Section 307.8 - Application of Standards(a) Flow conditions.(1) The following standards do not apply below critical low-flows:(A) site-specific criteria for dissolved oxygen, pH, temperature, and numerical chronic criteria for toxic materials, as listed in Appendices A, D, and E of § 307.10 of this title (relating to Appendices A - G);(B) numerical chronic criteria for toxic materials as established in § 307.6 of this title (relating to Toxic Materials);(C) total chronic toxicity restrictions as established in § 307.6 of this title;(D) maximum temperature differentials as established in § 307.4(f) of this title (relating to General Criteria); and(E) dissolved oxygen criteria for unclassified waters, as established in § 307.4(h) of this title and § 307.7(b)(3) of this title (relating to Site-Specific Uses and Criteria).(2) Critical low-flows for streams or rivers that are dominated by springflow are listed in the standards implementation procedures as amended and are calculated as follows: (A) for springflow-dominated streams or rivers that contain federally listed endangered or threatened aquatic or aquatic dependent species, the critical low-flow value is the 0.1 percentile value derived from a lognormal distribution for the period of record at the nearest United States Geological Survey (USGS) or International Boundary and Water Commission (IBWC) gauging station;(B) for springflow-dominated streams or rivers that do not contain federally listed endangered or threatened species, the critical low-flow value is the 5th percentile value of the flow data for the period of record at the nearest USGS or IBWC gauging station.(3) Numerical acute criteria for toxic materials and preclusion of total acute toxicity as established in § 307.6 of this title are applicable at stream flows that are equal to or greater than one-fourth of critical low-flows.(4) Harmonic mean flow is the applicable upstream flow when calculating wastewater permit limits for criteria that are assessed as long-term means, such as criteria for total dissolved solids, chloride, sulfate in Appendix A of § 307.10 of this title, and human health toxic criteria in Table 2 of § 307.6(d)(1) of this title. These criteria are applicable at all flow conditions except as specified for the applicability of assessment data in § 307.9 of this title (relating to Determination of Standards and Attainment).(5) Critical low-flows and harmonic mean flows for some classified segments are listed in the standards implementation procedures as amended. These critical low-flows are not for the purpose of regulating flows in water bodies in any manner or requiring that minimum flows be maintained in classified segments.(6) Critical low-flows and harmonic mean flows listed in the standards implementation procedures as amended apply only to river basin and coastal basin waters. They do not apply to bay waters, gulf waters, reservoirs, or estuaries.(7) Critical low-flows and harmonic mean flows in the standards implementation procedures as amended were calculated from historical USGS or IBWC daily streamflow records. If the calculated critical low-flow or harmonic mean flow value was equal to or less than 0.1 cubic foot per second (cfs), it was rounded up to 0.1 cfs.(8) Flow values are periodically recomputed to reflect alterations in the hydrologic characteristics of a segment, including reservoir construction, climatological trends, and other phenomena.(9) The general criteria are applicable at all flow conditions except as specified in this section or in § 307.4 of this title.(b) Mixing zones. A reasonable mixing zone is allowed at the discharge point of permitted discharges into surface water in the state, in accordance with the following provisions. (1) The following portions of the standards do not apply within mixing zones: (A) site-specific criteria, as defined in § 307.7 of this title and listed in Appendices A, D, E, F, and G of § 307.10 of this title;(B) numerical chronic aquatic life criteria for toxic materials as established in § 307.6 of this title;(C) total chronic toxicity restrictions as established in § 307.6 of this title;(D) maximum temperature differentials as established in § 307.4(f) of this title;(E) dissolved oxygen criteria for unclassified waters, as established in § 307.4(h) of this title;(F) dissolved oxygen criteria for intermittent streams, as established in § 307.4(h)(4) of this title;(G) aquatic recreation criteria for unclassified waters, as established in § 307.4(j) of this title and in § 307.7(b)(1) of this title;(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.(2) Numerical acute aquatic life criteria for toxic materials and preclusion of total acute toxicity as established in § 307.6 of this title 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 of permitted discharges, but there must be no lethality to aquatic organisms that move through a ZID. ZIDs must not exceed the following sizes: (A) 60 feet downstream and 20 feet upstream from a discharge point in a stream and river. In addition, ZIDs in streams and rivers must not encompass more than 25% of the volume of stream flow at or above seven-day, two-year low-flow conditions;(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(C) a 50-foot radius in all directions (or equivalent volume or area for diffuser systems) from a discharge point in a bay, a tidal river, an estuary, or the Gulf of Mexico.(3) Provisions of the general criteria in § 307.4 of this title remain in effect in mixing zones unless specifically exempted in this section.(4) Water quality standards do not apply to treated effluent 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) Where a mixing zone is defined in a valid permit of the Texas Commission on Environmental Quality, the Railroad Commission of Texas, or the United States Environmental Protection Agency, the mixing zone defined in the permit must apply.(6) Mixing zones must 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 as amended.(7) Mixing zones must not overlap unless it can be demonstrated that no applicable standards will be violated in the area of overlap. Existing and designated uses must not be impaired by the combined impact of a series of contiguous mixing zones.(8) Mixing zones must 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) Mixing zones must 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 that consist entirely of stormwater runoff). For domestic discharges with permitted monthly average flows less than one million gallons per day, a small mixing zone must be assumed in accordance with guidelines for mixing zone sizes specified in the standards implementation procedures as amended; and the commission may require specified mixing zones as appropriate.(10) Different mixing zone sizes for specific numeric criteria, such as for the protection of human health, aquatic life, and temperature, may be specified in a wastewater permit.(c) Minimum analytical levels. The specified definition of permit compliance for a specific toxic material must 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 Regulations Part 136. Minimum analytical levels are listed in the standards implementation procedures as amended.(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 that intake and discharge cooling water into different water bodies, this provision only applies if water quality and applicable water quality standards in the receiving water are maintained and protected.(e) Stormwater discharges. Pollution in stormwater must not impair existing or designated uses. Controls on the quality of stormwater discharges must be based on best management practices, technology-based limits, or both in combination with instream monitoring to assess standards attainment and to determine if additional controls on stormwater quality are needed. The standards implementation procedures as amended describe how water quality standards are applied to Texas Pollutant Discharge Elimination System stormwater discharges. The evaluation of instream monitoring data for standards attainment includes the effects of stormwater, as described in § 307.9 of this title.30 Tex. Admin. Code § 307.8
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; amended to be effective August 17, 2000, 25 TexReg 7722; amended to be effective July 22, 2010, 35 TexReg 6294; amended to be effective March 6, 2014, 39 TexReg 1450