U.S. Department of the Interior

U.S. Geological Survey

EFFECTS OF LAND USE AND TRAVEL TIME ON THE 

DISTRIBUTION OF NITRATE IN THE KIRKWOODCOHANSEY AQUIFER SYSTEM IN SOUTHERN NEW 

JERSEY

Water-Resources Investigations Report 01-4117

DELAWARE BAY

CAPE MAY

CUMBERLAND

ATLANTIC

SALEM

GLOUCESTER

CAMDEN

MONMOUTH

OCEAN

BURLINGTON

MERCER

PENNSYLVANIA

NEW JERSEY

DELAWARE RIVER

A T L A N T I C

O

C

E

A

N

OUTCROP AREA

OF THE KIRKWOOD-COHANSEY AQUIFER

SYSTEM

STUDY AREA

National Water-Quality Assessment Program
EFFECTS OF LAND USE AND TRAVEL TIME ON THE 

DISTRIBUTION OF NITRATE IN THE KIRKWOODCOHANSEY AQUIFER SYSTEM IN SOUTHERN NEW 

JERSEY

By Leon J. Kauffman, Arthur L. Baehr, Mark A. Ayers, and Paul E. Stackelberg

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 01-4117

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

West Trenton, New Jersey

2001
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For additional information Copies of this report can be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

Mountain View Office Park Box 25286

810 Bear Tavern Road Denver, CO 80225-0286

West Trenton, NJ 08628
iii

CONTENTS

Page

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

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

Approach..............................................................................................................................4

Previous investigations ........................................................................................................5

Description of study area .................................................................................................................5

Hydrogeology ......................................................................................................................5

Population and land use .......................................................................................................7

Water-supply issues and ground-water use .......................................................................10

Methods .........................................................................................................................................12

Simulation of ground-water flow.......................................................................................12

Model discretization ..............................................................................................14

Boundary conditions and model stresses ...............................................................14

Determination of ground-water age and contributing areas to wells and streams.............17

Determination of nitrate concentration at discharge points ...............................................17

Estimation of nitrate concentration in recharge over time.................................................18

Model simulation results................................................................................................................19

Model calibration and evaluation ......................................................................................19

Age of water in the aquifer and relation to land use in the recharge area..........................20

Age of ground-water discharge and relation to land use in the recharge area...................27

Effects of land use and travel time on distribution of nitrate.........................................................31

Comparison of simulated and measured nitrate concentrations ........................................33

Response to management alternatives ...............................................................................34

Spatial distribution of changes in nitrate concentrations ...................................................37

Summary and conclusions .............................................................................................................41

References cited.............................................................................................................................46

ILLUSTRATIONS

Figure 1. Map showing location of Glassboro study area, New Jersey.........................................3

2. Map showing extent of modeled area and approximate extent of New Jersey 

Department of Environmental Protection Water-Supply Critical Area II in the 

Glassboro study area, New Jersey............................................................................6

3. Graph showing population in the Glassboro study area, New Jersey, 1930-90.............7

4. Maps showing land use in the Glassboro study area, New Jersey, in 1950, 1973,

1986, and 1996.........................................................................................................8

5. Graph showing land use in the Glassboro study area, New Jersey, 1950-96...............10

6. Graph showing ground-water withdrawals from public-supply wells in the

Kirkwood-Cohansey aquifer system, Glassboro study area, New Jersey,

1981-96 ..................................................................................................................12
iv

ILLUSTRATIONS--Continued

Page

Figure 7. Map showing location and average annual volume of water withdrawals from

streams and from the Kirkwood-Cohansey aquifer system in the Glassboro

study area, New Jersey, 1992-96............................................................................13

8. Graph showing average reported ground-water withdrawals from KirkwoodCohansey aquifer system by water-use category, Glassboro study area, New 

Jersey, 1992-96 ......................................................................................................14

9. Map of ground-water flow model showing (a) horizontal discretization and

location of active, inactive, and stream cells and (b) vertical discretization..........15

10. Graph showing mean concentration of nitrate in recharge to aquifer from areas of 

four land-use types, 1940-2000, and for three water management alternatives 

 used in model simulations.....................................................................................18

11. Map showing zones of hydraulic conductivity used in model calibration and final 

calibrated horizontal and vertical hydraulic conductivities in the modeled

area.........................................................................................................................20

12. Map showing difference between the simulated and target water levels in the

modeled area ..........................................................................................................21

13. Graph showing ground-water age determined with geochemical methods and 

simulated ground-water age...................................................................................24

14. Graph showing distribution of land use at the point of recharge by groundwater age for water in the Kirkwood-Cohansey aquifer system, Glassboro

study area, New Jersey...........................................................................................25

15. Map showing travel time required for water that recharges the surficial aquifer

at a given location to reach its discharge point, Glassboro study area,

New Jersey .............................................................................................................26

16. Vertical section showing the distribution of simulated ground-water age along 

profile A-A′, Glassboro study area, New Jersey....................................................27

17-19. Maps showing:

17. Vertically averaged age of water in the Kirkwood-Cohansey aquifer system, 

Glassboro study area, New Jersey.................................................................28

18. Simulated contributing areas to wells and streams and selected simulated 

flowpaths in the Glassboro study area, New Jersey......................................30

19. Simulated contributing areas to selected public-supply wells in the

Glassboro study area and associated travel times from recharge

area to well....................................................................................................31

20. Graphs showing distribution of land use at the point of recharge by ground-water

age for water discharging to (a) all wells and ponds, (b) streams, (c) ponds,

(d) industrial wells, (e) public-supply wells, and (f) irrigation wells, Glassboro 

study area, New Jersey...........................................................................................32

21. Map showing location of ground-water and surface-water sampling sites and

land use in basins draining to surface-water sites, Glassboro study area,

New Jersey .............................................................................................................34
v

ILLUSTRATIONS--Continued

Page

Figures

22-24. Graphs showing:

22. Simulated and measured nitrate concentrations in public-supply wells in the 

Glassboro study area, New Jersey.................................................................35

23. Simulated and measured nitrate concentrations at three surface-water sites, 

Glassboro study area, New Jersey, 1987-99 .................................................36

24. Simulated nitrate concentrations in streams and public-supply wells 

determined from historical nitrate input and simulated response to three 

management alternatives for future nitrate input in recharge, Glassboro 

study area, New Jersey..................................................................................37

25-29. Maps showing:

25. Simulated nitrate concentration in streams in the Glassboro study area,

New Jersey, in 2000 and in 2050 with the assumption of no future

change in land use, withdrawals, or nitrate concentration in recharge .........38

26. Change in simulated nitrate concentration in streams in the Glassboro

study area, New Jersey, from 2000 to 2050 with the assumption of no

future change in land use, withdrawals, or nitrate concentration in

recharge.........................................................................................................39

27. Simulated nitrate concentration in a “typical” domestic well installed in the 

Kirkwood-Cohansey aquifer system in the Glassboro study area, New 

Jersey, in 2000 and in 2050 with the assumption of no future change in land 

use, withdrawals, or nitrate concentration in recharge..................................40

28. Simulated nitrate concentration in a “typical” public-supply well installed

in the Kirkwood-Cohansey aquifer system in the Glassboro study area, 

New Jersey, in 2000 and in 2050 with the assumption of no future change 

in land use, withdrawals, or nitrate concentration in recharge......................42

29. Change in simulated nitrate concentration from 2000 to 2050 in “typical” 

domestic and public-supply wells installed in the Kirkwood-Cohansey 

aquifer system in the Glassboro study area, New Jersey, with the assumption 

of no future change in land use, withdrawals, or nitrate concentration in 

recharge.........................................................................................................43

TABLES

Table 1. Land use and change in land use in the Glassboro study area, New Jersey, 1950-96 ....11

2. Geochemical and simulated ages of water samples from monitoring wells in the 

Glassboro study area, New Jersey................................................................................22

3. Relation of porosity to difference between simulated and geochemical groundwater ages in flow model of Glassboro study area, New Jersey..................................24
vi

CONVERSION FACTORS, VERTICAL DATUM, AND

ABBREVIATED WATER-QUALITY UNITS

Multiply By To obtain

Length

inch (in.) 25.4 millimeter

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square mile (mi2) 2.590 square kilometer

Volume

million gallons (Mgal) 3,785 cubic meter

Flow

gallon per day (gal/d) 0.003785 cubic meter per day

Temperature

degree Fahrenheit (°F) °C = 5/9 x (°F-32) degree Celsius (°C)

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929-- a 

geodetic datum derived from a general adjustment of the first-order level nets of the United 

States and Canada, formerly called Sea Level Datum of 1929.

Water-quality abbreviations:

mg/L -milligrams per liter

N -nitrogen
vii

FOREWORD

The U.S. Geological Survey (USGS) is 

committed to serve the Nation with accurate and 

timely scientific information that helps enhance 

and protect the overall quality of life, and facilitates effective management of water, biological, 

energy, and mineral resources. Information on the 

quality of the Nation’s water resources is of critical 

interest to the USGS because it is so integrally 

linked to the long-term availability of water that is 

clean and safe for drinking and recreation and that 

is suitable for industry, irrigation, and habitat for 

fish and wildlife. Escalating population growth and 

increasing demands for the multiple water uses 

make water availability, now measured in terms of 

quantity and quality, even more critical to the longterm sustainability of our communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program to

support national, regional, and local information 

needs and decisions related to water-quality management and policy. Shaped by and coordinated 

with ongoing efforts of other Federal, State, and 

local agencies, the NAWQA Program is designed 

to answer: What is the condition of our Nation’s 

streams and ground water? How are the conditions 

changing over time? How do natural features and 

human activities affect the quality of streams and 

ground water, and where are those effects most 

pronounced? By combining information on water 

chemistry, physical characteristics, stream habitat, 

and aquatic life, the NAWQA Program aims to 

provide science-based insights for current and 

emerging water issues. NAWQA results can contribute to informed decisions that result in practical 

and effective water-resource management and 

strategies that protect and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 

50 of the Nation’s most important river basins and 

aquifers, referred to as Study Units. Collectively, 

these Study Units account for more than 60 percent 

of the overall water use and population served by 

public water supply, and are representative of the 

Nation’s major hydrologic landscapes, priority 

ecological resources, and agricultural, urban, and 

natural sources of contamination.

Each assessment is guided by a nationally 

consistent study design and methods of sampling 

and analysis. The assessments thereby build local 

knowledge about water-quality issues and trends in 

a particular stream or aquifer while providing an 

understanding of how and why water quality varies 

regionally and nationally. The consistent, multiscale approach helps to determine if certain types 

of water-quality issues are isolated or pervasive, 

and allows direct comparisons of how human 

activities and natural processes affect water quality 

and ecological health in the Nation’s diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile 

organic compounds, trace metals, and aquatic 

ecology are developed at the national scale through 

comparative analysis of the Study-Unit findings.

The USGS places high value on the communication and dissemination of credible, timely, and 

relevant science so that the most recent and 

available knowledge about water resources can be 

applied in management and policy decisions. We 

hope this NAWQA publication will provide you 

the needed insights and information to meet your 

needs, and thereby foster increased awareness and 

involvement in the protection and restoration of 

our Nation’s waters.

The NAWQA Program recognizes that a 

national assessment by a single program cannot 

address all water-resource issues of interest. 

External coordination at all levels is critical for a 

fully integrated understanding of watersheds and 

for cost-effective management, regulation, and 

conservation of our Nation’s water resources. The 

Program, therefore, depends extensively on the 

advice, cooperation, and information from other 

Federal, State, interstate, Tribal, and local 

agencies, non-government organizations, industry, 

academia, and other stakeholder groups. The assistance and suggestions of all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water
1

EFFECTS OF LAND USE AND TRAVEL TIME ON THE 

DISTRIBUTION OF NITRATE IN THE KIRKWOOD-COHANSEY 

AQUIFER SYSTEM IN SOUTHERN NEW JERSEY

By Leon J. Kauffman, Arthur L. Baehr, Mark A. Ayers,

and Paul E. Stackelberg

ABSTRACT

Residents of the southern New Jersey Coastal 

Plain are increasingly reliant on the unconfined 

Kirkwood-Cohansey aquifer system for public 

water supply as a result of increasing population 

and restrictions on withdrawals from the deeper, 

confined aquifers. Elevated nitrate concentrations 

above background levels have been found in wells 

in the surficial aquifer system in agricultural and 

urban parts of this area. A three-dimensional 

steady-state ground-water-flow model of a 400-

square-mile study area near Glassboro, New 

Jersey, was used in conjunction with particle 

tracking to examine the effects of land use and 

travel time on the distribution of nitrate in ground 

and surface water in southern New Jersey.

Contributing areas and ground-water ages, or 

travel times, of water at ground-water discharge 

points (streams and wells) in the study area were 

simulated. Concentrations of nitrate were 

computed by linking land use and age-dependent 

nitrate concentrations in recharge to the discharge 

points. Median concentrations of nitrate in water 

samples collected during 1996 from shallow monitoring wells in different land-use areas were used 

to represent the concentration of nitrate in aquifer 

recharge since 1990. On the basis of upward trends 

in the use of nitrogen fertilizer, the concentrations 

of nitrate in aquifer recharge in agricultural and 

urban areas were assumed to have increased 

linearly from the background value in 1940 (0.07 

mg/L as N) to the 1990 (2.5-14 mg/L as N) concentrations.

Model performance was evaluated by comparing the simulation results to measured nitrate 

concentrations and apparent ground-water ages. 

