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Water: Mercury

Method 1631: Measurement of Mercury in Water; Revision B

Note: EPA no longer updates this information, but it may be useful as a reference or resource.

Fact Sheet; May 1999

EPA announces publication of a final rule approving the use of EPA Method 1631, Revision B, for the determination of mercury in EPA's wastewater program (40 CFR part 136). Approval of this method supports EPA's efforts to make available an additional analytical method capable of measuring mercury accurately at ambient water quality criteria levels.


The U.S. Environmental Protection Agency (EPA) publishes analytical testing methods that are used by industrial and municipal facilities to analyze chemical and biological components of wastewater, drinking water, sediment, and other environmental samples (for the purpose of data gathering and compliance monitoring under the Clean Water Act and the Safe Drinking Water Act).

Purpose of Final Rule for Method 1631

EPA proposed approval of Method 1631 on May 26, 1998 (63 FR 28867) and published a Notice of Data Availability (NODA) on March 5, 1999 (64 FR 10596). The NODA includes additional effluent data supporting the use of EPA Method 1631, Revision B, for effluent samples. In the final rule, EPA is formally approving Method 1631, Revision B, at 40 CFR Part 136.

This action provides an additional testing procedure for compliance monitoring under the NPDES program (CWA section 402); ambient water quality monitoring; CWA section 401 certifications; development of effluent limitations guidelines, pretreatment standards and new source performance standards in EPA's water programs; and general laboratory use.

This rulemaking does not repeal any of the currently approved test methods for mercury. Each permitting authority must decide which method is appropriate for its needs.

Highlights of Method 1631

Method 1631 allows determination of mercury at a minimum level (ML) of 0.5 ng/L (parts-per-trillion; ppt), approximately 400 times lower than the level achieved by other mercury methods previously approved at 40 CFR Part 136. Determination of mercury at low ppt levels is necessary to support measurements at the ambient water quality criteria for mercury published in the National Toxics Rule as 12 ppt (40 CFR 131.36) and in the Final Water Quality Guidance for the Great Lakes System as 1.3 ppt (60 FR 15366).

Benefits of Method 1631, Revision B
  • Allows determination of mercury at 0.5 ng/L
  • Improved accuracy and precision at low levels
  • Supports measurements at ambient water quality criteria levels
  • Performance-based
  • Includes standardized QC

Components of Method 1631, Revision B

Method 1631, Revision B, has four components:

  1. sample pretreatment,
  2. purge and trap,
  3. desorption, and
  4. detection of mercury by atomic fluorescence.

In the sample pretreatment step, bromine monochloride (BrCl) is added to the sample to oxidize all forms of mercury to Hg(ll). After oxidation, the sample is sequentially prereduced with NH2OH HCl to destroy free halogens, then reduced with SnCl2 to convert Hg(ll) to volatile Hg(0). The Hg(0) is purged from the aqueous solution onto a gold-coated sand trap. The trapped mercury is thermally desorbed from the gold trap into a flowing gas stream into the cell of a cold-vapor atomic fluorescence spectrometer. Quality is assured through calibration and testing of the oxidation, purging, and detection systems.

Method 1631, Revision B, contains QC acceptance criteria for all standardized QC tests. Compliance with these criteria will allow the data user to evaluate the quality of the results. This requirement increases the reliability of results produced and provides a means for laboratories and data users to monitor analytical performance, thereby providing a basis for sound, defensible data.

Changes Since Proposal

The Agency revised the proposed version of Method 1631 based on public comments. The following modifications are included in Method 1631, Revision B.

Changes included in Method 1631, Revision B
  • A change in sample holding time from 6 months to 28 days.
  • A change in MS/MSD performance criteria for recovery from 75-125% to 71-125%.
  • A change resulting in the removal of the option to report laboratory-determined MDLs and MLs.

Performance-based Features of Method 1631

Since Method 1631, Revision B, is performance-based, it encourages advances in analytical technology and may reduce the cost of analysis. The analyst is permitted to modify the method to overcome interferences or lower the cost of measurements, provided that all method equivalency and performance criteria are met. This performance-based approach is consistent with EPA's streamlining proposal (March 28, 1997; 62 FR 14976) and the Agency's performance-based measurement system policy (October 6, 1997; 62 FR 52098).

Additional Information and Copies

For additional information concerning this action you may contact the U.S. Environmental Protection Agency, Mail Code 4303T, 1200 Pennsylvania Avenue NW, Washington, DC 20460.

The Federal Register notice contains instructions on how to obtain additional information and how to review the public record for this rulemaking. The complete text of the Federal Register notice containing Method 1631, Revision B (June 08, 1999) is available.

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