ABSTRACT: The hydrogeologic framework underlying the 600-square-mile study area in Charlotte, De Soto, and Sarasota Counties, Florida, consists of the surficial aquifer system, the intermediate aquifer system, and the Upper Floridan aquifer. The hydrogeologic framework and the geochemical processes controlling ground-water composition were evaluated for the study area. Particular emphasis was given to the analysis of hydrogeologic and geochemical data for the intermediate aquifer system. Flow regimes are not well understood in the intermediate aquifer system; therefore, hydrogeologic and geochemical information were used to evaluate connections between permeable zones within the intermediate aquifer system and between overlying and underlying aquifer systems. Knowledge of these connections will ultimately help to protect ground-water quality in the intermediate aquifer system. The hydrogeology was interpreted from lithologic and geophysical logs, water levels, hydraulic properties, and water quality from six separate well sites. Water-quality samples were collected from wells located along six ground-water flow paths and finished at different depth intervals. The selection of flow paths was based on current potentiometric-surface maps. Ground-water samples were analyzed for major ions; field parameters (temperature, pH, specific conductance, and alkalinity); stable isotopes (deuterium, oxygen-18, and carbon-13); and radioactive isotopes (tritium and carbon-14).
The surficial aquifer system is the uppermost aquifer, is unconfined, relatively thin, and consists of unconsolidated sand, shell, and limestone. The intermediate aquifer system underlies the surficial aquifer system and is composed of clastic sediments interbedded with carbonate rocks. The intermediate aquifer system is divided into three permeable zones, the Tamiami/Peace River zone (PZ1), the Upper Arcadia zone (PZ2), and the Lower Arcadia zone (PZ3). The Tamiami/Peace River zone (PZ1) is the uppermost zone and is the thinnest and generally, the least productive zone in the intermediate aquifer system. The Upper Arcadia zone (PZ2) is the middle zone and productivity is generally higher than the overlying permeable zone. The Lower Arcadia zone (PZ3) is the lowermost permeable zone and is the most productive zone in the intermediate aquifer system. The intermediate aquifer system is underlain by the Upper Floridan aquifer, which consists of a thick, stratified sequence of limestone and dolomite. The Upper Floridan aquifer is the most productive aquifer in the study area; however, its use is generally restricted because of poor water quality. Interbedded clays and fine-grained clastics separate the aquifer systems and permeable zones.
The hydraulic properties of the three aquifer systems are spatially variable. Estimated trans-missivity and horizontal hydraulic conductivity varies from 752 to 32,900 feet squared per day and from 33 to 1,490 feet per day, respectively, for the surficial aquifer system; from 47 to 5,420 feet squared per day and from 2 to 102 feet per day, respectively, for the Tamiami/Peace River zone (PZ1); from 258 to 24,633 feet squared per day and from 2 to 14 feet per day, respectively, for the Upper Arcadia zone (PZ2); from 766 to 44,900 feet squared per day and from 10 to 201 feet per day, respectively, for the Lower Arcadia zone (PZ3); and from 2,350 to 7,640 feet squared per day and from 10 to 41 feet per day, respectively, for the Upper Floridan aquifer. Confining units separating the aquifer systems have leakance coefficients estimated to range from 2.3 x 10-5 to 5.6 x 10-3 feet per day per foot. Strata composing the confining unit separating the Upper Floridan aquifer from the intermediate aquifer system are substantially more permeable than confining units separating the permeable zones in the intermediate aquifer system or separating the surficial aquifer and intermediate aquifer systems.
In Charlotte, Sarasota, and western De Soto Counties, hydraulic head generally increases with depth indicating an upward flow potential. Elsewhere, head decreases with depth indicating a downward flow potential. During September 1998, relatively small head differences occurred between the Upper Floridan aquifer and Lower Arcadia zone (PZ3) in the intermediate aquifer system (up to 5 feet) whereas relatively larger head differences occurred between permeable zones of the intermediate aquifer system and the surficial aquifer system (as much as 8 feet).
The hydraulic connection between the surficial aquifer system, the intermediate aquifer system and associated permeable units, and the Upper Floridan aquifer is variable in the study area. Clay beds within the confining units can limit the degree of hydraulic connection between aquifer systems and permeable zones; however, facies changes within the units may result in local hydraulic connection with overlying and underlying aquifers. Generally, better hydraulic connection exists between the Upper Floridan aquifer and the Lower Arcadia zone (PZ3) than exists between the permeable zones of the intermediate aquifer system and the surficial aquifer system.
Several important findings about flow patterns in the intermediate aquifer system have been supported by combining geochemical modeling with hydraulic head data. Vertical flow from underlying aquifers is significant in the chemical evolution of water in the intermediate aquifer system. Flow patterns derived only from potentiometric-surface maps may be misleading because flow paths are presumed to be lateral. Recent flow patterns delineated strictly based on potentiometric-surface maps do not represent predevelopment ground-water flow paths. The chemical composition of intermediate aquifer waters most likely reflects predevelopment conditions. Areas where geochemical models indicate large amounts of upward flow may actually be areas where discontinuity of the permeable zone exists, thereby limiting lateral flow.
Water in the intermediate aquifer system varies widely in chemical composition, but generally fits into one of two categories. At inland sites, water is a mixed ion or mixed cation-bicarbonate type. Sites closer to the coast have a sodium-chloride or mixed cation-chloride type water. Water within the same permeable zone of the intermediate aquifer system does not have a distinct chemical composition throughout the study area. Water in the surficial aquifer system, which is a calcium bicarbonate type, is more dilute than water from underlying aquifers. The chemical composition of water from the Upper Floridan aquifer is variable in the study area with no dominant cation present.
Most water from the surficial aquifer system has isotopically lighter deuterium and oxygen-18 values than water from the intermediate aquifer system or the Upper Floridan aquifer. Water from the surficial aquifer system most likely represents a mixture of meteoric water, with an isotopically light composition, and ground water that has been recharged by water that has undergone evaporation, with an enriched isotopic composition. Water from the intermediate aquifer system and the Upper Floridan aquifer may be the result of recharge that occurred under different climatic conditions than present conditions. Water from the three aquifer systems have isotopically distinct carbon-13 signatures of dissolved inorganic carbon, which is related to the evolution of inorganic carbon.
Water from the intermediate aquifer system and Upper Floridan aquifer is probably greater than 10,000 years old. Age dating indicates that water in some parts of these aquifers may be greater than 20,000 years old. Thus, the aquifer systems may have been recharged under different hydraulic conditions than currently observed.