1. (a) Low-flow conditions.

   1. (1) The following standards do not apply below seven-day, two-year low-flows:

      1. (A) site-specific criteria, as defined in §307.7 of this title (relating to Site-Specific Criteria and Uses) and listed for each classified segment in Appendix A of §307.10 of this title (relating to Appendices A-E);
      2. (B) numerical chronic criteria for toxic materials as established in §307.6 of this title (relating to Toxic Materials);
      3. (C) total chronic toxicity restrictions as established in §307.6 of this title (relating to Toxic Materials);
      4. (D) maximum temperature differentials as established in §307.4(f) of this title (relating to General Criteria);
      5. (E) dissolved oxygen criteria for unclassified waters, as established in §307.4(h)(1) of this title (relating to General Criteria);
      6. (F) dissolved oxygen criteria for intermittent streams, as established in §307.4(h)(2) of this title (relating to General Criteria); and
      7. (G) fecal coliform criteria for unclassified waters, as established in §307.4(i) of this title (relating to General Criteria).
   2. (2) Numerical acute criteria for toxic materials and preclusion of total acute toxicity as established in §307.6 of this title (relating to Toxic Materials) are applicable at all flow conditions.
   3. (3) Low-flow criteria in Appendix B of §307.10 of this title are solely for the purpose of defining the flow conditions under which water quality standards apply to a given waterbody. Low-flow criteria listed in Appendix B of §307.10 of this title are not for the purpose of regulating flows in water bodies in any manner or requiring that minimum flows be maintained in classified segments.
   4. (4) Low-flow criteria defined in this section and listed in Appendix B of §307.10 of this title apply only to river basin and coastal basin waters. They do not apply to bay or gulf waters or reservoirs or estuaries.
   5. (5) Seven-day, two-year low-flows (7Q2) and harmonic mean flows in Appendix B of §307.10 of this title were calculated from historical U.S. Geological Survey (USGS) daily streamflow records. The low-flow criterion was set at 0.1 of one cubic foot per second (ft[sup]3[/sup]/s) when the calculated 7Q2 was equal to or less than 0.1 of one ft[sup]3[/sup]/s.
   6. (6) Flow values will be periodically recomputed to reflect alterations in the hydrologic characteristics of a segment, including reservoir construction, climatological trends, and other phenomena.
   7. (7) The general criteria are applicable at all flow conditions except as specified in this section or in §307.4 of this title (relating to General Criteria).
   8. (8) Specific human health criteria for concentrations in water to prevent contamination of fish and shellfish so as to ensure safety for human consumption, as established in §307.6 of this title (relating to Toxic Materials) do not apply at stream flows below the harmonic mean flow.

2. (b) Mixing zones. A reasonable mixing zone will be allowed at the discharge point of permitted discharges into surface water in the state, in accordance with the following provisions.

   1. (1) The following portions of the standards do not apply within mixing zones:

      1. (A) site-specific criteria, as defined in §307.7 of this title and listed for each classified segment in Appendix A of §307.10 of this title;
      2. (B) numerical chronic aquatic life criteria for toxic materials as established in §307.6 of this title (relating to Toxic Materials);
      3. (C) total chronic toxicity restrictions as established in §307.6 of this title (relating to Toxic Materials);
      4. (D) maximum temperature differentials as established in §307.4(f) of this title (relating to General Criteria);
      5. (E) dissolved oxygen criteria for unclassified waters, as established in §307.4(h)(1) of this title (relating to General Criteria);
      6. (F) dissolved oxygen criteria for intermittent streams, as established in §307.4(h)(2) of this title (relating to General Criteria);
      7. (G) fecal coliform criteria for unclassified waters, as established in §307.4(i) of this title (relating to General Criteria); and
      8. (H) specific human health criteria for concentrations in water to prevent contamination of drinking water, fish, and shellfish so as to ensure safety for human consumption, as established in §307.6 of this title (relating to Toxic Materials).

   2. (2) Numerical acute aquatic life criteria for toxic materials and preclusion of total acute toxicity as established in §307.6 of this title (relating to Toxic Materials) are applicable in mixing zones. Acute criteria and acute total toxicity levels may be exceeded in small zones of initial dilution (ZIDs) at discharge points, but there shall be no lethality to aquatic organisms which move through a ZID. ZIDs shall not exceed the following sizes:

      1. (A) 60 feet downstream and 20 feet upstream from a discharge point in a stream and river, and in addition, ZIDs in streams and rivers shall not encompass more than 25% of the volume of stream flow at or above seven-day, two-year low-flow conditions;
      2. (B) a 25-foot radius in all directions (or equivalent volume or area for diffuser systems) from a discharge point in a lake or reservoir; and
      3. (C) a 50-foot radius in all directions (or equivalent volume or area for diffuser systems) from a discharge point in a bay, tidal river, or estuary.
   3. (3) Provisions of the general criteria in §307.4 of this title (relating to General Criteria) remain in effect in mixing zones unless specifically exempted in this section.
   4. (4) Water quality standards do not apply to treated effluents at the immediate point of discharge--prior to any contact with either ambient waters or a dry streambed. However, effluent total toxicity requirements may be specified to preclude acute lethality near discharge points, or to preclude acute and chronic instream toxicity.
   5. (5) Where a mixing zone is defined in a valid commission and/or national pollutant discharge elimination system (NPDES) permit, the mixing zone defined in the permit will apply.
   6. (6) Mixing zones shall not preclude passage of free-swimming or drifting aquatic organisms to the extent that aquatic life use is significantly affected, in accordance with guidelines specified in the standards implementation procedures.
   7. (7) Mixing zones will not overlap unless it can be demonstrated that no applicable standards will be violated in the area of overlap. Existing and designated uses will not be impaired by the combined impact of a series of contiguous mixing zones.
   8. (8) Mixing zones will not encompass an intake for a domestic drinking water supply. Thermal mixing zones are excepted from this provision unless elevated temperatures adversely affect drinking water treatment.
   9. (9) Mixing zones will be individually specified for all permitted domestic discharges with a permitted monthly average flow equal to or exceeding one million gallons per day and for all permitted industrial discharges to water in the state (excepting discharges which consist entirely of stormwater runoff). For domestic discharges with permitted monthly average flows less than one million gallons per day, a small mixing zone will be assumed in accordance with guidelines for mixing zone sizes specified in the standards implementation procedures document; and the commission may require specified mixing zones as appropriate.
   10. (10) The size of mixing zones for human health criteria may vary from the size of mixing zones for aquatic life criteria.
3. (c) Minimum analytical levels. The specified definition of permit compliance for a specific toxic material will not be lower than established minimum analytical levels, unless that toxic material is of particular concern in the receiving waters, or unless an effluent specific method detection limit has been developed in accordance with 40 Code of Federal Regulation Part 136. Minimum analytical levels are listed in the standards implementation procedures.
4. (d) Once-through cooling water discharges. When a discharge of once-through cooling water does not measurably alter intake concentrations of a pollutant, then water-quality based effluent limits for that pollutant are not required. For facilities which intake and discharge cooling-water into different waterbodies, this provision only applies if water quality and applicable water quality standards in the receiving water are maintained and protected.

Source Note:The provisions of this §307.8 adopted to be effective July 10, 1991, 16 TexReg 3400; amended to be effective July 13, 1995, 20 TexReg 4701.