A multifunctional river-mouth lake (MRL) is a man-made lake at the river mouth to intercept river water for recreation, prevention from seawater intrusion, and reservation of surface water. Population and industry in the Taoyuan County have been growing and expanding significantly, water consumption and recreation will become critical for the continuous development. The purpose of this project is to make hydrogeological assessment for the feasibility of constructing MRLs in the coastal area of Taoyuan County. To understand the groundwater conditions and the hydrogeology of the coastal area in Taoyuan County, a research site is selected between Pushing stream and New Street stream for installing three test wells for pumping tests and relevant study. Continuous soil sampling is taken during drilling the three test wells. The geo-electric method is employed at fifteen locations in the research area to understand the area-wise hydrogeology. It is shown that an unconfined aquifer of 37-meter thickness lies under the research area. This aquifer is consisted of three layers of sandy sediments with different grain sizes. In the wet season from May to October, the water table is about 2 meters below the ground surface that is higher than the mean sea level. This high water-table condition is intrusive to prevent seawater intrusion. At a test well 250 meters away from the coastal line, tidal effects are noticed while the electric conductivity is considerably smaller than that of seawater. Thus, tidal effects reach inland at least 250 meters yet seawater does not encroach so far. Conservative assessment is made use of the maximum possible percolation rates at the bottom of the MRL and the minimum possible inflow rates from the nearby river. As such, three-dimensional numerical simulation indicates a man-made lake of an area of 1.2 acres and an water depth of 4 meters can be maintained by an inflow rate of 0.119 cubic meters per day, which is about 5% of the minimum daily flow rate in the nearby Pushing stream. If the actual inflow rate is larger than 0.119 cubic meters per day, more than one MRL or a MRL of a larger area can be constructed. Construction of MRLs in the coastal area of Taoyuan County is feasible by fully utilizing surface water and groundwater. No liner is warranted at the bottom of the MRL such that groundwater can freely move into and out of the MRL. If collapse will not occur, it is suggested that no liners be put around the wall of the MRL. The cost-effective inflatable rubber dam can be used to divert river water to the MRl. A preliminary design of the water conveyance channel and the water-taking unite associated with a rubber dam is given. The three-dimensional numerical simulation incorporates successfully the tidal effects into groundwater movement. Empirical relations for percolation rate and MRL surface area are developed, which can be used to acquire pertinent information. Single-well recovery tests are made and their data analyzed in detail. Results and data of this project are useful for relevant water resource development and irrigation.
