science for a changing world

Ground-Water Geochemistry at the South Ramp 

Jet Fuel Leak, Diego Garcia Atoll, 1993-94

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigation Report 01-4090

May 1994

Prepared in Cooperation with the

U.S. DEPARTMENT OF THE NAVY

NAVY SUPPORT FACILITY, DIEGO GARCIA

GROUND-WATER GEOCHEMISTRY AT THE SOUTH RAMP 

JET FUEL LEAK, DIEGO GARCIA ATOLL, 1993-94

By Gordon W. Tribble

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 01-4090

Prepared in cooperation with the

U.S. DEPARTMENT OF THE NAVY

NAVY SUPPORT FACILITY, DIEGO GARCIA

Honolulu, Hawaii 

2001
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey.

For additional information write to:

District Chief

U.S. Geological Survey

677 Ala Moana Blvd., Suite 415

Honolulu, HI 96813

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286
CONTENTS

Abstract.......................................................................................... 1

Introduction....................................................................................... 1

Purpose and Scope ............................................................................ 2

Setting...................................................................................... 2

Previous Investigations......................................................................... 2

Description of the Study Area......................................................................... 5

Geology .................................................................................... 5

Hydrology................................................................................... 5

Methods of Data Collection and Analysis................................................................ 5

Installation and Sampling of Monitor Wells, Lysimeters, and Vapor Points ................................ 6

S-Series Wells.......................................................................... 6

C-Series Wells ......................................................................... 6

BW21 Wells ........................................................................... 7

Field Analysis and Sample Treatment....................................................;........ 7

Laboratory Methods........................................................................... 8

Geochemical Calculations ...................................................................... 9

Geochemistry of Ground Water, Gases in the Unsaturated Zone, and Fuel Hydrocarbons .......................... 10

Chemical Overview ........................................................................... 10

Concentration of Gases in the Unsaturated Zone..................................................... 10

Major-Dissolved Ions.......................................................................... 13

Nutrients and Organic Carbon ................................................................... 13

Assessment of Fuel Contamination in the Aquifer ................................................... 15

Hydrocarbons in Ground Water and Soil Vapor. ..................................................... 18

Evaluation of Microbial Respiration and Fuel Degradation .................................................. 24

Excess DIC as an Indicator of Organic Respiration................................................... 24

Mechanisms of Microbial Oxidation .............................................................. 29

Stormwater Receiving Pond..................................................................... 30

Degradation of Aromatic Hydrocarbons ........................................................... 30

Summary and Conclusions ........................................................................... 33

References Cited ................................................................................... 34

Figures

1-2. Maps showing:

1. Diego Garcia Atoll in the Indian Ocean and location of the study area ............................ 3

2. South Ramp and locations of fuel leak and monitoring wells, Diego Garcia Atoll, 1993-94............ 4

3. Diagram of carbon dioxide (CO2) and methane (CH4) concentrations in the Unsaturated zone of the aquifer near

the South Ramp in May 1994, as a percentage of total gas concentration, Diego Garcia Atoll, 1993-94 .. 12

4-9. Graphs showing:

4. Concentrations of dissolved sodium (Na+), dissolved potassium (K+), and dissolved sulfate (SO42~)

compared with dissolved chloride (CL~) in ground-water samples from S-series wells and lysimeters

in February 1993, August 1993, and May 1994, Diego Garcia Atoll............................ 14

Contents iii
5. Concentrations of dissolved calcium (Ca ), dissolved magnesium (Mg ), and dissolved strontium (Sr2*) 

compared with dissolved chloride (Cl~) in ground-water samples from S-series wells and lysimeters 

in February 1993, August 1993, and May 1994, Diego Garcia Atoll. ........................... 14

6. Concentrations of dissolved phosphate (PC^3"1") and dissolved silica (Si) compared with total dissolved 

inorganic nitrogen (NO3~+NH4+) in ground-water samples from S-series wells and lysimeters in 

February 1993, August 1993, and May 1994, Diego Garcia Atoll.............................. 16

7. Concentrations of dissolved organic phosphorus (DOP) and dissolved organic nitrogen (DON) compared 

with dissolved organic carbon (DOC) in ground-water samples from S-series wells and lysimeters in 

February 1993, August 1993, and May 1994, Diego Garcia Atoll.............................. 16

8. Relation between dissolved organic carbon (DOC) to dissolved organic phosphorus (DOP) and dissolved 

organic carbon (DOC) to dissolved organic nitrogen (DON) compared with dissolved organic carbon 

(DOC) in ground-water samples from S-series wells and lysimeters in February 1993, August 1993, 

and May 1994, Diego Garcia Atoll...................................................... 17

9. Relation between benzene and dissolved organic carbon (DOC) in ground-water samples from S-series

wells in February 1993, August 1993, and May 1994, Diego Garcia Atoll....................... 20

10. Hydrologic sections of benzene concentration at well sites SI, S2, S3, and S5 in February 1993, August 1993,

and May 1994, Diego Garcia Atoll ........................................................ 21

11-12. Graphs showing:

11. Distribution of charge balance calculated using WATEQ4F for ground-water samples collected in February

1993, August 1993, and May 1994, Diego Garcia Atoll ..................................... 27

12. Relation between dissolved inorganic carbon (DIC) released from carbonate mineral dissolution and 

dissolved inorganic carbon (DIC) released from microbial respiration for ground-water samples 

collected in February 1993, August 1993, and May 1994, Diego Garcia Atoll.................... 27

13. Hydrologic section of dissolved inorganic carbon concentration at well sites SI, S2, S3, and S5 in May

1994, Diego Garcia Atoll................................................................ 28

Tables

1. Analytical precision, accuracy, and detection limits for selected chemical constituents in ground water from

Diego Garcia Atoll, 1993-94 ................................................................. 8

2. Field data and concentrations of dissolved hydrogen sulfide and oxygen in ground-water, rain, seawater,

and pond samples, Diego Garcia Atoll.......................................................... 36

3. Concentration of oxygen, carbon dioxide, and methane in vapor samples from the unsaturated zone, Diego

Garcia Atoll .............................................................................. 11

4. Concentrations of major dissolved ions in ground-water, rain, seawater, and pond samples, Diego Garcia

Atoll .................................................................................... 39

5. Concentrations of dissolved inorganic and organic nutrients in ground-water, rain, seawater, and pond samples,

Diego Garcia Atoll ......................................................................... 42

6. Approximate thickness of free-phase fuel at South Ramp wells and recovery pits, Diego Garcia Atoll.......... 19

7. Concentrations of benzene, dissolved organic carbon, oil and grease, and total petroleum hydrocarbons in

February 1993, Diego Garcia Atoll ............................................................ 19

8. Concentrations of volatile organic carbon compounds in vapor and water samples, Diego Garcia Atoll ......... 22

9. Concentrations of selected volatile organic carbon compounds found in ground water, Diego Garcia Atoll ...... 25

10. Charge balance, saturation index with respect to aragonite, and total dissolved inorganic carbon in ground-water

samples, Diego Garcia Atoll.................................................................. 45

11. Concentrations and relative percentage of three major groups of aromatic hydrocarbons in ground water, Diego

Garcia Atoll .............................................................................. 31

iv The Response of the lao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices, Maui, Hawaii
Conversion Factors

Multiply

foot (ft)

gallon (gal)

gallon per minute (gal/min)

million gallons per day (Mgal/d)

mile (mi)

inch (in.)

By

0.3048

0.003785

0.06308

0.04381

1.609

25.4

To obtain

meter

cubic meter

liter per second

cubic meter per second

kilometer

millimeter

Temperature is given in degrees Celsius ( Q, which can be converted to degrees Fahrenheit ( F) by using the equation:

°F = 1.8x°C + 32

Abbreviations used in water quality descriptions: 

DIG, dissolved inorganic carbon 

DOC, dissolved organic carbon 

DON, dissolved organic nitrogen 

DOP, dissolved organic phosphorus 

GC, gas chromatography

GC-MS, gas chromatography - mass spectroscopy 

IR, infrared

meq/L, milliequivalents per liter 

mg/L, milligrams per liter 

mM, millimoles per liter 

ppb, parts per billion 

ppm, parts per million 

u,g/L, micrograms per liter 

u,m, micron

|iS/cm, microsiemens per centimeter at 25 Celsius 

TPH, total petroleum hydrocarbons 

UV, ultraviolet 

VOC, volatile organic carbon

Contents

Ground-Water Geochemistry at the South Ramp Jet Fuel 

Leak, Diego Garcia Atoll, 1993-94

By Gordon W. Tribble

Abstract

In 1991, a leak of jet fuel from an underground 

pipe near the South Ramp of Diego Garcia was 

detected. The leaked fuel contaminated part of the 

shallow aquifer and threatened several nearby production wells. To assess the extent of contamination and microbial degradation of fuel, a 

geochemical analysis was used to characterize the 

prevailing chemical reactions in the aquifer. Data 

for this study were collected from a network of 

wells, vapor points, and lysimeters in February 

1993, August 1993, and May 1994. Chemical processes were deduced through analysis of major 

ions, gases, nutrients, and fuel constituents. The 

study determined the total amount of microbial respiration. Dissolved gases such as oxygen, hydrogen sulfide, and methane were used to provide 

insight into the respiratory processes of bacteria in 

the aquifer. In contaminated areas, the ratios of 

specific contaminant compounds gave a quantitative measure of the extent of aromatic degradation. 

Ground-water samples were anoxic, and gas from 

the unsaturated zone had low concentrations of 

oxygen. Gas samples near the contaminated area 

had high concentrations of methane. Dissolution of 

carbonate minerals and microbial oxidation of 

organic material, as measured by changes in 

geochemical indicators, were the principal reactions in the aquifer. Compared with uncontaminated sites, both processes occurred to a much 

higher degree at contaminated sites. The high levels of bacterial respiration at contaminated sites, 

compared with uncontaminated sites, showed that

petroleum hydrocarbons were actively degraded by 

naturally occurring bacteria in the aquifer.

Petroleum hydrocarbons were found dissolved 

in ground water, as a vapor phase in the unsaturated 

zone, and floating as a free-phase layer on top of 

the water table. Despite the recovery of more than 

134,000 gallons of fuel, substantial quantities of 

fuel remain in the aquifer. Benzene concentrations 

have shown a steady increase in both ground water 

and soil vapor. The ratio of benzene to more persistent aromatic compounds such as naphthalene, trimethyl benzene, and tetramethyl benzene, 

increased with depth in the aquifer, indicating that 

significant amounts of benzene were not being 

degraded under the reducing conditions in the aquifer.

INTRODUCTION

Diego Garcia atoll is part of the British Indian 

Ocean Territory and is a support base for U.S. Navy 

operations in the Indian Ocean. There are about 3,500 

inhabitants on the island and the demand for freshwater 

averages 0.8 to 1.2 Mgal/d. All of this water is pumped 

from the ground.

In May 1991, a rupture was discovered in an 

underground pipeline carrying JP-5 jet fuel. Fuel from 

this leak contaminated the soil and ground water near 

one of the major sources of freshwater on the island. 

The volume of leaked fuel is unknown. Work was initiated to recover free-phase fuel, retard the spread of fuel 

and contaminated ground water, and remediate the contaminated parts of the aquifer. The role of the U.S. Geological Survey (USGS) was to: (1) conduct computer 

simulations to aid in the containment of the contaminant

Abstract 1
plume and design of a fuel-recovery system; (2) monitor aquifer water levels to determine the direction of 

ground-water flow under different pumping scenarios; 

and (3) provide a characterization of chemical conditions and processes in both contaminated and uncontaminated parts of the aquifer.

Purpose and Scope

This report is part of a program to provide technical assistance to the U.S. Navy for remediation of the 

South Ramp fuel leak. This project started in June 1991 

with a site visit and preliminary characterization following discovery of the spill (C.D. Hunt, Jr., USGS, written 

commun., 1991). Since 1991, trips to Diego Garcia 

have been conducted to install wells for collecting water 

samples and monitoring water levels, service the waterlevel recording equipment, and conduct water-chemistry studies. The data in this report were collected during 

three trips in January and February 1993, August 1993, 

and May 1994.

The purpose of .this report is to describe the chemical characteristics of ground water near the South 

Ramp of Diego Garcia Atoll. This characterization 

includes the overall chemical composition of ground 

water in the shallow aquifer, the extent of chemical 

reactions in the aquifer, and factors that influence the 

breakdown of fuel contaminants dissolved in ground 

water. The evaluation of microbial activity and fuel 

degradation can assist in the selection of appropriate 

remediation methods following the removal of freephase fuel.

A general description of the water chemistry in the 

aquifer near the South Ramp is obtained from analysis 

of the ions and gases dissolved in ground water. Major 

reactions in the aquifer are deduced from chemical analysis of water and gas from contaminated and uncontaminated parts of the aquifer. A major component of the 

study is estimating, through increases in the amount of 

dissolved inorganic carbon (DIG), the amount of respiration that has occurred in contaminated and uncontaminated parts of the aquifer. Compounds such as O2, H2S, 

and CH4 are diagnostic to specific types of metabolism, 

and provide insight into microbial respiratory processes 

in the aquifer. Finally, ratios of specific contaminant 

compounds in contaminated areas give an independent 

measure of the extent of petroleum degradation.

Setting

Diego Garcia is a low carbonate atoll, located in 

the central Indian Ocean at latitude 7°S and longitude 

72°E (fig. 1). The atoll lagoon is nearly completely 

enclosed and is open to the ocean only along the northern edge. The climate is tropical, with an average annual 

temperature of 27°C and an average annual humidity of 

80 percent. Average annual rainfall from 1951 to 1990 

was 106 in., with most rain falling in September to February. Land surface elevation is typically less than 10 ft 

above mean sea level.

Diego Garcia has been a British territory since the 

late 18th century. It was used as a copra plantation, a 

coaling station, and a military outpost. U.S. Navy activities started in the 1970's. In the early 1980's, a large 

area (the South Ramp) for parking and fueling airplanes 

was constructed along the southern half of the airfield. 

The construction involved filling and grading of coastal 

wetlands with carbonate sediment dredged from the 

lagoon. An underground fuel pipeline with many subsurface hydrants was installed. The ramp was capped 

with packed carbonate sediment and about 18 in. of 

reinforced concrete. Several asphalt swales collect runoff from the ramp and parts of the taxi way. This runoff 

is discharged to a pond at the north end of the South 

Ramp through a covered storm drain. The general layout of the South Ramp is shown in figure 2.

Previous Investigations

The geohydrology of Diego Garcia was described 

in a series of unpublished reports prepared for the U.S. 

Navy (PRC Toups, 1983). More recently, Hunt (1991) 

showed that changes in the size of the freshwater lens 

could be related to changes in rainfall over both annual 

and interannual time scales.

Tribble and others (1994) describe the South Ramp 

fuel spill and outlined the work in progress for both fuel 

recovery and aquifer geochemistry. Concentrations of 

benzene, CH4, H2S, and CO2 at three sites during the 

January and February 1993 trip were presented. These 

data indicated the amount of microbial respiration (on 

the basis of CO2) was higher at sites contaminated by 

fuel than at uncontaminated sites. However, the amount 

of microbial activity was not quantitatively assessed.

2 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
72°25' E 72°30'

40° E

40° S

7°15'S

7°20'

7°25' 

80°

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ATOLL INDIAN 

OCEAN

Diego Garcia 

Lagoon

INDIAN 

OCEAN

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J________________|_

LOCATION MAP

Figure 1. Diego Garcia Atoll in the Indian Ocean and location of the study area.

Introduction 3
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EXPLANATION

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S5« CHEMISTRY WELL SITE AND NUMBER

D WATER-LEVEL WELL 

<g> RECOVERY PIT 

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Figure 2. South Ramp and locations of fuel leak and monitoring wells, Diego Garcia Atoll, 1993-94.
DESCRIPTION OF THE STUDY AREA 

Geology

Atolls are shoal-water structures that rise from the 

deep ocean floor and enclose a relatively shallow (less 

than 300 ft) lagoon. The base of the atoll is a basaltic 

volcano that has subsided. The amount of subsidence of 

the islands of the Chagos Archipelago is unknown. 

Drilling at the Marshall Islands in the Pacific reached 

basalt 4,500 ft below mean sea level (Emery and others, 

1954). Reef growth during the subsidence of the volcano results in a cap of calcium carbonate minerals that 

spans the distance from the top of the now-submerged 

volcanic edifice to the sea surface.

The shallow subsurface geology of atolls is determined by marine precipitation and deposition of carbonate minerals, the chemical alteration (or diagenesis)

fflfc

of these minerals, and changes in sea level. During the 

most recent ice age (18,000 years ago), sea level was at 

least 300 ft below its current height (Kennett, 1982) and 

atolls such as Diego Garcia were emergent structures 

subject to enhanced physical and chemical erosion by 

waves and rain. The rise in sea level since the last ice 

age has allowed resumption of reef growth at the top of 

the atoll and deposition of new carbonate minerals on 

top of the limestone that was recrystallized during the 

Pleistocene ice ages. Calcium carbonate from shallowwater reefs is originally precipitated principally as the 

minerals aragonite and magnesian calcite. But both aragonite and magnesian calcite are more soluble than pure 

calcite. Over time, diagenetic reactions transform the 

aragonite and magnesian calcite to a more consolidated 

and permeable calcitic limestone (Morse and Mackenzie, 1990).