Apparent ground-water ages at 32 monitoring 

wells in the study area determined from tritium/

helium-3 ratios and sulfur hexafluoride concentrations favorably matched simulated travel times to 

these wells. Simulated nitrate concentrations were 

comparable to concentrations measured in 27 

water-supply wells in the study area. A time series 

(1987-98) of nitrate concentrations at base-flow 

conditions in three streams that drain basins of 

various sizes and with various land uses was 

compared to simulated concentrations in these 

streams. In all three of the streams, a reasonable fit 

to the measured concentrations was achieved by 

multiplying the simulated concentration by 0.6. 

Because nitrate appeared to move conservatively 

(not degraded or adsorbed) in ground water to 

wells, the apparent non-conservative behavior in 

streams indicates that about 40 percent of the 

nitrate in aquifer recharge is removed by denitrification in the aquifer near the streams and (or) by 

in-stream processes.

The model was used to evaluate the effects of 

various nitrate management options on the concentration of nitrate in streams and water-supply wells. 

Nitrate concentrations were simulated under the 

following management alternatives: an immediate 

ban on nitrate input, reduction of input at a 

constant rate, and fixed input at the current (2000) 

level. The time required for water to move through 

the aquifer results in a time lag between the 

reduction of nitrate input in recharge and the 

reduction of nitrate concentration in streams and 
2

wells. In the gradual-reduction alternative, nitrate 

concentrations in streams and wells continued to

increase for several years after the reduction was 

enacted. In both the immediate-ban and gradualreduction alternatives, nitrate concentrations

remained elevated above background concentrations long after nitrate input ceased. In the fixeduse alternative, concentrations in streams and wells 

continued to increase for 30 to 40 years before

reaching a constant level.

The spatial distributions of simulated nitrate 

concentrations in streams in 2000 and 2050 were 

examined with the assumption of no change in land 

use, nitrate concentration in recharge, or groundwater withdrawals. As expected, nitrate concentrations were highest in agricultural areas and lowest 

in largely undeveloped areas. Changes in concentrations over time were greatest in streams in areas 

where the aquifer is thick and in streams that flow 

mostly through areas that are undeveloped but

whose contributing areas contain agricultural or

urban land distant from the stream. Results of the 

computer simulations indicate that nitrate concentrations in typical domestic or public-supply wells 

installed in most of the study area would increase 

over the next 50 years. The extremes in nitrate concentration (high and low) and magnitude of change 

in nitrate concentration occurred in domestic wells 

rather than public-supply wells because the

domestic wells intercept water derived from small 

contributing areas with fairly uniform land use and 

ground-water-age composition. Nitrate concentrations in water from public-supply wells were less 

extreme than in domestic wells because the publicsupply wells’ contributing areas supply water from 

multiple land uses and ground-water-age classes.

INTRODUCTION

Residents of the southern New Jersey Coastal 

Plain are increasingly reliant on the unconfined

Kirkwood-Cohansey aquifer system for public

water supply as a result of increasing population

and restrictions on withdrawals from the deeper,

confined aquifers. Elevated nitrate concentrations 

above background levels have been found in wells 

in the surficial aquifer system in agricultural and 

urban parts of this area.

The study area encompasses approximately 

400 mi2

 in the Philadelphia metropolitan area near 

Glassboro, New Jersey (fig. 1), and is underlain by 

the Kirkwood-Cohansey aquifer system. In this 

report, this area is referred to as the Glassboro 

study area. Population growth in this area has 

resulted in increased ground-water withdrawals 

from the Kirkwood-Cohansey aquifer system. The 

primary objective of the study was to provide a 

detailed understanding of the relation among land 

use, ground-water flow, and the evolution of water 

quality in a heavily used surficial aquifer at a scale 

applicable to the evaluation of water-management 

alternatives. Three-dimensional numerical simulations of ground-water flow and particle transport 

were linked to data sets in a geographical information system (GIS) to describe the source and age of 

water currently (2000) within and discharging from 

the aquifer system. (In this report, ground-water 

age refers to the time elapsed since the water was 

recharged to the saturated zone of the aquifer 

system.) Although the ground-water-flow model 

could be used to study the movement of any 

surface-introduced contaminant, the scope of the 

application of the model was limited to simulating 

nitrate concentrations in streams and wells given 

current land use and three hypothetical nitratemanagement alternatives.

The Kirkwood-Cohansey aquifer system is 

the principal surficial (unconfined) aquifer system 

in the New Jersey Coastal Plain (Zapecza, 1989). 

The aquifer, a major source of drinking water, is 

tapped by both domestic wells and large publicsupply wells (Nawyn and Clawges, 1995). Ground 

water affects stream ecology in this area because 

80 percent or more of streamflow is derived from 

ground-water discharge. The quality of the water 

in the aquifer, especially water discharging to wells 

and streams, is, therefore, of great interest for 

human health and aquatic life.

Surficial aquifers in urbanized and agricultural areas are vulnerable to contamination because 

they receive recharge across the entire land 
3

Base from U.S. Geological Survey, Wilmington,

Hammonton 1:250,000 quadrangles, 1969,

Universal Transverse Mercator Projection, Zone 18

SUSSEX

KENT

DELAWARE BAY

CAPE MAY

CUMBERLAND

ATLANTIC

SALEM

GLOUCESTER

CAMDEN

MONMOUTH

OCEAN

BURLINGTON

MERCER

BUCKS

PHILADELPHIA

DELAWARE

NEW

CASTLE

PENNSYLVANIA

NEW JERSEY

DELAWARE

DELAWARE RIVER

39°

30'

40°

00'

39°

00'

75°30' 75°00' 74°30' 74°00'

A T L A

N

T

I

C

O

C

E

A

N

0 10 20 MILES

0 10 20 KILOMETERS

Outcrop area of the KirkwoodCohansey aquifer system

Glassboro study area

EXPLANATION

Figure 1. Location of Glassboro study area, New Jersey.

surface, allowing for downward migration of compounds, such as nutrients, pesticides, and volatile 

organic compounds, that are used at the land 

surface. The Kirkwood-Cohansey aquifer system 

particularly is vulnerable because it consists of 

highly permeable, unconsolidated sands and

gravels, contains little organic matter, and

generally has a shallow water table (average 15 ft 

below land surface). Sources of nitrate (for

example, residential and agricultural fertilizers and 

rainfall) are widespread, and concentrations of 

nitrate greater than background levels were 

measured in ground water in observation wells 

(Stackelberg and others, 1997; Szabo and others, 

1997), domestic wells (MacLeod and others, 

1995), and public-supply wells (Stackelberg and 

others, 2000) throughout the Glassboro study area. 

The background concentration is considered to be 

the median concentration of nitrate in shallow 

wells in undeveloped areas. Similar findings were 
4

obtained in many other regions of the country in 

investigations conducted as part of the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program (Nolan and Stoner, 

2000).

To evaluate the effect of water quality in the 

shallow part of the Kirkwood-Cohansey aquifer 

system on the quality of water discharging to 

streams and wells, movement through the aquifer 

system must be considered. Because the path of 

the water through the aquifer varies in length and 

direction, the water discharging from the aquifer to 

streams and wells has a variety of sources and 

ages. Ground-water age can affect the concentration of a contaminant various ways. The concentration at the time of recharge (initial concentration) may change as a result of changes in chemical 

use over time, may be reduced through degradation 

or sorption, or may increase as the contaminant is 

formed through degradation of another contaminant.

This report (1) describes the ground-waterflow model of the study area; (2) describes the age 

and land-use signature of water in the KirkwoodCohansey aquifer system and water discharging to 

streams and wells in the study area; (3) presents the 

results of a simulation of the effects of several 

management alternatives on nitrate concentrations 

in the study area; and (4) evaluates the performance of the model by comparing measured and 

simulated water levels, ground-water ages, and 

nitrate concentrations.

Approach

This investigation was conducted as part of 

the Long Island-New Jersey (LINJ) NAWQA 

study unit. The data-collection and modeling 

approach used was a unique integration of groundwater components of NAWQA studies referred to 

as “land-use surveys” and “flow-path studies.” 

This integration allowed current water-quality conditions to be related to information on current and 

historical land and chemical use to determine 

potential future water-quality conditions in an 

aquifer that is important for water supply and 

whose recharge area encompasses multiple land 

uses.

In a land-use survey, water in shallow observation wells is sampled to assess the quality of 

recently recharged ground water associated with a 

particular land use. Results of land-use surveys 

conducted in the Glassboro study area (Stackelberg 

and others, 1997) were used to determine waterquality conditions associated with urban, agricultural, and undeveloped areas.

Flow-path studies completed as a part of 

NAWQA study units across the country generally 

can be characterized as cross-sectional studies (see, 

for example, Burow and others, 1999; Cowdery, 

1997; Mullaney and Grady, 1997; Saad and 

Thorstenson, 1998; Tesoriero and others, 2000) in 

which observation wells are located along a single 

perceived ground-water-flow path to conduct a 

detailed investigation of processes of transport 

from a single source. Wells sequenced in a crosssection provide water-quality information for 

ground water of different ages originating from a 

small recharge area.

The flow-path studies in the LINJ study 

differed from the cross-sectional studies used in 

many other NAWQA studies. In this study, a 10- 

to 15-year age group sampling program (Stackelberg and others, 2000) was designed based on 

simulated ground-water ages from a three-dimensional flow model to locate the screened depths of 

30 observation wells beneath urbanized land in the 

Glassboro study area. This flow-path study was 

similar to a land-use survey, except that water from 

these wells was sampled to assess the quality of 

ground water recharged in urban areas 10 to 15 

years prior to sampling. In contrast to the approach 

used in a typical NAWQA flow-path study, the 

approach used here results in water-quality information for ground water in a single age group over 

a large recharge area. In the second part of the 

LINJ flow-path study, water from public-supply 

wells in the area was sampled to assess the quality 

of water discharging from the aquifer (Baehr and 

others, 1999; Stackelberg and others, 2000).

The data-collection and modeling approach 

used in this study can be thought of as a regional 

flow-path study. Samples of water at the beginning 

(land-use surveys), middle (age-cohort sampling), 

and end (public-supply wells) of flow paths 

through the aquifer were collected and analyzed. 
5

The three-dimensional flow model was used to

relate the concentrations of contaminants in water 

from observation wells to the concentrations of 

contaminants in water discharged to streams and 

wells. The modeling approach provided a way to 

investigate past and future trends in water quality.

The ground-water modeling approach used in 

this study fundamentally is different from that of 

traditional quasi-three-dimensional plan-view

ground-water flow models, which are designed

primarily to estimate basin-wide water budgets.

These models typically use the vertical dimension 

to represent distinct geologic layers. Such a model 

encompassing most of the Glassboro study area, 

with a focus on the Maurice River Basin, has been 

constructed (Stephen Cauller, U.S. Geological

Survey, written commun., 2000). In the modeling 

approach used here, however, model layers in the 

vertical dimension are added, not to represent

distinct geologic layers, but to discretize the

vertical dimension in order to refine model definition of flow paths and travel times from point of 

recharge to point of discharge.

Previous Investigations

Zapecza (1989) described the hydrogeologic 

framework of the New Jersey Coastal Plain. Watt 

and Johnson (1992), Lacombe and Rosman (1995), 

Johnson and Charles (1997), and Charles and

others (2001) provide information on the hydrogeologic framework and geochemistry of, water 

levels, water use, and base flow in, and a general 

hydrologic budget for various parts of the surficial 

aquifer system underlying the study area. Martin 

(1998) simulated flow in the entire New Jersey 

Coastal Plain with an emphasis on water budget. 

Stephen Cauller (U.S. Geological Survey, written 

commun., 2000) simulated ground-water flow in 

the Kirkwood-Cohansey aquifer system in the

Glassboro study area to investigate the effect of 

ground-water withdrawals on base flow. This

model was designed to focus on ground-water flow 

to the Maurice River and its tributaries.

Rice and Szabo (1997) used two-dimensional 

ground-water-flow models of three vertical

sections in the Glassboro study area to simulate 

ground-water-flow paths and travel times. The

simulations were used to determine stratification of 

ground-water age and its relation to concentrations 

of nitrate and radium concentrations in the aquifer 

system.

Modica and others (1998) simulated groundwater flow in the Kirkwood-Cohansey aquifer 

system in the Cohansey River Basin to determine 

the source and residence time of ground-water 

flow to streams. In this study, contributing areas to 

six stream transects were delineated and the age 

distribution of the ground-water discharge along 

two transects was defined. Ground-water ages, 

estimated from chlorofluorocarbon (CFC) concentrations in water extracted from about 2 ft below 

the streambed, corroborated the simulated age distribution of ground-water flow to the stream 

transects. The relation of the fraction of groundwater flow to the stream transects affected by 

nitrate contamination to time was shown for the 

case where contamination continues and for the 

case where the source of contamination is eliminated.

DESCRIPTION OF STUDY AREA

The northwestern boundary of the study area 

closely follows the outcrop of the Kirkwood Formation. The remaining boundaries are formed by 

the basins that drain to the following surface-water 

data-collection sites: Cohansey River at Seeley, 

Maurice River at Millville, Hospitality Branch near 

Folsom, and Great Egg Harbor River at Folsom. 

Thirty-five municipalities lie at least partly within 

the study area (fig. 2). The largest towns in the 

area by population are Vineland City and 

Glassboro Borough.

