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Barrier systems are subsurface in nature; they fall into two categories: hydraulic barriers and physical barriers (Pool and Carrera 2010).
Hydraulic barriers can be divided into positive, negative and mixed barriers. In positive barriers freshwater is injected through recharge wells into the aquifer to raise the water table thus impeding the inland motion of saltwater. Negative barriers involve the interception of the intruding saltwater by pumping near the coast. A mixed hydraulic barrier combines a positive barrier and a negative barrier injecting freshwater inland to repulse the saltwater wedge and extracting saltwater near the shore to slow its advancement (Abdoulhalik et al 2017).
Physical barriers are constructed parallel to the coast and consist of a low-permeable material (such as steel or concrete) that acts to block the intrusion of saltwater into the aquifer. There are two types of physical barriers. Subsurface dams are imbedded in the impervious bottom layer of the aquifer only obstructing the lower part and allowing the natural discharge of freshwater to the ocean above. Cut-off walls extend from the top of the aquifer to a predefined depth (Abdoulhalik et al 2017).