Investigations and comparison of the ways to prevent saline water advancement in zones with shallow water table

One of the threating factors to freshwater resources is the advancement of saline water and intrusion into the groundwater aquifer. This problem occurs in coastal zones and desert margins and reduces freshwater quality. Evaporation from the soil surface and the water table depth are factors affecting the salinity and salt distribution in the saturated and unsaturated zones. HYDRUS and the accompanying software package provide numerical models used to simulate the movement of water, solute and heat in a porous medium for saturated and unsaturated conditions. According to the existing reports on the ability of the HYDRUS model to simulate moisture and salinity, using it can help decide and consider how to prevent the salinity advancement.

In this research, the advance of saline water with a concentration of 20 dS/m and a level of 25 cm towards freshwater with a concentration of 1 dS/m and a level of 10 cm in the domain of 360×70 cm is considered. Different scenarios were examined to prevent the progression of salinity using a validated model. The studied scenarios include inceptor pipe drainage, inceptor open drainage and subsurface barrier which were simulated using the HYDRUS-2D model. The parameter of the equations governing water flow and solute transport were estimated using observed moisture and salinity data and inverse solution tools in the HYDRUS-2D model. pipe and open drainage were considered at three distances of 270,180 and 90 cm from the freshwater reservoir and two depths of 15 and 5 cm from the impermeable layer. The effect of the subsurface dam on preventing the advance of saline water at three depths of 55, 65 and 70 were investigated.

Different scenarios of different drainage locations have been simulated to study salinity distribution and water table after 6 months. Regarding the location of the drainage site, three factors are important that have been studied: 1- The amount of water and salt outflow from the drainage 2- Controlling and preventing the advance of salinity 3- Its effect on the entry and exit of water from the freshwater aquifer. The location of surface and subsurface drainage showed different effects on salinity advancement. By changing the location of drainage from A to f the amount of drained water in pipe and open drainage decreased by 5.8 and 6.5 m3/m, respectively. Drainage location also affected actual evaporation from soil surface and salinity accumulation in the soil surface layer. In the cases of drainage where the lowest and highest evaporation from the soil surface occurs respectively, 15% difference was observed. In the case of open drainage at a distance of 90 cm from the freshwater reservoir and a depth of 5 cm from the impermeable layer, the amount of actual evaporation from the soil surface during the whole simulation period is greater than the actual evaporation in non-drained condition and also caused increased salinity between two reservoirs. The subsurface barrier has generally blocked the saline water flow only when it has reached the impermeable layer. The profile of the water table is broken due to very low hydraulic conductivity (about 0.1 m per day) when it reaches the subsurface barrier. The amount of failure and drop of the water table increased with increasing barrier depth.

The salinity distribution parameters in the area between two aquifers, discharged drain from drainage and its concentration and protection of freshwater aquifer are affected and can be considered according to the condition of each location. On the other hand, each of the scenarios has a positive and negative effect on these factors, so according to each sample and specific location, it must be decided how to prevent the progression of salinity (drainage or subsurface barrier) and their location.