Sterols and sterol ratios to trace fecal contamination: pitfalls and potential solutions

Fecal pollution in surface waters is a major threat to recreational and drinking water resources, with Escherichia coli being a primary concern. The best way to mitigate fecal pollutant loading is to identify the sources and tailor remediation strategies to reduce loading. Tracking E. coli back to its source is notoriously difficult in a mixed-use watershed where input from humans, wildlife, and livestock all contribute to E. coli loading. One proposed tracking method for E. coli contamination is the use of fecal sterols and sterol ratios. This study uses fecal sterol data published globally to assess how well sterol compositions for different species clusters along with the effectiveness of sterol ratios as tracking tools. Hierarchical cluster analysis produces stronger clusters based on sterol ratios than raw sterol concentration, but the global dataset results in clustering of the same species in different levels. The accuracy of the sterol ratios was also compared to understand the rate of false negatives and false positive assignments. Overall, these ratios did not have a high success rate for determining the correct source, which was also reflected in the poor clustering trends observed. Establishing local end-member sterol profiles is essential when using sterol signatures to unravel fecal loading.