Home Abstract Introduction Hydrogeology of S. Florida Ground-Water Movement - Movement Based on Natural Isotopes - Flow Patterns > Effects of Rising Sea Level - Upwelling Ground Water Subsurface Storage Summary and Conclusions References PDF Version

Ground-Water Movement in the Floridan Aquifer System in Southern Florida: Effects of Rising Sea Level on Ground-Water Movement

Sea level is generally accepted as the lower limit for hydraulic gradients in natural surface-water and groundwater flow systems. During periods of constant climate (no changes in rainfall and temperature) and rising sea level, freshwater in aquifer storage would gradually be displaced by seawater. During periods of constant climate and falling sea level, seawater in aquifer storage would gradually be displaced by freshwater.

Sea-level fluctuations during the past 150,000 yr (Cronin, 1983) range from about 23 ft above present sea level at about 140,000 yr B.P. (before present) to about 330 ft below present sea level at about 18,000 yr B.P. (fig. 20). High stands of sea level represent interglacial stages, and low stands represent glacial stages. The last rise in sea level, called the Holocene transgression, began about 18,000 yr ago and is continuing, although at a much slower rate. Predevelopment hydraulic gradients of the Floridan aquifer system in the late 1800's were, therefore, temporary and do not relate directly to antecedent flow conditions. However, flow rates based on radiocarbon dating (or the relative carbon-14 ages of water) reflect average antecedent hydraulic gradients. Therefore, estimates of hydraulic gradients, velocities, and transit times based on carbon-14 dating should consider the effects of changes in sea level, climate, topography, and perhaps permeability.

Data on paleoclimates, paleohydraulic gradients, paleotopography, and paleopermeability are generally lacking; however, data on changes in vegetative types during the past 8,500 yr in central Florida and southern Georgia (Watts, 1971) suggest that changes in forest types probably relate to rising sea level. Watts (1971, p. 676) reported that pollen

Figure 21. Comparisons of water level in central Florida, vegetation, and transit distance from subsea outcrop with sea level during the Holocene transgression. (Location of sites shown in fig. 2.) [larger version]

studies of bottom sediments in upland lakes in central Florida and southern Georgia showed that from about 8,500 to 5,000 yr B.P. (based on radiocarbon age) the vegetation was chiefly oak (Quercus) and that for the past 5,000 yr, the vegetation has chiefly been pine (Pinus). Prior to 8,500 yr B.P., a long hiatus occurred in sedimentation (about 27,000 yr) which probably resulted from greatly lowered water levels during the Wisconsin glaciation. Watts (1971, p. 686) estimated that during the hiatus, the water table was probably at least 40 ft below that of today, and he concluded that the changes in vegetation were chiefly caused by rising water tables and sea level rather than by increased rainfall.

The estimated rise in sea level during the Holocene transgression is shown in the conceptual cross section (fig. 21), along with the hypothetical rise of the water level in the Upper Floridan aquifer at the center of the Polk City high and the changes in forest vegetation according to Watts (1971). The hypothetical relationships suggest that at the beginning of the Holocene transgression, ground-water levels in central Florida were relatively high above sea level but low with respect to land surface. From 18,000 to 9,800 yr B.P., sea level was estimated to have risen about 0.03 ft/yr and water levels in central Florida were estimated to have risen about 0.008 ft/yr; water levels during this period in central Florida were too far below land surface for appreciable forest growth. From 9,000 to 5,000 yr B.P., sea level probably rose about 0.01 ft/yr and ground-water levels in central Florida rose about 0.006 ft/yr; water levels during this period in central Florida were sufficiently near land surface (within 20 to 40 ft) to sustain growth of oak forests. From 5,000 yr B.P. to the present, both sea level and water level in central Florida probably rose at the same rate, about 0.004 ft/yr; water levels in central Florida were sufficiently near land surface (within 20 ft or less) to sustain growth of pine forests. Dissolution of limestone during the Holocene transgression probably resulted in a slight increase in the permeability of the Floridan aquifer system.

The retreat of the glaciers (and hence the rise in sea level) during the Holocene transgression probably lagged behind the change (global warming) in worldwide climate, and rainfall over the Floridan Plateau probably was subtropical. Drainage in central peninsular Florida was chiefly subsurface through the Floridan aquifer system, and the part beneath the exposed land mass probably was filled with freshwater (fig. 22A). As sea level rose (rapidly at first), seawater moved inland through the Floridan aquifer system and displaced the stored freshwater (fig. 22B). Freshwater discharge to the Atlantic Ocean and the Gulf of Mexico by submarine springs decreased (principal discharge probably confined to the Straits of Florida). Continued subtropical rainfall maintained high water levels in central Florida so that the freshwater head was above land surface over most of the coastal areas. Circulation of freshwater in the Floridan aquifer system was restricted both laterally and vertically, and shallow circulation patterns developed in which karst features that earlier were sources of recharge to the system (during the low sea-level stand) became sources of discharge during the high sea-level stand. With high water tables, outflow (both surface water and ground water in the surficial aquifer) to the Atlantic Ocean and the Gulf of Mexico probably increased.

