Isotope Geochemistry and Chronology 

of Offshore Ground Water Beneath 

Indian River Bay, Delaware 

by John Karl Böhlke1 and David E. Krantz2

1 U.S. Geological Survey, 431 National Center, 12201 Sunrise Valley Drive, Reston, VA 20192

2

 Department of Earth, Ecological, and Environmental Sciences, University of Toledo, Toledo, OH 43606 

Water-Resources Investigations Report 03-4192 

In cooperation with the 

DELAWARE DEPARTMENT OF NATURAL RESOURCES AND 

ENVIRONMENTAL CONTROL 

Reston, Virginia 

2003 

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY
U.S. Department of the Interior 

GALE A. NORTON, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

The use of trade, product, or firm names in this report is for descriptive purposes only 

and does not imply endorsement by the U.S. Government. 

For additional information contact: 

Chief, Branch of Regional Research, Eastern Region 

U.S. Geological Survey 

431 National Center 

12201 Sunrise Valley Drive 

Reston, Virginia 20192 

Copies of this report can be purchased from: 

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286 

ii 
CONTENTS 

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

Introduction......................................................................................................................... 2 

Purpose and scope........................................................................................................... 2 

Description of study area ................................................................................................ 2 

Acknowledgments .......................................................................................................... 3 

Methods of investigation..................................................................................................... 3 

Drilling, geophysical logging, and well installation....................................................... 3 

Water sampling and analysis .......................................................................................... 4 

Calculation of recharge temperature, excess air, and excess nitrogen............................ 6 

Isotope geochemistry and chronology of ground-water samples ....................................... 7 

Major-element chemistry and water isotopes................................................................. 7 

Dissolved gases, nitrogen species, and nitrogen isotopes............................................... 8 

Ground-water dating by tritium.................................................................................... 10 

Ground-water dating by tritium-helium........................................................................ 11 

Ground-water dating by chlorofluorocarbons............................................................... 11 

Ground-water dating by sulfur hexafluoride ................................................................ 12 

Summary of ground-water ages.................................................................................... 12 

Implications for ground-water movement and nitrogen tranpsort .................................... 13 

Sources and fate of dissolved inorganic nitrogen in ground water............................... 14 

Patterns of ground-water flow ...................................................................................... 14 

Summary and conclusions ................................................................................................ 15 

References......................................................................................................................... 16 

FIGURES 

(all figures are placed at the end of the text) 

1. Map showing ground-water sampling sites and resistivity survey tracklines 

in Indian River Bay near White Neck, Delaware......................................19

2. Streaming resistivity section DE-R-05 from Indian River Bay near White Neck, 

Delaware....................................................................................20 

3. Site map and vertical profiles showing the distribution of pumped ground-water 

samples and geophysical logs.............................................................21

4. Graphs showing relations between chloride and other major ions........................22 

5. Graphs showing comparison of δ2H, δ18O, and chloride concentrations.................23 

6. Graph showing concentrations of argon and nitrogen in pumped ground waters.......24 

7. Vertical profiles showing the concentrations of ammonium, nitrate, initial nitrate, 

and oxygen in pumped ground-waters...................................................25 

8. Graph showing nitrogen concentrations and δ15N values of Holocene sediments, 

with hypothetical isotope fractionation curves for reactant and product............26 

9. Graphs showing documented and inferred records of tritium concentrations in 

precipitation and recharging ground water, compared with measured and 

calculated values for subestuarine ground-waters from Indian River Bay..........27 

10. Graphs showing comparison of environmental tracer data with hypothetical values..28 

iii 
11. Schematic diagrams showing the distribution of fresh and saline ground water 

beneath Indian River Bay near White Neck, Delaware, with the distribution 

of ground-water ages and nitrogen species............................................29

TABLES 

(all tables are placed at the end of the text, after the figures) 

1. Chemical and isotopic data for pumped ground waters and surface waters.............30

2. Concentration and isotopic composition of nitrogen in sediments.......................36

3. Summary of ground-water age interpretations..............................................37

iv 
CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS 

Multiply By To obtain

Length 

 

centimeter (cm) 0.3937 inch 

meter (m) 3.281 foot 

 

Volume 

 

liter (L) 0.2642 gallon 

liter (L) 0.03531 cubic foot 

 

Mass 

 

gram (g) 0.03527 ounce, avoirdupois 

kilogram (kg) 2.205 pound, avoirdupois 

 

Pressure 

 

kilopascal (kPa) 0.009869 atmosphere, standard 

kilopascal (kPa) 0.01 bar 

kilopascal (kPa) 0.2961 inch of mercury at 60 °F 

 

Radioactivity

 

tritium unit (TU) 0.118 becquerel per kg (Bq/kg) of water 

tritium unit (TU) 3.19 picocurie per kg (pCi/kg) of water 

 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F = (1.8 ⋅ °C) + 32 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C = (°F - 32) / 1.8 

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a

geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada,

formerly called Sea Level Datum of 1929. Elevations are given in meters (or feet) above sea level. 

v 
ABBREVIATIONS AND ACRONYMS 

km kilometer 

m meter 

cm centimeter 

μm micrometer 

ft feet 

kg kilogram

g gram

mg milligram

L liter 

mL milliliter 

mmol millimole 

μmol micromole 

fmol femtomole (10-15 mole)

mS/m millisiemens per meter 

μS/m microsiemens per meter 

ccSTP cubic centimeters of dry gas at standard temperature (0 °C) and pressure (1 atmosphere) 

‰ per mil (part per thousand)

pptv parts per trillion by volume (atmospheric mixing ratio of gas in equilibrium with a 

water sample at sea level) 

TU tritium unit (1 atom of 3

H per 1018 atoms of 1H) 

LDEO Lamont Doherty Earth Observatory (Noble Gas Laboratory)

MPTL USGS Menlo Park Tritium Laboratory

NWQL USGS National Water Quality Laboratory

RCFC USGS Reston Chlorofluorocarbon Laboratory

RSIL USGS Reston Stable Isotope Laboratory

RWCL USGS Reston Water Chemistry Laboratory

UDEL University of Delaware at Lewes 

USGS U.S. Geological Survey

vi 
Isotope Geochemistry and Chronology 

of Offshore Ground Water Beneath 

Indian River Bay, Delaware

by John Karl Böhlke1 and David E. Krantz2

1 U.S. Geological Survey, 431 National Center, 12201 Sunrise Valley Drive, Reston, VA 20192

2

 Department of Earth, Ecological, and Environmental Sciences, University of Toledo, Toledo, OH 43606 

ABSTRACT 

agricultural or mixed origin was transported 

laterally beneath the estuary in oxic and 

suboxic fresh ground water. Ammonium

produced by anaerobic degradation of organic 

matter in estuarine sediments was transported 

downward in suboxic saline ground water 

around the freshwater plumes. Many of the 

chemical and isotopic characteristics of the 

subestuarine ground waters are consistent with 

conservative mixing of the fresh (terrestrial) 

and saline (estuarine) endmember water types. 

These data indicate that freshwater plumes 

detected by geophysical surveys beneath Indian 

River Bay represent lateral continuations of the 

active surficial nitrate-contaminated freshwater 

flow systems originating on land, but they do 

not indicate directly the magnitude of fresh 

ground-water discharge or nutrient exchange 

with the estuary. There is evidence that some

of the terrestrial ground-water nitrate is reduced 

before discharging directly beneath the estuary. 

Local estuarine sediment-derived ammonium in 

saline pore water may be a substantial benthic 

source of nitrogen in offshore areas of the 

estuary. 

Results of geophysical surveys in Indian 

River Bay, Delaware, indicate a complex 

pattern of salinity variation in subestuarine 

ground water. Fresh ground-water plumes up 

to about 20 meters thick extending hundreds of 

meters offshore are interspersed with saline 

ground water, with varying degrees of mixing 

along the salinity boundaries. It is possible that 

these features represent pathways for nutrient 

transport and interaction with estuarine surface 

water, but the geophysical data do not indicate 

rates of movement or nutrient sources and 

reactions. In the current study, samples of 

subestuarine ground water from temporary 

wells with short screens placed 3 to 22 meters 

below the sediment-water interface were 

analyzed chemically and isotopically to 

determine the origins, ages, transport pathways, 

and nutrient contents of the fresh and saline 

components. Apparent ground-water ages 

determined from chlorofluorocarbons (CFCs), 

sulfur hexafluoride (SF6), tritium (3

H), and 

helium isotopes (3He and 4He) commonly were 

discordant, but nevertheless indicate that both 

fresh and saline ground waters ranged from a 

few years to at least 50 years in age. Tritiumhelium (3

H-3He) ages, tentatively judged to be 

most reliable, indicate that stratified offshore 

freshwater plumes originating in distant 

recharge areas on land were bounded by 

relatively young saline water that was 

recharged locally from the overlying estuary. 

Undenitrified and partially denitrified nitrate of 

1 
INTRODUCTION 

Submarine or subestuarine discharge of 

ground water could be a source of nitrogen in 

coastal waters, but little is known about its 

distribution or magnitude. Discharging fresh 

ground water might contain dissolved 

constituents derived from recharge areas on 

land, while discharging saline ground water 

might contain constituents derived from

recharge beneath the ocean or estuary. Both 

types also contain constituents derived from

reactions with aquifer materials, but these 

reactions may vary depending on the salinity 

and redox status in the recharge area and the 

hydrogeologic conditions along the groundwater-flow paths. Submarine or subestuarine 

ground-water discharge commonly is focused 

in a narrow zone near the shoreline, but 

offshore springs and other evidence indicate 

that freshwater also can be found far from

shore. 

Purpose and Scope 

This report presents results of part of a 

larger study of the distribution and nutrient 

status of subestuarine ground water in coastal 

bays of the Delmarva Peninsula (Bratton and 

others, in press; Krantz and others, in press; 

Manheim and others, in press). The larger 

study is aimed broadly at understanding the 

role of ground-water/surface-water interactions

in the health of coastal marine ecosystems. It 

includes seismic and streaming resistivity 

surveys of several coastal bays to determine the 

vertical and spatial distribution of sedimentary 

features and freshwater bodies beneath the 

estuaries. The larger study also includes coring 

and pore-water analyses to determine local 

vertical distributions of sediment properties 

along with salinity and concentrations of 

nitrogen, phosphorus, and silica in the shallow 

ground water. The current study was designed 

specifically to investigate the use of

environmental tracers and isotopes to determine 

sources of water and nutrients, transport 

directions and rates, and biogeochemical 

reactions affecting nutrient species in

subestuarine ground waters. This study was 

conducted in Indian River Bay, Delaware, in 

cooperation with the Delaware Department of 

Natural Resources and Environmental Control. 

This report presents a comprehensive set of 

chemical and isotopic analyses of fresh and 

saline ground-water samples that were pumped 

from representative parts of the complex flow 

system underlying Indian River Bay 75 to 300 

m offshore at depths of 3 to 22 m below the 

sediment-water interface. The chemical and 

isotopic data are compared with the 

geophysical results and evaluated with respect 

to the sources and ages of the ground waters 

and major dissolved constituents, including 

nitrogen species. The data, though derived 

from relatively few samples, can be used to test 

hypotheses about flow patterns and nutrient 

transport in offshore ground waters beneath a 

coastal estuary. 

Description of Study Area 

The current study was conducted near the 

southern shore of Indian River Bay in 

southeastern Delaware (fig. 1). The surficial 

aquifer in the study area is the Columbia 

aquifer, which includes the Pliocene Beaverdam

Formation and the overlying Pleistocene Omar 

Formation. These units have lithologies ranging 

from fluvial-deltaic sand and gravel (generally 

lower in the section) to marginal marine sand 

and silty sand (generally higher in the section). 

The Columbia aquifer is known to be 

contaminated with NO3

-

 (nitrate) in some areas 

(Hamilton and others, 1993). Concentrations of

NO3

-

 range from about 30 to 970 μmol/L 

(average = 460 ± 370 μmol/L) at depths of 

around 10-30 m within the “Holts Neck” 

subwatershed that discharges to Indian River 

Bay from the south (Andres, 1992) (A.S. 

Andres, Delaware Geological Survey, written 

commun., 2003). The base of the surficial 

aquifer is defined locally by a compacted silty 

layer at a depth of around 25-30 m, as indicated 

by drilling records and gamma logs in the study 

area (Krantz and others, in press). Along the 

margins of Indian River Bay within 100 m of 

shore, Holocene sediments overlying the aquifer 

commonly consist of less than 1 m of muddy 

sand. Small incised valleys in the pre-Holocene 

surface may be 2-3 m deep, and are generally 

filled with peat overlain by silt and capped with 

a thin veneer of muddy sand. The Holocene 

infill sequence thickens substantially offshore to 

approximately 15 m of silt in the incised valley 

of Indian River. 

