table 7). Total coliforms were detected in 33 percent of the samples from domestic wells and 16 percent of samples from public supply wells, and E. coli were detected in 8 and 3 percent of samples from domestic and public supply wells, respectively (fig. 12A).
Median concentrations of total coliforms and E. coli were at the detection limit of <1 CFU/100mL for all 6 classes of water use (fig. 12B, table 7); however, the concentrations in domestic wells were significantly higher (p-value <0.05) than concentrations in public-supply wells. In samples from domestic wells, the maximum concentrations of total coliforms and E. coli were 1,600 and 1,200 CFU/100mL, respectively. Maximum concentrations of total coliforms detected in samples from public-supply wells were >80 CFU/100mL for a well completed in the Floridan aquifer of the GAFL study unit, and 61 CFU/100mL for a well completed in the Glacial Deposits aquifer of the HPGW study unit. More than 75 percent of samples from domestic wells had concentrations of total coliforms of 2 CFU/100mL or less. In samples from public-supply wells, however, more than 75 percent of concentrations of total coliforms were less than the minimum report level of <1 CFU/100mL (fig. 12B).
Samples for wells completed in 10 principal aquifers were analyzed for the presence of coliphage (table 8). Wells in 9 of these 10 aquifers were sampled as part of the MAS networks and wells in 7 of the aquifers were sampled as part of the SWQA networks. In 4 aquifers (Floridan, Central Valley, Coastal Lowlands, and Columbia Plateau), from 2 to 6 wells tested positive for coliphage (fig. 7A and B). Although no coliphages were detected in public-supply wells sampled in the Coastal Lowlands aquifer, coliphages were detected in 3 of 24 public-supply wells in the Floridan aquifer, in 1 of 15 public-supply wells in the Central Valley aquifer, and in 1 of 34 public-supply wells in the Columbia Plateau aquifer. A few domestic wells also tested positive for coliphage, but the frequency of detection was less than 4 percent. Only 7 of all 200 domestic wells and 5 of all 183 (less than 3 percent) public-supply wells sampled tested positive for coliphage (table 7).
The presence or absence of coliphage viruses in samples did not show any relation to concentrations of total coliforms or E. coli. Specifically, coliphage could be present with total coliforms at low concentrations of 1 CFU/100mL or less (undetected), or they could be absent from samples when total-coliform concentrations were as high as 100 CFU/100mL (fig. 13). Only one of the coliphage-positive samples also tested positive for E. coli. This lack of co-occurrence of bacteria and coliphage is consistent with the understanding that no single organism can fully function as an indicator for the presence of different types of microorganisms (Cabelli, 1978; Havelaar and others, 1993). In a study of small public-supply systems, Francy and others (2004) also determined a lack of co-occurrence between concentrations or detections of coliform bacteria and coliphage.
The Kruskal-Wallis test indicated that one or more groups of well depths within study networks, uses of water, principal aquifers, and lithology differed (p-values <0.05) in their medians or distribution of values. Wells in the SWQA and MAS networks with median depths of 400 feet and 150 feet below land surface, respectively, were significantly deeper than wells of the LUS networks (median depth of about 50 feet below land surface), and public-supply wells (median depth of 427 feet) were deeper than wells used for domestic supply (median depth of 161 feet) or other purposes (fig. 14). For all 16 principal aquifers, the deepest wells sampled, with a median depth of 400 feet below land surface, were those in the Basin and Range aquifer (fig. 15). The depths of the public-supply wells in the SWQA network and the wells in the Basin and Range aquifer might explain, in part, the relatively low detection frequencies of the coliform bacteria in these wells. A thick unsaturated zone increases the potential for natural attenuation of microorganisms, thereby reducing the potential for or preventing the bacteria being transported into the ground water.
The principal aquifers were combined on the basis of median depths of sampled wells into three groups—shallow (median depths <100 feet), medium (median depths from 100 to 200 feet), and deep (median depths greater than 200 feet) to identify any relations between well depth and detection frequencies or concentrations of total coliform bacteria (fig. 16). Of a total of 317 detections of total coliform bacteria, 91 percent of the detections occurred in principal aquifers with median depths of sampled wells 200 feet deep or less, whereas only 9 percent of the detections occurred in the group of principal aquifers with median depths of sampled wells greater than 200 feet. The highest frequency of detection (50 percent) was in samples from wells in the medium group of aquifers with median well depths ranging from 100 to 200 feet (fig. 16).
For wells in which total coliform bacteria were detected, concentrations in wells in the deep group of principal aquifers were lower (p-value <0.05) than concentrations in wells in the shallow and medium groups of aquifers. The graphs of the data (fig. 16) also illustrate the poor relation between total coliform bacteria concentration and well depth, which indicates that other factors are likely controlling the occurrence and transport of coliform bacteria in the aquifers. Some of these other factors might be the geohydrologic characteristics of the aquifers (fractured and porous rock types), proximity of contaminating sources such as septic tanks (Francy and others, 2000), or age of the well (Bickford and others, 1996), along with other well-construction features.
The concentrations of selected chemical constituents (nutrients, major ions, dissolved oxygen, iron, manganese) and the measures of physical properties (specific conductance, water temperature) for samples collected from wells in the three major study networks also were evaluated for any relations to detections of total-coliform bacteria. There were significant differences (at the 95-percent confidence level) between median numbers of detections and nondetections of total coliforms with concentrations of dissolved oxygen, nitrite-plus-nitrate nitrogen, and orthophosphate (fig. 17). However, as with well depths, no strong correlation could be identified between the detection or nondetection of total‑coliform bacteria in a water sample and the values of water-quality constituents.
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