Irrigation wells in Florida are boreholes drilled to extract groundwater for agricultural fields, golf courses, and residential landscaping. The depth of these wells is not standardized but is a direct result of the local geology and the specific water source being targeted. Determining the necessary depth involves navigating underground layers of rock and sediment to find a reliable supply with suitable quality. This means a well drilled in the Panhandle will be fundamentally different from one drilled in Miami-Dade County.
The Primary Aquifers Used for Irrigation
Florida’s groundwater is stored in three major, layered aquifer systems that irrigation wells can access. The shallowest is the Surficial Aquifer System (SAS), found from just below the surface down to approximately 50 to 300 feet below land surface. This system, which includes the Biscayne Aquifer in South Florida, is generally unconfined and directly recharged by rainfall, making it the source for many small-scale residential irrigation wells.
The Intermediate Aquifer System (IAS) lies beneath the SAS and consists of discontinuous layers of water-bearing rock separated by dense clay. Wells targeting this system typically range from 50 to 400 feet deep. The IAS often provides a more dependable supply than the SAS, though its productivity is highly variable. It acts as a protective layer, providing confinement for the deepest source, the Floridan Aquifer System (FAS).
The FAS is a massive, confined body of limestone and dolomite that underlies the entire state, serving as the main source for large-scale agricultural and municipal water use. While the top of the FAS can be found at varying depths, wells must penetrate deep into the system, often between 100 and over 400 feet, to find a high-yield production zone.
Regional Depth Variation Across Florida
The depth required to tap into a suitable aquifer varies dramatically across Florida due to the slope and thickness of the overlying geological layers. In North and Central Florida, the Floridan Aquifer System (FAS) is often close to the surface. Wells here can access high-yield zones between 100 and 200 feet deep, allowing for relatively shallow agricultural wells.
Moving south and toward the coasts, the protective confining layers above the FAS thicken considerably, pushing the aquifer deeper and making it less accessible. In the northeastern and southeastern portions of the state, the top of the FAS can be found at depths up to 450 feet. In Southwest Florida, the top of the Floridan Aquifer can be found around 690 feet below land surface, with final well depths often exceeding 800 to 1,000 feet.
In South Florida, the FAS often contains brackish water, leading to a greater reliance on the shallow Surficial Aquifer System, particularly the Biscayne Aquifer. The pressure within the confined FAS, known as the potentiometric surface, also influences the required well depth. Where this pressure is high, the water level rises significantly in the wellbore, sometimes flowing freely at the surface, reducing the need for deep pumping equipment.
Well Construction and Water Quality Considerations
The final depth of an irrigation well is also a function of construction requirements designed to protect water quality. Wells must be constructed with a casing, a pipe inserted into the borehole, to prevent surface contaminants from reaching the target aquifer. The casing must be sealed in place with a cement or bentonite grout. This process prevents the mixing of water from different layers, especially poorer quality water from shallower zones.
The depth of the casing is determined by the presence of poor-quality water, such as high-mineral or saline zones. In coastal areas, the threat of saltwater intrusion forces drillers to set the casing deep enough to seal off upper zones where the freshwater-saltwater interface may rise due to pumping. If testing reveals the water quality at the target depth is too high in chlorides or other minerals, the well must either be drilled deeper to find a usable zone or be abandoned.
For smaller irrigation needs, a shallow well may suffice. However, deeper wells are less susceptible to contamination from the land surface and are more reliable during drought conditions. The final drilled depth is a balance between the geological availability of water, the necessary yield for the application, and the engineering requirements.