Investigations of the fluctuations of chloride content in wells that tap the zone of diffusion between fresh and salt water show that the salty well water behaves erratically when the well is pumped. Frequently, a static distribution of chloride content that ranges from less than 1,000 ppm at the top to more than 10,000 ppm at the bottom is present in the open‐hole part of a well. When the well is pumped, the discharge water tends to come from the upper part of the open hole because less energy is expended by removal of low‐density water from this region than by removal of high‐density water from the lower part of the open hole. Where the permeability of strata in the deep part of the open hole is greater than that in the shallow part, the tendency for natural selection of the less dense, shallow water is suppressed, and practically all the water in the blend comes from the deep part. As a result of this complex interrelation of hydraulics, distribution and density of the salt water, and permeability, the depth at which the pumped water enters the well bore is indeterminate. This deficiency leads to the use of multiple‐depth‐bottle, windshield‐wiper, and electrical‐conductivity sampling techniques for collection of data used in constructing maps and cross sections that show the areal‐depth relations of salt water in the Biscayne aquifer of the Miami area, Florida.