Spatial and temporal variability of bacterial indicators and pathogens in six California reservoirs during extreme drought

摘要

AbstractCalifornia has one of the largest systems of surface water reservoirs in the world, providing irrigation water to California's agriculturally productive Central Valley. Irrigation water is recognized as a vehicle for the microbial contamination of raw produce and must be monitored according to new federal regulation. The purpose of this study was to further understanding of the variability of fecal indicator bacteria (Escherichia coliand fecal coliforms) and pathogens (E. coliO157:H7 (O157), non-O157 Shiga toxin-producingE. coli(STEC) andSalmonella) along both horizontal and vertical profiles within California reservoirs. Monthly sampling was conducted in six reservoirs located in the foothills of the Western Sierra Nevada during the summer irrigation season and extreme drought conditions of 2014 (n = 257). Concentrations of fecal indicator bacteria were highly variable between reservoirs (p < 0.05) and along the horizontal profile (p < 0.001) from upstream to downstream, with higher concentrations typically found outside of the reservoirs than within. Though many of the reservoirs were thermally stratified, bacterial concentrations were not associated with water temperature (p > 0.05) or any one particular depth strata (p < 0.05). However, prevalence ofSalmonellaand STEC (16/70 and 9/70 respectively) was higher in the deep strata than in mid or surface layers. We found no statistical association between samples collected downstream of reservoirs and those from the reservoirs themselves. Continued monitoring and modeling of both bacterial indicators and enteric pathogens are critical to our ability to estimate the risk of surface irrigation water supplies and make appropriate management decisions.Graphical abstractDisplay OmittedHighlights•Downstream water quality was significantly different than within reservoirs.•Temperature or thermal stratification was not predictive of bacterial outcomes.•Bacterial outcomes varied significantly along horizontal spatial profiles.•Pathogens were highest in reservoirs with the lowest fecal indicator bacteria.