U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5235

Prepared in cooperation with the

Delaware Department of Natural Resources and Environmental Control

and the

Delaware Geological Survey

Residence Time, Chemical and Isotopic Analysis of 

Nitrate in the Groundwater and Surface Water of a 

Small Agricultural Watershed in the Coastal Plain, 

Bucks Branch, Sussex County, Delaware
Cover. The Bucks Branch watershed is in a relatively flat, predominantly agricultural setting, and most stream morphology has 

been channelized. Photograph by John Clune, U.S. Geological Survey.
Residence Time, Chemical and Isotopic 

Analysis of Nitrate in the Groundwater 

and Surface Water of a Small Agricultural 

Watershed in the Coastal Plain, Bucks 

Branch, Sussex County, Delaware

By John W. Clune and Judith M. Denver

Prepared in cooperation with the

Delaware Department of Natural Resources and Environmental Control

and the 

Delaware Geological Survey

Scientifc Investigations Report 2012–5235

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Clune, J.W., and Denver, J.M., 2012, Residence time, chemical and isotopic analysis of nitrate in the groundwater and 

surface water of a small agricultural watershed in the Coastal Plain, Bucks Branch, Sussex County, Delaware: 

U.S. Geological Survey Scientific Investigations Report 2012–5235, 15 p.
iii

Contents

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

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

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

Description of Study Area ...................................................................................................................2

Sampling and Analytical Methods..............................................................................................................2

Estimated Residence Time of Groundwater..............................................................................................5

Chemical and Isotopic Analysis of Nitrate ................................................................................................7

Major-Ion Chemistry.............................................................................................................................7

Isotopic Composition of Nitrate........................................................................................................11

Summary and Conclusions.........................................................................................................................12

Acknowledgments.......................................................................................................................................12

References Cited..........................................................................................................................................13
iv

Tables

1. Minimum, median, and maximum values of selected characteristics and 

constituents in groundwater and surface-water samples collected 

synoptically in the Bucks Branch watershed, Sussex County, Delaware, 

spring and summer 2011..............................................................................................................6

2. Chemistry of natural groundwater in the surfical aquifer of Southern 

Delaware........................................................................................................................................9

Figures

1. Map showing location of study area, surface-water sampling sites, and 

land use in the Bucks Branch watershed, Sussex County, Delaware ................................3

2. Aerial photographs of historical agricultural land use in Bucks Branch 

watershed, Sussex County, Delaware in (A) 1968, and (B) 1997...........................................4

3–4. Graphs showing—

3. Nitrogen inputs from fertilizer, manure, septic, and atmospheric 

sources in Sussex County, Delaware, 1982–2002...........................................................4

4. Concentration of silica in relation to the age of groundwater in 

Bucks Branch watershed, Sussex County, Delaware...................................................7

5. Boxplots showing distribution of selected characteristics and constituents 

in groundwater and surface-water samples, Bucks Branch watershed, 

Sussex County, Delaware............................................................................................................8

6–8. Graphs showing—

6. Concentrations of nitrate in relation to the concentrations of 

(A) calcium plus magnesium (Ca + Mg), (B) potassium plus 

chloride (K + Cl), and (C) specific conductance (SC) in 

groundwater samples collected in Bucks Branch watershed, 

Sussex County, Delaware, 2011.........................................................................................9

7. Relative abundance patterns of constituents in samples of 

(A) groundwater and (B) surface water, and (C) a trilinear 

diagram of major ions within Bucks Branch watershed, Sussex 

County, Delaware...............................................................................................................10

8. Relation of nitrogen isotopes to oxygen isotopes in groundwater 

and surface water, Bucks Branch watershed, Sussex County, 

Delaware.............................................................................................................................11

9. Map showing nitrate concentrations and nitrogen-isotope compositions 

in samples of surface water collected synoptically during high base-flow 

conditions, Bucks Branch watershed, Sussex County, Delaware, 

March 2011 ..................................................................................................................................12
v

Conversion Factors and Datums

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.004047 square kilometer (km2)

square mile (mi2) 2.59 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3) 

Flow rate

inch per year (in/yr) 2.54 centimeter per year (cm/yr)

foot per second (ft/s) 0.3048 meter per second (m/s)

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

 °C = (°F - 32) / 1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1988 

(NGVD 88). 

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Residence Time, Chemical and Isotopic Analysis of 

Nitrate in the Groundwater and Surface Water of a 

Small Agricultural Watershed in the Coastal Plain, 

Bucks Branch, Sussex County, Delaware

By John W. Clune and Judith M. Denver

Abstract

Nitrate is a common contaminant in groundwater and 

surface water throughout the Nation, and water-resource 

managers need more detailed small-scale watershed research 

to guide conservation efforts aimed at improving water quality. Concentrations of nitrate in Bucks Branch are among the 

highest in the state of Delaware and a scientific investigation 

was performed to provide water-quality information to assist 

with the management of agriculture and water resources. A 

combination of major-ion chemistry, nitrogen isotopic composition and age-dating techniques was used to estimate the 

residence time and provide a chemical and isotopic analysis of 

nitrate in the groundwater in the surficial aquifer of the Bucks 

Branch watershed in Sussex County, Delaware. The land 

use was more than 90 percent agricultural and most nitrogen 

inputs were from manure and fertilizer. The apparent median 

age of sampled groundwater is 18 years and the estimated 

residence time of groundwater contributing to the streamflow 

for the entire Bucks Branch watershed at the outlet is approximately 19 years. Concentrations of nitrate exceeded the U.S. 

