(a) Definitions. The following words and terms, when used in this chapter, have the defined meanings, unless the context clearly indicates otherwise.
  (1) Acute toxicity--Toxicity that 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) Ambient--Refers to the existing water quality in a particular water body.
  (3) Aquatic vegetation--Refers to aquatic organisms, i.e., plant life, found in the water and includes phytoplankton; algae, both attached and floating; and vascular and nonvascular plants, both rooted and floating.
  (4) Attainable use--A use that can be reasonably achieved by a water body in accordance with its physical, biological, and chemical characteristics whether it is currently meeting that use or not. Guidelines for the determination and review of attainable uses are provided in the standards implementation procedures. The designated use, existing use, or presumed use of a water body may not necessarily be the attainable use.
  (5) Background--Refers to the water quality in a particular water body that would occur if that water body were relatively unaffected by human activities.
  (6) 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.
  (7) Best management practices--Schedules of activities, maintenance procedures, and other management practices to prevent or reduce the pollution of water in the state from point and nonpoint sources, to the maximum extent practicable. Best management practices also include treatment requirements, operating procedures, and practices to control plant site runoff, spillage or leaks, sludge or waste disposal, or drainage from raw material storage.
  (8) Bioaccumulative--Describes a chemical that is taken up by aquatic organisms from water directly or through the consumption of food containing the chemical.
  (9) Bioaccumulation factor--A unitless value describing the degree to which a chemical can be concentrated in the tissues of an organism in the aquatic environment and that is absorbed through all routes of exposure, including the food chain. The bioaccumulation factor is the ratio of the concentration of a chemical in the tissue of an aquatic organism to the concentration of the chemical dissolved in ambient water at the site of sampling.
  (10) Bioconcentration factor--A unitless value describing the degree to which a chemical can be concentrated in the tissues of an organism in the aquatic environment and that is absorbed directly from the water. The bioconcentration factor is the ratio of a chemical's concentration in the tissue of an organism compared to that chemical's average concentration in the surrounding water.
  (11) Biological integrity--The species composition, diversity, and functional organization of a community of organisms in an environment relatively unaffected by pollution.
  (12) Biotic ligand model--A metal bioavailability model that uses receiving water body characteristics to develop site-specific water quality criteria.
  (13) Chronic toxicity--Toxicity that 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 the most common chronic toxicity test is seven days or more.
  (14) Classified--Refers to a water body that is listed and described in Appendices A and C of §307.10 of this title (relating to Appendices A - G). Site-specific uses and criteria for classified water bodies are listed in Appendix A of §307.10 of this title.
  (15) Coastal recreation waters--Marine coastal waters including oceans, coastal estuaries, and bays designated as primary contact recreation. Waters upstream of an unimpaired natural connection to the open sea or tidal inland waters are not considered coastal recreation waters (e.g., tidal rivers or streams).
  (16) Commission--Texas Commission on Environmental Quality.
  (17) Criteria--Water quality conditions that are to be met in order to support and protect desired uses, i.e., existing, designated, attainable, and presumed uses.
  (18) Critical low-flow--Low-flow condition that consists of the seven-day, two-year low-flow or the alternative low-flows for spring-fed streams as discussed in §307.8(a)(2) of this title (relating to Application of Standards) and below which some standards do not apply.
  (19) Designated use--A use that is assigned to specific water bodies in Appendix A, D, or G of §307.10 of this title (relating to Appendices A - G). Typical uses that may be designated for specific water bodies include domestic water supply, categories of aquatic life use, recreation categories, and aquifer protection.
  (20) Discharge permit--A permit issued by the state or a federal agency to discharge treated effluent or cooling water into waters of the state.
  (21) Dry weather flows--Sustained or typical dry, warm-weather flows between rainfall events, excluding unusual antecedent conditions of drought or wet weather.
  (22) EC50 --The concentration of a toxicant that produces an adverse effect on 50% of the organisms tested in a specified time period.
  (23) E. coli--Escherichia coli, a subgroup of fecal coliform bacteria that is present in the intestinal tracts and feces of warm-blooded animals. It is used as an indicator of the potential presence of pathogens.
  (24) Effluent--Wastewater discharged from any point source prior to entering a water body.
  (25) Enterococci--A subgroup of fecal streptococci bacteria (mainly Streptococcus faecalis and Streptococcus faecium that is present in the intestinal tracts and feces of warm-blooded animals. It is used as an indicator of the potential presence of pathogens.
  (26) Epilimnion--The upper mixed layer of a lake (including impoundments, ponds, and reservoirs).
  (27) Existing use--A use that is currently being supported by a specific water body or that was attained on or after November 28, 1975.
  (28) Fecal coliform--A portion of the coliform bacteria group that is present in the intestinal tracts and feces of warm-blooded animals; heat tolerant bacteria from other sources can sometimes be included. It is used as an indicator of the potential presence of pathogens.
  (29) Freshwaters--Inland waters that exhibit no measurable elevation changes due to normal tides.
  (30) 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.
  (31) Harmonic mean flow--A measure of mean flow in a water course that 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.
  (32) Incidental fishery--A level of fishery that applies to water bodies that are not considered to have a sustainable fishery but do have an aquatic life use of limited, intermediate, high, or exceptional.
  (33) Industrial cooling impoundment--An impoundment that is owned or operated by, or in conjunction with, the water rights permittee, and that is designed and constructed for the primary purpose of reducing the temperature and removing heat from an industrial effluent.
  (34) Industrial cooling water area--A designated area associated with a permitted wastewater discharge where numerical temperature criteria are not applicable in accordance with conditions and requirements specified in §307.4(f) of this title (relating to General Criteria) and §307.8(b) of this title (relating to Application of Standards).
  (35) Intermittent stream--A stream that has a period of zero flow for at least one week during most years. Where flow records are available, a stream with a seven-day, two-year low-flow of less than 0.1 cubic feet per second is considered intermittent.
  (36) Intermittent stream with perennial pools--An intermittent stream that maintains persistent pools even when flow in the stream is less than 0.1 cubic feet per second.
  (37) LC50 --The concentration of a toxicant that is lethal (fatal) to 50% of the organisms tested in a specified time period.
  (38) Main pool station--A monitoring station that is located in the main body of a reservoir near the dam and not located in a cove or in the riverine portion or transition zone of a reservoir.
Cont'd...