U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2017–5054

Water Quality in the Surficial Aquifer Near Agricultural 

Areas in the Delaware Coastal Plain, 2014

C&D Canal

Fall Line DELAWARE

DELAWARE

MARYLAND

MARYLAND

Chesapeake Bay

Wilmington

Dover

DELAWARE BAY

Delaware River

ATLANTIC OCEAN

NEW

CASTLE

KENT

SUSSEX

NEW

JERSEY

Indian River Bay

Rehoboth Bay

EXPLANATION

Delaware Coastal

Plain

Chesapeake Bay

watershed boundary

Fall Line

Sampled wells and

identifier

Eb35-23

Eb35-23

Ea44-13

Fb22-15

Fc12-26

Fa45-07

Gb11-07 Gc14-04

Gb21-10

Hb22-17

Hc14-15

Id14-03 Ic21-08

Ic43-01 Ie32-02

Je34-04

Jd55-10 Jb42-05

Kb32-29

Lb23-03

Lb52-07

Kd13-09

Ke33-22

Lb15-17

Le24-11

Ld33-10

Mc43-06

Nf24-05

Ng45-02

Oi13-06 Og15-07

Oi32-18

Oh42-07 Pg22-06

Pf41-02 Qc13-01

Qc22-04

Rf13-02

Ph34-15

Qh13-05

Qg32-18

Og32-07

Nd31-06

Nd55-06

Oc21-03

Od44-02

Od22-04 Nf52-02

Lc12-02

Pennsylvania

New

York Connecticut

New

Jersey

Maryland

Delmarva

Peninsula

Long Island

W

D.C.

est Virginia

Virginia

ATLANTIC OCEAN

Chesapeake Bay

Pamlico Sound

0 30 60 MILES

0 30 60 KILOMETERS

Northern Atlantic

Coastal Plain

N

North

Carolina

Delaware

Delaware Bay

100

80

60

40

20

0

0

20

40

60

80

100

100

80

60

40

20

0

0

20

40

60

80

100

100

80

60

40

20

0 0

20

40

60

80

100

100

80

60

40

20

0

80

60

40

20

0

100

20

40

60

80

20

40

60

Magnesium

80 Sulfate +

Chloride +

Nitrate

Calcium + Magnesium

Sulfate

Calcium Chloride + Nitrate

Sodium + Potassium

Bicarbonate

Percentage of total milliequivalents per liter

EXPLANATION

Geochemical group of well

Group 1

Group 2

Group 3
Cover. Map showing the Delaware Coastal Plain and location of wells (refer to figure 1); trilinear diagram of major ions within 

the surficial aquifer of the Delaware Coastal Plain by geochemical groups (refer to figure 6); and center-pivot irrigation system 

over soybean field. Photograph by John W. Clune, U.S. Geological Survey.
Water Quality in the Surficial Aquifer 

Near Agricultural Areas in the 

Delaware Coastal Plain

By Brandon J. Fleming, Laura L. Mensch, Judith M. Denver, Roberto M. Cruz, 

and Mark R. Nardi

Scientific Investigations Report 2017–5054

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Delaware Department of Agriculture
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

William H. Werkheiser, Acting Director

U.S. Geological Survey, Reston, Virginia: 2017

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment, visit https://www.usgs.gov or call 1-888-ASK-USGS

For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Fleming, B.J., Mensch, L.L., Denver, J.M., Cruz, R.M., and Nardi, M.R., 2017, Water quality in the surficial aquifer 

near agricultural areas in the Delaware Coastal Plain, 2014: U.S. Geological Survey Scientific Investigations Report 

2017–5054, 28 p., https://doi.org/10.3133/sir20175054.

ISSN 2328-0328 (online)
iii

Acknowledgments

The authors wish to extend special thanks to Ed Kee, formerly of the Delaware Department 

of Agriculture (DDA), for initiating and supporting this study. Thanks to Jessica Teunis of the 

U.S. Geological Survey (USGS) and Luke Myers, formerly of the USGS, for collecting much of the 

data for this report. The authors appreciate the technical reviews by Scott Ator of the USGS and 

Joshua Kasper of the Delaware Department of Natural Resources and Environmental Control 

(DNREC). 

v

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Description of Study Area ...................................................................................................................4

Water Use .....................................................................................................................................4

Hydrogeologic Setting ................................................................................................................5

Groundwater Chemistry..............................................................................................................6

Agricultural Practices, Nutrient Management, and Best Management 

Practices (BMPs) ....................................................................................................................8

Methods of Study...........................................................................................................................................8

Network Design.....................................................................................................................................9

Sample Collection and Analysis.........................................................................................................9

Quality Control.......................................................................................................................................9

Age Dating..............................................................................................................................................9

Data Analysis.......................................................................................................................................10

Factors Affecting Variability.......................................................................................................................10

Groundwater Age................................................................................................................................10

Land Use...............................................................................................................................................10

Soils.......................................................................................................................................................19

Well Depth and Aquifer Position ......................................................................................................19

Water Quality in the Surficial Aquifer.......................................................................................................19

Nitrate...................................................................................................................................................22

Chloride.................................................................................................................................................23

Summary and Conclusions.........................................................................................................................25

References Cited .........................................................................................................................................25
vi

Figures

1. Map showing the Delaware Coastal Plain and location of wells.........................................2

2. Pie chart showing nitrogen inputs to Delaware......................................................................3

3. Pie chart showing groundwater use by A, category for the Delaware

Coastal Plain, 2010, and B, scatter plot showing acres irrigated and

volume withdrawn for agricultural irrigation in the Delaware Coastal Plain.....................6

4. Map showing the thickness of the surficial aquifer in the Delaware

Coastal Plain..................................................................................................................................7

5. Results of correlation and cluster analysis, A, dendrogram showing the

grouping of wells by correlation and cluster analysis, and B, map showing

the spatial distribution of geochemical groups .....................................................................19

6. Trilinear diagram of major ions within the surficial aquifer of the Delaware

Coastal Plain by geochemical groups.....................................................................................21

7. Boxplots of A, nitrate, B, calcium, and C, magnesium concentrations by

geochemical groups...................................................................................................................22

8. Boxplots of A, agricultural land use, and B, developed land use by

geochemical groups...................................................................................................................23

9. Boxplots of A, chloride, B, chloride to bromide ratio by geochemical

groups, and C, scatter plot of chloride to bromide ratio compared to

chloride by latitude.....................................................................................................................24

Tables

1. Site information for wells sampled in the surficial aquifer of the

Delaware Coastal Plain, 2014 .....................................................................................................4

2. Water characteristics, major ion, nutrient, and atmospheric age tracer

results from groundwater in the surficial aquifer of the Delaware

Coastal Plain, 2014......................................................................................................................11

3. Summary statistics for selected physical properties and major ions and

elements of groundwater in the surficial aquifer of the Delaware Coastal

Plain, 2014 ....................................................................................................................................21
vii

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 0.4047 hectare (ha)

acre 0.004047 square kilometer (km2

)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

Flow rate

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

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

°F = (1.8 × °C) + 32

Conversion Factors

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

(NGVD 88).

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

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

Datums

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius 

(μS/cm at 25 °C).

Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Supplemental Information
viii

DDA Delaware Department of Agriculture

DGS Delaware Geological Survey

DNREC Delaware Department of Natural Resources and Environmental Control

EPA U.S. Environmental Protection Agency

NASS National Agricultural Statistics Service

NRCS Natural Resources Conservation Service

NWQL National Water Quality Laboratory

NWIS National Water Information System

TMDL Total Maximum Daily Load

USDA U.S. Department of Agriculture

USGS U.S. Geological Survey

Abbreviations
Water Quality in the Surficial Aquifer Near Agricultural 

Areas in the Delaware Coastal Plain, 2014

By Brandon J. Fleming1, Laura L. Mensch2, Judith M. Denver1, Roberto M. Cruz1, and Mark R. Nardi1

Abstract

The U.S. Geological Survey, in cooperation with the 

Delaware Department of Agriculture, developed a network of 

wells to monitor groundwater quality in the surficial aquifer of 

the Delaware Coastal Plain. Well-drained soils, a flat landscape, and accessible water in the Delaware Coastal Plain 

make for a productive agricultural setting. As such, agriculture 

is one of the largest industries in the State of Delaware. This 

setting enables the transport of chemicals from agriculture and 

other land uses to shallow groundwater. Efforts to mitigate 

nutrient transport to groundwater by the implementation of 

agricultural best management practices (BMPs) have been 

ongoing for several decades. To measure the effectiveness of 

BMPs on a regional scale, a network of 48 wells was designed 

to measure shallow groundwater quality (particularly nitrate) 

over time near agricultural land in the Delaware Coastal Plain. 

Water characteristics, major ions, nutrients, and dissolved 

gases were measured in groundwater samples collected from 

network wells during fall 2014. Wells were organized into 

three groups based on their geochemical similarity and these 

groups were used to describe nitrate and chloride concentrations and factors that affect the variability among the groups. 

The results from this study are intended to establish waterquality conditions in 2014 to enable comparison of future 

conditions and evaluate the effectiveness of agricultural BMPs 

on a regional scale. 

Introduction

More than 90 percent of Delaware is underlain by the 

Northern Atlantic Coastal Plain Aquifer system (fig. 1). Most 

of the Delaware Coastal Plain has a surficial aquifer with a 

shallow water table and is typically composed of sandy sediments. Abundant rainfall provides aquifer recharge, but can 

also carry chemicals from the land surface to the water table. 

This aquifer is an important source of drinking water for many 

residents of small towns and rural households with domestic 

use. The surficial aquifer also provides most of the streamflow 

for streams that originate in the Delaware part of the Coastal 

Plain. 

As plants grow, they extract nutrients from the soil. In 

order to maintain crop yields and plant health, farmers need 

to periodically replace these nutrients by adding either animal 

manure or commercial fertilizers to the soil. Most nitrogen is 

applied to the land in the form of organic nitrogen or ammonia 

in manures and commercial fertilizers. Much of the applied 

nitrogen is rapidly converted to nitrate by soil microbes. 

Nitrate dissolves in water and the amounts not used by crops 

can leach into the groundwater with recharge. Certain conditions increase the likelihood of nitrate moving into shallow 

groundwater. These conditions include the amounts and 

timing of nitrogen application to the land; the presence of 

well-drained, sandy soils that promote oxic soil and aquifer 

conditions (dissolved oxygen greater than 1 milligram per liter 

or mg/L); and a relatively high water table (Ator and Denver, 

2015; Rudolph, 2015). These conditions are common in many 

parts of the Delaware Coastal Plain and have resulted in 

widespread nitrate movement into the surficial aquifer in many 

areas. 

Nitrate, the major form of nitrogen that is soluble in 

water, is a common contaminant in groundwater and surface 

water in the Coastal Plain of Delaware (Ator and Denver, 

2015). Nitrate concentrations in the surficial aquifer are 

commonly far greater than would be expected under natural 

conditions, and often high enough to affect the suitability of 

water for human consumption (Debrewer and others, 2007). 

Nitrate from groundwater is the primary source of nitrogen 

to many streams, which often exceed the U.S. Environmental 

Protection Agency (EPA) Total Maximum Daily Load 

(TMDL) for surface-water quality (Delaware Department 

of National Resources and Environmental Control, 2015). 

Manure and fertilizer used for agriculture are the primary 

sources of nitrogen applied to the land in Delaware (fig. 2). 

The recognition of this water-quality concern has led to the 

implementation of agricultural nutrient management efforts 

designed to maximize yields while minimizing any potentially 

negative effects on environmental quality.

1

 U.S. Geological Survey.

2

 Delaware Department of Agriculture.
2 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014

75°50'

39°

38°30'

39°30'

75°

C&D Canal

Fall Line DELAWARE

DELAWARE

MARYLAND

MARYLAND

0 5 10 15 MILES

0 5 10 15 KILOMETERS

Chesapeake Bay

Wilmington

Dover

DELAWARE BAY

Delaware River

ATLANTIC 

OCEAN

Base from U.S. Geological Survey digital data, 2004,

1:24,000 projected to NAD 83 UTM Zone 18 NAD 83

from GCS NAD 1983

NEW

CASTLE

KENT

SUSSEX

NEW

JERSEY

Indian River

Bay

Rehoboth Bay

EXPLANATION

Delaware Coastal

Plain

Chesapeake Bay

watershed boundary

Fall Line

Sampled wells and

identifier

Eb35-23

Eb35-23

Ea44-13

Fb22-15

Fc12-26

Fa45-07

Gb11-07 Gc14-04

Gb21-10

Hb22-17

Hc14-15

Id14-03 Ic21-08

Ic43-01

Ie32-02

Je34-04

Jd55-10 Jb42-05

Kb32-29

Lb23-03

Lb52-07

Kd13-09

Ke33-22

Lb15-17

Le24-11

Ld33-10

Mc43-06

Nf24-05

Ng45-02

Oi13-06 Og15-07

Oi32-18

Oh42-07

Pg22-06

Pf41-02 Qc13-01

Qc22-04

Rf13-02

Ph34-15

Qh13-05

Qg32-18

Og32-07

Nd31-06

Nd55-06

Oc21-03

Od44-02

Od22-04

Nf52-02

Lc12-02

Pennsylvania

New

York Connecticut

New

Jersey

Maryland

Delmarva

Peninsula

Long Island

We

D.C.

st Virginia

Virginia

ATLANTIC

OCEAN

Chesapeake

Bay

Pamlico Sound

78°

72°

41°

36°

0 30 60 MILES

0 30 60 KILOMETERS

Northern Atlantic

Coastal Plain

N

North

Carolina

Delaware

Delaware Bay

Figure 1. The Delaware Coastal Plain and location of wells.
Introduction 3

Manure

(Agriculture)

(44.4%)

Fertilizer

(Agriculture)

(47.4%)

Atmospheric

deposition

(7.1%)

Septic

(1.1%)

Fertilizer applications or (for nitrogen) direct fixation from the

atmosphere by crops (Brakebill and Gronberg, 2017).

Manure (Gronberg and Arnold, 2017).

Atmospheric deposition (National Atmospheric Deposition

Program, 2015).

Septic (Estimated using method developed by Maizel and

others, 1997).

EXPLANATION

Figure 2. Nitrogen inputs to Delaware. (%, percent)

194,898 acres of Delaware farmland (Delaware Department 

of Agriculture, 2015). That is an average of 99 pounds of 

nitrogen per acre from commercial fertilizer (U.S. Department 

of Agriculture, 2012). Combined, an average of about 

232 pounds of nitrogen per acre annually are applied to 

farmland in Delaware.

Under State law (Delaware Code Title 3), every farm 

that applies nutrients to 10 or more acres must have a valid 

Nutrient Management Plan written by a Delaware certified 

nutrient consultant. Application of manure from poultry production is included in many Nutrient Management Plans for 

Delaware farms because of the value of manure as a nutrient 

source and its abundance. A fundamental goal of any nutrient 

management plan is to identify the most cost-efficient and 

environmentally sound way to provide plants with the optimum supply of nutrients. Success with conservation and nutrient application practices used in these plans should help to 

reduce concentrations of nitrate over time in Delaware waters. 

A groundwater-monitoring network was established 

in 2014 by the U.S. Geological Survey (USGS) and the 

Delaware Department of Agriculture (DDA) as a tool to 

measure changes in water quality over time in the Delaware 

Coastal Plain. Temporal trends in nitrate have been evaluated 

in groundwater networks in different hydrogeologic settings, 

including the Columbia Basin in Washington (Frans and 

Helsel, 2005), and the California Central Valley (Burow and 

others, 2012). In the Mid-Atlantic region, temporal trends in 

nitrate and pesticides were evaluated in the Valley and Ridge 

carbonate aquifers and the Delmarva Peninsula (Debrewer 

and others, 2008). Networks in similar hydrogeologic and 

land-use settings in Denmark (Hansen and others, 2011) have 

shown improvements in groundwater quality with changes in 

agricultural practices. Initial data from the network established 

by USGS and DDA demonstrated groundwater-quality conditions near agricultural lands in the Delaware Coastal Plain in 

2014. Repeated sampling of this network over time is intended 

to help track the effectiveness of agricultural best management 

practices (BMPs) aimed at reducing nitrogen transport to the 

water table in Delaware.

Purpose and Scope

The purposes of this report are to describe the design of 

an agricultural, shallow groundwater-quality monitoring network in the Delaware Coastal Plain and to present geochemical results of samples collected from the network in 2014. This 

study focuses on the 2014 water-quality conditions in shallow 

groundwater near agriculture, which is the predominant land 

use in Delaware. Results presented in this report are based on 

the analyses of samples collected at 48 shallow wells 

(fig. 1, table 1) in or adjacent to agricultural lands that are 

distributed throughout the Delaware Coastal Plain. These data 

allow comparison with future samples collected for a study 

of trends in nitrate and other chemicals in groundwater in the 

Delaware Coastal Plain. 

Agriculture is one of Delaware’s main industries, producing over $1 billion in annual sales. With 508,000 total acres 

of farmland in the State, roughly 2 out of every 5 acres in 

Delaware are dedicated to farming. Delaware’s average farm 

size of 200 acres is less than half the national average farm 

size of 438 acres, reflecting the prevalence of smaller farms. 

Poultry, predominantly broiler chickens, is one of the largest 

agricultural commodities in the State. The largest amount of 

farmland in the State is planted with the main crops: soybeans, 

corn, and wheat, with 415,000 total acres of these three 

crops combined (U.S. Department of Agriculture, National 

Agricultural Statistics Service, 2012). Produce, such as watermelons, sweet corn, lima beans, and cucumbers, also accounts 

for a part of the State’s agricultural production.

Between 2005 and 2013, an annual average of 

110,053 tons of poultry manure was applied to 49,268 acres 

of Delaware farmland (Delaware Department of Agriculture, 

2015). That is an average of 2.23 tons of poultry manure 

applied per acre. Since poultry manure is composed of 

approximately 3 percent nitrogen (Delaware Department of 

Agriculture, 2012), farmers applied an annual average of 

133 pounds of nitrogen per acre from poultry litter. Another 

major source of soil amendments used in the State is commercial fertilizer. In 2013, Delaware farmers reported applying 9,632 tons of nitrogen from commercial fertilizer on 
4 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014

Table 1. Site information for wells sampled in the surficial aquifer of the Delaware Coastal Plain, 2014.—Continued

[DNREC, Delaware Department of Natural Resources and Environmental Control; ft bls, feet below land surface; ft, feet]

Station number Local 

identifier

DNREC 

well identifier

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Well depth 

(ft bls)

Aquifer thickness 

(ft)

392959075435501 Fb22-15 106879 39.499834 -75.731599 23.34 67

392913075382001 Fc12-26 108632 39.487056 -75.638540 28 58

382932075221701 Rf13-02 155971 38.492083 -75.371417 12.9 19

384425075072401 Oi13-06 166167 38.740167 -75.123444 13 95

384411075150101 Og15-07 172328 38.736417 -75.250361 18.4 116

385730075321401 Ld33-10 166259 38.958361 -75.537111 17.4 80

391324075391901 Ic21-08 172352 39.223444 -75.655222 17.3 25

385956075303801 Lb15-17 172301 38.999 -75.510583 13.1 62

391814075435001 Hb22-17 172331 39.303889 -75.730472 12.3 41

392403075362101 Gc14-04 187638 39.4009 -75.605817 34 23

385817075265101 Le24-11 172300 38.971677 -75.447192 13.5 81

385129075370201 Mc43-06 155980 38.858083 -75.617167 12.8 67

384323075393201 Oc21-03 73085 38.723169 -75.658540 23 67

390634075433401 Jb42-05 172323 39.109444 -75.726194 11.1 11

391503075310401 Id14-03 155985 39.250917 -75.517722 18 15

384637075153201 Ng45-02 187640 38.777067 -75.258867 22 114

392324075445601 Gb21-10 106885 39.390111 -75.748544 14.75 61

383438075274201 Qc13-01 155972 38.577167 -75.46175 13.1 128

383221075182301 Qg32-18 166165 38.539222 -75.306472 11.8 19

384316075330501 Od22-04 155961 38.721 -75.551333 18.3 76

384201075185401 Og32-07 166198 38.700222 -75.315028 13.2 104

385830075423201 Lb23-03 172347 38.975 -75.708944 13.2 50

385515075431701 Lb52-07 166258 38.92075 -75.721444 16 51

390001075380101 Lc12-02 155982 39.000333 -75.633556 18.2 72

Description of Study Area

Most of Delaware lies on the Delmarva Peninsula within 

the Northern Atlantic Coastal Plain Physiographic Province 

(fig. 1). Topography in the Delaware Coastal Plain is relatively 

flat with the highest elevations located at the drainage divide 

between the Chesapeake Bay to the west and Delaware Bay 

and Atlantic Ocean to the east (fig. 1). On the Delaware 

Coastal Plain, land use is predominantly agriculture, forest and 

wetlands, and developed at 53 percent, 39 percent, and 

8 percent, respectively (Masterson and others, 2016). Average 

annual precipitation ranges from 41 to 45 inches per year 

(in/yr) (Sanford and others, 2012) and is typically evenly distributed throughout the year with 3–4 inches per month. 

Water Use

Total 2010 groundwater use in the Delaware Coastal 

Plain was 151 million gallons per day (Mgal/d), of which most 

(58 percent) was used for agriculture (Masterson and others, 

2016; fig. 3A). Over the last several decades, both irrigated 

land (U.S. Department of Agriculture, National Agricultural 

Statistics Service, 2012) and the volume of groundwater withdrawn for irrigation has increased substantially (Cheryl Dieter, 

USGS, written commun., 2016; fig. 3B). Most public water 

supply and all domestic water use in the Delaware Coastal 

Plain comes from confined aquifers or relatively thick parts of 

the surficial aquifer. Other uses of groundwater include commercial and industrial use. 
Introduction 5

Table 1. Site information for wells sampled in the surficial aquifer of the Delaware Coastal Plain, 2014.—Continued

[DNREC, Delaware Department of Natural Resources and Environmental Control; ft bls, feet below land surface; ft, feet]

Station number Local 

identifier

DNREC 

well identifier

Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

Well depth 

(ft bls)

Aquifer thickness 

(ft)

390409075311301 Kd13-09 176048 39.069111 -75.520361 13.4 47

390205075430901 Kb32-29 155978 39.034694 -75.719056 18.1 49

384550075304001 Nd55-06 166200 38.763944 -75.511028 18.2 79

384845075211901 Nf24-05 172295 38.812444 -75.355278 13.5 121

383308075382301 Qc22-04 73089 38.552338 -75.639373 29 98

392605075452801 Fa45-07 106882 39.434834 -75.757434 22.6 45

392428075445901 Gb11-07 106884 39.407889 -75.749377 23.58 56

393210075401601 Eb35-23 108633 39.536223 -75.670763 15 53

393126075460201 Ea44-13 108634 39.524001 -75.766879 17 53

383629075245601 Pf41-02 172327 38.607944 -75.415417 12.9 123

383412075125401 Qh13-05 166166 38.569889 -75.214917 18 104

383836075183001 Pg22-06 166189 38.643361 -75.308361 16.5 89

383749075110501 Ph34-15 155953 38.630194 -75.184611 12.9 92

384250075085001 Oi32-18 172294 38.71375 -75.147167 26.8 105

384737075342701 Nd31-06 172320 38.793639 -75.574056 13.8 74

384159075310801 Od44-02 90221 38.699833 -75.518833 14.6 81

384130075125801 Oh42-07 155951 38.691639 -75.216111 13.2 107

391112075380001 Ic43-01 155984 39.186667 -75.633417 17.1 36

390252075271301 Ke33-22 172318 39.047667 -75.453611 12.6 57

391936075363201 Hc14-15 106889 39.326778 -75.608538 13.12 15

384502075235301 Nf52-02 166168 38.750556 -75.397972 12.5 105

390544075300501 Jd55-10 166262 39.095556 -75.501472 15 45

390705075263201 Je34-04 172349 39.118139 -75.442083 13 46

391232075285401 Ie32-02 172350 39.208917 -75.481611 13.5 29

Hydrogeologic Setting

The Delaware Coastal Plain is underlain by an extensive 

unconfined surficial aquifer that is present at the land surface 

in most areas (fig. 4). This aquifer supplies most of the flow 

to streams and rivers that incise the land surface. It generally 

thickens from north to south (fig. 4) and overlies the subcrop 

areas of a series of confined aquifers and confining beds 

(Denver and Nardi, 2016). 

The recharge area for the surficial aquifer includes most 

of the land surface because of the sandy nature of the aquifer 

sediments. The mean annual estimated recharge ranges from 

about 14 to 17 in/yr, which is about one-third of the total 

precipitation (Sanford and others, 2012). Water in the surficial 

aquifer typically flows along relatively short flow paths 

(distances of several hundred feet to less than a few miles) 

towards discharge areas in streams and estuaries and reaches 

the discharge areas in less than 50 years (Denver and others, 

2004; Sanford and others, 2012). Groundwater flow is also 

intercepted by pumping wells. A small amount of the recharge, 

less than 2 percent, moves downward into the underlying confined aquifers (Leahy and Martin, 1993).

Geologic formations with predominantly sandy surficial 

sediments that compose the surficial aquifer in the Delaware 

Coastal Plain include the Parsonsburg Sand, Sinepuxent 

Formation (Fm.), and parts of the Omar Fm., the Columbia 

Fm., the Beaverdam Fm., and the Pennsauken Fm. (Ator and 

others 2005; Bachman and Wilson, 1984). Other formations 
6 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014

with mixed texture that have sandy lithology also can be 

part of the surficial aquifer. These formations include the 

Scotts Corner Fm. and Lynch Heights Fm., which occur on 

the eastern side and updip section of the Omar Fm. north of 

Indian River Bay (Ator and others, 2005; Mixon, 1985; Owens 

and Denny, 1979; Ramsey, 1997). These formations are of 

Quaternary through late Miocene age.

