Anthropogenic Sources of Contamination in the San Diego River and its Tributaries During Storm Events

Abstract:

Storm events have resulted in regular exceedances of benchmarks for fecal indicator bacteria in Southern California. Suspected sources of anthropogenic microbial contaminants in urban waterbodies include sewer exfiltration, sanitary sewer overflows, illegal connections and discharges, septic tanks, and wastes from homeless encampments. The intensity and duration of precipitation and watershed area experiencing interflow may influence concentrations and loadings of pollutants introduced to urban streams. The aim of this study was to analyze temporal variability in potential sources of river contamination by evaluating the temporal trends in traditional fecal indicator bacteria (i.e. E. coli and enterococcus), novel microbial (HF 183 and Pepper Mild Mottle Virus) and chemical (caffeine and sucralose) markers of human contamination during storm events during the 2018 and 2019 hydrologic years. Biological and chemical characteristics of different end-member environments were also analyzed to distinguish the different sources of pollution to the river.

The largest storm event sampled (January 2018) resulted in peak E. coli concentration of 1.7 x 10⁵ MPN/100 mL and loadings of 5x10¹⁴ MPN/day with caffeine concentrations reaching 5.2 µg/L. In four out of the five storm events sampled, increases in HF 183 and caffeine concentrations, and caffeine/sucralose ratios, demonstrated that untreated wastewater was most likely the main source of microbial pollution in the river. Chloride/bromide ratios aided in visualizing the transition from a saline groundwater at baseflow to rainwater and interflow inputs during storm events. Trends in specific UV absorbance and fluorescence index helped identify the timing of potential interflow conditions during storm events. This study is important in meeting water quality benchmarks for the San Diego River and informing efforts to improve water quality in other urban rivers.