KANSAS

SOUTH DAKOTA

OKLAHOMA

TEXAS

NEW MEXICO

COLORADO

WYOMING

Seward

SEWARD

COUNTY NEBRASKA

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2007–5088

Prepared in cooperation with the

City of Seward, Nebraska

Ground-Water Age and Quality in the High 

Plains Aquifer near Seward, Nebraska, 2003–04 

Ground-Water Age and Quality in the High 

Plains Aquifer near Seward, Nebraska, 

2003–04

By Jennifer S. Stanton, Matthew K. Landon, and Michael J. Turco

Prepared in cooperation with the City of Seward, Nebraska

Scientific Investigations Report 2007–5088

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:

Stanton, J.S., Landon, M.K., and Turco, M.J., 2007, Ground-water age and quality in the High Plains aquifer near 

Seward, Nebraska, 2003–04: U.S. Geological Survey Scientific Investigations Report 2007–5088, 37 p.

Back cover:

Top photograph shows USGS personnel drilling a monitoring well. Middle photograph shows USGS personnel sampling a monitoring well. Bottom photograph shows monitoring well protective cover.
iii

Contents

Glossary............................................................................................................................................................. vi

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

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

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

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

Previous Investigations.......................................................................................................................... 2

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

Methods............................................................................................................................................................. 4

Design of Well Network......................................................................................................................... 4

Well Installation ...................................................................................................................................... 4

Water-Level Measurements ................................................................................................................. 5

Collection and Analysis of Water Samples ........................................................................................ 5

Hydrogeologic Setting..................................................................................................................................... 6

Geology..................................................................................................................................................... 6

Ground-Water Levels ............................................................................................................................. 6

Ground-Water Age .......................................................................................................................................... 9

Ground-Water Quality ..................................................................................................................................... 15

Summary............................................................................................................................................................ 22

References Cited.............................................................................................................................................. 23

Appendix 1. Well Installation ......................................................................................................................... 28

Appendix 2. Estimation of Ground-Water Age ............................................................................................ 30

Figures

1. Map showing location of study area and monitoring-well nests near Seward, 

Nebraska, and observation wells measured in spring 2001 by Upper Big Blue Natural 

Resources District.......................................................................................................................... 3

2. Hydrogeologic sections along two ground-water flow paths upgradient from Seward 

west well field ................................................................................................................................. 7

3. Map showing water-level contours near Seward, Nebraska, spring 2001........................... 8

4. Graphs showing water levels measured in monitoring-well nests near Seward, 

Nebraska, July 2004 to March 2006............................................................................................. 10

5. Hydrogeologic sections showing best estimate of apparent mean recharge date and 

percentage of sample composed of young water for ground-water samples collected 

near Seward, Nebraska, 2003–04 ................................................................................................ 11

6. Graphs showing relations of depth below land surface to (A) apparent recharge date or 

mean recharge date of water, and (B) fraction of sample composed of young water for 

ground-water samples collected near Seward, Nebraska, 2003–04..................................... 12

7. Trilinear diagrams showing relative proportions, in percentage equivalents, of major 

ions in ground-water samples collected from monitoring and supply wells with respect 

to areal position and depth below land surface near Seward, Nebraska, 2003–04............ 17

8. Hydrogeologic sections showing nitrate concentrations in ground-water samples 

 collected from monitoring wells near Seward, Nebraska, August 2004, and supply 

well W11, November 2003 ............................................................................................................. 19
iv

9. Hydrogeologic sections showing dissolved-solids concentrations in ground-water 

samples collected from monitoring wells near Seward, Nebraska, August 2004, and 

supply well W11, November 2003 ................................................................................................ 20

10. Graphs showing relations between apparent recharge date and (A) measured nitrate 

concentrations and (B) nitrate concentrations of young water adjusted for mixing in 

ground-water samples collected near Seward, Nebraska, compared to historical 

nitrogen fertilizer applications in Seward County, Nebraska.................................................. 21

2–1. Graph showing ratio of measured tritium concentration to calculated initial tritium 

concentration before decay compared to tritium concentration in ground-water 

samples collected from monitoring wells near Seward, Nebraska, and values 

indicated by piston-flow and exponential-mixing models ....................................................... 33

2–2. Graph showing concentration of chloroflorocarbon-12 compared to tritium 

concentration in ground-water samples collected from monitoring and supply wells 

near Seward, Nebraska, and values indicated by piston-flow and exponential-mixing 

models .............................................................................................................................................. 34

2–3. Graph showing concentration of sulfur hexafluoride compared to tritium concentration 

in ground-water samples collected from monitoring wells near Seward, Nebraska, and 

values indicated by piston-flow and exponential-mixing models .......................................... 35

2– 4. Graph showing ground-water mean recharge dates determined using tritium/helium 

and chlorofluorocarbon-12 techniques in ground-water samples collected from 

monitoring wells near Seward, Nebraska.................................................................................. 37

Tables

1. Estimates of age and mixing fractions for ground-water samples collected near 

Seward, Nebraska, 2003–04.......................................................................................................... 13

2. Physical properties and concentrations of selected inorganic constituents and 

detected pesticide compounds in untreated ground water collected from supply 

wells near Seward, Nebraska, November 2003 ........................................................................ 16

3. Physical properties and concentrations of selected inorganic constituents in groundwater samples collected from monitoring wells near Seward, Nebraska, August 2004.... 18

4. Well-construction information for monitoring wells near Seward, Nebraska..................... 29

5. Dissolved-gas concentrations and estimated recharge temperatures and excess air 

quantities in ground-water samples collected from monitoring and supply wells near 

Seward, Nebraska, 2003–04.......................................................................................................... 31

6. Tracer data used in estimating ground-water age near Seward, Nebraska, 2003–04 ....... 32
Conversion Factors, Abbreviations, and Datums

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.004047 square kilometer (km2)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 0.003785 cubic meter (m3) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

Volume per unit mass

cubic centimeter per gram (cm3/g) 0.01602 cubic foot per pound (ft3/lb)

Mass

pound (lb) 0.4536 kilogram (kg) 

pound per acre (lb/acre) 1.120 kilogram per hectare (kg/ha)

pound per year (lb/yr) 0.4536 kilogram per year (kg/yr)

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 North American Vertical Datum of 1988 

(NAVD 88).

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

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

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

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

or micrograms per liter (μg/L).
vi

Glossary

Alluvial valley A valley filled with stream-deposited sediment.

Contributing area The area contributing recharge to a discharging well is the area at the water table where 

water entering the ground-water flow system will eventually be removed by the well.

Denitrification A process by which oxidized forms of nitrogen such as nitrate (NO3

-

) are reduced to form 

nitrogen oxides or free nitrogen, commonly brought about by the action of denitrifying bacteria. 

Hydraulic head The height of the free surface of a body of water above a given point beneath the surface.

Hydraulic-head gradient The change of hydraulic head per unit of distance in a given direction.

Oxic geochemical conditions The state of the geologic and chemical environment when ground water has a 

concentration of dissolved oxygen equal to or greater than 1.0 milligram per liter.

Terrigenic From the earth’s crust and mantle sources, nonatmospheric.

Upgradient Indicating a location with a higher ground-water hydraulic head along a ground-water flow 

path than a reference point along the flow path having a lower ground-water hydraulic head such as a discharging well.
Abstract

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

City of Seward, Nebraska, conducted a study of ground-water 

age and quality to improve understanding of: (1) traveltimes 

from recharge areas to public-supply wells, (2) the effects of 

geochemical reactions in the aquifer on water quality, and (3) 

how water quality has changed historically in response to landuse practices. Samples were collected from four supply wells 

in the Seward west well field and from nine monitoring wells 

along two approximate ground-water flow paths leading to the 

well field. Concentrations of three different chlorofluorocarbons (CFC–12, CFC–11, and CFC–113), sulfur hexafluoride 

(SF6), and ratios of tritium (3H) to helium-3 (3He) isotope 

derived from radioactive decay of 3H were used to determine 

the apparent recharge age of ground-water samples. Age interpretations were based primarily on 3

H/3He and CFC–12 data. 

Estimates of apparent ground-water age from tracer data were 

complicated by mixing of water of different ages in 10 of the 

13 ground-water samples collected.

Apparent recharge dates of unmixed ground-water samples or mean recharge dates of young fractions of mixed water 

in samples collected from monitoring wells ranged from 1985 

to 2002. For monitoring-well samples containing mixed water, 

the fraction of the sample composed of young water ranged 

from 26 to 77 percent of the sample. Apparent mean recharge 

dates of young fractions in samples collected from four supply 

wells in the Seward west well field ranged from about 1980 to 

1990. Estimated fractions of the samples composed of young 

water ranged from 39 to 54 percent. It is implicit in the mixing 

calculations that the remainder of the sample that is not young 

water is composed of water that is more than 60 years old 

and contains no detectable quantities of modern atmospheric 

tracers. Estimated fractions of the mixed samples composed of 

“old” water ranged from 23 to 74 percent. Although alternative 

mixing models can be used to interpret the results, the mean 

age and mixing fractions from the primary mixing models 

used were fairly similar.

Relations of ground-water age and nitrate concentrations 

to depth were not consistent across the study area. In some 

well nests, more young water and nitrate were present near 

the bottom than in the middle of the aquifer. These results 

probably reflect pumping from irrigation and supply wells, 

which are screened primarily in the lower part of the aquifer, 

and draw younger water downward in the aquifer. Substantial mixing probably occurs because the aquifer is relatively 

thin (50 feet) and has a relatively high density of wells (about 

five pumping wells per square mile). The most reliable estimate of horizontal traveltimes based on differences in groundwater ages between a shallow monitoring well at the upgradient end of the northwest well transect and the deep well at the 

downgradient end of the well transect was 9 years to travel 

a distance of about 2 miles. The general similarity of ages at 

similar depths between different well nests is consistent with 

the fact that horizontal flow in the aquifer is relatively rapid.

Concentrations of nitrate (as nitrogen) in untreated 

ground-water samples from supply wells in the well field were 

larger than the U.S. Environmental Protection Agency Maximum Contaminant Level for drinking water of 10 mg/L (milligrams per liter), ranging from 11.3 to 13.5 mg/L. It is unlikely 

that nitrate concentrations in the aquifer near the Seward west 

well field are decreased by denitrification in the aquifer due 

to oxic geochemical conditions that preclude this reaction. 

Nitrate concentrations coupled with water recharge dates were 

compared to historical estimated fertilizer application in an 

attempt to reconstruct historical trends in ground-water nitrate 

concentrations and their relation to land-use practices. Nitrate 

concentrations in young-water fractions, after adjustment for 

mixing, may be decreasing over apparent recharge dates of 

1980 to 2002, corresponding to a period of generally decreasing nitrogen fertilizer applications.

Introduction

Ground water is a source of drinking water for most communities in Nebraska. Ground-water supplies are vulnerable to 

contamination from agricultural, industrial, and other activities as a result of percolation of contaminants downward from 

the land surface to the water table. Once in the ground-water 

system, water and contaminants move from the recharge area 

to the discharge area, which may be a supply well. Evaluating 

the amount of time it takes water deposited at the land surface 

to reach a supply well can yield useful information regarding 

Ground-Water Age and Quality in the High Plains Aquifer 

near Seward, Nebraska, 2003–04

By Jennifer S. Stanton, Matthew K. Landon, and Michael J. Turco
contaminant transport and quality of local and regional ground 

water.

The citizens of Seward, in southeastern Nebraska, depend 

on ground water from Quaternary-age deposits of the High 

Plains aquifer for drinking water. Nitrate (as nitrogen) concentrations in the aquifer have been increasing over the past 

decade and have approached the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 mg/L in 

some wells (Paul Dammann, Water/Wastewater Superintendent, City of Seward, Nebraska, oral commun., July 17, 2002). 

Increased nitrate concentrations have been particularly problematic in the Seward west well field, located approximately 

3 mi west of Seward. The increase in nitrate concentrations 

has led city and county water managers to initiate a process to 

develop a nutrient management plan in the areas contributing 

recharge to supply wells.

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

of Seward, Nebraska, conducted a 2-year study from 2003 

through 2004 of ground-water age and quality to improve 

understanding of: (1) traveltimes from recharge areas to supply 

wells, (2) the effects of geochemical reactions in the aquifer on water quality, and (3) how water quality has changed 

historically in response to land-use practices. Age of ground 

water in supply wells in the Seward west well field and along 

two ground-water flow paths upgradient from the well field 

was determined to estimate time of travel from recharge areas 

to supply wells. Concentrations of nitrate, dissolved gases, and 

other geochemical constituents along flow paths to the supply 

wells were used to assess the extent to which denitrification 

affects nitrate concentrations in the aquifer. Ground-water age 

and nitrate concentrations were used to estimate historical 

changes in nitrate loading to the aquifer.

Purpose and Scope

The purpose of this report is to document ground-water 

age and selected water-quality characteristics along approximate ground-water flow paths in the High Plains aquifer near 

Seward, Nebraska. This report includes a brief description of 

the design of the local monitoring-well network and methods 

of data collection and analysis. The report describes the steps 

in estimating ground-water age using atmospheric tracers and 

describes the distribution of the estimated ground-water ages. 

General water-quality characteristics, with particular emphasis on nitrate, are described. Nitrate concentrations in ground 

water of different recharge ages are compared to estimated 

historical fertilizer applications. In addition, ground-waterlevel measurements were collected approximately monthly 

beginning in July 2004 to compare spatial and temporal variations in ground-water levels and flow patterns to ground-water 

age and chemistry data. The data through March 2006 are 

described in this report.

Description of Study Area

The study area is located west of the Big Blue River in 

central Seward County, Nebraska (fig. 1). Seward County is 

characterized by nearly flat or rolling upland dissected by a 

network of valleys formed from water erosion (Keech, 1978). 

Surficial geologic materials in Seward County include till (a 

glacially deposited mixture of clay, sand, gravel, and stones 

of all sizes), loess (windblown silt or clay, typically deposited 

during glacial periods), and stream-deposited sand and gravel. 

The study area is located in an area of relatively flat upland 

loess. Soils derived from this loess readily absorb and retain 

water but also allow water to move downward and recharge 

the aquifer.

The climate of Seward County is subhumid, receiving 

mean annual precipitation of 27.4 in. between 1949 and 2002 

(Nebraska Department of Natural Resources, 2002). Precipitation can vary substantially from year to year; mean annual 

precipitation ranged from 16.4 in. in 1955 to 41.4 in. in 1993. 

Most precipitation occurs during the frost-free period.

Land use in the area is dominated by agriculture. In 2002, 

about 83 percent of Seward County was used for growing 

crops (National Agricultural Statistics Service, 2002). About 

42 percent of cropland was irrigated. The primary crops grown 

were corn and soybeans. Agricultural chemicals, including 

nitrogen fertilizer, are applied in Seward County to maximize 

crop yields. There were more than 1,300 active irrigation wells 

in Seward County in 2006 (Nebraska Department of Natural 

Resources, 2006). Most irrigation wells are in the High Plains 

aquifer, west of the Big Blue River. In the study area, the average density of irrigation wells is about 4.75 wells per square 

mile (fig. 1).

Previous Investigations

Gottula (1990) described the hydrogeology and groundwater quality of the eastern part of the Upper Big Blue Natural 

Resources District (UBBNRD), which encompasses the study 

area. The spatial and temporal distribution of water quality in 

the aquifer in the UBBNRD was described by Verstraeten and 

others (1998). The basic hydrogeology of Seward County was 

described by Keech (1978). Regional hydrogeologic investigations including the study area were conducted by Johnson 

and Keech (1959) and Gutentag and others (1984). Several 

ground-water modeling investigations at various scales encompassing the study area (Emery, 1966; Huntoon, 1974; Cady 

and Ginsburg, 1979; Nebraska Natural Resources Commission, 1983; Alley and Emery, 1986; Luckey and others, 1986) 

contributed to the understanding of the ground-water hydrology in the area.

