Chemical and Microbial Markers for Discriminating Sanitary Sewer Contamination in Coastal, Urban Streams

Water quality benchmarks for fecal indicator bacteria (FIB) are often exceeded in many urban streams in southern California. Possible sources of elevated stream FIB concentrations within urban areas include sanitary sewer exfiltration, sanitary sewer overflows (SSOs), illegal discharges, and human or animal fecal material on the ground surface. Teasing apart the different sources remains a challenge, especially when untreated wastewater and runoff from open defecation sites both contain human fecal material. To distinguish the various sources of microbial contamination in an urban stream, temporal trends in biological and chemical markers of anthropogenic contamination were evaluated in the San Diego River and its tributaries during storm events in two consecutive hydrologic years. Temporal trends in FIB, HF183, pepper mild mottle virus (PMMoV), caffeine, sucralose, chloride, bromide, specific ultraviolet absorbance, and fluorescence index indicated that untreated wastewater flushed from the vadose zone was the main source of microbial pollution to the San Diego River, while open defecation near homeless encampments in the river margins was not a major source. We demonstrated that the combined use of caffeine/sucralose ratios and HF183 and PMMoV holds promise for identifying sewage inputs to surface waters. These findings highlight the need for maintenance and repair of aging sewer infrastructure.