Section 58.01.02.054 - BENEFICIAL USE SUPPORT STATUSIn determining whether a water body fully supports designated and existing beneficial uses, the Department shall determine whether all of the applicable water quality standards are being achieved, including any criteria developed pursuant to these rules, and whether a healthy, balanced biological community is present. The Department shall utilize biological and aquatic habitat parameters listed below and in the current version of the "Water Body Assessment Guidance," as published by the Idaho Department of Environmental Quality, as a guide to assist in the assessment of beneficial use status. Revisions to this guidance will be made after notice and an opportunity for public comment. These parameters are not to be considered or treated as individual water quality criteria or otherwise interpreted or applied as water quality standards. The Department shall employ a weight of evidence approach in evaluating a combination of water quality data types (including, but not limited to, aquatic habitat and biological parameters), when such a combination of data are available, in making its final use support determination.
01.Aquatic Habitat Parameters. These parameters may include, but are not limited to, stream width, stream depth, stream shade, measurements of sediment impacts, bank stability, water flows, and other physical characteristics of the stream that affect habitat for fish, macroinvertebrates or other aquatic life.02.Biological Parameters. These parameters may include, but are not limited to, evaluation of aquatic macroinvertebrates including Ephemeroptera, Plecoptera and Trichoptera (EPT), Hilsenhoff Biotic Index, measures of functional feeding groups, and the variety and number of fish or other aquatic life to determine biological community diversity and functionality.03.Use of Data Regarding pH, Turbidity, Dissolved Oxygen, and Temperature. In making use support determinations, the Department may give less weight to departures from criteria in Section 250 for pH, turbidity, dissolved oxygen, and temperature that are infrequent, brief, and small if aquatic habitat and biological data indicate to the assessor that aquatic life beneficial uses are otherwise supported. Unless otherwise determined by the Department, "infrequent" means less than ten percent (10%) of valid, applicable, representative measurements when continuous data are available; "brief" means two (2) hours or less; and "small" means conditions that avoid acute effects. Subsection 054.03 only applies to use of this data for determination of beneficial use support status. Subsection 054.03 does not apply to or affect the application of criteria for any other regulatory purpose including, but not limited to, determining whether a particular discharge or activity violates water quality standards.04.Natural Conditions. There is no impairment of beneficial uses or violation of water quality standards where natural background conditions exceed any applicable water quality criteria as determined by the Department, and such natural background conditions shall not, alone, be the basis for placing a water body on the list of water quality limited water bodies described in Section 055.05.Rigor, Quality and Relevance of Data. In making any use support determination, the Department shall consider the scientific rigor associated with the collection of samples or data (e.g., the scientific methods used to collect samples or data); the quality of measurements and/or analysis of the samples (e.g., methodology, instrumentation, accuracy, precision, and limits of detection where applicable); and the relevance of the data (e.g., the relationship to a water quality standard, beneficial use or cause of impairment, and how representative the samples or data are of the water body in question).Idaho Admin. Code r. 58.01.02.054