Research Areas > Beach Water Quality > Microbiological Water Quality at Non-Human Impacted Reference Beaches in Southern California During Wet Weather

Project: Microbiological Water Quality at Non Human-Impacted Beaches in Southern California During Wet Weather

Background and Objectives

Although wet weather discharges from urban watersheds often contain elevated concentrations of fecal indicator bacteria that may impact water quality at swimming beaches, not all of these bacteria arise from human sources. This study sought to quantify contributions of fecal indicator bacteria for coastal "reference" beaches in southern California known to have minimal human impacts. Reference beaches were defined as open beaches that have breaking waves and receive runoff from undeveloped (>93% open space) watersheds. This selection was further refined to represent a range of geographical conditions and watershed sizes.

Status

This study was conducted from 2004 to 2006.

Methods

Six marine reference beaches in southern California were selected for assessment of water quality during wet weather. Water samples were collected at these beaches during wet season storm events, both in discharge from the watershed and at the confluence of the discharge and surf. The primary "wave wash" sampling location was specifically at ankle- to knee-depth in the ocean, immediately in front of the freshwater input where the watershed discharge initially mixes with the ocean waves. The secondary sampling location was in the watershed discharge as it crossed the beach prior to mixing with the ocean, at the closest sampleable location. Flow measurements of watershed discharge were also taken at the secondary sampling site at each beach.

All samples were assayed for fecal indicator bacteria and salinity. Concentrations of total coliforms, E. coli, and Enterococci were enumerated using defined substrate methods. A subset of samples at secondary sites were also analyzed for human enteric virus, in order to detect or rule out the presence of human contributions of fecal pollution in the discharge, including discharge samples from the first day of storm flow.

Findings

• Samples collected during wet weather exceeded water quality thresholds at least 10 times more frequently than those collected during dry weather in summer or winter, although the frequency of exceedance differed among beaches.

• Exceedences were greatest less than 24 hours following recorded rainfall, then steadily declined over successive days.

• Early season storms exceeded water quality thresholds more frequently than late season storms, often for multiple fecal bacterial indicators, whereas most late-season storms exceeded water quality thresholds for only a single bacterial indicator.

• Large storms exceeded water quality thresholds more frequently than smaller-sized storms, partly due to the breaching of sand berms. Small storms could not breach sand berms, restricting watershed discharges from entering the surf zone.

• When discharges did reach the surf zone, bacterial concentrations in the "wave wash" were correlated with watershed bacterial flux.

Partners

City of Los Angeles
Weston Solutions, Inc.
Los Angeles Regional Water Quality Control Board