Notes on the geology and Water Resources of South Florida

Prof. Dean Whitman, Florida International University
Last Rev. 9/1997

General

The Florida peninsula and adjacent continental shelf occupies the Floridan Plateau

The Highest elevations in the state are in the central highlands, coincident with a long-standing geologic feature, the peninsular arch, which runs down the center of the state.

The Floridan plateau is composed of a thick section of mostly undeformed carbonate rocks. This carbonate platform that has been developing since the late Triassic opening of the North Atlantic (~180 MA)

The underlying igneous and metamorphic "basement" rocks are Jurassic and older.

Development of the platform was controlled by regional subsidence of the passive margin and eustatic sea level changes which allowed the deposition of a thick section of carbonate rock over many millions of years.

Various types of carbonate deposition
-- shells, limy mud, offshore sand bars, coral reefs
The peninsula has been built outward and upward over time by these processes.



South Florida


Topography and Physiography

Elevations are highest on the east coast and decrease westward (see handout)

Atlantic Coastal Ridge
The Everglades
  • -- regional trough, elevations less than 10 feet.
  • -- accommodates regional southward drainage from L. Okeechobe
  • Sandy flatlands
     
    Coastal glades
     
    Mangrove swamps and brackish estuaries

    Geology


    The surficial deposits that form these features are very young. All are less than 6 MY old. Most formed around 120,000 years ago (Pleistocene) during an interglacial period when sea level was ~ 25 ft higher -- pamlico shoreline

    Most surficial features are related to depositional environments formed during this latter sea level high stand. These environments are reflected by several different geological units (formations) -- see fig 2 in handout -- generally the of the same age. These formations gradually grade from one type to another. The relative age at a given site is determined by local stratagraphic relationships.

    Miami fm.
    Ft. Thompson fm.
    Anastasia fm.
    Key Largo limestone:

     

    Subsequent History

    Wisconsin Glacial period

    The present day productivity of the Biscayne aquifer is largely an effect of this secondary porosity.

    More recently, formation of sedimentary barrier islands: Miami Beach, Virginia key, Key Biscayne


    Florida Hydrogeology

    2 major aquifers in Florida: the Floridan (the whole state), and the Biscayne (south Florida)

    Floridan Aquifer:
    2 major recharge areas,

    In South Florida, the Floridan is brackish and is not useful as a water supply

    Biscayne Aquifer
    Forms the major water supply in Dade, Broward, Monroe Counties

    The Biscayne Aquifer is very susceptible to pollution for several reasons
    -- great permeability, unconfined, near surface, in urban area
    -- many landfills, leaking gasoline tanks, airports, industry

    Local water resource management

    Managed by the South Florida Water Management District (SFWMD) with system of levees and canals.

    Surface water is used mainly in the wet season
    Ground water flows towards coast

    The situation today is much different than it was before ~ 1920s

    Before development, the Atlantic coastal ridge formed the eastern rim of everglades. Regions as far east as Coral Gables were flooded during wet season and were grasslands in the dry season. Only the eastern areas on the coastal ridge were perennially dry with surface water flowing through low spots in coastal ridge during the largest floods. These were the areas that were settled originally.

    The water table was higher along the east coast than beneath the everglades, mirroring the topographic Atlantic coastal ridge. This high in the water table prevented the flow of ground water eastward out of the Everglades. The hydrostatic head of the groundwater was so large that there were fresh water springs beneath Biscayne Bay.

    Starting in 1920's canals were cut through the Atlantic coastal ridge (Miami, Tamiami, Coral Gables etc.) to drain water from the everglades and provide more land for farming and development. Canals were also constructed for purposes of flood control. The construction of these canals had several immediate consequences.
    -- allowed drainage of surface water of everglades
    -- lowered water table by several feet
    -- increased salt water intrusion along the coast and along the margins of the canals (see figure)

    The salt water intrusion eventually threatened several municipal well fields. Today, control structures are installed in the canals to prevent salt water from flowing upstream. Surface water is also released to resist intrusion and the advance of salt water into the Biscayne aquifer seems to have been stemmed for now.


    References:

    Fish, J. E., and Stewart, M., 1991, Hydrogeology of the surficial aquifer system, Dade County, Florida, U. S. Geological Survey Water Resources Investigation Report 90-4108, 50 p.

    Hoffmeister, J. E., 1974, Land from the Sea, the Geologic Story of South Florida, University of Miami Press, 143 p.

    Klein, H., and Waller, B. G., 1985, Synopsis of Saltwater Intrusion in Dade County, Florida, Through 1984., U.S. Geological SurveyWater Resources Investigation Report 85-410.

    Miller, J. A., 1986, Hydrogeologic framework of the Floridan aquifer system in Florida, and in parts of Georgia, Alabama, and South Carolina: U. S. Geological Survey Professional Paper 1403-B, 91 p., 33 pls.

    Randazzo, A. S., and Jones, D. S.,eds., 1997,The geology of Florida ., University Press of Florida, 327 p.

    Reese, R. S., 1994, Hydrogeology and the Distribution and Origin of Salinity in the Floridan Aquifer System, Southeastern Florida, U. S. Geological Survey Water Resources Investigation Report 94-4010, 56 p.