U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2006–5294

National Water-Quality Assessment Program

Vertical Gradients in Water Chemistry and Age in the

Northern High Plains Aquifer, Nebraska, 2003
Vertical Gradients in Water Chemistry and 

Age in the Northern High Plains Aquifer, 

Nebraska, 2003

By P.B. McMahon,1 J.K. Böhlke,1 and C.P. Carney2

1

U.S. Geological Survey

2

Nebraska Public Power District

National Water-Quality Assessment Program

Scientific Investigations Reports 2006–5294

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

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

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

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

U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

McMahon, P.B., Böhlke, J.K., and Carney, C.P., 2007, Vertical gradients in water chemistry and age in the northern 

High Plains aquifer, Nebraska, 2003: U.S. Geological Survey Scientific Investigations Report 2006–5294, 58 p.

ISBN 1-411-31773-4 
iii

Contents

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

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

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

Acknowledgments................................................................................................................................2

Description of Study Area ............................................................................................................................6

Geohydrologic Setting .........................................................................................................................6

Land Use and Water Use.....................................................................................................................6

Study Methods ...............................................................................................................................................6

Vertical Changes in Lithology ....................................................................................................................11

Vertical Hydraulic Gradients......................................................................................................................13

Vertical Gradients in Water Chemistry.....................................................................................................16

Tritium ...................................................................................................................................................17

Dissolved Oxygen................................................................................................................................17

Major Ions and Trace Elements........................................................................................................20

Nitrate...................................................................................................................................................24

Pesticides.............................................................................................................................................29

Vertical Gradients in Ground-Water Age.................................................................................................29

Mass-Balance Models.......................................................................................................................29

Radiocarbon Ages ..............................................................................................................................30

Summary and Conclusions.........................................................................................................................35

References Cited..........................................................................................................................................35

Appendix 1.....................................................................................................................................................41

Appendix 2.....................................................................................................................................................55

Figures

1–5. Maps showing:

 1. Land-surface elevations and locations of the northern, central, 

 and southern parts of the High Plains aquifer....................................................................3

 2. Ground-water elevations in the northern High Plains aquifer and 

 locations of the ground-water and surface-water sampling sites..................................4

 3. Saturated thickness of the northern High Plains aquifer and locations 

 of the ground-water and surface-water sampling sites ...................................................7

 4. Location of registered irrigation wells in the northern High Plains 

 and locations of the ground-water and surface-water sampling sites..........................8

 5. Land use overlying the northern High Plains aquifer and locations of the 

 ground-water and surface-water sampling sites ..............................................................9

6. Geohydrologic section between nested-well sites showing locations of the 

monitoring-well screens............................................................................................................12

7. Natural gamma and resistivity logs for boreholes at the nested-well sites.....................12

8. Water-level elevations and vertical hydraulic gradients at the nested wells..................16
iv

9. Concentrations of selected dissolved constituents in relation to well-screen 

depth below the water table.....................................................................................................20

10. Concentrations of selected dissolved constituents and mineral saturation 

indexes in relation to concentrations of dissolved inorganic carbon in 

water from the nested wells .....................................................................................................22

11. Strontium isotope ratios in relation to inverse strontium concentrations for 

water from the water-table and nested wells........................................................................22

12. Sulfur isotope ratios in relation to inverse sulfate concentrations for 

water from the water-table and nested wells........................................................................23

13. Concentrations of dissolved bromide in relation to concentrations of 

dissolved chloride in water from the nested wells and in precipitation ...........................23

14. (A) Oxygen isotope values for nitrate in relation to nitrogen isotope values 

for nitrate and (B) fraction of initial nitrate in recharge remaining in 

solution in relation to well-screen depth below the water table for 

water from the nested wells .....................................................................................................25

15. Adjusted radiocarbon age in relation to depth below the water table .............................32

16. Schematic diagram illustrating a method for calculating the maximum 

percentage of saturated thickness containing recently recharged water.......................33

17. Measured and modeled age distributions used to estimate long-term 

average recharge rates.............................................................................................................33

Tables

1. Construction information for monitoring wells........................................................................5

2. Lithologic description and mineralogy of selected drill cuttings .......................................14

3. Carbon content of sediments and δ13C and δ18O compositions of bulk carbonate.. ........15

4. Isotopic data from sampled monitoring wells screened in the northern High 

Plains aquifer and from the Platte River near Brady, Nebraska.........................................18

5. Dissolved-gas and related data, recharge temperatures, and estimates of initial 

nitrate concentration and isotope values in recharge.........................................................26

6. Summary of ground-water ages from tritium and radiocarbon dating .............................31

7. Apparent rates of dissolved-oxygen reduction and denitrification. ..................................34
v

Conversion Factors, Vertical datum, and abbreviations

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

square miles (mi2) 2.59 Square kilometers (km2)

Volume

gallon (gal) 3.785 liter (L) 

acre-foot (acre-ft) 1,233 cubic meter (m3) 

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

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

 °F=(1.8×°C)+32

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

 °C=(°F-32)/1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

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

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with credible scientific 

information that helps to enhance and protect the overall quality of life and that facilitates 

effective management of water, biological, energy, and mineral resources (http://www.usgs.

gov/). Information on the Nation’s water resources is critical to ensuring long-term availability 

of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish 

and wildlife. Population growth and increasing demands for water make the availability of that 

water, now measured in terms of quantity and quality, even more essential to the long-term 

sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 

to support national, regional, State, and local information needs and decisions related to 

water-quality management and policy (http://water.usgs.gov/nawqa). The NAWQA Program is 

designed to answer: What is the condition of our Nation’s streams and ground water? How are 

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 

and priorities. From 1991-2001, the NAWQA Program completed interdisciplinary assessments 

and established a baseline understanding of water-quality conditions in 51 of the Nation’s river 

basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.html). 

In the second decade of the Program (2001–2012), a major focus is on regional assessments 

of water-quality conditions and trends. These regional assessments are based on major river 

basins and principal aquifers, which encompass larger regions of the country than the Study 

Units. Regional assessments extend the findings in the Study Units by filling critical gaps in 

characterizing the quality of surface water and ground water, and by determining status and 

trends at sites that have been consistently monitored for more than a decade. In addition, the 

regional assessments continue to build an understanding of how natural features and human 

activities affect water quality. Many of the regional assessments employ modeling and other 

scientific tools, developed on the basis of data collected at individual sites, to help extend 

knowledge of water quality to unmonitored, yet comparable areas within the regions. The 

models thereby enhance the value of our existing data and our understanding of the hydrologic 

system. In addition, the models are useful in evaluating various resource-management scenarios 

and in predicting how our actions, such as reducing or managing nonpoint and point sources of 

contamination, land conversion, and altering flow and (or) pumping regimes, are likely to affect 

water conditions within a region.

Other activities planned during the second decade include continuing national syntheses of 

information on pesticides, volatile organic compounds (VOCs), nutrients, selected trace elements, and aquatic ecology; and continuing national topical studies on the fate of agricultural 

chemicals, effects of urbanization on stream ecosystems, bioaccumulation of mercury in stream 

ecosystems, effects of nutrient enrichment on stream ecosystems, and transport of contaminants to public-supply wells.

The USGS aims to disseminate credible, timely, and relevant science information to address 

practical and effective water-resource management and strategies that protect and restore 

water quality. We hope this NAWQA publication will provide you with insights and information 

to meet your needs, and will foster increased citizen awareness and involvement in the protection and restoration of our Nation’s waters. 
vii

The USGS recognizes that a national assessment by a single program cannot address all waterresource issues of interest. External coordination at all levels is critical for cost-effective management, regulation, and conservation of our Nation’s water resources. The NAWQA Program, 

therefore, depends on advice and information from other agencies—Federal, State, regional, 

interstate, Tribal, and local—as well as nongovernmental organizations, industry, academia, and 

other stakeholder groups. Your assistance and suggestions are greatly appreciated.

 Robert M. Hirsch

Associate Director for Water
Abstract

The northern High Plains aquifer is the primary source 

of water used for domestic, industrial, and irrigation purposes 

in parts of Colorado, Kansas, Nebraska, South Dakota, and 

Wyoming. Despite the aquifer’s importance to the regional 

economy, fundamental ground-water characteristics, such as 

vertical gradients in water chemistry and age, remain poorly 

defined. As part of the U.S. Geological Survey’s National 

Water-Quality Assessment Program, water samples from 

nested, short-screen monitoring wells installed in the northern 

High Plains aquifer were analyzed for major ions, nutrients, 

trace elements, dissolved organic carbon, pesticides, stable and 

radioactive isotopes, dissolved gases, and other parameters 

to evaluate vertical gradients in water chemistry and age in 

the aquifer. Chemical data and tritium and radiocarbon ages 

show that water in the aquifer was chemically and temporally stratified in the study area, with a relatively thin zone of 

recently recharged water (less than 50 years) near the water 

table overlying a thicker zone of older water (1,800 to 15,600 

radiocarbon years). In areas where irrigated agriculture was 

an important land use, the recently recharged ground water 

was characterized by elevated concentrations of major ions 

and nitrate and the detection of pesticide compounds. Below 

the zone of agricultural influence, major-ion concentrations 

exhibited small increases with depth and distance along flow 

paths because of rock/water interactions. The concentration 

increases were accounted for primarily by dissolved calcium, 

sodium, bicarbonate, sulfate, and silica. In general, the chemistry of ground water throughout the aquifer was of high quality. 

None of the approximately 90 chemical constituents analyzed 

in each sample exceeded primary drinking-water standards.

Mass-balance models indicate that changes in groundwater chemistry along flow paths in the aquifer can be 

accounted for by small amounts of feldspar and calcite dissolution; goethite and clay-mineral precipitation; organic-carbon 

and pyrite oxidation; oxygen reduction and denitrification; and 

cation exchange. Mixing with surface water affected the chemistry of ground water in alluvial sediments of the Platte River 

Valley. Radiocarbon ages in the aquifer, adjusted for carbon 

mass transfers, ranged from 1,800 to 15,600 14C years before 

present. These results have important implications with respect 

to development of ground-water resources in the Sand Hills. 

Most of the water in the aquifer predates modern anthropogenic activity so excessive removal of water by pumping is not 

likely to be replenished by natural recharge in a meaningful 

timeframe. Vertical gradients in ground-water age were used 

to estimate long-term average recharge rates in the aquifer. In 

most areas, the recharge rates ranged from 0.02 to 0.05 foot 

per year. The recharge rate was 0.2 foot per year in one part 

of the aquifer characterized by large downward hydraulic 

gradients.

Nitrite plus nitrate concentrations at the water table were 

0.13 to 3.13 milligrams per liter as nitrogen, and concentrations substantially decreased with depth in the aquifer. Dissolved-gas and nitrogen-isotope data indicate that denitrification in the aquifer removed 0 to 97 percent (average = 

50 percent) of the nitrate originally present in recharge. The 

average amount of nitrate removed by denitrification in the 

aquifer north of the Platte River (Sand Hills) was substantially greater than the amount removed south of the river (66 

as opposed to 0 percent), and the extent of nitrate removal 

appears to be related to the presence of thick deposits of 

sediment on top of the Ogallala Group in the Sand Hills that 

contained electron donors, such as organic carbon and pyrite, 

to support denitrification.

Apparent rates of dissolved-oxygen reduction and denitrification were estimated on the basis of decreases in dissolved-oxygen concentrations and increases in concentrations 

of excess nitrogen gas and ground-water ages along flow paths 

from the water table to deeper wells. Median rates of dissolved-oxygen reduction and denitrification south of the Platte 

River were at least 10 times smaller than the median rates 

north of the river in the Sand Hills. The relatively large denitrification rates in the Sand Hills indicate that the aquifer in that 

area may have a greater capacity to attenuate nitrate contamination than the aquifer south of the river, depending on rates of 

ground-water movement in the two areas. Small denitrification 

rates south of the river indicate that nitrate contamination in 

that part of the aquifer would likely persist for a longer period 

of time. 

Vertical Gradients in Water Chemistry and Age in the 

Northern High Plains Aquifer, Nebraska, 2003

By P.B. McMahon, J.K. Böhlke, and C.P. Carney
Introduction 

In 1991, the U.S. Geological Survey (USGS) began full 

implementation of the National Water-Quality Assessment 

(NAWQA) Program. The long-term goals of the NAWQA Program are to describe the status and trends in the quality of the 

Nation’s surface- and ground-water resources and determine 

the natural and anthropogenic factors affecting water quality 

(Gilliom and others, 1995). The High Plains Regional GroundWater study, which began in June 1998, is one of more than 50 

major river basins and aquifers being studied by the NAWQA 

Program. The High Plains aquifer is a nationally important 

water resource that underlies about 174,000 mi2 in parts of 

eight States (fig. 1). In 2000, about 30 percent of the ground 

water used for irrigation in the United States was pumped 

from this aquifer (Maupin and Barber, 2005). About 2 percent 

of the water pumped from the aquifer was used for drinking 

(Maupin and Barber, 2005). Because of the large areal extent 

of the High Plains aquifer, three geographic regions have 

been delineated for logistical purposes: northern High Plains 

(96,500 mi2), central High Plains (48,500 mi2), and southern 

High Plains (29,000 mi2) (fig. 1). The northern High Plains 

aquifer is the focus of this report. Reports similar to this one 

already have been published for the central and southern 

High Plains aquifers (McMahon, 2001; McMahon and others, 

2004a, b).

The northern High Plains aquifer is one of the primary 

sources of water used for domestic, industrial, and irrigation 

purposes in Nebraska, and in parts of Colorado, Kansas, South 

Dakota, and Wyoming. In addition, it contains the largest 

amount of ground water in storage and the largest saturated 

thicknesses of the entire High Plains aquifer system (McGuire 

and Sharpe, 1997). Despite the aquifer’s importance to the 

overall economy of the northern High Plains and the size of 

this resource, fundamental ground-water characteristics, such 

as vertical gradients in water chemistry and age, remain poorly 

defined. Understanding how water chemistry changes with 

depth in the aquifer is relevant because current or future waterlevel declines could increase the demand for water deeper in 

the aquifer. Contamination of shallow parts of the aquifer by 

sources at the land surface also could increase the demand for 

water deeper in the aquifer. Ground-water age distributions 

in aquifers are relevant because, when combined with other 

hydrogeologic and geochemical information, they can provide 

insights into recharge rates, an aquifer’s susceptibility to contamination from sources at the land surface, and biogeochemical reaction rates.

Numerous studies have examined water quality in the 

northern High Plains aquifer at various spatial scales, including those by Engberg and Spalding (1978), Krothe and others 

(1982), Engberg (1984), Johnson and others (1986), Exner and 

Spalding (1990), Verstraeten and others (1995), Druliner and 

others (1996), Gottula (1996), Gosselin and others (1997a), 

Litke (2001), and Stanton (2006). Those studies relied primarily on water samples collected from domestic and irrigation 

wells, which often have relatively long screened intervals that 

are not well suited for describing vertical gradients in water 

chemistry and age. Some studies have used water samples 

collected from short-screen monitoring wells to examine vertical gradients in water chemistry in the aquifer (Spalding and 

others, 1993; Steele and others, 1998; Emmons and Bowman, 

2000) and age in the aquifer (Druliner, 1997; Spalding and 

others, 2001; Verstraeten and others, 2001). Most of those 

studies focused on shallow zones in the aquifer or on parts of 

the aquifer that have relatively thin saturated thicknesses and 

short ground-water residence times. Even in those settings, 

however, gradients in ground-water chemistry and age were 

evident. Information like this is needed in parts of the aquifer 

where saturated thicknesses are large and development of the 

deep ground-water resource has not occurred so that baseline 

conditions in those areas can be established.

Purpose and Scope

The purpose of this report is to provide baseline information on vertical gradients in water chemistry and age in 

selected areas of the northern High Plains aquifer in Nebraska 

where saturated thicknesses are large but extensive development of the deep ground-water resource has not occurred. In 

2002 and 2003, 19 monitoring wells at 5 sites were completed 

in the aquifer (fig. 2 and table 1). The wells had 10-ft-long 

screens that were placed at various depths below the water 

table. In 2003 and 2004, water samples were collected from 

each well and from the Platte River and were analyzed for 

major ions; nutrients; trace elements; dissolved organic 

carbon (DOC); pesticides; dissolved helium (He), methane 

(CH4), neon (Ne), nitrogen (N2), oxygen (O2), and argon (Ar) 

gases; tritium (3H); stable isotopes of water (δ2H[H2O] and 

δ18O[H2O]); stable isotopes of dissolved nitrate (δ15N[NO3] 

and δ18O[NO3]) and nitrogen isotope of N2 gas (δ15N[N2]); 

stable and radioactive isotopes of dissolved inorganic carbon (δ13C[DIC] and 14C[DIC]); stable isotopes of dissolved 

sulfate (δ34S[SO4]); and the dissolved strontium isotope ratio 87Sr/86Sr. Alkalinity, pH, temperature, specific conductance, 

O2, and turbidity of the water were measured in the field prior 

to sample collection. Existing data from two additional wells, 

Imperial and Keith County (Co.) #1 (table 1), also were used 

in the analysis.

Acknowledgments

The cooperation of landowners who agreed to the 

installation of monitoring wells on their property is gratefully acknowledged. Borehole drilling and well installations 

were done in collaboration with the Platte River Cooperative 

Hydrology Study (COHYST). We thank Jim Goeke, Nebraska 

Conservation and Survey Division, for his contributions during the drilling phase of this project. We thank Glen Bowers, 

Twin Platte Natural Resources District, for help in procuring 

landowner permissions for well installations. X-ray diffrac  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
IMPERIAL

KEITH CO. 1

Colby RIVER SITE

Wayne

Odessa

Guymon

Wichita

Midland

Lubbock

Amarillo

Cheyenne

Garden

City

Scottsbluff

North

Platte

108° 104° 100° 96°

42°

34°

50 100 150 MILES

0 50 100 150 KILOMETERS

0

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian -96°

EXPLANATION

Elevation, in

feet

1,089

7,913

Detailed study

area shown in

figures 2-5

Colorado

New

Mexico

Texas

Oklahoma

Kansas

Nebraska

Wyoming South Dakota

Northern

High Plains

Central

High Plains

Southern

High Plains

Figure 1. Land-surface elevations and locations of the northern, central, and southern parts of the High Plains aquifer.

Introduction    
3,000

3,200

3,400

3,600

2,800

2,600

2,400

CUSTER

FURNAS

THOMAS

HARLAN

PHELPS

Republican River

Sutherland

KEITH CO. 1

LINCOLN CO. 3

LINCOLN

KEITH

PERKINS

Brady

North

Platte

RIVER SITE

LINCOLN CO. 4

LINCOLN CO. 2

LINCOLN CO. 1

DAWSON

Tryon LOGAN

MCPHERSON CO. 1

MCPHERSON

Geohydrologic

section

BLAINE

HOOKER

ARTHUR

Arthur

IMPERIAL CHASE

Imperial

DUNDY

HITCHCOCK

RED WILLOW

-102° -101°30' -101° -100°30' -100° -99°30'

40o

40°30'

41°

41°30'

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian -96°

20 MILES

A

EXPLANATION

Line of equal watertable elevation, in

feet. Contour interval

200 feet, based on

measurements from

2000

Nested wells

A'

Extent of Nebraska

Sand Hills region

River sampling site

Towns

Water-table wells

Approximate direction

of ground-water

movement

South Platte River

North Platte River

HAYES

FRONTIER

0

0 10 20 KILOMETERS

10 20 MILES

Figure 2. Ground-water elevations in the northern High Plains aquifer and locations of the ground-water and surface-water 

sampling sites. Elevations based on 2000 water-level measurements (V.L. McGuire, U.S. Geological Survey, written commun., 

2006).

 Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Table 1. Construction information for monitoring wells.

[All depths are in feet below land surface; casing and screen for all wells are made of threaded, flush-joint, Schedule 40 or Schedule 80 polyvinyl chloride 

(PVC) with a diameter of 2.0 or 2.5 inches; screen openings are 0.020 inch wide; --, not measured.]

Site 

identification

Well 

name

Construction 

date

Elevation 

of land 

surface 

(feet)

Depth to 

base of 

aquifer 

(feet)

Depth to 

water at 

time of 

sampling 

(feet)

Total 

depth 

of hole 

(feet)

Total 

depth 

of well 

(feet)

Depth 

of screened 

interval 

(feet)

Depth of 

screen 

midpoint

 below 

water 

table at 

time of 

sampling 

(feet)

403713101334401 Imperial 6/9/02 3,294 -- 91.60 109 109 95 to 104 7.90

410344101154201 Keith County #1 7/25/05 3,275 448 148.75 480 180 170 to 175 23.75

405937100183504 Lincoln County 

#1-25 4/17/02 2,633 380 12.80 420 25 15 to 25 7.20

405937100183503 Lincoln County 

#1-119 11/13/02 2,633 380 13.85 275 124 109 to 119 100.15

405937100183502 Lincoln County 

#1-270 11/13/02 2,633 380 13.05 275 275 260 to 270 251.95

405937100183501 Lincoln County 

#1-370 4/17/02 2,633 380 12.78 420 375 360 to 370 352.22

410944100192104 Lincoln County 

#2-255 4/20/02 2,929 770 208.75 260 260 245 to 255 41.25

410944100192102 Lincoln County 

#2-590 11/18/02 2,929 770 210.03 600 600 580 to 590 374.97

410944100192101 Lincoln County 

#2-760 4/20/02 2,929 770 209.19 800 765 750 to 760 545.81

405745100174504 Lincoln County 

#3-30 11/5/02 2,614 395 7.32 260 35 20 to 30 17.48

405745100174503 Lincoln County 

#3-110 11/5/02 2,614 395 7.28 475 115 100 to 110 97.54

405745100174502 Lincoln County 

#3-250 11/5/02 2,614 395 7.36 260 255 240 to 250 237.58

405745100174501 Lincoln County 

#3-390 11/5/02 2,614 395 7.48 475 395 380 to 390 377.45

412213100425304 Lincoln County 

#4-195 5/14/03 3,075 700 104.17 730 200 185 to 195 85.83

412213100425303 Lincoln County 

#4-359 5/15/03 3,075 700 107.80 371 364 349 to 359 246.20

412213100425302 Lincoln County 

#4-510 5/16/03 3,075 700 107.67 525 515 500 to 510 397.33

412213100425301 Lincoln County 

#4-695 5/14/03 3,075 700 107.74 730 700 685 to 695 582.26

413336101101004 McPherson 

County #1-45 5/24/02 3,326 775 8.25 800 50 35 to 45 31.75

413336101101003 McPherson 

County #1-330 11/23/02 3,326 775 21.39 560 340 320 to 330 303.61

413336101101002 McPherson 

County #1-550 11/23/02 3,326 775 21.20 560 555 540 to 550 523.80

413336101101001 McPherson 

County #1-770 5/24/02 3,326 775 22.10 800 780 760 to 770 742.90

Introduction 
tion (XRD) analyses of sediment samples were done by 

George Breit (USGS, Denver). We thank Rodney Caldwell 

(USGS, Helena, Montana) and David Gosselin (University of 

Nebraska, Lincoln, Nebraska) for comments on earlier drafts 

of this manuscript. 

Description of Study Area

The Nebraska study area includes four nested-well sites 

in Lincoln County, one nested-well site in McPherson County, 

water-table wells in Chase and Keith Counties, and a river 

sampling site on the Platte River in Lincoln County (fig. 2). 

The sampling sites north of the Platte River are located in the 

Sand Hills region of Nebraska, which is the largest sand dune 

area in the Western Hemisphere (Bleed and Flowerday, 1998). 

