模擬北投關渡地區砷之宿命及水文地質化學傳輸

Simulating Fate and Hydrogeochemical Transport of Arsenic in Beitou-Guandu Area

王勁壬(Chin-Jen Wang)

指導教授 ： 劉振宇

國立臺灣大學/生物資源暨農學院/生物環境系統工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.02171

摘要

北投關渡地區砷汙染主要來源為大屯火山，火山活動導致砷在北投地熱谷流出並經過地質化學循環過程中而逐漸累積在地下水體，此外，早期農民引用地熱谷之含砷地面水灌溉下游關渡平原而遭受大面積汙染，因此，地熱谷中的砷可能藉由地面水及地下水兩條路徑移流傳輸至下游地區。本研究應用水文地質化學傳輸模式模擬並解析砷在地面水及地下水路徑中之移動歷程，研究主要分為三部分，第一部分為模擬北投關渡地區地下水流場，模式率定及驗證之R2分別為0.975及0.992，RMSE則分別為2.75 m及1.73 m。應用率定及驗證後的模式其模擬結果顯示地下水流向主要由東北方往西南方，由上游地熱谷區流至下游關渡平原、關渡溼地及河口，隨著地勢變化而改變。第二部分為模擬砷移流傳輸路徑，模擬至第2,360天時，地面水與地下水路徑移動至河口之砷濃度分別為0.0042及7.45 μg/L，與環保署水質監測值(3 μg/L)比較，結果顯示地下水路徑是北投關渡地區砷之主要傳輸路徑。地面水路徑砷濃度明顯降低原因除了上游高砷水體被下游大量水體稀釋外，模擬的水體pH值<鐵氧化物pHzpc值導致砷與鐵氧化物帶電性不同，砷在氧化環境下容易被大量生成的鐵氧化物吸附而降低濃度及移動能力；地下水路徑遲滯砷傳輸原因包含砷被吸附在鐵氧化物及鐵硫化物的表面，錳氧化物與碳酸鹽類礦物則較不明顯。此外，本部分河口區域砷濃度高估的原因可能為未考慮砷與鐵硫化物的共沉澱反應或其他砷之源/匯項等機制。第三部分整合前二部分結果，模擬砷在地下水路徑之二維傳輸時空分布，模擬前3年內，砷濃度隨著地下水流往下游關渡平原及濕地方向逐漸增加，然而3年後砷因為吸附在鐵氧化物及鐵硫化物表面，傳輸現象明顯遲滯。總模擬時間 2,360天時，關渡平原靠近地熱谷的區域其砷濃度在250 μg/L以上，超過地下水第二類監測標準，與實際情形相符；關渡溼地區域砷濃度5-50 μg/L，靠近河口處因受潮汐影響，砷濃度可至10 μg/L以下。本研究依據第三部分建立的水文地質模式進行二種情境模擬，情境一「關渡濕地植物吸收對於砷的影響」中，結果推測關渡濕地主要紅樹林植物水筆仔可能為北投關渡地區砷的匯項之一，且可降低地下水體中5- 30 μg/L之砷濃度；情境二「氣候變遷下開發北投關渡地區地下水對砷污染潛勢之改變」中，模擬結果顯示抽水造成的洩降將會加速砷移動至下游地區，關渡平原北部區域受砷危害的潛勢將會增加，然而關渡濕地則幾乎不受影響。

關鍵字

砷 ； 北投關渡地區 ； 地下水 ； 水文地質化學傳輸 ； PHREEQC ； HYDROGEOCHEM

並列摘要

High arsenic (As) concentration of Beitou geothermal spring was up to 4,600 μg/L, and downstream Guandu Plain and Guandu Wetland may be affected by the movement of As, via the stream and groundwater flow. However, the distribution and hydrogeochemical processes of As from geothermal spring water to downstream of groundwater and Guandu Wetland were not well known. The purpose of the study is to assess the probable pathways (stream and groundwater flow) of As and evaluate fate and transport of As in Beitou-Guandu area. Firstly, the groundwater head and flow directions are simulated in this study area by using HYDROGEOCHEM- fluid flow model. The steady groundwater flow model is well-calibrated with the root mean square error (RMSE) 1.73 m and the R2 0.992. The simulated hydraulic head varies with terrain topography and the groundwater from the Beitou geothermal valley gradually move to the downstream of Guandu Wetland. Secondly, the As concentrations in the downstream river mouth area simulated by 1-Dimension PHREEQC of cases 1 (stream path) and 2 (groundwater flow path) are 0.0042 and 7.45 μg/L, respectively. However, the As concentration of river mouth area was 3 μg/L as observed value of EPA, which compared with results of two cases, the groundwater flow path (case 2) was the dominant advective-transport mobility of As. The major retarded mechanisms of As mobility in case 1 simulation were the surface adsorptions of the iron oxide minerals. Moreover, the major mechanism of As advective-transport mobility in case 2 governed by the adsorption reactions of iron oxide/ iron sulfide minerals. Thirdly, the spatial-temporal distributions of As in Beitou-Guandu area are simulated by the HYDROGEOCHEM - reactive transport model. The result shows that As concentration (>250 μg/L) in groundwater of Guandu Plain was caused by the movement of high As concentration from the Beitou geothermal spring water. In contrast, the low As concentration (5-50 μg/L) in Guandu Wetland was caused by the tidal water dilution. The simulated As concentrations increase in the first 3 years, then gradually decrease due to the adsorption of As on the iron oxide minerals and iron sulphide minerals. Fouthermore, the hydrogeochemical transport model is then applied to assess two scenarios including the effect on the mangrave plants of Guandu Wetland, and the application of groundwater development for adapting the climate change. For scenario 1, the dominant mangrove plants, Kandelia obovata, which can reduce about 5- 30 μg/L As concentration of groundwater. It may be one of the sinks of As in Beitou-Guandu area. For scenario 2, the application of permissible amount of groundwater withdrawn may speed up the As mobility in the downstream Guandu Plain. The result shows that As concentrations in northern Guandu Plain may increase about 200 μg/L. However, the As concentrations in Guandu Wetland are little affected.

並列關鍵字

Arsenic ； Groundwater ； Transport ； Hydrogeochemical model ； PHREEQC ； HYDROGEOCHEM

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