In several well-studied atolls in the Pacific (Vacher 

and Quinn, 1997), the result is a three-layer structure 

(from bottom to top) of: (1) dense, well-consolidated 

Pleistocene limestone; (2) unconsolidated lagoonal sediments of mud, sand, and gravel; and (3) unconsolidated 

and well-sorted beach sand and gravel. The upper two 

layers are relatively young, unaltered, and have moderate permeabilities. The lower limestone has a high permeability that formed during the subaerial exposure and 

recrystallization to calcite (Hamlin and Anthony, 1987). 

Drilling on Diego Garcia indicated that there was a 

slight ocean-to-lagoon dip in these layers, and the depth 

to the unconformity between the Pleistocene limestone 

and younger sediments appeared to be about 70 ft below

mean sea level (PRC Toups, 1983). At other atolls, this 

depth is typically 20 to 70 ft (Under-wood, 1990).

Hydrology

Atoll aquifers are recharged by rain infiltrating 

through a thin unsaturated zone. Recharge from rain 

typically forms a lens of freshwater that is buoyantly 

supported by dense saline ground water from the ocean. 

Mixing between the infiltrated rain and saltwater forms 

a zone of transitional salinity. The thickness of this transition zone is determined by the rate of recharge, tidal 

dynamics, and the hydraulic properties of the carbonate 

aquifer.

According to studies by Hunt (1991) and PRC 

Toups (1983), the freshwater part of the aquifer at 

Diego Garcia, as defined by the U.S. Environmental 

Protection Agency (USEPA) secondary drinking water 

standard (USEPA, 1989) of 250 mg/L dissolved Cl', is 

composed almost entirely of unconsolidated sand and 

gravel, with some coral and a few cemented layers of 

sandstone and conglomerate. The principal freshwater 

bodies are in the Cantonment and Air Operations areas, 

where the island is relatively wide. These two areas 

have provided greater than 99 percent of the supply of 

freshwater on Diego Garcia. Total ground-water withdrawals average 0.8 to 1.2 Mgal/d.

The aquifer in the Air Operations site is the smaller 

of the two principal water production areas. Since 1986, 

about one-third of the water supply for Diego Garcia 

was pumped from a series of shallow horizontal wells in 

the Air Operations area. The water table tends to be relatively flat, with a maximum elevation of about 2 ft 

above mean sea level. The maximum thickness of the 

freshwater lens is about 65 ft. Diurnal oscillations in the 

water level of the aquifer of several feet are caused by 

ocean tides propagating through the permeable matrix 

of the aquifer.

METHODS OF DATA COLLECTION AND 

ANALYSIS

The methods used in the collection and analysis of 

data in this report are given below. This includes a 

description of the well construction and field sampling 

methods, field and laboratory techniques for the chemical analysis of water and vapor samples, and numerical

Description of the Study Area 5
methods for the analysis of chemical data. Figure 2 

shows the Air Operations area, the location of nearby 

water-supply wells, and the monitoring wells used in 

this study.

Installation and Sampling of Monitor Wells, 

Lysimeters, and Vapor Points

A network of 37 wells was used to collect water 

chemistry samples from the saturated part of the aquifer. Vapor and water samples from the unsaturated zone 

were collected using five vapor points and six lysimeters installed at selected sites. The location of the sampling sites (some sites comprise more than one well) is 

shown in figure 2. Three types of wells, distinguished 

by their prefix and method of construction, were constructed and sampled as part of this chemical investigation. Differences in well construction and the type of 

sample collected required different sampling protocols 

to be used with each type of well. Water chemistry samples were not collected from wells used to measure 

water levels.

S-Series Wells

Twenty S-series wells at five sites (fig. 2) were 

drilled in October 1992 specifically for collecting water 

for chemical analysis. The number after the S designates the site. In this report the numeric suffix of a well 

designates the approximate elevation (in feet below 

mean sea level) of the screen midpoint. The wells were 

designed to permit the sampling of water from specific 

depth intervals below the water table. All wells were 

drilled using a truck-mounted 3 1/4-in. inside diameter 

hollow-stem auger to the target depth. Well casings (2- 

in. inside diameter flush-jointed schedule PVC pipe 

with a 5 ft screened interval) were assembled inside the 

auger, a wooden plate at the bottom of the auger was 

knocked out, and the auger sections were retracted, 

leaving the casing in the borehole. Water was added 

during retraction to minimize the upward flow of loose 

sand inside the auger. Native material was allowed to 

fill in around the casing as the auger was retracted. 

Wells were finished at the surface with a cement pad 

and a locking cap. Wells at sites contaminated by fuel 

had a temporary surface casing of 10-in. PVC set from 

the surface to 12 to 13 ft below ground during drilling; 

this casing prevented the direct contamination of the 

auger and well casing by free-phase fuel floating on the 

water table. An additional well (S2-38) apparently

cracked near the water table during installation and is 

contaminated by free-phase fuel. This well was not used 

to sample dissolved constituents.

The S-series wells were sampled with a batterypowered downhole pump with a packer. The packer was 

inflated 6 to 8 ft above the bottom of the well and 9 to 

12 gal of water were typically purged at a rate of 1 to 2 

gal/min prior to sampling. After sampling wells contaminated by fuel, the pump was cleaned by pumping 

about 5 gal of soapy water followed by a 10-gal rinse of 

potable water. The effectiveness of the decontamination 

procedure was confirmed by the negative detection of 

volatile aromatic hydrocarbons at the end of the potable 

water rinse.

A vapor point also was installed at each S site to 

permit collection and analysis of gaseous constituents in 

the unsaturated part of the aquifer. A 1-in. diameter hole 

was opened to a depth of about 3 ft below land surface 

using a metal rod and drop hammer. A slotted stainless 

steel tip with a length of 1/8-in. inside diameter 

polypropylene tubing was lowered down the hole. 

Small glass beads were used as packing around the slotted tip to maintain air flow. A grout seal above the beads 

was added to secure the tubing and prevent atmospheric 

vapors from being entrained down the sides of the hole. 

Vapor points were sampled using a peristaltic pump 

after purging about 4 L of vapor through the collection 

system.

To sample water from the unsaturated zone, lysimeters were installed to depths of 2 and 3 ft below land 

surface (elevation of 3 to 7 ft above mean sea level) at 

the three S sites not overlain by asphalt (S 1, S2, and S5). 

A pilot hole was drilled using the truck-mounted auger. 

The lysimeter was set into the hole and a slurry of native 

sand was poured around the ceramic cup. The hole was 

then backfilled with native material. Water from the 

lysimeters was collected with a hand pump after being 

held at vacuum for about 10 days. Yields typically were 

400 to 800 mL.

C-Series Wells

Seventeen C-series wells were drilled in June 1991 

following discovery of the leak to permit an areal survey of the contaminant plume. These exploratory wells 

were constructed of 2-in. inside diameter flush-jointed 

PVC pipe and installed to a depth of less than 10 ft 

below the water table. They had a broad screened interval that crossed the water table, and extended above

6 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
land surface at some wells. In this study, 14 of these 

wells were used to measure volatile aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and 

xylene concentrations in both the top of the water table 

and unsaturated parts of the aquifer. Concentrations of 

CO2, C>2, and CH4 in the unsaturated part of the aquifer 

also were measured.

Water for the C wells was collected using a disposable bailer. Prior to collecting water for analysis, the 

wells were purged by removing at least three well volumes. Water from the bailer was immediately transferred to vials. Wells that had free-floating fuel were not 

sampled for dissolved constituents. Vapor from the 

unsaturated part of the aquifer was collected in glass 

syringes as a battery-powered air blower pulled air from 

the aquifer through the well screen and up the well bore. 

About 30 L of vapor was purged through the collection 

system prior to sampling.

BW21 Wells

A site near the south end of the airfield (fig. 1), and 

far-removed from the fuel leak, was also sampled in this 

study. The three wells at this site were installed in 1982 

as part of a network of monitoring wells throughout 

both the Cantonment and Air Operations aquifers (PRC 

Toups, 1983). The wells were constructed of 1.75-in. 

inside diameter flush-jointed PVC pipe with a screened 

interval of 5 ft. These wells were purged by removing 

several well volumes of water with a gasoline-powered 

pump and collecting samples with a battery-powered 

peristaltic pump.

Field Analysis and Sample Treatment

At each of the S-series and BW21 wells, ground 

water was transferred from the pump to a closed cell 

using gas-impermeable tubing to minimize atmospheric 

contact during the measurement of pH, dissolved O2, 

and temperature. The pH electrode was calibrated 

against NBS (National Bureau of Standards)-traceable 

standards (±0.01). Dissolved O2 was determined by use 

of a polarographic electrode (±0.2 ppm, 0.1 ppm detection limit). Temperature (±0.1) was measured using a 

thermistor. Samples for the determination of total sulfide (H2S) were collected, without filtration and minimal atmospheric contact, into 60-mL glass bottles and 

immediately fixed with ZnSC>4 and NaOH. Samples for 

measurement of volatile aromatic hydrocarbons in the

field lab were collected into two 40-mL glass vials; an 

additional two vials were collected so that laboratory 

analysis by gas chromatograph-mass spectrometer 

(GC-MS) could be done if the field measurement indicated the presence of contaminants.

Samples of water from the unsaturated zone for 

major-ion and nutrient analysis were collected in disposable 1-gal semirigid polyethylene containers and filtered in a nearby laboratory within 30 minutes of 

collection. Samples were not filtered at the well head 

because of the potential contamination introduced by 

wind and squalls. Water was passed through a 42-mm 

diameter Watman GF/F filter (nominal pore size 0.7 

H,m) under vacuum filtration. Comparison of samples 

filtered using GF/C (nominal pore size 1 p,m) and ultrafiltration (nominal pore size 3 nm) found that ultrafiltered samples were not different in titration alkalinity, 

NO3~+NO2", and Si, 1 percent lower in NH4+, and 3 percent lower in PO43+. However, ultrafiltration did 

remove 30 to 70 percent of the organic C, N, and P; this 

was attributed to bacteria smaller than 1 (im (Hollibaugh and others, 1991). This indicates that the GF/F 

filters used in this study remove essentially the entire 

inorganic non-dissolved fraction from a water sample, 

but that bacterial biomass may be a component of what 

is reported as dissolved organic carbon, dissolved 

organic nitrogen, and dissolved organic phosphorus.

Prior to sample filtration, the filtering apparatus 

was washed with phosphate-free soap and rinsed with 

demineralized water, followed by two 200-mL aliquots 

of sample water. Filtered water for determination of 

titration alkalinity was stored in 175-mL high-density 

polyethylene bottles and kept chilled; the titration alkalinity samples were filtered to remove small particles of 

CaCC>3 that would otherwise affect the outcome of the 

alkalinity titration. Filtered water for determination of 

major ions was stored in 125-mL high-density, HNO3- 

washed polyethylene bottles, acidified to pH 2 with 

high-purity HNC>3, and kept chilled. Filtered water for 

determination of nutrients was stored in HCI-washed 

125-mL high-density polyethylene bottles. In general, 

field methods follow procedures described by Tribble 

and others (1990), Smith and others (1991), and Tribble 

(1997).

The concentrations of CC>2, C>2, and CH4 in the 

vapor phase of the unsaturated part of the aquifer were 

measured in the field lab using a portable gas chromatograph with a thermal conductivity detector (GC-TCD).

Methods of Data Collection and Analysis 7
The GC-TCD was calibrated against three standard reference gases before and after each field trip. The concentration of volatile aromatic hydrocarbons in water 

samples from the saturated zone and vapor samples 

from the unsaturated zone were measured at the field 

lab using a portable gas chromatograph with a photoionization detector (GC-PID). During the August 1993 

and May 1994 trips, wells that tested positive for aromatic hydrocarbons were subject to more complete 

analysis by GC-MS (see below). The GC-PID was calibrated daily against aromatic hydrocarbon standards 

and blanks, and had a detection limit of 0.1 ppb.

Laboratory Methods

Analyses for major ions and nutrients were performed at the University of Hawaii Department of 

Oceanography. This laboratory participates in the 

USGS standard reference water sample program, and 

has been approved by the USGS Branch of Quality 

Assurance. Ca2+, Mg2+, Sr2*, and Na* were determined 

by inductively coupled plasma/ optical emission spectroscopy. K+ was determined by atomic absorption

spectroscopy. Cl" was determined by titration with 

AgNO3 . SO42" was determined either by ion chromatography or gravimetrically as a BaSO4 precipitate, 

depending on the sample concentration. Titration alkalinity was determined by multi-point titration with HC1. 

H2S was determined by potassium iodide titration. 

PO43+, NO3>NO2", NH4+, and Si were determined colorimetrically on an autoanalyzer. DOP was calculated 

as the difference between PO43+ and total P following 

ultraviolet oxidation. DON was calculated as the difference between inorganic and total N following ultraviolet oxidation. POC was determined by infrared 

absorption following ultraviolet oxidation.

The analytical accuracy (agreement between the 

measured and the calculated or most probable value) of 

a determination was estimated by comparison with laboratory standards. Standards included both USGS standard reference samples and solutions prepared in the 

University of Hawaii laboratory. Analytical precision 

(reproducibility of a measured value) was determined 

by submission of blind duplicate samples to the laboratory. Values for the precision and accuracy are given in 

table 1.

Table 1. Analytical precision, accuracy, and detection limits for selected chemical constituents in ground water from Diego 

Garcia Atoll, 1993-94

[TA, titration alkalinity; DOP, dissolved organic phosphorus; DON, dissolved organic nitrogen; DOC, dissolved organic carbon; >, actual value is greater 

than value shown; mg/L, milligrams per liter; meq/L, milliequivalents per liter; Hg/L, micrograms per liter; --, not determined]

Precision2

Constituent

Ca

Mg

Sr

Na

SO4

K

Cl

H2S

TA 

PO43 '

NO3-

NH4+

Si(OH)4

DOP

DON

DOC

n1

13

13

13

13

13

13

13

13

13 

13

13

13

13

13

13

13

Average 

(percent)

0.60

0.85

1.10

0.52

1.02

0.06

1.04

0.01

0.06

7.8

64

2.0

0.7

33

27

1.0

Accuracy4

n3

72

72

72

56

77

19

103

-

49 

>50

>50

>50

>50

>50

>50

>50

Average 

(percent)

1.2

0.8

0.6

0.2

0.0

-1.1

-0.6

-

-0.02

3

3

3

3

5

5

5

Detection 

limit

0.002

0.002

0.002

0.1

0.1

0.01

0.2

0.1

0.04 

0.3

0.3

0.3

0.006

0.3

0.3

0.3

units

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

meq/L 

J-ig/L

l-ig/L

Hg/L

mg/L

jj.g/L

M-g/L

mg/L

'Number of paired samples

2 Precision calculated as the average percentage of difference between blind duplicate samples

3Number of analyses

4Accuracy calculated as the average percentage of difference between a measured value and a primary standard

8 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Analysis of volatile aromatic compounds by GCMS was done at the Central Laboratory of the USGS. 

The procedure is similar to USEPA method 524.2. Total 

petroleum hydrocarbons were analyzed according to 

USEPA method 418.1; oil and grease were analyzed 

according to USEPA method 413.2.

Geochemical Calculations

Calculation of charge balance, saturation state with 

respect to carbonate minerals, and total dissolved 

inorganic carbon (DIC=CO2(aq) + H2CO3 + HCO3 " + 

CC>32") were performed using the computer program 

WATEQ4F (Ball and Nordstrom, 1991). The charge 

balance, in percent, was used as an overall check of analytical accuracy for the major-ionic constituents, and 

calculated as:

ChargeBalance = 100 (Cat-An) 

[(Cat + An)'

(1)

where Cat and An are the measured total number of 

equivalents of cations and anions respectively. A 

negative charge balance indicates that one or more of 

the cations was underdetermined, one or more of the 

anions was overdetermined, or both. Conversely, a 

positive charge balance indicates that one or more of the 

cations was overdetermined, one or more of the anions 

was underdetermined, or both. A charge balance of zero 

indicates equivalence between the number of cations 

and anions, as demanded by the law of electrical 

neutrality, and strongly indicates accuracy in the 

analytical determinations (Hem, 1989). In practice, a 

charge balance of less than ±5 percent is considered 

good, and a balance in excess of ±10 percent is 

considered suspect.

