Theoretical values are not valid for the relation of fresh-water head to the depth of the fresh-water-salt-water interface beneath barrier islands when there is substantial motion of fresh groundwater and where the sedimentary deposits are not homogeneous or isotropic. Zones of fresh and salt water are stratified; fresh water occurs in zones of high permeability, whereas salty water occurs in zones of low permeability. Near Frisco, on Hatteras Island, North Carolina, the concentration of chloride in groundwater varies inversely with the relative permeability of the water-bearing zones to a depth of about 140 feet. Permeable zones generally contain water with a chloride content of less than 100 ppm, whereas relatively impermeable zones contain water with a chloride content greater than 100 ppm. The position of the contact of fresh water with salt water depends on the permeability of the sediments at depth and is affected by differential flushing by fresh groundwater as a result of the retreat of the sea water originally saturating the entire stratification, rather than by fresh-water head differences in the shallow aquifer system.