Impact of Sewage Line Spills on Pathogen Levels in Recreational Waters

Abstract

The objective of this study was to quantify the effect of City of Los Angeles sewage spills that reach the storm drain system on bacteria concentrations in Santa Monica Bay (SMB). The combined effect of dilution, transport, and die-off decreases bacteria concentrations with increasing distance downstream of the spill source. To assess the association between sewage spill events and bacteria levels in SMB, five specific spills were selected for detailed evaluation. Four of these spills occurred during dry weather and one during wet weather conditions. Each spill was characterized by spill date, volume, duration of release, transport path, distance to outfall, and measured concentrations of total coliform, E. coli/fecal coliform, and enterococcus at the source. Spill volumes ranged from 522 gallons to 560,000 gallons. Four of these spills entered SMB via Ballona Creek, and one was transported to the ocean via the Los Angeles River. The City of Los Angeles monitors bacteria levels daily at 18 different stations, located near major storm drains, along the SMB shoreline. By examining trends in the monitoring data at stations closest to the discharge point prior to and following the date of the spill and examining weather conditions as well as the occurrence of more than one spill that could affect the same area in the same time frame, it was possible to assess the impact of these spills on receiving waters. This approach was adopted for three of the five spills. For the remaining two spills, no monitoring data were available; for these spills, bacteria concentrations at the outfall were estimated using a simplified dilution and die-off model. The results indicate that there is little, if any, association between Los Angeles sewage line spills and the levels of pathogens measured in SMB. Even on days following the largest spills, bacteria levels in SMB in the vicinity of the discharge point did not increase significantly.