Hydrogeology

The Kirkwood-Cohansey aquifer system 

consists of a southeastward-dipping wedge of 

unconsolidated sediments, which include gravel, 

sand, silt, and clay. The thickness of the aquifer 

system in the study area ranges from less than 25 ft 

at the northwestern boundary to 300 ft in the southeast. Within the Glassboro study area, the 

Grenloch Sand Member of the Kirkwood Formation, the Cohansey Sand, and, where present, the 

overlying Bridgeton Formation are hydraulically 

connected and function together as the unconfined 

Kirkwood-Cohansey aquifer system (Zapecza, 
6

Great Egg

Harbor River

Hospitality

Bra nch

Maur ci e River

Cohan sey R vi er

Great Egg

Harbor River

Hospitality

Bra nch

Maur ci e River

Cohan sey R vi er

Deerfield 

Township

Vineland

City

Pittsgrove

Township

Upper

Deerfield

Township

Alloway

Township

Hopewell

Township

Elmer

Borough

Upper

Pittsgrove

Township Franklin Township

Newfield

Borough

Buena

Borough

Buena Vista

Township

Folsom

Borough

Monroe Township

Clayton

Borough Elk Township Pilesgrove 

Township

South Harrison

 Township

Harrison

Township

Mantua Township

Washington

Township

Pitman

Borough Winslow Township

Gloucester

Township

Evesham

Voorhees Township Township

Gibbsboro

Borough

Berlin

Township

Berlin

Borough

Pine

Valley

Borough

Lindenwold

Borough

Clementon

Borough

Pine

Hill

Borough

Glassboro

Borough

39

30'°

39

45'°

75 15' ° 75 °

Water-Supply Critical Area II--

From N.J. Department of

Environmental Protection

unpublished, 1:250,000.

Boundaries are approximate

and should not be used for

regulatory compliance

purposes.

Modeled area

EXPLANATION

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

CUMBERLAND

SALEM GLOUCESTER

CAMDEN

Figure 2. Extent of modeled area and approximate extent of New Jersey Department of 

Environmental Protection Water-Suppy Critical Area II in the Glassboro study area, 

New Jersey.
7

1989). The Grenloch Sand Member in this area is 

fine to medium sand and silty sand (Zapecza,

1989). The Cohansey Sand is predominantly

medium- to coarse-grained sand with some gravel 

and silt and interbedded clay (Rhodehamel, 1973). 

The Bridgeton Formation is discontinuous

throughout the study area, and is found on topographic highs. The Bridgeton Formation is coarsegrained sand and gravel. The Alloway Clay

Member of the Kirkwood Formation underlies the 

Kirkwood-Cohansey aquifer system throughout the 

study area and functions as a continuous and

competent confining bed (Nemickas and Carswell, 

1976).

Average annual precipitation is about 44 in/yr 

and is distributed nearly uniformly throughout the 

year. About 3 in/yr of precipitation runs off directly 

into streams; the remainder is accounted for by 

evapotranspiration (ET) or recharge to ground

water. The average annual ET is about 25 in/yr. ET 

is highest in the summer months, when the temperature is highest and plant growth is greatest. About 

18 in/yr of water is recharged to the ground water 

(Lacombe and Rosman, 1995). Streams in the 

study area generally are gaining streams--that is, 

ground water flows into the streams.

Population and Land Use

The population of the Glassboro study area 

during 1930-90 (fig. 3) was estimated from U.S. 

Census data (New Jersey Department of Labor, 

2000). Population data are available for individual 

municipalities at 10-year intervals. Many of the 

municipalities lie on the boundary of the study area; 

in these cases, the population of the municipality 

was scaled by the percentage of the municipality 

that lies within the study area.

Since 1960, the population of the townships in 

the study area has grown faster than the population 

of the boroughs and Vineland City (fig. 3). The 

boroughs and the city tend to be more densely populated, whereas townships historically are more 

rural and less densely populated.

Areas of urban, agricultural, and undeveloped 

land are distributed throughout the study area (fig. 

4); however, the western part of the study area is 

predominantly agricultural and the northeastern 

1930 1940 1950 1960 1970 1980 1990

YEAR

0

50

100

150

200

Entire study area

Boroughs

Townships

POPULATION, IN THOUSANDS

Figure 3. Population in the Glassboro study area, New Jersey, 1930-90. 

(Data from New Jersey Department of Labor, 2000)
8

Urban, residential

Urban, nonresidential

Cropland/pasture

Orchard/nursery

Forested

Water/wetlands

Barren

EXPLANATION

LAND-USE CLASS

(N.J. Department of Environmental Protection, 1996b,

modified with U.S.Geological Survey, 1986, and U.S.

Geological Survey, Rolla, Mo., unpub. data accessed

July 9,2000, on the World Wide Web at URL

ftp://ftpmcmc.er.usgs.gov/release/urban_dynamics/,

TIFF format)

(N.J. Department of Environmental Protection, 1996b,

modified with U.S.Geological Survey, 1986)

(N.J. Department of Environmental Protection, 1996b) (N.J. Department of Environmental Protection, 1996b,

modified with data on CD-ROM obtained from Steven

Carp, New Jersey Office of State Planning, Trenton,

N.J., 1999)

1950 1973

1986 1996

39

30'°

39

45'°

75 15' °

75 °

39

30'°

39

45'°

75 15' °

75 °

39

30'°

39

45'°

75 15' °

75 °

39

30'°

39

45'°

75 15' °

75 °

Figure 4. Land use in the Glassboro study area, New Jersey, 1950, 1973, 1986, and

1996.
9

part is predominantly urban land developed during 

the past 25 years. Vineland City in the southern 

part of the study area is another major tract of 

urban land. The largest tracts of undeveloped land 

are in the southeastern part of the study area.

The land-use data sets used in this investigation are based largely on the New Jersey Integrated 

Terrain Unit (ITU) GIS digital data set from 1986 

(N.J. Department of Environmental Protection, 

1996b). The ITU data set was based on interpretation of digital aerial photography. To look at 

change in land use over time, three additional landuse digital data sets were used to identify areas of 

land-use change: land cover from 1950’s USGS 

topographic quadrangle sheets (U.S. Geological 

Survey, Rolla, Mo., unpub. data accessed July 9, 

2000, on the World Wide Web at URL ftp://ftpmcmc.er.usgs.gov/release/urban_dynamics/, TIFF 

format); the Geographic Information Retrieval and 

Analysis System (GIRAS) land-use digital data set 

from the early 1970’s (U.S. Geological Survey, 

1986); and a digital data set of areas developed 

between 1986 and the mid-1990’s (data on CDROM obtained from Steven Carp, New Jersey 

Office of State Planning, Trenton, N.J., 1999).

The GIRAS land-use data were mapped at a 

scale of 1:250,000 and, therefore, are much coarser 

than the ITU data (mapped at a scale of 1:24,000). 

The coarse scale results in overestimation of the 

amount of agricultural and urban area and underestimation of the amount of undeveloped area in 

GIRAS. Because the difference in scale did not 

allow direct comparison of the ITU and GIRAS 

data sets, the GIRAS data set was used only to 

modify the ITU data set to represent 1973 land use. 

To create the 1973 land-use data set, the land-use 

designations from GIRAS were used when (1) ITU 

land use was urban and GIRAS was agricultural, 

forested, wetlands, or barren; (2) GIRAS land use 

was agricultural and ITU was brushland; (3) 

GIRAS land use was barren and ITU was agricultural, forested, or artificial lake; and (4) ITU land 

use was barren and GIRAS was agricultural or 

forested. For all other cases (areas in which land 

use likely did not change from 1973 to 1986), the 

land-use designations from the ITU data set were 

used.

The 1950 land-use data set was created by 

modifying the 1973 land-use data set described 

above on the basis of data from the scanned USGS 

topographic quadrangle sheets from the 1950’s. 

Areas were delineated as urban, forested, water, or 

open (assumed to be agricultural). Only land use 

classified as urban in 1973 that was not urban in 

the 1950 data set was changed to the land use from 

the 1950 data set. Otherwise, the 1973 land use 

was used. This method likely results in underestimating the amount of urban land and overestimating the amount of agricultural land because the 

method of extracting land use from topographic 

sheets identifies urban land only in the town 

centers and not in more rural areas. The method 

also does not take into account changes in land use 

other than the change to urban land.

Areas of new development were added to the 

1986 land-use data set to create the 1996 land-use 

data set. The digital data set of new development, 

created by using digital orthophoto quadrangles 

(DOQ’s) from 1995 and 1997, was obtained from 

the New Jersey Office of State Planning, Trenton, 

N.J. To create the 1996 land-use data set, the areas 

of new development were assumed to be residential land; all other areas were assigned the value 

from the 1986 data set.

Comparison of the four land-use data sets 

(figs. 4 and 5) shows an increase in the amount of 

urban land (21 percent) and a corresponding 

decrease in the amount of agricultural (12 percent) 

and undeveloped (forested and water/wetlands) 

land (9 percent). Not including water and 

wetlands, the study area changed from a fairly even 

mix of agricultural and forested land use in 1950 to 

a fairly even mix of agricultural, forested, and 

urban land use in the mid-1990’s. The large 

increase in urban land use between 1973 and 1986 

in areas distant from the town centers corresponds 

with the period of rapid growth in population of the 

townships (fig. 3). The land-use percentages for 

the study area shown in figure 4 are listed in 

table 1.
10

1950 1960 1970 1980 1990 2000

YEAR

0

10

20

30

40

50

Agricultural

Forested

Water/wetlands

Urban

PERCENT OF STUDY AREA

Figure 5. Land use in the Glassboro study area, New Jersey, 1950-96. [The asterisks 

denote times for which land-use data sets are available. (N.J. Department of Environmental 

Protection, 1996b; U.S. Geological Survey, Rolla, Mo., unpub. data accessed July 9, 2000, 

on the World Wide Web at URL ftp://ftpmcmc.er.usgs.gov/release/urban_dynamics/, TIFF 

format; U.S. Geological Survey, 1986; data on CD ROM obtained from Steven Carp, New 

Jersey Office of State Planning, Trenton, N.J., 1999)]

Water-Supply Issues and

Ground-Water Use

The New Jersey Department of Environmental Protection (NJDEP) has recommended

increased development of the Kirkwood-Cohansey 

aquifer system in the study area to meet a portion

of the water demand caused by suburban growth

and reduced pumping from deeper, confined

aquifers (N.J. Department of Environmental Protection, 1996a). Part of the study area lies within

NDJEP Water-Supply Critical Area II (fig. 2). The 

critical-area designation was made as a result of

the presence of a large water-level depression in

the underlying Potomac-Raritan-Magothy aquifer

system caused by years of appreciable groundwater withdrawals. As part of the critical-area

mandate, water allocations from the PotomacRaritan-Magothy aquifer system within the Critical 

Area were reduced by an average of 20 percent. 

As a result of increasing population and, to a lesser 

degree, restricted pumping from deeper aquifers in 

the 1990’s, withdrawals from the KirkwoodCohansey aquifer system in the study area 

increased by about 2,000 Mgal/yr from 1981 to 

1996 (fig. 6).

Ground-water-use data were obtained from 

computer files supplied by the NJDEP Bureau of 

Water Allocation. Water use from three sources is 

reported: wells, ponds created by excavating soil 

beneath the water table, and water that is part of the 

surface drainage system (streams and ponds 

created by impounding or diverting surface water). 

In this report, the first two sources are considered 

to be ground-water withdrawals. The locations of 

the withdrawals in the study area are shown in 
11

Table 1. Land use and change in land use in the Glassboro study area, New Jersey,

1950-96

[Data from N.J. Department of Environmental Protection, 1996b; U.S. Geological Survey, 

Rolla, Mo., unpub. data accessed July 9, 2000, on the World Wide Web at URL ftp://

ftpmcmc.er.usgs.gov/release/urban_dynamics/, TIFF format; U.S. Geological Survey, 

1986; data on CD-ROM obtained from Steven Carp, New Jersey Office of State Planning, 

Trenton, N.J., 1999]

Anderson Percent from year indicated to 

Level 2 Percent of study area 1996

classificaLand-use category tion1 1950 1973 1986 1996 1950 1973 1986

Total urban 1.79 11.75 20.80 24.63 18.04 10.58 3.83

Residential 11 1.33 8.78 15.54 19.36 1.30 .39 .00

Commercial 12 .26 1.17 1.56 1.56 .59 .25 .00

Industrial 13 .08 .42 .67 .67 1.16 .79 .00

Transportation 14 .01 .39 1.17 1.17 .02 .00 .00

Industrial/commercial 15 .01 .02 .02 .00 .00 .00

Mixed urban 16 .00 .00 .00 .00 .99 .57 .00

Other urban 17 .07 .49 1.06 1.06 .74 .30 .00

Recreational 18 .04 .49 .78 .78 -12.21 -7.50 -1.48

Total agricultural 42.18 37.48 31.46 29.98 -11.01 -6.31 -1.03

Cropland 21 37.37 32.67 27.40 26.37 -.88 -.88 -.37

Orchard 22 4.03 4.03 3.51 3.15 -.24 -.24 .00

Feedlot 23 .25 .25 .01 .01 -.08 -.08 -.08

Other agricultural 24 .54 .54 .54 .46 -1.64 -5.38 -2.35

Total undeveloped 56.03 50.77 47.74 45.39 -2.09 -2.09 -.90

Deciduous 41 13.75 13.75 12.56 11.66 -.52 -.52 -.25

Coniferous 42 4.59 4.59 4.32 4.07 -8.47 -3.35 -.85

Mixed forest 43 17.08 11.96 9.47 8.61 .93 .93 -.34

Brushland 44 2.03 2.03 3.31 2.96 .00 .00 .00

River 51 .08 .08 .08 .08 .00 .00 .00

Lake 52 .03 .03 .03 .03 -.09 .05 .00

Artificial lake 53 1.13 .98 1.03 1.03 -.28 -.28 .00

Wetland 62 15.88 15.88 15.60 15.60 .00 .00 .00

Beaches 71 .00 .00 .00 .00 .41 .41 .00

Mining 73 .40 .40 .81 .81 .13 .13 .00

Altered land 74 .17 .17 .30 .30 -.34 -.34 .00

Transitional 75 .42 .42 .09 .09 -.33 -.33 .00

Undifferentiated barren 76 .39 .39 .07 .07 .00 .00 .00

Modified wetlands 80 .07 .07 .07 .07 .00 .00 .00

1 Anderson and others, 1976.
12

1980 1984 1988 1992 1996

YEAR

0

1,500

3,000

4,500

6,000

ANNUAL GROUND-WATER WITHDRAWALS, 

IN MILLION GALLONS

Figure 6. Ground-water withdrawals from public-supply wells in the 

Kirkwood-Cohansey aquifer system, Glassboro study area, New Jersey, 

1981-96. (Unpublished data on file at U.S. Geological Survey office, West 

Trenton, N.J.)

