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A three-dimensional mathematical model of variable density groundwater flow and miscible solute transport is developed to investigate the saltwater contamination of a coastal aquifer in west Japan. The selected area is an agricultural region located on a coastal aquifer where the groundwater is being consumed continuously as the main water source for drinking, greenhouses and industries. The salinity of the pumped water has been analysed periodically and it has been found that the threat of saltwater intrusion is emerging due to the continuous exploitation of groundwater. Owing to the distributed nature of pumping-well locations and nonuniform geological conditions, a three-dimensional model is needed to understand the saline contamination of the coastal aquifer. A numerical model was built by using the finite difference framework, taking into account the development of a transition zone and the variation of fluid density within it. The model simulates the saturated and unsaturated zone groundwater flow under the influence of a variable density situation in the coastal aquifer. Moreover, it can be used as a numerical tool to simulate the salinity contamination of the freshwater in this coastal aquifer. NOTATION C concentration of water in the flow domain C w [1/L] specific moisture capacity D M [L 2 /T] fluid molecular diffusion coefficient D xx , D yy , D zz , D xy , D xz , D yx , D yz , D zx and D zy [L 2 /T] dispersion coefficients h [L] pressure head k(h) hydraulic conductivity k r unsaturated hydraulic conductivity k s ratio of saturated hydraulic conductivity S s [1/L] specific storage coefficient t time [T] u [L/T] Darcy's velocity in the x direction u9 [L/T] real pore velocities in the x direction V ¼ [L/T] the magnitude of the velocity vector v [L/T] Darcy's velocity in the y direction v9 [L/T] real pore velocities in the y direction w [L/T] Darcy's velocity in the z direction w9 [L/T] real pore velocities in the z direction AE dummy which takes 0 in the unsaturated condition and 1 in the saturated condition (Equation 4) AE L [L] longitudinal microscopic dispersion length AE T [L] transverse microscopic dispersion length Ł volumetric water content r [M/L 3 ] model domain water density r f [M/L 3 ] freshwater density r s [M 3 /L] density of saltwater 1. INTRODUCTION Anthropogenic activities are one of the main sources for saltwater encroachment in the coastal aquifers. Essink Gualbert (2001) quotes that almost 50% of the world's population lives within 60 km of the shoreline. Increasing demand for water in coastal areas causes high extraction rates and, as a consequence, substantial falls in aquifer levels in many coastal areas can be seen. The tremendous increase in population along with the associated increase in human activities has imposed additional demands on groundwater resources in these coastal aquifers. The phenomenon, which is referred to as saltwater intrusion, can have adverse and long-term impacts on coastal groundwater systems and limit their use as a supply of good-quality water for human...

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Saltwater intrusion, leading to the salinization of fresh groundwater, is the most challenging problem in coastal regions. Saltwater pumping from a barrier well is widely applied to prevent saltwater intrusion. Owing to its easy installation, many studies have investigated saltwater pumping. However, quantitative relationships between the barrier and inland production wells have not been revealed. In this study, lab-scale experiments were conducted to examine the effectiveness of a barrier well on the possible flow rate of freshwater from a production well. Moreover, a two-dimensional numerical model was created and simulated under the same conditions as those used in the experiments to analyze the experimental results. Consequently, a critical pumping ratio of 1.9 was obtained. In the numerical simulation, it was confirmed that an upconing of highly concentrated saltwater toward the barrier well was observed when the pumping ratio was less than the critical ratio. In conclusion, there is a critical pumping ratio between the barrier and the production well, and saltwater intrusion can be controlled by keeping the pumping rates under the critical ratio. Although further studies have yet to be conducted on a practical scale, this study showed the potential of the pumping ratio control to manage saltwater intrusion.

Effect of the fertilizer on chemical properties of ground water was investigated from March to November in a farm land of West of Fukuoka City. The variation of dissolved oxygen (DO) and nitrate (NO3-N) concentrations in ground waters with time was greatly different by the surface conditions of the land: paddy field or vegetable field. In the case of the former, the DO and NO3-N concentrations decreased gradually from May and showed a minimum at the end of August. While, in the case of the latter, the concentrations were highly constant. Despite that the large amount of phosphate was manured, its concentration in ground waters was low.