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Evaluating the reliability of continuous resistivity profiling to detect submarine groundwater discharge in a shallow marine environment Sarasota Bay, Florida
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Harrison, Arenll
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Marine resistivity
Geology
SGD
Seismics
Rapid reconnaissance methods
Dissertations, Academic -- Geology -- Masters -- USF
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theses   ( marcgt )
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Abstract:
ABSTRACT: Submarine groundwater discharge (SGD) can be an important pathway for nutrients entering coastal systems. However SGD flow paths can be difficult to identify and flow volumes difficult to quantify. This study assesses whether geophysical techniques are potentially cost effective methods for detecting the presence or lack of SGD within an estuary environment found in Sarasota Bay, Florida. In this area, a rapid increase in urbanization has led to increased nitrogen loading into the bay, with some 10% of this loading attributed to SGD. Discharging groundwater is expected to be fresher and hence higher resistivity, than "background" surface waters. Thus resistivity surveys sensitive to seafloor conductivities may be useful for identifying zones of SGD. However, terrain resistivities are influenced by matrix geology as well as pore water resistivity. In this study we compare the results of marine resistivity surveys against both geochemical measures of SGD (radon tra cers) and seismic profiles indicative of subsurface structure to better determine the relative impacts of geology and SGD on marine resistivity measurements in Sarasota Bay. On both regional (kilometers to tens of kilometers) and local scales (hundreds of meters) the relationship between marine resistivity and tracer-based SGD estimates does not follow the expected pattern of higher resistivities associated with higher SGD flux. Seafloor resistivities instead appear primarily influenced by stratigraphy, particularly the presence of a clay layer at ~10-15 m depth in the southern part of the bay. In the southern bay, resistivities decrease at the depths associated with the clay layer. On the local (hundreds of meters) scale, lateral variations in resistivities derived from inversions of resistivity data were not found to be reproducible; nearly-coincident lines collected 30 minutes apart in time show different local signatures. This apparent local lateral variability in the resistivi ty profiles is inferred to be a result of inversion of noisy streaming resistivity data.
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Thesis (M.A.)--University of South Florida, 2006.
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by Arnell Harrison.
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Evaluating the Reliability of Continuous Resistivity Prof iling to Detect Submarine Groundwater Discharge in a Shallow Mari ne Environment: Sarasota Bay, Florida by Arnell Harrison A thesis submitted in partial fulfillment of the requirement s for the degree of Master of Science Department of Geology College of Arts and Sciences University of South Florida Major Professor: Sarah Kruse, Ph.D. Mark Stewart, Ph.D. Stanley Locker, Ph.D. Date of Approval: July 7, 2006 Keywords: marine resistivity, geology, sgd, seismics, rapid reconnaissance methods Copyright 2006 Arnell Harrison

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i Table of Contents List of Figures ........................................................................................................ii Abstract ................................................................................................................iv 1. Introduc tion .......................................................................................................1 Regional Geology and Hydr ology ..............................................................3 St udy Area............................................................................................3 Regi onal Geol ogy.................................................................................4 Hydrology..............................................................................................7 Previous Marine Resistivity and Electromagnetic Studies.......................11 2. Met hods.......................................................................................................... 13 Resistivity Methods ..................................................................................13 Marine Resistivit y................................................................................15 Seismic Pr ofiling......................................................................................17 Radon and Continuous Radon Samplin g.................................................24 Estimating Pore water Resistivit y from Terrain Re sistivity.......................25 3. Results and Di scussion..................................................................................26 Regional Comparison of Resistivity and Radon.......................................26 Correlating Pore water Re sistivity and Terrain Resistivity Using Forma tion Factor s.....................................................................37 Local Scale Comparison of Re sistivity and Seis mic Data........................39 Discussio n...............................................................................................46 4. Conclu sions....................................................................................................48 Referenc es.........................................................................................................49 Appendice s.........................................................................................................53 Appendix A: Florida Geol ogical Survey Data..........................................54 Appendix B: U.S. Geol ogical Survey Data..............................................68 Appendix C: Electromagnet ic Survey Da ta.............................................76

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ii List of Figures Figure 1 Location map showing study area...................................................3 Figure 2 Stratigraphic secti ons found in st udy area......................................4 Figure 3 ROMP wells and formation factor lo cation ma p..............................6 Figure 4 Lithofacies and hydr ologic conductiv ity map...................................8 Figure 5 Dipole-dipole array diagr am..........................................................14 Figure 6 Marine resistivit y setup schem atic.................................................16 Figure 7 Location of resist ivity lines A and B ...............................................19 Figure 8 Location of resist ivity lines D and E ...............................................20 Figure 9 Location of resist ivity lines I, J and K............................................21 Figure 10 Location of resist ivity lines F, G and H ..........................................22 Figure 11 Location map showin g radon sampling sites.................................27 Figure 12 Location map showing ext ents of resistivit y surveys ....................29 Figure 13 Radon versus re sistivity at 5 m.....................................................30 Figure 14 Radon versus re sistivity at 10 m...................................................31 Figure 15 Radon versus re sistivity at 15 m...................................................32 Figure 16 Rainfall data plot s..........................................................................36

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iii Figure 17 Resistivity and seismi c lines near Phill ipi Creek ............................40 Figure 18 Resistivity and seismic lines from Littl e Sarasota Bay...................41 Figure 19 Resistivity and seismi c lines near New College............................43 Figure 20 Resistivity and seismi c lines near Bowl ees Creek.........................44

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iv Evaluating the Reliability of Continuous Resistivity Profiling to Detect Submarine Groundwater Discharge in a Shallow Marine Environment: Sarasota Bay, Florida Arnell Harrison ABSTRACT Submarine groundwater discharge (SGD ) can be an important pathway for nutrients entering coastal systems. Howeve r SGD flow paths can be difficult to identify and flow volumes difficult to quantify. This study assesses whether geophysical techniques are potentially cost effective methods for detecting the presence or lack of SGD within an estuar y environment found in Sarasota Bay, Florida. In this area, a rapid incr ease in urbanization has led to increased nitrogen loading into the bay, with some 10% of this loading attributed to SGD. Discharging groundwater is expected to be fresher and hence higher resistivity, than background surface waters. Thus re sistivity surveys sensitive to seafloor conductivities may be useful for identifyi ng zones of SGD. However, terrain resistivities are influenced by matrix geology as well as pore water resistivity. In this study we compare the results of marine resistivity surveys against both geochemical measures of SG D (radon tracers) and seismic profiles indicative of subsurface structure to better determine the relative impacts of geology and SGD on marine resistivity measurements in Sarasota Bay.

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v On both regional (kilometers to t ens of kilometers) and local scales (hundreds of meters) the relationship bet ween marine resistivity and tracer-based SGD estimates does not follow the expected pattern of higher resistivities associated with higher SGD fl ux. Seafloor resistivit ies instead appear primarily influenced by stratigraphy, particularly the presence of a clay layer at ~10-15 m depth in the southern part of the bay. In the southern bay, resistivities decrease at the depths associated with the clay laye r. On the local (hundreds of meters) scale, lateral variations in resistivities der ived from inversions of resistivity data were not found to be reproducible; nearly -coincident lines collected 30 minutes apart in time show different local signatur es. This apparent local lateral variability in the resistivity profiles is inferred to be a result of inversion of noisy streaming resistivity data.

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1. Introduction (Valiela and DElia 1990; Moore 1996; Horn 2002; Burnett 1999, 2001a; Burnett et al. 2003 and Swar zenski 2004;) have shown that in some cases, the direct discharge of groundwater into the coastal zone is an important pathway for nutrient and contaminant trans port from land to sea. Until fairly recently, investigations of submarine groun dwater discharge (SGD) have been accomplished using methods that tend to be labor intensive and time consuming. For example, prior to the development of equipment such as the RAD 7 (Burnett et al. 2003), which is used for continuous radon (Rn) sampling, radon and methane sampling and processing was a very labor intensive process. Another method that is commonly used in SGD investigations is the deployment of seepage meters, usually large meta l drums that have been cut and placed on the seafloor, with a plastic baggie attached to them to collect water discharging upwards across the seafloor. This met hod can be problematic for many reasons which include 1) surface water motion may drive flow into the meter, 2) only a few can be deployed at a time, 3) they assess seepage over a small spatial zone, and 4) they must be left out for long per iods of time (hours to days). New techniques for observing and quantif ying SGD in faster, more cost effective manners are desirable for bette r understanding the patterns of SGD. 1

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Geophysical techniques may offer the possibility of rapid reconnaissance for SGD. Marine-based continuous resistivity profiling (CRP) is one method that can be used for rapid data acquisition on a vari ety of scales ranging from meters to kilometers. This method works on the pr emise that dischargi ng groundwater is fresher than overlying surface waters and therefore sites of concentrated SGD will exhibit higher subseafloor resistivities. The principal complication in using th is method to potentially detect zones of fresher groundwater is that variations in ground resistivity reflect not only pore water variations but also differences in sediment porosity and type (i.e. clays are good conductors and hence show lower resi stivity but can have high porosities). Marine resistivity surveys were conducted in Sarasota Bay and compared with radon and seismic data. Comparison of the resist ivity and Rn data collected around the same time period does not show simple correlations, suggesting that porosity and sediment/rock type strongly in fluence the resistivity signal in Sarasota Bay. Direct information on s eafloor porosity and lithology would have required drilling into limestone and was beyond the scope of this project. However, indirect information on local geology was available through examination of seismic records collected in Sarasota Bay by the University of South Floridas (USFSP) Marine Science department in July 1996. In this thesis we examine the relationships between resistivity, Rn, and geology at two scales: 1) a regional scale using bot h radon concentration-derived SGD estimates that encompass the entir e bay, including Sarasota Bay in the north and Little Sarasota Bay to the s outh and regional geological data, and 2) a 2

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local scale (hundreds of meters) at predet ermined, site specific locations where nearby resistivity and seismic data were available. Regional Geology and Hydrogeology Study Area Located just south of Tampa Bay in southwest Florida, Sarasota Bay is an enclosed lagoon that is bounded to the west by shallow barrier islands and to the east by the mainland (Figure 1). The bay is approximately 400 km 2 and the watershed for the bay encompasses about 730 km 2 The bay is relatively shallow with an average depth of approximately 2-3 m and a maximum depth of 34 m. The tidal range for the bay is roughly 0.5 m (SBNEP, 2001). It is hydrologically connected to the Gulf of Mexico by several small passes (Longboat Pass, New Figure 1. Location map showing study area. Pass and Big Sarasota Pass) that subdivide the barrier island chain. Salinity in the bay is brackish to saline and is highly dependent on local ra infall and flushing within the bay. Sarasota Bay 3

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receives a majority of its freshwater i nput from several small tidal bayous and creeks which attain most of their input from storm water runoff, rainfall and groundwater seepage (Dillon, 2003). When present, groundwater seepage directly into the bay can be attributed to both artesian flow of deeper groundwater from underlying aquifers and to the re-circulated seawater moving across the sediment/surface water interface (Torres, 2001). Regional Geology In the vicinity of the bay, surface and near-surface sediments consist of quartz sand, consolidated and unconsolidated shell beds, clays, limestone and dolomites. These unconsolidated carbonates and siliclastic sediments represent a thin veneer (a few centimet ers to four meters thick) overlying an irregular base of Miocene limestone bedrock (H ine, 2003) (Figure 2). Figure 2. Stratigraphic sections found in study area. 4

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5 The Miocene bedrock has been classified by Scott (1988) as the Hawthorn Group, a unit consisting of t he Arcadia and Peace River formations. The Peace River formation (PRF), un like the Arcadia, is not contiguous throughout the study area. The Arcadia forma tion consists of, in ascending order, (1) Nocatee Member (2) Tampa Member and (3) Undifferentiated Arcadia Formation. It can be classified as a wh ite to tan colored quartz sandy limestone with a carbonate mud matrix. The Peace Ri ver formation, which is found in the southern portion of the bay, consists of sediments described as the Upper Hawthorn Clastics, which are distinguish able as yellowish-gray to light olive green interbedded phosphatic sands, clayey sands, clays and dolomite stringers (Campbell, 1985). The Avon Park forma tion, Ocala limestone and Suwannee limestone (ascending order) all reside underneath the previously mentioned Hawthorn Group. The southern portion of the bay di ffers geologically from its northern counterpart because of the presence of t he Peace River formation clay layers that form a semi-confining unit bet ween the undifferentiated deposits and the Arcadia limestone. Core samples taken from ROMP well TR 6-1 located on Siesta Key (Figure 3) show that the top of the Arc adia lies ~25.3 m below land surface, beneath surficial deposits and tens to hundreds of mete rs of the clays associated with the PRF. In contrast, ROMP well TR 7-1, located in northern Sarasota Bay (Figure 3) produced core samples that show the Arcadia formation approximately 9.10 meters below land surface and a very thin layer of unconsolidated sediments with no significant clay layers present.

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Figure 3. ROMP wells and Formation factor location map. 6

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7 In other areas in the northern bay, such as near New College (Figure 18), an even thinner sediment cover exists or is not present and the limestone outcrops at the seafloor or land surface. Hydrogeology The Sarasota area is underlain by Tertiary and Quaternary aged sediments and sedimentary rocks that cons titute the Surficial, Intermediate and Upper Floridan Aquifer Systems. Each aquifer contains one or more water producing zones separated by less pe rmeable units (Knochenmus and Bowman, 1998). The Surficial aquifer system comprises Pliocene to Holocene-age, unconsolidated to poorly indurated, clastic sediments, and is defined as a permeable unit contiguous with the l and surface (Southeastern Geological Society, 1986). The water-bearing capac ity of the aquifer system is largely dependent on grain size, sorting, and saturat ed thickness of sediments. There is a relationship between sediment type and hydraulic properties that can be seen in maps by Vacher et al. (1992) that s how an increase in hydraulic conductivities from north to south (To rres, 2001) (Figure 4).

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Figure 4. Lithofacies (a) and hydrau lic conductivity (b) of the surficial aquifer system, west central Florida (modified from Vacher and others, 1992). Recharge to this aquifer is provided by rainfall and by upward leakance from the Intermediate aquifer system in ar eas where there is a reversal in the regional head gradient. The Intermediate aquifer system is Oligocene to Miocene in age and consists of all rock units that lie bet ween the overlying aquifer system and the underlying Upper Floridan aquifer. It gener ally coincides with the previously mentioned stratigraphic unit designated as the Hawthorn Group which consists of interbedded clastic sediments and car bonate rocks. The Intermediate aquifer 8

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9 system averages approximately 120 m in thickness and contains an upper and lower confining unit as well as three water producing zones (Torres, 2001). The clays associated with the PRF are found in the Intermediate aquifer system with the top of the unit separati ng the Intermediate from the overlying Surficial aquifer system. There is no natural recharge from the overly ing aquifer system because of the upward head gradient that exists between all the aquifers. In certain areas; however, specifically those dominated by agricultural activities, changes to the natural potentiometric surface have caused reversals within the head gradients thereby inducing recharge to the underlying aquifers (Knochenmus and Bowman, 1998). Manatee County, for instance, is an agr icultural region that is highly dependant on constant groundwat er withdrawals from the underlying aquifers for both irrigation and domestic purposes espe cially during the dry season, which runs from December thru May. Ove r-pumping of these aquifers has caused a depression in the potentiometric surfac e and other adverse affects (SWFWMD, 1988). This change has in effect caused a change in the regional flow gradient and reversed the head gradient between the aquife rs (the coast in the vicinity of the Manatee and northern Sarasota County line now acts as both a discharge and recharge area), which may have allowe d for seepage of saline waters from the bay into the underlying aquifers. As previously stated, these reversals in head gradients occur mainly in the nort hern part of the bay, which has more agricultural areas, but head reversals have also been seen in the south in the vicinity of large well fields and pumping stations used for public supply.

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10 Finally, the Upper Floridan aquifer system consists of carbonate rocks primarily of Tertiary (Paleocene to O ligocene) age that ar e approximately 910 m thick. Recharge to this aquifer is by la teral flow from adjacent areas, whereas discharge is upward into the Intermediate aquifer system in the form of diffuse leakage or along perpendicular flow zones and fractures (Knochenmus and Bowman, 1998).

