BACTERIOLOGICAL QUALITY OF GROUND WATER USED FOR HOUSEHOLD SUPPLY
Relations between Bacterial Concentrations and Land
use, Physiography and Lithology
Relations between Bacterial Concentrations and Selected Well and Water-Quality
Characteristics
Other Possible Factors Related to Bacteria in Ground Water
Public health standards that define the potability of water have been set by the USEPA for the microbiological quality of drinking water used for public water supply. These standards use total coliform as the indicator bacteria for public drinking water supplies (U.S. Environmental Protection Agency, 1994). No national standards have been set for private, individual water supplies, therefore, USEPA's public supply standards are used in this report for comparison purposes only. The health effects of the presence of even one fecal coliform bacterium in a 100-mL sample of drinking water is viewed with concern by public health authorities (Francis and others, 1984).
The data for bacteriological testing in the Lower Susquehanna River Basin study unit are listed in the U.S. Geological Survey annual water data reports for the 1993-94 water years (Durlin and Schaffstall, 1994; 1996). The 1995 data used for analysis can be obtained from the USGS office in Lemoyne, Pa.
Total coliform, fecal coliform, and fecal streptococcus bacteria were not found in water samples from 31 of the 146 wells tested. Nearly 70 percent of the 146 water samples were positive for total coliform bacteria (fig. 2) and nearly 25 percent of the samples that were positive were too numerous to count (fig. 3). Approximately 25 percent of the samples were positive for fecal coliform bacteria (fig. 2) and nearly 15 percent of the samples that were positive were too numerous to count (fig. 3). These percentages are higher than those found in previous studies. Approximately 65 percent of the water samples were positive for fecal streptococcus bacteria (fig. 2) and almost 35 percent of the samples that were positive were too numerous to count (fig. 3).
E. coli testing was not conducted during 1993, the first year of the Lower Susquehanna River Basin study. Therefore, only 88 of the 146 water samples were tested for E. coli. About 30 percent of the samples from those 88 wells were positive for E. coli bacteria (fig. 2) and more than 10 percent of the samples that were positive were too numerous to count (fig. 3). The percentage of E. coli detections is higher than the percentage of fecal coliform detections (fig. 2). This is due to the fact that E. coli testing was only conducted on 88 of the 146 water samples.
Approximately one third of the samples that contained total coliform bacteria also contained fecal coliform bacteria, a percentage that agrees with the data of Breen and Dumouchelle (1991). Fecal streptococcus bacteria were present in more than three quarters of the samples that tested positive for total coliform bacteria and in all of the samples in which fecal coliform bacteria was present.
FIGURE
2. Bacteria presence in ground water from wells sampled as part of the
National Water-Quality Assessment in the Lower Susquehanna River Basin
study unit, Pennsylvania and Maryland.
FIGURE
3. Distribution of bacteria among count ranges in ground water from wells
sampled as part of the National Water-Quality Assessment in the Lower Susquehanna
River Basin study unit, Pennsylvania and Maryland.
FIGURE
4. Sampling locations and detections for total coliform in ground water
from wells sampled as part of the National Water-Quality Assessment in
the Lower Susquehanna River Basin study unit, Pennsylvania and Maryland.
FIGURE
5. Sampling locations and detections for fecal coliform in ground water
from wells sampled as part of the National Water-Quality Assessment in
the Lower Susquehanna River Basin study unit, Pennsylvania and Maryland.
The distribution of wells sampled in the Lower Susquehanna River Basin and the presence of total coliform and fecal coliform are shown in figures 4 and 5, respectively. Total coliform detections are distributed relatively evenly over all of the areas studied. The distribution of detections of fecal coliform, however, shows a more clustered pattern with almost one-half of the detections of fecal coliform being in the Great Valley carbonate subunit.
The Great Valley carbonate and Appalachian Mountain carbonate subunits had the highest percentages of the presence of bacteria (fig. 6). Fecal coliform and E. coli distributions shown in figure 6 are similar, as would be expected since E. coli is a subset of fecal coliform. The similarity between the distributions of total coliform and fecal streptococcus shown in figure 6 suggest that both of these bacteria types may originate from the same sources. The data were analyzed to determine variations in the concentrations of bacteria among environmental subunits, to determine relations between bacterial concentrations and well characteristics, and to determine relations between concentrations of bacteria and other water-quality constituents.
