science for a changing world

Prepared in cooperation with the 

UNIVERSITY OF NEBRASKA-LINCOLN 

and the CITY OF LOUP CITY, NEBRASKA

Use of Water Chemistry, Isotopes, and 

Chlorofluorocarbons to Investigate the Sources 

of Ground Water Beneath Loup City, Central 

Nebraska, 2000

Water-Resources Investigations Report 02-4049

U.S. Department of the Interior 

U.S. Geological Survey

U.S. Department of the Interior 

U.S. Geological Survey

Use of Water Chemistry, Isotopes, 

and Chlorofluorocarbons to 

Investigate the Sources of Ground 

Water Beneath Loup City, Central 

Nebraska, 2000

By G.V. STEELE and F. EDWIN HARVEY 

Water-Resources Investigations Report 02-4049

Prepared in cooperation with the 

UNIVERSITY OF NEBRASKA-LINCOLN and the 

CITY OF LOUP CITY, NEBRASKA
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute endorsement by the 

U.S. Geological Survey.

Lincoln, Nebraska: 2002

For additional information write to:

District Chief 

U.S. Geological Survey 

406 Federal Building 

100 Centennial Mall North 

Lincoln, NE 68508

Copies of this report can be purchased from:

U.S. Geological Survey 

Information Services 

Building 810

Box 25286, Federal Center 

Denver, CO 80225-0286
CONTENTS

Abstract..................................................................................................._ 1

Introduction............................................................................................_^ 2

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

Study Area .......................................................................... 2

Previous Investigations............................................................................................................................................. 4

Surface Water and Hydrogeology............................................................................................................................. 5

Acknowledgments.................................................................................................................................................... 6

Methods......................................................................................................................................................^ 7

Site Selection.....................................................................................................^^ 7

Site Identification...................................................................................................................................................... 7

Sample Collection..................................................................................................................................................... 13

Sample Analysis....................................................................................................................................................... 13

Chlorofluorocarbons and Ground-Water Age Dating..................................................................................... 13

Isotopes........................................................................................................................................................... 15

Oxygen-18 and Deuterium ................................................................................................................... 15

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

Quality Assurance and Quality Control.......................................................................................................... 17

Data-Analysis Procedures......................................................................................................................................... 17

Results of Data Analyses.................................................................................................................................................... 18

Physical Characteristics............................................................................................................................................ 18

Chemical Characteristics.......................................................................................................................................... 19

Age of Ground Water................................................................................................................................................ 26

Isotopic Data............................................................................................................................................................. 28

Oxygen and Hydrogen.................................................................................................................................... 28

Tritium............................................................................................................................................................ 31

Sources of Ground Water.................................................................................................................................................... 31

Summary............................................................................................................................................................................. 33

References...............................................................................................^^ 34

Appendix.. ..................................................................................................................................................................^ 37

FIGURES

1. Maps showing location of study area, central Nebraska........................................................................:............. 3

2. Map showing locations of surface-water and ground-water sampling sites and generalized geologic section

in study area......................................................................................................................................................... 8

3. Map showing configuration of water table in study area, before construction of Sherman Reservoir and

during 2000 .......................................................... 10

4. Schematic diagram of well-numbering system.................................................................................................... 12

5. Graphs showing atmospheric concentrations of Chlorofluorocarbons CFC-11, CFC-12, and CFC-113 in 

North American air and tritium concentrations in precipitation from Ottawa, Ontario, Canada, and Lincoln, 

Nebraska............................................................................................................................................................... 14

6. Graph showing mixing of ground-water source with an evaporated water source, such as a reservoir............... 16

7. Graph showing depth to water table in selected Bureau of Reclamation observation wells in study area.......... 19

8. Box plots showing distribution of physical properties and water-quality constituent concentrations by

source in study area, 2000.................................................................................................................................... 22

9. Trilinear diagram showing ionic composition of surface water and ground water in study area, 2000............... 25

10. Graphs showing oxygen (6 18O) and stable hydrogen (82H) composition of water samples from study area

for March and April 2000, and July 2000............................................................................................................ 29

11. Graphs showing tritium concentrations in water samples collected from study area in 2000 and back

calculated to estimate historic concentrations...................................................................................................... 32

Contents iii
TABLES

1. Geohydrologic characteristics of study area in central Nebraska......................................................................... 6

2. Sites selected for surface-water sampling and construction data for wells sampled in study area, central

Nebraska.......................................................................................................................................................^ 11

3. Results of measurements of physical properties in surface and ground water in study area, central Nebraska,

2000..................................................................................................^^ 20

4. Minimum, maximum, and quartile constituent concentrations by water source in study area, central Nebraska, 

2(X)0.........................................................................^^ 24

5. Results of chlorofluorocarbon age-dating analyses of water samples in study area, central Nebraska,

2(X)0....................................................................................................................................^ 27

6. Averaged modeled age of water samples from study area in central Nebraska analyzed for chlorofluorocarbons and tritium, 2000..................................................................................................................................... 28

7. Isotopic chemistry data for water samples collected in study area, central Nebraska, 2000................................ 30

8. Results of water-quality analysis for major ions in water samples from study area, central Nebraska,

2000...................................................................................................................................................................... 39

9. Results of water-quality analysis for nutrients in water samples from study area, central Nebraska,

2(X»................................................................_ 42

CONVERSION FACTORS AND VERTICAL DATUM

Multiply

inch (in.)

foot (ft)

mile (mi)

acre

square foot (ft2)

gallon (gal)

million gallons (Mgal)

acre-foot (acre-ft)

cubic foot per second (ft3/s)

gallon per minute (gal/min)

foot per day (ft/d)

By

Length

2.54

0.3048

1.609

Area

4,047

0.09290

Volume

3.785

3,785

1,233

Flow rate

0.02832

0.06309

0.3048

To obtain

centimeter

meter

kilometer

square meter

square meter

liter

cubic meter

cubic meter

cubic meter per second

liter per second

meter per day

Hydraulic gradient

foot per foot 0.3048 meter per meter 

________foot per mile (ft/mi)_____0.1894______meter per kilometer

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)71.8.

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 

1929) a geodetic datum derived from a general adjustment of the first-order level nets of both the 

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

Altitude, as used in this report, refers to distance above or below sea level.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (uS/cm at 25 °C).

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

micrograms per liter (jig/L).

iv Use of Water Chemistry, Isotopes, and Chlorof luorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to 

Investigate the Sources of Ground Water Beneath Loup City, 

Central Nebraska, 2000

ByG.V. Steele 1 and?. Edwin Harvey2 

Abstract

Ground-water levels near Sherman Reservoir 

have risen locally more than 85 feet since the 

Bureau of Reclamation (U.S. Department of the 

Interior) constructed the reservoir in 1962. Loup 

City, about 2.5 miles west-southwest of Sherman 

Reservoir, has experienced high ground-water 

levels in parts of the city. Local officials have 

attributed these high ground-water levels to 

seepage from Sherman Reservoir. In 2000, the 

U.S. Geological Survey, the University of 

Nebraska-Lincoln, and the city of Loup City 

began a cooperative study to investigate the 

sources of the ground water beneath Loup City. 

As part of this study, 24 surface- and groundwater sites were selected and sampled for physical 

properties, major ions, nutrients, and selected 

isotopes oxygen-18, deuterium, and tritium. In 

addition, water samples from a subset of these 

sites were analyzed for Chlorofluorocarbons to 

estimate the age of the water.

Results of analysis of surface-water and 

ground-water samples showed that water from 

both sources was similar and predominantly a 

calcium bicarbonate type. Evaporation from 

Sherman Reservoir does not appear to be concentrating chemical species in the lake. However, 

water varied more in chemistry between ground1 U.S. Geological Survey, Lincoln, Nebraska.

2 University of Nebraska-Lincoln, School of Natural 

Resource Sciences and Conservation and Survey Division, 

Lincoln, Nebraska.

water sites than between surface-water sites. 

Most domestic wells and the observation wells in 

the alluvial aquifer underlying Loup City had 

large calcium concentrations. Bureau of Reclamation observations wells, on the other hand, typically contained larger sodium concentrations.

Ground-water age dating indicated that the 

modeled age of the ground water underlying Loup 

City ranged from the mid-1940s to the early 

1960s. Analyses of water samples from a domestic well near Sherman Reservoir indicated that 

recharge occurred during the mid-1970s. Except 

for this single well, all other data indicated that 

although seepage of water from Sherman Reservoir locally has changed the regional groundwater flow paths, the ground water beneath Loup 

City predates construction of Sherman Reservoir.

Oxygen-18 and deuterium data indicated that 

there is substantial evaporative enrichment at 

Sherman Reservoir. However, this enrichment 

was not detected in most ground-water samples 

from wells in Loup City. In the two wells in Loup 

City that did show a strong evaporative signature, 

the source of the water likely was from a nearby 

canal and not Sherman Reservoir.

The sources of the water beneath Loup City 

cannot be accurately determined from available 

data. However, from analysis of the water chemistry and isotope data, it appears that the water 

underlying Loup City does not show evidence of a 

Sherman Reservoir component.

Abstract 1
INTRODUCTION

In 1962, the Bureau of Reclamation (BOR), 

U.S. Department of the Interior, completed Sherman 

Reservoir, about 2.5 mi east northeast of Loup City, 

Sherman County, Nebraska (fig. 1). Sherman Reservoir is used primarily to supply irrigation water to the 

Farwell Irrigation District and used secondarily for 

recreational purposes. Sherman Dam, which impounds 

the water in the reservoir, is "a homogeneous rolled 

earth-filled structure," 4,450 ft long at the crest and 

134 ft high (Nebraska Game and Parks Commission, 

2000). Sherman Reservoir creates about 65 mi of 

shoreline with a surface area of about 2,878 acres and 

a storage pool of about 69,076 acre-ft (Nebraska 

Game and Parks Commission, 2000). Typically, to fill 

Sherman Reservoir, water is diverted in March or 

April from the Middle Loup River at the Arcadia 

diversion dam south of Comstock, Nebraska (Karl 

Wadas and Harold 'Butch' Turek, Nebraska Department of Natural Resources, oral commun., 2000) into 

the 18-mi long Sherman Reservoir Feeder Canal. The 

diversions continue throughout the irrigation season 

(May through September) for subsequent release to 

irrigate about 50,000 acres of farmland downstream of 

Sherman Reservoir. Water also is diverted from the 

Sherman Reservoir Feeder Canal to the Middle Loup 

Public Power and Irrigation District's Canal Number 4 

(herein referred to as Canal 4) that flows through Loup 

City. Canal 4, completed in 1938 (Brown, 1955), is 

used to irrigate land in southwest Valley and Sherman 

Counties.

In 1974, BOR installed two subterranean drains in 

Loup City to address concerns the citizens of Loup 

City and the rural area had regarding rising groundwater levels in and around Loup City. These drains 

were installed to lower ground-water levels in Loup 

City to levels at or below those in existence before 

Sherman Reservoir was constructed.

BOR (Bureau of Reclamation, 1989) reported that 

a combination of above-normal rainfall, seepage 

losses from Canal 4 and its laterals, irrigation use, and 

the potential development of a seepage mound from 

Sherman Reservoir might have contributed to the high 

ground-water levels in the Middle Loup River Valley. 

BOR (Bureau of Reclamation, 1996) reported that 

seepage of surface water from Sherman Reservoir was 

causing ground-water levels to rise in the vicinity of 

Loup City; however, these ground-water-level rises 

were slight, and Loup City had had a history of high 

ground-water levels. The probable cause for the thencurrent high ground-water levels in Loup City was an 

above-normal amount of rainfall that had fallen since 

the early 1990s. BOR (Bureau of Reclamation, 1996) 

maintained that the drains that were installed were 

keeping ground-water levels in Loup City at or below 

pre-Sherman Reservoir construction levels. However, 

officials of Loup City maintain that these drains were 

not lowering ground-water levels sufficiently. The 

U.S. Geological Survey (USGS), in cooperation with 

the University of Nebraska-Lincoln and the city of 

Loup City, began a study in 2000 to investigate the 

sources of ground water beneath Loup City. The techniques used in this investigation could help communities having similar conditions.

Purpose and Scope

The purpose of this report is to present the results 

of the study to determine if the probable sources of 

ground water beneath Loup City, Nebraska, are from 

Sherman Reservoir or other surface-water features. 

This report describes (1) methods used to select and 

install monitoring wells for water-level and waterquality data collection, (2) results of the water-level, 

water-quality, age-dating, and water isotopic analyses, 

and (3) a discussion of the possible sources of the 

ground water beneath Loup City.

Study Area

The study area (fig. 1) is in the High Plains Section of the Great Plains Province (Fenneman, 1946) 

and is characterized by hot summers and severe 

winters. During 1999, air temperature extremes in 

Loup City ranged from -18 °F in January to 99 °F in 

July (National Oceanic and Atmospheric Administration, 2000). Precipitation during 1999 totaled 26.09 in. 

(National Oceanic and Atmospheric Administration, 

2000) or 0.37 in. above the 30-year normal (1961-90) 

for Loup City. From 1990 to 1999 the departure of 

total annual precipitation from normal varied from 

2.64 in. below normal in 1997 to 13.78 in. above 

normal in 1993 (V.L. McGuire, U.S. Geological 

Survey, written commun., 2001). During this same 

period, below-normal annual precipitation only 

occurred in 1994 and 1997. Precipitation departures 

from normal for 1990 to 1999 indicate a median of 

1.22 in. above normal and a mean of 2.22 in. above 

normal.

2 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska. 2000
98°55'

45°20'

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

Universal Transverse Mercator 

Zone 14

EXPLANATION

 Boundary of study area

 River, creek, canal or draw

Figure 1. Location of study area, central Nebraska.

0 0.5 1.0 MILE

I I-H ' 

0 0.5 1.0 KILOMETER

NEBRASKA

n I Valley County 

Blaine County'1 ' , -/ ,, .

comstock-M^Study area

North Platte. ArcadiaSherman County Lincoln

Index map

Introduction 3
Landforms in the study area are diverse and characterized by a nearly flat, 2-mi-wide alluvial plain of 

the Middle Loup River and rolling loess-mantled hills 

that make up the uplands. Loup City lies on the easternmost edge of the Middle Loup River Valley. However, the most prominent land feature in the study area 

is the loess-mantled hills that flank the Middle Loup 

River Valley. On the east-northeast side of Loup City, 

the loess-mantled hills start as bluffs and continue as 

rolling hills toward the northeast and east toward 

Sherman Reservoir (fig. 1), which lies in Oak Creek 

Valley. The only major surface-water feature lying 

between Sherman Reservoir and Loup City is Dead 

Horse Creek (fig. 1). Near the eastern edge of Loup 

City, Dead Horse Creek, which is ephemeral, has been 

filled in for crop cultivation. Most of the domestic 

wells that lie between Sherman Reservoir and Loup 

City are in Dead Horse Creek Valley.

Previous Investigations

Two reports that predate Sherman Reservoir 

(Brown, 1955, and Sniegocki, 1959) describe the 

regional geology, including the study area, and document the regional hydrogeology. Most of the other 

reports that incorporate the study area are limited to 

studies performed by BOR to document the groundwater conditions in and around Loup City (Bureau of 

Reclamation, 1989,1996,1999). Additional studies on 

general hydrology, geology, or ground-water-flow 

conditions include Miller and Associates Consulting 

Engineers, PC. (1988) and Olsson Environmental 

Sciences, Inc. (1997). Souders (2000) completed a 

regional geologic map (1° latitude by 2° longitude) 

and discussed geologic cross sections and depositional 

history of the region.

