Occurrence of Aeromonas spp. in a random sample of drinking water distribution systems in the USA.

Aeromonads are aquatic bacteria found in drinking water supplies worldwide. Some species, such as Aeromonas hydrophila, can cause disease in humans. For this survey, 293 United States public water systems were selected using random sampling, stratified by water source and system type. Water samples were collected during one year from three sites (six samples per site) in each system. Temperature, pH, turbidity, total and free chlorine were measured using standard methods. Aeromonads were detected in 130 of 5,042 valid samples (2.6%) from 42 (14.3%) systems using the ampicillin-dextrin agar with vancomycin culture method with oxidase, trehalose and indole confirmation tests. Concentrations of aeromonads in positive samples were 0.2 to 880 (median 1.6) colony-forming units (CFU) per 100 mL. Adjusted odds ratios of Aeromonas detection were 1.6 (95% confidence limits 1.0, 2.5) during the summer season, 3.3 (1.8, 6.2) for turbidity above 0.5 nephelometric units and 9.1 (3.5, 24) at 0 mg/L compared with 0.25 mg/L total chlorine. Geographic region, system size and type of water source were not significant predictors of Aeromonas detection in multivariate regression analysis. The results of this survey demonstrate the importance of maintaining adequate residual chlorine and low turbidity for preventing drinking water contamination with aeromonads.

Statistical procedures for determination and verification of minimum reporting levels for drinking water methods.

The United States Environmental Protection Agency's Office of Ground Water and Drinking Water has developed a single-laboratory quantitation procedure: the lowest concentration minimum reporting level (LCMRL). The LCMRL is the lowest true concentration for which future recovery is predicted to fall, with high confidence (99%), between 50% and 150%. The procedure takes into account precision and accuracy. Multiple concentration replicates are processed through the entire analytical method and the data are plotted as measured sample concentration (y-axis) versus true concentration (x-axis). If the data support an assumption of constant variance over the concentration range, an ordinary least-squares regression line is drawn; otherwise, a variance-weighted least-squares regression is used. Prediction interval lines of 99% confidence are drawn about the regression. At the points where the prediction interval lines intersect with data quality objective lines of 50% and 150% recovery, lines are dropped to the x-axis. The higher of the two values is the LCMRL. The LCMRL procedure is flexible because the data quality objectives (50-150%) and the prediction interval confidence (99%) can be varied to suit program needs. The LCMRL determination is performed during method development only. A simpler procedure for verification of data quality objectives at a given minimum reporting level (MRL) is also presented. The verification procedure requires a single set of seven samples taken through the entire method procedure. If the calculated prediction interval is contained within data quality recovery limits (50-150%), the laboratory performance at the MRL is verified.