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Clean Water Act Analytical Methods

2017 Methods Update Rule - Frequent Questions

Methods 608.3, 624.1 and 625.1

  1. All three methods contain the statement: “Results associated with a QC failure cannot be used to demonstrate regulatory compliance.” This appears to indicate no qualified data can be reported. Is this always the case?

    It is not acceptable to use data associated with failed QC tests to demonstrate compliance with a regulatory limit when no corrective action has been pursued, (e.g., cleanup procedures or method modifications). It is ultimately the responsibility of the permittee to report results for compliance monitoring required under its National Pollutant Discharge Elimination System (NPDES) permit. Permittees who choose to contract laboratories to perform the testing required for reporting results to demonstrate regulatory compliance should work closely with those laboratories to ensure that sufficient sample volumes for required QC testing, and re-analyses when necessary, are provided.

    EPA has provided consistent guidance on this issue for the NPDES compliance methods since the methods were first promulgated at 40 CFR Part 136 in 1984. For example, Section 8.1.7 of each of the methods states that the large numbers of analytes tested in performance tests in the methods present a substantial probability that one or more will fail acceptance criteria when many analytes are tested simultaneously. The methods allow for re-testing if this situation should occur. The methods also say that if continued re-testing results in further repeated failures, the laboratory is to document and report the failures with the data (data qualifiers may be used). In many cases, corrective action can be taken by the laboratory to correct the problem or solve the problem through re-testing. Laboratories should communicate with permittees who contract their services in a timely manner to inform them when corrective action or re-testing is necessary. If the holding time for a sample would be exceeded due to re-analysis of the sample, the permittee should consult with the regulatory/control authority for disposition.

    EPA’s 2007 guidance document should be consulted in cases where matrix interferences are observed. Please let your regulatory or permitting authority know about modifications that you have made that have worked, or not worked with difficult matrices. Reporting to the permitting authority that “the sample couldn't be analyzed” is not sufficient and will not be accepted as justification for a claim of matrix interference. The document, originally published in 1993 and known as the “Pumpkin Book,” was revised in 2007 and provides many examples of how to overcome quality control issues.
  2. Why does a continuing calibration verification sample need to use a second source standard?

    Methods 608.3, 624.1 and 625.1 do not distinguish between the initial and continuing calibration verification standards. The methods require verification of the working calibration curve or response factor (RF) (or the calibration factor [CF] in Method 608.3) immediately following calibration of the instrument. This initial calibration verification (ICV) must be performed using a standard (or standards) obtained from a source other than the source from which the calibration standards were obtained. Otherwise, the analysis of the verification standard would just show that the laboratory can get the same results for the same solution analyzed twice, which is not the intent of the calibration verification process.

    The methods also require subsequent verifications of the working calibration curve or RF (or the CF in Method 608.3) at the beginning of each 12-hour shift for Methods 624.1 and 625.1, or at the beginning and end of each 24-hour shift for Method 608.3. Method 608.3 also allows for an alternative approach in which calibration verification may be performed after a set number of injections (e.g., every 20 injections), to include injection of extracts of field samples, QC samples, instrument blanks, etc. (i.e., it is based on the number of injections performed, not sample extracts), provided that the time for the injections does not exceed 24 hours.

    It is acceptable for these subsequent verifications of the working calibration curve or RF (or the CF in Method 608.3), sometimes referred to as continuing calibration verification (CCV), to be performed using standards obtained from the same source as the calibration standards, as long as the ICV is from a second source. Alternatively, the ICV and the CCV can be from the same second source (e.g., not the same source as the multi-point calibration standards).

    The acceptance criteria for both ICV and CCV are the same. The most important distinction is that a second-source calibration verification standard (or standards) must be run immediately after each initial calibration to confirm that the instrument was calibrated correctly. The second-source standard may be obtained from a second manufacturer or a manufacturer’s batch (lot number) prepared independently from the batch (lot number) used for calibration.
  3. The QC check sample concentrate for Methods 608.3 (Section 6.8.3) and 625.1 (Section 8.2.1) must be prepared independently from the standards used for calibration. Does this mean that the QC sample concentrate must come from a different source than the calibration standards?

    The calibration standards are prepared in a solvent that is not miscible with water, whereas the spiking solution (QC sample concentrate) used for the LCS and the MS/MSD samples must be prepared in a water miscible solvent. The two solutions must be different (i.e., prepared independently), but they may be prepared from the same neat materials.
  4. The acceptance criteria for the calibration verification check are very wide for some compounds and can lead to poor quality data. Why are no fixed limits established?

    These acceptance limits were calculated from inter-laboratory validation studies that were conducted when the methods were first developed. EPA has no interlaboratory data to support changing these criteria at this time. However, a laboratory may establish internal limits that are more stringent than the limits included in the methods.
  5. Does the laboratory need to achieve the MDL and ML values published in the method? What if the permit limit is significantly higher than the published ML?

    This is a common question that is asked about all Clean Water Act methods that support NPDES permits. The NPDES rule, "Use of Sufficiently Sensitive Test Methods for Permit Applications and Reporting” (August 19, 2014) discusses this issue in detail. (Additional supporting documents are available in the docket at regulations.gov, docket no. EPA-HQ-OW-2009-1019.) The language in Methods 608.3, 624.1, and 625.1 is consistent with this rule. For the sake of brevity, only Method 608.3 is used as an example below. The other methods have nearly identical language.

    Section 1.6 of the method states “The MDL for an analyte in a specific wastewater may differ from those listed, depending upon the nature of interferences in the sample matrix.” Furthermore, EPA's 2007 guidance notes that MDLs are estimates based on performance studies, and the method may not achieve all of the stated performance characteristics in all possible sample matrices.

    Section 8.2 of the method states: “The laboratory must also establish MDLs for the analytes of interest using the MDL procedure at 40 CFR part 136, appendix B. The laboratory's MDLs must be equal to or lower than those listed in Tables 1 or 2, or lower than one-third the regulatory compliance limit, whichever is greater.”

    Therefore, EPA explicitly addresses the need to meet the method specific MDLs and MLs in all three methods.
  6. The published ML for Aroclor 1242 in Table 2 of Method 608.3 is 95. This is the only analyte where the ML is not three times the MDL. The MDL is 65, so should the ML be 195?

    Yes, the ML for Aroclor 1242 in Table 2 of Method 608.3 should be 195. This is a typographical error. If you need further written clarification, please request it using the “Contact Us” link below.