Some states require monitoring for chromium-VI in drinking water. Method 218.6 is not an approved drinking water method (i.e., it isn't listed at 40 CFR 141.23). Note: Chromium-VI is not regulated in drinking water at the federal level. When an EPA method is revised, EPA attempts to correct the preservation and holding time to be consistent with Table II. Also, when information becomes available about an improvement in a requirement for a holding time, it is more straightforward to change Table II than to revise all approved methods. Regarding disparities between holding times in Table II at Part 136 and in approved methods, holding time requirements in Table II apply to all approved methods, not just to EPA methods. Response: There may be disparities between requirements for drinking water and wastewater and disparities between requirements in the holding time table at 40 CFR Part 136 and in approved methods especially in microbiology methods because wastewater samples often differ significantly from drinking water samples. What this means is that drinking water and wastewater are regulated using the same methods but the holding times are different. Question: EPA Method 218.6 for drinking water requires a 24-hour holding time, whereas footnote 20 to Table II at Part 136 for wastewater allows 28 days when properly preserved, and the footnote supersedes the methods, so the footnote is applicable to Standard Methods also. " Disparity between holding times for drinking water and wastewater A permittee or monitoring laboratory is obligated to hold the sample for a shorter time if it knows that a shorter time is necessary to maintain sample stability. Also, as stated in footnote 4 to Table II, ". If desired, the sample may be held for no more than 24 hours unpreserved, as specified in the approved methods. Response: Footnote 20 allows the holding time to be extended to 28 days at the option of the permittee, discharger, or monitoring laboratory, provided the sample is preserved as specified in the footnote. analyze within 24 hours) then unpreserved is still acceptable? unable to analyze unpreserved within 24 hours)? Or does it mean that chemical preservation is an option available in order to achieve a longer hold time but if you do not apply the 28-day holding time (i.e. Question: Does footnote 20 to Table II mean that chemical preservation is the only option available and the resulting hold time is 28 days (i.e. Interpretation of holding time requirement ![]() " Therefore, if it is known that the chromium-6 concentration would change in less time than the 28 days allowed by footnote 20, or in less time than allowed in the approved method (e.g., 24 hours in Method 218.6), the sample must be held no longer than the time necessary to assure that the chromium-VI concentration does not change. "Some samples may not be stable for the maximum time period given in the table. The times listed are the maximum times that samples may be held before the start of analysis and still be considered valid (e.g., samples analyzed for fecal coliforms may be held up to 6 hours prior to commencing analysis). In addition, footnote 4 to Table II states, in part: "Samples should be analyzed as soon as possible after collection. ![]() The footnote applies to all approved methods (e.g., Standard Methods, ASTM methods). This footnote supersedes the holding time in the approved methods, unless the longer holding time would compromise measurement of chromium 6, in which case the holding time in the method must be used. The allowance in this footnote supersedes preservation and holding time requirements in the approved hexavalent chromium methods, unless this supersession would compromise the measurement, in which case requirements in the method must be followed." Response: Footnote 20 states: "To achieve the 28-day holding time, use the ammonium sulfate buffer solution specified in EPA Method 218.6. However, Section 1.4 of the same method states "analyzed within 24 hours of collection." The question is, which holding time is correct for Method 218.6? And, with respect to the non-EPA methods for chromium-VI listed in Table IB, is it also possible to have a 28 day holding time for these methods by using ammonium sulfate buffer as a preservative at the time of sample collection? Footnote 20 (page 11239) refers to the use of the ammonium sulfate buffer solution described in the method at Sections 7.9 and 8.2. ![]() In Table II (page 11236) the maximum holding time for chromium-VI is listed as 28 days with a reference to footnote 20. Question: Table IB (page 11218) lists the EPA Method for chromium-VI dissolved as 218.6, Rev. Hierarchy of holding times between footnotes to Table II and in approved methods
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