The fresh water in the Tidewater region floats on top of salt water; salt water is slightly denser than fresh water. The salt and brackish water intrudes from the ocean, Chesapeake Bay, and the northern extension of Albemarle/Pamlico Sound via the porous sediments underlying Tidewater.
If you calculate the Ghyben-Herzberg Lens, then there should be roughly 40 feet of fresh water underground for every foot of elevation that is saturated with fresh water. For example, a section of Virginia Beach that is 4 feet in elevation, with a groundwater level at 1 foot below the surface, should have 120 feet of fresh water underneath. (Math: 3 feet of elevation saturated with fresh water X 40 feet = 120 feet) Sink a well in the back yard beyond 120 feet, and you won't find "potable" (drinkable) water.
However, the groundwater levels around Newport News are at a confusing mix of depth, due to a very peculiar reason: