WRRCTR No.89 Eutrophication and Fish Toxicity Potentials in a Multiple-Use Reservoir

Abstract

A coordinated research effort was undertaken to investigate the impact of secondary-treated effluents from the Wahiawa and Whitmore Sewage Treatment Plants on the Wahiawa Reservoir and to evaluate alternatives to the present mode of effluent disposal. The research for the Department of Public Works, City and County of Honolulu, consisted of studies of the historical and institutional background of the reservoir, water and sediment quality, algal growth potential, fish toxicity of the secondary effluent, and reservoir and waste water management alternatives. Evaluation of the water quality data indicates a high level of nitrogen and phosphorus in the Wahiawa Reservoir waters as compared to upstream control points, such as the Ku Tree Reservoir. Sediment quality indicates a definite eutrophic condition downstream of the treatment plant discharges. Algal growth studies show that nitrogen may be the limiting growth factor for near-surface waters and phosphorus for deeper waters downstream of the treatment plant discharges and that phosphorus may be limiting upstream from those points. As expected, highest growth potential occurred with the sewage effluent or at the points of its discharge into the reservoir. The fish bioassays resulted in a 96-hr TLm at about 0.24 to 0.47 mg/l combined chlorine residual in continuous flow studies for tilapia, 0.28 mg/l for mosquito fish, and 0.05 mg/l for mollies. A number of management alternatives were evaluated and the most feasible of these alternatives appears to be secondary effluent reuse for sugarcane irrigation or tertiary waste water treatment. Although these warrant further detailed evaluation, both as to system design, costs and institutional constraints, preliminary estimates of costs and benefits derived favor the tertiary treatment alternative. The recommendation for management of the Wahiawa and Whitmore Village Sewage Treatment Plant effluents is the application of tertiary treatment, particularly for phosphorus removal. This level of treatment should only be necessary during the low flow period of the year, and the dilution and assimilation capacity of the reservoir waters can be utilized at higher reservoir water levels with the present secondary level of treatment. It is emphasized that the decision to implement tertiary treatment should not be a unilateral one, but should be made in consultation with all agencies involved in management and control of use of the reservoir waters so that the final management scheme is in the best interests of all.