FIGURE
6. Presence of total coliform, fecal coliform, E. coli, and fecal streptococcus
as a percentage of the number of wells sampled in each environmental subunit
in the Lower Susquehanna River Basin study unit, Pennsylvania and Maryland.
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Statistical tests were conducted to determine if relations existed between bacterial concentrations and land use, physiography, and bedrock lithology comprising aquifer. Testing the variation in bacterial concentrations among land-use categories was conducted because manure is applied to cropland and pastures in agricultural areas providing an additional source of bacteria that is not present in nonagricultural areas. Comparisons of bacterial concentrations to bedrock lithology help determine if certain types of aquifers are more susceptible to bacteria contamination than others. Statistical tests also were conducted to determine if the combination of land use, lithology and physiography was a significant factor affecting bacterial concentrations.
FIGURE
7. Distribution of concentrations of total coliform and fecal streptococcus
among land-use types, Lower Susquehanna River Basin study unit, Pennsylvania
and Maryland.
Table 4. Results of statistical analysis for relations between land use and
bacterial concentrations, Lower Susquehanna River Basin study unit,
Pennsylvania and Maryland.[Values followed by the same letter are
not significantly different as determined by Kruskal-Wallis Tests.]
--------------------------------------------------
Mean Rank
-----------------------------
Probability Agricultural Nonagricultural
--------------------------------------------------
Total Coliform
0.002 77-A 41-B
Fecal Coliform
.026 75-A 57-B
E. Coli
.059 45-A 31-A
Fecal Streptococcus
.028 75-A 51-B
--------------------------------------------------
For land-use relations, bacterial concentrations in agricultural and nonagricultural areas were compared. Boxplots show that total coliform and fecal streptococcus have a broader range of concentrations and higher median values in agricultural areas than in nonagricultural areas (fig. 7). Statistical tests showed significant differences among land-use categories for total coliform, fecal coliform, and fecal streptococcus (table 4). Although the differences between land-use categories are not statistically significant for E. coli at the selected 95 percent confidence level, the probability is very close to the level that would indicate significant differences existed between land-use categories. The data and statistics shown in figure 7 and table 4 indicate that higher bacterial concentrations are related to agricultural land use. Boxplots for fecal coliform and E. coli are not presented because of the small percentage of sites where those bacteria types were detected.
FIGURE
8. Distribution of concentrations of total coliform and fecal streptococcus
among physiographic provinces, Lower Susquehanna River Basin study unit,
Pennsylvania and Maryland.
Table 5. Results of statistical analysis for relations between physiography
and bacterial concentrations, Lower Susquehanna River Basin study
unit, Pennsylvania and Maryland. [Values followed by the same
letter are not significantly different as determined by
Kruskal-Wallis Tests.]
-----------------------------------------------
Mean Rank
--------------------------
Probability Ridge and Valley Piedmont
-----------------------------------------------
Total Coliform
0.008 81-A 62-B
Fecal Coliform
.022 78-A 66-B
E.coli
.004 48-A 35-B
Fecal Streptococcus
.001 83-A 59-B
-----------------------------------------------
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Physiographic province also is related to bacterial concentrations. Boxplots show that all four bacteria types have higher median concentrations and broader ranges of concentrations in the Ridge and Valley Physiographic Province than in the Piedmont Physiographic Province (fig. 8). The statistical test results also indicate significant differences between the physiographic provinces for all of the bacteria types (table 5). Equal numbers of agricultural and nonagricultural wells were sampled in both provinces. Moreover, a similar number of wells completed in carbonate and noncarbonate bedrock was sampled in both provinces. This variation in bacteriological quality of water by physiographic province could not be explained and could be related to regional variation in well characteristics, socioeconomic conditions, siting of septic systems, or agricultural practices (Francis and others, 1984).
FIGURE
9. Distribution of concentrations of total coliform and fecal streptococcus
among bedrock lithologies comprising aquifers, Lower Susquehanna River Basin
study unit, Pennsylvania and Maryland.
