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Effects of packstock use and backpackers on water quality in Yosemite National Park, California
AuthorForrester, Harrison A.
AdvisorHeyvaert, Alan C.
Natural Resources and Environmental Science
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Visitor use in designated Wilderness increases the potential for negative effects on water quality. However, effects from specific types of visitor use such as backpacker camping, packstock (horse and mule) trail use, and packstock grazing are difficult to quantify and isolate from background environmental processes. To determine the effects of visitor use on water quality in Wilderness in Yosemite National Park, we collected and analyzed surface-water samples for fecal indicator bacteria (Escherichia coli), nutrients (nitrogen, phosphorus, and carbon), suspended sediment concentration (SSC), and hormones (e.g. estrogen compounds) during the summers of 2012-2014. We collected samples upstream and downstream from different types of visitor use at routine intervals (weekly or biweekly) during steady flow (non-storm) conditions and during episodic storms. Additionally, we sampled upstream and downstream from meadows, and targeted different types of visitor use during a park-wide synoptic sampling campaign (n=63). Statistically significant (P≤0.05) increases in Escherichia coli (E. coli) and SSC occurred downstream from packstock stream crossings compared to upstream conditions during routine sampling (median difference: 3 CFU 100ml-1, and >0.3 mg l-1, respectively) and during storms (median difference: 32 CFU 100ml-1, and 2.9 mg l-1). During routine sampling, significant increases also occurred downstream from backpacker camping for E. coli (median difference: 1 CFU 100ml-1) and estrogen hormones were detected. No significant increases were detected for any of the measured water quality indicators downstream from packstock grazing. Most of the synoptic sample concentrations were near or below detection limits. Our results indicate that under current use levels: 1) packstock trail use and backpacker camping have detectable effects on water quality, which are most pronounced during storms; 2) increases in water quality indicators were not detected downstream from meadows where packstock were grazed; and 3) environmental processes in meadows might provide a valuable ecosystem service by reducing human related sources of microbial contamination.