Revised Hydrogeologic Framework of the Floridan Aquifer System 2016-03

Salt water intrusion inland is worse than you think, including the “Apalachicola salinity feature” up to the GA-FL line and east through Lowndes County, with a special additional brackish Valdosta feature. Central north Florida is an island of fresh groundwater surrounded by entire saline Florida coast around from Alabama plus across to Brunswick, GA, then again from Savannah up past Charleston. South of Lakeland, FL the map is all red for saline.

Figure 53. Estimated altitude of the 10,000-milligrams-per-liter (mg/L) total dissolved solids boundary, southeastern United States.
Figure 53. Estimated altitude of the 10,000-milligrams-per-liter (mg/L) total dissolved solids boundary, southeastern United States.

Apparently using the data preliminarily mapped earlier in the Florida Well Salinity Study, geologists from three states connected the dots in Revised Hydrogeologic Framework of the Floridan Aquifer System in Florida and Parts of Georgia, Alabama, and South Carolina, By Lester J. Williams and Eve L. Kuniansky, Professional Paper 1807, First release: April 2015, Revised March 2016 (ver. 1.1), Groundwater Resources Program, U.S. Department of the Interior, U.S. Geological Survey, Reston, Virginia: 2015.

Cover

Introduction, Page 3 [17 of 156]:

The regional extent and altitude of the freshwater-saltwater interface was mapped using geophysical logs, water-quality data from deep wells, and selected time-domain electromagnetic soundings. The interface is represented by the approximate location of the 10,000-milligrams-per-liter total-dissolved ­ solids concentration boundary that separates mostly fresh and brackish water from underlying saline water. Because the new map is based on well-log data rather than a calculated interface using a theoretical density contrast, geologically controlled salinity variations can be portrayed within the aquifer.

Figure 1. Location of the study area and approximate updip limit of the Floridan aquifer system, southeastern United States.
Figure 1. Location of the study area and approximate updip limit of the Floridan aquifer system, southeastern United States.

Additional salinity calculations from geophysical logs were used to create salinity profiles across the thick sequence of carbonate rocks that compose the aquifer system. Many of the profiles indicate zones of fresher water may be present beneath more saline water, along the deep transmissive part of the aquifer system.

Two subregional salinity features are identified from the salinity mapping. The first is informally named the “Apalachicola salinity feature” in the vicinity of a thick accumulation of fine-grained carbonate rocks in the Southwest Georgia embayment (also known as Apalachicola embayment). In that area, saltwater is contained in the lower part of the Floridan aquifer system and the effective thickness of the freshwater flow system is greatly reduced compared to that of previously published maps. The second feature is a previously unmapped disconnected zone of brackish to saline water that lies near the base of the aquifer system in the vicinity of the central part of the Georgia-Florida state line. Because of its shape and position, this disconnected zone is probably trapped connate water in fine-grained carbonate rocks near the base of the system isolated from higher perme-ability rocks above. High salinity zones are indicated in other parts of the aquifer system, such as near Brunswick, Georgia, and Fernandina Beach, Florida, or associated with previously mapped low-permeability units.

Methods of Investigation 7 [21 of 156]

Salinity Mapping

To simulate groundwater flow and the movement of the freshwater-saltwater interface, the approximate depth and extent of brackish and saline water was mapped in the aquifer system. The mapping primarily involved estimating varia-tions in TDS concentration using borehole geophysical logs supplemented with (1) water samples collected from deep test wells, (2) water samples collected from packer tests, or (3) water samples collected from the reverse-air discharge during the drilling of test holes. In addition, data from selected time-domain electromagnetic soundings were used in some areas to refine the position of salinity boundaries (Patrick Burger, St. Johns River Water Management District, written commun., 2013). Reese (2000) calculated log-derived TDS concentrations using deep induction and either density or sonic porosity logs in southwestern Florida. Sources of error in calculating TDS concentrations from well logs included the selection of representative porosity values and a cementation exponent for the intervals being analyzed, and possible errors associated with large-diameter boreholes common in the study area. Other studies in southeastern Florida produced similar results using similar methods (Reese, 1994; Reese and Memberg, 2000; and Reese, 2004).

Salinity variations are described in this report using a classification scheme modified from Reese (1994) and used in Reese (2000) and Reese and Memberg (2000). The scheme has five salinity zones and six salinity classes for describing salinity on the basis of estimated TDS concentration (table 1). However, only salinities greater than 10,000 mg/L TDS are mapped or shown in cross section.

Table 1. Salinity zones and classes used for describing salinity on the basis of estimated total dissolved solids concentration.
[mg/L, milligrams per liter; classification of water based on total dissolved solids modified from Reese (1994, 2000) and Reese and Memberg (2000)]
Salinity zone Salinity class Total dissolved solids
concentration
(mg/L)
Freshwater Fresh 0 -1,000
Brackish water Slightly brackish 1,000 -3,000
Brackish 3,000 -10,000
Salinity transition Moderately saline 10,000 -35,000
Saline water Saline 35,000 -100,000
Brine water Brine >100,000

