Hydrogeology and Groundwater Quality of Highlands ... - USGS

2 Hydrogeology and Groundwater Quality of Highlands County, Florida
The potentiometric surface of the Upper Floridan aquifer in
May 2007, which represents the hydrologic condi tions 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.
Introduction
Groundwater use in central Florida has increased
substantially over the past several decades. In Highlands
County, groundwater withdrawals have increased from
about 37 Mgal/d in 1965 to 107 Mgal/d in 2005. Some of
this increase is due to an expanding population, which has
increased in the county from about 21,000 in 1960 to about
99,000 in 2007. Much of this increase in water use, however,
is related to agricultural activities, especially citrus cultivation,
which increased more than 300 percent from 1965 to 2005.
The expansion of development and agriculture has placed
increased demands on limited water resources in the area.
As the demand for water in Highlands County increases,
additional information about the underlying aquifers is needed
to manage and develop the water supply effectively. With the
exception of the Lake Wales Ridge area, the hydrogeologic
framework, groundwater flow system, and water-quality
conditions are not well defined. The last comprehensive
countywide hydrogeologic investigation of Highlands County
was completed in 1956 (Bishop, 1956). Existing maps that
depict the tops, thicknesses, and lithologic characteristics
of the various hydrogeologic units are generally regional in
scope, are based on sparse data, and generally end at the water
management district boundaries. Similarly, existing maps
that show the areal distribution of water-quality characteristics
are based on limited data points. To address the need
for additional data, the U.S. Geological Survey (USGS), in
cooperation with Highlands County, the South Florida Water
Management District (SFWMD), and the Southwest Florida
Water Management District (SWFWMD), began a study in
2006 to evaluate the hydrogeology and groundwater quality
of the county. The study provides information useful for the
conservation, development, and management of the water
resources of Highlands County.
Purpose and Scope
The purpose of this report is to document a comprehensive
study of hydrogeology, hydrologic characteristics, and water
quality in Highlands County, Florida. The report includes data
that describe (1) the lithology, depth, thickness, and extent of
the surficial aquifer, intermediate aquifer system, and Floridan
aquifer system in Highlands County based on geologic,
geo physical, and drillers’ logs; (2) water levels and water-level
trends in the aquifer systems using maps and hydrographs; and
(3) areal and vertical water-quality characteristics within the
aquifer systems.
Data used in this report are derived from publications and
files of the USGS, SWFWMD, SFWMD, Florida Department
of Environmental Protection (FDEP), Florida Geological
Survey (FGS), and reports prepared by private consultants.
Additional data collected by the USGS from 2006 to 2008
were used to supplement previously available data.