2 
Streaming resistivity surveys in the 

Indian River estuary indicate complex patterns 

of salinity variation in subestuarine ground 

water between about 0 and 30 m below the 

sediment-water interface (Krantz and others, in 

press; Manheim and others, in press). Apparent 

interfaces between fresh and saline waters 

range from horizontal to vertical, and may be 

relatively abrupt or more gradual. The 3000-m

section of resistivity transect DE-R-05 from

north of Walter Point to Ellis Point (fig. 2) 

shows distinct zones of high resistivity that are 

interpreted as fresh ground-water bodies 

originating onshore, alternating with zones of 

low resistivity that are interpreted as saline 

water. The two most prominent freshwater 

zones, from 1,000 to 1,450 m and from 2,250 to 

2,800 m along the transect, coincide spatially 

with subestuarine incised valleys that are the 

offshore extensions of modern tributary streams 

occupied by tidal marshes. Another small 

freshwater zone, from 1,800 to 2,000 m, is 

aligned with a small stream that has been 

modified for a boat basin. Sections of the 

White Neck shoreline with low-relief headlands 

(generally less than 2-m elevation), such as 

Holts Landing from 2,000 to 2,250 m and a 

narrow headland between 1,450 and 1,750 m, are characterized by low-resistivity (saline) 

water in the upper 10 to 15 m of the transect. 

Saline water may also appear deep within the 

resistivity transect, for example, below 25 m in 

the section between 1,800 and 2,200 m. 

Acknowledgments 

The authors thank John Bratton (USGS, Woods 

Hole), Frank Manheim (USGS, Reston), Peter 

Swarzenski (USGS, St. Petersburg), Daniel 

Phelan and Robert Shedlock (USGS, 

Baltimore), and William Ullman (University of

Delaware) for field assistance, planning, 

discussions, and supporting data. The 

following people assisted with laboratory 

analyses at USGS in Reston, Virginia: Peggy 

Widman (major dissolved gases); Michael 

Doughten (inorganic chemistry); Wandee 

Kirkland and Eurybiades Busenberg (sulfur 

hexafluoride); Julian Wayland and Gerolamo

Casile (chlorofluorocarbons); Janet Hannon 

(nitrogen isotopes); and Tyler Coplen 

(hydrogen and oxygen isotopes). Brent Turrin 

(Lamont-Doherty Earth Observatory) assisted 

with the tritium-helium analyses. Karen 

Savidge (University of Delaware) provided 

nutrient analyses. Daniel Phelan, Eurybiades 

Busenberg, Scott Andres (Delaware Geological 

Survey), and Richard Greene (Delaware 

Department of Natural Resources and 

Environmental Control) provided constructive 

comments on earlier versions of the 

manuscript. 

METHODS OF INVESTIGATION 

Field geophysical studies to determine 

the distributions of fresh and saline ground 

waters beneath Indian River Bay are described 

in detail elsewhere (Krantz and others, in press; 

Manheim and others, in press). The following 

sections give a brief overview of the procedures 

used to select representative ground-water 

collection sites, obtain the ground-water 

samples, and analyze them chemically and 

isotopically. 

Drilling, Geophysical Logging, and 

Well Installation

The coring operation was conducted in 

October 2001 at four sites offshore from White

Neck near Holts Landing designated as WN1, 

WN2, WN3, and WN4 (fig. 3). The four sites 

were selected for coring, geophysical logging, 

and ground-water sampling, on the basis of the 

streaming resistivity data (figs. 2 and 3). The 

first two coring sites (WN1 and WN2) 

represent end-member conditions interpreted 

from the resistivity profiles; the third and fourth 

sites (WN3 and WN4) complete an onshoreoffshore transect from the margin to the center 

of Indian River Bay. Site WN1 was 

approximately 150 m from the shoreline in an 

area of low subsurface resistivity, which was 

interpreted to indicate an area with little or no 

fresh ground-water discharge. Sites WN2 (75 

m from the shoreline) and WN3 (300 m from

the shoreline) were in an area with high 

subsurface resistivity, interpreted as an area of 

potential fresh ground-water discharge, that 

coincided with a small incised valley identified 

on seismic profiles. Site WN4, in the middle of 

Indian River Bay approximately 1000 m

offshore from Ellis Point, was in the axis of the 

main stem of the incised valley of Indian River 

3 
identified from seismic data (Krantz and others, 

in press). The three main components of the 

coring operation at each site were (1) hydraulic 

vibracoring to recover sediments; (2) 

geophysical logging of the corehole; and (3) 

sampling of the pore fluids from the cores and 

in situ sampling of ground water at the core 

site. Vibracoring, geophysical logging, and 

ground-water sampling were completed at sites 

WN1, WN2, and WN3. Site WN4 was 

occupied for half a day before demobilizing, 

and only the borehole geophysical logging was 

completed at that site. 

The order of operations at WN1, WN2, 

and WN3 was to recover the core, run a gamma 

log of the hole, temporarily set polyvinyl 

chloride (PVC) casing in the hole to run an 

electromagnetic-induction (EM) log, interpret 

the gamma and EM logs, and choose specific 

depths for sampling ground water. A hydraulic 

vibracoring rig developed by MPI Drilling was 

used for coring, setting casing for geophysical 

logging, and advancing drivepoints for groundwater sampling. The drill rig is self-contained 

on a small trailer that was mounted over the 

edge of an 18-m long construction barge 

operated by the State of Delaware. The barge 

can operate in water as shallow as 0.5 m and 

has spuds to stabilize the vessel on the drilling 

site. The core barrel was standard NQ steel 

pipe with 7-cm outer diameter in 1.5-m long 

flush-threaded sections. Geophysical logs 

down the coreholes were recorded using a 

Mount Sopris Instruments MGX II portable 

digital logger running MSLog software, with a 

PGA-1000 gamma-logging tool and a PIA1000 EM probe. The gamma readings were 

made through the steel core barrel, which was 

advanced with a solid, conical drive point. The 

EM conductivity readings were made through 

5-cm diameter PVC casing that was closed at 

the bottom. The PVC casing was set by first 

drilling HQ steel core barrel (9-cm outer 

diameter) with a PVC knock-out drivepoint, 

inserting the PVC casing and filling it with 

freshwater, and then retracting the outer steel 

core barrel. The PVC casing filled with 

freshwater was pulled from the hole after the 

EM logging was completed. Ground-water 

samples for geochemical analyses were 

collected after moving the barge about 2 to 5 m

relative to the corehole location, then 

sequentially advancing the NQ steel core barrel 

with a 25-cm screened drive point to each of 

three depths selected by reviewing the gamma 

and EM logs. Individual drivepoint samples 

were named in the field by the corehole site and 

the mid-screen depth below the bottom of the 

bay in feet. For example, sample WN1-71 is 

from site WN1 at a mid-screen depth of 71 ft 

(feet) below the sediment-water interface. 

Water Sampling and Analysis 

Subestuarine ground-water samples 

were collected from discrete depths by 

pumping from the short-screened drivepoint. 

The geophysical logs were used to select 

sampling depths that represented different 

ground-water masses (varying salinities based 

on EM logs) and that were relatively likely to 

yield water at reasonable pump rates (avoiding 

clay-rich intervals indicated by high gamma 

activities). The drivepoint wells were 

developed and purged using a Waterra inertial 

lift pump with the inlet held above the screen 

until the water was clear of turbidity and 

dissolved oxygen measurements were stable. 

With the exception of Sites WN1-10 and WN3-

61, samples were pumped subsequently 

through two parallel discharge tubes (nylon and 

copper) from the level of the well screen using 

a submersible Bennett pneumatic piston pump. 

The copper discharge line was used mainly for 

sampling chlorofluorocarbons (CFCs) and 

other dissolved gases, and the nylon line was 

used for most of the other chemical and 

isotopic samples. Sites WN1-10 and WN3-61 

were sampled through a single discharge line 

consisting of nylon and Viton tubing with a 

peristaltic pump at the surface. Lubricants in 

the threads of the steel drivepoint casing were 

removed by steam cleaning after sampling was 

completed at Site WN1-25. For comparison to 

the ground-water samples collected in October 

2001, representative surface-water samples 

were collected from Indian River Bay at Holts 

Landing in October 2001 and September 2002, 

and from Rehoboth Beach in May 2002. 

In October 2001, specific conductance, 

pH, and O2 (dissolved oxygen) were measured 

in the field using WTW pH/conductivity and 

pH/O2 meters. One Winkler titration was 

performed to verify the calibration of the O2

meter. Specific conductance, pH, O2, and 

temperature were measured in a closed flowthrough chamber to avoid contact with air. 

4 
Samples to be analyzed for major 

dissolved gases argon (Ar), nitrogen (N2), 

oxygen (O2), and methane (CH4) were collected 

in 125-mL serum bottles, preserved with 

potassium hydroxide (KOH, 2 pellets, or 

approximately 200 mg), and sealed with blue 

butyl rubber stoppers (Bellco) without 

headspace. Gas analyses were done by gas 

chromatography (GC) on low-pressure 

headspace in the USGS Dissolved-Gas 

Laboratory in Reston, Virginia (Plummer, 

2003). Samples to be analyzed for CFCs 

(CFC11, CFC12, and CFC113) were collected 

through copper tubing into flame-sealed glass 

ampules under pure N2 headspace (Busenberg 

and Plummer, 1992). CFCs were extracted by 

a purge-and-trap procedure and analyzed by 

GC with an electron-capture detector in the 

USGS CFC Laboratory in Reston (Plummer, 

2003). For each sample, the measured 

concentrations of CFC11, CFC12, and CFC113 

were converted to equilibrium partial pressures 

at sea level after accounting for the measured 

salinity and for the equilibration temperature 

and excess air concentration indicated by major 

dissolved-gas data (Eurybiades Busenberg, 

USGS, written commun., 2003) (see section in 

this report on Calculation of Recharge 

Temperature, Excess Air, and Excess 

Nitrogen). The equilibrium partial pressures in 

the samples (given as atmospheric mixing 

ratios in parts per trillion by volume, pptv) 

were compared to the atmospheric CFC record 

to determine the apparent year in which the 

water last equilibrated with air, referred to as 

the recharge date (Plummer and Busenberg, 

2000). Samples for sulfur hexafluoride (SF6) 

analysis were collected unfiltered in 2-L glass 

bottles with polyseal caps without headspace. 

In the laboratory, an aliquot for analysis was 

taken from each of two bottles and the analyses 

were done by purge-and-trap GC (Busenberg 

and Plummer, 2000; Plummer, 2003). For each 

sample, the SF6 concentration was converted to 

an equilibrium partial pressure at sea level 

(atmospheric mixing ratio in pptv) after 

accounting for the measured salinity and for the 

equilibration temperature and excess air 

concentration indicated by the dissolved-gas 

data (Eurybiades Busenberg, USGS, written 

commun., 2003). 

Chloride concentrations were measured in the 

field with a Hach field testing kit. Field 

alkalinities were determined by incremental 

titrations using a WTW pH meter and Hach 

titrator. Salinities of surface-water samples 

collected in 2002 were determined in the 

laboratory with a YSI specific conductance 

meter calibrated against IAPSO standard 

seawater with a salinity of 35.00 g/kg. 

Samples for analysis of sodium (Na), 

potassium (K), magnesium (Mg), calcium (Ca), 

strontium (Sr), boron (B), iron (Fe), manganese 

(Mn), silicon (Si, reported as SiO2

2-

), chloride 

(Cl-), bromide (Br-), sulfate (SO4 ), nitrate 

(NO3

-

), and alkalinity were filtered in the field 

(0.45 μm) and analyzed in the USGS Water 

Chemistry Laboratory in Reston, Virginia 

(Michael Doughten, USGS, written commun., 

2002). Cation samples were acidified with 

Ultrex nitric acid and analyzed by directcurrent plasma spectrometry (ARL 

SpectraSpan V). Anions were analyzed by ion 

chromatography (Dionex DX-120), and 

alkalinities were measured with a Radiometer 

Analytical Titralab system autotitrator. 

Uncertainties are estimated to be approximately 

± 5-10 percent for the reported inorganic 

constituents (Michael Doughten, USGS, 

written commun., 2002). Calculated ion 

charges for the major species generally were 

balanced to within a few percent. 

Samples for dissolved nutrient analyses 

were filtered in the field (0.2 μm) and frozen 

before being analyzed at the University of 

Delaware (Karen Savidge, University of 

Delaware, written commun., 2001). Dissolved 

ammonium (NH4

+

), nitrate plus nitrite (NO3

- + 

NO2

-

, abbreviated NO2+3), and phosphate 

(PO4

3-), were analyzed by automated 

colorimetry using an O/I Analytical Flow 

Solution IV Analyzer. Concentrations of NH4

+

were determined by the phenol hypochlorite 

method (Grasshoff and Johansen, 1972; Glibert 

and Loder, 1977). NO2+3 concentration was 

determined by the sulphanilamide/N(1-napthyl) 

ethylene diamine method after cadmium

reduction of NO3

-

 to NO2

-

 (Glibert and Loder, 

1977). PO4

3- was determined by the phosphomolybdenum blue method (Strickland and 

Parsons, 1972). Typical precisions for analyses 

of NH4

+

, NO2+3, and PO4

3- were approximately 

± 0.2-1, ± 1-5, and ± 0.02-0.05 μmol/L, 

respectively (Karen Savidge, University of 

Delaware, written commun., 2002). 

Samples for helium (He), neon (Ne), 

and tritium-helium age determination were 

collected in flow-through copper tubes that 

5 
were crimp-sealed in the field. He and Ne were 

extracted for mass-spectrometric analysis, then 

degassed aliquots of the water were re-analyzed 

after several months to determine tritium (3

H) 

concentrations from helium-3 (3He) in-growth 

at the Lamont-Doherty Earth Observatory 

Noble Gas Laboratory (LDEO), Palisades, New 

York (Ludin and others, 1998). Additional 

samples for 3H analysis were collected 

unfiltered in bottles with polyseal caps. Some

of these samples (mainly ground-water 

replicates for the copper tubes) were analyzed 

by 3He in-growth at LDEO, and others (mainly 

surface waters) were analyzed by liquid 

scintillation counting after electrolytic 

enrichment at the USGS Tritium Laboratory in 

Menlo Park, California with uncertainties of

approximately ± 0.3-0.5 TU (R.L. Michel, 

USGS, written commun., 2002). 