The saturation state of a water sample is used to 

predict if a specific mineral is thermodynamically 

favored to dissolve or precipitate. For water samples 

from Diego Garcia, the saturation state with respect to 

the carbonate mineral aragonite was calculated because 

it is an abundant mineral in the reefs that build atolls and 

its solubility constant (Ksp) is better defined than that of 

the magnesian-calcite solid solution series. The degree 

of saturation is expressed as a saturation index (SI):

loglAP ~~j7 

Ks P

(2)

where IAP is the ion activity product of the appropriate 

ions and Ksp is the solubility constant for the mineral in 

question (Drever, 1988; Krauskopf, 1979). An SI of 

zero occurs when the solution is at equilibrium with the 

mineral. A positive SI indicates thermodynamic 

oversaturation and a tendency for the mineral to 

precipitate. A negative SI indicates thermodynamic 

undersaturation and a tendency for the mineral to 

dissolve. Analytic and thermodynamic uncertainty 

suggest that a range of ±0.2 be ascribed to the calculated 

value of SI for a particular sample.

Bacterial respiration and fuel degradation is evaluated in three ways. First, the concentration of dissolved 

inorganic carbon (DIG) was used to indicate the total 

amount of respiration. Because the dissolution of carbonate minerals also releases DIG to the aquifer, it is 

necessary to remove the contribution of inorganic reactions from the total DIG to calculate the amount of DIG 

released by bacterial respiration. For each sample, the 

amount of DIG contributed by the inorganic reaction of 

carbonate minerals was calculated from the concentrations of Ca2+, Mg2+, and Sr2"1". This amount was then 

subtracted (on a molar basis) from the total amount of 

DIG in the sample to calculate the amount of DIG contributed by microbial respiration. DIG from microbial 

respiration results from both the oxidation of naturally 

occurring organic matter and the degradation of petroleum hydrocarbons. Comparison of the levels of bacterial respiration between contaminated and 

uncontaminated areas gives an estimate of the amount 

of respiration from the microbial oxidation of hydrocarbons.

The amount of microbial respiration resulting from 

specific metabolic processes was evaluated by comparing the concentrations of either oxidants or by-products 

specific to the processes. Oxic respiration was evaluated by changes in C>2 concentration. Sulfate and nitrate 

reduction were evaluated by the presence of dissolved 

sulfide and the depletion of nitrate, respectively. The 

amount of methane in the unsaturated zone was used to 

indicate the degree of methanogenesis. This approach 

does not give a complete accounting of microbial activity, but does indicate the relative contributions of different metabolic processes to total microbial respiration.

The degradation of specific volatile aromatic 

hydrocarbons was evaluated by calculating the ratio of 

benzene to more persistent compounds such as trimethyl benzenes, tetramethyl benzenes, and naphthalene 

(Wilson and others, 1994).

Methods of Data Collection and Analysis 9
GEOCHEMISTRY OF GROUND WATER, 

GASES IN THE UNSATURATED ZONE, 

AND FUEL HYDROCARBONS

Chemical Overview

Field measurements during February 1993, August 

1993, and May 1994 of temperature, pH, and dissolved 

O2 are presented in table 2 (at end of report). Concentrations of dissolved I^S are also shown in table 2. 

Water from the saturated zone was typically warm and 

exhibited only a narrow range in temperature (27 to 

32°C). The average temperature was 29.0°C. The warm 

temperatures result from the warmth of rainwater (25 to 

29°C) in addition to solar warming of ground water. The 

warmest water was from the shallowest wells at sites S3 

and S4, which are in asphalt swales between the 

cement-covered parking ramp and taxi way. The constantly warm water is conducive to a steady rate of 

microbial metabolism because lag effects caused by 

changing temperatures are absent. The warmth of 

ground water also favors high rates of microbial metabolism (Chapelle, 1993).

Water from the unsaturated zone, collected by 

lysimeter, had an average temperature of 30.4°C and 

tended to be warmer than water from below the water 

table. This difference is especially obvious when water 

temperatures in the unsaturated zone are compared with 

the average temperature of water below the water table 

at sites with lysimeters (28.5°C). Although this may be 

somewhat a consequence of water from lysimeters heating during the lengthy (typically 10 minutes) sample 

collection, it also appears that water undergoes more 

solar heating while in the unsaturated zone than in the 

saturated zone.

The pH of ground water from the saturated part of 

the aquifer was 6.7 to 7.8, the average pH was 7.2. 

Water that had an odor of fuel hydrocarbons (sites SI, 

S2, and S3) tended to have the lowest pH values. There 

was no large or consistent difference in pH between 

water from the saturated zone and water from the unsaturated zone. However, because the unsaturated zone 

samples were collected by suction lysimeter, it is likely 

that the loss of CC>2 resulted in the measured pH values 

being somewhat too high.

Dissolved C>2 concentrations in water from wells 

below the water table were low. Values ranged from 0.9 

to greater than 0.1 ppm. The average concentration was

0.2 ppm, which is the instrument level of precision and 

near the detection limit. Because it is possible that trace 

amounts of atmospheric C>2 contaminated the sample 

during collection, it is reasonable to consider the saturated part of the aquifer as essentially anoxic. Low concentrations of dissolved C>2 are common in ground 

water (White and others, 1990). The concentrations in 

ground water were significantly different than those 

measured in rain (5.6 ppm) and seawater (4.1 ppm). The 

depletion of C>2 in ground water results from metabolic 

consumption by bacteria in the aquifer. Oxygen and 

H2S concentrations were not measured in water samples from the unsaturated zone because sample water 

was exposed to the atmosphere during collection.

H2S concentrations in ground water were variable, 

and ranged from 0.1 to 10.2 ppm. H2S was not detected 

in any of the rain samples. One seawater sample registered 0.1 ppm H2S, which is the level of detection and 

may be a false positive. Samples that had high concentrations of H2S were from the deepest wells at sites S3 

and S4. These wells tended to be the most saline (see 

below) and consequently had a larger pool of SO42" 

available for bacterial sulfate reduction.

Concentration of Gases in the Unsaturated 

Zone

The concentration of O2, CO2, and CH4 in the 

unsaturated zone during the three sampling trips was 

measured at each the five S-sites using a permanently 

installed gas point. In the May 1994 trip, additional 

sampling was done on selected C-wells. Oxygen concentrations were low in all samples, typically about 3 

percent (table 3), in contrast with an atmospheric value 

of about 16 percent. Carbon dioxide and CH4 concentrations were quite variable; C02 concentrations ranged 

from 0.1 percent to 13.2 percent (average=1.9 percent) 

and CH4 concentrations ranged from 0 to 55.4 percent 

(average=8.1 percent). This is in sharp contrast to atmospheric values of 0.03 percent for CO2 and 1.6 ppb for 

CH4 . Samples from sites contaminated by fuel hydrocarbons, such as SI, S2, S3, and C2-1 had the highest 

values of CO2 and CH4 (fig. 3). The lines in figure 3 

near the leak are speculative because samples could not 

be collected from beneath the cement ramp. However, 

it is clear that sites closest to the leak have the most elevated concentrations of CO2 and CH4 in the unsaturated 

zone. These values result from the microbial degrada10 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 3. Concentration of O2, CO2, and CH4 in vapor samples from the unsaturated zone, Diego Garcia Atoll

[Values in percent]

Well site

SI

SI

S2

S2

S2

S3

S3

S3

S4

S4

S4

S5

S5

S5

Cl-1

Cl-2

C2-1

C3-1

C3-3

C3-4

C3-5

C3-6

C4-1

C4-3

C5-1

C6-1

Date

1-30-93

5-7-94

2-2-93

8-20-93

5-7-94

2-1-93

8-20-93

5-7-94

1-30-93

8-20-93

5-7-94

2-2-93

8-20-93

5-6-94

5-9-94

5-9-94

5-12-94

5-12-94

5-12-94

5-9-94

5-9-94

5-9-94

5-9-94

5-9-94

5-9-94

5-9-94

Time

1400

1600

1000

1108

1600

1130

1000

1600

1530

928

1600

1500

1025

1655

1540

1708

1327

1230

1320

1725

1718

1536

1528

1420

1412

1400

02

2.6

0.2

3.5

2.9

0.3

2.1

8.9

2.2

13.2

9.4

2.2

16.2

1.5

2.9

3.4

3.5

3.4

3.3

3.3

3.1

3.4

3.3

3.0

3.4 .

3.5

3.4

C02

3.6

1.2

3.4

1.2

3.0

13.2

6.8

3.1

0.4

0.4

0.7

4.1

0.5

0.1

0.7

0.2

2.3

0.3

0.1

1.3

0.2

0.1

2.3

0.1

0.1

0.1

CH4

33.6

26.1

24.0

12.5

55.4

0.1

0.5

35.2

2.4

2.6

5.6

0

0.2

0

0

0

11.7

0

0

1.4

0

0

0

0

0

0

Geochemistry of Ground Water, Gases in the Unsaturated Zone, and Fuel Hydrocarbons 11
EXPLANATION

LINE OF EQUAL CO, CONCENTRATION 

Interval 0.5 percent Dashed where uncertain

EXPLORATION WELL SITE

CHEMISTRY WELL SITE

WATER-SUPPLY WELL

CONCRETE

ASPHALT

GRASS

SOUTH RAMP

EXPLANATION

LINE OF EQUAL CH4 CONCENTRATION 

Interval 0.5 percent Dashed where uncertain

EXPLORATION WELL SITE

CHEMISTRY WELL SITE

WATER-SUPPLY WELL

CONCRETE

ASPHALT

GRASS

SOUTH RAMP

DIEGO 

GARCIA 

ATOLL

1°

Figure 3. Carbon dioxide (CO2) and methane (CH4) concentrations in the unsaturated zone of the aquifer near 

the South Ramp in May 1994, as a percentage of total gas concentration, Diego Garcia Atoll, 1993-94.

12 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
tion of fuel in the unsaturated zone and near the top of 

the water table.

The water table in the Air Operations area is quite 

shallow (typically less than 6 ft below ground surface) 

and aquifer water levels exhibit diurnal tidal changes of 

3 to 4 ft (this is about 60 to 80 percent of the tide range 

in the ocean). Consequently, about half of the volume of 

gas in the unsaturated zone is displaced or compressed 

with every tide cycle. Thus, it is likely that the concentrations of gases in the unsaturated zone are highly variable and follow a tidal pattern. Preliminary data (R.W. 

Lee, USGS, written commun., 1995) shows 5- to 8-fold 

tidal fluctuations in the concentrations of O2, CO2, and 

CH4. The dynamic physical processes in the unsaturated zone may explain much of the variability seen at a 

given site over time.

Major-Dissolved Ions

For the purposes of this report, the major-dissolved 

ions are considered to include the cations Ca2+, Mg2"1", 

Sr2*, Na+, and K+ and the anions Cl', SO42-, and HCO3 ' 

. Previous work at other atolls (for example, Kwajalein 

Atoll, Tribble, 1997) has shown that as recharge water 

from rain mixes with deep saline water from the ocean, 

the ions Cl", Na"*", and K* undergo little if any chemical 

reaction and mix conservatively. In contrast, Ca+2, 

Mg*2 , Sr"*"2 and HCO3~ showed a significant excess over 

what would be expected from conservative mixing. 

This excess was attributed to both the dissolution of carbonate minerals and, for DIG, oxidation of organic matter.

Ground-water samples from Diego Garcia had low 

Cl" concentrations (table 4, at end of report). The highest value measured (site S3 at a depth of 40 ft below 

mean sea level) was 54.6 mg/L, or less than 0.3 percent 

the concentration of seawater. The two rain samples had 

Cl" concentrations of 0.1 mg/L. However, the average 

Cl" concentration of water from the unsaturated zone 

collected by lysimeters was 8.6 mg/L (n=13), suggesting that the salinity of water recharging the aquifer is 

higher than the two rain samples collected. Two reasons 

for this are the loss of water (but not salt) from evapotranspiration and the dry deposition of marine aerosols.

The concentrations of Na+, K+, and SC>42~ were 

also low in the ground-water samples. When the concentrations of these ions are plotted against dissolved 

Cl" concentrations, there is broad scatter around the

lines for conservative mixing between rain and seawater 

(fig. 4). This scatter is somewhat greater than would be 

expected from analytical uncertainty alone. SC>42~ 

showed the greatest deviation from conservative mixing, with both excesses and deficiencies. Several wells 

also appeared to have excesses of K*. Na+ shows the 

closest correspondence to the conservative mixing line, 

but also exhibited some departures. The departures in 

these elements is probably a result of biological activity 

(especially with respect to SC>42~, which is removed by 

dissimilatory sulfate reduction and added by the subsequent oxidation of the resulting sulfide) and the dry deposition of marine aerosols having ion ratios different 

from seawater (Andrae and others, 1986; Mouri and 

Okada, 1993).

The bivalent cations Ca2"1", Mg2"1", and Sr2"1" showed 

extreme enrichments over what would be expected from 

conservative mixing between rain and seawater (fig. 5). 

These excesses are a result of the dissolution of the carbonate minerals aragonite and magnesium-calcite. The 

ratio of the excess cations (on a molar basis) indicates 

the net dissolution of a carbonate mineral having the 

composition Ca0^Mgo^Sfy.oiCC^. Although this 

composition is close to that of the mineral dolomite, the 

geochemical setting of Diego Garcia suggests that the 

presence of dolomite is unlikely because dolomite is a 

very rare mineral in modem reef and atoll sediments 

(Morse and Mackenzie, 1990). More likely, the 

observed excesses result from the dissolution of highMg calcites and partial reprecipitation as a less soluble 

calcite with a lower Mg content. The 1 percent enrichment in Sr also indicates the dissolution of aragonite, 

which typically has a Sr content of less than 1 percent 

(Morse and Mackenzie, 1990).

Nutrients and Organic Carbon

Ground-water (saturated and unsaturated) nutrient 

concentrations tended to be elevated over both rain and 

seawater (table 5, at end of report). PO43+ concentrations in ground water averaged 16.6 ^ig/L, in contrast 

with rain and seawater concentrations of 2.8 and 6.7 

U£/L respectively. NO3~ concentrations in ground water 

averaged 20.3 u.g/L, in contrast with rain and seawater 

concentrations of 14.5 and 5.1 (ig/L respectively. 

NH4+concentrations in ground water averaged 796 

H-g/L, in contrast with rain and seawater concentrations 

of 9 and 16 |ig/L respectively. Samples from the unsaturated zone had much more of the inorganic N as

Geochemistry of Ground Water, Gases in the Unsaturated Zone, and Fuel Hydrocarbons 13
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9 =
NO3 " compared with NH4+ (average 30 percent NO3 ") 

than samples from below the water table (average 2 percent NO3 "). This is a result of the reducing conditions 

below the water table and loss of NO3 ~ to denitrification. The average concentration of dissolved Si in 

ground-water samples was 1,098 H-g/L. This is much 

higher than the concentrations measured in rain (6 

(ig/L) and seawater (average=87 |lg/L). Dissolved Si is 

considered a nutrient because the absence of Si-containing minerals means that the chemistry of dissolved Si is 

controlled by biologic, rather than geologic, reactions. 

The elevated concentrations of dissolved inorganic N, 

P, and Si are consistent with the microbial oxidation of 

organic matter and the concomitant release of nutrients.

A plot of dissolved PO43+ as a function of dissolved inorganic N (NO3~+NH4+, fig. 6) shows no relation between the release of N and P in the organic matter 

undergoing oxidation in the ground-water samples. 

Similarly, dissolved Si as a function of dissolved inorganic N (fig. 6) also shows little relation between the 

release of N and Si from the oxidation of organic matter. 

Possible reasons for this are multiple sources of organic 

matter with different N:P ratios, removal of P by sorbtion onto carbonate minerals, and removal of N by denitrification.

Concentrations of organic C, N, and P in ground 

water were variable and reflected the bulk composition 

of natural dissolved organic material and dissolved fuel 

constituents. Dissolved organic carbon (DOC) concentrations averaged 5.2 mg/L and ranged from 1.2 to 17.6 

mg/L. DOC concentrations in ground water were much 

higher than the concentrations measured in either rain 

(average 0.8 mg/L) or seawater (average 0.4 mg/L). 

Dissolved organic phosphorus (DOP) concentrations 

averaged 5.3 p,g/L and ranged from 0 to 13.6 |ig/L. 

Ground-water concentrations of DOP were close to 

those measured in rain (average=4.3 p.g/L) and seawater 

(average=6.0 |ig/L). Dissolved organic nitrogen (DON) 

concentrations averaged 197 |ig/L and ranged from 45 

to 621 |ig/L. DON concentrations in ground water were, 

on average, about twice the concentrations measured in 

both rain (DON=95 ^ig/L) and seawater (106 |ig/L).

Within a water sample, there was no consistent 

relation between the concentrations of DOC, DON, and 

DOP (fig. 7). The ratios of organic C, N, and P were 

extremely variable, ranging from 110 to 44,800 (average=2,160) for DOCDOP and 3.9 to 97 (average=30) 

for DOCDON. When the organic C:N and C:P ratios

are plotted against the DOC concentration, there is a 

trend towards organic matter with lower N and P ratios 

in samples having high DOC concentrations (fig. 8). 

Thus, ground-water samples having elevated DOC concentrations do not show a concomitant increase in DOP 

or DON. Hydrocarbon contamination was evident in 

samples with high concentrations of DOC (see below). 