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11 Previous Marine Resistivity and Electromagnetic Studies Using electrical resistivity techniques to study seafloor terrain resistivity is such a new method that ther e is a limited quantit y of literature on the topic. Two studies that have used electrical resistivit y with the aim of identifying zones of submarine groundwater discharge are t hose by Manheim et al. (2004) and Krantz et al. (2004). Manheim et al. (2004) used streami ng resistivity along with other adjunct methods (core and pore water samples) to detect fresh ground water located in the subsurface below coastal bays of the Delmarva Peninsula, which consists of both fine-grained surficial sediments and permeable sands. Manheim et al. (2004) showed fresh wate r lenses that extend from a few hundred meters to more than 2 km from sh ore. Hypersaline brines were also detected in the subsurface at shallow (<20 m) as well as deeper (>300 m) depths. Their work showed that stream er resistivity systems can be effective tools for locating fresh and/or brackish waters in specific types of coastal environments. This technique can pr ovide continuous regional/local scale profiling and allow for fast er (~30 times) data colle ction than comparable land based studies (Manheim et al., 2004). Krantz et al. (2004) used electrical re sistivity in conjunction with drilling and geochemical methods to establish the hydrogeologic setting and groundwater flow beneath Indian River Bay, Delaware, an area that is primarily composed of organic-rich silts. In this study, the resistivity profiles helped show

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12 submarine groundwater discharge, comp lex ground water flow patterns, and various modes of mixing occurring in the underlying aquifer systems. In particular, the shore parallel resistivit y profiles showed alternating subsurface zones of high and low resistivity, which we re interpreted as saline water from the estuary moving down into the aquifer. Shore perpendicular profiles showed fresh water coming from the land margin and flowing beneath the bay to discharge near the center of the bay. Their combined met hods provided results that illustrated the flow of fresh ground wate r that produced plumes 20 m thick and 400 to 600 m wide that may extend 1 km or more from the shore beneath the estuary. These plumes underlie small in cised valleys which were filled with 1 to 2 m of silt and peat that act as a semi-c onfining layer to re strict the downward flow of salt water from t he estuary (Krantz et al., 2004).

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2 Methods To evaluate the utility of geophysical methods for detecting SGD, these methods were compared to geochemical tr acers on both a larger regional scale (kilometers to tens of kilometers) and a small local scale (tens to hundreds of meters). For the regional scale surve ys radon advection rates were measured and converted to SGD rates and then compared to continuous resistivity profiles collected throughout the entire bay. For the local scale surveys, coincident marine based resistivity and seismic profiles were compared. Resistivity Methods Marine resistivity follows the same basic principles of land based resistivity surveys with only a few variations. We not e for reference that resistivity is the inverse of conductivity, and that terrain resist ivity is the resistivity of the volume of material sampled by the instrument. Thus the terrain resistivity below the seafloor is the resistivity of the combined matrix plus porewaters, while above the seafloor the terrain resistivity is just t he surface water resistivity, or 1/surface water conductivity. For a single measurement of terrain resistivity, four electrodes are positioned at a given distance from each other. A constant direct current is introduced between the two (current/s ource) electrodes and the resulting potential difference is measured between t he other two (potent ial) electrodes. 13

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The measured potential difference is a func tion of the terrain resistivity and the electrode geometry. Certain electrode geometries are utiliz ed for ease of data collection and interpretation (See Figure 5). The geometry (also known as an array) used in this study was the dipole-dipole. Figure 5. Dipole-dipole array diagram. The dipole-dipole array has both current and potential electrode pairs oriented in a straight line with the potential electrode pair offset from the current electrode pair. For simplicity, the spac ing between electrodes in each of the potential and current electr ode pairs is set equal; this setup is sometimes referred to as axial dipole. The spacing between the current and potential electrode pairs is then varied. When t he two pairs are closely spaced, the instrument is sensitive to the shallo w subsurface; as the offset between the electrode pairs is increased, the depth of sensitivity increases. By sampling a range of offsets, terrain re sistivities structure can be measured as a function of depth. The only constraint of using the dipole-dipole setup is that it requires 14

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more power than other geometries to accommodate large offsets between the current and potential electrode pairs. The potential differences measured at the potential electrode pair for the various geometries must be combined an d inverted for a best-fitting terrain resistivity model. The final best-fi tting model will depend on a number of parameters that control the inversion procedure. Because marine resistivity data sets are so large, inversions must be r un separately on subsets of the profiles. Marine Resistivity Our resistivity surveys were adapt ed for marine deployment using streamer resistivity techni ques which had been previously tested in other coastal bay environments (Manheim, 2004). Resi stivity data for regional comparisons with radon data were collected by the Florida Geological Survey (FGS), in June 2002, and the U.S. Geological Survey (USGS), in May 2003 and February 2004 using the Zonge Streaming Resistivit y/IP system of Zonge Engineering and Research Organization, Inc. and the AGI SuperSting of Advanced Geosciences, Inc., respectively (because the USGS data were collected around the same time as the radon data that data set was used for comparison instead of the FGS data, however, the FGS data can be found in the appendix) Both systems continually record and store data using a multi-channel resistivity receiver as well as collect position coordinates from a GPS receiver. The 100-m streaming resistivity cables used in both systems c ontain a current electrode pair and nine potential electrodes set up to be used in the dipole-dipole array with a 10 m 15

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spacing. The streamers are towed across the water's surface at a speed of ~3-5 knots. Operating in continuous mode, the syst em injects current in the first two electrodes and then measures eight voltage potentials in the trailing electrode pairs (Figure 6). Figure 6. Marine resistivity setup schematic. Streaming resistivity data were co llected once every few seconds. Measurement intervals were determined by the user and depth of penetration was equal to approximately 0.20-0.33 the length of the electrode array. Postprocessing of the resistiv ity data involved several in verse modeling iterations using the software provided with the systems and modeling software such as Golden Surfer for creating final plots. In addition to the regional surveys a fi nal set of small scale surveys was conducted with collaborators at the USGS in February 2006 to compare geologic features found in seismic profiles and resi stivity on a smaller, local scale. The 16

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resistivity cable used had a custom AGI 100-m dipole-dipole array setup with 10meter electrode spacing, marine wate r seal, Kevlar strength member and stainless steel anchors. There were 9 stainless steel potential electrodes and 2 graphite current electrodes. A sea anchor with sufficient tension for the survey speed (survey speeds varied from 4-6 kph) was also used. To obtain correct global positioning, a WAAS differential G PS was available throughout the survey area from a NMEA 0183 2.0 stream of the Lowrance 480M GPS/sonar. The transducer was set to 200 kHz and had a dept h of resolution of 5 cm (system has maximum resolution up to 50 m with th is setting). The entire set of smallscale surveys was conducted over a peri od of approximately 9 hours. Also the use of graphite current electrodes allow ed for less aggregation on the electrodes which produced less noisy data and a greater depth of penetration in this survey compared to the previous surveys. Seismic Profiling The seismic data used in this study were collected by Locker et al., (2001) during a July 23-26, 1996 survey conducted from Tampa bay to Venice, Fl. For data collection over the course of the su rvey they used a high-resolution single channel Huntec Boomer seismic acqui sition system. Seismic data were acquired using low power levels ranging from 100-200 J to maximize vertical resolution of the thin Holocene section that is found in northern Sarasota Bay. Along with the Boomer set up, a 10 Element Innovative transducer, streamer, and Elics Delph2 Digital seismic acquisition and processing software were also used. 17

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In cases where digital data were not av ailable paper records were taken using an ORE Geopulse boomer system. Two seismic lines were used for co mparison with the resistivity data collected in February 2006, line 2 in the nor th and line 24 in the south (Figures 710). Because of its length, line 24 was br oken up into multiple lines but for our purposes the lines that were closest to our resistivity surveys, lines 24f and 24i, were used (Figures 9-10). 18

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Figure 7. Location of resistivity lines A and B. 19

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Figure 8. Location of resistivity lines D and E. 20

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Figure 9. Location of resistivity lines I, J and K. 21

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Figure 10. Location of resistivity lines F, G and H. 22

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The acquisition parameters for all lines were set with a shot interval = 400 ms, sampling frequency = 8000 Hz and high/l ow filters set to 960 Hz/3200 Hz, respectively. Lines 2 and 24f both had re cording lengths = 140 ms and line 24i had a record length = 240 ms. For comparison with resistivity surveys, selected sections just a few hundred meters long ar e examined. To estimate depths from seismic travel times we assumed the velo city of the seismic wave to be equal to 1700 m/s in the sediments on all lines. 23

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Radon and Continuous Radon Sampling Methods developed by collaborators at FSU (Burnett, et al., 2003) were used as the approach for quantifying SGD. This method used geochemical tracers and a mass balance model to identify areas of potentially high groundwater seepage in Sarasota Bay. Water and sediment samples collected from July 2002-July 2004 were analyzed for radon-222 using the radon emanation method. Surface water radon concentrations were converted to inventories and adjusted for di ffusive flux to model advec tive flow. The Advection Calculation program used to calculate the flow rates was based on a radon mass balance and assumed steady-state conditi ons. Radon loss to the atmosphere was incorporated into the program, how ever, loss of radon via mixing and/or flushing was not accounted for. The following parameters were used in calculating advection rates within t he model (M. Murray, pers. comm.): Rn concentration in surfac e water (dpm/L or pCi/L) Total water depth (m) Wind speed (m/s) Rn concentration in air (dpm/L or pCi/L) Water temperature Rn concentration in groundwater Porosity Area of measurement (m 2 ) 24

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Estimating Pore Water Resistiv ity from Terrain Resistivity During this study of SGD, it was im portant to differ entiate between the pore water ( W ) and subsurface/terrain resistivities ( T ). The relationship between these two parameters can be descr ibed using a formation factor (F), where F = T / W The formation factors are impor tant because once this value is known for a given lithology; it can then be used to extrapolate the pore water resistivities extending over various lithographic units. Formation factors could not be directly determined along the marine resistivity surveys, as boat-based cori ng was beyond the scope of this project. To estimate formation factors in sedi ments, however, pore water samples were collected with drive-point samplers at onshore and offshore sites within tens of meters of the coast. Pore water sample s were extracted with a peristaltic pump and resistivities were measured in the fi eld. At these same sites terrain resistivities were measured with various combinations of the EM-31, EM-34 and the small Schlumbeger marine resistiv ity array, as access permitted. 25

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3. Results and Discussion Regional Comparison of Resistivity and Radon The feasibility of identifying zones of SGD in a shallow estuary environment using a marine resistivity system was tested using a continuous resistivity profiling setup in three su rveys collected in June 2002, May 2003 and February 2004 by the FGS and USGS, respectively. The survey lengths were approximately 14, 30 and 17 km long, respectively wit h a depth of penetration around 25 m. The USGS surveys were conducted clos e to the same time as the radon surveys (Figure 11) which is why t he FGS data are excluded from the comparisons that follow below. 26

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Figure 11. Location map showing radon sampling sites. Radon advection rates were converted to estimated SGD rates for comparison with regional resistivity data. 27

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The southern bay was characterized using the 2003 survey which covered mostly the middle and souther n portions of the bay and consisted of resistivity data ranging from 0.1-3 -m and the northern bay wit h the 2004 data that concentrated on the northern portion of t he bay with values of approximately 0.130 -m. In some places t he results of the resistivity inversions are clearly unreasonable. For example, at some site s inverted terrain resistivities between 1 and 3 m depth gave unreasonably high resistivity values. At these depths, values should reflect the highly conductive (very low resistivity) surface waters input into the bay by the Gulf of Mexico An example is shown at 10 km along the May 2003 survey line on Figure 12. Such locations clearly represent inversion artifacts associated with noisy or sparse data; therefore, these values were disregarded during the interpretation process (see Appendix B for original data sets). In order to visualize and interpret t he data at various levels it was parsed by depth below sea level ranging from 515 m and plotted using ArcGIS software on the following maps (Figures 13-15). 28

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Figure 12. Location map showing extents of resistivity surveys. 29

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Figure 13. Radon versus resistivity at 5 m. 30

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Figure 14. Radon versus resistivity at 10 m. 31

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Figure 15. Radon versus resistivity at 15 m. 32

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In the southern part of the bay, resist ivities generally increase with depth from 5-15 m below the water su rface (from approximately 0.3-1 -m at 5 and 10 m to 1-3 -m at 15 m). Data collected at the 10 m depth are being influenced by a conductive clay layer (clay = 1 to 100 -m (Beck, 1981)) which produce resistivity values similar to those found in the shallow depths (< 5 m below sea level). At 15 m, the lithology is dominat ed by limestone a naturally more resistive (wet limestone = 10 2 to 10 3 -m, (Beck, 1981)) ma terial than clay that produces consistently higher resistivity values. In comparison to the southern bay, t he northern bay at a depth of 5 m has resistivity values that are unifo rmly high, ranging between 3 and 30 -m, which may be explained by the proxim ity of the limestone to t he sediment/surface water interface. At 10 m depth resistivity values are between 0.3 and 30 -m and decrease even more to approximately 0.1 and 30 -m at 15 m below sea level. Thus on a regional scale, the northern bay differs from the southern bay because resistivities decrease with in creasing deptha result that is counterintuitive to what would be ex pected to occur in a region where a freshwater lens is expe cted to extend offshore. Even though resistivities decrease with depth they are still significantly higher than those found in the southern bay. So what would cause the sediments/pore waters to become more saline with depth? There are two possible explanations for the lower resistivity rates found at 15 m compared to resistiv ities at 10 or 5 m depth. The area between 3035000N and 342000 E in Manatee County (Figure 15) is a relatively unpopulated region, which is used primar ily for agricultural purposes. One 33

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explanation is that the area is no longer just a discharge area (from the Intermediate aquifer) but also a recharge area and because of this two way head gradient, now saline surface waters may penetrate to greater depths through conduits found in the limestone and move into the underlying aquifers. The other possible explanation is that over pumping from the underlying Floridan aquifer for irrigation and other purposes has altered the extent and thickness of the freshwater lens and thereby reduced the amount of readily available freshwater. There appears to be an inverse relationship regionally between advection rates and resistivity values in the nort hern and southern portion s of the bay. The northern portion of the bay is dominated by lower flow rates ranging between 0.71 and 5.9 cm/day and higher resistivity values between 3.1 and 30.0 -m. The south displays generally higher flow rates ranging from 5.9 to 24.0 cm/day with lower resistivity values between 0.31 and 3.1 -m. This inverse relationship is the opposite of what would be expect ed if SGD were the dominant cause of resistivity variability throughout the bay. Since SGD does not appear to be the only factor affecting resistivity signals then what else could also account for these changes? Both surveys were collected using the same equipment and processing software therefore acquisition and processing par ameters are consistent for both the 2003 and 2004 surveys. One factor that was variable betw een the 2003 and 2004 surveys were the time of data collection. Both data se ts were collected during the dry season (December thru May) but they differ by the year and how far into the dry season 34

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they were when collected. The May surv ey occurs at the very end of the dry season so aquifer levels would be at t heir lowest and the region would have gone for the longest periods with little to no ra infall, which would produce less available freshwater and ther efore lower resistivity values. In comparison, the February survey would have been collected earlier in the season when aquifer levels would hav e be higher than those found in Maywhen more ground water is needed for irri gation and public supply during the hotter, summer period. When compared with historical readings, February 2004 was characterized in a hydrologic condi tions study conducted by the Southwest Water Management District as wetter then normal (SWFWMD, 2004) while May 2003 was characterized as normal (SWFWMD, 2003). This same pattern can also be seen in Figure 16, which shows data taken from three different rainfall gauges located in Sarasota Bay. The additi onal freshwater input during this time period may be another explanatio n for the higher overall re sistivities found in the northern bay during the 2004 survey. 35

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RNF-117 0 2 4 6 8 10 12 149/1/2002 11/ 1 /2 0 02 1 /1/ 2 00 3 3/1 / 20 0 3 5/1/ 2 003 7/1 / 20 0 3 9/1/2003 11/ 1 /2 0 03 1 /1/ 2 00 4 3/1 / 20 0 4 5 /1/ 2 00 4 7/1/ 2 00 4 9/1/2004 11/1/2004 1 /1/ 2 00 5 3 /1/ 2 00 5 5/1/ 2 00 5 7/1/2005 9/1/ 2 005 11 /1 / 20 05 1/1/ 2 006rainfall (cm) RNF-562 0 2 4 6 8 10 12 149 /1/2002 11/1/2002 1 / 1 /2003 3/1 / 20 0 3 5/1 / 20 0 3 7/1/20 0 3 9/1 / 20 0 3 1 1 /1 /2 0 0 3 1/1 / 20 0 4 3 /1/2004 5 /1/2004 7 /1/2004 9/1/2004 11/1/ 2 004 1 /1/2005 3/1/20 0 5 5/1/20 0 5 7/1/20 0 5 9/1 / 20 0 5 1 1 /1/ 2 0 0 5 1/1/20 0 6rainfall (cm) RNF-415 0 2 4 6 8 10 12 149 /1 / 2 0 02 11 / 1/2002 1/1/ 2 003 3 /1 / 2 0 0 3 5/1/2003 7 /1 / 2 0 0 3 9 /1 / 2 0 03 11 / 1/2003 1 / 1/ 2 004 3 /1 / 2 0 0 4 5 / 1/ 2 004 7/1/2004 9/1 / 2 0 04 11 / 1/20 0 4 1 / 1/ 2 005 3 / 1/ 2 005 5/1/2005 7 /1 / 2 0 05 9/1/2 0 05 11/1/2005 1/1/2006rainfall (cm) Figure 16. Rainfall data plots. 36

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As previously stated, the flow rates which were calculated with an advection model did not take into acc ount flushing and/or mixing which may explain the lower flow rates found in the north as opposed to the south. The north is privy to more flus hing and mixing with gulf wate rs carried in daily with the tides by the various inlets that are found here. Results from the final resistivity su rvey conducted in February 2006 also show consistently higher resistivity val ues in the north com pared to the south. This survey was conducted over the cour se of a day which means there is an absence of seasonal variability in this survey. Thus at least some of the difference between the nor thern 2004 and southern 2003 surveys appear due to differences in lithology and seasonally-i ndependent porewater variation. The consistency between overall resistivit y patterns and lithology (more resistive limestone near the seafloor in the north and the conducti ve clay layer at ~10 m depth in the south) suggests that lithologic variations are perhaps the dominant contribution to the regiona l resistivity signatures in Sarasota Bay. Correlating Pore Wate r Resistivity and Terra in Resistivity Using Formation Factors Formation factors that were calculated from resistivity surveys taken along with pore water samples can be found in the following table. Table 1 below shows the results from three sites tak en throughout the entire bay. These sites are found at New College in the north, 10 th Street Park around the middle of the bay and finally Stickney Bridge in the sout h (Figure 3). As would be expected, 37

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the formation factor decreases toward t he south because of the presence of the highly conductive clays associated with the Peace River formation, which is found only in the southern part of the bay. Site Terrain Resistivity (m/S) Pore Water Resistivity (m/S) Formation Factor ( T / W ) New College 0.0018 0.00030 5.4 10th St. Park 0.0022 0.00050 4.5 Stickney Bridge 0.0013 0.00030 4.4 Table 1. Formation factor results from the three regional sites sampled in Sarasota bay. 38

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Local Scale Comparison of Resistivity and Seismic Data Examining the bay to see if resistiv ity variations were found on both the regional and local scale was an essential part of this thesis. To conduct the local scale surveys, four site specific locati ons (chosen because of their proximity to geologic features found in the seismic record) were visited in February 2006 (Figures 7-10). Multiple resistivity survey lines (between 2 and 3 lines per site) ranging in length from ~ 300 m to 1100 m were all collected throughout the bay over the period of a few hours (approximately 9-10 hours). Overall results from these local scale surveys support the conclusions hypothesized in preceding sections relat ed to the regional differences between the north and south, which assumed t hat a high conductivity (possibly semiconfining) clay layer is present in the s outh but not in norther n Sarasota Bay. The southern bay resembles the nor thern bay because both have a lower resistivity zone found at depth; however, t he south differs from the north due to the presence of a highly conductive z one found below the sediment/surface water interface approximately 9 m below sea level (Figures 17 and 18). 39

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Figure 17. Resistivity and seismic lines near Phillipi Creek. Locat ion shown in Figure 9. Interpretations show a thickening of Holocene deposits compared to the northern bay. The clay-rich PRF is approximately 9 m thick. Higher resistivity limestone associated with the Arcadia is found at ~15 m depth. A stra tigraphic section schematic of ROMP 6-1, located on Siesta Key (location shown in Figure 3) is shown to the right of the seismic profile. 40

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Figure 18. Resistivity and seismic lines from Little Sarasota Ba y. Location shown in Figure 10. Here Holocene deposits are t hickest, up to 3 meters. Clays associated with the PRF are seen here and may act as a semi-confining unit between the over and underlying aquifers. Notice the high resistivity zones located above and below the conductive zone in the resistivity profiles. 41

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This conductive layer is part of th e clays associated with the PRF that have been mentioned previously in the regional geology section. Line K does not show the presence of a hi ghly conductive layer (~0.25 -m for surface waters) above the sediment/surface water interfac e that can be seen in both I and J. This may be due to additional noise that was present during data collection of line K (i.e. boat traffic in channel and/or proximity to the channel). The high resistivity areas in I and J are found appr oximately 15 m below sea level, which is deeper than the high resistivit y zone found in the north ( < 10 m). Lines F-H all easily indicate the position of a conduc tive layer of clays found below the sediment/surface water interface. These clays are resting on top of a high resistive zone located approximately 15-20 m below sea level (see figure 18). Smaller lateral changes in resistivity (factors of less than 4 or 5) are not consistent between surveys collected less than a half hour apart. A possible explanation for this is t hat this horizontal inhomogeneity could be artifacts of inversion of noisy data. (Note repeat surv eys do not perfectly duplicate positions, but are closer than the lateral dimensi ons of the features observed.) For example, lines A and B (See Figure 19 for locations) show no obvious similarity between runs except in the underlying fr esh/high resistivity features located approximately 10 m below sea level. Sm all lateral variations are not depicted consistently in either line, i.e. between 360 and 460 m line B shows a high resistivity area that is not present in line A. 42

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Figure 19. Resistivity and seismic lines near New College. Locat ion shown in Figure 7. This interpretation shows the top of the limestone at shallow depths and in some areas outcropping at the seafloor. There appears to be two distinct layers (A and A2) in the Arcadia limestone. Holocene deposits are very thin or non-existent throughout the survey line. 43

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Figure 20. Resistivity and seismic lines near Bowlees Creek. Loc ation shown in Figure 8. Interpretations show the presence of the limestone here is within 5 meters of the seafloor. Sags in the Arcadia are f illed with Holocene or possibly PRF sediments ROMP well 7-1, near Bowlees Creek (location shown in Figure 3) has no significant clays layers. Note that no clays (PRF) are observed above the limestone (A) in the stratigraphic section schematic. 44

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Consistent with results found in lines A and B, lines D and E (Figure 20) show a conductive layer above the sediment/surface water interface with a gradual freshening with depth. Similar to lines A and B, lines D and E show that lateral variability between surveys is not ro utinely reproduced in both of the runs, however, a high resistivity zone is present in the subsurface approximately 10 m below sea level that coincides with the one found in lines A and B. In lines I-K resistivity patterns are similar to those the north with small scale horizontal changes not being reci procated over all three lines and the fractured appearance to the underlying limestone. Although changes in the lateral direction are not reliable, changes found in the vertical section are consistently reproduced during all su rveys, therefore changes by a factor of 4 or 5 (or greate r) that are not reliabl e in the horizontal direction can be believed in the vertical dire ction. As previously stated, there is a high resistivity zone found at depth in all su rvey lines with the only variation found in the south where a highly conductive ar ea is located (this vertical feature is present in all southern survey lines exc ept where noted in previous section) above the higher resistivity zone. In both the north and south, smaller-sca le seismic features (such as the bowl-shaped sag feature located at Bowlees Creek approximately 9 m below sea level or the v shaped sag feature found ~ 12 m below sea level at Phillipi Creek) do not have coincident resistivity signatur es. So again, smaller scale features are probably the result of noise, and do not reflect geologic heterogeneities. This lack of a signal could have occurred because the CRPs resolution is incapable 45

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of detecting small scale (few meters) feat ures so it instead depicts an overall picture of larger scale geometries. Discussion It appears that some of the high resist ivity areas found in the north are right at the sediment/surface water in terface and this may be explained by the close proximity of the lim estone to the surface in this area where often the amount of sediment cover (sand) is very th in or non-existent. It should also be noted that breaks in the resistivity highs (these may represent conduits/fractures in the limestone) appear to be drawing down the more conductive surface waters to lower depths. A possible ex planation for this is that the northern bay is acting not as an area of discharge but as one of discharge and recharge, which most likely has been created by a reversal in the head gradients due to over-pumping of the underlying aquifers for agricultura l purposes (namely in Manatee county) for the past few years (See Hydrogeology section). The south appears to have less variability (more contiguous) in the underlying limestone unlike t he north, which had a discontinuous almost broken appearance to its limestone. Unlike lines A-E in the north, lines F-H show a small zone of higher resistivity between the sediment/surface water interface and the top of the clay layer that is found about 10 m below sea level. This high resistivity area is probably a zone of fresh water associated with the Surficial Aquifer system. If this high resistivity area were due to channel effects (from the nearby Intercoastal 46

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Waterway) then there would have been a gradational increase in resistivity values with depth similar to that of t he north, not a zone of higher resistivity (freshwater saturated sediments) then a high conductivity zone (clay layer) and finally freshening with depth (freshwater saturated limestone). Because a zone of higher resistivity is found below 15 m, it is my belief that the clay layer is acting as a semi-confining unit that is restricting any seepage from underlying and/or overlying aquifers (this area hasnt had any head gradient reversals so the regional gradient makes this an area of discharge). 47

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4. Conclusions Continuous resistivity profiling in kars tic environments such as Sarasota Bay, appears to be a viable method for rapid, large scale surveys that yield information on the overall underlying geology and potential areas of interest. For smaller, more detailed investigations of SGD it would appear that radon sampling is the most appropriate and proven me thod for detecting and quantifying these changes. Small scale (tens to hundreds of mete rs) surveys do not show consistent correlations with geological features imaged in seismic lines, and further show that horizontal variations in resistivity of a factor of < 4-5 are likely to be the product of inversion artifacts created duri ng processing of the data, not changes in resistivity created by buried featur es. Sag features had no distinct corresponding resistivity signal. Vertic al variability in survey lines appears consistent and is hence probably indicative of real features and/or facies changes occurring below the sediment/surfa ce water interface (i.e. changes from saltwater saturated sands to limes tone saturated with freshwater). 48

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References Beck, A.E., 1981, Physical Principles of Exploration Methods, Halsted Press, New York. Burnett, W.C., Bokuniewicz, H., Huettel, M., Moore, W.S., and Taniguchi, M., 2003, Groundwater and pore water inputs to the coastal zone: Biogeochemistry, v.66, p.3-33. Campbell, K.M., 1985, Geology of Sarasota County, Florida: Florida Geological Survey Open-File Report 10, 15 p. Duerr, A.D., and Wolansky, R.M., 1986, Hydrogeology of the Surficial and intermediate aquifers of central Sarasota C ounty, Florida: U.S. Geological Survey Water Resources Investi gation Report 86-4068, 48 p. Hine, A.C., Brooks, G.R., Davis, Jr., R. A., et al., 2003, The West-Central Florida inner shelf and coastal system: A geologic conceptual overview, Marine Geology 200, 1-17 p. Horn, D.P., 2002, Beach groundwater dynamics, Geomorphology 48, 121-146 p. Knochenmus, L.A., and Bowman Geronia (Moe), 1998, Transmissivity and water quality of water-producing zones in the intermediate aquifer system, Sarasota County, Florida: U.S. Geol ogical Survey Water Resources Investigation Report 98-4091, 27p. 49

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Krantz, D.E., Manheim, F.T. Bratton, J.F. and Phelan D.J., 2004, Hydrogeologic setting and ground-water flow beneath a secti on of Indian River Bay, Delaware, Ground Water 42, no. 7, 1035-1051 p. Kwon, H.-S., Kim, J.-H., Ahn, H.-Y., et al., 2005, Deli neation of a fault zone beneath a riverbed by an electrical resistivity survey using a floating streamer cable, Exploration Geophysics 36, 5058 p. Locker, S.D., Davis, R.A., Brooks, G.R., Hine A.C., Twitchell, D.C., 2001g, WestCentral Florida Coastal Transect #7; Longboat Key: U.S. Geological Survey Open File Report 99-511. Locker, S.D., Davis, R.A., Brooks, G.R., Hine A.C., Twitchell, D.C., 2001h, WestCentral Florida Coastal Transect #8; Sies ta Key: U.S. Geological Survey Open File Report 99-512. Locker, S.D., Davis, R.A., Brooks, G. R., Hine A.C., Twitchell, D.C., 2001j, Compilation of geophysical and sedimentological data se ts for the West-Central Florida studies project: U.S. Geological Survey Open File Report 99-539. Locker, S.D., Hine A.C., Brooks, G.R., 2003, Regional stratigraphic framework linking continental shelf and coastal sedimentary deposits of west-central Florida, Marine Geology 200, 351-378 p. Manheim, F.T., Krantz, D.E., Manheim, F.T. Bratton, J.F., 2004, Studying ground water under Delmarva Coastal bays using electrical resistivity, Ground Water 42, no. 7, 1052-1068 p. Moore, W.S., 1996, Large groundwater inputs to coastal waters revealed by 226Ra enrichments, Nature 380, 6112-614 p. 50

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Scott, T.M., 1988, The lithostratigraphy of the Hawthorn Group (Miocene) of Florida: Tallahassee, Flor ida Geological Survey bulletin no. 59, 148 p. Southeastern Geological Society, 1986, Hydrogeological units of Florida: Tallahassee, Florida Bureau of Geol ogy Special Publication 28, 9 p. Southwest Water Management District, 1988, Ground-water resource availability inventory: Sarasota County, Florida: Brooksville, Fl., 200 p. Swarzenski, P.W., Bratton, J.F. and Crusius, J., 2004, Submarine ground water discharge and its role in coastal processes and ecosystems: U.S. Geological Survey, Open File Report xxxxxx. Swarzenski, P.W., Burnett, W.C., Reich, C.D., Dulaiova, H., Peterson, R. and Meunier, J., 2004, Novel geophysical and geochemical techniques used to study submarine groundwater discharge in Bisca yne Bay, Florida: U.S. Geological Survey, Fact Sheet 2004-3117. Swarzenski, P.W., Charette, M. and Langevin, C., 2004, An autonomous, electromagnetic seepage meter to study coastal groundwater/surface water exchange: U.S. Geological Surv ey, Open File Report 20041369. Torres A.E., L.A. Sacks, Yobbi, D.K., Knochenmus L.A. and Katz B.G., 2001, Hydrogeologic framework and geochemistry of the interm ediate aquifer system in parts of Charlotte, De Soto, and Saraso ta Counties: U.S. Geological Survey Water Resources Investi gation Report 01-4015, 74 p. Twitchell, D.C. and Paskevich, V., 1999, Bathymetry, Sidescan Sonar Image, Surface Sediments, and Surficial Geologica l Map of the Inner Shelf off Sarasota, Florida: Preliminary discussion and GIS database release: U.S. Geological 51

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Survey Open File Report 99-396. http://pubs.usgs.gov/of/of99396/htmldocs/geology.htm Vacher, H.L., Jones, G.W., and Stebnisky, R.J., 1992, Heterogeneity of the Surficial aquifer system in west central Fl orida: Tallahassee, Florida Geological Survey Special Publicat ion No. 36, p. 93-99. Valiela, I., and DElia, C., 1990, Groundwater inputs to coastal waters, Biogeochemistry 10, no. 3: 175. 52

