Using in situ measurements of optical brighteners for rapid reconnaissance of wastewater inputs to water resources.

Untreated wastewater entering the environment through leaking infrastructure and sewer overflows threatens both human and aquatic health. Water managers therefore need low cost, in situ methods to detect sewage contamination in real time to promptly employ mitigation strategies. However, wastewater has traditionally been identified in waterbodies using chemical and microbial tracers and indicators that can be non-unique to wastewater and often require complex and expensive analyses. Optical brighteners (synthetic brightening compounds present in laundry detergents and paper products) are emerging as ideal tracers of wastewater because of their quick and inexpensive field detection using handheld fluorometers. To test the efficacy of optical brighteners as standalone, in situ wastewater tracers, field readings of their fluorescence were compared with traditional wastewater analytes (e.g., B, F, microbial indicators) at multiple points in time and space for a suburban watershed (Fishpot Creek, Saint Louis, Missouri, United States). We also used chemical tracers in three mixing models of endmembers to assess the wastewater fraction across the watershed. Compared to other analytes, optical brightener fluorescence measurements had the strongest correlation with wastewater infrastructure density (r = 0.71, p < 0.05), indicating their utility as tracers. All our endmember mixing models employing optical brightener readings predicted positive and significant correlations between the untreated wastewater fraction in streamflow and sewer pipe density at each site (r ≥ 0.77, p < 0.05). While using optical brightener readings for wastewater detection has some limitations (e.g., minor photodegradation), we found them to be more robust tracers than other analytes. Thus, optical brightener fluorescence measurements are an ideal initial screening tool for identifying wastewater contributions to the environment.