Brown (1955) completed a comprehensive preSherman Reservoir study of the Middle Loup River 

Valley from the southern edge of Blaine County to St. 

Paul. Brown reported that the Middle Loup River was 

predominantly a gaining stream in this area. Ground 

water typically moved from the upland areas to the 

Middle Loup River Valley. The only time the Middle 

Loup River was not a gaining stream was during times 

of high stage. Prior to the construction of Sherman 

Reservoir, depths to water in the alluvial sediment 

ranged from less than 10 ft near the Middle Loup 

River to about 60 ft near the margins of the Middle 

Loup River Valley (Brown, 1955). Brown reported that

locally depths to water in the upland areas were 

greater than 150ft.

Sniegocki (1959) reported geologic and groundwater conditions in the Loup River drainage basin, 

which includes the study area and covered a greater 

areal extent than Brown (1955). In addition, Sniegocki 

(1959) identified both water-table and confined conditions in the ground-water systems in the study area. 

Both Brown (1955) and Sniegocki (1959) indicated 

that the Middle Loup River was a gaining river and 

that the regional ground-water-flow direction predominantly was west-northwest to east-southeast. The 

water-table contour maps created by Brown (1955) 

and Sniegocki (1959) used sufficient data to document 

ground-water-flow directions in the Middle Loup 

River Valley. However, both reports lacked sufficient 

wells in the upland areas to characterize upland 

ground-water-flow directions.

In 1973 BOR (Chief, BOR, Drainage Branch, 

written commun., 2000) conducted a study to determine the feasibility of a drainage system in Loup City 

to lower ground-water levels to pre-Sherman Reservoir construction levels. One previous investigation 

that is of particular importance to this study was 

completed by BOR (Bureau of Reclamation, 1989). In 

that report, BOR evaluated ground-water conditions 

from the Middle Loup River to Sherman Reservoir and 

the seepage loss from Canal 4. The BOR study 

included irrigated lands along the Middle Loup River 

Valley several miles north and south of Loup City. Key 

components of the 1989 BOR report included documentation of a ground-water-flow model used by BOR 

to simulate ground-water levels around Sherman 

Reservoir and Loup City. The ground-water-flow 

model used by BOR was a modified version of the 

Illinois State Water Survey's two-dimensional, finitedifference ground-water-flow model developed by 

Prickett and Lonnquist (1971). Results of the BOR 

ground-water-flow model indicated that seepage from 

Sherman Reservoir had developed a mound of water 

that "has traveled out from the reservoir several miles 

and now may, or may not, be influencing the water 

table levels in the study area." In addition, losses of 

water from Canal 4 and recharge to the aquifer from 

local precipitation were evaluated. Constant-head 

ponding tests were used to quantify the loss of water 

from Canal 4 at two separate locations along the canal 

in the vicinity of Loup City. BOR concluded that loss 

of water from Canal 4 was occurring, and this loss was 

contributing seepage to ground water at Loup City.

4 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska. 2000
BOR (Bureau of Reclamation, 1996) evaluated the 

effects of seepage from Sherman Reservoir on the 

water table at Loup City. BOR updated the original 

ground-water-flow model of 1989 by simulation of 

ground-water-flow conditions in the study area 

through construction of a transient, two-dimensional 

ground-water-flow model using the USGS's modular, 

finite-difference ground-water-flow model, 

MODFLOW (McDonald and Harbaugh, 1988). BOR 

concluded that the simulations using the groundwater-flow model indicated that temporal changes in 

ground-water levels in the Loup City area are affected 

by several factors, including local precipitation, the 

Middle Loup River, BOR-installed drains, sewer-line 

infiltration, and Sherman Reservoir. For the current 

study, all of these effects, excluding precipitation, 

were evaluated using chemical and isotopic analyses 

of water samples. During the current study, precipitation amounts were insufficient for collection of 

samples for a complete water-quality analysis.

Surface Water and Hydrogeology

Perennial surface-water features in the study area 

are limited primarily to the Middle Loup River and 

Sherman Reservoir. However, during the irrigation 

season, surface water also is present in Canal 4. The 

Middle Loup River is a braided, sand-bed stream that 

derives most of its flow from ground water. The annual 

mean flow of the Middle Loup River at Arcadia, 

upstream from the study area, from 1962 to 1993, was 

719 ft3/s (Boohar and others, 1994). Flow of the 

Middle Loup River at Loup City was about 1,180 ft3/s 

during March and April 2000 and about 188 ft3/s 

during July 2000.

The Farwell Irrigation District operates Sherman 

Reservoir and maintains a pool (forebay) elevation of 

about 2,155 ft above sea level. The average pool elevation was 2,155 ft in March and April, and 2,157 ft in 

July. Typical depths in the main pool can be as great as 

50 ft. The 2,155-ft pool elevation is about 85 ft higher 

than the ground-water elevation at Loup City. Therefore, a hydraulic gradient of about 0.0064 ft/ft or 

34 ft/mi exists between Sherman Reservoir and 

Loup City.

The principal hydrogeologic units within the study 

area (table 1) are the saturated alluvial deposits of 

Holocene and Pleistocene (Quaternary) age that make 

up the valley fill, the alluvial deposits of Pliocene 

(Tertiary) age that underlie the loess deposits on the

uplands, and the bedrock deposits (Ogallala Formation) of Miocene (Tertiary) age. The Ogallala 

Formation, which underlies the entire study area 

(fig. 2fl), serves as the basal aquifer (herein referred to 

as the Ogallala aquifer), and overlies the Pierre Shale 

of Cretaceous age.

The Ogallala Formation predominantly is fluvial 

material consisting of clay, silt, clayey silt, siltstone, 

sand, sandstone, pebbly sandstone, and gravelly sandstone (Souders, 2000). The sand and sandstone beds of 

the Ogallala Formation can be well sorted, very fine to 

fine, and fine to medium sand, whereas the gravelly 

sandstone tends to be poorly sorted (Souders, 2000).

In the study area, depths to water in wells open to 

the Ogallala aquifer vary from less than 6 ft under 

Loup City to more than 150 ft in the uplands between 

Loup City and Sherman Reservoir. Olsson Environmental Sciences, Inc. (1997) reported a specific yield 

of 0.30, a hydraulic conductivity of 107 ft/d, and an 

effective porosity of 0.30 for the Ogallala aquifer 

(table 1), which likely are typical of the shallowest 

unconsolidated material in the Ogallala aquifer. 

Typical specific-yield and hydraulic-conductivity 

values (0.15 to 0.16 and 14 to 19 ft/d, respectively) of 

the Ogallala aquifer likely are similar to those reported 

by Peckenpaugh and Dugan (1983) in an area about 

18 mi south of the study area. The predominant 

ground-water-flow direction in the upland areas prior 

to the completion of Sherman Reservoir was southsoutheasterly to east (fig. 3) (Brown, 1955). Groundwater flow in upland areas near the Middle Loup River 

Valley was south to southwesterly and toward the 

Middle Loup River. Following the completion and 

filling of Sherman Reservoir, the predominant groundwater-flow direction changed to west to southwesterly 

in areas between Sherman Reservoir and Loup City 

(fig. 3) (Bureau of Reclamation, 1989). In areas 

directly downstream of Sherman Reservoir, groundwater flow was south-southeasterly to east. During this 

study, water levels in the Ogallala aquifer rose locally 

about 30 ft above the contact between the Ogallala 

aquifer and the overlying alluvial sand. This resulted 

in a small, but measurable (0.20 ft), upward groundwater head difference between the Ogallala and alluvial aquifers directly underlying Loup City.

In the uplands between Loup City and Sherman 

Reservoir undifferentiated loess, eolian sand, and alluvial deposits of Quaternary age and undifferentiated 

loess and alluvial deposits of Tertiary age (table 1) 

directly overlie the Ogallala Formation. The loess

Introduction
Table 1. Geohydrologic characteristics of study area in central Nebraska

[Geohydrologic characteristics from Souders (2000); ft, feet; <, less than; >, greater than; gal/min, gallons per minute; ft/d, feet per day; specific-yield and 

hydraulic-conductivity values from Peckenpaugh and Dugan (1983) and Olsson Environmental Services, Inc. (1997)]

System Series Geologic unit

Maximum 

thickness (ft) Hydrologic characteristics

Holocene Alluvium 50 to 100

Quaternary

Silt to very coarse sand and some gravel. Yields to 

wells about 1,000 gal/min. Typical values for 

specific yield, 0.20; hydraulic conductivity, 

54 ft/d; and effective porosity, 0.25.

Pleistocene Eolian sand <100 Not considered a major water source.

Loess <40 Not considered a major water source.

Loess <50 Not a major water source, could serve as a local 

confining unit.

Pliocene

Alluvium <50

Tertiary

Silt to very coarse sand and some gravel. Yields to 

wells may range from 500 to 1,000 gal/min. 

Values for specific yield, hydraulic conductivity, and effective porosity probably similar to 

alluvium of Quaternary age.

Miocene Ogallala Formation > 300

Silt to sand and some gravel and sandstone. Yields 

to wells about 500 to 1,000 gal/min. Typical 

values may range from 0.15 to 0.30 for specific 

yield; 14 to 100 ft/d for hydraulic conductivity, 

and 0.20 to 0.30 for effective porosity. Largest 

values of specific yield and hydraulic conductivity likely associated with shallowest unconsolidated material.

Cretaceous Upper Cretaceous Pierre Shale <100 Not a major water source, serves as a basal 

confining unit.

deposits typically are unsaturated, but locally seepage 

from Sherman Reservoir probably has filled some of 

the fractures in the loess. The thickness of the 

Pliocene-age alluvium has not been measured, but it is 

thought to be less than 50 ft. Depths to water in these 

alluvial deposits could not be quantified because 

driller's logs were unavailable or not sufficiently 

detailed for the domestic wells between Loup City and 

Sherman Reservoir. However, because some observation wells in the uplands are only about 150 ft deep, it 

is likely that they were screened in the first saturated 

sand and gravel deposits. No determination has been 

made for the specific-yield and hydraulic-conductivity 

values for these alluvial deposits. However, because 

these alluvial deposits are similar to the deposits of the 

alluvial aquifer in the Middle Loup River Valley, they 

likely have similar hydraulic properties.

The alluvial deposits that provide the fill for the 

Middle Loup River Valley consist of gravel, sand, and 

lesser amounts of silt and clay (Souders, 2000). Depth 

to water in these deposits, which underlie most of 

Loup City, ranges from less than 3 ft near the Middle 

Loup River to more than 10 ft near the break in slope

of the uplands at Loup City. In March 2000, the depth 

to ground water was about 6 ft at the western edge of 

Loup City. Olsson Environmental Services, Inc. 

(1997) reported a specific yield of 0.20, a hydraulic 

conductivity of 54 ft/d, and an effective porosity of 

0.25 for the alluvial aquifer (table 1). Although the 

values reported by Olsson Environmental Services, 

Inc. (1997) are mostly site specific (in Loup City), 

these values are considered typical of the alluvial 

aquifer as a whole. The predominant ground-waterflow direction both before and since the completion of 

Sherman Reservoir is along and toward the Middle 

Loup River.

Acknowledgments

The authors acknowledge the citizens and staff of 

Loup City and the staff of the Farwell Irrigation 

District for their help during the study. Special appreciation is extended to the Honorable Mayor Norman 

Woitalewicz, Mark Eureck, and Robert Piontkowski, 

Loup City, and Darwin Lee, Farwell Irrigation District

6 Use of Water Chemistry. Isotopes, and Chlorof luorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
Superintendent, for their help in locating observation 

wells and multi-level monitoring-well sites.

Special appreciation also is extended to the landowners who provided access to their domestic wells 

for the collection of water samples: Harold and Evelyn 

Janulewicz, Lanny and Sharon Kizer, Norman and 

Donna Lewandowski, and Mrs. Mildred Norseen. 

Appreciation is extended to Kenneth Martin, Supervisor, Nebraska Game and Parks Commission, Sherman 

Reservoir Recreation Area, for providing access to the 

domestic well near Sherman Reservoir.

METHODS

Surface-water and ground-water samples were 

collected concurrently from 24 selected sites during 

two sampling periods March/April and July 2000. 

This section describes the site selection and analytical 

methods used to collect these data during this study.

Site Selection

Four surface-water sites were selected for 

sampling Middle Loup River at Loup City, Sherman 

Reservoir (two sites), and Canal 4. These sites were 

chosen because water from one or more of them possibly contributes to ground water beneath Loup City. 

Because Sherman Reservoir is large, two sites were 

selected for sampling one in the primary pool, and 

another in an arm located close to Loup City. In addition, each of these two sites was sampled at three 

depths near the bottom of the reservoir (deep), near 

the top of the pool (shallow), and approximately halfway between these two depths (intermediate).

Twenty-four ground-water wells (table 2) (including two monitoring-well sites and Loup City's sewerlift station discussed next) at 20 sites were selected on 

the basis of specific well types and location domestic 

(5 wells), observation (BOR, 6 wells, and Loup City, 

5 wells), monitoring-well sites (2), and public supply 

(1 well). All five domestic wells are in the uplands 

between Loup City and Sherman Reservoir. Seven of 

the 11 BOR and Loup City observation wells and the 

single public-supply well are in Loup City. The 

remaining four BOR observation wells are between 

Loup City and Sherman Reservoir.

In addition to the existing wells, two sites in Loup 

City were selected for installation of multi-level monitoring wells. The multi-level monitoring wells each

contained three wells open at different depths deep, 

intermediate, and shallow. Each deep well was open 

just below the top of the Ogallala Formation, whereas 

the intermediate and shallow monitoring wells in each 

cluster were open at the base (intermediate) and the 

top (shallow) of the overlying saturated alluvial 

deposits. These multi-level monitoring wells enabled 

the collection of vertically distributed ground-water 

samples.

One additional site was chosen for sampling. Loup 

City officials stated that ground water seeps through 

cracks in the storm-sewer system, and their system is 

overtaxed with respect to the volume of water the 

system treats. Therefore, Loup City's sewage lift 

station was chosen to represent the ground water 

leaving the storm-sewer drainage system. The lift 

system drains both the sanitary sewer and the stormsewer systems of Loup City.

To differentiate the sample sites for comparison, 

they were divided into three classifications or 

sources surface water, alluvial aquifer (which 

included the Loup City sewer), and Ogallala aquifer. 

On the basis of its location and depth, each groundwater site was classified either as alluvial or Ogallala. 

The alluvial classification was used to designate those 

monitoring wells that were open to the alluvium in the 

Middle Loup River Valley. The Ogallala classification 

was used to designate the predominant formation of 

the uplands. BOR observation wells located in the 

uplands may not be open to the Ogallala aquifer; 

however, they were included in the Ogallala classification because: (1) they are in the uplands and overlie 

the Ogallala aquifer and (2) their open interval and the 

unit in which they are open are not known. Ogallala 

monitoring wells also included the deeper monitoring 

wells open to the Ogallala aquifer but located in 

Loup City, as well as Loup City's public-supply well.

Site Identification

This report uses three different methods of site 

identification (table 2). The first method uses the 

USGS site identification number. USGS assigns a 

unique number to each site (such as 06779500 for 

surface-water sites and 411706098582201 for groundwater sites).