The age of each ground-water sample 

(in years since the time it left contact with air) 

was assumed to be equal to the time indicated 

by radioactive decay of 3H to 3He in a closed 

system, after adjusting the measured 3He 

concentration for atmospheric gas contributions 

and for excess terrigenic He (Schlosser and 

others, 1998) with an assumed 3He/4He ratio of 

2 x 10-8 (Mamyrin and Tolstikhin, 1984). 

Temperatures used to calculate atmospheric 

contributions were determined from the 

concentrations of Ar and Ne, assuming those 

gases had only atmospheric sources (see 

section on Calculation of Recharge 

Temperature, Excess Air, and Excess 

Nitrogen). For each sample, the concentrations

of 3

H and tritiogenic 3He (3He[trit]) were 

converted to ratios of 3H/3H°, where 3H° is the 

initial 3H concentration (3H° = 3H + 3He[trit]), and used to calculate the closed-system 3

H-3He 

radioactive decay age (t): 

t = -1/λ · ln(

3

H/3H°), [1] 

where λ is the 3H decay constant

(0.055764/year). 

Stable isotopic compositions of H, O, 

and N were analyzed as δ2H, δ18O, and δ15N 

values, respectively (generalized as δiE, 

expressed in per mil or ‰): 

δiE = (Rx/Rref – 1) · 1000 ‰, [2] 

where R refers to the isotopic mole ratio of

2

H/1H, 18O/16O, or 15N/14N, x refers to the 

sample, and ref is the primary reference 

material for which the δ value is defined as 0. 

The primary reference materials are VSMOW 

(Vienna Standard Mean Ocean Water) for H 

and O, and “air” (atmospheric N2) for N. 

Samples to be analyzed for H and O isotopes of 

water were collected in 60-mL glass bottles and 

analyzed at the USGS Reston Stable Isotope 

Laboratory in Reston, Virginia (Coplen, 2003) 

by H2 and CO2 equilibration and mass 

spectrometry. δ2H and δ18O values were 

normalized to the VSMOW-SLAP scale 

(Coplen, 1988) with uncertainties of about ± 1 

‰ and ± 0.1 ‰ (1σ), respectively. For N 

isotope analyses of NH4

+

, filtered water 

samples were treated with magnesium oxide 

(MgO) and the resulting ammonia (NH3) was 

distilled from the water and collected in a dilute 

acid solution that was dried and then oxidized 

chemically to produce N2. The N2 was 

analyzed by mass spectrometry for the USGS 

National Water Quality Laboratory by the 

ZymaX Isotope Laboratory, San Luis Obispo, 

California. For N isotope analyses of NO3

-

, 

filtered freshwater samples were freeze-dried 

and the salts were baked in evacuated sealed 

glass tubes with Cu+Cu2O+CaO, then the N2

gas produced in the tubes was expanded into a 

dual-inlet isotope-ratio mass spectrometer in 

the USGS Reston Stable Isotope Laboratory 

(Böhlke and Denver, 1995). Sediment samples 

were dried, ground, and homogenized, then 

baked and analyzed like the NO3

-

 samples. N 

isotope data for NH4

+

, NO3

-

, and sediment were 

normalized to δ15N values of +0.4 ‰ for 

IAEA-N1 and +180 ‰ for USGS-32 (Böhlke 

and Coplen, 1995). For N isotope analyses of 

dissolved ground-water N2, the headspace gas 

remaining in each 125-mL serum bottle after 

GC analysis was sealed into a pair of evacuated 

glass tubes with reagents, baked, and analyzed 

against samples of N2 in air (δ15N = 0.0 ‰) and 

laboratory-equilibrated water (δ15N = +0.7 ± 

0.1 ‰). The overall uncertainties of the 

normalized N isotope analyses are estimated to 

be approximately ± 0.2-0.3 ‰ for NH4

+

 and 

NO3

-

, and ± 0.1 ‰ for N2. 

Calculation of Recharge Temperature, 

Excess Air, and Excess Nitrogen 

The apparent recharge temperature and 

amount of excess air were calculated for each 

6 
ISOTOPE GEOCHEMISTRY

AND CHRONOLOGY OF 

GROUND-WATER SAMPLES 

sample from the concentrations of Ar and Ne 

by assuming two atmospheric dissolved-gas 

components (Aeschbach-Hertig and others, 

1999; Stute and Schlosser, 2000): (1) from air 

equilibration at the elevation of the water table 

and 100-percent relative humidity; and (2) from

unfractionated excess air introduced during 

recharge or sampling. This calculation was 

completed for each sample by solving 

iteratively an equation relating the measured 

concentrations of Ar and Ne to a common 

equilibration temperature and concentration of 

airexcess (excess air): 

Results of analyses and calculations are 

summarized in table 1 and table 2 for water 

samples and sediment samples, respectively. 

For some of the analytes in table 1, such as the 

major dissolved gases, CFCs, and SF6, individual results are given for field replicates 

collected in multiple containers. For some

analytes and calculated values such as salinity, 3

H, and recharge temperature, results are given 

for more than one analytical method. In both 

of these situations, “selected” values are also 

given. The “selected” values represent either 

averages or choices based on critical evaluation 

of the data. An overview of the ground-water 

dating results is given in table 3. 

airexcess in ccSTP/L 

 = {[Ar]meas – [Ar(T,S,elev)]asw} / 

 0.417 μmolAr/ccSTPair

 = {[Ne]meas – [Ne(T,S,elev)]asw} / 

 0.000811 μmolNe/ccSTPair, [3] 

where [Ar,Ne]meas is the measured 

concentration of Ar or Ne in the sample (in 

μmol/L) and [Ar,Ne(T,S,elev)]asw is the 

concentration of Ar or Ne in air-saturated water 

at the appropriate temperature, salinity, and 

elevation (sea level for these samples), 

calculated using Ar and Ne solubility data 

(Weiss, 1970, 1971). The quantity ccSTP 

refers to cubic centimeters of gas at standard 

temperature (0 °C, or degrees Celsius) and 

pressure (1 atmosphere). 

Major-Element Chemistry and Water 

Isotopes 

Patterns of salinity variation in 

subestuarine ground water were inferred from

electromagnetic logging in the vertical 

boreholes (fig. 3). The measured salinities in 

the pumped ground waters confirmed the 

geophysical results indicating that saline 

ground water similar to Indian River Bay 

surface water was present throughout most of 

the sampled interval at site WN1, whereas large 

intervals of freshwater were present at 

intermediate depths at sites WN2 and WN3 

(fig. 3; table 1). Freshwater with Clconcentrations less than 20 mmol/L (millimole 

per liter) was sampled 75 and 300 m offshore 

beneath the bay at depths of around 10-16 m

below the sediment-water interface at sites 

WN2-35, WN2-53, and WN3-34. None of the 

ground-water samples had salinity higher than 

Indian River Bay surface water, which was 

between 80 and 90 percent of standard seawater 

salinity (35 g/kg) in samples collected during 

October 2001 and September 2002. Thus, there 

is no evidence in these samples for intrusion or

discharge of undiluted seawater or brine. 

Concentrations of N2 that are in excess 

of the amounts attributable to atmospheric 

sources may be the result of denitrification. 

The concentration of N2,excess (excess N2

attributable to denitrification) was calculated 

for each sample by using the recharge

temperature and airexcess value derived from Ar 

and Ne data: 

N2,excess in μmol/L 

 = [N2]meas – [N2(T,S,elev.)]asw - 

 [airexcess in ccSTP/L] · 34.8 μmolN2/ccair. [4] 

The detection limit for N2,excess based on these 

calculations and data from this study is 

estimated to be approximately 10 μmol/L 

(equivalent to 20 μmol/L denitrified NO3

-

). For 

some samples lacking reliable Ne data, 

estimates of recharge temperatures, airexcess, and 

N2,excess were based on Ar and N2

concentrations and inter-sample comparisons, 

resulting in larger detection limits of around 

20-30 μmol/L for N2,excess. 

Ratios of major inorganic species 

(Na/Cl-, K/Cl-, Mg/Cl-, Ca/Cl-, Sr/Cl-, B/Cl-, 

Br/Cl-, and SO4

2-/Cl-

) in the ground waters 

generally were consistent with two-component 

mixtures of freshwater and estuarine surface 

7 
water (fig. 4). Even alkalinity is roughly 

correlated with salinity, and that trend is 

consistent with generally small amounts of 

sulfate reduction and methanogenesis, though 

both processes appear to have occurred to a 

limited degree in the saline ground waters. 

SiO2 concentrations are inversely correlated 

with salinity, consistent with mixing of water 

sources and minor additions attributable to 

water-rock interactions (fig. 4). 

The δ2H and δ18O values of fresh ground 

waters are consistent with a local meteoric water 

line defined by δ2H = 8 · δ18O + 14 (fig. 5A). 

Andres (1991) reported 27 analyses of fresh 

ground waters from onshore wells in the 

Rehoboth Bay and Indian River Bay watersheds 

with average values of –38.0 ± 3.6 ‰ for δ2H 

and –6.27 ± 0.58 ‰ for δ18O, which are similar 

to the freshwater endmember values in the 

Indian River Bay subestuarine ground-water 

dataset (fig. 5A). These data are consistent with 

results from other shallow aquifers in the MidAtlantic Coastal Plain, including some from an 

agricultural site near Fairmount, Delaware (fig. 

5) (Dunkle and others, 1993). Correlations 

among δ2H, δ18O, and Cl- for all of the 

subestuarine ground waters are consistent with 

mixing between fresh meteoric water and saline 

estuarine surface water (fig. 5). Furthermore, 

the estuarine surface-water samples plot on a 

mixing line between fresh meteoric water and 

standard open-ocean seawater (δ2H = 0.0, δ18O 

= 0.0, Cl- = 560 mmol/L). These data are 

interpreted to mean that the saline ground water 

was recharged locally beneath the estuary, and 

the surface water in the bay was a brackish 

mixture of seawater and fresh meteoric-water 

discharge from streams and ground water. The 

isotope data do not indicate substantial amounts 

of evaporation of the estuarine surface water 

before it entered the subsurface. A single sample 

collected May 12, 2002 from the ocean at 

Rehoboth Beach was on the same mixing line 

with δ2

H, δ18O, and Cl- values slightly higher 

than those of the estuary. 

Dissolved Gases, Nitrogen Species, 

and Nitrogen Isotopes 

Concentrations of Ne, Ar, and N2 were 

used to estimate recharge temperatures, 

amounts of airexcess attributable to incorporation 

of air during recharge or sampling, and 

amounts of N2,excess attributable to 

denitrification. In samples from sites WN1-71, 

WN2-53, and WN2-68, all three gases yielded 

internally consistent temperatures and airexcess

concentrations, with no evidence of 

denitrification. Samples from site WN2-35 

indicated a significant amount of N2,excess with 

values of temperature and airexcess that were 

similar to the values at other sites. In contrast, 

samples from site WN3-34 (and possibly WN3-

61) yielded discordant results with anomalous 

temperatures indicating that the He-Ne samples 

from those sites may have been partially 

degassed, possibly during sample collection. 

Overall, the dissolved gas data 

generally indicate that the fresh ground waters 

were recharged at approximately 10-11 °C with 

airexcess concentrations of 0-2 ccSTP/L, whereas 

saline ground waters were recharged at 

approximately 14-16 °C with 0-1 ccSTP/L of 

airexcess (table 1; fig. 6). By comparison, the 

mean annual air temperature at Georgetown, 

Delaware (12 miles northwest of Indian River 

Bay) from 1976-1995 was approximately 12.9 

°C (mean monthly averages ranged from 0.8-

24.7 °C) (UDCANR, 2003); the mean annual 

soil temperature at 2 m depth at Cape 

Henlopen, Delaware from 1997-2001 was 

approximately 14 °C (monthly values ranged 

from about 11-17 °C) (Wehmiller and others, 

2000); and the mean annual water temperature 

in Indian River Bay from 1989-1992 was about 

17.5 °C (ranging from about 5-30 °C) (William

Ullman, University of Delaware, written 

commun., 2002). The average difference 

between the inferred recharge temperatures of 

fresh and saline ground-water samples is 

qualitatively consistent with the slightly higher 

average temperature of Indian River Bay water 

compared to air and soil temperatures on land, 

but the recharge temperatures in both groundwater types apparently are biased slightly to 

lower values. The small difference in the 

average airexcess concentrations may be 

attributed to differences in the recharge 

processes of the fresh and saline ground waters. 

Fresh ground water recharging beneath an 

unsaturated zone on land may be expected to 

incorporate more air from dissolution of 

trapped bubbles than saline ground water 

recharging beneath the standing water and 

saturated sediments in the estuary. 

