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Summary of the Hydrology of the Floridan Aquifer System In Florida and In Parts of Georgia, South Carolina, and Alabama
By Richard H. Johnson and Peter W. Bush
Professional Paper 1403-A
The chemistry of water in the Floridan aquifer system is briefly discussed here; the geochemistry of the system is discussed in detail in Professional Paper 1403-I.
DISSOLVED SOLIDS AND MAJOR CONSTITUENTS
In general, the dissolved-solids concentrations of water at any point in the Upper Floridan are related to flow and proximity to the freshwater-saltwater interface. In the unconfined or semiconfined areas where flow is vigorous, dissolved-solids concentrations are low.
Where the system is tightly confined, flow is more sluggish and concentrations are higher. In Florida south of Lake Okeechobee and in parts of the St. Johns River valley, residual saltwater remains unflushed from the system and dissolved- solids concentrations are high. Concentrations also become increasingly higher in coastal areas as the freshwater-saltwater interface is approached.
In the Upper Floridan, dissolved-solids concentrations vary from less than 25 mg/L near outcrop areas to more than 25,000 mg/L along the coasts. Within the system the dominant cations are Ca, Mg, Na, and K; the dominant anions are HCO3, Cl, and SO4. Locally, smaller amounts of dissolved iron, manganese, nitrate, phosphate, fluoride, strontium, sulfide, and silica contribute to the dissolved-solids concentration.
Figure 7 shows the general distribution of dissolved-solids concentrations in water produced from wells that yield from the entire Upper Floridan. Throughout most of the Upper Floridan, dissolved-solids concentrations are maintained at less than 500 mg/L by saturation with calcite and dolomite, and by the limited occurrence of more soluble minerals like gypsum. Higher concentrations are generally due to the presence of seawater in the system. In coastal areas of Florida, northeast Georgia, and South Carolina, seawater should occur in the lower part of the Upper Floridan where predevelopment heads were low; high chloride concentrations in fully penetrating coastal wells have indicated that seawater is present in these areas. High dissolved-solids concentrations along the coast of southeast Georgia and northeast Florida cannot be attributed to seawater, however, because chloride concentrations in the Upper Floridan are not high. Declining heads in the Upper Floridan apparently have induced highly mineralized, low-chloride water from the Lower Floridan to move upward, gradually increasing dissolved-solids concentrations over a large area. According to Brown (1984), highly mineralized, low-chloride water may have been present throughout most of the Fernandina permeable zone of the Lower Floridan prior to development.
Several distinct hydrochemical facies characterize the water chemistry in the Upper Floridan as shown on plate 3. The principal chemical processes leading to the development of the hydrochemical facies in the Upper Floridan are: (1) dissolution of aquifer minerals toward equilibrium as ground water moves from recharge to discharge areas; (2) mixing of ground water with seawater along the freshwater- saltwater interface in coastal areas, or with residual saline water in low-permeability rocks where the Upper Floridan is unflushed, or with recharge water; (3) cation exchange between water and aquifer minerals. Each of these processes exhibits distinctive chemical traits; thus the hydrochemical facies map, when combined with hydrogeologic data, can generally identify the predominant processes in any part of the Upper Floridan system.
In most of Georgia and central Florida, dissolution of calcite produces low to moderate increases in dissolved solids; ground water in these areas is generally calcium-bicarbonate dominated. Where incongruent dissolution of dolomite adds sufficient Mg to the water, a calcium-magnesium-bicarbonate facies develops. Along the coast of Georgia and northeast Florida, gypsum dissolution adds SO4 and a calcium-magnesium-bicarbonate-sulfate facies develops. In southwest Florida, residual gypsum occurs in moderate quantities in the Upper Floridan; in this area of sluggish flow, sulfate is the predominant anion, and a calcium-magnesium-sulfate facies occurs. This same facies has developed in the Gulf Trough area of southwest Georgia and in adjacent northwest Florida. The Gulf Trough is a narrow band of low-permeability rocks extending northeastward across south-central Georgia that impedes ground-water circulation and slows dissolution of residual gypsum in the Upper Floridan. Other data presented in Professional Paper 1403-I indicate that small amounts of residual seawater also contribute to the high dissolved-solids concentrations occurring in Gadsden County, Fla., although the quantities of seawater are insufficient to affect the hydrochemical facies mapped in the area.