For surface-water sites, the eight-digit number, 

sometimes extended up to 11 digits, is assigned by 

downstream direction along the main stream. The first 

two digits "06" represent the major river basin; which

Methods
98°55'

45°20'

45°15'

Canal Number 4

LCPS1 

BOB LC-20 

LCMW2

LC11-J

BOR ROL-4 

LC10-H I / 

riVBOR LC-52 5lC9-H-Ax ' 

LC9-H

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

Universal Transverse Mercator 

Zone 14 EXPLANATION ° °-5 1.0 KILOMETER

 Trace of section shown in Sampling sites and identifiers 

figure 2B Canal Number 4A Surface-water site

DW-Sa Domestic well 

^ Boundary of study area LC11 _j , ., , .. LL "^1 * Loup City observation well

LCMW1 o Multi-level monitoring well 

BORRR-2 a Bureau of Reclamation observation well 

LCPS1 T Public-supply well

River, creek, canal or draw

Well used for generation 

of cross section

Figure 2. (A) Locations of surface-water and ground-water sampling sites and (B) generalized geologic section in study area.

8 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
(feet) 

2,250 -i

2,200 -

2,150-

2,100-

2,050 -

2,000 -

1,950

Middle Loup 

River Valley

Vertical scale greatly exaggerated

Datum is sea level

1.0 MILE

2,050

2,000

1,950

1.0 KILOMETER

System 

Quaternary

L

Tertiary

L

EXPLANATION

Geologic unit

Alluvium Holocene and Pleistocene age

Undifferentiated loess, eolian sand, and alluvium Pleistocene age

Undifferentiated loess and alluvium Pliocene age

Ogallala Formation Miocene age

. _ _ Geologic contact Dashed where inferred 

^ _ _ Water level (2000) Dashed where inferred

* Location of test hole from unpublished Bureau of Reclamation 

data on file with University of Nebraska-Lincoln Conservation 

and Survey Division

BIS Bend in section

Note: Depths to Pliocene-aged deposits in the uplands were derived from Bureau of Reclamation cores drilled prior to

construction of Sherman Reservoir (unpublished data on file with University of Nebraska-Lincoln Conservation and 

Survey Division). Top of Ogallala Formation modified from Souders (2000).

Figure 2. (A) Locations of surface-water and ground-water sampling sites and IB) generalized geologic section in study 

area Continued.

in this case is the Missouri River Basin. The next six 

digits represent the downstream order number. 

The USGS ground-water site identification 

number is a 15-character number that contains no 

blanks or alphabetical characters and generally is used

as an internal control number. Although the site identification number was initially formed from a latitude 

and longitude, the number is an identifier and has no 

locational significance (Mathey, 1990, p. 2-10). Wells 

with the first 13 characters of the site identification

Methods
98°55'

45°20' V

45° 15

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

Universal Transverse Mercator 

Zone 14

EXPLANATION

|_____\ No data for 2000 water levels 

_^__ Boundary of study area

 * Generalized direction of ground-water flow, 2000

 River, creek, canal, or draw

 2080 Water-table contour Shows the altitude of the water 

table before construction of Sherman Reservoir 

(Brown, 1955; Sniegocki, 1959). Contour interval 

20 feet. Datum is sea level

-2160 Water-table contour Shows the altitude of water 

table, 2000. Dashed where inferred. Contour 

interval 20 feet. Datum is sea level

0 0.5 1.0 KILOMETER

 Ground-water-level control point, 

2000

	Sampling sites

A Surface-water site

® Domestic well

4 Loup City observation well

O Multi-level monitoring well

D Bureau of Reclamation observation well

 » Public-supply well

A Sewer

Figure 3. Configuration of water table in study area, before construction of Sherman Reservoir and during 2000.

10 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
Table 2. Sites selected for surface-water sampling and construction data for wells sampled in study area, central Nebraska

[all Bureau of Reclamation (BOR) well depths are estimated from soundings made during study; USGS, U.S. Geological Survey; ID, identification number 

or identifier; fig., figure; D, deep; M, intermediate; S, shallow; --, not measured or not applicable; EWI, equal-width integrated sample; No., number; <, less 

than; SW, southwest; PSG, thought to be screened in the Pliocene-age sand and gravel deposits]

USGS 

site ID

Field ID 

(fig. 2) Local ID Well type

Depth of 

sample or 

well (feet 

below land 

surface)

Open 

interval 

(feet) Aquifer

Well 

diameter 

(inches)

Depth to 

water 

(feet 

below 

land 

surface)

Surface-water sites

06779500

06778450

06784195001

06784195002

06784195003

06784190001

06784190002

06784190003

Middle Loup River

Canal 4

Sherman Reservoir 1-D

Sherman Reservoir 1-M

Sherman Reservoir 1-S

Sherman Reservoir 2-D

Sherman Reservoir 2-M

Sherman Reservoir 2-S

Middle Loup River at Loup City

Canal No. 4 at Loup City

Sherman Reservoir 1,200 feet

Sherman Reservoir 1 ,200 feet

Sherman Reservoir 1,200 feet

Sherman Reservoir SW Branch

Sherman Reservoir SW Branch

Sherman Reservoir SW Branch

-

~

~

-

-

--

-

EWI

EWI

45

25

<5

25

15

<5

-

-

~

-

--

 . ~

-

-

--

-

--

--

--

-

--

--

-

--

--

..

--

--

-

-

--

--

--

-

-

Ground-water sites

411706098582201

411700098580201

411912098544801

411728098574001

411833098555701

411649098541401

411747098562001

411723098562801

411701098565001

411648098542401

411709098540101

411656098581701

411648098582301

411654098581501

411652098581201

411648098581301

411637098581001

411637098581002

411637098581003

411656098582701

411656098582702

411656098582703

411706098581001 

411619098581001

BOR LC-20

BOR LC-52

BOR ROL-3

BOR ROL-4

BOR RR-2

BOR RR-3

DW-1

DW-2

DW-3

DW-4

SRGP-1

LC10-H

LC11-J

LC9-H-A

LC9-I

LC9-J

LCMW1-D

LCMW1-M

LCMW1-S

LCMW2-D

LCMW2-M

LCMW2-S

LCPS-1 

LCLS-1

15N 14W 7CBDA1

15N14W7CA 1

16N 14W34BBBB1

15N14W7AB 1

16N 14W33CCCC1

15N 14W10DCCD1

15N 14W 5DDAA1

15N 14W 8ADB 1

15N 14W 8DCBB1

15N 14W10DCCA1

15N 14W10ADDD1

15N 14W 7CDA 1

15N14W7CCBD1

15N 14W 7CDBD1

15N 14W 7CDCA1

15N 14W 7CDCD1

15N 14W18BADB1

15N 14W18BADB2

15N 14W18BADB3

15N14W7CCAB1

15N 14W 7CCAB2

15N14W7CCAB3

15N14W7CABB1 

15N 14W18BDDC1

Observation

Observation

Observation

Observation

Observation

Observation

Domestic

Domestic

Domestic

Domestic

Domestic

Observation

Observation

Observation

Observation

Observation

Monitoring

Monitoring

Monitoring

Monitoring

Monitoring

Monitoring

Public supply 

Sewer2

25

30

140

90

150

140

202

184

194

200

194

30

30

20

24

25

55

36

23

56

36

23

336

-

--

--

~

--

--

-

150-194

180-200

174-194

23-28

25-30

15-20

15-20

20-25

50-55

31-36

18-23

51-56

31-36

18-23

196-336

Alluvial

PSG1

PSG

PSG

PSG

PSG

Ogallala

Ogallala

Ogallala

Ogallala

Ogallala

Alluvial

Alluvial

Alluvial

Alluvial

Alluvial

Ogallala

Alluvial

Alluvial

Ogallala

Alluvial

Alluvial

Ogallala

1.25

1.25

1.25

1.25

1.25

1.25

4.0

4.0

4.0

4.0

4.0

2.0

2.0

2.0

2.0

2.0

4.0

4.0

4.0

4.0

4.0

4.0

16

8.53

16.64

42.08

56.35

66.59

113.60

--

-

--

-

-

5.12

5.86

5.87

7.61

6.74

6.84

7.01

6.97

5.26

6.06

6.04

-

*May be screened in loess deposits. 

Lift station with subterranean storm and sanitary sewage ports.

Methods 11
number in common are distinguished by adding a 

sequential digit to the number. Sequential numbers 

are assigned by order of inventory such as 

411637098581001 and 411637098581002.

The second method, field identifier, is associated with the location of the surface-water or 

monitoring-well site, the geographical name, or 

the owner or type. For surface-water sites, the 

geographical name was used such as Middle Loup 

River and Canal 4. Because Sherman Reservoir is 

large, two sampling sites were selected on the reservoir Sherman Reservoir 1 and Sherman Reservoir 

2. The number T represents the site located in the 

main pool of the reservoir, whereas the number '2' 

represents the site in an arm off of the main pool but 

toward Loup City.

For ground-water sites, field identification 

numbers assigned by BOR kept their original field 

identification number (such as BOR LC-20 and 

BOR LC-52). All domestic wells, except for the 

Nebraska Game and Parks well, were assigned DW 

to indicate domestic wells. The terminal numeric 

sequence for the domestic wells was assigned by 

order of inventory DW-1 to DW-4. The remaining domestic well was assigned SRGP-1 because it 

is near Sherman Reservoir and is owned by the State 

of Nebraska Game and Parks Commission. Observation wells owned by Loup City were assigned a 

preceding LC and terminated with a number-letter 

sequence representing the nearest street intersection. 

Thus, LC9-J represented an observation well located 

in Loup City at the intersection of 9th and J Streets. 

One observation well, LC9-H-A, was in an alley near 

the intersection of 9th and H Streets and was assigned 

the terminal letter 'A.' Loup City's public-supply well 

and sewer lift station were assigned LCPS-1 and 

LCLS-1, respectively.

The two multi-level monitoring-well sites that 

were installed for this study were assigned a field identification number similar to all other Loup City 

wells LCMW1 and LCMW2. The field identification 

numbers started with LC to designate the owner, but 

MW was added to designate them as multi-level monitoring wells. Terminal designations for the sites were 

1 or 2 to designate the site number and S (shallow), M 

(intermediate), or D (deep) to designate the depth. 

Thus, LCMW2-S was the shallowest multi-level 

monitoring well at the second site.

The third method, local identifier, is the official 

USGS location description. For surface-water sites,

number 

14W33CCCC1

Figure 4. Well-numbering system.

this was the name of the site. For ground-water sites, it 

was based on the land subdivisions in the Bureau of 

Land Management's (BLM) survey of Nebraska 

(fig. 4). The number preceding N (north) indicates the 

township or tier, the numeral preceding W (west) indicates the range, and the numbers preceding the terminal letters indicate the section in which the well is 

located. The terminal letters designated A, B, C, and 

D, denote the quarter section, the quarter-quarter 

section, the quarter-quarter-quarter section, and the 

quarter-quarter-quarter-quarter section. The designation is given in a counterclockwise direction beginning 

with "A" in the northeast comer of each subdivision. 

Each well also is assigned a terminal number "1" 

designating the order of inventory. Like the USGS site 

identification number, two or more wells are distinguished further by adding a sequential digit to the well 

number, assigned by order of inventory.

12 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
Sample Collection

Surface-water samples from the Middle Loup 

River and Canal 4 were collected using equal-width 

increment sampling procedures as described by Wells 

and others (1990). The discharges for the Middle Loup 

River and Canal 4 were measured concurrently with 

sample collection. Water samples from Sherman 

Reservoir were collected from a boat using a submersible pump at the desired depth. All sample collections 

used standard USGS procedures as described by Wells 

and others (1990) and U.S. Geological Survey (1997 

to present).

Ground-water samples were collected from the 

monitoring-well sites using standard USGS procedures as described by U.S. Geological Survey (1997 to 

present). A submersible pump was used to pump water 

through a flow-through chamber to monitor the physical properties (specific conductance, pH, temperature, 

and dissolved oxygen concentration) prior to the 

collection of a sample. Water samples from domestic 

wells were collected using a hose connecting an 

outside spigot (that was independent of a pressure 

tank) to the flow-through chamber. Physical property 

measurements were read at 3-minute intervals and 

recorded after these readings stabilized within preset 

limitations or after at least three well volumes of water 

had been removed from the well. Some wells did not 

yield sufficient volumes of water or, in the case of the 

BOR observation wells, they were too narrow to allow 

collection of ground-water samples with a submersible 

pump. In these wells, disposable bailers were used to 

extract the required three casing volumes before the 

wells were sampled. In some cases, BOR observation 

wells, such as well LC-20, were bailed dry and then 

sampled the following day after ground-water levels 

had recovered. For these wells it was presumed that 

because the well was bailed dry, the water yielded by 

the well the following day represented the water in 

the aquifer.

Sampling for analyses of concentrations of chlorofluorocarbons (CFCs) (CFC-12, dichlorodifluoromethane, CF2C12; CFC-11, trichlorofluoromethane, 

CFC13 ; and CFC-113, trichlorotrifluoroethane, 

C2F3Cl3) in ground water requires special sampling 

techniques and equipment. This equipment was 

designed to ensure that the ground-water sample 

did not come in contact with the atmosphere, as 

described by Busenberg and Plummer (1992) and 

U.S. Geological Survey (1997 to present). This 

special equipment facilitates sealing water samples

into borosilicate-glass ampoules prior to transport to 

the laboratory. Analyses for CFCs also require the 

collection of dissolved-gas samples in serum bottles, 

which are preserved on ice immediately following 

collection of the water sample. Dissolved-gas samples 

help define a recharge temperature that is used in 

calculating the ground-water age (Busenberg and 

Plummer, 1992).

Sample Analysis

Water samples from all sites were analyzed for 

physical properties and major ions (such as calcium, 

magnesium, sodium, potassium, sulfate, and chloride), 

nutrients, and selected isotopes oxygen-18 ( 18O), 

deuterium (2H), and tritium (3H). Samples from a 

subset of the sites (one of the domestic wells, all six of 

the multi-level monitoring wells, the single publicsupply well, as well as the Middle Loup River, and the 

intermediate depth at both Sherman Reservoir 

sampling sites) were analyzed for CFC and selected 

dissolved gases (argon, carbon dioxide, methane, 

nitrogen, and oxygen). Water samples collected for 

analyses of major ions and nutrient concentrations 

were sent to the USGS National Water-Quality 

Laboratory (NWQL) in Denver, Colorado, for 

analysis. Water samples collected for analysis of CFCs 

and concentrations of dissolved gases were sent to the 

USGS CFC Laboratory in Reston, Virginia. Water 

samples collected for analyses of 180, 2H, and 3H 

were sent to the University of Waterloo (Ontario, 

Canada) and Purdue University (Lafayette, Indiana), 

respectively.

Chlorofluorocarbons and Ground-Water Age Dating

Previous studies using the CFC age-dating techniques have determined the age of recently (less than 

about 50 years) recharged ground water with an accuracy of plus-or-minus 1 to 3 years (Busenberg and 

Plummer, 1992; Plummer and Busenberg, 2000). The 

CFC age-dating technique is more precise, and analytical costs are less than other techniques, such as 

tritium or 3H/3He (helium), used for dating recent 

ground water. CFC ground-water ages also agree 

favorably with ages determined using other dating 

techniques, such as tritium, 3H/3He, and krypton-85 

(Ekwurzel and others, 1994). However, because CFCs 

can and do degrade under certain conditions (such as

Methods 13
1940

5 E

C 13 S

10,000

5,000

1,000 

= 500

100

50

10

(B)

anoxic conditions), 

overall accuracy 

of ground-water age 

dating can diminish.

