1. (a) Definitions. The following words and terms, when used in this chapter, shall have the defined meanings, unless the context clearly indicates otherwise.

   1. (1) Acute toxicity--Toxicity which exerts a stimulus severe enough to rapidly induce an effect. The duration of exposure applicable to acute toxicity is typically 96 hours or less. Tests of total toxicity normally use lethality as the measure of acute impacts. (Direct thermal impacts are excluded from definitions of toxicity.)
   2. (2) Ambient--Refers to the existing water quality in a particular waterbody.
   3. (3) Background--Refers to the water quality in a particular waterbody that would occur if that waterbody were relatively unaffected by human activities.
   4. (4) Bedslope--Stream gradient, or the extent of the drop in elevation encountered as the stream flows downhill. One measure of bedslope is the elevation decline in meters over the stream distance in kilometers.
   5. (5) Best management practice--A practice or combination of practices determined to be the most practicable means of preventing or reducing, to a level compatible with water quality goals, the amount of pollution generated by point and nonpoint sources.
   6. (6) Bioaccumulative toxic--A chemical which is taken up by aquatic organisms from water directly or through the consumption of food containing the chemicals.
   7. (7) Chronic toxicity--Toxicity which continues for a long-term period after exposure to toxic substances. Chronic exposure produces sub-lethal effects, such as growth impairment and reduced reproductive success, but it may also produce lethality. The duration of exposure applicable to chronic toxicity is normally seven days or more.
   8. (8) Commission--The Texas Natural Resource Conservation Commission.
   9. (9) Contact recreation--Recreational activities involving a significant risk of ingestion of water, including wading by children, swimming, water skiing, diving, and surfing.
   10. (10) Criteria--Water quality conditions which are to be met in order to support and protect desired uses.
   11. (11) Critical low-flow--Low-flow condition (e.g., 7Q2 flow) below which some standards do not apply. The impacts of permitted discharges are analyzed at critical low-flow.
   12. (12) Discharge permit--A permit issued by the state to discharge treated effluent or cooling water into waters of the state.
   13. (13) EC[sub]50[/sub]--The concentration of a toxicant that produces an adverse effect on 50% of the organisms tested in a specified time period.
   14. (14) Effluent--Wastewater discharged from any point source prior to entering a waterbody.
   15. (15) Epilimnion--The upper mixed layer of a lake (including impoundments, ponds, and reservoirs).
   16. (16) Fecal coliform--That portion of the coliform bacteria group which is present in the intestinal tracts and feces of warm-blooded animals.
   17. (17) Freshwaters--Inland waters which exhibit no measurable elevation changes due to normal tides.
   18. (18) Halocline--A vertical gradient in salinity under conditions of density stratification that is usually recognized as the point where salinity exhibits the greatest difference in the vertical direction.
   19. (19) Harmonic mean flow--A measure of mean flow in a water course which is calculated by summing the reciprocals of the individual flow measurements, dividing this sum by the number of measurements, and then calculating the reciprocal of the resulting number.
   20. (20) Industrial cooling impoundments--An impoundment which is owned or operated by, or in conjunction with, the water rights permittee, and which is designed and constructed for the primary purpose of reducing the temperature and removing heat from an industrial effluent.
   21. (21) Intermittent stream--A stream which has a period of zero flow for at least one week during most years. Where flow records are available, a stream with a 7Q2 flow of less than 0.1 ft[sup]3[/sup]/s is considered intermittent.
   22. (22) LC[sub]50[/sub]--The concentration of a toxicant that is lethal (fatal) to 50% of the organisms tested in a specified time period.
   23. (23) Marine waters--Coastal waters which have measurable elevation changes due to normal tides. Marine waters are considered to be saltwater for purposes of standards application. In the absence of tidal information, marine waters are generally considered to be coastal waters which typically have salinities of two parts per thousand or greater in a significant portion of the water column.
   24. (24) Method detection limit--The minimum concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero and is determined from analysis of a sample in a given matrix containing the analyte. The method detection limit (MDL) is estimated in accordance with 40 Code of Federal Regulations, 136, Appendix B.
   25. (25) Minimum analytical level--The lowest concentration at which a particular substance can be quantitatively measured with a defined precision level, using approved analytical methods. The minimum analytical level is not the published method detection limit for an EPA-approved analytical method, which is based on laboratory analysis of the substance in reagent (distilled) water. The minimum analytical level is based on analyses of the analyte in the matrix of concern (i.e., wastewater effluents). The commission will establish general minimum analytical levels that will be applicable when information on matrix-specific minimum analytical levels is unavailable.
   26. (26) Mixing zone--The area contiguous to a discharge where mixing with receiving waters takes place and which may not meet certain criteria applicable to the receiving water.