Acknowledgments

The authors would like to thank Paul Dammann, the 

now retired former Director of the City of Seward Public 

   Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
Figure 1. Location of study area and monitoring-well nests near Seward, Nebraska, and observation wells measured in spring 2001 by Upper Big 

Blue Natural Resources District.

Introduction   

Base from U.S. Geological Survey digital data, 1:100,000, 1999

Universal Transverse Mercator projection, Zone 14

Horizontal coordinate information is referenced to the North American

 Datum of 1983 (NAD 83)

A

A’, B’

B

FP4

FP1

FP2

FP3

NEBRASKA

Big Blue River

SEWARD COUNTY

Seward

Milford

Garland

Pleasant Dale

Seward

Lincoln Creek

Milford Beaver Crossing

Utica

Goehner

Cordova

Beaver Crossing

Utica

Goehner

Staplehur Staplehurst

Bee

Garland

Pleasant Dale

Tamora

Cordova

Unnamed tributary

Unnamed tributary

0 0.5 1.0 1.5 2.0 MILES

0 0.5 1.0 1.5 2.0 KILOMETERS

Big Blue River

34

34

Seward

Study area

SEWARD

COUNTY

Index map

STUDY AREA

97°14' 12'

40°56'

54'

40°52'

10' 8' 97°6'

EXPLANATION

A A’

FP2

W11

Extent of High Plains aquifer

in Nebraska

Trace of section shown in figures 2, 5, 8, and 9

Monitoring-well nest location

 and well-nest identifier 

Supply well and identifier

Well

W9

W11

W7

W10
Utilities Department, for his pivotal role in initiating this 

study, his willingness to provide institutional knowledge about 

the ground-water resources in the Seward area, and his unwavering dedication to the protection of the valuable resource. The 

authors would also like to thank the landowners who allowed 

monitoring-well nests to be installed on their property without 

which the study would not have been possible.

Methods

Design of Well Network

Monitoring-well nests were installed along estimated 

ground-water flow paths leading to the Seward west well field. 

Ground-water flow paths were estimated from water-table 

altitude maps from different sources including output from 

a simple mathematical model for delineating the wellhead 

protection areas (WHPAs) for the west well field (Bill Lukash, 

Nebraska Department of Environmental Quality (NDEQ), 

written commun., 2001), published maps (Keech, 1978; Cady 

and Ginsberg, 1979), and data compiled for this study (see 

section on “Ground-Water Levels”). Well nests were installed 

at different distances from the well field along the estimated 

ground-water flow paths to represent different traveltimes 

from the monitoring well to the well field. The selected locations represented traveltimes of less than 30 years and were 

based on NDEQ model output. Within this constraint, specific 

monitoring-well locations were selected on the basis of accessibility.

The four supply wells in the Seward west well field are 

located within 0.5 mi of each other (fig. 1), have contributing 

recharge areas that are directly affected by ground-water withdrawals, and have had historically similar water chemistry. 

Therefore, the sampling results from monitoring wells along 

flow paths leading to the well field can be considered representative of conditions for all four supply wells.

A nest of three monitoring wells each was installed 

at four sites for the collection of water levels and samples 

(fig. 1). Each nest of wells consisted of three wells screened at 

different depths in the aquifer and located within a few feet of 

each other. These well nests allowed measurement of vertical 

differences in ground-water levels, chemistry, and age. One 

well nest (FP1) was located within 100 ft of one of the supply 

wells (well W11) (probably within the estimated 1-year traveltime to the well) to characterize ground-water age at different 

depths in the aquifer in the vicinity of a supply well. A second 

nest (FP2) was installed about 1 mi upgradient from well W11 

at about the estimated location of the 10-year traveltime to the 

well. The third nest (FP4) was located at about the estimated 

location of the 30-year traveltime, about 2 mi upgradient from 

supply well W11. A fourth nest (FP3) was installed about 

1 mi to the southwest of supply well W11 along a different 

potential ground-water flow path to the well field. This well 

nest was installed because analysis of ground-water altitudes 

indicated that ground-water flow might be converging on the 

well field from the southwest in addition to the prevailing 

regional ground-water flow from the northwest (see section on 

“Ground-Water Levels”).

The three wells at each nest were constructed with one 

well screen placed at the water table (about 70 ft below land 

surface), one screen placed near the bottom of the aquifer 

(about 120 ft below land surface), and one screen placed 

about halfway between the water table and the bottom of the 

aquifer (about 100 ft below land surface). The exact vertical 

placement of the screens depended on the location of highly 

permeable sand and gravel layers in the aquifer, which were 

determined from lithologic and geophysical logging during 

well installation.

Well Installation

Well installation methods were selected to ensure that 

water-quality samples represented aquifer conditions and 

that equipment and construction materials were not a source 

for leaching or sorption of sampled chemicals. Wells were 

installed with a mud rotary rig by Layne-Western Incorporated 

(Kearney, Nebraska) under a drilling contract with the City of 

Seward. Drilling was completed during January 14–24, 2004. 

While drilling monitoring wells, samples of the aquifer materials were collected at 5-ft intervals to describe the physical 

characteristics of the aquifer. In addition, geophysical (natural 

gamma and electrical resistivity) logs were obtained before 

well casings were installed. Natural gamma and electrical 

resistivity measurements can identify the relative permeability 

of the aquifer throughout the well borehole.

Well casings were constructed of 2-in diameter polyvinyl-chloride (PVC) pipe. Wells were constructed with either 

5- or 10-ft screens with a 5-ft blank casing and end cap below 

the screen (Appendix 1, table 4). The annular space (the space 

between the well casing and the borehole wall) from the bottom of the well to 12 ft below land surface was filled with a 

silica sand and gravel pack. The remaining annular space was 

filled with a 10-ft bentonite pellet seal and 2 ft of bentonite/

cement grout. A concrete pad and protective steel cover were 

placed at the land surface around the well.

After installation, the wells were developed to remove 

drilling mud from the borehole introduced during the drilling process and to remove fine particles near the well screen 

that might clog the screen. Immediately after installation, 

the wells were air surged and pumped for at least 2 hours 

by the driller or until the water pumped from the well was 

clear. In June 2004, the wells were revisited with a Waterra 

purge pump, which surges (introduces pressure waves into) 

the well, thereby mobilizing fine particles from near the well 

screen. The monitoring wells were surged and pumped for 

2 to 10 hours; 67 to 337 casing volumes were pumped from 

each well to remove drilling fluid residuals and aquifer fine 

sediment. Specific conductance and turbidity were monitored 

during pumping.

   Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
Water-Level Measurements

Ground-water levels were measured approximately 

monthly in the monitoring-well nests beginning in July 2004 

to compare spatial and temporal variations in ground-water 

levels and flow patterns to ground-water age and chemistry 

data. Measurements made through March 2006 are included 

in this report. Ground-water levels could not be measured at 

the shallowest monitoring well (FP4–90) in well nest FP4 

because the water level was deeper than the bottom of the well 

screen; at times the shallowest well at well nests FP2 and FP3 

also were dry. Water levels were measured using methods 

described by Garber and Koopman (1968). Data are available in the USGS National Water Information System (NWIS) 

(U.S. Geological Survey, 2007).

Collection and Analysis of Water Samples

Water samples were collected from the four supply wells 

in November 2003 and analyzed for physical properties and 

concentrations of nitrogen and phosphorus compounds, major 

ions, trace elements, and selected pesticides. Ground-water 

age was estimated from concentrations of tritium, chlorofluorocarbons (CFCs), and sulfur hexafluoride (SF6

) coupled with 

interpretation of dissolved gas data (methane, nitrogen, and 

argon) to estimate recharge temperature. The primary purposes 

for collecting these samples were (1) to determine if it was 

feasible to apply ground-water age-dating techniques to water 

that has recharged the aquifer in the past 60 years and (2) to 

determine the quality of untreated water from supply wells.

Water samples were collected from 9 of the 12 monitoring wells in August and September of 2004 and analyzed for 

physical properties, concentrations of nitrogen and phosphorus 

compounds, and major ions. Ground-water age was determined with concentrations of tritium, selected noble gases 

[helium-3 (3He), helium-4 (4He), and neon], dissolved gases, 

CFCs, and SF6. Three wells were not sampled because the 

water table was below the well screen at the time of sampling.

Ground-water samples were collected from wells using 

procedures described by the USGS National Field Manual 

(U.S. Geological Survey, variously dated) and the USGS Chlorofluorocarbon Laboratory (U.S. Geological Survey, 2005). 

Water samples were processed onsite in a mobile laboratory 

using methods designed to minimize changes to the watersample chemistry. Water was pumped from wells with a submersible pump and delivered to the mobile laboratory through 

Teflon tubing with stainless-steel connections. Prior to sample 

collection, stagnant water was flushed from the well by purging at least three casing volumes from the well. While purging, 

specific conductance, pH, water temperature, turbidity, and 

dissolved oxygen were measured until readings were stable 

(U.S. Geological Survey, variously dated). Once readings had 

stabilized, water samples were collected in precleaned bottles 

within an enclosed chamber to prevent sample contamination. CFC, SF6

, noble gas, and dissolved gas samples were 

collected separately from copper tubing attached to the submersible pump; these samples did not contact the atmosphere 

(U.S. Geological Survey, 2005). To prevent degradation of 

water samples and maintain the initial concentration of compounds between the time of sample collection and laboratory 

analyses, contents of the bottles were preserved according to 

laboratory requirements. Preservation practices differ among 

chemical constituents and may include chilling, filtration, and 

(or) chemical treatment (U.S. Geological Survey, variously 

dated).

Nitrogen and phosphorus compounds (Fishman, 1993; 

Patton and others, 2003), major ions (Fishman and Friedman, 

1989; Fishman, 1993; American Public Health Association, 

1998), trace elements (Fishman and Friedman, 1989; Faires, 

1993; Garbarino, 1999; and pesticides (Zaugg and others, 1995; Lindley and others, 1996; Sandstrom and others, 

2001; Madsen and others, 2003) were analyzed at the USGS 

National Water Quality Laboratory (NWQL) in Lakewood, 

Colorado. For samples collected from supply wells in November 2003, tritium analysis was completed by the University of 

Miami, Tritium Laboratory in Miami, Florida (Östlund, 1987). 

Dissolved gases, CFCs, and SF6 were analyzed at the USGS 

Chlorofluorocarbon Laboratory in Reston, Virginia (U.S. Geological Survey Chlorofluorocarbon Laboratory, 2006a,b,c). For 

monitoring-well samples collected in August 2004, noble gas 

and tritium analyses were performed at the Lamont-Doherty 

Earth Observatory of Columbia University in Palisades, New 

York (Clark and others, 1976; Schloesser and others, 1989).

Quality-control samples were collected to assess the 

reliability of sample processing and analytical methods. One 

field-blank sample was collected and analyzed for nitrogen 

and phosphorus compounds and major ions. Field-blank samples are collected to determine the occurrence and magnitude 

of sample contamination during sample collection, processing, and transport. Field-blank results indicated that sample 

contamination was minimal. The only analytes detected 

in the field-blank sample were calcium (0.02 mg/L), silica 

(0.04 mg/L), and iron (estimated 4 μg/L). Calcium and silica 

concentrations in the field-blank sample were substantially 

less than environmental sample concentrations. However, the 

iron concentration was greater than all but two environmental 

sample concentrations. Therefore, at small concentrations, it 

was not possible to determine if concentrations of iron in environmental samples were representative of aquifer conditions or 

the result of sampling contamination. One sequential replicate 

sample was collected and analyzed for nitrogen and phosphorus compounds, and major ions. Sequential replicate samples 

are two environmental samples collected one after another 

within a short period of time to represent a duplication of the 

same sample water. They are collected to determine variability 

of the data as a result of sampling and analytical procedures. 

Replicate sample results support the reliability of the data; all 

relative differences between constituent concentrations were 

less than 3 percent.

Methods 
Hydrogeologic Setting

The hydrogeologic setting of the study area includes 

the composition and distribution of water-bearing layer(s) in 

the subsurface and the paths followed by ground water from 

recharge to discharge areas, primarily withdrawals by irrigation and supply wells.

Geology

 The Seward west well field and installed monitoringwell nests are located west of the Big Blue River within the 

High Plains aquifer (fig. 1). Sand and gravel deposited during 

the Pleistocene Epoch (0.01 to 1.6 million years ago) are the 

primary water-bearing sediment permitting withdrawal of 

economically viable quantities of ground water in this area 

(Keech, 1978; Gottula, 1990; Verstraeten and others, 1998). 

This sediment is relatively coarse-grained and yields large 

amounts of water where saturated. Depth to the water table 

ranges from 75 to 96 ft below land surface in the study area. 

Loess overlies the sand and gravel and provides a relatively 

thick layer of fine-grained sediment that may inhibit the downward movement of water to the High Plains aquifer. Beneath 

the sand and gravel are glacial till, silt, and clay that generally 

do not yield large quantities of water. Bedrock underlying the 

study area is the Dakota Sandstone that was deposited during 

the Early Cretaceous Period (about 96 to 138 million years 

ago). The Dakota Sandstone is not used extensively for irrigation or public supply in the study area because of high salinity 

and the requirement of drilling deep wells to reach it.

The physical characteristics of aquifer sediment can 

affect ground-water flow rates, ground-water age, and groundwater chemistry within the aquifer. Aquifer sediment samples 

and geophysical logs indicated that the aquifer materials in 

the immediate vicinity of the monitoring wells were primarily fine to medium sand with intervening layers of clay or silt. 

Figure 2 shows hydrogeologic sections of the sediment collected from monitoring-well boreholes. A thick layer (between 

about 40 and 70 ft) of clay lies between the land surface and 

the sand deposits. Within the sand deposits, another clay layer 

about 10 ft thick extends from monitoring-well nest FP1 to 

well nests FP2 and FP3. Well boreholes did not extend deeper 

than the top of the glacial till as the overlying sand is the 

water-bearing layer exclusively screened in the Seward west 

well field. The saturated thickness of the sand overlying the 

glacial till is about 50 ft or less, and supply wells generally are 

screened in the bottom 20 ft of that thickness.

Ground-Water Levels

Ground-water levels can indicate the vertical and horizontal ground-water flow directions within an aquifer. The 

ground-water altitude, or hydraulic head in an unconfined 

aquifer, is a measure of ground-water potential energy. 

Because ground water will flow from places with high energy 

to places with low energy, differences in hydraulic head, or 

hydraulic-head gradients, are the driving forces that cause 

water to move from one place to another. The velocity of 

ground-water flow is proportional to the hydraulic gradient. In 

the horizontal dimension, ground water will flow from areas 

with higher hydraulic head to lower hydraulic head. In the 

vertical dimension, downward gradients cause ground water 

and contaminants to move downward from the water table into 

the aquifer.

Several sources of information on ground-water flow 

direction were considered. Direction of ground-water flow 

and placement of monitoring wells were determined primarily using ground-water levels measured in spring 2001 

by UBBNRD as part of their long-term ground-water-level 

monitoring program (fig. 3). These data were retrieved from 

the USGS NWIS (U.S. Geological Survey, 2007) and used to 

plot ground-water altitudes and draw lines of equal groundwater-level altitude. Ground water moves perpendicular to 

contour lines of equal ground-water-level altitude. The resulting spring 2001 ground-water-altitude map shows that ground 

water converged toward the well field from the northwest and 

southwest. This convergent flow field may be caused by local 

reductions in hydraulic head within the well field caused by 

withdrawals from supply wells and nearby irrigation wells. If 

larger withdrawals of ground water are occurring within the 

well field than areas to the north and south, ground water will 

be drawn to the well field.