The low N and P organic content of hydrocarbons tends 

to raise the organic C:N and C:P ratios in water with 

high levels of DOC.

Assessment of Fuel Contamination in the 

Aquifer

Because high priority was given to immediate 

remediation and recovery of free-phase fuel while 

maintaining the operational status of the South Ramp, a 

formal site characterization was not done. Following 

the initial characterization of June 1991 (C.D. Hunt, Jr., 

USGS, written commun., 1991), the chemical work 

done during this study was designed to provide an overview of geochemical process in both contaminated and 

uncontaminated parts of the aquifer. The goal of this 

work was to determine the amount of bacterial activity 

in both uncontaminated and hydrocarbon-affected areas 

so that post-fuel recovery remedial methods could be 

selected. Sampling for contaminant hydrocarbons was 

not designed to show the spatial distribution of all fuel 

constituents in the aquifer. However, the data does permit a generalized characterization of the plume. Hydrocarbons were found dissolved in ground water, as a 

vapor phase in the unsaturated zone, and floating as a 

free-phase layer on top of the water table.

Following discovery of the leak in May 1991, a 

reconnaissance trip was performed by USGS personnel 

(C.D. Hunt, Jr., USGS, written commun., 1991). During 

this trip, 17 C-wells were installed to map the areal distribution of free-phase fuel. On the three subsequent 

trips that form the basis for this report (February 1993, 

August 1993, and May 1994) 14 remaining C-wells 

(three were destroyed during repair of the fuel line and 

groundskeeping activities), two fuel recovery pits, and 

well S2-50 (casing cracked) were examined for freephase fuel.

Fuel thickness in a well is not a direct measure of 

the thickness of the fuel layer in the surrounding aquifer. Typically, the fuel column in a well is several times 

larger than the actual thickness in the surrounding aquiGeochemistry of Ground Water, Gases in the Unsaturated Zone, and Fuel Hydrocarbons 15
at 

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ORGANIC CARBON (DOC) CONCENTRATION, IN MILLIGRAMS PER LITER

18

Figure 8. Relation between dissolved organic carbon (DOC) to dissolved organic phosphorous (DOP) and 

dissolved organic carbon (DOC) to dissolved organic nitrogen (DON) compared with dissolved organic carbon 

(DOC) in ground-water samples from S-series wells and lysimeters in February 1993, August 1993, and May 

1994, Diego Garcia Atoll.

Geochemistry of Ground Water, Gases in the Unsaturated Zone, and Fuel Hydrocarbons 17
fer (Ballestero and others, 1994; Huntley and others, 

1994) because of differences in specific gravity, physical properties of the aquifer, temporal changes in pressure, and flow dynamics of both the fuel and water 

capillary fringes. At Diego Garcia, this is compounded 

by the large tidal oscillations in the water table. Fuelrecovery operators report that fuel thickness in the 

recovery pits varies with tidal stage, and is thickest at 

high tide. Consequently, measurements of free-fuel 

thickness do not give an accurate representation of 

thickness in the aquifer. However, the limited data 

available (table 6) suggest that substantial fuel remains 

in the aquifer as a free-phase layer on the water table, 

despite the recovery of 134,000 gal of fuel from the 

recovery pits between May 1991 and April 1994 (BJS 

International Associates, Diego Garcia, written commun., 1994). However, since the initial characterization 

in June 1991, free-phase fuel has not been observed in 

wells that were initially sited outside the free-phase 

plume, suggesting that the free-phase portion of the 

plume does not appear to have spread over an extensive 

area during the period of observation.

Hydrocarbons in Ground Water and Soil Vapor

The extent of hydrocarbon contamination was 

assessed through (1) field measurements of volatile 

organic carbon (VOC) compounds in water and soil 

vapor using a GC-PID, (2) laboratory measurements of 

total petroleum hydrocarbons (TPH) and oil and grease 

in ground water, (3) laboratory measurements of dissolved organic carbon (DOC), and (4) laboratory GCMS scans of VOC compounds in ground water. Special 

attention was paid to determination of VOC because 

several aromatic compounds, such as benzene, are both 

highly soluble (relative to aliphatic hydrocarbons) and 

toxic. The GC-MS scans were not initially part of the 

sampling protocol, but were started after the February 

1993 sampling trip. The laboratory GC-MS more accurately measures VOC compounds than the GC-PID 

used in the field, and by characterizing the entire VOC 

inventory, can be used to infer microbial activity and 

preferential degradation of selected compounds.

The analysis of TPH and oil and grease was discontinued after the February 1993 trip because the 

detection levels were high relative to the observed concentrations. Of 11 wells that pumped obviously contaminated water (as indicated by a strong fuel odor and 

subsequent detection of volatile aromatic compounds),

the TPH value was above the detection limit at only 

three well sites (table 7). The oil and grease analysis 

confirmed contamination in 10 of the 11 wells, but at a 

low precision and gave what appeared to be a false positive for a well about 1 mi from the site of the leak that 

exhibited no other signs of contamination. Also, the 

DOC measurements provide a reasonable proxy for the 

presence of hydrocarbons. Figure 9 shows the relation 

between DOC and field GC measurements of benzene. 

Typically, wells with DOC concentrations less than 3 

ppm were uncontaminated by hydrocarbons, whereas 

nearly all samples with DOC concentrations greater 

than 4 ppm had detectable levels of benzene.

High concentrations of DOC are not a reasonable 

predictor of benzene concentrations; however, samples 

from a given site tend to have similar DOC:benzene 

ratios. For example, S3 had high levels of DOC while 

maintaining relatively low benzene concentrations 

whereas sites SI and S2 tended to have higher benzene 

and lower DOC concentrations. If the elevated DOC 

concentrations result from petroleum hydrocarbons, 

differences between sites may be a result of plume maturation and different amounts of aliphatic compared 

with aromatic hydrocarbons in ground water. Because 

benzene was found in all wells having a fuel odor, the 

combination of detectable benzene and elevated DOC 

concentrations is taken as a reliable indicator of fuel 

contamination.

Benzene concentrations in ground water and in soil 

vapor steadily increased during the course of the three 

sampling trips. Hydrologic sections of benzene concentrations measured with the GC-PID in the field at sites 

SI, S2, S3, and S5 are shown in figure 10. From February 1993 to May 1994, maximum measured concentrations in water have increased from about 10 to 30 ppb. 

Because the hydrologic section does not show the center of the plume nor is it aligned along a flow path, it is 

unclear if the increase is a result of advective flow of 

more contaminated water moving from the center of the 

plume past the wells, or if the increase is a result of continued partitioning from the free-phase fuel layer in 

ground water.

Despite the increasing concentration of contaminants within the defined plume, the extent of dissolved 

aromatic hydrocarbons does not appear to have spread 

substantially since the first sampling in February 1993. 

Table 8 shows the concentrations of benzene in ground 

water and soil vapor measured during the three trips.

18 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 6. Approximate thickness of free-phase fuel at South Ramp wells and recovery pits, Diego Garcia Atoll

[--, not measured; D, destroyed during pipeline repair; >, actual values is greater than value shown; -, approximately]

Fuel thickness 

(feet)

Well site

C2-1

C2-2

C2-3

S2-37

S3 recovery pit

S 1 recovery pit

June 1 991 1

1.8

0.9

0.02

not installed

1.5

not installed

February 19932

~2

D

D

>3

--

--

August 19933

1.91

D

D

3.94

0.95

0.69

May 19943

1.78

D

D

4.74

0.67

0.27

1 Measurements made using fuel paste on a tape measure

2 Measurements made using a bailer lowered into the well

3 Measurements made using a Keck free-product interface probe

Table 7. Concentrations in ground water of benzene, dissolved organic carbon, oil and grease, and total petroleum 

hydrocarbons, February 1993, Diego Garcia Atoll

[All quantities in milligrams per liter except benzene in ppb; ft, feet; ppb, parts per billion; ppm, parts per million; nd, no detection; <, actual value 

is less than value shown]

Well site

SI

SI

SI

SI

S2

S2

S2

S3

S3

S3

S3

S4

S4

S4

S4

S5

S5

S5

S5

S5

BW21

BW21

BW21

Altitude 

(ft)

-9

-20

-31

-39

-9

-19

-29

-11

-21

-30

-40

-11

-21

-31

-41

-11

-18

-27

-37

-46

-8

-23

-48

Benzene 

(ppb)

7.2

7.5

6.3

5.7

11.5

4.9

10.2

0.8

1.9

2.0

1.6

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

Dissolved organic 

carbon 

(ppm)

13.9

6.8

5.9

4.1

10.9

8.3

11.6

17.6

11.0

9.9

4.6

4.1

4.5

2.8

2.5

3.3

2.8

2.1

1.9

1.8

2.1

1.9

1.3

Oil and grease 

(ppm)

3

3

3

2

2

2.5

2

5

2

3

<1.3

<1.3

<1.3

<1.3

<1.3

<1.3

<1.3

<1.3

<1.3

<1.3

2

<1.3

<1.3

Total petroleum 

hydrocarbons 

(ppm)

<0.7

1

<.7

<.7

<.7

<.7

<.7

<.7

<.7

1.5

1

<.7

<.7

<.7

<.7

<.7

<.7

<.7

<.7

<.7

<.7

<.7

<.7

Geochemistry of Ground Water, Gases in the Unsaturated Zone, and Fuel Hydrocarbons 19
35

30

§ 25

CD

CC 

HI

C/3 

DC

LU 

N

UJ 

CO

20

15

10

8

 WELL SITE SI

 WELL SITE S2

A WELL SITE S3

O WELL SITE S4, 

S5.0RBW21

10 12 14 16

ORGANIC CARBON (DIG) CONCENTRATION, IN MILLIGRAMS PER LITER

18

Figure 9. Relation between benzene and dissolved organic carbon (DOC) in ground-water samples from Sseries wells in February 1993, August 1993, and May 1994, Diego Garcia Atoll.

20 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
FEET

i°

Sea level 

-10-

-20-

-30-

-40-

-50

S1 S2

A'

February 1993

/- Water table

Vertical exaggeration x 9 

A'

vertical exaggeration x 9 

A'

Midpoint

of screened

interval

Vertical exaggeration x 9

EXPLANATION

10 LINE OF EQUAL BENZENE CONCENTRATION 

Interval, in parts per billion, is variable

200

i

400 FEET

j_____i

100 METERS

Figure 10. Hydrologic sections of benzene concentration at well sites S1, S2, S3, and S5 in February 1993, 

August 1993, and May 1994, Diego Garcia Atoll. Line of section shown in figure 2.

Geochemistry of Ground Water, Gases in the Unsaturated Zone, and Fuel Hydrocarbons 21
Table 8. Concentrations of volatile organic carbon compounds in vapor and water samples, Diego Garcia Atoll 

[Values are in parts per billion; measurements by gas chromatograph with a photoionization detector; , no sample collected; nd, no detection; 

n.a., not applicable; tr, trace]

Well or type 

of sample

SI 

SI

SI

SI

SI

S2 

S2

S2

S2

S3 

S3

S3

S3

S3

S4 

S4

S4

S4

S4

S5 

S5

S5

S5

S5

S5

BW21

BW21

BW21

Seawater

Pond

Cl-1 

Cl-1

Cl-2 

Cl-2

C2-1 

C2-1

C3-1 

C3-1

C3-2 

C3-3 

C3-3

Altitude 

(feet)

n.a. 

-9

-20

-31

-39

n.a. 

-9

-19

-29

n.a. 

-11

-21

-30

-40

n.a. 

-11

-21

-31

-41

n.a. 

-11

-18

-27

-37

-46

-8

-23

-48

n.a.

n.a.

n.a. 

n.a.

n.a. 

n.a.

n.a. 

n.a.

n.a. 

n.a.

n.a. 

n.a. 

n.a.

Water or 

vapor 

sample

vapor 

water

water

water

water

vapor 

water

water

water

vapor 

water

water

water

water

vapor 

water

water

water

water

vapor 

water

water

water

water

water

water

water

water

water

water

vapor 

water

vapor 

water

vapor 

water

vapor 

water

vapor 

vapor 

water

Benzene

7.2

7.5

6.3

5.7

high 

12

5

10

2.6 

0.8

1.9

2.0

1.6

2.8 

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

4.9

high 

fuel

nd 

tr

February 1993

Xylene1

14.3

13.7

13.4

7.1

0.4

1.2

0.6

2.3 

0.3

1.0

1.4

0.6

nd 

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

~

~

;;

nd

Toluene1

nd

nd

nd

nd

0.8

0.2

6.3

nd 

0.8

nd

0.4

0.1

nd 

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

~

~

;;

nd

August 1993

Benzene

6.6 

10

11

8.1

3.8

7 

18

19

15

2.2 

2.8

2.7

2.2

2.7

tr 

nd

nd

nd

nd

nd 

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd 

nd

nd 

5.0

high 

fuel

nd 

tr

nd 

tr 

tr

May 1994

Benzene

5.0 

22

16

8.7

8.4

45 

32

24

22

3.8 

3.9

3.7

3.2

2.4

nd 

nd

nd

nd

nd

nd 

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd 

nd

nd 

0.5

32 

fuel

nd 

tr

nd 

tr 

tr

22 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 8. Concentrations of volatile organic carbon compounds in vapor and water samples, Diego Garcia AtollContinued

[Values are in parts per billion; measurements by gas chromatograph with a photoionization detector; , no sample collected; nd, no detection; 

n.a., not applicable; tr, trace]

Well or type 

of sample

C3-4

C3-4

C3-5

C3-5

C3-6

C3-6

C4-1

C4-1

C4-2

C4-2

C4-3

C4-3

C5-1

C5-1

C6-1

C6-1

Altitude 

(feet)

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

Water or 

vapor 

sample

vapor

water

vapor

water

vapor

water

vapor

water

vapor

water

vapor

water

vapor

water

vapor

water

Benzene

--

nd

--

nd

--

nd

--

nd

--

--

--

nd

--

nd

--

--

February 1993

Xylene1

--

nd

--

nd

--

nd

--

nd

--

--

--

nd

--

nd

--

--

Toluene1

--

nd

--

nd

--

nd

--

nd

--

--

--

nd

--

nd

--

--

August 1993

Benzene

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

--

nd

-

nd

May 1994

Benzene

nd

nd

--

nd

nd

nd

nd

nd

--

--

nd

nd

nd

nd

nd

nd

Xylene and toluene quantified by gas chromatograph-mass spectrometer (see table 9)

Notes:

SI

51 -vapor

52

53

Cl-2

C2-1

C3-1

C3-2

C3-3

C3-5

C4-2

All samples

Feb. 1993

All samples

All samples

May 1994

All samples

May 1994 and Aug. 1993

All samples

All samples

May 1994

May 1994

Many hydrocarbons with unresolved high-level peaks

Line blocked, no sample

Many hydrocarbons with unresolved high-level peaks

Many hydrocarbons with unresolved low-level peaks

Water with few small peaks

Fuel in well, no water sample; many high-level peaks in vapor

Water with one low-level peak

No vapor sample, casing screened above ground

Unresolved low-level peaks in water and vapor

Hole in casing above ground, no vapor sample

Casing broken, plant roots in well, not sampled

Geochemistry of Ground Water, Gases in the Unsaturated Zone, and Fuel Hydrocarbons 23
Wells that were contaminated in February 1993 

remained contaminated and showed increasing benzene 

concentrations. However, wells that were uncontaminated in February 1993 remained free of detectable contamination. The relatively coarse spatial distribution of 

wells prevents observing small-scale movement of the 

plume.

During the August 1993 and May 1994 trips, samples from wells that showed detectable benzene were 

subject to laboratory GC-MS analysis of VOC. These 

results are shown in table 9, as are the benzene concentrations measured in the field by GC-PID. For a given 

well, the laboratory GC-MS analysis gave lower benzene values than the field GC-PID because field GCPID is less efficient at separating compounds, so the 

benzene peak contains some non-benzene compounds.

Site S2 had the highest concentration of contaminants, followed closely by SI. Site S3 had the lowest 

concentration of volatile organic compounds.Typically, 

aromatic compounds dominated the VOC pool. 

Because JP-5 is a relatively heavy distillate, it is not surprising that benzene was in much lower concentrations 

than several other aromatic compounds, and typically 

constituted less than 5 percent of the total mass of identified aromatic compounds. Naphthalene was consistantly the most common aromatic compound identified, 

and typically constituted 30 to 40 percent of the total 

mass of identified aromatic compounds; other common 

compounds were ethyl benzene, propylbenzenes (isoand n-), trimethyl benzenes, and tetramethyl benzenes.

EVALUATION OF MICROBIAL 

RESPIRATION AND FUEL DEGRADATION

Excess DIG as an Indicator of Organic 

Respiration

The concentration of DIG in water samples from 

unsarurated and saturated parts of the aquifer was calculated using WATEQ4F (Ball and Nordstrom, 1991) and 

measured concentrations of major ions, pH, and temperature. These results are shown in table 10 (at end of 

report), along with the calculated charge balance and 

saturation state with respect to the mineral aragonite.

