Publication Summary

A low-flow water quality survey of five miles of the South Fork of the Palouse River (SFPR) was conducted concurrently with a Class II inspection at the Pullman wastewater treatment plant (WWTP).

A low-flow water quality survey of five miles of the South Fork of the Palouse River (SFPR) was conducted concurrently with a Class II inspection at the Pullman wastewater treatment plant (WWTP).

Physical and chemical data indicated very few improvements in water quality conditions above the WWTP since a 1978 survey, especially with fecal coliform, nutrients, and pH. However, benthic macroinvertebrate results indicated improved water quality conditions. Instream ammonia had markedly decreased with the removal of a major discharger.

The Pullman WWTP effluent was poorly diluted during the survey. Effluent quality had improved since 1978; chlorine and ammonia toxicity problems were not present. Effluent created Class A pH, temperature and dissolved oxygen (D.O.) criteria violations, and excessive nutrient load.

A total maximum daily load (TMDL) analysis concluded: (1) the SFPR above the WWTP is at its TMDL for nutrients and bacteria, (2) the SFPR has inadequate discharge for nine months of the year to dilute the WWTP effluent to meet 20:1 dilution guidelines, (3) current and maximum permitted loads of ammonia and BOD would probably create Class A D.O., temperature, and pH violations for 1.5 to 2.5 miles below the WWTP during low-flow conditions (7Q10). Identification of fecal coliform source(s) above the WWTP; monitoring of pH, temperature, and ammonia above and below the WWTP; and consideration of seasonal effluent removal from the SFPR were recommended.