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Clean Water Act Analytical Methods

Hexavalent Chromium: Questions and Answers

Hierarchy of holding times between footnotes to Table II and in approved methods

Question: Table IB (page 11218) lists the EPA Method for chromium-VI dissolved as 218.6, Rev. 3.3 (1994). In Table II (page 11236) the maximum holding time for chromium-VI is listed as 28 days with a reference to footnote 20. 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. However, Section 1.4 of the same method states [that samples must be] "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?

Response: Footnote 20 states: "To achieve the 28-day holding time, use the ammonium sulfate buffer solution specified in EPA Method 218.6. 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."

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 footnote applies to all approved methods (e.g., Standard Methods, ASTM methods).

In addition, footnote 4 to Table II states, in part: "Samples should be analyzed as soon as possible after collection. 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)... "Some samples may not be stable for the maximum time period given in the table. 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. See §136.3(e) for details... " 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.

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Interpretation of holding time requirement

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. 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. analyze within 24 hours) then unpreserved is still acceptable?

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. If desired, the sample may be held for no more than 24 hours unpreserved, as specified in the approved methods. Also, as stated in footnote 4 to Table II, "... 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... "

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Disparity between holding times for drinking water and wastewater

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. What this means is that drinking water and wastewater are regulated using the same methods but the holding times are different.

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.

Regarding the disparity between requirements for chromium 6 for drinking water and wastewater, EPA received a request and data supporting a change of holding time to 28 days for chromium-VI in wastewater, proposed the change in the Federal Register on April 6, 2004 (69 FR 18165), and approved the change on March 12, 2007 (72 FR 11199). EPA did not receive a request and data supporting a change of holding time for drinking water and, therefore, did not change the holding time for drinking water.

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. 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. When an EPA method is revised, EPA attempts to correct the preservation and holding time to be consistent with Table II.

Note: Chromium-VI is not regulated in drinking water at the federal level. Method 218.6 is not an approved drinking water method (i.e., it isn't listed at 40 CFR 141.23).

Some states require monitoring for chromium-VI in drinking water. In those cases the Primacy Agencies will need to determine how to handle the holding time difference between the method and Table II at Part 136.

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Holding time for colorimetric methods

Question: We use Standard Methods 3500-Cr D, which is a colorimetric method, and are not sure if our method is compatible with the change in preservation requirement in Table II. We assume that it is compatible because the method we use is still approved for NPDES and there is no alternative for Cr6+ in Table II. We have done some preliminary work using the buffer with our method and what immediately stands out is a shift in the curve. Is there any supporting information about using the buffer from Method 218.6 with other Cr6+ methods?

Response: At present, there is no supporting information about using the buffer with methods other than ion chromatography Method 218.6. We suggest to test this modification prepare standard solutions using the buffer, and produce a calibration curve with the buffered solutions. If that does not work, do not use the buffer and return to requirements in the method (24-hour holding time, unpreserved), as provided for in footnote 20 to Table II.

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Information supporting holding time change

Question: Where can I get a copy of the information and/or data used to make the holding time change for hexavalent chromium?

Response: The information was provided in the docket for the March 12, 2007 final rule. Available at, docket number EPA-HQ-OW-2003-0070.
Sample filtration

Question: There is a requirement in EPA Method 218.6 to filter the sample prior to adding the buffer. However, the Methods Update Rule (MUR) makes no mention of filtration when preserving the sample. It does however, say that the "new" requirements supersede the method requirements. Is filtration required prior to preservation for EPA Method 218.6 to enable the 28 day holding time?

Response: Yes, filtration is required immediately (within 15 minutes of collection) to prevent interconversion of chromium species. The suppression requirement applies to preservation and holding time only.

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Correct pH range

Question: What about the disparity in the pH range specified in footnote 20 in Table II at 40 CFR 136.3, and in EPA Method 218.6?

Response: The correct pH range is 9.3–9.7 as specified in footnote 20, and must be met when the holding time is extended by use of either sodium hydroxide (NaOH), or the buffer solution in Section 7.9 of the method. We suggest that you use the buffer described in Section 7 of EPA Method 218.6, if the pH range can be achieved with sample dilution by no more than 10 percent. If this pH range cannot be achieved without diluting the sample by more than 10 percent, use a minimum amount of NaOH solution, supplemented by buffer solution as necessary.

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