Environmental Protection Agency drinking-water standard of 

10 milligrams per liter (as nitrogen) in 60 percent of groundwater samples and 42 percent of surface-water samples. The 

overall geochemistry in the Bucks Branch watershed indicates 

that agriculture is the predominant source of nitrate contamination and the observed patterns in major-ion chemistry are 

similar to those observed in other studies on the Mid-Atlantic 

Coastal Plain. The pattern of enrichment in nitrogen and 

oxygen isotopes (δ15N and δ18O) of nitrate in groundwater and 

surface water indicates there is some loss of nitrate through 

denitrification, but this process is not sufficient to remove all 

of the nitrate from groundwater discharging to streams, and 

concentrations of nitrate in streams remain elevated. 

Introduction

Nitrogen is a common contaminant in groundwater and 

surface water throughout the Nation (Dubrovsky and others, 

2010). Sources of nitrate include atmospheric deposition, 

municipal and residential wastewater treatment systems, 

and the widespread agricultural use of commercial fertilizer 

and manure (Puckett, 1995). Previous studies have shown 

that elevated concentrations of nitrate in groundwater and 

surface water are often attributable to urban development 

or agriculture (Ator and Ferrari, 1997; Mueller and Spahr, 

2006; Dubrovsky and others, 2010). Regional assessments 

and modeling have shown that the predominant impairment 

to water quality on the Delmarva Peninsula is due to agriculture (Hamilton and others, 1993; Shedlock and others, 

1999; Debrewer and others, 2007; Ator and others, 2011). The 

substantial nitrogen inputs from manure and fertilizer associated with intensive agricultural areas have been reflected 

in the chemical composition of groundwater in the surficial 

aquifer of this region (Denver, 1986, 1989). Bucks Branch, a 

small agricultural watershed in Sussex County, Delaware, has 

some of the highest measured concentrations of total nitrogen 

in any stream in the State (Delaware Department of Natural 

Resources and Environmental Control, 2010) and most of the 

nitrogen is in the form of nitrate.

Although the association of groundwater and surfacewater chemistry with land use can be indicative of nitrogen 

sources, more specific investigations have been conducted 

using chemical, isotopic, and age-dating techniques to provide 

a better understanding of the nitrogen sources, biogeochemical 

processes, and the residence time associated with nitrate transport through groundwater. For instance, major-ion chemistry 

in relation to nitrate concentrations in groundwater has shown 

a marked contrast between water that recharged the surficial 

aquifer through areas of agriculture compared to other land 
2 Residence Time, Chemical and Isotopic Analysis of Nitrate in Bucks Branch Watershed, Sussex County, Delaware

uses (Denver, 1986, 1989: Hamilton and others, 1993). The 

nitrogen and oxygen isotopes of nitrate have been used to indicate the sources of nitrogen, and tracing the fractionation of 

these isotopes can provide evidence of denitrification (Heaton, 

1986; Clark and Fritz, 1997; Kendall and McDonnell, 1998; 

Böhlke, 2003). Age dating of groundwater has also been used 

successfully in determining the overall residence time of water 

within aquifers (Busenberg and Plummer, 1992; Dunkle and 

others, 1993). 

Recent approaches to describe temporal changes in the 

concentrations of nitrate in groundwater have used a combination of the chemical, isotopic, and age-dating techniques 

(Böhlke and Denver, 1995; Stoner and others, 1997; Denver 

and others, 2010). The research described in this report builds 

upon this multidisciplinary approach at a small watershed 

scale. At this level, the results will serve the needs of resource 

managers by providing a more practical perspective on the 

traveltime of groundwater to streams, a detailed understanding 

of the sources of nitrogen for targeting management efforts, 

and better quantification of the outcomes of water-quality 

improvements (National Research Council, 2011; Weller 

and others, 2010). This study was conducted by the U.S. 

Geological Survey (USGS) in cooperation with the Delaware 

Geological Survey (DGS) and the Delaware Department of 

Natural Resources and Environmental Control (DNREC) and 

is part of a regional effort to better understand small watershed 

processes in the Chesapeake Bay watershed.