The nested-well site south of the Platte River is located in the 

alluvial system of the Platte River Valley.

Geohydrologic Setting

The High Plains aquifer in the study area primarily 

consists of alluvial sediments of the Ogallala Group, which are 

Miocene in age (Swinehart and Diffendal, 1998). In general, 

those sediments are composed of variably consolidated clay, 

silt, sand, and gravel. Detailed descriptions of the aquifer 

sediments in Lincoln and McPherson Counties, which include 

unconsolidated sediments and shale, siltsone, and sandstone, 

can be found in the reports by Goeke (2000, 2001). Other 

important water-bearing units of the High Plains aquifer in the 

study area include alluvial gravels of the Broadwater Formation (Pliocene in age) and alluvial sands and gravels in the 

Platte River Valley (Pleistocene and Holocene in age). In the 

study area, the aquifer is underlain by siltstones or clayey fine 

sands of the Brule Formation (Oligocene in age).

The largest saturated thicknesses in the entire High Plains 

aquifer occur in the Nebraska Sand Hills. In 2000, the average 

depth to water below land surface in the Sand Hills was about 

90 ft and the average saturated thickness was about 520 ft 

(V.L. McGuire, U.S. Geological Survey, written commun., 

2005). At the nested-well sites, depths to water ranged from 

about 10 to 200 ft and saturated thicknesses ranged from about 

400 to 725 ft (fig. 3). Hydraulic gradients in the aquifer north 

of the Platte River indicate ground-water movement generally 

is from northwest to southeast (fig. 2). Ground-water movement south of the river generally is from west to east. The 

Platte River is the regional discharge area for the aquifer in the 

study area, but discharge to local streams such as Birdwood 

Creek also is important. Discharge to irrigation wells is important in the Platte River Valley and in agricultural areas south of 

the river (fig 4). There is relatively little irrigated agriculture in 

the Sand Hills.

Recharge to the northern High Plains aquifer may occur 

by infiltration of irrigation water, areally diffuse infiltration of 

precipitation, focused recharge of precipitation under depressions in the land surface, infiltration of stormwater runoff 

through streambeds, and leakage from irrigation canals and 

reservoirs in the Platte River Valley. Luckey and others (1986) 

estimated predevelopment recharge rates ranging from about 

0.005 to 0.1 ft/year on the basis of a regional model of groundwater movement; the largest recharge rates occurred in the 

Sand Hills and the smallest recharge rates occurred south and 

west of the Sand Hills. McMahon and others (2006) estimated 

a recharge rate of 0.2 ft/year on the basis of tritium profiles 

in the unsaturated zone in a nonirrigated area of southwestern 

Nebraska with sandy soils. 

Climate in the study area is characterized by cold and 

relatively dry winters and hot and relatively wet summers. 

Mean annual precipitation in the study area is about 20 in. 

(1971–2000), with about 60 percent of the precipitation occurring from May through August (High Plains Regional Climate 

Center, 2005). Mean annual air temperature in the study area 

is about 48°F (9°C). 

Land Use and Water Use

Land in the Sand Hills is rangeland used primarily for 

grazing. Irrigated agriculture is the primary land use in the 

Platte River Valley and in areas south of the river (fig. 5). In 

2002, corn (6.8 million acres), wheat (3.3 million acres), forage crops (3.2 million acres), and soybeans (2.9 million acres) 

were the primary crops grown in the northern High Plains in 

terms of acres planted (U.S. Department of Agriculture, 2002). 

Of the approximately 5.1 million acre-ft of water withdrawn 

from the High Plains aquifer in Nebraska in 1990, 93 percent 

was used for irrigation (McGuire and Sharpe, 1997). In 1990, 

about 12 percent of the northern High Plains area was irrigated 

(Qi and others, 2002). Lincoln Co. #3 was the only nestedwell site at which an irrigated crop (corn) was grown (fig. 5).

Study Methods

In 2002 and 2003, nested monitoring wells were installed 

at each of five sites for the collection of water levels and water 

samples for chemical analysis. The well sites north of the 

river generally are located along a regional flow path from 

northwest to southeast in the aquifer (fig. 2). Water samples 

for chemical analysis were collected from the wells and from 

the Platte River by the USGS in 2003 and 2004. The following 

section of the report describes the methods used for the installation of monitoring wells, geophysical logging, water-level 

measurements, sediment-sample collection, water-sample collection, chemical analyses of sediment and water, and quality 

control of water-quality data.

Well installation. Monitoring wells were installed by the 

USGS and contract drillers in 2002 and 2003 using mud-rotary 

drilling techniques. In general, two boreholes were drilled at 

each site, and two wells were placed in each borehole using 

established well-installation protocols (Lapham and others, 

 Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian -96°

-102° -101°30' -101° -100°30' -100° -99°30'

40°

40°30'

41°

41°30'

EXPLANATION

Nested wells

Extent of Nebraska

Sand Hills region

Saturated Thickness

0 to 250 ft

250 to 500 ft

500 to 750 ft

750 to 1,000 ft

River sampling site

Towns

Water-table wells

IMPERIAL

DUNDY

HAYES

HITCHCOCK RED WILLOW

Imperial

LINCOLN CO. 3

LINCOLN

PERKINS

KEITH CO. 1

RIVER SITE

LINCOLN CO. 4

LINCOLN CO. 2

LINCOLN CO. 1

DAWSON

Brady

North

Platte

Sutherland

Tryon

ARTHUR

Arthur MCPHERSON CO. 1

HOOKER THOMAS BLAINE

CUSTER

Republican River

South Platte River

North Platte River

CHASE

KEITH

LOUP

FURNAS

LOGAN

MCPHERSON

0 10 20 KILOMETERS

0 10 20 MILES

Figure 3. Saturated thickness of the northern High Plains aquifer and locations of the ground-water and surfacewater sampling sites. Thicknesses based on water-level measurements made in 2000 and the U.S. Geological Survey 

base-of-aquifer map (V.L. McGuire, U.S. Geological Survey, written commun., 2006; Gutentag and others, 1984).

Study Methods 
HARLAN

PHELPS

-102° -101°30' -101° -100°30' -100° -99°30'

EXPLANATION

Nested wells

Extent of Nebraska

Sand Hills region

Registered irrigation

well

River sampling site

Towns

Water-table wells

RIVER SITE

LINCOLN CO. 4

LINCOLN CO. 2

LINCOLN CO. 1

LINCOLN CO. 3

CUSTER

DAWSON

BLAINE LOUP THOMAS

LOGAN

Brady

North

Platte

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian -96°

IMPERIAL

LINCOLN

DUNDY

HAYES

PERKINS

FRONTIER

HITCHCOCK RED WILLOW

Imperial

KEITH Sutherland

ARTHUR

CHASE

KEITH CO. 1

40°

40°30'

41°

41°30'

Arthur

Tryon

MCPHERSON CO. 1

HOOKER

MCPHERSON

FURNAS Republican River

South Platte River

North Platte River

0 10 20 KILOMETERS

0 10 20 MILES

Figure 4. Location of registered irrigation wells in the northern High Plains and locations of the ground-water and 

surface-water sampling sites. 

 Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
KEITH

FURNAS

HARLAN

PHELPS

MCPHERSON

GOSPER

Arthur MCPHERSON CO. 1

ARTHUR

HOOKER THOMAS

Tryon

-102° -101°30' -101° -100°30'

41°

41°30'

KEITH CO. 1

LINCOLN CO. 4

LINCOLN CO. 2

LINCOLN CO. 1

CUSTER

LINCOLN

DAWSON

PERKINS

LOGAN

Brady

North

Platte

Sutherland

IMPERIAL

CHASE

HAYES

FRONTIER

Imperial

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection

Standard Parallels 29°30' and 45°30', central meridian -96°

40°

40°30'

DUNDY

HITCHCOCK RED WILLOW

Nested wells

Extent of Nebraska

Sand Hills region

Forest

River sampling site

Towns

Water-table wells

EXPLANATION

Urban

Agriculture

Rangeland

Water

Wetland

Barren

Land Use

LOUP BLAINE

0 10 20 KILOMETERS

0 10 20 MILES

-100° -99°30'

LINCOLN CO. 3

RIVER SITE

Figure 5. Land use overlying the northern High Plains aquifer and locations of the ground-water and surface-water 

sampling sites. Land-use data from Feagas and others (1983) collected from about 1977 to 1982.

Study Methods 
1995). Wells were installed by placing polyvinyl chloride 

(PVC) well screen and casing in the borehole after drilling 

to the target depth, placing a sand filter in the annular space 

around the well screen and bentonite plug on top of the sand 

filter, then sealing the annular space from the top of the plug to 

the bottom of the next screened interval using a high-density 

bentonite grout. Well screen and casing for the shallower well 

then were placed in the borehole, followed by a sand filter 

around the well screen, bentonite plug on top of the sand filter, 

and bentonite grout from the top of the plug to land surface. 

At each site, the shallowest well was installed near the water 

table and the deepest well was installed near the bottom of 

the aquifer. The wells were developed by bailing and pumping (water-table wells) or by air lifting and pumping (deeper 

wells) immediately after installation and again at least 1 month 

before sample collection. During the second well-development 

period, pH, temperature, specific conductance, and turbidity of the water were monitored to determine when the well 

began producing water representative of the aquifer. Water 

was considered representative of the aquifer after a minimum 

of five casing volumes of water had been pumped and the 

change in pH, temperature, specific conductance, and turbidity 

of the water between three successive measurements (taken at 

30-minute intervals) was less than 5 percent. Well-construction information is listed in table 1. The well name includes 

site and well-depth information, except for Imperial and Keith 

Co. #1. For example, Lincoln Co. #1-270 refers to a well at 

the Lincoln County #1 site, with a maximum depth of the 

screened interval of 270 ft below land surface.

Geophysical logging. Natural gamma and electrical 

resistivity logs were obtained for the open boreholes. Only the 

deepest borehole at each site was logged. The boreholes were 

logged by Sargent Irrigation Company, Broken Bow, Nebr.

Water-level measurements. Water levels in all wells 

were measured before water-sample collection by using an 

electric tape. In addition, water levels in the nested wells were 

measured monthly for 13 months. The latitude and longitude 

of the well locations were determined by using a Global Positioning System (GPS) with a precision of about ±20 ft. The 

vertical position of the measuring point for each nested well 

was determined relative to nearby benchmarks using surveying 

equipment with a precision of about 0.2 ft.

Sediment-sample collection. Sediment cuttings were 

collected from the drilling-mud return flow every 5 ft by 

using a wire-mesh screen. The cuttings were rinsed in clean 

tap water to remove excess drilling mud and stored in sealed 

canvas bags after the lithology was determined.

Water-sample collection. Ground-water and surfacewater samples were collected and processed in a mobile waterquality laboratory using methods described by Koterba and 

others (1995) and McMahon and others (2004a). Water was 

pumped from the wells by using a submersible pump made of 

stainless steel and Teflon. To minimize the risk of sample contamination, sample collection and preservation took place in 

dedicated environmental chambers consisting of clear polyethylene bags supported by tubular PVC frames. Samples from 

the river were collected by hand from the center of flow using 

Teflon bottles and then filtered and preserved in the mobile 

laboratory. All samples analyzed for dissolved constituents 

(except dissolved gases) were filtered through 0.45-micrometer capsule filters. Sampling equipment was thoroughly 

cleaned after sampling each site. Measurements of alkalinity, 

pH, temperature, specific conductance, O2, and turbidity were 

done in the field using the techniques described in the USGS 

field manual (variously dated). Dissolved hydrogen sulfide 

(H2S) also was measured in the field using a portable spectrophotometer and the methylene blue method (Hach Company, 

1992).

Mineralogy and chemical analysis of sediment. Mineralogy of selected sediment samples was estimated by examination of loose grains under a petrographic microscope and XRD 

analysis. Analyses of the inorganic carbon, organic carbon, 

and total carbon contents of sediment were done at the USGS 

National Water Quality Laboratory in Lakewood, Colo.

Chemical analysis of water. Chemical analyses for 

major ions, nutrients, trace elements, DOC, and pesticides 

were done at the USGS National Water Quality Laboratory in 

Lakewood, Colo. (Fishman, 1993; Zaugg and others, 1995). 

The charge balance error (calculated relative to the sum of the 

equivalents of cations and anions) was less than 5 percent for 

all analyses except those from wells Lincoln Co. #2-255 and 

McPherson County #1-770, which were 6.1 and 14.7 percent, 

respectively. The water from well Lincoln Co. #2-255 was 

relatively dilute, so a charge balance error slightly larger than 

5 percent is considered acceptable for the purposes of this 

study. The sample from McPherson County #1-770 was not 

used for geochemical modeling. 

Analyses of dissolved CH4, N2, O2, and Ar were done 

at the USGS Chlorofluorocarbon Laboratory in Reston, 

Va. (Busenberg and others, 1993; http://water.usgs.gov/lab/

dissolved-gas/, accessed on November 21, 2005). Analyses of 

dissolved He and Ne were done at the Lamont-Doherty Earth 

Observatory, Noble Gas Laboratory in Palisades, N.Y. (Ludin 

and others, 1998).

Analyses of δ2H[H2O], δ18O[H2O], δ15N[NO3], 

δ18O[NO3], δ15N[N2], and δ34S[SO4] were done at the USGS 

Stable Isotope Laboratory in Reston, Va. (Coplen and others, 

1991; Epstein and Mayeda, 1953; Böhlke and Denver, 1995; 

Carmody and others, 1997; Casciotti and others, 2002; Böhlke 

and others, 2003). The delta “δ” notation of isotope X is 

defined by equation 1. 

R is the ratio of the rare isotope to the abundant isotope (for 

example, 15N/14N) in the sample and standard. Values of 

δ2H[H2O] and δ18O[H2O] are reported in per mil (‰) relative 

to Vienna Standard Mean Ocean Water (VSMOW), normalized to VSMOW and Standard Light Antarctic Precipitation 

(Coplen, 1988). The 2-sigma precisions of hydrogen (H) and 

oxygen (O) isotopic results are 2 and 0.2‰, respectively. 

δX = Rsample – Rstandard

Rstandard

× 1000 (1)

10 Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Values of δ15N[NO3

] and δ18O[NO3] were measured using 

the denitrifier method (Casciotti and others, 2002), and are 

reported in ‰ relative to atmospheric N2 and VSMOW, 

respectively (Böhlke and others, 2003). The 2-sigma precisions of nitrogen (N) and O isotopic results are 0.5 and 

1.0‰, respectively. Values of δ15N[N2] are reported with 

respect to atmospheric N2 and were calibrated by analyses 

of air (0‰). The procedures were verified by analyses of 

air-saturated water samples that were collected at 16°C under 

controlled laboratory conditions, prepared as samples, and 

yielded δ15N[N2] values equal to +0.72±0.03‰, consistent 

with air-water equilibrium (Knox and others, 1992; Hübner, 

1986). Values of δ34S[SO4] are reported with respect to Vienna 

Cañon Diablo troilite (VCDT), defined by assigning a value of 

–0.3‰ exactly (Coplen and Krouse, 1998) to IAEA-S-1 Ag2S. 

Analyses of IAEA-SO-5, IAEA-SO-6, and National Bureau of 

Standards (NBS) 127 BaSO4 reference materials in the USGS 

laboratory yield +0.5, -34.05, and +21.10‰, respectively. The 

2-sigma precision of sulfur (S) isotopic results is 0.4‰. 

Analyses of 3H were done at the USGS Tritium Laboratory in Menlo Park, Calif. (Thatcher and others, 1977). The 

average 2-sigma precision of the analyses was 0.3 tritium units 

(TU).

Measurements of δ13C[DIC], reported relative to Vienna 

Peedee belemnite (VPDB), and 14C[DIC] sample preparation 

were done at the University of Waterloo in Ontario, Canada. 

Measurements of 14C[DIC] were done at the IsoTrace Radiocarbon Laboratory in Toronto, Canada, using accelerator mass 

spectrometry. Reported 14C values, in percent modern carbon 

(pmc), are not adjusted for the difference between –25‰ and 

the δ13C value of the sample. 

Strontium isotope analyses were done at the USGS SolidSource Mass Spectrometry Laboratory in Menlo Park, Calif. 

Ratios of 87Sr/86Sr were analyzed by the positive ion thermal 

mass spectrometry method with a precision of 0.00002.

Quality control of water-quality data. Quality of 

sample collection, processing, and analysis was checked by 

collecting quality-control samples. The number of qualitycontrol samples was approximately 30 percent of the number 

of environmental samples. Quality-control samples included 

field blanks, replicates, and pesticide spikes. Field blanks verified that decontamination procedures were adequate and that 

field and laboratory procedures did not contaminate samples. 

The source solution for blank samples was specially prepared 

organic- and inorganic-free water provided by the USGS. 

Blank solution was passed through all sampling equipment, 

and then a sample of the blank solution was collected using 

the same procedures used to collect environmental samples. 

Blank samples were collected for major ions, nutrients, trace 

elements, DOC, and pesticides. Replicate samples assessed 

the combined effects of field and laboratory procedures on 

measurement variability. Replicate samples were collected 

sequentially for all analyses. Pesticide spikes are environmental samples collected and spiked in the field with known 

quantities of the pesticide compounds of interest. Pesticide 

spike samples were used to test for analytical recoveries of 

the spiked compounds and for matrix interferences from the 

other chemicals in the water sample. Analytical recoveries are 

expressed in percentages of the expected concentrations. 

Results from the field-blank and replicate sample analyses indicate that generally there were no systematic problems 

with contamination or analytical precision. In addition, all but 

two to the samples had ion charge-balance errors less than 

5 percent. The sample from Lincoln Co. #2-255 had a charge 

balance error of 6.1 percent, which was considered to be 

acceptable given the small concentration of dissolved solids 

in that sample. The sample from McPherson Co. #1-770 had 

a charge balance error of 14.7 percent. It is likely that the 

measured field alkalinity for that sample was too low. Nevertheless, that sample was not used for mass-balance modeling 

because of the relatively large charge balance error. Analytical 

recoveries for the pesticide compounds averaged 107±30 percent of the expected concentrations. The quality-control data 

are available online at http://co.water.usgs.gov/nawqa/hpgw/

datarep/TOC.html (accessed on 2/1/2007).

Vertical Changes in Lithology

Lithologic variability in the aquifer may affect water 

chemistry and ground-water age distributions because of its 

effect on rock/water interactions and rates and directions of 

ground-water movement. Therefore, vertical changes in lithology in the aquifer were examined using lithologic descriptions of sediment cuttings collected during drilling, borehole 

geophysical (gamma and resistivity) logs, XRD analyses of 

sediments, and elemental carbon (C) analyses. Gamma logs 

record the natural gamma radiation emitted by radioactive elements in sediments (Keys, 1990). Shale and clay tend to have 

larger proportions of these radioactive elements than sand; 

therefore, an increase in gamma activity recorded by gamma 

logs generally is attributed to increased proportions of clay 

in the aquifer or changes in clay mineralogy. Resistivity logs 

record the electrical conductivity or resistivity of the rocks and 

water surrounding the borehole. Electrical conductivity and 

resistivity are affected by the porosity, permeability, and clay 

content of the rocks and by the dissolved-solids concentration of the water within the rocks. In freshwater aquifers like 

the one studied here, decreasing resistivity readings normally 

indicate increasing clay content of the sediments.

The northern High Plains aquifer at the study sites is 

underlain by sediment of the Brule Formation that consists of 

dense reddish-brown siltstone and clayey fine sand that generally was characterized by relatively high natural gamma counts 

and decreased resistivity in geophysical logs (figs. 6 and 7). 

The presence of siltstone and clayey fine sand at the base of 

the aquifer indicates that there probably was not substantial 

hydrologic interaction between the High Plains aquifer and 

underlying water-bearing units.

Aquifer sediments in the study area were heterogeneous 

vertically and horizontally on the basis of natural gamma 

Vertical Changes in Lithology 11
A’ A McPherson Co. #1 Lincoln Co. #4 Lincoln Co. #2 Lincoln Co. #1

Lincoln Co. #3

ELEVATION, IN FEET

4,000

3,500

3,000

2,500

2,000

0 10 20 30 40 50 60 70

DISTANCE, IN MILES

Platte

River

Valley

Water

table

Land

Surface

Vertical scale greatly exaggerated

Figure 6. Geohydrologic section between nested-well sites showing locations of the 

monitoring-well screens. The section location A-A’ is shown in figure 2.

DEPTH BELOW LAND SURFACE, IN FEET

NATURAL GAMMA

LOG, IN COUNTS

PER SECOND

RESISTIVITY LOG,

IN OHM-METERS

NATURAL GAMMA

LOG, IN COUNTS

PER SECOND

RESISTIVITY LOG,

IN OHM-METERS

NATURAL GAMMA

LOG, IN COUNTS

PER SECOND

RESISTIVITY LOG,

IN OHM-METERS

NATURAL GAMMA

LOG, IN COUNTS

PER SECOND

RESISTIVITY LOG,

IN OHM-METERS

NATURAL GAMMA

LOG, IN COUNTS

PER SECOND

RESISTIVITY LOG,

IN OHM-METERS

Figure 7. Natural gamma and resistivity logs for boreholes at the nested-well sites.

12 Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
and resistivity log responses and lithologic descriptions. At the 

Lincoln Co. #2 site, for example, sediment near the water table 

was characterized by thick grey-brown clay from about 190 to 

235 ft below land surface (fig. 7). Such a thick low-permeability 

unit was not observed in any of the other boreholes. The general 

absence of thick confining layers in the aquifer at the other locations indicates that water-table drawdowns caused by pumping 

large-capacity wells could draw anthropogenic contaminants near 

the water table deeper into the aquifer. At the present time (2005), 

the low density of irrigation wells in the Sand Hills (fig. 4) indicates that this process is not likely to be regionally important. 

Sediments near the base of the aquifer at Lincoln Co. #2 and 

at McPherson Co. #1 were characterized by 50- to 75-ft-thick 

deposits of very coarse sand and fine gravel, whereas sediments 

near the base of the aquifer at the other sites were characterized 

by interbedded fine sandstone and siltstone. Thick intervals of 

relatively coarse sediment were observed at shallower depths 

at the other sites. At Lincoln Co. #4, for example, intervals of 

coarse sand and fine gravel were present in the upper half of 

the aquifer at depths of about 200 and 240 ft below land surface 

(fig. 7). The shallowest well at Lincoln Co. #3 was screened in 

coarse sand and fine gravel of the Platte River alluvium. Coarsegrained deposits like these are primary water-bearing intervals 

in the aquifer; therefore, their three-dimensional distribution 

and connectivity in the aquifer has important consequences with 

respect to ground-water quality and age distributions (Bethke 

and Johnson, 2002; Weissmann and others, 2002). Weissmann 

and others (2002) showed that “young” ground-water samples 

can contain a large fraction of “old” water. If the young fraction 

in such samples contains contaminants, then it is possible that 

contaminant levels in those samples will increase in the future as 

the fraction of young water increases.