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Appendices 53

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Appendix A: Florida Geological Survey Data In June 2002 a resistivity survey was collected by the Florida Geological Survey (FGS) in Sarasota Bay using t he Zonge Streaming Resistivity/IP system of Zonge Engineering and Research Organization, Inc. This system continually recorded data from a GPS receiver and used a 10 m dipole-dipole array. Current was injected through a line of streaming electrodes, in tow behind the research vessel, at a preset interval and apparent resistivity values representing various depths were read for each injection. Af ter data collection is complete, the data were inverted by Zonge, Inc. using 2-D inversion software. The survey line extends 14 km beginni ng in Philippi Creek and ending at Stephens Point (see figure A for location). To facilitate ease during the inversion process the single survey line was broken up into multiple segments referred to as sections A through N, which are pr ovided and labeled accordingly below. Because the survey line was divided a fter data collection was complete, only beginning and ending line navigation points were provided therefore only one line could be produced for the location map. 54

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Appendix A: (Continued) 220Jun07 A, Section A Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0.5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 aa.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 55

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Appendix A: (Continued) 220Jun07 A, Section B Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0.5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 ab.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 56

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Appendix A: (Continued) 220Jun07 A, Section C Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0.5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 ac.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300 2350 2400 2450 2500 2550 2600 2650 2700 2750 2800 2850 2900 2950 3000 3050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300 2350 2400 2450 2500 2550 2600 2650 2700 2750 2800 2850 2900 2950 3000 3050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300 2350 2400 2450 2500 2550 2600 2650 2700 2750 2800 2850 2900 2950 3000 3050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 57

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Appendix A: (Continued) 220Jun07 A, Section E Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0.5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 ae.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350 4400 4450 4500 4550 4600 4650 4700 4750 4800 4850 4900 4950 5000 5050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350 4400 4450 4500 4550 4600 4650 4700 4750 4800 4850 4900 4950 5000 5050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 3850 3900 3950 4000 4050 4100 4150 4200 4250 4300 4350 4400 4450 4500 4550 4600 4650 4700 4750 4800 4850 4900 4950 5000 5050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 58

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Appendix A: (Continued) 220Jun07 A, Section F Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0.5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 af.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300 5350 5400 5450 5500 5550 5600 5650 5700 5750 5800 5850 5900 5950 6000 6050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300 5350 5400 5450 5500 5550 5600 5650 5700 5750 5800 5850 5900 5950 6000 6050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 4850 4900 4950 5000 5050 5100 5150 5200 5250 5300 5350 5400 5450 5500 5550 5600 5650 5700 5750 5800 5850 5900 5950 6000 6050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 59

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Appendix A: (Continued) 220Jun07 A, Section G Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0. 5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 ag.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 5850 5900 5950 6000 6050 6100 6150 6200 6250 6300 6350 6400 6450 6500 6550 6600 6650 6700 6750 6800 6850 6900 6950 7000 7050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 5850 5900 5950 6000 6050 6100 6150 6200 6250 6300 6350 6400 6450 6500 6550 6600 6650 6700 6750 6800 6850 6900 6950 7000 7050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 5850 5900 5950 6000 6050 6100 6150 6200 6250 6300 6350 6400 6450 6500 6550 6600 6650 6700 6750 6800 6850 6900 6950 7000 7050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 60

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Appendix A: (Continued) 220Jun07 A, Section H Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0.5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 ah.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 6850 6900 6950 7000 7050 7100 7150 7200 7250 7300 7350 7400 7450 7500 7550 7600 7650 7700 7750 7800 7850 7900 7950 8000 8050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 6850 6900 6950 7000 7050 7100 7150 7200 7250 7300 7350 7400 7450 7500 7550 7600 7650 7700 7750 7800 7850 7900 7950 8000 8050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 6850 6900 6950 7000 7050 7100 7150 7200 7250 7300 7350 7400 7450 7500 7550 7600 7650 7700 7750 7800 7850 7900 7950 8000 8050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 61

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Appendix A: (Continued) 220Jun07 A, Section I Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0.5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 ai.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550 8600 8650 8700 8750 8800 8850 8900 8950 9000 9050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550 8600 8650 8700 8750 8800 8850 8900 8950 9000 9050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 7850 7900 7950 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 8550 8600 8650 8700 8750 8800 8850 8900 8950 9000 9050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 62

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Appendix A: (Continued) 220Jun07 A, Section J Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0. 5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 aj.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 8850 8900 8950 9000 9050 9100 9150 9200 9250 9300 9350 9400 9450 9500 9550 9600 9650 9700 9750 9800 9850 9900 9950 10000 10050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 8850 8900 8950 9000 9050 9100 9150 9200 9250 9300 9350 9400 9450 9500 9550 9600 9650 9700 9750 9800 9850 9900 9950 10000 10050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 8850 8900 8950 9000 9050 9100 9150 9200 9250 9300 9350 9400 9450 9500 9550 9600 9650 9700 9750 9800 9850 9900 9950 10000 10050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 63

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Appendix A: (Continued) 220Jun07 A, Section K Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0. 5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 ak.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 9850 9900 9950 10000 10050 10100 10150 10200 10250 10300 10350 10400 10450 10500 10550 10600 10650 10700 10750 10800 10850 10900 10950 11000 11050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 9850 9900 9950 10000 10050 10100 10150 10200 10250 10300 10350 10400 10450 10500 10550 10600 10650 10700 10750 10800 10850 10900 10950 11000 11050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 9850 9900 9950 10000 10050 10100 10150 10200 10250 10300 10350 10400 10450 10500 10550 10600 10650 10700 10750 10800 10850 10900 10950 11000 11050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 64

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Appendix A: (Continued) 220Jun07 A, Section L Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0. 5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 al.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 10850 10900 10950 11000 11050 11100 11150 11200 11250 11300 11350 11400 11450 11500 11550 11600 11650 11700 11750 11800 11850 11900 11950 12000 12050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 10850 10900 10950 11000 11050 11100 11150 11200 11250 11300 11350 11400 11450 11500 11550 11600 11650 11700 11750 11800 11850 11900 11950 12000 12050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 10850 10900 10950 11000 11050 11100 11150 11200 11250 11300 11350 11400 11450 11500 11550 11600 11650 11700 11750 11800 11850 11900 11950 12000 12050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 65

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Appendix A: (Continued) 220Jun07 A, Section M Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0.5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 am.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 11850 11900 11950 12000 12050 12100 12150 12200 12250 12300 12350 12400 12450 12500 12550 12600 12650 12700 12750 12800 12850 12900 12950 13000 13050 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 11850 11900 11950 12000 12050 12100 12150 12200 12250 12300 12350 12400 12450 12500 12550 12600 12650 12700 12750 12800 12850 12900 12950 13000 13050 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 11850 11900 11950 12000 12050 12100 12150 12200 12250 12300 12350 12400 12450 12500 12550 12600 12650 12700 12750 12800 12850 12900 12950 13000 13050 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 66

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Appendix A: (Continued) 220Jun07 A, Section N Florida State University Marine Resistivity Survey Line 220Jun07 A 2D Smooth-Model Inversion DipoleDipole Resistivity DataSurvey Parameters: 10 m DipoleDipole data 4.0 hertz repetition rate Inversion control parameters: ResSmth=1, dpW=0.5, dxW=1, dzW=1AUTHORDRAWNDATESCALEREPORT REF: ZongeZonge01/07/071:4500Job 200220 an.s2d Inversion Model Resistivity (ohm-m) Elevation (m) 12850 12900 12950 13000 13050 13100 13150 13200 13250 13300 13350 13400 13450 13500 13550 13600 13650 13700 13750 13800 13850 -30 -25 -20 -15 -10 -5 0 Calculated Apparent Resistivity (ohm-m) n-spacing 12850 12900 12950 13000 13050 13100 13150 13200 13250 13300 13350 13400 13450 13500 13550 13600 13650 13700 13750 13800 13850 1 2 3 4 5 6 Observed Apparent Resistivity (ohm-m) n-spacing 12850 12900 12950 13000 13050 13100 13150 13200 13250 13300 13350 13400 13450 13500 13550 13600 13650 13700 13750 13800 13850 1 2 3 4 5 6 0m60m120m180m240m 0.01 0.126 0.2 0.316 0.501 0.794 1.26 2 3.16 5.01 7.94 12.6 20 31.6 50.1 79.4 158 251 398 631 1000 67