Table 6. Results of statistical analysis for relations between bedrock
lithology comprising aquifer and bacterial concentrations,
Lower Susquehanna River Basin study unit, Pennsylvania and
Maryland. [Values followed by the same letter are not
significantly different as determined by Kruskal-Wallis Tests.
A double dash (--) indicates that no E. coli samples were
collected in areas underlain by sandstone and shale.]
------------------------------------------------------------
Mean Rank
---------------------------------------
Probability Carbonate Sandstone and Crystalline
shale
------------------------------------------------------------
Total Coliform
0.070 80-A 64-A 63-A
Fecal Coliform
.065 78-A 68-A 65-A
E.coli
.004 49-A -- 35-B
Fecal Streptococcus
.170 78-A 66-A 63-A
------------------------------------------------------------
Bedrock lithology comprising the aquifer also was evaluated as a factor related to bacterial concentrations. Boxplots show that total coliform and fecal streptococcus have a broader range of concentrations and higher median values in areas underlain by carbonate bedrock than in areas underlain by other bedrock types (fig. 9). Previous studies have shown that ground water is more susceptible to contamination by nitrate and herbicides in areas underlain by carbonate bedrock than in areas underlain by noncarbonate bedrock (Fishel and Lietman, 1986). Water from carbonate bedrock has the highest mean rank of bacterial concentrations for all of the bacteria types; however, statistical tests show that the differences among the mean ranks for bedrock lithology categories are not statistically significant for total coliform, fecal coliform, or fecal streptococcus at the 95 percent confidence level (table 6). The probabilities for fecal coliform (0.065) and total coliform (0.070) are both close to the 0.05 level that would indicate significant differences among lithology categories for these bacteria types. Apparent differences shown for E. coli among categories could be because of the fact that E. coli samples were only analyzed in two of the lithology categories. The data and statistics shown in table 7 and figure 10 indicate that some differences exist among lithology categories, with higher concentrations of bacteria detected in areas underlain by carbonate bedrock. This relation is not statistically significant for all bacteria types.
FIGURE
10. Distribution of concentrations of 
total coliform
and 
fecal streptococcus
among land-use settings within environmental subunit, Lower Susquehanna
River Basin study unit, Pennsylvania and Maryland.
Table 7. Results of statistical analysis for relations between land-use
settings within environmental subunits and bacterial
concentrations, Lower Susquehanna River Basin study unit,
Pennsylvania and Maryland. [Values followed by the same
letter(s) are not significantly different as determined by
Kruskal-Wallis Tests. A doubledash (--) indicates that no
E. coli samples were collected in that setting.]
Setting abbreviations:
AMCA=App. Mtn. Carbonate, agricultural setting
GVCA=Great Valley Carbonate, agricultural setting
PDXA=Piedmont Crystalline, agricultural setting
AMSA=App. Mtn. Sandstone and shale, agricultural setting
PDCA=Piedmont Carbonate, agricultural setting
AMSF=App. Mtn. Sandstone and shale, nonagricultural setting
PDXF=Piedmont Crystalline, nonagricultural setting
------------------------------------------------------------------
Mean Rank
----------------------------------------------------
Probability AMCA GVCA PDXA AMSA PDCA AMSF PDXF
(a) (b)
------------------------------------------------------------------
Total
Coliform
0.002 93-A 85-A 74-AB 69-AB 61-B 51-B 34-B
Fecal
Coliform
.001 74-A 98-A 73-B 66-B 64-B 57-B 57-B
E. coli
.014 46-A 52-A 37-B -- -- -- 31-B
Fecal
Streptococcus
.001 90-A 91-A 70-AB 68-AB 54-B 61-AB 43-B
------------------------------------------------------------------
The land-use setting within the environmental subunit was evaluated to determine the combined effects of land use, physiography, and lithology on bacterial concentrations. Combining land use, physiography, and lithology specifically identifies areas where bacterial concentrations are highest. Boxplots show that the broadest range of concentrations and the highest medians for total coliform and fecal streptococcus are in the Appalachian Mountain and Great Valley carbonate agricultural settings (fig. 10). The Appalachian Mountain and the Great Valley carbonate agricultural settings are both in the Ridge and Valley Physiographic Province (table 1). Statistical analyses showed significant differences among settings for every bacteria type (table 7). The Appalachian Mountain carbonate and Great Valley carbonate agricultural settings consistently had the highest mean ranks. Nonagricultural settings in the Piedmont crystalline subunit consistently had the lowest mean ranks.