Quartz is the primary detrital mineral in the aquifer sediments (table 2). Quartz is a relatively stable mineral phase that is 

not readily dissolved in water; therefore, quartz dissolution is not 

a substantial source of dissolved solids in ground water. Volcanic ash apparently was present in one sample from the aquifer 

(Lincoln Co. #4, 655 to 600 ft) that was analyzed by XRD, and 

its presence in sediment cuttings was noted several times in the 

lithologic logs. Volcanic ash is relatively soluble compared to 

quartz, and its dissolution can be a source of dissolved arsenic 

(As) and uranium (U). Calcite and feldspars (sodium plagioclase 

and potassium feldspar) generally were less abundant than quartz 

in the aquifer, but they are relatively reactive mineral phases. Dissolution of these minerals can be a source of dissolved calcium 

(Ca2+), potassium (K+), sodium (Na+), bicarbonate (HCO3

–

), 

and silica (SiO2). Calcite occurred in various forms in cuttings, 

including pore-filling cements in sandstone rock fragments and 

caliche. The δ13C values of bulk carbonate in sediment samples 

ranged from –9.1 to –1.4 ‰ (table 3). Bulk carbonate in the 

undifferentiated sediments of Tertiary and Quaternary age was 

most enriched in 13C and bulk carbonate in or near the Brule 

Formation was most depleted in 13C. Illite, kaolinite, and smectite 

were the primary clay minerals identified in the sediment samples 

(table 2). Smectite has a relatively large cation-exchange 

capacity, and cation exchange can be an important process 

affecting ground-water chemistry.

Only one sample (McPherson County #1, 73 to 75 ft), 

a black siltstone from the undifferentiated sediments of 

Tertiary and Quaternary age overlying the Ogallala Group, 

contained detectable sedimentary organic carbon (table 3). 

Other sampled siltstones and sandstones did not contain detectable quantities of organic carbon. Despite the 

apparent lack of organic carbon in the sampled sediments, 

electron donors like sedimentary organic carbon or pyrite 

must have been present in the aquifer to account for some 

of the changes in water chemistry with depth at all of the 

sites (see the section “Dissolved Oxygen”).

Vertical Hydraulic Gradients

Water-level data were collected at all of the nestedwell sites on about a monthly basis from December 2002 

to December 2003 (fig. 8), except at the Lincoln Co. #4 

site, where measurements were made three times from 

August 2003 to April 2004. Measured water levels fluctuated by less than 4 ft at all of the sites except Lincoln 

Co. #2, where irrigation pumping in the area resulted in 

seasonal changes in water level of about 34 ft (fig. 8). The 

pattern of water-level fluctuations at Lincoln Co. #3 also 

indicates seasonal effects from irrigation pumping in the 

area. Maximum fluctuations at Lincoln Co. #3 were about 

14 times smaller than those at Lincoln Co. #2, even though 

Lincoln Co. #3 is located closer to irrigated fields than 

Lincoln Co. #2. Lincoln Co. #3 is located in the Platte 

River Valley; therefore, water-level declines resulting from 

irrigation at that site could be moderated by the river. As 

will be discussed in the section “Major Ions and Trace 

Elements,” there is chemical evidence for infiltration of 

river water into the aquifer in the vicinity of Lincoln Co. 

#3.

Vertical hydraulic gradients are based on differences in water-level elevations among nested wells and 

are shown in figure 8. The gradients indicate the vertical 

direction of water movement in the aquifer and were calculated for vertically adjacent pairs of nested wells using 

equation 2.

 

 

where Hs is water-level elevation in the shallow well, 

Hd is water-level elevation in the deep well, and Δz is 

the distance between the two well screens. Downward 

gradients near the water table indicate recharge conditions 

and the potential for contaminants that enter the aquifer 

from land surface to move deeper into the aquifer. Upward 

hydraulic gradients near the water table indicate dis­charge 

conditions. All of the gradients near the water table were 

downward, except at Lincoln Co. #3 (fig. 8). The 

gradient = 

Hs – Hd

∆z

(2)

Vertical Hydraulic Gradients 13
Table 2. Lithologic description and mineralogy of selected drill cuttings.

[All depths are in feet below land surface; mineralogy determined by X-ray diffraction analysis; relative mineral abundances are listed in 

decreasing order; Na-plagioclase, sodium plagioclase; K-feldspar, potassium feldspar; ?, questionable presence.]

Geologic 

unit

Well 

name

Depth 

interval 

(feet)

Lithologic description Mineralogy

Undifferentiated sediments 

of Tertiary and 

Quaternary age

Lincoln 

County #2 204 to 205

Well sorted, light reddish brown clayey fine sand to silt. Some of the 

sample is marked by white to very pale reddish-brown cement. Few 

plant fragments preserved. 

Whole sample: quartz, smectite,

 illite, Na-plagioclase, K-feldspar,

 calcite, zeolite

White fragments: calcite, quartz,

 smectite, Na-plagioclase

Lincoln 

County #2 225 to 229

Fine to medium pale reddish brown clayey sand. Poorly sorted, rounding decreases with grain size, numerous fragments of black soft grains 

– may be charcoal or heavily carbonized woody fragments, other plant 

fragments are relatively tan to brown with cream colored interiors.

Whole sample: quartz, smectite, 

 K-feldspar, Na-plagioclase, illite

Lincoln 

County #4 190 to 195

Poorly sorted, coarse to medium sand. Few pebble-sized aggregates 

cemented by sparse pale red-brown clay/carbonate. Several plant fragments are brown and relatively fresh in appearance.

Whole sample: quartz, Na-

 plagioclase, K-feldspar, calcite,

 illite

Ogallala Group Lincoln 

County #1 205 to 210 (Sample ground prior to description)

Whole sample: quartz, Na-plag

 clase, K-feldspar, calcite, illite,

 amphibole?

Lincoln 

County #2 735 to 740

Moderately sorted granules with two main components a) pale yellow-green rock fragments composed of feldspar and quartz from 

crystalline rocks and b) aggregates of fine sand cemented by white, soft 

intergranular cement.

Whole sample: quartz, K-feldspar,

 Na-plagioclase, calcite, illite

White aggregates of fine sand

 calcite, quartz

Pale yellow-green aggregates:

 quartz, Na-plagioclase, K-feldspar,

 calcite, smectite, illite

Lincoln 

County #3 120 to 125 Pale gray-green, sand, clayey silt. Abundant clay forms matrix containing rounded silt to medium sand.

Whole sample: calcite, quartz, 

 Na-plagioclase, K-feldspar, 

 smectite, illite, kaolinite?

Lincoln 

County #3 130 to 135

Poorly sorted, coarse sand to pebble-sized grains mixed with small, 

fine-grained aggregates bound by pale brown clay intergranular fill; one 

clast of replace wood? Soft, white, fine-grained material around more 

massive siliceous core forms one grain. Large grains are commonly red, 

mainly quartz and granitic to gneissic rock fragments

Whole sample: quartz, K-feldspar,

 Na-plagioclase, calcite, illite

White grain: tridymite, calcium

 clinoptilolite, calcite

Lincoln 

County #3 205 to 210

Poorly sorted, light brown clayey sand in large aggregates. Grains 

range from silt to granule. Fine matrix of silt and clay is estimated to be 

10–20 percent of the sample. 

Whole sample: quartz, calcite, 

 K-feldspar, Na-plagioclase, illite

Lincoln 

County #3 225 to 230

Poorly sorted, medium sand to pebble sized grains, small grains are 

cemented by white to light-brown intergranular pore filling. Some 

grains of white cementing phase appear to be 80 percent cement, these 

grains are massive and some have small cylindrical cavities 0.02–0.04 

in. diameter (rhizoliths?). Larger grains are rock fragments of granite 

(gneiss).

Whole sample: quartz, K-feldspar,

 Na-plagioclase, calcite, illite

Grains with white cement: quartz,

 Na-plagioclase, calcite

Lincoln 

County #4 355 to 360

Poorly sorted coarse to fine sand. Fine sand is cemented by pale graybrown cement. A few large aggregates appear to be yellow-brown mud 

armored by coarse to medium sand grains. A few fragments of white 

porcelaneous aggregates. Trace plant fragments.

Whole sample: quartz, Na-plag

 clase, smectite

White grain aggregate: smectite,

 Na-plagioclase, trace zeolite

Lincoln 

County #4 500 to 505

Pale gray-green clayey fine sand aggregates with scattered medium 

sand to granule-sized grains. Few plant fragments embedded in the 

aggregates

Whole sample: quartz, smectite,

 Na-plagioclase, K-feldspar, calcite,

 illite, trace amphibole?

White aggregate: quartz, smectite,

 Na-plagioclase, illite

Lincoln 

County #4 655 to 660 Very pale gray-brown mud. Examination with petrographic microscope 

detected mainly angular isotropic, clear fragments – volcanic ash?

Whole sample: amorphous solid,

 quartz, plagioclase, calcite

Lincoln 

County #4 690 to 695

Moderately sorted clayey coarse to fine sand. Sand interstices are 

infilled with pale, brown argillaceous matrix. Sand grains are clear, red, 

pale yellow-green. A few plant fragments are visible.

Whole sample: quartz, Na-plagio

 clase, K-feldspar, calcite

Brule Formation Lincoln 

County #4 720 to 725 Well sorted, pale reddish brown, clayey fine sand. Abundant plant 

fragments.

Whole sample: quartz, smectite,

 K-feldspar, Na-plagioclase, calcite,

 illite

Clay-sized grains: smectite, illite

McPherson 

County #1 779 to 785 Pale red brown, poorly sorted, clayey fine sand. Trace plant fragments.

Whole sample: quartz, smectite, Na-

 plagioclase, K-feldspar

Clay-sized grains: smectite, illite,

 chlorite?

14  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Table 3. Carbon content of sediments and δ13C and δ18O compositions of bulk carbonate. 

[Carbon contents are for size fraction less than 2 millimeters; all carbon values are estimated, so the total carbon content may not equal the sum of inorganic 

plus organic carbon; depths are in feet below land surface; g/kg, grams of carbon per kilogram of sediment; ‰, per mil; VPDB, Vienna Peedee belemnite; 

VSMOW, Vienna Standard Mean Ocean Water; --, not measured; <, less than.]

Geologic 

unit Well name

Depth 

interval 

(feet)

Lithologic

description

Total 

carbon 

(g/kg)

Inorganic 

carbon 

(g/kg)

Organic 

carbon 

(g/kg)

δ13C, 

inorganic 

carbon

(‰, VPDB)

δ18O, 

inorganic 

carbon

(‰, VSMOW)

Undifferentiated 

sediments of 

Tertiary and 

Quaternary 

age

Lincoln County #2 204 to 

205

Clayey fine sand 

to silt -- -- -- -3.62 22.52

Lincoln County #2 225 to 

229

Fine to medium 

grained clayey sand 0.31 < 0.2 < 0.2 -- --

Lincoln County #4 185 to 

190 Clayey siltstone -- -- -- -2.02 24.72

Lincoln County #4 190 to 

195

Poorly sorted, coarse 

to medium sand -- -- -- -1.44 24.60

McPherson 

County #1 73 to 75 Black siltstone 1.72 .52 1.00 -- --

McPherson 

County #1

100 to 

105 Clayey siltstone 1.42 .64 < .2 -- --

McPherson 

County #1

163 to 

165 Sandy siltstone .45 < .2 < .2 -- --

Ogallala 

Group Lincoln County #1 205 to 210

Interbedded sandstone and siltstone 26.62 25.80 < .2 -7.36 21.13

Lincoln County #1 225 to 

230 Clayey sandstone 21.75 21.00 < .2 -7.36 19.93

Lincoln County #3 120 to 

125 Sand, clayey silt -- -- -- -7.06 21.08

Lincoln County #3 205 to 

210

Poorly sorted clayey 

sand -- -- -- -7.75 21.04

Lincoln County #3 355 to 

360 Fine sandstone -- -- -- -7.24 22.30

Lincoln County #4 500 to 

505 Clayey fine sand -- -- -- -- --

Lincoln County #4 685 to 

690 Medium to fine sand -- -- -- -8.28 21.42

Brule 

Formation Lincoln County #3 470 to 475 Siltstone -- -- -- -9.10 25.44

Lincoln County #4 720 to 

725

Well sorted clayey 

fine sand 1.35 1.00 < .2 -- --

Vertical Hydraulic Gradients 15
Figure 8. Water-level elevations and vertical hydraulic gradients at the nested wells. Vertical hydraulic gradients were 

calculated using equation 2 (see text). 

largest downward gradient near the water table was measured 

at McPherson Co. #1, the site located farthest from the Platte 

River, and the downward gradients in other wells decreased 

toward the river along geohydrologic section A-A’. Gradients 

near the water table at Lincoln Co. #3 were upward, except 

during the irrigation season when the gradients were reversed. 

Upward gradients are expected at Lincoln Co. #3 because of 

its location in the Platte River Valley, which is the regional 

ground-water discharge area for the aquifer. Lincoln Co. #1 is 

located just above the valley, and gradients indicate that deep 

ground water was moving upward in that area even though a 

small downward gradient existed near the water table.

Irrigation pumping in the vicinity of Lincoln Co. #3 

effectively reversed hydraulic gradients at that site for 1 to 

3 months (fig. 8). During that time period, the direction of 

ground-water movement was changed from the natural condition of upward movement to a condition of downward movement. This reversal in flow direction has the potential to draw 

contaminants near the water table deeper into the aquifer. A 

reversal in gradient was not observed at Lincoln Co. #2 even 

though water levels declined by as much as 34 ft during the 

irrigation season. A gradient reversal did not occur at that site 

because water levels at each of the three monitored depths in 

the aquifer responded in a similar fashion to irrigation pumping, indicating that the aquifer may be hydraulically well 

connected in the vertical direction. This is in sharp contrast 

to water-level responses to pumping observed in areas of the 

High Plains aquifer in Kansas where water-level declines near 

the water table were much less than those deeper in the aquifer 

(McMahon, 2001). The Kansas and Lincoln Co. #2 sites had 

similar saturated thicknesses (~500 ft) and water-level declines 

(greater than 30 ft). At that Kansas site, several relatively thick 

clay layers in the aquifer helped to isolate individual waterbearing sand and gravel units. The apparent vertical connectivity of water-bearing units at Lincoln Co. #2, in addition to 

large seasonal water-level declines, could enhance downward 

movement of contaminants in the aquifer if they were to reach 

the water table. 

Vertical Gradients in Water Chemistry

Concentrations of dissolved constituents varied with 

depth in the northern High Plains aquifer. Some of those 

changes were the result of spatial and temporal variability in 

inputs from sources at the land surface and others were the 

result of chemical reactions along flow paths in the aquifer. 

This section presents chemical data and discusses processes 

that might account for the observed vertical gradients in water 

chemistry. 

Irrigation season (June 1 through August 31)

Potential for downward ground-water movement

Potential for upward ground-water movement

Well identification number (table 1)

DATE DATE DATE DATE DATE

EXPLANATION

16  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Tritium 

Tritium is a radioactive isotope of hydrogen with a halflife of about 12.3 years. It is an excellent tracer of water movement because it substitutes for stable hydrogen isotopes in the 

water molecule. Because 3H is radioactive, its concentration 

in ground water decreases over time because of radioactive 

decay. In general, 3H in ground water originates from precipitation. Before the onset of atmospheric testing of nuclear 

weapons in 1953 (prebomb), the 3H content of precipitation 

in the central United States probably ranged from about 3 to 8 

TU (Kaufman and Libby, 1954; Thatcher, 1962). As a result of 

radioactive decay, ground water derived from precipitation that 

fell before 1953 would have contained no more than about 0.5 

TU of 3H in 2003. The 3H content of precipitation increased 

substantially after the onset of atmospheric nuclear weapons 

testing but has slowly decreased from its peak in the early 

1960s. Even with the variability in 3H content of precipitation 

over time, ground water totally derived from precipitation that 

fell since 1953 (postbomb) contained more than 0.5 TU in 

2003. 

Tritium was detected in water from three of the five 

nested wells screened near the water table (Lincoln Co. #1-25, 

Lincoln Co. #3-30, McPherson Co. #1-45) (table 4). The depth 

to water at the time of sampling at each of those three sites 

was less than 13 ft (table 1). The two nested wells screened 

near the water table that produced water without detectable 3H 

had depths to water greater than 100 ft (Lincoln Co. #2-255, 

Lincoln Co. #4-195). The data indicate that water moved from 

the land surface to the relatively shallow water tables in less 

than about 50 years, whereas transits times of water through 

the thicker unsaturated zones were greater than 50 years. 

None of the deeper nested wells produced water with detectable 3

H concentrations, except for McPherson Co. #1-770 (see 

discussion below; table 4), indicating that water from those 

deeper wells was recharged prior to 1953. The tritium data 

show that under undisturbed conditions, the aquifer contained 

a relatively thin layer of recently recharged ground water (less 

than 50 years) overlying a thicker layer of older ground water. 

Radiocarbon dating of dissolved inorganic carbon (DIC) was 

used to estimate ground-water ages in those deeper wells to 

obtain better resolution of vertical gradients in ground-water 

age in the aquifer. The radiocarbon age estimates are discussed 

in the section “Vertical Gradients in Ground-Water Age.” 

The presence of postbomb 3H in water from McPherson 

Co. #1-770 is evidence for downward leakage of shallow 

ground water in that well. Subsequent investigations using a 

downhole camera and packers showed that a leaking joint in 

the PVC casing at a depth of about 40 ft below land surface 

was allowing shallow ground water into the well at a rate of 

about 0.06 gal/min. Water from McPherson Co. #1-770 may 

have contained about 30 percent shallow ground water at the 

time of sampling on the basis of 3H concentrations measured 

in water from that well (1.8 TU) and from the water-table well 

at that site (McPherson Co. #1-45, 5.8 TU) and assuming that 

background ground water at the depth of the McPherson Co. 

#1-770 screen did not contain 3H. 

Dissolved Oxygen 

Some chemicals of environmental concern in the aquifer, 

such as nitrate (NO3

–

), U, and sulfate (SO4

2–), may be reduced 

to less oxidized forms by naturally occurring microorganisms 

under suboxic to anoxic conditions. Microbial reduction of 

these chemicals is beneficial because it may convert the chemicals into less soluble or less toxic forms. Reducing conditions, 

however, also may lead to the mobilization of toxic elements 

like As or nuisance elements like iron (Fe). In oxygenated 

ground water, microorganisms may oxidize some chemicals, 

such as atrazine, ultimately to carbon dioxide (CO2) (McMahon and others, 1992). The role of dissolved O2

 in microbial 

processes and ground-water chemistry makes measuring its 

distribution in ground water important if the fate of potentially 

harmful chemicals is to be understood.

Concentrations of O2 in water from the nested wells 

ranged from less than 0.5 to 7.8 mg/L (Appendix 1). For the 

sites located in the Sand Hills, O2 concentrations generally 

were largest at the water table and decreased with depth (for 

example, see O2 data for Lincoln Co. #4 in Appendix 1). In the 

Sand Hills, O2 concentrations were less than 0.5 mg/L in all of 

the deep wells, except for Lincoln Co. #1-370 and Lincoln Co. 

#2-760. These data indicate that sufficient supplies of electron 

donors, such as organic carbon or sulfide minerals, were present in the aquifer to support microbial reduction of O2

 within a 

relatively short travel distance from the water table. The presence of O2

 in water from Lincoln Co. #1-370 and Lincoln Co. 

#2-760 could indicate shallow ground water mixing in the well 

bore at those sites. If so, the small 14C concentrations and lack 

of detectable 3H in water from those wells indicate that the 

fraction of shallow water in those samples could be small.

Profiles of dissolved O2 concentrations in ground water 

from Lincoln Co. #3 were distinctly different from those at the 

sites located in the Sand Hills north of the river. Concentrations of O2

 were larger than 2.5 mg/L in water from each of 

the Lincoln Co. #3 wells (Appendix 1), indicating the presence of less reducing conditions south of the river compared 

to conditions north of the river. The water-table contour map 

in figure 2 indicates that water in the Lincoln Co. #3 wells 

was recharged in areas south of the river, possibly in western 

Lincoln County and parts of Keith and Perkins Counties. The 

aquifer in those areas primarily consists of sediments of the 

Ogallala Group (Diffendal and Goeke, 2000; Dreeszen, 2000; 

Goeke, 2001), as opposed to the combination of Ogallala 

Group and younger sediments north of the river (fig. 6). Those 

younger sediments overlying the Ogallala Group apparently 

contain a sufficient supply of electron donors to support relatively rapid O2

 reduction. Swinehart (1998) noted the presence 

of organic carbon-rich fine sands in the Sand Hills north of the 

study area and an organic carbon-rich siltstone was recovered 

from the McPherson Co. #1 bore hole (table 3). The presence 

Vertical Gradients in Water Chemistry 17
Table 4. Isotopic data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near 

Brady, Nebraska.[Multiple entries indicate replicate samples or measurements; TU, tritium units; ‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; VPDB, Vienna Peedee Belemnite; Vienna Cañon Diablo Troilite; pmc, percent modern carbon; <, less than; --, no data; nr, near]

Well/

site 

name

Sampling 

date

Tritium

(3H) 

(TU)

δ2H[H2O] 

(‰, 

VMOW)

δ18O[H2O]

(‰, 

VSMOW)

δ13C[DIC]

(‰, 

VPDB)

14C[DIC]

(pmc)

δ15N[NO3]

(‰, AIR)

δ18O[NO3]

(‰, 

VMOW)

δ15N[N2]

(‰, AIR)

δ34S[SO4]

(‰, 

VCDT)

87

Platte River nr. Brady, Nebr. 10/12/04 5.0 –73.0 –9.20 –7.03 98.5 -- -- -- -- Imperial 8/19/03 2.8 –75.2 –10.40 4.8 -1.9 -- -- 

Keith County #1 9/20/05 < .5 –79.1 –10.90 -- -- -- -- -- –0.6 Lincoln County #1-25 9/6/03 8.5 8.4 –65.6 –9.30 –8.89 45.1 14.5 7.1 0.81 .98 –1.6 Lincoln County #1-119 9/5/03 < .5 –62.1 –8.88 –9.58 39.5 8.6 9.2 .89 –13.4 Lincoln County #1-270 9/5/03 < .5 –64.1 –9.14 –8.71 41.6 5.8 2.0 .76 .58 –10.6 Lincoln County #1-370 9/6/03 < .5 –68.9 –9.57 –9.70 –11.50 12.7 14.2 5.6 5.5 –.1.2 .64 .62 –4.7 –5.1 Lincoln County #2-255 9/7/03 < .5 –66.5 –9.35 –7.27 73.4 4.9 –3.3 .59 .54 –4.2 Lincoln County #2-590 9/7/03 < .5 –68.1 –9.38 –11.00 13.8 7.8 7.9 .80 –7.2 Lincoln County #2-760 9/7/03 < .5 –67.3 –67.2 –9.29 –9.36 –9.62 –9.98 24.6 24.3 6.7 6.6 .2.3 .75 .72 –7.9 –7.6 Lincoln County #3-30 9/4/03 3.0 –73.4 –10.20 –7.72 63.9 8.1 3.3 .53 .54 –8.3 Lincoln County #3-110 9/3/03 < .5 –72.7 –10.10 –7.42 23.9 5.1 –2.5 .67 .66 –6.5 Lincoln County #3-250 9/4/03 < .5 –81.8 –10.90 –14.00 21.1 5.1 –2.2 .43 .51 –4.6 Lincoln County #3-390 9/3/03 < .5 –83.9 –11.30 –9.68 7.9 6.2 –1.5 .69 .68 –5.8 Lincoln County #4-195 10/22/03 < .5 –73.6 –10.20 –7.35 91.6 4.4 –2.9 .54 .51 .5 
Table 4. Isotopic data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, Nebraska

[Multiple entries indicate replicate samples or measurements; TU, tritium units; ‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; VPDB, Vienna Peedee BelemniteTroilite; pmc, percent modern carbon; <, less than; --, no data]

Well/

site name

Sampling 

date

Tritium

(3H) 

(TU)

δ2H[H2O] 

(‰, 

VSMOW)

δ18O[H2O]

(‰, 

VSMOW)

δ13C[DIC]

(‰, 

VPDB)

14C[DIC]

(pmc)

δ15N[NO3]

(‰, AIR)

δ18O[NO3]

(‰, 

VSMOW)

δ15N[N2]

(‰, AIR)

δ34S

(

VC

Lincoln County #4-359 10/23/03 < .5 –69.5 –9.73 –7.08 42.4 10.9 10.8 5.7 5.5 .67 –Lincoln County #4-510 10/22/03 < .5 –69.6 –69.5 –9.66 –9.69 –8.03 26.1 -- -- .72 .74 Lincoln County #4-695 10/22/03 < .5 –70.3 –9.73 –9.62 –10.30 11.0 11.9 17.8 6.9 .79 .76 McPherson County #1-45 9/8/03 5.8 –78.5 –10.90 –8.70 82.9 3.2 3.1 –2.4 –2.4 .49 .28 McPherson County #1-330 9/9/03 < .5 –69.8 –9.53 –8.94 66.3 -- -- .97 –McPherson County #1-330 5/18/04 -- -69.8 -9.53 -8.62 63.4 -- -- -- McPherson County #1-550 9/9/03 < .5 –64.8 –8.54 –6.29 –6.22 75.9 76.1 -- -- .86 .86 McPherson County #1-550 5/19/04 -- -64.7 -8.50 -6.42 75.7 -- -- -- McPherson County #1-770 10/13/04 1.8 –77.8 –10.30 –7.58 40.8 4.4 –2.1 -- –
of relatively reducing conditions in the aquifer north of the 

river, but not south of the river, has important implications 

with respect to spatial variability in the persistence of contaminants such as NO3

–

 in the aquifer. This is discussed in the 

section “Nitrate.”