Purpose and Scope

The purpose of this report is to present (1) estimated residence times of groundwater, and (2) provide a chemical and 

isotopic analysis of nitrate in the groundwater of the surficial 

aquifer and surface-water system for Bucks Branch watershed 

on the Delmarva Peninsula in Sussex County, Delaware. These 

objectives were met by the collection of stream-water samples 

in 2011 during periods when the dominant contribution of 

streamflow was from groundwater during high (March) and 

low (August) base-flow conditions, and with the collection 

of groundwater samples in June 2011. Major-ion chemistry 

and the nitrogen and oxygen isotopic composition of nitrate 

in groundwater and stream water during base flow were used 

to evaluate the potential sources of nitrogen and to describe 

biogeochemical processes affecting concentrations of nitrate 

in water. Concentrations of sulfur hexafluoride (SF6

) and 

dissolved gases in groundwater, and concentrations of silica 

in groundwater and surface water were used to determine the 

apparent age of groundwater in the aquifer and to estimate the 

average residence time of groundwater discharging to streams. 

Description of Study Area

Bucks Branch drains 7.02 square miles (mi2), and is a 

tributary to the Nanticoke River on the Delmarva Peninsula, 

which eventually drains into the Chesapeake Bay. The study 

area is located approximately 32 miles (mi) west of the 

Atlantic Ocean shoreline in Sussex County, Delaware 

(fig. 1). The area is underlain by sediments of the Beaverdam 

Formation that consists mainly of fluvial and estuarine deposits of sand, gravel, silt, and clays (Andres, 1994). The soil is 

predominantly well-drained sandy loam (U.S. Department 

of Agriculture, 2012). The unconfined surficial aquifer is 

approximately 100 feet (ft) thick and the water table is 5 to 

10 feet below land surface (Denver, 1986). In this area of the 

Coastal Plain Physiographic Province, groundwater provides 

the largest contribution of freshwater flow in streams and is 

the predominant influence on surface-water quality and the 

major source of nitrate (Shedlock and others, 1999; Lindsey 

and others, 2003; Spahr and others, 2010). 

Land use is predominantly agricultural, consisting mainly 

of cropland (90.1 percent) and concentrated animal feeding 

operations (0.8 percent), along with wetland (4.1 percent), 

residential dwellings (2.6 percent), forest (1.9 percent) and 

other settings (0.5 percent) (fig. 1). Most wetlands are located 

in the headwaters along the main stem of Bucks Branch, 

where the soils are poorly drained and less suited for agriculture. Agriculture has been primarily poultry production 

and varieties of row-crop cultivation during the past 50 years 

(fig. 2). Manure and fertilizer are applied to corn fields from 

February to April and to fields of small grains from February 

to March. Corn is planted from April to May and harvested for 

silage and grain from August through October. The vast majority of nitrogen inputs in this part of Sussex County, Delaware 

are from manure and fertilizer (fig. 3).

The topography in the Bucks Branch watershed is relatively flat, with elevations that range from 30 to 50 ft above 

sea level. The local climate provides for a wet season of 

increased precipitation from April through June, with a mean 

annual rainfall of 44 inches per year (in/yr), of which approximately 14 inches (in.) recharges the aquifer (Johnston, 1973). 

The watershed consists of three tributaries where the stream 

morphology has been extensively altered. Channelization has 

created steep banks and berms along the streams that are often 

hydraulically disconnected from flood plains and riparian 

zones. This channelization and dredging (ditching) of streams 

affects the nearby water table and impairs the functional properties of the aquatic ecosystem (Williams and others, 2004). 

The adjacent access roads used for maintaining the ditches 

reduce canopy cover over the stream channels. 

Sampling and Analytical Methods

Sampling sites included 7 stream locations throughout 

the watershed (fig. 1), and a random selection of 10 shallow 

domestic wells (median depth of 63 ft below land surface) 

screened within the surficial aquifer (locations of homeowners’ 

wells not shown). Streams were sampled synoptically during 

high (March) and low (August) base flows and groundwater 

was sampled once in June 2011. The USGS and DNREC 
Sampling and Analytical Methods 3

Figure 1. Location of study area, surface-water sampling sites, and land use in the 

Bucks Branch watershed, Sussex County, Delaware.
4 Residence Time, Chemical and Isotopic Analysis of Nitrate in Bucks Branch Watershed, Sussex County, Delaware

Figure 2. Aerial photographs of historical agricultural land use in Bucks Branch watershed, Sussex County, Delaware in (A) 1968, 

and (B) 1997.

Figure 3. Nitrogen inputs from fertilizer, manure, septic, and atmospheric sources in Sussex County, 

Delaware, 1982–2002 (Values are based on data from Ruddy and others, 2006; U.S. Bureau of the Census, 

1993; and calculations described in Lindsey and others, 2009).
Estimated Residence Time of Groundwater 5

worked together to perform the data collection and analysis. 

The USGS collected samples for analysis of isotopes of nitrate 

at the surface-water sites and sampled groundwater for both 

isotopes and age dating at the domestic wells. This work was 

completed in coordination with DNREC scientists from the 

Division of Water who provided the reconnaissance and landowner permission along with the field collection of groundwater and surface-water samples for analysis of nutrients and 

major ions.

