TCEQ Assessment Methodology

The provisions of sections 305(b) and 303(d) of the Clean Waters Act require the TCEQ to identify water bodies in the state that do not satisfy uses or meet numerical criteria (parameter concentrations) defined in the Texas Surface Water Quality Standards (TSWQS). The TCEQ provides a report to the EPA (the Texas Integrated Report for Clean Water Act Sections 305(b) and 303(d), hereinafter referred to as IR) every two years. The report includes identification of impaired waters (the 303(d) List), a list of water bodies evaluated, identification of water bodies either newly listed or removed from the 303(d) List and other supporting information. The assessment providing the basis of the 2010 IR relied upon the 2000 TSWQS.

The TCEQ approved revised water quality standards that become effective as a state rule in July 2010, although the EPA has not yet approved the revisions. The 2010 IR was submitted to EPA September 17, 2010, and has not yet been formally approved at that level. TCEQ has developed a guidance document that outlines the IR assessment methodology titled, Guidance for Assessing and Reporting Surface Water Quality in Texas.

TCEQ has divided the water bodies of the state into distinct segments that generally represent natural watersheds. Each water body is given a number that indicates the river basin and segment. If Appendix A of the TSWQS (found in the Texas Administrative Code [TAC], Title 30, §307) specifies use and numerical criteria for the segment, it is considered a classified segment. Tributaries that flow into a classified segment are termed unclassified segments, and are generally assessed against the standards that apply to the classified segment. An exception would be tributaries that are listed in Appendix D of the TSWQS.

Each segment is subdivided into hydrologically-distinct units, or assessment units (AU). The boundaries of an AU are typically the points at which a tributary merges with the parent segment. Each segment may be divided into more than one AU, and each AU may contain more than one monitoring station. Ideally, an AU should be no more than 25 miles in length, and the monitoring station should be located at the lower end of the AU. For obvious reasons, this pattern does not neatly apply to reservoirs, bays, estuaries and the Gulf of Mexico. Water quality conditions in each assessment unit are the basis for determination of standards attainment or non-attainment.

Numerical criteria assigned to a segment are tied to TSWQS defined uses, which include aquatic life use (ALU), recreational use (contact or noncontact), domestic water supply use and other presumed uses. Each classified segment is assigned an ALU based on physical, chemical and biological characteristics. There are five ALU categories: limited, intermediate, high and exceptional.

A number of methods are used to assess support of the ALU, including dissolved oxygen criteria, toxic substances in water criteria, ambient water and sediment toxicity test results, and indices for habitat, benthic macroinvertebrate and fish communities. Biological assessment procedures are used to determine support of the ALU category assigned to a water body. This assessment involves an evaluation of the available instream habitat and the diversity and distribution of fish and macrobenthic invertebrates in a water body, which can vary as a result of changes in the aquatic environment caused by natural and/or non-natural events.

The chief non-natural agents of change are anthropogenic pollution, instream habitat alteration and disruption of the instream flow conditions to which the biological community is adapted. Field assessment involves collecting, counting and identifying these organisms, and using these findings to calculate the biotic integrity index (IBI). Calculation of the IBI is complex and will not be discussed in this document. For more information refer to Surface Water Quality Monitoring Procedures, Volume 2: Methods for Collecting and Analyzing Biological Assemblage and Habitat Data.

TCEQ uses data collected during the most recent seven-year period (December 1 through November 30 of the preceding year) for the periodic assessment required by the CWA. An attempt is made to compile a spatially and temporally representative dataset. Samples must be collected over at least two years, no more than 2/3 of the data can represent a single year, and no more than 1/3 of the samples should be from the same season. Samples obtained during low flow events (as defined in the TCEQ guidance document) are excluded. At least 10 samples are required to determine standard attainment or nonattainment, but four or more samples can be used to establish a concern. If there is insufficient data to determine standards attainment and/or support of the designated use, that information is noted in the IR. Nonsupport for most designated uses is established by evaluating the number of times a water quality standard was exceeded. Statistical methods are applied to the calculation of exceedance thresholds to ensure that the chance of claiming that an AU is impaired when in reality it is not (a Type I error), remains fixed. These exceedance thresholds (the number or exceedances required to consider an AU impaired) vary with sample size and parameter.

Calculation of the threshold for delisting an AU follows the same logic, but the number of exceedances required to change the status of the AU is not the same in each case. Standards may also be stated in terms of the median concentration or geometric mean (the average of the logarithms of sample results, converted back to a natural number). In the case of bacterial parameters (E. coli, enterococci), the most common evaluation of standards attainment involves comparison of the seven-year geometric mean to the WQS, although single-sample exceedances have also been used to establish impairment. Beginning with the 2012 assessment, the seven-year geometric mean will be the only basis for determining impairment

An impairment category is assigned to each identified impairment, and those AU that are classified as category 5 are added to the 303(d) list of impaired waters. Category 5 includes:

  • Impairments of water quality parameters for which a total maximum daily load (TMDL) project is under way, scheduled or will be scheduled (5a)

  • Cases where review of water quality standards for that AU is appropriate (5b)

  • Cases where additional data must be collected before a TMDL is scheduled (5c)

H-GAC staff will address the 2008 and 2010 impairment status of the watershed in the individual water quality reviews, and map impairments at the AU level from impairment information contained in the draft 2010 IR.

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