Major Ions and Trace Elements

Dissolved-solids concentrations in undisturbed aquifers 

usually increase with depth below the water table as the residence time of ground water increases and rock/water reactions 

progress. That trend is evident in water samples from Lincoln 

Co. #4 (fig. 9), the site north of the river that is considered to 

be the most representative of undisturbed conditions in the 

aquifer. Dissolved-solids concentrations in water from that 

site increased from 118 mg/L near the water table to 263 mg/L 

near the base of the aquifer. Much of the increase in dissolved 

solids with depth is accounted for by increases in concentrations of dissolved Ca2+, Na+

, alkalinity, SO4

2–, and SiO2

(fig. 9). In general, the chemistry of ground water throughout 

the aquifer was of high quality. None of the approximately 

90 chemical constituents analyzed in each sample exceeded 

primary drinking-water standards (U.S. Environmental Protection Agency, 2006). Water samples collected from undisturbed 

parts of the aquifer were primarily of a Ca2+-HCO3

-

 type, 

although a few samples were of a Ca2+-Na+-HCO3

-

 type. 

Concentrations of most constituents at the base of the aquifer 

also were relatively small, indicating that interactions with 

the underlying geologic formations probably had only small 

effects on water quality in the aquifer. This was not always the 

case in the central and southern High Plains aquifers, where 

interactions with underlying geologic formations containing 

nonpotable water adversely affected water quality in the aquifer (McMahon and others, 2004a, b). 

Agricultural activity at the land surface can increase 

the concentration of dissolved solids in recharge above what 

would be expected under natural conditions (Böhlke, 2002; 

McMahon and others, 2004b), resulting in a reversed gradient in dissolved-solids concentrations near the water table. 

Irrigated agriculture appears to have produced that effect at 

the Lincoln Co. #3 site. At that site, the concentration of dissolved solids in water from the well screened near the water 

table (433 mg/L) was larger than the concentration in water 

from the next deepest well (286 mg/L) (fig. 9). The presence 

of modern recharge at that site is evident from postbomb 3H 

in water from the water-table well, Lincoln Co. #3-30, and 

effects from agriculture also are evident from the detection of 

a pesticide compound (deethylatrazine) and relatively large 

concentrations of NO3

–

, SO4

2–, and chloride (Cl–

) (table 4 and 

Appendix 1).

Given that HCO3

–

 accounted for 84 percent, on average, 

of the anion equivalents in the water samples, concentrations 

of selected dissolved constituents were plotted relative to 

concentrations of DIC to better understand what geochemical 

reactions or other processes in the aquifer might account for 

the observed changes in dissolved-constituent concentrations 

(fig. 10). Dissolved inorganic carbon was calculated from 

Figure 9. Concentrations of selected dissolved constituents in relation to 

well-screen depth below the water table.

EXPLANATION

Lincoln Co. #1 Lincoln Co. #2 Lincoln Co. #3 Lincoln Co. #4 McPherson Co.#1

20 Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
alkalinity and pH data using the geochemical code NETPATH 

(Plummer and others, 1994). In those plots, concentrations are 

reported on a molar basis to facilitate comparisons between 

concentrations and chemical reactions that might account for 

them, as discussed below. Dissolved Ca2+ concentrations were 

strongly correlated to DIC concentrations. Calcite dissolution in the presence of CO2

 is one possible reaction that could 

produce a positive correlation between Ca2+ and DIC concentrations 

(eq. 3). 

 

Calcite was relatively abundant in the aquifer (table 2), and 

CO2 could be produced from the coupled oxidation of organic 

carbon and reduction of dissolved O2. Other electron acceptors, such as NO3

–

 and SO4

2–, are discussed later in the report. 

Equation 3 produces Ca2+ and DIC in a molar ratio of 1:2. 

The data actually plot along a line with a 1:3 slope (fig. 10), 

indicating that the water is depleted in Ca2+ relative to what 

would be expected on the basis of equation 3. This apparent 

depletion could result from Ca2+ uptake from solution or from 

other reactions that also contributed Ca2+ and DIC to solution 

in ratios other than 1:2. One possible sink for dissolved Ca2+ is 

cation exchange on clay minerals, such as smectite (table 2), 

in the aquifer. The composition of exchangeable cations on the 

clays is unknown, but if Na+ is abundant on the exchange sites, 

then a reaction similar to equation 4 could account for the 

apparent Ca2+ depletion.

Concentrations of dissolved Na+ and SiO2 also exhibit 

generally positive correlations with DIC concentrations 

(fig. 10). Those relations could result, in part, from dissolution 

of sodium-bearing silicate minerals. The XRD analyses show 

that sodium plagioclase was relatively abundant in the aquifer 

(table 2). Dissolution of albite (pure sodium plagioclase) or 

sodium plagioclase (also containing calcium) in the presence 

of CO2 could produce positive Na+-DIC and SiO2-DIC correlations (eqs. 5 and 6). 

The combined reactions described by equations 3 and 6 would 

add Ca2+ and DIC to solution in a ratio of 1:2.7, which is close 

to the observed ratio of 1:3 (fig. 10). The small additional Ca2+

depletion could be accounted for by cation exchange. The 

87Sr/86Sr ratios of dissolved strontium (Sr2+) in ground water, 

excluding Lincoln Co. #3-30, ranged from 0.70965 to 0.71052 

(table 4). Those values could be consistent with the interpretation that the dissolved Sr2+ was derived from plagioclase 

dissolution (Gosselin and others, 1997b; McNutt, 2000). More 

precise information on the chemical and isotopic compositions 

of the carbonate and silicate minerals and exchangeable cation 

compositions, in addition to petrographic observations of 

mineral dissolution or precipitation, would help to refine our 

understanding of geochemical processes in the aquifer. Nevertheless, the reactions presented here are qualitatively consistent with the measured ground-water chemistry and the trends 

in saturation indexes for calcite and albite (fig. 10; Appendix 

2). Saturation index is defined by equation 7:

where, IAP is the ion activity product for mineral dissolution 

and Keq is the equilibrium constant for mineral dissolution 

(Stumm and Morgan, 1981). Saturation indexes were calculated from chemical data in Appendix 1 using the geochemical 

code NETPATH (Plummer and others, 1994). The saturation 

indexes in figure 10 show that calcite and albite have the 

potential to dissolve along flow paths from the recharge areas 

(small DIC concentrations in figure 10) to the downgradient 

wells (large DIC concentrations in figure 10).

Other processes that may affect water chemistry in the 

aquifer are mixing with surface water, sulfide-mineral oxidation, SO4

2– reduction, and possibly evaporation. Mixing, as 

used in this report, refers to the convergence of flow paths 

from different recharge sources. At Lincoln Co. #3, Sr2+

concentrations and isotopic compositions indicate that mixing 

occurred where flow paths originating in regional recharge 

areas south of the river converged with flow paths that originated at the river as a result of induced infiltration (fig. 11). 

Water from Lincoln Co. #3-30 may have contained about 30 

percent river water on the basis of the Sr2+ data. The composition of water from Lincoln Co. #3-110 was used to represent 

regional ground water because the hydraulic-head data show 

Lincoln Co. #3-110 to be upgradient from Lincoln Co. #3-30 

(fig. 8). The elevated SO4

2– and U concentrations in water from 

Lincoln Co. #3-30 provide additional evidence for mixing with 

river water at that site (Appendix 1). Excluding water from 

Lincoln Co. #3-30, the Sr2+ data for the other water samples 

are consistent with chemical evolution in the aquifer along 

flow paths from the regional recharge areas (fig. 11).

Sulfate concentrations increased with depth at some 

of the nested well sites (fig. 9). The concentration increases 

generally were accompanied by decreases in the δ34S values 

for SO4

2– (fig. 12). For example, the median concentration 

and isotopic composition of SO4

2– in recharge waters north 

of the Platte River were 2.4 mg/L and –0.53 ‰, respectively, 

whereas the median concentration and isotopic composition of 

SO4

2– in the downgradient waters were 11 mg/L and –9.8 ‰, 

excluding the sample from McPherson Co. #1-550 (fig. 12). 

This relation between SO4

2– concentration and δ34S values 

indicates that the increase in SO4

2– concentrations with depth 

may be related to the oxidation of sulfide minerals, such as 

pyrite, a source of 34S-depleted sulfur (Krouse and Mayer, 

CaCO (3) 3 (calcite) + H2O + CO2 = 

 Ca2+ + 2HCO3

-

Ca2+ + 2Na-clay = 2Na+ + Ca-clay (4)

(5)

(6)

Saturation inde (7) x = IAP

Keq

 4NaAlSi3O8 (albite) + 6H2O + 4CO2 = 2Al2Si2O5 (OH)4

 (kaolinite) + 8SiO2 + 4Na+ + 4HCO3

-

-

 1.77Na0.62Ca0.38Al1.38Si2.62O8 (sodium plagioclase) + 

 3.67H2O + 2.44CO2 = 1.23Al2Si2O5(OH)4 (kaolinite) + 

 2.20SiO2 + 1.10 Na+ + 0.67Ca2+ + 2.44HCO3

-

Vertical Gradients in Water Chemistry 21
Figure 10. Concentrations of selected dissolved constituents and mineral saturation 

indexes in relation to concentrations of dissolved inorganic carbon in water from the 

nested wells. 

Figure 11. Strontium isotope ratios in relation to 

inverse strontium concentrations for water from the 

water-table and nested wells. 

EXPLANATION

0.7110

0.7108

0.7106

0.7104

0.7102

0.7100

0.7098

0.7096 0 0.3 0.6 0.9 1.2 1.5

87STRONTIUM/86STRONTIUM

1/STRONTIUM (1/MICROMOLES PER LITER)

North of the Platte River

South of the Platte River

Percentage of river water in a mixture of waters

 from the Platte River and from

 Lincoln Co. #3-110 (LC3-110)

Recharge wells (Lincoln Co. #2-255, Lincoln Co.

 #4-195, McPherson Co. #1-45)

Recharge well (Keith Co. #1)

Downgradient wells (Lincoln Co. #3 wells)

Downgradient wells (All remaining wells north of river)

1:2

1:3

Equilibrium

Mineral dissolution

CONCENTRATION OF DISSOLVED INORGANIC

CARBON, IN MICROMOLES PER LITER

Calcite

Albite

CONCENTRATION OF DISSOLVED

SODIUM, IN MICROMOLES 

PER LITER

CONCENTRATION OF DISSOLVED

CALCIUM, IN MICROMOLES 

PER LITER

CONCENTRATION OF DISSOLVED

SILICA, IN MICROMOLES 

PER LITER LOG SATURATION INDEX

22 Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
2000), in the aquifer. The water sample from McPherson Co. 

#1-550 was the only one to have a smaller SO4

2– concentration ( about 0.1 mg/L) and larger δ34S value (3.94 ‰) than 

the recharge waters (fig. 12). The decrease in concentration 

and increase in δ34S value probably is the result of microbial 

SO4

2– reduction, which could occur in the absence of dissolved 

O2 and NO3

–

 in ground water, which was the case at that well 

(Appendix 1). Sulfate reduction produces hydrogen sulfide 

(H2S), but none was detected in the sample from McPherson 

Co. #1-550. The produced H2S may have reacted with dissolved iron (Fe2+) or manganese (Mn2+) and precipitated from 

solution. Water from McPherson Co. #1-550 did contain a 

relatively elevated Mn2+ concentration (Appendix 1).

Precipitation is the ultimate source of water in the aquifer 

and evaporation of that precipitation could increase the concentration of dissolved constituents in recharge. Chloride and 

bromide (Br-) concentrations are expected to be conservative 

in the aquifer; therefore, they could be useful indicators of 

by Harvey and Welker (2000). The mean Cl- concentration in 

North Platte precipitation was calculated from the analyses of 

wet deposition collected near North Platte from 1984 to 2004 

(National Atmospheric Deposition Program, 2005). Bromide 

was not measured in the precipitation, so Br- concentrations in 

precipitation were calculated on the basis of the mean Cl- concentration in precipitation and the mean Br/Cl ratio (.00642) 

in water from Lincoln Co. #2, Lincoln Co. #4, and McPherson Co. #1. Those water-table wells contained the smallest 

Cl- concentrations and are considered to be the closest to 

precipitation in composition. This approach for estimating the 

Br- concentration in precipitation assumes that the Br/Cl ratio 

in ground water near the water table is representative of the 

ratio in precipitation.

EXPLANATION

North of the Platte River

South of the Platte River

Recharge wells (Lincoln Co. #2-255, Lincoln Co.

 #4-195, McPherson Co. #1-45)

Recharge well (Keith Co. #1)

Downgradient wells (Lincoln Co. #3 wells)

General composition of recharge

Downgradient wells (All remaining wells north of river)

Figure 12. Sulfur isotope ratios in relation to inverse 

sulfate concentrations for water from the water-table 

and nested wells. 

EXPLANATION

Ground water

Mean precipitation

Evaporation trend

CONCENTRATION OF DISSOLVED CHLORIDE,

IN MICROMOLES PER LITER

CONCENTRATION OF DISSOLVED BROMIDE,

IN MICROMOLES PER LITER

Figure 13. Concentrations of dissolved bromide 

in relation to concentrations of dissolved 

chloride in water from the nested wells and in 

precipitation. Precipitation data for chloride from 

North Platte, Nebraska (National Atmospheric 

Deposition Program, 2005). The precipitation 

value of bromide/chloride was assumed to be 

equal to that in water from the shallow wells at 

Lincoln Co. #2, Lincoln Co. #4, and McPherson 

Co. #1. 

For the most part, trace element concentrations were 

small and did not exhibit any systematic changes with depth 

below the water table (Appendix 1). None of the trace elements exceeded a Federal primary drinking-water standard, 

although the secondary drinking-water standard for manganese 

(50 μg/L) was exceeded in water from three wells (Lincoln Co. 

#1-25, Lincoln Co. #1-119, and McPherson Co. #1-550) (U.S. 

Environmental Protection Agency, 2006). The elevated Mn2+

concentrations in those wells probably resulted from microbial 

evaporation during recharge. Concentrations of Cl- and Br- in 

ground water generally plotted along a trend line originating 

near the mean concentration of precipitation at North Platte 

(fig. 13), indicating that evaporation may have influenced 

the concentration of dissolved constituents in recharge or 

shallow ground water. Evaporation effects, if they occurred, 

did not have a large effect on the δ2H[H2O] and δ18O[H2O] 

compositions for ground water (table 4), which plotted along 

the meteoric water line developed for North Platte, Nebraska, 

Vertical Gradients in Water Chemistry 23
Mn(IV) reduction associated with anoxic conditions in those 

waters. The two largest arsenic (As) concentrations, 9.2 and 

9.8 μg/L, occurred in oxygenated water from water-table wells 

Lincoln Co. #2-255 and Lincoln Co. #4-195 (Appendix 1). 

The Federal primary drinking-water standard for As is 10 μg/L 

(U.S. Environmental Protection Agency, 2006). Two common 

sources of As in ground water are the oxidation of sulfide minerals, such as pyrite, and the reductive dissolution of Mn and 

Fe oxides (Smedley and Kinniburgh, 2002). The fact that the 

two largest As concentrations occurred in oxygenated ground 

water indicates that oxidation of sulfide minerals could be an 

important mechanism for mobilizing As in the aquifer. This 

hypothesis is consistent with the interpretation that increases 

in concentrations of dissolved SO4

2– in the aquifer were the 

result of sulfide-mineral oxidation (fig. 12).

Radon concentrations generally exhibited increased concentrations with depth at the Lincoln Co. #1, #3, and #4 nested 

well sites (Appendix 1). Concentrations at the base of the 

aquifer at each of those sites exceeded 300 pCi/L, which is one 

option in the USEPA’s proposed drinking-water standard for 

radon (U.S. Environmental Protection Agency, 1999). Overall, 

radon concentrations exceeded the proposed standard in water 

samples from 9 of the 19 nested wells.

Nitrate

Nitrogen (N) is naturally present in relatively small concentrations in rocks, soils, organic material, and the water that 

comes in contact with them (Holloway and Dahlgren, 1999). 

Nitrogen also is present in relatively large concentrations in 

animal manure and fertilizers applied to agricultural fields 

and in sewage effluent. Nitrate, the most commonly occurring form of N in ground water, is of environmental concern 

because its discharge to surface water can support eutrophication. Nitrate also is a health concern because ingestion of large 

concentrations of NO3

–

 in drinking water can lead to low oxygen levels in the blood of infants (Fan and Steinberg, 1996). 

The USEPA has established a primary drinking-water standard 

for NO3

–

 of 10 mg/L as N (U.S. Environmental Protection 

Agency, 2006).

Nitrite plus nitrate concentrations at the water table 

were 0.13 to 3.13 milligrams per liter as nitrogen, and concentrations substantially decreased with depth in the aquifer 

(Appendix 1). In all cases, NO3

–

 concentrations were larger 

than NO2

–

 concentrations (median NO2

–

/ NO2

–

 + NO3

–

 ratio is 

0). The relatively large NO3

–

 concentration and postbomb 3H 

concentration in water from Lincoln Co. #3-30 indicates that 

recent recharge affected by agricultural activity was present 

near the water table at that site. Land use at the other nestedwell sites was undeveloped rangeland; therefore, NO3

–

 concentrations near the water table at those sites may by representative of natural recharge. The smaller concentrations of NO3

–

 in 

water farther below the water table might be attributed to 

either a lack of NO3

–

 in recharge to those wells (3H data indicate those waters were recharged prior to the start of modern 

agriculture in the area) or to denitrification along flow paths in 

the aquifer. Knowledge about denitrification in the aquifer is 

important because this process is the primary natural-attenuation mechanism for NO3

–

 contamination in ground water.

Denitrification is a microbially mediated process that 

reduces NO3

–

 to N2 gas in suboxic environments (eq. 8). 

In equation 8, reduction of NO3

–

 is coupled to the oxidation of organic carbon, which produces N2

 and bicarbonate. In 

some environments, denitrification may be coupled to the oxidation of sulfide minerals instead of, or in addition to, organic 

carbon (Postma and others, 1991; Böhlke and others, 2002), 

which produces N2 and dissolved SO4

2– (eq. 9).

 

One way to evaluate whether denitrification has occurred 

in the aquifer is to compare the δ15N and δ18O values for NO3

–

in downgradient waters to the values for NO3

–

 in more recent 

recharge. This approach assumes steady-state conditions with 

respect to the isotopic composition of NO3

–

 in recharge, which 

may not be valid for ground water that spans a broad range of 

ages. Denitrification fractionates N and O isotopes such that 

the δ15N and δ18O values for residual NO3

–

 become progressively larger as the reaction proceeds. The isotopic values 

for NO3

–

 that has been reduced should plot along trend lines 

with slopes of 1:2 to 1:1 (Böttcher and others, 1990; Granger 

and others, 2004) (fig. 14A). The data in fig. 14A roughly 

plot within these trend lines given the indicated isotopic 

compositions of NO3

–

 in recharge, indicating that denitrification probably has removed NO3

–

 from solution in the aquifer. 

This interpretation is consistent with the small concentrations 

of dissolved O2 in the aquifer. The modest fit of the isotopic 

data to the fractionation trend lines indicates that the isotopic 

composition of NO3

–

 in recharge may have been more variable 

than indicated in fig. 14A. 

Another way to evaluate whether denitrification has 

occurred in the aquifer is to calculate the amount of N2 in 

ground water from denitrification, referred to here as “excess 

N2” (eq. 10).

This approach has the added benefit of providing information on denitrification reaction progress. In equation 10, 

N2-total represents the measured concentration of N2 dissolved 

in water (table 5), N2-air saturation represents the amount of 

N2 dissolved in water as a result of air-water equilibration at 

recharge conditions, and N2-excess air represents the amount 

of N2 in water from dissolution of excess air (air bubbles) in 

water. 

The value of N2-air saturation is dependent on water 

temperature and elevation of the water table in the recharge 

area at the time of recharge. The latter factor is referred to as 

(8) 4NO3 + 5C + 3H2O = 

 2N2 + 5HCO3 + H+ -

-

(9) 6NO3 + 2FeS2 + 2H2O = 3N2 + 

 2FeOOH + 4SO4

2 + 2H+

-

-

- (10)

-

excess N - 2 = [N2 total] – [N2 air saturation] 

 – [N2 excess air]

24  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
EXPLANATION EXPLANATION

A. B.

Figure 14. (A) Oxygen isotope values for nitrate in relation to nitrogen isotope values for nitrate and (B) 

fraction of initial nitrate in recharge remaining in solution in relation to well-screen depth below the water 

table for water from the nested wells.

“recharge elevation.” Recharge temperatures were calculated 

using measured concentrations of dissolved Ne and Ar (Stute 

and Schlosser, 1993) and assuming complete dissolution of 

excess air during recharge. Recharge elevation depends on 

several factors, including depth of the well screen below the 

water table, location of the well in the regional flow system, 

and recharge rate (Cook and Böhlke, 2000). Recharge elevation increases relative to the local water-table elevation as 

the depth of the well screen below the water table increases. 

At each well nest, the elevation of the local water table was 

assumed to be representative of the recharge elevation of the 

shallowest well. Recharge elevations were increased in increments of 200 ft, relative to the local water-table elevation, for 

successively deeper wells. This represents a 600-ft increase 

in recharge elevations for each well nest, which is similar to 

the total measured gain in water-table elevations from Lincoln 

Co. #1 to McPherson Co. #1 of 700 ft (fig. 6). The calculated 

amount of excess N2 was not particularly sensitive to uncertainties in recharge elevation. For example, an elevation uncertainty of ±200 ft only produced uncertainties of about ±0.3°C 

and ±1 μM (±0.028 mg/L as N) in recharge temperatures and 

excess N2 concentrations.