Samples from only two sites had 

substantial concentrations of O2 and NO3

-

(table 1; fig. 7): WN2-53 and WN3-34 had 26 

8 
to 115 μmol/L O2 (GC analyses) and 51 to 108 

μmol/L NO3

-

 with δ15N[NO3

-

] values of 5.3 to 

6.4 ‰. Both of these samples were freshwater 

and neither appears to have had N2,excess; thus, 

these samples provide evidence for transport of 

NO3

-

 from onshore recharge areas to 

subestuarine positions in the aquifer without 

substantial NO3

-

 reduction. The δ15N values 

are within the range commonly observed in 

NO3

-

 beneath fertilized agricultural fields in the 

Mid-Atlantic region, but they are lower than 

most values observed in areas dominated by 

manure or septic system sources of NO3

-

(Böhlke and Denver, 1995; Kendall and 

Aravena, 2000; Böhlke, 2002, in press). 

The freshwater sample from site WN2-

35 had no measurable O2 or NO3

-

, but contained 

an estimated 29 μmol/L of N2,excess (equivalent 

to 58 μmol/L of denitrified NO3

-

). This sample 

also had an anomalously high value of δ15N[N2] 

(+1.1 ‰) compared to the other samples, which 

had δ15N[N2] = +0.6 to +0.9 ‰ (approximately 

consistent with air equilibration). The gas 

isotopes confirm that the sample from site 

WN2-35 had a substantial component of nonatmospheric N2, which is interpreted as a 

product of denitrification. This sample provides 

evidence that some of the NO3

-

 transported 

beneath the estuary was reduced within the 

aquifer before mixing with saline ground water 

or discharging. The δ15N value of the nonatmospheric N2,excess component is poorly 

defined because of the much larger component 

of atmospheric N2 in the sample, but it could be 

consistent with the δ15N values of the NO3

-

 in 

the oxic samples, indicating a similar source of 

NO3

- in the corresponding recharge. In the 

WN2 profile, denitrified ground water occurred 

above undenitrified ground water within the 

freshwater wedge; however, the location of 

active denitrification and the identity of the 

electron donor(s) are not known. There is no 

evidence for N2,excess attributable to 

denitrification in the saline ground waters, given 

a detection limit of around 10 to 20 μmol/L (20 

to 40 μmol/L of NO3

-

 equivalent). 

Concentrations of NH4

+

 were 49 to 76 

μmol/L in the pumped saline ground waters, 

but less than 3 μmol/L in the fresh ground 

waters. Similarly, concentrations of CH4 were 

0.3 to 0.7 μmol/L in the saline ground waters, 

but less than 0.1 μmol/L in the fresh ground 

waters. These differences may be attributed to 

the contrasting origins of the fresh and saline 

pumped ground waters: (1) the fresh ground 

waters were recharged on land with O2 and 

NO3

-

 and subsequently were reduced partially 

in the aquifer, but not to the point of NH4

+

 or 

CH4 production; and (2) the saline ground 

waters were recharged beneath the estuary and 

acquired NH4

+

 and CH4 while passing through 

reduced organic-rich sediments beneath the 

estuary. Pore waters squeezed from shallow 

sediment cores at sites WN2 and WN3 have a 

wide range of NH4

+

 concentrations from about 

160 to 930 μmol/L (fig. 7; Bratton et al., in 

press). The highest concentrations are in the 

saline pore waters overlying the freshwater 

plume, but some of the freshwaters near the 

upper boundary of the plume also appear to 

have substantial amounts of NH4

+

. These data 

may indicate a source of NH4

+

 in the fresh 

ground water near the top of the plume, 

possibly from recharge in marginal wetlands or 

reactions in shallow nearshore aquifer 

materials. 

Three of the deep saline ground-water 

samples had δ15N[NH4

+

] values of +2.8 to +4.5 

‰ (table 1). Shallow Holocene sediment 

samples had N concentrations ranging from

about 60 to 12,000 mg/kg with δ15N[N-total]

values of -1 to +5 ‰, while deeper Pleistocene 

sediments had N concentrations generally less 

than about 20 mg/kg with unmeasurable δ15N 

(table 2). The overall similarity of the δ15N 

values of aqueous NH4

+

 and Holocene 

sediment N indicates that the NH4

+

 in the saline 

ground water may have been produced by 

mineralization of organic matter in shallow 

sediments undergoing diagenesis. It is 

commonly observed or assumed that NH4

+

released from organic matter by microbial 

degradation has a δ15N value about equal to or 

slightly lower than the δ15N value of the 

organic N source, which may increase as a 

result (Fogel and Cifuentes, 1993; Macko and 

others, 1993; Nadelhoffer and Fry, 1994). 

However, Lehman and others (2002) show that 

the δ15N values of bulk organic pools can also 

decrease during anoxic degradation and 

bacterial growth. A semi-logarithmic inverse 

correlation between N concentration and δ15N 

in the Holocene sediments from Indian River 

Bay could be consistent with varying degrees 

of N loss by a process yielding a product such 

as NH4

+

 that is slightly depleted in 15N (fig. 8). 

If it were assumed that the variations in the N 

concentrations and δ15N values of the 

9 
sediments were caused by progressive N loss, 

then an apparent isotope fractionation factor for 

the loss process could be derived by fitting the 

array of sediment data to an approximation of 

the Rayleigh fractionation equation: 

δ15N ≈ δ15N° + ε · ln(C/C°), [5] 

where C° and δ15N° are the average N 

concentration and δ15N value of the leastreacted samples with the most N (fig. 8). The 

apparent isotope fractionation factor (ε) derived 

from the sediment data is –1.8 ‰. The 

δ15N[NH4

+

] values of 3 of the deeper groundwater samples are about 4-5 ‰ higher than the 

δ15N values of the most N-rich sediments, but 

similar to those of sediments with low-tointermediate N concentrations (fig. 8). These 

data could indicate that (1) the deeper groundwater NH4

+

 was derived from the shallower 

organic-rich sediments and was partly oxidized 

during or after the mineralization process, 

resulting in slight 15N enrichment of the NH4

+

before it moved downward (Hübner, 1986); (2) 

the deeper ground-water NH4

+

 was produced 

mainly in sediments with intermediate or 

relatively low N concentrations; (3) the few 

analyzed sediment samples are not fully 

representative; or (4) the apparent fractionation 

factor derived from the sediment analyses is 

misleading. A single saline ground-water 

sample from site WN2-68 yielded a relatively 

high value of δ15N[NH4

+

] (+13 ‰). If this 

value is not an artifact, then it could indicate a 

different source of NH4

+

 or a relatively large 

amount of isotopic fractionation by partial 

oxidation of the NH4

+

. 

Ground-Water Dating by Tritium (3H) 

Of the ground-water dating methods 

applied in this study, the one based on 3H 

concentrations is considered to be the least 

vulnerable to contamination, degradation, and 

degassing problems, but it may have relatively 

large age uncertainties because of the complex 

history of 3H in the atmosphere and the 

relatively poorly known 3H concentrations in 

coastal waters in comparison to the long-term

record for Washington, D.C. (fig. 9). 

Concentrations of 3H in the ground waters 

pumped from beneath Indian River Bay ranged 

from 0.3 to 9.3 TU (tritium unit, or 1018 · 

3

H/1H); however, six of the eight samples had a 

relatively narrow range of 3H concentrations 

between 3.2 and 5.6 TU (average = 4.5 ± 0.9 

TU). Though there are no long-term records of 

3

H concentrations in waters near the Delaware 

coast, there is evidence that 4-5 TU may be 

approximately equal to the average value of 

“modern” precipitation and recent groundwater recharge in the study area (figs. 9A-B). 

Since large-scale thermonuclear bomb 

testing began in 1952, 3H concentrations at 

Cape Hatteras, North Carolina have been 

substantially lower than in Washington, D.C., 

partly because of the greater influence of low- 3

H marine moisture near the coast. In 2000 and 

2001, the average concentration of 3H in 

precipitation at Cape Hatteras was 

approximately 5.0 TU, roughly half the average 

concentration in Washington, D.C. (R.L. 

Michel, USGS, written commun., 2002). Data 

reported by Lindsey and others (in press) 

indicate that four shallow ground-water 

samples from the headwaters of the Pocomoke 

River basin with apparent 3H-3He recharge 

dates in the late 1990s had initial 3H 

concentrations ranging from 3.9 to 6.3 TU 

(averaging 5.4 ± 0.9 TU) (fig. 9B). These data 

indicate that precipitation contributing to 

ground-water recharge near the southern border 

of Delaware had 3H concentrations about half 

the average value in precipitation at 

Washington, D.C. during the same period 

(approximately 11.5 TU from 1996 to 1999) 

(IAEA, 1999) (R.L. Michel, USGS, written 

commun., 2002). Ekwurzel and others (1994) 

reported 3

H concentrations and 3H-3He ages of 

ground-water samples indicating that recharge 

in the Fairmount watershed (west of Rehoboth 

Bay) had 3H concentrations ranging from 0.5 to 

1.0 times the values recorded in Washington, 

D.C. in the 1970s and 1980s (fig. 9B). It is 

possible that the 3H concentrations in 

Fairmount ground waters were slightly higher 

than contemporary values at the coast because 

of less marine moisture or because the recharge 

at Fairmount included older irrigation water 

with bomb 3H. Assuming the local 3H 

concentrations in precipitation and atmospheric 

moisture near the east coast of Delaware were 

correlated with those in Washington, D.C., but 

lower by a factor of about 2 because of 

proximity to the ocean, it can be shown that 

ground waters recharged from local 

precipitation at any time since about the mid10 
1970s would have had an average of around 5 

TU if sampled in 2001 (fig. 9A). In addition, 

four samples of saline surface water (29.1 to 

31.5 g/kg) collected from the Indian River Bay 

and Rehoboth Beach in 2001 and 2002 had 3

H 

concentrations ranging from 3.5 to 7.6 TU 

(average = 4.9 ± 1.8 TU) (table 1), similar to 

the inferred values for modern precipitation. 

Therefore, the available data indicate that 

estuarine and shallow coastal marine waters 

may have had 3H concentrations that were 

similar to those of local atmospheric moisture 

and precipitation. Thus, for most of the 

subestuarine pumped ground-water samples, 

including both fresh and saline endmembers, 3

H concentrations between about 3 and 6 TU 

could be interpreted to indicate apparent ages 

between about 0 and 30 years (post-bombpeak), 45-50 years (pre-bomb-peak), or 

possibly mixtures including bomb-peak and 

pre-bomb waters (fig. 9A). 

The highest 3H concentration (9.3 TU) 

was obtained for the freshwater sample from

site WN2-35. This value is consistent with 

recharge beneath the land surface by

precipitation that fell during the early 1970s, 

slightly earlier than some of the other samples 

with 3-6 TU (fig. 9). The lowest 3H 

concentration (0.3 TU in the brackish-water 

sample from site WN3-61) was substantially 

lower than modern values in precipitation or 

surface water in the Mid-Atlantic region. This 

low 3H concentration indicates that the mixed 

(brackish) ground water near the lower 

boundary of the freshwater plume at this 

offshore site was mainly recharged before 

1952, when large-scale atmospheric 

thermonuclear bomb testing began; that is, the 

bulk of the water in this sample was more than 

50 years old (fig. 9). 

Ground-Water Dating by TritiumHelium (3

H-3He) 

In comparison to the other 

environmental gas tracer methods used for 

ground-water dating, the 3H-3He method is not 

affected by chemical contamination and 

degradation, but it is relatively susceptible to 

errors caused by degassing in pumps and 

discharge tubing. The 3H-3He data indicate 

apparent radioactive decay ages ranging from

about 2 to 37 years (corresponding recharge 

dates from 1965 to 1999). The sample from

site WN3-61 had a substantial amount of 

terrigenic He (29 percent of the total He in the 

sample) and yielded an adjusted apparent age 

of 56 years, but should be considered undatable 

by this method because of its low (pre-bomb) 3

H concentration. No adjustments were made 

for terrigenic He in the other samples, for 

which the average of the apparent calculated 

concentrations of terrigenic He was –0.6 ± 1.9 

percent of the total He. Analyses of replicate 

samples from site WN1-71 were essentially 

identical, indicating that site yielded water only 

2 ± 1 years old. Ages derived from replicate 

samples from site WN3-34 were in reasonably 

good agreement (19 years compared to 25 

years), despite the fact that the samples 

apparently were degassed by different amounts 

(both were substantially undersaturated with 

respect to Ne). The apparent age of the second 

replicate (25 years) is considered more reliable 

because the second replicate was not degassed 

as much as the first. For most of the samples, 

the apparent 3H-3He ages are concordant with 

both the measured 3H concentrations and the 

reconstructed initial 3H concentrations (3H°), 

allowing for minor dispersion of the 1960s 

bomb peak (fig. 10). For example, the sample 

from site WN2-35 almost certainly contained 

recharge from the 1960s with high 3H°, and the 

sample from site WN3-61 contained mainly 

pre-bomb water. The sample from site WN2-

53 also apparently contained recharge from the 

1960s, but that sample appears to be slightly 

older than the one from site WN2-35. 

Ground-Water Dating by

Chlorofluorocarbons (CFCs) 

Concentrations of CFC12 indicate 

ground-water apparent ages of about 26 to 53 

years (recharge dates from 1949 to 1976), 

except at site WN2-53, where the sample 

apparently was contaminated with CFC12. 

