Saltwater can’t be used for irrigation or drinking. When saltwater intrusion affects existing groundwater supplies, there are no happy outcomes. So sea-level rise poses more than just a flooding threat to Florida. It can also contaminate groundwater supplies for many communities. When this happens then local water wells that provide both drinking water and irrigation become unusable.

Geologists and hydrologists refer to the invasion of saltwater into fresh groundwater as “saltwater intrusion.” Groundwater includes the water in the clays and soils below our feet and water occupying pore spaces, or open spaces, in rocks like sandstone or limestone. When these rock and soil zones filled with water, they are called aquifers. 

Under pristine and undisturbed conditions, the rain that falls on Florida soaks into the ground and creates an underground reservoir of fresh water. But freshwater from the rain occupies the same aquifers that extend offshore, where they fill with salty ocean water. Since freshwater is less dense than seawater, the freshwater floats on the saltwater and creates a layer of potable water that cities and farms both use. 

Where has all the freshwater gone?

Wells that pump water to the surface of the ground are used to remove freshwater from an aquifer. Whenever some water is pumped from an aquifer, it creates a small bit of “space” in the form of lowering the local water pressure. Surface water from rain can then sink in and fill that space. But, if wells pump the water out too fast or remove too much water, then the rain recharge can’t keep up.  In those circumstances, the saltwater, lying under the freshwater layer, will rise to fill the space. This process is how most saltwater intrusion occurs. 

Over 90 percent of people in the northeast and east-central portions of Florida get their water from the Floridian aquifer. If this aquifer becomes contaminated, then they will feel the effects. Traditionally, the biggest culprit for inducing saltwater intrusion is over-pumping; using more water than the rain can replace. However, sea-level rise will introduce additional stress on that system in two fundamental ways:

Surface flooding from the ocean will introduce more saltwater into the aquifer recharge system, contaminating the existing freshwater supplies.

Increased water pressure from higher sea-levels will force more saltwater into the aquifers and decrease the space available for freshwater.   

Once aquifers become contaminated from saltwater intrusion, a return to uncontaminated freshwater supplies is difficult to impossible. Water management in low-lying and coastal communities is the most viable solution for combating the undesirable effects that over-pumping and sea-level rise have on valuable freshwater aquifers. But without good aquifer management, many communities will struggle with water problems.

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Sources

Saltwater Intrusion (USGS) – https://www.usgs.gov/mission-areas/water-resources/science/saltwater-intrusion?qt-science_center_objects=0#qt-science_center_objects

Invading seawater jeopardizes South Florida’s delicate drinking water source, but we can lessen the threat – https://www.miamiherald.com/opinion/editorials/article212844644.html#storylink=cpy

It’s Not Just Flooding – https://www.wuft.org/specials/water/saltwater-intrusion/

Florida’s Aquifers – https://www.sjrwmd.com/water-supply/aquifer/

White. Jr & David Kaplan – https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ehs2.1258