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Statistical tests were conducted to determine the relations between bacterial
concentrations and other continuous variables. The results of the tests
of selected well characteristics and selected water-quality constituents
are discussed.
In tests of bacterial concentrations and selected well characteristics, total coliform, fecal coliform, and fecal streptococcus were all statistically related to well age as indicated by probabilities less than 0.05 (table 8). However, these relations were weak. Total coliform, for example, is statistically related to well age as indicated by a probability of 0.001. The correlation coefficient, however, is small (0.298), which means that the relation is poor and probably insignificant in predicting the concentration of total coliform from the well age. There were no statistically significant relations between bacterial concentrations and the following: well depth, depth to bedrock, casing length, specific capacity, and depth to the first water bearing zone.
Statistically significant relations exist between bacterial concentrations and certain water-quality constituents as indicated by probabilities less than 0.05 (table 9). E. coli, for example, is associated with the concentration of total dissolved solids as indicated by a probability of 0.005. The correlation coefficient is small (0.298), and the relation is probably insignificant in predicting the concentration of E. coli from the concentration of total dissolved solids. Concentrations of dissolved organic carbon were related with concentrations of total coliform, fecal coliform, E. coli, and fecal streptococcus. Similarly, relations were identified between concentrations of total dissolved solids and concentrations of total coliform and fecal coliform; between concentrations of ammonia plus organic nitrogen and concentrations of fecal coliform; and between concentrations of chloride and concentrations of total coliform. Although these relations are statistically significant based on the probability, the corresponding correlation coefficients (Spearman's rho) are all quite small. The small coefficients mean that the correlations are probably insignificant in predicting the concentration of bacteria from the concentration of the other water-quality constituents.
Table 8. Summary of statistical correlations between bacterial concentrations
and selected well characteristics, Lower Susquehanna River Basin
study unit, Pennsylvania and Maryland. [SCAP, Specific Capacity;
WBZ, Water Bearing Zone.]
Constituent Total Coliform Fecal Coliform E.Coli Fecal Streptococcus
(Spearman's correlation coefficient/Probability value/Number of Samples)
--------------------------------------------------------------------------------
Well Depth -0.016/0.851/146 0.104/0.212/146 0.100/0.359/87 0.084/0.317/145
Depth to Bedrock -.117/ .253/97 .072/ .486/ 97 -.044/ .742/58 .063/ .540/96
Casing length -.041/ .622/145 .032/ .703/145 .109/ .315/87 -.030/ .719/144
SCAP .044/ .595/146 -.016/ .844/146 .015/ .892/87 .090/ .282/145
Well Age .273/ .001/144 .206/ .013/144 .038/ .72/85 .298/ .000/143
Depth to WBZ -.102/ .33/93 .083/ .427/93 .038/ .783/54 -.034/ .745/92
---------------------------------------------------------------------------------
Table 9. Summary of statistical correlations between bacterial concentrations
and selected water-quality constituents, Lower Susquehanna River
Basin study unit, Pennsylvania and Maryland. [MBAS, Methylene Blue
Active Substance; TDS, Total Dissolved Solids; DOC, Dissolved
Organic Carbon; DO, Dissolved Oxygen.]