The value of N2-excess air was calculated using 

equation 11:

 

where [N2/Ar]-air is the ratio of N2 and Ar in air, Ar-total 

represents the measured concentrations of Ar dissolved in 

water (table 5), and Ar-air saturation represents the amount of 

Ar dissolved in water as a result of air-water equilibration, as 

determined from the Ne and Ar data. Equation 11 assumes that 

the gas composition of the dissolved air bubbles was identical 

to that of air. 

Recharge temperatures calculated using the Ne and Ar 

data ranged from 5.6 to 12.4°C and were consistently warmer 

for water sampled near the water table than for water sampled 

near the base of the aquifer (table 5). The average difference 

in recharge temperature between the top and bottom of the 

aquifer was 4.2°C. This difference in recharge temperature, 

in part, is due to the increase in recharge elevation with depth 

below the water table. The gradient in mean annual air temperature in relation to land-surface elevation for the Sand Hills 

is estimated to be about –0.07°C/100 ft (High Plains Regional 

Climate Center, 2005). This temperature-elevation gradient 

and the 600-ft increase in recharge elevation from the top to 

the bottom of the aquifer could account for about 0.5°C of the 

temperature difference. The remaining temperature difference 

presumably is related to changes in climatic conditions during 

the recharge history of the aquifer.

The amount of excess N2 in each sample can be calculated once recharge temperature and excess air are determined. 

Concentrations of excess N2 ranged from less than 30 to 

113 μM (less than 0.8 to 3.2 mg/L as N) (table 5). Excess N2

- - (11)

-

[N2 excess air] = [N2/Ar] air {[Ar-total] 

 [Ar air saturation]}

× –

Vertical Gradients in Water Chemistry 25
Table 5. Dissolved-gas and related data, recharge temperatures, and estimates of initial nitrate concentration and isotope values in recharge.

[Multiple entries indicate replicate samples or measurements; μM, micromoles per liter; <, less than; >, greater than; ‰, per mil; °C, degrees Celsius; 

cm3STP/L, cubic centimeters per liter at standard temperature and pressure; ±, plus or minus 1 standard deviation; --, no data]

Well/

site 

name

Estimated 

elevation 

of water 

table in 

recharge 

area 

(feet)

Oxygen

(O2)

(μM)

Methane

(CH4)

 (μM)

Nitrogen

(N2)

 (μM)

δ15N[N2] 

(‰, 

AIR)

Argon

(Ar)

(μM)

Neon 

(Ne)

(μM×

10-3)

Recharge 

temp­‐

erature 

(°C)

Excess 

air (cm3

STP/L)

Nitrogen (N2

) 

from 

air saturation 

(μM)

Lincoln County

 #1-25 2620 32

< 1

< 1

739

739

0.81

.98 

15.77

15.71 9.425 11.7±0.1 1.7 569

Lincoln County

 #1-119 2820 25

< 1

< 1

878

875 .89 17.5417.45 12.40 10.7±0.2 5.4 577

Lincoln County

 #1-270 3020 82

< 1

< 1

785

775

 .76

.58

17.08

17.02 11.68 10.7±0.1 4.5 573

Lincoln County

 #1-370 3220 5048

< 1

< 1

766

758

 .64

.62

17.50

17.35

 10.97

11.05 8.2±0.3 3.5±0.1 600

Lincoln County

 #2-255 2720 248246

< 1

< 1

646

643

 .59

.54

15.82

15.73 -- 12.4±0.21 2.5 559

Lincoln County

 #2-590 3120 22

< 1

< 1

770

766 .80 17.1517.09 -- 8.31

 2.7±0.1 601

Lincoln County 

 #2-760 3320 8281

< 1

< 1

754

756

 .75

.72

17.17

17.19

 10.19

10.50 7.8±0.3 2.7±0.2 603

Lincoln County

 #3-30 2610 105104

< 1

< 1

701

695

 .53

.54

16.23

16.13 10.33 11.8±0.2 2.8 568

Lincoln County

 #3-110 2810 226225

< 1

< 1

713

705

 .67

.66

16.78

16.58 -- 11.5±0.41 4.1 567

Lincoln County

 #3-250 3010 182182

< 1

< 1

705

712

 .43

.51

17.19

17.35 10.24 7.8±0.3 2.5 610

Lincoln County

 #3-390 3210 8686

< 1

< 1

746

746

 .69

.68

17.57

17.56

 11.37

10.18 7.5±1.0 3.2±1.0 610

Lincoln County

 #4-195 2970 294275

< 1

< 1

692

635

 .54

.51

16.52

15.74 9.458 10.1±1.7 1.7±0.2 582

Lincoln County

 #4-359 3170 4 < 1 747 .67 16.50 10.43 10.1 3.0 574

Lincoln County

 #4-510 3370 24

< 1

< 1

725

719

 .72

.74

16.03

15.92 9.663 10.4±0.2 2.2 569

Lincoln County

 #4-695 3570 74

< 1

< 1

790

787

 .79

.76

17.56

17.60

 9.726

9.842 5.6±0.1 1.9±0.1 629

McPherson

 County 

#1-45

3320 217

213

< 1

< 1

664

666

 .49

.28

15.72

15.79 10.23 12.1±0.2 3.0 549

McPherson

 County 

#1-330

3520 1

2

< 1

< 1

679

676

 .97 15.63

15.57 9.226 10.6±0.1 1.7 563

McPherson

 County 

#1-550

3720 2

2

< 1

< 1

657

654

 .86

.86

15.48

15.40 9.150 10.8±0.2 1.7 557

McPherson

County 

#1-770

3920 -- < 1

< 1

706

683 -- 15.6115.30 -- -- -- --

26  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Table 5. Dissolved-gas and related data, recharge temperatures, and estimates of initial nitrate concentration and isotope values in 

recharge.—Continued

[Multiple entries indicate replicate samples or measurements; μM, micromoles per liter; <, less than; >, greater than; ‰, per mil; °C, degrees Celsius; cm3STP/L, 

cubic centimeters per liter at standard temperature and pressure; ±, plus or minus 1 standard deviation; --, no data]

Well/

site

name

Nitrogen 

(N2) 

from 

excess 

air 

(μM)

Excess 

nitrogen 

(N2) 

(μM)

δ15N

[excess-N2]

(‰, AIR)

Nitrate 

(NO3), 

measured 

(μM)

Nitrate 

(NO3), 

measured 

(mg/L 

as N)

δ15N[NO3], 

measured(‰, 

AIR)

Nitrate 

(NO3), 

initial 

(μM)

Nitrate 

(NO3), 

initial 

(mg/L as 

N)

δ15N[NO3], 

initial 

(‰, AIR)

Fraction 

of initial 

nitrate in 

recharge 

removed by 

denitrification2

Lincoln 

County 

#1-25

 60 111±1 2.2 9 0.13 14.5 231±2 3.23 2.7 0.96

Lincoln 

County 

#1-119

187 113 3.1 60 .84 8.6 286±1 4.00 4.3 .79

Lincoln 

County 

#1-270

158 49±6 2.0±0.2 64 .89 5.8 162±12 2.27 3.5±0.2 .60

Lincoln 

County 

#1-370

123 39±2 .6 78 1.09 5.6

5.5 156±5 2.18 3.1±0.1 .50

Lincoln 

County 

#2-255

 86 < 30 -- 79 1.11 4.9 79 1.11 4.9 0.0

Lincoln 

County 

#2-590

 96 72±1 2.2 35 .49 7.8 179±1 2.51 3.3 .80

Lincoln 

County 

#2-760

 94 57±5 1.7±0.2 41 .57 6.7

6.6 155±10 2.17 3.0±0.2 .74

Lincoln 

County 

#3-30

 98 < 30 -- 224 3.13 8.1 224 3.13 8.1 0.0

Lincoln 

County 

#3-110

142 < 30 -- 184 2.57 5.1 184 2.57 5.1 0.0

Lincoln 

County 

#3-250

 86 < 30 -- 114 1.59 5.1 114 1.59 5.1 0.0

Lincoln 

County 

#3-390

110 < 30 -- 101 1.42 6.2 101 1.42 6.2 0.0

Lincoln 

County 

#4-195

 60 < 30 -- 75 1.05 4.4 75 1.05 4.4 0.0

Lincoln 

County 

#4-359

105 66 1.0 9 .12 10.9

10.8 141 1.97 1.7 .94

Lincoln 

County 

#4-510

 75 78±2 1.3 < 4 < .06 -- 156±4 2.18 1.3 > .97

Lincoln 

County 

#4-695

 65 94±3 1.3 6 .09 17.8 194±6 2.72 1.8 .97

Vertical Gradients in Water Chemistry 27
Table 5. Dissolved-gas and related data, recharge temperatures, and estimates of initial nitrate concentration and isotope values in 

recharge.—Continued

[Multiple entries indicate replicate samples or measurements; μM, micromoles per liter; <, less than; >, greater than; ‰, per mil; °C, degrees Celsius; 

cm3STP/L, cubic centimeters per liter at standard temperature and pressure; ±, plus or minus 1 standard deviation; --, no data]

Well/

site 

name

Nitrogen 

(N2) from 

excess 

air (μM)

Excess 

nitrogen (N2

) 

(μM)

δ15N

[excessN2

]

(‰, AIR)

Nitrate 

(NO3), 

measured 

(μM)

Nitrate 

(NO3), 

measured 

(mg/L 

as N)

δ15N[NO3], 

measured(‰, 

AIR)

Nitrate 

(NO3), 

initial 

(μM)

Nitrate 

(NO3), 

initial 

(mg/L as 

N)

δ15N[NO3], 

initial 

(‰, 

AIR)

Fraction 

of initial nitrate 

in recharge 

removed by 

denitrification2

McPherson 

County 

#1-45

 105 < 30 -- 196 2.75 3.2

3.1 196 2.75 3.23.1 0.0

McPherson 

County 

#1-330

 59 56±1 4.2±0.1 < 4 < .06 -- 112±2 1.57 4.2±0.1 > .96

McPherson 

County 

#1-550

 59 40±1 3.7±0.1 < 4 < .06 -- 80±1 1.12 3.7±0.1 > .95

McPherson 

County 

#1-770

 -- -- -- 36 .50 4.4 -- -- -- --

1

Recharge temperature estimated from nitrogen and argon data.

2

Fraction removed by denitrification = [1- (measured NO3–/initial NO3–)].

was present in all of the samples containing less than 16 μM 

(0.5 mg/L) of O2 (table 5). Excess N2 was not detected in 

samples containing more than 62 μM (2 mg/L) of O2. The one 

exception was the sample from Lincoln Co. #2-760, which 

contained 82 μM (2.6 mg/L) of O2 and 57 μM of excess N2. 

As previously discussed, that sample may have contained a 

fraction of shallow ground water. The presence of excess N2 in 

ground water and its inverse correlation to O2 concentrations 

supports the interpretation that denitrification occurred in the 

aquifer.

Denitrification reaction progress is defined as the fraction, f, of initial NO3

–

 in recharge removed by denitrification 

(eq. 12). 

 

 

where [NO3

–

]measured is the measured NO3

–

 concentration in the 

water sample and [NO3

–

]

initial is the initial NO3

–

 concentration 

in recharge. The initial NO3

–

 concentration in recharge (in 

molar units) was calculated using equation 13.

Initial NO3

–

 concentrations ranged from 75 to 286 μM 

(1.05 to 4.00 mg/L as N) (table 5). Denitrification removed 

0 to more than 97 percent of the initial NO3

–

 in recharge 

(average = 50 percent) (fig. 14B), on the basis of the initial 

and measured NO3

–

 concentrations. These results indicate 

that denitrification was an important process for attenuating 

NO3

–

 concentrations in some parts of the aquifer. 

Average denitrification reaction progress north of the 

river (66 percent) was substantially greater than it was for the 

single well nest south of the river (0 percent) (fig. 14B), and 

it appears to be related to the presence of thick deposits of 

sediment on top of the Ogallala Group north of the river (fig. 

6). The importance of those sediments in terms of denitrification is evident from the changes in denitrification reaction 

progress with depth at Lincoln Co. #4 and McPherson Co. #1 

(fig. 14B). Nearly all of the NO3

–

 removal by denitrification at 

those sites occurred between the two shallowest wells, which 

are screened above or near the top of the Ogallala Group 

(fig. 14B). There was little additional NO3

–

 removal in the 

Ogallala Group, which is similar to what was observed south 

of the river at Lincoln Co. #3. The change in denitrification 

reaction progress with depth at Lincoln Co. #1 was unique 

among the nested-well sites. Reaction progress at that site 

actually decreased with depth. That site is characterized by 

a shallow depth to water and soils composed of silt and fine 

sand. That combination of factors, and possibly others, apparently supported substantial denitrification near the water table. 

The decrease in reaction progress below the shallow well may 

be related to the convergence of flow paths from multiple 

depths in the regional discharge area of the river valley. In gen-

(12)

(13) - - [NO3 ]initial = [NO3]measured 

 +2×[excess N2]

28  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003

-

ƒ = 1

[NO3 ]measured

[NO3 ]initial

– -
eral, the sediments on top of the Ogallala Group in the Sand 

Hills provide the aquifer a level of protection from NO3

–

 contamination that is not present south of the river.

Measured concentrations and isotopic values for NO3

–

and N2 were used to reconstruct the N isotopic composition of 

initial NO3

–

 in recharge. The initial values ranged from 1.3 to 

4.9‰ (average = 3.2‰) for wells north of the river (table 5). 

These are relatively small δ15N values for NO3

–

 that presumably was derived from soil N (Kreitler, 1979; Heaton, 1986). 

Interestingly, the δ15N values for natural NO3

–

 in ground 

water systematically increase from the northern High Plains 

to the southern High Plains (McMahon and Böhlke, 2006). 

McMahon and Böhlke (2006) discuss possible mechanisms 

to explain this observation. It is important to understand the 

variability in δ15N values for natural NO3

–

 if isotopes are 

to be used as a tracer of NO3

–

 contamination in semiarid 

regions such as the High Plains. This is because large natural 

NO3

–

 deposits in the subsoil, which are known to exist in the 

High Plains (Boyce and others, 1976; McMahon and others, 

2006), can be mobilized by irrigation return flow. Differentiating between agricultural and natural NO3

–

 in return flow on 

the basis of N isotopes can be complicated if there is large 

variability in the δ15N values of natural NO3

–

.

Pesticides

The water-table well at Lincoln Co. #3 was the only one 

to produce water that contained a detection of a pesticide compound. Deethylatrazine was detected in water from that well at 

an estimated concentration of 0.006 μg/L (Appendix 1). Deethylatrazine is a degradation product of atrazine, a common corn 

herbicide, and corn is grown in that area.

Vertical Gradients in Ground-Water 

Age

As discussed in the “Tritium” section, water from 

some of the water-table wells was recharged after the early 

1950s, whereas water from the deeper wells at most sites was 

recharged before the early 1950s. These data indicate vertical 

gradients in ground-water age were present at those sites. The 

magnitude of the gradient, however, cannot be determined 

using 3H data alone. Measurements of 14C[DIC] have been 

used by several researchers to estimate ground-water ages 

greater than about 1,000 years. Applying 14C[DIC] measurements to ground-water age-dating issues requires that those 

measurements be adjusted for the addition or removal of DIC 

along flow paths leading to the sampled wells. Construction of 

mass-balance models to account for changes in concentrations 

of dissolved constituents between initial and final waters along 

flow paths is one approach for making those adjustments 

(Plummer and others, 1990; Aravena and others, 1995, Kalin, 

2000; McMahon and others, 2004a).

Mass-Balance Models

Mass-balance models require chemical and isotopic 

data for initial (upgradient) and final (downgradient) waters. 

The initial and final waters are from wells assumed to be 

located on the same flow path, and it is further assumed that 

the chemistry of the initial water has not changed over time. 

In other words, the chemistry of the initial water should be 

representative of the chemistry of the final water at the time it 

was recharged. For radiocarbon dating purposes, the samples 

should be free of postbomb 14C. For the High Plains aquifer, 

water containing less than 0.5 TU 3H is assumed to be free 

of postbomb 14C. McMahon and others (2004a) constructed 

mass-balance models for the central High Plains aquifer by 

using samples from the most upgradient water-table wells in 

a regional transect as initial waters because simulations of 

ground-water movement indicated those areas were the most 

likely recharge areas to downgradient wells and because they 

contained less than 0.5 TU 3H. A similar approach was used in 

this study.

Even with the steps taken to match final waters with 

appropriate initial waters, it is difficult to establish whether 

initial and final waters selected for mass-balance modeling 

actually were aligned along the same flow path. Thus, chemical analyses from the shallowest wells at Lincoln Co. #2, Lincoln Co. #4, and McPherson Co. #1 were used to represent the 

composition of initial waters for nested wells located north of 

the river (table 4 and Appendix 1). In other words, each of the 

final waters was modeled using three different initial waters. 

The shallowest well at McPherson Co. #1 was included in this 

group, even though the sample from that well contained 5.8 

TU 3H, because its 14C[DIC] composition was not enriched in 14C compared to water that was recharged prior to the early 

1950s (table 4). For nested wells located south of the river 

(Lincoln Co. #3), the chemical analysis from Keith Co. #1 was 

used to represent the composition of initial waters (fig. 2). 

The computer code NETPATH (Plummer and others, 

1994) was used to construct mass-balance models using the 

initial waters described above and final waters defined by the 

deeper wells in each well nest. The constraints used in the 

models were concentrations of aluminum (Al), Ca, DIC, Fe, 

N, Na, sulfur (S), silicon (Si), and redox state of the water. 

Additional constraints were imposed by using mineralogic 

data, mineral saturation indices, and isotopic data for dissolved 

species and solid phases containing C. The following phases 

were included in the mass-balance models: calcite, lignite 

(CH2O), Ca-Na exchange, albite, sodium plagioclase (Na0.62Ca

0.38Al1.38Si2.62O8

), kaolinite, SiO2, pyrite, goethite, CO2 gas, and 

N2 gas. Lignite and pyrite were included in the mass-balance 

models to provide a source of electrons for the reduction of 

dissolved O2 and NO3

–

 (eqs. 8 and 9). Pyrite also was a source 

of dissolved S.

Several mass-balance models were identified for each 

pair of initial and final waters; however, constraints imposed 

by the saturation indices and isotopic data eliminated many 

of them. The mass-balance results listed in Appendix 2 

Vertical Gradients in Ground-Water Age 29
satisfied most of the constraints imposed by the data. Most of 

the models for wells north of the river required the dissolution 

of sodium plagioclase and calcite; the precipitation of kaolinite 

and SiO2; and cation exchange. All but one model required the 

coupling of lignite or pyrite oxidation to the reduction of O2

or NO3

–

, thereby producing CO2 in the case of lignite oxidation. It was not possible to know the O2

 concentration in the 

final water at the time of recharge; therefore, the amount of O2

reduction was calculated as the difference in O2 concentrations 

between the initial and final waters. The amount of denitrification was assumed to equal the amount of excess N2

-N 

present in the final waters, as previously discussed (table 5). 

Thus, for calculating mass balances between initial and final 

waters, the NO3

–

 concentration in the initial water was set 

equal to the NO3

–

 concentration in the final water at the time 

it was recharged (table 5). Models for wells south of the river 

(Lincoln Co. #3) required calcite precipitation and little or no 

lignite oxidation. Calcite precipitation was required because of 

the relatively large DIC and Ca2+ concentrations in the initial 

water (Keith Co. #1) relative to the final waters. Very little lignite oxidation was required because the final waters contained 

substantial amounts of O2 and no excess N2. It should be noted 

that the mass-balance models are considered to be nonunique 

because small variations in several of the model parameters 

could provide similar matches between measured and calculated values. However, none of the variations in model parameters that satisfied the model constraints produced substantially different results in the radiocarbon ages. 

Very small additions of CO2 were required to achieve 

mass balance in a few models, even though the system below 

the water table is considered closed to exchange of CO2 gas 

with the soil atmosphere. Most of the models also required 

a small component of evaporation to achieve mass balance. 

The Cl- and Br- data indicate that evaporation of precipitation 

or shallow ground water may have occurred (fig. 13), but the 

CO2 additions and evaporation also could reflect uncertainties in (a) selecting the appropriate initial water for the final 

waters, (b) the DIC analyses, and (c) the charge imbalance of 

the major-ion analyses (Plummer and others, 2004). Selected 

final waters also were modeled using the computer code 

PHREEQC (Parkhurst and Appelo, 1999) because it allows the 

user to assign uncertainties to the chemical and isotopic data 

for initial and final waters. Those models did not require CO2

additions and resulted in slightly larger radiocarbon ages than 

the ages calculated by using the NETPATH models, as will be 

discussed in the next section.

Radiocarbon Ages

Adjusted radiocarbon ages of ground water were calculated using NETPATH and the radioactive decay equation:

 

where Δt (in years) is the time since ground water was isolated 

from the atmosphere, 5,730 is the half-life of 14C (in years), 

And (in pmc) is the 14C[DIC] value in the final water adjusted 

for C mass transfers in the absence of radioactive decay, and A 

(in pmc) is the measured value of 14C[DIC] in the final water. 

The value of And is dependent on the value of Ao (the 14C[DIC] 

value in the initial water, table 4) and the C mass transfers 

calculated in the mass-balance models (Appendix 2).

Values of 14C[DIC] from the shallowest wells at Lincoln 

Co. #2, Lincoln Co. #4, and McPherson Co. #1 were used to 

represent Ao values north of the river. Those 14C[DIC] values 

range from 73.4 to 91.6 pmc (table 4). Radiocarbon was not 

measured in water from Keith Co. #1, the initial water south 

of the river; therefore, initial 14C activity models available in 

NETPATH were used to estimate Ao at that site (Plummer 

and others, 1994). The activity models presented by Fontes 

and Garnier (1979) and Plummer and others (1990) provided 

estimates of Ao that ranged from 50 to 55 pmc. These values 

are similar to the measured Ao values for recharge in the central and southern High Plains (McMahon and others, 2004a, 

b), but they are smaller than the values north of the river. This 

difference may be due to less carbonate-mineral dissolution 

by water in the unsaturated zone as it moved from the land 

surface to the water table north of the river compared to south 

of the river. Dissolution of 14C-free carbonate minerals in the 

unsaturated zone would decrease Ao values. This interpretation 

is consistent with the larger calcite saturation index at Keith 

Co. #1 (-0.15) compared to the average saturation index for 

the three initial waters north of the river (-1.14), indicating that 

recharge south of the river was near equilibrium with respect 

to calcite but recharge north of the river was undersaturated 

with respect to calcite.

Unadjusted and adjusted radiocarbon ages calculated 

using these Ao values are listed in table 6. The radiocarbon 

ages presented here do not account for the possibility that 

14C[DIC] of recharge varied over time because of variations in 

the Earth’s geomagnetic field strength and solar fluctuations 

(Mazaud and others, 1991; Bard and others, 1993). Age corrections for matrix diffusion (molecular diffusion of 14C[DIC] 

from sand to clay layers) were not made in this study but could 

produce younger adjusted ages than those reported in table 6 

(Bethke and Johnson, 2002). Average adjusted radiocarbon 

ages from NETPATH ranged from 1,800 to 15,600 years, and 

they were from 0 to 4,300 years younger than the unadjusted 

ages. On average, radiocarbon ages calculated using mass 

transfers from PHREEQC were 900 years older than the ages 

calculated using mass transfers from NETPATH. This difference is similar to the 1-sigma precision of the age estimates 

from NETPATH (table 6). 