Aquifers that are otherwise confined that underlie the 

surficial aquifer may increase its thickness in areas where they 

subcrop beneath the surficial aquifer and are under water-table 

conditions. Older formations that contain aquifers and subcrop 

the surficial aquifer include the Tertiary age formations of the 

Chesapeake Group, and the Vincentown and Hornerstown 

Fms.; the Cretaceous age Mt. Laurel, Englishtown, and 

Magothy Fms., and the sandy sediments of the Potomac Group 

(Ator and others, 2005).

Groundwater Chemistry

The chemical constituents measured in groundwater 

reflect the rock types of the aquifer sediments, redox 

conditions in the aquifer, and chemicals applied to the land 

surface that are soluble in water and available to leach into 

the groundwater system. The surficial aquifer in the Delaware 

Coastal Plain is composed mostly of siliciclastic sediments 

in which quartz minerals are the major component (Jordan, 

1964). Quartz is resistant to weathering, and under natural 

conditions in well-drained areas with minimal inputs of 

anthropogenic chemicals, the water in the surficial aquifer 

system is very dilute with specific conductance of less than 

about 60 microsiemens per centimeter at 25 degrees Celsius 

(μS/cm) and nitrate concentrations of less than 0.4 mg/L as N 

(Denver, 1989; Hamilton and others, 1993). Many of the 

common constituents in fertilizers that are typically applied 

to fields also dissolve in water and can travel to groundwater 

if they leach below the root zone. Plants need a variety of 

nutrients to grow, including nitrogen, phosphorus, potassium, 

magnesium, and calcium. With the exception of phosphorus, 

these chemicals typically contribute to the major ions in 

groundwater affected by agricultural activities (Hamilton and 

others, 1993). In well-drained soils, phosphorus is typically 

bound to soils and sediments and not dissolved in water 

(Ator and Denver, 2015).

Nitrate is widespread throughout the surficial aquifer 

of the Delmarva Peninsula (Debrewer and others, 2007) and 

nitrate concentrations in natural groundwater rarely exceed 

0.4 mg/L in the surficial aquifer of the Delmarva Peninsula 

(Hamilton and others, 1993). Concentrations of nitrate above 

natural background levels are likely impacted by anthropogenic activity (Debrewer and others, 2007). Recent studies on 

the Delmarva Peninsula have shown that nitrate concentrations 

in groundwater increased about 2 mg/L as N in parts of the 

surficial aquifer used for domestic supply from 1988 to 2001 

in response to increased nitrogen applications in previous 

decades (Denver and others, 2004; Ator and Denver, 2015). 

Headwater streams on the Delmarva Peninsula, which derive 

most of their flow as groundwater discharge from the surficial 

aquifer, also show nitrate concentrations above natural levels 

during base flow (Ator and Denver, 2015, Denver and others, 

2004). 

Agriculture

(58%)

Public and domestic supply

(36%)

Commercial

and industrial

(6%)

A BIrrigated acres

151 million gallons per day Year

Withdrawals, in million gallons per day

140,000 110

100

90

80

70

60

50

40

30

20

120,000

100,000

80,000

60,000

40,000

20,000

1950 1960 1970 1980 1990 2000 2010

0

Figure 3. Groundwater use by A, category for the Delaware Coastal Plain 2010 (Masterson and others, 2016), and B, acres irrigated 

(U.S. Department of Agriculture, National Agricultural Statistics Service, 2012) and volume withdrawn for agricultural irrigation in the 

Delaware Coastal Plain (Cheryl Dieter, USGS, written commun., 2016). (%, percent)
Introduction 7

Fall Line DELAWARE

DELAWARE

MARYLAND

MARYLAND

Chesapeake Bay

DELAWARE BAY

ATLANTIC OCEAN

NEW JERSEY C&D Canal

NEW

CASTLE

KENT

SUSSEX

Wilmington

Dover

75°50'

39°

38°30'

39°30'

75°

0 5 10 15 MILES

0 5 10 15 KILOMETERS

Base from U.S. Geological Survey digital data, 2004,

1:24,000 projected to NAD 83 UTM Zone 18 NAD 83

from GCS NAD 1983

Delaware River

EXPLANATION

Fall Line

Thickness of the surficial

aquifer, in feet

High: 198

Low: 0

Figure 4. Thickness of the surficial aquifer in the Delaware Coastal Plain (Denver and Nardi, 

2016).
8 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014

Agricultural Practices, Nutrient Management, 

and Best Management Practices (BMPs)

Farmers use tools such as soil tests, Pre-Sidedress Nitrate 

Tests (PSNTs), plant tissue sample analyses, and crop yield 

production history to determine a crop’s nutrient needs for 

each soil type (Haering and Evanylo, 2006). When plants 

receive the correct balance of nitrogen, phosphorus, and potassium throughout the growing season, they are able to use these 

compounds with optimal efficiency, leaving less in the soil. 

Providing inadequate nutrient balance not only reduces yield, 

but it also reduces the plant’s efficiency and can lead to nutrients going unused by the crop (Plaster, 1997). A healthier plant 

promotes more efficient utilization of nutrients.

Nitrogen, in particular, is an essential nutrient for plants; 

without it, a plant cannot grow normally. Nitrogen is a major 

component of chlorophyll, the molecule used by plants during 

photosynthesis to convert sunlight energy into sugars (Plaster, 

1997). Nitrogen is also an essential element of amino acids, 

the building blocks of protein. Delaware has predominantly 

sandy soils, particularly in parts of Kent and Sussex Counties 

(Soil Survey Staff, 2016). Sandy soils often contain low 

amounts of organic materials and can be deficient in nitrogen 

(Shober, 2015). Because of these two factors, it is especially 

important to provide nitrogen in its most bioavailable form, 

nitrate, to crops.

Delaware farmers have been using an increasing array 

of BMPs to reduce nutrient loading to surface water and 

groundwater. For example, the use of cover crops has become 

increasingly widespread in Delaware, taking up nutrients from 

the soil during the winter months to reduce nutrient leaching to 

the groundwater. Modified tillage practices are used to reduce 

the movement of sediment and nutrients away from fields. 

Cover crops are grasses, legumes, or small grains planted 

between crop cycles to protect and improve soil health. They 

protect soil by reducing erosion, improving stability, and 

managing soil moisture. Cover crops also benefit the larger 

ecosystem by increasing biodiversity, attracting pollinators, 

suppressing weeds, and providing forage (Nolan and others, 

2002). Lastly, cover crops also promote soil health by increasing organic matter, redistributing nutrients within the soil, 

fixing nitrogen, and removing excess nutrients (Nolan and others, 2002). Legumes such as field peas, clover, and vetch are 

excellent at fixing nitrogen in soil, and grass cover crops such 

as wheat, rye, and barley are particularly good at removing 

excess nutrients from soil (U.S. Department of Agriculture, 

Natural Resources Conservation Service, 2014). 

Farmers have also altered their tillage practices in order 

to minimize the movement of nutrients away from their 

intended use locations. Tillage is the agricultural practice of 

cultivating or preparing soil for planting. Conventional or 

traditional tillage practices involve the mechanical disturbance 

of the top layer of soil, and the mixing of plant residues left 

after harvest. Conventional tillage is defined as leaving 15 

percent or less crop residue on a field after harvest (Delaware 

Department of Natural Resources and Environmental Control, 

2015). Although this practice has benefits, such as insect and 

weed control, conventional tillage leaves soil exposed. This 

can increase soil erosion and sediment runoff, and increases 

soil compaction, which facilitates surface-water runoff.

In contrast, conservation tillage practices involve the 

minimal disturbance of soil and plant residues that remain on 

a field after harvest. Essentially, the soil remains undisturbed 

after harvest and is minimally disturbed during the planting 

of the next crop. Conservation tillage is defined as leaving 

a minimum of 30 percent crop residue on a field (Delaware 

Department of Natural Resources and Environmental Control, 

2015). To adapt to this different practice, farmers use specifically designed equipment that enables them to drill or plant 

seeds while minimizing the disturbance of soil and existing 

crop residue. This practice has many benefits, including the 

preservation of soil moisture; the reduction of soil erosion 

from wind, rain, or other forces; and an increase in the organic 

matter content in soil. By minimizing soil erosion, conservation tillage decreases the movement of nutrients bound with 

this soil off of the field in overland runoff. By increasing 

organic matter content in the soil, conservation tillage provides increased material to bind nutrients to the soil, minimizing the leaching of nutrients through the soil into the shallow 

groundwater.

In Delaware, these BMPs are being implemented in all 

three counties. For example, in 2015, conventional tillage was 

used on 11.6, 21.5, and 22.4 percent of New Castle County, 

Kent County, and Sussex County farmland, respectively. In 

contrast, conservation tillage practices were used in 81.7, 

72.5, and 65.9 percent of New Castle County, Kent County, 

and Sussex County farmland, respectively. Cover crops were 

used on 24, 43, and 32.5 percent of New Castle County, Kent 

County, and Sussex County farmland, respectively (Delaware 

Department of Natural Resources and Environmental Control, 

2015).

The goal of using cover crops and conservation tillage, 

along with estimating nutrient budgets prior to the application 

of fertilizer, is to minimize the amount of land-applied nutrients available to move into surface water and groundwater in 

Delaware.

Methods of Study

A new network of wells was designed from wells in two 

active groundwater-monitoring networks maintained by the 

DDA and the USGS in the unconfined surficial aquifer of the 

Delaware Coastal Plain. Wells were included in the network 

on the basis of depth, previously known oxic conditions, and 

the proximity to agricultural land use. Water from wells was 

analyzed to establish geochemical conditions in 2014 and 

identify patterns in water quality in the unconfined surficial 

aquifer of the Delaware Coastal Plain. Land use and soils near 

the wells are summarized concurrently with the groundwaterquality results, creating a snapshot of land use and water quality that will be used to compare with future conditions. 
Methods of Study 9

Network Design

In 1995, the DDA designed a shallow groundwater-monitoring network with the assistance of the Delaware Geological 

Survey (DGS) (Blaier and Baxter, 2000). The network consists 

of 104 dedicated monitoring wells located throughout the State 

south of the Chesapeake and Delaware Canal (C&D Canal, 

fig. 1). The wells are primarily used to monitor the State’s 

shallow groundwater for pesticides of interest that are registered for use in the State.

With a few exceptions, the DDA pesticide monitoring 

wells are located on roadsides in State rights-of-way. Wells 

are screened in the Columbia aquifer. All wells are considered 

shallow, with the bottom of screen depths less than 40 feet (ft) 

below ground surface. Well depths range from 8.35 ft to 

38.70 ft. The average completion depth for all 104 currently 

active monitoring wells is 16.10 ft. The monitoring wells were 

screened across the water-table surface at the time of drilling. 

Since the network was initially designed to monitor 

groundwater for agricultural herbicides, all of the monitoring 

wells are located below the C&D Canal in the Delaware 

Coastal Plain, where most of the State’s agricultural land is 

located. This requirement generally excludes land above the 

C&D Canal, areas within incorporated towns and cities, and 

areas along the coast where land is largely marsh and other 

wetlands. 

A subset of wells from the DDA network and from 

an agricultural land-use network developed for the USGS 

National Water-Quality Assessment (NAWQA) Project were 

included in the new network. The existing USGS network was 

designed to assess water quality in the surficial aquifer of the 

Delmarva Peninsula (Debrewer and others, 2007; Koterba and 

others, 1990; Shedlock and others, 1993) as part of NAWQA. 

A subset of wells from the NAWQA network that are located 

in Delaware, and screened within oxic parts of the shallow 

unconfined aquifer, were selected for inclusion in the new network. In total, 8 wells from the USGS network and 40 wells 

from the DDA pesticide network were sampled during fall 

2014 (fig. 1, table 1). Well depths ranged from 11 ft to 34 ft. 

Sample Collection and Analysis

Water samples were collected using methods outlined in 

the USGS National Field Manual for the Collection of WaterQuality Data (U.S. Geological Survey, variously dated). The 

sample collection was designed to represent shallow, oxic 

groundwater conditions in the Delaware Coastal Plain from 

October through December 2014. Prior to sampling, at least 

three well volumes were purged using a Fultz sp300 submersible pump to remove standing water, and geochemical and 

physical field parameters were monitored with a YSI 6920 

sonde until stable conditions were reached. For this study, 

stable geochemical conditions were defined as five successive 

5-minute measurements of pH (± 0.1 units), water temperature 

(± 0.2 degrees Celsius), specific conductance (± 3 percent), 

dissolved oxygen (± 0.3 mg/L), and turbidity (± 10 percent). 

