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Notice of Availability, “Understanding and Accounting for Method Variability in WET Applications Under the NPDES Program”

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Information about this document as published in the Federal Register.

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Environmental Protection Agency (EPA).


Notice of availability of document.


On June 30, 2000, EPA issued the final document, entitled “Understanding and Accounting for Method Variability in Whole Effluent Toxicity (WET) Applications Under the NPDES Program” in response to questions on WET test method variability. WET applications are implemented under the National Pollutant Discharge Elimination System (NPDES) Program.


Final document issued June 30, 2000.


A copy of the final document and supporting documents including the public comments received by EPA on the July 26, 1999 draft document are available for review at the EPA's Water Docket, Room EB57, 401 M Street, S.W., Washington, D.C. 20460. For access the Docket materials, call (202) 260-3027 between 9 a.m. and 3:30 p.m. Eastern Time for an appointment.

The complete text of this Federal Register notice and “Understanding and Accounting for Method Variability in Whole Effluent Toxicity (WET) Applications Under the NPDES Program” may be viewed or downloaded on the Internet at​owm/​npdes.htm.

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For technical questions on this document, contact Debra Denton, (415-744-1919) or Laura Phillips (202-260-9522), Water Permits Division, (4203), USEPA, Office of Wastewater Management, 1200 Pennsylvania Avenue, N.W., Washington, D.C. 20460. Copies of the document may be requested from the Office of Water's Resource Center at (202-260-1827) or by contacting the National Center for Environmental Publications and Information (NCEPI) at (513-489-8190).

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The Whole Effluent Toxicity (WET) approach to protection of water quality is the focus of this document. In 1989, EPA defined whole effluent toxicity as “the aggregate toxic effect of an effluent measured directly by an aquatic toxicity test.” At the same time, EPA promulgated regulations requiring NPDES permit limitations for WET under certain circumstances. [54 FR 23868 at 23895, June 2, 1989]. Aquatic toxicity tests are laboratory experiments that measure the biological effect (e.g., growth, survival, and reproduction) of effluents or receiving waters on aquatic organisms. In aquatic toxicity tests, groups of organisms of a particular species are held in test chambers and exposed to different concentrations of an aqueous test sample, for example, a reference toxicant, an effluent, or a receiving water. Observations are made at predetermined exposure periods. At the end of the test, the responses of test organisms are used to estimate the effects of the toxicant or effluent. In the early 1980s, EPA published methods (USEPA 1985, 1988, 1989) for estimating the short-term acute and chronic toxicity of effluents and receiving waters to freshwater and marine organisms.

Effect of This Document

EPA is providing this document to clarify several issues regarding WET variability and reaffirm EPA's earlier guidance and recommendations published in the Technical Support Document for Water Quality-Based Toxics Control (TSD, USEPA 1991). Today's document is intended to provide NPDES regulatory authorities and all stakeholders, including permittees, with guidance and recommendations on how to understand and account for measurement variability in WET testing.

Three Goals of Today's Document

Today's document describes three goals EPA has defined to address issues surrounding WET variability. In addition, the document is intended to satisfy the requirements of a settlement agreement to resolve litigation over rulemaking to standardize WET testing procedures. These three goals are:

1. To quantify the variability of the promulgated test methods and report a coefficient of variation (CV) as a measure of test method variability.

2. To evaluate the statistical methods described in the Technical Support Document for Water Quality-Based Toxics Control (TSD) for determining the need for and deriving WET permit conditions.

3. To suggest guidance for regulatory authorities on approaches to address and to minimize test method variability. In addition, the document is intended to provide guidance to regulatory authorities, permittees, and WET testing laboratories on conducting the biological and statistical methods and evaluating test effect concentrations.

Principal Conclusions

The principal conclusions of this document in response to the three document goals follow.

Evaluation of Test Method Variability

  • Comparisons of WET method precision with method precision for analytes commonly limited in NPDES permits demonstrate that the variability of the promulgated WET methods is within the range of variability experienced in other types of analyses. Several researchers also noted that method performance improves when prescribed methods are followed closely by experienced analysts.
  • The document provides interim CVs for promulgated WET methods in Appendix A of the final document pending completion of between-laboratory studies, which may affect these interim CV estimates.

Evaluation of Approach To Incorporate Test Method Variability

  • EPA's Technical Support Document for Water Quality-based Toxics Control (TSD) presents guidance for developing effluent limits that appropriately protect water quality, regarding both effluent variability and analytical variability, provided that the WET criteria and waste load allocation (WLA) are derived correctly.
  • EPA's analysis of data gathered in the development of today's document indicates that the TSD approach appropriately accounts for both effluent variability and method variability. EPA does not accept that a reasonable alternative approach is available to determine a factor that would discount the effects of method variability in TSD procedures based on CVs because the approach would not assure adequate protection of water quality.

Development of Guidance to Regulatory Authorities

  • EPA recommends that NPDES permitting authorities implement the statistical approach as described in the Start Printed Page 44529TSD to evaluate effluent and to derive WET limits or monitoring triggers.
  • EPA recommends that NPDES permitting authorities calculate the facility-specific CVs using point estimate techniques to determine the need for and to derive a permit limit for WET, even if self-monitoring data are to be determined using hypothesis testing techniques, for example, to determine a “no effect concentration (NOEC)”. The document describes such facility-specific calculation procedures.

Additional Recommendations and Guidance

This document also provides recommendations and guidance on minimizing variability in three specific areas in order to generate sound WET test results: (1) Obtaining a representative effluent sample; (2) conducting the toxicity tests properly to generate the biological endpoints; and (3) conducting the appropriate statistical analysis to determine the effect concentrations (IC25, NOEC). If these recommendations are addressed, the reliability of the test endpoint values should improve.

  • Permitting authorities should design a sampling program that collects representative effluent samples to fully characterize effluent variability for a specific facility over time.
  • Permitting authorities should ensure proper application of WET statistical procedures and test methods.
  • EPA recommends that NPDES permitting authorities incorporate both the upper and lower bounds using the percent minimum significant difference (PMSD) to control and to minimize within-test method variability and increase test sensitivity. To achieve the PMSD upper bound, either the replication should increase or within-test method variability should decrease, or both.
  • EPA recommends that WET testing laboratories maintain control charts for PMSD and the control mean and report the PMSD with all WET test results.
  • NPDES permitting authorities should develop a quality control checklist to assist in evaluating and interpreting toxicity test results.
  • EPA recommends that permitting authorities and laboratories participate in the National Environment Laboratory Accreditation Program and conduct routine performance audit inspections to evaluate laboratory performance.
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Dated: July 12, 2000.

Michael B. Cook,

Director, Office of Wastewater Management.

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[FR Doc. 00-18102 Filed 7-17-00; 8:45 am]