In this section
- Groundwater quality in the St. Johns River Water Management District
The resource assessment section evaluates and interprets hydrogeologic data from the St. Johns River Water Management District monitoring well construction and from other sources to identify and correlate the Cenozoic clastic and carbonate sedimentary deposits of varying composition, hydraulic characteristics, thickness, and extent into lithostratigraphic and hydrostratigraphic units. The lithostratigraphic units of varying composition and hydraulic characteristics are associated with hydrostratigraphic units that subdivide the hydrogeologic system based on permeability and provide a framework for the modeling of flow systems.
A lithostratigraphic unit is defined as the rock strata or sediments with a predominant lithology that were deposited in a similar environment within a given geologic period. The rock strata are recognized as a unit with respect to any of the characteristics, properties, or attributes that rocks or sediments may possess for the purpose of description, correlation, and mapping. Stratigraphy is concerned not only with the chronologic order and age relations of rock strata, but also with their distribution, lithologic composition, fossil content, geophysical characteristics, and geochemical properties. The lithostratigraphic units have regional areal extent and sufficient thickness to be mappable.
The hydrostratigraphic units are the aquifer and confining units of regional extent that subdivide the hydrogeologic system based on permeability. This multilayered sequence of aquifer and confining units are comprised of all or parts of specific lithostratigraphic units. An aquifer is a body of rock or sediments that contains sufficient saturated permeable material to conduct groundwater and to yield significant quantities of water to wells and springs. A confining unit is a body of relatively impermeable or significantly less permeable material stratigraphically adjacent to one or more aquifer units. The characteristics of the hydrostratigraphic units can vary significantly from north to south and east to west within the district and adjacent areas. Variations in the extent, thickness, and dip of the units are due to geologic structure, deposition, erosion, and karst features.
One of Florida’s many springs, Croaker Hole Spring in Putnam County.
A hydrostratigraphic aquifer or confining unit is composed of sediments or rocks that have similar lithologic and hydrogeologic characteristics, with each unit having distinct hydraulic properties and permeability characteristics that determine the potential for water supply. The hydrostratigraphic units in the district include the surficial aquifer, the intermediate confining unit and intermediate aquifer units, and the units within the Floridan aquifer system. The Floridan aquifer system includes the Upper Floridan aquifer (UFA), the middle confining units I and II, and the Lower Floridan aquifer. The UFA includes the Ocala permeable zone (upper part of the UFA), the Ocala low permeable zone, and the Avon Park permeable zone (lower part of the UFA). However, not all units are present in all areas of the district.
Recharge to the Upper Floridan aquifer
Maps of groundwater recharge areas are useful planning tools for groundwater resource management. Groundwater recharge to the Floridan aquifer, as mapped by the district, is the addition of water to the Floridan aquifer from the overlying surficial aquifer, or the more direct infiltration of rainfall in areas where the surficial sediments are thin or absent and where the limestones of the Floridan aquifer are at or near land surface. The amount of water available as recharge to the Floridan aquifer is that part of rainfall within the district and adjacent areas, after losses to runoff and evapotranspiration, which infiltrates through the soil zone to the water table and continues to move downward to underlying aquifer systems.
Soils having high infiltration potential with little or no runoff due to poorly developed surface drainage features are most conducive to recharging groundwater systems. However, most soil borings or other soil information provide data on soil characteristics down to a depth of about five to six feet. To determine recharge to hydrogeologic units below the soil zone, such as the Floridan aquifer, an evaluation of geologic and hydrologic characteristics is needed.
The hydrogeologic characteristics of the surficial, intermediate, and Floridan aquifer systems determine areas and rates of recharge to the Floridan aquifer. The surficial aquifer extends from the land surface downward to the top of the intermediate confining unit separating the surficial and Floridan aquifer systems. The water table of the surficial aquifer is the upper surface of the zone of saturation. The intermediate confining unit, where present, retards the movement of water between the surficial and Floridan aquifers and confines the Floridan aquifer under artesian conditions. The water level in a well tapping the Floridan aquifer is called the potentiometric level, and this water level can be higher, lower, or at the same elevation as the water table. Leakance of water through the confining unit is dependent on its thickness and hydraulic conductivity.
Recharge to the Floridan aquifer occurs in areas where the water table elevation is higher than the Upper Floridan potentiometric elevation, creating downward hydraulic pressure to move water through the confining unit. In areas where the Upper Floridan potentiometric surface is at a higher elevation than the water table, leakance is upward from the Floridan to the surficial aquifer. The areas of upward leakance define the discharge areas of the Floridan aquifer. Where Floridan potentiometric elevations are above the land surface, springs and free-flowing artesian wells occur.
Recharge rates to the Floridan aquifer were calculated based on an analysis of the hydraulic pressure differences between the surficial aquifer water table and the Floridan potentiometric surface, and on the leakance of water through the confining unit separating the aquifers. Recharge also occurs directly from infiltrating rainfall where the limestones of the Floridan aquifer are at or near land surface. Significant recharge may also occur where the confining unit is breached by sinkholes and sinkhole-related features, depending on the hydrogeologic relationships between the surficial and Floridan aquifers. Recharge rates were mapped at a 4 inch/year contour interval.
For additional information, see the GIS map titled “Recharge Areas of the Floridan Aquifer in the St. Johns River Water Management District.”