Well Pf41-02 recharged too slowly for standard purging 

procedures. This well was purged and allowed to recover to 

90 percent of the original water level before measuring field 

parameters and sample collection. Samples were collected 

using Teflon tubing and a 0.45-micrometer capsule filter inside 

a clean sampling chamber. Filtered water samples for inorganics analysis were preserved using nitric acid to a pH below 2. 

All samples were maintained at a temperature below 4 degrees 

Celsius in a sealed cooler during shipment to the laboratory.

Samples from all wells were analyzed for major ions 

and nutrients at the USGS National Water Quality Laboratory 

(NWQL) in Denver, Colorado using methods described in 

Fishman (1993). Samples from 25 wells also were analyzed 

for trace atmospheric gases, including sulfur-hexafluoride, 

nitrogen, argon, carbon dioxide, methane, and oxygen. Gas 

analyses were conducted at the USGS Chlorofluorocarbon 

Laboratory in Reston, Virginia and recharge age determined 

using methods described by Busenberg and Plummer (2000). 

Alkalinity and bicarbonate concentrations were determined 

in samples with pH greater than 4.5 using field electrometric 

titrations on filtered samples using the inflection point method 

(Rounds, 2006). Where pH was below 4.5, alkalinity and 

bicarbonate concentrations were assumed to be zero.

Quality Control

Quality-control samples were collected to evaluate and 

estimate potential contamination bias and measurement variability from water-quality data-collection processes (Koterba 

and others, 1995). An equipment blank was collected prior 

to sampling; four field blanks and four replicates were collected during field activities at selected wells. Field collection 

procedures for quality-control samples were established using 

the USGS National Field Manual (U.S. Geological Survey, 

variously dated) in a manner consistent with procedures for 

the acquisition of environmental samples. This evaluation of 

quality-control samples included the review of analytical data, 

field practices, reported environmental concentrations, and 

timing of quality-control activities.

Age Dating

Recharge dates for groundwater samples were estimated 

on the basis of measured concentrations of sulfur hexafluoride 

(SF6), and dissolved nitrogen, argon, carbon dioxide, methane, 

and oxygen (Busenberg and Plummer, 2000). Industrial 

production of SF6 started in the 1950s and increasing concentrations in the atmosphere have been documented since 1978 

(Maiss and Brenninkmeijer, 1998). SF6 can be used to date 

groundwater that is in equilibrium with the atmosphere when 

it is recharged. SF6 is a good tracer for young groundwater 

(recharged after 1970), however, its usefulness as a groundwater age tracer decreases for older waters. For this study, field 

conditions at 25 wells were suitable to sample for SF6. 
10 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014

Data Analysis

Statistical methods were selected to identify patterns 

in shallow, oxic groundwater from unconfined wells near 

agricultural land use. Nonparametric Spearman correlation 

analysis (Helsel and Hirsch, 2002) was used to identify wells 

with similar geochemical characteristics based on analytical 

results of pH, specific conductance, dissolved oxygen, silica, 

chloride, bicarbonate, sulfate, magnesium, calcium, potassium, 

sodium, and nitrate. The Spearman correlation matrix for all 

wells was used as input for an agglomerative hierarchical 

cluster analysis using Ward’s method to minimize the variance 

between clusters (Ward, 1963). The Python library SciKitLearn (Pedregosa and others, 2011) was used to apply the 

cluster analysis. Groups of wells determined by cluster 

analysis were used to summarize water-quality results and 

describe the variability of relevant physical properties (land 

use, soils, and aquifer thickness). Non-parametric KruskalWallis multiple comparison tests were done using the pgirmess 

package in the R software (Giraudoux, 2017) to determine 

if populations of selected constituents and land cover were 

significantly different by geochemical groups derived by correlation and cluster analysis. Land use and land cover from 

the State of Delaware (2007) and soils were documented on a 

well-by-well basis within an approximately 1,640-ft radius of 

each well through a Geographic Information System buffering 

and overlay procedure that extracted land-use polygons for 

summary. Selected water-quality constituents were compared 

directly to physical properties. Trilinear diagrams were constructed to illustrate major cations and ions in various water 

types identified within the network. Chloride to bromide mass 

ratios were calculated to identify possible sources of chloride 

in groundwater samples.

Factors Affecting Variability

The quality-control procedures included five blank 

samples to test for potential equipment contamination and 

four replicate samples to test for reproducibility of results. 

Detectable concentrations of ammonia, chloride, and sulfate 

were found in blank samples, but were within twice the 

respective laboratory detection limits. Manganese was 

detected at 0.54 micrograms per liter (μg/L) in the pre-study 

equipment blank, which is greater than the laboratory detection limit of 0.20 μg/L. For replicate samples, a relative 

percent difference (RPD) between environmental and replicate 

results of 20 percent was used as an indication of variability 

from sampling procedures for this study. A single constituent, 

iron, exceeded the 20-percent RPD criteria, at 6.1 μg/L for the 

replicate and < 4 μg/L for the environmental sample, however 

the mean RPD for all replicate iron samples was 8.9 percent 

and within the study criteria. Quality-control results show 

there was not significant variability or bias that would affect 

the interpretation of water-quality results for this study. 

The quality of water in the surficial aquifer of the 

Delaware Coastal Plain is influenced by the availability 

of dissolved ions from natural and human sources, and by 

geochemical factors that affect the fate and transport of these 

ions. Dissolved oxygen concentrations can control geochemical transformations, and in the surficial aquifer, waters are 

typically considered oxic, with dissolved oxygen concentrations above 0.5 mg/L (McMahon and Chapelle, 2008). 

Understanding redox conditions is important because under 

oxic conditions redox-sensitive parameters (such as nitrate) 

are stable. In poorly drained settings in the surficial aquifer, 

anoxic conditions (dissolved oxygen less than 0.5 mg/L) exist, 

although these were avoided by design in this study. Land use 

and chemicals applied on the land surface play an important 

role in surficial groundwater quality. Soil conditions and aquifer properties also can affect recharge rates and groundwater 

quality. Groundwater age can influence the quality of groundwater, depending on the land use at the time recharge occurs. 

These and other factors combine to determine the spatial 

variability in groundwater quality in the surficial aquifer of the 

Delaware Coastal Plain. 

Groundwater Age

Apparent groundwater ages of samples for which dissolved gases were measured ranged from 1 to 20 years 

(table 2). These results are consistent with other age tracer 

results in similar settings (Clune and Denver, 2012; Debrewer 

and others, 2008) and demonstrate that shallow wells screened 

just below the water table typically produce relatively young 

groundwater. Recently recharged groundwater, such as the 

groundwater observed in this study, is where expected changes 

in nitrate concentrations would be observed first, since it 

represents the start of the groundwater-flow paths to streams in 

the Delaware Coastal Plain. 

Land Use

Agriculture, followed by forests, is typically the predominant land-cover type near the network wells. Land-cover 

data used in this analysis were from the State of Delaware 

(2007). The land-cover data for the entire State are made up 

of 33 classes. These data were used to summarize the land use 

in a 1,640-ft-radius circular buffer around each well. When 

summarized and grouped into four categories by well location, 

of the 48 wells sampled, 38 had a majority land-cover type of 

cropland, 6 had urban, 3 had forest, and 1 had wetland.
Factors Affecting Variability 11

Table 2. Water characteristics, major ion, nutrient, and atmospheric age tracer results from groundwater in the surficial aquifer of the Delaware Coastal Plain, 2014.—Continued[--, no data; <, less than; d, sample was diluted; i, result may be affected by interference; n, below the reporting level but at or above the detection level; r, value verified by rerun, same method; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; mL, millimeters; fmol/kg, femtomoles per kilogram; CaCO3, calcium carbonate; SiO2, silicon dioxide; SF6, sulfur hexafluoride; pptv, parts per trillion by volume]

Station number Local identifier Geochemical group

Dissolved 

oxygen, 

water, 

unfiltered 

(mg/L)

pH, water, 

unfiltered, 

field, 

standard 

units

Specific 

conductance, 

water, 

unfiltered 

(μS/cm at 

25 °C)

Temperature, 

water 

(degrees 

Celsius)

Dissolved 

solids dried at 

180 °C, water, 

filtered 

(mg/L)

Calcium, 

water, 

filtered 

(mg/L)

Magnesium, 

water, 

filtered 

(mg/L)

Potassium, 

water, 

filtered 

(mg/L)

Sodium, 

water, 

filtered 

(mg/L)

Alkalinity, 

water, filtered, 

inflection-point 

titration method 

(incremental titration 

method), field 

(mg/L as CaCO3)

Bicarbonate, 

water, filtered, 

inflection-point 

titration method 

(incremental titration 

method), field 

(mg/L)

392959075435501 Fb22-15 1 7.6 5.0 201 15.8 116 12.6 11.1 2.12 2.70 3.0 4.4392913075382001 Fc12-26 1 8.2 5.8 259 14.1 166 19.4 11.4 1.11 8.88 23.3 28.4382932075221701 Rf13-02 1 9.1 4.5 192 18.9 117 10.7 3.62 8.06 8.19 0.2 2.6384425075072401 Oi13-06 1 8.8 5.8 141 17.8 89 11.5 3.39 1.46 6.86 8.8 10.9

384411075150101 Og15-07 1 9.3 4.5 658 15.6 409 56.9 23.5 5.03 18.7 0.5 2.5

385730075321401 Ld33-10 1 4.1 4.9 312 15.5 208 18.7 15.0 3.76 11.2 0.3 1.0391324075391901 Ic21-08 1 5.9 6.0 297 16.8 189 22.4 16.8 7.99 5.72 35.9 43.8385956075303801 Lb15-17 1 7.8 5.0 163 13.9 100 10.1 6.79 2.49 4.80 1.8 2.9391814075435001 Hb22-17 1 7.7 4.8 114 11.6 75 9.16 3.43 1.19 2.67 0.5 1.7392403075362101 Gc14-04 1 2.3 5.2 150 11.6 94 11.6 5.19 2.97 4.06 10.6 13.2385817075265101 Le24-11 1 4.3 5.0 352 14.9 213 29.4 r 17.1 r 1.81 r 5.11 r 6.8 9.0385129075370201 Mc43-06 1 5.8 5.5 335 15.2 237 35.5 9.32 3.18 8.34 14.7 18.0384323075393201 Oc21-03 1 9.9 5.2 197 14.7 136 13.3 8.70 1.85 5.52 2.3 3.2390634075433401 Jb42-05 1 1.2 6.0 180 14.8 139 19.5 r 3.88 r 3.27 r 6.24 r 20.7 25.3391503075310401 Id14-03 1 9.0 5.1 414 14.8 274 32.0 20.9 1.57 9.45 4.0 5.6384637075153201 Ng45-02 1 9.6 5.5 199 15.6 151 18.1 5.86 2.68 5.20 2.5 <0.1392324075445601 Gb21-10 2 4.0 5.0 592 16.8 340 18.4 12.3 3.26 69.4 10.1 12.8383438075274201 Qc13-01 2 7.6 5.1 197 17.8 120 9.85 2.85 2.67 18.0 2.5 3.7383221075182301 Qg32-18 2 2.7 4.3 169 20.7 105 d 7.50 2.12 2.22 11.2 -- --384316075330501 Od22-04 2 6.5 4.8 132 14.9 74 5.95 3.02 5.93 7.82 1.0 2.8384201075185401 Og32-07 2 8.0 4.4 208 19.6 112 r 5.31 r 2.77 r 5.23 r 21.0 r -- --385830075423201 Lb23-03 2 <1.0 5.3 262 17.4 185 20.2 6.66 2.18 14.2 6.6 8.3385515075431701 Lb52-07 2 6.8 5.0 109 18.0 86 5.06 3.27 1.90 6.55 3.3 4.8390001075380101 Lc12-02 2 7.1 4.7 235 16.1 128 10.9 7.09 1.60 16.5 1.8 3.5

390409075311301 Kd13-09 2 5.6 4.8 158 13.8 102 6.02 3.63 2.77 13.9 0.8 2.3

390205075430901 Kb32-29 2 7.1 4.8 513 13.9 298 8.58 r 5.65 r 5.18 r 72.9 r 0.8 2.0384550075304001 Nd55-06 2 10.2 4.8 95 17.0 52 5.33 r 0.826 r 1.26 r 7.28 r 0.0 1.5

384845075211901 Nf24-05 2 8.7 4.5 292 17.6 153 9.41 r 5.77 r 7.39 r 25.8 r -- --

383308075382301 Qc22-04 2 6.4 4.8 171 13.7 104 6.07 7.09 3.00 9.37 2.3 3.6
12 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014 Table 2. Water characteristics, major ion, nutrient, and atmospheric age tracer results from groundwater in the surficial aquifer of the Delaware Coastal Plain, 2014. —Continued [--, no data; <, less than; d, sample was diluted; i, result may be affected by interference; n, below the reporting level but at or above the detection level; r, value verified by rerun, same method; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; mL, millimeters; fmol/kg, femtomoles per kilogram; CaCO3, calcium carbonate; SiO2, silicon dioxide; SF6, sulfur hexafluoride; pptv, parts per trillion by volume] Station number Local identifier Geochemical group Bromide, water, filtered (mg/L) Chloride, water, filtered (mg/L) Fluoride, water, filtered (mg/L) Silica, water, filtered (mg/L as SiO2