A ground-water altitude map of Seward County from 

1972 (Keech, 1978) shows that regional ground-water flow in 

the area west of Seward is primarily from west to east but with 

a substantial southwest-to-northeast component of flow in the 

vicinity of the Seward west well field. Water-level altitude 

maps of the UBBNRD from 1975 (Cady and Ginsberg, 1979) 

and 1996 (Verstraeten and others, 1998) as well as statewide 

water-level maps from 1979 and 1995 (Conservation and Survey Division of University of Nebraska, 1980, 2000) indicated 

similar flow directions. Ground-water flow directions in the 

study area may be affected by the geometry of Lincoln Creek, 

a tributary to the Big Blue River, and the Big Blue River itself. 

Both of these streams probably serve as discharge areas for 

regional ground-water flow (Keech, 1978). These streams, 

particularly Lincoln Creek, have a northwest-to-southeast 

orientation immediately to the east and north of the study area 

(fig. 1), which may cause ground water in the study area to 

sometimes flow to the northeast towards Lincoln Creek. Thus, 

the southwest-to-northeast flow component towards the well 

field may not be related entirely to withdrawals from the well 

field but may reflect, partially or entirely, a natural shift in the 

regional ground-water direction close to discharge areas along 

Lincoln Creek.

It is also likely that temporal changes in recharge and discharge as a result of varying climatic and pumping conditions 

result in shifts in regional and local ground-water flow directions. Broadly, it appears that there is always regional flow 

towards the well field from the northwest or west as indicated 

 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
Figure 2. Hydrogeologic sections along two ground-water flow paths upgradient from Seward west well field.

Hydrogeologic Setting 

1,400

1,420

1,440

1,480

1,500

1,520

1,540

1,560

2.1 2.0 1.5 1.0 0.5 0

Miles upgradient from well field

Altitude, in feet above North American

Vertical Datum of 1988

Altitude, in feet above North American

Vertical Datum of 1988

Miles upgradient from well field

1,400

1,420

1,440

1,460

1,480

1,500

1,520

1,540

1,560

1.0 0.5 0

FP1

FP1

FP2

(bend in

section)

(bend in

section)

(bend in

section)

FP4

FP3

CLAY

W11

W11

CLAY

SAND

CLAY

SAND

TILL TILL

Water table

Land surface

Land surface

A

(northwest)

B

(southwest)

B’

(northeast)

A’

(southeast)

Water table

TILL

SAND

SAND

CLAY

CLAY

1,460

(95)

(105)

(129)

(90)

(105)

(125)

(90)

(105)

(125)

(90)

(105)

(123)

(90)

(105)

(123)

(90)

FP3

W11

EXPLANATION

Monitoring-well screen interval and

 well-nest identifier—Number in

 parenthesis ( ) is approximate depth

 of individual well, in feet below land

 surface

Supply-well screen interval and

 well identifier

Trace of sections shown in figure 1

Vertical scale greatly exaggerated
by the 2001 and other historical water-level maps. Because 

there is some uncertainty in local ground-water flow directions 

to the Seward west well field, one of the four monitoring-well 

nests was installed along a possible ground-water flow path 

(B–B’, fig. 1) from the southwest to compare and contrast 

ground-water age, quality, and hydraulic head along this flow 

path with those along a flow path from the northwest (A–A’, 

fig. 1).

Depths to water in the monitoring wells ranged from 

74.7 to 95.9 ft below land surface (U.S. Geological Survey, 

2007). Water levels changed seasonally, declining during the 

summer (peak irrigation months) and recovering during the 

fall, winter, and spring (fig. 4). Seasonal water-level declines 

were between about 10 and 12 ft in the FP3 wells, between 

about 6 and 7 ft in the FP2 wells, and about 4 ft in the FP1 and 

FP4 wells.

Figure 3. Water-level contours near Seward, Nebraska, spring 2001. Water-level measurements made by Upper Big Blue Natural Resources 

District.

 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04

Seward

Big

Blue

River

1470.60

1472.50

1459.63

1457.53

1470.24

1481.34

1463.75

1449.16

1455.73

1452.96 1463.79

1487.14

1480

1480

1500

1470

1470

1460

1460

1450

1440

1430

1450

1440

1430

1460

0 0.5 1.0 1.5 2.0 MILES

0 0.5 1.0 1.5 2.0 KILOMETERS

97°14' 12'

40°56'

54'

40°52'

10' 8' 97°6'

EXPLANATION

Water-level contour−Shows altitude of water

 level, spring 2001. Dashed where approximately

 located. Contour interval 10 feet. Datum is

 North American Vertical Datum of 1988

Approximate direction of ground-water flow

Monitoring-well nest

Observation well−Number is water-level altitude

 in feet above North American Vertical Datum

 of 1988

1472.50

1470

Base from U.S. Geological Survey digital data, 1:100,000, 1999

Universal Transverse Mercator projection,

Zone 14

Horizontal coordinate information is referenced to the North American

 Datum of 1983 (NAD 83)

Lincoln Creek
Vertical and horizontal ground-water flow patterns were 

determined by converting water-level measurements from 

depth below land surface to depth above the North American 

Vertical Datum of 1988 (NAVD 88) to achieve comparability using a common reference datum. Vertical gradients were 

determined by comparing ground-water-level measurements 

at the three depths in each well nest. The vertically stratified 

water levels indicated that water generally flowed horizontally 

but had an upward flow component from the bottom to the 

middle and top of the sand at well nest FP3 and flowed horizontally (had little or no vertical gradient) at well nests FP4, 

FP2, and FP1. Beginning in July 2005, the upward gradient at 

well nest FP3 became steeper but otherwise showed little seasonal change (fig. 4). Minor downward and upward gradients 

also occurred at well nest FP2 during the summer of 2004 and 

summer of 2005, respectively.

Horizontal gradients indicated hydraulic-head gradients 

from well nest FP4 to well nest FP2 to well nest FP1, consistent with ground water flowing from the northwest towards 

well nest FP1 and the supply wells. Hydraulic heads at well 

nest FP3 also were generally higher than at well nest FP1, 

consistent with movement of ground water to the well field 

from the southwest (fig. 4). However, during the late summer of 2004, ground-water altitudes at well nest FP3 were 

only slightly higher than at well nest FP1, indicating very low 

hydraulic gradients. Such low gradients would be consistent 

with a lessening of the flow of ground water towards the well 

field from the southwest, perhaps as a result of seasonal shifts 

in ground-water recharge and irrigation pumping. During 

other periods, hydraulic-head differences between well nests 

FP3 and FP1 (higher hydraulic heads at well nest FP3) were 

similar to hydraulic-head differences between well nests FP2 

and FP1; because the distances between these well-nest pairs 

are similar, the hydraulic-head differences indicate very similar horizontal hydraulic-head gradients towards the well field 

from the northwest and southwest.

Ground-Water Age

Concentrations of atmospheric trace gases and isotopic 

tracers dissolved in ground water were used to estimate the 

approximate time since the water entered the aquifer. Concentrations of three different chlorofluorocarbons (CFC–12, 

CFC–11, and CFC–113), sulfur hexafluoride (SF6), and ratios 

of tritium (3H) to the helium-3 (3He) isotope derived from 

radioactive decay of 3H were used to estimate the apparent 

recharge age of the ground-water sample. Because each agetracer technique has associated uncertainties and is affected in 

different ways by aquifer processes, ages estimated using these 

techniques are referred to as “apparent ages.” The details of 

analytical and interpretative techniques used to estimate apparent ages are described by the U.S. Geological Survey (2005) 

and have been described previously for CFCs (Busenburg and 

Plummer, 1992), SF6 (Busenburg and Plummer, 2000), and 

3

H/ 3He (Schlosser and others, 1988, 1989; Solomon and 

others, 1992). Brief descriptions of the CFC, SF6, and 3H/ 3He 

methods are provided in Appendix 2.

Estimates of apparent ground-water age from tracer data 

were complicated by mixing of water of different ages in 10 

of the 13 ground-water samples collected. In addition, the 

apparent ages indicated by CFC and SF6 methods often were 

older than those indicated by the 3H/3He method; this is probably because the depths from land surface to the water table 

of 75 to 95 ft are large enough to cause CFC and SF6 apparent ages to lag behind 3

H/3He apparent ages, as 3H/3He is not 

affected by time for atmospheric gases to reach the bottom of 

deep unsaturated zones. Finally, geologic sources of 3He and 

SF6 in some ground water apparently affected 3H/3He and SF6

estimates. The effect of these geologic sources was to add 

uncertainty to age estimates compared to ideal conditions. 

Summary estimates of ground-water ages and mixtures are 

reported in the following paragraphs. Additional details of the 

estimates are discussed in Appendix 2.

Multiple interpretations of concentrations of atmospheric 

tracers to estimate ground-water ages are possible using different models or approximations of how ground water recharged 

during a particular year moves and mixes with water of other 

ages. Comparisons of tracer concentrations to expected historical values in recharge and comparisons of different tracers 

indicate whether or not substantial mixing of water of different 

ages occurs. Estimates of ground-water age from environmental tracer data may be relatively straightforward for a water 

sample with a single age; estimation is more complex for a 

sample that is a mixture of water of varying ages (Cook and 

Böhlke, 2000). A mixture can be the result of convergence of 

ground-water flow to a pumping well, for example, or it can be 

an artifact of sampling a well with a long screen that intersects 

water of variable age.

On the basis of different tracer concentrations plotted 

in relation to each other in comparison with curves predicted 

with different mixing models (see Appendix 2), it was interpreted that the samples collected were a mixture of relatively 

young water containing modern atmospheric tracers and 

relatively old water that did not contain modern atmospheric 

tracers. Although alternative mixing scenarios are possible, it 

is not possible to quantify more complex mixing of more than 

two bodies of water of different age with the data collected for 

the study.

Best estimates of apparent recharge dates of unmixed 

ground-water samples or mean age of young-water fractions 

in samples collected from monitoring wells ranged from 1985 

to 2002 (figs. 5 and 6, table 1). For monitoring-well samples 

containing mixed-aged water, best estimates of the fraction 

of the sample consisting of young water ranged from 26 to 

77 percent. Best estimates of apparent mean recharge dates of 

young-water fractions in samples collected from four supply 

wells in the Seward west well field ranged from about 1980 

to 1990. The best estimate of the fraction of the supply-well 

samples consisting of young water ranged from 39 to 54 percent. It is implicit in the mixing calculations that the remainder 

Ground-Water Age 
10 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04

Figure 4. 

9/1/2004

Water levels measured in monitoring-wells nests near Seward, Nebraska, July 2004 to March 2006.

7/1/2004

11/1/2004

1/1/2004

3/1/2005

5/1/2005

7/1/2005

9/1/2005

11/1/2005

1/1/2005

3/1/2006

5/1/2006

9/1/2004

7/1/2004

11/1/2004

1/1/2004

3/1/2005

5/1/2005

7/1/2005

9/1/2005

11/1/2005

1/1/2005

3/1/2006

5/1/2006

9/1/2004

7/1/2004

11/1/2004

1/1/2004

3/1/2005

5/1/2005

7/1/2005

9/1/2005

11/1/2005

1/1/2005

3/1/2006

5/1/2006

1,4701,4681,4661,4641,4621,4601,4581,4561,4541,4521,450

Water level, in feet above North American Vertical Datum of 1988

Water level, in feet above North American Vertical Datum of 1988

Monitoring-well nest FP4 Monitoring-well nest FP2 Monitoring-well nest FP1

Monitoring-well nest FP3 Monitoring-well nest FP1

1,4801,4761,4721,4681,4641,4601,4561,4521,4481,4441,440

EXPLANATIO

N

Shallow monitoring wellMiddle monitoring wellDeep monitoring well
Figure 5. Best estimate of apparent mean recharge date and percentage of sample composed of young water (less than 60 years old) for 

ground-water samples collected near Seward, Nebraska, 2003–04.

Ground-Water Age 11

1,400

1,420

1,440

1,460

1,480

1,500

1,520

1,540

1,560

2.1 2.0 1.5 1.0 0.5 0

Miles upgradient from well field

Altitude, in feet above North American

Vertical Datum of 1988

Altitude, in feet above North American

Vertical Datum of 1988

Miles upgradient from well field

1,400

1,420

1,440

1,460

1,480

1,500

1,520

1,540

1,560

1.0 0.5 0

FP1

FP1

FP2

FP4

FP3

W11

W11

EXPLANATION

CLAY

CLAY

SAND

CLAY

SAND

TILL TILL

Water table

Land surface

Land surface

Water table

TILL

SAND

SAND

CLAY

CLAY

1996

2002

1987 (54%) 1993

1985 (26%) 

1987 (38%) 

1985 (34%) 

1987 (68%) 1987 (77%) 

2002

1993

1987 (77%) 

1980 (54%) 

1980 (54%) 

Approximate direction of ground-water

 flow

Monitoring-well screen interval and

 well-nest identifier−Number by screen

 interval is apparent recharge date

 determined from piston-flow model

Monitoring-well screen interval and

 well-nest identifier−Numbers by screen

 interval are apparent mean recharge date

 and percentage (%) of sample composed

 of young water determined from binary-

 mixing model

Supply-well screen interval and well

 identifier−Numbers by screen interval

 are apparent mean recharge date and

 percentage (%) of sample composed of

 young water determined from exponential

 binary-mixing model 

Trace of sections shown in figure 1

Vertical scale greatly exaggerated

FP4

1996 

FP3

1985 (26%) 

W11

1980 (54%) 

A

(northwest)

B

(southwest)

B’

(northeast)

A’

(southeast)

(bend in

section) (bend in

section)

(bend in

section)
Figure 6. Relations of depth below land surface to (A) apparent recharge date or mean recharge date of water, and (B) fraction of sample composed of young water (less 

than 60 years old) for ground-water samples collected near Seward, Nebraska, 2003–04.

12 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04 FP1−90 FP2−105 FP1−105 FP4−105 FP3−105 FP1−123 FP2−125 W10 FP3−125 FP4−129 W7 W9 W11 W9 W10 (A) (B) FP1−90 FP2−105 FP1−105 FP4−105 FP3−105 FP1−123 W7 FP2−125 FP3−125 W11 FP4−129 1960 1970 1980 1990 2000 2010 0 0.2 0.4 0.6 0.8 1.0 100 110 120 1308090

Depth to bottom of well screen, in feet

Apparent recharge date or mean recharge date Fraction of sample composed of young water

EXPLANATIO

N

FP3-125 Piston-flow model using ratio of

 tritium to helium-3 (

3H/3He)

Binary mixture using ratio of tritium to helium-3 (

3H/3He)

Exponential mixture using chlorofluorocarbon-12 (CFC–12)Binary mixture usingchlorofluorocarbon-12 (CFC–12)

Well identifier
Table 1. Estimates of age and mixing fractions for ground-water samples collected near Seward, Nebraska, 2003–04.

