EFFECT OF FRESHWATER RECHARGE ON SALTWATER INTRUSION THROUGH AQUIFER

Abstract

The management of salt water intrusion in the coastal aquifer is the major environmental problem faced by water resource planners worldwide. Saltwater intrusion is the process of movement of saline water into the fresh water aquifer, which contaminate the drinking water resources. This contamination process is occurring because of high demand of water during summer months when people extract more groundwater from aquifer. Once saltwater intrusion has begun, the continued irrigation of crops using water with high salt content produces a progressively higher salt residue in soil which accelerates the desertification of soils. In order to control the process of salt water intrusion, various laboratory experiments were performed to study the pattern of saltwater transport in freshwater aquifer. All experiments were conducted in a sand box. The sand box was divided into three distinct chambers: a central flow chamber containing the porous medium and two constant-head chambers containing salt water and freshwater. There were several ports present on the back side of the sand box. These ports were arranged at a 14 cm apart from each other but first port was at 7 cm from left side of sand box and second port was at 7cm ahead from first port and remaining ports were 14cm apart from each other. Each port was connected with syringe from which the sample of saltwater was collected. In first experiment the sand box was allowed to transmit freshwater from left to right at a fixed gradient condition. Subsequently saltwater was introduced in the sand box to see the progression of saline water concentration in the aquifer system. The saltwater concentration was measured with the help of conductivity meter. After that a series of three experiments was performed with three different freshwater recharging fluxes (900mm/day, 2000mm/day and 3300mm/day) by keeping the similar hydrological conditions. The breakthrough curves were obtained at different locations by sampling the soil water from various ports. After 6 hours, It was observed that under no recharge of freshwater a salt concentration was observed upto 40% of the boundary condition (35000ppm) at port 2, while it was further delayed in ports located to down gradient locations. On the other hand, under recharging cases of 900mm/day, 2000mm/day and 3300mm/day the saltwater concentration in port 2 increased only upto 37%, 33.6% and 28.6% of initial concentrations, respectively. This shows the positive role of surface water recharge on ingress of saline water in fresh aquifer zones. Before simulating our experiments datasets first we took literature datasets and simulate these datasets in SEAWAT simulator of iv visual MODFLOW for model testing. After that, simulation experiments were conducted for these cases using SEAWAT simulator in visual MODFLOW integrated with PCG solver. The simulated and observed breakthrough curves were compared under different recharging conditions and compared well by calculating normalized root mean square value for all model domains. The results of this research are direct use in controlling the saline water intrusion by surface water recharging techniques at field level.