Publication Summary

An abbreviated Class II inspection of the Kittitas wastewater treatment plant (WTP) was concurrently conducted with a three-day, low-flow, receiving water survey of Cooke Creek in August 1987. The purpose was to: (1) determine the efficiency of the WTP during the irrigation season when infiltration problems are greatest, (2) determine the impact of the effluent on Cooke Creek, and (3) compare 1987 receiving water quality to that in 1978-79.

An abbreviated Class II inspection of the Kittitas wastewater treatment plant (WTP) was concurrently conducted with a three-day, low-flow, receiving water survey of Cooke Creek in August 1987. The purpose was to: (1) determine the efficiency of the WTP during the irrigation season when infiltration problems are greatest, (2) determine the impact of the effluent on Cooke Creek, and (3) compare 1987 receiving water quality to that in 1978-79. The WTP was hydraulically overloaded. Both influent and effluent were very weak; e.g., 43 mg/L and 8 mg/L BOD, respectively. The BOD removal was less than 70 percent. Effluent impacts observed on creek water quality were primarily a result of poor dilution and mixing. Residual chlorine was detected, and dissolved oxygen (D.O.) was depressed just below the outfall. Water quality, benthic macroinvertebrate, and fish population data suggested the WTP, non-point sources, or poor irrigation management practices continue to degrade creek water quality. The total maximum daily load analysis used a typical creek low-flow condition and reduced infiltration with resultant increases in WTP effluent strength. Predicted were: D.O. depression, ammonia toxicity, chlorine toxicity, and nutrient enrichment for at least 0.5 mile downstream. Several recommendations were made.