Salinity mapping units include the freshwater, brackish-water, salinity-transition, saline-water, and brine-water zones (table 1). Of greatest interest to this study was the definition of the brackish-water zone between 1,000 and 10,000 mg/L TDS concentration, the salinity transition zone between 10,000 and 35,000 mg/L TDS concentration, and the saline-water zone between 35,000 and 100,000 mg/L TDS concentration. Because salinity increases rapidly through the salinity transition zone, usually across tens of feet to several hundred feet, as described by Reese (1994), the base of the brackish-water zone was used to approximate the boundary of the freshwater-saltwater interface. This approach differs slightly from that used to develop the freshwater-saltwater interface map of Sprinkle (1989) who used chloride data from widely spaced deep wells and approximated the remaining areas of the interface using the Hubbert (1940) formula and known (or estimated) predevelopment freshwater hydraulic heads. Sprinkle (1989) defined the base of freshwater to be the midpoint of the freshwater-saltwater transition zone or at a chloride concentration of 10,000 mg/L. Although salinity boundaries other than the base of the brackish-water zone were calculated from well logs used in this study, these other zones were not mapped because of the similar configuration of these zones on a regional scale. It should be noted that the base of the brackish-water zone may or may not approximate the freshwater-saltwater interface because of density equilibrium and it is assumed that the salinity boundaries have changed little during the last 40 to 60 years when the well-log and water-sample data used to develop the map were collected. This seems true for southern Florida (Reese, 2004), as well as the coastal areas of the Floridan aquifer system in Georgia (Peck and others, 2011), where little change in chloride concentrations have been reported over the past few decades.

Figure 2. Locations of hydrogeologic cross-section lines and selected wells in the Floridan aquifer system, southeastern United States.
Figure 2. Locations of hydrogeologic cross-section lines and selected wells in the Floridan aquifer system, southeastern United States.

Brackish and Saline Zones in the Floridan Aquifer System 122 [136 of 156]

As a result of preferential flow through the permeable parts of the aquifer system, salinity reversals (or inversions) are not only common but suggest freshwater may be moving beneath more saline water in areas previously excluded from the active flow system. These inversions, (verified by a limited amount of sampling data) could be the result of fresher groundwater moving preferentially through more transmissive zones deep within the aquifer system either as part of the modern flow system or as a result of freshwater movement during lower sea-level stands. Although the extent of salinity inversion in the Floridan aquifer system is not presently known, preferential movement of groundwater through highly transmissive zones into offshore areas may represent undocumented offshore groundwater movement.

Figure 25. Thickness of the Floridan aquifer system, southeastern United States (see plate 6 for more detail; mg/L, milligrams per liter).
Figure 25. Thickness of the Floridan aquifer system, southeastern United States (see plate 6 for more detail; mg/L, milligrams per liter).

Apalachicola salinity feature

Page 126 [140 of 156]

In the updip part of the Floridan aquifer system, fresh-water is present throughout the entire vertical extent of the system, with a notable exception of extreme southwestern Georgia and the east-central Florida panhandle where a saline zone is present in the lower part of the aquifer system. In that area, brackish and saline waters are present in the Floridan aquifer system and the area is informally named the “Apalachicola salinity feature” herein (fig. 53). Based on salinity mapping conducted as part of this study, this feature appears to extend from the southern part of several Georgia counties along the Georgia-Florida state line (Seminole, Decatur, Grady, Thomas, Brooks, and Lowndes Counties), into the Florida panhandle, including all of Leon and Wakulla Counties and parts of Gadsen, Jefferson, Liberty, Franklin, and Gulf Counties (see cross section H—H’ on pl. 14). The exact source of this feature is unknown; however, it seems to be associated with fine-grained sediments of the Southwest Georgia embayment (Kellam and Gorday, 1990; Schmidt, 1984) that may have resulted in trapped or incompletely flushed connate water.

Brackish Valdosta feature

Page 126 [140 of 156]

Another area of increased salinity in the Floridan aquifer system is a newly mapped, disconnected zone of brackish to saline water that lies near the base of the aquifer system near Valdosta, Ga., and shown as part of the Apalachicola salinity feature (fig. 53 and pl. 14). Because of its shape and position, the area of increased salinity is probably connate water trapped in carbonate rocks near the base of the aquifer system and isolated from greater permeability rocks above. This area also is coincident with the location of lower permeability rocks associated with evaporitic rocks of the MAPCU in the vicinity of Valdosta, Ga., and may explain its presence there. High-salinity zones also are present in other parts of the aquifer system and mostly associated with previously mapped low-permeability units, confining beds, and high-permeability zones in the coastal areas of the aquifer system. These zones include saline connate water apparently trapped in the evaporite unit of the MAPCU in southwestern Florida and in saline parts of the APPZ in coastal areas.

Figure 22. Generalized altitude of the top of the Floridan aquifer system, southeastern United States (see plate 4 for more detail).
Figure 22. Generalized altitude of the top of the Floridan aquifer system, southeastern United States (see plate 4 for more detail).

Using the approximate altitude of the 10,000-mg/L TDS concentration boundary, the thickness of the freshwater part of the aquifer system can be approximated (fig. 54). The term “freshwater” is loosely used to describe the thickness of the groundwater in the aquifer system containing 10,000 mg/L TDS or less. The freshwater thickness map was constructed by subtracting the altitude of the top of the aquifer system (fig. 22, pl. 4) from the altitude of the 10,000-mg/L horizon (fig. 53).

Figure 54. Estimated thickness of the fresh- and brackish-water zones of the Floridan aquifer system, southeastern United States.
Figure 54. Estimated thickness of the fresh- and brackish-water zones of the Floridan aquifer system, southeastern United States.

There’s much more in the paper, which is well (pun intended) worth reading.

 -jsq, John S. Quarterman, Suwannee RIVERKEEPER®

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