CFC11 and CFC113 generally indicate 

relatively old recharge dates and were not 

detected in some samples that had substantial 

concentrations of CFC12. These results are 

consistent with partial degradation of CFCs, 

beginning with CFC11 and CFC113, as is 

commonly observed in chemically reduced 

environments elsewhere (Oster and others, 

1996; Plummer and Busenberg, 2000). Oster 

11 
and others (1996) presented evidence that the 

degradation rate of CFC11 is an order of 

magnitude higher than that of CFC12 in some

reduced ground waters. Despite evidence for 

selective CFC11 and CFC113 degradation 

beneath Indian River Bay, the CFC12 apparent 

recharge dates of freshwaters from sites WN2-

35 and WN3-34 are roughly consistent with the 

corresponding 3

H-3He ages and 3H 

concentrations (fig. 10), and the CFC12 

apparent recharge date of saline water from site 

WN3-61 (late 1940s) is qualitatively consistent 

with the low (pre-bomb) concentration of 3H in 

that sample. Furthermore, in each of the three 

profiles, CFC12 apparent ages increase 

downward, and the CFC12 apparent ages of the 

deep saline samples increase slightly with 

distance offshore, as do the CFC12 apparent 

ages of the freshwater samples. On the basis of 

these data and comparisons, it is concluded that 

CFC11 and CFC113 were partially to 

completely degraded and should not be used in 

the evaluation of the ground-water ages. The 

CFC12 data may be useful, but the CFC12 may 

also be partially degraded. 

Ground-Water Dating by Sulfur 

Hexafluoride (SF6) 

Concentrations of SF6 indicate groundwater apparent ages of about 2 to 24 years 

(recharge dates from 1977 to 2000), except for 

site WN1-25, which was contaminated with SF6. 

The SF6 ages were slightly to moderately 

younger than the CFC12 and 3

H-3He ages in two 

fresh ground-water samples, and much younger 

in three saline ground waters. Mixing ground 

waters of varying ages could account for the 

relatively small discrepancies in the freshwater 

samples, but cannot account for the larger 

discrepancies in the saline samples (fig. 10). 

Other possible reasons for SF6 ages being 

younger than CFC12 ages include CFC12 

degradation and excess SF6. Observations that 

could be interpreted as evidence for CFC12 

degradation include (1) the apparent age 

discrepancies are larger in saline samples, which 

are anoxic and have relatively high 

concentrations of NH4

+

 and CH4, conditions that 

may be associated with degradation of CFC12 

(Oster and others, 1996; Rowe and others, 

1999); and (2) the other analyzed CFCs (CFC11 

and CFC113) generally appear to have been 

degraded substantially relative to CFC12. 

Observations that could be interpreted as 

evidence of excess SF6 include (1) CFC12 has 

been shown to be relatively stable in many other 

aquifers under reducing conditions, even when 

CFC11 and CFC113 have been degraded 

(Plummer and Busenberg, 2000); (2) at least one 

of the samples had a SF6 concentration 

substantially higher than that of modern water 

equilibrated with uncontaminated air; (3) low 

levels of natural or anthropogenic background 

SF6 have been observed in ground water from a 

variety of hydrogeologic settings including the 

Mid-Atlantic Coastal Plain (Busenberg and 

Plummer, 2000); and (4) regionally elevated 

atmospheric SF6 concentrations have been 

documented in some areas (Busenberg and 

Plummer, 2000). Four samples had relatively 

uniform SF6 concentrations averaging 0.33 ± 

0.06 fmol/L (femtomoles per liter), indicating 

apparent recharge dates in the late 1970s. One 

of those samples (from site WN3-61, with SF6 = 

0.23 fmol/L and 3H = 0.3 TU) must have been 

recharged mainly before thermonuclear bomb 

testing in the early 1950s. It would be possible 

to account for the low concentrations of 3H and 

SF6 in that sample by assuming it was a mixture 

with about 10 percent young water (less than 5 

years old, containing modern concentrations of 

3

H and SF6) and about 90 percent old water 

(more than 40-50 years old, containing no 3H or 

SF6); however, the ratio of 3H/3H° would not be 

consistent with such a mixture. Therefore, the 

data seem to indicate that there was a consistent 

low background or contaminant level of SF6 in 

the samples and that SF6 apparent ages of 

around 22-24 years should be regarded as 

minimum ages. 

Summary of Ground-Water Ages

Ground-water samples generally consist 

of mixtures of waters with a range of recharge 

dates. In this study, sampling from drive-point 

wells with short screens was intended to 

minimize artificial mixing of ages in the 

samples, but natural dispersion in the aquifer is 

likely to have mixed the dating tracers to some

degree. Nevertheless, although it is not possible 

to give a definitive summary of the age 

distribution of the pumped ground waters 

because of potential mixing and observed 

discordance among the various ground-water 

12 
Apparent deficits of CFC12 in 

comparison to 3

H-3He recharge dates can occur 

as a result of mixing young water (containing 

CFC12, 3H, and 3He[trit]) with old (pre-tracer) 

water. The Cl- and δ18O data indicate mixing of 

saline and fresh ground-water components in 

some samples, but some of the largest apparent 

age discrepancies are in samples that are 

relatively close to the saline endmember (sites 

WN1-71 and WN2-68). Discordance between 

the CFC12 and 3H-3He data in these samples 

could be attributed to mixing of young and old 

(pre-tracer) saline waters, but this would require 

that the initial 3H concentrations of the young 

fractions were substantially higher than those 

estimated for the coastal region, and perhaps 

even slightly higher than the measured values in 

precipitation in Washington, D.C. (table 1; fig. 

10). Although this cannot be ruled out entirely, 

it would be difficult to reconcile with the other 

3

H data from surface waters and young ground 

waters in the region. Because the saline ground 

waters also contained substantial amounts of 

CH4 and NH4

+

, and because CFC11 and 

CFC113 were essentially gone, it is considered 

likely that CFC12 was at least partially 

degraded, as observed elsewhere in some anoxic 

ground-water environments (Oster and others, 

1996; Rowe and others, 1999). Therefore, the 

preferred interpretation of the ground-water 

ages in this study is based mainly on the 3H and 

3

H-3He results (table 3), but the overall 

uncertainties of these ages or age mixtures are 

difficult to quantify. 

dating techniques, it can be concluded that all of 

the samples (fresh and saline) except the one 

from site WN3-61 were dominated by water that 

recharged within the last 50 years (table 3). The 

most reliable results were obtained from the 

freshwaters, where apparent age discordance is 

minimal. For example, site WN2-35 yielded 

apparent ages of about 30 to 40 years from 3

H, 

28 years from CFC12, 23 years from SF6, and 

34 years from 3H-3He. Data from all three 

freshwater samples indicate ages at depth that 

are at least qualitatively consistent with those of 

freshwater surficial aquifers beneath the 

Delmarva Peninsula (Dunkle and others, 1993). 

The two freshwater samples from site WN2 

appear to have been stratified, with the deeper 

sample slightly older than the shallower sample. 

Brackish water from site WN3-61 

(approximately two-thirds fresh and one-third 

saline) was dominated by water more than 50 

years old, which is substantially older than the 

water from site WN3-34, consistent with age 

stratification in the freshwater plume at site 

WN3. Therefore, the age data are consistent

with the hypothesis that freshwater underlying 

Indian River Bay was essentially a continuation 

of the surficial aquifer that was recharged 

onshore. More detailed sampling, especially at 

shallower depths, would be required to define 

the age gradients and discharge patterns in the 

subestuarine freshwater. 

For some of the saline ground waters, 

which had relatively high concentrations of CH4

and NH4

+

, substantial discordance among the 

dating techniques is not completely understood. 

The most important discrepancies are in the 

deep saline samples from sites WN1-71 and 

WN2-68. If the 3

H-3He data and interpretations 

for these samples are accurate, then it would 

appear that CFC12 was degraded by about 64 to 

73 percent and SF6 had a low level of 

contamination or natural background equivalent 

to an atmospheric partial pressure of about 0.6 

pptv. Anthropogenic contamination could have 

resulted from drilling and sampling, or it could 

have been present in Indian River Bay when the 

saline ground waters were recharged. If the 

CFC12 results are assumed to be reliable, as 

they are in many other ground-water systems, 

then the SF6 would appear to be contaminated 

and the concentration of 3He[trit] would be too 

low by a factor of about 4 to 20. However, 

there is no obvious reason to suspect such a bias 

in the He isotope data. 

IMPLICATIONS FOR GROUNDWATER MOVEMENT AND 

NITROGEN TRANSPORT 

The results of this study have important 

implications with respect to fresh and saline 

ground-water movement and nutrient transport 

beneath Indian River Bay. In addition, despite 

the limited number of samples, and some

uncertainty in the interpretations of the data, the 

results provide useful guidance for future 

studies of offshore freshwater/salt-water 

interactions in coastal estuaries like Indian River 

Bay. 

13 
Sources and Fate of Dissolved 

Inorganic Nitrogen in Ground Water 

Though limited in number, the data from

the pumped subestuarine ground waters indicate 

that the major dissolved inorganic N species 

(NO3

-

 and NH4

+

) were produced independently 

under contrasting physical conditions (fig. 11). 

NO3

-

 occurs only in the freshwater plumes and 

almost certainly was derived from nitrification 

in unsaturated soils on land. The observed or 

inferred initial NO3

-

 concentrations (51 to 108 

μmol/L) were substantially higher than what are 

normally considered to be natural in the region 

(less than 30 μmol/L), but not as high as 

concentrations observed in young ground waters 

recharged directly beneath fertilized agricultural 

fields or septic systems in the region (commonly 

several hundred to more than one thousand 

μmol/L) (Hamilton and others, 1993; Böhlke 

and Denver, 1995). The ground-water ages, and 

the concentrations and isotopic compositions of

the NO3

-

, could be consistent with old 

agricultural ground waters or with mixed land 

uses in the recharge areas. Parts of the plumes 

containing younger ground water might have 

higher concentrations of [NO3

-

]°. 

The only evidence for denitrification 

was in freshwater at site WN2, where the redox 

gradient in the freshwater plume was inverted 

(where anoxic, denitrified water was above oxic, 

NO3

-

 -bearing water). It is not known where 

denitrification occurred, but it must have been 

downgradient from the recharge area on land, 

and it apparently was not a result of mixing with 

reduced saline water. The inverted redox 

gradient in the freshwater plume at site WN2 

might be a local phenomenon related to 

upgradient wetland or channel-filling sediments, 

or it may be a more widespread phenomenon 

related to the overall lithostratigraphic sequence 

of the pre-Holocene aquifer section. 

In contrast, NH4

+

 in the pumped ground 

water samples was consistently associated with 

elevated salinity and was most likely derived 

from diagenesis of sedimentary organic matter 

under suboxic to anoxic conditions beneath the 

estuary. Surficial Holocene sediments have the 

highest concentrations of organic N and porewater NH4

+

, whereas the underlying preHolocene sediments have much less organic N 

and uniformly moderate concentrations of 

dissolved NH4

+

. Though it is possible that NH4

+

was produced throughout the anoxic parts of the 

flow system, it is considered likely that most of 

the NH4

+

 in the deeper saline ground waters was 

produced near the sediment-water interface and 

carried downward with the water. NH4

+

 was 

also abundant in some of the fresh pore waters 

squeezed from sediment cores near the upper 

boundary of the freshwater plume, possibly 

indicating recharge in coastal freshwater 

wetlands or relatively reducing conditions in the 

uppermost parts of the aquifer. More detailed 

study of the distributions and isotopic 

compositions of coexisting organic N and NH4

+

in the shallow parts of the flow system could 

provide more definitive evidence on these 

issues. 

Patterns of Ground-Water Flow

Despite uncertainties in the interpretation 

of the age results, it may be concluded that both 

the fresh and saline pumped subestuarine ground 

waters were recharged mainly within the last 50 

years. The freshwater beneath the Bay can be 

considered as a continuation of the surficial 

aquifer with sources of water and nitrate beneath 

the upgradient land surface (fig. 11). Like 

surficial aquifers on land, the fresh subestuarine 

ground-water plume appears to be stratified, with 

deeper waters having relatively old recharge 

dates and distant recharge areas. The apparent 

ages of the fresh subestuarine ground-water 

samples are similar to or slightly older than those

of ground waters at comparable depths in 

surficial aquifers beneath the Delmarva Coastal 

Plain. 

Although these data indicate that fresh 

ground water beneath the estuary at least 300 m

from shore is part of an active flow system, they 

do not indicate directly the pattern or rate of 

fresh ground-water discharge to the bottom of 

the estuary. It is possible that fresh groundwater discharge occurs offshore in focused 

discharge areas (springs) that were not 

intercepted by the geophysical or coring 

operation, or that discharge is preceded by 

thorough mixing with saline ground water 

below the sediment-water interface. McKenna 

and others (2001) reported thermal features in 

Rehoboth and Indian River Bays that may 

include areas of rapid focused discharge. 

Additional work on this issue could be 

important because of the different effects these 

modes of discharge might have on nutrient 

14 
transport. The organic-rich Holocene sediments 

are considered to represent a potential 

biogeochemical barrier to slow, pervasive 

movement of NO3

-

 into the estuary from below, 

but perhaps not to rapid, focused discharge of 

NO3

-

. The distribution of younger fresh ground 

waters (0 to 20 years old) in the discharge area 

would be especially relevant because of their 

higher potential NO3

-

 concentrations (Böhlke 

and Denver, 1995) and faster potential discharge 

rates. 