CFC concentrations 

in the water samples are 

compared to calculated 

and measured values in 

the air (fig. 5). These 

comparisons then are 

used to determine a 

"modeled" or "apparent" age for the water. 

Modeled ages are provided because of the 

uncertainty involved in 

quantifying the actual 

ground-water flow rates. 

Ground-water age, 

when using CFC concentrations in water 

samples, are based on 

the minimum estimate 

of time that has elapsed 

since the newly 

recharged water has 

come into contact with 

the saturated zone and, 

thus, became isolated 

from the soil atmosphere (Szabo and 

others, 1996; Plummer 

and Busenberg, 2000). 

The age of the water 

also is based on the 

assumption of piston 

flow and does not 

account for mixing 

scenarios that can occur 

in wells with large open 

intervals or multiple 

producing fracture 

zones, or diffusion or 

dispersion resulting

from ground-water flow and transport. Therefore, the 

results are referred to as "modeled" or "apparent" 

ages. For this report, all ages that were determined by 

CFC data are referred to as modeled. The assumption 

of piston flow is reasonable considering the open intervals of the wells are much shorter than the thickness of

1950 1960 1970 1980 1990 2000

J______I______I_______I______I______L

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Figure 5. ^Atmospheric concentrations of Chlorofluorocarbons CFC-11, CFC-12, and CFC-113 in North 

America air (modified from Plummer and Busenberg, 2000) and (B) tritium concentrations in precipitation 

from Ottawa, Ontario, Canada, and Lincoln, Nebraska (International Atomic Energy Agency, 2001).

the aquifer. Szabo and others (1996) report that the 

difference between the minimum traveltime at the top 

of the open interval and the maximum traveltime at the 

bottom of the open interval is most significant at depth 

where ground-water flow lines are closely spaced. 

Near the water table the ground-water flow lines are

14 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground.Water Beneath Loup City, Central Nebraska, 2000
not as closely spaced, and the small open intervals of 

the wells intersect fewer flow lines. Water that is determined to be of "modern" age refers to water with CFC 

concentrations that are within the possible range of 

modern air for the CFC concentration. For this report, 

"modern" refers to water whose modeled age is 0 to 

10 years.

Uncertainties in CFC ages can arise from factors 

such as thickness of the unsaturated zone, mixture of 

young and old water, errors in estimating recharge 

temperature, degradation of CFCs (redox conditions 

from anoxic, sulfate-reducing, or methanogenic environments), dispersion of ground-water flow, and CFC 

contamination (Plummer and Busenberg, 2000). 

Unsaturated zone thicknesses that exceed 30 ft cause 

lag-time errors. Cook and Solomon (1995) presented 

calculated lag times for CFC concentrations. They 

suggest that lag times from unsaturated zone thickness 

that is less than 30 ft will be less than 2 years and, 

thus, can be ignored. Mixture of young and old water 

or errors in estimating recharge temperature can make 

the modeled age of the water appear either too young 

or too old. Errors in estimating recharge altitudes can 

cause the modeled age to appear too young or too old 

because CFC dissolves less at higher altitudes than at 

lower altitudes. However, recharge estimates of altitudes that are less than 300 ft in error are not important 

(Plummer and Busenberg, 2000). Degradation of 

CFCs can cause the modeled age of the water to 

appear too old. Dispersion of ground-water flow can 

cause younger water (1975 to 1993) to appear too old 

or older water (older than 1975) to appear too young. 

Contamination of CFCs generally produces impossibly young water (water that has recharged in the 

future) or water that appears too young.

Because the previously mentioned processes can 

affect CFC concentrations, concentrations of CFCs 

from water samples were tested for sensitivity. Thickness of the unsaturated zone was not considered 

because none of the sites where CFC data were 

collected had water tables (where recharge occurred) 

more than 30 ft below the land surface. In addition, 

dispersion of ground-water flow is not expected to 

have a significant effect on CFC concentrations 

(Plummer and Busenberg, 2000). Therefore, hydrodynamic dispersion was assumed to be negligible. 

However, the remaining processes were tested. Testing 

included reviewing mixing scenarios of binary water 

(old mixed with young) and testing CFC concentrations for piston, binary, or exponential flow characteristics. Degradation of CFCs was evaluated through 

analysis of redox, sulfate-reducing, and methanogenic 

conditions from physical-property, water-chemistry, 

and dissolved-gases data collected during the study. 

CFC contamination was evaluated by comparing CFC 

air-mixture concentrations with concentrations in 

water samples.

Isotopes

A brief overview of isotopes and their use related 

to the Loup City study is presented here. Much of the 

material and a comprehensive discussion of isotopes 

and their uses in hydrologic studies are summarized in 

Clark and Fritz (1997).

A number of isotopes, both stable and radioactive, 

are used in hydrologic studies and have been termed 

"environmental isotopes." For this study, these include 

the stable isotopes of water 18O and H and the 

radioactive isotope of hydrogen 3H. Generally 

speaking, stable isotopes typically are used to identify 

the source (origin) of the water or its dissolved constituents and any physical, chemical, and biological 

processes that may have affected the water since its 

origin. Hydrogen and oxygen are the naturally occurring stable isotopes of water and, therefore, are ideal 

tracers. Radioactive isotopes are used to determine the 

"age" of water, that is, when recharge water loses 

contact with the atmosphere.

Oxygen-18 and Deuterium

Stable isotopes are measured as the ratio of the 

two most abundant isotopic forms of the element. 

For oxygen, it is the ratio of 18O, with an abundance 

of 0.204 percent, to 16O (oxygen-16, the more 

common form), with an abundance of 99.796 percent. 

Thus, the 18O/16O ratio is about 0.00204. For hydrogen, it is the ratio of 2H (deuterium, also written as 

"D"), with an abundance of 0.015 percent, to the more 

common form, 1H (protium), with an abundance of 

99.985 percent, giving a 2W1H ratio of about 0.00015.

Stable isotope concentrations are expressed as the 

difference between the measured ratio of the sample 

and the measured ratio of a reference standard. Isotope 

results are reported as parts per thousand (per mil) 

with respect to the standard using the '5' (pronounced 

delta) notation expressed as:

} sample

R.

Rstandard

-1 - 1000, (D

Methods 15
where Rsampie is the ratio of 18O/16O or 2W1H in 

the sample and Rstandard ls tne rati° °ftne 

international standard for oxygen and hydrogen, Vienna Standard Mean Ocean Water 

(VSMOW).

When using the 5 notation, seawater has a 518O 

value of 0 per mil. Thus, water samples that have negative 818O (and 82H as well) values (such as is typical 

with precipitation or ground-water samples) are said to 

be depleted in 18O relative to seawater. Water samples 

that have positive 8 values are said to be enriched in

10

O relative to seawater. These terms also can be used 

to describe two isotope compositions relative to each 

other. For example, a precipitation sample that has a 

8O value of -25 per mil is said to be depleted relative 

to a precipitation sample with a 8 18O value of -11 per 

mil. Likewise, a ground-water sample with a 818O 

value of -5 per mil is said to be enriched relative to a 

ground-water sample that has a 818O value of -12 per 

mil. In the literature, the terms "heavy" and "light" 

also may be used interchangeably with enriched and 

depleted when referring to stable isotope 818O and 

82H values. For example, a precipitation sample with a 

8 O value of -25 per mil is said to be isotopically 

lighter (or simply light) when compared to a precipitation sample having a 8 18O value of -8 per mil. A surface-water sample with a 818O value of -5 per mil is 

considered isotopically heavier (heavy) when compared to a ground-water sample with a 

818O value of -10 per mil. A simple 

rule to follow with regard to the terminology is the more negative the 

value...the 'lighter' or more 'depleted' 

it is; the more positive the value...the 

'heavier' or more 'enriched' the sample is said to be.

A plot of 818O versus 82H can be 

used to determine probable groundwater sources (Craig, 1961). The 818O 

and 82H values of global precipitation 

generally plot close to a straight line, 

called the global meteoric water line 

(GMWL). The equation of this line is 

82H = 8 818O + 10. Local meteoric 

water lines (LMWLs), which have 

slightly different slopes and intercepts 

than the GMWL, result from differences in altitude, local climate, and distance from the moisture source. This 

study used LMWLs developed from

20

-20

-40

' -60

-80

-100

-120

data collected at two nearby National Atmospheric 

Deposition Project locations in Nebraska Mead 

(Harvey, 2001) and North Platte (Harvey and Welker, 

2000). If ground-water 818O and 82H values plot near 

the LMWL for the sampling area, the water likely is 

meteoric in origin, that is to say, derived from precipitation without subsequent modification. If they do not 

plot along this line, they have been affected by some 

physical or chemical process prior to recharge or during residence in the aquifer.

Once meteoric water is precipitated, a number of 

processes may alter the water's isotopic signature (the 

combined 8 18O and 82H values). The process most 

relevant to this study is evaporation. As water evaporates from an open surface (river, lake, or irrigated 

field), its signature moves away from the meteoric 

water line along a line having a slope of between 2 and 

about 5 (depending on the effect of humidity). This 

shift off the meteoric water line occurs because of the 

difference in the vapor pressures of H2 18O and 2HHO, 

which imparts disproportional enrichments in the 

water phase during evaporation (Clark and Fritz, 

1997). For a given type of water, the greater the degree 

of evaporation, the farther from the meteoric water line 

the resulting signature will be. Rivers and lakes typically undergo greater evaporation than ground water 

and generally plot off the meteoric water line along an 

evaporation line (fig. 6).

Global meteoric water line

Evaporation line

Meteoric end member 

(ground water or precipitation)

Evaporated end 

member 

(surface water)

Advective or diffusive mixing line 

between two end members

Q = aA + bB 

a + b= 1

-140 -25 -20 -15 -10 

818 0 (per mil VSMOW)

-5

Figure 6. Mixing of ground-water source with an evaporated water source, such as a 

reservoir (VSMOW = Vienna Standard Mean Oceanic Water; A is meteoric end member; 

B is evaporated end member and Q is sum of A and B, where a = percent of A, 

b = percent of B, and a + b = 1).

16 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska. 2000
If a sample of ground water plots off the meteoric 

water line, along an evaporation line, one of two 

conclusions can be reached: (1) the sample has been 

affected by evaporation prior to recharge (water reaching the saturated zone), or (2) the sample represents a 

mixture of two "end member" types of water one of 

meteoric origin (plotting on the water line) and one 

having undergone evaporation (a river or lake, for 

example, plotting off the line) (fig. 6). Ground water 

having undergone mixing typically will plot somewhere between the two end members. The degree and 

ratio of mixing determines the proximity of the 

ground-water sample to an end member. The closer the 

ground-water sample is to a particular end member, 

the larger the ratio of the end member in the groundwater sample.

Tritium

Tritium (3H) is the naturally occurring, short-lived 

radioactive isotope of hydrogen and is produced in the 

upper atmosphere by the reaction of cosmic neutrons 

(n) with 14N (Faure, 1986):

14 3 n

N + «-> H+ 12C , (2)

where N is the isotope nitrogen-14 and 12C is the 

isotope carbon-12.

Tritium combines with oxygen to form tritiated 

water (3HHO), which is dispersed throughout the 

hydrosphere. Tritium's half-life, 4,500 days 

(12.32 years) (Lucas and Unterweger, 2000), allows it 

to be used as a dating tool for relatively young water 

(less than 50 years) (Mann and others, 1982). The natural background activity of 3H in precipitation prior to 

1952 was about 1 to 10 tritium units (TU) (Davis and 

DeWiest, 1966). Beginning in 1952, large amounts of 

3H were added to this natural background value from 

aboveground testing of thermonuclear weapons. At the 

maximum in 1963, the quantity of human-generated 

tritium produced was 100 to 1,000 times greater than 

that of natural 3H (Fontes, 1980).

Tritium concentrations in precipitation have been 

monitored at numerous locations throughout North 

America since the early 1950s. The nearest location to 

the study area is Lincoln, Nebraska; however, Ottawa, 

Ontario, Canada (not shown in figures) has a more 

complete record. Tritium concentrations were 

recorded in precipitation samples in Lincoln from 

1962 to 1986, and at Ottawa from 1953 to the present 

(fig. 55). Figure 5B shows a plot of the average yearly 

3H activity in precipitation at both of these sites. The

major increase in 3H concentration in 1963 reflects the 

peak of aboveground thermonuclear testing. Since that 

time, 3H concentrations in Ottawa precipitation have 

decreased gradually to a current value of about 20 TU 

(International Atomic Energy Agency, 1992). Precipitation in east-central Nebraska has similar values 

ranging from 10 to 30 TU (unpublished precipitation 

data from this study and others that currently are being 

conducted by Dr. F. Edwin Harvey, University of 

Nebraska-Lincoln, School of Natural Resource Sciences and Conservation and Survey Division).

The significant difference in 3H concentrations 

between precipitation before 1952 and precipitation 

after 1952, and the significant increase in 3H concentrations in 1963 precipitation allow 3H to be used as a 

natural tracer. Tritium has been used to determine 

ground-water age and flow velocities (von Buttlar, 

1959; Egboka and others, 1983), dispersion in the 

ground-water zone (Atakan, 1972; Robertson and 

Cherry, 1989), diffusion in the ground-water zone 

(Foster, 1975), and ground-water recharge (Allison 

and Hughes, 1975; Larson and others, 1987; Daniels 

and others, 1989).

In this study, 3H was used to determine the age of 

surface-water and ground-water samples. Generally, 

water samples containing measurable 3H concentrations likely have been recharged since the early 1950s. 

Water samples that do not have detectable concentrations of H likely recharged prior to the 1950s.

Quality Assurance and Quality Control

To ensure that each sample collected represented 

actual constituent concentrations, quality-assurance 

(QA) and quality-control (QC) measures were instituted. Consequently, six replicate and blank samples, 

which represent about 10 percent of the total samples, 

were collected during the study. All QA/QC samples 

were collected concurrently, processed, and shipped 

with the regular samples. In addition, samples for 

major-ion analysis were reanalyzed if their anion and 

cation concentrations did not balance within 5 percent 

of each other.

Data-Analysis Procedures

Data-analysis procedures included general statistical analyses and the generation of maps, box plots, 

bilinear diagrams, scatter plots, and time-series plots. 

This report contains only selected maps and plots

Methods 17
appropriate for the discussion. The generalized geologic section (fig. 2B) was created using a compilation 

of available data. The contact of the Ogallala Formation was developed from nearby BOR core analyses 

used for designing Sherman Reservoir Dam, and the 

contours from Sounders (2000). The contacts of the 

Pliocene-aged sediment are inferred. No data were 

available to map the contact accurately, but Souders 

(2000) reports its presence. The water table before 

Sherman Reservoir construction shown in figure 3 was 

generated using historical data from Brown (1955) and 

Sniegocki (1959). The 2000 water table (fig. 3) was 

constructed from data collected during this study. The 

water levels shown in figure 2 were generated from the 

2000 water-table map (fig. 3).

For water-quality analyses, the generation of 

trilinear diagrams required a concentration of bicarbonate from the water samples. Bicarbonate concentrations were calculated by dividing carbonate values 

(estimated from the acid-neutralizing capacity) by 

0.8202 as described by Hem (1985). Some data were 

analyzed and described in terms of statistical quartiles 

(percentiles) data values less than 25, 50, or 

75 percent of the sample observations. Percentiles are 

more descriptive than minimum and maximum 

because they more accurately represent the distribution of the data.

Nonparametric statistical analysis of the data was 

performed to determine variations between data sets. 

Nonparametric statistical methods rely on the conversion of data to ranked values and do not require that 

the data be distributed normally (Dowdy and 

Wearden, 1991).

Data were grouped into appropriate subgroups, 

and each "censored datum" was assigned a value equal 

to one-half the reporting limit. Censored data have 

values less than the reporting limit of the analytical 

procedure. A value of one-half the reporting limit 

assures that the censored data remain smaller than the 

smallest number greater than the reporting limit. 

Because the tests performed are nonparametric and 

compare two independent populations by ranking the 

data, assigning a minimum value to the censored data 

does not affect the test results (Helsel and Hirsch, 

1992).

The Mann-Whitney rank-sum test (Helsel and 

Hirsch, 1992) was used to determine if median chemical concentrations were significantly different between 

independent populations, such as sulfate concentrations from ground-water samples at Loup City

compared with sulfate concentrations in ground-water 

samples from the domestic wells. All statistical analyses used a confidence level of 95 percent (a = 0.05). A 

confidence level is the probability that the test statistic 

generated for the test accurately defines the populations being tested. The p-value associated with this 

particular test is the probability of getting a test statistic equal to, or more extreme than, the value computed 

from the data when the null hypothesis (difference in 

the medians of the populations is zero) is true. If the 

p-value was less than or equal to 0.05, the null hypothesis was rejected, and it was assumed that a significant 

difference existed between the two populations. If the 

p-value was greater than 0.05, the null hypothesis was 

accepted, and a significant difference between the two 

populations could not be concluded.

RESULTS OF DATA ANALYSES

This section presents the results of the analyses of 

the physical and chemical data collected in the study 

area. Analyses of the physical characteristics were 

limited primarily to hydrographs and water-table 

configuration maps. Analyses of the chemical characteristics are presented in graphs and tables. This 

section also describes the analyses of the ground-water 

ages and the analysis of the isotope data.

Physical Characteristics

Although ground-water-level data collected before 

Sherman Reservoir construction in the Loup City area 

are limited, analysis of historical water-level data 

collected from BOR observation wells shows that 

ground-water levels between Loup City and Sherman 

Reservoir rose as much as 85 ft since the completion 

of Sherman Reservoir (fig. 7). A comparison of the 

water table before Sherman Reservoir construction 

with the 2000 water table (fig. 3) shows that the 

ground-water-flow direction between Sherman Reservoir and Loup City has changed. This indicates that 

Sherman Reservoir is losing water to the ground-water 

system and locally affecting ground-water levels in 

wells between Sherman Reservoir and Loup City.

Following the completion of the multi-level monitoring wells, ground water in wells open to the Ogallala aquifer (wells LCMW1-D and LCMW2-D) and 

directly beneath the alluvial aquifer rose about 30 ft 

above the top of the contact of the alluvial and Ogal18 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
30

(-C

DD

80

105

130

Observation well 

BOR ROL-4

Observation well 

BOR RR-2

2000 water level.

2000 water level.

1959 1965 1972 1980 1987 1994 2001

Figure 7. Depth to water table in selected Bureau of Reclamation observation wells in study area. Location of 

observation wells shown in figure 2.

lala aquifers. The cause of the hydrostatic pressure has 

not been determined, but it is likely the result of both 

hydraulic pressure placed on the ground-water reservoir both by locally confined conditions and by 

Sherman Reservoir. Sniegocki (1959) reported that 

ground water in the study area is under both confined 

and unconfined conditions. Ground water likely is 

under locally confined conditions in the uplands east 

of Loup City. However, during this study, the presence 

of confined or unconfined conditions was not determined for ground water in the upland areas. With 

respect to the hydrostatic pressure exerted by Sherman 

Reservoir, the volume of water in Sherman Reservoir 

creates a tremendous downward force on the groundwater system, which, in turn, must be distributed elsewhere in the system. Both of these conditions could 

account for the hydrostatic pressure that was evident in 

the two monitoring wells.

In Loup City, measured ground-water levels 

between the shallow and deep monitoring wells and 

the intermediate-depth and the deep monitoring wells 

showed a slight upward ground-water head difference 

of about 0.20 ft. Alone, this small upward head difference beneath Loup City does not identify the pressure 

from the reservoir as the cause of the higher groundwater levels. However, the mound development and 

the increased lateral hydraulic gradient from the reservoir to the city make it appear likely that those conditions may have increased the upward head difference 

after reservoir construction.

Chemical Characteristics

With some exceptions, temporal variations in 

specific-conductance measurements were fairly 

consistent (within 10 percent) between water samples 

collected in March/April and July (table 3). Specific 

conductance of water samples from surface-water sites 

typically was within 5 percent from the March/April 

sample to the July sample; whereas specific conductance in ground-water samples typically was within 

20 percent. The specific conductance in water samples 

from one well (LC9-J) decreased about 46 percent 

between March/April and July. No reason was determined for the large reduction in specific conductance 

at this site; however, the July measurement

Results of Data Analyses 19
Table 3. Results of measurements of physical properties in surface and ground water in study area, central Nebraska, 2000

[ID, identifier; fig., figure; SC, specific conductance in microsiemens per centimeter at 25 degrees Celsius; pH, in standard units; T, water temperature in 

degrees Celsius; DO, dissolved oxygen in milligrams per liter; SW, surface water; , not measured; Alluvium, alluvial deposits; Ogallala, Ogallala Formation]

Field ID 

(fig. 2)

Middle Loup River

Canal 41

Sherman Reservoir 1-D

Sherman Reservoir 1-M

Sherman Reservoir 1-S

Sherman Reservoir 2-D

Sherman Reservoir 2-M

Sherman Reservoir 2-S 

BOR LC-202

BOR LC-52

BOR ROL-3

BOR ROL-^

BOR RR-2

BOR RR-32

DW-1

DW-2

DW-3

D\\M

LC10-H

Date

3/30/2000 

7/17/2000

7/20/2000

3/30/2000 

7/19/2000

3/30/2000 

7/19/2000

3/30/2000 

7/19/2000

3/30/2000 

7/19/2000

3/30/2000 

7/19/2000

3/30/2000 

7/19/2000

3/31/2000

4/04/2000 

7/18/2000

4/05/2000 

7/19/2000

4/05/2000 

7/26/2000

4/05/2000 

7/19/2000

4/05/2000

4/05/2000 

7/20/2000

4/05/2000 

7/20/2000

4/06/2000 

7/20/2000

4/06/2000 

7/20/2000

4/04/2000 

7/21/2000

Time

1630 

0910

1445

1220 

1415

1230 

1520

1240 

1415

1350 

1550

1400 

1610

1410 

1610

1200

1230 

1520

1000 

1050

1430 

0730

1100 

0850

1215

1600 

1050

1645 

1230

09:00 

1340

0945 

0800

1330 

1340

SC

Surface-water sites

217 

252

204

211

233

210 

220

210

221

211 

219

211 

220

211 

218 

Ground-water sites 

1,130

660

677

253 

297

676 

579

540

438

119

512 

513

508

485

548 

527

531 

438

592 

546

pH

8.2 

8.4

9.0

8.0

7.4

8.0 

8.3

8.0 

8.3

7.8 

8.6

8.0 

8.6

8.0 

8.6

7.2

7.3 

7.3

7.1 

7.4

7.4 

7.3

9.6

8.3

9.0

7.2 

7.5

7.2 

7.6

7.1 

7.5

7.1 

7.5

7.3 

7.5

T

15.0 

21.5

--

7.0 

22.0

8.0 

25.0

8.5 

25.0

7.5 

25.0

8.0 

25.0

8.5 

25.0

10.5

12.5

11.5 

18.5

12.0 

15.0

13.0

12.0

13.0 

13.5

11.5 

12.5

12.5 

13.0

12.0 

15.0

11.5 

20.0

DO

9.1 

8.1

8.8

8.9 

.1

9.8 

5.4

10.1 

6.1

9.2 

6.1

9.9 

6.1

9.8

7.3

--

--

«

-

--

8.9 

9.1

11.3 

10.1

8.4 

8.8

9.1 

9.0

--

Source

SW

SW

SW

SW

SW

SW

SW

SW 

Alluvium

Ogallala

Ogallala

Ogallala

Ogallala

Ogallala

Ogallala

Ogallala

Ogallala

Ogallala

Alluvium

20 Use of Water Chemistry, Isotopes, and Chlorof luorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
Table 3. Results of measurements of physical properties in surface and ground water in study area, central Nebraska, 2000 Continued

Field ID 

(fig. 2) Date Time SC pH DO Source

LC11-J

LC9-H-A

LC9-I2

Ground-water sites Continued

3/31/2000 

7/21/2000

3/31/2000 

7/21/2000

3/31/2000

0835

0800

1030

1220

1330

621

634

550

550

565

7.2

7.3

7.2 

7.2

7.0

12.5

13.0

13.0

13.5

13.0

1 Canal 4 was dry during March/April 2000 sample period. 

2Not sampled in July 2000.

0.2

.2

3.0

2.3

Alluvium

Alluvium

Alluvium

LC9-J

LCLS-1

LCMW1-D

LCMW1-M

LCMW1-S

LCMW2-D

LCMW2-M

LCMW2-S

LCPS-1

SRGP-1

3/31/2000 

7/21/2000

4/06/2000 

7/24/2000

3/28/2000 

7/24/2000

3/28/2000 

7/24/2000

3/28/2000 

7/24/2000

3/29/2000 

7/24/2000

3/29/2000 

7/24/2000

3/29/2000 

7/24/2000

3/29/2000 

7/21/2000

3/30/2000 

7/20/2000

1235 

1300

1030 

1420

1240 

1210

1525 

1250

1615 

1330

0925 

0850

1040 

0940

1155 

1020

1450 

1450

0900 

0930

835 

445

1,580 

1,220

549 

535

549

542

570 

567

577 

576

615 

604

663 

638

590 

592

314

323

7.1 

7.4

7.4 

7.5

7.4 

7.2

7.4 

7.3

7.4 

7.2

7.1

7.2

7.2 

7.3

7.3 

7.5

7.3 

7.2

7.4 

7.7

13.0 

13.5

11.0 

19.0

13.5 

14.5

14.0 

14.0

13.5 

14.0

12.5 

13.5

12.5 

13.5

13.0 

13.0

12.5 

15.0

13.0 

14.5

--

5.5 

2.8

5.6

7.4

3.4 

6.0

.2

.2

5.6 

5.6

3.1

.2

2.2 

.2

5.6

5.2

.5 

.2

Alluvium

Alluvium

Ogallala

Alluvium

Alluvium

Ogallala

Alluvium

Alluvium

Ogallala

Ogallala

(445 ^iS/cm) is closer to measurements in water from 

the other wells in Loup City. pH values varied little in 

water samples from surface-water and ground-water 

sites. Most pH measurements in samples from these 

sites were within 1.0 standard unit between March/ 

April and July. For the most part, the temperature of 

the ground water does not change substantially 

through the year. Temperatures of ground water from 

samples collected in July typically stayed within 

10 percent of those collected in March/April. Some 

exceptions are a 61-percent change in temperature in 

water from well BOR ROL-3 (11.5 to 18.5 °C) and a

74-percent change in water from well LC10-H 

(11.5 to 20 °C). No determinations were made as to 

why the water temperatures at these two sites changed 

so much.

Most of the water-quality constituents varied 

among the three sources sampled (fig. 8). The largest 

difference among the three sources, with respect to 

physical properties, was in concentrations of dissolved 

oxygen. Dissolved-oxygen concentrations in groundwater samples from the alluvial aquifer were much 

smaller and significantly different (p-value = 0.00) 

than those from surface water or from the Ogallala

Results of Data Analyses 21
1,600 (15) (19)

03 CO E .= I ;s 1,400 

1,200 

1,000

E 3

o E

CJ vp

" 600 03

400 

$ 200

27

CO

 - 24 -

_(0 ff*

03

" 21 -

CD 

CD

I 18-

T3

~ 15 -

0)

I 12 -

CD

E 9-

(14) (19)

-

i

i

i

i i

* 

* *

 =p ^ :

* i 

i i

Surface Alluvial Ogallala 

water aquifer aquifer

aquifer. Water samples 

from observation 

well LC11-J and multilevel monitoring 

wells LCMW1-S, 

LCMW2-M, and 

LCMW2-S had at least 

one dissolved-oxygen 

measurement less than 

0.5 mg/L (table 3). For 

the surface-water sites 

and Ogallala aquifer, only 

water from Sherman 

Reservoir 1-D and 

domestic well SRGP-1 

had at least one 

dissolved-oxygen 

concentration less than 

0.5 mg/L. Dissolvedoxygen concentrations in 

samples from the surfacewater sites and the Ogallala aquifer were not 

significantly different 

(p-value = 0.38). pH of 

water samples varied 

significantly between the 

surface-water and 

ground-water samples 

(surface water and 

alluvial aquifer, 

p-value = 0.00; surface 

water and Ogallala aquifer, p-value = 0.00) but 

did not vary significantly 

between samples 

collected from the alluvial and Ogallala aquifers (p-value = 0.53).

Minimum, maximum, and quartile concentrations 

were computed for the water-quality analyses from the 

three sources (table 4). Median concentrations for constituents in water samples from the alluvial and Ogallala monitoring wells were similar. However, the 

median concentration of sulfate in water samples from 

the alluvial aquifer (15 mg/L) was nearly twice the 

median concentration of sulfate measured in water 

samples from the Ogallala aquifer (8.0 mg/L). Concentrations of hydrogen sulfide (H2S) coupled with 

low concentrations of dissolved oxygen typically are 

found in areas that are at the end of ground-water flow

(26) 10 05) (19) (26)

(24)

12 (15) (14) (16)

8=4 

£ E

Surface 

water

Alluvial 

aquifer

Ogallala 

aquifer

EXPLANATION

(17) Number of samples

* Outlier

90th percentile

75th percentile

50th percentile or median

25th percentile 

10th percentile

Figure 8. Distribution of (A) physical properties and (B) water-quality constituent concentrations by 

source in study area, 2000.

paths (Domenico and Schwartz, 1990). Although not 

measured directly, H2S was detected by smell in water 

from monitoring wells LCMW1-S and LCMW2-S.

Analysis of the water-chemistry data (tables 8 

and 9 in appendix) from all sampling sites shows that, 

except for the sewer lift station (LCLS-1), the water 

quality at all sites was similar and predominantly a 

calcium bicarbonate type (fig. 9). Because Loup City's 

sanitary sewer contributes to the total effluent, the 

water chemistry at site LCLS-1 (Loup City's 

sewer sanitary and storm combined) had much larger 

sodium and chloride concentrations than did all of the 

other sites. One well, BOR LC-20, contained water

22 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City. Central Nebraska, 2000
(B)

(9) (16) (23) (9) (16) (23)

IUU

O3

- 80

CD

CL 

v>

1 60

'g

E 

3 20

CD

O

0

en

liter Iligrams per

CO -p» CCO

E 20

c

E 

- 13 10

-a u

0

co

0

ou

CD 

= 

+- n« 24

(U 

Q. 