Within the Upper Floridan vertical mixing of fresh-water and seawater occurs naturally in the zone of dispersion along the transition zone between freshwater and saltwater. In this zone the water chemistry gradually changes from calcium-magnesium-bicarbonate type with low dissolved solids near the top, to seawater at the bottom. Lateral changes in hydrochemical facies also occur along the coasts of central Florida and southeast South Carolina because of increasing amounts of seawater in the Upper Floridan. Inland a few miles from the coast, a calcium-magnesium-bicarbonate facies generally occurs. Nearer the sea, the Upper Floridan contains some seawater at depth, and the small amounts of seawater change the water from calcium-magnesium-bicarbonate dominated to water with approximately equal proportions of all major constituents (designated "Mixed" on plate 3). As the seawater content of the Upper Floridan increases, there is a change to sodium-chloride facies. The effects of freshwater-saltwater mixing on deep wells in coastal areas is dependent on the depth (position) of the interface, the depth of the well, and the rate of pumpage. Water from lightly pumped, shallow wells in most coastal areas would not be as affected by the seawater, and might be quite similar to freshwater from wells farther inland.
Mixing of freshwater with residual saline water produces changes in hydrochemical facies and dissolved-solids concentrations similar to mixing freshwater and present day seawater. In south Florida flow is very sluggish due to very thick confinement of the system, and residual saline water occurs in the Upper Floridan.
Mixing of residual seawater in the Upper Floridan with freshwater could also produce the high dissolved-solids concentrations and sodium-chloride facies mapped in the valley of the St. Johns River. However, an alternative explanation for the highly mineralized water occurring in that area was offered by Wyrick (1960) and Leve (1983). They suggested that saline water from deeper units is rising into the Upper Floridan along fault zones in northeast Florida. Invasion of the Upper Floridan by more mineralized water has been documented in Valdosta, Ga. (Krause, 1979), Brunswick, Ga. (Wait, 1965), and Nassau County, Fla. (Fairchild and Bentley, 1977). Only in the Brunswick area, however, has invasion of saline water caused higher dissolved-solids and a change to sodium-chloride facies in the Upper Floridan. The narrow band of calcium-magnesium-sulfate facies mapped along the valley in northeast Florida supports the theory that upwelling of mineralized water may occur along fault zones. Prior to development the system was discharging to the St. Johns River where the narrow band is mapped; the 1980 potentiometric-surface map (p1. 2) indicates that the current system is discharging to the river and to a center of pumping on the south side of Jacksonville. These discharge patterns seem to have prevented the spread of the calcium-magnesium-sulfate facies eastward toward the coast.
Recharge to the Upper Floridan mixes with ground water in the aquifer and has variable chemical effects depending on both chemistry and amount of the recharge. Recharge from underlying sand aquifers in southwestern Georgia locally affects both Ca and HCO3 concentrations, but as discussed in Professional Paper 1403-I, the rate of upward leakage is believed to be very small as there is no change detected in the predominant ions. A change in hydrochemical facies suggests that recharge of Ca and HCO3 type water occurs immediately southeast of the Gulf Trough. This recharge is probably low in dissolved solids, as dissolved-solids concentrations change from 250 to 500 mg/L in the vicinity of the Trough to 0 to 250 mg/L down gradient (southeast of the trough). Upward leakage from underlying sand aquifers also occurs in east-central Georgia, where it has lowered Ca concentrations and raised Na/Cl molal ratios (Sprinkle, in press) but has had no effect on dissolved-solids concentrations or hydrochemical facies. Nearer the coast in east-central Georgia, a mixed-bicarbonate facies is mapped. In this area, the Lower Floridan is probably discharging small amounts of water to the Upper Floridan because this was a predevelopment discharge area for the Upper Floridan. Upward leakage of water from the Lower Floridan increases Na concentrations in the Upper Floridan, indicating Na concentrations may be relatively high in the Lower Floridan.
Only in the western Florida panhandle does ion exchange seem a probable mechanism for developing a unique hydrochemical facies. In that area, the typical calcium-magnesium-bicarbonate water of recharge areas evolves into a sodium-bicarbonate facies. The source of additional Na is not well established, although preliminary chemical data indicate sodium-silicate dissolution is not the sole source. Infiltration of Na rich water is not possible because this is a discharge area for the Upper Floridan, nor is upward leakage a plausible source, because, except in western Okaloosa County, the almost impermeable Bucatunna Formation separates the Upper and Lower Floridan aquifers in most of the area. Taken together the chemical and hydrologic data strongly imply that ion exchange is the mechanism for producing the sodium-bicarbonate facies occurring in the Upper Floridan in the western Florida panhandle.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:04 PM(TJE)