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Saltwater Intrusion

"Water is life; without water, we have nothing. Without water, we die."

So said, Garald G. "Jerry" Parker, Sr. (1905—2000) known as the "Father of Florida groundwater hydrology". A renowned hydrologist, Parker also named the principal artesian aquifer the Floridan Aquifer. Perhaps his most significant legacy for us who reside in the region administered by the Southwest Florida Water Management District is his definition of the Florida Hydrologic Divide of Florida which separates the geographic region where aquifer recharge is solely from rainwater from that to the north of the divide.

Hydrologic Divide

Aquifer recharge to the south of the hydrologic divide is solely from rainfall. For information, north of the divide some groundwater within the Floridan aquifer system is recharged from Georgia and some from Alabama. (Personal communication from Rick Copeland, 2011).

W. Badon-Ghijben (1888, 1889) and A. Herzberg (1901), derived analytical solutions to approximate the behavior of salt water intrusion. Today the relation is called the Ghyben-Herzberg relation and is of profound importance to regional environmental systems.

In west central Florida three factors combine to ensure application of the Ghyben Herzberg relation to systems governing behavior of fresh water resources and salt water intrusion:

  1. A consequence of the divide is that rainfall is the sole source of recharge to the aquifer systems,
  2. due to the lack of an impermeable land cover to the aquifer systems, rainwater falling upon the land surface percolates readily down and accumulates over time in the aquifer rocks,
  3. the bounding to the west by the Gulf of Mexico ensures a ready source for salt water intrusion postulated by that Ghyben Herzberg relation.
  4. Note in particular that this discussion applies to unconfined aquifer systems. For confined aquifers an analogy can be made with the Ghyben-Hertzberg model, albeit the more complex;

The following is taken from FGS Bulletin 69, Page 9, and explains the origin of Florida's lens system:

"Most of the Florida land mass is a peninsula that is surrounded by saltwater. Relict saltwater also underlies the entire state.

The reason for this is that the Florida Platform consists of carbonate rocks that were deposited in a shallow ocean. At the time of deposition, saltwater existed in their intergranular pore spaces. Gradually over geologic time, sea level was lowered relative to its position when the carbonate sediments were deposited. Through compaction and down warping of sediments on both sides of the Platform, a series of complex fracture patterns developed. The patterns are often reflected at land surface and have actually influenced the pathways of many of Florida's streams.

Over geologic time, as sea level lowered, the central portion of the Florida Platform was exposed to the atmosphere. As rainfall percolated downward it eventually replaced the upper portion of saltwater in the developing aquifers with a freshwater "lens."

Today, the irregularly shaped "lens" is generally thickest in the central portion of the state, where it is over 610 m (2,000 ft) thick (Klein, 1975). It becomes narrow toward Florida's coastline. The base of the "lens" is typically a transitional rather than a sharp boundary.

Groundwater in the deeper portion of the "lens", and along the coasts, is mixed with saltwater and has relatively high concentrations of saline indicators such as sodium (Na), chloride (Cl), and sulfate (SO4). "

Salt water intrusion and the Ghyben Herzberg Relation

Saltwater intrusion happens when saltwater penetrates underground from the sea into freshwater aquifer systems. This behavior is caused by sea water having a higher density than freshwater (due to carrying more solutes). Note also that elevation of the sea level due to warming of the sea increases pressure at depth and increases the tendency of the saltwater to intrude. Sea level rise also diminishes the fresh water depth above sea level increasing the tendency toward up-coning.

The pressure under a column of saltwater is thus greater than the pressure under a column of the same height of freshwater. When these two columns are connected at the bottom, then the pressure difference would cause a flow from the column of saltwater to the freshwater column until the pressures equalize.

The mixing is inhibited only so long as the mass of the fresher water remains sufficient to resist the upwelling (up-coning) tendency of the saltier water from lower down.

Depletion of the mass of fresh water in an aquifer system above sea level reduces its relative pressure and intensifies the tendency for salt water to mix with the fresh water.

The flow of saltwater inland happens in the coastal areas. Further inland, the freshwater column is higher due to the increasing altitude of the land and enables the relative fresh and salt water pressures to equalize and stabilize the salt water intrusion.

The higher water levels inland have another effect of causing water to flow seaward, as freshwater flows out, in the lower parts saltwater flows in.

The Ghyben-Herzberg relation (from Wikipedia)

Ghyben-Herzberg relation

In the equation,
z = (ρf / (ρs - ρf) ) * hwhere the thickness of the freshwater zone above sea level is represented as h and that below sea level is represented as z. The two thicknesses h and z, are related by ρ f and ρs where ρf is the density of freshwater and ρs is the density of saltwater.

Freshwater has a density of about 1.000 grams per cubic centimeter (g/cm3) at 20 °C, whereas that of seawater is about 1.025 g/cm3. The equation can be simplified to z = 40 * h

The Ghyben-Herzberg ratio states, for every foot of fresh water in an unconfined aquifer above sea level, there will be forty feet of fresh water in the aquifer below sea level.

Environmentally, these Ghyben-Herzberg "lens" systems are crucially important not only to healthy stream flows and the biotic health of the protected waterways but also to the region's potable water supply.

The lens system assures the water supply only so long as its lens' mass remains sufficient to inhibit mixing of non-potable water from lower aquifer systems.

Florida Geological Survey have advised that the lens' depths have reduced since last being surveyed by Howard Klein et. al., in 1975 but is not due to be determined again for some time to come. Klein reported upon areas of the east coastal and southern regions of Florida which had become completely denuded of the potable water lens due to over-pumping.

Moreover, the 2010 report to SWFWMD by Vanasse Hengen Brustlin, Inc. graphically illustrates the discharge of non-potable water from spring vents fed from a contaminated lens system co-located with an underground tributary into the southern section of Kings Bay.

Periods of drought, returns to the atmosphere, high rates of pumping, and lateral underground infusions from the Gulf of Mexico (Fretwell and Causseaux,1983), seek to reduce the mass and precipitate the mixing

Modeling

Modeling of saltwater intrusion is considered difficult. Some typical difficulties that arise are:

  • The possible presence of fissures, cracks and fractures in the aquifer systems whose precise positions are unknown but which have great influence on the development of the saltwater intrusion.

  • The possible presence of small scale heterogenities in the hydraulic properties of the aquifer, which are too small to be take into account by the model but which may also have great influence on the development of the saltwater intrusion.

  • The change of hydraulic properties by the saltwater intrusion. A mixture of saltwater and freshwater is often under-saturated with respect to calcium, triggering dissolution of calcium in the mixing zone and changing hydraulic properties.

  • The process known as cation exchange, which slows the advance of a saltwater intrusion and also slows the retreat of a saltwater intrusion.

  • The fact that saltwater intrusions are often not in equilibrium makes it harder to model. Aquifer dynamics tend to be slow and it takes the intrusion cone a long time to adapt to changes in pumping schemes, rainfall, etc. So the situation in the field can be significantly different from what would be expected based on the sea level, pumping scheme etc.

  • For long-term models, the future climate change and sea level rise will effect Model results in the future.

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