Groundwater and surface-water samples for the majorion and nutrient analysis were collected following protocols 

outlined in the USGS National Field Manual (U.S. Geological 

Survey, variously dated). Groundwater samples were collected 

using a brass connection to a sampling point (outside spigot) 

prior to any water treatment. Wells were purged until field 

parameters (pH, specific conductance, water temperature, 

and dissolved oxygen) stabilized and the final readings were 

recorded. Samples were then collected for laboratory 

analysis of nutrients, major ions, nitrate isotopes, and age 

dating. Surface-water sampling included field measurements 

(discharge, pH, specific conductance, dissolved oxygen, and 

temperature) in which the mean of stabilized readings was 

taken at six-tenths of the vertical stream depth to represent the 

average velocity of the water column and provide a homogenous mixture. Isokinetic samples were collected using a 

depth-integrating vertical sampler and composited in a churn 

following USGS protocols for equal-width-increment sampling (U.S. Geological Survey, variously dated). Grab samples 

were collected at the centroid of flow for streams that had a 

velocity of less than 1.5 feet per second. 

The samples for major ion and nutrient chemistry 

were filtered onsite through a disposable filter with a 

0.45-micrometer (μm) pore size and analyzed by the DNREC 

laboratory in Dover, Delaware. Samples were analyzed for 

major ions by either the inductively coupled plasma method 

with atomic emission spectrometry, ion chromatography, or 

automated colorimetry. Nutrient analyses were performed 

by automated colorimetry (with persulfate digestion when 

required) with uncertainties of less than 10 percent (less than 

20 percent for silica) (Ben Pressly, DNREC, written commun., 

2012).

Samples collected for analysis of nitrogen isotopes were 

filtered onsite using a sterile Nalgene nitrate-free 0.2-millimeter (mm) filter, frozen until shipped to the USGS Stable 

Isotope Laboratory in Reston, Virginia, and analyzed using 

the bacterial reduction method (Sigman and others, 2001; 

Casciotti and others, 2002). The nitrogen isotopic delta (δ15N) 

values obtained represent the isotopic ratio of 15N/14N of the 

sample relative to the standard of atmospheric N2

, whereas 

δ18O values represent the isotopic ratio of 18O/16O of the 

sample relative to the standard of Vienna Standard Mean 

Ocean Water. The isotopic ratios in this study are reported in 

parts per thousand (‰), normalized to reference values given 

in Böhlke and others (2003), with analytical uncertainties 

(1-sigma) of approximately ± 0.25 ‰ for δ15N and ± 0.5 ‰ for 

δ18O (Révész and Casciotti, 2007). 

Samples for age dating were processed, preserved, and 

shipped to the USGS Chlorofluorocarbon Laboratory in 

Reston, Virginia and the apparent age of groundwater was 

determined from concentrations of dissolved gases (N2

, Ar, 

CH4

, O2, CO2) and gaseous SF6 using methods described in 

Busenburg and Plummer (1992). SF6

 is soluble in groundwater, and the increasing rate of anthropogenic concentrations of 

SF6

 has been well established for the atmosphere over time. 

Detectable amounts of SF6 are often present in younger waters 

(approximately 1970s) such as the surficial aquifer of Bucks 

Branch, and can be used for age dating. The apparent age of 

groundwater reflects atmospheric concentrations of SF6

 at the 

time the water reached the water table.

The quality assurance of sampling included both replicate and blank samples. There was no significant variability 

or bias apparent in the sampling or laboratory techniques for 

the major ions, nutrients, nitrogen isotopes, or age dating. 

An internal USGS laboratory evaluation program (LEP) was 

completed for the major ions and nutrient samples analyzed by 

the DNREC laboratory, from which a comparison and analysis 

of recent standard reference samples were performed. The ion 

balance was calculated to check the analytical results, and was 

considered to be of good quality (Hem, 1985). The LEP was 

approved for the objectives of this report and entry of the data 

into the USGS National Water Information System (NWIS). 

After results were received from the laboratories, graphical 

and statistical analyses using relative abundance patterns, 

trilinear diagrams, correlation matrices, and linear correlations 

were generated to describe the data collected. 

Estimated Residence Time of 

Groundwater

The apparent age of groundwater sampled in the Bucks 

Branch watershed during June 2011 ranged from 13 to 30 

years, with a median age of 18 years (table 1). This range is 

similar to groundwater ages reported for other areas of the 

Delmarva Coastal Plain (Dunkle and others, 1993). Wells 

were selected randomly throughout the watershed and most 

wells had long screened intervals that intercept flow paths of 

varying ages; therefore, no clear relation between apparent 

age and well depth was observed. Since the samples are likely 

to be mixed, the apparent ages may not be equal to the actual 

mean ages of the samples; nevertheless, the aggregated data 

are considered to indicate the approximate residence time of 

groundwater in the surficial aquifer. 