Ground-water ages increased substantially with depth 

below the water table at all of the sites except McPherson Co. 

#1 (fig. 15), indicating that the aquifer in most areas contained 

a stratified sequence of ground-water ages that was characterized by a relatively thin zone of recently recharged water (less 

than 50 yr, as indicated by 3H concentrations larger than 0.5 

TU) overlying a thicker zone of water that was recharged from 

∆t = 5,730

1n2

1n (And

A ) (14)

30 Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Table 6. Summary of ground-water ages from tritium and radiocarbon dating.

[Multiple entries indicate replicate samples or measurements; TU, tritium units; pmc, percent modern carbon; <, less than; >, greater than; ±, plus or minus 1 

standard deviation; --, not applicable]

Well name

Tritium 

(

3

H),

measured

(TU)

14

C[DIC], 

measured

(pmc)

Average 

unadjusted radiocarbon age1

(

14

C years)

Average adjusted radiocarbon 

age2

(

14

C years)

(NETPATH)

Average adjusted 

radiocarbon age2

(

14

C years)

(PHREEQC/NETPATH)

Tritium 

age

(years)

Lincoln County #1-25 8.5

8.4 45.1 -- -- -- < 50.

Lincoln County #1-119 < .5 39.5 6,100±900 1,800±900 -- > 50.

Lincoln County #1-270 < .5 41.6 5,600±900 2,500±900 2,900±1,000 > 50.

Lincoln County #1-370 < .5 12.7

14.2

15,400±900

14,600±900 12,600±900 14,100±1,100 > 50.

Lincoln County #2-255 < .5 73.4 2,600 -- -- > 50.

Lincoln County #2-590 < .5 13.8 14,800±900 11,700±900 -- > 50.

Lincoln County #2-760 < .5 24.6

24.3

10,000±900

10,100±900 7,500±900 -- > 50.

Lincoln County #3-30 3.0 63.9 -- -- -- < 50.

Lincoln County #3-110 < .5 23.9 6,500±600 6,200±400 -- > 50.

Lincoln County #3-250 < .5 21.1 7,500±600 7,300±400 7,500±200 > 50.

Lincoln County #3-390 < .5 7.90 15,600±600 15,600±400 15,600 > 50.

Lincoln County #4-195 < .5 91.6 700 -- -- > 50.

Lincoln County #4-359 < .5 42.4 5,500±900 2,100±300 -- > 50.

Lincoln County #4-510 < .5 26.1 9,500±900 5,800±700 6,900±500 > 50.

Lincoln County #4-695 < .5 11.0

11.9

16,600±900

16,000±900 12,600±800 14,900±1,200 > 50.

McPherson County #1-45 5.8 82.9 -- -- -- < 50.

McPherson County #1-330 < .5 66.3 1,800±900 -- -- > 50.

McPherson County #1-550 < .5 75.9

76.1

700±900

600±900 -- -- > 50.

McPherson County #1-770 1.8 40.8 -- -- -- --

1

Assumes initial 14C values of 73.4, 82.9, and 91.6 pmc for wells north of the river and 14C values of 50 and 55 pmc for wells south of the river (Lincoln Co. 

#3), except for wells Lincoln Co. #2–255 and Lincoln Co. #4–195 which were assumed to have an initial 14C value of 100 pmc. Does not account for addition or 

removal of carbon along flow paths.

2

Assumes initial 14C values of 73.4, 82.9, and 91.6 pmc for wells north of the river and 14C values of 50 and 55 pmc for wells south of the river (Lincoln Co. 

#3). Accounts for addition and removal of carbon along flow paths.

Vertical Gradients in Ground-Water Age 31
about 1,800 to 15,600 14C years ago. No recently recharged 

water was detected at the two sites (Lincoln Co. #2 and #4) 

with unsaturated zones thicker than 100 ft (table 4). At Lincoln Co. #1 and #3, the aquifer contained, at most, 24 to 26 

percent recently recharged water on the basis of screen depths 

below the water table and saturated thicknesses and assumwith shallow ground water also appears to have occurred in 

McPherson Co. #1-770. Consequently, ages for water from 

those wells were not plotted in fig. 15. 

Vertical gradients in age were calculated using 

equation 15:

 

where agebottom refers to the radiocarbon age (NETPATH) 

for the deepest wells in fig. 15, and agetop refers to the age 

for the shallowest wells. Depthbottom and depthtop refer to the 

well-screen depths below the water table for those wells. Age 

gradients ranged from 24 to 42 yr/ft at all the sites except 

McPherson Co. #1. The age gradient at McPherson Co. #1 was 

1.3 yr/ft, which is much smaller than the gradients observed at 

the other sites (fig. 15). The small gradient at McPherson Co. 

#1 could be indicative of relatively rapid recharge that might 

be related to the large downward head gradient at that site 

(fig. 8). The age gradient at McPherson Co. #1 corresponds 

to a recharge rate of about 0.2 ft/yr, assuming a sediment 

porosity of 0.3 and a linear age gradient. The assumption of 

a linear age gradient may not be valid for McPherson Co. #1, 

but it is difficult to determine on the basis of the existing data 

and their associated precisions. Nevertheless, this is the same 

recharge rate that was calculated for sandy soils at the Imperial site (fig. 2) on the basis of a tritium profile in the unsaturated zone (McMahon and others, 2006), and it is only about 

20 percent larger than the largest recharge rate estimated for 

the Sand Hills on the basis of a regional ground-water flow 

model (Luckey and others, 1986). It is worth noting that 14C 

samples were collected again at McPherson Co. #1–330 and 

–550 about 8 months after the initial samples were collected to 

verify those earlier results. Both sample sets had very similar 

14C concentrations (table 4). 

Age gradients at Lincoln Co. #1 and 4 (and possibly #3) 

did not appear to be linear, so long-term average recharge rates 

at those sites were calculated using equation 16:

 

where R is recharge rate (ft/yr), Z is aquifer thickness (ft), z

is depth below the water table (ft), n is porosity (assumed to 

equal 0.3), and age is ground-water age at depth z (yr). This 

approach assumes that the age-depth profiles follow an exponential age distribution (Cook and Böhlke, 2000). The exponential model appears to match the Lincoln Co #1 and #4 data 

reasonably well (fig. 17), resulting in recharge rates of 0.03 

and 0.05 ft/yr, respectively. These values are about 2 to 6 times 

smaller than the recharge rates estimated for the Sand Hills 

on the basis of a regional ground-water flow model (Luckey 

and others, 1986). The exponential model match to data from 

Lincoln Co. #3 was less satisfactory, possibly as a result of 

pumping in the valley (fig. 4), which could vertically mix 

water in the aquifer. Nevertheless, the inferred recharge rate at 

that site (0.02 ft/yr) is similar to those at Lincoln Co. #1 and 

#4. On average, recharge rates derived from age gradients in 

Gr (15) adient = 

agebottom agetop

depthbottom depthtop

–

–

R = (16) Zn

age

1n Z

Z – z ( )

Figure 15. Adjusted radiocarbon age in relation to depth 

below the water table. Lincoln Co. #1–25, Lincoln Co. #3–30, and 

McPherson Co. #1–45 were assigned ages of 25 years on the 

basis of tritium data, which indicate ages less than 50 years 

(table 6). Unadjusted radiocarbon ages were assigned to Lincoln 

Co. #2–255, Lincoln Co. #4–195, and the two deeper wells at 

McPherson Co. #1 (table 6).

ing that only recent recharge was present in the aquifer above 

the shallowest well with 3H concentrations less than 0.5 TU 

(fig. 16). The actual percentages are not well defined because 

of the large vertical spacing between wells, but they could 

be smaller than these. These results have important implications with respect to development of ground-water resources 

in the Sand Hills. Most of the water in the aquifer predates 

modern anthropogenic activity so excessive removal of water 

by pumping is not likely to be replenished by natural recharge 

in a meaningful timeframe. Excessive ground-water pumping 

generally has not occurred in the Sand Hills, but it has resulted 

in large water-level declines in parts of the aquifer south of the 

river (fig. 4) (McGuire and Sharpe, 1997).

The decrease in ground-water age near the base of the 

aquifer at Lincoln Co. #2 (Lincoln Co. #2-760, table 6) most 

likely is the result of mixing with shallow (young) ground 

water during or after well construction and is not considered to 

be representative of natural conditions in the aquifer. Mixing 

EXPLANATION

ADJUSTED RADIOCARBON AGE, IN YEARS

DEPTH OF WELL-SCREEN MIDPOINT

BELOW WATER TABLE, IN FEET

32 Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Monitoring-well test

Land surface

Water table

Base of aquifer

Shallowest well

producing water

with tritium

concentration less

than 0.5 Tritium Units

(TU)

Hypothetical tritium

concentration in

ground water

Interval in which

water is assumed to

contain more than

0.5 TU of tritium

10 TU

Less than 0.5 TU

Less than 0.5 TU

Less than 0.5 TU

A

Saturated thickness (B)

Percentage of saturated thickness containing recently recharged water 

(greater than 0.5 TU of tritium) = (A/B) × 100

Figure 16. Schematic diagram illustrating a method for calculating the maximum percentage of 

saturated thickness containing recently recharged water.

Figure 17. Measured and modeled age distributions used 

to estimate long-term average recharge rates.

EXPLANATION

ADJUSTED RADIOCARBON AGE, IN 14C YEARS

DEPTH BELOW WATER TABLE, IN FEET

Vertical Gradients in Ground-Water Age 33
the Sand Hills were about 8 times larger than those in the central High Plains aquifer (McMahon and others, 2004a). This 

apparent increase in recharge rates likely results from sandier 

soils (increased infiltration) and colder recharge temperatures 

(reduced evapotranspiration) in the Sand Hills compared to the 

central High Plains because mean annual precipitation in both 

study areas were within 5 percent of each other (High Plains 

Regional Climate Center, 2005). 

Apparent rates of O2 reduction and denitrification were 

estimated on the basis of decreases in O2 concentrations and 

increases in concentrations of excess N2 and ground-water 

ages along flow paths from the water table to deeper wells. 

Decreases in O2 concentrations with depth at the nested-well 

sites indicate that many of the samples had lost some O2

(Appendix 1). For O2, calculations of apparent reduction rates 

assume ground water was recharged with 7.4 mg/L of dissolved O2

, which is the average O2 concentration in water from 

the shallowest wells at Lincoln Co. #2 and #4, McPherson Co. 

#1, and Keith Co. #1 (Appendix 1). Using Lincoln Co. #4-359 

as an example, the apparent O2 reduction rate would be at 

least 0.003 mg/L/yr on the basis of its measured O2 concentration (less than 0.5 mg/L) and radiocarbon age (2,100 years). 

Apparent O2 reduction rates are listed in table 7. Increases 

in excess N2 concentrations with depth at the nested-well 

sites indicate that many of the samples also underwent some 

denitrification (table 5). Calculations of apparent denitrification rates assume denitrification began where the traveltime 

was equal to one-half the adjusted radiocarbon age of water 

from the second shallowest well in each nest. This approach 

was used because denitrification in aquifer sediments probably 

does not begin until O2 concentrations have been reduced to 

about 2 mg/L or less (McMahon and others, 2004a, b), which 

typically occurred between the shallowest and second shallowest wells. Using Lincoln Co. #4-359 as an example again, 

the apparent denitrification rate would be about 0.002 mg/L/yr 

as N on the basis of the calculated excess N2 concentration 

(66 μmol/L or 1.85 mg/L as N) and the radiocarbon age of the 

sample (2,100 years) minus the age when denitrification is 

assumed to have begun (1,050 years). Apparent denitrification 

rates are listed in table 7.

Median rates of O2 reduction and denitrification south of 

the river (Lincoln Co. #3) were 0.0002 mg/L/yr and less than 

0.0002 mg/L/yr as N, respectively (table 7). These rates are 

similar to those calculated for the central and southern High 

Plains aquifers (McMahon and others, 2004a, b), but they are 

at least 10 times smaller than the median rates north of the 

river (Lincoln Co. #1 and #4 and McPherson Co. #1). The 

much faster rates of O2 reduction and denitrification in sediments north of the river probably are due to the relative abundance of electron donors, such as organic carbon and pyrite, 

in sediments overlying the Ogallala Group in the Sand Hills 

(fig. 6), as discussed previously in the section “Dissolved Oxygen” and as indicated by the mass-balance models (Appendix 

2). The relatively large denitrification rates in the Sand Hills 

indicate that the aquifer in that area may have a greater capacity to attenuate NO3

–

 contamination than the aquifer south of 

the river, depending on rates of ground-water movement in the 

two areas. Small denitrification rates south of the river indicate 

that NO3

–

 contamination in that part of the aquifer would 

likely persist for a longer period of time, as indicated by the 

presence of dissolved O2 and NO3

–

 and the absence of excess 

N2, in water from Lincoln Co. #3-390, which has an apparent 

age of 15,600 14C years (table 6).

Table 7. Apparent rates of dissolved-oxygen reduction and denitrification.

[ΔAge, assumed reaction time; ΔO2, dissolved-oxygen loss; N2, excess nitrogen gas; N, nitrogen; mg/L, milligrams per liter; mg/L/yr, milligrams 

per liter per year; <, less than; >, greater than]

Well 

name

ΔAge

(years)

ΔO2

(mg/L)

O2 reduction 

rate (mg/L/yr)

ΔAge

(years)

Excess N2

(mg/L as N)

Denitrification 

rate (mg/L/yr as N)

Lincoln County #1-119 1,800 > 6.9 > 0.004 900 3.16 0.004

Lincoln County #3-110 6,200 0.1 .00002 3,100 < .84 < .0003

Lincoln County #3-250 7,300 1.2 .0002 4,200 < .84 < .0002

Lincoln County #3-390 15,600 4.7 .0003 12,500 < .84 < .00007

Lincoln County #4-359 2,100 > 6.9 > .003 1,050 1.85 .002

McPherson County #1-330 1,800 > 6.9 > .004 900 1.57 .002

34  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Summary and Conclusions

The northern High Plains aquifer is the primary source 

of water used for domestic, industrial, and irrigation purposes 

in parts of Colorado, Kansas, Nebraska, South Dakota, and 

Wyoming. Despite the aquifer’s importance to the regional 

economy, fundamental ground-water characteristics, such as 

vertical gradients in water chemistry and age, remain poorly 

defined. As part of the U.S. Geological Survey’s National 

Water-Quality Assessment Program, water samples from 

nested, short-screen monitoring wells installed in the northern 

High Plains aquifer were analyzed for major ions, nutrients, 

trace elements, dissolved organic carbon, pesticides, stable and 

radioactive isotopes, dissolved gases, and other parameters 

to evaluate vertical gradients in water chemistry and age in 

the aquifer. Chemical data and tritium and radiocarbon ages 

show that water in the aquifer was chemically and temporally stratified in the study area, with a relatively thin zone of 

recently recharged water (less than 50 years) near the water 

table overlying a thicker zone of older water. In areas where 

irrigated agriculture was an important land use, the recently 

recharged ground water was characterized by elevated concentrations of major ions and nitrate and the detection of pesticide 

compounds. Below the zone of agricultural influence, majorion concentrations exhibited small increases with depth and 

distance along flow paths because of rock/water interactions. 

The concentration increases were accounted for primarily by 

dissolved calcium, sodium, bicarbonate, sulfate, and silica. In 

general, the chemistry of ground water throughout the aquifer 

was of high quality. None of the approximately 90 chemical 

constituents analyzed in each sample exceeded primary drinking-water standards.

Nitrite plus nitrate concentrations at the water table 

were 0.13 to 3.13 milligrams per liter as nitrogen, and concentrations substantially decreased with depth in the aquifer. 

Dissolved-gas and nitrogen-isotope data indicate that denitrification in the aquifer removed 0 to 97 percent (average = 

50 percent) of the nitrate originally present in recharge. The 

average amount of nitrate removed by denitrification in the 

aquifer north of the Platte River (Sand Hills) was substantially greater than the amount removed south of the river (66 

as opposed to 0 percent), and the extent of nitrate removal 

appears to be related to the presence of thick deposits of 

sediment on top of the Ogallala Group in the Sand Hills that 

contained electron donors, such as organic carbon and pyrite, 

to support denitrification.

Mass-balance models indicate that changes in groundwater chemistry along flow paths in the aquifer can be 

accounted for by small amounts of sodium plagioclase and 

calcite dissolution; kaolinite and silica precipitation; lignite 

and pyrite oxidation; oxygen reduction and denitrification; and 

cation exchange. Mixing with surface water affected the chemistry of ground water in alluvial sediments of the Platte River 

Valley. Radiocarbon ages in the aquifer, adjusted for carbon 

mass transfers, ranged from 1,800 to 15,600 14C years before 

present. These results have important implications with respect 

to development of ground-water resources in the Sand Hills. 

Most of the water in the aquifer predates modern anthropogenic activity so excessive removal of water by pumping is not 

likely to be replenished by natural recharge in a meaningful 

timeframe. Vertical gradients in ground-water age were used 

to estimate long-term average recharge rates in the aquifer. In 

most areas, the recharge rates ranged from 0.02 to 0.05 foot 

per year. The recharge rate was 0.2 foot per year in one part of 

the aquifer characterized by large downward hydraulic gradients. On average, recharge rates in the Sand Hills were about 8 

times larger than those in the central High Plains aquifer. This 

difference in recharge rates likely results from sandier soils 

and colder recharge temperatures in the Sand Hills compared 

to the central High Plains because mean annual precipitation 

in both study areas are within 5 percent of each other.

Apparent rates of dissolved-oxygen reduction and denitrification were estimated on the basis of decreases in dissolved-oxygen concentrations and increases in concentrations 

of excess nitrogen gas and ground-water ages along flow paths 

from the water table to deeper wells. Median rates of dissolved-oxygen reduction and denitrification south of the Platte 

River were at least 10 times smaller than the median rates 

north of the river in the Sand Hills. The relatively large denitrification rates in the Sand Hills indicate that the aquifer in that 

area may have a greater capacity to attenuate nitrate contamination than the aquifer south of the river, depending on rates of 

ground-water movement in the two areas. Small denitrification 

rates south of the river indicate that nitrate contamination in 

that part of the aquifer would likely persist for a longer period 

of time.

Results from this study show that measurements of vertical gradients in water chemistry and age can provide fundamental baseline information on water chemistry, recharge 

rates, and biogeochemical reaction rates in aquifers. Initiating 

similar studies in other principal aquifers of the United States 

would be a good first step toward developing a geochemical framework for these aquifers based on these important 

variables. Such a framework could be used to develop better 

understanding of aquifer susceptibility to contamination and 

better forecasts of contamination of important receptors of 

ground water, such as drinking-water wells, streams, lakes, 

and estuaries. 

References Cited

Aravena, R., Wassenaar, L.I., and Plummer, L.N., 1995, Estimating 14C ground-water ages in a methanogenic aquifer: 

Water Resources Research, v. 31, p. 2307–2317.

Bard, E., Arnold, M., Fairbanks, R.G., and Hamelin, B., 1993, 230Th-234U and 14C ages obtained by mass spectrometry on 

corals: Radiocarbon, v. 35, p. 191–199. 

References Cited 35
Bethke, C.M., and Johnson, T.M., 2002, Ground-water age: 

Ground Water, v. 40, p. 337–339.

Bleed, A.S., and Flowerday, C.A., 1998, An atlas of the Sand 

Hills: University of Nebraska, Institute of Agriculture and 

Natural Resources, Conservation and Survey Division 

Resource Atlas No. 5b, 260 p.

Böhlke, J.K., and Denver, J.M., 1995, Combined use of 

ground-water dating, chemical, and isotopic analyses to 

resolve the history and fate of nitrate contamination in two 

agricultural watersheds, Atlantic Coastal Plain, Maryland: 

Water Resources Research, v. 31, p. 2319–2339.

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

contamination: Hydrogeology Journal, v. 10, p. 153–179. 

[Erratum, 2002, v. 10, p. 438–439.] 

Böhlke, J.K., Wanty, R., Tuttle, M., Delin, G., and Landon, 

M., 2002, Denitrification in the recharge area and discharge 

area of a transient agricultural nitrate plume in a glacial outwash sand aquifer, Minnesota: Water Resources Research, 

v. 38, 10.1029/2001WR00063.

Böhlke, J.K., Mroczkowski, S.J., and Coplen, T.B., 2003, 

Oxygen isotopes in nitrate: new reference materials for 

18O:17O:16O measurements and observations on nitrate-water 

equilibration: Rapid Communications in Mass Spectrometry, v. 17, p. 1835–1846.

Böttcher, J., Strebel, O., Voerkelius, S., and Schmidt, H.L., 

1990, Using isotope fractionation of nitrate-nitrogen and 

nitrate-oxygen for evaluation of microbial denitrification in 

a sandy aquifer: Journal of Hydrology, v. 114, p. 413–424.

Boyce, J.S., Muir, J., Edwards, A.P., Seim, E.C., and Olson, 

R.A., 1976, Geologic nitrogen in Pleistocene loess of 

Nebraska: Journal of Environmental Quality, v. 5, p. 93–96.

Busenberg, E., Weeks, E.P., Plummer, L.N., and Bartholomy, 

R.C., 1993, Age dating ground water by use of chlorofluorocarbons (CCl3

F and CCl2F2) and distribution of chlorofluorocarbons in the unsaturated zone, Snake River Plain 

aquifer, Idaho National Engineering Laboratory, Idaho, 

USA: U.S. Geological Survey Water-Resources Investigations Report 93–4054, 47 p.

Carmody, R.W., Plummer, L.N., Busenberg, E., and Coplen, 

T.B., 1997, Methods for collection of dissolved sulfate and 

sulfide and analysis of their sulfur isotopic compositions: 

U.S. Geological Survey Open-File Report 97–234, 91 p.

Casciotti, K. L., Sigman, D. M., Hastings, M., Böhlke, J.K., 

and Hilkert, A., 2002, Measurement of the oxygen isotopic 

composition of nitrate in seawater and freshwater using the 

denitrifier method: Analytical Chemistry, v. 74, 

p. 4905–4912.

Cook, P.G., and Böhlke, J.K., 2000, Determining timescales 

for groundwater flow and solute transport, in Cook, P.G., 

and Herczeg, A.L., eds., Environmental tracers in subsurface hydrology: Boston, Kluwer Academic Publishers, 

p. 1–30.

Coplen, T.B., 1988, Normalization of oxygen and hydrogen 

isotope data: Chemical Geology, v. 72, p. 293–297.

Coplen, T.B., Wildman, J.D., and Chen, J., 1991, Improvements in the gaseous hydrogen-water equilibration technique for hydrogen isotope ratio analysis: Analytical 

Chemistry, v. 63, p. 910–912.

Coplen, T.B., and Krouse, H.R., 1998, Sulfur isotope data 

consistency improved: Nature, v. 392, p. 32.

Diffendal, R.F., Jr., and Goeke, J.W., 2000, Keith County testhole logs: University of Nebraska, Institute of Agriculture 

and Natural Resources, Conservation and Survey Division 

Resource THR–51, 115 p. 

Dreeszen, V.H., 2000, Perkins County test-hole logs: University of Nebraska, Institute of Agriculture and Natural 

Resources, Conservation and Survey Division Resource 

THR–68, 112 p. 

Druliner, A.D., Chen, H.H., and McGrath, T.S., 1996, Relations of nonpoint-source nitrate and atrazine concentrations 

in the High Plains aquifer to selected explanatory variables 

in six Nebraska study areas: U.S. Geological Survey WaterResources Investigations Report 95–4202, 51 p. 