The average charge balance for all samples was 3.1 

percent, with a range of -2.5 percent to 20.6 percent. A 

histogram of the calculated charge balances is shown in 

figure 11. Most samples are in the range of 2 to 4 percent. The average positive charge balance indicates a 

slight bias in the analysis of major ions, either through 

overdetermination of cations (predominantly Ca2+ and 

Mg2+) or underdetermination of anions (predominantly 

titration alkalinity). The charge balance for most samples was quite reasonable, and indicates that the analytical results are accurate. Samples collected by lysimeter 

at site SI had the worst charge balances; the results of 

this analysis must be considered suspect.

High concentrations of DIG were found in all 

water samples. This high DIG results from both the dissolution of carbonate mineral and the oxidation of 

organic carbon to inorganic carbon. The dissolution of 

carbonate minerals can be calculated from the stoichiometric release of bivalent cations (Ca2+, Mg2+, and 

Sr2"1"). The difference between the total DIG and that 

released from mineral dissolution is attributed to 

organic respiration. For each water sample, the amount 

of organic respiration and mineral dissolution is given 

in table 10.

The relation between the release of DIG from respiration and the release of DIG from mineral dissolution 

is shown in figure 12. The data appear to define a reaction path whereby the release of DIG (and acidity) from 

the oxidation of organic material stimulates the dissolution of carbonate minerals. Although data from the 

unsaturated part of the aquifer (sampled by lysimeter) 

shows slightly greater scatter, the trend for all data can 

be described by a line having the regression (Model II 

in Sokal and Rohlf, 1969) equation: Org-C = 1.7 * 

Carb-C - 1.7 (n=85, r^O.91).

The saturation state of the water with respect to 

aragonite (table 10) or calcite (not shown) does not 

change with increasing amounts of carbonate mineral 

dissolution. Because most water samples were slightly 

oversaturated with respect to aragonite, it appears that 

the reactive phase is a high-Mg calcite. This is supported by the high Mg+2 :Ca+2 ratios in ground-water 

samples.

The highest concentrations of DIG were found in 

samples in the contaminated part of the aquifer. A 

hydrologic section of DIG from respiration is shown in 

figure 13. As with figure 10, some care must be taken in 

the interpretation because the section does not fall 

directly on a flow path and samples were not collected 

in the presumed center of the plume. However, when 

considered with measured contaminant concentrations, 

the data indicate that the presence of fuel contaminants

24 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 9. Concentrations of selected volatile organic carbon compounds found in ground water, Diego Garcia Atoll

[Analysis by laboratory gas chromatograph-mass spectrometer method using U.S. Environmental Protection Agency method 524.2; all concentrations in parts per 

value shown]

m

g

o

3

2.

5 o'

5 

o;

sL

30

o

D>

a 

 n

2. 

o

<B 

(O

0>

O.

A

O

Well site (altitude of sample, in feet)

Methyl-/erf-butyl ether

Chloroform

Bromodichloromethane

Dibromochloromethane

Bromoform

Benzene

Benzene (field) 1

Toluene

Ethyl benzene

m- and /7-Xylene

0-Xylene

Isopropylbenzene

n-Propylbenzene

1 ,3,5-Trimethylbenzene

2-Ethyl toluene

terf-Butylbenzene

1 ,2,4-Trimethylbenzene

sec-Butylbenzene

p-Isopropyltoluene

1 ,2,3-Trimethylbenzene

H-Butylbenzene

1 ,2,3,4-Tetramethylbenzene

1 ,2,3,5-Tetramethylbenzene

Naphthalene

S1

8-93

0.1

0.8

0.7

1.3

0.8

8.9

10.0

0.3

35.0

1.2

0.1

15.3

17.0

0.6

2.4

0.9

9.3

15.8

2.9

2.4

19.5

98.0

41.0

170

(-9)

5-94

>0.2

1.2

0.4

2.0

>0.2

7.0

22.0

0.4

34.3

4.4

0.9

15.2

45.6

0.2

2.5

1.0

10.4

16.8

8.4

3.1

22.1

101

35.0

196

S1 (-20)

8-93

0.5

0.4

0.5

0.9

0.6

9.7

10.5

>0.2

30.4

0.1

0.1

10.9

26.2

0.2

0.5

0.7

2.6

10.7

0.3

0.8

12.4

59.0

18.9

140

5-94

0.5

0.4

0.4

0.8

0.6

10.1

16.0

0.2

33.2

1.0

0.2

11.8

28.9

0.2

0.9

0.8

3.5

11.8

0.8

24.0

15.6

62.8

19.8

163

S1 (-31)

8-93

1.1

0.3

0.4

0.9

0.7

7.2

8.1

>0.2

13.3

0.1

0.0

6.2

13.8

0.2

0.2

0.4

1.1

4.1

0.4

0.1

>0.2

30.6

11.3

94

5-94

0.9

0.6

0.8

1.1

>0.2

7.6

8.7

0.2

14.0

0.2

0.2

6.5

5.2

0.1

0.6

0.6

0.9

3.6

2.5

4.7

1.5

49.4

15.3

138

S1 (-39)

8-93

0.8

>0.2

0.3

0.8

0.6

3.4

3.8

>0.2

3.8

>0.2

>0.2

1.8

3.5

0.3

0.2

>0.2

1.1

1.3

0.2

>0.2

0.9

11.6

3.5

39

5-94

0.9

0.4

0.2

0.4

0.1

5.3

8.4

0.2

4.0

0.1

0.0

1.9

3.7

0.3

0.2

0.2

0.3

1.5

0.3

0.5

1.1

19.2

6.2

76

8-9

0

1

1

1

1

14

18

0

39

0

0

16

48

0

1

0

40

14

4

2

20

120

22

211
o

o

c

10>

O

(D

O 

O

(D

(D 

W

3)

D) 

XI

q 5'

(Q

o 

O

D) 

O5'

to 

<o

CO

Table 9. Concentrations of selected volatile organic carbon compounds found in ground water, Diego Garcia Atoll-Continued

[Analysis by laboratory gas chromatograph-mass spectrometer method using U.S. Environmental Protection Agency method 524.2; all concentrations in parts pe

value shown]

Well site (altitude of sample, in feet)

S2 (-29)

Methyl-rerf-butyl ether

Chloroform

Bromodichloromethane

Dibromochloromethane

Bromoform

Benzene

Benzene (field) 1

Toluene

Ethyl benzene

m- and p-Xylene

0-Xylene

Isopropylbenzene

n-Propylbenzene

1 ,3,5-Trimethylbenzene

2-Ethyl toluene

terf-Butylbenzene

1 ,2,4-Trimethylbenzene

sec-Butylbenzene

/7-Isopropyltoluene

1 ,2,3-Trimethylbenzene

H-Butylbenzene

1 ,2,3,4-Tetramethylbenzene

1 ,2,3,5-Tetramethylbenzene

Naphthalene

8-93

0.8

0.6

0.6

0.9

0.6

15.0

14.6

0.1

33.0

0.3

0.1

15.1

38.0

0.0

1.0

0.9

1.5

13.8

0.7

2.1

17.3

81.0

26.0

180

5-94

0.9

0.6

1.2

1.3

0.5

11.2

22.0

0.5

33.9

0.3

0.1

15.3

39.8

0.5

0.7

1.3

1.8

13.9

1.0

2.5

14.6

81.6

28.6

193

S3 (-11)

8-93

0.6

0.2

0.6

1.5

1.1

1.7

2.8

0.2

2.8

0.1

0.3

1.9

4.5

>0.2

1.2

0.8

1.8

5.4

0.3

1.1

5.7

56.0

23.0

66

5-94

0.6

0.8

1.2

1.7

0.4

1.8

3.9

0.3

3.0

0.3

0.5

1.8

4.8

0.3

0.7

0.8

1.2

5.9

0.2

0.8

5.9

70.7

23.2

49

S3 (-21)

8-93

0.6

>0.2

0.2

0.5

0.5

1.5

2.7

>0.2

1.5

0.1

0.2

1.2

2.5

>0.2

0.7

0.6

0.8

4.4

0.3

0.6

4.0

54.0

17.0

58

5-94

0.6

0.5

0.8

1.1

0.3

0.9

3.7

0.2

1.3

0.4

0.5

1.3

2.7

>0.2

0.7

0.6

0.3

4.7

0.3

0.6

4.5

59.4

18.5

47

S3 (-30)

8-93

0.6

>0.2

0.5

1.1

0.9

1.2

2.2

>0.2

0.9

0.1

0.1

0.7

1.3

>0.2

0.5

0.3

0.2

2.5

>0.2

0.4

1.9

36.0

9.9

39

5-94

0.6

0.5

0.6

0.8

0.2

1.2

3.2

0.2

1.3

0.2

0.3

1.2

2.4

>0.2

0.5

0.5

0.2

4.4

0.1

0.5

3.8

54.7

14.8

44

Measured on-island by gas chromatograph with a photoionization detector (GC-PID)
CO111

CO

35

30

25

20

£ 15

CO

10

-2 4 6 8 10 

CHARGE BALANCE, IN PERCENT

12 18 22

Figure 11. Distribution of charge balance calculated using WATEQ4F (Ball and Nordstrom, 1991) for 

ground-water samples collected in February 1993, August 1993, and May 1994, Diego Garcia Atoll.

ID

Z" 14 

O

CC

CL 12

CO _. HI CC

0 "^ 10

CC LLJ

U- Q.

§ES e

z|

CC =d 6

< ^

O ^

o -

Z 4

<

O

CC

o

Z 2

0

I I I I I I I I I

 WATER SAMPLE FROM WELL

© WATER SAMPLE FROM LYSIMETER

t 

*

 / 

©

 ©

-

**

 0

© 9 4^0

. © . ©

O 8*O

 0 * 

f 9 

%*§ * A^

<* 0*0

I I I I I I I I I

0123456789 10 

INORGANIC CARBON (DIG) FROM MINERAL DISSOLUTION, IN MILLIMOLES PER LITER

Figure 12. Relation between dissolved inorganic carbon (DIG) released from carbonate mineral dissolution and 

dissolved inorganic carbon (DIG) released from microbial respiration for ground-water samples collected in 

February 1993, August 1993, and May 1994, Diego Garcia Atoll.

Evaluation of Microbial Respiration and Fuel Degradation 27
A'

FEET

Sea level 

-10-

-20-

-30-

-40-

-50

IS' en, w

Midpoint 

of screened 

interval \

Vertical exaggeration x 9

EXPLANATION

10 LINE OF EQUAL DISSOLVED INORGANIC 

CARBON CONCENTRATION-Interval 

2 millimoles per liter

200

i

400 FEET

i

100 METERS

Figure 12. Hydrologic section of dissolved inorganic carbon concentration at well sites S1, S2, S3, and S5 in 

May 1994, Diego Garcia Atoll. Line of section shown in figure 2.

28 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
leads to much higher levels of organic respiration, and 

that bacterial activity in the aquifer is actively degrading the fuel.

At well sites SI and S2, slightly more than half of 

the increase in DIG from respiration appeared to occur 

in the unsaturated zone. Although the highest levels of 

respired DIG were observed in the water table, it may be 

that most activity occurs in the unsaturated zone or very 

close to the water table. The tidal oscillations in the 

water table may provide a favorable environment for 

microbial respiration, with rewetting over a large interval followed by reexposure to gas phases in the unsaturated zone. At site S5, the unsaturated zone waters had 

higher levels of respired DIG than water from below the 

water table. It may be that transient storms with large 

rainfall create pulses of recharge than do not undergo 

substantial reaction in the unsaturated zone, and that 

these fluids were not sampled by the lysimeters.

Mechanisms of Microbial Oxidation

Heterotrophic bacteria oxidize organic carbon as a 

source of energy; this oxidation requires the reduction 

of a terminal electron acceptor. Different terminal electron acceptors have different thermodynamic efficiencies, and yield differing amounts of energy. In order of 

decreasing yield, the most common terminal electron 

acceptors (and their associated metabolic pathways) 

are: oxygen (oxic respiration), nitrate (nitrate reduction), iron (iron reduction), sulfate (sulfate reduction), 

and other organic molecules (methanogenesis). Per 

molecule of glucose oxidized, the energy yield from 

methanogenesis is about 11 percent of the energy 

released during oxic respiration (Froelich and others, 

1979). Given a sufficient amount of organic carbon and 

nutrients for growth, bacteria will sequentially utilize 

the most energy-yielding terminal electron acceptor 

until it is depleted, then switch to the next most efficient 

acceptor.

At an atoll such as Diego Garcia, this process is 

somewhat simplified by the absence of appreciable 

quantities of iron. Furthermore, nitrate tends to be found 

in relatively low concentrations. Thus, the primary 

mechanisms of microbial oxidation are oxic respiration, 

sulfate reduction, and methanogenesis.

Oxygen concentrations in the unsaturated zone 

were considerably lower than in the atmosphere, indicating high amounts of oxic respiration in the unsaturated zone. Vapor samples from the most contaminated 

sites (SI and S2) tended to have the lowest C>2 concentrations. It appears as if metabolic activity in the unsaturated part of the aquifer is high enough to consume 

most of the oxygen. The flow of oxygen into the aquifer 

is enhanced by the diurnal oscillations in water level, 

which displace about half of the volume of the unsaturated zone with every tide cycle. However, this exchange 

may be retarded throughout much of the area by the 

thick cement paving of the South Ramp and taxiway 

and also by the asphalt-covered stormwater collection 

areas between the ramp and taxiway.

Because oxygen concentrations in the unsaturated 

part of the aquifer are low, water recharging the basal 

aquifer arrives with little oxygen. Concentrations of dissolved oxygen from well samples were mostly at or near 

the level of detection. It is unlikely that significant 

amounts of organic carbon are respired by oxic respiration below the water table. If recharge water was in 

equilibrium with a soil vapor C>2 concentration of 5 percent (table 3), the consumption of this oxygen would 

produce less than 0.06 mM of DIG. Although most of 

the respiration may be occurring in the unsaturated 

zone, there appears to be significant respiration within 

the saturated aquifer. Because 62 concentrations are 

low, the oxidation of organic carbon in the saturated 

part of the aquifer is occurring principally by mechanisms other than oxic respiration.

Concentrations of f^S in water from wells at contaminated sites (SI, S2, and S3) typically exceeded 5 

mg/L. Assuming a 2:1 reaction stoichiometry between

f\

organic carbon and SO4 , the observed rise would contribute about 0.3 mM of DIG due to respiration. 

Although this is more than can be attributed to oxic respiration, it is not enough to explain the apparent 

increase in DIG from respiration in the saturated zone.

The high levels of CH4 observed in the soil vapor 

at the most active (and contaminated sites) indicate that 

methanogenesis is an important contributor to total 

microbial respiration. Because methanogenesis is a 

strictly anaerobic process, it is unlikely to be a significant process in the unsaturated part of the aquifer, 

except perhaps in the most contaminated or microbially 

active parts of the unsaturated zone.

Taken as a whole, the data demonstrate that microbial activity exerts a major control on the aquifer 

geochemistry at the Air Operations area. Sites contaminated by fuel have much higher levels of microbial

Evaluation of Microbial Respiration and Fuel Degradation 29
activity than uncontaminated sites, indicating that the 

fuel is being oxidized as an energy source for microbial 

metabolism. Most activity appears to be in the unsaturated part of the aquifer. The rate of this activity significantly lowers the concentration of oxygen. Oxygen 

concentrations typically were about 4 percent in much 

of the unsaturated zone. This may represent a metabolic 

threshold below which oxic respiration of organic matter in the aquifer slows or may reflect the average concentration of C>2 across oxic and anoxic 

microenvironments. It is likely the overall amount of 

oxic respiration is determined by the physics of oxygen 

delivery to the aquifer. Following oxygen depletion, 

denitrification and sulfate reduction account for minor 

amounts of the microbial respiration. However, because 

the pools of nitrate and sulfate are small, methanogenesis must be the predominant pathway for microbial respiration following the depletion of oxygen. The high 

concentrations of CH4 in the soil vapor at the most 

active sites support this interpretation. Because methanogenesis is a low-energy yielding process and the most 

easily degraded compounds will be oxidized first, it is 

likely that although microbial oxidation is clearly 

enhanced at sites contaminated by fuel, the breakdown 

of compounds that are resistant to oxidation will proceed slowly if at all.

Stormwater Receiving Pond

A pond at the north end of the South Ramp 

receives rain runoff from the ramp and parts of the taxiway. Rainwater reaches the pond through an underground culvert and storm drains located in asphalt 

swales between the ramp and taxi way. The pond is connected with the lagoon by a valve that allows discharge 

of rainwater but nominally prevents inflow of seawater. 

The pond is brackish, with a Cl" concentration corresponding to 1 to 3 percent that of seawater. The pond 

contains an active population of phytoplankton, as indicated by the green residue on filter paper, the high concentrations of dissolved C>2, the high pH values, and the 

low concentrations of DIG.

