ABSTRACT

GEOCHEMISTRY OF THE PLEISTOCENE AQUIFER, NORTHEASTERN ANDROS ISLAND, BAHAMAS

By Derek Wayne Dice

In this study, the geochemistry and mineralogy of several rock cores from the upper 12 meters from Andros Island were evaluated along with water samples collected at specific depths from within the aquifer. Mineralogy was determined by x-ray diffraction, and the elemental composition (major and minor elements) was examined in selected rock samples by DCP spectrometry. The elemental composition of the groundwater was determined by DCP spectrometry, high-performance liquid chromotography, and in-situ pH. By comparing the Ca/Sr ratios of both the groundwater and the limestone along with saturation states of the groundwater, we concluded that dissolution was the dominant process operating in the upper, freshwater lens. By linking petrology, rock geochemistry, water geochemistry, and the location of sea-level, we were able to gain a better understanding of the progress and stages of diagenesis that are presently ongoing in this Pleistocene aquifer.

GEOCHEMISTRY OF THE PLEISTOCENE AQUIFER, NORTHEASTERN ANDROS ISLAND, BAHAMAS

A Thesis

Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Master of Science Department of Geology

by Derek Wayne Dice Miami University Oxford, Ohio 2003

Advisor______(Mark Boardman)

Reader______(Cindy Carney) TABLE OF CONTENTS Section Page

I. Introduction 1 A. Carbonate Diagenesis 1 B. Meteoric Diagenesis 2 C. Scope of this Project 3 II. Study Area 5 A. Overview of Bahamas Geology 5 1. Geographical Background of the Bahamas 5 2. Geologic History of the Bahamas 5 3. Quaternary History of the Bahamas 6 B. Field Area Lithology 7 C. Field Area Hydrology 9 1. Island Hydrology 9 2. Controls on the Freshwater Lens of Andros 10 III. Methods of Investigation 11 A. The Study Area 11 B. Construction and Installation of the Multi-level Samplers 11 C. Sampling of the Wells 12 D. Core Samples 13 E. Laboratory Analysis 13 IV. Results 14 A. Chemistry of the Groundwater 14 B. Alkalinity 15 C. pH 15 D. Saturation States 15 E. Chemistry of the Rainwater 15 F. Rock Specimens 15 G. X-ray Diffraction of the Bulk Rock 16

ii V. Discussion 17 A. Water Chemistries 17 B. Proof of Reactivity 18 C. Reaction-derived Values 19 1. Non-reactive with the Aquifer 19 2. Reactive with the Aquifer (Reduction) 20 3. Reactive with the Aquifer (Dissolution within the Aquifer) 20 D. Evidence of Precipitation and Dissolution within the Aquifer 21 E. Quantifying the Dissolution and Precipitation 22 F. Rate of Mineralogical Transformation 24 G. Geologic Implications of this Rate of Transformation 26 VI. Conclusions 28 VII. References 29

iii LIST OF TABLES Number Page 1. Groundwater Classification Based on Total Dissolved Solids 34 2. Measured Constituents in the Groundwater 35 3. Measured and Calculated Constituents in the Groundwater 37 4. Calculated IAP/K’s in the Groundwater 38 5. Average Rainwater Composition 39 6. Measured Constituents in the Rock 40 7. Calculated Values for Precipitation and Dissolution 42 8. Stabilization Rates 43 9. Stabilization Rates of Related Carbonate Environments 44 10. Half-lives of Aragonite Stabilization 45

iv LIST OF FIGURES Number Page

1. Meteoric Environment 46 2. Map of the Bahamas 47 3. Pleistocene Sea-level Curve 48 4. Sea-level History of Northeastern Andros 49 5. Ghyben-Herzberg Principle for a Coastal Aquifer 50 6. Map of North Andros Island Field Area 51 7. Map of North Andros Island Study Area 52 8. Design of the Multi-level Sampler 53 9. Packing Sequence of the Multi-level Sampler 54 10. Well 69 Multi-level Sampler 55 11. Well C2N Multi-level Sampler 56 12. Well 9N Multi-level Sampler 57 13. Well 4NN Multi-level Sampler 58 14. Well 4SA Multi-level Sampler 59 15. Graph of Chloride in the Groundwater with Depth 60 16. Graph of Sulphate in the Groundwater with Depth 61 17. Graph of Calcium in the Groundwater with Depth 62 18. Graph of Strontium in the Groundwater with Depth 63 19. Graph of Sodium in the Groundwater with Depth 64 20. Graph of Magnesium in the Groundwater with Depth 65 21. Graph of Potassium in the Groundwater with Depth 66 22. Graph of Alkalinity in the Groundwater with Depth 67 23. Graph of pH in the Groundwater with Depth 68 24. Graph of log IAP/KT for Aragonite in the Groundwater with Depth 69 25. Graph of log IAP/KT for Calcite in the Groundwater with Depth 70 26. Graph of Calcium in the Rocks with Depth 71 27. Graph of Strontium in the Rocks with Depth 72 28. Graph of Magnesium in the Rocks with Depth 73

v 29. Graph of Potassium versus Chloride in the Groundwater 74 30. Graph of Sodium versus Chloride in the Groundwater 75 31. Graph of Magnesium versus Chloride in the Groundwater 76 32. Graph of Calcium versus Chloride in the Groundwater 77 33. Graph of Strontium versus Chloride in the Groundwater 78 34. Graph of Sulphate versus Chloride in the Groundwater 79 35. Graph of Reaction-derived Magnesium in the Groundwater with Depth 80 36. Graph of Reaction-derived Potassium in the Groundwater with Depth 81 37. Graph of Reaction-derived Sodium in the Groundwater with Depth 82 38. Graph of Reaction-derived Sulphate in the Groundwater with Depth 83 39. Graph of Reaction-derived Calcium in the Groundwater with Depth 84 40. Graph of Reaction-derived Strontium in the Groundwater with Depth 85 41. Graph of Strontium versus Calcium in the Groundwater 86 42. Graph of Calcium and Strontium Ratios in Carbonate Rocks 87 43. Graph of the Remaining Aragonite versus the Duration of Diagenesis 88

vi INTRODUCTION

Carbonate Diagenesis Diagenesis includes the natural changes which occur in sediments and sedimentary rocks from the time of initial deposition to the time when the changes caused by elevated temperatures and pressures are considered metamorphic. The ultimate end-point of the diagenetic process affecting carbonate sediments (comprised of aragonite, high-magnesium calcite, and low-magnesium calcite) is chemical and mineralogical stabilization of an initially heterogeneous mixture of carbonate particles. With increasing time, unstable carbonate particles dissolve and stable carbonate particles precipitate. Thus, structural order (calcite) and chemical purity is attained (Morse and Mackenzie, 1990). The major factors affecting the overall diagenetic process are the mineralogy of the original grains, the chemistry of the interstitial fluids flushing through the system, the duration of these processes, and the physical and chemical constants involved (Scoffin, 1987). The purpose of this paper is to report on the early diagenesis of a Pleistocene aquifer. By definition, carbonate rocks contain > 50% carbonate minerals. The most abundant of these minerals are aragonite, calcite, and dolomite. In these minerals, the = CO3 anions (carbonate) are an equilateral triangle, with oxygen atoms on the corners and a carbon atom in the center. These rock-forming minerals are either rhombohedral (calcite) or orthorhombic (aragonite) in crystal habit, depending on the incorporated cation. In pure calcite, the carbonate ions are in layers that alternate with layers of = calcium cations. Each calcium ion has six CO3 anions in octahedral (6-fold) coordination, building rhombohedral crystals. Divalent cations smaller than calcium (1.0 Å), such as sodium (.97 Å), manganese (.74 Å), iron (.72 Å), or magnesium (.63 Å), may easily be substituted into the cation layers of calcite (Ford and Williams, 1989). In aragonite, the calcium and carbonate ions form unit cells of cubic coordination (9-fold coordination) forming orthorhombic crystals (Scoffin, 1987). This structure will not accept cations smaller than calcium (1.0 Å), and could incorporate barium ( 1.32 Å),

1 lead ( 1.35 Å), or strontium ( 1.06 Å) (Prothero and Schwab, 1996). In aragonite precipitated from seawater, strontium is the most common substitute cation. Both ancient and modern shallow water carbonates are composed predominantly of mixtures of aragonite, high-Mg calcite (>4 mole%Mg+2), and low-Mg calcite (<4 mole %Mg+2). This mineralogy depends upon the composition of the calcium carbonate-secreting organisms and the mineralogy of the abiotic precipitates at that time. The most common tropical, shallow water carbonate grains are composed of aragonite and high-Mg calcite. However, both of these minerals are unstable relative to low- magnesium calcite. At some point, mineralogically unstable carbonates will be converted to the most stable form of CaCO3, low-Mg calcite. Unless dolomitized, the final product of carbonate diagenesis is low-Mg calcite, as seen in most of the ancient carbonate rocks (Tucker and Bathurst, 1990).

Meteoric Diagenesis The meteoric environment consists of rainfall-derived water in contact with sediment or rock. Within the meteoric environment, there are two major zones of interest. These are the vadose and phreatic zones, separated by the water-table (Figure 1). Water drains through the vadose zone due to gravity. The pores within this zone are characteristically filled with air, but are subjected to wetting and drying depending upon climatic conditions (Tucker and Wright, 1990). Below the vadose zone and the water-table is the phreatic zone. This zone is characterized by fluid filling all of the pore spaces, and typically includes a horizontal groundwater flow direction. In an island or coastal setting, this meteoric zone is underlain by a denser water (highly saline or marine water); i.e. the less dense freshwater lens rests on top of the more saline water. A more specific account of this salinity difference along with more characteristic island hydrology will be discussed later. Freshwater entering the meteoric zone is undersaturated with respect to all of the carbonate minerals and will immediately begin to dissolve the surrounding limestone (Tucker and Bathurst, 1990). Rainfall-derived water (with its initial CO2 concentration) picks up additional CO2 derived from the oxidation of organic matter in the soil. This

2

addition of CO2 leads to an increase in the ability of the water to dissolve the surrounding sediment. Aragonite is more soluble than low-Mg calcite; therefore in the presence of water, aragonite will preferentially dissolve. Continued dissolution of ++ = aragonite may elevate the IAP of Ca and CO3 to the point at which low-Mg calcite can precipitate. The dissolution of aragonite often includes liberation of Sr++; whereas the precipitation of calcite excludes Sr++. This “exclusion” of strontium in precipitated calcite is an important property of diagenetic limestones and results in a low-Sr calcite. It is possible that aragonite will continue to dissolve, while calcite cementation occurs.

