ABSTRACT

Groundwater is the main source of water supply in Highlands County, Florida. As the demand for water in the county increases, additional information about local groundwater resources is needed to manage and develop the water supply effectively. To address the need for additional data, a study was conducted to evaluate the hydrogeology and groundwater quality of Highlands County.

Total groundwater use in Highlands County has increased steadily since 1965. Total groundwater withdrawals increased from about 37 million gallons per day in 1965 to about 107 million gallons per day in 2005. Much of this increase in water use is related to agricultural activities, especially citrus cultivation, which increased more than 300 percent from 1965 to 2005.

Highlands County is underlain by three principal hydrogeologic units. The uppermost water-bearing unit is the surficial aquifer, which is underlain by the intermediate aquifer system/intermediate confining unit. The lowermost hydrogeologic unit is the Floridan aquifer system, which consists of the Upper Floridan aquifer, as many as three middle confining units, and the Lower Floridan aquifer.

The surficial aquifer consists primarily of fine-to-medium grained quartz sand with varying amounts of clay and silt. The aquifer system is unconfined and underlies the entire county. The thickness of the surficial aquifer is highly variable, ranging from less than 50 to more than 300 feet. Groundwater in the surficial aquifer is recharged primarily by precipitation, but also by septic tanks, irrigation from wells, seepage from lakes and streams, and the lateral groundwater inflow from adjacent areas.

The intermediate aquifer system/intermediate confining unit acts as a confining layer (except where breached by sinkholes) that restricts the vertical movement of water between the surficial aquifer and the underlying Upper Floridan aquifer. The sediments have varying degrees of permeability and consist of permeable limestone, dolostone, or sand, or relatively impermeable layers of clay, clayey sand, or clayey carbonates. The thickness of the intermediate aquifer system/ intermediate confining unit ranges from about 200 feet in northwestern Highlands County to more than 600 feet in the southwestern part. Although the intermediate aquifer system is present in the county, it is unclear where the aquifer system grades into a confining unit in the eastern part of the county. Up to two water-bearing units are present in the intermediate aquifer system within the county. The lateral continuity and water-bearing potential of the various aquifers within the intermediate aquifer system are highly variable.

The Floridan aquifer system is composed of a thick sequence of limestone and dolostone of Upper Paleocene to Oligocene age. The top of the aquifer system ranges from less than 200 feet below NGVD 29 in extreme northwestern Highlands County to more than 600 feet below NGVD 29 in the southwestern part. The principal source of groundwater supply in the county is the Upper Floridan aquifer. As of 2005, about 89 percent of the groundwater withdrawn from the county was obtained from this aquifer, mostly for agricultural irrigation and public supply. Over most of Highlands County, the Upper Floridan aquifer generally contains freshwater, and the Lower Floridan aquifer contains more mineralized water. The potentiometric surface of the Upper Floridan aquifer is constantly fluctuating, mainly in response to seasonal variations in rainfall and groundwater withdrawals. The potentiometric surface of the Upper Floridan aquifer in May 2007, which represents the hydrologic conditions near the end of the dry season when water levels generally are near their lowest, ranged from about 79 feet above NGVD 29 in northwestern Highlands County to about 40 feet above NGVD 29 in the southeastern part of the county. The potentiometric surface of the Upper Floridan aquifer in September 2007 was about 3 to 10 feet higher than that measured in May 2007.

Groundwater samples collected from 129 wells by the U.S. Geological Survey, State, and County agencies between 2000 and 2008 were used to characterize groundwater quality in Highlands County. Water-quality samples from 58 wells were collected specifically for this study by the U.S. Geological Survey and analyzed for common inorganic constituents and nutrients.

Water quality in the surficial aquifer can be highly variable. This variability results from several factors, including the lithology of the sediments, interaction with the Upper Floridan aquifer, and most importantly, effects of land use. Concentrations of specific conductance and major ions are generally low. Specific conductance ranged from 32 to 723 microsiemens per centimeter (µS/cm), chloride concentrations ranged from 2.6 to 54 milligrams per liter (mg/L), sulfate concentrations ranged from 0.2 to 87 mg/L, and hardness (as CaCO₃) ranged from 4.0 to 159 mg/L. Of the samples collected in the surficial aquifer, only nitrate concentrations exceeded the Florida primary drinking-water standard of 10 mg/L. The application of fertilizers related to citrus farming is the most likely source of nitrate in groundwater in this area.

Specific conductance of water in the intermediate aquifer system in Highlands County ranged from 66 to 11,500 µS/cm, and concentrations of chloride and sulfate ranged from 3.8 to 3,770 and 0.12 to 111 mg/L, respectively. With only a few exceptions, concentrations of total dissolved solids, chloride, sulfate, and nitrate were below State drinking-water standards.

Specific conductance of water in the Upper Floridan aquifer ranged from 133 to 1,900 µS/cm, and concentrations of chloride and sulfate ranged from 4.4 to 403 and less than 0.18 to 255 mg/L, respectively. Water from the Upper Floridan aquifer in most of the county is hard, ranging from 64 to 410 mg/L. Nitrate concentrations ranged from less than 0.04 to 0.22 mg/L; however, most of the water samples collected from the Upper Floridan aquifer had concentrations less than 0.04 mg/L.

Concentrations of chemical constituents in the Upper Floridan aquifer vary both areally and with depth. Inorganic constituent concentrations in water from the Upper Floridan aquifer generally were below State and Federal drinking-water standards, except in the southeastern and southwestern parts of the county where the water is more mineralized. The sources of the mineralized water are from relict seawater that entered the aquifer during a higher stand of sea level in the geologic past and has not been completely flushed from the aquifer, and from the dissolution of sulfur-bearing minerals in the aquifer.

First posted September 7, 2010