MODELLING SALINE WATER INTERFACE UNDER VARYING GROUND WATER FLOW CONDITIONS

Abstract

Saltwater intrusion is the phenomenon which occurs in the coastal regions where the saline water from water bodies moves towards the freshwater aquifer due to its high density. This process of saline water intrusion accelerated due to excessive pumping of groundwater from coastal areas. Main impacts of saltwater intrusion are: salinization of drinking water resources in coastal regions, reduction in fertility of soil due to salt residues, reduction in crop yields, and desertification of coastal zones. At the same time, changing climatic conditions contributes in groundwater table dynamics resulting in variation of groundwater flow patterns. Thus, a significant variation in groundwater velocities is expected in regions where saline water interacts with the surrounding land mass. The main focus of this study is to investigate impact of varying groundwater velocities on dynamics of saline water interface using a series of sand tank practical and numerical experiments under controlled conditions. Change in concentration of saline water at different location and at different times in the sand tank was observed under different groundwater velocities. Finally observed results were compared with simulation data obtained from Visual Modflow software. The entire study consists of two parts: 1) Conducting a series of practical experiments using two dimensional sand tank setup and 2) Numerical solution of the designed experiments using visual Modflow. Dimensions of sand tank used for this study is 150cm length×55cm height×12.5cm thickness. The tank setup consists of two 8 cm diameter wells fixed at the both sides of the steel box integrated with affront glass sheet. Freshwater was allowed to enter to the tank setup from the left side well with help of a peristaltic pump and a saltwater reservoir was attached to the right side well. Four different groundwater velocity regimes were created in the tank setup for investigating the dynamics of seawater interface. Equal volume of water was pumped out from top of right well to maintain a steady state water table for a considered regime. Throughout the experiment, the saltwater head was kept constant and freshwater head was changed for the considered different groundwater regimes. Subsequently, concentration of saline water at different time period and at different locations was measured using conductivity meter. Further, simulation runs were conducted considering the all four different groundwater flow regimes using Modflow. Experiment and simulation results were compared well at the end. 2 It was observed that in one case when the freshwater level was raised, the groundwater velocity towards the saline water reservoir increased accordingly, resulting in a slow movement of saline water into freshwater aquifer. A lower salinity concentration was observed at higher ground water velocities towards ocean as compared to the lesser groundwater velocities at a particular point. In other words, it took more time for saline water flow under high groundwater velocity conditions to reach at the same concentration as compared with low groundwater velocity regimes. For example, at the location of 14cm from saltwater boundary after time period of 2 hrs, salinity concentration of 8010 ppm was observed for zero groundwater velocity, 5890 ppm for 0.227 m/day, 4102 ppm for 0.455 m/day and 953 ppm for 0.683 m/day groundwater velocity, respectively. Similarly at other time periods, decreasing trend of salinity concentration for higher groundwater velocity towards salt water reservoir was observed. It can be concluded that as long as freshwater level remains quite higher than the seawater level, movement of saline water towards aquifer will be less and thus, extent of salt contaminant in coastal region can be checked effectively. Therefore, results of this study can be used directly in modifying the coastal flow regimes of shallow fresh water aquifers to mitigate the salt water intrusion.