True and false positive rates in maximum contaminant level tests

Area of Science:

• Environmental Science
• Microbiology
• Public Health

Background:

• The U.S. EPA is revising drinking water regulations for total coliform bacteria.
• Current regulations lack quantification of coliform levels that exceed maximum contaminant levels (MCLs).
• Existing interim rules (NIPDWR) show significant variability in stringency and detection capabilities.

Purpose of the Study:

• To develop a method for evaluating the coliform level an MCL detects with a specific likelihood.
• To quantify the stringency of MCLs based on error rates (Type I and Type II).
• To assess EPA's proposed monthly MCL against a reformulated "reasonably safe" standard.

Main Methods:

• Treating MCLs as decision rules with defined Type I and Type II error rates.
• Quantifying MCL stringency by the mean coliform level detected with a given likelihood.
• Comparing stringency of interim and proposed MCLs using empirical data and statistical models.

Main Results:

• Interim rules (NIPDWR) exhibit wide variations in stringency.
• The proposed monthly MCL may not adequately protect smaller water systems.
• Empirical data challenges the assumption of identically distributed coliform levels, questioning the utility of monthly MCLs.

Conclusions:

• A new method quantifies MCL stringency, revealing inconsistencies in current and proposed regulations.
• The proposed monthly MCL may offer less protection for smaller systems.
• An alternative single-sample MCL with repeat sampling is proposed for more effective and cost-efficient monitoring.