Scope of this Project Studies of freshwater diagenesis of carbonates have been performed where Holocene sediment of known age was analyzed along with its groundwater. In these studies it was safely assumed that no diagenetic precursors were formed by previous episodes of freshwater diagenesis (Halley and Harris, 1979; Budd, 1988; Budd and Land, 1990; McClain et al., 1992). This assumption allows a direct correlation between the mineralogical diagenesis in the rocks and its associated groundwater chemistries over a known time of freshwater exposure. From these studies, measurements were made of the magnitude, efficiency, and rate of mineralogical transformation from aragonite to calcite. Other studies (Plummer et al., 1976; Back et al., 1979; Evans and Ginsburg, 1987; and Saller and Moore, 1991) have investigated older (Quaternary) rocks which contained more advanced diagenetic alteration from aragonite to calcite. The carbonates rocks in these studies had experienced previous episodes of freshwater diagenesis because sea-level fluctuations have raised and lowered the water table (along with the phreatic freshwater lens) throughout the Quaternary. On Andros Island, the upper lithology is of the Pleistocene age and has experienced previous fluctuations in sea-level (Carney and Boardman, 1991), and during the last 125,000 years its upper lithology has experienced primarily vadose diagenesis (~3,000 to 8,000 years of phreatic diagenesis and ~120,000 years of vadose diagenesis). In this study, we examined the diagenesis within in the meteoric zone on north Andros Island, Bahamas. In particular, we compared the present geochemistry of the rocks and

3 groundwater with the presumed rock and water precursors in order to determine the rate of transformation from aragonite to the more stable low-Mg calcite.

4 STUDY AREA Overview of Bahamas Geology

Geographical Background of the Bahamas The Bahamas is an archipelago of carbonate islands and shallow banks (<10 m) bounded by several deep-water channels (>800 m) (Figure 2). This archipelago is located in the western North Atlantic Ocean (21° to 26°30’ N and 69° to 80°30’W) and covers a total area of 260,000 km2, 14,300 km2 of which is subaerially exposed (Sealey, 1985). Of this subaerial exposure, there are 29 land masses which are considered islands and 661 minor islands called cays. The islands and cays are typically low lying with the topography dominated by eolinian ridges which may extend to 30 meters above sea level (Sealey, 1985). Two large banks, Great Bahama Bank and Little Bahama Bank, host many of the islands in the northwestern section of the archipelago. Great Bahama Bank is dissected by two deep troughs, the Tongue of the Ocean (1400-2000 m) in the middle and Exuma Sound (1700-2000 m) in the eastern section of the bank. The Northwest and Northeast Providence Channels separate Little Bahama Bank from Great Bahama Bank (Newell and Rigby, 1957). The majority of the islands found on these banks are along the eastern, or windward, edges of the banks due to the predominant northeast trade winds (Shinn, et al., 1969). Andros Island is located 220 km east-southeast of Florida and is part of the Great Bahama Bank. Being the largest of the Bahamian Islands, Andros occupies an area of 5700 km2 and trends north-south (Sealey,1985), with the Tongue of the Ocean bounding the eastern edge of the island.

Geologic History of the Bahamas The carbonate platform known as the Bahamas has existed for at least 135 million years, dating into the Cretaceous Period. From a series of deep cores taken from the Tongue of the Ocean, across Andros Island, through the Straits of Florida, Beach and Ginsburg (1980) found that shallow marine sedimentation produced the limestones and dolomites seen throughout the lengths of the cores. The production of sediment along

5 with the subsidence of the bank has allowed for the nearly continuous accumulation of carbonates throughout most of the geologically history of the Bahamas (Sealey, 1985).

Quaternary History of the Bahamas The evolution of the Bahamas during the Quaternary was controlled by sea- level fluctuations caused by massive glaciation in the northern latitudes. During periods of glaciation, the ice caps and glaciers grew and sequestered some of the water once in the ocean. This removal of water caused a lowering of the sea-level, exposing the banktops in the Bahamas. These “drops” in sea-level can be quite significant, as seen in the last glaciation in which sea-level dropped nearly 120 m (Sealey, 1985). As ice caps and glaciers melted, the water returned to the oceans and sea-level rose (interglacial period). Much of this glacioeustasy during the Quaternary can be inferred by deep-sea oxygen isotope data calibrated against exposed reef material in Barbados and New Guinea (Hays, 1976). From the isotope-derived sea-level curve (Kindler and Hearty, 1997), it appears that sea-level was approximately 6 m below its present level throughout much of the Quaternary. The Quaternary consisted of long periods of island exposure punctuated by short intervals of submergence and carbonate sediment production (Bloom et al., 1974). The most recent sea-level highstand, besides the present-day highstand, occurred approximately 125,000 years ago during the last interglacial period (Sangamon interglacial or substage 5e) (Figure3). Evidence shows that the sea-level during this time was nearly 6 m above its current elevation (Boardman & Carney, 1991). A majority of the banktops were flooded during this time with only a few eolian ridges existing above the water surface (Figure 4). This highstand resulted in vast sediment accumulation of shallow-water carbonates on the banktops. It is this sediment package which makes up the upper 6-7 m of lithology in my field area on Andros Island (Fitzgerald, 1997).

Field Area Lithology The lithology of the field area is known from studies of cores recovered over the past several years (Carney and Boardman, 1991; Boardman et al, 1993; and Boardman et

6 al, 1994). Three of these cores (69, C2N, and 4SA) were described both megascopically and microscopically in detail, and the descriptions are summarized below (Carney and Boardman, 1991; Troska, 1992; Boardman et al, 1993; Boardman et al, 1994; Dacyk, 1996; and Fitzgerald, 1997). Borehole core 69. Core 69 can be divided into four distinct sections (with a total depth of 12.6 m). The upper 5.0 m is an oolitic/peloidal grainstone. This grainstone consists of approximately 60% ooids and 30% peloids (% of total grains) with skeletal and algal fragments comprising the remaining 10%. Dissolved laminae of many of the ooids indicate selective dissolution and micritization of the grains (nearly 10%). The cement found includes both meniscus (at the grain contacts) and void filling space. There is a gradual change from vadose type cements (e.g. meniscus) at the surface to phreatic dominated cement (equant, blocky calcite crystals) at depth. An interesting feature is the abundance of holes and sediment-filled holes (mottles), interpreted to be relict burrows and infilled burrows. The lack of large amounts of mud, good sorting and rounding of the grains (ooids and peloids), and the presence of skeletal grains supports the interpretation of Carney and Boardman (1991) that the upper several meters was originally a stabilized sand flat. Within this oolitic/peloidal grainstone at about 2 meters depth is a 2 cm thick, brownish-red, micritic zone with wavy laminations. This feature may be a paleosol or exposure surface (Fitzgerald, 1997). Below the mottled grainstone is a mottled skeletal packstone, which is approximately 2 meters thick (5.8 m to 7.8 m). Pelloids are common and skeletal grains of foraminifera, green algae, and molluscs are poorly sorted and not rounded. There are few holes, and ooids are rare. There is approximately 20 to 25% micrite matrix in this section, and all grains show at least partial recrystallization to calcite or complete micritization. Cementation occurs primarily as equant void-fillings and occasionally at grain contacts. This lithology is interpreted as a normal marine lagoon (Carney and Boardman, 1991). Below the mottled skeletal packstone (approximately 7 m below the ground surface) a 50 cm thick red to brown, very well cemented, and dense layer is present. This layer is interpreted as a paleosol (Carney and Boardman, 1991). Many clasts were

7 found embedded in a dense, micritic matrix along with calcified rootlets in void spaces. These are features were created during times of exposure (Carney and Boardman, 1991). Below the paleosol, a 3 m thick packstone can be found (7.8 m to 11.8 m). The skeletal material found in this section consists of branching corals, molluscs, algae, and forams. Large calcite crystals are found in void spaces and this section is clearly much better cemented than the units above. The lithology (the presence of coral in this packstone) suggest that it could have originated in a lagoon mainly due to the presence of coral in this packstone. The remaining 1.5 m (11.8 m to 13.3 m)of this core consists of a mottled, skeletal wackestone/ packstone. Ostracodes and small forams make up much of the skeletal fraction of the grain, while a lime mud matrix is abundant. Carney and Boardman (1991) suggest the origin of this lithology is a restricted lagoon environment. Borehole core C2N. Core C2N can be divided into four distinct sections (with a total depth of 12.6 m). The upper 3.5 m is an oolitic/peloidal packstone. This packstone consists of approximately 27% ooids and 64% peloids (% of total grains) with skeletal and algal fragments comprising the remaining 9%. With micrite lacking in this section, the cement contributes to approximately 30% to the overall rock composition. Porosity is moderate, ranging between 6-19%. This material could have been deposited in an oolitic sand flat (Fitzgerald, 1997). Below the oolitic/peloidal packstone is peloidal packstone, which is approximately 2.25 meters thick (3.5 m to 5.75 m). With ooids absent in this section, peloids are dominant (averaging 88% of the grains), with clasts and skeletal grains making up the remainder of the grains. Bioturbation and cement are also prevalent in this section. Cement averages 27%, with porosity averaging 15% of the total rock composition. This lithology indicates deposition in a peloidal-rich lagoon (Fitzgerald, 1997). Below the mottled peloidal packstone is a 1.15 m thick (from 5.75 m to 6.9 m) is a dense, heavily mottled (bioturbated), blackened-grain dominated section. These features indicated a paleosol (Fitzgerald, 1997). Below the paleosol is 4.1 m (from 6.9 m to 11.0 m) of vuggy, well-cemented , micrite-dominated zone. With moderate porosity, averaging 10% of the total rock

8 composition, this section is considered a wackestone. An open muddy lagoon is the likely depositional agent for this section (Fitzgerald, 1997). The final zone is 1.6 m in length (from 11.0 m to 12.6 m). This dense, matrix- dominated zone is very mottled and contains very few grains. With micrite making up over 50% of the rock and cement making up of over 35%, this zone is classified as a wackestone. This lithology could have been deposited in a restricted lagoon (Fitzgerald, 1997). Borehole core 4SA. Borehole core 4SA reached a depth of only 4.5 meters and consists entirely of an oolitic-peloidal grainstone. Overall, grain micritization was extensive. Porosity is mainly interparticle porosity and is patchily distributed, while some of the porosity occurs within partially dissolutioned oolitic laminae. Cementation is intergranular and consists of a fine to coarse mosaic of equant blocky low Mg calcite.

Field Area Hydrology Island Hydrology The hydrology of carbonate islands is unique in that the groundwater involves water derived from rainfall, from the ocean, and water composed of a mixture of the two sources (brackish water). The existence of a freshwater lens on an island setting means that the recharge from precipitation exceeds the amount of both evaporation and the outflow of freshwater to the ocean. The location and stability of the freshwater lens, the brackish transition zone, and the saline groundwater are mainly determined by the salinity of the waters which determine the density. Water temperature can also affect the density, but the differences in water temperature are small (0° to 3°) relative to the salinity (0 to 35 ppt) differences. Because of these density differences, rainfall-derived freshwater must rest on top of a more dense ocean water. For a freshwater-seawater system, the Ghyben-Herzberg Principle predicts that for every meter of freshwater above sea-level there must be 40 meters of freshwater below sea level (Figure 5). Assumptions behind this principle are quite problematic in that the lithology of the aquifer, the transition zone due to the miscibility of the waters, and the hydrostatic nature of the system are all not taken into account (Vacher, 1997).

