Hydrogeology and Groundwater Quality of Highlands ... - USGS

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16 Hydrogeology and Groundwater Quality of Highlands County, Florida Quality-Assurance Samples Quality-assurance samples were collected to ensure the integrity of water-quality samples. Analysis of qualityassurance samples provides information about the potential for sample contamination during collection, processing, and laboratory analysis. Quality assurance for water samples included replicate groundwater samples and field equipment blank samples. Replicate samples consist of two or more sets of samples collected concurrently so that both samples are assumed to have identical chemical compositions. Replicate samples were used to measure variability and precision characteristics associated with sampling and analytical procedures. Field equipment blanks provide information on possible contamination introduced to the sample during cleaning, collection, and processing of the sample in the field (Francy and others, 2005). About 10 percent of the water samples collected during this study was quality-assurance samples consisting of both equipment blanks and replicate samples. Figure 12. Relation of stratigraphic and hydrogeologic units. Modified from Spechler and Kroening (2007). Geologic Framework Highlands County is underlain by sediments of Cenozoic age to a depth of about 5,000 ft below land surface (Bishop, 1956; Chen, 1965). These sediments consist primarily of sand, clay, phosphate grains, carbonates (limestone and dolostone), and evaporites (gypsum and anhydrite). Geologic units corresponding to these sediments, from oldest to youngest, are the Cedar Keys Formation of late Paleocene age, the Oldsmar Formation of early Eocene age, the Avon Park Formation of middle Eocene age, the Ocala Limestone of late Eocene age, the Suwannee Limestone of early Oligocene age, the Hawthorn Group of late Oligocene to Miocene age, and the undifferentiated surficial deposits of Pliocene to Holocene ages (fig. 12). All of the geologic units that are exposed (crop out) at the surface are composed of unconsolidated siliciclastic sediments of Pliocene to Holocene age (fig. 13). GEOLOGY SERIES STRATIGRAPHIC UNIT HYDROGEOLOGIC UNIT AND LITHOLOGY Holocene and Pleistocene Undifferentiated surficial deposits Sand Surficial aquifer Pliocene Miocene Oligocene Eocene Paleocene Hawthorn Group Suwannee Limestone Ocala Limestone Avon Park Formation Oldsmar Formation Cedar Keys Formation Cypresshead Formation Peace River Formation Arcadia Formation Nocatee Member Sand, clay Phosphate, clay, sand, limestone, and dolostone Limestone and dolostone Limestone and dolostone with some intervals containing inclusions of gypsum and anhydrite Limestone and dolostone with beds of gypsum and anhydrite Intermediate aquifer system or Intermediate confining unit Floridan aquifer system Upper Floridan aquifer Confining unit upper Arcadia aquifer (Zone 2) Confining unit lower Arcadia aquifer (Zone 3) Confining unit Upper permeable zone Semi-confining unit Lower permeable zone (Avon Park permeable zone) Middle confining units I, II, and/or VI Lower Floridan aquifer Sub-Floridan confining unit Boulder Zone
27°30’ 27°15’ 27°00’ HARDEE COUNTY DESOTO COUNTY CHARLOTTE COUNTY TQuc Some differences are present between the geologic nomenclature used in earlier water-resource assessments of Highlands (Bishop, 1956) and adjacent counties (Pride and others, 1966; Stewart, 1966; Wilson, 1977) and those used in this report. Miller (1986) determined that the Avon Park Limestone and the Lake City Limestone could not be distinguished from each other on the basis of either lithology or fauna. Therefore, the term Lake City Limestone was abandoned, and all of the lithologic units within the Lake City Limestone were reclassified as Avon Park Limestone. Tc 81°30’ 81°15’ 81°00’ TQsu POLK COUNTY TQd Base modified from U.S. Geological Survey digital data, 1:100,000, 1985 Albers NAD projection 1983 HIGHLANDS COUNTY TQd GLADES COUNTY Figure 13. Geologic map of Highlands County. From Scott and others (2001). Qu 0 0 TQsu Geologic Framework 17 OSCEOLA COUNTY OKEECHOBEE COUNTY EXPLANATION PLEISTOCENE / HOLOCENE Qu Undifferentiated sediments PLIOCENE / PLEISTOCENE TQsu Shelly sediments of Plio-Pleistocene age TQd Dunes TQuc Reworked Cypresshead Formation PLIOCENE Tc Cypresshead Formation 5 10MILES 5 10KILOMETERS Miller (1986) also renamed the unit the Avon Park Formation, because it contained appreciable amounts of rock types other than limestone. The Ocala Group, which included the Inglis, Williston, and Crystal River Formations, is referred to as the Ocala Limestone, and the unit previously referred to as the Hawthorn Formation is now recognized as the Hawthorn Group (Scott, 1988). Two formations were introduced to the Hawthorn Group (Scott, 1988)—the Arcadia Formation and the Peace River Formation. Two members were named in the Arcadia Formation—the Tampa Member and Nocatee Member.
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