figure 7. The water-use data are compiled for four 

types of users: agricultural irrigation wells, publicsupply systems with allocations greater than 

100,000 gal/d, other ground-water users with allocations greater than 100,000 gal/d (mostly industrial), and users with allocations less than 100,000 

gal/d (small public-supply systems, fire companies, schools, and small industry). Water use is not 

reported for domestic wells serving individual 

households; however, the amount of water

withdrawn from domestic wells was estimated by 

multiplying the number of people served by private 

wells, determined from 1990 census data

(U.S. Bureau of the Census, 1992), by a per capita 

coefficient of 82 gal/d (Nawyn, 1998).

Public (38 percent) and domestic (13 percent) 

water supply together account for about half of the 

water withdrawn from the Kirkwood-Cohansey 

aquifer system (fig. 8). The remaining 49 percent is 

withdrawn for industrial and irrigation purposes.

METHODS

In this section, the methods used to simulate 

ground-water flow and nitrate concentrations are 

described. Results of the ground-water-flow simulation were used as input to a particle-tracking 

program which, together with results of GIS landuse analysis, allowed simulation of nitrate concentrations in ground water from wells and streams.

Simulation of Ground-Water Flow

A numerical model, the USGS three-dimensional finite-difference code MODFLOW-96 

(Harbaugh and McDonald, 1996a, 1996b), was 

used to simulate ground-water flow and head distributions across the study area. The results of this 

simulation were used as input to the particletracking program MODPATH (Pollock, 1994) that 

is used to delineate recharge areas and compute 

travel times through the aquifer.
13

°

°

°

°
14

Public supply

14.1(38%)

Domestic supply 

5.3 (14%)

Agricultural 

(Irrigation) 

8.9 (24%)

Industrial

8.8 (24%)

Figure 8. Average reported ground-water withdrawals from the Kirkwood-Cohansey 

aquifer system by water-use category, Glassboro study area, New Jersey, 1992-96. 

(Values are in million gallons per day; %, percent; unpublished data on file at the U.S. 

Geological Survey office, West Trenton, N.J.)

Model Discretization

The aquifer is represented in the numerical 

model by a three-dimensional grid of cells that 

consists of 343 columns, 214 rows, and 12 layers. 

The cells are 492 ft by 492 ft in the horizontal 

dimension and range from 20 to 80 ft in the vertical 

dimension (fig. 9) depending on their position in 

the aquifer system. The cells near the top of the 

model are thin for the purpose of simulating the 

vertical component of flow induced by recharge 

and topographic relief. Model layers in the vertical 

dimension are added not to represent geologic 

layers but to discretize the model in the vertical 

direction to refine the locations of flow paths in the 

aquifer system originating at land surface. The 

grid was aligned approximately with the northeastern study-area boundary to minimize the total 

number of model cells required.

Each cell in the model was determined to be 

either active or inactive. The modeled area 

consists of all active cells. Ground water is not 

simulated to flow through inactive cells. Active 

cells meet the following three conditions: (1) the 

cell is within the study-area boundary, (2) the 

elevation of the top of the cell is greater than the 

elevation of the bottom of the Kirkwood-Cohansey 

aquifer system, and (3) the elevation of the bottom 

of the cell is lower than the land-surface elevation.

Boundary Conditions and Model 

Stresses

All external boundaries of the modeled area 

are modeled as no-flow boundaries. On the northwestern perimeter of the modeled area, the 

boundary closely follows the outcrop of the 

Kirkwood Formation (fig. 9a). The boundary was 
15

Outcrop of the Kirkwood Formation

Stream cells

Inactive cells

Active cells

Model boundary

EXPLANATION

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

214 Rows

343 Columns

(a)

(b)

-50

-250

-200

-150

-100

0

50

100

150

200

ALTITUDE, IN FEET ABOVE OR BELOW SEA LEVEL

Layer 1

Layer 2

Layer 11

Layer 12

Layer 3

Layer 4

Layer 5

Layer 6

Layer 7

Layer 8

Layer 9

Layer 10

Figure 9. Ground-water-flow model of the Glassboro study area, New Jersey, showing 

(a) horizontal discretization and location of active, inactive, and stream cells, and (b) 

vertical discretization.
16

established by combining the drainage boundaries 

for the points on each stream where the stream 

elevation was equal to the elevation of the bottom 

of the aquifer system. On the other three sides, the 

boundaries of the modeled area are surface-water 

divides. These surface-water divides are considered to be surrogates for ground-water divides. The 

assumption of the co-location of the surface- and 

ground-water divides may be inappropriate under 

some conditions. Major pumping near the divides 

may cause the ground-water divide to shift away 

from the surface-water divide. If the vertical head 

gradient at the ground-water divide is sufficiently 

large, ground-water flow may have a regional 

component and water would flow under the local 

ground-water divide. Given the aquifer thickness 

in this area, this type of flow likely occurs only if a 

locally extensive layer of low-permeability 

material is present.

Recharge to the aquifer is modeled by 

applying the source to the uppermost active layer 

of the model. The location of the uppermost active 

layer depends on the simulation. If the head in a 

cell in the uppermost active layer is lower than the 

elevation of the bottom of the cell, then the cell is 

dry and is designated as an inactive cell. Recharge 

then is applied to the next lower cell. The base of 

the surficial aquifer system, defined as the 

elevation of the bottom of the sand layer of the 

Kirkwood Formation, is assumed to be a no-flow 

boundary. Although flow may occur across this 

boundary to or from the underlying confined 

aquifer, this component of flow is considered to be 

negligible.

Recharge to the aquifer was specified as a 

uniform flux of 0.004 ft/d (17.5 in/yr). Lacombe 

and Rosman (1995) reported a recharge value of 

18.6 in/yr for the Maurice River Basin. Watt and 

Johnson (1992) reported a recharge value of 18.3 

in/yr for the Great Egg Harbor River Basin. 

Charles and others (2001) computed a value of 

16.3 in/yr for the Maurice River Basin and 14.1 

in/yr for the Cohansey River Basin. Rice and 

Szabo (1997) used a value of 18 in/yr in their 

models of flow in the Maurice and Cohansey River 

Basins. Modica and others (1998) used a value of 

15 in/yr for the Cohansey River Basin. Martin 

(1998) used 20 in/yr for the entire New Jersey 

Coastal Plain.

All wells with pumping rates greater than 6 

Mgal/yr based on average yearly withdrawals from 

1992 to 1996 reported to the NJDEP Bureau of 

Water Allocation were included in the simulations 

(fig. 7). Water extracted from ponds not directly 

connected to streams also was included in the simulations. Ponds of this type were modeled as 

shallow, large-diameter wells. The ponds are constructed by excavating beneath the water table. 

Similar to rates of pumping from wells, the 

pumping rates were set equal to the average yearly 

withdrawals from 1992 to 1996 reported to the 

NJDEP Bureau of Water Allocation. The locations 

of the wells were determined from latitudes and 

longitudes obtained from the NJDEP Bureau of 

Water Allocation. These locations were verified 

with 7.5-minute USGS topographic quadrangles 

(1:24,000 scale) and aerial photography. If the 

reported position was not at a pond, the location of 

the nearest pond was used. The topographic quadrangles also were used to determine whether the 

pond was connected to a stream.

Streams in the study area were modeled with 

the MODFLOW drain package. A drain simulates 

base flow by allowing water to enter a stream when 

the head in the aquifer is greater than the stage of 

the stream. The elevation or stage of the stream is 

held constant and the discharge to the stream is 

determined by multiplying the difference between 

the head in the aquifer and stream stage by the conductance of the streambed. The conductance of the 

streambed is the hydraulic conductivity of the 

material in the streambed multiplied by the stream 

area in the cell divided by the thickness of the streambed. The drain package does not allow for flow 

from a stream into the aquifer.

A model cell was designated a stream cell if a 

stream passed through any part of the model cell. 

The stage for stream cells that coincided with lakes 

or places where the 10-ft topographic contours 

crossed streams on the 7.5-minute USGS topographic quadrangles was set equal to the value of 

the elevation of lake or the contour line crossing 

the stream; all other stream cells were assigned 

stages on the basis of linear interpolation between 

these points.
17

Determination of Ground-Water Age 

and Contributing Areas to Wells and 

Streams

The particle-tracking model MODPATH

(Pollock, 1994) was used to calculate travel times 

and discharge points for water entering the aquifer 

at the water table. MODPATH uses the cell-by-cell 

flow values from MODFLOW to simulate the path 

of a particle of water through the aquifer. Travel 

times along the paths are computed by using the 

magnitude of the cell-by-cell flows, the porosity of 

the aquifer, and the model cell dimensions. A 

porosity of 0.3 was used in this simulation.

Both forward and backward tracking

approaches were used in simulation. In the

forward tracking approach, one particle was started 

at the water table in the center of each cell that 

intersected the water table. The particle then was 

tracked forward until it discharged at a stream or 

well. Particles were stopped at weak sinks (see 

discussion of weak sinks below). Given uniform 

recharge and cell size, each particle represents the 

same volume of water. This approach was used to 

determine the contributing areas and travel times 

for water entering streams and wells with high 

pumping rates, such as public-supply and irrigation 

wells.

In the backward tracking approach, particles 

were started at locations within the aquifer and 

tracked backward until they reached the water 

table, or location of recharge (contributing area). 

Particles were allowed to pass through weak sinks. 

The backward tracking approach was used to

determine the contributing areas and travel times 

for points in the aquifer or water entering wells 

with low pumping rates, such as domestic or monitoring wells.

Weak sinks are cells that contain a sink that 

does not capture all of the water entering the cell; 

water flows out at least one of the cell faces. When 

the backward particle tracking approach was used, 

particles were allowed to pass through weak sinks 

so they eventually would reach the water table. 

When the forward tracking approach was used, 

however, particles were stopped at weak sink cells. 

This approach may cause the simulated contributing area for that cell to be larger than the actual 

contributing area. Subsequently, simulated contributing areas of cells downgradient from a weak 

sink may be smaller than the actual contributing 

area. Particles were stopped at weak sinks in 

forward-tracking runs to be conservative with 

respect to wells (which are more likely than 

streams to be weak sinks)--that is, overestimating 

rather than underestimating the size of the contributing area.

Determination of Nitrate 

Concentration at Discharge Points

The concentration of nitrate in recharge 

entering the aquifer at the water table was assumed 

to be a function of the land use at the site of 

recharge and the year in which the recharge took 

place. The results of the MODPATH simulations 

provide information on the recharge location and 

time required for each particle to reach its 

discharge location. Each particle is assigned a concentration corresponding to the land use in the year 

that the recharge took place. For example, if the 

concentration is being computed for the year 2000 

and MODPATH simulates a travel time of 20 

years, and the starting location of the particle as the 

cell at column 200, row 100, the particle is given 

the concentration corresponding to the land use at 

that point in 1980. If the concentration in 2020 is 

being computed, the particle would be given the 

concentration corresponding to the land use at that 

point in 2000. The percentage of each land use in 

1950, 1973, 1986, and 1996 was calculated for all 

cells in the model. Land-use percentages between 

these years were linearly interpolated. Land-use 

data for 1950 were used for all years before 1950 

and land-use data for 1996 were used for all years 

after 1996.

To compute the concentration at a particular 

discharge point, the average of the concentrations 

associated with all the particles that flow to that 

particular discharge point is used. Because 

recharge and cell size are uniform, each particle 

represents an equal amount of water and, thus, an 

average is appropriate. If recharge and cell size 

were not uniform, a weighted average would be 

required.
18

The transport of nitrate in the aquifer is 

assumed to be conservative. Because nitrogen 

neither forms insoluble minerals that could precipitate nor is appreciably adsorbed under aquifer conditions, the only means of in-situ nitrate removal 

from ground water is by reduction (Appelo and 

Postma, 1996). Dissolved oxygen concentrations 

in the ground water generally are greater than 

3 mg/L (Szabo and others, 1997); therefore, 

reduction of nitrogen is unlikely to occur. Nitrate 

reduction does take place on a local scale in this 

aquifer. Samples from 2 of 37 monitoring wells 

sampled for dissolved gas in the study area 

contained excess dissolved nitrogen greater than 

1 mg/L (unpublished data on file at the U.S. Geological Survey office in West Trenton, N.J.). The 

amount of dissolved argon is used to determine the 

amount of nitrogen expected to be dissolved in 

water in equilibrium with the atmosphere; any 

nitrogen in excess of this amount is attributed to 

nitrate reduction (Dunkle and others, 1993; Rowe 

and others, 1999).

