潮汐效應對海岸水層中污染物傳輸之影響

Abstract

傳統上，對海岸水層中污染物傳輸及海水入侵地下水的問題，做穩態或非穩態模擬時，多數採用固定水頭處理海岩的邊界。事實上，在海岸地區，由於潮汐水位週期性的擾動，對臨海地下含水層會產生壓力波，當此壓力波向內陸傳遞時，臨海含水層中地下水流之流速及污染物之移行，將會產週期性的變化。由於潮汐水位的擾動，模擬、分析海岸含水層的流速場時，應採取隨時間變動的定水頭邊界條件，以符合現地的物理狀況。本論文的目的，在探討受潮汐漲落的影響下，臨海地下含水層中地下水流場、污染物遷移及海水入侵之變化。藉由擴展葉高次的地下水流及傳輸模式2DFEMFAT程式，使包含可模擬潮位變動的調和函數，當隨時間變化的定水頭邊界值，以進行模式模擬分析。 在相同的水文地質條件下，模擬有潮汐效應情況6000天海水入侵地下水結果，與Frind[1982]、sego1.[1975]、yehetal.[1995]和Huyakornet al. [1987]等人在不考慮潮汐效應下，所模擬結果比較，尚稱合理。模擬結果顯示，潮汐效應會改變流場狀況，高潮時，海水向內陸移動，低潮時，則往海岸移動；此現象不但造成海水╱淡水過渡區的擴散速率有增大的趨勢，並且有增加海水入侵地下水及污染物向海岸遷移的效果。

The problem of contaminatnt transport and seawater intrusion has been widely recognized in groundwater utilization for many aquifers in various parts of the world. Traditionally, steady state and transient simulations of seawater intrusion have been performed using an approach while the boundary condition along the seaside is assumed to be a constant head. In reality, in coastal ares\as the periodic rise and fall of the tidewater stage in the ocean do produce progressive pressure waves in adjacent aquifers. As there pressure waves propagate inland. the groundwater flow field as well as the transient position of seawater front and contaminant plume continuously changes accordingly. Thus, there is a need to include a tidal boundary in the simulation model that accounts for tidal flucuation. One realistic way is to use the harmonic function for representing the real tidal cycles. The object of this study is to investigate the impact of the tidal fluctuation on the mixing of freshwater and seawater as well as contaminant transport in coastal aquifers. The model 2DFEMFAT is modified to include a harmonic function, as the time-varying constant head boundary. This model is utilized to simulate and analyze the contaminant transport problem in coastal aquifers. Comparisons of seawater intrusion at the simulation time of 6000days from the modified 2DFEMFAT with that of Frind [1982], segol et al.[1975],Yeh et al.[1995] and Huyakorn et al. [1987] under the same hydrogeological conditions show reasonable match. The simulated results indicate that the tidal fluctuations do influence the groundwater flow field and cause seawater intrudes inward at high tide period and outward at low tide period. Also, the rate of expansion of the mixing zone is faster than the one without considering tidal boundary. It is concluded that the tidal fluctuations do enhance he seawater migration and mixing as well as the contaminant transport toward the sea.