[3H/3He, tritium/helium, CFC–12, chlorofluorocarbon-12; PFM, piston-flow model; EBMM, exponential binary-mixing model; BMM, binary-mixing model; NP, not possible; NC, not collected; bold date and mixing fraction, best estimate of mean age and young-water mixing fraction; young water, ground water recharged after 3H and CFC–12 were present in atmosphere (younger than about 60 years); old water, ground water recharged before 3H and CFC–12 were present in atmosphere (older than about 60 years)]

Well 

identifier 

(fig. 1)

Sampling

date

(month/

day/year)

3H/3He tracer method CFC–12 tracer method Youngest

possible 

estimate

of mean 

age of 

young 

fraction

(date)

Best 

estimate 

of

mean 

age of 

young 

fraction

(date)

Oldest

possible 

estimate

of mean 

age of 

young 

fraction

(date)

Age description

Method used

for best 

estimate

of age date

PFM

age

(date)

EBMM

mean 

age of 

young 

fraction

(date)

EBMM

percentage of 

young 

water

BMM

mean 

age of 

young 

fraction

(date)

BMM

percentage of 

 young 

water

PFM

mean 

age

(date)

EBMM

mean 

age of 

young 

fraction

(date)

EBMM

percentage of

young 

water

BMM

mean 

age of 

young 

fraction

(date)

BMM

percentage 

of 

young 

water

Monitoring wells

FP1–90 8/23/2004 2002 2002 100 2002 100 1984 1991 84 1984 100 2002 2002 1984 early 2000s 3H/3He PFM

FP1–105 8/23/2004 1993 1998 100 1996 86 1973 1980 52 1973 100 1998 1993 1973 mid-1990s 3H/3He PFM

FP1–123 8/23/2004 1984 1998 60 1987 77 1969 1980 34 1979 40 1998 1987 1969 majority late 1980s to 

late 1990s water

3H/3He BMM

FP2–105 8/24/2004 1962 1994 34 1983 34 1970 1990 29 1985 34 1990 1985 1970 one-third late 1980s to 

early 1990s mixed with old water

CFC–12 BMM

FP2–125 8/24/2004 1980 1998 52 1987 68 1971 1980 40 1979 47 1998 1987 1980 majority late 1980s to 

late 1990s water

3H/3He BMM

FP3–105 8/24/2004 2003 NP NP NP NP 1968 1990 24 1985 26 2003 1985 1968 one-fourth late 1980s 

to early 1990s mixed with old water

CFC–12 BMM

FP3–125 8/25/2004 1971 1998 29 1987 38 1963 1990 12 1985 14 1998 1987 1963 one-third late 1980s to 

early 1990s mixed with old water

3H/3He BMM

FP4–105 8/25/2004 1996 1999 100 1997 95 1987 1998 87 1992 87 1999 1996 1987 late 1990s 3H/3He PFM

FP4–129 8/25/2004 1976 1998 42 1987 54 1986 NP NP NP NP 1998 1987 1976 one-half late 1980s to 

1990s water

3H/3He BMM

Supply wells

W7 11/14/2003 NC NC NC NC NC 1973 1980 50 1979 59 1980 1980 1973 mixed with one-half 

young with mean age of late 1970s or early 1980s and old water

CFC–12 EBMM

W9 11/13/2003 NC NC NC NC NC 1971 1980 39 1979 46 1980 1980 1971 mixed with one-half 

young with mean age of late 1970s early 1980s and old water

CFC–12 EBMM

W10 11/14/2003 NC NC NC NC NC 1974 1990 48 1985 55 1990 1990 1974 mixed with one-half 

young with mean age of mid-1980s to early 1990s and old water

CFC–12 EBMM

W11 11/14/2003 NC NC NC NC NC 1974 1980 54 1979 64 1980 1980 1974 mixed with one-half 

young with mean age of late 1970s early 1980s and old water

CFC–12 EBMM

Ground-Water Age 13
of the sample that is not young water consists of water that is 

more than 60 years old and contains no detectable quantities 

of modern atmospheric tracers. This “old” water accounted for 

23 to 74 percent of water in the samples.

Ground-water ages and mixing fractions were estimated 

primarily on the basis of CFC–12 and 3H/3He data (table 1). 

The three CFC tracers (CFC–12, CFC–11, and CFC–113) generally indicated very similar ages and mixing fractions. Of the 

three CFC tracers, CFC-12 is the least susceptible to sorption 

and degradation, although in the case of this study, these 

processes did not appear to affect CFC concentrations. Therefore, only ages estimated from CFC–12 data are reported here. 

The 3H/3He method also was used as a primary dating technique. There were consistent differences between the CFC–12 

and 3H/3He tracer methods. Apparent ground-water ages 

estimated using the 3H/3He method were as much as 20 years 

younger than those estimated using the CFC–12 method with 

an average difference of 9 years (see Appendix 2). The differences between these methods probably occur because the relatively thick unsaturated zone (75 to 96 ft) of the High Plains 

aquifer in the study area causes CFC–12 concentrations in soil 

gas at the bottom of the unsaturated zone to lag several years 

behind atmospheric CFC–12 concentrations. Consequently, 

CFC–12 ages are older than true ages or ages estimated using 

the 3H/3He tracer method, which is not affected by unsaturated-zone lag times. For two monitoring wells (FP2–105 and 

FP3–105), 3H/3He sampling results were anomalous. For well 

FP3–105, the mean age and fraction of young water in the 

sample could not be estimated. For well FP2–105, apparent 

mean age was older, and the fraction of young water in the 

sample was less than that for other samples at comparable 

depths. For these two samples, estimates were based on results 

using the CFC–12 method instead. For the four supply wells 

sampled in November 2003, 3H/3He samples were not collected. Therefore, estimates were based solely on results from 

the CFC–12 method. The differences between CFC–12 and 

3

H/3He ages were greater for relatively shallow water samples 

and did not occur for all samples (Appendix 2). Therefore, in 

the wells screened in the middle to lower part of the aquifer 

where ages were based on CFC–12 results, the ages were not 

adjusted to compensate for the fact that CFC–12 ages may be 

older than ages determined using the 3H/3He method.

At three of the nine monitoring wells sampled, groundwater ages were estimated using a piston-flow model (PFM), 

which is based on the concept that ground water that recharges 

during a particular year does not mix with water of other ages 

along the flow path to the sampled well. The relatively shallow 

ground-water samples from wells FP1–90, FP1–105, and 

FP4–105 may have not been subjected to as much mixing 

as deeper samples because most irrigation and supply wells 

are screened near the bottom of the aquifer. Samples from 

the shallowest well (FP1–90), with the water table within 

the screened interval, had an apparent recharge date of 2002 

(

3

H/3He method) (table 1, fig. 5). Samples collected from the 

upper-to-middle part of the aquifer at wells FP1–105 and 

FP4–105 had PFM apparent recharge dates of 1993 and 1996, 

respectively, indicating traveltimes from the recharge area 

to the well at the time of sampling of 8 to 11 years (table 1, 

fig. 6A).

In six of the nine monitoring wells sampled, discrepancies between tracers, specifically the comparison of CFC–12 

or 3H/3He results to 3H, indicated that water from the wells 

consists of a mixture of water of different ages (see 

Appendix 2 for selected tracer-tracer plots, figs. 2-1–2-3). 

Some of the samples collected from intermediate depths and 

all of the samples collected from the lower parts of the aquifer 

contained mixed water (fig. 6B). Apparent mean recharge 

dates of young-water fractions of these mixed samples (referring to monitoring wells) were all in the mid-1980s, indicating 

traveltimes of 17 to 19 years from the recharge area to the 

monitoring well at the time of sampling for the mean youngwater fraction (table 1). Because this water is mixed, it also 

contains “old” water more than 60 years old. In addition, the 

supply wells appear to represent a mixture of two water types: 

(1) exponentially mixed young water with a mean age of 13 to 

23 years, and (2) “old” water. Such mixing is not surprising considering that the High Plains aquifer is relatively thin 

(50 ft) in the area of the supply wells and has a relatively high 

density of wells (4.75 irrigation wells per square mile plus 

supply wells), indicating that withdrawals are relatively large. 

Such pumping stress has the effect of increasing ground-water 

velocities and mixing of ground water. Although alternative 

mixing models can be used to interpret the results, the mean 

ages and mixing fractions for the primary mixing models used 

were similar. Results from multiple models are presented in 

table 1 for comparison.

 All of the samples collected contained at least some 

water that recharged the aquifer in the last 25 years. Relations of ground-water recharge dates or mean recharge dates 

of water mixtures with depth were not consistent (fig. 6A). In 

well nests FP1 and FP4, changes in mean recharge dates from 

the shallowest to the deepest sample were 15 and 9 years, 

respectively. However, mean recharge dates in the deepest 

wells at the FP2 and FP3 well nests were slightly younger and 

very similar to recharge dates in the shallowest wells (fig. 5). 

In these well nests, the fractions of young water in the sample 

also were less in the shallow samples than in the deep wells 

(fig. 6B). The occurrence of more young water in the deep 

rather than intermediate parts of the aquifer at these locations 

is consistent with the effects of pumping from irrigation and 

supply wells, which are screened primarily at the bottom or 

throughout the entire aquifer. Withdrawals from the lower part 

of the aquifer have the effect of drawing younger water from 

shallower depths to the deeper part of the aquifer, perhaps 

leaving less young ground water at intermediate depths away 

from the depths of supply-well screens.

Horizontally, water age along estimated ground-water 

flow paths did not follow a clear pattern of younger to older 

with distance along the flow path towards the supply wells. 

Such a horizontal pattern is common in relatively simple 

ground-water systems without a large amount of mixing of different-aged water. The complex pattern of water age probably 

14 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
is affected at least partially by pumping in the aquifer. Consequently, in most cases, meaningful horizontal traveltimes 

from upgradient wells to wells farther downgradient (near 

the supply wells) could not be calculated or were so small 

as to be questionable. Probably the most reliable estimate of 

horizontal traveltimes can be calculated from comparison of 

the mean recharge date at monitoring well FP4–105 (1996) 

to well FP1–123 (1987). The difference in age implies that 

ground water, were it to flow along a path between these two 

wells, would take an average of 9 years to travel this distance. 

Although hydraulic heads indicate such a ground-water flow 

path is conceptually reasonable, the presence of older water in 

between these wells at well nest FP2 and the likely complexity 

of ground-water flow due to a highly pumped system suggest that it is not certain that these two wells are along a true 

ground-water flow path. These uncertainties need to be considered when using the apparent 9-year traveltime as an estimate 

of horizontal traveltime. However, the general similarity of 

water ages at similar depths between different well nests is 

consistent with the fact that horizontal flow in the aquifer is 

relatively rapid. These results indicate that traveltimes in the 

study area may be much less than the 30-year traveltime estimated from simple analytical modeling to delineate wellhead 

protection areas.

Ground-Water Quality

Concentrations of constituents were compared with U.S. 

Environmental Protection Agency (USEPA) drinking-water 

standards (U.S. Environmental Protection Agency, 2004) to 

place the quality of the untreated supply- and monitoring-well 

water in the context of nationally recognized public-supply 

criteria. Regular testing is conducted by public-water 

suppliers to ensure that drinking-water standards are not 

exceeded. If needed, the water is treated to reduce contaminant 

concentrations to levels that are safe for human consumption, 

or well usage is discontinued. Concentrations in this report 

are untreated ground water and, therefore, do not reflect water 

quality delivered to consumers; drinking-water standards do 

not apply to untreated ground water. Comparisons of concentrations in ground-water samples to drinking-water standards 

are made only to illustrate ground-water quality relative to 

established drinking-water benchmarks.

USEPA has established several kinds of drinking-water 

standards (U.S. Environmental Protection Agency, 2004). The 

Maximum Contaminant Level (MCL) is the highest level of 

a contaminant that is allowed in publicly supplied drinking 

water. The Secondary Drinking-Water Regulation (SDWR) 

is a nonenforceable guideline regarding cosmetic or esthetic 

effects of drinking water. The treatment technique (TT) specifies a constituent concentration above which a public water 

supplier is required to physically remove the constituent from 

delivered water.

Analytical results from water samples collected from 

the four supply wells in the study area in November 2003 

indicated that water quality was generally suitable for drinking with regard to major ions, trace elements, and pesticide 

compounds (table 2). However, nitrite plus nitrate as nitrogen 

concentrations in water from all four supply wells were larger 

than the USEPA MCL of 10 mg/L, ranging from 11.3 to 

13.5 mg/L. Nitrite as nitrogen concentrations were less than 

the laboratory reporting level of 0.008 mg/L; therefore, nitrate 

plus nitrite as nitrogen concentrations are referred to as nitrate 

concentrations in this report. Dissolved-solids concentrations 

in water from three of the four wells also exceeded USEPA 

nonenforceable SDWR of 500 mg/L. Five pesticides or pesticide degradation products were detected in the supply wells 

at concentrations much smaller than the maximum concentration allowed by USEPA drinking-water regulations. Most 

pesticide compound detections were reported as estimated 

by the laboratory. Although the compounds were positively 

identified, concentrations were estimated when the concentration was less than the laboratory reporting level, less than 

the lowest calibration standard, uncertain because of a matrix 

interference, or because the compound was detected in instrument blanks (Childress and others, 1999). Deethylatrazine, 

a degradation product of the herbicide atrazine, was always 

reported as an estimated value because the analytical method 

does not meet method performance criteria (Zaugg and others, 

1995). The pesticide compound with the largest concentrations 

was atrazine and was detected in water from all four wells in 

the west well field at concentrations ranging from 0.103 to 

0.346 μg/L.

Complete tables of the analytical results from monitoring-well water samples are published in Hitch and others 

(2005). The relative proportions of cations and anions in water 

samples collected from the nine monitoring wells in the study 

area exhibited patterns related to depth and position along flow 

paths. When elements dissolve in water, they dissociate into 

either positively charged ions (cations) or negatively charged 

ions (anions). Major cations in ground water typically are 

calcium, magnesium, sodium, and potassium. Major anions 

in ground water typically are carbonate alkalinity [reported as 

calcium carbonate (CaCO3) equivalent], sulfate, chloride, and 

fluoride. When nitrate is present in large quantities, it also can 

be a major anion. The total electrical charge (or milliequivalents) of all cations and anions in a solution must balance to 

maintain electrical neutrality. One way to represent the relative 

percentage of the major ions in water is to plot them on a 

trilinear diagram. The relative proportions of ions as a function of areal position and depth below land surface are shown 

in figure 7. A shift in anion position indicates that the relative amount of carbonate alkalinity decreases and the relative 

amount of sulfate increases as water moves along the groundwater flow paths (fig. 7A). A shift in cations with depth below 

land surface indicates that the relative amount of calcium 

decreases and the relative amount of sodium increases from 

deep to shallow wells (fig. 7B).

Ground-Water Quality 15
It is unlikely that nitrate concentrations in the aquifer 

near the Seward west well field are reduced by denitrification 

in the aquifer because oxic geochemical conditions do not 

favor the occurrence of this reaction. Denitrification can naturally reduce nitrate concentrations in aquifers. In the process 

of denitrification bacteria consume nitrate as an energy source, 

converting it to inert nitrogen gas through a series of reactions. 

Denitrification generally will occur in anaerobic conditions; 

therefore, dissolved oxygen concentrations can indicate 

whether denitrification is likely to be an important factor. 

Dissolved oxygen concentrations less than 1 mg/L generally 

are associated with conditions favorable for denitrification 

(Trevors, 1985).

Dissolved oxygen concentrations in all ground-water 

samples collected during this study indicated that denitrification probably is not an important factor near the Seward well 

field; dissolved oxygen concentrations in samples ranged 

from 4.9 to 6.5 mg/L in supply wells (table 2) and from 4.4 to 

8.1 mg/L in monitoring wells (table 3). Because a product 

of denitrification is nitrogen gas, quantities of nitrogen gas 

present in ground-water samples at levels in excess of those 

expected in water in equilibrium with the atmosphere also can 

indicate that denitrification is occurring. However, groundwater samples collected as part of this study did not contain 

excess nitrogen gas (Appendix 2, table 5).

Nitrate and dissolved-solids concentrations typically 

are related to depth below the water table. However, neither 

was consistently related to depth in this study. At monitoringwell nests FP3 and FP4, nitrate concentrations were largest 

near the water table. Elsewhere, nitrate concentration was 

largest in the deeper well of nest FP2 (well FP2–125) and 

largest at the intermediate depth of nest FP1 (well FP1–105) 

(fig. 8). Dissolved-solids concentrations exhibited the same 

patterns as nitrate concentrations (fig. 9). Concentrations 

16  Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04

Table 2. Physical properties and concentrations of selected inorganic constituents and detected pesticide compounds in untreated ground water 

collected from supply wells near Seward, Nebraska, November 2003.