Estimation of Nitrate Concentration 

in Recharge Over Time

In order to compute the concentrations of 

nitrate as outlined above, it was necessary to 

develop a history of nitrate concentrations in 

recharge for the land uses of interest (fig. 10). The 

current value for nitrate concentration was based 

on median values from three networks of shallow 

monitoring wells designed to characterize the 

quality of water beneath agricultural, urban, and 

undeveloped land. These median values were 

14 mg/L as N for cropland and pasture, 10 mg/L as 

N for orchards and nurseries, 2.5 mg/L as N for 

urban land, and 0.07 mg/L as N for undeveloped 

Orchard/nursery

1940 1960 1980 2020 2040 2060

YEAR

0

2

4

6

8

10

12

14

16

1940 1960 1980 2000

YEAR

0.0E0

1.0E7

2.0E7

3.0E7

4.0E7

Cropland/pasture

Urban

Undeveloped/background

Historical

Historical

Historical

Gradual reduction

Fixed use

Fixed use

Gradual reduction

Gradual reduction

Total ban

Fixed use

2000

NITRATE CONCENTRATION, 

IN MILLIGRAMS PER LITER AS NITROGEN

SALES OF NITROGREN FERTILIZER, 

IN TONS OF NITROGEN

Figure 10. Mean concentration of nitrate in recharge to aquifer from four land-use 

types, 1940-2000 (solid lines), and for three water-management alternatives (dotted 

lines) used in model simulations, Glassboro study area, New Jersey. [Inset shows 

reported sales of nitrogen fertilizer in New Jersey, 1945-98 (Alexander and Smith, 

1990; Battaglin and Goolsby, 1994; David Lorenz, U.S. Geological Survey, written 

commun., 2000)]
19

land (Stackelberg and others, 1997). On the basis 

of trends in the use of nitrogen fertilizer and reconstructed concentrations of nitrate in recharge in the 

Coastal Plain of New Jersey (Modica and others, 

1998) and Maryland (Bohlke and Denver, 1995), 

the concentrations of nitrate in aquifer recharge in 

agricultural and urban areas were assumed to 

increase linearly from the background value of 

0.07 mg/L as N in 1940 to the respective 1990 concentrations. Prior to 1940, the nitrate concentration in recharge from all land uses was assumed to 

be at the background level.

MODEL SIMULATION RESULTS

In this section, results of the model simulations are presented. The calibration procedure is 

discussed and model performance is evaluated. 

Ground water, both in the aquifer and discharging 

from the aquifer, is characterized with respect to 

age and land use in the recharge area.

Model Calibration and Evaluation

The flow model was calibrated by adjusting 

horizontal, vertical, and streambed hydraulic conductivities to achieve the closest possible match 

between simulated and observed heads. A single 

value of streambed hydraulic conductivity of

50 ft/d was used throughout the model. The horizontal and vertical conductivities were assigned on 

the basis of zones in areas with similar stratigraphy 

(see fig. 11) determined by using geophysical and 

driller’s logs (Stephen Cauller, written commun., 

2000). Additional, smaller zones were created in 

the northeastern part of the study area because satisfactory calibration could not be achieved with 

larger zones. Hydraulic conductivity was assumed 

to be constant with depth.

A map of the water table in the study area was 

created by using a GIS from water levels in 592 

wells (data on file at U.S. Geological Survey, West 

Trenton, N.J.), the elevations of perennial streams, 

and water-level contours from results of surficialaquifer studies in the area (Lacombe and Rosman 

1995; Watt and Johnson 1992; Johnson and 

Charles 1997; Charles and others, 2001). This map 

was used to establish a target water level for each 

cell within the model. The hydraulic conductivities then were adjusted to minimize the sum of 

squares of the difference between the target water 

level and simulated water level in each cell. The 

final calibrated hydraulic conductivities and the 

locations of the conductivity zones are shown in 

figure 11.

The difference between the targeted and 

simulated water levels is shown in figure 12. 

Simulated water levels generally are within 5 ft of 

the target water levels. Most of the areas in which 

the difference is large are near the edge of the 

model and, therefore, are subject to boundary 

effects. In some of these areas, however, no 

measured water level is available and the poor 

match may be the result of the process used to 

create the water-table map.

The base flow simulated with the model was 

compared with measured base flows to evaluate 

the accuracy of the recharge value used. Charles 

and others (2001) report ground-water discharge to 

streams to be 15.6 and 13.1 in/yr for the Maurice 

and Cohansey Rivers, respectively. Base flow in 

the Great Egg Harbor River is 17.3 in/yr (Watt and 

Johnson, 1992). The simulated values for these 

rivers at the boundary of the model were 15.3 in/yr 

for the Maurice River, 16.1 in/yr for the Cohansey 

River, and 15.4 in/yr for the Great Egg Harbor 

River. The simulated ground-water discharge 

nearly was equal to the reported discharge for the 

Maurice River but was high for the Cohansey 

River and low for the Great Egg Harbor River. If 

the values are weighted by drainage area and 

combined, the measured ground-water discharge is 

15.7 in/yr and the simulated ground-water 

discharge is 15.4 in/yr.

To evaluate the performance of the model 

with respect to simulation of travel time and to 

calibrate the value of porosity, the model-simulated 

ground-water age and the age determined by 

analyses for environmental tracers in ground water 

from monitoring wells were compared (table 2, fig. 

13). The travel times to monitoring wells were 

estimated by using the ratios of tritium to helium-3 

(3H/3He) (for example, Eckwurzel and others, 

1994; Szabo and others, 1996) and sulfur hexafluoride (SF6) concentrations (Busenberg and 

Plummer, 2000). Ages were determined by

3

H/3He and (or) SF6 analyses for 55 monitoring 

wells in the study area. Simulated travel times to 
20

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5

Zone 6

Zone 7

Zone 8

Zone 9

Zone 10

Zone 11

Zone 12

Zone 13

Model boundary

ZONES OF HYDRAULIC CONDUCTIVITY--

Values are in feet per day

EXPLANATION

Kh = 45

Kv = 45

Kh = 10

Kv = 0.11

Kh = 6

Kv = 0.08

Kh = 30

Kv = 0.57

Kh = 33

Kv K = 0.24 h = 28

Kv = 0.28

Kh = 81

Kv = 0.75

Kh = 80

Kv = 0.56

Kh = 90

Kv = 1.25

Kh = 60

Kv = 0.42

Kh = 100

Kv = 0.56

Kh = 90

Kv = 0.77

Kh = 80

Kv = 0.52

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

Figure 11. Zones of hydraulic conductivity used in model calibration and final 

calibrated horizontal (K ) and vertical (K ) hydraulic conductivities in the modeled h v

area, Glassboro study area, New Jersey.

the monitoring wells were determined by starting 

particles at the center of the screened interval and 

tracking them backward to the water table.

The value of porosity to be used in the model 

was the value at which the sum of squares of the 

difference between the 3

H/3He ages and the 

simulated ages was minimal. This value was determined to be 0.30 (table 3). Use of this value 

resulted in a reasonable match between the 

simulated and geochemically interpreted ages.

Age of Water in the Aquifer and 

Relation to Land Use in the 

Recharge Area

The model was used to simulate the length of 

time that the water has been in the aquifer and the 

land use in the area where the water entered the 

aquifer in order to obtain a general indication of 

current (2000) water-quality conditions in the 

aquifer. The Kirkwood-Cohansey aquifer system 

in the study area contains about 3 trillion gallons of 

water if a porosity of 0.3 is assumed. The average 

age of this water is 49 years; however, the small 
21

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

DIFFERENCE BETWEEN SIMULATED AND 

TARGET WATER TABLE--Values are in feet 

EXPLANATION

< - 10

> -10 - -5

> -5 - 5

> 5 - 10

> 10

Stream used to create 

target water table

Well used to create 

target water table

Less than or equal to

Greater than

39

30'°

39

45'°

75 15' °

75 °

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

<

>

Figure 12. Difference between simulated and target water levels in the modeled area,

Glassboro study area, New Jersey.
22

Table 2. Geochemical and simulated ages of water samples from monitoring wells in the Glassboro study area, New 

[--, data not available]

Well name

U.S. 

Geological 

Survey 

well 

number Latitude Longitude

Depth to 

top of 

screened 

interval

(in feet)

Depth to 

bottom of 

screened 

interval

(in feet)

Date 

heliumtritium 

sample 

collected

Heliumtritium age

(in years)

Date sulfur 

hexafluoride 

sample 

collected

Sulfur 

hexafluoride 

age

(in years)

Simul

age to 

of scr

(in ye

AG06

AG08AG09AG11

AG12NU01-TEN

NU02NU02-TEN

NU06NU06-TEN

NU08

NU09NU09-TEN

NU10-TEN

NU11

NU11-TENNU13-TENNU16-TENNU19-TENNU22-TEN

NU26-TENNU27-TEN

NU29

NU29-TENNU30NU30-TEN

OU01-TEN

OU02-TEN

OU04

OU04-TEN

11-0692

33-082233-082033-0819

11-088915-1266

15-121015-1267

7-0841

7-0867

15-1220

15-121915-1264

15-1277

7-0836

7-08687-08707-08697-0887

15-1280

7-08797-0886

15-1258

15-126815-126015-1271

33-0844

11-0937

15-1214

15-1262

39°30’58.5”

39°35’24.9”39°37’10.9”39°35’41.7”

39°31’58.9”39°43’26.7”

39°43’42.6”39°43’42.6”39°45’27.1”

39°45’27.1”39°43’38.9”

39°40’22.5”39°40’22.4”

39°39’48.2”39°46’04.5”

39°46’04.4”39°43’46.7”39°42’33.8”39°42’54.5”

39°41’50.2”

39°47’04”39°49’42”39°44’42.9”

39°44’42.9”39°45’08.9”39°45’08.9”

39°35’32.5”

39°29’19.9”

39°39’17.1”

39°39’16.9”

75°12’19.1”

75°13’23.2”75°12’09.8”75°11’02.9”

75°15’02.2”75°04’57.2”

75°04’00.6”75°04’00.7”75°00’39.5”

75°00’39.5”75°01’26.3”

74°59’09.1”74°59’09.3”

74°58’28.8”75°00’33.5”

75°00’33.4”74°59’49.6”74°57’42.4”74°59’03”

74°59’00.8”

74°56’16.2”74°55’08.8”75°03’07.4”

75°03’07.4”75°02’35”75°02’34.9”

75°10’11”

75°01’16.8”

75°05’35.2”

75°05’35.3”

33

291720

3755.5

17.540.552

77.528.5

2950.5

4730

67.552.545.549.5

43

38.533.517

32.55064.5

37

67

25

47

38

311922

3958

19.54354

8030.5

3153

4937

70554852

454135.519

355267

39.5

69.5

27

49.5

4/28/1992

2/18/19982/19/199812/4/1996

1/21/19981/09/1998

2/11/1998

11/11/1997

1/29/1998

6/06/1997

12/09/1996

12/16/199712/16/1997

1/22/19989/09/1997

11/06/1997

1/06/19981/08/19981/08/1998

1/12/1998

12/17/199712/09/199712/11/1996

12/04/1997

2/11/19981/07/1998

2/18/1998

11/18/1997

--

1/22/1998

6.1

1.54.71.8

.6

23.0

1.4

21.0

--

11.5

5.3

.3

13.3

21.3

7.5

30.1

5.62.6

20.5

36.926.2

6.84.3

2.13.2

14.3

10.9

6.3

--

8.6

--

2/23/19992/19/1998

--

1/21/1998

--

2/11/19982/11/1998

1/29/1998

1/29/1998

--

1/27/19981/27/1998

1/22/19982/22/1999

1/29/1998

------

2/24/19992/21/19992/21/1999

--

2/12/19981/07/19982/11/1998

2/18/1998

1/21/1998

1/22/19982/23/1999

--

5.61.1

--

4.2

--

1.7

13.7

4.1

7.6

--

1.0

15.6

14.0

4.2

29.1

------

30.013.1

4.6

--

1.64.1

10.7

12.1

17.0

3.1

11.7

1.1.2.

2.1.

6.2.

9.1.

9.

1.2.

7.

8.4

2.

14.

6.8.6.

8.

10.

17.

1.5.2.

8.

9.

5.

1.

7.
23

Table 2. Geochemical and simulated ages of water samples from monitoring wells in the Glassboro study area, Ne

Continued

Well name

U.S. 

