Risk factors for coliform bacteria in backcountry lakes and streams in the Sierra Nevada mountains: a 5-year study.

OBJECTIVE: To provide a 5-year longitudinal assessment of risk of acquiring disease from Sierra Nevada Wilderness area lakes and streams. This study examines the relative risk factors for harmful water microorganisms, using coliforms as an indicator.
METHODS: Streams and lakes in the backcountry of Yosemite and Kings Canyon National Parks and neighboring wilderness areas were selected and water was analyzed each year over a 5-year period. A total of 364 samples from lakes or streams were chosen to statistically differentiate the risk categories based on land usage, as follows: 1) areas rarely visited by humans (Wild), 2) human day-use-only areas (Day Hike), 3) areas used by backpackers with overnight camping allowed (Backpack), 4) areas primarily impacted by horses or pack animals (Pack Animal), and 5) cattle and sheep grazing tracts (Cattle). Water was collected in sterile test tubes and Millipore coliform samplers. Water was analyzed at the university microbiology lab, where bacteria were harvested and then subjected to analysis using standardized techniques. Statistical analysis to compare site categories was performed utilizing Fisher exact test and analysis of variance.
RESULTS: A total of 364 sampling sites were analyzed. Coliforms were found in 9% (4/47) of Wild site samples, 12% (5/42) of Day Hike site samples, and 18% (20/111) of Backpacker site samples. In contrast, 63% (70/111) of Pack Animal site samples yielded coliforms, and 96% (51/53) of samples from the Cattle areas grew coliforms. Differences between Backpacker vs Cattle or Pack Animal areas were significant at P <or= .05. All samples grew normal aquatic bacteria.
CONCLUSION: Surface water from watersheds below cattle areas and those used by pack animals is at high risk for containing coliform organisms. Water from Wild, Day Hike, or Backpack sites poses far less risk for contamination by coliforms.