Publication Summary

A water quality modeling study was initiated to develop waste load allocations (WLAs) for point sources for biochemical oxygen demand, ammonia, and chlorine in the Puyallup River basin. The dischargers permitted through the National Pollutant Discharge Elimination System (NPDES) include ten municipalities, four industrial discharges, and four fish hatcheries. Background and nonpoint source loads were also assessed. In addition to considering existing discharges and nonpoint loads, the potential impact of future dischargers was examined.

A water quality modeling study was initiated to develop waste load allocations (WLAs) for point sources for biochemical oxygen demand (BOD), ammonia, and chlorine in the Puyallup River basin. The dischargers permitted through the National Pollutant Discharge Elimination System (NPDES) include ten municipalities, four industrial discharges, and four fish hatcheries. Background and nonpoint source loads were also assessed. In addition to considering existing discharges and nonpoint loads, the potential impact of future dischargers was examined.

This study approach is consistent with Ecology's Total Maximum Daily Load (TMDL) process, which was recently approved by EPA to fulfill the requirements of Section 303(d) of the federal Clean Water Act.

Dissolved oxygen standards in the lower Puyallup River would not be met if significant additional BOD sources were introduced, unless currently permitted BOD loads (nitrogenous BOD from ammonia and/or carbonaceous BOD) are reduced. Reductions in ammonia loading were found to have greater potential to improve dissolved oxygen than reductions of carbonaceous BOD. Ammonia loads from existing permittees are likely to be in excess of proposed mixing zone limits for most municipal discharges. Implementation of mixing zone regulations is expected to protect the water quality criteria in all segments of the Puyallup River basin.

Modeling of toxic parameters shows that assigning WLAs based on allowable dilution flows in mixing zones would not result in cumulative excess of water quality criteria for protection of aquatic life for existing and currently proposed discharges.

A TMDL is recommended that establishes WLAs based on mixing zone limitations for existing dischargers with an allocation for future growth or water quality protection. The critical condition for the TMDL is the 7-day-10-year low river flow. The proposed TMDLs are 19,500 lbs/day of 5-day BOD (BOD5), 3,330 lbs/day of ammonia as N, and 45.9 lbs/day of total residual chlorine. With existing technology-based limits for BOD5 and proposed mixing zone limits for ammonia and chlorine, a reserve for protection of water quality or additional loading from future growth was estimated.

Link to EIM data for User Study ID GPEL0002