The impact of tunneling construction on the hydrogeological environment of “Tseng-Wen Reservoir Transbasin Diversion Project” in Taiwan Feng-Rong Yang a , Cheng-Haw Lee a,b, ⁎, Wen-Jui Kung a , Hsin-Fu Yeh a a Department of Resources Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC b Sustainable Environment Research Center, National Cheng Kung University, Tainan 701, Taiwan, ROC abstract article info Article history: Received 7 August 2007 Received in revised form 4 July 2008 Accepted 25 July 2008 Available online 15 August 2008 Keywords: Tunnel inflow Hydrogeological conceptual model Groundwater flow Fracture rock MODFLOW FEMWATER Groundwater flow and the associated surface water flow are potential negative factors on underground tunnels. Early detection of environmental impacts on water resources is of significant importance to planning, design and construction of tunnel projects, as early detection can minimize accidents and project delays during construction. The groundwater modeling software package Groundwater Modeling System (GMS), which supports the groundwater numerical codes MODFLOW and FEMWATER, was utilized to determine the impact of tunneling excavation on the hydrogeological environment in a regional area around the tunnel and a local hot springs area, at the “Tseng-Wen Reservoir Transbasin Diversion Project”, in Taiwan. A hydrogeological conceptual model was first developed to simplify structures related to the site topography, geology and geological structure. The MODFLOW code was then applied to simulate groundwater flow pattern for the hydrogeological conceptual model in the tunnel area. The automated parameter estimation method was applied to calibrate groundwater level fluctuation and hydrogeological parameters in the region. Calibration of the model demonstrated that errors between simulated and monitored results are smaller than allowable errors. The study also observed that tunneling excavation caused groundwater to flow toward the tunnel. No obvious changes in the groundwater flow field due to tunnel construction were observed far away in the surrounding regions. Furthermore, the FEMWATER code for solving 3-D groundwater flow problems, in which hydrogeological characteristics are integrated into a geographic information system (GIS), is applied to evaluate the impact of tunnel construction on an adjacent hot spring. Simulation results indicated that the groundwater drawdown rate is less than the groundwater recharge rate, and the change to the groundwater table after tunnel construction was insignificant for the hot spring area. Finally, the groundwater flow obtained via the GMS indicated that the hydrogeological conceptual model can estimate the possible quantity of tunnel inflow and the impact of tunnel construction on the regional and local groundwater resources regime of the transbasin diversion project. © 2008 Elsevier B.V. All rights reserved. 1. Introduction As industry and commerce have increased in southern Taiwan, public water demand has grown gradually, and new water supplies are needed to meet the demand. Tai receives considerable precipitation (~2500 mm/year), which is 3.5 times the worldwide average. Precipita- tion during May to October constitutes about 80% of total annual precipitation. However, due to significant topographical relief and unevenly distributed rainfall, most precipitation becomes runoff and drains into the ocean within a very short distance. The mountainous regions may experience annual precipitation as high as 4900 mm, whereas the plain regions may receive less than 1200 mm. The ratio of precipitation in the wet and dry seasons is also depending on regional properties. For instance, the precipitation ratio during wet and dry seasons is 6:4 in northern Taiwan and 9:1 in southern Taiwan. The available water quota for each person in Taiwan is only 12.5% of the world average. Efficient management is extremely important in Taiwan, since water resources are very limited (TPWCB, 1998; WRA, 2004). To balance the seasonal variations of rainfall, many reservoirs and water diversion tunnels have been constructed in Taiwan. The “Tseng- Wen Reservoir Transbasin Diversion Project” is one such project currently underway in southern Taiwan. The diversion project will include a water tunnel, diverting water from the Lao-Nong Creek, a tributary of the Kao-Ping River (Fig. 1), to the Tseng-Wen Reservoir, thus increasing inflow and improving the potential of the reservoir for the needs of future long-term water demand in southern Taiwan. When the project is completed, an additional of approximately 600,000 m 3 of water can be supplied daily to southern Taiwan users. The project has been approved and is scheduled for completion before 2012. The Tseng-Wen Reservoir is the largest reservoir in Taiwan and can store 609 million m 3 . The watershed area is 481 km 2 and the historical average annual inflow is only 1080 million m 3 (CGS, 2002; Engineering Geology 103 (2009) 39–58 ⁎ Corresponding author. Department of Resources Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC. E-mail address: leech@mail.ncku.edu.tw (C.-H. Lee). 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.07.012 Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo