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# LOCATION OF PROJECT SAMPLING SITES

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# LEON CO. WAKULLA CO. WAKULLA CO. LEON CO.



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# B A Y A P A L A C H E E St. Marks National Wildlife Refuge 329 351 391 544 564 607 650 653 658 663

674 717 749 801 713 641

318 372 635 705 774 900 6493 1020 1056 1176 1261 1399 1427 2562 2749

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# Y District Boundary Lakes Incorporated Areas Public Lands County Boundary U.S. Highways Rivers Conduit System State/County Roads 01-06-015 

# FIGURE 9

# 14 through 2000, it was only following Tropical Storm Helene in September 2000 that conditions were appropriate for high-flow sampling. The project wells, Middle River Sink and Wakulla Springs were sampled again in October 2000 for that purpose. Nitrogen isotope data gathered to establish the ratio of organic to inorganic nitrogen in the samples were inconclusive for four wells. This was due to nitrate concentrations being below detection limits. Due to this, four alternate wells with elevated nitrate concentrations were selected for additional sampling. These wells were sampled on April 25, 2001. Surface Water Monitoring The study plan included collection of continuous hydrologic data to help characterize the dynamics of the hydrologic cycle and the interaction between surface water and ground water resources in the study area. The automated monitoring equipment utilized on the project included an automated rainfall station, a current meter (water velocity) and in-situ water-quality data-sonde meters (water temperature and conductivity). Continuous streamflow monitoring was beyond the scope of the project. Site locations are shown in Figure 10. Rainfall An automated rainfall station was established at a central location in Wakulla County (at the River Sink Water Tower located on Highway 319 near the intersection with SR 267 (30 ° 16‘38”/ 84 ° 21’22” NAD27). The station consisted of a rainfall tipping bucket sensor and a data logger, which recorded rainfall data on a ten-minute time interval. The tipping bucket rain gage includes a funnel and a tipping mechanism. Rainfall passes through the funnel onto the tipping mechanism which causes a switch contact closure for each 0.01 inches of accumulated rainfall. The data logger stores the number of tips and records the accumulated rainfall depth. The tipping bucket has an accuracy of ± 2 percent for rainfall rates of less than 1 inch per hour and ± 3 percent for rates above 1 inch per hour. The station was visited each month to retrieve data, perform routine maintenance and insure proper functioning. Annual field calibrations were performed to insure sensor accuracy. The station was installed in May 15, 1999. Rainfall data was also utilized from two long-term rainfall stations located in Leon County. These included a station operated by NWFWMD located near Lake Munson in southern Leon County and data collected by the National Weather Service at the Tallahassee Regional Airport. Current Meter The NWFWMD operates and maintains a self-contained electro-magnetic current meter in the main spring vent of Wakulla Springs. InterOcean Systems, Inc manufactured the meter (S4 model). It has the capability to continuously measure and record velocity and direction components of water movement, water temperature and conductivity. The meter is installed in the main discharge tunnel (A-tunnel) at a depth of approximately 190 ft below the surface of the spring. The meter was installed in May 1997. The geometry of the vent opening has been measured (Figure 11) and the opening area used, with the velocity data and a calibration factor, to calculate continuous discharge data. The current meter is a ten-inch diameter sphere with four electro-magnetic sensors located symmetrically around the central axis of the sensor. The meter has an internal compass to provide directional information with the velocity data. Conductivity and temperature sensors are also located along the center axis of the meter. The current meter collects data on a three-hour

# PRINCIPAL KARST FEATURES

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Rise Ames

Sink Hydrolab
Lower River Sink Upper River Sink Go Between Sink 1 Fisher Sink Hydrolab

Sally Ward Spring
Lost Creek Hydrolab

Spring Creek #1 Spring Creek #2 Hydrolab Fish-Eye

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USGS Gaging Station, Lost Creek at Arran USGS Gaging Station, St. Marks R. near Newport Lake Munson NWFWMD Gaging

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# SESF

NWFWMD Riversink Rainfall Station NWS Tallahassee Airport

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# Y Y



# N Incorporated Areas Major Lakes

Public Lands Cody Scarp Conduit System County Boundary Rivers 

# Y Springs/Karst 

# "  ́ Gaging Stations 

# 0 5 Miles

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# 0 10 20 30 40 50 60 70 80 74 65 25 15 5

0 10 20 30 40 50 60 70 80 74 65 25 15 5 0 5 10 15 20

-5 -10 -15 -20 0 5 10 15 20

-5 -10 -15 -20 17' 17'        	    	 
     
          	    	 
     
  



#        	    4" DIA. SUPPORT PIPE

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FIGURE 11 CROSS-SECTION OF WAKULLA SPRINGS PRINCIPLE CONDUIT SHOWING S4 EMPLACEMENT

# 17 recording cycle and stores it internally. The meter is brought to the surface every eight to twelve months to retrieve data and service the meter. The meter is installed in the center of a limestone constriction at the mouth of the main spring vent (Figure 11). The mooring system consists of telescoping aluminum pipes that extend between the roof and floor of the spring vent. Two arms extend from the side of the aluminum pipes to support the meter. The mooring system was designed to minimize disturbance of the spring flow, prevent damage to the limestone surfaces of the spring vent opening and facilitate easy removal and installation of the current meter. The cross-sectional area of the conduit at the point where the meter is installed is 741 ft 2



# . Main vent discharge has been estimated as the product of observed point velocity in A-tunnel and A-tunnel cross-sectional area at the point of velocity measurement. These estimates have been verified with five conventional channel discharge measurements at the CR 365 bridge and the other three major inflows (Sally Ward Spring and two McBrides Slough inflows) that discharge to the river above the CR 365 bridge. The conventional channel discharge measurements were completed during one-day measuring events. A variable time lag was assumed between the main vent and the CR 365 bridge based on a distance of 2.75 miles and an average channel velocity for the measuring period. Discharges from Sally Ward Spring and the McBrides Slough inflow were subtracted from the Wakulla River discharge at CR 365 to approximate discharge from the main vent. Discharge from small springs and seeps along the river was estimated to contribute little to the total river flow and were excluded. Observed main vent discharge versus calculated discharge is given in Figure 12.

Figure 12. Wakulla Springs Main Vent Measured Discharge versus Calculated Discharge. In-situ Water Quality Meters Self-contained water quality data loggers were installed at six locations (stations 1—6 below, Figure 10) to continuously measure surface water conductivity and temperature. The water quality data loggers were programmed to collect data on an hourly time interval at the six stations. The station locations are described below. Three stations, Middle River Sink Spring, 0 100 200 300 400 500 600

0 100 200 300 400 500 600 S4 Calculated Discharge (cfs)Measured Vent Discharge (cfs)

# 18 Wakulla Springs and Spring Creek #2 had moving water for the entirety of their respective data collection periods. Due to drought conditions, Fisher Creek, Lost Creek and Ames Sink intermittently had conditions of very low to zero flow. However, there was continuous standing water at each of these sites. Time series data for these stations should be evaluated with this in mind. Continuous mean daily flow data for Lost Creek during 1999 and 2000 were obtained from the USGS and are presented later in this report. Collection of continuous stream flow data at Fisher Creek Sink and Ames Sink was beyond the scope of this investigation. 1. Fisher Creek Sink at Leon Sink Geological Area —Fisher Creek is a black-water stream thatoriginates in the Apalachicola National Forest. Fisher Creek disappears underground and reemerges at several locations along its length below the Springhill Road Bridge. The meter was installed on a walkway in a six-inch diameter PVC support pipe at the point where Fisher Creek disappears underground for the last time (30 ° 18’35.115”/84 ° 21’22.783” NAD83). 2. Ames Sink —Ames Sink is located about eight miles south of Tallahassee and funnels water from Munson Slough underground as it flows south. The meter was installed in a six-inch diameter PVC pipe mounted on the SW bank of the sink near the inflow channel into the sink (30 ° 19’08.859”/84 ° 17’54.916” NAD83). 3. Lost Creek below FR 13 —Lost Creek is a black-water stream originating in the Apalachicola National Forest. The creek meanders through the forest before disappearing underground south of Crawfordville. The meter was installed in a six-inch PVC pipe extending into the creek about 0.5 miles upstream from the point where it flows underground (30 ° 10’33.014”/ 84 ° 24’00.696” NAD83). 4. Wakulla River at Boat Dock —The Wakulla River begins at a basin created by the main Wakulla Spring vent located at the Wakulla Springs State Park. The meter was installed on a concrete slab, with a vertical support pipe, below the tour boat dock located about 500 feet downstream of the main spring vent (30 ° 14’08.381”/84 ° 18’05.199” NAD83). 5. Spring Creek Vent #2 —Spring Creek Vent #2 is one of several springs that discharge into the estuary near the Spring Creek community. The meter was installed in a six-inch diameter pipe on the upper edge of the sink in the path of the spring flow (Figure 13). The meter was installed in about 13 feet of water (30 ° 04’53.687”/84 ° 19’47.141” NAD83). A schematic representation of the geometry of Spring Creek Vent #2 sink and pool is also given in Figure 13. This figure is adapted from Lane (2001). Lane also gives a map depicting the location of the 13 mapped vents in the Spring Creek group. Specific conductivity versus time data for this station is given in Figure 14. These data show that, for most of the data collection period, the conductivity at the measurement point was rarely less than 2,000 μ mhos/cm. Specific conductivity values daily ranged between about 2,000 μ mhos/cm to well in excess of 9,000 μ mhos/cm, apparently reflecting the mixing of saline surface water with fresher ground water discharge within the spring pool itself. Only once were waters in the spring pool consistent with what should be expected for Floridan Aquifer water uncontaminated by saline surface water. This occurred for about a week in early November 1999 following heavy rains associated with Hurricane Floyd. 6. River Sink Group —The River Sink area is a series of sinks and karst features in northern Wakulla County. The meter was installed on a concrete slab, with a vertical support pipe, in

#  	 

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# FIGURE 13 CROSS-SECTION OF SPRING CREEK VENT #2 SHOWING HYDROLAB EMPLACEMENT20Figure 14. Specific Conductivity and Rainfall versus Time for Spring Creek Vent #2, 1999.0100020003000400050006000700080009000 

# 3/19/99

4/2/99 4/16/ 9

9 4/30/ 9

9 5/14/ 9

9 5/28/ 9

9 6/11/ 9

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# Specific Conductivity (umhos/cm)012345 

# River Sink Station Rainfall (inches/day)RainfallSpring CreekNo Rainfall DataNo Data 

# No Data No Data

No Data

No Data

# 21 7. the center of a section of collapsed conduit where ground water emerges to create a stream that flows about 200 feet before disappearing underground into a sink (30 ° 16’36.24” 84 ° 20’27.396” NAD83). Data from three additional stations (Figure 10) were also used in this investigation. 8. USGS Lost Creek Gaging Station —The USGS maintains a continuous stream flow gaging station on Lost Creek. The station is located on the downstream side of the Highway 368 bridge, 0.5 miles east of Arran (30 ° 11‘17”/84 ° 24’30” NAD27). This station is located about 1.8 miles north (linearly) of the point where Lost Creek goes underground and about one mile (linearly) above the NWFWMD water quality data logger site (station #3 above). Continuous daily mean flow versus conductivity data from the NWFWMD Lost Creek station are given in Figure 15, for the period 3/99 to 11/00. For most of this period, conductivities range between 40 to 70 μ mhos/cm. During three low-flow periods (04/99 to 06/99, 12/99, and 04/00 to 08/00) surface water conductivities rose to as high as 200 μ mhos/cm. Conductivities this high appear to reflect base flow discharge from the Floridan Aquifer into Lost Creek. Along the lower reach of Lost Creek, Floridan Aquifer heads are above stream channel elevations. Accordingly, it is quite plausible that Floridan Aquifer waters are discharging to the channel above the point where Lost Creek goes underground. 9. USGS St. Marks River Gaging Station —The USGS maintains a continuous streamflow gaging station on the St. Marks River about 0.65 miles below the St. Marks River rise. The station is located on the east bank of the river, about 0.4 miles south of the Leon/Wakulla county line (30 ° 16‘00”/84 ° 09’00” NAD27). 10. Munson Slough north of Lake Munson (S3) —The NWFWMD maintains a continuous stream flow station on Munson Slough. The station is located at the point where Munson Slough crosses under Highway 319 (30 ° 23‘14”/84 ° 18’49” NAD27). This station is located about 0.95 mile north of Lake Munson and about 5.24 miles north of the continuous recording water quality logger in Ames Sink (station #2). Continuous daily mean flow (station #9) versus conductivity data from the Ames Sink hydrolab station (station #2) are given in Figure 16, for the period 3/99 to 9/00. For most of this period, conductivities range between 57 and 261 μ mhos/cm. During relatively dry periods surface water conductivities rose. High conductivities within Ames Sink may reflect re-circulation of Floridan Aquifer water into the sink or discharge of Floridan Aquifer water into lower Munson Slough during periods of minimal surface water inflow. Four sites had meters installed within six-inch diameter PVC support pipes. For these stations PVC support pipes were installed to protect the meters. Each pipe had vent holes drilled around the perimeter of the pipe to ensure good water flow around the sensors. Two stations were installed on concrete anchor slabs having two-inch diameter PVC support pipes vertically mounted in the center. The two-inch diameter support pipe extended about three feet above the channel bottom. Water quality meters were installed on the two-inch vertical support pipe within the water column. The water quality meter stations were visited monthly to retrieve data and re-calibrate the meters. The sensor arrays were cleaned monthly and the meters were monthly re-calibrated using appropriate conductivity calibration standards. The temperature probes were monthly22Figure 15. Lost Creek Specific Conductivity, Rainfall and Daily Mean Flow versus Time (flow data from USGS).050100150200250 

# 0 3 /18/ 9 9 0 4 /1

7 / 99 05/

1 7 / 99 06/16/ 9 9 0 7 /16/ 9 9 0 8 /1

5 / 9 9 09/

1 4 / 99 10/14/ 9 9 1 1 /13/ 9 9 1 2 /1

3 / 99 1/12/0 0 2/11/00 3 /1

2 / 00 4 /1

1 /0

0 05/

1 1 / 00 6 / 10/00 7 /1

0 / 00 8 / 9 /0

0 9 / 8/

0 0 10/8/00 1 1 /7

/ 00Date 

# Specific Conductivity (umhos/cm)01002003004005006007008009001000 

# Daily Mean Flow (cfs)/Rainfall River Sink Station (inches/day) X 100Daily Mean FlowRainfallSpecific ConductivityNo Rainfall Data23Figure 16. Ames Sink Specific Conductivity, Rainfall and Munson Slough @ Highway 319 Daily Mean Flow versus Time.050100150200250300 

# 03/16/9 9 04/15/9 9 05/15/99 06/14/99 07/14/99 08/13/ 9

9 09/12/ 9

9 10/12/ 9

9 11/11/ 9

9 12/11/ 9

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0 6/

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00 7/

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# Specific Conductivity (umhos/cm)0100200300400500600700 

# Daily Mean Flow (cfs)/Rainfall Airport Station (inches/day) X 100Daily Mean FlowRainfallSpecific Conductivity

# 24 calibrated to an external thermometer reading. The meters experienced minimal sensor fouling at all six locations throughout the data collection period. They also experienced minimal drift with respect to both temperature and conductivity. Calibration adjustments for temperature ranged from 0.01 to 4.86 degree Celsius with an average adjustment of 0.35 degrees. The adjustments required for conductivity ranged from zero to 29 μ mhos/cm with an average of 6.1 μ mhos/cm. Temperature readings from the hydrolabs under identical conditions were compared once to assure data consistency. Results are as follows: Fisher Creek @ Leon Sink Serial #15588 19.6 ° C Ames Sink #15591 19.8 ° C Lost Creek below FR13 #18253 19.4 ° C Middle River Sink Spring #15589 19.8 ° C External thermometer reading 19.7 ° C

# 25 HYDROLOGY OF THE ST. MARKS-WAKULLA RIVERS WATERSHED Physiography and Geology The study area principally lies within the Tallahassee Hills and the Gulf Coastal Lowlands physiographic divisions (Figure 17). The northern two-thirds of Leon County lie within the Tallahassee Hills. The southern third of Leon County and all of Wakulla County lie within the Gulf Coastal Lowlands. The northern portion of Jefferson County lies in the Tallahassee Hills, the southern portion in the coastal lowlands. Elevations in the Tallahassee Hills are quite variable, being as high as 250 ft above sea level and as low as 50 ft above sea level. Elevations on the Gulf Coastal Lowlands are much more uniform and lower, being generally less than 50 ft above sea level. The Cody Scarp divides these two physiographic divisions and is defined by a fairly abrupt decrease in elevation from the Tallahassee Hills onto the coastal lowlands (Figure 17). It is most evident in eastern Leon County. In the western part of the county, the break in topography is more gentle and the scarp more difficult to map. It represents the landward edge of a transgressional marine erosion event. South of the Tallahassee Hills, the Woodville Karst Plain forms a subdivision of the Gulf Coastal Lowlands. The Woodville Karst Plain lies in southeast Leon County and eastern Wakulla County. It is an erosional surface bounded on the north by the Cody Scarp and on the west by a second, lower scarp (Figure 17). Land surface elevations on the Woodville Karst Plain are low, rarely more than 50 ft above sea level. The overlying confining units have been stripped away by sea level fluctuations, leaving only a thin veneer of unconsolidated sediments covering the St. Marks Formation. Sinkholes, shallow closed depressions, cenotes, springs, other karst landforms, and an almost complete lack of surface streams dominate the landscape. Most of the Tallahassee Hills in Leon County lie within one of four large closed lake basins (Figure 18). These are the Lake Iamonia, Lake Jackson, Lake Lafayette and Lake Miccosukee basins. The basins within which Lakes Jackson and Iamonia lie were originally tributary to the Ochlockonee River. Erosion improved the hydraulic connection to the underlying Floridan Aquifer to the point where the drainage became internal and the basins closed. Lake Jackson is effectively completely closed. Lake Iamonia is still hydraulically connected to the Ochlockonee River floodplain under high flow conditions. Under low flow conditions, it is a closed basin. Both lakes have active sinkholes in their beds. Similarly, Lakes Lafayette and Miccosukee were originally tributary to the St. Marks River. Erosion and internal drainage have more or less closed both of these basins. An intermittent overland hydraulic connect exists between the Lake Lafayette basin and the St. Marks River floodplain. The Lake Miccosukee basin is effectively completely closed by a system of sinks that capture any outflow along the relic stream channel. These include Lake Drain sink and Creek sink. Both lakes have active sinks in their beds as well. Smaller closed basins lying in the Tallahassee Hills include the Fred George basin draining to Fred George sink, the Patty Sink basin, and the Black Creek basin draining to Bird and Copeland sinks. The Lake Munson basin lies on both the Tallahassee Hills and on the Gulf Coastal Lowlands. Surface waters originating in the Lake Munson basin flow south onto the coastal lowlands, through Lake Munson and eventually drain underground at Ames Sink.

# PRINCIPAL PHYSIOGRAPHIC FEATURES

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St. Marks Crawfordville JEFFERSON CO. 

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# N U.S. Highways Conduit System State/County Roads Rivers County Boundary 

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# 0 5 Miles

01-06-004 

# FIGURE 17

# SURFACE WATER BASINS AND PRINCIPAL SURFACE WATER FEATURES

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# GEORGIA

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# A P A L A C H E ENWFWMD JEFFERSON CO.



# B A Y WAKULLA CO.



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01-06-006 

# FIGURE 18

# 28 The Tallahassee Hills are underlain by (in descending order) the Miccosukee Formation, the Torreya Formation of the Hawthorn Group, the St. Marks Formation, the Suwannee Limestone and older, more deeply lying units. Hendry and Sproul (1966) first described the geology of Leon County. Scott (1988) describes the lithostratigraphy of the Hawthorn Group in Florida, including Leon County. The Hawthorn Group is absent in southeast Leon County and eastern Wakulla County, having been removed by the marine transgression that formed the Cody Scarp. Within the Tallahassee Hills the Miccosukee Formation is limited to the higher elevations found on inter-stream divides (Figures 19 and 20). Elsewhere, it has been removed by erosion. The Torreya Formation is near the surface along lower elevations within the Tallahassee Hills. These include the many stream channels that dissect the hills and in the bottoms of the large lakes that dot the landscape. Torreya Formation is frequently seen in scoured stream channels and drainage ditches found throughout Tallahassee and the surrounding urbanized area. Lying immediately south of the Tallahassee Hills, fairly thick deposits of undifferentiated sand mantle the northern edge of the Gulf Coastal Lowlands. These consist of relict beach and dune deposits. One portion of this sequence is known as the Lake Munson Hills. Undifferentiated sands, Jackson Bluff Formation, Intracoastal Formation, Torreya Formation, St. Marks Formation, Suwannee Limestone and older, more deeply lying units underlie the Gulf Coastal Lowlands south of the Tallahassee Hills. Rupert and Spencer (1988) describe the geology of Wakulla County. The area of occurrence of the Intracoastal and Torreya Formations is limited to approximately the western half of the county (Figure 21). The relatively low hydraulic conductivity of the Intracoastal and Torreya formations hydraulically isolates the surficial sands from the underlying Floridan Aquifer. Given the flatness of the terrain, the thinness of the surficial sands and the low permeability of the underlying sediments, broad expanses of wet pine flatwoods dominate the landscape. The Ochlockonee River and Fisher, Black and Lost creeks drain these flatwoods. Fisher, Black and Lost creeks flow from west to east. Once they encounter the western edge of the Woodville Karst Plain, they disappear underground. Their waters drain either to Wakulla Springs or to the Gulf of Mexico. On the karst plain itself, the stratigraphy consists of the thin veneer of sands lying directly on top of the St. Marks and older formations (Figures 20 and 21). Hydrology of the Floridan Aquifer The work of Davis (1996) provides a comprehensive overview of the Floridan Aquifer flow system beneath Leon and Wakulla counties. Davis calibrated and applied a Floridan Aquifer flow model to a large study area in southwest Georgia and north Florida (including Leon and Wakulla counties). Based on potentiometric surface mapping, Davis defined an approximate recharge area for the coastal Wakulla County discharge features (Wakulla Springs, Spring Creek, lower St. Marks River and other submerged lands beneath the Gulf of Mexico). That area includes all or portions of Leon, Gadsden and Jefferson counties in Florida and Thomas, Grady, Colquitt, Mitchell and Decatur counties in Georgia. The Florida portion of this area is delineated in Figure 1. In both Georgia and Florida the total recharge area for these features is about 2,200 mi 2



# . As a part of his conceptual model development, Davis divided Leon County into three areas: unconfined where the Floridan Aquifer is near land surface; unconfined where the Floridan Aquifer is overlain but not confined by low-permeability sediments; and confined. The first two of these areas generally correspond to areas identified as unconfined and semi-confined, respectively, in Figure 1. In Wakulla County, Davis conceptualized the Floridan Aquifer as being either unconfined and near land surface, or confined. Davis’ unconfined area generally

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# SURFACE GEOLOGY

Source: Scott (2001)

01-06-005 

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# GEOLOGIC CROSS-SECTION B-B' IN LEON AND WAKULLA COUNTIES

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# GEOLOGIC CROSS-SECTION A-A' IN WAKULLA COUNTY

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# 32 corresponds to what is mapped as unconfined on Figure 1. This also corresponds to what was mapped as the Woodville Karst Plain by Hendry and Sproul (1966), Lane (1986), Rupert and Spencer (1988) and Rupert (1988). Davis calibrated his model to conditions in October and November 1991. Based on the calibration results, Davis developed Floridan Aquifer recharge rates for each of these three areas. For that portion of Leon County overlain by low-permeability sediments but not confined, he calibrated a recharge rate of 7.9 in/yr. For that portion of Leon and Wakulla counties under unconfined conditions, he calibrated a recharge rate of 18 in/yr. For the two sprayfields operated by the City of Tallahassee he applied recharge rates of 62 in/yr (reflecting the volume of effluent applied to the sprayfield). For confined portions of both counties, he calibrated fluxes into the Floridan Aquifer (recharge across the confining unit) as high as 10 in/yr and out of the aquifer (discharge) as high as 10 in/yr. Discharge across the confining unit was simulated to occur in the immediate vicinity of the Ochlockonee River. The highest recharge rates occurred immediately adjacent to the unconfined parts of the model. Davis measured discharge from the Floridan Aquifer at several sites in Wakulla County. On November 1, 1991 the following flows were measured: Spring Creek sub-aqueous spring group—307 cfs, Wakulla Springs—350 cfs, St. Marks River south of Leon/Wakulla county line— 602 cfs. The total discharge from these three features is 1,259 cfs, virtually all of which comes from the Floridan Aquifer. Davis’ calibrated model simulated a discharge from these features of 1,246 cfs. Diffuse discharge to the Gulf of Mexico (the only other discharge sink for the coastal Wakulla County recharge area) was simulated as being minimal. Ground water flowing beneath Leon County in the Floridan Aquifer flows in a generally north to south direction. Because of extremely high transmissivities, pumping by the City of Tallahassee has relatively little effect on the potentiometric surface and no significant cone of depression has formed. The potentiometric surface at the State line stands at an elevation of about 70 ft above sea level. This declines to about 10 ft above sea level at the Leon-Wakulla county line. Water that flows across the state line is augmented by water that leaks into the Floridan Aquifer within the county. In spite of fact that the Floridan Aquifer rarely outcrops within Leon County, it is well connected to the overlying land surface. Downward leakage occurs in the base of closed depressions, through lakebeds and sinkholes, and through the confining unit. Sediments of the St. Marks Formation are rarely observed in Leon County, but they are not far beneath the surface. Sediments that comprise the Torreya Formation are relatively thin. In the eastern two-thirds of the county, they are typically 100-ft thick or less. In many low-lying areas they have been significantly thinned by erosion. In the northern part of the county the top of the St. Marks Formation stands at elevations as high as 100 to 120 ft above sea level, resulting in a significant thickness of unsaturated carbonate rock north of the Cody Scarp. Unsaturated thicknesses of St. Marks sediments in this area are as great as 80 ft. Katz et al. (1997a) conducted an investigation into ground water/surface water interactions at two sites in Leon County, near Fred George Sink within the Tallahassee Hills and near Lake Bradford, on the Gulf Coastal Lowlands. They also collected samples from 11 City of Tallahassee public supply wells open to significant portions of the highly permeable uppermost Floridan Aquifer. Katz et al. observed that ground waters from five City wells are, in part, composed of surface waters enriched with 18 

