| Summary: | 博士 === 國立臺灣大學 === 環境工程學研究所 === 93 === Recently, the concentrations of total phosphorus (TP) in the reservoirs in Taiwan have increased at a rate sufficient to cause public concern. The understanding of the mechanisms and factors influencing both the external and internal P input in the reservoir is needed. The objective of this research was to evaluate the integrated effect of hydrological processes, biogeochemical processes of P and the human activities in the watershed on the fate of P in the reservoir. The study areas include both the Techi Reservoir and Feitsui Reservoir, respectively.
The chemical P fractionation and geochemical simulations in the sediments and surface water were used to clarify the speciation of P forms in order to verify the source of P in the watershed. The results showed that the distribution of P forms in the sediments in Feitsui Reservoir had been affected by the Ca or Ca bound P export from the debris from tunnel construction and groundwater inflow in the watershed, subsequently influencing the chemical composition of porewater and the internal loads of P. Dissolved organic P in the surface runoff from the Techi watershed, accounting for 30 % of TP, was the major source of P serving for the prosperity of the dominant algal species, Peridinium spp., in the surface water.
A sediment P transport model (2 Box-Sediment Phosphorus Transport and Flux Model, 2B-SEPF), which considers the sediment water interaction (vertical turbulent diffusion, overlying water temperature difference, overlying dissolved oxygen difference), microbial reaction and precipitation-dissolution dynamics of P minerals, has been developed and verified. The predicted internal dissolved total phosphorus (DTP) loads was almost equal to the external DTP loads in Feitsui Reservoir, a subtropical and deep reservoir. However, the water quality simulation results by CE-QUAL-W2 model revealed that the hydrologically induced density currents in winter together with the external loads of P were the main cause of the surface water quality deterioration, even though the bottom water carrying abundant P internal loads might be lifted to the middle layer by the water momentum force by the density current.
The apparatus of the in-situ tracer experiment integrated with a submerged video camera was developed in this study. The estimated vertical turbulent diffusion coefficient,εz , at the sediment water interface with a depth of 85m at dam in Feitsui Reservoir was in the range of 0.3-2.5 cm2/sec. It makes the prediction of the sediment P flux more accurately.
The spatial and temporal variations of the suspended solids (SS) brought in by the
turbidity currents were also used to estimate the horizontal and vertical dispersion coefficients in the reservoir by applying the Gaussian distribution theory. They were useful for predicting the hydrodynamics and transport of P in the reservoir.
In summary, the integrated sediment–water simulated model (2B-SEPF and CE-QUAL-W2 model) could be a useful tool to predict the fate of P in the reservoir with complex hydrological problems like density currents. The chemical P fractionation and geochemical simulations techniques could be used to clarify the effects of human activities on the biogeochemical processes of P and the linkage of P species with the speciation of algae. They are also useful for the BMP management in the watershed and the downstream water treatment planning for public health.
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