AWWA WQTC57037

This paper provides background information on Tacoma Water's water supply system, and summarizes Tacoma's involvement to date at monitoring the impacts on water quality resulting from a test of using the Corps of Engineers' Howard Hanson Reservoir for municipal water storage. During the spring and summer of 2002, the Corps and Tacoma conducted a test of the Additional Water Storage Project by storing and holding 20,000 acre feet of water behind Howard Hanson Dam (HHD) above the normal high pool of 1147 ft., taking the high pool elevation to 1167 ft. In addition to monitoring the structural integrity of the dam during the test pool, the Corps and Tacoma cooperated in monitoring the quality of the water stored to the new elevation. Seven monitoring stations were established within the reservoir. An eighth station was located at Tacoma's Headworks located about 3-1/2 miles downstream of HHD. Water quality parameters measured at these locations included turbidity, flavor profile analysis/flavor rating analysis, total coliform/fecal coliform, giardia and cryptosporidium, temperature, dissolved oxygen, conductivity and pH, total organic carbon, color, UV 254, ozone kinetics, total trihalomethane/haloacetic acid, algae, geosmin and MIB. All parameters were measured just below the water surface, and several were also taken at 10 ft, 20 ft, and 25 ft depths. Samples were analyzed either by Tacoma Water staff, or taken to private laboratories, or to the Seattle Water Department laboratory for analysis. In addition to monitoring water quality parameters, the twenty-foot elevation of newly-inundated reservoir shoreline was monitored for evidence of sloughing, a potential source of turbidity, during reservoir drawdown. Sampling began on April 15, 2002, when the pool was at elevation 1157 ft., ten feet above normal high pool. The reservoir reached its target high pool elevation of 1167 ft. on June 3, and remained at approximately this level through July 1,2002. Samples were taken in the reservoir from a boat every other week from April through November 2002. Samples obtained at the Tacoma Headworks were taken in front of the intake.