CO

E 18 ca 

en

1 12

c

ro c

13

co 

0

1 nnn

Dissolved solids, liter milligrams in per 

en ro*»ooc

CO CO

_, 1

1 1

-

JL

(9) (16) (23)

1

- -*- ^ i -

(9) (16) (23)

i i

-

*

. i .

i i

(9) (16) (22)

i

^x ^ 

^ ^

X

Surface Alluvial Ogallala 

water aquifer aquifer

/o

Q)

±i

5. bU

E

CD

o) 45

mi Magnesium, in CO * 3 CJ1 CO

cn

liter nilligrams perS ^

.E 20

E 

o 

'co in

o

Q.

0

^u

O)

~ ??

0} 

Q. 

CO

| 24

1C

Q)"

t Q

-C

0

1C

litern milligrams Nitrate, per

>^>cnoocoho^a

^

0

I 1 1

-

-+. ^ ^ "

~%~ ' *

(9) (16) (23)

i i i

X 

X

X

1 1 1

(9) (16) (23)

i i i

*

*

1

-r- T "T3

(9) (18) (25)

1

¥ L^^^^ ^^^

Surface Alluvial Ogallala 

water aquifer aquifer

Figure 8. Distribution of 14;* physical properties and (B) water-quality constituent concentrations by 

source in study area, 2000 Continued.

Results of Data Analyses 23
g Table 4. Minimum, maximum, and quartile constituent concentrations by water source in study area, central Nebraska, 2000

SF [all concentrations in milligrams per liter unless otherwise noted; Min, minimum; Max, maximum; %, percentile; CaCO3, calcium carbonate; ANC, acid-neutralizin

^ per liter]

a> 

S

^ Constituent

1^ Hardness (as CaO3)

§ Calcium

5T Magnesium

.§ Sodium

CD

-*" Potassium a>

a

o

 Bicarbonate

O

| ANC

| Sulfate

|_ Chloride

i Fluoride

3

1 Silicate

& 

c5' Dissolved solids

D

* Nitrate

£, Ammonium

e

§ Phosphorus

CA

« Phosphate

I Iron (u,g/L)

5 Manganese (ng/L)

Surface-water concentrations

Min

81

26

3.8

6.9

6.2

120

94

6.1

1.1

.3

40

159

<.05

<.02

.01

.04

< 10

<2.2

25th %

84

27

4.1

7.4

6.7

120

110

6.2

1.2

.3

40

160

<.05

<.02

.01

.04

20

<2.2

Median

87

28

4.2

7.4

6.8

130

110

6.2

1.2

.4

40

162

<.05

<.02

.02

.04

20

2.0

75th %

87

28

4.2

7.5

7.1

130

110

6.6

1.5

.4

55

166

<.05

<.02

.06

.05

20

3.0

Max

100

34

5.1

7.7

10

130

110

8.9

1.7

.4

60

195

.61

<.02

.14

.43

30

5.0

Min

240

77

9.9

8.1

8.3

330

270

10

2.9

.1

<.l

349

<.05

<.02

.04

.07

< 10

3.0

Alluvial aquifer concentrations1

25th %

240

78

12

9.0

10

330

270

12

4.1

.2

54

358

<.05

<.02

.08

.24

< 10

40

Median

260

83

13

9.4

11

370

300

15

4.5

.3

59

394

.56

.03

.10

.29

15

170

75th %

270

85

14

18

13

390

320

23

5.8

.4

60

20

1.0

.07

.13

.36

44

230

Max

320

91

21

180

40

540

440

28

260

.5

68

886

1.6

9.5

1.8

5.7

220

1,660

Min

27

10

0.4

4.9

4.1

100

82

1.9

1.2

.1

<.l

90

<.05

<.02

<.01

.08

< 10

<2.2

O

tS i 

" Includes data from Loup City's sewer.

o

O
60 40

Calcium

20 40 60 80 

Chloride plus nitrate as nitrogen

Percentage of milliequivalents per liter

EXPLANATION

Surface-water site Ground-water site

100

X Middle Loup River

T. Sherman Reservoir

Canal Number 4

® Domestic well

^ Loup City observation well

© Multi-level monitoring well

U Bureau of Reclamation observation well

v Public-supply well LCPS-1

A Sewer LCLS-1

Figure 9. Ionic composition of surface water and ground water in study area, 2000. Location of sampling sites shown in 

figure 2.

with a larger magnesium concentration than did all 

other sites. This well is at the northern edge of Loup 

City and is only about 20 ft deep. The reason for the 

larger magnesium concentration was not determined, 

and the complexity of the ground-water system 

precludes definitive explanation.

Surface-water samples exhibited similar concentrations for ionic compositions (fig. 9) during both 

sampling periods. This suggests that the chemical 

concentrations in Sherman Reservoir did not change 

much, if at all, following diversions from the Middle 

Loup River. The water-chemistry data at distinct 

depths in the lake were nearly identical (tables 8

Results of Data Analyses 25
and 9), probably indicating that (1) the system was 

well mixed, (2) evaporative process had little effect on 

the overall chemistry of the lake, and (3) chemical 

species were not concentrating in the lake.

The ground-water sites varied more in water 

chemistry than did the surface-water sites. Water 

samples from the domestic wells, the Loup City observation wells, and the multi-level monitoring wells all 

had a large calcium component. However, water 

samples from most of the BOR observation wells 

showed larger sodium concentrations (fig. 8) than did 

water samples from the other wells. This could indicate that the BOR observation wells are open to the 

shallower alluvial deposits overlying the Ogallala 

Formation whose water typically contains larger 

concentrations of calcium. The large concentrations of 

calcium in water samples from the domestic, Loup 

City observation, and multi-level monitoring wells 

indicate that (1) all of the domestic wells are open to 

the deeper Ogallala aquifer and (2) the Loup City 

observation and multi-level monitoring wells all 

contained water that currently is in the Ogallala aquifer (wells LCMW1-D and LCMW2-D) or once was 

in the Ogallala aquifer.

Significant differences in concentrations of 

calcium, magnesium, dissolved solids, and nitrate in 

samples from the alluvial and Ogallala aquifers 

(p-values ranged from 0.00 to 0.02) and from surfacewater and ground-water sources (p-values ranged from 

0.00 to 0.01) were detected. In addition, significant 

differences in sodium, potassium, and sulfate concentrations were detected in samples collected from the 

surface-water sites and the alluvial aquifer, and from 

the Ogallala aquifer and the alluvial aquifer (all 

p-values less than 0.02). However, differences in 

concentrations of these constituents were insignificant 

in samples collected from the surface-water sites and 

the Ogallala aquifer (p-values ranged from 0.23 to 

0.88). Differences in concentrations of chloride in 

surface-water samples were significant with respect to 

ground-water samples (p-value = 0.00). However, 

concentrations of chloride in samples from the alluvial 

and Ogallala aquifers were insignificant (p-value = 

0.39). These differences further support evidence of 

the variance in water chemistry between samples from 

the alluvial and Ogallala aquifers, and from surfacewater and ground-water sources.

Age of Ground Water

Analysis of the CFC data shows that CFC-11 and 

CFC-113 concentrations (table 5) in some of the water 

samples could have degraded or were contaminated 

CFC-11 and CFC-113 typically are not as stable as 

CFC-12 (Plummer and Busenberg, 2000). Therefore, 

the modeled age of the water is reported herein from 

CFC-12 data, which was not contaminated, and is the 

maximum age of the young fraction of water.

Testing of the CFC data for sensitivity showed that 

the assumptions were valid and that the modeled 

recharge age of the water was realistic. Testing the 

CFC data for differing mixing scenarios of binary 

water (old mixed with young) was inconclusive but did 

show that the water underlying Loup City was either 

not a mixture of young and old water or it was a 

mixture of old and very old water. Likewise, testing 

for piston, binary, or exponential-flow characteristics 

did not refute the assumption that piston flow dominated the ground-water flow regime. Although some 

of the water samples showed anoxic conditions from 

low dissolved-oxygen measurements and the smell of 

H2S, sulfate-reducing conditions appeared limited, 

and no methanogenic conditions were indicated by the 

water chemistry or dissolved-gases data. Therefore, 

except for the possible degradation of CFC-11 or 

CFC-113 in some of the water samples, degradation 

of CFC-12 probably was negligible.

Age dates of the surface-water sites indicate that 

they contain modern water (less than 10 years old) 

(table 6). The modeled recharge dates of the ground 

water underlying Loup City ranged from the mid1940s to early 1960s. Specifically, ground water in the 

alluvial aquifer and at the top of the Ogallala aquifer 

was about 50 to 55 years old. The water sample from 

300 ft below the top of the Ogallala aquifer and 

directly underlying Loup City (well LCPS-1; mid- to 

late 1950s) was about 10 years younger than water 

samples from the top of the Ogallala aquifer [wells 

LCMW1-D and LCMW2-D; mid- to late 1940s (near 

the detection limit of CFC analysis)]. Water samples 

from the multi-level monitoring wells open at the base 

of the alluvial aquifer (wells LCMW1-M and 

LCMW2-M) show modeled ages of mid- to late 1940s 

(table 6). Water samples collected from the multi-level 

monitoring wells open to the top of the alluvial aquifer 

(wells LCMW1-S and LCMW2-S) show modeled 

ages of late 1950s to early 1960s. Analysis of samples 

from the only domestic well that was analyzed for 

CFC data (well SRGP-1; mid-1970s) indicates water

26 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City. Central Nebraska, 2000
5?

S.

o 

at

S

Table 5. Results of chlorofluorocarbon (CFC) age-dating analyses of water samples in study area, central Nebraska, 2000

[modeled dates are averaged, recharge elevation and temperature are values used for chlorofluorocarbon calculations; ID, identifier; fig., figure; ft, feet; ASL, abov

grams per kilogram; pptv, parts per trillion, volume; Mod., modern; Cont., contaminated with modem concentrations]

Field ID 

(fig. 2)

Middle Loup River

Sherman Reservoir 1-M

Sherman Reservoir 2-M

Date

3/30/2000

3/30/2000

3/30/2000

Time

1630

1230

1400

Recharge 

elevation 

(ft ASL)

2,100

2,100

2,100

Recharge Concentration in solution 

temperature (pg/kg)

(°C)

15.0

7.9

8.1

CFC-11 CFC-12 CFC-113

Surface-water sites 

536 285

12,600

5,150

338

341

69.6

99.6

98.8

Calculated atmosphere partial 

pressure (pptv)

CFC-11

226

5,290

2,160

CFC-12

518

616

621

CFC-1

70.

100

99.

Ground-water sites

LCMW1-D

LCMW1-M

LCMW1-S

LCMW2-D

LCMW2-M

LCMW2-S

LCPS-1

SRGP-1

3/28/2000

3/28/2000

3/28/2000

3/29/2000

3/29/2000

3/29/2000

3/29/2000

3/30/2000

1400

1530

1130

0930

1045

1200

1450

0900

2,500

2,100

2,100

2,500

2,100

2,100

2,500

2,100

11.5

13.0

12.3

11.5

12.1

12.2

11.1

11.5

7.4

5.3

539

2.8

5.8

3.3

25.0

97.6

1.8

1.3

69.8

2,8

3.5

15.6

14.8

113

4.3

2.0

13.9

1.3

6.0

1.4

2.7

12.8

3.2

2.4

223

1.2

2.5

1.4

10.4

36.8

3.3

2.5

125

5.3

6.3

27.6

26.7

187

4.

2.

13.

1.

6.

1.

2.

11.

 s
Table 6. Averaged modeled age of water samples from study area 

in central Nebraska analyzed for chlorofluorocarbons (CFCs) and 

tritium, 2000

[CFC ages were based on average of three dichlorodifluoromethane values 

and are the modeled date of recharge; ID, identifier; fig., figure; CFC-12, 

dichlorodi-fluoromethane; SW, surface water; Modern, less than 10 years 

old; , not sampled; colored table cells indicate ages that are post (younger 

than) Sherman Reservoir construction]

Age of Age of water

Field ID Hydrologic water based on based on tri-

(fig. 2} system CFC-12 data tium data

Sherman Reservoir 1 

Sherman Reservoir 2- 

Middle Loup River

LCMW1-D

Surface-water sites

-M SW Modern

-M SW Modern 

SW Modern 

Ground-water sites

Ogallala mid- to late 

1940s

pre-1960

LCMW1-M

LCMW1-S

LCMW2-D

LCMW2-M

LCMW2-S

SRGP-1

LCPS-1

Alluvial

Alluvial

Ogallala

Alluvial

Alluvial

Ogallala

Ogallala

mid- 1940s

early 1960s

late 1940s

late 1940s

late 1950s

mid-1970s

mid-to-late 1950s

pre-1960

pre-1960

pre-1960

pre-1960

pre-1960

post- 1960

pre-1960

young enough to originate from Sherman Reservoir. 

Therefore, evaluation of the CFC data indicates that 

the age of the ground water beneath Loup City 

predates the construction of Sherman Reservoir.

Further evaluation of the age of ground water 

directly beneath Loup City indicates that the sample 

from multi-level monitoring well LCMW1-S was the 

only water sample that contained water young enough 

to have resided in Sherman Reservoir. The dates of 

water in all other samples collected from the monitoring wells in Loup City or Loup City's public-supply 

well predate Sherman Reservoir. Therefore, because 

the ages of ground water in almost all monitoring 

wells predate construction of Sherman Reservoir, 

the ground water underlying Loup City likely did 

not originate from Sherman Reservoir or from 

recent precipitation.

Isotopic Data

Results of the analyses for 18O and 2H in water 

samples show that water in Sherman Reservoir has 

been affected by evaporation for at least March 

through July and likely throughout the remainder of

the irrigation and growing season. This is indicated by 

data representing Sherman Reservoir samples plotting 

below the LMWL (fig. 10). Ground water beneath 

Loup City does not show this relation to the LMWL. 

Therefore, the water from which these ground-water 

samples originated did not undergo evaporative 

processes such as those that occurred in Sherman 

Reservoir, indicating that the ground water beneath 

Loup City has not resided in Sherman Reservoir.

Oxygen and Hydrogen

Several observations can be made related to the 

isotopic data from analyses of surface-water and 

ground-water samples for stable oxygen and hydrogen 

isotopes (fig. 10, table 7). First, both graphs in 

figure 10 show that water in Sherman Reservoir plots 

below the LMWL and has an enriched 818O and 52H 

signature (-7.4 per mil and -60 per mil, respectively) 

relative to local meteoric ground water, which is indicative of evaporation. The stable isotope shift is more 

than 2 per mil in 518O indicating that stable isotopes 

of water provide a useful tracer to determine if reservoir water is present in the monitoring wells. Water 

samples from the Middle Loup River and Canal 4 also 

show evidence of evaporation (fig. 10).

Water samples from domestic well SRGP-1 

[because of its relatively close position to the plotting 

location of water from Sherman Reservoir and because 

it is off the meteoric water line along an evaporation 

line (fig. 10)] consist of a large proportion of reservoir 

water either by direct hydraulic connection or as a 

result of reservoir seepage to the shallow ground-water 

system. Water from observation wells LC-20 and 

LC-52 also has been affected substantially by evaporation, but the source of that water is more likely 

seepage from nearby Canal 4 and not from Sherman 

Reservoir.

