3.2 Floridan Aquifer, Wells & Saltwater Intrusion

3.2 Floridan Aquifer, Wells & Saltwater Intrusion

Quick Answer: Florida drinking water is predominantly groundwater. The Floridan Aquifer System supplies much of the state; wells must be constructed with casing, grout, and sanitary seals; coastal saltwater intrusion from overpumping is tracked with chloride; and wells under the direct influence of surface water (GWUDI) are treated as surface water.

Florida’s Groundwater Dominance

Unlike many northern states that lean on large surface reservoirs, Florida’s public water supply is overwhelmingly groundwater-based. Operators may work at:

• Single-well community systems with chlorination only
• Multi-well wellfields feeding ground storage and high-service pumps
• Utilities that blend groundwater with surface water or RO product water
• Coastal systems managing brackish zones and chloride constraints

Exam framing: if a question contrasts source stability, expect groundwater to show more stable temperature and turbidity than rivers, but often higher hardness, iron, manganese, sulfide, and CO₂, and in coastal corridors chloride/TDS risk.

The Floridan Aquifer System

The Floridan Aquifer System is a thick sequence of permeable limestone and dolomite that underlies all of Florida and extends into neighboring states. Key operator concepts (not a geology thesis):

Confined vs Unconfined Behavior

In northern and central Florida, the Floridan is often closer to the surface, feeds springs, and participates in classic karst hydrology. In southern Florida, the productive zones may be deeper or more complex, and many coastal utilities also use the Biscayne or other surficial/intermediate aquifers—still groundwater, still subject to saltwater intrusion and contamination pathways.

Why Aquifer Type Matters on the Exam

• Vulnerability: unconfined and karst settings need stronger wellhead protection and faster response to spill or septic failures.
• Water quality signature: anoxic confined zones favor dissolved Fe/Mn and H₂S; oxygenated shallow zones may show more microbial variability.
• Pumping impacts: confined systems can show wide cones of depression in the potentiometric surface; unconfined systems show water-table drawdown that can induce recharge from rivers, canals, or the ocean.

Well Construction Basics

A well is both a production tool and a potential short-circuit for contamination. FDEP and water-well construction rules (and licensed drillers) exist so the borehole is not an open pipe from the surface to the aquifer.

Essential Components

1. Casing — Steel or other approved materials lining the borehole. Seals off shallow contaminated zones and unstable formations. Casing should extend above finished grade so surface water cannot enter the top.
2. Annular seal / grout — Cement or bentonite filling the annulus between casing and borehole wall. A failed annular seal is a classic pathway for coliform contamination after rain.
3. Sanitary well seal (cap) — Vermin-proof seal on the casing top with a screened, downturned vent so the well can breathe without admitting insects or debris.
4. Well screen / open hole interval — In sand aquifers, screens and gravel packs admit water while excluding formation sand. In competent limestone, open-hole intervals are common; sand production still must be controlled.
5. Surface completion — Concrete pad, grading that drains away from the well, protected electrical conduits, and secured enclosures. Ponding around a wellhead is an exam red flag.
6. Pump and drop pipe — Submersible or line-shaft turbines sized to the well’s sustainable yield—not to “whatever the motor can pull.”

Hydraulic Measurements Operators Use

• Static water level — Resting level after recovery
• Pumping (dynamic) water level — Level while pumping at a stated rate
• Drawdown = pumping level − static level (using consistent reference points)
• Specific capacity = yield (gpm) ÷ drawdown (ft)

Declining specific capacity with a stable static level often points to screen encrustation or biofouling. Falling static and pumping levels over years point more toward aquifer decline or regional overpumping. Sudden sand production suggests overpumping or screen/pack failure.

Worked idea: Static 45 ft, pumping 95 ft at 500 gpm → drawdown 50 ft → specific capacity 10 gpm/ft. If next year the same 500 gpm requires 145 ft pumping level with the same static, drawdown is 100 ft and specific capacity is 5 gpm/ft—a rehabilitation investigation is warranted.