)

Sulfate, 

water, 

filtered 

(mg/L)

Ammonia, 

water, 

filtered 

(mg/L as 

nitrogen)

Nitrate plus 

nitrite, 

water, 

filtered 

(mg/L as 

nitrogen)

Nitrite, 

water, 

filtered 

(mg/L as 

nitrogen)

Orthophosphate, 

water, 

filtered 

(mg/L as 

phosphorus)

Total nitrogen 

[nitrate + nitrite 

+ ammonia + 

organic-N], 

water, filtered, 

analytically 

determined 

(mg/L)

Iron, 

water, 

filtered 

(μg/L)

Manganese, 

water, filtered 

(μg/L)

392959075435501 Fb22-15 1 <0.030 20.3 0.04 13.8 33.5 <0.01 5.60 d <0.001 0.006 6.02 d 4.4 n 66.0392913075382001 Fc12-26 1 <0.030 14.2 0.02 n 13.9 38.6 <0.01 9.20 d <0.001 0.013 10.7 d 7.8 n 0.62382932075221701 Rf13-02 1 0.032 n 9.09 0.19 8.05 10.2 0.01 13.9 d <0.001 0.007 14.9 d 4.5 n 40.9384425075072401 Oi13-06 1 <0.030 13.6 <0.01 5.17 3.93 <0.01 7.53 d <0.001 <0.004 8.03 d <4.0 0.90

384411075150101 Og15-07 1 0.073 54.6 0.07 6.91 56.8 <0.01 41.5 d <0.001 <0.004 46.7 d <4.0 13.5

385730075321401 Ld33-10 1 <0.030 15.1 0.01 n 20.6 64.5 <0.01 12.3 d <0.001 0.010 12.3 d <4.0 451391324075391901 Ic21-08 1 <0.030 7.72 0.03 12.3 65.3 <0.01 5.98 d <0.001 0.031 6.30 d <4.0 0.44385956075303801 Lb15-17 1 <0.030 11.0 0.02 9.39 25.0 <0.01 6.85 d <0.001 <0.004 7.37 d <4.0 38.0391814075435001 Hb22-17 1 <0.030 5.66 0.06 10.7 25.2 <0.01 3.00 <0.001 0.005 n 3.42 <4.0 6.74392403075362101 Gc14-04 1 <0.030 16.4 0.03 8.95 14.6 <0.01 3.41 <0.001 0.057 3.62 6.8 n 39.5385817075265101 Le24-11 1 0.062 26.7 0.09 12.8 r 48.0 <0.01 16.5 d <0.001 0.006 n 16.9 d <4.0 r 26.0 r385129075370201 Mc43-06 1 <0.030 20.8 0.15 5.91 25.5 <0.01 22.1 d <0.001 <0.004 22.8 d <4.0 8.44384323075393201 Oc21-03 1 <0.030 6.90 <0.01 15.6 14.4 <0.01 15.2 d <0.001 0.007 n 16.5 d <4.0 13.6390634075433401 Jb42-05 1 <0.030 10.7 0.05 20.9 r 29.1 0.03 4.95 <0.001 0.042 5.16 d 73.3 r 40.9 r391503075310401 Id14-03 1 <0.030 32.3 0.04 18.0 99.3 d 0.01 n 11.1 d <0.001 0.016 11.5 d 4.3 n 21.3384637075153201 Ng45-02 1 0.030 n 16.8 0.01 n 9.29 8.72 0.01 n 13.1 d <0.001 <0.004 13.0 d <4.0 1.82392324075445601 Gb21-10 2 0.108 137 d 0.03 15.8 36.6 <0.01 6.80 d 0.003 0.007 7.26 d <4.0 98.3383438075274201 Qc13-01 2 <0.030 31.9 0.04 5.93 10.1 <0.01 5.12 d <0.001 <0.004 5.54 d <4.0 23.4383221075182301 Qg32-18 2 <0.030 25.1 0.12 6.95 9.12 0.02 4.90 0.004 <0.004 5.08 d 77.4 20.1384316075330501 Od22-04 2 <0.030 14.4 0.03 5.68 7.62 <0.01 5.55 d <0.001 <0.004 6.10 d 7.4 n 14.2384201075185401 Og32-07 2 <0.030 29.6 0.12 9.52 r 13.1 <0.01 6.89 d <0.001 <0.004 7.22 d <4.0 6.97 385830075423201 Lb23-03 2 <0.030 32.9 0.03 19.3 24.9 <0.01 8.44 d 0.016 0.010 8.87 d <4.0 64.4385515075431701 Lb52-07 2 <0.030 13.3 0.01 n 22.0 5.73 <0.01 4.05 <0.001 0.010 4.16 <4.0 24.2390001075380101 Lc12-02 2 <0.030 i 51.7 0.06 12.3 9.45 <0.01 2.74 <0.001 <0.004 2.91 4.5 n 12.8

390409075311301 Kd13-09 2 <0.030 12.4 0.04 10.6 24.7 <0.01 4.92 <0.001 <0.004 5.09 d <4.0 40.4

390205075430901 Kb32-29 2 <0.030 i 129 d 0.03 8.80 r 10.7 <0.01 6.77 d <0.001 <0.004 7.04 d <4.0 r 130 r384550075304001 Nd55-06 2 <0.030 9.35 0.02 6.97 r 12.5 <0.01 2.21 <0.001 0.009 2.40 <4.0 36.9 r

384845075211901 Nf24-05 2 <0.030 46.4 0.12 5.79 r 12.1 <0.01 9.43 d <0.001 <0.004 9.29 d 41.4 r 24.9 r

383308075382301 Qc22-04 2 0.036 n 13.0 0.02 n 11.6 7.11 <0.01 10.2 d <0.001 0.007 n 10.6 d <4.0 59.8
Factors Affecting Variability 13

Table 2. Water characteristics, major ion, nutrient, and atmospheric age tracer results from groundwater in the surficial aquifer of the Delaware Coastal Plain, 2014.—Continued[--, no data; <, less than; d, sample was diluted; i, result may be affected by interference; n, below the reporting level but at or above the detection level; r, value verified by rerun, same method; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; mL, millimeters; fmol/kg, femtomoles per kilogram; CaCO3, calcium carbonate; SiO2, silicon dioxide; SF6, sulfur hexafluoride; pptv, parts per trillion by volume]

Station number Local identifier Geochemical group Excess air (mL) (1 bottle) Excess air (mL) (2 bottles) Excess air (mL) (both) Average excess air (mL) (average)

Estimated 

recharge 

temperature 

(degrees 

Celsius)

(1 bottle)

Estimated 

recharge 

temperature 

(degrees 

Celsius)

(2 bottles)

Estimated 

recharge 

temperatures 

(degrees 

Celsius)

(both)

Estimated 

recharge 

temperatures 

(degrees 

Celsius)

(average)

SF6

concentration 

(fmol/kg)

(1 bottle)

SF6

concentration 

(fmol/kg)

(2 bottles)

SF6

concentration 

(fmol/kg)

(both)

392959075435501 Fb22-15 1 4.3 4.3 4.3 4.3 12.0 12.0 12.0 12.0 3.3 3.4 3.3 to 3.4392913075382001 Fc12-26 1 -- -- -- -- -- -- -- -- -- -- --382932075221701 Rf13-02 1 0.0 0.0 0.0 0.0 9.2 9.2 9.2 9.2 2.2 2.1 2.1 to 2.2384425075072401 Oi13-06 1 -- -- -- -- -- -- -- -- -- -- --

384411075150101 Og15-07 1 -- -- -- -- -- -- -- -- -- -- --

385730075321401 Ld33-10 1 -- -- -- -- -- -- -- -- -- -- --391324075391901 Ic21-08 1 1.8 1.3 1.3 to 1.8 1.6 11.6 11.2 11.2 to 11.6 11.4 2.4 2.6 2.4 to 2.6385956075303801 Lb15-17 1 1.4 1.4 1.4 1.4 11.5 11.5 11.5 11.5 2.4 2.5 2.4 to 2.5391814075435001 Hb22-17 1 3.3 3.3 3.3 3.3 10.6 10.6 10.6 10.6 3.0 3.1 3 to 3.1392403075362101 Gc14-04 1 2.9 3.5 2.9 to 3.5 3.2 9.7 10.7 9.7 to 10.7 10.2 3.2 3.2 3.2 to 3.2385817075265101 Le24-11 1 -- -- -- -- -- -- -- -- -- -- --385129075370201 Mc43-06 1 1.9 1.9 1.9 1.9 11.8 11.8 11.8 11.8 2.2 2.5 2.2 to 2.5384323075393201 Oc21-03 1 2.2 2.2 2.2 2.2 12.0 12.0 12.0 12.0 2.6 2.7 2.6 to 2.7390634075433401 Jb42-05 1 0.5 0.5 0.5 0.5 12.5 12.5 12.5 12.5 1.3 1.3 1.3 to 1.3391503075310401 Id14-03 1 1.6 1.6 1.6 1.6 11.8 11.8 11.8 11.8 2.1 2.0 2 to 2.1384637075153201 Ng45-02 1 1.6 1.6 1.6 1.6 13.6 13.6 13.6 13.6 2.5 2.7 2.5 to 2.7392324075445601 Gb21-10 2 -- -- -- -- -- -- -- -- -- -- --383438075274201 Qc13-01 2 1.2 1.2 1.2 1.2 13.5 13.5 13.5 13.5 2.2 2.2 2.2 to 2.2383221075182301 Qg32-18 2 0.6 0.6 0.6 0.6 13.5 13.5 13.5 13.5 2.3 2.4 2.3 to 2.4384316075330501 Od22-04 2 -- -- -- -- -- -- -- -- -- -- --384201075185401 Og32-07 2 -- -- -- -- -- -- -- -- -- -- --385830075423201 Lb23-03 2 1.4 1.3 1.3 to 1.4 1.4 11.7 11.8 11.7 to 11.8 11.8 2.6 2.7 2.6 to 2.7385515075431701 Lb52-07 2 2.3 2.3 2.3 2.3 12.2 12.2 12.2 12.2 2.1 2.1 2.1 to 2.1390001075380101 Lc12-02 2 -- -- -- -- -- -- -- -- -- -- --

390409075311301 Kd13-09 2 2.6 2.6 2.6 2.6 11.1 11.1 11.1 11.1 2.8 2.7 2.7 to 2.8

390205075430901 Kb32-29 2 -- -- -- -- -- -- -- -- -- -- --384550075304001 Nd55-06 2 1.1 1.1 1.1 1.1 8.8 8.8 8.8 8.8 2.7 2.4 2.4 to 2.7