Figure 23. Theoretical velocity distribution during the Holocene transgression, Boulder Zone of the Lower Floridan aquifer. [larger version]

Results of radiocarbon dating of seawater from the Boulder Zone (in the Lower Floridan aquifer) suggest that the rise of movement inland may be directly related to the rise in sea level. Comparison of the estimated inland transit curve based on apparent carbon-14 ages of samples at sites 9 and 10 (see inset A in fig. 21) with the sea-level curve (fig. 21) suggests that seawater moved inland about 1 mi for each 1-ft rise in sea level. The velocity of seawater moving inland in south Florida probably was greatest at the beginning of the Holocene transgression and least at the end (present). The velocities, based on assumed aquifer characteristics (T=2.5 x 10⁷ ft²/d, m=650 ft, and = 0.3) ranged from about 172 to 5 ft/yr (fig. 23). The corresponding hydraulic gradients would, therefore, range from 3.7 x 10^-6 to 1.1 x 10^-7 (table 10), and the corresponding differences in head between sites 9 and 10 (44.5 mi apart) would be about 0.87 and 0.026 ft. Again, these values are relatively small compared with transient effects of tides and changes in fluid density.

The distance traveled by a water particle moving inland through the Lower Floridan aquifer from the Straits of Florida during the Holocene transgression (330-ft rise x inland movement of 1 mi per 1 ft of rise = 330 mi) would greatly exceed half the width of the Floridan Plateau (width (fig. 22A), 270 mi; half the width, 135 mi), suggesting that the movement of seawater inland must be accompanied by a circulation system, as proposed by Kohout (1965) (that is, movement upward through the middle confining unit and seaward through the Upper Floridan aquifer). The scenario of groundwater movement in southern Florida would, therefore, include (1) the continued rise in sea level and concurrent displacement of stored freshwater by inland-moving cool seawater chiefly through the Boulder Zone (the Lower Floridan aquifer), (2) heating of the seawater during inland transit (lowering of the density), (3) upward movement of the seawater through fractures and sink holes that transect the middle confining unit, and (4) dilution and transport of the seawater back to the ocean by seaward-flowing freshwater in the Upper Floridan aquifer. However, the combined effects of heating and dilution alone probably would provide sufficient loss of head in the Boulder Zone to drive the circulation regardless of changes in sea level.

Table 10. Estimated maximum and minimum velocities, hydraulic gradients, and head loss between site 9 and 10 during the Holocene transgression, Lower Floridan aquifer [Based on radiocarbon dating and theoretical relation between sea level rise and inland transit in fig. 23. Site locations shown in fig. 2]
	Velocity (feet per year)	Hydraulic gradient		Distance (miles)	Head loss (feet)
		(feet per foot)	(feet per mile)
Maximum	172	3.7 x 10-6	1.9 x 10-2	44.5	0.87
Minimum	5	1.1 x 10-7	5.8 x 10-4	44.5	0.026

The time involved in the circulation is short by geologic standards but extremely long by man's standards. The circulation is best shown by flow lines in figure 24. Fractures and sinkholes, which transect the middle confining unit, hydraulically connect the Upper Floridan aquifer with the Boulder Zone (Lower Floridan aquifer) near the coastline. Relatively young, cold seawater flows into the Boulder Zone through the sinkholes and fractures, then is heated as it flows inland. Some saltwater moves upward into the Upper Floridan aquifer through sinkholes and fractures, where it is diluted and carried seaward by the flowing freshwater. The relative carbon-14 activities and apparent ages of the ground waters in the Floridan aquifer system support the theory of this circulation, as does the uranium isotope data and the occurrence of anomalously cool saltwater in the Lower Floridan aquifer. Oxygen isotope data (table 4) show enrichment of oxygen-18 with respect to oxygen-16 (¹⁸O) in the saltwater in the Lower Floridan aquifer and a possible relation between oxygen-18 concentration and radiocarbon age. However, the data are insufficient to draw conclusions on paleotemperatures of the ocean during the Holocene transgression. The role that rising sea level plays in circulation is not well documented, and more research is needed to fully evaluate such an effect.