The residence time of groundwater contributing to the 

stream during base flow was based on an empirical model 

using simple linear regression of groundwater ages and silica 

concentrations in groundwater within the Bucks Branch 

watershed (fig. 4). Similar relations have been shown to be 

useful surrogates for age on the local scale throughout parts 

of the Atlantic Coastal Plain because concentrations of silica 

increase over time as minerals containing silica, including 
6 Residence Time, Chemical and Isotopic Analysis of Nitrate in Bucks Branch Watershed, Sussex County, Delaware

Table 1. Minimum, median, and maximum values of selected characteristics and constituents in groundwater and surface-water 

samples collected synoptically in the Bucks Branch watershed, Sussex County, Delaware, spring and summer 2011.

[ft, feet; ft3/s, cubic feet per second; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; μg/L, micrograms per liter; min, 

minimum; max, maximum; n, number of samples; <, less than; --, no data; N, nitrogen; CaCO3, calcium carbonate; NO3, nitrate, laboratory; nitrogen isotopic 

delta (δ15N) values of nitrate represent the isotopic ratio of 15N/14N of the sample relative to the standard of atmospheric N2; oxygen isotopic delta (δ15O) values 

of nitrate represent the isotopic ratio of 18O/16O of the sample relative to the standard of Vienna Standard Mean Ocean Water]

Constituent (units)

Groundwater Surface water

June 2011 

(n=10)

High base flow Low base flow

March 2011 

(n=7)

August 2011 

(n=5)

Min Median Max Min Median Max Min Median Max

Physical properties

Well depth (ft below land surface) 54 63 87 -- -- -- -- -- --

Discharge (ft3/s) -- -- -- 0.05 2.74 11.8 0.05 0.31 0.76

Temperature (°C) 14.7 16.0 17.0 7.1 13.1 14.0 20.0 22.9 23.8

Specific conductance (μS/cm at 25°C) 76 144 544 147 175 219 120 196 220

Dissolved oxygen (mg/L) 2.3 4.8 9.9 10.1 13.3 17.0 5.4 7.0 8.0

pH (standard units) 4.8 5.0 5.4 6.0 6.2 6.5 6.4 6.5 6.9

Alkalinity (mg/L CaCO3) 2.0 5.8 12.0 8.8 9.5 11.5 23.7 24.5 29.9

Nutrients

Nitrate (mg/L as N) 3.7 10.7 46.6 4.9 10.2 12.9 7.1 8.2 11.1

Ammonia (mg/L as N) < 0 0.01 0.03 < 0.01 0.01 0.18 0.06 0.14 0.16

Phosphorus (mg/L) 0.01 0.01 0.02 0.01 0.02 0.03 0.02 0.04 0.09

Major ions or elements

Calcium (mg/L) 3.0 13.1 51.2 9.3 11.9 15.6 13.7 17.3 19.5

Magnesium (mg/L) 0.7 3.2 14.9 2.4 3.7 5.6 4.3 5.4 6.9

Potassium (mg/L) 1.3 3.5 9.0 2.2 2.6 3.2 3.6 3.7 4.2

Sodium (mg/L) 3.6 8.0 16.9 8.7 9.1 9.7 9.2 9.7 9.9

Bicarbonate (mg/L) 2.5 7.1 14.6 10.7 11.6 14.0 28.9 29.9 36.5

Chloride (mg/L) 6.0 14.0 31.0 14.0 15.0 17.0 13.0 14.0 15.0

Sulfate (mg/L) < 0.75 0.75 3.30 8.4 12.0 15.9 4.8 7.7 13.8

Bromide (mg/L) < 0.5 < 0.5 < 0.5 < 0.2 < 0.2 < 0.2 < 0.05 < 0.05 < 0.05

Silica (mg/L) 13.0 15.8 24.1 10.3 16.4 17.6 18.0 18.8 22.1

Iron (μg/L) < 0.1 1.0 35 35 35 118 53 220 344

Isotopes 

δ15N (per mil in NO3) 1.4 5.7 13.4 6.5 7.7 10.4 9.3 10.5 11.9

δ18O (per mil in NO3) 1.1 2.3 5.7 3.4 3.9 5.7 5.2 6.1 6.5

Dissolved gases

Argon (mg/L) 0.7 0.7 0.8 -- -- -- -- -- --

Carbon dioxide (mg/L) 23.3 41.7 95.8 -- -- -- -- -- --

Dissolved oxygen, lab (mg/L) 2.4 5.1 9.5 -- -- -- -- -- --

Methane (mg/L) 0.0 0.0 0.0 -- -- -- -- -- --

Nitrogen gas (mg/L) 16.8 20.2 23.8 -- -- -- -- -- --

Residence time

Estimated age (years) 13 18 30 13 19 21 21 22 26
Chemical and Isotopic Analysis of Nitrate 7

Figure 4. Concentration of silica in relation to the age of 

groundwater in Bucks Branch watershed, Sussex County, 

Delaware.

quartz and feldspars, are dissolved by water flowing through 

the groundwater system (Denver, 1986; Denver and others, 

2010). A surface-water sample collected at base flow at a 

particular point in the stream system represents an aggregate 

of all groundwater discharging from upstream of that point 

in the watershed. A preliminary estimate of the aggregate 

residence time of groundwater contributing to surface-water 

samples can be made by comparing concentrations of silica in 

surface water to the data shown in figure 4. Using this method, 

the aggregate groundwater residence times contributing to 

streamflow range from 13 to 26 years (table 1). The estimated 

residence time of groundwater contributing to streamflow for 

the entire Bucks Branch watershed represented by data at the 

outlet (SW1; fig. 1), is approximately 19 years. This value is 

comparable to the overall median age (18 years) for groundwater. Surface-water samples were not available in Gilbert 

Trivitis Ditch; therefore, the groundwater sample from this 

subwatershed was not used for calculating the age of surface 

water. These estimates might be refined with more explicit 

consideration of mixing effects on the concentrations of silica 

and age tracers, but that was beyond the scope of this study. 

