An improved fluorescence-based tracer of wastewater effluent for use in urban coastal waters

Dissolved organic matter (DOM) associated with wastewater has specific fluorescence properties that can be used to facilitate the tracing of effluent in urban waters. A large number of field-based fluorescence sensors for DOM are now commercially available and can be used on ships (e.g., underway or profiling) to facilitate the rapid monitoring of effluent. However, most field-based sensors only measure fluorescence at a single excitation/emission wavelength pair (e.g., 370 nm/ 460 nm) making the interpretation of the data vulnerable to confounding effects by other sources of DOM. In urban coastal waters, DOM from runoff in particular can severely limit the reliability of this single fluorescence measurement as a tracer of effluent. Here, excitation-emission fluorescence matrices (EEMs) measured on samples collected before, during, and after a planned nearshore diversion of wastewater in Santa Monica Bay (Southern California) facilitated the development of an improved tracer for use in urban coastal waters influenced by runoff and phytoplankton blooms. The new tracer used a combination of three fluorescence measurements made at different excitation-emission pairs carefully selected to maximize sensitivity to effluent, while minimizing sensitivity to DOM from other sources. The improved fluorescence-based tracer was able to reliably detect effluent during the diversion, while remaining largely insensitive to DOM from runoff and blooms. The results are used to make recommendations for the configuration of fluorescence sensors designed to trace effluent in urban coastal waters.