[Constituents are from filtered samples and represent dissolved phase. USEPA, U.S. Environmental Protection Agency; NTU, nephelometric turbidity unit; 

TT, treatment technique; mg/L, milligrams per liter (parts per million); SDWR, Secondary Drinking-Water Regulations; μS/cm, microsiemens per centimeter; o

C, degrees Celsius; MCL, Maximum Contaminant Level; N, nitrogen; <, less than; μg/L, micrograms per liter (parts per billion); E, estimated; --, not 

applicable]

Constituent or property Supply well (fig. 1) USEPA drinking-water standard / type of 

standard1

W7 W9 W10 W11

Physical properties

Water level, in feet below land surface 95.00 92.00 87.50 82.20 --

Specific conductance, in μS/cm at 25 oC 799 892 730 837 --

pH, in standard units 6.8 6.8 6.9 6.8 6.5–8.5 / SDWR

Temperature, in oC 12.1 12.0 12.0 12.1 --

Turbidity, in NTU .1 .2 .2 .1 5 / TT

Oxygen, in mg/L 6.4 4.9 6.5 5.5 --

Selected inorganic constituents

Dissolved solids, in mg/L 517 559 462 541 500 / SDWR

Sulfate, in mg/L 110 149 67.1 127 250 / SDWR

Chloride, in mg/L 5.80 6.69 7.06 6.60 250 / SDWR

Fluoride, in mg/L .3 .2 .3 .3 4 / MCL

Nitrite as N, in mg/L <.008 <.008 <.008 <.008 --

Nitrite plus nitrate as N, in mg/L 12.8 11.3 12.7 13.5 10 / MCL

Arsenic, in μg/L 3.5 3.9 5.9 4.8 10 / MCL

Uranium, in μg/L 8.42 9.58 8.00 7.16 30 / MCL

Pesticide compounds detected

Alachlor, in μg/L <.005 <.005 <.005 E.003 2 / MCL

Atrazine, in μg/L .242 .148 .103 .346 3 / MCL

Deethylatrazine2, in μg/L E.027 E.016 E.129 E.225 --

Metolachlor, in μg/L E.006 <.013 <.013 E.007 100 / MCL

Simazine, in μg/L E.004 <.005 <.005 E.003 4 / MCL 1

U.S. Environmental Protection Agency, 2004.

2

A degradation product of atrazine.
Figure 7. Relative proportions, in percentage equivalents, of major ions in ground-water samples collected from monitoring and supply wells with respect to (A) areal position and (B)

depth below land surface near Seward, Nebraska, 2003–04.

Ground-Water Quality 17

EXPLANATION

Deep monitoring wellMiddle monitoring wellShallow monitoring wellSupply well

Percent

Calcium

Percent Percent

Calcium plus magnesium

Sulfate

0

 40

 60

 80

 100

Chloride, fluoride, nitrite plus nitrate

100 

80 

60 

40 

20 

 0 

EXPLANATION

FP4 monitoring-well nestFP3 monitoring-well nestFP2 monitoring-well nestFP1 monitoring-well nestSupply well

(A) Areal position (B) Depth

 20

Sodium plus potassium Carbonate plus bicarbonate

Sulfate plus chloride

Magnesium

20

40

60

80

100

100

80

60

40

20

0

100

80

60

40

20

0

100 80 60 40 20 0 100

100

80

60

40

20

0

100

100

80

60

40

20

0

80

60

40

20

0

80 60 40 20 0

0

Percent

Calcium

Percent Percent

Calcium plus magnesium

Sulfate

0

 40

 60

 80

 100

Chloride, fluoride, nitrite plus nitrate

100 

80 

60 

40 

20 

 0 

 20

Sodium plus potassium Carbonate plus bicarbonate

Sulfate plus chloride

Magnesium

20

40

60

80

100

100

80

60

40

20

0

100

80

60

40

20

0

100 80 60 40 20 0 100

100

80

60

40

20

0

100

100

80

60

40

20

0

80

60

40

20

0

80 60 40 20 0

0
of nitrate, dissolved solids, and some major ions in water 

sampled from supply well W11 were intermediate between 

concentrations in the deep- and medium-depth monitoring 

wells at well nest FP1. These results are consistent with water 

from well W11 being a mixture of water from deep- and 

medium-depth parts of the saturated sand layer.

Nitrate concentrations coupled with apparent recharge 

dates can be used to reconstruct historical trends in groundwater nitrate concentrations (Böhlke, 2002). These interpreted 

nitrate trends then can be used to examine relations between 

historical land-use practices such as fertilizer applications 

and nitrate in ground water. Measured nitrate concentrations 

corresponding to the estimated sample age or mean age of the 

young-water fraction in mixed samples are shown in figure 10. 

Relations of measured nitrate concentrations corresponding to 

estimated recharge ages and historical fertilizer applications 

are not evident. However, if the ground-water samples are 

mixtures of young water and old water, the measured nitrate 

Table 3. Physical properties and concentrations of selected inorganic constituents in ground-water samples collected from monitoring wells near 

Seward, Nebraska, August 2004.

[Constituents are dissolved. USEPA, U.S. Environmental Protection Agency; NTU, nephelometric turbidity unit; >, TT, treatment technique; mg/L, milligrams 

per liter (parts per million); SDWR, Secondary Drinking-Water Regulations; μS/cm, microsiemens per centimeter; oC, degrees Celsius; MCL, Maximum Contaminant Level; <, less than; μg/L, micrograms per liter (parts per billion); E, estimated; --, not applicable]

Constituent or 

property

Monitoring-well nest 

USEPA drinkingwater standard / 

type of standard1

FP1 well nest FP2 well nest FP3 well nest FP4 well nest

FP1–90 

(shallow)

FP1–105

(middle)

FP1–123 

(deep)

FP2–105

(middle)

FP2–125

(deep)

FP3–105

(middle)

FP3–125

(deep)

FP4–105

(middle)

FP4–129

(deep)

Physical properties

Water level, in 

feet below 

land surface

81.59 81.69 81.68 85.39 85.32 88.32 89.34 95.67 95.64 --

Specific conductance, 

in μS/cm at 

25 oC

589 820 768 594 657 617 556 643 568 --

pH, in standard 

units

6.5 6.7 6.7 6.8 6.8 6.9 6.7 6.8 7.0 6.5–8.5 / SDWR

Temperature, 

in oC

15.2 15.0 14.2 14.1 15.2 16.3 14.3 14.7 14.6 --

Turbidity, in 

NTU

1.2 1.5 3.3 .9 1.3 .3 .3 .3 .6 5 / TT

Oxygen, in 

mg/L

5.8 7.0 4.8 7.9 4.4 5.6 5.0 8.1 5.9 --

Selected inorganic constituents

Dissolved solids, in mg/L386 553 531 383 435 389 354 427 377 500 / SDWR

Sulfate, in 

mg/L

60.9 129 119 31.2 58.5 30.4 39.0 32.2 32.1 250 / SDWR

Chloride, in 

mg/L

4.84 4.96 4.81 5.53 10.5 3.36 3.20 2.87 4.21 250 / SDWR

Fluoride, in 

mg/L

.3 .3 .3 .3 .2 .3 .3 .3 .3 4 / MCL

Nitrite as N, in 

mg/L

<.008 <.008 <.008 <.008 <.008 <.008 <.008 <.008 <.008 --

Nitrite plus 

nitrate as N, 

in mg/L

8.96 14.5 11.6 11.1 14.0 8.48 3.68 16.7 7.83 10 / MCL

1

U.S. Environmental Protection Agency (2004).

18  Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
Figure 8. Nitrate concentrations in ground-water samples collected from monitoring wells near Seward, Nebraska, August 

2004, and supply well W11, November 2003.

Ground-Water Quality 19

2.1 2.0 1.5 1.0 0.5 0

Miles upgradient from well field

Altitude, in feet above North American

Vertical Datum of 1988

Altitude, in feet above North American

Vertical Datum of 1988

Miles upgradient from well field

1.0 0.5 0

FP1

FP1

FP2

FP4

FP3

W11

W11

EXPLANATION

CLAY

CLAY

SAND

CLAY

SAND

TILL TILL

Water table

Land surface

Land surface

Water table

TILL

SAND

SAND

CLAY

CLAY

16.7

8.96

14.5

13.5

7.83

8.48

3.68

11.1

14.0 11.6

8.96

14.5

11.6 13.5

Approximate direction of ground-

 water flow

Monitoring-well screen interval and

 well-nest identifier—Number is nitrate

 concentration in milligrams per liter

Supply-well screen interval and well

 identifier—Number is nitrate concen-

 tration in milligrams per liter

Trace of sections shown in figure 1

Vertical scale greatly exaggerated

FP3

8.5 

13.5

W11

A

(northwest)

B

(southwest)

B’

(northeast)

A’

(southeast)

(bend in

section) (bend in

section)

(bend in

section)

1,400

1,420

1,440

1,480

1,500

1,520

1,540

1,560

1,400

1,420

1,440

1,460

1,480

1,500

1,520

1,540

1,560

1,460
concentrations should reflect this mixing as well. Mixedsample nitrate values should reflect mixing of young water 

containing nitrate with old water containing little or no nitrate. 

Ground water that entered the aquifer more than 60 years ago 

(that is, “old” water) should have smaller nitrate concentrations than modern recharge because commercial fertilizer 

commonly was not applied to agricultural fields before the 

1950s. National studies have found that nitrate concentrations 

in ground water minimally affected by human activities were 

less than 2 or 3 mg/L (Madison and Brunett, 1985; Mueller 

and Helsel, 1996). As a simplification to provide an upper 

bound for nitrate concentrations adjusted for mixing, nitrate 

Figure 9. Dissolved-solids concentrations in ground-water samples collected from monitoring wells near Seward, Nebraska, 

August 2004, and supply well W11, November 2003.

20 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04

2.1 2.0 1.5 1.0 0.5 0

Miles upgradient from well field

Altitude, in feet above North American

Vertical Datum of 1988

Altitude, in feet above North American

Vertical Datum of 1988

Miles upgradient from well field

1.0 0.5 0

FP1

FP2

FP4

FP3

W11

W11

EXPLANATION

CLAY

CLAY

SAND

CLAY

SAND

TILL TILL

Water table

Land surface

Land surface

Water table

TILL

SAND

SAND

CLAY

CLAY

427

386

377 553

389

354

383

435 531 541

541

541

386

553

531

Approximate direction of ground-

 water flow

Monitoring-well screen interval and 

 well-nest identifier—Number is dis-

 solved-solids concentration, in milli-

 grams per liter

Supply-well screen interval and well

 identifier—Number is dissolved-solids 

 concentration in milligrams per liter

Trace of sections shown in figure 1

Vertical scale greatly exaggerated

FP3

389 

W11

A

(northwest)

B

(southwest)

B’

(northeast)

A’

(southeast)

(bend in

section)

(bend in

section)

FP1

(bend in

section)

1,400

1,420

1,440

1,480

1,500

1,520

1,540

1,560

1,400

1,420

1,440

1,460

1,480

1,500

1,520

1,540

1,560

1,460
concentrations in old water were assumed to be zero. The 

measured nitrate concentrations then were divided by the fraction of young water in the sample from age tracers (table 1, 

fig. 6B) to estimate the nitrate concentration in young water. 

These estimated nitrate concentrations of young recharge 

water from the last three decades adjusted for mixing are 

shown in figure 10B.

The nitrate concentration adjusted for mixing (hereinafter 

called mixing-adjusted nitrate concentrations) appear to be 

related to the historical nitrogen fertilizer applications more 

strongly than the measured nitrate concentrations (fig. 10). 

Although there is a wide range in mixing-adjusted nitrate 

concentrations for mixed water samples from the mid- to 

late-1980s, the general pattern is one of decreases in mixingadjusted nitrate concentrations from apparent recharge dates 

of about 1980 to 2002 (Kendall’s Tau = -0.42, p = 0.0408). 

However, there are relatively few data points, and only one 

sample had an apparent recharge date after 1996 making this 

conclusion subject to some uncertainties. The decreases in 

mixing-adjusted nitrate concentrations occur during a period 

Figure 10. Relations between apparent recharge date and (A) measured nitrate concentrations and (B) nitrate concentrations of young water 

adjusted for mixing in ground-water samples collected near Seward, Nebraska, compared to historical nitrogen fertilizer applications in Seward 

County, Nebraska (Alexander and Smith, 1990; David Lorenz, U.S. Geological Survey, written commun., 1999).

Ground-Water Quality 21

Nitrogen fertilizer use, in millions of pounds per year Nitrogen fertilizer use, in millions of pounds per year

Adjusted nitrate concentration, in milligrams per liter Nitrate concentration, in milligrams per liter

1940 1950 1960 1970 1980 1990 2000 2010

0

5

10

15

20

25

30

35

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

(A)

(B) 

FP2–105

FP1–105

FP4–105

FP3–105

FP1–123

FP2–125

W10

FP3–125

FP4–129

W11

W7

W9

W11

W7

W9

FP3–105

FP2–105

FP3–125

FP4–129

FP1–123

W10

FP2–125

FP1–105

FP4–105

FP4–105

FP1–90

EXPLANATION

Apparent recharge date

Fertilizer use

Monitoring well−

 Nitrate adjusted

 for mixing

Supply well−Nitrate

 adjusted for mixing

Well identifier

FP1–90

W10

EXPLANATION

Fertilizer use

Monitoring well and

 identifier

Supply well and

 identifier

FP1–90
of generally decreasing nitrogen fertilizer applications. Total 

annual nitrogen fertilizer use in Seward County was estimated 

from fertilizer sales to be about 19,700,000 lb in 1998, or 

about 53 lb/acre (Alexander and Smith, 1990; David Lorenz, 

U.S. Geological Survey, written commun., 1999). Historical 

nitrogen fertilizer use in Seward County increased from 1945 

to 1980, decreased between 1980 and 1990, and then remained 

somewhat steady between 1990 and 2001.

The interpretation that mixing-adjusted nitrate concentrations in young ground water have decreased in response 

to decreases in nitrogen fertilizer applications over the past 

25 years is subject to the following uncertainties and assumptions: (1) it is assumed that countywide nitrogen fertilizer 

applications estimated from fertilizer sales reflect the 

historical pattern of fertilizer applications in the local study 

area; (2) ground-water samples indicating recharge dates from 

the 1980s and early 1990s are mixtures of young-water 

fractions having modern atmospheric tracers and nitrate concentrations and old-water fractions containing no modern tracers or nitrate; (3) mixing fractions (or percentages of young 

water) are accurately calculated using the CFC–12 and 3H/3He 

tracers (table 1); (4) estimates of the apparent recharge dates 

are unbiased; (5) all of the nitrate in the mixed samples is from 

the young-water fraction; if nitrate concentrations in “old” 

water are non-zero, the mixing-adjusted nitrate concentrations 

would be less and relations of age to nitrate concentrations 

might be weaker; and (6) historical nitrogen fertilizer applications have been uniform spatially across the study area, and 

the applications only change through time; substantial lateral 

variations in fertilizer application rates affecting different 

wells in the study area could invalidate the assumptions of the 

historical nitrate reconstructions.

Age-dating techniques can estimate the amount of time 

that water and solutes have resided in the saturated zone. 

However, the time it takes for recharging water to percolate 

through the unsaturated zone and reach the aquifer also needs 

to be considered in the historical reconstructions of nitrate. 

Traveltimes for water and solutes through the unsaturated 

zone can range from days to centuries depending upon factors 

such as depth to water, physical properties of the unsaturated zone, climate, and land-use practices. Recent studies 

of unsaturated zone traveltimes in the High Plains aquifer in 

western Nebraska, Kansas, and Texas indicate that water and 

solutes moved along both fast and slow paths (McMahon and 

others, 2006); traveltimes along fast paths can range from days 

to years; traveltimes along slow paths can range from decades 

to centuries. The study sites of McMahon and others (2006) 

were sites with less precipitation and greater evapotranspiration than the Seward, Nebraska, study area sites. Therefore, 

it is expected that traveltimes through the unsaturated zone in 

the Seward study area would be less than those determined by 

McMahon and others (2006). Nevertheless, multiyear delays 

between fertilizer application and arrival of resulting nitrate at 

the water table can create unknown offsets between applications and ground-water nitrate concentrations.