9 Controls on the Freshwater Lens of Andros Rainfall is the sole source of freshwater on many tropical islands, including Andros Island. With no rivers feeding the subsurface aquifer, the freshwater lens will thicken or thin corresponding to the magnitude and frequency of rainfall events versus evaporation. The average rainfall over the period from 1977-1984 on north Andros was 120 cm (Cant and Weech, 1986). This amount of annual rainfall appears to be enough to provide Andros with a continuous, cross-island freshwater lens. This annual amount of rainfall is not the actual amount of water infiltrating into the aquifer. Direct evaporation, transpiration, and runoff remove some of the rainwater and the subsequent amount of water recharging the aquifer is much smaller. Little et al. (1973) found evaporation and transpiration on Andros to be approximately 75% of the precipitation. With no rivers on North Andros, runoff is considered to be zero. Thus, the average annual recharge to the aquifer is approximately 90 cm/year (or 25% of precipitation). Even though much of the precipitation is consumed before infiltrating, the freshwater lens on Andros is the largest of any Bahamian island. The Bahamian Water Corporation is thinning the lens by extracting vast quantities for domestic use. New Providence Island, only 45 km to the east of Andros, receives approximately 5 million gallons per day of freshwater extracted from well fields on north Andros. This daily amount of freshwater is enough to meet 60% of the drinking water demand in the city of Nassau (Bukowski, 1996). Abstraction of this magnitude may cause salt water encroachment within the freshwater lens and trouble for the Bahamians of Andros who use this valuable resource.

10 METHODS OF INVESTIGATION

The Study Area The field area is located in the northeast section of Andros Island and encompasses a 3 km x 1 km area along Charlies Blue Hole Road (Figure 6). With Queens Highway to the west and Cantor Sound to the east, our study area lies in a section of land delineated by logging roads 1 through 9, along with a N (north) or S (south) which cross Charlies Blue Hole Road (running east and west) (Figure 7). Well 69 is located at the intersection of road 9 and Charlies Blue Hole Road. In order to evaluate the geochemistry of the groundwater of northern Andros, multi-level wells were fabricated and installed into pre-existing coreholes. These pre- existing coreholes (denoted as 9N, 69, 4SA, C2N, and 4NN) were originally drilled and examined by Troksa (1992), McGavern (1995), and Dacyk (1996) and are 5 cm in diameter and up to 12.6 m in length.

Construction and Installation of the Multi-level Samplers The multi-level samplers were constructed in the field by attaching 1/8” inner- diameter nylon tubing to the outside of a 1/2” PVC pipe with electrical tape (Figure 8). The ends of the tubing were melted shut and many small holes (1/32”) were drilled into the tubing over a 10 cm length to allow water to be pumped into the tubing. Sampling ports were spaced at 1 meter intervals (starting one meter up from the bottom of the PVC). The base of each corehole was filled with 5 cm of gravel (1/4” - 1/8”) before the pvc/nylon tubing was installed. Once the pvc/nylon tubing was set in the hole, the wells were packed with alternating layers of fine sand and coarse gravel, with each layer 50 cm in depth (Figure 9). The sampling level was surrounded with the conductive gravel 20 cm above the sampling port and 20 cm below the inlet, along with surrounding the sampling port which was 10 cm. This “layer-cake” packing was to discourage vertical movement of water within the corehole when sampling began and to encourage sampling water from within the aquifer at that particular depth. The difference (3 orders of magnitude) in the hydraulic conductivity of the packing material was enough to deter the vertical

11 4 2 movement of the water within the corehole (Kcoarse gravel = 10 gpd/ft , while Kfine 1 2 sand = 10 gpd/ft ) (Freeze and Cherry, 1979). Every well, except for C2N (filled to ground surface with gravel), is capped with a bentonite seal (0.5 to 2 m thick). The PVC and nylon tubing were trimmed so that no part of the well was exposed above the ground surface. All of the wells were “closed off” with threaded, plastic well caps. Well logs for each well are given in Figures 10, 11, 12, 13, and 14.

Sampling the Wells Water from each sample tube was purged by pumping about 2 liters of water out of the tube. Most sampling ports produced clear water after only 1 liter of purging. pH was monitored during purging and stabilized after 1 liter was extracted. Sampling of the groundwater was achieved through a peristaltic pump along with an in-line flow through chamber and sampling port. The water was initially pumped through an in-line flow through chamber which consisted of an Orion pH electrode. The groundwater was only sampled after the pH had stabilized. The groundwater was then diverted to a 50 ml syringe for sample collection. For each sample the water was “pushed” directly into a 50 ml syringe and thus was never exposed to the atmosphere. The water in the syringe was then filtered with a 0.4 µm in-line filter and placed into a 60 ml plastic bottle. Two 50 ml samples were collected in this manner. One of the 50 ml samples was treated with two drops of highly concentrated nitric acid for cation preservation, while the other 50 ml sample was not acidified and was used for anion analysis. A third sample was taken for determination of alkalinity. The alkalinity sample was filtered with a 0.4 µm in-line filter and transferred directly into a 50 ml syringe and placed in a cooler. The entire sampling procedure for alkalinity did not expose any of the water to the atmosphere in order to help deter the effects of CO2 exchange. Alkalinity measurements were conducted at the end of each sampling day, but the method used to determine total alkalinity was flawed and no actual alkalinity values obtained in this manner were used.

12 Core Samples Samples of the rock (from cores 69, 4SA, and C2N) were obtained by drilling (with a portable hand drill) into the cores and removing several grams of rock powder. Enough powder was obtained in order to make X-ray diffraction mounts and for dissolution for direct-current spectroscopy samples. Specimens were taken throughout the entire lengths of the cores at approximately 20-30 cm intervals.

Laboratory Analysis Water samples were brought back to Miami University and analyzed for both cations and anions. The cations measured were calcium, magnesium, sodium, potassium, and strontium. Cation analysis was done with the direct-current plasma spectrometer. Anions were measured using a Dionex high-performance liquid chromograph. Anions measured were chloride and sulfate. Powders from core samples (69, 4SA, and C2N) were diluted (~ 1:12,500) with 5N HNO3 and analyzed by the direct-current plasma spectrometer. This analysis measured the cations of the bulk solid (a combination of the grains and cement within the rock). Powders from the same samples were mixed with deionized water and mounted onto a glass plate and were analyzed by X-ray diffractometer in order to evaluate the abundance of carbonate minerals within the core.

13 RESULTS

Chemistry of the Groundwater Chemical analysis of the groundwater samples indicate that 29 of 31 samples are freshwater (< 1000 mg/l TDS, from Freeze & Cherry, 1979) (Table 1). The bottom two samples from well 69 (at 11.18 and 12.18 meters below the surface) showed higher chemical concentrations and are part of the brackish, or mixing zone.

Anions Chloride. Chloride in the freshwater lens (Table 2 and Figure 15) averages 92.8 ppm, with a minimum concentration of 6.0 ppm and a maximum concentration of 453.8 ppm. There is a general increase in chloride with depth in all of the wells. Within the mixing zone, chloride values attained 3961.8 ppm. Sulfate. In the freshwater lens, sulfate concentrations (Table 2 and Figure 16) averaged 9.4 ppm, with a minimum of 1.8 ppm and a maximum of 32.4 ppm. There is a general increase in sulfate with depth in all of the wells. Within the mixing zone, sulfate concentrations reached 310.6 ppm.

Cations Calcium. Calcium in the freshwater lens (Table 2 and Figure 17) averaged 62.1 ppm, with a minimum concentration of 33.8 ppm and a maximum concentration of 106.2 ppm. In general, there is an increase in calcium concentrations with depth. Within the mixing zone, calcium values reached 244.3 ppm. Strontium. Strontium in the freshwater lens (Table 2 and Figure 18) averaged 1.6 ppm, with a minimum concentration of 0.6 ppm and a maximum concentration of 4.2 ppm. Strontium in the mixing zone reached a concentration of 7.8 ppm. Sodium. Within the freshwater lens, sodium (Table 2 and Figure 19) averaged 50.7 ppm, with a minimum concentration of 5.8 ppm and a maximum concentration of 243.2 ppm. In general, there is an increase in sodium with depth in each of the wells. Sodium in the mixing zone reached 2406.9 ppm.

14 Magnesium. Magnesium in the freshwater lens (Table 2 and Figure 20) averaged 7.3 ppm, with a minimum concentration of 1.4 ppm and a maximum concentration of 29.5 ppm. Magnesium in the mixing zone attained a value of 283.8 ppm. Potassium. Within the freshwater lens, potassium (Table 2 and Figure 21) averaged 1.9 ppm, with a minimum concentration of 0.1 ppm and a maximum concentration of 9.3 ppm. There was an increase in potassium with depth seen in each of the wells. Potassium in the mixing zone reached a concentration of 109.8 ppm.

Alkalinity Alkalinities were calculated by subtracting the sum of the cations from the sum of the anions. The alkalinities (Table 3) ranged from 1.811 mM to 5.208 mM, with an average of 3.194 mM (Figure 22). The lowest alkalinities were found in well C2N, where the average alkalinity was 2.081 mM. pH In-situ pH values (Table 3) ranged from 7.038 to 7.636, with an average of 7.310 (Figure 23). There appeared to be a general decrease in pH with depth in all of the wells.

Saturation States Within the freshwater lens, most samples are very close to saturation (Table 4) with respect to both aragonite (Figure 24) and calcite (Figure 25).

Chemistry of the Rainwater Rainwater was collected from our field site during the sampling period and was analyzed for chemical composition. The average concentrations of the rainwater (4 samples) constituents are given in Table 5.

Rock Specimens Bulk rock samples from cores 69, 4SA, and C2N were analyzed for cations (calcium, strontium, and magnesium ) (Table 6). Calcium ranged from 371,100 ppm to 434,021 ppm, and averaged 395,654 ppm (Figure 26). Strontium ranged from 893 ppm

15 to 5,492 ppm, averaging 3,111 ppm (Figure 27). There appears to be an overall decrease in strontium with depth in all of the cores. In the upper 7 m (from 0 m to 7 m) strontium averaged 3873 ppm, and in the lower 5 m (from 7.01 m to 12.6m) strontium averaged 1865 ppm. Magnesium in the upper 7 m (from 0 m to 7 m) averaged 1255 ppm, and in the lower 5 m (from 7.01 m to 12.6m) magnesium averaged 2026 ppm (Figure 28). Laboratory-pure calcium carbonate was also measured and found to contain on average 410,840 ppm calcium and 392 ppm strontium.

X-ray Diffraction of Bulk Rock Boardman and Carney (1997) found that the upper 6 m of the lithology from core 69 contains nearly 60% of the original aragonite, while there is essentially no aragonite remaining in the lower lithology below 6 m. From this study, the upper 6 m from core C2N averaged 14.0% aragonite, while the lower 6.5 m averaged only 4.4% aragonite.

