Because freshwater is lighter than salt water, it floats on top of it. Along the coast, you find what is called a “fresh water lens” where the fresh water floats on top of the salt water.
Private wells along the coast will tap into this fresh water lens as a drinking water supply. Salt water can get into the fresh water lens, making it unusable for drinking. This mixing or intrusion can happen for many reasons, such as hurricanes or storms or when too much fresh water is withdrawn from wells along the coast.