384845075211901 Nf24-05 2 -- -- -- -- -- -- -- -- -- -- --

383308075382301 Qc22-04 2 1.0 1.0 1.0 1.0 8.2 8.2 8.2 8.2 2.5 2.6 2.5 to 2.6
14 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014 Table 2. Water characteristics, major ion, nutrient, and atmospheric age tracer results from groundwater in the surficial aquifer of the Delaware Coastal Plain, 2014. —Continued [--, no data; <, less than; d, sample was diluted; i, result may be affected by interference; n, below the reporting level but at or above the detection level; r, value verified by rerun, same method; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; mL, millimeters; fmol/kg, femtomoles per kilogram; CaCO3, calcium carbonate; SiO2, silicon dioxide; SF6, sulfur hexafluoride; pptv, parts per trillion by volume] Station number Local identifier Geochemical group SF6 concentration (fmol/kg) (average) Calculated SF6 partial pressure (pptv) (1 bottle) Calculated SF6 partial pressure (pptv) (2 bottles) Calculated SF6 partial pressure (pptv) (both) Calculated SF6 partial pressure (pptv) (average) Piston flow model SF6 recharge age (years) (1 bottle) Piston flow model SF6 recharge age (years) (2 bottles) Piston flow model SF6 recharge age (years) (both) Piston fow model SF6 recharge age (years) (average) 392959075435501 Fb22-15 1 3.3 6.0 6.1 6 to 6.1 6.0 8.8 8.3 8.3 to 8.8 8.6 392913075382001 Fc12-26 1 -- -- -- -- -- -- -- -- -- 382932075221701 Rf13-02 1 2.1 5.4 5.1 5.1 to 5.4 5.2 11.3 12.8 11.3 to 12.8 12.1 384425075072401 Oi13-06 1 -- -- -- -- -- -- -- -- -- 384411075150101 Og15-07 1 -- -- -- -- -- -- -- -- -- 385730075321401 Ld33-10 1 -- -- -- -- -- -- -- -- -- 391324075391901 Ic21-08 1 2.5 5.3 5.9 5.3 to 5.9 5.6 11.9 9.4 9.4 to 11.9 10.7 385956075303801 Lb15-17 1 2.5 5.7 5.7 5.7 to 5.7 5.7 9.9 9.9 9.9 9.9 391814075435001 Hb22-17 1 3.1 5.7 5.9 5.7 to 5.9 5.8 9.9 9.4 9.4 to 9.9 9.7 392403075362101 Gc14-04 1 3.2 6.1 6.0 6 to 6.1 6.1 8.4 8.9 8.4 to 8.9 8.7 385817075265101 Le24-11 1 -- -- -- -- -- -- -- -- -- 385129075370201 Mc43-06 1 2.4 5.0 5.5 5 to 5.5 5.2 12.9 10.9 10.9 to 12.9 11.9 384323075393201 Oc21-03 1 2.6 5.7 5.8 5.7 to 5.8 5.8 9.9 9.4 9.4 to 9.9 9.7 390634075433401 Jb42-05 1 1.3 3.4 3.4 3.4 to 3.4 3.4 20.5 20.5 20.5 20.5 391503075310401 Id14-03 1 2.0 4.7 4.6 4.6 to 4.7 4.7 14.5 15.0 14.5 to 15 14.7 384637075153201 Ng45-02 1 2.6 6.1 6.7 6.1 to 6.7 6.4 8.5 6.5 6.5 to 8.5 7.5 392324075445601 Gb21-10 2 -- -- -- -- -- -- -- -- -- 383438075274201 Qc13-01 2 2.2 5.6 5.6 5.6 to 5.6 5.6 10.3 10.8 10.3 to 10.8 10.6 383221075182301 Qg32-18 2 2.3 6.4 6.4 6.4 to 6.4 6.4 7.3 7.3 7.3 7.3 384316075330501 Od22-04 2 -- -- -- -- -- -- -- -- -- 384201075185401 Og32-07 2 -- -- -- -- -- -- -- -- -- 385830075423201 Lb23-03 2 2.7 6.1 6.4 6.1 to 6.4 6.2 8.4 7.4 7.4 to 8.4 7.9 385515075431701 Lb52-07 2 2.1 4.6 4.6 4.6 to 4.6 4.6 14.9 14.9 14.9 14.9

390001075380101 Lc12-02 2 -- -- -- -- -- -- -- -- --

390409075311301 Kd13-09 2 2.8 5.6 5.6 5.6 to 5.6 5.6 10.4 10.4 10.4 10.4

390205075430901 Kb32-29 2 -- -- -- -- -- -- -- -- --384550075304001 Nd55-06 2 2.5 5.8 5.3 5.3 to 5.8 5.5 9.9 11.9 9.9 to 11.9 10.9

384845075211901 Nf24-05 2 -- -- -- -- -- -- -- -- --

383308075382301 Qc22-04 2 2.6 5.3 5.6 5.3 to 5.6 5.5 11.9 10.4 10.4 to 11.9 11.2
Factors Affecting Variability 15

Table 2. Water characteristics, major ion, nutrient, and atmospheric age tracer results from groundwater in the surficial aquifer of the Delaware Coastal Plain, 2014.—Continued[--, no data; <, less than; d, sample was diluted; i, result may be affected by interference; n, below the reporting level but at or above the detection level; r, value verified by rerun, same method; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; mL, millimeters; fmol/kg, femtomoles per kilogram; CaCO3, calcium carbonate; SiO2, silicon dioxide; SF6, sulfur hexafluoride; pptv, parts per trillion by volume]

Station number Local identifier Geochemical group

Dissolved 

oxygen, 

water, 

unfiltered 

(mg/L)

pH, water, 

unfiltered, 

field, 

standard 

units

Specific 

conductance, 

water, 

unfiltered 

(μS/cm at 

25 °C)

Temperature, 

water 

(degrees 

Celsius)

Dissolved 

solids dried at 

180 °C, water, 

filtered 

(mg/L)

Calcium, 

water, 

filtered 

(mg/L)

Magnesium, 

water, 

filtered 

(mg/L)

Potassium, 

water, 

filtered 

(mg/L)

Sodium, 

water, 

filtered 

(mg/L)

Alkalinity, 

water, filtered, 

inflection-point 

titration method 

(incremental titration 

method), field 

(mg/L as CaCO3)

Bicarbonate, 

water, filtered, 

inflection-point 

titration method 

(incremental titration 

method), field 

(mg/L)

392605075452801 Fa45-07 3 8.8 6.0 1,290 17.0 698 36.4 21.2 3.79 185 38.4 46.9392428075445901 Gb11-07 3 9.1 5.5 580 15.3 311 r 5.88 2.27 1.82 98.9 10 12.5393210075401601 Eb35-23 3 4.0 6.0 1,500 17.3 791 46.2 11.1 1.97 221 37.5 45.8393126075460201 Ea44-13 3 7.8 5.9 1,250 19.1 626 11.5 4.17 2.76 223 34.4 40.7383629075245601 Pf41-02 3 -- 4.5 358 -- 203 11.1 3.47 11.0 40.2 2.3 5.2383412075125401 Qh13-05 3 3.8 4.6 175 17.7 126 1.44 4.04 2.51 21.6 0.2 5.9383836075183001 Pg22-06 3 2.7 4.9 114 16.4 87 1.89 1.76 2.47 13.7 2.9 4.9

383749075110501 Ph34-15 3 4.8 5.6 120 21.6 65 r 6.76 2.85 1.93 10.3 21.2 26.1

384250075085001 Oi32-18 3 8.2 5.2 189 18.8 125 3.96 5.45 2.37 20.1 3.8 5.1384737075342701 Nd31-06 3 3.8 6.1 546 18.9 293 r 31.9 5.96 2.00 71.8 8.1 9.9384159075310801 Od44-02 3 7.4 4.7 118 16.7 68 1.21 2.60 6.30 8.85 2.5 4.9384130075125801 Oh42-07 3 5.3 5.1 457 18.8 248 r 9.24 4.20 4.09 65.5 6.6 8.7

391112075380001 Ic43-01 3 8.2 5.3 176 16.6 108 3.43 2.40 1.31 25.8 5.1 6.5

390252075271301 Ke33-22 3 3.8 6.2 428 13.2 227 r 27.8 11.3 3.17 35.5 63.3 77.2391936075363201 Hc14-15 3 5.7 5.9 748 12.8 428 22.1 r 7.82 r 5.71 r 106 r 13.4 16.5384502075235301 Nf52-02 3 5.7 6.7 470 14.4 264 15.6 4.14 1.42 75.6 73.3 89.3390544075300501 Jd55-10 3 3.1 5.8 277 16.6 169 13.1 r 5.40 r 2.50 r 29.6 r 24.6 30.3390705075263201 Je34-04 3 7.9 5.7 226 14.0 133 2.41 r 1.67 r 0.30 r 38.7 r 21.2 26.1391232075285401 Ie32-02 3 5.2 6.3 1,250 13.2 704 59.5 32.9 4.22 139 121 148
16 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014 Table 2. Water characteristics, major ion, nutrient, and atmospheric age tracer results from groundwater in the surficial aquifer of the Delaware Coastal Plain, 2014. —Continued [--, no data; <, less than; d, sample was diluted; i, result may be affected by interference; n, below the reporting level but at or above the detection level; r, value verified by rerun, same method; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; mL, millimeters; fmol/kg, femtomoles per kilogram; CaCO3, calcium carbonate; SiO2, silicon dioxide; SF6, sulfur hexafluoride; pptv, parts per trillion by volume] Station number Local identifier Geochemical group Bromide, water, filtered (mg/L) Chloride, water, filtered (mg/L) Fluoride, water, filtered (mg/L) Silica, water, filtered (mg/L as SiO2

)

Sulfate, 

water, 

filtered 

(mg/L)

Ammonia, 

water, 

filtered 

(mg/L as 

nitrogen)

Nitrate plus 

nitrite, 

water, 

filtered 

(mg/L as 

nitrogen)

Nitrite, 

water, 

filtered 

(mg/L as 

nitrogen)

Orthophosphate, 

water, 

filtered 

(mg/L as 

phosphorus)

Total nitrogen 

[nitrate + nitrite 

+ ammonia + 

organic-N], 

water, filtered, 

analytically 

determined 

(mg/L)

Iron, 

water, 

filtered 

(μg/L)

Manganese, 

water, filtered 

(μg/L)

392605075452801 Fa45-07 3 <0.060 d 334 d 0.07 n, d 7.09 65.7 d <0.01 6.83 d <0.001 0.006 7.38 d <4.0 0.84392428075445901 Gb11-07 3 <0.030 117 d 0.02 7.66 46.4 <0.01 5.42 d <0.001 0.005 5.86 d <4.0 25.3393210075401601 Eb35-23 3 <0.060 d 430 d <0.07 d 7.85 8.15 d <0.01 4.96 0.001 0.014 5.05 d 8.6 13.7393126075460201 Ea44-13 3 <0.060 d 336 d 0.05 n, d 3.22 25.1 d <0.01 3.02 <0.001 0.004 2.86 <4.0 18.0383629075245601 Pf41-02 3 0.045 n 89.7 0.14 6.57 15.9 <0.01 0.373 <0.001 <0.004 0.47 156 18.0383412075125401 Qh13-05 3 <0.030 27.6 0.03 12.3 21.5 <0.01 1.56 <0.001 <0.004 1.73 <4.0 3.07383836075183001 Pg22-06 3 0.037 n 22.4 0.03 9.88 5.77 <0.01 0.930 <0.001 <0.004 1.04 <4.0 1.68

383749075110501 Ph34-15 3 <0.030 12.8 <0.01 2.85 3.50 <0.01 3.33 <0.001 <0.004 3.45 <4.0 31.3

384250075085001 Oi32-18 3 0.066 21.1 0.02 n 8.09 32.9 <0.01 3.55 <0.001 <0.004 3.67 4.1 n 12.0384737075342701 Nd31-06 3 <0.030 108 d 0.04 3.15 21.0 <0.01 1.05 <0.001 0.007 1.20 <4.0 1.24384159075310801 Od44-02 3 0.045 n 23.8 0.05 7.53 7.56 <0.01 0.562 <0.001 <0.004 0.60 <4.0 4.40384130075125801 Oh42-07 3 0.049 n 117 d 0.07 7.66 25.8 <0.01 0.340 <0.001 <0.004 0.41 <4.0 4.42

391112075380001 Ic43-01 3 <0.030 30.3 0.07 13.7 26.7 <0.01 1.12 <0.001 0.004 n 1.25 <4.0 4.78

390252075271301 Ke33-22 3 0.042 n 62.4 0.04 3.73 37.2 <0.01 2.15 <0.001 <0.004 2.35 <4.0 15.1391936075363201 Hc14-15 3 <0.030 i 196 d 0.02 n, d 4.84 r 17.8 d <0.01 2.29 <0.001 <0.004 2.48 5.4 n 1.94 r384502075235301 Nf52-02 3 <0.030 98.9 0.05 5.86 5.23 <0.01 0.839 <0.001 0.050 0.95 <4.0 <0.20390544075300501 Jd55-10 3 <0.030 50.2 0.03 4.77 r 25.1 0.03 1.65 <0.001 0.006 n 1.74 <4.0 r 6.38 r390705075263201 Je34-04 3 <0.030 39.8 0.03 6.76 r 17.0 0.01 n 0.361 <0.001 <0.004 0.42 4.3 1.90391232075285401 Ie32-02 3 <0.060 d 322 d 0.07 n, d 5.39 8.10 d 0.02 n 0.825 <0.001 0.005 n 0.91 5.2 n 7.81
Factors Affecting Variability 17

Table 2. Water characteristics, major ion, nutrient, and atmospheric age tracer results from groundwater in the surficial aquifer of the Delaware Coastal Plain, 2014.—Continued[--, no data; <, less than; d, sample was diluted; i, result may be affected by interference; n, below the reporting level but at or above the detection level; r, value verified by rerun, same method; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; mL, millimeters; fmol/kg, femtomoles per kilogram; CaCO3, calcium carbonate; SiO2, silicon dioxide; SF6, sulfur hexafluoride; pptv, parts per trillion by volume]

Station number Local identifier Geochemical group Excess air (mL) (1 bottle) Excess air (mL) (2 bottles) Excess air (mL) (both) Average excess air (mL) (average)

Estimated 

recharge 

temperature 

(degrees 

Celsius)

(1 bottle)

Estimated 

recharge 

temperature 

(degrees 

Celsius)

(2 bottles)

Estimated 

recharge 

temperatures 

(degrees 

Celsius)

(both)

Estimated 

recharge 

temperatures 

(degrees 

Celsius)

(average)

SF6

concentration 

(fmol/kg)

(1 bottle)

SF6

concentration 

(fmol/kg)

(2 bottles)

SF6

concentration 

(fmol/kg)

(both)

