Monitoring bathing beach water quality using composite sampling

Abstract

Michigan water quality standards for public bathing beaches require local health departments to collect and analyze a minimum of three water samples for Escherichia coli during each sampling event. The geometric mean number of E. coli colonies is then compared to the 300 colonies per 100 ml standard to determine compliance. This article compares the results of the currently mandated procedure to a composite sampling method, whereby the three samples are mixed in equal volumes and analyzed once. This effectively replaces the geometric mean of the individual sample results with an arithmetic mean. Although arithmetic means are more affected by outliers, this sensitivity to high concentrations is more health conservative than the geometric mean. During the 2007 sampling season, nine bathing beaches were monitored once each week. Three individual point samples and a composite sample were analyzed for each sampling event. No statistically significant differences in bacteria concentrations were found between composite sample analysis and the arithmetic mean of individual point sample analyses. No violations were detected in the 2007 sampling season, so using historical data, a retrospective analysis was performed on samples gathered at nine bathing beaches in Kalamazoo County, Michigan during the years 2001-2007. The arithmetic mean of the three samples taken at each site served as a surrogate composite sample. The benefits of compositing the three samples were investigated assuming a 2/3 reduction in analytical costs. In the traditional sampling method, three individual samples were obtained and analyzed once in every 3-week period during the summer season, whereas compositing was simulated by taking the arithmetic mean of each week's results. The results of this retrospective cost analysis indicates that ten to 14 violations would have been missed using the less frequent traditional sampling and analysis methodology. Composite sampling is a cost-saving alternative to traditional sampling techniques that can be more protective of public health, particularly when the savings are applied to increased numbers of samples in time or space.

Optimal strategies for monitoring irrigation water quality

“…Composite sampling can significantly reduce the analytical costs by reducing the number of samples while increasing the likelihood of capturing the high variation inherent to microbe concentrations in water that would likely be missed with a single sample. Previous surface water studies have shown that composite samples provide better protection of human exposure to elevated bacteria concentrations compared to a single sample (Reicherts and Emerson, 2010;. However, even using composite samples, a sampling strategy requiring five sampling events per year will not adequately protect produce safety considering the high variability potential of bacteria in water (Boehm, 2007;Verhougstraete and Rose, 2014).…”

“…Thus, a single sample may not provide an adequate representation of the true microbial water quality in an irrigation water canal. Water quality scientists have also noted the implications of a single sample versus multiple samples for management actions (e.g., opening or closing of a beach) (Bertke, 2007;Reicherts and Emerson, 2010). Kinzelman et al (2006) determined that compositing multiple lake water samples and assaying with a single test was not statistically different (P > 0.02) than analyzing multiple individual samples and reporting an average; both approaches called for similar management actions.…”

An experimental comparison of composite and grab sampling of stream water for metagenetic analysis of environmental DNA

“…Alternatively, analysis samples can be aggregated from numerous smaller volumes or aliquots (“composite” samples) collected over spatial or temporal intervals. The benefits and limitations of composite sampling have been examined in a number of microbiological applications (Jarvis, 2007; Reicherts & Emerson, 2010) that seek reduced cost or increased representativeness of the data.…”

Comparison of the Multiple-Sample Means with Composite Sample Results for Fecal Indicator Bacteria by Quantitative PCR and Culture

“…In enumeration of FIB by culture-based methods in water samples, results from composite samples have been shown to be significantly correlated to the arithmetic means of single-sample results and to yield similar beach closure decisions (1,6,10). Moreover, analysis of a single composite sample has been estimated to be nearly 40% less expensive than multiple single-sample culture analyses (10). Recent studies have indicated that qPCR-based measurements of FIB are statistically associated with gastrointestinal illness among swimmers (15,16,17), and composite sampling could generate significant savings for water quality monitoring programs using qPCR.…”

“…Compositing involves mixing equal volumes of spatially discrete samples and analyzing the homogenate. In enumeration of FIB by culture-based methods in water samples, results from composite samples have been shown to be significantly correlated to the arithmetic means of single-sample results and to yield similar beach closure decisions (1,6,10). Moreover, analysis of a single composite sample has been estimated to be nearly 40% less expensive than multiple single-sample culture analyses (10).…”