Free-phase fuel was observed discharging along 

the margin of the pond closest to the ramp in June 1991, 

February 1993, August 1993, and May 1994. Fuel also 

appears to be entering the pond from the culvert. The 

bottom of the pond is covered with a thick layer of 

organic material that appears to be a microbial mat. Disturbing this mat near the edge causes release of both 

free-phase fuel and gas bubbles. Analysis of the gas 

bubbles by GC-PID in the field revealed high concentrations of VOC compounds. It appears that there is 

both a surface and subsurface flux of fuel contaminants 

into the pond.

Analysis with GC-PID in February 1993, August 

1993, and May 1994 did not detect any VOC compounds in water collected from a depth of 6 in. in the 

pond (pond depth estimated at 6 to 10 ft). Samples were 

collected about 50 ft from the edge of the pond. In 

August 1993, a series of five samples was collected 

from the pond edge to a distance of 50 ft. The sample 

closest to the edge showed a trace of VOC contamination; all others were clean. Despite having received a 

steady flux of fuel for several years, it appears the contaminants are not accumulating in pond waters. It is 

likely that volatilization and degradation prevent this 

accumulation. Although the assimilative capacity of the 

pond is unknown, the discharge of contaminants to 

pond waters is not necessarily detrimental to pond ecology. Contaminants do not appear to persist in the pond 

or to be transported to the lagoon.

Degradation of Aromatic Hydrocarbons

Concentrations of DIG and other compounds associated with microbial metabolism demonstrate that fuel 

is actively being degraded by naturally occurring bacteria within the aquifer. Although aromatic compounds 

are a minor component in JP-5, they pose the greatest 

health risk due to the combination of their high toxicity 

and solubility. Also, benzene is considered resistant to 

microbial degradation, particularly under the anaerobic 

conditions that prevail at Diego Garcia. Consequently, 

the degradation of aromatic compounds merits special 

attention. Degradation was evaluated using the GC-MS 

analysis for VOC conducted at contaminated wells in 

August 1993 and May 1994.

Table 11 summarizes the concentrations and abundances of selected aromatic compounds. Also shown 

are various ratios within a sample between compounds, 

as well as the changes in compounds and ratios between 

compounds at different wells at a given site. Finally, the 

concentration of benzene is calculated by assuming 

conservative behavior of compounds considered less 

susceptible to degradation than benzene (Wilson and 

others, 1994).

30 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 11 . Concentrations and relative percentage of three major groups of aromatic hydrocarbons in ground water, Diego Garcia At

[ppb, parts per billion; 1-ring, monocyclic aromatic compounds; n.a., not applicable; VOC, volatile organic compounds-data from table 9]

o

o 

tr

si! 

x

(0

v> o

I 

o

3 

0)

Q.

O 

(0 

(D

5

a

to

o'

3

S1 (-9)

8-93

Concentrations (ppb)

Benzene 8.9

Other 1-ring 262

Naphthalene 1 70

Percentage of total VOC

Benzene 2.0

1-ring 59

Naphthalene 39

Ratios

1 -ring/benzene 29

Ethylbenzene/benzene 3.9

Naphthalene/benzene 19

1 -ring/naphthalene 1 .5

Change relative to shallowest well 1

Benzene n.a.

Other monocyclic aromatic n.a. 

compounds

Naphthalene n.a.

1 -ring/benzene n.a.

Ethylbenzene/benzene n.a.

Naphthalene/benzene n.a.

1 -ring/naphthalene n.a.

5-94

7.0

301

196

1.4

60

39

43

4.9

28

1.5

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

S1 (-20)

8-93

9.7

174

140

3.0

54

43

18

3.1

14

1.2

1.1

0.7

0.8

0.6

0.8

0.8

0.8

5-94

10.1

215

163

2.6

55

42

21

3.3

16

1.3

1.4

0.7

0.8

0.5

0.7

0.6

0.9

S1 (-31)

8-93

7.2

82

94

3.9

45

51

11

1.8

13

0.9

0.8

0.3

0.6

0.4

0.5

0.7

0.6

5-94

7.6

105

138

3.0

42

55

14

1.8

18

0.8

1.1

0.3

0.7

0.3

0.4

0.7

0.5

S1 (-39)

8-93

3.4

28

39

4.8

. 40

55

8

1.1

11

0.7

0.4

0.1

0.2

0.3

0.3

0.6

0.5

5-94

5.3

39

76

4.4

33

63

7

0.7

14

0.5

0.8

0.1

0.4

0.2

0.2

0.5

0.3

S2 (-

8-93

14.9

329

211

2.7

59

38

22

2.6

14

1.6

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

Predicted benzene concentration if ** is conservative

** = Naphthalene 8.9

** = Tri-methyl benzene 8.9

** = Tetra-methyl benzene 8.9

7.0

7.0

7.0

7.3

2.6

5.0

5.8

14.1

4.2

4.9

1.0

2.7

5.0

2.9

3.3

2.0

1.0

1.0

2.7

0.5

1.3

14.9

14.9

14.9
o

o

c

Q.

O

(0

o 

o

(D

(0 

V)

o

c

3D

I

(D 

(Q 

O

O

m 

o5'

<o

(O 

CO

Table 11. Concentrations and relative percentage of three major groups of aromatic hydrocarbons in ground water, Diego Garcia Ato

[ppb, parts per billion; 1-ring, monocyclic aromatic compounds; n.a., not applicable; VOC, volatile organic compounds-data from table 9]

S2 (-29)

Concentrations (ppb)

Benzene

Other 1-ring

Naphthalene

Percentage of total VOC

Benzene

1-ring

Naphthalene

Ratios

1 -ring/benzene

Ethylbenzene/benzene

Naphthalene/benzene

1 -ring/naphthalene

Change relative to shallowest well

Benzene

Other monocyclic aromatic 

compounds

Naphthalene

1 -ring/benzene

Ethylbenzene/benzene

Naphthalene/benzene

1 -ring/naphthalene

8-93

15.0

231

180

3.5

54

42

15

2.2

12

1.3

i

1.0

0.7

0.9

0.7

0.8

0.8

0.8

5-94

11.2

236

193

2.5

54

44

21

3.0

17

1.2

1.0

0.6

1.0

0.6

0.8

1.0

0.6

S3 (-11)

8-93

1.7

105

66

1.0

61

38

62

1.6

39

1.6

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

5-94

1.8

120

49

1.1

70

29

66

1.6

27

2.5

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

S3 (-21)

8-93

1.5

88

58

1.0

60

39

59

1.0

39

1.5

0.9

0.8

0.9

0.9

0.6

1.0

1.0

5-94

0.9

96

47

0.7

67

32

102

1.4

49

2.1

0.5

0.8

1.0

1.5

0.8

1.8

0.8

S3 (-30)

8-93

1.2

55

39

1.3

58

41

46

0.8

33

1.4

0.7

0.5

0.6

0.7

0.5

0.8

0.9

5

8

4

6

3

7

3

0

0

0

1

0

1

0

Predicted benzene concentration if ** is conservative

** = Naphthalene

** = Tri-methyl benzene

** = Tetra-methyl benzene

12.7

1.3

11.2

11.4

1.0

9.7

1.7

1.0

1.7

1.8

1.8

1.8

1.5

0.5

1.5

1.7

0.7

1.5

1.0

0.2

1.0

1

0

1
The concentrations of the predominant aromatic 

groups and their relative percentages showed a pattern 

with depth at all three sites. Although concentrations 

tended to decrease with depth, it appears the relative 

proportion of naphthalene increased with depth. The 

relative percentage of benzene also showed a slight tendency to increase with depth at given site and time, suggesting that the other monocyclic aromatics are being 

selectively degraded. This can also be seen by looking 

at the ratios of major groups and compounds at different 

depths and sites. Relative to benzene and naphthalene, 

other monocyclic aromatic hydrocarbons as a whole, as 

well as ethylbenzene, show a decrease with depth (ethylbenzene:naphthalene ratio not shown in table 11). 

Relative to benzene, naphthalene also shows a decrease 

with depth at sites SI and S2, but not at S3. Using other 

compounds to predict a benzene concentration also 

indicates that benzene is persistent relative to naphthalene as well as other monocyclic aromatic hydrocarbons 

(including trimethyl- and tetramethyl benzenes). The 

predicted concentration is consistently higher than measured, indicating that benzene is recalcitrant relative to 

all other compounds considered.

Data from table 9 show a clear decrease in toluene 

and xylene both with depth at a site and with decreasing 

levels of contamination. The preferrential degradation 

of these compounds relative to benzene is well documented (Chapelle, 1993). However, trimethyl-, tetramethyl-, and ethylbenzene constituted the bulk of the pool 

of monocyclic aromatics. The apparent degradation of 

these compounds relative to benzene is surprising 

because they are considered less susceptible to microbial breakdown (Wilson and others, 1994). The same is 

true with respect to naphthalene. It is possible that differential partitioning between the free-fuel phase and 

water (and perhaps selective transport in the aquifer) of 

specific compounds explains this result. Nevertheless, 

the data do not indicate that the biodegradation of benzene is a significant phenomenon. Factors controlling 

the distribution and concentration of aromatic hydrocarbons need to be clarified by additional monitoring over 

time and removal of the free-phase fuel from the aquifer.

SUMMARY AND CONCLUSIONS

A jet fuel leak, detected in 1991, contaminated part 

of the aquifer near the South Ramp of Diego Garcia. 

Analyses of major ions, gases, nutrients, and fuel constituents were done to chemically characterize the contamination. Organic and inorganic reactions in the 

aquifer were deduced from chemical analysis of water 

and gas from the contaminated and the uncontaminated 

parts of the aquifer. The study determined the amount of 

respiration that has occurred at sampling locations. 

Chemicals such as oxygen, hydrogen sulfide, and methane that are diagnostic to specific types of metabolism 

provide insight into respiratory processes in the aquifer. 

In contaminated areas, the ratios of specific contaminant compounds give an independent measure of the 

extent of contaminant degradation. A network of 23 

wells was used to collect water samples for study of the 

chemistry of the saturated part of the aquifer. Unsaturated zone vapor and water samples were collected using 

five vapor points and six lysimeters installed at selected 

sites. Samples were collected in February 1993, August 

1993, and May 1994.

Ground-water samples were warm, and had low 

concentrations of oxygen and high concentrations of 

hydrogen sulfide relative to atmospheric equilibrium. 

Gas from the unsaturated zone also had low concentrations of oxygen; samples near contaminated area had 

high levels of methane. Ground-water samples had high 

concentrations of bivalent cations and inorganic nutrients, resulting from inorganic dissolution of carbonate 

minerals and from microbial oxidation of organic material. Both processes occurred to a much higher degree at 

contaminated sites. The high levels of bacterial respiration at contaminated sites indicated that hydrocarbons 

were being actively degraded by naturally occurring 

bacteria in the aquifer.

Hydrocarbons were found dissolved in ground 

water, as a vapor phase in the unsaturated zone, and 

floating as a free-phase layer on top of the water table. 

Limited data indicate that, despite the recovery of more 

than 134,000 gallons, substantial fuel remained in the 

aquifer as a free-phase layer on the water table. Benzene 

concentrations have shown a steady increase in both 

ground water and soil vapor. Consideration of various 

aromatic compounds such as naphthalene, trimethyl 

benzene, and tetramethyl benzene, indicates that while 

hydrocarbons as a whole were being degraded, there 

was no evidence for the degradation of benzene under 

the reducing conditions observed in the aquifer.

Summary and Conclusions 33
REFERENCES CITED

Andrae, M.O., Charlson, R.J., Bruynseels, F., Storms, H., 

Van Grieken, R., and Maenhaut, W., 1986, Internal mixture of sea salt, silicates, and excess sulfate in marine 

aerosols: Science, v. 232, p. 1620-1623.

Ball, J.W., and Nordstrom, O.K., 1991, WATEQ4F: User's 

manual with revised thermodynamic data base and test 

cases for calculating speciation of major, trace and redox 

elements in natural waters: U.S. Geological Survey 

Open-File Report 91-183, 208 p.

Ballestero, T.P., Fiedler, F.R., and Kinner, N.E., 1994, An 

investigation of the relationship between actual and 

apparent gasoline thickness in a uniform sand aquifer: 

Ground Water, v. 32, p. 708-718.

Chapelle, F.H., 1993, Ground-water microbiology and 

geochemistry: New York, John Wiley, 424 p.

Drever, J.I., 1988, The geochemistry of natural waters (2d 

ed.): Englewood Cliffs, N.J, Prentice-Hall, 437 p.

Emery, K.O., Tracy, J.I., Jr., and Ladd, H.S., 1954, Geology 

of Bikini and nearby atolls: U.S. Geological Survey 

Professional Paper 260-A, 256 p.

Froelich, P.N., Klinkhammer, G.P., Bender, M.L., Luedtki, 

N.A., Heath, R., Cullin, P., Dauphin, P., Hammond, D., 

Hartman, B., and Maynard, V., 1979, Early oxidation of 

organic matter in pelagic sediments of the eastern equatorial Atlantic: Suboxic diagenesis: Geochimica et Cosmochimica Acta, v. 43, p. 1075-1090.

Hamlin, S.N., and Anthony, S.S., 1987, Ground-water 

resources of the Laura area, Majuro atoll, Marshall 

Islands: U.S. Geological Survey Water-Resources Investigations Report 87-4047, 69 p.

Hem, J.D., 1989, Study and interpretation of the chemical 

characteristics of natural waters: U.S. Geological Survey 

Professional Paper 2254, 263 p.

Hollibaugh, J.T., Buddemeier, R.W., and Smith, S.V., 1991, 

Contributions of colloidal and high molecular weight 

dissolved material to alkalinity and nutrient concentrations in shallow marine and estuarine ecosystems: 

Marine Chemistry, v. 34, p. 1-27.

Hunt, C.D., Jr., 1991, Climate-driven saltwater intrusion in 

atoll, in Peters, H.J., ed, Ground Water in Pacific Rim 

Countries: Symposium of the Irrigation and Drainage 

Division - American Society of Civil Engineers, Honolulu, Hawaii, July 23-25, 1991, p. 43-49.

Huntley, D., Wallace, J.W., and Hawk, R.N., 1994, Nonaqueous phase hydrocarbon in a fine-grained sandstone: 2. 

Effect of local sediment variability on the estimation of 

hydrocarbon volumes: Ground Water, v. 32, p. 778-783.

Kennett, J.P., 1982, Marine geology: Englewood Cliffs, N.J., 

Prentice-Hall, 752 p.

Krauskopf, K.B., 1979, Introduction to geochemistry (2d 

ed.): New York, McGraw-Hill, 617 p.

Morse, J.W., and Mackenzie, F.T., 1990, Geochemistry of 

sedimentary carbonates in Developments in Sedimentology 48: Amsterdam, Elsevier, 707 p.

Mouri, H., and Okada, K., 1993, Shattering and modification 

of sea-salt particles in the marine atmosphere: Geophysical Research Letters, v. 20, p. 49-52.

PRC Toups, 1983, Final submittal engineering study to evaluate potable water supply alternatives and groundwater 

yield at Diego Garcia, British Indian Ocean Territory, 

Volume II (prepared for U.S. Navy, Naval Facilities 

Engineering Command, Pacific Division, Pearl Harbor, 

Hawaii): Orange, Calif., PRC Toups, variously paged, 

7app.

Smith, S.V., Hollibaugh, J.T., Dollar, S.J., and Vink, S., 

1991, Tomales Bay metabolism: C-N-P stoichiometry 

and ecosystem heterotrophy at the land-sea interface: 

Estuarine, Coastal and Shelf Science, v. 33, p. 223-257.

Sokal, R.R., and Rohlf, F.J., 1969, Biometry: San Francisco, 

Calif., W.H. Freeman, 776 p.

Tribble, G.W., 1997, Ground-water geochemistry of Kwajalein Island, Republic of the Marshall Islands, 1991: U.S. 

Geological Survey Water-Resources Investigations 

Report 97-4184, 47 p.

34 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Tribble, G.W., Sansone, F.J., and Smith, S.V., 1990, Stoichiometric modeling of carbon diagenesis within a coral 

reef framework: Geochimica et Cosmochimica Acta, 

v. 54, p. 2439-2449.

Tribble, G.W., Lee, R.W., Wydoski, D.S., Torikai, J.D., 

Hunt, C.D., Jr., 1996, Preliminary assessment of jet fuel 

contamination of an atoll aquifer, in Morganwalp, D.W., 

and Aronson, D.A., eds.: U.S. Geological Survey Toxic 

Substances Hydrology Program- Proceedings of the 

Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993: U.S. Geological Survey WaterResources Investigations Report 94-4014.

Underwood, M.R., 1990, Atoll island hydrogeology: Conceptual and numerical models: Honolulu, Hawaii, University of Hawaii, Ph.D. dissertation, 205 p.

U.S. Environmental Protection Agency, 1989, Proposed rule, 

National primary and secondary drinking water regulations: U.S. Federal Register, v. 54, no. 97, May 22,1989, 

p. 22, 062-22, 160.

