Salt water encroachment in limestone at Silver Bluff, Miami, Florida

Abstract

Silver Bluff is part of a low coastal ridge that averages approximately 8 feet above sea level. To a depth of 20 feet it is composed of oolitic limestone, which, in turn, is underlain by coral limestone, calcareous sandstone, and sandy limestone of extremely high permeability to a depth of 120 feet. Shelly sandy marl of low permeability occurs from 120 to 160 feet and relatively impermeable clayey silty marl below a depth of 160 feet.Construction of drainage canals was begun about 1910 and as a result the water table has been lowered several feet. This has allowed salt water to encroach inland 8,000 to 9,000 feet. A balance between fresh and salt water, in accordance with the Ghyben-Herzberg Theory (1:40 ratio) appears to be established for a zone between the shore and 2,500 feet inland. Beyond this zone the actual contact between fresh and salt water is lower than the theoretical contact computed on a 1:40 ratio. It is probable that a balance has not been reached beyond 2,500 feet because of insufficient time since the water table was lowered by drainage, but as time goes on and the salt wedge continues its encroachment, equilibrium will be established farther and farther inland. The salt wedge should finally come to rest where a sufficient weight of fresh water above mean sea level will force the salt water to the bottom of the highly permeable aquifer.