Survey of Pathologic Microorganisms in the Streams Along the Tahoe Rim Trail

Introduction This study aimed to estimate the contamination of water sources along the Tahoe Rim Trail (TRT) through evaluation of the presence and concentration of Escherichia coli , Giardia, and Cryptosporidium spp. Methods Sample sites were selected from 6 of the 8 sections of the TRT. Each stream was sampled 3 or 4 times during the summer and early fall of 2023. Samples were collected and analyzed for E. coli using Standard Methods 9223B and Giardia and Cryptosporidium spp. using EPA Method 1623. Results E. coli counts ranged from <1 to 29.2 most probable number (MPN)/100 mL. There was no difference in contamination with E. coli between streams ( P =0.287) or sampling rounds ( P =0.413). Giardia counts ranged from 0 to 7 count/10 L. There was a difference in contamination with Giardia between the highest and lowest concentration streams ( P =0.028) but no difference in contamination between other streams or between sampling rounds ( P =0.924). Cryptosporidium counts ranged from 0 to 1 count/10 L. There was no difference in contamination with Cryptosporidium between streams ( P =0.589) or between sampling rounds ( P =0.888). Conclusions E. coli , Giardia, and Cryptosporidium are present in the streams along the TRT. Results from this study show that the risk of illness from occasional direct consumption of water from the streams sampled is low; however, it is always advisable to filter or treat water from any backcountry source prior to consumption. More extensive studies are recommended to evaluate backcountry water sources in the Tahoe Basin to further evaluate the risk to human health from direct consumption .