Geological 

Survey 

well 

number Latitude Longitude

Depth to 

top of 

screened 

interval

(in feet)

Depth to 

bottom of 

screened 

interval

(in feet)

Date 

heliumtritium 

sample 

collected

Heliumtritium age

(in years)

Date sulfur 

hexafluoride 

sample 

collected

Sulfur 

hexafluoride 

age

(in years)

Simul

age to 

of scr

(in ye

OU05-TENOU06OU06-TENOU07-TENOU08-TENOU09-TENOU10OU10-TENOU14-TENOU15-TENOU16-TENOU17-TENOU18-TENOU19-TENOU20-TENUN09RUTGERS-MEDRUTGERS-DEEPTPE-SHALLOWTPE-MED-SHTPE-MED-DETPE-DEEPWTMUA-SHALLOWWTMUA-MEDIUMWTMUA-DEEP

1-1243

15-124815-126511-0938

7-0880

15-126311-092711-0936

7-0885

11-093515-1270

7-08827-0884

15-127915-1282

7-0842

11-069311-069415-105715-106315-105815-105915-105115-105215-1053

39°35’30.3”39°41’04.1”39°41’04.1”39°29’29.3”39°47’49.3”39°42’09.9”39°29’17.7”39°29’17.6”39°46’45.7”39°28’27.8”39°45’01.7”39°48’19.9”39°48’41.6”39°41’37.3”39°42’25.5”39°39’39.3”39°31’04”39°31’04”39°42’42”39°42’42”39°42’42”39°42’42”39°43’14”39°43’14”39°43’14”

74°52’37.8”74°59’30.4”74°59’30.6”75°01’59”74°55’53.8”75°06’37.5”75°00’36.7”75°00’36.9”74°59’19.9”75°01’38.4”75°02’04.7”74°57’01.7”74°56’25.1”75°00’04”75°00’38.9”74°53’41.5”75°12’21”75°12’21”75°03’29”75°03’29”75°03’29’75°03’29”75°01’44”75°01’44”75°01’44”

37.519.5465337.5403047.544.567.567.5686759.5501273

105

22357095226092

4021.548.555.54042.53250477069.5706961.5521478

110

274075

100

276597

12/05/199712/18/199712/18/1997

--

12/17/1997

2/12/1998

12/16/1996

----

11/18/1997

1/07/1998

12/10/199712/09/1997

2/12/19981/08/1998

--

4/28/19924/28/19924/29/19924/29/19924/29/19924/29/19924/30/19924/30/19924/30/1992

19.5

.2

14.8

--

7.6

20.0

4.4

----

8.6

12.427.926.814.1

9.1

--

24.336.3

3.57.2

24.433.3

2.5

12.939.8

2/12/19981/27/19981/27/19981/20/19982/24/19992/12/1998

--

1/20/19982/22/1999

--

2/22/19992/21/19992/21/1999

--

2/24/19992/17/1998

------------------

11.1

2.5

12.117.129.010.1

--

7.1

26.0

--

30.028.031.0

--

7.02.1

------------------

14.

1.7.6.7.8.1.5.

10.

9.

26.12.12.10.

8.1.

11.22.

1.5.

18.444.

2.

15.46.
24

1 2 3 4 5 6 7 89 10 20 30 40 50 60 70 80 100

100

10

1

0

0.2

0.3

0.5

0.6

0.7

0.8

2

3

4

5

6

7

8

20

30

40

50

60

70

80

0.4

GROUND-WATER AGE DETERMINED WITH GEOCHEMICAL 

METHODS, IN YEARS

SIMULATED GROUND-WATER AGE DETERMINED WITH FLOW MODEL, IN YEARS

Tritium-helium ( 3H / 3He )

Sulfur-hexafluoride ( SF6 )

1:1 line

Figure 13. Ground-water age determined with geochemical methods ( H- He 3 3

and (or) SF ) and simulated ground-water age, where a sample was collected, 6

Kirkwood-Cohansey aquifer system, Glassboro study area, New Jersey.

Table 3. Relation of porosity to difference between simulated and geochemical groundwater ages in flow model of Glassboro study area, New Jersey

Sum of squares of the difference 

Porosity between simulated and geochemical 

ages

0.25 3,289

.26 3,197

.27 3,124

.28 3,072

.29 3,038

.301 3,025

.31 3,031

.32 3,057

.33 3,102

.34 3,167

.35 3,252

1 Value used in model simulation.
25

percentage of very old water (>210 years) skews 

the average age. Fifty percent of the water is less 

than 22 years old, and 25 percent of the water is 

less than 10 years old.

A histogram of the age classes of water 

currently in the Kirkwood-Cohansey aquifer 

system in the study area is shown in figure 14. 

Each bar in the histogram is shaded to demonstrate 

the amount of water that is being contributed from 

different land-use classes. The age classes are 

equal intervals of the logarithm of age. The landuse class was assigned on the basis of the land use 

at the time the water recharged the aquifer.

The spatial variability in the time required for 

water to travel from the point of recharge to the 

point of discharge is shown in figure 15. Water 

that is recharged near streams moves relatively 

quickly to the streams, whereas water that is 

recharged at locations distant from streams, such as 

along surface-water divides, can take hundreds of 

years to move through the aquifer. Changes in land 

use and (or) chemical use (for example, establishing riparian buffer zones) in areas near streams 

can have a relatively quick effect on streamwater 

quality. On the other hand, chemicals entering the 

aquifer at areas distant from the streams can affect 

water quality for tens to hundreds of years.

The simulated age of water in vertical 

sections along columns 230 and 231 of the model 

is shown in figure 16. The age increases with 

depth in the aquifer. As the aquifer thickens, in 

general, the amount of water in a given age class 

increases, especially for the older age classes. 

25

Undeveloped

Orchard/nursery

Cropland/pasture 20

Urban, nonresidential

Urban, residential

> Greater than

PERCENT

15

10

5

0

0.0 -1.0

> 1.0 - 2.1> 2.1 - 4.6

> 4.6 - 10.0

>10.0 - 21.0

> 21.0 - 46.0

> 46.0 - 100.0

> 100.0 - 210.0

> 210.0

AGE, IN YEARS

Figure 14. Distribution of land use at the point of recharge by ground-water 

age for water in the Kirkwood-Cohansey aquifer system, Glassboro study area, 

New Jersey.
26

0 50 100 150 200 250

TRAVEL TIME, IN YEARS

EXPLANATION

Stream

39

30'°

39

45'°

75 15' °

75 °

Figure 15. Travel time required for water that recharges the surficial aquifer at a 

given location to reach its discharge point (stream or well), Glassboro study area, 

New Jersey.

Near streams, water from all age classes converges 

upward to discharge to the stream, with younger 

water entering near the edge of a stream and older 

water discharging near the center (Modica and 

others, 1998).

Near wells, water from all age classes 

converges toward the well screen. In general, old 

water enters near the bottom of the screen, whereas 

younger water enters at the top. A “cone of age 

depression” is present around wells that withdraw 

large amounts of water from an unconfined aquifer. 

This cone could be important where, for example, 

a low-volume domestic well is near a large-volume 

well. The domestic well might be installed at a 

depth that normally would intercept 25- to 50-yearold water; however, the effects of pumping from a 
27

nearby large-volume well might cause the 

domestic well to intercept water that is 0 to 10 

years old instead.

The spatial distribution of the vertically 

averaged age of water in the aquifer is shown in 

figure 17. The average age was computed by (1) 

simulating the ground-water age at 10-ft intervals, 

(2) integrating age values within each 10-ft interval 

by assuming an exponential model (Solomon and 

others, 1995) for increase in age with depth, and 

finally (3) summing the integrated ages for each 

10-ft interval and dividing by the total depth. The 

average age increases with the thickness of the 

aquifer. The average age also is greatest beneath 

streams because the older water is converging 

upward to the stream. High-volume withdrawals 

can increase or decrease the average age of water 

in the aquifer around the well depending on the 

location of the well screen. If the screen is near the 

bottom of the aquifer, young water will be drawn 

downward and the average age will decrease; if the 

screen is in the upper part of the aquifer, old water 

will be pulled upward and the average age will 

increase.

Age of Ground-Water Discharge and 

Relation to Land Use in the 

Recharge Area

The model was used to characterize ground 

water discharging from the aquifer to wells and 

streams with respect to age and land use in the 

recharge area. This water is a mixture of water of 

various ages that was recharged in areas with 

0 2 4 6 8 10 12 14 15.7

-200

200

-100

0

100

ALTITUDE, IN FEET ABOVE OR 

BELOW SEA LEVEL

DISTANCE, IN MILES

A

Confining unit

Pumped wells

Confining unit

Stream cell Stream cells

A'

0 - 10 A

> 10 - 25

> 25 - 50

> 50 - 100

> 100 +

Greater than

GROUND-WATER AGE, 

IN YEARS

EXPLANATION

>

VERTICAL SCALE GREATLY EXAGGERATED

A'

Figure 16. Vertical section showing the distribution of simulated groundwater age along profile A-A', Glassboro study area, New Jersey.
28

39

30'°

39

45'°

75 15' °

75 °

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

0 - 10

> 10 - 21

> 21 - 46

> 46 - 100

> 100

Greater than

AVERAGE AGE OF WATER, 

IN YEARS

EXPLANATION

>

Figure 17. Vertically averaged age of water in the Kirkwood-Cohansey aquifer 

system, Glassboro study area, New Jersey.
29

different land uses. The endpoint files from 

MODPATH were linked to digital land-use data 

sets with a GIS to quantify the amount of water 

from different land-use and age classes for each 

discharge location. The combination of land use 

and age was used to estimate concentrations of 

nitrate in the aquifer. In addition to characterizing 

the current quality of water discharging to wells 

and streams, the model was used to simulate the 

future response to possible changes in land use and 

nitrate concentration in recharge.

The first step was to determine the areas 

where the water was recharged to the aquifer, or 

the contributing areas, for various wells and 

streams (fig. 18). Ninety-three percent of the water 

that recharges the aquifer discharges to streams. 

Contributing areas to wells in surficial aquifers 

tend to be elongated areas that extend upgradient 

from each well (fig. 19). Contributing areas to 

wells near a ground-water divide are more 

rounded. For the most part, the effect of pumping 

wells in an unconfined, highly permeable aquifer is 

to slightly lower the water table over a broad area 

rather than to radically change the hydraulic-head 

gradient in the vicinity of the well. In some cases, 

the contributing area to a well does not include the 

actual well location. This case is most likely to 

occur where pumping rates are low, the well is 

screened near the bottom of the aquifer, and (or) 

the well is near a stream. To simulate this case 

with a ground-water-flow model, the vertical discretization must be sufficiently fine. Given a 

uniform recharge rate, the size of the contributing 

area to a well is linearly proportional to the 

pumping rate of the well.

Simulating contributing areas with a 

numerical model is subject to certain limitations. 

Model accuracy may be affected by the discretization of the model, generalization of aquifer properties, and changes in source and sink strength over 

time (for example, changing pumping rates or variations in recharge). The accuracy with which a 

contributing area can be simulated increases with 

the size of the contributing area. For example, the 

simulated contributing area to the entire Maurice 

River in the study area will be more accurate than 

the contributing area to Still Run (a tributary to the 

Maurice River), which likely will be more accurate 

than the contributing area to a public-supply well, 

which, in turn, will be more accurate than the contributing area to a domestic well. Despite these 

limitations, a ground-water-flow model can 

provide a more accurate approximation of a contributing area than other methods in which circular 

buffer zones, simple analytical models, or surfacewater divides, for example, are used.

The fraction of water entering streams and 

wells from various land-use classes is shown by 

age class in figure 20. The age classes again are 

equal intervals of logarithm of age. Water discharging to all streams and water discharging to all 

wells is grouped to allow a general comparison of 

the age and land-use derivation of water entering 

wells and streams. Water that discharges to wells 

(fig. 20a) is more likely to originate in urban and 

agricultural areas and travel longer through the 

aquifer than water that discharges to streams (fig. 

20b) because most wells are located in urban and 

agricultural areas and generally are screened in the 

bottom part of the aquifer.

The differences in the composition of water 

in different types of wells reflect the differences in 

the location and construction characteristics of the 

wells (fig. 20c-f). Public-supply wells and industrial wells contain water of similar composition. A 

large proportion of water withdrawn by irrigation 

wells originates from agricultural land, reflecting 

the use of the water. In addition, water in irrigation 

wells is younger than water in public-supply wells 

because the screened interval is shallower. Water 

from ponds in the study area is used mostly for 

agricultural purposes (especially orchards), and its 

composition reflects the large proportion of water 

recharged in agricultural areas. In contrast to the 

wells, the ponds contain younger water because 

they are shallower with respect to ground-water 

discharge. Compared to the streams (similar depth 

of discharge), little water older than 20 years discharges to the ponds because, unlike streams, they 

are not necessarily located at natural topographic 

lows, where water of all ages converges.

Older water generally is of better quality than 

younger water in the aquifer system, although 

exceptions can be found. Compounds that degrade 

over time are less likely to be detected the longer 

the water has been in the aquifer. Most contaminants have been available only in the last 50 years; 
30

39

30'°

39

45'°

75 15' °

75 °

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

CONTRIBUTING AREA TO

Stream cell

Tributary to Delaware River

Maurice River

Great Egg Harbor River

Cohansey River

Public-supply well

Other well

Flow path

EXPLANATION

Figure 18. Simulated contributing areas to wells and streams and selected 

simulated flow paths in the Glassboro study area, New Jersey.
31

39

30'°

39

45'°

75 15' °

75 °

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

0 1 2 MILES

0 1 2 3 KILOMETERS

TRAVEL TIME TO WELL, 

IN YEARS

EXPLANATION

0 - 13

> 13 - 25

> 25 - 50

> 50 - 100

> 100

Greater than

Public-supply well

Contributing area to 

public-supply wells

Active stream cell

>

Figure 19. Simulated contributing areas to selected public-supply wells in the Glassboro 

study area, New Jersey, and associated travel times from recharge area to well.

thus, concentrations of these compounds tend to be 

higher in recently recharged water than in older 

water. Concentrations of compounds that are 

breakdown products of another compound and 

compounds that were used in greater amounts in 

the past also tend to be higher in older water than 

in younger water.