One of the most interesting results of 

this study is that the deep saline ground waters 

at locations WN1 and WN2 appear to be 

relatively young. According to the 3H-3He data, 

the apparent ages of the deep saline samples 

increase offshore, and in WN2, the deep saline 

water appears to be younger than the overlying 

freshwater. If these tentative age interpretations 

are even qualitatively correct, they would imply 

relatively rapid downward movement of saline 

water around the edges of the freshwater plume, 

with the youngest deep ground-water ages (as at 

site WN1) perhaps indicating areas of 

preferential downwelling. At site WN1-71, the 

apparent age (2.2 years) and depth (21.6 m) of 

the saline ground water yield an apparent 

downward net vertical component of velocity of 

approximately 10 m/year. For an estimated 

porosity of 40 percent, this would correspond to 

a net local subestuarine recharge rate of 4 

m/year. However, these data do not address the 

magnitudes of water and chemical fluxes near 

the sediment-water interface and over short time

scales, which could include flow reversals 

caused by changes in surface-water levels as a 

result of tides and winds. Upward diffusion or 

advection of NH4

+

 formed by degradation of 

organic matter in the Holocene sediments could 

be a substantial source of N to the overlying 

estuary. 

SUMMARY AND CONCLUSIONS 

Stable isotopes and other environmental 

tracers were analyzed in samples of subestuarine 

ground water beneath Indian River Bay, 

Delaware to determine patterns of flow and 

sources and fate of nitrogen. These analyses 

were done through a cooperative agreement 

between the U.S. Geological Survey and the 

Delaware Department of Natural Resources and 

Environmental Control as part of a larger study 

of ground-water discharge and nutrient 

enrichment in coastal estuaries. 

Geophysical studies indicated freshwater 

plumes beneath the estuary at depths between 0 

and 30 meters and extending at least 500 to 

1,000 meters offshore. Analyses of groundwater samples from 3 to 22 meters beneath the 

bay confirmed the geophysical results, 

indicating that freshwater plumes were bounded 

by saline water with salinity similar to the 

estuarine surface water. Though the pumped 

ground-water sample set was small, and some of 

the analyses yielded complicated results, the 

isotopes and environmental tracer data from the 

vicinity of a plume north of White Neck, 

Delaware, indicate the following: 

(1) Much of the ground water beneath the 

estuary was recharged within the last 50 

years. This includes both fresh and saline 

endmembers at depths ranging to at least 22 

meters below the sediment-water interface. 

(2) The fresh ground-water plume to at least 

300 meters offshore beneath the estuary is 

an actively flowing continuation of the 

surficial aquifer with recharge areas on 

land. 

(3) Nitrate transported offshore in the fresh 

ground-water plume was formed in aerated 

soils in agricultural or mixed land-use 

areas. 

(4) Denitrification occurred in the upper part of 

the aquifer where nitrate-bearing fresh 

ground water encountered reducing 

conditions within the aquifer. 

(5) Saline ground water was recharged from the 

estuary and moved downward relatively 

rapidly around the freshwater plume. 

(6) Ammonium transported downward in 

saline ground water was formed in anoxic 

to suboxic environments by degradation 

of organic matter in estuarine sediments. 

Though it is evident that the subestuarine 

ground-water flow system is active and 

complex, these limited data do not provide 

evidence for discharge of fresh ground water in 

offshore areas of the estuary. Direct 

subestuarine discharge of fresh ground water 

containing nitrate that has not been denitrified is 

considered to be most likely near the shoreline 

or in offshore focused-discharge areas such as 

springs, which were not sampled in this study. 

Ammonium produced by degradation of organic 

matter in subestuarine sediments may be a 

substantial source of fixed nitrogen near the 

15 
sediment-water interface in much of the 

offshore area of the estuary. 

This study establishes two general 

features of the subestuarine ground water that 

could guide future studies: (1) anthropogenic 

atmospheric environmental tracers including 

tritium (and decay-product helium), 

chlorofluorocarbons, and sulfur hexafluoride are 

appropriate dating tools for the time scale of 

interest, but multi-component analyses may be 

required to establish the reliability of derived 

ages; and (2) the speciation of nitrogen at depth 

is complex and spatially variable, so that multicomponent chemical and isotopic analyses are 

required to resolve nitrogen sources and 

transformations. More complete information 

about the vertical and lateral gradients in 

ground-water ages of the freshwater plumes, 

especially along their upper boundaries, might 

yield more information about the distribution of 

freshwater discharge. Confirmation of the 

apparent pattern of rapid saline ground-water 

movement around the plumes could be 

especially valuable for testing complex variabledensity models of subestuarine flow. More 

detailed analysis of the boundaries between 

freshwater plumes and surrounding saline 

ground waters might yield new information 

about relations between ammonium and nitrate 

and the distribution of denitrification beneath 

Indian River Bay and other similar subestuarine 

environments. 

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Figure 1. Ground-water sampling sites (WN-1 to WN-4) and resistivity survey tracklines (DE-R-05

and DE-R-08) in Indian River Bay near White Neck, Delaware (modified from Krantz and others, in 

press). 

19 
Figure 2. Streaming resistivity section DE-R-05 from Indian River Bay near White Neck, Delaware 

(modified from Krantz and others, in press) [m = meters; ohm-m = ohm-meters]. Approximate 

locations of ground-water sampling sites WN-1 (little or no freshwater) and WN-2 (thick freshwater 

plume) are shown on the resistivity section. 

20 
Figure 3. Site map and vertical profiles showing the distribution of pumped ground-water samples 

and geophysical logs (modified from Bratton and others, in press; Krantz and others, in press) [depths 

in ft (feet) and m (meters)]. The site map includes resistivity trackline DE-R-05 and the corehole sites 

WN-1 to WN-4. Vertical profiles include natural gamma logs (cps = counts per second; low in sand 

and high in clay) and electromagnetic induction (EM) conductivity logs (mS/m = millisiemens per 

meter; low in freshwater and high in saline water). Measured salinities (in ‰, or grams per kilogram) 

of pumped ground-water samples are shown in the profiles for comparison with the geophysical data. 

“TD cored” indicates the total depth of the cored sediments at sites WN-1, WN-2, and WN-3. 

21 
0 200 400 600

0

20

40

60

Mg (

m

m

ol/L)

1/10

1/25

1/71

2/35

2/53

2/68

3/34

3/61

"sw"

Subestuarine ground water

"2/35" refers to site WN2-35'

Surface water

ir = Indian River Bay

reh = Rehoboth Beach

0 200 400 600

0

5

10

C

a (mmol/L)

1/10

1/25

1/71

2/35

2/53

2/68

3/34

3/61

"sw"

0 200 400 600

0

10

20

30

SO

42- (mmol/L)

1/10

1/25

1/71

2/35

2/53

2/68

3/34

3/61

"sw"

0 200 400 600

0

1

2

3

Alk

alinity (

m

m

ol/L a

s H

C

O

3

-)

1/10

1/25

1/71

2/35

2/53

2/68

3/34

3/61

"sw"

0 200 400 600

Cl (mmol/L)

0

5

10

K (mmol/L)

1/10

1/25

1/71

2/35

2/53

2/68

3/34

3/61

"sw"

0 200 400 600

Cl (mmol/L)

0

0.1

0.2

0.3

Si

O

2 (m

m

ol/L)

1/10

1/25

1/71

2/35

2/53

2/68

3/34

3/61

ir1

A

B

C

D

E

F

Figure 4. Relations between chloride and other major ions. [A] magnesium (Mg) and chloride (Cl-); 

[B] calcium (Ca) and Cl-

; [C] potassium (K) and Cl-; [D] sulfate (SO4

2-) and Cl-

; [E] alkalinity (as HCO3

-

or bicarbonate equivalents) and Cl-

; [F] silica (SiO2) and Cl- [data from table 1; mmol/L = millimole per 

liter]. Closed symbols indicate laboratory analyses from the Reston water chemistry laboratory 

(RWCL in table 1); open symbols in [E] indicate field alkalinity analyses; open symbols in [F] indicate 

laboratory SiO2 analyses from the University of Delaware (UDEL in table 1). The label “sw” refers to 

standard mid-ocean seawater, “ir1” indicates Indian River bay water, and the pumped ground-water 

samples are identified by site and depth in feet (for example 2/35 is from site WN-2, 35 feet below the 

sediment-water interface) (see table 1). 

22 
-7 -6 -5 -4 -3 -2 -1 0

-40

-30

-20

-10

0

δ

2H (

‰)

1/10

1/25

1/71

2/35

2/53

2/68

3/34

3/61

ir1

ir2

ir3

reh

"sw"

Subestuarine ground water "2/35" refers to site WN2-35'

Ground water in

Indian River Bay watershed

Ground water near

Fairmount, Delaware

Surface water

ir = Indian River Bay

reh = Rehoboth Beach

meteoric water line

δ2H = 8⋅δ18O + 14

meteoric waterbay water

mixing line

-7 -6 -5 -4 -3 -2 -1 0

δ18O (‰)

0

100

200

300

400

500

600

Cl (

m

m

ol/L)

1/10

1/25

1/71

2/35

2/53

2/68

3/34

3/61

ir1

ir2

ir3

reh

"sw"

A

B

meteoric waterbay water

mixing line

Figure 5. Comparison of δ

2

H, δ

18O, and chloride concentrations. [A] δ2

H and δ

18O; [B] chloride (Cl-

) 

and δ

18O [data from table 1; ‰ = per mil; mmol/L = millimole per liter]. The explanation in [B] applies 

to both panels. Landward ground-water data are included for areas near Indian River Bay (Andres, 

1991; Dunkle and others, 1993). The label “sw” refers to standard mid-ocean seawater, “ir1” to “ir3” 

indicate Indian River bay samples, “reh” indicates ocean water from Rehoboth Beach, and the 

pumped ground-water samples are identified by site and depth in feet (for example 2/35 is from site 

WN-2, 35 feet below the sediment-water interface) (see table 1). 

23 
400 500 600 700 800 900

N2 (μmol/L)

10

15

20

Ar (

μmol/L)

1/10

1/25

1/71

2/35 2/53

2/68

3/34

3/61

5

10

15

20

10

15

20

5

10

15

20

Subestuarine ground water "2/35" refers to site WN2-35'

Air-saturated water at temperatures indicated +0 ccSTP/L

+4 ccSTP/L

air-saturated

water at

0 m elev.

at T°C

excess

air

excess

N2

freshwater

seawater

Figure 6. Concentrations of Ar (argon) and N2 (nitrogen) in pumped ground-water samples [μmol/L 

= micromoles per liter]. Air-saturation curves were calculated using solubilities from Weiss (1970) 

assuming elevation = 0 meters, salinity = 0 (freshwater) and 35 grams per kilogram (seawater), 

airexcess = 0 and +4 ccSTP/L (cubic centimeters of gas, at standard temperature and pressure, per 

liter), and temperature = 5 to 20 °C (degrees Celsius). The pumped ground-water samples are

identified by site and depth in feet (for example 2/35 is from site WN-2, 35 feet below the sedimentwater interface) (see table 1). 

24 
0 500 1000

EM Conductivity (mS/m)

30

20

10

0

Depth below

s

edi

ment-w

ater i

nterfa

ce (

m)

0 500 1000

EM Conductivity (mS/m)

0 500 1000

EM Conductivity (mS/m)

0 200 400 600 800 1000

NH4+ in pore water (μmol/L)

0 200 400 600 800 1000

NH4+ in pore water (μmol/L)

EM Conductivity

Pumped ground-water sample

Pore-water ammonium concentration

0 500 1000

EM Conductivity (mS/m)

30

20

10

0

D

ept

h belo

w

s

e

dim

ent-w

ater i

nterfa

c

e (

m)

(75 m offshore)

(150 m offshore)

(300 m offshore) (1000 m offshore)

fresh Ù saline

fresh Ù saline

WN2 WN3

NH4+ = 52

NO3

-

 = 4

O2 < 3

NH4+ = 56

NO3

-

 = 1

O2 < 3

NH4+ = 76

NO3

- = 1

O2 < 3

WN1-25'

WN1-10'

WN1-71'

WN2-35'

WN2-53'

WN2-68'

WN3-34'

WN3-61'

NH4+ = 2

NO3

-

 < 1 [NO3]° = 58

O2 < 3

NH4+ = 1

NO3

-

 = 108

O2 = 115

NH4+ = 63

NO3

- = 6

O2 < 3

NH4+ = 2

NO3

-

 = 51

O2 = 26

NH4+ = 49

NO3

-

 < 1

O2 < 3

WN1

WN4

Figure 7. Vertical profiles showing the concentrations of NH4

+

 (ammonium), NO3

-

 (nitrate), [NO3

-

]° 

(initial nitrate), and O2 (oxygen) in pumped ground-water samples. The pumped samples are plotted 

at the sample depths (m = meters) in relation to the measured or interpolated values of 

electromagnetic induction (EM) conductivity given in mS/m (millisiemens per meter) from the 

geophysical logs (solid curves; see fig. 3). Dashed curves indicate concentrations of NH4

+

 in pore 

waters squeezed from shallow sediment cores at sites WN2 and WN3 (pore-water data from Bratton 

and others, in press). 

