Technical Report No. 103
Leighton W.K. Lum and Reginald H.F.Young
In recent times the increased influx of nutrients and organic matter as a result of man’s activities into some waterways has resulted in the accelerated eutrophication of these waters. This has been found to be the case in Wahiawa Reservoir. Secondarily-treated sewage effluent discharges into the reservoir have been implicated as the primary cause of the eutrophic conditions present in the reservoir.
Studies have been undertaken and management alternatives are being considered to improve the trophic status of the reservoir. The nutrient mass balance has been determined and the source of the major portion of the nutrient influx to be from the Wahiawa Sewage Treatment Plant (STP). Corrective measures will probably be directed toward further treatment or diversion of this source.
The relative importance of in situ nutrient sources, such as N fixation and the sediment, have not, however, been determined. The eutrophication potential of the sediment and its relative importance and impact on the selection of a management alternative was the objective of this study. To ascertain this effect, sediment-water contact studies were carried out.
Sediment was incubated with four different types of water simulating aerobic and anaerobic conditions in the hypolimnion of the Wahiawa Reservoir. Samples were withdrawn periodically and analyzed for nutrients, chemical oxygen demand (COD), and other constituents. After parameters reached a steady-state, the algal growth potential (AGP) of the waters was determined.
Under aerobic conditions, the sediment tended to have an oligotrophic effect on the water. Phosphorus was adsorbed by the sediment, N release from the sediment was inhibited by the development of a surface-oxidized layer, and the COD of the water was decreased. This effect was verified by the poor AGP of the water, which was P deficient.
The sediment was found to be capable of deoxygenating dissolved 02-saturated water. Substances causing a cop were released from the sediment under anaerobic conditions and the measured COD increase was about 10 mg/l, exceeding the increase resulting from the STP effluent and other sources. The COD levels maintained verified the capability of sediments to deoxygenate saturated waters.
Under anaerobic conditions, considerable N release from the sediment occurred. The increase alone far exceeded critical levels and was attributed to the release Of NH3 (5.7 to 8.2 mg/l NH3-N) from sediments. The level of P04= in most systems -far exceeded critical Limits but particulate PO4 was generally unavailable for algal growth according to test results. Results of dissolved PO4 from the sediment occurred only in low-turbidity waters under reducing conditions. The concentration of dissolved PO4 exceeded by 3 to 4 times the critical levels established in the reservoir. The high AGP in some reactor waters was verification of the eutrophication potential of the sediment. However, considering the dilution capacity of the reservoir, the concentration of P, but no N, maintained by release from the sediment would be rendered insignificant during periods of high influx and mixing.
The loading rates for sediment N and P under anaerobic conditions were of significance when compared to established guidelines. However, the sediment was of minor importance relative to the nutrient Loading from other sources to the reservoir. Even with the removal of a majority of the nutrient inputs by diversion or further treatment of the Wahiawa STP effluent, sediment still accounted for only 18% of the N and less than 1% of the P loading to the reservoir.
Sediments were found to be able to cause or maintain eutrophic conditions in the test reactors under anaerobic conditions. However, the nutrient contribution was of minor importance relative to other sources. Corrective measures with respect to nutrients released from the sediment are not required, but stabilization of the 02 demand from substances released from the sediment may be required to improve dissolved 02 levels in the deeper portions of the reservoir.
A nutrient model of the reservoir shows that diversion of the Wahiawa STP effluent will result in a 65% reduction of N and a 46% reduction of P in the epilimnion of the reservoir. New steady-state conditions were expected to be attained within 30 days, while new steady-state conditions for the hypolimnion and sediments may require about two years.