Ab initio and in situ comparison of caffeine, triclosan, and triclocarban as indicators of sewage-derived microbes in surface waters

Three organic wastewater compounds (OWCs) were evaluated in theory and practice for their potential to trace sewage-derived microbial contaminants in surface waters. The underlying hypothesis was that hydrophobic OWCs outperform caffeine as a chemical tracer, due to their sorptive association with suspended microorganisms representing particulate organic carbon (POC). Modeling from first principles (ab initio) of OWC sorption to POC under environmental conditions suggested an increasing predictive power: caffeine (0.2% sorbed) < triclosan (9-60%; pH 6-9) < triclocarban (76%). Empirical evidence was obtained via analysis of surface water from three watersheds in a rural-to-urban gradient in Baltimore, MD. Mass spectrometric OWC detections were correlated to microbial plate counts for 40 monitoring sites along 14 streams, including multiple chronic sewage release sites and the local wastewater treatment plant. Consistent with ab initio calculations, correlation analyses of 104 observations for fecal coliforms, enterococci, and Escherichia coli in natural surface waters showed that the particle-active antimicrobials triclosan and triclocarban (R² range, 0.45-0.55) were indeed superiorto caffeine (0.16-0.37) fortracking of microbial indicators. It is concluded that chemical monitoring of microbial risks is more effective when using hydrophobic OWCs such as triclosan and triclocarban in place of, or in conjunction with, the traditional marker caffeine.