Many of the remaining ground-water samples 

appear to have been affected by slight evaporation, 

but this possibly results from evaporation at the time 

of recharge and not from seepage from Sherman 

Reservoir. Data are inconclusive, but the slight evaporative signature also may reflect the effect of seepage 

locally to the shallow aquifer from the Middle Loup 

River or Canal 4. On the basis of plots in figure 10, it 

is estimated that unaffected ground water in the region 

should have 8 18O and 5 2H signatures of about 

-10 and -72 per mil, respectively. These values are 

similar to the average weighted yearly mean values 

reported for precipitation in central Nebraska (-9.6 per

28 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
-55

-60

-65

-70

-75

(A) March and April 2000

Area of multiple 

evaporation lines

July 2000

Area of multiple 

evaporation lines

jg -90

1 -55

% -60

-65

-70

-75

-80

-85

-90 

-13 -12 -11 -10 -9 -8 -/

518 0 (per mil VSMOW)

EXPLANATION

 - Global meteoric water line (GMWL)

 Local meteoric water line (LMWL) at Mead, Nebraska

 Local meteoric water line (LMWL) at North Platte, Nebraska

Surface-water sites Ground-water sites and identifiers

 Sherman Reservoir DW-1 $ Domestic well

X Middle Loup River LCID-H^ Loup City observation well

+ Canal Number 4 O Multi-level monitoring well

ROL-4Q Bureau of Reclamation observation well

v Public-supply well LCPS-1

A Sewer LCLS-1

 Precipitation

Figure 10. Oxygen (8180) and stable hydrogen (82H) composition of water samples 

from study area for W March and April 2000, and (B) July 2000 (VSMOW = Vienna 

Standard Mean Oceanic Water). Location of sampling sites shown in figure 2.

Results of Data Analyses 29
Table 7. Isotopic chemistry data for water samples collected in study area, central Nebraska, 2000

[change from Vienna Standard Mean Oceanic Water (VSMOW) unless otherwise indicated, in per mil; ID, identifier; fig., figure; 8, delta; 

18O, oxygen-18; 2H, deuterium; 3H, tritium; TU, tritium units; ±, plus or minus; --, not sampled]

March and April

Field ID

(fig. 2)

Precipitation

8 180

--

8 2H

~

JH 

ITU)

-

JH error 

(±TU) 8 180

-9.6

July

JH JH error 

8 2H (TU) (+TU)

-59 12.6 1.1

Surface-water sites

Middle Loup River

Sherman Reservoir 1-D

Sherman Reservoir 1-M

Sherman Reservoir 1-S

Sherman Reservoir 2-D

Sherman Reservoir 2-M

Sherman Reservoir 2-S

Canal Number 4

-9.5

-7.3

-7.5

-7.4

-7.3

-7.4

-7.3

-

-72

-60

-60

-60

-60

-59

-58

-

4.58

5.99

5.61

5.26

6.25

6.04

5.13

~

0.93

.91

.87

.96

.89

.95

.95

-

-7.9

-7.7

-7.7

-7.7

-7.5

-7.7

-7.6

-8.9

-63

-63

-61

-61

-60

-59

-57

-68

Ground-water sites

BOR LC-20

BOR LC-52

BOR ROL-3

BOR ROL-4

BOR RR-2

BOR RR-3

DW-1

DW-2

DW-3

D\\M

SRGP-1

LC9-H-A

LC9-I

LC9-J

LC10-H

LC11-J

LCLS-1

LCMW1-S

LCMW1-M

LCMW1-D

LCMW2-S

LCMW2-M

LCMW2-D

LCPS-1

-8.7

-8.7

-11.9

-9.6

-9.9

-9.5

-10.8

-10.1

-9.8

-9.6

-7.7

-10.1

-10.4

-9.1

-9.6

-9.9

-10.1

-10.2

-10.7

-10.5

-10.1

-10.2

-10.1

-9.9

-68

-68

-85

-71

-72

-67

-74

-71

-70

-74

-60

-74

-75

-71

-67

-69

-74

-73

-75

-77

-69

-69

-71

-73

4.21

3.92

0.29

1.51

3.51

12.28

.69

8.46

5.06

7.53

6.22

1.05

1.06

1.70

.97

.90

1.94

.13

.39

.53

.95

.81

.88

1.82

.85

.80

.64

.81

.83

1.01

.73

.94

.89

1.04

.91

.80

.79

.78

.76

.75

.87

.66

.85

.73

.73

.71

.75

.89

~

-8.8

-12.3

-9.5

-10.7

_

-10.7

-9.8

-9.8

-10.0

-7.7

-10.1

~

-9.3

-9.5

-10.0

-10.3

-10.2

-10.4

-10.7

-9.9

-10.1

-10.1

-10.1

..

-64

-87

-69

-76

_

-77

.74

-70

-68

-60

.74

..

-71

-71

-69

-72

-75

-75

-75

-72

-72

-77

-75

30 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
mil for 5 18O and -69 per mil for 8 2H) reported by 

Harvey and Welker (2000), and with ground-water 

values from other studies in the region (Schellpeper 

and Harvey, 1998).

Tritium

Tritium was detected in all of the water samples, 

with concentrations ranging from 0.13 to 12.6 TU 

(table 7). Although several general observations can 

be made, it is difficult to interpret the 3H values in this 

study without additional information because the 

values are close to the mean detection limit of 

0.85 TU. The precipitation sample collected in July 

had a value of 12.6 TU, which is consistent with 

modem precipitation values determined elsewhere in 

the region (unpublished data, F. Edwin Harvey, School 

of Natural Resource Sciences and Conservation and 

Survey Division, University of Nebraska-Lincoln, 

2000). The Sherman Reservoir values, which represent 

a more time-integrated sample than the other samples, 

generally were about 5 to 6 TU (table 7). This indicates that a more likely local average yearly precipitation value may be 6 TU or that the Sherman Reservoir 

samples may be a mixture of precipitation and groundwater base flow, which is believed to contain little or 

no 3H on the basis of water samples from wells in the 

area. Without additional information on the relative 

contribution of ground water to upstream sources, it is 

not clear which scenario is correct. The majority of the 

samples had very low 3H values (less than 6 to 12 TU) 

(table 7), indicating that the water was recharged prior 

to the 1960s (table 6).

Assuming pre-1950 3H concentrations in ground 

water are no greater than 6 to 7 TU, water containing 

greater than 0.5 TU today (2000) can be used to indicate some component of young (post-1952) water in 

the sample (Solomon and Cook, 2000). To verify the 

concordance of the 3H and CFC dates, all 3H concentrations from the water samples were back calculated 

from the 2000 collection date (fig. 11). The results 

shown in figure 11 indicate that all water samples 

collected in wells from Loup City have decayed from 

water that was present before 1960 and, thus, are 

concordant with the CFC data, which provide for a 

more accurate date of recharge.

SOURCES OF GROUND WATER

Analysis of the physical and chemical data from 

the water samples precludes determination of a definitive source of the ground water beneath Loup City. 

The data presented here show that Sherman Reservoir 

is not the direct source of the ground water because the 

isotopic signature of the ground-water samples from 

monitoring wells in Loup City do not show evidence 

of a Sherman Reservoir component, and the age of the 

ground water beneath Loup City predates Sherman 

Reservoir. The age of the ground water also rules out 

local precipitation as its source. However, it is possible 

that (1) the water seeping from Sherman Reservoir has 

not moved as far as Loup City, (2) the ground water 

underlying Loup City is older water whose flow paths 

have been changed by the ground-water mound underlying Sherman Reservoir, or (3) Sherman Reservoir is 

not affecting the ground-water-flow paths beneath 

Loup City. An argument can be made against option 1 

because, on the basis of figure 7, ground-water levels 

between Sherman Reservoir and Loup City have stabilized, thus indicating that the water from Sherman 

Reservoir has had time to reach Loup City.

An estimate of the average linear horizontal 

ground-water-flow velocity in the shallowest parts of 

the Ogallala aquifer (which, for the purposes of this 

report, also include the Pliocene-age alluvial deposits 

overlying the Ogallala Formation) can be calculated 

by using Darcy's law as described by Hall and others 

(1991, p. 172) plus the hydraulic conductivity, effective porosity (Olsson Environmental Services, Inc., 

1997) (100 ft/d and 0.30, respectively), and groundwater head differences reported previously for the 

Ogallala aquifer (0.0064 ft/ft). Darcy's law, including 

effective porosity, is written as:

(3)

where

V = average linear ground-water-flow velocity, in feet per day; 

K = horizontal hydraulic conductivity, in feet

per day; 

/ = horizontal hydraulic gradient, in feet per

foot; and

n = effective porosity, as a decimal fraction. 

Using equation 3, the average linear velocity for 

the Ogallala aquifer is about 2.3 ft/d. Therefore, if an 

assumption is made that surface-water seepage started 

shortly after Sherman Reservoir was completed (1962)

Sources of Ground Water 31
Surface-water sites Domestic wells

10"

103

102

10'

10°

ID' 1

Calculated biannual 

tritium concentration

1 Sherman Res 1-D 

Sherman Res 1-M 

Sherman Res 1-S

 Sherman Res 2-D 

Sherman Res 2-M 

Sherman Res 2-S

 Middle Loup River (Res, Reservoir)

10"

103

.E 102

10 1

:e 10°

Loup City observation and 

public-supply wells

ID' 1

Calculated biannual 

.tritium concentration

10"

103

102

101

10°

Bureau of Reclamation observation wells

10' 1

Calculated biannual 

tritium concentration BORLC-20

 BOR LC-52

- BOR ROL-3

 BORROW

 BOR RR-2

- BOR RR-3

Calculated biannual

ition

 DW-1

 DW-2

- DW-3

 DW-4

 SRGP-1

Multi-level monitoring wells

Calculated biannual 

tritium concentration LCMW1-D 

LCMW 1-M 

LCMW1-S 

LCMW 2-D 

LCMW 2-M 

LCMW 2-S

Calculated and measured tritium 

concentrations in precipitation

Ottawa, Canada 

Lincoln, Nebraska 

Calculated tritium value

-Calculated tritium concentration 

for latitude 41° North and 

longitude 99° West

1953 1961 1969 1977 1985 1993 2001 1953 1961 1969 1977 1985 1993 2001

Figure 11. Tritium concentrations in water samples collected from study area in 2000 and back calculated to 

estimate historic concentrations. Location of sampling sites shown in figure 2.

32 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
and water from Sherman Reservoir traveled in the 

Pliocene-age alluvial deposits and the shallowest part 

of the Ogallala Formation, it would take ground water 

about 9,900 days (about 27 years) to travel the 4.3 mi 

(22,700 ft) distance from Sherman Reservoir (at the 

thalweg of Oak Creek Valley) to Loup City. This is 

well within the time frame between the construction of 

Sherman Reservoir and the present day (2000).

An argument also can be made against option 3 

because a hydraulic gradient exists from Sherman 

Reservoir toward Loup City, which results from water 

seeping from Sherman Reservoir. Therefore, it is 

likely that if Sherman Reservoir is affecting the 

ground water beneath Loup City, the pressure head 

that is exerted by water in Sherman Reservoir has 

changed the regional ground-water-flow path.

A small upward ground-water head difference of 

about 0.20 ft was measured between water levels in 

wells in the Ogallala aquifer (wells LCMW1-D 

and LCMW2-D) and wells in the alluvium 

(wells LCMW1-M and LCMW1-S, and LCMW2-M 

and well LCMW2-S) at the two multi-level monitoring-well sites. This indicates that ground water has the 

potential to move from the underlying Ogallala aquifer 

into the alluvial aquifer beneath Loup City. Therefore, 

it is likely that, although the ground water underlying 

Loup City did not reside in Sherman Reservoir, the 

pressure head from Sherman Reservoir feasibly could 

have affected ground-water levels beneath Loup City 

and, thus, caused an increase in the hydraulic pressure 

between the two aquifers.

SUMMARY

Locally, ground-water levels near Sherman Reservoir have risen since the Bureau of Reclamation 

constructed Sherman Reservoir in 1962. Loup City, 

about 2.5 mi west-southwest of Sherman Reservoir, 

also has experienced high ground-water levels in parts 

of the city. Local officials have attributed these high 

ground-water levels to seepage from Sherman Reservoir. In 2000, the USGS, in cooperation with the 

University of Nebraska-Lincoln and the city of Loup 

City, began a study to investigate the sources of 

ground water beneath Loup City, Nebraska. Waterquality samples collected in March, April, and July 

2000 from 24 surface-water and ground-water sites 

were analyzed to quantify the source of ground water 

beneath Loup City. These water samples were 

analyzed for physical properties, major ions, nutrients,

and selected isotopes ( 18 0, 2H, and 3H). Samples from 

a subset of the sites were analyzed for CFC and 

selected dissolved gases.

Hydrographs of historical ground-water levels 

from BOR observation wells indicate that groundwater levels have risen about 85 ft since Sherman 

Reservoir was completed in 1962. Also, a comparison 

of the water table before Sherman Reservoir construction with the 2000 water table shows that the groundwater-flow direction between Sherman Reservoir and 

Loup City has changed. This indicates that Sherman 

Reservoir is losing water to the ground-water system 

and locally affecting ground-water levels in wells 

between Sherman Reservoir and Loup City.

Water-quality data show that the only physical 

property that was substantially different between the 

surface water and the ground water in the alluvial 

and the Ogallala aquifers was the concentration of 

dissolved oxygen. Dissolved-oxygen concentrations in 

water samples from the alluvial aquifer were much 

smaller and significantly different than those in water 

samples from the Ogallala aquifer or the surfacewater sites.

Water-chemistry data indicate that the water types 

of all three sources were similar and predominantly 

were a calcium bicarbonate type. Water chemistry at 

the surface-water sites shows that the compositions of 

major ions were tightly grouped. Therefore, chemical 

concentrations in Sherman Reservoir did not change 

much following diversions from the Middle Loup 

River. Evaporative processes that occurred in 

Sherman Reservoir did not concentrate chemical 

species in the lake.

Water varied more in chemistry between groundwater sites than between surface-water sites. Median 

sulfate concentrations in water samples from the alluvial aquifer were nearly twice the median concentration of sulfate measured in water samples from the 

Ogallala aquifer. Water samples from the domestic 

wells, the Loup City observation wells, and the multilevel monitoring wells all seem to have a large calcium 

component. Water samples from BOR observation 

wells, which likely are open to the alluvial deposits 

overlying the Ogallala aquifer, typically contained 

larger sodium concentrations. The large calcium 

concentrations in water samples from the domestic 

wells, the Loup City observation wells, and the multilevel monitoring wells indicate that (1) all of the 

domestic wells are open to the deeper Ogallala aquifer 

and (2) the Loup City observation and multi-level

Summary 33
monitoring wells all contain water that presently is in 

the Ogallala aquifer or once resided in the Ogallala 

aquifer.

Age-dating techniques indicate that the surfacewater sites contain modern water (less than 10 years 

old). However, modeled recharge dates of the ground 

water underlying Loup City generally ranged from 

mid-1940s to early 1960s. Water samples collected 

from Loup City's public-supply well contained a 

modeled recharge date of mid- to late 1950s. Water 

samples collected from the shallowest multi-level 

monitoring wells had modeled recharge ages from late 

1950s to early 1960s. Only water samples collected 

from domestic well SRGP-1 indicated water young 

enough, modeled recharge age of mid-1970s, to originate from Sherman Reservoir. Therefore, the age of 

the ground water beneath Loup City probably predates 

construction of Sherman Reservoir. Tritium values 

collected from wells in Loup City support the CFC 

data.