Constituent Total Coliform Fecal Coliform E.Coli Fecal Streptococcus
(Spearman's correlation coefficient/Probability value/Number of Samples)
-----------------------------------------------------------------------------------
Nitrate 0.150/0.072/145 0.117/0.162/145 0.077/0.479/86 0.047/0.576/144
Chloride .176/ .034/145 .158/ .059/145 .170/ .117/86 .081/ .335/144
MBAS .047/ .575/145 .033/ .695/145 -.088/ .420/86 -.064/ .444/144
TDS .247/ .003/145 .192/ .021/ 145 .298/ .005/86 .124/ .138/144
DOC .251/ .002/145 .276/ .001/145 .256/ .017/86 .215/ .010/144
Ammonia + Organic .037/ .659/146 .183/ .027/146 .098/ .366/87 .017/ .836/145
Nitrogen
Nitrite -.044/ .595/146 -.054/ .518/146 .006/ .954/87 .020/ .810/145
DO -.026/ .753/146 .034/ .683/146 -.091/ .402/87 .073/ .383/145
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The statistical analysis of the data shows that land use, physiographic province, and probably bedrock type are factors influencing the concentration of bacteria in ground water. Other factors that were not quantified in this study also may affect bacterial concentrations in well water. High concentrations of bacteria in well water may result when there is a bacteria source and a pathway for the bacteria to enter the aquifer or the well. Larger numbers of bacteria in the environment increase the chance that some bacteria will enter a well. Application of sludge from sewage treatment plants is not common and sites were chosen such that there were no feedlots within a half mile radius. Therefore, the most likely cause of increased quantities of bacteria in the environment near the wells sampled would be failed septic systems or manure applied to fields. Some well-construction characteristics could allow a direct path for bacteria to enter a well. Hydrogeologic structures that do not filter out all bacteria can also provide a path for bacteria to enter the aquifer. These factors could influence whether or not bacteria are detected in water from a well.
Wells can be constructed to reduce or eliminate the number of pathways for contaminants to enter the water supply, however, deterioration of old wells and improper siting and installation of new wells could allow pathways for bacteria to enter a well directly (fig. 11). Bacteria could enter a well if 1) the top of the casing is not sealed and vented properly, 2) the annulus around the outside of the casing is not grouted, 3) there is leakage around the pitless adaptor, or 4) the casing is cracked or otherwise deteriorating. As previously noted, some differences in well-construction regulations exist, however, only six wells in Maryland and four wells in Chester County, Pa. were sampled. Therefore, information on the effect of regulations was inadequate to conduct statistical analysis for this study. Casing caps, regulations and standards for well construction, well deterioration and other age factors are discussed here as important considerations to minimize the number of pathways into the well.
Most wells sampled had a loose-fitting well cap on the top of the casing instead of a sanitary seal. Many of these wells had spiders, ants, earwigs, or other insects inside the well at the top of the casing. These insects can fall down the inside of the casing and introduce bacteria into the well (Wisconsin Department of Natural Resources, 1993). Some casings were cut off flush with the ground. This situation, combined with a loose well cap, could increase the chance that dirt or surface water could enter the well. Sanitary seals have an expanding rubber gasket that can prevent such things as dirt, spiders, insects, and surface water from entering the well through the top of the casing. Because only three of the 146 wells sampled had sanitary seals, analyzing the significance of this factor statistically was not possible. Nevertheless, the field observations made for this study show that casing caps were generally not sealed and represent a potential pathway for contaminants to enter the well.
Ungrouted wells are vulnerable to contamination. Regulations for well construction are written to include detailed requirements and standards for grouting (Chester County Health Department, 1993). Well-completion records for 108 wells indicated that grout was installed in 29 wells, 23 in Pennsylvania and 6 in Maryland. The annulus of loose dirt or fill around the casing of an ungrouted well may be a pathway for bacteria-laden water to move down along the casing.
Physical defects such as leakage around the pitless adaptor may have existed in some of the wells sampled. Wells in Maryland and Chester County, Pa. that do not pass the initial potability test must be retested, checked to ensure that there are no physical defects in the construction of the well, and evaluated for treatment options. Leakage around the pitless adaptor is another possible pathway for bacteria to enter a well.
The competence of a well casing is commonly related to the age of a well. The median age of the wells sampled was from 7 to 10 years old and only 3 of the 146 wells sampled were more than 20 years old. Because so few "old" wells were sampled, problems commonly associated with older wells such as well pits with standing water, inadequate casing length, and cracked or corroded casings were minimized. Lack of grouting could increase the deterioration rate of the well casing. Any crack or hole in the casing that is in the saturated zone provides a pathway for bacteria to enter into the well.
FIGURE
11. Pathways for bacterial contamination into enter a well.
Well-construction practices such as installing grout and a sanitary seal reduce the chance that bacteria will enter the well from surface sources of contamination, but that does not guarantee that bacteria will not be present in the well. If there is a pathway for the bacteria to enter the bedrock aquifer, well-construction practices cannot prevent bacteria from entering the well. Bacteria sources and pathways into the aquifer are illustrated in figure 12.