Summary

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

of Seward, Nebraska, conducted a 2-year study from 2003 to 

2004 of ground-water ages and quality to improve understanding of: (1) traveltimes from recharge areas to supply wells, 

(2) the effects of geochemical reactions in the aquifer on water 

quality, and (3) how water quality has changed historically 

in response to land-use practices. Three monitoring wells 

screened at different depths in the aquifer were installed in 

each of four monitoring-well nests within 2 mi of the Seward 

west well field along estimated ground-water flow paths 

towards the well field from the northwest and southwest. Historical ground-water altitude data indicate that ground-water 

flow may converge on the area of the well field from these 

directions. 

The High Plains aquifer in the study area has a depth to 

water table ranging from 75 to 96 ft below land surface. The 

aquifer consists of coarse sand and gravel with a saturated 

thickness of about 50 ft or less. Supply wells typically are 

screened in the bottom 20 ft of the aquifer. The aquifer is 

underlain by glacial till and overlain by loess and clay soils.

Ground-water levels were measured in monitoring wells 

beginning in July 2004 to allow comparison of spatial and 

temporal variations in ground-water levels and flow patterns 

to ground-water age and chemistry data. Ground-water-level 

data collected for this study indicated seasonal changes in 

water levels, with summer irrigation season water levels 4 to 

12 ft below winter levels. Vertical hydraulic-head gradients in 

well nests generally were very small, consistent with mostly 

horizontal ground-water flow within the aquifer towards the 

Seward west well field.

Concentrations of three different chlorofluorocarbons 

(CFC–12, CFC–11, and CFC–113), sulfur hexafluoride (SF6), 

and ratios of tritium (3H) to helium-3 (3He) isotope derived 

from radioactive decay of 3H were used to estimate the apparent recharge age of ground-water samples. Age estimates 

were based primarily on CFC–12 and 3H/3He data. Estimates 

of apparent ground-water age from tracer data were complicated by mixing of water having different ages in 10 of 

the 13 ground-water samples collected, differences between 

results for different tracers because of lag effects caused by 

relatively large depths to water, and apparent geologic sources 

of 3He and SF6.

Estimated apparent recharge dates of unmixed groundwater samples or mean ages of young fractions of mixed water 

in samples collected from monitoring wells ranged from 1985 

to 2002. For monitoring-well samples containing mixed water, 

the fraction of the sample consisting of young water ranged 

from 26 to 77 percent of the sample. Estimated mean apparent 

recharge dates of young fractions of mixed water in samples 

collected from four supply wells in the Seward west well field 

ranged from about 1980 to 1990. Estimated fractions of the 

supply-well samples consisting of young water ranged from 

39 to 54 percent. It is implicit in the mixing calculations that 

22 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
the remainder of the sample that is not young water consists of 

water that is more than 60 years old and contains no detectable 

quantities of modern atmospheric tracers. Estimated fractions 

of the mixed samples composed of “old” water ranged from 

23 to 74 percent. Although alternative mixing models can be 

used to interpret the results, the mean ages and mixing fractions for the primary mixing models used were fairly similar.

All of the samples collected contained at least some 

water that recharged the aquifer in the last 25 years. Relations of ground-water apparent ages or mean ages of water 

mixtures with depth were not consistent. In two of the well 

nests, changes in mean apparent age from the shallowest to 

the deepest sample were 15 and 9 years. However, mean ages 

in the deepest wells in two other well nests were slightly 

younger, and fractions of young water in the sample were less 

in the shallow samples. The occurrence of more young water 

in the deep rather than intermediate parts of the aquifer at 

these locations is consistent with the effects of pumping from 

irrigation and supply wells, which are screened primarily in 

the bottom of or throughout the entire aquifer. Withdrawals 

from the lower part of the aquifer have the effect of drawing 

younger water from shallower depths to the deeper part of the 

aquifer, perhaps leaving less young ground water at intermediate depths away from the depths of supply-well screens. Such 

mixing is not surprising considering that in the study area the 

High Plains aquifer is relatively thin (50 ft) and has a relatively 

high density of wells (4.75 irrigation wells per square mile 

plus supply wells), indicating that withdrawals are relatively 

large. Such pumping stress has the effect of increasing groundwater velocities and mixing of ground water.

Horizontally, water age along estimated ground-water 

flow paths did not follow a clear pattern of younger to older 

with distance along the flow path towards the supply wells. 

The complex pattern of ages probably is affected at least 

partially by pumping in the aquifer and precludes calculating 

meaningful horizontal traveltimes in most cases. The most 

reliable estimate of horizontal traveltimes, made on the basis 

of differences in ages between the shallow monitoring well at 

the upgradient end of the northwest well transect and the deep 

well at the downgradient end of the well transect, was 9 years 

to travel this distance of about 2 mi. Although hydraulic 

heads indicate such a ground-water flow path is conceptually 

reasonable, the presence of water of older ages in between 

these wells and the likely complexity of ground-water flow 

due to a highly pumped system suggest that it is not certain 

that these two wells are along a true ground-water flow path. 

The general similarity of water ages at similar depths between 

different well nests is consistent with the fact that horizontal 

flow in the aquifer is relatively rapid. These results may indicate that traveltimes in the studied area are much less than the 

30-year traveltime estimated from simple analytical modeling 

to delineate wellhead protection areas.

Results from sampling four supply wells indicated that 

water quality was generally suitable for drinking with regard 

to major ions, trace elements, and pesticide compounds. 

However, nitrate concentrations in untreated water from all 

four of the supply wells were larger than the USEPA MCL of 

10 mg/L, ranging from 11.3 to 13.5 mg/L. Nitrate and dissolved-solids concentrations in water from monitoring wells in 

the aquifer were not consistently related to depth in this study, 

probably because of mixing related to ground-water pumping. 

It also is unlikely that nitrate concentrations in the aquifer near 

the Seward west well field are decreased by denitrification in 

the aquifer because the oxic geochemical conditions do not 

favor the occurrence of this reaction; there was no evidence 

of excess nitrogen gas that might indicate the reaction is 

occurring.

Nitrate concentrations coupled with water recharge dates 

were compared to historical estimated fertilizer application 

in an attempt to reconstruct historical trends in ground-water 

nitrate concentrations and their relation to land-use practices. 

Historical nitrogen fertilizer use in Seward County increased 

from 1945 to 1980, decreased between 1980 and 1990, and 

then remained somewhat steady between 1990 and 2001. 

Relations of measured nitrate concentrations corresponding to 

estimated recharge ages and historical fertilizer applications 

were not evident. However, nitrate concentration in youngwater fractions after adjustment for mixing appeared to be 

related to the historical nitrogen fertilizer applications more 

strongly than the measured nitrate concentrations. Although 

there is a wide range in mixing-adjusted nitrate concentrations for mixed-water samples from the mid- to late-1980s, the 

general pattern is one of decreases in mixing-adjusted nitrate 

concentrations from apparent recharge dates of about 1980 to 

2002, which corresponds to a period of generally decreasing 

nitrogen fertilizer applications. The interpretation that nitrate 

concentrations in ground water have decreased in response 

to decreases in nitrogen fertilizer applications over the past 

25 years is subject to several uncertainties and assumptions.

References Cited

Alexander, R.B., and Smith, R.A., 1990, County-level estimates of nitrogen and phosphorus fertilizer use in the 

United States, 1945 to 1985: U.S. Geological Survey OpenFile Report 90–130, available on Web, accessed July 14, 

2005, at http://pubs.usgs.gov/of/1990/ofr90130/report.html

Alley, W.M., and Emery, P.A., 1986, Groundwater model of 

the Blue River Basin, Nebraska—twenty years later: Journal 

of Hydrology, v. 85, p. 225–249.

American Public Health Association, 1998, Standard methods 

for the examination of water and wastewater (20th ed.): 

Washington, D.C., American Public Health Association, 

American Water Works Association, and Water Environment Federation, p. 3–37 to 3–43.

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

contamination: Hydrogeology Journal, v. 10, p. 153–179.

References Cited 23
Böhlke, J.K., in press, TRACERMODEL1—Excel workbook 

for calculation and presentation of environmental tracer data 

for simple groundwater mixtures, in IAEA Guidebook on 

the use of chlorofluorocarbons in hydrology: Vienna, International Atomic Energy Agency.

Busenberg, Eurybiades, and Plummer, L.N., 1992, Use of 

chlorofluorocarbons (CCl3F and CCl2F2) as hydrologic tracers and age-dating tools—the alluvium and terrace system 

of central Oklahoma: Water Resources Research, v. 28, 

no. 9, p. 2257–2283.

Busenberg, Eurybiades, and Plummer, L.N., 2000, Dating 

young ground water with sulfur hexafluoride—natural 

and anthropogenic sources of sulfur hexafluoride: Water 

Resources Research, v. 36, no. 10, p. 3011–3030.

Cady, R.E., and Ginsburg, M.H., 1979, Interpretative study 

and numerical model of the hydrogeology of Upper Big 

Blue Natural Resources District: Lincoln, Conservation and 

Survey Division, University of Nebraska-Lincoln, 185 p.

Childress, C.J. Oblinger, Foreman, W.T., Connor, B.F., and 

Maloney, T.J., 1999, New reporting procedures based on 

long-term method detection levels and some considerations 

for interpretations of water-quality data provided by the 

U.S. Geological Survey National Water Quality Laboratory: 

U.S. Geological Survey Open-File Report 99–193, 19 p.

Clark, W.B., Jenkins, W.J., and Top, Z., 1976, Determination of tritium by mass spectrometric measurement of 3

He: 

International Journal of Applied Radiation and Isotopes, 

v. 27, p. 515–522.

Conservation and Survey Division of the University of 

Nebraska, 1980, Configuration of the water table, 1979: 

Lincoln, Conservation and Survey Division of the University of Nebraska, information available on the Web, 

accessed October 4, 2006, at http://csd.unl.edu/general/

gis-datasets.asp#Configuration_of_the_Water_Table,_1979

Conservation and Survey Division of the University of 

Nebraska, 2000, Configuration of the water table, 1995: 

Lincoln, Conservation and Survey Division of the University of Nebraska, information available on the Web, 

accessed October 4, 2006, at http://csd.unl.edu/general/

gis-datasets.asp#Configuration_of_the_Water_Table,_1995

Cook, P.G., and Böhlke, J.K., 2000, Determining timescales 

for groundwater flow and solute transport, in Cook, P.G., 

and Herczeg, A., eds., Environmental tracers in subsurface 

hydrology: Boston, Kluwer Academic Publishers, p. 1–30.

Cook, P.G., and Solomon, D.K., 1995, The transport of 

atmospheric trace gases to the water table—implications for 

groundwater dating with chlorofluorocarbons and krypton 

85: Water Resources Research, v. 31, no. 2, p. 263–270.

Emery, P.A., 1966, Use of analog model to predict streamflow depletion, Big and Little Blue River Basin, Nebraska: 

Ground Water, v. 4, no. 4, p. 13–20.

Faires, L.M., 1993 Methods of analysis by the U.S. Geological 

Survey National Water Quality Laboratory—determination of metals in water by inductively coupled plasmamass spectrometry: U.S. Geological Survey Open-File 

Report 92–634, 28 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.

Fishman, M.J., and Friedman, L.C., 1989, Methods for 

determination of inorganic substances in water and fluvial 

sediments: U.S. Geological Survey Techniques of WaterResources Investigations, book 5, chap. A1, 545 p.

Garbarino, J.R., 1999, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—determination of dissolved arsenic, boron, lithium, selenium, 

strontium, thallium, and vanadium using inductively 

coupled plasma-mass spectrometry: U.S. Geological Survey Open-File Report 99–093, 31 p.

Garber, M.S., and Koopman, F.C., 1968, Methods of measuring water levels in deep wells: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 8, 

chap. A1, 23 p.

Gottula, J.J., 1990, A study of nonpoint source ground water 

contamination in the eastern portion of the Upper Big 

Blue Natural Resources District—a special area report: 

Lincoln, Nebraska Department of Environmental Control, 

Water Quality Division, Ground Water Section, variously 

paginated.

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.

High Plains Climate Center, 2005, National Weather Service 

surface observations and automated weather data network 

data: Information available on Web, accessed July 1, 2005, 

at http://www.hprcc.unl.edu/data.htm

Hitch, D.E., Walczyk, V.C., Miller, J.D., Drudik, R.A., and 

Hull, S.H, 2005, Water resources data—Nebraska—

water year 2004: U.S. Geological Survey Water-Data 

Report NE–04–1, 673 p.

24 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
Huntoon, P.W., 1974, Predicted water-level declines for 

alternative groundwater developments in the upper Big Blue 

River basin, Nebraska: Lincoln, Conservation and Survey 

Division, University of Nebraska, Nebraska Geological 

Survey Resource Report No. 6, 106 p.

Johnson, C.R., and Keech, C.F., 1959, Geology and ground 

water resources of the Big Blue River Basin above 

Crete, Nebraska: U.S. Geological Survey Water-Supply 

Paper 1474, 94 p.

Keech, C.F, 1978, Water resources of Seward County, 

Nebraska: Lincoln, Conservation and Survey Division of the 

University of Nebraska, Water Survey Paper 46, 88 p.

Lindley, C.E., Stewart J.T., and Sandstrom, M.W., 1996, 

Determination of low concentrations of acetochlor in water 

by automated solid-phase extraction and gas chromatography with mass selective detection: Journal of AOAC 

International, v. 79, no. 4, p. 962–966.

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, 63 p.

Madison, R.J., and Brunett, J.O., 1985, Overview of the 

occurrence of nitrate in ground water of the United 

States, in National Water Summary 1984—hydrologic 

events, selected water-quality trends, and groundwater resources: U.S. Geological Survey Water-Supply 

Paper 2275, p. 93–105.

Madsen, J.E., Sandstrom, M.W., and Zaugg, S.D., 2003, 

Methods of analysis by the U.S. Geological Survey National 

Water Quality Laboratory—a method supplement for the 

determination of fipronil and degradates in water by gas 

chromatography/mass spectrometry: U.S. Geological Survey Open-File Report 02–462 11 p.

McMahon, P.B., Dennehy, K.F., Bruce, B.W., Böhlke, J.K., 

Michel, R.L., Gurdak, J.J., and Hurlbut, D.B., 2006, Storage and transit time of chemicals in thick unsaturated 

zones under rangeland and irrigated cropland, High Plains, 

United States: Water Resources Research, v. 42, W03413, 

doi:10.1029/2005WR004417.

Mueller, D.K., and Helsel, D.R., 1996, Nutrients in the 

Nation’s water—too much of a good thing?: U.S. Geological Survey Circular 1136, 24 p.

National Agricultural Statistics Service, 2002, County summary highlights, 2002: U.S. Department of Agriculture, 

information available on Web, accessed February 8, 2006, at 

http://www.nass.usda.gov/Data_and_Statistics/Quick_Stats/

Nebraska Department of Natural Resources, 2002, Monthly 

precipitation in inches—station 25–7715: Information available on Web, accessed February 9, 2006, at http://dnrdata.

dnr.state.ne.us/Rainfall/

Nebraska Department of Natural Resources, 2006, Registered groundwater wells database: Nebraska Department of 

Natural Resources, information available on Web, accessed 

February 9, 2006, at http://dnrdata.dnr.ne.gov/wellssql/

Nebraska Natural Resources Commission, 1983, Report on the 

Big and Little Blue River Basins area planning study, Technical Appendix A—groundwater resources, development, 

calibration, verification and utilization of the groundwater 

models: Lincoln, Nebraska Natural Resources Commission, 

variously paginated.

Östlund, H.G., 1987, Tritium: GEOSECS Atlantic, Pacific, 

and Indian Ocean Expeditions, v. 7, p. 7–10.

