Storm water runoff from an urban watershed in southern California: Transport of suspended particles and fecal pollution, and coastal impact

Jong Ho Ahn, Ph. D. Candidate

Environmental Engineering, Henry Samueli School of Engineering

University of California, Irvine, CA. 92697

Global human migration toward the ocean has fueled an urbanization of the earth's coastal regions, replacing natural landscapes (rivers, fields, forests, and estuaries) with urban civil infrastructure (canals, roadways, residential communities, and commercial land-uses). Coastal urbanization has the potential to dramatically alter the flow of material from the land into the ocean, with consequent impacts on coastline stability, biogeochemical cycling, and the health of near-shore ecosystems. Recently, surface water runoff and associated pollution have been emerging as the primary source of pollutant loading to the urban ocean due to improvement in civil infrastructure, pollutant source control, and disposal/treatment technology. In this study we investigate the transport of particles and microbial indicators of fecal pollution in storm water in the Santa Ana River -- a human-impacted “urban” river -- in southern California, and identify the coastal water quality impact of storm water runoff.

Suspended particle transport in storm water runoff from the Santa Ana River can be divided into channel erosion and slope erosion depending flow scaling of suspended particle size spectra. The observation that fecal indicator bacteria are not diluted during periods of peak flow, together with the apparent paucity of human viral pathogens, suggest that most of the fecal indicator bacteria originate from sources other than human sewage. The coastal water quality impact of storm water runoff from the Santa Ana River depends on prevailing ocean currents, within-plume processing of particles and pathogens, and the timing, magnitude and nature of runoff discharged from river outlets over the course of a storm. The results demonstrate that storm water runoff negatively impacts coastal water quality, both in the surf zone and offshore. In the surf zone, entrainment of storm water runoff leads to very poor water quality, with fecal indicator bacteria concentrations exceeding State standards, but cross-shore currents apparently dilute contaminated surf zone water with cleaner water from offshore. Offshore of the surf zone, storm water runoff ejected from the river mouth spreads out over a large area. Fecal indicator bacteria concentrations in the offshore plumes are below State standards, even in cases where offshore samples test positive for pathogenic viruses and fecal indicator viruses. Fecal indicator bacteria concentration is inversely correlated with average particle size, and multiple lines of evidence indicate that bacteria and viruses are either not particle-associated, or associated with relatively small particles.