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Appendix B: U.S. Geological Survey Data First, we will examine the USGS 2003 cruise that extends 30 km and began just south of Whitaker Bayou and ends in Little Sarasota Bay just north of Casey Key. For ease during the inversion process the survey line was broken up into 3 segments which shall be referred to as line 1, line 2 and line 3 (Figure B). Line 1 extends from Whitaker Bayou north to about the Manatee/Sarasota County line and covers the distances rangi ng from 0 to 5090 m along the survey line. The resistivities in this line r ange from a minimum of approximately 0.92 to a maximum of approximately 8.8 Ohm-meters ( -m). Coloring irregularities found above the sediment/surface water inte rfaces in all three lines may be due to discrepancies within the inversion softw are and for this reason some of the lower resistivity values observed in t hese areas were omitted from the given resistivity ranges for each line segment. Line 2 begins at the Manatee/Sarasota border then goes west until it terminates approximately 0.5 km from Longboat Key. The distances covered are ~5179 to 6718 m along the survey line. Re sistivity ranges here are between 0.53 and 2.8 -m. Line 3 begins due south of the end of line 2 (approximately 0.5 m from the landward side of Longboat key) and extends down through Little Sarasota Bay just north of Osprey, Fl. The distanc es covered are ~6746 to 29745 m along the survey line. Resistivity r anges here are between 2.8 and 8.4 -m. 68

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Appendix B: (Continued) The USGS 2004 survey extends out for approximately 17 km beginning at New College and continues north-northwe st before ending approx imately 1.5 km east from the top of Longboat Key. T he similarity between the 2003 and 2004 cruises is the seepage pattern illustrat ed, which mocks that of line 1 with its point-like distribution that appears to approach the surface/sediment water interface. The obvious distinction between the two surveys is observed in the resistivity values collected from the 2004 survey, which shows values higher than those in 2003. Resistivity ranges her e are between approximately 60 and 0.95 -m with a maximum of 808 -m, which is probably again due to inversion software issues that were previously mentioned. 69

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Appendix B: (Continued) Figure A. Location map for all three resistivity surveys collected throughout Sarasota Bay. The single FGS survey was collected i n June 2002 (red line) from Philipi Creek to Stephens Point. The two USGS surveys were collected in May 2003 (blue) and February 2004 (purple) and covered Sarasota and Little Sarasota Bays. 70

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Appendix B: (Continued) Figure B. Location map from 2003 USGS survey showing locations of lines 1-3. 71

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Appendix B: (Continued) Figure C. Original plot of resistivity profile line 1 showing inverted section from May 2003 USGS survey conducted in Sarasota Bay near New College. Location of this line segment is shown in Figure B. Inversion artifacts created during the various iterations are seen above the sediment/surface water interface as resistivity highs (orange/red) ranging between 2.9 and 8.8 Ohm-m. 72

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Appendix B: (Continued) Figure D. Original plot of resistivity profile line 2 showing inverted section from May 2003 USGS survey conducted in Sarasota Bay near the Manatee/Sarasota county line. Location of this line segment is shown in Figure B. Inversion artifacts created during the iteration process are seen above the sediment/surface water interface as resistivity highs ranging between 1.2 and 2.8 Ohm-m. 73

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Appendix B: (Continued) Figure E. Original plot of resistivity profile line 3 showing inverted section from May 2003 USGS survey conducted in Sarasota Bay. The survey line ran from Longboat Key in the north/mid bay down to Casey Key in the south. Location of this line segment is shown in Figure B. Inversion artifacts created during the multiple inversions are seen above the sediment/surface water interface as resistivity highs ranging between 2.8 and 8.4 Ohm-m. 74

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Appendix B: (Continued) Figure F. Original plot of resistivity profile showing inverted section from February 2004 USGS survey conducted in northern Sarasota Bay. Location of this line segment is shown in Figure A. Modifications to the inversion software used to create this profile have eliminated many of the inversion artifacts seen in previous survey lines collected in the 2003 survey. Unreasonab ly high values between 85 and 808 Ohm-m can still be attributed to noisy data and/or inversion discrepancies. 75

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Appendix C: Electromagnetic Survey Data Electromagnetic (EM) data were also co llected for this thesis, however, because of the proximity of power lines and other electrical sources the data were too noisy to be used effectively. Similar to resistivity, EM methods work by using electricity to induce current; however, the difference is f ound in the current source, which is an alternating current provided by an internal, self-contain ed transmitter coil. This transmitter coil generates an EM field that penetrate s the subsurface before being picked up by a receiver coil. The magni tude of the resulting field is directly proportional to the terrain conductivity. The system used here was the Geonics EM-31 which operates at a frequency of 9800 kHz with an inter-coil spacing of 3.67 m. Readings are collected in quadrature phase vertical dipole mode (VMD) and inverted for seabed conductivity assuming a simple 2-layered earth model (algorithm by S. Sandber g, pers. comm.). Surf ace water conductivity ( 1 ) and depth (d 1 ) are measured directly, so the only unknown is seabed conductivity ( 2 ). This method is sensitive only to t he conductivity of the uppermost few meters of sediment/rock, and only wor ks in saline water when surface water depths are less than ~1-1.5 meters (Greenwood et al., 2006). 76

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Appendix C: (Continued) Greenwood et al. (2004) used elec tromagnetic methods to map pore water salinity over land and shallow marine waters in a coastal wetland located in Tampa Bay, Florida. Using the EM-31 and EM-34 of Geonics, Ltd., it was shown that information on seabed conductivity can be obtained in saline waters with depths equal to < 1.5 m. The EM met hod offers access to very shallow water and difficult coastal wetlands; however, fi eld trials and models show that the towed EM technique is probably not suit able for imaging subtle conductivity anomalies beneath Tampa Bay (Greenw ood et al., 2006). Figure G. Schematic showing setup geometry for EM-31 in canoe. Surface water conductivity ( 1 ), depth (d 1 ) and seabed conductivity ( 2 ) are variables needed by the inversion algorithm to infer seabed conductivity. 77

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Appendix C: (Continued) Figure H. Location map showing electromagnetic data collected with the Geonics EM-31 in August 2004 from Roberts and Little Sarasota Bay. The proximity of power lines and other electrical interference to the instrument at the time of data collection created noisy data that could not be effectively used. 78


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Evaluating the reliability of continuous resistivity profiling to detect submarine groundwater discharge in a shallow marine environment :
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[Tampa, Fla] :
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ABSTRACT: Submarine groundwater discharge (SGD) can be an important pathway for nutrients entering coastal systems. However SGD flow paths can be difficult to identify and flow volumes difficult to quantify. This study assesses whether geophysical techniques are potentially cost effective methods for detecting the presence or lack of SGD within an estuary environment found in Sarasota Bay, Florida. In this area, a rapid increase in urbanization has led to increased nitrogen loading into the bay, with some 10% of this loading attributed to SGD. Discharging groundwater is expected to be fresher and hence higher resistivity, than "background" surface waters. Thus resistivity surveys sensitive to seafloor conductivities may be useful for identifying zones of SGD. However, terrain resistivities are influenced by matrix geology as well as pore water resistivity. In this study we compare the results of marine resistivity surveys against both geochemical measures of SGD (radon tra cers) and seismic profiles indicative of subsurface structure to better determine the relative impacts of geology and SGD on marine resistivity measurements in Sarasota Bay. On both regional (kilometers to tens of kilometers) and local scales (hundreds of meters) the relationship between marine resistivity and tracer-based SGD estimates does not follow the expected pattern of higher resistivities associated with higher SGD flux. Seafloor resistivities instead appear primarily influenced by stratigraphy, particularly the presence of a clay layer at ~10-15 m depth in the southern part of the bay. In the southern bay, resistivities decrease at the depths associated with the clay layer. On the local (hundreds of meters) scale, lateral variations in resistivities derived from inversions of resistivity data were not found to be reproducible; nearly-coincident lines collected 30 minutes apart in time show different local signatures. This apparent local lateral variability in the resistivi ty profiles is inferred to be a result of inversion of noisy streaming resistivity data.
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Marine resistivity.
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Rapid reconnaissance methods.
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