Patton, C.J., and Kryskalla, J.R., 2003, Methods of analysis by 

the U.S. Geological Survey National Water Quality Laboratory—evaluation of alkaline persulfate digestion as an alternative to Kjeldahl digestion for determination of total and 

dissolved nitrogen and phosphorus in water: U.S. Geological Survey Water-Resources Investigations Report 03–4174, 

33 p.

Sandstrom, M.W., Stropple, M.E., Foreman, W.T., and Schroeder, M.P., 2001, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—determination of moderate-use pesticides and selected degrates in 

water by C-18 solid-phase extraction and gas chromatography/mass spectrometry: U.S. Geological Survey WaterResources Investigations Report 01–4098, 70 p.

Schlosser, Peter, Stute, Martin, Dörr, H., Sonntag, Christian, 

and Münnich, K.O., 1988, Tritium/3He dating of shallow 

groundwater: Earth and Planetary Science Letters, v. 89, 

p. 353–362.

Schlosser, Peter, Stute, Martin, Sonntag, Christian, and Münnich, K.O., 1989, Tritiogenic 3

He in shallow groundwater:

Earth and Planetary Science Letters, v. 94, p. 245–256.

Solomon, D.K., Cook, P.G., and Stanford, W.E., 1998, Dissolved gases in subsurface hydrology, chapter 9, in Kendall, 

C., and McDonnell, J.J., eds., Isotope tracers in catchment 

hydrology: Amsterdam, Elsevier Science B.V., p. 291–318.

Solomon, D.K., Poreda, R.J., Schiff, S.L., and Cherry, J.A., 

1992, Tritium and helium-3 as groundwater age tracers in 

the Borden aquifer: Water Resources Research, v. 28, no. 3, 

p. 741–755.

Trevors, J.T., 1985, The influence of oxygen concentrations on 

denitrification in soil: Applied Microbiology and Biotechnology, v. 23, no. 2, p. 152–155.

References Cited 25
U.S. Environmental Protection Agency, 2004, 2004 Edition of 

the drinking water standards and health advisories: Washington, D.C., Office of Water, EPA 822–R–04–005, 12 p.

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 on the Web, accessed January 17, 2003, at http://pubs.water.usgs.gov/twri9A

U.S. Geological Survey, 2005, Sampling instructions: U.S. 

Geological Survey information available on the Web, 

accessed February 10, 2006, at http://water.usgs.gov/lab/

U.S. Geological Survey, 2007, National Water Information System Web interface, USGS ground-water data for 

Nebraska: U.S. Geological Survey data, accessed March 12, 

2007, at http://waterdata.usgs.gov/ne/nwis/gw/

U.S. Geological Survey Chlorofluorocarbon Laboratory, 

2006a, Analytical procedures for dissolved gases: U.S. 

Geological Survey information available on Web, accessed 

May 14, 2007, at http://water.usgs.gov/lab/dissolved-gas/

lab/analytical_procedures/

U.S. Geological Survey Chlorofluorocarbon Laboratory, 

2006b, Analytical procedures for CFCs: U.S. Geological 

Survey information available on Web, accessed May 14, 

2007, at http://water.usgs.gov/lab/chlorofluorocarbons/lab/

analytical_procedures/

U.S. Geological Survey Chlorofluorocarbon Laboratory, 

2006c, Analytical procedures for SF6: U.S. Geological Survey information available on Web, accessed May 14, 2007, 

at http://water.usgs.gov/lab/sf6/lab/analytical_procedures/

Verstraeten, I.M., McGuire, V.L., and Heckman, K.L., 1998, 

Hydrogeology and subsurface nitrate in the Upper Big Blue 

Natural Resources District, central Nebraska, July 1995 

through September 1997: U.S. Geological Survey WaterResources Investigations Report 98–4207, 81 p.

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.

26  Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
Appendixes
Appendix 1. Well Installation

Well-construction information for monitoring wells installed for this study by the City of Seward is given in table 4.

28  Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
Table 4. Well-construction information for monitoring wells near Seward, Nebraska.

[Well identifier includes monitoring-well nest identifier (see fig. 1) and approximate well depth. NAVD88, North American Vertical Datum of 1988]

Wellnest 

identifier 

(fig. 1)

Well 

identifier

Latitude 

(degrees, 

minutes, 

seconds)

Longitude 

(degrees, 

minutes, 

seconds)

U.S. Geological Survey site 

number

Measuring-point 

altitude, 

in feet 

above 

NAVD 88

Landsurface 

altitude, 

in feet 

above 

NAVD 88

Height of 

measuring 

point, 

in feet 

above 

NAVD 88

Depth to 

bottom 

of well, 

in feet below land 

surface

Length 

of 

screen, 

in feet

Length 

of well 

sump, 

in feet

Depth 

of top of 

screen, 

in feet 

below 

land 

surface

Depth of 

bottom of 

screen, 

in feet 

below 

land 

surface

Altitude of 

top of 

screen, 

in feet 

above 

NAVD 

88

Altitude 

of bottom of 

screen, 

in feet 

above 

NAVD 88

FP1 FP1–90 40o53’40.5” 97o09’44.5” 405340097094501 1,537.1 1,534.1 3.0 88.9 5.0 5.0 78.9 83.9 1,455.3 1,450.3

FP1–105 40o53’40.5” 97o09’44.5” 405340097094502 1,537.0 1,534.2 2.8 106.0 10.0 5.0 91.0 101.0 1,443.2 1,433.2FP1–123 40o53’40.5” 97o09’44.5” 405340097094503 1,537.1 1,534.2 2.9 120.6 10.0 5.0 105.6 115.6 1,428.6 1,418.6

FP2 FP2–90 40o54’16.4” 97o10’37.7” 405330097110001 1,547.8 1,544.9 2.9 88.0 5.0 5.0 78.1 83.1 1,466.9 1,461.9

FP2–105 40o54’16.4” 97o10’37.7” 405330097110002 1,547.7 1,544.6 3.1 102.0 5.0 5.0 92.0 97.0 1,452.7 1,447.7

FP2–125 40o54’16.4” 97o10’37.7” 405330097110003 1,547.6 1,544.6 3.0 125.7 5.0 5.0 115.7 120.7 1,428.9 1,423.9

FP3 FP3–90 40o53’26.4” 97o10’38.6” 405212097110001 1,542.0 1,539.4 2.6 91.2 5.0 5.0 81.2 86.2 1,458.3 1,453.3

FP3–105 40o53’26.4” 97o10’38.6” 405212097110002 1,542.1 1,539.4 2.7 105.5 5.0 5.0 95.5 100.5 1,443.9 1,438.9FP3–125 40o53’26.4” 97o10’38.6” 405212097110003 1,541.9 1,539.2 2.7 127.5 5.0 5.0 117.5 122.5 1,421.7 1,416.7

FP4 FP4–95 40o54’32.1” 97o11’46.0” 405420097114001 1,559.8 1,560.0 -.2 95.2 5.0 5.0 85.2 90.2 1,474.8 1,469.8

FP4–105 40o54’32.1” 97o11’46.0” 405420097114002 1,559.8 1,560.0 -.2 107.6 5.0 5.0 97.6 102.6 1,462.4 1,457.4FP4–129 40o54’32.1” 97o11’46.0” 405420097114003 1,559.8 1,560.0 -.2 129.2 5.0 5.0 119.2 124.2 1,440.8 1,435.8

Appendix 1 29
Appendix 2. Estimation of GroundWater Age

Three different methods were used to estimate groundwater age in the study—the CFC, SF6

, and 3H/3He methods. 

The CFC and SF6 estimates of ground-water age are based 

upon the increase in concentrations of these trace gases in the 

atmosphere since the 1940s and 1950s because of industrial 

use of these compounds (Busenburg and Plummer, 1992, 

2000; U.S. Geological Survey, 2005). CFCs (or freons) are stable organic compounds first produced in the 1930s as refrigerants but also were used as aerosol propellants, cleaning agents, 

solvents, and blowing agents in the production of foam rubber 

and plastics. All CFCs eventually are released to the atmosphere and then are transported through the atmosphere to the 

hydrosphere. Concentrations of these gases in the atmosphere 

have increased in a nearly linear fashion since the 1940s, 

although concentrations of CFCs in the atmosphere have leveled off or declined in the last 10 years following implementation of an international treaty to ban CFC production in 1992. 

Substantial production of SF6 began in the 1960s for use in 

high-voltage electrical switches, and atmospheric concentrations of SF6

 have continued increasing since that time; SF6 is 

very stable in the atmosphere.

The concentrations of CFCs and SF6 in the atmosphere 

are proportional to concentrations in recharge water entering 

an aquifer at that time, with the proportionality determined by 

the recharge temperature, which is estimated from concentrations of dissolved nitrogen and argon gas. If there are no other 

sources or sinks for these compounds in the aquifer and the 

effects of dispersion are minimal, the concentrations of these 

compounds in the ground water at the time of recharge are preserved and indicate the approximate date that the water entered 

the aquifer. The three individual CFC compounds (CFC–12, 

CFC–11, and CFC–113), each having a different historical 

atmospheric concentration trend, provide three independent 

age estimates.

The 3H/3He method of ground-water age dating is based 

upon the increase in 3H concentrations in precipitation that 

occurred following atmospheric hydrogen bomb testing in the 

1950s and 1960s (Schlosser and others, 1988, 1989; Solomon 

and others, 1992; U.S. Geological Survey, 2005). Peak 3H 

concentrations occurred in 1963 and have declined substantially since then. The presence of 3

H indicates that some of the 

water in the sample has recharged the aquifer since the early 

1950s. However, concentrations of 3H alone cannot provide 

an unambiguous apparent age because similar 3H values in 

precipitation have occurred over multiple years. However, 3H 

radioactively decays to 3He with a half-life of 12.43 years. By 

determining the ratio of 3H to 3He derived from decay of 3H, 

the apparent time that the water has resided in the aquifer can 

be determined precisely. Helium also can be produced naturally by sediment; the amount of 3

He derived from 3H decay 

rather than geologic sources is estimated using 3He/4He ratios 

and neon (Ne) gas concentrations.

Concentrations of dissolved gases and estimated recharge 

temperatures are shown in table 5. Recharge temperatures 

calculated from dissolved nitrogen and argon concentrations 

generally compared well with ground-water temperatures 

measured at the time of sampling; measured ground-water 

temperatures are subject to some uncertainties depending 

upon the length of sampling lines and environmental conditions at the time of sampling. A few dissolved gas recharge 

temperatures were a few degrees higher than the average 

annual air temperature of 13 oC (High Plains Climate Center, 

2005) or onsite-measured water temperatures of 14 to 15 oC 

in monitoring wells; these variations in recharge temperature 

were considered real variations in recharge temperatures. 

Recharge temperatures and quantities of excess air calculated 

from dissolved-gas concentrations were used directly as input 

to estimate CFC and SF6 concentrations expected in recharge 

water on the basis of historical atmospheric concentrations. 

Resulting age estimates were not very sensitive to small 

adjustments in recharge temperature. In the most extreme case 

of uncertainty in recharge temperatures, changing the recharge 

temperature from 17.9 to 12.0 oC changed the piston-flow 

model recharge age for CFC–11 and CFC–12 by 6 years. 

Concentrations of age-dating tracers and related data used for 

correcting or estimating ages are shown in table 6.

To estimate ground-water ages from atmospheric tracer 

data, measured tracer and 3H values were compared to those 

expected from different lumped-parameter models including 

a piston-flow model (PFM), binary-mixing model (BMM), 

exponential-mixing model (EMM), and binary mixing of 

exponentially mixed water (EBMM) (Cook and Böhlke, 

2000). Excel workbooks for calculation and presentation of 

environmental tracer data for simple ground-water mixtures of 

Böhlke (in press) were used for analysis of the data. Example 

plots are shown in figures 2–1, 2–2, and 2–3. Measured concentrations of particular tracers also were compared to those 

of other tracers. This estimation process was iterative, and 

refinements were made on the basis of estimates of multiple 

tracers (CFC–12, CFC–11, CFC–113, 3H/3He, SF6, 3H) and 

samples. Historical atmospheric concentrations of tracers 

were obtained from the USGS Chlorofluorocarbon Laboratory. Monthly concentrations of tritium in precipitation since 

1953 for the Seward, Nebraska, area were estimated using a 

program that calculates tritium concentrations for a user-specified latitude/longitude by extrapolating results from stations 

where tritium in precipitation has been measured (Robert L. 

Michel, Research Hydrologist, U.S. Geological Survey, written commun., August 31, 2004). Tritium concentrations then 

were corrected for radioactive decay to the time of sampling in 

2003 and 2004.

The simplest model is a piston-flow model (PFM). The 

PFM presumes that water entering the aquifer moves through 

the aquifer system without mixing and retains the original 

concentration of the water at the time of recharge (Cook and 

Böhlke, 2000). This theoretical ideal is a simplification but 

can be a reasonable approximation in some simple aquifer systems. If tracer results suggest mixing, several different mixing 

30 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
Table 5. Dissolved-gas concentrations and estimated recharge temperatures and excess air quantities in ground-water samples 

collected from monitoring and supply wells near Seward, Nebraska, 2003–04. 

[Replicate analyses are shown for most samples. NAVD 88, North American Vertical Datum of 1988; mg/L, milligrams per liter]

Well identifier 

(fig. 1)

Date

collected

(month/day/

year)

Time

collected

(24-hour)

Water

temperature

measurements

(degrees 

Celsius)

Estimated

recharge

altitude

(feet above

NAVD 88)

Dissolved gas concentrations Calculated recharge

temperature from 

dissolved nitrogen 

and argon gas

concentrations 

(degrees Celsius)

Calculated

excess

air

(mg/L)

Calculated

excess

nitrogen 

gas

(mg/L)

Nitrogen

gas (N2)

(mg/L)

Argon

gas (Ar)

(mg/L)

Methane

gas

(mg/L)

Monitoring wells

FP1–90 8/23/2004 1200 15.2 1,540 17 0.59 0 17.9 2.4 0

8/23/2004 1200 15.2 1,540 18 .60 0 17.8 3.1 0

FP1–105 8/23/2004 1600 15.0 1,540 22 .71 0 12.0 5.1 0

8/23/2004 1600 15.0 1,540 22 .71 0 12.2 5.4 0

FP1–123 8/23/2004 1800 14.2 1,540 22 .70 0 13.2 5.8 0

FP2–105 8/24/2004 1000 14.2 1,540 20 .69 0 10.8 3.4 0

8/24/2004 1000 14.2 1,540 20 .70 0 10.4 3.1 0

FP2–125 8/24/2004 1400 15.2 1,540 21 .67 0 14.9 5.5 0

8/24/2004 1400 15.2 1,540 21 .68 0 14.8 5.6 0

FP3–105 8/24/2004 1700 16.3 1,540 20 .69 0 12.1 3.9 0

8/24/2004 1700 16.3 1,540 20 .68 0 12.5 3.9 0

FP3–125 8/25/2004 1100 14.3 1,540 20 .65 0 14.2 3.9 0

8/25/2004 1100 14.3 1,540 20 .65 0 14.3 3.9 0

FP4–105 8/25/2004 1500 14.7 1,540 18 .62 0 14.8 2.2 0

8/25/2004 1500 14.7 1,540 17 .60 0 15.0 1.3 0

FP4–129 8/25/2004 1800 14.6 1,540 19 .65 0 13.8 3.1 0

8/25/2004 1800 14.6 1,540 20 .67 0 13.5 3.9 0

Supply wells

W7 11/14/2003 0900 12.1 1,595 23 .73 0 11.8 6.5 0

11/14/2003 0900 12.1 1,595 23 .74 0 11.3 6.5 0

W9 11/13/2003 1500 12.0 1,595 23 .72 0 12.9 6.5 0

11/13/2003 1500 12.0 1,595 22 .71 0 13.1 6.1 0

W10 11/14/2003 1500 12.0 1,595 22 .71 0 12.4 5.8 0

11/14/2003 1500 12.0 1,595 22 .70 0 12.9 5.7 0

W11 11/14/2003 1300 12.1 1,595 26 .79 0 12.1 10 0

11/14/2003 1300 12.1 1,595 24 .75 0 11.9 7.8 0

Appendix 2 31
Table 6. Tracer data used in estimating ground-water age near Seward, Nebraska, 2003–04. 

