Florida Surficial

The shallow Florida aquifer that isn't carbonate — and where most private wells in much of the state actually live

States

FL, GA, SC, NC, AL

Type

Shallow Quaternary sands and limestones of the surficial aquifer system

Status

Stable in most of the system; nitrate and pesticide concerns in agricultural and residential sub-regions; PFAS at military and industrial sites

If you have a private well in Florida, there is a very good chance it taps the surficial aquifer rather than the deep Floridan. This is one of the most common geological misunderstandings in private-well water — the Floridan aquifer gets all the press because it supplies almost every Florida municipality, but most Floridan wells are 500–2,000+ feet deep. Drilling that depth costs tens of thousands of dollars and is mostly a municipal-utility undertaking. The typical Florida private well is 30–150 feet deep, screened in the shallow Quaternary sands and surficial limestones — that is, the surficial aquifer system, a separate hydrogeologic unit with separate hydrology and separate water-quality concerns.

The surficial aquifer matters for the same reason it gets less attention: it is what most rural Florida actually drinks. It is also where the contamination problems Florida private-well users actually face mostly live. The Floridan, sealed away under hundreds of feet of confining clay (the Hawthorn Group across most of the state), is mostly protected from surface contamination. The surficial aquifer is, by definition, exposed to it.

What it is, geologically

The surficial aquifer system in Florida is the saturated portion of the unconsolidated and lightly consolidated sediments at and near the land surface — generally Quaternary sands, sandy clays, and shelly limestones deposited as Florida emerged and re-emerged from sea level over the Pleistocene. Thicknesses vary from a few feet to several hundred feet. The aquifer is generally unconfined (meaning the water table is the upper surface of the saturated zone), recharged directly by rainfall, and discharges to wetlands, springs, streams, and (along the coasts) to the ocean.

The surficial system has several locally-named sub-units in Florida:

• Surficial sand aquifer — the most widespread; thin in north Florida, thicker in the central peninsula, very thick in coastal southeast Florida (where it merges with the Biscayne aquifer in Miami-Dade and Broward counties).
• Sand-and-gravel aquifer (Florida Panhandle west of the Apalachicola River) — primary supply for Pensacola and Escambia/Santa Rosa counties; geologically distinct, sometimes treated as its own principal aquifer.
• Shallow secondary aquifers — in the central Florida ridge, a series of perched or semi-confined aquifers above the Hawthorn Group.

The surficial system also extends north into the Atlantic coastal plain of Georgia, the Carolinas, and southeastern North Carolina, and is mapped (with regional variations) along most of the Atlantic and Gulf coastal margin.

Population using it

This is harder to count than for confined-aquifer systems because the surficial aquifer is usually the private-well source while the Floridan is the municipal source. Roughly:

• Florida Panhandle (Pensacola area) — sand-and-gravel aquifer is the primary supply for both municipal and private wells, ~500,000 people.
• Rural and unincorporated Florida statewide — most private wells; estimates suggest 1.5–2 million Floridians on private wells, the great majority of which are in the surficial system.
• Southeastern Atlantic coastal plain — Brunswick GA, Wilmington NC, parts of coastal SC — surficial wells supplement deeper Floridan or coastal-plain confined aquifers.
• Coastal southeast Florida — the surficial system merges into the Biscayne aquifer; Miami-Dade and Broward county supply is technically Biscayne, which is itself a highly-permeable surficial limestone.

Water quality

Because the surficial aquifer is unconfined and shallow, it inherits the surface contamination footprint of whatever sits above it. The water-quality story is therefore much more locally variable than the deep Floridan.

• Nitrates — the most consistent issue. Sources: residential septic systems (very common in surficial-aquifer regions), agricultural fertilizer (citrus, sugarcane, vegetables, sod), dairy and poultry operations. Documented nitrate hotspots include parts of Suwannee County, the Wekiwa Springs basin, and the Indian River Lagoon contributing area. See nitrates.
• Bacteria — shallow well plus septic-tank density plus warm climate plus high water table = chronic coliform risk. Annual testing is the floor; semi-annual is reasonable in rural septic-density areas. See bacteria.
• Pesticides — legacy organochlorines from citrus, current-use herbicides from sugarcane and row crops, residential lawn-care compounds. State and county monitoring data exist but private-well coverage is patchy. See pesticides.
• PFAS — Florida has multiple major source sites (Patrick Space Force Base, Eglin AFB, Tyndall AFB, MacDill, and several airports and chemical-industry sites). The surficial aquifer is the first system contaminated when PFAS reaches the water table. Florida Department of Environmental Protection has expanded PFAS testing in recent years; off-base private-well plumes have been documented at multiple sites. See PFAS.
• Iron and manganese — extremely common in the surficial aquifer due to organic-rich reducing conditions. Many Florida surficial wells require iron/manganese treatment. See iron and manganese.
• Saltwater intrusion — the coastal margin of the surficial aquifer is vulnerable, particularly in the Panhandle, the lower Keys, and the southeast Atlantic coast where the surficial-Biscayne system meets the ocean. Sea-level rise compounds this. See saltwater intrusion.
• Hardness — generally moderate to soft in the sandy sub-aquifers, harder where the surficial system includes shelly limestones.
• Hydrogen sulfide — common in deeper portions of the surficial aquifer, particularly in central and southwest Florida. See hydrogen sulfide.

Surficial vs. Floridan: the practical question

If your well is less than ~200 feet deep in most of Florida, you are almost certainly in the surficial aquifer system. If your well is deeper than ~400 feet, you are probably in the Floridan. The 200–400 foot zone is where local geology decides — your well log (filed with the Florida Department of Environmental Protection's water-management district at the time of drilling) will tell you which formation is screened. The water-quality testing recommendations are different enough that knowing matters: a Floridan well's standard panel is light on nitrate/bacteria/pesticide concern and heavier on hardness, sulfate, and naturally-occurring radium; a surficial well's panel is the reverse.

Known contaminant concerns

Communities on this aquifer