Shallow groundwater flow paths can influence the age 

of surface water both temporally and spatially (Lindsey and 

others, 2003). The estimated ages of stream water are younger 

during high base flow (March) than low base flow (August) 

(table 1). This observation is consistent with the stream 

having received more contributions from shallow flow paths 

containing water with shorter residence times when the water 

table was higher in the spring. Although the median ages of 

groundwater and surface water were similar, the median age 

of all surface-water samples (20 years) was slightly older than 

the median age of groundwater samples (18 years), possibly 

because the aggregate groundwater contributing to streamflow 

included older water that was not intercepted in the domestic 

wells (table 1, fig. 5H). 

Chemical and Isotopic Analysis of 

Nitrate

The isotopic analysis of nitrate along with the major-ion 

chemistry of the groundwater and surface water provided useful insight into the sources of nitrogen and a better understanding of the biogeochemical processes that occur in the Bucks 

Branch watershed. The geochemical characteristics of the surficial aquifer are presented through summary statistics of the 

water quality, correlations of nitrate with other major constituents and a graphical representation of the type of water found 

in this setting. The nitrogen and oxygen isotopes of nitrate are 

compared with those from previous studies and the pattern of 

isotopic enrichment of nitrate in the groundwater and surface 

water is discussed in the following sections. 

Major-Ion Chemistry

Under natural conditions, dissolved ions in the groundwater of the surficial aquifer are derived largely from atmospheric 

deposition and dissolution of silicate minerals (Denver, 1986). 

This results in dilute acidic groundwater in which sodium, 

bicarbonate, and chloride are the major ions and concentrations of nitrate are less than 1 milligram per liter (mg/L) as N 

(table 2). The surficial aquifer in the Bucks Branch watershed 

is susceptible to contamination because of the combination 

of well-drained soils, sandy aquifer sediments, and a shallow 

water table. As a result, constituents from chemicals applied to 

enhance crop production in excess of plant needs are present 

in groundwater and concentrations of ions are generally much 

greater than those present under natural conditions (table 1, 

fig. 5). 

In the wells sampled in this study, the surficial aquifer 

had oxic conditions with dissolved oxygen levels greater than 

0.5 mg/L (McMahon and Chapelle, 2007). Microbes will 

utilize nitrate as an energy source only if free oxygen is not 

available (Drever, 1997). The lack of these reducing conditions for effective removal of nitrate by microbes coupled 

with the high rate of nitrogen input often leaves any effects of 

possible attenuation of nitrate indistinguishable in the surficial 

aquifer (Böhlke and Denver, 1995). Nitrate concentrations 

exceeded the U.S. Environmental Protection Agency (USEPA) 

drinking-water standard (10 mg/L as N) in 60 percent of the 

groundwater wells and at 42 percent of the surface-water sites 

(fig. 5B) (U.S. Environmental Protection Agency, 2009).

In comparison to natural conditions, the pH was slightly 

more acidic, possibly as a result of nitrification of ammonia 

and organic nitrogen from fertilizer, manure, and soil organic 

matter (tables 1 and 2). For arable soil to remain fertile, the 

acidity is neutralized with the application of crushed dolomite 

[CaMg(CO3

)2], referred to locally as “lime.” The dolomite 

applications are reflected in the groundwater by the increased 

levels of calcium (Ca2+) and magnesium (Mg2+) ions and 

their significant relation with nitrate (table 1, fig. 6A). The 
8 Residence Time, Chemical and Isotopic Analysis of Nitrate in Bucks Branch Watershed, Sussex County, Delaware

Figure 5. Distribution of selected characteristics and constituents in groundwater and surface-water 

samples, Bucks Branch watershed, Sussex County, Delaware.

plant nutrient potash (KCl) that is applied to agricultural soils 

results in elevated levels of potassium (K+

) and chloride (Cl-) 

ions in groundwater and also has a significant relation with 

nitrate (table 1, fig. 6B). 

Specific conductance was a good predictor of nitrate 

concentrations in the Bucks Branch watershed because the 

overall ionic strength in groundwater is dominated by ions that 

are commonly related to agricultural applications of fertilizer, 

manure, and lime (fig. 6C). The specific conductance intercept 

of 34.5 microsiemens per centimeter (μS/cm) is comparable 

to what would be expected under natural conditions (Denver, 

1986; Denver and others, 2010) (fig. 6C). 