[Results for replicate analyses for some analytes and samples are shown. CFC, chlorofluorocarbon; pptv, parts per trillion by volume; TU, tritium units; SF6, 

sulfur hexafluoride; He, helium; d3He, helium-3/helium-4 isotopic ratio relative to standard reference; 3He/4He, helium-3/helium-4 absolute ratio; 4He, helium-4 

concentration; Ne, neon concentration; cm3STP/g, cubic centimeters per gram at standard temperature and pressure; NA, not analyzed]

Local 

well 

name

(fig. 1)

Sample 

date

(month/

day/year)

Calculated atmospheric mixing 

ratio corrected for excess air and 

recharge temperature1

Tritium2

,

He ingrowth

(TU)

Tritium2,

bottle

(TU)

Tritium3,

bottle

(TU)

d3He2

(per mil)

3

He/4He2

sample

(x10-6)

4

He2

(cm3STP/g

x10-8)

Ne2

(cm3STP/g

x10-8)

Percentage

helium3 from

terrigenic 

sources

CFC–11

(pptv)

CFC–12

(pptv)

CFC–113

(pptv)

SF6

(pptv)

Monitoring wells

FP1–90 8/23/2004 211 387 51 0.41 8.22 NA NA 2.29 1.416 5.43 23.16 -6

8/23/2004 209 378 50 .33 NA NA NA NA NA NA NA NA

FP1–105 8/23/2004 74 184 14 .75 6.75 8.20 NA 6.72 1.477 7.96 28.87 7

8/23/2004 73 180 14 .81 NA NA NA NA NA NA NA NA

FP1–123 8/23/2004 40 113 4.9 .59 4.92 NA NA 3.36 1.431 8.92 28.18 20

8/23/2004 40 121 5.0 .65 NA NA NA NA NA NA NA NA

FP2–105 8/24/2004 42 129 7.3 .48 2.54 3.15 NA 3.49 1.432 25.95 80.87 14

8/24/2004 43 131 8.2 .43 NA NA NA NA NA NA NA NA

FP2–125 8/24/2004 44 142 7.7 .98 4.38 4.51 NA 1.98 1.411 8.48 25.49 24

8/24/2004 48 141 8.8 .81 NA NA NA NA NA NA NA NA

FP3–105 8/24/2004 42 103 13 .49 2.19 NA NA -3.08 1.341 7.48 28.42 3

8/24/2004 43 105 14 .54 NA NA NA NA NA NA NA NA

FP3–125 8/25/2004 13 53 1.5 1.23 NA 1.19 NA -14.11 1.189 8.66 25.19 27

8/25/2004 10 54 1.2 1.27 NA NA NA NA NA NA NA NA

FP4–105 8/25/2004 187 447 60 .70 6.59 NA NA 12.59 1.558 5.14 21.89 -4

8/25/2004 189 439 60 .72 NA NA NA NA NA NA NA NA

FP4–129 8/25/2004 204 337 39 1.90 2.58 2.92 NA -8.29 1.269 7.74 21.73 32

8/25/2004 261 404 51 1.20 NA NA NA NA NA NA NA NA

8/25/2004 230 460 60 NA NA NA NA NA NA NA NA NA

Supply wells

W7 11/14/2003 70 175 12 .63 NA NA 6.39 NA NA NA NA NA

11/14/2003 68 172 11 .59 NA NA NA NA NA NA NA NA

W9 11/13/2003 55 144 7.3 .68 NA NA 4.54 NA NA NA NA NA

11/13/2003 54 133 7.0 .63 NA NA NA NA NA NA NA NA

W10 11/14/2003 86 206 11 .63 NA NA 5.15 NA NA NA NA NA

11/14/2003 86 213 11 .61 NA NA NA NA NA NA NA NA

W11 11/14/2003 76 196 15 NA NA NA 6.23 NA NA NA NA NA

11/14/2003 75 185 15 NA NA NA NA NA NA NA NA NA

1

 Analyzed by U.S. Geological Survey Chlorofluorocarbon Laboratory, Reston, Virginia.

2

 Analyzed by Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York.

3

 Analyzed by University of Miami Tritium Laboratory, Miami, Florida.

32 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04
models can be considered. For this study binary-, exponential-, 

and exponential-binary mixing models were used to estimate 

the fraction of the sample consisting of young water having a 

particular mean age. In the binary-mixing model (BMM), the 

sample is a mixture of two subparcels, one of which is young 

and contains measurable tracer and one of which is too old 

to have measurable tracer. In the exponential-mixing model 

(EMM), the sample consists of a number of subparcels, each 

having its own tracer concentration and age, which have an 

exponential age distribution and an overall mean age. The 

mean of the combined subparcel ages affects the weight that 

new recharge water mixing with existing ground water has 

on the composition of the mixture as a whole. The EMM 

represents a highly mixed sample composed of water with 

many different ages along a distribution. The EMM has been 

found to be an appropriate model for ground-water ages in 

situations such as discharge from a spring or long-screen 

well in an aquifer that collects a mixture of most or all of the 

water in the aquifer (Cook and Böhlke, 2000). The exponential-binary mixing model (EBMM) is a binary mixture of a 

subparcel of water mixed according to the EMM and having 

a particular mean age and a subparcel that is too old to have 

measurable tracer.

Tritium concentrations of mixed samples were less than 

PFM or EMM lines (figs. 2–1, 2–2, and 2–3). However, the 

mixed-sample tritium concentrations can be interpreted as 

Appendix 2 33

Figure 2–1. Ratio of measured tritium concentration to calculated initial tritium concentration before decay compared to tritium concentration in ground-water samples collected from monitoring wells near Seward, Nebraska, and values indicated by piston-flow and exponentialmixing models.

FP1–90

FP2–105

FP1–105

FP3–105

FP4–105

FP1–123

FP2–125

FP3–125 FP4–129

1987

1983

0

10

20

30

40

0.0 0.2 0.4 0.6

Ratio of measured tritium/calculated initial tritium concentration

Tritium concentration, in tritium units

0.8 1.0

EXPLANATION

1987

FP4–105

Piston-flow model (PFM)

Exponential-mixing model (EMM)

Binary-mixing line (BMM)

Ground-water sample and well identifier

 (table 1)

PFM and binary-mixing line intersection

 point selected for mixing calculations
plotting along a linear mixing line drawn through the samples 

from the origin of the graph (zero concentration, old water 

recharge before tracers were in the atmosphere) to the point 

where this mixing line intersects the PFM or EMM curves. 

The mixing fraction is calculated from the distance that a sample plots along the mixing line between the two end members.

Monitoring-well samples showing evidence of mixing 

were treated as being mixtures of young piston-flow water 

recharged during a particular year and old water containing no 

tracers (BMM). The BMM was selected for monitoring wells 

because, away from the immediate effects of pumping wells, 

ground-water flow is unlikely to be a mixture of ground water 

of all ages in the aquifer. Rather, it was thought to be more 

hydrologically realistic that samples from monitoring wells 

may reflect mixing of young water with a narrow distribution 

of ages, approximated as a piston-flow age, and “old” water 

that may linger in parts of the aquifer with relatively slow flow 

or in clayey sediment.

In the case of supply wells, which pump at high rates 

from screens that cover nearly half the saturated thickness, 

mixing of water of many different ages drawn to the supply 

well is expected. For this situation, the exponential-mixing 

model (EMM) could approximate the mixing of different 

water that would occur in the supply well. However, tracertracer plots indicated that samples from supply wells did not 

34 Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04

Figure 2–2. Concentration of chloroflorocarbon-12 compared to tritium concentration in ground-water samples collected from monitoring 

and supply wells near Seward, Nebraska, and values indicated by piston-flow and exponential-mixing models.

FP1–90

FP2–105

FP1–105

FP3–105

FP4–105

FP1–123

FP2–125

FP3–125

FP4–129

1979

1985

1980

1990

FP4–129

FP4–129

W07

W09 W10

W11

0

10

20

30

40

0 100 200 300

Chlorofluorocarbon-12 concentration, in parts per trillion by volume

Tritium concentration, in tritium units

400 500 600

EXPLANATION

1979

1980

FP4–105

Piston-flow model (PFM)

Exponential-mixing model (EMM)

Binary-mixing line (BMM)

Ground-water sample and well identifier

 (table 1)

PFM and binary-mixing line intersection

 point selected for mixing calculations

EMM and binary-mixing line intersection

 point selected for mixing calculations
plot along lines predicted by the EMM model (fig. 2–2). However, the supply-well samples plot along mixing lines between 

the EMM and zero point of the graphs; these data are consistent with tracer concentrations representing mixtures of 

exponentially mixed young water mixed with “old” water 

(EMM with binary mixing, EBMM). Such mixing is not surprising considering that the aquifer is relatively thin (50 ft) and 

has a relatively high density of wells (4.75 irrigation wells per 

square mile plus supply wells), indicating that withdrawals are 

relatively large. High pumping stress has the effect of increasing ground-water velocities and mixing of ground water. 

Because a higher degree of mixing for supply wells was considered more reasonable than for monitoring wells, which are 

pumped infrequently and at very low rates, the EBMM model 

was considered to be conceptually most reasonable for supply 

wells, whereas the BMM model was considered most reasonable for monitoring wells. In reality, the mean ages and mixing 

fractions for these two models were fairly similar. Results for 

both models are presented in table 1.

Near Seward, the resolution of the CFC method may not 

be optimal because the depth to the water table of 75 to 96 ft 

was large enough that CFC and SF6 concentrations at the water 

table may be in equilibrium with atmospheric concentrations 

from previous years rather than the current year (Solomon 

and others, 1998). The apparent CFC age is based on the 

assumption that infiltration water is in equilibrium with the 

atmosphere at the time of recharge. As the thickness of the 

unsaturated zone increases, there is increasing likelihood that 

the CFC concentration in soil gas at the bottom of the unsaturated zone reflects an older atmospheric concentration rather 

than the current atmospheric concentration. This lag time for 

atmospheric signals to move through the unsaturated zone 

may cause apparent CFC ages to be older than the true age of 

recharge. The 3H/3He technique is not affected by the lag time 

for atmospheric air to move through the unsaturated zone. The 

results of Cook and Solomon (1995) indicate that for similar 

depth to water the lag time for CFC–12 and CFC–113 could 

be 6 to 9 years and for CFC–11 could be 9 to 13 years.

Figure 2–3. Concentration of sulfur hexafluoride compared to tritium concentration in ground-water samples collected from monitoring 

wells near Seward, Nebraska, and values indicated by piston-flow and exponential-mixing models.

Appendix 2 35

FP1–90

FP2–105

FP1–105

FP3–105

FP2–125

FP3–125

FP4–129 FP4–129

0

10

20

30

40

0 1.0 2.0 3.0

Tritium concentration, in tritium units

4.0 5.0 6.0

EXPLANATION

FP4–105

Piston-flow model (PFM)

Exponential-mixing model (EMM)

Ground-water sample and well identifier

 (table 1)

Sulfur hexafluoride concentration, in parts per trillion by volume
Comparison of interpreted ages from CFC–12 and 3

H/3He 

tracers indicated that CFC–12 mean ages range from -1 to 

20 years older than 3H/3He ages, with an average difference 

of 9 years (fig. 2–4). The differences between CFC–12 and 

3

H/3He ages were greater for relatively shallow water samples. 

However, the magnitude of the difference varied between sites, 

and there was not a difference in age using these two tracers 

from one sample. The average difference in age of 9 years 

between these tracers may reflect primarily the effect of thick 

unsaturated zones in causing CFC–12 ages to be older than 

3

H/3He ages. However, the difference in PFM-based ages of 

as much as 20 years in some shallow and intermediate depth 

samples was not expected and may not be caused entirely by 

the thick unsaturated-zone effect.

The differences between the two methods indicate that 

ages have uncertainties of approximately 10 years and as 

much as 20 years in some samples. Although 3H/3He ages are 

preferred when available because they are less susceptible to 

depth-to-water effects, other 3He sources or processes could 

be affecting 3H/3He ages, and uncertainties need to be kept 

in mind.

Additional He sources, terrigenic He, may be present in 

aquifers where the rocks are enriched in uranium or thorium, 

or in ground-water samples in which young water has mixed 

with relatively old water containing terrigenic, and in some 

cases, mantle He (Schlosser and others, 1989). Terrigenic He 

was detected in seven of the nine Seward ground-water samples submitted for analysis (table 6, rightmost column, positive 

values indicate terrigenic He present in quantifiable amounts; 

negative values indicate terrigenic He less than quantifiable 

36  Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04

amounts). Terrigenic He was indicated by concentrations of 

4

He greater than about 7 x 108 cm3/g at Standard Temperature 

and Pressure. Terrigenic He accounted for 2.7 to 27.4 percent 

of the He in the samples where it was detected. The amount 

of terrigenic He generally increased with depth in the aquifer. 

Corrections to the amount of 3He derived from decay of 3H 

rather than terrigenic 3He were made by the Lamont-Doherty 

Laboratory; however, these corrections have uncertainties and 

have the effect of increasing the amount of uncertainty in the 

estimated age (table 6). Sample FP2–105 had a large neon 

concentration suggesting air contamination of the sample. 

3

H/3He data consequently were not used for age interpretation. 

Sample FP3–105 plotted off of curves of the ratio of measured 

tritium to calculated initial tritium concentration compared 

to 3H concentration (fig. 2–1); the sample had smaller concentrations of 3

H and 3He derived from tritium than would be 

expected for samples with its measured CFC and SF6 concentrations. The CFC–12 ages were preferred over the 3

H/3He 

estimates for this sample because the sample results were 

anomalous for 3H/3He.

Naturally occurring SF6 sources may have affected 

some of the SF6 results, particularly in deeper ground-water 

samples. SF6 occurs naturally in fluid inclusions in some 

minerals and igneous rocks, and in some volcanic and igneous fluids (Busenberg and Plummer, 2000). Concentrations 

of SF6 in wells FP3–125 and FP4–129, near the bottom of the 

aquifer, were higher than values expected on the basis of other 

tracers (fig. 2–3). Similar enrichment of SF6 in deeper parts of 

the aquifer has been detected in samples collected from York, 

Nebraska, as well (unpublished data on file with the USGS, 

Lincoln, Nebraska). 
Appendix 2 37

Figure 2–4. Ground-water mean recharge dates determined using tritium/helium and chlorofluorocarbon-12 

techniques in ground-water samples collected from monitoring wells near Seward, Nebraska.

1960 1970 1980 1990 2000 2010

130

120

110

100

90

80

Apparent recharge date of sample or mean recharge date of mixture

Depth to bottom of well, in feet below land surface

EXPLANATION

Sample age−Piston-flow model using

 ratio of tritium to helium-3 (

3

H/3He)

Sample age–Piston-flow model using

chlorofluorocarbon-12 (CFC–12)

Binary mixture–Mean age of young fraction

 using ratio of tritium to helium-3 (

3

H/3He)

Binary mixture–Mean age of young fraction

 using chlorofluorocarbon-12 (CFC–12)
Prepared by Lawrence Publishing Service Center.

Edited by Lanna Combs.

Illustrations and cover design by Jeff Hartley and Mike Kemppainen.

Layout and design by Kristi Hartley.

For more information concerning the research described in this report, 

contact:

U.S. Geological Survey

5231 South 19th Street

Lincoln, NE 68512

(402) 328–4100

http://ne.water.usgs.gov
   Report Title Printed on recycled pape

r

Stanton and others—Ground-Water Age and Quality in the High Plains Aquifer near Seward, Nebraska, 2003–04 —Scientific Investigations Report 2007–5088