392605075452801 Fa45-07 3 -- -- -- -- -- -- -- -- -- -- --392428075445901 Gb11-07 3 -- -- -- -- -- -- -- -- -- -- --393210075401601 Eb35-23 3 1.4 1.4 1.4 1.4 11.5 11.5 11.5 11.5 2.7 2.9 2.7 to 2.9393126075460201 Ea44-13 3 -- -- -- -- -- -- -- -- -- -- --383629075245601 Pf41-02 3 -- -- -- -- -- -- -- -- -- -- --383412075125401 Qh13-05 3 -- -- -- -- -- -- -- -- -- -- --383836075183001 Pg22-06 3 2.3 2.3 2.3 2.3 7.0 7.0 7.0 7.0 2.6 2.3 2.3 to 2.6

383749075110501 Ph34-15 3 -- -- -- -- -- -- -- -- -- -- --

384250075085001 Oi32-18 3 -- -- -- -- -- -- -- -- -- -- --384737075342701 Nd31-06 3 -- -- -- -- -- -- -- -- -- -- --384159075310801 Od44-02 3 -- -- -- -- -- -- -- -- -- -- --384130075125801 Oh42-07 3 0.9 0.9 0.9 0.9 13.4 13.4 13.4 13.4 2.2 2.2 2.2 to 2.2

391112075380001 Ic43-01 3 2.3 2.3 2.3 2.3 9.2 9.2 9.2 9.2 2.7 2.5 2.5 to 2.7

390252075271301 Ke33-22 3 -- -- -- -- -- -- -- -- -- -- --391936075363201 Hc14-15 3 -- -- -- -- -- -- -- -- -- -- --384502075235301 Nf52-02 3 0.4 0.1 0.1 to 0.4 0.2 16.1 16.3 16.1 to 16.3 16.2 2.2 2.2 2.2 to 2.2390544075300501 Jd55-10 3 2.5 2.5 2.5 2.5 13.6 13.6 13.6 13.6 2.7 2.7 2.7 to 2.7390705075263201 Je34-04 3 -- -- -- -- -- -- -- -- -- -- --391232075285401 Ie32-02 3 -- -- -- -- -- -- -- -- -- -- --
18 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014 Table 2. Water characteristics, major ion, nutrient, and atmospheric age tracer results from groundwater in the surficial aquifer of the Delaware Coastal Plain, 2014. —Continued [--, no data; <, less than; d, sample was diluted; i, result may be affected by interference; n, below the reporting level but at or above the detection level; r, value verified by rerun, same method; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; μg/L, micrograms per liter; mL, millimeters; fmol/kg, femtomoles per kilogram; CaCO3, calcium carbonate; SiO2, silicon dioxide; SF6, sulfur hexafluoride; pptv, parts per trillion by volume] Station number Local identifier Geochemical group SF6 concentration (fmol/kg) (average) Calculated SF6 partial pressure (pptv) (1 bottle) Calculated SF6 partial pressure (pptv) (2 bottles) Calculated SF6 partial pressure (pptv) (both) Calculated SF6 partial pressure (pptv) (average) Piston flow model SF6 recharge age (years) (1 bottle) Piston flow model SF6 recharge age (years) (2 bottles) Piston flow model SF6 recharge age (years) (both) Piston flow model SF6 recharge age (years) (average) 392605075452801 Fa45-07 3 -- -- -- -- -- -- -- -- -- 392428075445901 Gb11-07 3 -- -- -- -- -- -- -- -- -- 393210075401601 Eb35-23 3 2.8 6.2 6.8 6.2 to 6.8 6.5 7.8 6.3 6.3 to 7.8 7.1 393126075460201 Ea44-13 3 -- -- -- -- -- -- -- -- -- 383629075245601 Pf41-02 3 -- -- -- -- -- -- -- -- -- 383412075125401 Qh13-05 3 -- -- -- -- -- -- -- -- -- 383836075183001 Pg22-06 3 2.5 4.8 4.2 4.2 to 4.8 4.5 13.8 16.3 13.8 to 16.3 15.1 383749075110501 Ph34-15 3 -- -- -- -- -- -- -- -- -- 384250075085001 Oi32-18 3 -- -- -- -- -- -- -- -- -- 384737075342701 Nd31-06 3 -- -- -- -- -- -- -- -- -- 384159075310801 Od44-02 3 -- -- -- -- -- -- -- -- -- 384130075125801 Oh42-07 3 2.2 5.8 5.7 5.7 to 5.8 5.7 9.8 9.8 9.8 9.8 391112075380001 Ic43-01 3 2.6 5.3 4.9 4.9 to 5.3 5.1 11.9 13.9 11.9 to 13.9 12.9 390252075271301 Ke33-22 3 -- -- -- -- -- -- -- -- -- 391936075363201 Hc14-15 3 -- -- -- -- -- -- -- -- -- 384502075235301 Nf52-02 3 2.2 6.8 7.3 6.8 to 7.3 7.0 6.4 4.4 4.4 to 6.4 5.4 390544075300501 Jd55-10 3 2.7 6.0 5.9 5.9 to 6 5.9 9.0 9.5 9 to 9.5 9.2 390705075263201 Je34-04 3 -- -- -- -- -- -- -- -- -- 391232075285401 Ie32-02 3 -- -- -- -- -- -- -- -- --
 Water Quality in the Surficial Aquifer 19

Soils

Soils data were extracted on an individual well basis 

using a similar buffer and extract methods as those used for 

the land-cover data. Soils data came from the Gridded Soil 

Survey Geographic (gSSURGO) database (Soil Survey Staff, 

2016) and were downloaded from the U.S. Department of 

Agriculture’s Geospatial Data Gateway. Surficial soils around 

the wells were mostly sandy with 46 of the 48 sampled wells 

having sand content greater than 50 percent. Soils at all wells 

had low clay content with none having more than 18.5 percent 

clay.

Well Depth and Aquifer Position

Surficial aquifer thickness at each well was determined 

using a simple overlay where well locations were intersected 

with an aquifer thickness dataset (Denver and Nardi, 2016). 

The dataset maps the thickness of the surficial unconfined 

aquifer, including from the land surface and unsaturated zone 

to the bottom of sediments of geologic units identified as part 

of the surficial aquifer. The thickness of the surficial aquifer 

may have impacts on groundwater-flow paths and the potential 

for denitrification. Wells in this study were mostly located in 

areas of relatively thick aquifer (table 1), with a mean thickness for all wells of 68.9 ft. The thickness range of all wells 

was 117 ft, with a minimum of 11 ft and a maximum of 128 ft.

 Water Quality in the Surficial Aquifer

Three groups of wells with similar properties were 

derived using Spearman’s rank correlation and cluster analysis 

(table 2, fig. 5A). A geographic pattern (fig. 5B) is not evident 

from this analysis; however, land use and geochemical properties are similar within groups. These groups are useful in 

describing some of the variability in groundwater quality and 

may facilitate more detailed statistical analysis in future studies. The patterns observed in these three groups are described 

below.

The major-ion composition of groundwater (table 3) in 

agricultural areas of Delaware consists of two end-member 

water types (fig. 6) that reflect the spatial variability associated 

with land use and land cover, practices applied on the land, 

soils, and aquifer properties. Overall, the predominant cations 

are sodium and calcium, whereas the predominant anions are 

chloride and nitrate, consistent with previous studies in the 

Coastal Plain (Ator, 2008; Ator and Denver, 2015). However, 

within the geochemical groups, there are strong differences 

between major ions in groups 1 and 3. Group 1 is a calciummagnesium-nitrate water type (fig. 6), typically identified as 

an agricultural signature in the Delmarva Peninsula (Böhlke, 

2002). Group 3 is a sodium-potassium-chloride water type and 

group 2 appears to be a mixture of groups 1 and 3 (fig. 6). 

Sampled well identifiers

Group 1

Group 2

Group 3

Gc14-04

Fb22-15

Hb22-17

Fc12-26

Gb21-10

Gb11-07

Fa45-07

Hc14-15

Ea44-13

Eb35-23

Id14-03

Ic21-08

Ie32-02

Ic43-01

Og15-07

Mc43-06

Lb15-17

Le24-11

Oi13-06

Jb42-05

Ng45-02

Rf13-02

Ld33-10

Oc21-03

Qc13-01

Og32-07

Nd55-06

Kd13-09

Nf24-05

Kb32-29

Qc22-04

Od22-04

Lc12-02

Qg32-18

Lb52-07

Lb23-03

Pg22-06

Oh42-07

Ke33-22

Je34-04

Qh13-05

Oi32-18

Pf41-02

Nf52-02

Nd31-06

Jd55-10

Od44-02

Ph34-15

A

Figure 5. Results of correlation and cluster analysis, A, dendrogram showing the grouping of wells 

by correlation and cluster analysis, and B, the spatial distribution of geochemical groups.
20 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014

75°50'

39°

38°30'

39°30'

75°

C&D Canal

Fall Line DELAWARE

DELAWARE

MARYLAND

MARYLAND

0 5 10 15 MILES

0 5 10 15 KILOMETERS

Chesapeake Bay

Wilmington

Dover

DELAWARE BAY

Delaware River

ATLANTIC OCEAN

NEW JERSEY

Base from U.S. Geological Survey digital data, 2004,

1:24,000 projected to NAD 83 UTM Zone 18 NAD 83

from GCS NAD 1983

NEW

CASTLE

KENT

SUSSEX

Indian River

Bay

Rehoboth Bay

Eb35-23

Ea44-13

Fb22-15

Fc12-26

Fa45-07

Gb11-07 Gc14-04

Gb21-10

Hb22-17

Hc14-15

Id14-03 Ic21-08

Ic43-01

Ie32-02

Je34-04

Jd55-10 Jb42-05

Kb32-29

Lc12-02

Lb23-03

Lb52-07

Kd13-09

Ke33-22

Lb15-17

Le24-11

Ld33-10

Mc43-06

Nf24-05

Ng45-02

Oi13-06 Og15-07

Oi32-18

Oh42-07

Pg22-06

Pf41-02 Qc13-01

Qc22-04

Rf13-02

Ph34-15

Qh13-05

Qg32-18

Og32-07

Nd31-06

Nd55-06

Oc21-03

Od44-02

Od22-04

Nf52-02

EXPLANATION

Chesapeake Bay

watershed boundary

Fall Line

Group 1

Group 2

Group 3

Geochemical group of wells

Delaware Coastal

Plain

B

Figure 5. Results of correlation and cluster analysis, A, dendrogram showing the grouping of wells by 

correlation and cluster analysis, and B, the spatial distribution of geochemical groups.—Continued
 Water Quality in the Surficial Aquifer 21

Table 3. Summary statistics for selected physical properties and major ions and elements of groundwater in the surficial aquifer of 

the Delaware Coastal Plain, 2014.

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; N, nitrogen; <, less than]

Constituents 

(units)

Geochemical Group 1 Geochemical Group 2 Geochemical Group 3

Minimum Median Maximum Minimum Median Maximum Minimum Median Maximum

Physical properties

pH (standard units) 4.65 5.69 6.57 4.41 5.15 6.08 4.85 5.95 7.48

Specific Conductance (μS/cm) 114 200 658 95 197 592 114 428 1,496

Dissolved Oxygen (mg/L) 1.23 7.71 9.89 1 6.78 10.24 2.67 5.68 9.06

Major ions or elements

Bicarbonate (mg/L) <0.1 5.00 43.80 <0.1 2.8 12.8 4.90 16.50 147.90

Calcium (mg/L) 9.157 18.39 56.85 5.06 7.50 20.25 1.21 11.11 59.53

Chloride (mg/L) 5.655 14.65 54.60 9.35 29.64 136.85 12.80 89.68 430.31

Magnesium (mg/L) 3.385 9.01 23.53 0.83 3.63 12.26 1.67 4.17 32.93

Nitrate (mg/L as N) 3.000 10.15 41.47 2.21 5.55 10.25 0.34 1.56 6.83

Potassium (mg/L) 1.111 2.59 8.06 1.26 2.77 7.39 0.30 2.50 10.98

Silica (mg/L) 5.169 11.49 20.85 5.68 9.52 21.96 2.86 6.76 13.71

Sodium (mg/L) 2.674 5.98 18.73 6.55 14.17 72.92 8.85 40.23 223.00

Sulfate (mg/L) 3.928 27.30 99.30 5.73 10.71 36.59 3.50 21.03 65.69

100

80

60

40

20

0

0

20

40

60

80

100

100

80

60

40

20

0

0

20

40

60

80

100

100

80

60

40

20

0

0

20

40

60

80

100

100

80

60

40

20

0

80

60

40

20

0

100

20

40

60

80

20

40

60

Magnesium

80

Sulfate + Chloride + Nitrate

Calcium + Magnesium

Sulfate

Calcium Chloride + Nitrate

Sodium + Potassium

Bicarbonate

Percentage of total milliequivalents per liter

EXPLANATION

Geochemical group of well

Group 1

Group 2

Group 3

Figure 6. Trilinear diagram of major ions within the surficial aquifer of the Delaware Coastal Plain by geochemical groups.
22 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014

Nitrate

Nitrate is the predominant nutrient found in the surficial 

aquifer of the Delaware Coastal Plain, with a median concentration of 4.95 mg/L in samples collected from the 48 wells. 

