U.S. Geological Survey

Bacteriological Quality of Ground Water Used for Household Supply, Lower Susquehanna River Basin, Pennsylvania and Maryland

Tammy M. Bickford and Bruce D. Lindsey
Ground water is an important source of household supply in the Lower Susquehanna River Basin. About 800,000 households and approximately 300,000 private wells are in the study area; approximately 40 percent of the households depend on ground water from private wells for water supply. In this study, we found that bacteriological quality of untreated ground water used for household water supply in the basin was related to land use and physiographic province. No significant relation existed between bacteria concentrations and selected water-quality constituents. Differences in bacteria concentrations among bedrock types were only statistically significant for Escherichia coli (E. coli). Bacteria concentrations were not related to well characteristics such as total well depth or casing length, although further study is needed.

Water samples collected from 146 household supply wells in the Lower Susquehanna River Basin were analyzed from 1993 to 1995 for fecal-indicator organisms including total coliform, fecal coliform, E. coli, and fecal streptococcus concentrations. Bacteria were not detected in water samples from 31 of the 146 wells sampled. Of the 146 water samples, 101 tested positive for total coliforms, 34 tested positive for fecal coliforms, and 92 tested positive for fecal streptococci. E. coli concentrations were determined in water from 88 of the 146 wells sampled. Of those 88 water samples, 26 tested positive for E. coli. Nearly one-third of the samples that contained total coliforms also contained fecal coliforms. Fecal streptococci were present in more than three quarters of the samples that tested positive for total coliforms and in all of the samples in which fecal coliforms were present.

Land use was related to the presence of bacteria in water from household supply wells. Bacterial concentrations in agricultural and nonagricultural areas were compared. Concentrations of total coliform, fecal coliform, and fecal streptococcus were higher in agricultural areas than they were in nonagricultural areas; these differences in concentrations for different land uses were statistically significant. Differences in E. coli concentrations are not statistically significant among land-use categories.

Physiographic province was also related to the presence of bacteria in water from household supply wells. Bacterial concentrations in the Ridge and Valley and Piedmont Physiographic Provinces were compared. Water from wells sampled in the Ridge and Valley Physiographic Province was more likely to have bacteria than water from wells in the Piedmont Physiographic Province.

It is uncertain whether the aquifers sampled have widespread contamination or the bacteriological contamination is a site-specific occurrence. Other factors may exist, besides land use and physiographic province, that could affect bacteria concentrations. These factors include 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. Hydrogeologic structures, such as sinkholes and fractures in the bedrock, transport the water rapidly from the land surface to the well and could affect bacteria concentrations. 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 characteristics. Further study with an assessment designed to compare different well-construction practices would provide the data needed to determine the relation between well construction and the bacteriological quality of well water.

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