Vacher, H.L., and Quinn, T.M., eds., 1997, Geology and 

hydrogeology of carbonate islands. Developments in 

Sedimentology 54: Amsterdam, Elsevier, 933 p.

White, A.F., Peterson, M.L., and Solbau, R.D., 1990, Measurement and interpretation of low levels of dissolved 

oxygen in ground water: Ground Water, v. 28, p. 584- 

590.

Wilson, J.T., Pfeffer, P.M., Weaver, J.W., Kampbell, D.H., 

Wiedemeier, T.H., Hansen, I.E., and Miller, R.N., 1994, 

Intrinsic bioremediation of JP-4 jet fuel, in Symposium 

on Intrinsic Bioremediation of Ground Water, Denver, 

Colorado, August 30-September 1, 1994: U.S Environmental Protection Agency Office of Research and Development EPA/540/R-94/515., August 1994, p. 60-72.

References Cited 35
Table 2. Field data and concentrations of dissolved H2S and O2 in ground-water, rain, seawater, and pond samples, Diego 

Garcia Atoll

[Samples from above sea level at well sites SI, S2, and S5 collected from the unsaturated zone using lysimeters; ft, feet; gal, gallon; °C, degrees Celsius; ppm, 

parts per million; --, not measured; n.a., not applicable]

Well site or 

type of 

sample

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

Altitude 

(ft)

6

5

5

5

-9

-9

-9

-20

-20

-20

-31

-31

-31

-39

-39

-39

7

7

7

6

6

6

-9

-9

-9

-19

-19

-19

-29

-29

-29

-11

-11

-11

-21

-21

-21

-30

-30

-30

-40

-40

Date

2-2-93

2-2-93

8-21-93

5-15-94

1-30-93

8-17-93

5-8-94

1-30-93

8-17-93

5-8-94

1-30-93

8-17-93

5-8-94

1-30-93

8-17-93

5-8-94

2-2-93

8-21-93

5-15-94

2-2-93

8-21-93

5-15-94

2-2-93

17-Aug-93

8-May-94

2-2-93

17-Aug-93

8-May-94

2-2-93

8-17-93

5-8-94

2-1-93

8-16-93

5-7-94

2-1-93

8-16-93

5-7-94

2-1-93

8-16-93

5-7-94

2-1-93

8-16-93

Time

1555

1610

1440

1700

1105

1120

1158

1210

1015

1117

1310

930

1021

1415

840

928

1700

1455

1640

1720

1505

1620

1100

1402

1543

1005

1315

1505

920

1222

1424

1350

1440

1311

1110

1348

1234

1000

1147

1148

1145

1026

Volume (gal)

0.04

0.2

0.2

0.2

9

12

9

6

9

9

8

12

12

8

9

9

0.2

0.2

0.2

0.2

0.2

0.3

12

9

9

12

9

9

9

9

9

12

12

12

12

15

12

10

15

12

12

15

Temperature (°C)

30.9

29.8

30.2

31.5

28.9

29.3

29.0

29.9

29.1

29.0

29.2

28.8

28.6

29.3

28.7

28.2

30.5

30.1

32.3

30.5

30.3

31.9

29.2

29.2

29.8

29.2

29.8

29.8

28.7

29.6

29.7

31.4

32.0

32.1

30.3

31.1

31.3

29.9

30.2

30.7

29.3

29.1

PH

7.12

7.21

7.07

7.03

6.88

6.90

6.76

7.32

7.29

6.98

7.47

7.54

7.29

7.58

7.59

7.48

7.10

6.85

7.90

7.16

7.11

6.96

6.79

6.85

6.72

6.90

6.93

6.82

6.98

6.96

6.84

6.92

6.97

6.90

7.10

7.07

7.00

7.12

7.09

7.02

7.46

7.33

H2S 

(ppm)

~

_

-

2.3

2.4

2.4

3.4

5.4

4.2

5.2

6.4

8.4

3.8

5.2

7.6

_

~

__

--

2.0

2.6

2.3

2.4

2.6

2.4

3.0

3.7

3.4

7.2

7.5

7.4

8.7

8.8

8.5

10.0

9.6

9.4

7.8

8.9

02 

(ppm)

-

-

~

0.6

0.1

0.1

0.4

0.2

0.1

0.3

0.2

0.1

0.4

0.1

0.1

_

-

_

-

0.1

0.1

0.1

0.2

0.2

0.1

0.2

0.2

0.1

0.1

0.2

0.1

0.2

0.1

0.1

0.1

0.1

0.1

0.2

0.1

36 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 2. Field data and concentrations of dissolved H2S and 02 in ground-water, rain, seawater, and pond samples, Diego 

Garcia Atoll-Continued

[Samples from above sea level at well sites SI, S2, and S5 collected from the unsaturated zone using lysimeters; ft, feet; gal, gallon; °C, degrees Celsius; ppm, 

parts per million; --, not measured; n.a., not applicable]

Well site or 

type of 

sample

S3

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

BW21

BW21

BW21

BW21

BW21

BW21

BW21

BW21

BW21

Altitude 

(ft)

-40

-11

-11

-11

-21

-21

-21

-31

-31

-31

-41

-41

-41

4

4

4

3

3

3

-12

-12

-12

-18

-18

-18

-27

-27

-27

-37

-37

-37

-46

-46

-46

-8

-8

-8

-23

-23

-23

-48

-48

-48

Date

5-7-94

1-30-93

8-14-93

5-7-94

1-30-93

8-14-93

5-7-94

1-30-93

8-14-93

5-7-94

1-30-93

8-14-93

5-7-94

2-2-93

8-21-93

5-15-94

2-2-93

8-21-93

5-15-94

1-31-93

8-15-93

5-6-94

1-31-93

8-15-93

5-6-94

1-31-93

8-15-93

5-6-94

1-31-93

8-15-93

5-6-94

1-31-93

8-15-93

5-6-94

2-3-93

8-21-93

5-9-94

2-3-93

8-21-93

5-9-94

2-3-93

8-21-93

5-9-94

Time

1103

1745

1646

1019

1705

1602

941

1600

1450

907

1520

1340

825

1520

1518

1555

1535

1529

1515

1315

1325

1617

1215

1242

1435

1100

1130

1456

1000

1044

1408

900

946

1317

1420

1032

1101

1400

1126

1112

1320

1218

1117

Volume (gal)

12

9

9

12

9

11

9

10

9

12

8

9

12

0.2

0.2

0.2

0.2

0.2

0.3

12

12

12

12

12

12

12

12

10

10

10

10

10

12

10

60

60

12

60

70

24

60

84

36

Temperature (°C)

29.8

30.6

30.0

30.4

30.1

30.0

30.1

29.5

28.9

29.4

28.9

28.5

28.9

31.3

28.7

29.4

30.6

28.2

30.9

27.5

27.4

28.1

27.8

27.5

28.0

28.0

27.4

28.0

27.7

27.2

28.3

27.4

27.0

28.3

27.6

27.6

29.5

27.7

27.8

28.9

27.4

27.5

28.4

PH

7.23

7.34

7.35

7.35

6.88

7.10

7.18

7.21

7.31

7.33

7.50

7.40

7.48

7.00

7.47

7.44

6.91

7.47

7.34

7.36

7.46

7.44

7.16

7.25

7.22

7.34

7.39

7.39

7.41

7.41

7.40

7.48

7.42

7.40

7.41

7.39

7.40

7.49

7.44

7.50

7.82

7.78

7.82

H2S 

(ppm)

10.1

3.6

3.1

2.8

4.2

3.8

3.4

8.2

6.5

5.2

10.2

7.7

8.6

__

~

-

1.0

0.9

0.7

1.1

1.3

1.1

2.4

2.2

2.2

1.3

1.2

1.4

1.3

0.9

1.2

0.3

0.2

0.1

0.4

0.3

0.3

1.3

1.3

1.3

02

(ppm)

0.1

0.1

0.0

0.1

0.2

0.0

0.1

0.2

0.1

0.1

0.1

0.1

0.1

_

~

--

--

0.1

0.1

0.1

0.1

0.1

0.1

0.2

0.1

0.1

0.1

0.1

0.1

0.2

0.1

0.1

0.2

0.9

0.4

0.2

0.6

0.3

0.2

0.6

0.2

Table 2 37
Table 2. Field data and concentrations of dissolved H2S and O2 in ground-water, rain, seawater, and pond samples, Diego 

Garcia Atoll-Continued

[Samples from above sea level at well sites SI, S2, and S5 collected from the unsaturated zone using lysimeters; ft, feet; gal, gallon; °C, degrees Celsius; ppm, 

parts per million; --, not measured; n.a., not applicable]

Well site or 

type of 

sample

Rain

Rain

Seawater

Seawater

Seawater

Seawater

Seawater

Pond

Pond

Pond

Altitude 

(ft)

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

Date

2-4-93

2-4-93

2-3-93

2-7-93

8-22-93

8-22-93

5-9-94

2-3-93

8-20-93

5-9-94

Time

740

1705

950

1700

1000

1056

1630

910

1521

1800

Temperature 

Volume (gal) (°C)

25.4

29.4

27.5

28.9

26.7

27.1

29.0

29.8

27.6

29.1

pH

6.44

6.87

8.16

8.18

8.20

8.19

8.25

9.15

8.86

9.23

H2S 

(ppm)

0.0

0.0

0.0

0.1

0.0

0.0

0.0

0.1

0.0

0.0

02

(ppm)

5.7

5.4

2.8

4.2

4.5

4.4

4.4

5.4

6.1

7.4

38 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 4. Concentrations of major dissolved ions in ground-water, rain, seawater, and pond samples, Diego Garcia Atoll

[ft, feet; mg/L, milligrams per liter; meq/L, milliequivalents per liter; TA, titration alkalinity; n.a., not applicable]

Well site 

or type of 

sample

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

Altitude 

(ft)

6

5

5

5

-9

-9

-9

-20

-20

-20

-31

-31

-31

-39

-39

-39

7

7

7

6

6

6

-9

-9

-9

-19

-19

-19

-29

-29

-29

-11

-11

-11

-21

-21

-21

-30

-30

-30

-40

-40

-40

Date

2-93

2-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

Ca2+ 

(mg/L)

123.0

128.0

147.0

168.0

112.0

114.3

136.0

56.4

52.2

82.8

37.9

35.7

47.8

29.7

31.2

36.0

58.3

47.7

47.4

95.7

116.0

145.0

142.0

116.9

140.0

108.0

95.3

110.0

81.1

85.1

95.9

99.8

94.1

98.8

80.2

79.9

89.0

69.8

76.7

78.8

42.1

45.7

54.1

Mg2+ 

(mg/L)

40.8

48.7

64.2

72.3

102.0

99.0

108.0

69.8

79.0

109.0

62.9

62.6

86.2

54.5

60.9

74.2

38.6

40.0

28.6

43.7

43.9

66.7

99.9

108.7

111.0

99.9

101.6

110.0

107.0

123.9

135.0

65.5

71.6

71.7

59.4

64.4

69.3

59.5

67.4

71.9

66.8

70.6

78.9

Sr2* 

(mg/L)

2.74

2.75

2.84

2.82

2.47

2.55

2.81

2.29

2.21

3.54

1.89

1.74

2.48

1.58

1.61

1.91

0.97

0.68

0.81

2.24

2.65

2.50

3.04

2.86

3.15

3.74

3.52

3.78

4.98

5.17

5.60

3.29

3.26

3.31

3.11

3.21

3.28

3.19

3.14

3.40

1.88

2.24

2.65

Na* 

(mg/L)

18.5

22.3

35.4

15.4

21.8

18.8

20.3

15.7

21.7

20.8

21.4

21.5

23.5

24.6

27.0

24.6

39.2

41.5

19.8

18.6

21.7

14.5

16.4

17.3

17.2

15.8

16.3

15.8

19.1

17.9

17.8

37.5

37.5

39.6

39.4

42.4

41.4

40.7

43.3

42.3

44.8

44.6

45.5

K* 

(mg/L)

1.4

1.0

2.4

0.7

1.4

0.9

0.5

1.1

1.3

1.2

1.6

1.7

1.6

1.7

1.8

1.8

0.8

0.6

0.1

0.7

1.1

0.6

1.2

1.6

1.3

2.6

3.2

3.0

1.5

2.0

2.1

13.4

14.3

14.6

14.0

14.7

14.9

12.8

14.4

14.3

11.3

12.6

13.1

ci-

(mg/L)

2.6

3.1

32.4

7.6

16.6

9.6

10.4

9.8

10.1

9.6

14.8

11.7

10.9

17.4

16.2

10.7

4.7

15.6

9.3

4.7

26.7

9.3

11.4

8.9

7.9

11.2

7.4

7.3

11.1

8.6

8.4

39.3

37.1

36.3

39.2

38.9

38.7

41.6

41.5

41.4

54.6

46.3

46.2

SO42' 

(mg/L)

4.5

2.0

6.2

9.8

1.0

0.8

0.9

4.1

1.9

1.0

14.7

12.7

6.7

27.4

25.9

20.1

11.0

9.2

6.4

0.3

4.6

13.4

0.8

0.8

0.8

0.9

0.6

0.6

2.7

1.5

1.0

2.1

2.7

2.3

3.2

2.2

2.2

4.6

1.7

2.4

12.6

10.3

6.8

TA 

(meq/L)

8.48

10.38

11.23

13.75

14.43

14.16

16.33

8.70

9.55

13.69

7.11

7.08

9.81

6.25

6.78

7.72

7.48

7.03

5.25

8.80

10.22

13.08

15.31

15.08

16.74

13.71

13.66

14.93

13.62

14.66

16.18

10.83

11.18

11.74

9.57

10.10

10.67

9.15

10.06

10.66

7.89

8.56

9.62

Table 4 39
Table 4. Concentrations of major dissolved ions in ground-water, rain, seawater, and pond samples, Diego Garcia AtollContinued

Well site 

or type of 

sample

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

BW21

BW21

BW21

BW21

BW21

BW21

BW21

BW21

BW21

Rain

Rain

Altitude 

(ft)

-11

-11

-11

-21

-21

-21

-31

-31

-31

-41

-41

-41

4

4

4

3

3

3

-12

-12

-12

-18

-18

-18

-27

-27

-27

-37

-37

-37

-46

-46

-46

-8

-8

-8

-23

-23

-23

-48

-48

-48

n.a.

n.a.

Date

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

2-93

Ca2+ 

(mg/L)

60.6

59.4

59.8

88.7

85.3

80.0

47.5

48.9

51.0

37.9

36.7

35.9

74.6

100.2

122.0

80.8

88.9

110.0

69.6

66.0

68.9

74.2

74.0

82.5

59.6

57.2

59.4

59.3

61.9

63.5

60.8

58.0

62.5

69.1

69.1

73.4

59.6

64.3

62.4

26.2

27.0

25.4

0.32

0.35

Mg2+ 

(mg/L)

27.2

27.1

24.7

37.1

35.5

32.9

50.0

51.6

49.0

67.7

68.4

63.8

13.9

27.2

24.7

18.1

38.9

25.1

13.3

13.1

11.9

25.6

25.7

26.2

38.5

38.0

36.4

24.6

26.1

24.7

26.8

24.0

24.7

17.6

16.9

16.9

24.3

21.2

22.8

47.7

49.2

46.0

0.04

0.12

Sr2* 

(mg/L)

0.78

0.75

0.74

1.33

1.25

1.18

2.91

2.63

2.80

4.60

4.55

4.55

0.79

0.90

1.13

0.84

0.90

0.87

1.21

1.08

1.10

1.86

1.82

1.94

3.24

2.95

2.99

3.14

2.95

3.01

3.93

3.76

3.58

1.47

1.46

1.41

1.69

1.66

1.68

1.41

1.57

1.41

0.01

0.00

Na+ 

(mg/L)

15.4

15.0

12.6

19.6

19.4

16.4

26.5

22.7

19.5

37.0

33.5

29.1

61.1

21.5

14.4

16.6

6.9

3.4

6.2

5.0

4.8

6.1

7.1

6.3

8.6

9.2

8.0

5.9

6.6

6.4

6.5

6.7

8.1

7.2

8.7

8.5

7.4

8.1

8.4

26.4

32.2

23.6

0.42

1.03

K*

(mg/L)

12.8

12.6

11.8

13.6

13.6

12.5

10.5

12.1

12.5

6.4

8.2

10.6

1.9

1.7

0.2

2.0

0.2

0.2

2.5

2.2

1.7

1.7

1.9

1.6

1.6

1.9

1.9

2.1

2.4

2.4

2.3

2.9

3.0

1.1

1.2

1.5

1.1

1.2

1.3

2.2

2.4

2.1

0.12

0.14

ci-

(mg/L)

