Spatial and temporal analysis of fecal indicator bacteria concentrations in beach water in San Diego, California

Abstract: With millions of yearly beach visitors in southern California, beach water quality represents an important factor in public health and ocean dependent economy. Fecal indicator bacteria (FIB) in water is much easier to measure than disease causing organisms but correlation exists between the two. For this reason, FIB concentrations are used to measure recreational water quality and related health risk of body contact with the water. The source of FIB is often related, but not limited, to surface runoff. This study analyzed beach water FIB concentrations at two San Diego beaches, Ocean Beach and Tourmaline Surfing Park, that are affected by the river and storm water discharges, and explored the land use in their adjacent watersheds. The study distinguished between wet and dry weather and compared bacterial concentrations against the watershed land use. Furthermore, the study analyzed temporal and spatial dynamics of bacterial concentrations during and after storm weather at Ocean Beach. Finally, the study examined the relationship between bacterial concentrations at seven sampling sites of San Diego River tributaries and the land use within the tributaries. Fecal indicator bacteria concentrations showed a significant rise during rainfall. Ocean Beach had significantly higher FIB concentrations during dry weather, compared to Tourmaline Surfing Park, with significantly higher FIB concentrations at the sampling location near the river discharge in wet weather. Bacterial concentrations generally decreased with the distance from the closest surface water discharge. The peak in FIB concentration rise was reached already on the first day of the storm weather at Tourmaline Surfing Park and on the second day at Ocean Beach. As little as 1 mm of rainfall was needed for a significant raise in bacterial concentrations in beach water. Watersheds with a higher percentage of residential and transport area had lower mean bacterial concentrations in beach water at the surface water discharge, compared to watershed with lower percent of residential and transport area. Higher percentage of land, used for parks or open space, undeveloped land, commercial and public services, recreation, industry and agriculture in the watershed, corresponded to higher mean indicator bacteria at the beach where the river discharges. No correlation was found between Enterococcus concentration in San Diego River and land use in the tributaries.