In the three groups of wells determined by correlation and 

cluster analysis, geochemical group 1 had the highest median 

nitrate concentration at 10.15 mg/L, exceeding the EPA 

drinking-water standard for public water systems of 

10 mg/L (U.S. Environmental Protection Agency, 2009) 

(fig. 7A; table 3). Nitrate concentrations in group 1 were different than the nitrate concentrations in group 3 at the 

(p < 0.05) level of significance. Group 1 also had the highest 

median values of calcium and magnesium (figs. 7B and 7C, 

table 3) and differs from group 2 at the (p < 0.05) level of 

significance. Calcium and magnesium in group 3 were not different than groups 1 or 2 at the (p < 0.05) level of significance. 

Group 1 had the highest percentage of agricultural land in the 

1,640-ft-radius land buffer analysis (fig. 8A), but was not 

different from groups 2 or 3 at the (p < 0.05) level of significance. Groundwater collected from wells in group 1 exhibited 

characteristics described as having an agricultural signature 

(calcium-magnesium-nitrate water type; fig. 6) in previous 

studies on the Delmarva Peninsula (Böhlke, 2002; Denver, 

1989; Hamilton and others, 1993). 

Group 3, which had the highest percentage of developed 

land near the wells (fig. 8B), had the lowest median nitrate 

concentrations (1.55 mg/L, table 3). Nitrate concentrations in 

the surficial aquifer have been shown to be lower in urban and 

developed areas compared to agricultural areas (Denver and 

others, 2014). The median concentration of nitrate in group 2 

(5.55 mg/L) is between groups 1 and 3. The overall geochemical signature of group 2 (fig. 6, table 3) appears to be a blend 

of the group 1 (high nitrate, low chloride) and group 3 

(low nitrate, high chloride) end members. 

Nitrate as N concentration,

in milligrams per liter

Calcium concentration,

in milligrams per liter

Magnesium concentration,

in milligrams per liter

Geochemical groups Geochemical groups

Geochemical groups

50

40

30

20

10

0

70

60

50

40

30

20

10

-10

40

35

30

25

20

15

10

5

-5

0

0

1 1

1

16 16

16

A A

A

B

B

AB

AB

A B

13 13

13

19 19

19

2 2

2

3 3

3

EXPLANATION

25th percentile (25 percent of the

data values are less than this value.)

Sample populations that share the same

letter symbol are not significantly different

at the 0.05 level of significance.

Median

Data value within 1.5 times the IQR

outside the quartile

Concentration in groundwater samples from wells

16 Number of samples

B

75th percentile (75 percent of the

data values are less than this value.)

Interquartile

range (IQR)

A B

C

Figure 7. Boxplots of A, nitrate, B, calcium, and C, magnesium concentrations by geochemical groups.
 Water Quality in the Surficial Aquifer 23

Chloride

Chloride concentrations in natural groundwater are typically low (Hem, 1985), but elevated concentrations of sodium 

and chloride in groundwater are common in the United States 

(Panno and others, 2006). Common sources of chloride in 

groundwater are from deicing activities, commercial fertilizer, 

sewage and animal wastes, landfill leachate, seawater intrusion, and migration of formation brines (Mullaney and others, 

2009). In Delaware, likely sources of chloride in groundwater 

are from deicing, potassium chloride fertilizers, and septic and 

animal wastes. Seawater intrusion may be a source near the 

coast, but because all wells in this study are at least several 

miles from the coast, it is unlikely that it affected the chloride 

concentrations in shallow groundwater that was sampled. The 

highest median concentrations of chloride (89 mg/L) were 

measured in samples collected from wells in group 3 (table 3, 

fig. 9A). These differ from chloride concentrations in group 1 

at the (p < 0.05) level of significance. The ratio of chloride to 

bromide, often used to identify potential sources of chloride, 

was also much higher in group 3 than in groups 1 or 2, with 

a median value of over 2,000. Ten of the 19 wells in group 3 

had chloride to bromide ratios above 2,000 (fig. 9B), typical 

of chloride with a halite source (Mullaney and others, 2009). 

Samples from wells in the northern part of the study area 

showed the highest chloride to bromide ratios and the highest 

chloride concentrations (fig. 9C). Wells in group 3 also had 

the highest percentage of developed land use near them 

(fig. 8B), which may be related to increased chloride from 

deicing activity. In general, more deicing products are used 

on roads in urban and suburban areas, and in Delaware, that 

includes more of the northern part of the Coastal Plain where 

ice is a greater problem. Although it is difficult to identify the 

exact sources of chloride, the chloride to bromide ratios in 

group 3 are significantly different from the chloride to bromide 

ratios in group 1 (fig. 9B). 

Geochemical groups Geochemical groups

A B

Percent agricultural land use

Percent developed land use

100 100

80 80

60 60

40 40

20 20

0 0

1 2 3 1 2 3

16

AAA AAA

13 19 16 13 19

EXPLANATION

25th percentile (25 percent of the

data values are less than this value.)

Median

Data value within 1.5 times the IQR

outside the quartile

Percent agricultural land use around the wells

75th percentile (75 percent of the

data values are less than this value.)

Interquartile

range (IQR)

Sample populations that share the same

letter symbol are not significantly different

at the 0.05 level of significance.

16 Number of samples

A

EXPLANATION

25th percentile (25 percent of the

data values are less than this value.)

Median

Data value within 1.5 times the IQR

outside the quartile

Percent developed land use around the wells

75th percentile (75 percent of the

data values are less than this value.)

Interquartile

range (IQR)

Sample populations that share the same

letter symbol are not significantly different

at the 0.05 level of significance.

16 Number of samples

A

Figure 8. Boxplots of A, agricultural land use, and B, developed land use by geochemical groups.
24 Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014

Geochemical groups

Chloride concentration, in milligrams per liter

Geochemical groups

A

C

B

Chloride to bromide ratio

Chloride to bromide ratio

Latitude

500 10,000

10,000

8,000

6,000

4,000

2,000

0

0 100 200 300 400 500

400 8,000

300 6,000

200 4,000

100 2,000

0 0

1 1 2 2 3 3

Chloride concentration,

in milligrams per liter

39.45°

39.30°

39.15°

39.00°

38.85°

38.70°

38.55°

16 16

A A AB B B AB

13 19 13 19

EXPLANATION

25th percentile (25 percent of the

data values are less than this value.)

Median

Data value within 1.5 times the IQR

outside the quartile

Chloride concentration in groundwater

samples from wells

75th percentile (75 percent of the

data values are less than this value.)

Interquartile

range (IQR)

Sample populations that share the same

letter symbol are not significantly different

at the 0.05 level of significance.

16 Number of samples

B

EXPLANATION

25th percentile (25 percent of the

data values are less than this value.)

Median

Data value within 1.5 times the IQR

outside the quartile

Chloride to bromide ratio in groundwater

samples from wells

75th percentile (75 percent of the

data values are less than this value.)

Interquartile

range (IQR)

Sample populations that share the same

letter symbol are not significantly different

at the 0.05 level of significance.

16 Number of samples

B

Figure 9. Boxplots of A, chloride, B, chloride to 

bromide ratio by geochemical groups, and C, scatter 

plot of chloride to bromide ratio compared to chloride 

by latitude.
References Cited 25

Summary and Conclusions

The U.S. Geological Survey, in cooperation with the 

Delaware Department of Agriculture designed a network of 

wells to monitor groundwater quality in the surficial aquifer 

of the Delaware Coastal Plain. Wells in existing groundwater 

networks throughout the Delmarva Peninsula were selected 

to assess water-quality conditions over the entire Delmarva 

Peninsula and many water-quality parameters, including 

nitrate. Because of the broad nature of these existing network 

designs, the number of wells located in settings where nitrate 

is present was small, limiting the use of statistical methods 

to determine whether changes in nitrate concentrations are 

occurring over time. The network designed for this study 

takes advantage of what was learned about the factors affecting spatial variability in nutrients and targets those settings 

(shallow wells at the water table, young groundwater [recently 

recharged], well-drained soils, and oxic groundwater) where 

changes in nitrate are most likely to be observed. Forty-eight 

wells were identified from two existing groundwater-monitoring networks in the surficial aquifer of the Delaware Coastal 

Plain and sampled for water quality in 2014. 

The wells sampled in 2014 were grouped based on 

their similarity in geochemistry using correlation and cluster 

analysis. These groups were effective in explaining some of 

the variability within the network, especially for nitrate and 

chloride concentrations. Although spatial patterns were not 

evident for nitrate, land-use patterns near wells appear to be a 

factor in the geochemistry of shallow groundwater. Results of 

this study showed that the highest median nitrate concentrations are in the group with the highest percentage of agricultural land use and the highest median chloride concentrations 

are in the group with the highest percentage of developed land 

use. There appears to be a spatial pattern in chloride concentrations, with higher values in the northern part of the study 

area that may be related to deicing activities. These groups of 

wells can be evaluated for trends independently of the entire 

network with future sampling, which could be useful in identifying where agricultural best management practices (BMPs) 

are most effective at reducing the leaching of nutrients to 

shallow groundwater. 

The implementation of agricultural BMPs across 

Delaware over the last several decades has had a goal of 

reducing the leaching of nitrate into shallow groundwater. In 

the surficial aquifer of the Delaware Coastal Plain, changes 

in water quality that may be attributed to agricultural BMPs 

may only become apparent with repeated monitoring over 

long periods of time. The results of this study are intended 

to establish water-quality conditions in 2014 to allow future 

comparison and evaluate the effectiveness of BMPs on a 

regional scale.

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quality in a shallow regional unconsolidated aquifer, 

Northern Atlantic Coastal Plain: U.S. Geological Survey 

Scientific Investigations Report 2008–5190, 21 p., 

accessed September 15, 2016, at https://pubs.usgs.gov/

sir/2008/5190/.

Ator, S.W. and Denver, J.M., 2015, Understanding nutrients 

in the Chesapeake Bay watershed and implications for 

management and restoration—The Eastern Shore: U.S. 

Geological Survey Circular 1406, 72 p., accessed April 7, 

2017, at http://dx.doi.org/10.3133/cir1406.

Ator, S.W., Denver, J.M., Krantz, D.E., Newell, W.L., and 

Martucci, S.K., 2005, A surficial hydrogeologic framework 

for the Mid-Atlantic Coastal Plain: U.S. Geological Survey 

Professional Paper 1680, 44 p., accessed April 7, 2017, at 

https://pubs.usgs.gov/pp/1680/report.pdf.

Bachman, L.J. and Wilson, J.M., 1984, The Columbia aquifer 

of the Eastern Shore of Maryland: Maryland Geological 

Survey Report of Investigations No. 40, 144 p., accessed 

April 7, 2017, at http://www.mgs.md.gov/publications/

report_pages/RI_40.html.

Blaier, S.C., and Baxter, S.J., 2000, The occurrence and 

distribution of several agricultural pesticides in Delaware’s 

shallow ground water: Delaware Geological Survey 

Report of Investigations No. 61, 23 p., accessed April 7, 

2017, at http://www.dgs.udel.edu/publications/ri61-occurrence-and-distribution-several-agricultural-pesticidesdelaware%E2%80%99s-shallow-gro.

Böhlke, J.K., 2002, Groundwater recharge and agricultural 

contamination: Hydrogeology Journal, v. 10, no. 1, 

p. 153–179, DOI: 10.1007/s10040-001-0183-3, accessed 

April 7, 2017, at http://link.springer.com/article/10.1007/

s10040-001-0183-3.

Brakebill, J.W. and Gronberg, J.M., 2017, County-level 

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fertilizer for the conterminous United States, 1987–2012: 

U.S. Geological Survey data release, accessed November 

15, 2016, at https://doi.org/10.5066/F7H41PKX. 

Burow, K.R., Jurgens, B.C., Belitz, Kenneth, and Dubrovsky, 

N.M., 2012, Assessment of regional change in nitrate concentrations in groundwater in the Central Valley, California, 

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no. 8, p. 2,609–2,621, accessed April 7, 2017, at http://

dx.doi.org/10.1007/s12665-012-2082-4. 
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Busenberg, Eurybiades, and Plummer, L.N., 2000, Dating 

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and anthropogenic sources of sulfur hexafluoride: Water 

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

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pesticide concentrations in the surficial aquifer on the 

Delmarva Peninsula: U.S. Geological Survey Scientific 

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Edited by Valerie M. Gaine.

Graphics and layout by Timothy W. Auer.

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Fleming and others—Water Quality in the Surficial Aquifer Near Agricultural Areas in the Delaware Coastal Plain, 2014—Scientific Investigations Report 2017–5054

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20175054