Saltwater Intrusion | Geology Optional Notes for UPSC PDF Download

Saline Water Intrusion

• Definition of Saline Water Intrusion: Saline water intrusion refers to the increase in the salinity of groundwater either laterally or in depth. This phenomenon is primarily caused by human activities that disrupt the natural groundwater system.
• Cause and Impact: The intrusion of saltwater into extensively exploited aquifers poses a significant challenge, especially in coastal regions and areas where fresh and saline water aquifers are in close proximity. Coastal aquifers are particularly vulnerable to sea water intrusion, with the invading water predominantly exhibiting seawater characteristics. On the other hand, inland areas may experience a range of water qualities varying from brackish to highly concentrated brines.
• Significance: Saline water intrusion is a pressing issue with far-reaching consequences, affecting both coastal and inland regions. The contamination of freshwater sources with saline water can have detrimental effects on agriculture, ecosystems, and human populations reliant on these resources.
• Examples: An illustrative example of saline water intrusion is the over-extraction of groundwater for agricultural irrigation near coastal areas. This excessive pumping can create a pressure gradient that draws in saltwater from the nearby sea, contaminating the freshwater aquifers. Similarly, in inland regions, improper waste disposal practices can lead to the infiltration of saline water into underground water sources, compromising their quality.

Saltwater Intrusion

• Definition of Interface: The boundary separating saltwater and freshwater is known as the interface. This boundary shifts towards the freshwater area when a hydraulic gradient is established, typically from the saline water region towards the freshwater zone. Factors like groundwater development or changes in water levels, such as during high tide, can influence this movement.
• Variation in Groundwater Disposition: The distribution of freshwater and saline groundwater bodies can vary significantly based on hydrogeological and hydrological conditions. This variability plays a crucial role in understanding and managing saltwater intrusion issues.
• Challenges of Saltwater Intrusion: Once saltwater intrusion occurs, it becomes challenging to mitigate its adverse impacts. Therefore, careful planning and management are essential when developing groundwater resources in areas where freshwater overlays or adjoins saline zones.
• Managing the Interface: It is important to establish a safe limit for the movement of the interface beforehand. Neglecting this precaution can lead to complacency, especially when extraction points are distant, as the interface's movement may seem slow. Notably, in coastal aquifers, a small change in the water table elevation can result in a significant shift in the interface's position.

Controlling Sea-Water Intrusion

• Reducing Pumpage and Implementing Cyclical Pumping:

• One effective method involves reducing the amount of water extracted and adopting a cyclical pumping approach. By rearranging the pumping pattern or redistributing extraction points, a sea-ward hydraulic gradient can be restored. For example, instead of continuous extraction, pumping can be done intermittently to allow the aquifer to recharge naturally.

• Artificial Recharge of Aquifers:

• To maintain proper water levels and hydraulic gradients and eliminate overdraft conditions, aquifers can be artificially recharged. This involves replenishing the intruded aquifer with water. For instance, excess surface water can be directed into the aquifer through various methods such as percolation ponds or injection wells.

• Creation of Fresh-Water Ridge:

• Creating a fresh-water ridge can help combat sea-water intrusion. This can be achieved through the use of injection wells or other artificial recharge methods. For instance, excess rainwater can be stored underground to form a barrier against saltwater intrusion.

• Construction of Artificial Subsurface Barriers:

• To prevent sea-water intrusion, artificial subsurface barriers can be constructed. This may involve building dikes or injecting materials like cement grout or bentonite mud through boreholes. For example, impermeable barriers can be created underground to block the movement of saltwater into freshwater aquifers.

Applicability to Inland Fresh-Water Aquifers

The measures mentioned above are not limited to coastal areas but are also applicable to inland fresh-water aquifers that are adjacent to saline zones. By employing these strategies, the intrusion of saltwater can be effectively managed in various geological settings.