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Eect of Sampling Frequency on Shoreline Microbiology Assessments MOLLY K. LEECASTER 1 , STEPHEN B. WEISBERG * Southern California Coastal Water, Research Project, 7171 Fenwick Lane, Westminster, CA 92683, USA More than 80,000 shoreline bacteriological samples are collected annually in southern California to protect beachgoer health, but sampling frequency varies from daily to monthly among sampling sites. To assess the ef- fectiveness of various sampling frequencies, we used ®ve years of data from 24 Los Angeles area sites that have been monitored daily to simulate ®ve alternative sampling strategies: ®ve weekdays, ®ve days per week including a weekend day, three days per week, weekly, and monthly. For each of these sampling strategies, we included in the simulation the local custom of adaptive sampling, in which a site is resampled the following day if bacterial concen- trations exceed the State of California's beach water quality standards. We found that sampling ®ve times per week resulted in observing about 80% of the events in which State standards were exceeded. This frequency dropped to 55%, 25%, and 5% for three times per week, weekly, and monthly sampling, respectively. Adaptive sampling was ineective because nearly 70% of the water quality exceedences were single-day events, even at the most frequently contaminated sites. This high frequency of single-day events is of concern because the public is typi- cally noti®ed about water quality conditions 24±48 h after samples are collected, meaning that most warnings are out-of-date when they are issued. Ó 2001 Elsevier Sci- ence Ltd. All rights reserved. Introduction Water quality is measured at beaches throughout the country to protect swimmers from the potential in- ¯uence of human activity, such as surface runo, wastewater discharge, and industrial input. Although there are federal initiatives to standardize these eorts, most of the programs are presently conducted inde- pendently by numerous county health departments, resulting in inconsistencies in program implementa- tion. A review of southern California beach monitor- ing programs found dierences in the number of indicators measured, measurement methods used, and frequency of monitoring Schi et al., 2001a). A sur- vey of beach monitoring throughout the country USEPA, 2000) found similar kinds of programmatic dierences. One of the greatest dierences among programs is sampling frequency. Most monitoring is conducted on a weekly to monthly basis, although no studies have been conducted to evaluate the eectiveness of these dierent temporal strategies. Twenty-four beach sites in Los Angeles County are monitored daily, providing a un- ique opportunity to assess the eectiveness of less fre- quent sampling strategies. Here we sample from those data to assess the extent to which public health is pro- tected using ®ve lesser measurement frequencies pres- ently being employed at other beaches in southern California. Methods Five years of data 1 January 1995±31 December 1999) from 24 Los Angeles County beach sites that are sampled seven days per week were used in the analysis. Eleven of these sites are located on coastal beaches 50 yards from freshwater outlets that drain urban runo, eleven are located on coastal beaches away from urban drains, and two are on a protected beach located inside of the Los Angeles Harbor breakwater. Total coliforms were measured using membrane ®ltration APHA, 1995) at each of these sites, while fecal coli- forms were measured using membrane ®ltration at 20 of the sites. We simulated four sampling frequencies: ®ve days per week, three days per week, weekly, and monthly. The ®ve days per week simulation was conducted using two dierent selectors: 1) all weekdays and 2) Monday± Thursday plus Saturday. The three-days-per-week sim- ulation was conducted using Monday, Wednesday, and Friday as sampling days. The once-per-week sample was taken on Thursday and the monthly sample was taken on the ®rst day of every month. The sampling days were chosen to mimic sampling strategies employed in southern California, although examination of the data revealed little dierence among the day of the week in the frequency of bacterial standard exceedences. In each of these simulations, we mimicked the local custom of Marine Pollution Bulletin Vol. 42, No. 11, pp. 1150±1154, 2001 Ó 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0025-326X/01 $ - see front matter PII: S0025-326X01)00130-8 1150 * Corresponding author. 1 Present Address: IDAHO National Engineering and Environmen- tal Labs, P.O. Box 1625 IDAHO Falls, ID 83415, USA.

Effect of Sampling Frequency on Shoreline Microbiology Assessments

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