Oxygen-18 and 2H isotopic data show an enriched 

evaporative signature for water samples collected 

from Sherman Reservoir, which indicates that water 

samples from Sherman Reservoir have undergone 

evaporation. Ground-water samples from two BOR 

observation wells in Loup City also have been affected 

substantially by evaporation, but the source of their 

water likely is from seepage from the nearby Canal 4 

and not Sherman Reservoir. Many of the remaining 

ground-water samples appear to have been affected by 

slight evaporation, but this possibly results from evaporation at the time of recharge and not from seepage 

from Sherman Reservoir.

The sources of the water beneath Loup City 

cannot be determined accurately from available data. 

However, from analysis of the water chemistry and 

isotope data, it appears that the water underlying Loup 

City does not show evidence of a Sherman Reservoir 

component. Furthermore, it is likely that, although the 

water underlying Loup City predates construction of 

Sherman Reservoir, the pressure head from Sherman 

Reservoir feasibly could have affected ground-water 

levels beneath Loup City and caused an increase in the 

hydraulic pressure between the Ogallala and alluvial 

aquifers.

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E.G., 1983, Migration of contaminants in groundwaters at a landfill a case study, part 3. Tritium as an 

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of environmental isotope geochemistry: Amsterdam, 

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Foster, S.S.D., 1975, The chalk groundwater tritium 

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Hall, S.H., Luttrell, S.P., and Cronin, W.E., 1991, A method 

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determine the isotope composition of precipitation characterizing the meteoric input function for 

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composition of precipitation in the semi-arid northcentral portion of the U.S. Great Plains: Journal of 

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water resources: Amsterdam, Elsevier Publications, 

522 p.

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characteristics of natural water (3d ed.): U.S. Geological Survey Water-Supply Paper 2254, 263 p.

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http://isohis.iaea.org/Projects.asp

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of the tritium interface method for determining 

recharge rates to unconfined drift aquifers, volume I, 

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review and critical evaluation of the half-life of tritium: 

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Comments of the NBS tritiated- water standards and 

their use: International Journal of Applied Radiatomical Isotopes, v. 33, p. 383-386.

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sherman.html

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of parts of the Central Platte and Lower Loup Natural 

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36 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
APPENDIX

Appendix 37
38 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
Table 8. Results of water-quality analysis for major ions in water samples from study area, central Nebraska, 2000

[all values in milligrams per liter unless noted; ID, identifier; fig., figure; ANC, acid-neutralizing capacity; ng/L, micrograms per liter; --, not analyzed or no d

Reservoir]

Field ID 

(fig. 2)

Middle Loup 

River

Canal 4

SR1-D

SR1-M

SR1-S

SR2-D

SR2-M

SR2-S

Date Time

3/30/2000 1630 

7/17/2000 0910

7/20/2000 1445

3/30/2000 1220

3/30/2000 1230

7/19/2000 1520

3/30/2000 1240

3/30/2000 1350

3/30/2000 1400

7/19/2000 1610

3/30/2000 1410

Calcium 

carbonate

83 

100

81

87

86

83

87

87

92

82

87

Calcium

27 

34

26

28

28

27

28

28

30

26

28

Magnesium

4 

5.1

3.8

4.2

4.1

4.0

4.2

4.1

4.3

4.1

4.2

Sodium

7.4 

7.7

6.9

7.5

7.5

7.5

7.5

7.5

7.3

7.4

7.4

Sodium 

adsorption

Potassium ANC

Surface-water sites 

0.4 7.1 97

.3 7.2

.3

.4

.4

.4

.4

.3

.3

.4

.3

6.2 94

6.8 109

10 109

7.0 104

6.9 109

6.7 109

6.8 109

7

6.6 108

Carbondioxide

1.2

~

2.1

2.1

--

2.1

3.3

2.1

-

2.1

Sulfate

6.1 

8.9

6.6

6.2

6.2

7.0

6.2

6.2

6.2

6.9

6.5

Chloride

1.5

1.7

1.5

1.2

1.2

1.1

1.2

1.3

1.2

1.1

1.5

Fluoride

0.35

.28

.28

.39

.38

.30

.38

.38

.38

.31

.36

Ground-water sites

BOR LC-20

BOR LC-52

-

3/31/2000 1200

4/04/2000 1230

7/18/2000 1520

480

220

240

92

67

74

61

12

13

23

41

39

.5

1.0

1.0

44 832

12 345

11 333

102

33

27

3.5

2.7

18

13

12

.80

.40

.37

BORROL-3 4/05/2000 1000 34 11 1.5

BORROL-^ 4/05/2000 1430 170 52 10 20

.7

.7

5.2 82 13 1.9

8.6 289

BORRR-2 4/05/2000 1100 47 14 2.6 7.4 .5 5.2 204

BORRR-3 4/05/2000 1215 27 10 .36 4.9 .4 8.1 240

20

2.3

2.8 .25

6.9 .46

5.3 .28

5.8 18 .30 
Table 8. Results of water-quality analysis for major ions in water samples from study area, central Nebraska, 2000 Continued

UseofWatei

5

to

1g-

 o 

to

C/I

Q>

o. 

o

=r 

o

3

luorocarbor

is to Invest!

CO 

CB

S

CO

CO

e

CB 

CA

O

|

a.

i

3

 Beneath 1

£

 a

n

o

CP

1Gtt

(D

er

3 

If

i

Field ID 

(fig. 2) Date Time

Calcium 

carbonate Calcium

Magnesium Sodium

Sodium 

adsorption

Potassium ANC

Carbondioxide Sulfate Chloride Fluoride 

Ground-water sites Continued

DW-1

DW-2

DW-3

DW-4

LC10-H

LC11-J

LC9-H-A

LC9-I

LC9-J

LCLS-1

LCMWl-D

LCMW1-M

LCMW1-S

4/05/2000

7/20/2000

4/05/2000

7/20/2000

4/06/2000

7/20/2000

4/06/2000 

7/20/2000

4/04/2000

3/31/2000

3/31/2000

7/21/2000

3/31/2000

3/31/2000

4/06/2000

3/28/2000

7/24/2000

3/28/2000

7/24/2000

3/28/2000

7/24/2000

1600

1050

1645

1230

0900

1340

0945 

0800

1330

0835

1030

1220

1330

1235

1030

1240

1210

1525

1250

1615

1330

230

250

220

230

240

250

200 

210

240

270

240

250

270

320

280

240

240

240

240

260

77

84

72

74

81

84

66 

69

77

86

81

83

87

91

86

72

76

77

77

84

83

9.5

9.9

11

11

10

10

8.3 

8.4

12

13

9.9

10

13

21

16

14

12

12

12

11

10

6.9

6.5

7.2

7

7

6.2

7 

6.7

8.7

14

9.2

9.4

9.3

48

180

10

7.5

8.2

8.1

9.3

8.6

0.2

.2

.2

.2

.2

.2

.2 

.2

.2

.4

.3

.3

.2

1.0

5.0

.3

.02

.2

.2

.3

4.7

4.7

7.4

6.8

6.2

5.8

4.2 

4.1

11

14

10

8.3

11

21

12

11

7.9

11

9.7

10

8.8

261

260

248

236

271

260

219

222

386

317

274

273

280

443

375

311

270

275

272

272

269

33

28

~

43

--

34

37

39

35

~

54

68

29

24

-

21

_

21

..

6.8

7.3

9.1

10

6.1

6.4

9.1 

10

13

16

10

12

22

10

28

11

8

13

11

26

27

3.8

4.7

3.8

4.5

3.8

4.3

4

4.8

5.5

4.7

4.4

4.5

4.9

30

260

3.6

4.2

2.9

3.9

3.6

4.5

0.20

.18

.19

.18

.20

.19

.27 

.24

.32

.39

.31

.23

.35

.41

.22

.24

.14

.22

.16

.26
Table 8. Results of water-quality analysis for major ions in water samples from study area, central Nebraska, 2000 Continued

Field ID 

(fig. 2)

LCMW2-D

LCMW2-M

LCMW2-S

LCPS-1

SRGP-1

Date

3/29/2000

7/24/2000

3/29/2000

7/24/2000

3/29/2000

7/24/2000

3/29/2000

7/21/2000

3/30/2000

7/20/2000

Time

0925

0850

1040

0940

1155

1020

1450

1450

0900

0930

Calcium 

carbonate

260

250

270

260

260

250

270

--

130

130

Calcium

81

80

83

83

80

77

89

89

40

41

Magnesium

13

13

14

13

16

14

12

12

7.5

7.6

Sodium

16

11

13

13

24

22

9.4

9.7

7.6

8

Sodium 

adsorption

Potassium ANC

Carbondioxide

Ground- water sites Continued

0.4 11 285 44

.3

.3

.4

.7

.6

.3

~

.3

.3

9.8

11

10

18

40

6.2

6.8

7.5

7

283

308

303

329

314

286

285

156

162

--

38

--

32

 --

28

-

12

-

Sulfate

16

15

15

12

24

20

17

18

5.6

6.2

Chloride

3.8

4.7

3.8

4.3

6

6.1

3.7

4.9

1.3

1.2

Fluoride

0.26

.17

.23

.14

.48

.37

.19

.15

.46

.36
Table 9. Results of water-quality analysis for nutrients in water samples from study area, central Nebraska, 2000

[all concentrations in milligrams per liter; USGS, U.S. Geological Survey; ID, identification number or identifier; fig., figure; N, nitrogen; <, less than; , no 

result]

USGS site ID

06779500

06778450

06784195001

06784195002

06784195003

06784190001

06784190002

06784190003

Field ID 

(fig. 2)

Middle Loup River

Canal 4

Sherman Reservoir 1-D

Sherman Reservoir 1-M

Sherman Reservoir 1-S

Sherman Reservoir 2-D

Sherman Reservoir 2-M

Sherman Reservoir 2-S

Date

3/30/2000

7/17/2000

7/20/2000

3/30/2000

3/30/2000

7/19/2000

3/30/2000

3/30/2000

3/30/2000

7/19/2000

3/30/2000

Time Nitrate as N

Surface-water sites

1630

0910

1445

1220

1230

1520

1240

1350

1400

1610

1410

0.611

<.050

<.050

<.050

<.050

<.050

<.050

<.050

<.050

<.050

<.050

Ammonia

< 0.020

<.020

<.020

<.020

<.020

<.020

<.020

<.020

<.020

<.020

<.020

Nitrite as N Phosphorus

<0.01 0.139

< .01 .084

< .01 .095

< .01 .014

<.01 .013

< .01 .058

<.01 .015

< .01 .017

< .01 .014

< .01 .06

< .01 .014

Phosphate

0.43

.26

.29

.04

.04

.18

.05

.05

.04

.18

.04

Ground-water sites

411706098582201

411700098580201

411912098544801

411728098574001

411833098555701

411649098541401

411747098562001

411723098562801

411701098565001

BOR LC-20

BOR LC-52

BOR ROL-3

BOR ROL-4

BOR RR-2

BOR RR-3

DW-1

DW-2

DW-3

3/31/2000

4/04/2000

7/18/2000

4/05/2000

7/19/2000

4/05/2000

7/26/2000

4/05/2000

7/19/2000

4/05/2000

4/05/2000

7/20/2000

4/05/2000

7/20/2000

4/06/2000

7/20/2000

1200

1230

1520

1000

1050

1430

0730

1100

0850

1215

1600

1050

1645

1230

0900

1340

15

<.050

<.050

.338

3.42

2.15

1.54

.075

1.17

<.050

1.19

1.14

2.53

2.54

2.91

2.87

.187

1.46

1.53

.025

.289

.038

.44

.047

.16

.101

<.020

<.020

<.020

<.020

<.020

<.020

< .01 .712

<.01 1.48

<.01 1.57

< .01 < .010

.12 <.010

.01 . < .010

.47 < .010

.02 < .010

.01 < .010

< .01 < .010

< .01 .03

< .01 .041

< .01 .055

< .01 .073

< .01 .044

< .01 .04

2.2

4.5

4.8

__

--

-

--

--

.09

.13

.17

.22

.13

.12

42 Use of Water Chemistry, Isotopes, and Chlorofluorocarbons to Investigate the Sources of Ground Water Beneath Loup City, Central Nebraska, 2000
Table 9. Results of water-quality analysis for nutrients in water samples from study area, central Nebraska, 2000 Continued

USGS site ID

411648098542401

411656098581701

411648098582301

411654098581501

411652098581201

411648098581301

411619098581001

411637098581001

411637098581002

411637098581003

411656098582701

411656098582702

411656098582703

411706098581001

411709098540101

Field ID 

(fig. 2}

DW-4

LC10-H

LC11-J

LC9-H-A

LC9-I

LC9-J

LCLS-1

LCMW1-D

LCMW1-M

LCMW1-S

LCMW2-D

LCMW2-M

LCMW2-S

LCPS-1

SRGP-1

Date

Ground4/06/2000 

7/20/2000

4/04/2000 

7/21/2000

3/31/2000 

7/21/2000

3/31/2000 

7/21/2000

3/31/2000

3/31/2000 

7/21/2000

4/06/2000 

7/24/2000

3/28/2000 

7/24/2000

3/28/2000 

7/24/2000

3/28/2000 

7/24/2000

3/29/2000 

7/24/2000

3/29/2000 

7/24/2000

3/29/2000 

7/24/2000

3/29/2000 

7/21/2000

3/30/2000 

7/20/2000

Time Nitrate as N

water sites Continued 

0945 0.993 

0800 .857

1330 

1340

0835 

0800

1030 

1220

1330

1235 

1300

1030 

1420

1240 

1210

1525 

1250

1615 

1330

0925 

0850

1040 

0940

1155 

1020

1450 

1450

0900 

0930

.521 

.929

<.050 

<.050

1.37 

1.21

.06

.113 

<.050

.67 

.598

1.29 

1.31

.698

.875

<.050 

<.050

1.5 

1.48

1.55 

1.5

<.050 

<.050

2.24 

2.55

.104 

.073

Ammonia

< 0.020 

<.020

.061 

<.020

.03 

.033

<.020 

<.020

.071

.53 

.43

8.58 

9.49

.033 

<.020

.022 

<.020

.063 

.055

<.020 

<.020

<.020 

<.020

.037 

.03

<.020 

<.020

<.020 

<.020

Nitrite as N Phosphorus

< 0.01 0.025 

< .01 .025

.01 .096 

.01 .079

<.01 .106 

<.01 .116

< .01 .089 

< .01 .094

<.01 .121

<.01 .181 

< .01 .259

.05 1.84 

.04 1.61

.02 .082 

< .01 .053

< .01 .055 

< .01 .045

< .01 .088 

< .01 .068

.01 .056 

< .01 .054

.02 .067 

.01 .068

<.01 .119 

<.01 .128

< .01 .047 

< .01 .049

< .01 .201 

<.01 .178

Phosphate

0.08 

.08

.29

.24

.33 

.36

.27 

.29

.37

.55 

.79

5.7 

4.9

.25 

.16

.17 

.14

.27 

.21

.17 

.17

.21 

.07

.36 

.39

.14 

.15

.62

.55

Appendix 43
G.V. Steele and F.E. Harvey USE OF WATER CHEMISTRY, ISOTOPES, AND CHLOROFLUOROCARBONS TO INVESTIGATE THE SOURCES 

OF GROUND WATER BENEATH LOUP CITY, CENTRAL NEBRASKA, 2000 USGS/WRIR 02-4049

i5u

s