關渡平原位於臺灣北部，為一片沖積平原並是台灣少數完整之生態濕地系統。先前研究指出位於關渡平原東北方之北投地熱谷溫泉之砷含量高達4.6 mg/L，其值遠超過世界衛生組織(WHO)訂定之飲用水水質污染標準0.01 mg/L，而含砷之溫泉水可能沿著地下水進入下游之關渡平原、關渡濕地及淡水河河口生態系統，進而影響用水安全與造成生態危害。本研究以MODFLOW建立關渡平原水文地質模式模擬地下水的流動，再以MODPATH推測砷於地下水中傳輸路徑。模式率定以關渡平原22口鑽探井歷史水位資料為依據，率定及驗證後之地下水位均方根差為0.778公尺，達到預定均方根差小於1公尺之目標值，且觀測與模擬水位間之決定係數為0.9994並達顯著水準。模擬結果顯示地下水流方向由東北方往西南方，由山區向關渡平原方向流動，水位隨著地表高程向關渡平原遞減，與觀測水位之趨勢符合，可藉此驗證模擬與實際地下水流狀況。最後由MODPATH模擬中推測砷可能沿著地下水水流注入磺港溪進而匯入關渡平原，未來建議應用追蹤劑試驗或同位素分析以驗證本研究成果之準確性。 Guandu plain is located in northern Taiwan, where is an alluvial plain and few complete wetland system in Taiwan. Previous studies indicated that arsenic (As) concentration of geothermal spring in Beitou area was up to 4.6, significantly exceeding the World Health Organization (WHO) maximum contamination limit (MCL) of 0.01 in drinking water. High As contents may move by groundwater to downstream of coastal plain, wetland, and estuary ecosystem, causing the concern of the domestic safe use of spring water and the potential hazard of ecological environment. The objective of the study is to develop a hydrogeologic model of Guandu plain using MODFLOW and to estimate the flow path of arsenic in groundwater using MODPATH. The simulated groundwater level is calibrated and validated against the measured groundwater level with the root mean square error (RMSE) of 0.778m and the R-square is upon 0.9994 that is extremely significant. The analyzed results show that groundwater flow direction is from the northeast to the southwest which water table decrease with terrain. Furthermore, we estimated that the flow path of arsenic in groundwater was from Huang Gang creek to Guandu plain , and thus requires further research and discussion.