Saltwater Intrusion

The Freshwater–Saltwater Balance

Near the coast, denser seawater underlies or borders fresher groundwater. The interface position is a dynamic balance between:

• Inland freshwater head (recharge + limited pumping)
• Ocean/estuary saltwater head
• Aquifer permeability and geometry

Overpumping lowers freshwater head, allows the saltwater interface to move inland and/or upward (upconing beneath a well), and can permanently degrade a well or wellfield.

Mechanisms Exam Items Emphasize

1. Lateral coastal intrusion — Cone of depression from coastal wellfields pulls the interface landward.
2. Upconing — Deep pumping pulls saline water from below into the screened interval.
3. Canal and drainage effects — In South Florida especially, drainage canals and water-management stages can alter heads and salt positions.
4. Drought compounding — Less recharge + same or higher pumpage accelerates intrusion.
5. Abandoned or improperly constructed wells — Vertical conduits that mix saline and fresh zones.

Chloride Monitoring

Chloride is the practical operator tracer for saltwater intrusion (often paired with conductivity/TDS and sometimes sodium).

Operator practices:

• Establish baseline chloride for each well when new or healthy
• Trend chloride with pumpage and water levels (not one-off samples only)
• Watch for step changes after hurricanes, canal operations, or new nearby pumping
• Compare wells: rising chloride in seaward wells first is a classic pattern
• Coordinate with engineers/hydrogeologists and the Water Management District when trends threaten CUP limits or treatment capability

Secondary drinking-water regulations commonly list chloride at 250 mg/L as an aesthetic SMCL; utilities may set lower internal triggers because blending, RO recovery, and corrosion/taste issues appear well before customer revolt. Primary health concerns at high salinity are broader (TDS, sodium-sensitive populations, treatment limits)—exam answers should focus on source management and intrusion control, not “chlorinate harder.”

Operational Responses to Rising Chloride

• Reduce pumping rate and daily volume from affected wells
• Rotate to inland or less-impacted wells
• Shorten continuous run times to limit upconing
• Blend with lower-chloride sources within finished-water targets
• Evaluate aquifer storage and recovery (ASR), alternative supplies, or brackish RO
• Repair cascading wells and abandon failed wells properly
• Revisit wellfield expansion plans with the WMD consumptive use permit in mind

GWUDI — Groundwater Under the Direct Influence of Surface Water

GWUDI is a regulatory determination that a groundwater source is influenced by surface water quickly enough that surface-water pathogens can be present. Indicators include:

• Insects, algae, Giardia, Cryptosporidium, or other large-diameter pathogens in the source
• Rapid shifts in turbidity, temperature, conductivity, or pH that track rainfall or nearby surface water
• Shallow construction, karst features, springs, or proximity to rivers/canals/lakes

Why GWUDI Changes Everything

A GWUDI source is regulated as surface water. That typically means:

• Filtration and disinfection requirements under the Surface Water Treatment Rule framework
• Combined filter effluent turbidity performance standards
• CT-based inactivation credits for Giardia and viruses
• More intensive monitoring and operator attention

Florida’s karst and spring terrain makes GWUDI a realistic Class C topic even for “well” operators. If turbidity in a well spikes hours after a thunderstorm, do not stop at “increase chlorine”—start the surface-influence investigation path.

Well Abandonment

Unused wells that are not maintained become hazards:

• Direct conduits for surface contamination
• Inter-aquifer mixing of poor-quality water into potable zones
• Physical safety hazards
• Hidden pathways for saltwater or contaminated shallow water

Proper abandonment (plugging) uses approved sealing materials placed from the bottom up (or per method required for the construction type) so the borehole is permanently sealed. Abandonment is not “cap it and forget it.” Utilities should inventory old wells on wellfield property and retire them through licensed procedures required in Florida.

Putting It Together for FDEP Exams

Master aquifer vulnerability, sanitary construction, chloride trends, and GWUDI triggers. Those four ideas carry most Florida well-and-intrusion items on the operator exams.