16

DISCUSSION

Water Chemistries The amount of any element entering the groundwater of Andros Island includes 1) ions entering the aquifer with the rainwater and dry aerosols and 2) ions from seawater (mixing from below). This initial chemistry is modified by reactions with the aquifer: 1) the addition of ions resulting from dissolution reactions with the aquifer and/or 2) reduction of ions by the precipitation of minerals. One of the goals of this study is to understand the reactions within the aquifer. Both the ions in rainwater and from mixing with seawater have the same ultimate origin (the ocean). Calcium, for example, can be modeled in the following manner: Catotal = Cainput from rain, aerosols, or seawater mixing + Careaction- derived Reaction-derived calcium is the calcium that has been added to or removed from the groundwater due to the dissolution and/or precipitation of the host rock. To determine this quantity, the calcium derived from rain and seawater must be estimated and subtracted from the total calcium. Careaction-derived = Catotal - Cainput from rain, aerosols, or seawater mixing We use chloride as a conservative element in order to estimate the amount of each element derived from rain or seawater. Since carbonate rocks have essentially no chloride in their crystal lattice (Morse and MacKenzie, 1990), and no other rock in the Bahamas has chloride in it, then the only source for chloride in the water would be from rainwater or seawater. Because the nuclei of rain droplets are mostly evaporated droplets of sea spray, a coastal setting rainwater should mimic the relative chemical composition of the nearby seawater.. The amount of any element derived in this way (from seawater or from rain) can be calculated by multiplying the chloride concentration of the groundwater by the element/Cl ratio in normal seawater. For example, typical Ca/Cl ratio (in ppm) in seawater is 411/19350 (or .0212403). The amount of calcium derived from seawater would be:

17 Caderived from seawater = (Ca/Cl)seawater x Clgroundwater ,or

Caderived from seawater = .0212403 x Clgroundwater

This same type of calculation can be performed with any of the major elements (Mg++, + + ++ = K , Na , Sr , and SO4 ) in the groundwater. This method of estimation assumes that chloride is non-reactive in limestones and that the element/Cl ratio of seawater is constant. Both of these assumptions are reasonable for the major elements, and not reasonable for biogenically reactive components (carbon, nitrate, nitrite, and phosphate). If there is less of a particular element than expected from this chloride analysis, then the reaction-derived amount (i.e. Careaction-derived) would be negative, indicating consumption or precipitation of this constituent. Catotal - Caderived from seawater = Careaction-derived If the reaction-derived values are near zero, then the element was not reactive with the aquifer. Positive values for reaction-derived concentrations indicate the element was added to the groundwater by the limestone aquifer (i.e. that dissolution has taken place).

Proof of Reactivity A plot of the concentration of each constituent versus chloride provides a visual representation of the reactivity within the aquifer. Data which plots above a seawater/freshwater mixing line would indicate enrichment (dissolution of the aquifer material), plots below the line would suggest reduction (precipitation), and plots on this line may indicate simple mixing of the waters (no reactivity within the aquifer). When plotted against chloride and compared to the respective freshwater/seawater mixing lines, we were able to reconfirm our reaction-derived values. It appears potassium (Figure 29), sodium (Figure 30), and magnesium (Figure 31) are primarily derived from mixing of fresh and seawater. All groundwater samples plotted above the mixing lines for calcium (Figure 32) and strontium (Figure 33), suggesting enrichment. While only sulfate (Figure 34) showed evidence of reduction by plotting below the mixing line.

18 Reaction-derived Values Three cations (Mg++, K+, and Na+) had reaction-derived values near zero, indicating that there was no net dissolution or precipitation occurring within the aquifer (denoted as “non-reactive with the aquifer”). The concentrations of these cations are completely explained by mixing with seawater or by addition via rainfall. Calcium and strontium had high, positive reaction-derived values, meaning that their concentrations in the groundwater result from the dissolution of the aquifer material. Sulfate generally has reaction-derived values near zero, but at the bottom of the = core the reaction-derived SO4 was negative. This indicates precipitation of sulfur within the aquifer

Non-reactive with the Aquifer Reaction-derived Magnesium. Reaction-derived magnesium (Figure35) ranged from -0.73 ppm to 3.26 ppm, with an average of 1.21 ppm. Total magnesium ranged from 1.4 to 29.5 ppm. On average, 16.5% of the magnesium was reaction-derived. These findings are interesting because many modern carbonates are comprised of high-Mg calcite, yet in our samples there appears to be very little magnesium derived from the host rock. The lack of magnesium is consistent with the lack of high-Mg calcite in the X-ray diffraction analysis and from the near zero concentrations of Mg++ seen in both the bulk rock and the ooids from Joulters Cay. This means that any of the original high-Mg calcite that might have once been in the rock has since been altered to a more stable low-Mg calcite. Reaction-derived Potassium. Reaction-derived potassium (Figure 36) ranged from -0.70 ppm to 0.43 ppm, with an average of 0.00 ppm. Total potassium ranged from 0.1 to 9.3 ppm. On average, 0.1% of the potassium was reaction-derived. It appears that nearly all of the potassium was derived from rainwater or seawater, not from the host material. Potassium does not fit well into any carbonate lattice site because of its size and charge. Its ionic radius (1.40Å) is much larger than the preferred size of 0.97Å for calcite and 1.10Å for aragonite, and its charge is +1 rather than +2.

Reaction-derived Sodium. Reaction-derived sodium (Figure 37) ranged from -9.13 ppm to 7.59 ppm, with an average of -0.46 ppm. Total sodium ranged from 5.8 to 243.2 ppm.

19 On average, less than 1% of the sodium was reaction-derived. Sodium in the aquifer originates from rainwater or seawater, not from interactions with the aquifer material. This comes as no surprise given that sodium was found only in trace concentrations in the bulk rock because it does not fit well into the lattice sites of either calcite or aragonite

Thus, the concentrations of magnesium, potassium, and sodium appear to be the product of freshwater/seawater mixing. Very little of these constituents were formed from rock/water interactions. These conclusions are not surprising because these elements exist in trace quantities in the bulk rock and would not add a significant concentration into the groundwater upon dissolution of the limestone.

Reactive with the Aquifer (Reduction) Reaction-derived Sulfate. Reaction-derived sulfate (Figure 38) ranged from -15.47 to 3.55 ppm, with an average of -2.88 ppm. Total sulfate ranged from 1.8 to 32.4 ppm. There is an overall reduction in sulfate at the base of the cores. The formation of CaSO4 is not likely because the IAP/K is , which is far from saturation. Reduction to HS- and precipitation as FeS2 is another possibility. Troksa (1992) found pyrite in several of her cores on Andros Island.

Reactive with the Aquifer (Dissolution within the Aquifer) Calcium and strontium are the only two constituents that are reactive with the aquifer, meaning that the limestone aquifer dissolves and adds calcium (Careaction-derived) and strontium (Srreaction-derived) into the groundwater.

Reaction-derived Calcium. Reaction-derived calcium (Figure 39) ranged from 32.06 ppm to 96.19 ppm, with an average of 60.12 ppm. Total calcium ranged from 33.8 to 106.2 ppm. On average, 96.8% of calcium was reaction-derived. This suggests a significant amount of dissolution of calcium carbonate.

Reaction-derived Strontium. Reaction-derived strontium (Figure 40) ranged from 0.53 ppm to 4.2 ppm, with an average of 1.75 ppm. Total strontium ranged from 0.6 to 4.2

20 ppm. On average, nearly 100% of the strontium was reaction-derived, suggesting a significant amount of dissolution of a Sr-rich calcium carbonate. The principle elements added to the groundwater by reactions (i.e. excess relative to mixing) were calcium and strontium. This was expected since the rocks are calcium carbonate and much of the original grains were aragonite containing high amounts of strontium. From petrographic analysis, Carney and Boardman (1991) found that ooids (aragonite) comprise nearly 70% of the grains in the upper 7 meters of this lithology. The high concentration of ooids is consistent with the present bulk rock mineralogy which shows that 60% of the rock in this section is composed of aragonite. On average, the strontium concentration in the upper 7 meters was 3873 ppm. In modern ooids found at Joulters Cay, the average strontium concentration was 9349 ppm.

Evidence of Precipitation and Dissolution within the Aquifer If the reactive Ca and Sr are derived from dissolution of the bulk rock, then the Ca/Sr ratios of the groundwater should be identical to the rock Ca/Sr ratios. The average Ca/Sr ratio of the rock in the upper 6 m is 105 (394827 ppm/ 3873 ppm). The average Ca/Sr in the water from the upper 6 m is 44 (55.8 ppm /1.3 ppm) (Figure 41). Compared to the bulk rock, the groundwater is depleted in calcium and/or enriched in strontium. Something other than simple dissolution of the bulk rock is occurring. Selective dissolution of the strontium-rich carbonate may explain the Ca/Sr ratio of the groundwater. The bulk rock is comprised of both aragonite (60%) and low-Mg calcite (40%). Studies show that ooid make up approximately 70% of the grains in the bulk rock (Carney and Boardman, 1991). Ooids are made up of aragonite and are Sr-rich; whereas low-Mg calcite is Sr-poor (Figure 42). Perhaps more of the Sr-rich aragonite from ooids is preferentially dissolving over the low-Mg calcite. Dissolution of aragonite is expected because aragonite is thermodynamically more soluble than low-Mg calcite. Petrographically, dissolution of ooid laminae is clearly evident in this upper lithology (Carney and Boardman, 1991). If the ooids on Andros have a calcium and strontium concentration similar to Joulters (a modern ooid sand shoal), then the Ca/Sr ratio added to the groundwater should be 45 (418228 ppm /9349 ppm). This value of 45 is very similar to the Ca/Sr we measured in the groundwater; 44. This concurrence of

21 Ca/Sr in solution with Ca/Sr from ooids suggests that the aragonite (from the ooids) is preferentially dissolving from the bulk rock. The petrographic evidence of precipitated low-Mg calcite cement (Carney and Boardman, 1991) and the mineralogic evidence of low-Mg calcite suggest that precipitation of low-Mg calcite occurred. Precipitation of low-Mg calcite would decrease the calcium concentration without altering the strontium concentration in the water because strontium (Sr++) is too big to fit into the precipitating calcite lattice. If significant amounts of precipitation of low-Mg calcite were occurring in these rocks, a lower Ca/Sr would be expected in the groundwater due to the removal of calcium by the precipitating calcite. Water samples from cores 69, C2N, and 4NN are enriched in Sr in relation to Ca (if dissolution of ooids alone produced the Ca/Sr ratios). This data can be explained by the precipitation of low-Mg calcite.

Quantifying the Dissolution and Precipitation We can model the amount of dissolution and/or precipitation by considering the fluxes of calcium and strontium to and from the aquifer fluids in a series of mass-balance equations. ++ Strontium (Sr reaction-derived) should equal the amount of strontium from the dissolution of aragonite minus that amount of strontium incorporated in the precipitated ++ ++ calcite cement. The Sr reaction-derived and Ca reaction-derived are known from the measurement of Sr++, Ca++, and Cl- in the water. This can be shown by the following equations:

++ ++ ++ Sr reaction-derived(moles) = Sr dissolution - Sr precipitation

Dissolution ++ ++ Sr dissolution (moles) = (y)(molar fraction of Sr in aragonite) where y = moles of aragonite dissolved

++ ++ Sr dissolution (moles) = (y)(.00935, the molar fraction of Sr in Joulters’ ooids)

22 Precipitation ++ ++ Sr precipitation(moles) = (z)(molar fraction of Sr in precipitated calcite) where z = moles of calcite precipitated

++ Sr precipitation(moles) = (z)[( Sr/Ca water sample)(distribution coefficient of .055)], this distribution coefficient for Sr into calcite is from Katz et al. 1972.

++ Thus, Sr reaction-derived = y(.00935) - z[(Sr/Ca water sample)(.055)].

The same process is done with calcium:

++ ++ ++ Ca reaction-derived(moles) = Ca dissolution - Ca precipitation

Dissolution ++ Ca dissolution (moles) = (moles of aragonite dissolved)(molar fraction of Ca++ in aragonite)

++ Ca dissolution (moles) = (y)(.99065, normalized to the ooid concentration of Joulters’ ooids of both calcium and strontium to 1; i.e. 1 - .00935) Precipitation ++ Ca precipitation (moles) = (moles of calcite precipitated)(molar fraction of Ca++ in precipitated calcite)

++ Ca precipitation (moles) = (z)[(1- (Sr/Ca water sample)(.055)], assuming only calcium and strontium are in the precipitated calcite.

