Abstract
Contamination of surface waters with microbial pollutants from fecal sources is a significant human health issue. Identification of relative fecal inputs from the mosaic of potential sources common in rural watersheds is essential to effectively develop and deploy mitigation strategies. We conducted a cross-sectional longitudinal survey of fecal indicator bacteria (FIB) concentrations associated with extensive livestock grazing, recreation, and rural residences in three rural, mountainous watersheds in California, USA during critical summer flow conditions. Overall, we found that 86% to 87% of 77 stream sample sites across the study area were below contemporary Escherichia coli-based microbial water quality standards. FIB concentrations were lowest at recreation sites, followed closely by extensive livestock grazing sites. Elevated concentrations and exceedance of water quality standards were highest at sites associated with rural residences, and at intermittently flowing stream sites. Compared to national and state recommended E. coli-based water quality standards, antiquated rural regional policies based on fecal coliform concentrations overestimated potential fecal contamination by as much as four orders of magnitude in this landscape, hindering the identification of the most likely fecal sources and thus the efficient targeting of mitigation practices to address them.
Highlights
Contamination of surface waters with pathogens from fecal sources is a significant human health issue [1,2,3]
There were 595 stream water samples collected with concentrations determined for E. coli and fecal coliform
While by no means perfect [4], E. coli-based water quality standards have clearly been determined to be biologically and statistically superior to standards based on fecal coliforms (FCs) at any benchmark concentration (e.g., [11,12,16,43])
Summary
Contamination of surface waters with pathogens from fecal sources is a significant human health issue [1,2,3]. There is a high correlation between the presence of FIB at certain concentrations (i.e., microbiological water quality standards), direct fecal contamination, waterborne pathogens, and human health risk This assumption has become increasingly challenged by a growing body of research (e.g., [4,5,7]), and the scientific search for improved proxies to detect microbial health risks continues with a focus on molecular and other methods (e.g., [8,9,10]). Despite their shortcomings, FIB-based microbial water quality standards remain the primary policy approach to microbial water quality protection [11,12]. Contemporary research demonstrates that E. coli is a superior FIB to FCs as a proxy for fecal pollution from warm-blooded mammals [12,13] primarily due to excessive detections of non-fecal, environmental thermotolerant coliform bacteria by the FC assay (e.g., [14,15,16]), but some rural and developing water quality authorities still rely on FCs
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