Aquatic Life Water Quality Standards Draft Technical Support Document for Total Suspended Solids (Turbidity)

The existing turbidity water quality standard (WQS) has been in use since the late 1960's. The standard has several weaknesses, including being a statewide standard and, having Nephelometric Turbidity Units, is not concentration-based and not amenable to load-based studies. In addition, issues include having too much variation in measurements because of particle composition in water, variation among meters, and poor quantitative documentation of what a turbidity unit is. Although recognized earlier, these weaknesses became a significant problem when EPA's and the Agency's Total Maximum Daily Load (TMDL) program became fully realized in the early 2000's. Once the TMDL studies began, it became clear that the existing standard was only indirectly related to biotic community health. In addition, TMDL development was challenging because the studies needed to be developed using Total Suspended Solids (TSS), which has concentration-based units (mg/L). As a result, a committee of Agency staff across several Divisions met for over a year to develop TSS criteria. These draft TSS criteria are regional in scope and based on a combination of both biotic sensitivity to TSS concentrations and reference streams/least impacted streams as data allow. The Criteria table contains the recommended TSS criteria, utilizing multiple lines of evidence when available. TMDL development is also challenging due to the lack of frequency and duration in the current turbidity standard. Without frequency and duration, it is very difficult to determine critical conditions and to develop a load and wasteload allocation for the critical conditions, both of which are required elements of a TMDL.