Beach sands along the California coast are diffuse sources of fecal bacteria to coastal waters

Abstract

Fecal indicator bacteria (FIB) are nearly ubiquitous in California (CA) beach sands. Sands were collected from 55 beaches along the CA coast. Ninety-one percent of the beaches had detectable enterococci (ENT) while 62% had detectable E. coli (EC) in their sands. The presence of a putative bacterial source (such as a river), the degree of wave shelter, and surrounding land use explained a significant (p < 0.05) fraction of the variation in both ENT and EC densities between beaches. Sand characteristics including moisture content, organic carbon, and percentfines, significantly (p < 0.05) influenced only EC densities in beach sand. We assayed 34 of 163 sand samples for salmonellae, but did not detect this bacterial pathogen. The potential for FIB to be transported from the sand to sea was investigated at a single wave-sheltered beach with high densities of ENT in beach sand: Lovers Point, CA (LP). We collected samples of exposed and submerged sands as well as water over a 24 h period in order to compare the disappearance or appearance of ENT in sand and the water column. Exposed sands had significantly higher densities of ENT than submerged sands with the highest densities located near the high tide line. Water column ENT densities began low, increased sharply during the first flood tide and slowly decreased over the remainder of the study. During the first flood tide, the number of ENT that entered the water column was nearly equivalent to the number of ENT lost from exposed sands when they were submerged by seawater. The decrease in nearshore ENT concentrations after the initial influx can be explained by ENT die-off and dilution with clean ocean water. While some ENT in the water and sand at LP might be of human origin because they were positive for the esp gene, others lacked the esp gene and were therefore equivocal with respect to their origin. Follow-up sampling at LP revealed the presence of the human specific Bacteroides marker in water and sand.