++ Thus, Ca reaction-derived = y(.99065) - z[1- (Sr/Ca water sample)(.055)].

23 Therefore, the two equations of interest are: ++ Sr reaction-derived = y(.00935) - z[(Sr/Ca water sample)(.055)], and

++ Ca reaction-derived = y(.99065) - z[1- (Sr/Ca water sample)(.055)].

For each freshwater sample in the upper 6 m, we were able to use the calcium and strontium concentrations derived from the rocks and apply the above equations to quantify how much dissolution and precipitation could have occurred in order to produce each of the groundwater samples’ specific chemistries. These results are displayed in Table 7. The magnitude of aragonite dissolution ranged from 0.7 mM/l to 3.1 mM/l in the freshwater lens, and averaged 1.6 mM/l. The amount of calcite precipitation ranged from 0.0 mM/l to 1.3 mM/l, and averaged 0.3 mM/l.

Rate of Mineralogical Transformation From the above mass-balance equations, the data show that dissolution was the dominant process within the aquifer, and it is possible to calculate the mass of aragonite dissolved per liter of groundwater. It would be insightful to investigate the rate at which this process was transforming the aragonite into a more stable carbonate mineral phase. McClain et al. (1992), from Vacher et al. (1990), was able to describe this rate of transformation by the following equation:

1000η Rs = Ma ρm τa

3 3 In this equation, Rs is the stabilization rate (cm /m /year) of aragonite, Ma is the mass of aragonite previously calculated to be dissolved per liter of groundwater (mg/l), 1000n converts liters of groundwater in the saturated zone to cubic meters of rock with a 3 porosity of η, ρm is the density of aragonite (2.93 g/cm ), and τa is the average age of the groundwater (years) within the chemically reactive zone of the aquifer. This rate of

24 stabilization can be thought of as the rate at which aragonite is transformed to calcite per cubic meter of groundwater within the saturated zone (Vacher and others, 1990). Bukowski (1996) modeled the groundwater flow in this area of North Andros Island and found that groundwater originating near our farthest inland well would reside in the chemically reactive zone (the upper 7 m) for approximately two years if there was 20% porosity in this zone. Therefore, the average rate of aragonite stabilization from this reactive region with a residence time of two years would be 53 cm3/m3/year, and ranged from 25 cm3/m3/year to 105 cm3/m3/year (Table 8). Halley and Harris (1979) investigated the freshwater zone on South Joulters Cay (an oolite approximately 670 years old) and found an average stabilization rate of 61 cm3/m3/year (Table 9). Budd (1988) conducted a similar study on a 700 year old oolite on Schooner Cays and found a stabilization rate of 49 cm3/m3/year. Vacher (1990) calculated the stabilization rate for the Pleistocene-aged island of Bermuda (from data in Plummer et al., 1976) to be 8.1 cm3/m3/year. McClain et al., (1992) investigated biogenic-rich carbonates in Holocene sediments at Ocean Bight, Bahamas where high- Mg calcite was abundant in addition to high-Sr aragonite. They found that the average rate of metastable mineral (high-Mg calcite plus aragonite) stabilization was 106 cm3/m3/year.

As seen when comparing the various environments, the rates of aragonite-to- calcite transformation are quite variable. There does not appear to be a clear trend based on the age of the sediment and the corresponding rate. The accuracy in estimating variables such as residence time, recharge, and mineralogy of the carbonate minerals could lead to the variation in transformation rates. However, Vacher et al., (1990), proposed that these differences might be controlled by the total amount of aragonite remaining in the rock. They offer no clear physical or chemical reason for their proposal. Their idea is: if there is very little aragonite remaining in the rock, then one should expect the rate of transformation to be low. The rates of transformation (Tables 8,9) did not incorporate how much aragonite per cubic meter was presently remaining in the rock. The idea of the half-life of aragonite was proposed (Plummer et al., 1976; Halley and Harris, 1979; Vacher et al., 1990) to empirically fit the data to a mathematical model.

25 The remainder of this section follows the half-life of aragonite argument from Vacher et al., (1990). If the rate of aragonite stabilization is assumed to be proportional to the amount of aragonite present, then the half-lives of aragonite from any of these environments should be similar. To calculate the half-life of aragonite, the following equations were followed (Vacher et al., 1990): where:

dA = - kA, dA is the rate of aragonite stabilization (R ); dt dt s A is the amount of aragonite (cm3/m3) at time t (yr)

-1 A = A0 exp (-kt), k is the rate constant (yr ) of the reaction A0 and t 1/2 is the corresponding half-life (yr) − ln( 2 ) t = 1/2 k

The half-lives of aragonite stabilization were calculated from the various environments and are shown in Table 10.

From this study on North Andros, the value of 6,278 years for a half-life of aragonite fits well with the other studies. This value was associated with a residence time of two years within the chemically reactive zone of the aquifer.

Geologic Implications of this Rate of Transformation If we accept the half-life calculated for aragonite in the freshwater lens in this study, the amount of remaining aragonite corresponds to the duration of phreatic freshwater diagenesis experienced by the aquifer. The upper 5-6 m of this lithology was interpreted to have formed ~ 125 ka ago when the sea-level was approximately 5-6 m higher than present. Using the half-life of aragonite found from this study (6,278 years) and the total age of the aquifer, there

26 should be approximately 9.5 x 10-5% of aragonite remaining in the rocks and there should be complete calcification. Today, the aquifer contains approximately 60% aragonite, and we assume the aquifer was originally 100% aragonite. However, how can this discrepancy be explained? A possible resolution to this problem would be that the freshwater lens over the past 125 ka was not always at its present position. Even though Andros Island has been subaerially exposed for nearly 125 ka, the rocks from the upper 5-6 m have only contained a freshwater lens during the 5e highstand and again during the last 2000-3000 years (Boardman et. al., 1993). During the 5e highstand the sediments of the aquifer were deposited and then possibly subjected to a freshwater lens like the present environment of Joulters (Halley & Harris, 1979), Schooner (Budd, 1988), and Ocean Bight (McClain et at, 1992). Thus, besides the late Holocene, the latter portion of the 5e highstand was the only time when these rocks could have been exposed to a freshwater phreatic zone because sea-level was much lower during this time interval (between the 5e highstand and the present highstand). Beginning about 3 ka ago, the Holocene rise in sea-level placed the freshwater lens into this upper 6 m of rock, near its current location (Boardman et. al., 1993). Therefore, the upper 5 m of the rocks on Andros Island have only experience two periods of freshwater lens exposure. In terms of diagenesis, we estimate that this package of rocks could have experienced a total of ~5,000 years of phreatic diagenesis. Using an aragonite half-life of 6,278 years and the fact that this upper section contains 60% of its original 100% aragonite, we estimate that the upper 5 m of North Andros should have been exposed to a freshwater, phreatic environment for 5,500 years (Figure 43). The estimate based on sea-level fluctuations and the estimate based on the half-life of aragonite are in excellent agreement.

27 CONCLUSIONS

1. Using the simple multi-level samplers, along with our packing scheme, we measured an increase in the concentrations of the chemical constituents (Ca, Sr, Na, and Cl) with depth in all of the wells. We were very successful in sampling the groundwater from the specific depths of interest, which was a major goal of this study. We were also able to locate the transition, or brackish zone at about 11 meters in our deepest well (69), reconfirming the extent of the freshwater lens.

2. By comparing the ratios of the major elements to Cl, we determined that the majority of the Ca and Sr is derived from dissolution of the aquifer; whereas the majority of Mg, Na, K, SO4, and Cl came from seawater via aerosols in rain or by mixing from below.

3. With the ability to quantify how much dissolution had taken place in each of the water samples, understanding some of the aquifers’ physical characteristics, and estimating an average residence time of the groundwater in the chemically active freshwater zone, we could calculate a stabilization rate (53 cm3/m3/year). This rate gave us an idea of how much aragonite was transformed to calcite per cubic meter of groundwater.

4. Our calculated half-life of aragonite agrees with other published half-lives. From this important finding, we now understand how an 125 ka island, like Andros Island, can still have nearly 60% of its original aragonite. Our data shows that the current state of carbonate diagenesis on Andros is heavily dependent upon the presence or absence of a freshwater lens in the upper lithology. Thus, the keys to understanding freshwater diagenesis is to know where sea-level (along with the water-table) has been over the islands history, to have a good idea of the mineralogy of the original material, and to understand the current status, or how far along in the diagenetic process it is today.

28 References

Back, William, Hanshaw, B.B., Pyle, T.E., Plummer, L.N. and Weidie, A.E., 1979, Geochemical significance of ground-water discharge and carbonate solution to the formation of Caleta Xel Ha, Quintata Roo, Mexico: Water Resources Research, v.15, p. 1521-1535.

Bloom, A.L., Broecker, W.S., Chappell, J.M.A., Matthew, R.K., and Mesollela, K.J., 1974, Quaternary seal level fluctuations on a tectonic coast: New 230Th/234U Dates on the Huon Peninsula, New Guinea: Quaternary Research, v.4, p. 185-205.

Boardman, M.R., and Carney, C., 1991, Origin and accumulation of lime mud in ooid tidal channels, Bahamas: Journal of Sedimentary Petrology, v. 61, p. 661-680.

Boardman, M.R., Carney, C., and Bergstrand, P., 1993, A Quaternary analog for interpretation of Mississippian oolites, in Ketih, B.D., and Zuppman, C.W., eds., Mississippian oolites of North America and their modern analogs: American Association of Petroleum Geologists, Studies in Geology, v. 35, Tulsa, OK, p. 227-241.

Boardman, M.R., McCartney, R.F., and Eaton, M.R., 1995, Bahamian Paleosols: Origin, relation to paleoclimate, and stratigraphic significance, in Curran, H.A., and White, B., eds. Terrestrial and shallow marine geology of the Bahamas and Bermuda: Geological Society of America Special Paper 300, p. 33-47.

Budd, D.A., 1988, Aragonite-to-calcite transformation during fresh-water diagenesis of carbonates: Insights from pore-water chemistry: Geological Society of America Bulletin, v. 3, p. 143-163.

Budd, D.A., and Land, L.S., 1990, Geochemical imprint of meteoric diagenesis in Holocene ooid sands, Schooner Cays, Bahamas: correlation of calcite cement

29 geochemistry with extant groundwaters: Journal of Sedimentary Petrology, v. 60, p.361-378.

Bukowski, J., 1996, A regional hydrogeologic study: Two-dimensional profile model of the hydrodynamics of the Pleistocene aquifer, North Andros Island, Bahamas: Unpubl. M.S. Thesis, Wright State University, Dayton, OH, 103 p.

Cant, R.V., and Weech, P.S., 1986, A review of the factors affecting the development of Ghyben-Hertzberg lenses in the Bahamas: Journal of Hydrology, v. 84, p. 333- 343.

Carney, C.K., and Boardman, M.R., 1991, Petrographic comparison of oolitic sediment from Joulters Cays and Andros Island, Bahamas, in Bain, ed., Proceedings of the fifth symposium on the geology of the Bahamas: Bahamian Field Station, San Salvador, Bahamas, p. 37-55.

Dacyk, P.I., 1996, Examination of diagenesis within an active meteoric/marine water mixing zone in a Pleistocene carbonate aquifer; North Andros Island, Bahamas. Unpubl. M.S. Thesis, Wright State University, Dayton, OH, 174 p.