EFFECTS OF LAND USE AND 

TRAVEL TIME ON DISTRIBUTION 

OF NITRATE

Concentrations of nitrate in water discharging 

to streams and wells were simulated to demonstrate the effects of land use and travel time on 

water quality. Nitrate concentrations commonly 

are elevated above background levels in shallow 

ground water in the Kirkwood-Cohansey aquifer 
32

0

5

10

15

20

25

30

35

0.0 - 1.0

>1.0 - 2.1>2.1 - 4.6

>4.6 - 10.0

>10.0 - 21.0>21.0 - 46.0

>46.0 - 100.0

>100.0 - 210.0

>210.0

AGE, IN YEARS

A. WELLS AND PONDS

PERCENT

0.0 - 1.0

>1.0 - 2.1>2.1 - 4.6

>4.6 - 10.0

>10.0 - 21.0>21.0 - 46.0

>46.0 - 100.0

>100.0 - 210.0

>21.0

PERCENT

AGE, IN YEARS

B. STREAMS

0

5

10

15

20

25

30

35

C. PONDS

PERCENT

0

5

10

15

20

25

30

35

0.0 - 1.0

>1.0 - 2.1

>2.1 - 4.6

>4.6 - 10.0

>10.0 - 21.0>21.0 - 46.0

>46.0 - 100.0

>100.0 - 210.0

>210.0

AGE, IN YEARS

0

5

10

15

20

25

30

35

0.0 - 1.0

>1.0 - 2.1>2.1 - 4.6

>4.6 - 10.0

>10.0 - 21.0

>21.0 - 46.0

>46.0 - 100.0

>100.0 - 210.0

>210.0

D. INDUSTRIAL WELLS

AGE, IN YEARS

PERCENT

0

5

10

15

20

25

30

35

0.0 - 1.0

>1.0 - 2.1>2.1 - 4.6

>4.6 - 10.0

>10.0 - 21.0

>21.0 - 46.0

>46.0 - 100.0

>100.0 - 210.0

>210.0

AGE, IN YEARS

PERCENT

E. PUBLIC-SUPPLY WELLS

0

5

10

15

20

25

30

35

F. IRRIGATION WELLS

PERCENT

0.0 - 1.0

>1.0 - 2.1>2.1 - 4.6

>4.6 - 10.0

>10.0 - 21.0>21.0 - 46.0

>46.0 - 100.0

>100.0 - 210.0

>210.0

AGE, IN YEARS

Undeveloped

Orchard/nursery

Cropland/pasture

Urban, nonresidential

Urban, residential

Undeveloped

Orchard/nursery

Cropland/pasture

Urban, nonresidential

Urban, residential

Undeveloped

Orchard/nursery

Cropland/pasture

Urban, nonresidential

Urban, residential

Undeveloped

Orchard/nursery

Cropland/pasture

Urban, nonresidential

Urban, residential

Undeveloped

Orchard/nursery

Cropland/pasture

Urban, nonresidential

Urban, residential

Undeveloped

Orchard/nursery

Cropland/pasture

Urban, nonresidential

Urban, residential

Figure 20. Distribution of land use at the point of recharge by ground-water age for 

water discharging to (a) all wells and ponds, (b) streams, (c) ponds, (d) industrial 

wells, (e) public-supply wells, and (f) irrigation wells, Glassboro study area, New 

Jersey. (>, greater than)
33

system beneath agricultural and urban areas in 

southern New Jersey (Stackelberg and others, 

1997). Over the past 60 years, the use of nitrogen 

fertilizers in agricultural and urban areas has 

increased, and nitrate concentrations in recharge in 

the Atlantic Coastal Plain also have increased over 

the same time period (Bohlke and Denver, 1995). 

In an aerobic ground-water-flow system like most 

of the Kirkwood-Cohansey aquifer system, nitrate 

behaves conservatively. Its widespread and 

increasing use, relevance to ground- and surfacewater quality, largely conservative behavior, and 

availability of data make nitrate a good choice for 

demonstrating effects of age and land use on water 

quality in this study.

Comparison of Simulated and 

Measured Nitrate Concentrations

Nitrate concentrations were simulated for 27 

public-supply wells in the study area (locations 

shown in fig. 21) that were sampled in 1998 (Baehr 

and others, 1999; Stackelberg and others, 2000) 

and were compared to the measured values (fig. 

22). Generally, the simulated nitrate concentrations were within 2.0 mg/L as N of the measured 

concentrations, a reasonable fit for the intended 

purpose. Five of the sampled wells with larger 

errors were near the boundary of the model and the 

simulated contributing areas likely are affected by 

the model boundary conditions. The close 

agreement between the simulated and measured 

values indicates that nitrate, for the most part, 

moves conservatively through the aquifer to these 

wells.

The comparison to measured data from 

streams is less straightforward than from wells. 

The model simulates only the portion of streamflow that is derived from ground water; therefore, 

the stream samples whose nitrate concentrations 

are most appropriate for comparison with the 

simulated nitrate concentrations are those collected 

when flow and chemical input derived from 

sources other than ground water (for example, 

runoff or discharge from a wastewater-treatment 

plant) are minimal. Three sites in the study area 

were used to compare simulated and measured 

nitrate concentrations: Great Egg Harbor River 

near Sicklerville, Maurice River at Norma, and 

Cohansey River at Seeley. The basins that drain to 

these three sites differ in size and land-use composition (see fig. 21). Nitrate concentrations for these 

sites were retrieved from the U.S. Geological 

Survey’s National Water Information System 

database with two restrictions: (1) streamflow at 

the time the sample was collected was classified as 

base flow, and (2) no appreciable point-source contributions of nitrate or flow were being made to the 

stream when the sample was collected. Streamflow was classified as base flow if the mean flow at 

both the Maurice River at Norma and Great Egg 

Harbor River at Folsom (sites for which long-term 

daily flow values were available) had not increased 

by more than 1 percent or decreased by more than 

10 percent in the previous 3 days.

Comparison of nitrate concentrations for the

three streams showed that the simulated nitrate 

concentrations consistently exceeded the measured 

concentrations. Given the conservative behavior 

of nitrate in water withdrawn by public-supply 

wells, this difference likely can be attributed to 

non-conservative behavior of nitrate in and (or) 

near streams. Reasons for non-conservative 

behavior may include denitrification in the relatively organic-rich sediments that make up the 

streambed or uptake by aquatic plants and algae in 

the water column. For the three streams, a 40-

percent reduction in the simulated concentrations 

was needed to match the measured concentrations 

in the streams (fig. 23). The scatter in the 

measured concentrations may be a result of the 

variation in water temperature. Biological activity 

in the water column increases with the temperature 

of the streamwater; therefore, nitrate concentrations decrease with increasing temperature. A 40-

percent loss in nitrate seems to be representative of 

the average amount of nitrate that is removed by 

in- or near-stream processes in the study area since 

1990.

Because the method described above uses 

nitrate concentration in recharge based on the mean 

concentration in water associated with each landuse type, the reliability of the method for simulating nitrate concentration increases with the size 

of the contributing area to the discharge location—

that is, the larger the area over which this “mean” 

is applied, the more likely it is to represent the 

actual concentration in the recharge.
34

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

39

30'°

39

45'°

75 15' °

75 °

Basin draining to surfacewater sampling site

Study-area boundary

Surface-water-sampling site

Public-supply well

Percent

EXPLANATION

Maurice River at Norma N.J.

Basin area: 112 square miles

Land use: 24% agricultural

 26% urban

 50% undeveloped Cohansey River at Seely N.J.

Basin area: 25 square miles

Land use: 74% agricultural

 6% urban

 20% undeveloped

Great Egg Harbor River

near Sicklerville, N.J.

Basin area: 15 square miles

Land use: 8% agricultural

 42% urban

 50% undeveloped

%

Figure 21. Location of ground-water and surface-water sampling sites and land use 

in basins draining to surface-water sites, Glassboro study area, New Jersey. [Land-use 

data are for 1996 (N.J. Department of Environmental Protection, 1996b; data on CDROM obtained from Steven Carp, New Jersey Office of State Planning, Trenton, N.J., 

1999)]

Response to Management 

Alternatives

The model was used to evaluate the effects of 

various management alternatives on the concentration of nitrate in streams and public-supply wells. 

Nitrate concentrations were simulated under the 

following conditions: an immediate ban on nitrate 

input, a gradual reduction in input, and fixed input. 

Under the ban alternative, the nitrate concentration 

in recharge immediately would decrease to the 

background concentration. Under the gradualreduction alternative, the nitrate concentration in 

recharge would be reduced at a constant rate over 
35

0

1

2

3

4

5

6

7

8

012345678

MEASURED NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER AS NITROGEN

SIMULATED NITRATE CONCENTRATION, IN MILLIGRAMS 

PER LITER AS NITROGEN

Figure 22. Simulated and measured nitrate concentrations in public-supply wells

in the Glassboro study area, New Jersey. (Open circles represent sites near the 

model boundary. The solid line shows where the simulated concentration equals 

the measured concentration. The dotted lines show where the simulated concentration differs from the measured concentration by plus or minus 2 milligrams per 

liter as nitrogen.)

50 years until reaching the background concentration. Under the fixed-input alternative, the nitrate 

concentration in recharge would remain constant at 

the concentration in 2000. The response of the 

aquifer system to these management alternatives is 

shown in figure 24.

Immediate Ban. Under the immediate-ban 

alternative, the concentration at the discharge 

locations will start to decrease almost immediately 

but will not be reduced to background levels for 

about 15 years for streams and about 30 years for 

wells. The decrease in concentration in discharge 

will begin when the water recharged after the ban 

reaches the discharge point. For streams, this time 

will be very short, because water recharged near 

streams flows quickly to the stream. The time 

likely will be longer for wells, especially those 

with low pumping rates and (or) screened near the 

bottom of the aquifer.

Gradual reduction. Under the gradualreduction alternative, the nitrate concentration 

continues to increase for about 10 years in streams 

and 15 years in wells before eventually decreasing 

to the background concentration. The continued 

increase in concentration during the initial years 

after the reduction in input is a result of the influx 

of water that was recharged before the reduction. 

The presence of older water will cause the concentration in the discharge to remain above background levels (<1 mg/L as N) for about 50 years 

for streams and about 70 years for wells.

Fixed Input. Under a fixed-input alternative, 

concentrations eventually will approach a constant 

value. The time until the constant value is reached 

will depend on the age composition of the discharge. In the short term, concentrations will 

continue to increase over time as the proportion of 

post-1950 water increases. As more of the dis-
1988 1990 1992 1994 1996 1998 2000

6

5

4

3

2

1

0

YEAR

EXPLANATION

NITRATE CONCENTRATION, IN MILLIGRAMS

PER LITER AS NITROGEN

SIMULATED CONCENTRATION--

MEASURED CONCENTRATION--

Cohansey River at Seely, N.J. (60% of simulated value)

Maurice River at Norma, N.J. (60% of simulated value)

Great Egg Harbor River near Sicklerville, N.J. (60% of simulated value)

Water temperature

<15 degrees Celsius

Water temperature

>15 degrees Celsius

Great Egg Harbor River near Sicklerville, N.J. 

Cohansey River at Seely, N.J.

Maurice River at Norma, N.J.

> Greater than, < Less than, % Percent

Figure 23. Simulated and measured nitrate concentrations at three

surface-water sites, Glassboro study area, New Jersey, 1987-99.

36
37

1940 1960 2000 2040 2080 2100

0

5

1

2

3

4

Historical input

Fixed input

Gradual reduction

Immediate ban

1980 2020 2060

Streams Public-supply wells

Gradual reduction

Immediate ban

NITRATE CONCENTRATION, IN 

MILLIGRAMS PER LITER

YEAR YEAR

1940 1960 2000 2040 2080 2100

0

5

1

2

3

4

Historical input

1980 2020 2060

Fixed input

NITRATE CONCENTRATION, IN 

MILLIGRAMS PER LITER

Figure 24. Simulated nitrate concentrations in streams and public-supply wells determined 

from historical nitrate input and simulated response to three management alternatives (fixed

input, gradual reduction, and immediate ban) for future nitrate input in recharge, Glassboro 

study area, New Jersey.

charging water reaches the constant recharge concentration, the overall concentration will approach 

a constant value. The higher the percentage of 

young water at the discharge point, the more 

rapidly the concentration will approach the 

constant value. The constant concentration of 

nitrate that eventually is reached will be the nitrate 

concentration in recharge from each land use multiplied by the percentage of that land use in the 

contributing area.

Spatial Distribution of Changes in 

Nitrate Concentrations

Concentrations of nitrate in streams (reduced 

by 40 percent to account for non-conservative 

behavior in and near streams) in 2000 and 2050 

were simulated with the assumption of constant 

land use, withdrawal rates and locations, and 

recharge concentration during the period (fig. 25). 

