Saline water intrusion can be considered an issue of growing importance in coastal-belt regions around the world. A large number of coastal aquifers are affected by this saltwater intrusion in the southern part of Bangladesh. This process is aggravated due to both climatic and anthropogenic causes which include: reduced freshwater discharge, faulty management practices in case of coastal polders, tide penetration, sea level rise, backwater flow and low flow conditions in rivers. Consequently, due to high demand of fresh water, over extraction of groundwater has accelerated the problem in many folds. As Bangladesh lies in the low deltaic zone, the nation faces the extremities of salinity intrusion associated with socio-economic impacts, since agriculture roughly employs about half the nations

The impacts of saline water intrusion are inclusive of chemical imbalances in the topsoil inclusive of changes in ion concentrations and pH value of the soil which inevitably will lead to reductions in crop yields, crop size and increase in amount of defective crops. All of these impacts critically influence socio-economic as well as environmental dynamics of the region. It is important to not overlook the importance of the increase in river water salinity as a contributor to the increasing salinity of the soil in the Southern part of Bangladesh; nevertheless it does appear that saline water intrusion through groundwater plays a key role here.

Numerous measures, for both hydraulic and engineering approaches, that can be taken to control saltwater intrusion in groundwater (coastal aquifer system) and subsequent increase in salinity of agricultural soil are available but only some of them are feasible in the context of Bangladesh when considering the fiscal and technological inputs involved.

One feasible method that could be used to control salt water intrusion into groundwater sources (i.e. the coastal aquifers) is artificial recharge. In this context, artificial recharge refers to the injection of water into a low water quality or contaminated aquifer with no intent to withdraw the injected water. The injected water then produces a hydrodynamic barrier that has the effect of a physical barrier to sea water intrusion. The hydrodynamic barrier is created by raising the piezometric head of the fresh water aquifer and preventing the salt water from moving inland. Hydrodynamic barriers are created with either recharge basins or recharge wells. It is imperative to realize that this process involving injection wells could also be modified so that instead of just managed recharging of these coastal aquifers through freshwater collected or diverted from other sources, sufficiently treated wastewater could be injected near the salt water interface to aid in maintaining a sufficient head of freshwater. Another possible solution presents itself in the process of extracting seawater through extraction wells from the aquifer and provides a means by which the interface between fresh and saline water can be drawn down and to reduce the loss of freshwater by outflow to the sea. Likewise a seemingly interesting solution is also found in the combination of extracting/injecting wherein both the processes are used simultaneously at strategic points for greater degree of effect. Examples can be conveniently found by observing the saltwater intrusion barrier wells data from the USEPA.

On a more geological note, installing impermeable barriers such as grout and steel sheet piles is normally limited to areas where the contaminated aquifer is relatively shallow and the

subsurface geology allows for a proper seal. One of the new technologies that are being developed overseas uses what scientists are calling crystallization technology. The "CRYSTECHSALIN" process reduces the permeability of the geological rock formations that act as a barrier for the coastal aquifer. By artificially inducing a crystallized solution of slightly soluble minerals from over saturated solution, the porosity of the rock formations can be sufficiently altered and can prevent the saltwater from seeping through the rock into the aquifer. This crystallization process interacts with soil and geologic rock formations turning into a grout like binding agent. The over saturated crystallized solution used has flow and seal properties that allow it to penetrate through the soil and rock layers while creating a seal that diminishes the permeability of the rock barriers, hence preventing saltwater from intruding into the aquifer. It is essential to note that since this is a new technique a proper study, to ensure that the solution is applicable to geological conditions of Southern Bangladesh, is necessary.

Apart from salinity intrusion barriers which have been discussed so far, proper farming practices are also essential in case of the saline groundwater affecting the agricultural soil. Drainage to intercept and redirect groundwater is important for the agricultural practices in the southern regions, since locating the drain to intercept good quality water across the slope, and discharging this water safely into a waterway, drainage line, and dam or storage area in essential to managing the problematic effects of salinity on agricultural lands. This would lower the water table in the salt affected areas and thereby promote the natural leaching of salts to reduce salinity level in the soil.

The methods and strategies mentioned and reviewed here, are quite aptly feasible in terms of the technology being deployed in its use, but it is important to clarify here that solutions to the problem of increasing salinity in the agricultural soil due to saline water intrusion in groundwater have only been considered from an engineering point of view. And as per that criterion, the hydraulic and engineering controlling measures and modifications herein are solutions of a more or less technical order.

References:

1. Lanbo Liu, SALINE WATER INTRUSION, in Natural and Human Hazards, from Encyclopaedia of Life Support Systems (EOLSS), Developed under the Auspices of the UNESCO, Eolss Publishers, Oxford, UK, [http://www.eolss.net][Retrieved February 3, 2006]

2. Lubna Seal and Mohammed Abdul Baten, SALINITY INTRUSION IN INTERIOR COAST: A NEW CHALLENGE TO AGRICULTURE IN SOUTH CENTRAL PART OF BANGLADESH, Working Draft. [www.unnayan.org][Retrieved June 16, 2014]

3. USEPA, THE CLASS V UNDERGROUND INJECTION CONTROL STUDY, VOLUME 20, SALT WATER INTRUSION BARRIER WELLS, [http://water.epa.gov][Retrieved June 16, 2014]