Patterns and sources of fecal coliform bacteria in three streams in Virginia, 1999-2000

Surface-water impairment by fecal coliform bacteria is a water-quality issue of national scope and importance.
In Virginia, more than 175 stream segments are on the Commonwealth's 1998 303(d) list of impaired waters
because of elevated concentrations of fecal coliform bacteria. These fecal coliform-impaired stream segments
require the development of total maximum daily load (TMDL) and associated implementation plans, but accurate
information on the sources contributing these bacteria usually is lacking. The development of defendable fecal
coliform TMDLs and management plans can benefit from reliable information on the bacteria sources that are
responsible for the impairment. Bacterial source tracking (BST) recently has emerged as a powerful tool for
identifying the sources of fecal coliform bacteria that impair surface waters. In a demonstration of BST
technology, three watersheds on Virginia's 1998 303(d) list with diverse land-use practices (and potentially
diverse bacteria sources) were studied. Accotink Creek is dominated by urban land uses, Christians Creek by
agricultural land uses, and Blacks Run is affected by both urban and agricultural land uses. During the 20-month
field study (March 1999?October 2000), water samples were collected from each stream during a range of flow
conditions and seasons. For each sample, specific conductance, dissolved oxygen concentration, pH, turbidity,
flow, and water temperature were measured. Fecal coliform concentrations of each water sample were determined
using the membrane filtration technique. Next, Escherichia coli (E. coli) were isolated from the fecal coliform
bacteria and their sources were identified using ribotyping (a method of 'genetic fingerprinting').

Study results provide enhanced understanding of the concentrations and sources of fecal coliform bacteria in
these three watersheds. Continuum sampling (sampling along the length of the streams) indicated that elevated
concentrations of fecal coliform bacteria (maximum observed concentration of 290,000 colonies/100 milliliters
(col/100mL) could occur along the entire length of each stream, and that the samples collected at the downstream
monitoring station of each stream were generally representative of the entire upstream reach. Seasonal patterns
were observed in the base-flow fecal coliform concentrations of all streams; concentrations were typically highest
in the summer and lowest in the winter. Fecal coliform concentrations were lowest during periods of base flow
(typically 200?2,000 col/100mL) and increased by 3?4 orders of magnitude during storm events
(as high as 700,000 col/100mL). Multiple linear regression models were developed to predict fecal coliform
concentrations as a function of streamflow and other water-quality parameters. The source tracking technique
provided identification of bacteria contributions from diverse sources that included (but were not limited to) humans,
cattle, poultry, horses, dogs, cats, geese, ducks, raccoons, and deer. Seasonal patterns were observed in the
contributions of cattle and poultry sources. There were relations between the identified sources of fecal coliform
bacteria and the land-use practices within each watershed. There were only minor differences in the distribution of
bacteria sources between low-flow periods and high-flow periods. A coupled approach that utilized both a large
available source library and a smaller, location-specific source library provided the most success in identifying the
unknown E. coli isolates. BST data should provide valuable support and guidance for producing more defendable and
scientifically rigorous watershed models. Incorporation of these bacteria-source data into watershed management
strategies also should result in the selection of more efficient source-reduction scenarios for improving water quality.