| Summary: | 碩士 === 國立臺灣大學 === 生物環境系統工程學研究所 === 106 === Groundwater containing arsenic has always been one of the most important environmental issues in the world, In Southeast Asia, due to the marine transgression and regression during the ice age, sedimentary formations were found with high arsenic content in previous studies. A large amount of groundwater extraction containing arsenic not only endangers human health and environment, but also causes land subsidence. Consolidation causes inter-layer water containing high arsenic released from clay or impermeable layer further worsening the groundwater quality. The mechanism of arsenic release and the transport process of arsenic to the groundwater have not been well studied. Erban et al. (2013) explored the land subsidence in the Southeast Asia, Mekong delta, Vietnam and found that the arsenic concentrations in wells may increase with the depth in deep groundwater, which was originally low in arsenic content. They speculated that high arsenic interlayer water may be expelled to the lower aquifer caused by land subsidence. Groundwater in the southern Choushi River Alluvial Fan of Taiwan is abundant. Due to intensive pumping over decades, severe subsidence and deterioration of groundwater quality were found. Government invested a large fund to set up groundwater/subsidence monitoring stations, which may provide valuable spatiotemporal data of subsidence and groundwater arsenic concentration. This study first analyzes the correlation of the land subsidence and groundwater arsenic changes in the Choushi River Alluvial Fan of Yunlin area. Secondly, a three-dimensional groundwater model MODFLOW coupled with the compaction mode software complex process of INTERBED (IBSI) and solute transport mode (MT3D) was developed to simulate the groundwater flow, land subsidence and arsenic transport in Yunlin. The simulation results demonstrate that the intensive pumping leads to subsidence and the increase of arsenic concentration in the deep groundwater aquifer. The inter-related processes among groundwater pumping, subsidence and increase of arsenic concentration identified herein may be useful to the management of groundwater resource in the Yunlin area and other Southeast Asian countries.
The results of the study also show that the arsenic concentration in the aquifer one significantly increases in the major subsidence area, while the aquifer two only with less noticeable increase. The implication of the developed model can further explore the consequences of subsidence caused by groundwater pumping and an increase the arsenic concentration in the deep aquifer using Scenario Analysis.
Scenario Analysis One: If arsenic concentration level is similar in both aquifer and aquitard, the results of the arsenic concentration do not vary much in the aquifer one and the aquifer two after intensive pumping and land subsidence. Scenario One confirmed that the clay or the impermeable layer containing high arsenic concentration plays an important role to furnish stable arsenic sources to lower aquifer after compaction.
Scenario Analysis Two: Increase or decrease the total pumping volume of 15% and 30%. In the results of increase or decrease pumping affects the subsidence area and arsenic concentration, which confirmed the physical process of arsenic release from aquitard to the lower aquifer was attributed to subsidence. Also, intensive pumping causes an expanding of subsidence area which is the driving force of arsenic release. The affecting time of arsenic to lower aquifer may be shorter than 10 years proposed by Erban et al. (2013).
Scenario Analysis Three: The climate changes may delay or shorten the rainy season causing insufficient domestic water supply by the Hushan Reservoir in Yunlin. The groundwater is considered as a supplement water resource. Two pumping schemes are proposed, one is by placing the 60% increase of total pumpage in mid-fan and distal-fan, the other is by placing the 60% increase of total pumpage in proximal-fan. The result shows increasing pumping in proximal-fan is most effective in reducing subsidence and arsenic concentration in the main subsidence area. This study proposes a sound groundwater pumping strategy in Taiwan and other Southeast Asian countries is to select the pumping area with free or loss aquitard layer, which can effectively reduce the subsidence and arsenic release and warrant the safe use of groundwater.
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