25 
100 1,000 10,000

N in sediment (mg/kg)

-3

-2

-1

0

1

2

3

4

5

6

δ15N (

‰)

Holocene

sediments

Rayleigh

fractionation

(ε = -1.8 ‰)

instantaneous

product

accumulated

product

residual

reactant range of δ

15N 

of NH4

+ in

deeper saline

ground water

Figure 8. Nitrogen concentrations and δ

15N values of Holocene sediment samples (data from table 

2), with hypothetical isotope fractionation curves for reactant (solid curve) and product (dashed 

curves) [mg/kg = milligrams per kilogram in sediment; ‰ = per mil]. The range of δ

15N values of 

dissolved NH4

+

 in deeper saline ground waters (table 1, minus 1 outlier) is shown for comparison. 

The apparent isotope fractionation factor ε for N loss from the sediments is defined by ε = (α – 1) · 

1000 ‰, where α = [15N/14N]product/[15N/14N]reactant (see also equation 6). 

26 
1950 1960 1970 1980 1990 2000 2010

0

10

20

30

40

50

3H (

TU,

d

e

c

a

yed t

o O

ctober, 2001)

1/71

2/35

2/53 2/68 3/34

3/61 ir1 ir2

ir3

reh

1950 1960 1970 1980 1990 2000 2010

Year

0

20

40

60

80

100

3

H° (

TU, initial)

Precipitation in Washington,D.C.

Precipitation in Washington,D.C.

multiplied by 0.55

Surface water

(year = collection date) ir = Indian River Bay

reh = Rehoboth Beach

Ground water

(year = 3H-3He age) "2/35" refers to site WN2-35'

Ground water near

Fairmount, Delaware

Ground water in

Pocomoke River basin

1/71

2/35

2/53

2/68

3/34

3/61

ir1ir2

ir3

reh

A

B

Figure 9. Documented and inferred records of 3H (tritium) concentrations in precipitation and 

recharging ground water, compared with measured and calculated values for subestuarine groundwater samples from Indian River Bay, Delaware (table 1) [TU = tritium units]. The pumped groundwater samples are identified by site and depth in feet (for example 2/35 is from site WN-2, 35 feet 

below the sediment-water interface) (see Table 1). Shown for comparison are data for Washington, 

D.C. (IAEA, 1999, R.L. Michel, USGS, written commun., 2002). The hypothetical curve for the 

Delaware coast was drawn by assuming it was correlated with the Washington, D.C. curve but 

systematically lower (multiplied by 0.55). [A] 3

H values as measured at the time of sampling in 

October, 2001 (after radioactive decay). [B] Initial (undecayed) values. The explanation in [A] 

applies to both panels. Additional ground-water data from Fairmount, Delaware, are from Ekwurzel 

and others (1994). Data from the Pocomoke River basin are from Lindsey and others (in press). 

27 
0 200 400 600

CFC12 (pptv)

0

100

200

300

CFC11 (

p

ptv)

40

20

10

10

20

40

1/71 1/25 2/35

2/68

3/34

3/61

0 200 400 600

CFC12 (pptv)

0

20

40

60

80

CF

C113 (

ppt

v)

40

20

10

10

20

40

1/25

1/71

2/35

2/68

3/34

0 200 400 600

CFC12 (pptv)

0

1

2

3

4

5

SF

6 (p

ptv)

40

20

10

10

20

40

1/71

2/35

2/68

3/34 3/61

0.0 0.2 0.4 0.6 0.8 1.0

3H/3H° (TU)

0

1

2

3

4

5

SF

6 (p

ptv)

40

20

10

10

20

40

1/71

2/35

2/53

2/68

3/34

3/61

0.0 0.2 0.4 0.6 0.8 1.0

3H/3H° (TU)

0

200

400

600

CFC12 (

p

ptv)

40

20

10

10

20

40 1/71

2/35

3/34 2/68

3/61

0.0 0.2 0.4 0.6 0.8 1.0

3H/3H° (TU)

0

20

40

60

3H° (

T

U)

20

10

10

20

40

(sampled in 2001.8)

Piston-flow

Exponential mixture

Bimodal mixture

Ground-water sample "2/35" refers to site WN2-35'

1/71

2/35

2/53

2/68

3/34

3/61

A

B

C F

E

D

Figure 10. Comparison of environmental tracer data with hypothetical values. In each panel, the 

solid curve indicates concentrations and ratios of tracers that would yield concordant ages in the 

absence of mixing, contamination, or degradation (pptv = parts per trillion by volume of gas phase in 

equilibrium with the water sample; TU = tritium units). The long-dashed curve indicates concordance 

of mean ages for exponential mixtures (Cook and Böhlke, 2000; Böhlke, 2002). Short-dashed lines 

indicate concordance of binary mixtures in which one endmember is > 50 years old and contains no 

tracer. Curves are labeled with mean ages of hypothetical samples with representative concordant 

tracer concentrations. Light crosses in [D] indicate 3

H° concentrations that would be required to 

explain the CFC12 deficits by mixing (see text and table 1, last column). 

28 
(N2,denit.)

NH4

+

NH4+

NO3- 37

34

NH4+

NH4

+

3

H/3He

apparent age

(years)

37

17

25

50+

NO3-

34

17

2

(N2,denit)

A

B

Figure 11. Schematic diagrams showing the distribution of fresh and saline ground water beneath 

Indian River Bay near White Neck, Delaware, with the distribution of ground-water ages (numbers in 

italics) and nitrogen species (NH4

+

 = ammonium; NO3

-

 = nitrate; N2,denit = excess N2 attributed to 

denitrification) (modified from Bratton and others, in press; Krantz and others, in press). [A] Section 

perpendicular to the shoreline. [B] Section parallel to the shoreline. 

29 
Table 1. Chemical and isotopic data for pumped ground waters and surface waters 

[Units of measure are: m, meter; μm, micrometer; g, gram; mg, milligram; kg, kilogram; mmol, millimole; μmol, micromole; fmol, femtomole (10-15mole); mS, millisiemen; μS, microsiemen; ccSTP, cubic centimeters of gas at standard temperature (0 °C) and pressure (1 atmosphere); ‰, per mil (part per thousand); pptv, parts per trillion by volume, atmospheric mixing ratio of gas in equilibrium with a water sample; TU, tritium unit (1 atom of 3H per

1018 atoms of 1H). Chemical symbols are: O2, molecular oxygen; H O, 2 water; Na, sodium; K, potassium; Mg, magnesium; Ca, calcium; Sr, strontium;

Cl-, chloride; NO3-, nitrate; SO42-, sulfate; SiO2, silica; PO43-, phosphate; CFC11, chlorofluorocarbon CCl3F; CFC12, chlorofluorocarbon CCl2F2; 

CFC113, chlorofluorocarbon C Cl 2 3F3; SF6, sulfur hexafluoride; 3H, tritium; He, helium; Ne, neon. Sources of data are: LDEO, Lamont-Doherty Earth

Observatory (Noble Gas Laboratory); UDEL, University of Delaware at Lewes; MPTL, USGS Menlo Park Tritium Laboratory; NWQL, USGS National

Water Quality Laboratory; RCFC, USGS Reston Chlorofluorocarbon Laboratory; RWCL, USGS Reston Water Chemistry Laboratory; RSIL, USGS 

Reston Stable Isotope Laboratory. “Select” indicates values that represent either averages or choices based on critical evaluation; shaded entries have 

relatively large uncertainties] Annotations:

[1] Samples are identified by the corehole site, followed by the depth of the well screen beneath the sediment-water interface in feet. [2] BLS, “below land surface” (below the sediment-water interface) [3] Fit, Cl- concentrations calculated from specific conductance data based on a regression with available titration Cl- values

[4] X, fraction of young water in a binary mixture required to account for 3H, 3H/3H°, and CFC12 data if CFC12 is not degraded; one component of the 

mixture is tracer-free (pre-1940) and the other component has its age determined by the 3H/3H° ratio. 

[5] Standard seawater composition from Stumm and Morgan (1996)

30 
Table 1. Chemical and isotopic data for pumped ground waters and surface waters (continued)

Screen Log Field Field Field Field Field Field Field Field Select Select RSIL RSIL

Sample Figure Sample Field depth EM specific pH O2 O2 alkalinity Cl Cl salinity density salinity H O H 2 O2

site labels date sample comments BLS conduct. conduct. (std. meter titration as CaCO3 titration fit calc. calc. calc. δ2Η δ18Ο

[1] (m) (mS/m) (mS/cm) units) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (g/L) (kg/L) (g/kg) (‰) (‰)

[2] [3]

Ground water

WN1-10' 1/10 10/24/2001 peristaltic pump, low yield 3.0 930 27,700 6.28 0.48 na na na 9,821 17.74 1.0107 17.56 -20.5 -3.39

yellowish muddrillpipe not cleaned

WN1-25' 1/25 10/25/2001 Bennett pump, low yield 7.6 978 39,200 7.06 0.53 na 132.0 na 14,132 25.53 1.0167 25.11 -14.4 -2.12

drillpipe not cleaned

WN1-71' 1/71 10/25/2001 Bennett pump, high yield 21.6 na 44,300 7.66 0.10 na 128.0 na 16,080 29.05 1.0194 28.50 -9.0 -1.52

Fe oxide in 3H bottle 1.0194

WN2-35' 2/35 10/27/2001 Bennett pump, high yield 10.7 313 2,510 5.34 0.07 na 8.9 715 771 1.39 0.9981 1.40 -32.9 -5.87WN2-53' 2/53 10/27/2001 Bennett pump, high yield 16.2 270 431 5.52 3.20 3.6 8.9 112 48 0.09 0.9971 0.09 -36.2 -6.16WN2-68' 2/68 10/27/2001 Bennett pump, high yield 20.7 996 38,200 6.24 0.28 na 86.0 13,750 13,753 24.84 1.0161 24.45 -15.8 -2.43WN3-34' 3/34 10/29/2001 Bennett pump, high yield 10.4 299 1,822 5.44 1.72 na 37.0 510 532 0.96 0.9977 0.96 -35.7 -6.26

Fe oxide in 3H bottle 0.9977

WN3-61' 3/61 10/29/2001 peristaltic pump, bubbles 18.6 583 16,900 5.40 0.15 na 50.0 5,890 5,875 10.61 1.0052 10.56 -25.7 -4.39

mud in serum bottlesFe oxide in 3H bottle

Surface Water

(lab) (lab)

Indian River Bay ir1 10/24/2001 from Holts Landing 31.92 1.0216 31.25 -6.4 -1.15

Indian River Bay irb 10/26/2001 from Holts LandingIndian River Bay ir2 9/23/2002 from Holts Landing 29.72 1.0199 29.14 -8.6 -1.28Indian River Bay ir3 9/25/2002 from Holts Landing 30.38 1.0204 29.77 -8.1 -1.11

Rehoboth Beach reh 5/12/2002 32.14 1.0217 31.46 -5.5 -0.91

std seawater "sw" 35.86 1.0246 35.00 0.0 0.00

[5]

31 
Table 1. Chemical and isotopic data for pumped ground waters and surface waters (continued)

RWCL RWCL RWCL RWCL RWCL RWCL RWCL RWCL RWCL RWCL RWCL Field RWCL RWCL RWCL Field RWCL UDEL UDEL UDEL NWQL UDEL RSIL

Sample Na K Mg Ca Sr Fe Mn SiO2 B Br Cl Cl SO42- NO3- alkalinity alkalinity Si Si PO43- NH4+ NH4+ NO2+3- NO2+3-

site (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ as HCO3- as HCO3- (μmol/ (μmol/ (μmol/ (μmol/ δ15Ν (μmol/ (μmol/

[1] L) L) L) L) L) L) L) L) L) L) L) L) L) L) (mmol/L) (mmol/L) L) L) L) L) (‰) L) L)

Ground water

WN1-10' 223 5.40 27.20 5.61 0.05 0.04 -0.01 0.14 0.21 0.41 279 277 12.91 < 0.3 1.13 na 142 158 0.9 51.6 na 4.5 naWN1-25' 324 7.55 39.21 7.93 0.07 0.01 -0.01 0.07 0.31 0.63 406 399 18.74 < 0.3 2.11 2.64 75 77 0.5 56.5 4.5 0.9 naWN1-71' 367 8.95 43.20 8.56 0.07 0.03 -0.01 0.08 0.35 0.69 446 454 20.92 < 0.3 2.29 2.56 82 85 1.4 76.1 3.6 1.1 naWN2-35' 16 0.29 2.17 0.58 0.01 0.02 0.00 0.27 0.00 0.03 20 22 0.78 < 0.02 0.08 0.18 272 316 0.6 2.5 na 0.6 naWN2-53' 3 0.08 0.24 0.13 0.00 0.01 0.00 0.27 0.00 0.01 3 1 0.07 0.11 0.11 0.18 272 329 0.6 0.9 na 108.1 105WN2-68' 304 7.24 36.49 7.16 0.06 0.00 -0.01 0.15 0.28 0.60 389 388 19.99 < 0.8 1.41 1.72 153 157 0.6 63.4 13.0 5.8 naWN3-34' 11 0.19 1.56 0.61 0.01 0.02 0.00 0.26 0.00 0.02 14 15 0.71 < 0.06 0.11 0.74 264 303 0.5 2.3 na 50.8 40WN3-61' 122 2.30 15.76 4.27 0.03 0.03 0.05 0.32 0.05 0.25 155 166 7.18 <0.2 0.45 1.00 317 323 0.7 49.2 2.8 0.4 na

Surface Water

(calc)

Indian River Bay 500 <1 0.9 2.0 <5Indian River Bay <1 0.8 2.4 <5Indian River Bay 466Indian River Bay 476