Bacteria from a septic system could enter the aquifer directly if the septic system is not functioning properly. This is a factor that could not be quantified in this study but may have had an influence on the presence of bacteria. Any septic system can introduce bacteria into the aquifer if the depth to the fractured bedrock is very shallow. Depth to bedrock generally ranged from 10-90 ft in the areas studied with a median bedrock depth of 30 ft (Risser and Siwiec, in press). Commonly, however, bedrock outcrops are evident in fields near the sampling locations.Bacteria from agricultural application of manure could enter the aquifer through paths such as sinkholes and areas with shallow bedrock (fig. 12). The timing of manure application with respect to a rainfall event may also increase the likelihood that bacteria would infiltrate the aquifer directly. In the above scenarios, the bacteria would be in the ground water, and any well that produced water from that aquifer would contain that bacteria, despite the well-construction characteristics.
FIGURE
12. Pathways for bacterial contamination to enter into a bedrock aquifer.
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The bacteriological quality of raw ground water used for household water supply was assessed for this study. Water samples were collected from selected areas in the Lower Susquehanna River Basin study unit in 1993-95 from 146 wells in 17 counties in Pennsylvania and 2 counties in Maryland. Each well within the five subunits was sampled once. Subunit criteria include the land use, physiography, and bedrock lithology.
Bacteria were not found in water samples from 31 of the 146 wells sampled. Of the water samples collected, 101 tested positive for total coliform bacteria, and 34 tested positive for fecal coliform bacteria. Of the 145 samples that were collected for fecal streptococcus, 91 tested positive. E. coli testing was not conducted during 1993, the first year of the Lower Susquehanna River Basin study. Therefore, water from 88 of the 146 wells was tested for E. coli. Of those 88 water samples, 26 tested positive for E. coli bacteria. Nearly one-third of the samples that contained total coliform bacteria also contained fecal coliform bacteria. Fecal streptococcus bacteria was present in more than three quarters of the samples that tested positive for total coliform bacteria and in all of the samples in which fecal coliform bacteria was present. The Great Valley and Appalachian Mountain carbonate subunits had the highest percentages of bacterial presence.
Statistical analyses were conducted to determine the relation between concentrations of bacteria and characteristics of environmental subunits. Statistical test results showed that land use and physiographic province are the variables that have the greatest affect on bacterial concentrations. The statistical analyses show that concentrations of total coliform, fecal coliform, and fecal streptococcus are significantly higher in agricultural areas than they are in nonagricultural areas. E. coli concentrations are not significantly different among land-use categories. All four bacteria types have concentrations that are higher in the Ridge and Valley Physiographic Province than the concentrations are in the Piedmont Physiographic Province. E. coli concentrations are significantly higher in areas underlain by carbonate bedrock. Other differences among bedrock types are not statistically significant at the 95 percent confidence level, yet are at the 93 percent confidence interval, which indicates a probable relation between bacterial concentrations and bedrock type.
Statistical tests also were conducted to determine if bacterial concentrations were related to selected well characteristics and concentrations of selected water-quality constituents. Some correlations exist between bacterial concentrations and well characteristics. Correlations also exist between bacterial concentrations and selected water-quality constituents. The results of the tests indicate the correlations are probably insignificant in predicting the concentration of bacteria from well characteristics or the concentration of the other water-quality constituents.
It is uncertain whether the aquifers sampled have widespread contamination or the bacteriological contamination is the result of site-specific factors. Other factors may exist, besides land use and physiography, that could affect bacterial concentrations. These factors include hydrogeologic structures and whether or not 1) the septic system is functioning, 2) manure has been applied to nearby fields, or 3) the well has been protected from surface contamination by grout and a sanitary seal. The large number of wells that did not have sanitary seals and were not grouted made it difficult to determine if bacterial contamination was a result of aquifer contamination or well construction. Further study with an assessment designed to compare different well characteristics would provide the data needed to determine whether the aquifers sampled have widespread contamination or the bacteriological contamination is the result of site-specific factors.
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