好氧降解對地下水中BTEX污染團之影響

Abstract

在加油站和大型油庫的石油碳氫化合物(LNAPL)滲漏污染中，LNAPL在地下飽和層中會隨地下水流進行傳輸作用並同時進行自然衰減程序，由於石油碳氫化合物在地下水環境中主要衰減機制為好氧性生物衰減，故透過現場地下水環境的溶氧量以及入滲所改變的氧氣濃度，即可評估場址是否具有發生生物降解的優勢條件，而適度的添加釋氧化劑除了可以使污染物的降解反應持續進行，也可以增加地下飽和層的含氧量，促進好氧性生物衰減率。 在本研究中利用MODFLOW程式模擬地下水流情況並配合RT3D模擬污染物的現地衰減與傳輸。本研究並嘗試模擬現地不同的背景環境和入滲分佈，在假設降雨入滲完全到達地下飽和層的前提下給予不同的氧氣含量以及氧化劑的應用來增加評估BTEX污染範圍的準確性。 研究結果顯示，在加油站BTEX滲漏污染場址中入滲造成BTEX污染物降解所佔總量百分比約為20%，而大型油庫BTEX場址入滲造成BTEX污染物降解的總量百分比約為5%。在入滲分佈影響上，於污染源區域降雨入滲所造成的污染物降解量最高。於污染源下游降雨的入滲造成下游降解速率較快，使的污染團往下游移動較緩。其中在C=0.60的場址以污染源區域降雨場址降解最快，在C=0.75的場址以污染源下游降雨場址降解最快。而在氧化劑的注入方面，在加油站污染場址注入氧化劑90天後可降解BTEX總量百分比約50%-80%；在大型油庫污染場址注入氧化劑180天後可降解BTEX總量百分比約20%-30%。

Petroleum hydrocarbons leaked from storage tanks of gas stations and tank farms can contaminate soil and groundwater. However, the constituents of gasoline can be biodegraded naturally. Aerobic biodegradation is the major attenuation process for these constituents such as BTEX. If the concentration of dissolved oxygen in the groundwater is enhanced whether by natural infiltration or artificially induced, the biodegradation can be accelerated. Furthermore, the oxidants introduced in the remediation process for oxidizing contaminants can also promote biodegradation by increasing the content of oxygen in the underground saturated layer. In this study, the aerobic biodegradation of BTEX plume in aquifer is modeled with Visual MODFLOW. While the groundwater flow is simulated with MODFLOW, the transport and attenuation of pollutants is computed with RT3D. The effect of location of infiltration region and oxygen content of infiltrated water on overall degradation rate of BTEX was analyzed. In addition, the effectiveness of injection of oxidant on enhancing the degradation rate was also looked into. The results show that BTEX degradation resulted from elevated oxygen level induced by infiltration is about 20% in the simulated gas stations leakage scenario. On the other hand, only about 5% of the total amount of BTEX in tank farm site has been degraded. Furthermore, the results showed that the infiltration near the point of leakage has resulted in most significant degradation of BTEX. In contrast, infiltration at the downstream of the point of leakage led to degradation of the leading front of the plume and effectively restrained the plume from moving further downstream. For a runoff coefficient, C, of 0.6 the infiltration in the source region led to fastest degradation, while the infiltration at downstream of source region caused most degradation when C = 0.75. Injection of oxidants into aquifer can lead to degradation 50 – 80% of BTEX within 90 days in the simulated gasoline station leakage case. On the other hand, only 20 – 30% of BTEX degraded in 180 days in the tank farm case.