Druliner, A.D., 1997, Water quality and chemical evolution 

of ground water in the Long Pine Creek area, Brown and 

Rock Counties, Nebraska, 1993–94: U.S. Geological Survey 

Water-Resources Investigations Report 96–4095, 47 p.

Engberg, R.A., and Spalding, R.F., 1978, Ground-water quality atlas of Nebraska: University of Nebraska, Institute 

of Agriculture and Natural Resources, Conservation and 

Survey Division Resource Atlas No. 3, 38 p.

Engberg, R.A., 1984, Appraisal of data for ground-water 

quality in Nebraska: U.S. Geological Survey Water-Supply 

Paper 2245, 54 p. 

Emmons, P.J., and Bowman, P.R., 2000, Ground-water flow 

and water quality of the Indian Island well field near Grand 

Island, Nebraska, 1994–95: U.S. Geological Survey Fact 

Sheet FS–179–99, 4 p.

Epstein, S., and Mayeda, T., 1953, Variations of O–18 content 

of water from natural sources: Geochimica et Cosmochimica Acta, v. 4, p. 213–224.

Exner, M.E., and Spalding, R.F., 1990, Occurrence of pesticides and nitrate in Nebraska’s ground water: University of 

Nebraska, Institute of Agriculture and Natural Resources, 

Water Center Report WC–1, 34 p.

36  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Fan, A.M., and Steinberg, V.E., 1996, Health implications of 

nitrate and nitrite in drinking water––An update on methemoglobinemia occurrence and reproductive and developmental toxicity: Regulatory Toxicology and Pharmacology, 

v. 23, p. 35–43. 

Feagas, R.G., Claire, R.W., Guptill, S.C., Anderson, K.E., and 

Hallam, C.A., 1983, Land use and land cover digital data: 

U.S. Geological Survey Circular 895–E, 21 p.

Fishman, M.J., 1993, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory––Determination of inorganic and organic constituents in water and 

fluvial sediments: U.S. Geological Survey Open-File Report 

93–125, 217 p.

Fontes, J.-Ch., and Garnier, J.-M., 1979, Determination of the 

initial 14C activity of the total dissolved carbon: a review of 

the existing models and a new approach: Water Resources 

Research, v. 15, p. 399–413.

Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995, Design of 

the National Water-Quality Assessment Program––Occurrence and distribution of water-quality conditions: U.S. 

Geological Survey Circular 1112, 33 p.

Goeke, J.W., 2001, Lincoln County test-hole logs: University 

of Nebraska, Institute of Agriculture and Natural Resources, 

Conservation and Survey Division Resource THR–56, 

200 p. 

Goeke, J.W., 2000, McPherson County test-hole logs: 

University of Nebraska, Institute of Agriculture and Natural 

Resources, Conservation and Survey Division Resource 

THR–60, 54 p.

Gosselin, D.C., Headrick, J., Tremblay, R., Chen, X.H., and 

Summerside, S., 1997a, Domestic well-water quality in 

rural Nebraska––Focus on nitrate-nitrogen, pesticides, and 

coliform bacteria: Ground Water Monitoring and Remediation, v. 17, p. 77–87.

Gosselin, D.C., Nabelek, P.E., Peterman, Z.E., and Sibray, S., 

1997b, A reconnaissance study of oxygen, hydrogen, and 

strontium isotopes in geochemically diverse lakes, Western 

Nebraska, USA: Journal of Paleolimnology, v. 17, p. 51–65. 

Gottula, J.J., 1996, A study of nonpoint source ground-water 

contamination in the northern portion of the middle Republican Natural Resources District, Nebraska––A Special Protection Area report: Nebraska Department of Environmental 

Quality, Water Quality Division, Ground Water Section, 

March 1996, 80 p.

Granger, J., Sigman, D.M., Needoba, J.A., and Harrison, P.J., 

2004, Coupled nitrogen and oxygen isotope fractionation 

of nitrate during assimilation by cultures of marine phytoplankton: Limnology and Oceanography, v. 49, 

p. 1763–1773.

Gutentag, E.D., Heimes, F.J., Krothe, N.C., Luckey, R.R., and 

Weeks, J.B., 1984, Geohydrology of the High Plains aquifer 

in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming: U.S. Geological 

Survey Professional Paper 1400–B, 63 p.

Hach Company, 1992, Water analysis handbook: Loveland, 

Colo., Hach Chemical Co., 835 p.

Harvey, F.E., and Welker, J.M., 2000, Stable isotope composition of precipitation in the semi-arid north-central portion of 

the US Great Plains: Journal of Hydrology, v. 238, 

p. 90-109.

Heaton, T.H.E., 1986, Isotopic studies of nitrogen pollution in 

the hydrosphere and atmosphere–a review: Chemical Geology, v. 59, p. 87–102.

High Plains Regional Climate Center, 2005, Historical climate 

data for Nebraska: available online at http://www.hprcc.unl.

edu/ (accessed on 12/21/2005).

Holloway, J.M., and Dahlgren, R.A., 1999, Geologic nitrogen 

in terrestrial biogeochemical cycling: Geology, v. 27, 

p. 567–570.

Hübner, H., 1986, Isotope effects of nitrogen in the soil and 

biosphere, in Fritz, P., and Fontes, J.C., eds., Handbook of 

environmental isotope geochemistry: Amsterdam, Elsevier, 

v. 2, p. 361–425.

Johnson, M.S., Engberg, R.A., and Goeke, J.E., 1986, Hydrologic data for the southern Sand Hills area, Nebraska: U.S. 

Geological Survey Open-File Report 86–411, 136 p.

Kalin, R.M., 2000, Radiocarbon dating of groundwater systems, in Cook, P.G., and Herczeg, A.L., eds., Environmental 

tracers in subsurface hydrology: Boston, Kluwer Academic 

Publishers, p. 111–144.

Kaufman, S., and Libby, W.F., 1954, The natural distribution 

of tritium: Physics Review, v. 93, p. 1337–1344.

Keys, W.S., 1990, Borehole geophysics applied to groundwater investigations: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 2, chap. E2, 150 p.

Knox, M., Quay, P.D., and Wilbur, D., 1992, Kinetic isotopic 

fractionation during air-water gas transfer of O2, N2, CH4, 

and H2: Journal of Geophysical Research, v. 97, issue C12, 

p. 20,335–20,343.

Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, 

Ground-water data-collection protocols and procedures for 

the National Water-Quality Assessment Program––Collection and documentation of water-quality samples and related 

data: U.S. Geological Survey Open-File Report 95–399, 

113 p.

References Cited 37
Kreitler, C.W., 1979, Nitrogen-isotope ratio studies of soils 

and groundwater nitrate from alluvial fan aquifers in Texas: 

Journal of Hydrology, v. 42, p. 147–170. 

Krothe, N.C., Oliver, J.W., and Weeks, J.B., 1982, Dissolved 

solids and sodium in water from the High Plains aquifer in 

parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming: U.S. Geological 

Survey Hydrologic Investigations Atlas 658, 2 sheets.

Krouse, H.R., and Mayer, B., 2000, Sulfur and oxygen 

isotopes in sulfate, in Cook, P.G., and Herczeg, A.L., eds., 

Environmental tracers in subsurface hydrology: Boston, 

Kluwer Academic Publishers, p. 195–231. 

Lapham, W.W., Wilde, F.D., and Koterba, M.T., 1995, 

Ground-water data-collection protocols and procedures for 

the National Water-Quality Assessment Program––Selection, installation, and documentation of wells, and collection 

of related data: U.S. Geological Survey Open-File Report 

95–398, 69 p.

Litke, D.W., 2001, Historical water-quality data for the High 

Plains Regional Ground-Water Study Area in Colorado, 

Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, 

Texas, and Wyoming, 1930–98: U.S. Geological Survey 

Water-Resources Investigations Report 00–4254, 65 p.

Luckey, R.R., Gutentag, E.D., Heimes, F.J., and Weeks, J.B., 

1986, Digital simulation of ground-water flow in the High 

Plains aquifer in parts of Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming: 

U.S. Geological Survey Professional Paper 1400–D, 57 p.

Ludin, A., Weppernig, R., Bönisch, G., and Schlosser, P., 

1998, Mass spectrometric measurement of helium isotopes 

and tritium in water samples: Lamont-Doherty Technical 

Report 98–06, [http://www.ldeo.columbia.edu/~etg/ms_ms/

Ludin_et_al_MS_Paper.html].

Maupin, M.A. and Barber, N.L., 2005, Estimated withdrawals from principal aquifers in the United States, 2000: U.S. 

Geological Survey Circular 1279, 46 p.

Mazaud, A., Laj, C., Bard, E., Arnold, M., and Tric, E., 1991, 

Geomagnetic field control of 14C production over the last 

80 ky–implications for the radiocarbon time scale: Geophysical Research Letters, v. 18, p. 1885–1888. 

McGuire, V.L., and Sharpe, J.B., 1997, Water-level changes 

in the High Plains aquifer––Predevelopment to 1995: U.S. 

Geological Survey Water-Resources Investigations Report 

97–4081, 2 sheets.

McMahon, P.B., Chapelle, F.H., and Jagucki, M.L., 1992, 

Atrazine mineralization potential of alluvial aquifer sediments under aerobic conditions: Environmental Science & 

Technology, v. 26, p. 1556–1559.

McMahon, P.B., 2001, Vertical gradients in water chemistry 

in the central High Plains aquifer, southwestern Kansas and 

Oklahoma Panhandle, 1999: U.S. Geological Survey WaterResources Investigations Report 01–4028, 47 p.

McMahon, P.B., Böhlke, J.K., and Christenson, S.C., 2004a, 

Geochemistry, radiocarbon ages, and paleorecharge conditions along a transect in the central High Plains aquifer, 

southwestern Kansas, USA: Applied Geochemistry, v. 19, 

p. 1655–1686.

McMahon, P.B., Böhlke, J.K., and Lehman, T.M., 2004b, Vertical gradients in water chemistry and age in the southern 

High Plains aquifer, Texas, 2002: U.S. Geological Survey 

Scientific Investigations Report 2004–5053, 53 p.

McMahon, P.B., Dennehy, K.F., Bruce, B.W., Böhlke, J.K., 

Michel, R.L., Gurdak, J.J., and Hurlbut, D.A., 2006, Storage 

and transit time of chemicals in thick unsaturated zones 

under rangeland and irrigated cropland in the High Plains, 

United States: Water Resources Research, v. 42, W03413, 

doi:10.1029/2005WR004417.

McMahon, P.B., and Böhlke, J.K., 2006, Regional patterns 

in the isotopic composition of natural and anthropogenic 

nitrate in ground water, High Plains, USA: Environmental 

Science & Technology, v. 40, p. 2965–2970.

McNutt, R.H., 2000, Strontium isotopes, in Cook, P.G., and 

Herczeg, A.L., eds., Environmental tracers in subsurface 

hydrology: Boston, Kluwer Academic Publishers, 

p. 233–260.

National Atmospheric Deposition Program, 2005, National 

Trends Network: available online at http://nadp.sws.uiuc.

edu (accessed on 1/4/2006). 

Parkhurst, D.L., and Appelo, C.A.J., 1999, User’s guide to 

PHREEQC (ver. 2)––A computer program for speciation, 

batch reaction, one-dimensional transport, and inverse 

geochemical calculations: U.S. Geological Survery WaterResources Investigations Report 99–4259, 312 p. 

Plummer, L.N., Busby, J.F., Lee, R.W., and Hanshaw, B.B., 

1990, Geochemical modeling of the Madison aquifer in 

parts of Montana, Wyoming, and South Dakota: Water 

Resources Research, v. 26, p. 1981–2014. 

Plummer, L.N., Prestemon, E.C., and Parkhurst, D.L., 1994, 

An interactive code (NETPATH) for modeling NET geochemical reactions along a flow PATH, version 2.13: U.S. 

Geological Survey Water Resources-Investigations Report 

94–4169, 130 p.

Plummer, L.N., Bexfield, L.M., Anderholm, S.K., Sanford, 

W.E., and Busenberg, E., 2004, Geochemical characterization of ground-water flow in the Santa Fe Group aquifer 

system, Middle Rio Grande Basin, New Mexico: U.S. 

Geological Survey Water-Resources Investigations Report 

03–4131, 395 p. 

Postma, D., Boesen, C., Kristiansen, H., and Larsen, F., 1991, 

Nitrate reduction in an unconfined sandy aquifer–water 

chemistry, reduction processes, and geochemical modeling: 

Water Resources Research, v. 27, p. 2027–2045. 

38  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
Qi, S.L., Konduris, A., Litke, D.W., and Dupree, J., 2002, 

Classification of irrigated land using satellite imagery, the 

High Plains aquifer, nominal date 1992: U.S. Geological 

Survey Water-Resources Investigations Report 02–4236, 

35 p.

Smedley, P.L., and Kinniburgh, D.G., 2002, A review of the 

source, behavior, and distribution of arsenic in natural 

waters: Applied Geochemistry, v. 17, p. 517–568.

Spalding, R.F., Exner, M.E., Martin, G.E., and Snow, D.D., 

1993, Effects of sludge disposal on ground-water nitrate 

concentrations: Journal of Hydrology, v. 142, p. 213–228.

Spalding, R.F., Watts, D.G., Schepers, J.S., Burbach, M.E., 

Exner, M.E., Poreda, R.J., and Martin, G.E., 2001, Controlling nitrate leaching in irrigated agriculture: Journal of 

Environmental Quality, v. 30, p. 1184–1194.

Stanton, J.S., 2006, Ground-water quality of the northern High 

Plains aquifer, 1997, 2002-04: U.S. Geological Survey 

Scientific Investigations Report 2006-5138, x p.

Steele, G.V., Cannia, J.C., and Stanton, J.S., 1998, Geohydrology and water quality of the North Platte River alluvial 

aquifer, Garden County, western Nebraska, 1993–94: U.S. 

Geological Survey Water-Resources Investigations Report 

98–4033, 75 p.

Stumm, W., and Morgan, J.J., 1981, Aquatic Chemistry: John 

Wiley & Sons, New York, New York, 780 p.

Stute, M., and Schlosser, P., 1993, Principles and applications 

of the noble gas paleothermometer, in Swart, P.K., Lohmann, K.C., McKenzie, J., and Savin, S., eds., Climate change 

in continental isotope records: Geophysical Monograph 78, 

p. 89–100.

Swinehart, J.B., and Diffendal, R.F., 1998, Geology of the 

pre-dune strata, in Bleed, A.S., and Flowerday, C.A., eds., 

An Atlas of the Sand Hills: University of Nebraska, Institute 

of Agriculture and Natural Resources, Conservation and 

Survey Division Resource Atlas No. 5b, p. 29–42.

Swinehart, J.B., 1998, Wind-blown deposits, in Bleed, A.S., 

and Flowerday, C.A., eds., An Atlas of the Sand Hills: 

University of Nebraska, Institute of Agriculture and Natural 

Resources, Conservation and Survey Division Resource 

Atlas No. 5b, p. 43–56.

Thatcher, L.L., 1962, The distribution of tritium fallout in precipitation over North America: Bulletin of the International 

Association of Scientific Hydrology, v. 7, p. 48–58.

Thatcher, L.L., Janzer, V.J., and Edwards, K.W., 1977, Methods for determination of radioactive substances in water and 

fluvial sediments: U.S. Geological Survey Techniques of 

Water-Resources Investigations, chap. A–5, 95 p.

U.S. Department of Agriculture, 2002, 2002 Census of 

Agriculture: available online at http://www.nass.usda.gov/

Census_of_Agriculture/index.asp (accessed in 1/12/2006). 

U.S. Environmental Protection Agency, 1999, National Primary Drinking-Water Regulations; Radon-222: available 

online at http://www.epa.gov/safewater/radon/radfr2.html

(accessed on 12/19/2005).

U.S. Environmental Protection Agency, 2006, National 

Primary Drinking-Water Regulations: available online at 

http://www.epa.gov/safewater/mcl.html#listmcl (accessed on 

10/20/2006).

U.S. Geological Survey, variously dated, National field manual 

for the collection of water-quality data: U.S. Geological 

Survey Techniques of Water-Resources Investigations, book 

9, chaps. A1–A9, available online at http://pubs.water.usgs.

gov/twri9A (accessed on 12/19/2005). 

Verstraeten, I.M., Sibray, S.S., Cannia, J.C., and Tanner, 

D.Q., 1995, Reconnaissance of ground-water quality in the 

North Platte Natural Resources District, western Nebraska, 

June-July 1991: U.S. Geological Survey Water-Resources 

Investigations Report 94–4057, 114 p.

Verstraeten, I.M., Böhlke, J.K., Kraemer, T.F., and Cannia, J.C., 2001, Use of environmental tracers and isotopes 

to evaluate sources of water, nitrate, and uranium in an 

irrigated alluvial valley, Nebraska: U.S. Geological Survey 

Fact Sheet FS-100-01, 4 p.

Weissmann, G.S., Zhang, Y., LaBolle, E.M., and Fogg, 

G.E., 2002, Dispersion of ground-water age in an alluvial 

aquifer system: Water Resources Research, v. 38, 1198, 

doi:10.1029/2001WR000907.Zaugg, S.D., Sandstrom, 

M.W., Smith, S.G., and Fehlberg, K.M., 1995, Methods 

of analysis by the U.S. Geological Survey National Water 

Quality Laboratory––Determination of pesticides in water 

by C–18 solid-phase extraction and capillary-column gas 

chromatography/mass spectrometry with selected-ion monitoring: U.S. Geological Survey Open-File Report 95–181, 

60 p. 

References Cited 39
Appendix 1

Water-Quality Data from Sampled Monitoring 

Wells Screened in the Northern High Plains 

Aquifer and from the Platte River

Appendix 1  41
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, 

[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephμg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Sampling 

date

Turbidity

(NTU)

Oxygen 

(O2), dissolved 

(mg/L)

pH, 

field

(standard 

units)

Specific 

conductance 

(μS/cm)

Water 

temperature 

(°C)

Hydrogen 

sulfide 

(H2S), 

field, total 

(mg/L)

Calcium (Ca), 

dissolved 

(mg/L)

Magn

(M

diss

(m

Platte River near Brady, Nebr. 10/12/04 -- 9.5 8.4 620 19.3 -- 63.0 16Imperial 9/4/02 1.8 9.3 7.49 318 15.7 -- 46.2 7Keith County #1 9/20/05 0.6 7.4 7.4 393 16.3 -- 52.6 10Lincoln County #1-25 9/6/03 .2 < .5 7.75 301 13.7 -- 37.7 6Lincoln County #1-119 9/5/03 .2 < .5 7.77 284 14.0 < 0.01 39.7 6Lincoln County #1-270 9/5/03 .2 < .5 7.79 275 15.3 < .01 38.1 6Lincoln County #1-370 9/6/03 3.8 1.5 7.78 354 15.4 -- 35.3 6Lincoln County #2-255 9/7/03 .3 7.8 7.43 151 14.9 -- 16.2 2Lincoln County #2-590 9/7/03 7.2 < .5 7.65 360 16.7 < .01 47.6 6Lincoln County #2-760 9/7/03 5.1 2.9 7.78 293 16.8 -- 37.7 4Lincoln County #3-30 9/4/03 .1 3.6 7.55 610 11.4 -- 71.7 15Lincoln County #3-110 9/3/03 .7 7.3 7.69 386 12.2 -- 48.1 10Lincoln County #3-250 9/4/03 .5 6.2 7.68 393 13.3 -- 41.0 7Lincoln County #3-390 9/3/03 .9 2.7 7.65 490 13.7 -- 47.4 10
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, Nebraska. —Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephμg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Sampling 

date

Turbidity

(NTU)

Oxygen 

(O2), dissolved 

(mg/L)

pH, 

field

(standard 

units)

Specific 

conductance 

(μS/cm)

Water 

temperature 

(°C)

Hydrogen 

sulfide 

(H2S), 

field, total 

(mg/L)

Calcium (Ca), 

dissolved 

(mg/L)

Magne

(M

disso

(mg

Lincoln County #4-195 10/22/03 .2 7.3 7.25 112 14.7 -- 12.8 1Lincoln County #4-359 10/23/03 .2 < .5 7.87 188 15.3 < .01 26.5 3Lincoln County #4-510 10/22/03 .2 < .5 7.76 233 16.6 < .01 33.8 4Lincoln County #4-695 10/22/03 .5 < .5 7.61 346 17.5 < .01 37.3 5McPherson County #1-45 9/8/03 .2 7.1 7.58 131 11.9 -- 15.5 1McPherson County #1-330 9/9/03 .4 < .5 7.84 226 13.2 < .01 27.8 3McPherson County #1-550 9/9/03 1.4 < .5 7.48 393 14.2 < .01 56.2 7McPherson County #1-770 10/13/04 .2 < .5 7.37 378 14.3 < .01 48.8 5
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near 

Brady, Nebraska. —Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephity units; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Sodium 

(Na), dissolved 

(mg/L)

Alkalinity, 

dissolved 

(mg/L as 

CaCO3)

Bromide

(Br), dissolved 

(mg/L)

Chloride 

(Cl), dissolved 

(mg/L)

Fluoride 

(F), dissolved 

(mg/L)

Silica 

(SiO2), 

dissolved 

(mg/L)

Sulfate 

(SO4), 

dissolved 

(mg/L)

Dissolved 

solids, 

residue 

at 180°C 

(mg/L)

Nitrogen, 

ammonium 

(NH4), dissolved (mg/

as N)

Platte River near Brady, Nebr. 47.3 177 0.067 15.8 0.5 36.6 120. 427 < 0.04 Imperial 6.29 142 E.03 2.03 .6 58.0 5.8 238 < .04 Keith County #1 11.7 157 .22 8.26 .5 58.7 17.9 284 < .04 Lincoln County #1-25 10.4 138 E.010 1.51 .5 56.7 11.0 220 < .04 Lincoln County #1-119 7.67 134 .025 1.08 .4 61.2 6.6 215 < .04 Lincoln County #1-270 6.91 130 .025 .93 .3 62.8 7.1 208 < .04 Lincoln County #1-370 25.3 150 .025 1.15 .6 67.5 21.2 264 < .04 Lincoln County #2-255 6.06 65.6 .015 .92 .3 48.6 1.9 124 < .41 Lincoln County #2-590 17.5 162 .025 1.37 .3 65.3 16.6 422 < .41 Lincoln County #2-760 14.8 133 .021 1.28 .3 59.5 11.4 213 < .04 Lincoln County #3-30 23.8 194 .078 10.7 .3 56.9 93.1 433 < .04 Lincoln County #3-110 14.2 168 .050 2.45 .5 56.9 17.4 286 < .04 Lincoln County #3-250 26.2 159 .042 4.08 .6 64.1 29.7 291 < .04 Lincoln County #3-390 35.1 170 .064 7.15 .6 59.8 60.2 349 < .04 
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, Nebraska. —Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephity units; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Sodium 

(Na), dissolved 

(mg/L)

Alkalinity, 

dissolved 

(mg/L as 

CaCO3)

Bromide

(Br), dissolved 

(mg/L)

Chloride 

(Cl), dissolved 

(mg/L)

Fluoride 

(F), dissolved 

(mg/L)

Silica 

(SiO2), 

dissolved 

(mg/L)

Sulfate 

(SO4), 

dissolved 

(mg/L)

Dissolved 

solids, residue at 

180°C 

(mg/L)

Nitrogen, 

ammonium 

(NH4), dissolved (mg/

as N)

