Updated Requirements from NELAC and
US EPA on TTHMs and Arsenic

Article written by Wade DeLong, Lab Manager, Analytical Products Group

Recent Updates to the Stage 2 Disinfection Byproduct Rule

The Federal Register dated January 4, 2006 details the Final Rule of 40CFR Parts 9, 141 and 142 the “National Primary Drinking Water Regulations: Stage 2 Disinfectants and Disinfection Byproducts Rule”. http://www.epa.gov/fedrgstr/EPA-WATER/2006/January/Day-04/

The rule is effective on March 6, 2006 and is promulgated as of January 4, 2006. This update will change, among other things, the manner in which proficiency testing (PT) samples are evaluated. The changes could potentially make passing a PT study more difficult for the affected parameters. The acceptance range calculation used by PT providers will be changing from using interlaboratory statistics to fixed percent limits. The individual trihalomethane (THM) compounds will be evaluated by an acceptance range of the true value ± 20 percent. The Total Trihalomethane (TTHM) acceptance criteria will still be based on successfully meeting all four individual THM compounds acceptance limits.

The individual Haloacetic Acid (HAA) compounds will be evaluated using an acceptance range calculated using the true value ± 40 percent. Chlorite and Bromate acceptance ranges will be calculated using the true value ± 30 percent. Table IV.0-1 on page 52 in the link listed above (use the pdf view) provides a consolidated list of the PT (a.k.a. PE) acceptance criteria. There are some important dates within the final rule document, page 438 (CFR page number) “The new acceptance limits are effective 60 days after promulgation. Laboratories that were certified under the Stage 1 DBPR PE acceptance criteria will be subject to the new criteria when it is time for them to analyze their annual DBP PE sample(s)” Then on page 480 …the laboratory must: … “(ii) Until March 31, 2007, in these analyses of PE samples, the laboratory must achieve quantitative results within the acceptance limit on a minimum of 80% of the analytes included in each PE sample. The acceptance limit is defined as the 95% confidence interval calculated around the mean of the PE study between a maximum and minimum acceptance limit of +/- 50% and +/- 15% of the study mean. (iii)Beginning April 1, 2007 the laboratory must achieve quantitative results on the PE sample analyses that are within the following acceptance limits:” which refers to a table detailing the fixed percent limits.

APG’s study schedule is such that the fixed percent limit acceptance criteria will be used beginning in the March 2007 WS study. In the WS studies beginning March 2006 through January 2007 we will provide a sample evaluation using the fixed percent limit acceptance criteria. We hope this will help our laboratories ease through this transition by identifying any areas of concern before the change in acceptance criteria affects certification requirements.