水文變化、生物地質化學作用及集水區人為活動對水庫磷質量平衡及藻類消長之影響－以台灣亞熱帶深水水庫為例

The Effects of Hydrological Processes, Biogeochemistry and Human Activities in the Watershed on Phosphorus Mass Balance and the Subsequent Growth of Predominant Algal Species: A Case Study on the Subtropical and Deep Reservoirs in Taiwan

陳怡靜(Yi-Jing Chen)

指導教授 ： 吳先琪

國立臺灣大學/工學院/環境工程學研究所/博士(2004年)

https://doi.org/10.6342/NTU.2004.00123

摘要

磷是水生物重要營養鹽，近年來台灣水庫總磷濃度升高受到大眾關注，需瞭解影響水庫磷的內、外部負荷之重要機制及因子，進行改善。本論文以亞熱帶深水水庫－德基及翡翠水庫為研究對象，探討集水區人為活動、水文變化造成之水庫密度流及生物地質化學作用對水庫磷的傳輸及流布影響。
應用磷的化學分選方法及地化模式模擬於翡翠水庫底泥及土壤研究得知，翡翠集水區原生磷礦物是鋁結合磷，但受到坪林隧道工程棄土及地下湧水引入之鈣結合磷及鈣離子濃度影響，底泥磷物種組成改變，孔隙水磷濃度降低，遂降低磷內部負荷。德基集水區釋出之溶解有機磷佔總磷量30%，1998年之後地表逕流水中溶解有機磷降低是德基二角多甲藻(Peridinium spp.)優勢消失的重要因子。
本研究依據現地採集數據，發展納入底層水溶氧、底層水溫、垂直向紊流擴散、生物反應及底泥主要含磷礦物溶解沉澱平衡動力等機制於底泥傳輸及通量模式(2 box sediment phosphorus transport and flux model, 2B-SEPF)中，模擬底泥溶解總磷通量。研究結果顯示，翡翠水庫底泥磷通量顯著受到邊界處之底層水溶氧、水溫及垂直向紊流擴散影響。模擬之底泥溶解總磷負荷和溶解總磷外部負荷相當。然而以CE-QUAL-W2二維水理水質模式模擬結果，翡翠水庫年初表水總磷濃度突增主要是受到冬季冷水密度流造成的水體抬升及前一年停留在中層水之磷外部負荷聯合貢獻結果，相對地磷內部負荷雖可被抬升至中層水處，對表水的貢獻有限。
本論文發展出深水攝影暨現地示蹤劑實驗技術，有助於更準確模擬底泥磷通量。茲於翡翠水庫底泥－水界面處實驗，示蹤劑煙團變化配合高斯分布理論推求出水底部垂直紊流擴散係數約為0.3-2.5 cm2/sec。以翡翠水庫發生高濁度引起的異重流時水中濁度隨時空之變化做為天然示蹤劑，有助於模式解析翡翠水庫水理及磷傳輸動態。所推估出翡翠水庫縱向水體延散係數， 為0.188 至0.493 m2/sec，垂直向延散係數， 為 0.11 cm2/sec。
綜合結論，整合磷的內、外部負荷所發展之底泥連結水質模式 (2B-SEPF 和CE-QUAL-W2 模式)配合水下攝影暨示蹤劑技術，可有效預測水體磷的宿命。磷的化學分選技術及地化模式應用可協助釐清磷的生地化作用受到人為活動影響及磷與藻類消長之關係。它們將有助於回溯可能污染磷源進行有效的集水區管理，用於下游淨水廠水質處理規劃策略參考以維護用水品質及大眾健康。

關鍵字

密度流 ； 生物地質化學作用 ； 紊流擴散係數 ； 底泥 ； 水庫 ； 水質模式 ； 藻類 ； 質量平衡 ； 磷

並列摘要

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.

並列關鍵字

sediments ； reservoir ； biogeochemical processes ； density currents ； water quality model ； mass balance ； turbulent diffusion coefficient ； algae ； Phosphorus

參考文獻

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