Rehoboth Beach 503

std seawater 480 10.46 53.57 10.51 0.09 0.42 0.87 560 560 28.96 2.38 2.38

[5]

32 
Table 1. Chemical and isotopic data for pumped ground waters and surface waters (continued)

RWCLRWCLRWCLRWCLRWCLRWCL RSIL Field Aver. Aver. Aver. Aver. Aver. Aver. LDEO LDEO LDEO LDEO LDEO Ar-N2 Ar-N2 Ne-Ar Ne-Ar Ne-Ar-N2 Select Select Select

Sample O2 CO2 CH4 Ar N2 Ar/N2 N2 O2 O2 CH4 Ar N2 Ar/N2 N2 He He He Ne Ne T air T air N2 T air N2

site (μmol/ (μmol/ (μmol/ (μmol/ (μmol/(μmol) δ15Ν (μmol/ (μmol/ (μmol/ (μmol/ (μmol/(μmol) δ15N δ3He (ccSTP/ (μmol/ (ccSTP/ (μmol/ (°C) excess °C excess excess (°C) excess excess

[1] L) L) L) L) L) (‰) L) L) L) L) L) (‰) (per- g) L) g) L) (ccSTP/ (ccSTP/ (μmol/ (ccSTP/ (μmol/

cent) x 10-8 x 10-8 L) L) L) L) L)

Ground water

WN1-10' 2 0 0.32 16.16 635 2.55 0.99 15 2 0.33 15.57 606 2.57 0.96 na na na na na 11.1 1.3 na na na 11.1 1.3 <20

2 0 0.34 14.98 577 2.60 0.93

WN1-25' 2 0 0.68 13.52 527 2.57 0.74 17 2 0.70 13.52 524 2.58 0.79 na na na na na 15.2 0.9 na na na 15.2 0.9 <20

2 0 0.71 13.51 521 2.59 0.84

WN1-71' 2 0 0.61 12.77 477 2.68 0.84 3 1 0.61 12.73 477 2.67 0.86 0.9 4.03 0.0018 16.72 0.00761 15.7 0.0 16.2 0.3 -6 16.0 0.2 <10

1 0 0.61 12.70 476 2.67 0.88 1.2 4.03 0.0018 16.52 0.00752 16.0 0.2 -4

WN2-35' 2 0 0.00 17.78 732 2.43 1.02 2 2 0.00 17.85 735 2.43 1.06 173.5 5.37 0.0024 23.39 0.01042 11.5 3.3 10.0 1.8 29 10.0 1.8 29

2 0 0.00 17.92 737 2.43 1.09

WN2-53' 116 0 0.07 18.01 715 2.52 0.64 100 115 0.04 17.99 713 2.52 0.61 93.3 5.74 0.0026 24.39 0.01086 10.1 1.9 10.5 2.4 -8 10.3 2.2 <10

115 0 0.00 17.96 712 2.52 0.57

WN2-68' 2 0 0.52 13.72 530 2.59 0.60 9 2 0.54 13.73 530 2.59 0.65 22.9 4.47 0.0020 18.23 0.00827 14.5 0.8 14.4 0.8 1 14.5 0.8 <10

2 0 0.56 13.75 530 2.60 0.70

WN3-34' 28 0 0.00 18.15 731 2.48 0.71 54 26 0.00 18.05 724 2.49 0.71 73.3 2.43 0.0011 11.73 0.00523 10.2 2.4 3.1 -5.4 159 10.2 2.4 <20

24 0 0.00 17.94 717 2.50 0.71 73.2 3.95 0.0018 17.58 0.00783 6.1 -1.8 86

He-Ne samples partially degassed?

WN3-61' 1 0 0.50 15.86 588 2.70 na 5 1 0.51 15.96 595 2.68 0.93 -13.1 7.11 0.0032 21.27 0.00955 10.3 -0.1 12.2 1.5 -32 10.3 -0.1 <20

1 0 0.51 16.06 602 2.66 0.93

Surface Water

Indian River BayIndian River BayIndian River BayIndian River Bay

Rehoboth Beach

std seawater

[5]

33 
Table 1. Chemical and isotopic data for pumped ground waters and surface waters (continued)

RCFC RCFC Aver. σ RCFC RCFC RCFC RCFC Aver. σ RCFC RCFC RCFC RCFC Aver. σ RCFC RCFC RCFC RCFC Aver. σ RCFC RCFC

Sample SF6 SF6 SF6 SF6 SF6 SF6 cfc11 cfc11 cfc11 cfc11 cfc11 cfc11 cfc12 cfc12 cfc12 cfc12 cfc12 cfc12 cfc113 cfc113 cfc113 cfc113 cfc113 cfc113

site (fmol/ (pptv) (pptv) (pptv) recharge age (fmol/ (pptv) (pptv) (pptv) recharge age (fmol/ (pptv) (pptv) (pptv) recharge age (fmol/ (pptv) (pptv) (pptv) recharge age

[1] L) date (years) L) date (years) L) date (years) L) date (years)

Ground water

WN1-10' na na na na na na na na na na na na na na na na na na na na na na na naWN1-25' 2.67 8.71 8.89 0.25 contaminated? 122 9.7 18.9 8.0 1963 39 809 237.8 237.4 1.3 1976 26 0 0.0 7.7 6.8 1972 30

2.78 9.07 299 23.9 803 235.9 41 10.7

287 23.0 811 238.5 48 12.5

WN1-71' 1.34 4.84 4.85 0.01 2000 2 26 2.2 2.8 0.8 1954 48 602 190.7 190.6 0.2 1974 28 0 0.0 0.0 0.0 <1955 >46

1.34 4.85 39 3.4 601 190.4 0 0.0

WN2-35' 0.31 0.64 0.68 0.05 1979 23 49 2.4 2.1 0.8 1954 48 1059 195.6 196.8 2.6 1974 28 0 0.0 0.0 0.0 <1955 >46

0.34 0.71 55 2.6 1081 199.8 0 0.0

25 1.2 1056 195.1 0 0.0

WN2-53' 0.34 0.68 0.70 0.03 1979 23 1688 82.1 82.3 0.2 1973 29 7402 1367 1401 7.9 contaminated? 0 0.0 0.0 0.0 <1955 >46

0.36 0.72 1694 82.4 7466 1379 0 0.0

1694 82.4 7483 1382 0 0.0

WN2-68' 0.96 2.99 3.04 0.06 1993 9 15 1.1 1.0 0.1 1952 50 382 108.5 110.9 4.7 1969 33 0 0.0 0.0 0.0 <1955 >46

0.99 3.08 13 1.0 410 116.4 0 0.0

12 0.9 380 107.9 0 0.0

WN3-34' 0.41 0.81 0.77 0.06 1980 22 663 32.3 32.2 0.2 1966 36 675 124.8 117.0 11.0 1970 32 0 0.0 0.0 0.0 <1955 >46

0.37 0.73 655 32.0 591 109.2 0 0.0

WN3-61' 0.23 0.61 0.62 0.01 1978 24 136 7.3 4.8 2.2 1956 46 20 4.3 4.9 1.6 1949 53 1265 222.1 129 87.4 contaminated

0.23 0.63 67 3.6 18 3.7 668 117.2

64 3.4 32 6.7 277 48.6

Surface Water

Indian River BayIndian River BayIndian River BayIndian River Bay

Rehoboth Beach

std seawater

[5]

34 
Table 1. Chemical and isotopic data for pumped ground waters and surface waters (continued)

No terrigenic correction With terrigenic correction

MPTL LDEO LDEO Select LDEO LDEO LDEO Select Select Select Select Select CFC12 in Young 3H° in

Sample 3H 3H 3H 3H 4He[terr] 3He[trit] 3H° 3H/3H° 3H-3He 3H-3He 3He[trit] 3H° 3H/3H° 3H-3He 3H-3He 3He[trit] 3H° 3H/3H° 3H-3He 3H-3He 3H-3He fraction young

site bottle tube bottle (TU) (percent) (TU) (TU) (TU) age recharge (TU) (TU) (TU) age recharge (TU) (TU) (TU) age recharge year (X) fraction

[1] (TU) (TU) (TU) (years) date (years) date (years) date (pptv) [4] (TU)

Ground water

WN1-10' 3.20 na na 3.20 na na na na na na na na na na na na na na na na na na naWN1-25' 4.80 3.97 4.20 4.32 na na na na na na na na na na na na na na na na na na naWN1-71' 4.72 4.68 na 4.68 -0.4 0.6 5.3 0.89 2.1 2000 0.5 5.2 0.91 1.8 2000 0.6 5.3 0.88 2.2 2000 537 0.35 14.9

4.68 1.0 0.7 5.3 0.88 2.4 1999 0.9 5.6 0.84 3.1 1999

WN2-35' na 9.33 10.52* 9.33 -4.0 52.3 61.7 0.15 33.9 1968 51.2 60.5 0.15 33.5 1968 52.3 61.7 0.15 33.9 1968 99 > 1 na

WN2-53' na 4.25 4.54 4.39 -2.3 30.2 34.6 0.13 37.0 1965 29.5 33.9 0.13 36.6 1965 30.2 34.6 0.13 37.0 1965 67 na naWN2-68' na 4.71* 4.48 4.48 1.1 6.2 10.7 0.42 15.6 1986 6.5 11.0 0.41 16.1 1986 6.2 10.7 0.42 15.6 1986 408 0.27 39.3

WN3-34' na 5.27 5.90 5.58 8.4 10.4 16.0 0.35 18.8 1983 11.5 17.1 0.33 20.0 1982 16.5 22.1 0.25 24.7 1977 259 0.45 48.9

na 5.58 0.8 16.5 22.1 0.25 24.7 1977 16.7 22.3 0.25 24.8 1977

He-Ne samples partially degassed? He-Ne samples partially degassed?

WN3-61' na 0.73* 0.31 0.31 28.7 -4.8 -4.5 -0.07 na na 6.5 6.8 0.05 55.6 1946 6.5 6.8 0.05 55.6 1946 2 > 1 na

* high He blank,

3 H suspect

Surface Water

(collection)

Indian River Bay 3.50 3.50 3.5 2002

Indian River Bay 2002Indian River Bay 4.24 4.24 4.2 2003Indian River Bay 4.41 4.41 4.4 2003

Rehoboth Beach 7.60 7.60 7.6 2002

std seawater

[5]

35 
Table 2. Concentration and isotopic composition of nitrogen in sediments 

[mg/kg, milligrams per kilogram; ‰, per mil; na, not analyzed]

Corehole and depth Sample Sediment Nitrogen Nitrogen

below sediment-water depth age concentration δ15N 

interface, in feet (meters) (mg/kg) (‰) 

WN1-6.0' 1.8 Pleistocene 12 na

WN1-8.6' 2.6 Pleistocene 11 na

WN1-10.8' 3.3 Pleistocene 12 na

WN1-17.2' 5.2 Pleistocene 12 na

WN1-19.1' 5.8 Pleistocene 10 na

WN1-24.0' 7.3 Pleistocene 13 na

WN1-27.2' 8.3 Pleistocene 10 na

WN1-33.1' 10.1 Pleistocene 11 na

WN1-43.0' 13.1 Pleistocene 12 na

WN2-6.5’ (< 63 micron) 2.0 Holocene (?) 14,972 -0.89 (4.5 percent of sample)

WN2-6.5’ (> 63 micron) 2.0 Holocene (?) 10,732 -1.05 (95.5 percent of sample)

WN2-6.5’ (composite) 10,923 -1.04

WN3-0.6' 0.2 Holocene 296 5.3

WN3-2.0' 0.6 Holocene 59 na

WN3-3.5' 1.1 Holocene 901 3.5

WN3-9.1’ 2.8 Holocene 4,319 1.0

WN3-11.0' 3.4 Holocene (?) 2,698 1.5

WN3-12.8' 3.9 Pleistocene 45 na

WN3-15.3' 4.7 Pleistocene 20 na

WN3-21.0' 6.4 Pleistocene 21 na

WN3-23.1' 7.0 Pleistocene 9 na

36 
37 

Table 3. Summary of ground-water age interpretations

 

 [3H, tritium; 3H°, initial tritium; He, helium; Ne, neon; CFC12, chlorofluorocarbon CCl2F2; 

SF6, sulfur hexafluoride] 

Sample Sample Age Summary 

name date (Apparent 3H-3He ages are underlined) 

WN1-10' 10/24/01 Age uncertain, could be 0-30 years 

from 3

H only 

WN1-25' 10/25/01 Age uncertain, could be 0-30 years 

from 3

H and CFC12? 

SF6 contaminated, no He data 

WN1-71' 10/25/01 About 2 years 3

H, 3H-3He, and SF6 concordant 

CFC12 degraded? 

WN2-35' 10/27/01 28-34 years 3

H, 3H-3He, and CFC12 roughly concordant 

SF6 close but slight background? 

WN2-53' 10/27/01 37-45? years (late 1950s, early bomb, mixed?) 

low 3

H/3H°, moderate 3H° 

CFC12 contaminated, SF6 background? 

WN2-68' 10/27/01 9-16 years? 3

H, 3H-3He, and SF6 poorly concordant 

CFC12 degraded? 

WN3-34' 10/29/01 22-25 years? 3

H, 3H-3He, and SF6 concordant 

He+Ne degassed, CFC12 degraded? 

WN3-61' 10/29/01 ≥ 50 years (> 90 percent pre-bomb 3

H) 

undatable by these methods 

SF6 background?