Saltwater intrusion can naturally occur in coastal aquifers, owing to the hydraulic connection between groundwater and seawater. Because saline water has a higher mineral content than freshwater, it is denser and has a higher water pressure. As a result, saltwater can push inland beneath the freshwater
There are several reasons to saline, the most important of them is seawater intrusion. This case occur in the coastal areas. Also some of sedimentary rocks is cause to saline "as limestone", that is through dissolving.
There are several methods. Sea water intrusion through rivers and lakes is the most common. Further, there are lands which had been under the sea, long before, but later converted to highlands due to sedimentation of volcanic actions. We can see such lands even in the interior locations.