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Procedures for Detection and Quantitation
Technical Support Document for Water Quality-Based Toxics Control
States and Regions, with guidance on procedures for use in water quality-based control of toxic pollutants
Document Type: Policy and Guidance Documents
Date Published: 03/01/1991
The full document is available (PDf, 26,600 kB).
The text the advisory committee is probably most interested in is the discussion in Section 5.7.3 starting on page 111 under the heading "Detection Level Limits." The excerpt of that discussion follows.
Detection Level Limits
A commonly encountered problem is the expression of calculated limits for specific chemicals where the concentration of the limit is below the analytical detection level for the pollutant of concern. This is particularly true for pollutants that are toxic in extremely low concentrations or that bioaccumulate.
The recommended approach for these situations is to include in Part 1 of the permit the appropriate permit limit derived from the water quality model and the WLA for the parameter of concern, regardless of the proximity of the limit to the analytical detection level. The limit also should contain an accompanying requirement indicating the specific analytical method that should be used for purposes of compli- ance monitoring. The requirement should indicate that any sample is analyzed in accordance with the specified method and found to be below the compliance level will be deemed to be in compliance with the permit limit unless other monitoring information (as discussed below) indicates a violation. Sample results reported at or above the compliance level should be reported as observed whereas samples below the compliance level should be reported as less than this level.
The level of compliance cited in the permit must be clearly defined and quantified. For most NPDES permitting situa- tions, EPA recommends that the compliance level be de- fined in the permit as the minimum level (ML). The ML is the level at which the entire analytical system gives recog- nizable mass spectra and acceptable calibration points. This level corresponds to the lowest point at which the calibra- tion curve is determined based on analyses for the pollutant of concern in a reagent water. The ML has been applied in determinations of pollutant measurements by gas chromatog- raphy combined with mass spectrometry. The concept of a minimum level recently was used in developing the Organic Chemicals, Plastics, and Synthetic Fibers effluent guidelines.
The minimum level is not equivalent to the method detection level, which is defined in 40 CFR Part 136 Appendix 6 as the minimum concentration of a substance that can be measured and reported with 99-percent confidence that the analyte concentration is greater than zero and is determined from the analysis of a sample in a given matrix containing the analyte. EPA is not recommending use of the method detection level because quantitation at the method detection level is not as precise as at the ML. It is not similar to the practical quantitation limit (PQL), which is typically set as a specific (and sometimes arbitrary) multiple of the method detection level. Because the PQL has no one definition, EPA is not recommending its use in NPDES permit- ting. Nor is it similar to other terms such as the limit of detection, limit of quantitation, estimated quantitation limit, or instrument detection limit.
The permitting authority may choose to specify another level at which compliance determinations are made. Where the permit- ting authority so chooses, the authority must be assured that the level is quantifiable, defensible, and close as possible to the permit level.
Where water quality-based limits below analytical detection levels are placed in permits, EPA recommends that special conditions also be included in the permit to help ensure that the limits are being met and that excursions above water quality standards are not occurring. Examples of such special conditions include fish tissue collection and analyses, limits and/or monitoring requirements on internal waste streams, and limits and/or monitoring for surrogate parameters. This information can be used to help support reopening the permit to establish more stringent effluent limits if necessary.