Lincoln County #4-195 5.67 47.4 0.014 0.74 .2 54.6 3.0 118 < .04 Lincoln County #4-359 6.55 89.6 .016 0.75 .3 58.4 3.1 152 < .04 Lincoln County #4-510 7.2 108 .009 0.76 .2 63.8 9.9 189 < .04 Lincoln County #4-695 29.9 163 .021 1.69 .3 76.0 17.5 263 < .04 McPherson County #1-45 4.66 39.9 .011 1.34 .2 49.3 6.2 114 < .04 McPherson County #1-330 8.75 108 .014 1.08 .5 56.5 2.3 175 < .04 McPherson County #1-550 15.8 201 .028 2.02 .3 58.8 E.1 271 < .04 McPherson County #1-770 12.8 114 .049 1.18 .2 65.8 19.7 236 < .04 
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, 

Nebraska.—Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephelometric turbidity units; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Nitrogen, 

nitrite + nitrate 

(NO2+NO3), dissolved (mg/L 

as N)

Nitrogen, 

nitrite (NO2), 

dissolved 

(mg/L as N)

Phosphorus, 

ortho (PO4), 

dissolved 

(mg/L as P)

Carbon, 

organic, 

dissolved 

(DOC) 

(mg/L)

Aluminum 

(Al), dissolved 

(μg/L)

Antimony 

(Sb), 

dissolved 

(μg/L)

Arsenic 

(As), dissolved 

(μg/L)

Barium 

(Ba), dissolved 

(μg/L)

Platte River near Brady, Nebr. 0.38 < 0.008 0.012 -- E1.23 E0.15 5.5 83Imperial 2.40 < .008 E.01 1.0 -- -- -- --Keith County #1 2.23 < .008 0.01 -- E1.1 E.1 5.8 121Lincoln County #1-25 0.13 < .008 .18 0.6 E1.08 E.2 7.3 117Lincoln County #1-119 0.85 .012 < .18 .4 E1.03 E.2 5.4 168Lincoln County #1-270 0.89 < .008 < .18 E.2 < 1.6 <.3 7.1 161Lincoln County #1-370 1.09 < .008 < .18 E.2 4.1 <.3 8.7 82Lincoln County #2-255 1.11 < .008 .28 E.3 < 1.6 <.3 9.2 65Lincoln County #2-590 0.49 < .008 < .18 .4 5.1 <.3 5.5 161Lincoln County #2-760 0.57 < .008 < .18 E.2 E.95 E.2 7.9 143Lincoln County #3-30 3.13 < .008 < .18 .5 < 1.6 <.3 3.3 85Lincoln County #3-110 2.57 < .008 < .18 E.2 < 1.6 <.3 5.6 71Lincoln County #3-250 1.59 < .008 < .18 E.2 < 1.6 <.3 7.3 64Lincoln County #3-390 1.42 < .008 < .18 .4 E.90 <.3 6.8 45Lincoln County #4-195 1.05 < .008 .33 E.3 E1.28 E.2 9.8 41Lincoln County #4-359 .12 < .008 < .18 E.3 E1.55 E.1 4.7 155Lincoln County #4-510 < .06 < .008 < .18 E.2 E.94 <.2 5.0 131Lincoln County #4-695 .09 < .008 < .18 .6 E1.49 <.2 6.1 150McPherson County #1-45 2.75 < .008 .22 .5 E1.11 .4 5.0 51McPherson County #1-330 < .06 < .008 < .18 .9 1.81 <.3 5.4 71McPherson County #1-550 < .06 < .008 < .18 1.3 E.85 <.3 5.2 151McPherson County #1-770 .50 < .008 < .18 .5 E1.08 .2 5.3 168
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near 

Brady, Nebraska.—Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephity units; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Beryllium 

(Be), 

dissolved 

(μg/L)

Boron 

(B), 

dissolved 

(μg/L)

Cadmium 

(Cd), dissolved 

(μg/L)

Chromium (Cr), 

dissolved 

(μg/L)

Cobalt 

(Co), 

dissolved 

(μg/L)

Copper

 (Cu), 

dissolved 

(μg/L)

Iron

 (Fe), 

dissolved 

(μg/L)

Lead

 (Pb), 

dissolved 

(μg/L)

Platte River near Brady, Nebr. < 0.06 88 < 0.04 < 0.8 0.24 1.8 < 6. < 0.08 Imperial -- -- -- -- -- -- -- -- Keith County #1 < .06 56 < .04 1.7 .04 E.3 < 6. < .08 Lincoln County #1-25 < .06 38 < .04 < .8 .33 1.8 26. < .08 

Lincoln County #1-119 < .06 39 < .04 < .8 .15 .4 < 8. < .08 Lincoln County #1-270 < .06 39 < .04 < .8 .12 .5 < 8. < .08 Lincoln County #1-370 < .06 77 < .04 2.6 .15 1.0 < 8. < .08 Lincoln County #2-255 < .06 18 < .04 3.4 .05 6.6 22 < .08 

Lincoln County #2-590 < .06 45 < .04 < .8 .21 7.3 < 8. < .08 Lincoln County #2-760 < .06 45 < .04 1.5 .12 3.4 < 8. < .08 Lincoln County #3-30 < .06 44 < .04 2.2 .22 5.6 < 8. < .08 Lincoln County #3-110 < .06 45 < .04 3.7 .11 .4 < 8. < .08 Lincoln County #3-250 < .06 53 E.02 2.0 .11 2.1 < 8. < .08 Lincoln County #3-390 < .06 78 < .04 1.9 .12 .9 E5. < .08 Lincoln County #4-195 < .06 14 < .04 3.8 .11 E.3 < 6. < .08 Lincoln County #4-359 < .06 22 < .04 < .8 .18 2.9 < 6. < .08 Lincoln County #4-510 < .06 32 < .04 < .8 .18 2.6 6. < .08 

Lincoln County #4-695 < .06 70 < .04 E.6 .17 .4 < 6. < .08 McPherson County #1-45 < .06 10 < .04 2.5 .10 5.8 < 8. < .08 McPherson County #1-330 < .06 31 < .04 < .8 .20 .4 < 8. < .08 McPherson County #1-550 < .06 40 < .04 < .8 .29 .5 < 8. < .08 McPherson County #1-770 < .06 34 E.02 < .8 .15 .6 < 6. E.04 
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, 

Nebraska.—Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephelometric turbidity units; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Manganese 

(Mn), dissolved (μg/L)

Molybdenum 

(Mo), dissolved

(μg/L)

Nickel

 (Ni), 

dissolved 

(μg/L)

Radon

 (Rn)

 (pCi/L)

Selenium 

(Se), 

dissolved 

(μg/L)

Silver

 (Ag), 

dissolved 

(μg/L)

Strontium 

(Sr), 

dissolved 

(μg/L)

Thallium (Tl), 

dissolved 

(μg/L)

U

Platte River near Brady, Nebr. 10.6 4.5 1.2 -- 1.3 < 0.20 504 < 0.04 Imperial -- -- -- -- -- -- -- -- Keith County #1 .3 3.6 .4 -- 3.0 < .20 370 < .04 Lincoln County #1-25 104. 2.0 .8 270. 2.3 < .20 341 E.02 Lincoln County #1-119 177. 2.1 .3 380. 1.1 < .20 357 < .04 Lincoln County #1-270 37.9 1.1 .3 380. 3.1 < .20 371 < .04 Lincoln County #1-370 21.1 3.7 .7 870. 1.8 < .20 456 < .04 Lincoln County #2-255 < .2 1.3 .9 230. 1.1 < .20 89.9 < .04 Lincoln County #2-590 30.2 4.0 1.3 770. E.5 < .20 350 < .04 Lincoln County #2-760 .9 2.6 .7 580. E.4 < .20 278 < .04 Lincoln County #3-30 < .2 1.4 1.0 280. 2.0 < .20 650 < .04 Lincoln County #3-110 < .2 4.0 .6 510. 1.5 < .20 474 < .04 Lincoln County #3-250 < .2 3.6 .6 470. 1.7 < .20 423 < .04 Lincoln County #3-390 1.7 3.9 .5 880. 1.6 < .20 647 < .04 Lincoln County #4-195 1.0 .7 .3 220. .6 < .20 63.4 < .04 

Lincoln County #4-359 20.0 1.5 .8 140. E.4 < .20 122 < .04 Lincoln County #4-510 35.1 1.6 .7 100. < .4 < .20 188 < .04 Lincoln County #4-695 16.9 4.2 .6 460. < .4 < .20 408 E.02 McPherson County #1-45 0.2 E.3 .8 180. .5 < .20 74.5 < .04 McPherson County #1-330 43.8 1.7 .6 280. < .5 < .20 148 < .04 McPherson County #1-550 95.4 1.9 .8 190. < .5 < .20 293 < .04 McPherson County #1-770 20.7 5.3 .4 190. < .4 < .20 249 < .04 
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, 

[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephμg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Vanadium 

(V), dissolved 

(μg/L)

Zinc 

(Zn), dissolved 

(μg/L)

2,6-Diethyl 

aniline, 

dissolved

(μg/L)

Acetochlor, 

dissolved 

(μg/L)

Alachlor, 

dissolved 

(μg/L)

Alpha BHC, 

dissolved 

(μg/L)

Atrazine, 

dissolved 

(μg/L)

Azinphosmethyl, dissolved (μg/L) B

Platte River near Brady, Nebr. 6.3 0.7 -- -- -- -- -- -- Imperial -- -- < 0.006 < 0.006 < 0.004 < 0.005 < 0.007 < 0.05 

Keith County #1 15.3 .8 -- -- -- -- -- -- 

Lincoln County #1-25 4.0 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 

Lincoln County #1-119 20.7 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #1-270 17.7 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #1-370 15.3 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #2-255 15.8 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #2-590 9.5 < 1.6 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #2-760 14.5 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #3-30 11.2 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #3-110 14.2 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #3-250 15.0 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #3-390 10.6 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #4-195 8.3 < .6 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #4-359 12.9 .6 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #4-510 13.4 < .6 < .006 < .006 < .004 < .005 < .007 < .05 Lincoln County #4-695 8.8 E.4 < .006 < .006 < .004 < .005 < .007 < .05 McPherson County #1-45 10.7 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 McPherson County #1-330 8.7 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 McPherson County #1-550 14.0 < 1.0 < .006 < .006 < .004 < .005 < .007 < .05 McPherson County #1-770 9.7 9.5 < .006 < .006 < .005 < .005 < .007 < .05 
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, 

Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephunits; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Carbaryl, 

dissolved 

(μg/L)

Carbofuran, 

dissolved 

(μg/L)

Chlorpyrifos, 

dissolved 

(μg/L)

Cyanazine, 

dissolved 

(μg/L)

DCPA, 

dissolved 

(μg/L)

Deethylatrazine, 

dissolved 

(μg/L)

Diazinon, 

dissolved 

(μg/L)

Dieldri

dissolv

(μg/L)

Platte River near Brady, Nebr. -- -- -- -- -- -- -- -- Imperial < 0.041 < 0.02 < 0.005 < 0.018 < 0.003 < 0.006 < 0.005 < 0.005 Keith County #1 -- -- -- -- -- -- -- -- Lincoln County #1-25 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #1-119 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #1-270 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #1-370 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #2-255 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #2-590 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #2-760 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #3-30 < .041 < .02 < .005 < .018 < .003 E.006 < .005 < .005 Lincoln County #3-110 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #3-250 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #3-390 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #4-195 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #4-359 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #4-510 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 Lincoln County #4-695 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 McPherson County #1-45 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 McPherson County #1-330 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 McPherson County #1-550 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .005 McPherson County #1-770 < .041 < .02 < .005 < .018 < .003 < .006 < .005 < .009 
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, 

Nebraska.—Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephelometric turbidity units; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

EPTC, 

dissolved 

(μg/L)

Ethalfluralin, 

dissolved 

(μg/L)

Ethoprophos, 

dissolved

(μg/L)

Fipronil, 

dissolved 

(μg/L)

Fipronil 

sulfide, dissolved 

(μg/L)

Fipronil 

sulfone, 

dissolved 

(μg/L)

Fonofox, 

dissolved 

(μg/L)

Lindane, 

dissolved 

(μg/L)

Platte River near Brady, Nebr. -- -- -- -- -- -- -- --Imperial < 0.002 < 0.009 < 0.005 < 0.007 < 0.005 < 0.005 < 0.003 < 0.004Keith County #1 -- -- -- -- -- -- -- --Lincoln County #1-25 <.002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #1-119 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #1-270 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #1-370 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #2-255 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #2-590 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #2-760 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #3-30 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #3-110 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #3-250 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #3-390 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #4-195 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #4-359 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #4-510 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004Lincoln County #4-695 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004McPherson County #1-45 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004McPherson County #1-330 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004McPherson County #1-550 < .002 < .009 < .005 < .007 < .005 < .005 < .003 < .004McPherson County #1-770 < .004 < .009 < .005 < .016 < .013 < .024 < .003 < .004
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near 

Brady, Nebraska.—Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephelometric turbidity units; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Linuron, 

dissolved 

(μg/L)

Malathion, 

dissolved 

(μg/L)

Metolachlor, 

dissolved

(μg/L)

Metribuzin, 

dissolved 

(μg/L)

Molinate, 

dissolved 

(μg/L)

Napropamide, 

dissolved

(μg/L)

p,p’-DDE, 

dissolved 

(μg/L)

Parath

dissol

(μg/

Platte River near Brady, Nebr. -- -- -- -- -- -- -- -Imperial < 0.035 < 0.027 < 0.013 < 0.006 < 0.002 < 0.007 < 0.003 < 0.Keith County #1 -- -- -- -- -- -- -- -Lincoln County #1-25 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #1-119 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #1-270 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #1-370 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #2-255 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #2-590 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #2-760 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #3-30 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #3-110 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #3-250 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #3-390 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #4-195 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #4-359 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #4-510 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .Lincoln County #4-695 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .McPherson County #1-45 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .McPherson County #1-330 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .McPherson County #1-550 < .035 < .027 < .013 < .006 < .002 < .007 < .003 < .McPherson County #1-770 < .035 < .027 < .006 < .006 < .003 < .007 < .003 < .
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near Brady, 

Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephelometric turbidity units; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Parathionmethyl, 

dissolved 

(μg/L)

Pebulate, 

dissolved 

(μg/L)

Pendimethalin, 

dissolved

(μg/L)

Permethrin, 

dissolved 

(μg/L)

Phorate, 

dissolved 

(μg/L)

Prometon, 

dissolved 

(μg/L)

Pronamide, 

dissolved 

(μg/L)

Propachlor, 

dissolved 

(μg/L)

Platte River near Brady, Nebr. -- -- -- -- -- -- -- -- Imperial < 0.006 < 0.004 < 0.022 < 0.006 < 0.011 < 0.01 < 0.004 < 0.01 

Keith County #1 -- -- -- -- -- -- -- -- Lincoln County #1-25 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #1-119 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #1-270 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #1-370 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #2-255 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #2-590 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #2-760 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #3-30 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #3-110 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #3-250 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #3-390 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #4-195 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #4-359 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #4-510 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

Lincoln County #4-695 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

McPherson County #1-45 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

McPherson County #1-330 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

McPherson County #1-550 < .006 < .004 < .022 < .006 < .011 < .01 < .004 < .01 

McPherson County #1-770 < .015 < .004 < .022 < .006 < .011 < .01 < .004 < .025 
Appendix 1. Water-quality data from sampled monitoring wells screened in the northern High Plains aquifer and from the Platte River near 

Brady, Nebraska.—Continued[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; P, phosphorus; NTU, nephturbidity units; μg/L, micrograms per liter; pCi/L, picocuries per liter; --, no data; E, estimated]

Well/

site name

Propargite, 

dissolved 

(μg/L)

Simazine, 

dissolved

(μg/L)

Tebuthiuron, 

dissolved

(μg/L)

Terbacil, 

dissolved 

(μg/L)

Terbufos, 

dissolved

(μg/L)

Thiobencarb, 

dissolved

(μg/L)

Tri-allate, 

dissolved 

(μg/L)

T

Platte River near Brady, Nebr. -- -- -- -- -- -- -- Imperial < 0.02 < 0.005 < 0.02 < 0.034 < 0.02 < 0.005 < 0.002 Keith County #1 -- -- -- -- -- -- -- Lincoln County #1-25 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #1-119 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #1-270 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #1-370 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #2-255 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #2-590 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #2-760 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #3-30 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #3-110 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #3-250 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #3-390 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #4-195 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #4-359 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #4-510 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

Lincoln County #4-695 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

McPherson County #1-45 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

McPherson County #1-330 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

McPherson County #1-550 < .02 < .005 < .02 < .034 < .02 < .005 < .002 

McPherson County #1-770 < .02 < .005 < .02 < .034 < .02 < .005 < .006 
Appendix 2

Mass-Balance Model Results

Appendix 2  55
Appendix 2. Average mass transfers, mineral saturation indices, isotope balances, and adjusted radiocarbon ages for final waters. 

[Mass-balance calculations were performed with NETPATH (Plummer and others, 1994). Lincoln Co. #2-255, Lincoln Co. #4-195, and McPherson Co. #1-45 were used as inin the mass-balance models for each of the final waters, except those from Lincoln Co. #3. Keith Co. #1 was used as the initial water for the final waters from Lincoln Co. #3. mass transfers, isotope values, and radiocarbon ages represent averages of all plausible models. Mass transfers are in millimoles per kilogram of water. Positive mass transfers mineral dissolution. Negative numbers indicate mineral precipitation; SI, saturation index; Positive SI values indicate the water is supersaturated with respect to the indicated mphase; Negative SI values indicate the water is undersaturated with respect to the indicated mineral phase; --, no models found; ±, plus or minus one standard deviation]

Final 

water Calcite Lignite (CH2O)

Calciumsodium 

exchange

Sodium 

plagio

clase

Kaolinite Silica, (SiO2) Pyrite Goethite

Lincoln County #1–119 0.298 0.487 –0.033 0.093 –0.064 –0.212 0.014 –0.014 Lincoln County #1–270 .168 .341 –.048 .070 –.048 –.243 .021 –.021 Lincoln County #1–370 .117 .234 .090 .103 –.077 –.286 .033 –.033 Lincoln County #2–590 .209 .266 .038 .086 –.060 –.432 .025 –.025 Lincoln County #2–760 .162 .264 .015 .174 –.120 –.470 .023 –.023 Lincoln County #3–110 –.165 .019 .032 .000 .000 –.096 .011 –.011 Lincoln County #3–250 –1.22 .057 –.198 1.06 –.732 –1.57 .012 –.012 Lincoln County #3–390 –1.77 .000 –.396 1.59 –1.10 –2.49 .051 –.051 Lincoln County #4–359 .231 .393 –.026 .137 –.094 –.069 .003 –.003 Lincoln County #4–510 .265 .337 –.014 .063 –.044 –.055 .025 –.025 Lincoln County #4–695 .309 .347 .210 .082 –.057 –.234 .035 –.035 McPherson County #1–330 -- -- --

--

-- -- -- -- 

McPherson County #1–550 -- -- -- -- -- -- -- -- 
Appendix 2. Average mass transfers, mineral saturation indices, isotope balances, and adjusted radiocarbon ages for final waters.—

Continued[Mass-balance calculations were performed with NETPATH (Plummer and others, 1994). Lincoln Co. #2-255, Lincoln Co. #4-195, and McPherson Co. #1-45 were used as initial waters in the mass-balance models for each of the final waters, except those from Lincoln Co. #3. Keith Co. #1 was used as the initial water for the final waters from Lincoln Co. #3. The listed mass transfers, isotope values, and radiocarbon ages represent averages of all plausible models. Mass transfers are in millimoles per kilogram of water. Positive mass transfers indicate mineral dissolution. Negative numbers indicate mineral precipitation; SI, saturation index; Positive SI values indicate the water is supersaturated with respect to the indicated mineral phase; Negative SI values indicate the water is undersaturated with respect to the indicated mineral phase; --, no models found; ±, plus or minus one standard deviation]

Final 

water

Nitrogen

(N2)

gas

Evaporation 

factor Calcite SI Albite SI Anorthite SI Kaolinite SI Chalcedony SI

Lincoln County #1–119 –0.036 1.36 0.010 -1.213 -3.988 2.856 0.690Lincoln County #1–270 .000 1.58 .021 -1.326 -4.049 2.641 .685Lincoln County #1–370 .000 2.17 .023 -.078 -2.840 3.909 .715Lincoln County #2–590 .000 2.10 .071 -.264 -2.572 4.180 .685Lincoln County #2–760 .000 1.66 .035 -1.192 -4.196 2.369 .642Lincoln County #3–110 .000 1.08 .057 -.938 -3.973 3.154 .682Lincoln County #3–250 .000 1.50 -.030 -.588 -3.973 3.137 .720Lincoln County #3–390 .000 2.17 .012 -.636 -4.107 2.979 .685Lincoln County #4–359 –.001 1.02 -.192 -1.244 -3.855 2.810 .652Lincoln County #4–510 –.007 1.27 -.113 -1.400 -4.172 2.492 .676Lincoln County #4–695 –.030 1.85 -.056 -.423 -3.640 3.217 .741McPherson County #1–330 -- -- -- -- -- -- --McPherson County #1–550 -- -- -- -- -- -- --
Appendix 2. Average mass transfers, mineral saturation indices, isotope balances, and adjusted radiocarbon ages for final waters.—Continued

[Mass-balance calculations were performed with NETPATH (Plummer and others, 1994). Lincoln Co. #2-255, Lincoln Co. 

#4-195, and McPherson Co. #1-45 were used as initial waters in the mass-balance models for each of the final waters, except 

those from Lincoln Co. #3. Keith Co. #1 was used as the initial water for the final waters from Lincoln Co. #3. The listed mass 

transfers, isotope values, and radiocarbon ages represent averages of all plausible models. Mass transfers are in millimoles per 

kilogram of water. Positive mass transfers indicate mineral dissolution. Negative numbers indicate mineral precipitation; SI, 

saturation index; Positive SI values indicate the water is supersaturated with respect to the indicated mineral phase; Negative SI 

values indicate the water is undersaturated with respect to the indicated mineral phase; --, no models found; ±, plus or minus one 

standard deviation]

Final 

water

δ13C

[calcite]

δ13C[organic 

carbon]

δ13C[DIC], 

measured

δ13C[DIC], 

modeled

Adjusted 

radiocarbon age

(

14

C years)

Lincoln County #1–119 –4 –20 -9.6 -9.9 1,800±900

Lincoln County #1–270 –4 –20 -8.7 -9.7 2,500±900 

Lincoln County #1–370 –4 –20 -9.7 -9.6 12,600±900

Lincoln County #2–590 –6 –25 -11.0 -9.6 11,700±800

Lincoln County #2–760 –7 to –2 –25 -9.6 -9.3 7,500±900 

Lincoln County #3–110 -- –25 -7.4 -8.1 7,400±400 

Lincoln County #3–250 -- –25 -14.0 -9.2 7,400±400 

Lincoln County #3–390 -- –25 -9.7 -8.6 15,700±500

Lincoln County #4–359 –4 to –2 –20 -7.1 -9.6 2,100±300

Lincoln County #4–510 –4 to –2 –20 -8.0 -9.5 5,800±700

Lincoln County #4–695 –4 to –2 –20 -10.0 -9.3 12,600±800

McPherson County #1–330 -- -- -- -- --

McPherson County #1–550 -- -- -- -- --

58  Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003
McMahon and others—Vertical Gradients in Water Chemistry and Age in the Northern High Plains Aquifer, Nebraska, 2003—SIR 2006–5294

Printed on recycled paper