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City of San Marcos TPDES Permit No. WQOO 1 0273002 24-HOUR ACUTE BIOMONITORING REOUIREMENfS: FRESHWATER The provisions of this Section apply to Outfall 001 for whole effluent toxicity testing (biomonitoring). 1. Scope, Frequency and Methodology a. The permittee shall test the effluent for lethality in accordance with the provisions in this Section. Such testing will determine compliance with the Surface Water Quality Standard, 30 TAC g307.6(e)(2)(B), of greater than 50% survival of the appropriate test organisms in 100% effluent for a 24-hour period. b. The toxicity tests specified shan be conducted once per six months. The permittee shall conduct the following toxicity tests utilizing the test organisms, procedures, and quality assurance requirements specified in this section of the permit and in accordance with "Methods for Measuring the Acute Toxicity of Effluents and Receiving Waters to Freshwater and Marine Organisms, Fifth Edition" (EPA-82l-R-02-0 12), or the most recent update thereof: 1) Acute 24-hour static toxicity test using the water flea (Daphnia pulex or Ceriodaphnia dubia). A minimum of five replicates with eight organisms per replicate shall be used in the control and in each dilution. 2) Acute 24-hour static toxicity test using the fathead minnow (Pimephales promelas). A minimum of fiye replicates with eight organisms per replicate shall be used in the control and in each dilution. A valid test result must be submitted for each reporting period. The permittee must report, then repeat, an inyalid test during the same reporting period. The repeat test shall include the control and the 100% effluent dilution and use the appropriate number of organisms and replicates, as specified above. An inyalid test is herein defmed as any test failing to satisfY the test acceptability criteria, procedures, and quality assurance requirements specified in the test methods and permit. c. In addition to an appropriate control, a 100% effluent concentration shall be used in the toxicity tests. The control and/or dilution water shall consist of a standard, synthetic, moderately hard, reconstituted water. d. This permit may be amended to require a Whole Effluent Toxicity (WET) limit, a Best Management Practice (BMP), Chemical-Specific (CS) limits, additional toxicity testing, and/or other appropriate actions to address toxicity. The permittee may be required to conduct additional biomonitoring tests and/or a Toxicity Reduction Evaluation (TRE) ifbiomonitoring data indicate multiple numbers of unconfirmed toxicity events. 2. Required Toxicity Testing Conditions a. Test Acceptance - The permittee shall repeat any toxicity test, including the control, if the control fails to meet a mean survival equal to or greater than 90%. b. Dilution Water - In accordance with item l.c., the control and/or dilution water shall consist of a standard, synthetic, moderately hard, reconstituted water. c. Samples and Composites 1) The permittee shall collect one flow-weighted 24-hour composite sample from Outfall 001. A 24-hour composite sample consists of a minimum of 12 effluent portions collected at equal time intervals representative of a 24-hour operating day and combined proportional to flow, or a sample continuously collected proportional to flow over a 24-hour operating day. Page 36