Passage II

High concentrations of fecal coliforms in the water of a lake are a hazard to humans swimming in the lake. Fecal coliforms are bacteria found in the intestines of all endothermic animals. Two studies in 2001 examined fecal coliform concentrations in surface water samples from a large freshwater lake and its tributary streams.

Study 1

Around 10 A.M. on Monday of every week in June, July, and August, 100 mL water samples were collected from 3 separate locations within the area of the lake desig­nated for swimming. The samples were analyzed to deter­mine the average fecal coliform concentration, in colony-forming units (CFU) per 100 mL (see Figure 1). A CFU value is the number of distinct colonies formed on a growth plate by the bacteria in a sample after the sample has been placed on the growth plate and incubated for 24 hr.

Study 2

In July, on the last of 14 consecutive days of fair weather, 100 mL water samples were collected from each of the following sites—the designated swimming area, 6 lake sites that were outside the designated swimming area, and each of the lake's 5 tributary streams. Several 100 g samples of sand from the beach at the designated swimming area were also collected. A second set of samples was collected at the same sites on a rainy July day, the last of 7 consecutive days of rain. The samples were analyzed to determine the average fecal coliform concen­tration (see Figure 2). The heavy, dashed, horizontal line in Figure 2 represents the fecal coliform water quality stan­dard for the lake. If the average fecal coliform concentra­tion for water samples collected from the lake on a particular day exceeds chis level, all swimming in the lake is prohibited for that day.

Figures 1 and 2 adapted from Michele Zimmerman, "Occurrence of Bacteria in Blue Marsh Lake and Selected Tributaries, Berks County, Pennsylvania: September-October 2001." ©2002 by the United States Geological Survey.