Dateline: 09/19/00 Edited down by John Ensworth 1/15/01
Fresh water is lighter than salt water. Therefore, fresh water "floats" on top of salt water. This principle becomes extremely important when considering the drilling of a well in order to tap into the ground water of any island. The weight of the rain water that percolates into the ground depresses the salt water beneath it forming a profile that has the appearance of a lens. This is called the Ghyben-Herzberg lens. The principle of this relationship was discovered independently by a Dutch scientist named Baden-Ghyben and a German scientist named Herzberg.
The underground boundary that separates the fresh water layer from the salt water is not a sharp boundary line. In reality, this boundary is a transition zone of brackish water (fresh/salt mixture). This is caused by seasonal fluctuations in rainfall, tidal action, and the amount of water being withdrawn either by humans or by natural discharge.
Fresh water has a density of 1.0 while salt water has a density of 1.025. From this, you can see that salt water is slightly heavier than fresh water. The ratio between the two is 41:40. The formation of the Ghyben-Herzberg lens has a profound effect upon the availability of fresh water on an island. This principle essentially states that for every foot of ground water above sea level there are forty feet of fresh water below sea level!
Understand that this applies only to fresh ground water that is sitting directly on an intruded body of salt water. It has no meaning on a large island where an inland body of ground water may be confined by layers and dikes of lava rock. On islands that are largely composed of dense lava rock, little if any salt water intrudes very far into the underlying rock structure. It is generally only in shoreline regions that salt water intrudes into the cracks, crevices, and loose rock spaces.
On low, small islands that are largely composed of coral or other porous materials, salt water intrusion into the underlying interior is quite common. The drilling or digging of wells on these islands and especially on along the shoreline must be done with care. Going too deeply will penetrate the transition zone and result in salt water infiltration and the contamination of the fresh water in the well. I have seen such a well dug in solid lava rock along the shoreline in the Ka' u District on the island of Hawai‘i. This well was located a few feet above the high tide line approximately a hundred feet from the ocean. The opening was rectangular, about 3 1/2 feet wide, and 6+ feet long. On one end a set of stairs had been cut into the rock allowing one to walk down five or six feet to the level of the well water. This well was unused as there are no dwellings or settlements in the immediate area. In fact, the area was considered to be "range" land where a few head of cattle managed to find enough to eat in this dry, desert-like environment. The size and construction of the well indicated that at one time it may have served a considerable number of people and possibly even an old Hawaiian village. Unfortunately the well had been abandoned and neglected. It contained a fair amount of decaying leaves and grass. There was also a considerable amount of broken glass from beer bottles that had been thrown against the interior sides of the well. This was probably done by shore fishermen who frequented the area from time to time and used the well as a convenient trash dump during evening camping parties. Nonetheless, the well serves as an excellent example of the Ghyben-Herzberg lens's importance to the availability of fresh water in a place where one might think that none can be found.
This was transferred from: http://geography.about.com/science/geography/library/misc/ucghyben.htm?rnk=r1&terms=fresh+water+lens
Another interesting link: http://www.arm.gov/docs/education/background/impacts.html