Comparison and Verification of Bacterial Water Quality Indicator Measurement Methods Using Ambient Coastal Water Samples

John F. Griffith 1, Larissa A. Aumand 2, Ioannice M. Lee 3, Charles D. McGee 4, Laila Othman 5, Kerry J. Ritter 1, Kathy O. Walker 6 and Stephen B. Weisberg 1

1 Southern California Coastal Water Research Project (SCCWRP), Westminster, California
2 MEC Analytical Systems, Inc., Carlsbad, California
3 City of Los Angeles, Department of Sanitation, Los Angeles, California
4 Orange County Sanitation District, Fountain Valley, California
5 City of San Diego, Metropolitan Waste Water Department, San Diego, California
6 Los Angeles County Sanitation District, Whittier, California

More than 30 different groups routinely monitor water along southern California's beaches for bacterial indicators of fecal contamination. Data from these efforts are frequently combined and compared even though three different methods (membrane filtration (MF), multiple tube fermentation (MTF) and chromogenic substrate substrate (CS) methods) are used in these programs. To assess interchangeability of these data and quantify variability within method, 26 laboratories participated in an intercalibration exercise. Each laboratory processed three replicates from eight ambient water samples employing the method or methods they routinely use for water quality monitoring. Verification analyses were conducted also on a subset of wells from the CS analysis. Enterococci results were generally comparable across methods. There was a 9% false positive rate and a 4% false negative rate in the CS verifications, though these errors were small in context of within and among laboratory variability. Fecal coliforms were also comparable across all methods, though CS underestimated the other methods by about 10% because it measures only E. coli, rather than the larger fecal coliform group measured by MF and MTF. CS overestimated total coliforms relative to the other methods by several fold and was found to have a 40% false positive rate in verification.

Keywords: microbiology, intercalibration, variability, bacterial indicators, environmental samples