Median concentrations of chloride and other ions that 

are not affected by reducing conditions (calcium, magnesium, 

potassium, and sodium) were similar in groundwater and surface water. The range of these values is greater in groundwater 

than in surface water because surface water is a mixture of 

all the groundwater flow paths discharging from the upstream 

watershed (table 1). 

Concentrations of nitrate, iron, and dissolved oxygen are 

potentially affected by geochemical conditions and differed 

in groundwater and surface-water environments (figs. 5B, D, 

E). The differences are most likely related to changes in water 

chemistry that occur as discharging groundwater flows through 
Chemical and Isotopic Analysis of Nitrate 9

Table 2. Chemistry of natural groundwater in the surfical 

aquifer of Southern Delaware.

[μS/cm, microsiemens per centimeter; mg/L, milligrams per liter]

Chemical constituent Median value1

Silica 23 mg/L

Specific conductance 55 μS/cm

pH 5.8

Bicarbonate 15.6 mg/L

Calcium 2.6 mg/L

Magnesium 0.6 mg/L

Potassium 1.2 mg/L

Sodium 6.7 mg/L

Nitrate as N 0.9 mg/L

Chloride 5.4 mg/L

Sulfate 1.1 mg/L

Iron 0.03 mg/L

1

 Source: Denver, 1986, 1989.

near-stream riparian-zone sediments where it may encounter 

reducing conditions and increase concentrations of dissolved 

iron. When the discharging groundwater is reintroduced to 

the atmosphere in the stream, the dissolved oxygen increases. 

Higher concentrations of iron in surface water compared to 

groundwater provide evidence that some of the nitrate could 

be lost prior to discharge to the stream through denitrification 

if groundwater passed through the shallow riparian zone 

sediments. These effects were more apparent in the summer 

(August) during low base flow than in the spring (March) during high base flow.

Besides inputs of nitrogen from local land-use sources, 

a combination of factors contribute to high concentrations of 

nitrate in surface water all year. The thick surficial aquifer in 

the Bucks Branch watershed and ditching of stream channels 

has potentially disconnected much of the stream network 

from the riparian zone. As a result, many groundwater flow 

paths containing high concentrations of nitrate may bypass 

the anoxic conditions associated with the riparian zone and 

discharge directly through the sandy streambed. 

Overall, the groundwater and surface water under natural 

settings reflect a Ca-Na-HCO3

 water type compared to the 

agricultural Ca-Mg-NO3

 water type found in the Bucks Branch 

watershed (fig. 7). The relative abundance of cations and 

anions in groundwater and surface water present a pattern 

commonly seen in water affected by agriculture (figs. 7A and 

7B) (Denver, 1989). Also, spatial and temporal differences are 

apparent in the chemical composition as groundwater moves 

to surface water during high and low base-flow conditions, 

as shown in the trilinear diagram in figure 7C. The collective 

geochemistry in the Bucks Branch watershed is similar to the 

geochemistry described in other water-quality studies in the 

Figure 6. Concentrations of nitrate in relation to the 

concentrations of (A) calcium plus magnesium (Ca + Mg), (B)

potassium plus chloride (K + Cl), and (C) specific conductance 

(SC) in groundwater samples collected in Bucks Branch 

watershed, Sussex County, Delaware, 2011.
10 Residence Time, Chemical and Isotopic Analysis of Nitrate in Bucks Branch Watershed, Sussex County, Delaware

Figure 7. Relative abundance patterns of constituents in samples of (A) groundwater and (B) surface water, and (C) a trilinear 

diagram of major ions within Bucks Branch watershed, Sussex County, Delaware.
Chemical and Isotopic Analysis of Nitrate 11

Delmarva Coastal Plain and is an indication that agriculture 

is the predominant source of nitrate contamination (Denver, 

1986, 1989; Hamilton and others, 1993; Böhlke and Denver, 

1995; Ator and others, 2004). 

Isotopic Composition of Nitrate

The major sources of nitrogen in the Bucks Branch 

watershed, based on county estimates, are inorganic fertilizer, 

manure, and atmospheric deposition (Ruddy and others, 2006) 

(fig. 3). Most of the δ15N values of nitrate in groundwater 

shown in figure 8 are similar to those commonly reported for 

nitrate recharged beneath natural soils or fertilized fields with 

values around +2 to +6 ‰ (Heaton, 1986; Böhlke and others, 

2002). The δ15N values of nitrate in groundwater samples were 

lower than those in the stream-water samples collected during 

base flow (fig. 5F, table 1). Surface water at base flow represents the cumulative contributions of upstream groundwater 

recharged from all possible sources of natural soil, fertilized 

soil, and manure/septic areas, and any geochemical changes 

(figs. 8 and 9). Although the δ15N values from surface water at 

base flow were similar to data observed for nitrate recharged 

beneath manure/septic areas (+8 to +20 ‰), the major-ion 

chemistry of groundwater and surface water described in the 

previous section provides evidence that δ15N enrichment is 

more likely from the loss of nitrate through denitrification 

(Kreitler and Jones, 1975; Gormly and Spalding, 1979; Böhlke 

and others, 2002).

