This just showed up at one of the forums I watch, "Coliform and Pathologic Bacteria in Sierra Nevada National Forest Wilderness Area Lakes and Streams", Wilderness and Environmental Medicine: Vol. 15, No. 4, pp. 245–249. While it specifically did not look at giardia, it did look at illness-causing bacteria.

"Objective.—To analyze backcountry-area water quality in US Department of Agriculture Forest Service–designated wilderness areas for the presence of coliform and potentially pathogenic bacteria.

Methods.—Thirty-one backcountry lakes and streams were selected that would stratify the risk based on use by backpackers, pack animals, commercial grazing animals, or natural unaffected wilderness areas. Sites included Desolation Wilderness (10 sites), Carson-Iceberg Wilderness (4 sites), Emigrant Wilderness (3 sites), Hoover Wilderness (6 sites), John Muir Wilderness (3 sites), and Golden Trout Wilderness (5 sites). Water was collected in sterile tubes and quantification was performed through Millipore bacterial samplers. On return to the laboratory, bacteria were harvested from the samplers and subjected to qualitative analysis that identified species according to standard laboratory methods.

Results.—Coliform bacteria were detected in 14 of 31 sites (45%). Eight sites had high levels of coliforms. All 8 of these sites correlated with heavy human use or commercial grazing. Coliforms were identified as Escherichia coli. In addition, 1 sample contained Yersinia entercolitica. All samples contained expected amounts of normal aquatic bacteria, including Pseudomonas spp, Rahnella aquatilis, Serratia spp, and other nonpathogenic species of Yersinia in concentrations of 600 to 10000 colony-forming units per 100 mL.

Conclusions.—In this study, coliform bacteria were found at nearly half of the sampling sites. High coliform levels correlated with high-impact human use or cattle grazing."

"Many wilderness lakes and streams studied contained low to moderate levels of coliforms. The highest levels of coliforms occurred in watersheds heavily used by humans, livestock, or cattle grazing. The low levels of coliform bacteria found in a very few streams and lakes may be part of a natural ecological environment or may be secondary to occasional contamination from humans, pack animals, or natural wild animals. Noncoliform and nonpathogenic aquatic bacteria were present in counts consistent with other studies on environmental bacteria."

http://www.wemjournal.org/wmsonline/?req...p;amp;page=0245

Scroll down to the bottom of the above reference to see the list of locations sampled.

FB