   27. (27) No significant aquatic life use--The instream use that is typically assigned to a waterbody, such as an intermittent stream, which is not appropriate for an aquatic life use category of limited or greater. There can be some aquatic life present in a waterbody which is designated as having no significant aquatic life use. Basic water quality standards--such as the general criteria in §307.4 of this title (relating to General Criteria), the numerical acute aquatic life criteria in §307.6(c) of this title (relating to Toxic Materials), and the biomonitoring requirements to preclude acute toxicity to aquatic life in §307.6(e) of this title--apply to waterbodies with no significant aquatic life use.
   28. (28) Noncontact recreation--Recreational pursuits not involving a significant risk of water ingestion, including fishing, commercial and recreational boating, and limited body contact incidental to shoreline activity.
   29. (29) Nonpersistent toxic--A toxic substance that readily degrades in the aquatic environment, exhibits a half-life of less than 96 hours, and does not have a tendency to accumulate in organisms.
   30. (30) Oyster waters--Waters producing edible species of clams, oysters, or mussels.
   31. (31) Persistent toxic--A toxic substance that is not readily degraded and exhibits a half-life of 96 hours or more in an aquatic environment.
   32. (32) Salinity--The total dissolved solids in water after all carbonates have been converted to oxides, all bromide and iodide have been replaced by chloride, and all organic matter has been oxidized. For most purposes, salinity is considered equivalent to total dissolved salt content. Salinity is normally expressed in parts per thousand.
   33. (33) Settleable solids--The volume or weight of material which will settle out of a water sample in a specified period of time.
   34. (34) Seven-day, two-year low-flow--The lowest average flow for seven consecutive days with a recurrence interval of two years, as statistically determined from historical data. It is the flow used for determining the allowable discharge load to a stream.
   35. (35) Shellfish--Clams, oysters, mussels, crabs, crayfish, lobsters, and shrimp.
   36. (36) Standard Methods for the Examination of Water and Wastewater--A document describing sampling and analytical procedures, which is published by the American Public Health Association, American Water Works Association, and Water Environment Federation. The most recent edition of this document is to be followed whenever its use is specified by these rules.
   37. (37) Standards--The designation of water bodies for desirable uses and the narrative and numerical criteria deemed necessary to protect those uses.
   38. (38) Standards implementation procedures--Procedures entitled Implementation of the Texas Natural Resource Conservation Commission Standards via Permitting.
   39. (39) Stream order--A classification of stream size, where the smallest, unbranched tributaries of a drainage basin are designated first order streams. Where two first order streams join, a second order stream is formed; and where two second order streams join, a third order stream is formed, etc. For purposes of water quality standards application, stream order is determined from USGS topographic maps with a scale of 1:24,000.
   40. (40) Surface water in the state--Lakes, bays, ponds, impounding reservoirs, springs, rivers, streams, creeks, estuaries, wetlands, marshes, inlets, canals, the Gulf of Mexico inside the territorial limits of the state, and all other bodies of surface water, natural or artificial, inland or coastal, fresh or salt, navigable or nonnavigable, and including the beds and banks of all water-courses and bodies of surface water, that are wholly or partially inside or bordering the state or subject to the jurisdiction of the state; except that waters in treatment systems which are authorized by state or federal law, regulation, or permit, and which are created for the purpose of waste treatment are not considered to be waters in the state.
   41. (41) Sustainable fisheries--Descriptive of waterbodies which potentially have sufficient fish production or fishing activity to create significant long-term human consumption of fish. Sustainable fisheries include perennial streams and rivers with a stream order of three or greater; lakes and reservoirs greater than or equal to 150 acre-feet and/or 50 surface acres; all bays, estuaries, and tidal rivers. Waterbodies which are presumed to have sustainable fisheries include all designated segments listed in Appendix A unless specifically exempted.
   42. (42) Total dissolved solids--The amount of material (inorganic salts and small amounts of organic material) dissolved in water and commonly expressed as a concentration in terms of milligrams per liter. The term is equivalent to the term filterable residue, as used in the publication entitled Standard Methods for the Examination of Water and Wastewater.
   43. (43) Total suspended solids--Total suspended matter in water, which is commonly expressed as a concentration in terms of milligrams per liter. The term is equivalent to nonfilterable residue, as used in the publication entitled Standard Methods for the Examination of Water and Wastewater.
   44. (44) Total toxicity--Toxicity as determined by exposing aquatic organisms to samples or dilutions of instream water or treated effluent. Also referred to as whole effluent toxicity or biomonitoring.
   45. (45) Toxicity--The occurrence of adverse effects to living organisms due to exposure to toxic materials. Adverse effects caused by conditions of temperature and dissolved oxygen are excluded from the definition of toxicity. With respect to the provisions of §307.6(e) of this title (relating to Toxic Materials), which concerns total toxicity and biomonitoring requirements, adverse effects caused by concentrations of dissolved salts (such as sodium, potassium, calcium, chloride, carbonate) in source waters are excluded from the definition of toxicity. Source water is defined as surface water or groundwater that is used as a public water supply or industrial water supply (including a cooling-water supply). Source water does not include brine water that is produced during the extraction of oil and gas, or other sources of brine water that are substantially uncharacteristic of surface waters in the area of discharge. In addition, adverse effects caused by concentrations of dissolved salts which are added to source water by industrial processes are not excluded from the requirements of §307.6(e) of this title, except as specifically noted in §307.6(e)(2)(B) of this title, which concerns requirements for toxicity testing of 100% effluent. This definition of toxicity does not affect the standards for dissolved salts in this chapter other than §307.6(e) of this title. The standards implementation procedures contain provisions to protect surface waters from adverse effects of dissolved salts and methods to address the effects of dissolved salts on total toxicity tests.
