Biscayne

A 4,000-square-mile sponge of limestone underneath South Florida — the most productive aquifer in the country and the one most directly threatened by sea-level rise

States

FL

Type

Highly transmissive Pleistocene karst limestone (Miami Limestone, Fort Thompson Formation)

Status

Saltwater intrusion advancing inland; sea-level-rise-driven retreat of freshwater lens; injection-well disposal contamination concerns

The Biscayne aquifer is the shallow karstified limestone aquifer underneath Miami-Dade County, Broward County, and the southern half of Palm Beach County, Florida. It is the principal water supply for about 4 million South Floridians and is widely regarded as one of the most productive aquifers in the world by transmissivity per unit thickness — wells routinely produce thousands of gallons per minute from depths of just 50-100 feet. The aquifer's productivity is the foundation of South Florida's modern existence; without it, Miami would not have grown to be Miami.

The Biscayne is also the US aquifer most directly threatened by sea-level rise. The aquifer is hydraulically connected to the ocean along the Atlantic shoreline; the freshwater "lens" that floats above the deeper saltwater is held in place by the freshwater hydraulic head. As sea level rises and as inland pumping continues, that lens thins and retreats. Saltwater intrusion has been advancing inland for over 80 years, and the rate is accelerating. The City of Hallandale Beach lost three of its eight production wells to saltwater between 2014 and 2019; other coastal Miami-Dade and Broward municipalities face similar trajectories.

What it is, geologically

The Biscayne is built from late Pleistocene limestone — primarily the Miami Limestone (oolitic, very young, ~125,000 years old) and the underlying Fort Thompson Formation (mostly fossiliferous limestone with some interbedded sand). Both units are highly porous and karstified. The aquifer is essentially unconfined — there is no significant overlying confining layer — which makes it vulnerable to surface contamination on a scale matched only by the most extreme karst aquifers like the Edwards.

The aquifer is hydraulically connected to the Everglades to the west; the historical freshwater flow from the Everglades through the aquifer to Biscayne Bay was a key part of the regional hydrology before drainage and development reshaped it. The Comprehensive Everglades Restoration Plan (CERP) is partly aimed at restoring some of those historical freshwater flows, which would also help maintain the aquifer's freshwater head.

Saltwater intrusion: the inland march

The freshwater-saltwater interface in the Biscayne aquifer has been monitored continuously since the 1940s. The interface position is mapped annually by the USGS and the South Florida Water Management District. The pattern over the past half-century:

• Hallandale Beach — saltwater has advanced inland by over 4 miles since 1950, retiring multiple production wells.
• Hollywood and surrounding southern Broward County — similar pattern.
• Northern Miami-Dade — multiple coastal cities have moved their wellfields inland multiple times.
• Florida Keys — Biscayne aquifer wells in the upper Keys have largely been abandoned; the Keys are supplied by surface water piped from the mainland.

For private well owners — relatively rare in Miami-Dade and Broward, but more common in agricultural southern Miami-Dade (the Homestead/Florida City area) — chloride trend monitoring is essential. See saltwater intrusion.

Injection-well contamination concerns

South Florida disposes of treated wastewater through deep-injection wells that inject the effluent into the brackish "boulder zone" below the freshwater aquifer. The injection wells were designed on the assumption that the boulder zone was hydraulically isolated from the overlying Biscayne. Multiple lines of evidence accumulated through the 1990s-2000s showed that this assumption is incorrect in some areas — treated wastewater has been documented migrating upward into the Biscayne aquifer, particularly through fault zones and karst conduits. The implications for drinking water quality are debated and the regulatory response has been gradual.

Water quality

• Saltwater intrusion markers (chloride, sodium, sulfate) — the dominant concern. See saltwater intrusion.
• Bacteria — karst conduits and the lack of a confining layer mean surface contamination reaches wells quickly. See bacteria.
• Nitrates — agricultural and septic-source; relevant in southern Miami-Dade ag areas.
• Hardness — high (limestone-derived). Most homes have softeners.
• Iron, hydrogen sulfide — present in deeper portions of the aquifer.
• Pharmaceuticals and personal-care products (PPCPs) — emerging concern from the injection-well migration story; not regulated; detection at trace concentrations.

Known contaminant concerns

Communities on this aquifer