Bias in fecal indicator bacteria tests in wood-associated wastewaters: Findings from a robust, multi-facility dataset

Fecal indicator bacteria (FIB) are surrogate measures of fecal contamination and the basis of recreational water quality criteria to protect human health, but are prone to interference from non-fecal, environmental bacteria. We used a robust dataset of paired FIB (fecal coliform, Escherichia coli, and enterococci) and microbial source tracking (MST) data (Enterococcus speciation, human and animal markers) in wood-associated wastewaters (n = 22 woodyards and pulp and paper facilities, 13 US states) to evaluate the consistency in FIB outcomes with respect to test type (i.e., quantification method) and FIB, and the frequency with which elevated FIB was attributable to fecal sources. Agreement in sample bacterial characterization was poor between paired measurements of fecal coliform and enterococci (65% agreement) and E. coli and enterococci (53% agreement), with significantly more test failures based on regulatory thresholds occurring using fecal coliform and enterococci compared to E. coli. All three FIB were poor indicators of human fecal contamination with human markers absent in 73-100% of samples with elevated FIB. Among these, fecal contamination from avian and ruminant sources was rarely observed. Similarly, elevated FIB measured as enterococci was due to plant-associated Enterococcus casseliflavus or other environmental Enterococcus sp. in 96% of samples with paired speciation data. These results demonstrate that FIB are unreliable surrogate measures of fecal contamination in wood-associated wastewaters, and alternative approaches are needed to assess risk to human health and fecal contamination of recreational waters.