The current assessment method of the Narrative Criteria ORS 340-041-0007(9) lacks specific details and protocols. To support a determination of impairment by aquatic weeds or algae, terms such as “excessive growths” and “dominate the assemblage” must be clarified and a weight of evidence approach defined. This update will add clarity to the existing methodology.
DEQ historically listed waterbodies based on excessive amounts of algae (i.e. benthic/floating algae). DEQ is exploring multiple lines of evidence that may be used to assess attainment of this criteria. This update will provide clarity and a clear weight of evidence approach to remove “excessive algae” listings. Lines of evidence may include: methods for visual estimation of algae, periphyton biomass thresholds, algae community composition, and other supporting WQ information such as nutrients, diel dissolved oxygen and/or pH.
In conjunction with protocols for excessive algae, DEQ plan to develop a weight of evidence approach for nutrient impairment. Possible lines of evidence could be diel DO/pH, phosphorus/nitrogen concentrations together with chlorophyll (water column and benthic), phytoplankton community structure, and macroinvertebrates.
Assessment of TMDL benchmarks for Category 2
DEQ is coordinating with EPA to determine, where no numeric criteria exist (i.e. phosphorus, nutrients, sediment), whether attainment of TMDL targets/benchmarks may be used to identify where waterbodies have been restored and beneficial uses are currently being supported.
Sedimentation is a significant cause of impaired waters in Oregon. Development of a sedimentation benchmark and corresponding assessment methodology would require peer review. A methodology to assess suspended and bedded sediment would build on current knowledge and procedures for implementing the narrative criterion.