When rainfall occur and the water moves from rock and Salt in the sea comes from rocks on land.

The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid which forms from carbon dioxide and water.

As the rain erodes the rock, acids in the rainwater break down the rock. This process creates ions, or electrically charged atomic particles. These ions are carried away in runoff to streams and rivers and, ultimately, to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water.

Thus the water from rainfall mixed with sea and thus it becomes salty.