   46. (46) Toxicity biomonitoring--The determination of total toxicity. Documents which describe procedures for toxicity biomonitoring are cited in §307.6 of this title.
   47. (47) Water-effects ratio--The quantifiable difference in the toxicity of a substance at an instream site, in comparison to the toxicity that was measured in experiments using laboratory water. The water-effects ratio provides an estimate of the bioavailability and toxicity of a substance in a particular waterbody. It may be used to establish site-specific criteria for aquatic life protection. The water-effects ratio is calculated as the toxic concentration (LC[sub]50[/sub]) of a substance in water at a particular site, divided by the toxic concentration of that substance as reported in lab toxicity tests. The site-specific criterion is equal to the water-effects ratio times the statewide aquatic life criterion in §307.6(c) of this title.
   48. (48) Water quality management program--The commission's overall program for attaining and maintaining water quality consistent with state standards, as authorized under the Texas Water Code, the Texas Administrative Code, and the Clean Water Act, §§106, 205(j), 208, 303(e), and 314 (33 United States Code §§1251 et seq).
   49. (49) Wetland--An area (including a swamp, marsh, bog, prairie pothole, or similar area) having a predominance of hydric soils that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support and that under normal circumstances supports the growth and regeneration of hydrophytic vegetation. The term "hydric soil" means soil that, in its undrained condition, is saturated, flooded, or ponded long enough during a growing season to develop an anaerobic condition that supports the growth and regeneration of hydrophytic vegetation. The term "hydrophytic vegetation" means a plant growing in: water or a substrate that is at least periodically deficient in oxygen during a growing season as a result of excessive water content. The term "wetland" does not include irrigated acreage used as farmland; a man-made wetland of less than one acre; or a man-made wetland for which construction or creation commenced on or after August 28, 1989, and which was not constructed with wetland creation as a stated objective, including but not limited to an impoundment made for the purpose of soil and water conservation which has been approved or requested by soil and water conservation districts. If this definition of wetland conflicts with the federal definition in any manner, the federal definition prevails.
   50. (50) Zone of initial dilution--The small area at the immediate point of discharge where initial dilution with receiving waters occurs, and which may not meet certain criteria applicable to the receiving water. A zone of initial dilution is substantially smaller than a mixing zone.
   51. (51) Bioconcentration factor (BCF)--A unitless value describing the degree to which a chemical can be concentrated in the tissues of an organism in the aquatic environment. The BCF is the concentration of a chemical in one or more tissues of the organism divided by the average exposure concentration the organism received.

2. (b) Abbreviations. The following abbreviations apply to this chapter.

   1. (1) AP--Aquifer protection.
   2. (2) BMP--Best management practices.
   3. (3) AS--Agricultural water supply.
   4. (4) CFR--Code of Federal Regulations.
   5. (5) Cl[sup]-1[/sup]--chloride.
   6. (6) CR--contact recreation.
   7. (7) DO--Dissolved oxygen.
   8. (8) E--exceptional aquatic life use.
   9. (9) EPA--United States Environmental Protection Agency.
   10. (10) degree(s) F--Degree(s) Fahrenheit.
   11. (11) ft[sup]3[/sup]/s--Cubic feet per second.
   12. (12) H--high aquatic life use.
   13. (13) I--intermediate aquatic life use.
   14. (14) IS--Industrial water supply.
   15. (15) L--limited aquatic life use.
   16. (16) mg/l--Milligrams per liter.
   17. (17) ml--Milliliter.
   18. (18) N--Navigation.
   19. (19) NCR--Noncontact recreation.
   20. (20) NPDES--National pollutant discharge elimination system, as set out in the Clean Water Act, §402 (33 United States Code 1342).
   21. (21) O--Oyster waters.
   22. (22) PS--Public water supply.
   23. (23) 7Q2--Seven-day, two-year low flow.
   24. (24) SO[sub]4[/sub][sup]-2[/sup]--sulfate.
   25. (25) TDS--total dissolved solids.
   26. (26) USFDA--U.S. Food and Drug Administration.
   27. (27) USGS--U.S. Geological Survey.
   28. (28) WF--waterfowl habitat.
   29. (29) WQM--water quality management.
   30. (30) microgram/L--micrograms per liter.
   31. (31) ZID--zone of initial dilution.

Source Note:The provisions of this §307.3 adopted to be effective July 10, 1991, 16 TexReg 3400; amended to be effective July 13, 1995, 20 TexReg 4701.