Currently (2000), nitrate concentrations are highest 

in streams in the agricultural areas, mostly in the 

western part of the study area. Concentrations are 

lowest in the Great Egg Harbor River and in some 

other small tributaries that drain mostly undeveloped areas. As discussed in the fixed-input alternative above, concentrations generally will 

increase over the next 50 years. The magnitude of 

these changes is shown in figure 26. The absolute 

changes are largest in the western and south-southeastern parts of the study area, where agricultural 

inputs of nitrate are high. The percent changes are 

largest in the eastern and southern parts of the 

study area, where streams flow through wetlands 

and the flow paths of recharge that originates in 

agricultural and urban areas are longer.

Nitrate concentrations in 2000 and 2050 in a 

“typical” domestic well installed in the KirkwoodCohansey aquifer system were simulated (fig. 27). 

A “typical” domestic well was defined as a well 

anywhere in the study area screened at a depth 

from 90 to 100 ft below land surface or over the 

bottom 10 ft of the aquifer if the total thickness is 

less than 100 ft. Higher nitrate concentrations 

indicate where agricultural land and to a lesser 

degree urban land is located. Nitrate concentrations generally are highest in agricultural and, to a 

smaller degree, urban areas, and along surfacewater divides, where the unsaturated zone is 

thickest, because water moves more rapidly 

through the unsaturated zone than through the 

saturated zone of the aquifer. If land use, withdrawal rates and locations, and recharge concentration are assumed to be constant, the area in which 

nitrate concentrations exceed the maximum contaminant level (10 mg/L as N) is considerably 
38

0 - 1

> 1 - 3

> 3 - 5

> 5 - 8.4

Greater than

NITRATE CONCENTRATION--

In milligrams per liter as nitrogen

EXPLANATION

2000

2050

>

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

39

30'°

39

45'°

75 15' °

75°

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

39

45'°

75 15' °

75°

39

30'°

Figure 25. Simulated nitrate concentration in streams in the Glassboro study area, 

New Jersey, in 2000 and in 2050 with the assumption of no future change in land use,

withdrawals, or nitrate concentration in recharge during the period.
39

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

39

30'°

39

45'°

75 15' °

75°

39

45'°

75 15' °

75°

39

30'°

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

-65 - 0.001

 0 - 20

> 20 - 100

> 100 - 1800

Greater than

CHANGE IN NITRATE CONCENTRATION--In percent

EXPLANATION

-0.5 - 0

 > 0 - 0.5

> 0.5 - 1

> 1 - 2.34

Greater than

CHANGE IN NITRATE CONCENTRATION--In milligrams per liter as 

nitrogen

EXPLANATION

>

>

Figure 26. Change in simulated nitrate concentration in streams in the Glassboro 

study area, New Jersey, from 2000 to 2050 with the assumption of no future change 

in land use, withdrawals, or nitrate concentration in recharge during the period.
40

0.07 - 1

>1 - 3

> 3 - 10

> 10 - 14

Greater than

NITRATE CONCENTRATION--

In milligrams per liter as nitrogen

EXPLANATION

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

39

45'°

75 15' °

75°

39

30'°

39

45'°

75 15' °

75°

39

30'°

2000

2050

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

>

Figure 27. Simulated nitrate concentration in a "typical" domestic well installed in the

Kirkwood-Cohansey aquifer system in the Glassboro study area, New Jersey, in 2000

and in 2050 with the assumption of no future change in land use, withdrawals, or nitrate 

concentration in recharge during the period. (A "typical" domestic well is defined as having 

a screened interval from 90 to 100 feet, or at the bottom 10 feet of the aquifer if the aquifer 

is less than 100 feet thick.)
41

higher in 2050 than in 2000. Moreover, the area in 

which nitrate concentrations are at background 

levels (<1 mg/L as N) is much lower in 2050 than 

in 2000. In some areas where land use changed 

from agricultural to urban, however, concentrations would be lower in 2050 than in 2000 because 

the nitrate inputs associated with urban land use 

are lower than those associated with agricultural 

land use.

Nitrate concentrations in 2000 and 2050 in a 

“typical” public-supply well installed in the 

Kirkwood-Cohansey aquifer system were 

simulated in the model (fig. 28). A “typical” 

public-supply well was defined as a well screened 

in the bottom 30 ft of the aquifer system anywhere 

in the study area and with a pumping rate of 0.6 

Mgal/d. The nitrate concentrations reflect the 

density of agricultural and, to a lesser degree, 

urban land use. The simulated areas of high and 

low nitrate concentrations in 2000 and 2050 

generally are similar to those simulated for 

domestic wells. Because their contributing areas 

are larger, extreme concentrations in the publicsupply wells are less common than in the domestic 

wells. Compared to streams, wells receive a higher 

percentage of “older” water; therefore, nitrate concentrations in wells will change more dramatically 

than those in streams over the 50-year period (with 

no change in input).

The change in nitrate concentration from 

2000 to 2050 for typical public- and domesticsupply wells is shown in figure 29. The changes 

are largest in domestic wells, especially in the agricultural areas. In some areas, nitrate concentration 

decreases in water from both public-supply and 

domestic wells as a result of a change in land use 

from agricultural to urban. The changes in concentration at public-supply wells are largest in the 

thick parts of the aquifer where some agricultural 

land is present.

The results demonstrate the effect of the characteristics of the contributing areas to domestic and 

public-supply wells on water quality. The 

extremes in nitrate concentration (high and low) 

and the largest changes in nitrate concentration 

from 2000 to 2050 occurred in domestic wells 

rather than public-supply wells because the 

domestic wells intercept water derived from small 

contributing areas with fairly uniform land use and 

ground-water-age composition. Nitrate concentrations in public-supply wells were less extreme and 

increased less compared to domestic wells because 

contributing areas supply water derived from 

multiple land uses and ground-water-age classes 

for public-supply wells.

SUMMARY AND CONCLUSIONS

The Kirkwood-Cohansey aquifer system is 

the principal unconfined aquifer system in the 

southern New Jersey Coastal Plain. The properties 

of the soils and aquifer materials (sandy, permeable, and low organic-matter content) contribute to 

the vulnerability of the aquifer system to contamination. The presence of contaminants in recharge 

commonly is related to the land use in the area in 

which precipitation recharges the aquifer. The land 

use in the recharge area, along with travel time 

through the aquifer, is used to explain the quality 

of water at streams and wells, the locations of 

ground-water discharge. As part of the USGS’s 

NAWQA Program, a data-collection and groundwater-modeling study was conducted to characterize ground-water age and nitrate concentration 

in the Glassboro area in southern New Jersey with 

respect to land use in the recharge area at the time 

and place of recharge. Nitrate concentrations were 

simulated as an example of how the land-use 

signature and age of water affect water quality.

A three-dimensional ground-water flow 

model was developed for a 400-mi2 area of the 

Kirkwood-Cohansey aquifer system in the 

southern New Jersey Coastal Plain. The aquifer 

system consists of unconfined, unconsolidated 

sands and gravels with some interbedded clays. 

Land use in the study area currently (2000) is a 

mixture of urban, agricultural, and forest (25-30 

percent each), with the remainder consisting 

mostly of wetlands. Most of the urban development has occurred since 1950 on land that previously was either agricultural or forested. An 

increase in urban land and the corresponding 

increase in population created the need for 

increased water supply. For many years most of 

the water used for public supply came from deeper, 

confined aquifers below the Kirkwood-Cohansey 

aquifer system. In recent years, however, restric-
42

39

45'°

75 15' °

75°

39

30'°

39

45'°

75 15' °

75°

39

30'°

2000

2050

0.07 - 1

>1 - 3

> 3 - 10

> 10 - 14

Greater than

NITRATE CONCENTRATION--

In milligrams per liter as nitrogen

EXPLANATION

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

>

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

Figure 28. Simulated nitrate concentration in a "typical" public-supply well installed in the

Kirkwood-Cohansey aquifer system in the Glassboro study area, New Jersey, in 2000 and 

in 2050 with the assumption of no future change in land use, withdrawals, or nitrate concentration in recharge during the period. (A "typical" public-supply well is defined as having a 

screened interval at the bottom 30 feet of the aquifer with a pumping rate of 0.6 million 

gallons per day. Areas where the "typical" well would dewater the aquifer are shown in white.)
43

< 0

 0 - 2

> 2 - 5

> 5 - 13.5

Greater than

Less than

CHANGE IN NITRATE CONCENTRATION--

In milligrams per liter as nitrogen

EXPLANATION

Public-supply wells

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

>

<

39

45'°

75 15' °

75°

39

30'°

Domestic wells

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

39

45'°

75 15' °

75°

39

30'°

CAMDEN

GLOUCESTER

SALEM

CUMBERLAND

Figure 29. Change in simulated nitrate concentration from 2000 to 2050 in "typical" domestic

and public-supply wells installed in the Kirkwood-Cohansey aquifer system in the Glassboro 

study area, New Jersey, with the assumption of no future change in land use, withdrawals, or 

nitrate concentration in recharge during the period. (A "typical" domestic well is defined as having 

a screened interval from 90 to 100 feet, or at the bottom 10 feet of the aquifer if the aquifer is less 

than 100 feet thick. A "typical" public-supply well is defined as having a screened interval at the 

bottom 30 feet of the aquifer with a pumping rate of 0.6 million gallons per day. Areas where the 

"typical" well would dewater the aquifer are shown in white.)
44

tions placed on withdrawals from the confined 

aquifers have necessitated increased withdrawals 

from the Kirkwood-Cohansey aquifer system.

Results of numerical simulations of groundwater flow and transport were integrated with a 

GIS to characterize ground water with respect to its 

land-use source at the time and place of recharge 

and the age of the water. Ground-water flow was 

simulated with the computer program 

MODFLOW. These results were used as input to 

the computer program MODPATH, whose output 

consists of contributing areas and travel times from 

locations of recharge to locations of discharge. A 

GIS was used to organize the model input and 

output data and relate the recharge locations to 

land-use data sets. Simulated ages approximately 

matched the ages of water samples from monitoring wells measured by using tritium-helium and 

sulfur hexafluoride age-dating techniques.

The average age of water in the aquifer 

system in the study area is approximately 50 years; 

however, most of the water is younger. Volumetrically, 50 percent of the water is less than 20 years 

old and 25 percent is less than 10 years old. The 

time required for water to move from recharge to 

discharge is shortest near streams. Water entering 

the aquifer system near wells from which large 

volumes of water are withdrawn also has a relatively short travel time through the system. Travel 

times are longest for water recharged near groundwater divides. The age of water increases with 

depth in the aquifer, with the exception of water in 

areas near streams where the water flows upward 

to discharge to the stream. The vertically averaged 

age of water increases as the aquifer thickens and 

in the vicinity of streams.

Water that discharges to streams and wells is 

a mixture of water of different ages derived from 

areas with different land uses. Water that discharges to streams generally is younger than water 

that discharges to wells. Wells draw a greater percentage of water from agricultural and urban areas 

than streams because they typically are located in 

those areas.

Nitrate was chosen to demonstrate the effects 

of age and land use on water quality because of its 

widespread and increasing use, presence in 

ground- and surface-water systems, mostly conservative (not degraded or adsorbed) behavior, and 

availability of data. Based on estimates of nitrate 

concentrations in recharge over time for each landuse type together with the age and land-use distributions for various discharge points, nitrate concentrations were computed. Simulated nitrate 

concentrations in water from 27 public-supply 

wells compared favorably to measured concentrations, indicating that nitrate transport to these wells 

is conservative. Simulated and measured nitrate 

concentrations compared favorably for time-series 

data at three surface-water sites after simulated 

concentrations were decreased by 40 percent. This 

difference is attributed to the non-conservative 

behavior of nitrate in the aquifer near streams (for 

example, denitrification) and in the water column 

of the stream itself (for example, uptake by aquatic 

plants).

The model was used to evaluate the effects of 

various management alternatives on the concentration of nitrate in streams and public-supply wells 

under the assumption of no future changes in land 

use or withdrawal rates or locations. The time 

required for water to move through the aquifer 

results in a time lag between the reduction of 

nitrate input in recharge and the reduction of nitrate 

concentrations in streams and water from wells. In 

the gradual-reduction alternative, nitrate concentrations in streams and wells continued to increase 

for 10 to 15 years after the reduction was enacted. 

In both the immediate-ban and gradual-reduction 

alternatives, nitrate concentrations remained 

elevated for decades after nitrate input ceased. In 

the fixed-use alternative, concentrations in streams 

and wells continued to increase for 30 to 40 years 

before reaching a constant level.

The simulated spatial distribution of nitrate in 

streams in 2000 and 2050 was examined under the 

assumption of no future changes in land use, withdrawal rates or locations, or recharge concentration. As expected, concentrations were highest in 

agricultural areas and lowest in largely undeveloped areas. In general, the simulated nitrate con-
45

centrations increased over time, especially in 

streams in areas where the aquifer is thick and in 

streams with mostly undeveloped land nearby 

(short flow paths) and agricultural or urban land in 

the more distant parts of the contributing area 

(longer flow paths).

Results of the computer model simulations 

indicate that in most of the study area, given the 

same assumption of no future changes in land use, 

withdrawal rates or locations, or recharge concentration, nitrate concentrations in a typical domestic 

or public-supply well will increase over the next 50 

years (from 2000 to 2050). Nitrate concentrations 

likely will decrease in some areas where agricultural land has been converted to urban land. 

Nitrate concentrations in domestic wells will vary 

most because these wells intercept water derived 

from small contributing areas with fairly uniform 

land use and ground-water travel times. Nitrate 

concentrations in public-supply wells are between 

the highest and lowest concentrations in domestic 

wells because their contributing areas are larger 

and water is integrated from multiple age classes 

and land uses.
46

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