Research Areas > Beach Water Quality > Efficacy of Shallow Water Sampling to Determine Exposure of Surfers to Indicator Bacteria at Marine Beaches

Project: Efficacy of Shallow Water Sampling to Determine Exposure of Surfers to Indicator Bacteria at Marine Beaches

Background and Objectives

Beach water quality in Southern California is the most intensively monitored in the nation, with over 185,000 water samples collected and analyzed each year during routine shoreline monitoring. All shoreline water quality monitoring occurs at a depth of about 0.3 m, since breaking waves make sampling inconvenient and potentially dangerous in deeper water. This is also a common depth of exposure for small children, who are the most immunologically susceptible swimmers. It is unclear how well sampling at this depth protects surfers, though, as they typically are exposed at locations 20 m or more offshore. Surfers are often avid users of recreational waters and among the most vulnerable to waterborne illnesses because of repeated, sudden, and substantial (head to toe) exposure. Compounding this exposure, prime surfing locations are often found at the mouths of creeks and rivers where sand bars formed by sediment deposition cause waves to break farther offshore, offering surfers a longer ride. These creeks and rivers may transport high concentrations of pathogens from upstream, creating greater potential exposure. This study examined the relationship between the microbial water quality of samples taken at the 0.3-m depth and water quality of samples obtained offshore, where surfers typically line up to catch waves. The goal of the study was to determine if samples taken along the shoreline adequately characterize microbiological water quality in deeper waters.

Status

This study was conducted from 2003 to 2005.

Methods

Paired water samples were collected at the shallow depth where sampling typically occurs, and outside the breaking surf offshore, where surfers typically line up to catch surfable waves. Sampling was conducted at 12 beaches in the summer dry season and at 9 beaches following winter rainstorms. Beaches selected for study all had a flowing freshwater creek, surfers present at the site, and a history of microbial water quality standards exceedences. Seven pairs of samples at different distances from the freshwater outlets were collected at all beaches. Water samples were analyzed for enterococci using the defined substrated method (IDEXX).

Findings

• The nearshore and offshore samples were correlated during both the wet and dry sampling periods, but the correlation was higher following rainstorms.

• Concentrations of enterococci were typically higher in shoreline samples than offshore samples, with the difference being nearly three-fold under dry conditions and only 25% higher under wet conditions. Thus, shoreline sampling is protective of surfers exposed to waters farther offshore.

• For less than 1% of total samples (one sample pair in dry weather and three sample pairs during wet weather), shoreline samples met water quality standards while a corresponding offshore sample failed the standard.

Partners

Bight '03 Shoreline Microbiology participants