# O and D. In the case of two wells (CW-19 and CW26), and based on mixing model studies, up to 32 percent lake water (subjected to evaporation prior to recharging the Floridan Aquifer) is required to provide the chemical and isotopic

# 33 composition observed in ground water from these wells. They postulate Fred George Sink and Lake Jackson as plausible sources for this evaporated surface water. Floridan Aquifer water that flows into Wakulla County flows in a southerly direction toward points of discharge. These include Wakulla Springs, Spring Creek springs group, the lower St. Marks River and the Gulf of Mexico. Water that flows into the county subterraneanly is augmented by downward leakage of local rainfall and sinking streams. These include Lost, Black and Fisher creeks and Munson Slough. Lost, Black and Fisher creeks drain pine flatwoods lying west of the Woodville Karst Plain. Munson Slough drains the Lake Munson basin, which lies on both the Tallahassee Hills and the Woodville Karst Plain. Beginning with an elevation of 10 ft at the Leon-Wakulla county line, water levels in the Floridan Aquifer decline to near zero at the coastline. Potentiometric Surface of the Floridan Aquifer on the Woodville Karst Plain Based on water-level data collected during the term of the project, three Floridan Aquifer potentiometric surface maps of the Woodville Karst Plain were prepared. Maps (Figures 22 through 24) are representative of conditions in January 1999, August 1999, and March 2000. Numbers of control points included on the maps ranged from 42 to 48. All wellhead elevations were leveled to NGVD. Water level values given on these maps are reliable to ± 0.1 ft, NGVD. All three maps show basically the same potentiometric surface contour pattern. Contours are closest together immediately south of the Cody Scarp. Within about four miles of the scarp, the potentiometric surface begins to flatten. From the point where the flattening becomes apparent (just north of the Leon/Wakulla County line), water levels decline very gently toward the south. Over eastern Wakulla County they decline as little as a foot over distances of as much as 10 miles (0.0001). Wakulla Springs is situated more or less at the center of this flattening, although it imparts no visible (from these data) perturbation to the potentiometric surface. Rather, the potentiometric surface continues to slope away from the spring to the southwest. The Big Dismal—Turner Sink conduit system is imbedded in the zone of low hydraulic gradient. The extremely high Floridan Aquifer transmissivities associated with this feature undoubtedly facilitate potentiometric surface flattening. However, flatness is not limited to the axis of the conduit system, as it also extends to the north and to the northeast, in the direction of Woodville. These data imply that an eight to 10-mile wide highly transmissive band of the Floridan Aquifer is situated in central Wakulla County. This zone of high conductivity locally perturbs the general north to south flow regime and funnels water to Wakulla Springs from the northwest, north and northeast. It includes the mapped conduit system, a paleo surface drainage channel connecting Munson Slough and McBride Slough, and presumes the existence of other, unmapped conduits north and northeast of Wakulla Springs. Using other lines of evidence, Werner (2000) postulates the existence of a second, significant conduit system due north of Wakulla Springs. The calibrated transmissivity distribution of Davis (1996) is consistent with this interpretation, as it shows a band of very high transmissivity (as high as 10M ft 2 

# d) aligned north/south through the center of Wakulla County. The potentiometric surface data also imply that Wakulla Springs incompletely captures ground water flow moving from north to south to the west of the spring. The prevailing wisdom is that the Big Dismal—Turner Sink conduit system is connected to Wakulla Springs. However, this hypothesis has yet to be proved by direct exploration. While there is, likely, some connection, the completeness with which the conduit system captures and conveys ground water flow to Wakulla Springs is unknown. These head data imply some significant quantity of bypass flow.