Evan, C.C., and Ginsburg, R.N., 1987, Fabric-selective diagenesis in the Late Pleistocene Miami Limestone: Journal of Sedimentary Petrology, v. 57, p. 311-318.

Fitzgerald, T.M., 1997, Comparison of the depositional and diagenetic histories of two cores from Andros Island, Bahamas, Unpubl. M.S. Thesis, Wright State University, Dayton, OH, 97 p.

Halley, R.B. and Harris, P.M., 1979, Fresh-water cementation of a 1,000-year-old oolite: Journal of Sedimentary Petrology, v. 49, p. 969-988.

30 Hays, J.D., Imbrie,J., and Shackleton, N.J., 1976, Variations in the Earth’s orbit: Pacemaker of the ice ages: Science, v.194, p. 1121-1132.

Katz, A., Sass, E., Starinsky, A. and Holland, H.D., 1972, Strontium behavior in the aragonite-calcite transformation: An experimental study at 40-98o C: Geochimica et Cosmochimica Acta, v. 36, p. 481-496.

Little, B.G., Buckley, D.K., Jefferiss, A., Stark, J. and Young, R.N., 1973, Land resources of the Commonwealth of the Bahamas, v. 4a Andros Island: Land Resources Division, Tolworth Tower, Surry, England, KT6 7DY, p. 40-110.

McClain, M.E., Swart, P.K., and Vacher, H.L., 1992, The hydrogeochemistry of early meteoric diagenesis in a Holocene deposit of biogenic carbonates: Journal of Sedimentary Petrology, v. 62, p. 1008-1022.

McGavern, B.A., 1995, The Pleistocene oolite of North Andros Island, Bahamas: A model for depositional and diagenetic history: Unpubl. M.S. Thesis, Wright State University, Dayton, OH, 289 p.

Morse, J.W. and Mackenzie, F.T., 1990. Geochemistry of Sedimentary Carbonates. Elsevier, Amsterdam, 707 pp.

Newell, N.D., and Rigby, K., 1957, Geologic Studies of the Great Bahama Bank, in LeBlanc, R.J., and Breeding, J.G., eds., Regional aspects of carbonate deposition: Society of Economic Paleontologists and Mineralogists, Special Publication, v.5, p. 15-22.

Plummer, L.N., and others, 1976, Hydrogeochemistry of Bermuda: A case history of ground-water diagenesis of biocalcarentites: Geological Society of America Bulletin, v. 87, p. 1301-1316.

31

Saller, A.H., and Moore, C.H., 1991, Geochemistry of meteoric calcite cements in some Pleistocene limestones: Sedimentology, v. 38, p. 601-621.

Scoffin, T.P., 1987, An Introduction to Carbonate Sediments and Rocks, Blackie & Sons Limited, London, 274 p.

Sealey, N.E., 1985, Bahamian Landscapes: an Introduction to the Geology of the Bahamas: Collins Caribbean, London, 96 p.

Shinn, E.A., Lloyd, R.M., and Ginsburg, R.N., 1969, Anatomy of a modern carbonate tidal flat Andros Island Bahamas: Journal of Sedimentary Petrology, v. 30, p. 1202-1228.

Troska, M.R., 1991, Variability of lithologic characteristics of a Pleistocene ooid sand shoal, Andros Island, Bahamas: Links to the past: Unpubl. M.S. Thesis, Miami University, Oxford, OH, 124 p.

Tucker, M.E. and Wright, V.P., 1990, Carbonate sedimentology: Blackwell Scientific Publications, Oxford, 482 p.

Tucker, M., Wright, P., 1990, Carbonate Sedimentology: Blackwell Scientific Publications, Inc., Cambridge, MA, 482 p.

Vacher, H.L., and Ayers, J.F., 1980, Hydrology of small oceanic islands-utility of an estimate of recharge inferred from the chloride concentration of the freshwater lenses: Journal of Hydrology, v. 45, p. 21-37.

Vacher , H.L., Bengstsson, T.O. and Plummer, L.N., 1990, Hydrology of meteoric diagenesis - Residence time of meteoric ground water in island fresh-water lenses

32 with application to aragonite-calcite stabilization rate in Bermuda: Geological Society of America Bulletin, v.102, p. 223-232.

Vacher , H.L., and Wallis, T.N., 1992, Comparative hydrology of fresh-water lenses of Bermuda and Great Exuma Island, Bahamas: Groundwater, v.30, p. 15-20.

Whitaker, F.F., and Smart, P.L., 1997, Groundwater circulation and geochemistry of a karstified bank-marginal fracture system, South Andros Island, Bahamas: Journal of Hydrology, v. 197, p. 293-315.

33

Classification Total Dissolved Solids (mg/l) Freshwater 0 – 1,000 Brackish water 1,000 - 10,000 Saline water 10,000 - 100,000 Brine water More than 100,000

Table 1. Groundwater Classification Based on Total Dissolved Solids (Freeze and Cherry, 1979)

34

Well Depth (m) pH field Mg K Na Ca Sr Ocean values 1290 399 10760 411 8 9N skinny blue 3.1 8.200 12.1 3.1 83.3 73.5 2.2 9N black 4.1 7.283 11.8 3.0 78.6 71.8 2.1 9N skinny red 5.1 7.38 16.7 4.4 123.5 74.3 2.4 9N PVC 6.1 7.256 16.8 4.5 124.6 76.1 2.4

69 white 3.18 7.23 1.7 0.1 6.4 60.9 1.0 69 red 4.18 7.203 2.6 0.4 15.0 60.3 1.1 69 green 5.18 7.229 2.5 0.4 14.5 59.0 1.0 69 blue 6.18 7.23 4.3 0.8 29.0 63.5 1.2 69 brown 7.18 7.206 7.3 1.6 55.6 70.0 1.5 69 black 8.18 7.101 15.5 4.0 123.9 90.1 2.4 69 skinny red 9.18 7.123 12.6 3.2 94.2 84.3 2.1 69 skinny blue 10.18 7.038 29.5 9.3 243.2 106.2 3.0 69 skinny green 11.18 6.791 64.2 32.7 628.2 152.0 3.4 69 PVC 12.18 7.137 283.8 109.8 2406.9 244.3 7.8

4SA red 2.75 7.344 1.4 0.3 5.8 63.3 1.0 4SA blue 3.75 7.135 2.1 0.4 10.4 68.1 1.2 4SA PVC 4.75 7.128 2.1 0.4 10.3 67.1 1.3

C2N orange 0.61 7.636 2.4 0.6 7.1 33.8 0.6 C2N white 1.61 7.549 2.5 0.6 7.4 34.9 0.6 C2N red 2.61 7.465 2.6 0.5 7.0 35.3 0.6 C2N green 3.61 7.427 2.3 0.5 8.8 43.0 0.7 C2N blue 4.61 7.431 2.5 0.5 8.4 39.2 0.6 C2N brown 5.61 7.48 2.5 0.5 8.7 40.9 0.7 C2N black 6.61 7.385 3.0 0.8 16.3 49.6 0.9 C2N PVC 7.71 7.423 2.1 0.7 11.5 39.1 0.6

4NN red 5.28 7.548 2.6 0.8 17.9 41.1 1.4 4NN green 6.28 7.416 4.0 1.0 22.2 54.5 2.2 4NN blue 7.28 7.252 1.9 0.3 9.0 65.1 2.6 4NN brown 8.28 7.302 3.8 0.8 20.2 64.7 4.2 4NN black 9.28 7.254 15.2 4.1 111.2 82.6 2.8 4NN PVC 10.28 7.306 26.2 6.9 195.3 90.1 2.3

FW Average FW average FW average FW average FW Average FW average 7.343 7.3 1.9 50.7 62.1 1.6

Table 2. Measured Constituents in the Groundwater.

35 % Seawater Well F Cl Br NO3 SO4 (from Cl-) Ocean values 1.3 19350 67 1 2710 100.0 9N skinny blue 0.0 148.3 0.3 29.5 16.1 0.8 9N black 0.0 148.6 0.3 31.9 15.7 0.8 9N skinny red 0.0 230.7 0.5 41.3 20.9 1.2 9N PVC 0.0 236.4 0.5 48.0 21.5 1.2

69 white 0.1 9.3 0.0 1.3 2.5 0.0 69 red 0.1 21.9 0.0 1.5 4.7 0.1 69 green 0.1 24.0 0.1 1.4 4.6 0.1 69 blue 0.1 52.6 0.1 1.6 7.7 0.3 69 brown 0.0 112.4 0.3 6.4 11.3 0.6 69 black 0.0 229.4 0.5 7.1 19.6 1.2 69 skinny red 0.0 175.7 0.4 28.1 15.2 0.9 69 skinny blue 0.0 453.8 1.0 6.4 28.8 2.3 69 skinny green 0.0 935.4 2.2 0.0 64.4 4.8 69 PVC 0.0 3961.8 8.9 0.0 310.6 20.5

4SA red 0.1 6.0 0.0 0.3 2.5 0.0 4SA blue 0.1 15.8 0.0 0.5 3.2 0.1 4SA PVC 0.1 16.0 0.0 0.4 3.3 0.1

C2N orange 0.0 13.1 0.0 0.0 1.9 0.1 C2N white 0.0 12.5 0.0 0.0 1.8 0.1 C2N red 0.1 12.1 0.0 0.1 1.8 0.1 C2N green 0.1 14.6 0.0 0.1 2.0 0.1 C2N blue 0.1 14.8 0.0 0.0 1.9 0.1 C2N brown 0.1 15.9 0.0 0.1 2.2 0.1 C2N black 0.1 30.5 0.1 0.2 3.9 0.2 C2N PVC 0.0 23.2 0.0 0.2 1.8 0.1

4NN red 0.2 18.6 0.0 0.2 5.3 0.1 4NN green 0.4 29.4 0.1 0.3 7.6 0.2 4NN blue 0.4 13.7 0.0 0.6 4.0 0.1 4NN brown 0.7 35.3 0.1 0.5 8.6 0.2 4NN black 0.0 202.2 0.4 1.9 20.8 1.0 4NN PVC 0.0 344.0 0.7 3.5 32.4 1.8

FW Average FW Average FW Average FW Average FW Average 0.1 91.8 0.2 7.4 9.4

Table 2 Continued. Measured Constituents in the Groundwater.

36

Well Depth (m) pH Field expected HCO3 mM pCO2 (Wateq)

9N skinny blue 3.10 8.200 3.881 -2.824 9N black 4.10 7.283 3.570 -1.918 9N skinny red 5.10 7.380 3.652 -1.992 9N PVC 6.10 7.256 3.624 -1.873

69 white 3.18 7.230 3.145 -1.951 69 red 4.18 7.203 3.168 -1.915 69 green 5.18 7.229 3.017 -1.964 69 blue 6.18 7.230 3.159 -1.938 69 brown 7.18 7.206 3.166 -1.902 69 black 8.18 7.101 4.433 -1.640 69 skinny red 9.18 7.123 4.184 -1.691 69 skinny blue 10.18 7.038 5.208 -1.487

4SA red 2.75 7.344 3.331 -2.039 4SA blue 3.75 7.135 3.531 -1.812 4SA PVC 4.75 7.128 3.478 -1.808

C2N orange 0.61 7.636 1.811 -2.581 C2N white 1.61 7.549 1.904 -2.477 C2N red 2.61 7.465 1.926 -2.479 C2N green 3.61 7.427 2.288 -2.281 C2N blue 4.61 7.431 2.091 -2.318 C2N brown 5.61 7.480 2.151 -2.357 C2N black 6.61 7.385 2.518 -2.197 C2N PVC 7.71 7.423 1.960 -2.331

4NN red 5.28 7.548 2.450 -2.372 4NN green 6.28 7.416 3.083 -2.145 4NN blue 7.28 7.252 3.359 -1.943 4NN brown 8.28 7.302 3.316 -1.994 4NN black 9.28 7.254 4.231 -1.813 4NN PVC 10.28 7.306 4.987 -1.789

Table 3. Measured and Calculated Constituents in the Groundwater.