Further evidence for denitrification as an important 

process affecting the differences in isotopic composition of 

groundwater and surface water is seen in the comparison of 

the nitrogen and oxygen isotopes of nitrate. Denitrification 

converts NO3

 to N2 gas and the microbial preference of lighter 

isotopes (14N and 16O) during this isotope fractionation leaves 

the residual NO3

 more enriched in heavier isotopes (15N and 18O) and the lighter molecules escape into the vapor phase 

(Kendall and McDonnell, 1998; Eby, 2004). An enrichment 

trend of the heavier isotopes in nitrate is shown in figure 8 

with a 2:1 slope of δ15N to δ18O. One groundwater sample is 

not shown because of degassing during shipment. This linear 

relation between the N and O isotopes in flow-path studies has 

been shown to be an indication of denitrification in groundwater (Böttcher and others, 1990; Groffman and others, 2006). 

Although the samples collected from Bucks Branch are not 

along a flow path, the values of δ18O observed in the stream 

samples also indicate denitrification, because they are higher 

than typical δ18O values of nitrate formed by nitrification 

in this region (+1 to +3 ‰) (J.K. Böhlke, U.S. Geological 

Survey, written commun., 2012). Excess N2

 gas produced by 

denitrification was also detected in three of the groundwater 

samples indicating denitrification is also possible in some parts 

of the groundwater system prior to reaching stream discharge 

areas. 

Figure 8. Relation of nitrogen isotopes 

to oxygen isotopes in groundwater and 

surface water, Bucks Branch watershed, 

Sussex County, Delaware (Heaton, 1986; 

Böhlke and others, 2002).
12 Residence Time, Chemical and Isotopic Analysis of Nitrate in Bucks Branch Watershed, Sussex County, Delaware

Figure 9. Nitrate concentrations and nitrogen-isotope compositions in samples of surface water collected synoptically during high 

base-flow conditions, Bucks Branch watershed, Sussex County, Delaware, March 2011.

Summary and Conclusions

Bucks Branch in Sussex County, Delaware has some of 

the highest concentrations of total nitrogen measured in any 

stream in the State. Analysis of major-ion chemistry, the isotopic composition of nitrate, and age-dating techniques were 

used to improve the understanding of the chemical and isotopic composition of nitrate in the groundwater in this watershed 

and provide a range for the residence time of groundwater 

in the surficial aquifer. Land use in the Bucks Branch watershed is predominantly agricultural and nitrate concentrations 

exceeded the U.S. Environmental Protection Agency drinkingwater standard (10 milligrams per liter as nitrogen) in samples 

from 60 percent of the groundwater wells and 42 percent of 

the surface-water sites used in this study. Results of analysis of 

the geochemistry of groundwater and surface water indicates 

that agriculture is the predominant source of nitrate contamination. The pattern of enrichment in nitrogen and oxygen 

isotopes (δ15N and δ18O) in groundwater and surface water is 

consistent with loss of nitrate through denitrification, but rates 

of denitrification were not sufficient to remove most of the 

nitrate from groundwater discharging to streams. The estimated aggregate residence time of groundwater contributing 

to surface water for the entire Bucks Branch watershed in the 

most downstream sample is approximately 19 years. Further 

study along specific groundwater flow paths could lead to a 

better understanding of the rate of denitrification, the relative 

importance of fertilizer and manure sources, and the influence 

of shallow groundwater on stream quality. The preliminary 

analysis in this report provides information about the transport 

and fate of nitrate in small scale watersheds that can be useful 

for better management of agriculture and water resources. 

Acknowledgments

This project was funded under a cooperative agreement 

between the Delaware Geological Survey (DGS) and the 

U.S. Geological Survey (USGS), in support of the Delaware 

Department of Natural Resources and Environmental Control 

(DNREC). 

The authors would like to thank Hassan Mirsajadi, Amber 

Joseph, Blair Venables, Xie Xia, Kathy Knowles, Sergio 

Huerta, and Ben Pressly of DNREC for overall project support. Thanks are extended to Dr. Joanna York of the University 

of Delaware, and Deborah Bringman, Christopher Lewis, John 

Karl Böhlke, Scott Ator, Brook Connors, Douglas Chambers, 

Bruce Lindsey, Valerie Gaine, Timothy Auer, and Ruth Larkins 

of USGS for their assistance with this study. The authors also 

sincerely appreciate the generosity of local landowners who 

provided access for sampling. 
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Prepared by USGS West Trenton Publishing Service Center.

Edited by Ruth M. Larkins.

Graphics and layout by Timothy W. Auer.

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Clune, J.W., and Denver, J.M.—Residence Time, Chemical and Isotopic Analysis of Nitrate in the Groundwater and Surface Water of Bucks Branch Watershed, Sussex County, Delaware—Scientific Investigations Report 2012–5235

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