15.2

11.0

9.1

10.9

13.9

11.5

21.7

15.8

12.9

36.8

30.8

21.4

7.5

9.0

4.3

6.4

6.0

3.4

7.5

6.0

5.3

8.6

7.6

7.1

9.5

8.7

7.7

7.2

7.6

6.2

7.7

6.9

7.4

9.5

10.7

10.5

9.1

9.8

9.7

29.7

38.3

31.3

0.05

0.05

SO42' 

(mg/L)

1.2

1.1

1.1

2.6

0.9

0.8

2.2

1.4

0.8

4.3

2.9

1.6

9.0

10.0

3.5

3.8

5.4

3.6

1.6

2.0

1.8

3.1

3.1

3.4

5.4

4.0

2.6

1.7

1.5

2.0

3.0

3.0

2.9

6.2

6.4

7.3

7.0

7.3

7.2

27.9

25.1

23.8

0.50

0.51

TA 

(meq/L)

5.63

5.74

5.48

8.04

7.79

7.11

7.59

7.44

6.96

8.07

7.95

7.52

7.11

7.93

8.03

5.86

7.62

7.33

4.54

4.36

4.42

5.62

5.95

6.01

5.98

5.98

5.84

5.28

5.40

5.31

5.17

5.04

5.24

4.79

4.79

4.87

4.83

4.83

4.87

5.03

4.92

4.84

0.00

0.02

40 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 4. Concentrations of major dissolved ions in ground-water, rain, seawater, and pond samples, Diego Garcia AtollContinued

Well site 

or type of 

sample

Seawater

Seawater

Seawater

Seawater

Seawater

Pond

Pond

Pond

Altitude 

(ft)

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

Date

2-93

2-93

8-93

8-93

5-94

2-93

8-93

5-94

Ca2* 

(mg/L)

404

392

425

420

402

18.6

27.0

26.3

Mg2+

1,263

1,230

1,318

1,298

1,267

29.0

52.9

59.3

(mg/L)

6.00

6.97

7.92

7.84

7.29

0.34

0.49

0.44

Na* 

(mg/L)

10,913

10,438

11,280

11,260

11,610

145.0

253.0

354.0

K* 

(mg/L)

410

405

408

410

397

9.1

13.0

15.8

ci- 

(mg/L)

19,040

18,110

19,271

19,379

21,212

235.0

447.3

634.5

so42-

(mg/L)

2,675

2,586

2,705

2,724

2,806

5.8

94.0

10.0

TA 

(meq/L)

2.31

2.59

2.35

2.27

2.24

2.01

2.99

2.13

Table 4 41
Table 5. Concentrations of dissolved inorganic and organic nutrients in ground-water, rain, seawater, and pond samples, Diego 

Garcia Atoll

[ft, feet; ng/L, micrograms per liter; mg/L, milligrams per liter; DOP, dissolved organic phosphorus; DON, dissolved organic nitrogen; DOC, dissolved 

organic carbon; n.a., not applicable]

Well site or 

type of 

sample

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

S2-

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

Altitude 

(ft)

6

5

5

5

-9

-9

-9

-20

-20

-20

-31

-31

-31

-39

-39

-39

7

7

7

6

6

6

-9

-9

-9

-19

-19

-19

-29

-29

-29

-11

-11

' -11

-21

-21

-21

-30

-30

-30

-40

-40

-40

Date

2-93

2-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

PO "^

1.2

0.3

0

0

15.8

27.3

14.9

12.7

29.7

24.8

11.5

16.1

18.6

11.8

11.8

14.9

5.9

2.5

1.2

0.3

0

0

16.1

35.9

19.8

14.9

29.7

22.3

15.5

23.5

27.3

35.6

50.8

35.9

18.0

28.5

21.1

18.0

18.6

13.6

14.6

16.1

14.9

NO?- (ng/L)

2.0

0.8

3.4

2.8

0.4

2.2

3.4

0.8

7.8

3.9

0.8

6.2

4.2

0.7

6.7

4.5

1,345

141

7.3

7.3

1.1

2.8

6.4

1.1

2.8

1.1

1.1

2.2

0.3

1.1

2.8

0.8

2.8

3.4

0.6

5.0

2.8

0.3

6.7

2.8

0.1

6.2

3.4

NH4+

(ng/L)

3,290

2,670

2,150

1,270

440

569

87.0

1,390

1,090

1,160

990

982

928

537

656

900

9.0

11.0

31.0

1,730

6,020

5,220

434

744

239

1,300

832

310

1,030

860

363

472

698

590

375

502

447

388

487

488

287

356

411

Si(OH)4

1,640

674

2,050

1,310

2,070

937

892

643

816

912

584

531

764

542

515

720

836

1,000

578

1,110

1,910

1,180

1,350

1,340

1,270

1,670

,750

,380

,050

,240

,430

2,200

2,260

2,210

1,860

2,190

2,180

1,960

2,160

2,200

1,700

1,890

2,150

DOP 

(MP/L)

9.9

3.7

13.6

11.1

0.3

12.4

0

0.9

13.6

0

2.2

7.4

5.0

3.4

6.2

6.2

1.9

0

8.7

2.8

9.9

8.7

0

0

6.2

1.9

0

8.7

1.5

3.7

8.7

2.8

1.2

8.7

2.8

2.5

8.7

1.9

7.4

6.2

2.5

5.0

6.2

DON (ng/L)

430

116

621

376

328

298

370

132

175

226

129

122

203

166

104

338

45.0

127

318

189

320

436

112

337

436

182

189

205

295

180

178

537

245

225

279

186

160

146

183

177

72

123

150

DOC

(mg/L)

15.2

2.8

5.5

4.4

13.9

7.6

5.9

6.8

8.0

8.3

5.9

5.7

5.0

4.1

3.7

3.4

1.4

1.6

1.3

7.9

6.1

8.7

10.9

8.5

8.9

8.3

6.3

5.7

11.6

8.4

6.9

17.6

15.4

13.0

11.0

11.3

9.4

9.9

11.2

9.7

4.6

7.0

7.4

42 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 5. Concentrations of dissolved inorganic and organic nutrients in ground-water, rain, seawater, and pond samples, Diego 

Garcia Atoll-Continued

[ft, feet; \igfL, micrograms per liter; mg/L, milligrams per liter; DOP, dissolved organic phosphorus; DON, dissolved organic nitrogen; DOC, dissolved 

organic carbon; n.a., not applicable]

Well site or 

type of 

sample

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

BW21

BW21

BW21

BW21

BW21

BW21

BW21

BW21

BW21

Altitude 

(ft)

-11

-11

-11

-21

-21

-21

-31

-31

-31

-41

-41

-41

4

4

4

3

3

3

-12

-12

-12

-18

-18

-18

-27

-27

-27

-37

-37

-37

-46

-46

-46

-8

-8

-8

-23

-23

-23

-48

-48

-48

Date

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

p<v

16.4

21.1

11.1

17.3

23.5

12.4

16.4

14.9

11.1

18.3

18.6

14.9

7.4

7.4

5.0

6.2

9.9

13.6

28.8

39.6

43.4

24.2

24.8

21.1

15.5

19.8

12.4

12.1

18.6

9.9

7.7

11.1

6.2

16.4

24.8

18.6

10.8

8.7

11.1

8.1

7.4

7.4

N0?-

(ng/t)

0.1

0.6

2.8

0.1

1.7

2.8

0.1

7.8

2.2

0.4

7.3

2.8

109

1.1

5.0

72.0

2.8

3.9

0.6

1.1

1.7

0.3

1.1

1.1

0.3

1.7

1.1

0.1

1.7

0.6

0.3

0.0

0.0

0.1

0.0

134

1.3

0.0

2.8

1.3

2.2

2.8

NH4*

(ng/t)

1,500

1,280

848

2,770

3,000

2,000

2,360

2,540

1,860

2,010

2,180

2,200

3.0

25.0

36.0

2.0

7.0

11.0

51.0

104

60

92.0

125

94

194

250

217

196

227

178

227

167

153

13.0

18.0

6.0

19.0

21.0

6.0

148

165

110

Si(OHj4 

(H9/L)

1,060

969

893

1,600

1,510

1,210

2,070

1,750

1,380

2,410

2,210

2,050

781

608

343

335

169

229

479

378

387

387

539

442

498

498

528

502

563

496

524

531

631

486

596

448

461

385

484

629

549

608

DOP (ng/L)

1.9

5.0

7.4

0.9

5.0

7.4

2.2

12.4

13.6

2.2

3.7

6.2

2.5

11.1

8.7

3.4

6.2

7.4

0

9.9

9.9

0.3

5.0

7.4

0.6

8.7

8.7

1.9

1.2

8.7

2.2

3.7

7.4

3.7

2.5

6.2

4.0

5.0

11.1

3.4

9.9

11.1

DON

45

209

214

70

150

148

56.0

144

83.0

64.0

137

79.0

188

168

246

177

161

256

168

172

218

153

176

195

115

187

201

94.0

132

183

86.0

114

161

162

170

290

159

160

262

109

101

220

DOC 

(mg/L)

4.1

4.5

3.4

4.5

4.9

3.8

2.8

3.2

2.3

2.5

2.9

2.3

2.1

2.3

2

2.7

2.6

3.3

3.3

3.4

2.4

2.8

3.1

2.4

2.1

2.6

2.1

1.9

2.5

1.7

1.8

2.2

1.8

2.1

2.7

2.4

1.9

2.4

1.8

1.3

1.7

1.2

Rain n.a. 2-93 3.1 12.5 7.0 6.0 3.4 0.7

Tables 43
Table 5. Concentrations of dissolved inorganic and organic nutrients in ground-water, rain, seawater, and pond samples, Diego 

Garcia Atoll-Continued

[ft, feet; jig/L, micrograms per liter; mg/L, milligrams per liter; DOP, dissolved organic phosphorus; DON, dissolved organic nitrogen; DOC, dissolved 

organic carbon; n.a., not applicable]

Well site or 

type of 

sample

Rain

Seawater

Seawater

Seawater

Seawater

Seawater

Pond

Pond

Pond

Altitude 

(ft)

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

Date

2-93

2-93

2-93

8-93

8-93

5-94

2-93

8-93

5-94

5$2.5

8.4

5.3

6.8

7.7

5.3

0.6

0.9

1.1

NO "

16.5

3.2

2.5

5.6

11.5

2.8

0.4

0.8

0.6

NHd*

10.0

4.0

56.0

4.0

11.0

3.0

44.0

2.0

28.0

Si(OH)4

6.0

99.0

206

23.0

42.0

63

389

634

397

DOP

5.3

5.0

8.1

7.4

4.6

5.0

5.9

5.6

2.3

DON

102

77.0

139

88.0

118

109

312

386

357

DOC 

(mg/L)

0.8

0.4

0.5

0.3

0.3

0.3

4.9

5.3

5.8

44 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94
Table 10. Charge balance, saturation index with respect ot aragonite, and total dissolved inorganic carbon in groundwater samples, Diego Garcia Atoll

[Calculations done using WATEQ4F (Ball and Nordstrom, 1991)]

Calculated contributions to 

total dissolved inorganic 

carbon from:

Well 

site

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

SI

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S2

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

S3

Altitude of 

sample 

(feet)

6

5

5

5

-9

-9

-9

-20

-20

-20

-31

-31

-31

-39

-39

-39

7

7

7

6

6

6

-9

-9

-9

-19

-19

-19

-29

-29

-29

-11

-11

-11

-21

-21

-21

-30

-30

-30

-40

-40

-40

Date

2-93

2-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

Charge 

balance 

(percent)

20.6

10.3

16.2

6.9

0.9

2.3

0.0

3.8

3.3

1.5

3.9

4.4

3.9

-2.2

1.2

7.7

-1.3

-2.2

-0.8

4.8

-1.8

1.4

3.2

2.2

0.1

3.5

1.2

1.7

-0.6

3.4

2.6

3.9

3.4

1.8

3.0

3.2

5.2

1.5

4.1

2.4

2.1

3.4

4.7

Dissolved 

inorganic 

carbon 

(mM)

9.6

11.5

12.9

15.9

17.7

17.4

21.2

9.3

10.2

16.1

7.3

7.1

10.3

6.3

6.8

7.8

8.6

8.9

5.3

9.9

11.6

15.5

19.6

17.4

22.2

16.7

16.6

18.8

16.0

17.5

20.1

12.8

12.9

14.1

10.5

11.2

12.2

9.9

11.0

12.0

7.9

8.8

10.1

Saturation 

index with 

respect to 

aragonite

0.33

0.49

0.43

0.53

0.19

0.19

0.19

0.19

0.10

0.14

0.07

0.07

0.09

0.04

0.04

0.21

-0.04

-0.40

0.57

0.28

0.35

0.39

0.22

0.37

0.18

0.18

0.14

0.14

0.13

0.13

0.12

0.12

0.16

0.12

0.14

0.14

0.14

0.08

0.14

0.11

0.13

0.08

0.08

Carbonate 

mineral 

dissolution 

(mM)

4.8

5.2

6.3

7.2

7.0

7.0

7.9

4.3

4.6

6.6

3.6

3.5

4.8

3.0

3.3

4.0

3.1

2.8

2.4

4.2

4.7

6.4

7.7

7.4

8.1

6.8

6.6

7.3

6.5

7.3 . 8.0

5.2

5.3

5.5

4.5

4.7

5.1

4.2

4.7

5.0

3.8

4.1

4.6

Organic 

respiration 

(mM)

4.8

6.3

6.6

8.7

10.7

10.4

13.3

5.0

5.6

9.5

3.7

3.6

5.5

3.3

3.5

3.8

5.5

6.0

2.9

5.7

6.9

9.1

11.9

10.0

14.1

9.9

10.0

11.5

9.5

10.2

12.1

7.6

7.6

8.6

6.0

6.5

7.1

5.7

6.3

7.0

4.1

4.7

5.5

Table 10 45
Table 10. Charge balance, saturation index with respect ot aragonite, and total dissolved inorganic carbon in groundwater samples, Diego Garcia Atoll-Continued 

[Calculations done using WATEQ4F (Ball and Nordstrom, 1991)]

Well 

site

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S4

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

S5

BW21

BW21

BW21

BW21

BW21

BW21

BW21

BW21

BW21

Altitude of 

sample 

(feet)

-11

-11

-11

-21

-21

-21

-31

-31

-31

-41

-41

-41

4

4

4

3

3

3

-11

-11

-11

-18

-18

-18

-27

-27

-27

-37

-37

-37

-46

-46

-46

-8

-8

-8

-23

-23

-23

-48

-48

-48

Date

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

1-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

2-93

8-93

5-94

Charge 

balance 

(percent)

4.9

3.2

3.1

5.9

4.9

5.9

-1.3

3.9

7.9

3.1

5.5

8.0

1.0

-1.9

6.5

3.0

0.7

2.9

3.1

2.4

1.9

3.9

-0.3

5.6

4.5

3.3

5.2

-2.5

0.5

1.8

4.2

0.5

2.9

1.6

1.1

3.0

2.4

2.1

2.5

0.5

5.7

-0.8

Dissolved 

inorganic 

carbon 

(mM)

5.9

6.0

5.8

9.8

8.7

7.8

8.0

7.7

7.3

8.0

8.1

7.5

8.4

8.4

8.5

7.2

8.1

7.9

4.9

4.6

4.7

6.3

6.5

6.6

6.4

6.3

6.2

5.6

5.7

5.6

5.4

5.3

5.6

5.1

5.1

5.2

5.1

5.1

5.2

5.1

5.0

4.9

Saturation 

index with 

respect to 

aragonite

0.10

0.14

0.10

-0.07

0.15

0.14

-0.05

0.07

0.07

0.13

0.05

0.07

-0.04

0.55

0.61

-0.17

0.47

0.46

0.08

0.15

0.16

-0.02

0.10

0.11

0.08

0.12

0.13

0.11

0.14

0.14

0.18

0.10

0.13

0.14

0.13

0.19

0.16

0.16

0.20

0.12

0.09

0.11

Calculated contributions to 

total dissolved inorganic 

carbon from:

Carbonate 

mineral 

dissolution 

(mM)

2.6

2.6

2.5

3.8

3.6

3.4

3.3

3.4

3.3

3.8

3.8

3.6

2.4

3.6

4.1

2.8

3.8

3.8

2.3

2.2

2.2

2.9

2.9

3.2

3.1

3.0

3.0

2.5

2.7

2.6

2.7

2.5

2.6

2.5

2.4

2.5

2.5

2.5

2.5

2.6

2.7

2.5

Organic 

respiration 

(mM)

3.3

3.4

3.3

6.1

5.1

4.4

4.8

4.3

4.0

4.2

4.3

3.9

6.0

4.8

4.4

4.4

4.2

4.1

2.6

2.4

2.4

3.4

3.6

3.5

3.2

3.3

3.1

3.1

3.1

3.0

2.7

2.9

2.9

2.7

' 2.7

2.7

2.6

2.6

2.6

2.4

2.2

2.4

46 Ground-Water Geochemistry at the South Ramp Jet Fuel Leak, Diego Garcia Atoll, 1993-94

* U.S. GOVERNMENT PRINTING OFFICE: 2001 689-085 / 16009 Region No. 10