37

log IAP/K log IAP/K cal log IAP/K dol (d) log IAP/K Well Depth (m) wateq arag wateq wateq dol wateq

9N skinny blue 3.1 0.960 0.820 0.950 1.500 9N black 4.1 0.03 -0.110 -0.920 -0.370 9N skinny red 5.1 0.15 0.010 -0.540 0.010 9N PVC 6.1 0.04 -0.100 -0.770 -0.220

69 white 3.18 -0.12 -0.260 -1.990 -1.440 69 red 4.18 -1.53 -0.300 -1.870 -1.320 69 green 5.18 -0.15 -0.300 -1.880 -1.330 69 blue 6.18 -0.11 -0.250 -1.580 -1.030 69 brown 7.18 -0.09 -0.230 -1.350 -0.800 69 black 8.18 0.02 -0.120 -0.920 -0.370 69 skinny red 9.18 -0.01 -0.150 -1.030 -0.480 69 skinny blue 10.18 0.08 -0.060 -0.590 -0.040 69 skinny green 11.18 0.26 0.110 -0.050 0.500 69 PVC 12.18 0.93 0.790 1.750 2.300

4SA red 2.75 0.03 -0.120 -1.790 -1.240 4SA blue 3.75 -0.12 -0.270 -1.960 -1.410 4SA PVC 4.75 -0.14 -0.290 -1.990 -1.440

C2N orange 0.61 -0.16 -0.300 -0.166 -1.110 C2N white 1.61 -0.22 -0.370 -1.800 -1.250 C2N red 2.61 -0.29 -0.440 -1.920 -1.370 C2N green 3.61 -0.18 -0.330 -1.830 -1.280 C2N blue 4.61 -0.26 -0.400 -1.910 -1.360 C2N brown 5.61 -0.18 -0.320 -1.780 -1.230 C2N black 6.61 -0.13 -0.270 -1.690 -1.140 C2N PVC 7.71 -0.29 -0.440 -2.050 -1.500

4NN red 5.28 -0.06 -0.210 -1.520 -0.970 4NN green 6.28 0.01 -0.130 -1.310 -0.760 4NN blue 7.28 -0.05 -0.190 -1.840 -1.290 4NN brown 8.28 -0.01 -0.160 -1.460 -0.910 4NN black 9.28 -0.03 0.120 -0.700 -0.150 4NN PVC 10.28 0.26 0.120 -0.210 0.340

Table 4. Calculated IAP/K’s in the Groundwater.

38

Chemical Constituent Concentration (ppm) Cl- 0.40 NO3- 4.78 SO4= 0.16 Na+ < 2 Ca++ < 2 Mg++ < 1

Table 5. Average rainwater composition.

39

Rock Depth Mg ppm Na ppm Ca ppm Sr ppm

4sa3 2.25 974 1170 386893 5114 4sa4 2.3 896 1066 392750 4986 4sa5 2.5 817 1131 398559 4994 4sa6 2.7 695 1046 393971 4864 4sa7 2.95 675 1005 397332 4774 4sa8 2.3 864 892 403339 4467 4sa9 3.4 773 991 398274 4761 4sa10 3.5 800 938 398782 4576 4sa11 3.6 1005 522 405545 3724 4sa12 3.7 989 676 400087 4260 4sa13 3.9 1107 583 390728 3725 4sa14 4 942 591 372593 3673 4sa15 4.1 1003 825 374212 4237

69-20 1.6 955 1292 386779 5492 69-25 1.8 1807 0 371106 1237 69-78 5 1441 549 385544 3524 69-95 6.2 1164 829 395859 3933 69-116 7.25 2502 0 407501 1602 69-122 7.3 1909 0 402795 1340 69-129 8 1879 0 406069 1503 69-139 9.2 1429 0 412690 1027 69-159 11.3 1493 0 383431 1099 69-173 11.8 1455 0 389847 893 69-179 12.2 1453 0 388596 1547

Table 6. Measured Constituents in the Rock.

40

Rock Depth Mg ppm Na ppm Ca ppm Sr ppm

c2n1 0.2 1043 879 399238 4218 c2n3 0.45 947 926 407211 4150 c2n6 0.7 1325 799 406863 3901 c2n7 0.8 1226 959 397580 4358 c2n16 1.25 950 938 396274 4541 c2n22 1.75 979 716 392114 3810 c2n35 2.75 908 798 393683 4271 c2n40 3.5 906 755 409654 4312 c2n54 4.4 1119 711 396660 4532 c2n57 4.7 1188 662 398899 4273 c2n59 4.8 1317 598 401068 4192 c2n60 5 1257 418 388371 3772 c2n62 5.25 1013 303 385014 2970 c2n68 5.5 1288 404 391963 3857 c2n 74 5.75 972 255 386034 3866 c2n 76 6 2509 0 410112 1648 c2n 77 6.1 4836 0 408988 1387 c2n 79 6.4 2772 0 399088 1611 c2n 80 6.6 1719 0 382125 1424 c2n 82 7 3221 437 388089 3308 c2n 89 7.5 3262 462 380707 3376 c2n 96 8 2911 369 392464 3064 c2n 110 8.6 2221 328 403926 2862 c2n 114 8.9 2281 235 401540 2418 c2n 120 9.5 1668 206 399413 2158 c2n 130 10.1 2397 401 385495 3144 c2n 137 10.7 1203 0 426539 2275 c2n 139 10.9 1344 0 434021 1940 c2n 144 11.2 1910 207 388836 1095 c2n 145 11.4 2109 0 419259 1029 c2n 149 11.7 2166 0 382830 1471 c2n 152 12.4 2020 0 395015 1482 c2n 154 12.5 1952 0 381639 1448 c2n 156 12.6 1793 236 373935 947

4na top 1480 0 428930 2100 4na mid 1401 0 427003 1913 4na bot 1425 0 427078 1722

Table 6 Continued. Measured Constituents in the Rock.

41

Well Depth (m) Precipitation Dissolution (mM/l) (mM/l) 9N skinny blue 3.1 0.8 2.7 9N black 4.1 0.8 2.6 9N skinny red 5.1 1.1 3.1 9N PVC 6.1 1.0 3.0

69 white 3.18 0.0 1.3 69 red 4.18 0.0 1.3 69 green 5.18 0.0 1.3 69 blue 6.18 0.0 1.4

4SA red 2.75 0.0 1.2 4SA blue 3.75 0.0 1.5 4SA PVC 4.75 0.0 1.5

C2N orange 0.61 0.0 0.7 C2N white 1.61 0.0 0.7 C2N red 2.61 0.0 0.7 C2N green 3.61 0.0 0.9 C2N blue 4.61 0.0 0.8 C2N brown 5.61 0.0 0.8 C2N black 6.61 0.0 1.0

4NN red 5.28 0.7 1.8 4NN green 6.28 1.3 2.8

Table 7. Calculated Values for Precipitation and Dissolution

42 Stabilization Rate (cm3/m3/ x years) Well Depth for 1 year for 2 years for 3 years (m) 9N skinny blue 3.1 184 92 61 9N black 4.1 180 90 60 9N skinny red 5.1 210 105 70 9N PVC 6.1 207 104 69

69 white 3.18 86 43 29 69 red 4.18 91 46 30 69 green 5.18 86 43 29 69 blue 6.18 99 49 33

4SA red 2.75 85 42 28 4SA blue 3.75 101 51 34 4SA PVC 4.75 104 52 35

C2N orange 0.61 49 25 16 C2N white 1.61 51 25 17 C2N red 2.61 48 24 16 C2N green 3.61 62 31 21 C2N blue 4.61 53 27 18 C2N brown 5.61 56 28 19 C2N black 6.61 70 35 23

4NN red 5.28 123 62 41 4NN green 6.28 190 95 63

Table 8. Stabilization Rates.

43

Location Rate of Stabilization Age of Deposit Carbonate Mineral (cm3/m3/year) (yrs. old) Assemblage Joulters Cay 61 670 High-Sr aragonite Schooner Cay 49 700 High-Sr aragonite Bermuda 8.1 80-250,000 Ocean Bight 106 < 2000 High-Mg calcite & High-Sr aragonite North Andros 53 125,000 High-Sr aragonite

Table 9. Stabilization Rates of Related Carbonate Environments.

44

Location Porosity (n) % Aragonite Half-life of Aragonite Remaining Today (yrs) Joulters Cay 0.45 92.5 6,000 Schooner Cay 0.4 90 4,852 Bermuda 0.2 10 6,900 Ocean Bight 0.3 80 3,662 North Andros 0.2 60 6,278

Table 10. Half-lives of Aragonite Stabilization

45

Meteoric Environment

Vadose Zone

Freshwater Phreatic Zone <1,000 TDS Mixing Zone 1,000-10,000 TDS

Marine Phreatic Zone >10,000 TDS

Figure 1. Meteoric Environment.

46

North Florida

100

Northern Water Andros Island depth < 20 m

San Salvado

Great Baham a

Figure 2. Map of the Bahamas.

47

Pleistocene Sea-Level Curve

0 MSL

)

m

(

l 30 e

v

e

l

-

a

e

s

60 n

a

e

m

w o 90

l e

b

h

t p e 120 D Sangamon Wisconsin Illinoian Glacial Interglacial Glacial

0 50000 100000 150000

Years before present

Figure 3. Pleistocene Sea-level Curve.

48

Sea-level History of Northeastern Andros

dune Pleistocene 125 ky +6 132,000 to 120,000 ybp reef sea-level

c

ii Pleistocene highstand t shoal

a

e (oxygen isotope stage 5e)

r present

0 h

p sea-level

e

n

ii

r Earlier a Pleistocene -6 m

+6 cemented shoal 120,000 to 3000 ybp

e

n complex Sea-level remained lower o present

z 0 than present sea-level. e sea-level s Maximum lowering was

o

d probably ~100m lower

a

v than present. -6 Late Pleistocene sea-level

+6

e

s

o cemented shoal

d In the Holocene, sea-level

a complex v present rose to within 6 m of the 0 sea-level Pleistocene highstand

c

ii t where it remains today.

a

e r (Boardman et. al., 1993)

h

-6 p

Subsidence History paleo subsidence +6 dune palea reef paleo shoal present North Andros has 0 sea-level subsided approximately

c

i

t 3 m in 125 ky. Paleo sea-

a

e Earlier

r level is about 3 m above h Pleistocene -6 p present sea-level.

( difi d f T k 1992)

Figure 4. Sea-level History of Northeastern Andros (modified from Troska, 1991).

49

Ghyben-Herzberg Principle for a Coastal Aquifer

Land Surface

Water Table

h Ocean

Fresh Marine groundwater groundwater 3 Density= ρ Density= ρw =1.000 g/cm 1.025g/cm

ρ z = w Z h ρ s − ρw

Figure 5. Ghyben-Herzberg Principle for a Coastal Aquifer.

50

Tongue of the Ocean

9 Field Area 1

4N C2 9 4S 6 1 CB

N

1000 m

Figure 6. Map of North Andros Island Field Area.

51

N 4NN

C2N 9N

Charlies Blue Hole Charlie's Road

4SA 69

Well Locations df f Main Road

Logging Road .

Figure 7. Map of North Andros Island Study Area

52

Nylon tubing

Inlet holes

PVC

Figure 8. Design of the Multi-level Sampler.

53

Fine Sand

Gravel

Fine Sand

Gravel

Fine Sand

Gravel

Fine Sand

Figure 9. Packing Sequence of the Multi-level Sampler.

54

Well 69 0

Bentonite 1

2 Gravel 2.18 ) Bentonite m

(

3 Gravel 3.18

e c Bentonite a 4 f Gravel 4.18

r Bentonite u

s

5 Gravel 5.18

d Sand

n a 6 Gravel 6.18 l

Sand w o 7 Gravel 7.18

l e Sand

b 8 Gravel 8.18

h t Sand p e 9 Gravel 9.18

D Sand

10 Gravel 10.18 Sand Gravel 11 11.18 Mud 12

13 Figure 10. Well 69 Multi-level Sampler

55

Well C2N

0 Gravel Sand Gravel .61 1 Sand

Gravel 1.61

2 Sand

Gravel 2.61 Depth below 3 Sand land surface Gravel 3.61 (m) 4 Sand Gravel 4.61 5 Sand Gravel 5.61 6 Sand Gravel 6.61 7 Sand Gravel

Figure 11. Well C2N Multi-level Sampler

56 Well 9N

0 Gravel Sand Gravel .61 Sand 1 Gravel 1.61 2 Sand

Gravel 2.61 Depth below 3 Sand land surface Gravel 3.61 (m) 4 Sand

Gravel 4.61 5 Sand Gravel 5.61 6 Sand Gravel 6.61 7 Sand Gravel

Figure 12. Well 9N Multi-level Sampler

57

Well 4NN

0

1

Bentonite

2

)

m Sand

(

3 Gravel 3.28

e c Sand a 4 f Gravel 4.28

r

u Sand

s

5 Gravel 5.28

d

n Sand a 6 l Gravel 6.28

w Sand o 7 7.28 l Gravel

e Sand b 8 Gravel h 8.28

t Sand p

e 9 Gravel Sand 9.28

D Gravel 10

11

12

13

Figure 13. Well 4NN Multi-level Sampler

58

Well 4SA 0 Bentonite

1 Sand

Gravel 1.75

2 Sand ) Gravel 2.75 m

(

3 Sand

e Gravel c 3.75 a 4 f Sand r Gravel

u

s

5

d n a 6

l

w o 7 l

e b 8 h

t p e 9

D

10

11

12

13 Figure 14. Well 4SA Multi-level Sampler.

59 Cl- vs. Depth

0

2.5

5

Depth (m) below ground surface 7.5

10

12.5 0 100 200 300 400 500

9N Cl-(ppm)

69

4SA

C2N

4NN

Figure 15. Graph of Chloride in the Groundwater with Depth

60

= SO4 vs. Depth

0

2.5

5

Depth (m) below ground surface 7.5

10

12.5 0 10203040

= 9N SO4 (ppm) 69

4SA

C2N

4NN

Figure 16. Graph of Sulphate in the Groundwater with Depth

61

Ca++ vs. Depth

0

2.5

5

) below ground surface

Depth (m 7.5

10

12.5 0 25 50 75 100 125

9N Ca++ (ppm) 69 4SA C2N 4NN

Figure 17. Graph of Calcium in the Groundwater with Depth

62

Sr++ vs. Depth

0

1

2

3

4

5

6

7 Depth (m) below ground surface

8

9

10

11

12

012345

9N Sr++ (ppm) 69 4SA

C2N 4NN

Figure 18. Graph of Strontium in the Groundwater with Depth

63

Na+ vs. Depth

0

2.5

e c

5

ground surfa low

) be pth (m e

D 7.5

10

12.5 0 50 100 150 200 250

9N Na+ (ppm)

69 4SA

C2N

4NN

Figure 19. Graph of Sodium in the Groundwater with Depth

64

Mg++ vs. Depth

0

2.5

5

Depth (m) below ground surface 7.5

10

12.5 0 5 10 15 20 25 30

9N Mg++ (ppm) 69

4SA

C2N 4N

Figure 20. Graph of Magnesium in the Groundwater with Depth

65

K+ vs. Depth

0

2.5

5

Depth (m) below ground surface 7.5

10

12.5 0 2.5 5 7.5 10

K+ (ppm) 9N

69

4SA C2N

4NN

Figure 21. Graph of Potassium in the Groundwater with Depth

66

Alkalinity vs. Depth

0

2.5

5

Depth (m) below ground surface 7.5

10

12.5 123456

- 9N Alkalinity (as HCO 3) 69

4SA C2N

4NN

Figure 22. Graph of Alkalinity in the Groundwater with Depth

67

pH vs. Depth

0

2.5

ace f r u 5 ound s

ow gr bel ) m h (

Dept 7.5

10

12.5 7 7.5 8 8.5

pH 9N

69 4SA

C2N

4NN

Figure 23. Graph of pH in the Groundwater with Depth

68 log IAP/KT for Aragonite

0

1

2

3

4

5

6

Depth (m) below ground surface 7

8

9

10

11

12 -2 -1.5 -1 -0.5 0 0.5 1

log IAP/KT 9N

69 4SA

C2N

4NN

Figure 24. Graph of log IAP/KT for Aragonite in the Groundwater with Depth

69

log IAP/KT for Calcite

0

1

2

3

4

5

6

Depth (m) below ground surface 7

8

9

10

11

12 -3 -2 -1 0 1

log IAP/KT 9N 69

4SA

C2N 4NN

Figure 25. Graph of log IAP/KT for Calcite in the Groundwater with Depth.

70

Calcium vs. Depth

0

1

2

3

4 ace

f

5

ound sur

6 below gr ) m 7 Depth ( 8

9

10

11

12

300000 350000 400000 450000

Ca++ (ppm) in the rocks 4SA 69

C2N

Figure 26. Graph of Calcium in the Rocks with Depth.

71

Strontium vs. Depth

0

1

2

3

4

5

6

7 Depth (m) below ground surface 8

9

10

11

12

0 2500 5000 7500 10000

Sr++ (ppm) in the rock 4SA 69

C2N

Figure 27. Graph of Strontium in the Rocks with Depth.

72

Magnesium vs. Depth

0

1

2

3

4

5

6

7 Depth (m) below ground surface 8

9

10

11

12

0 2500 5000 7500 10000

Mg++(ppm) in the rock 4SA

69

C2N

Figure 28. Graph of Magnesium in the Rocks with Depth.

73

+ - K vs. Cl

12.5

10

7.5

(ppm)

+ K

5

2.5

0 0 100 200 300 400 500

- Cl (ppm)

Water samples Diluted seawater

Figure 29. Graph of Potassium versus Chloride in the Groundwater.

74

Na+ vs. Cl-

300

250

200

150

(ppm) +

Na

100

50

0 0 100 200 300 400 500

Cl-(ppm)

Water samples

Diluted seawater

Figure 30. Graph of Sodium versus Chloride in the Groundwater.

75

++ - Mg vs. Cl

40

30

(ppm) 20

++

Mg

10

0 0 100 200 300 400 500

Cl- (ppm)

Water samples

Diluted seawater

Figure 31. Graph of Magnesium versus Chloride in the Groundwater.

76

++ - Ca vs. Cl

125

100

75

ppm) (

++ Ca 50

25

0 0 100 200 300 400 500

- Cl (ppm)

Water samples Diluted seawater

Figure 32. Graph of Calcium versus Chloride in the Groundwater.

77

Sr++ vs. Cl-

5

4

3

(ppm)

++ Sr 2

1

0 0 100 200 300 400 500

Cl-(ppm)

Water samples

Diluted seawater

Figure 33. Graph of Strontium versus Chloride in the Groundwater

78

= - SO4 vs. Cl

80

60

40 (ppm)

= 4

SO

20

0 0 100 200 300 400 500

Cl- (ppm)

Water samples

Diluted seawater

Figure 34. Graph of Sulphate versus Chloride in the Groundwater.

79 Mg++(excess) vs. Depth

0

2.5

5

Depth (m) below ground surface 7.5

10

12.5 -101234

9N Mg++ excess (ppm) 69

4SA

C2N

4NN

Figure 35. Graph of Reaction-derived Magnesium in the Groundwater with Depth

80

K+ (excess) vs. Depth

0

2.5

5

Depth (m) below ground surface 7.5

10

12.5 -1 -0.5 0 0.5

+ 9N K excess (ppm) 69 4SA

C2N

4NN

Figure 36. Graph of Reaction-derived Potassium in the Groundwater with Depth

81

Na+ (excess) vs. Depth

0

2.5

5 w ground surface o

h (m) bel

Dept 7.5

10

12.5 -10 -5 0 5 10

9N Na+ excess (ppm) 69

4SA C2n

4NN

Figure 37. Graph of Reaction-derived Sodium in the Groundwater with Depth

82

= SO4 (excess) vs. Depth

0

2.5

5

Depth (m) below ground surface 7.5

10

12.5 -0.4 -0.3 -0.2 -0.1 0 0.1

= 9N SO4 excess(ppm) 69

4SA C2N

4NN

Figure 38. Graph of Reaction-derived Sulphate in the Groundwater with Depth

83

Ca++(excess) vs. Depth

0

2.5

5

Depth (m) below groumd surface 7.5

10

12.5 20 40 60 80 100

9N Ca++ excess (ppm) 69

4SA C2N

4NN

Figure 39. Graph of Reaction-derived Calcium in the Groundwater with Depth

84 Sr++(excess) vs. Depth

0

2.5

5

Depth (m) below ground surface 7.5

10

12.5 012345

9N Sr++ excess (ppm) 69

4SA

C2N

4NN

Figure 40. Graph of Reaction-derived Strontium in the Groundwater with Depth

85

Sr++ versus Ca++

3

Joulters 2.5 Ooids

Ca/Sr=44

2

(ppm)

++

Sr 1.5

Bulk

1 Rock Ca/Sr=104

0.5 Calcite Ca/Sr=200

0 0 20 40 60 80 100 120 140

Ca++ (ppm) Bulk rock (60% aragonite)

Freshwater samples

Ooid line (100% aragonite) Calcite line (0% aragonite)

Figure 41. Graph of Strontium versus Calcium in the Groundwater

86

10,000 Ooids Sr is found abundantly in ooids Ca/Sr = 40

8,000

6,000

4,000

ppn’t Calcite 2,000 Ca/Sr = 200 Sr does not fit into calcite lattice

400,000 200,000 Ca (ppm)

Figure 42. Graph of Calcium and Strontium Ratios in Carbonate Rocks.

87

1

Amount of remaining Aragonite (log scale)

Joulters

Schooners Ocean Bight

0.1 0 1000 2000 3000 4000 5000 6000 Duration of Phreatic Diagenesis (yr)

Figure 43. Graph of Remaining Aragonite versus the Duration of Diagenesis.

88