Loess (Unsaturated)

Sand and gravel

(Unconfined aquifer)

Clayey silt

(Upper confining unit)

Clayey silt

(Lower confining

unit)

Sand

(Upper confined aquifer)

Sand and silt

(Lower confined

lenses)

Shale

Recharge

Recharge

Recharge Recharge

Generalized flow path

Water table

Supply

well

Discharge

Urban

area

Scientific Investigations Report 2008-5050

U.S. Department of the Interior

U.S. Geological Survey

National Water-Quality Assessment Program

Transport of Anthropogenic and Natural Contaminants (TANC) to Public-Supply Wells

Hydrogeology, Chemical Characteristics, and Transport

Processes in the Zone of Contribution of a Public-Supply

Well in York, Nebraska

Hydrogeology, Chemical Characteristics, 

and Transport Processes in the Zone of 

Contribution of a Public-Supply Well in 

York, Nebraska 

By Matthew K. Landon, Brian R. Clark, Peter B. McMahon, Virginia L. McGuire, 

and Michael J. Turco

National Water-Quality Assessment Program 

Transport of Anthropogenic and Natural Contaminants (TANC) 

to Public-Supply Wells

Scientific Investigations Report 2008–5050

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

For product and ordering information:

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

Telephone: 1-888-ASK-USGS

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natural hazards, and the environment:

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

U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Landon, M.K., Clark, B.R., McMahon, P.B., McGuire, V.L., and Turco, M.J., 2008, Hydrogeology, chemical characteristics, and transport processes in the zone of contribution of a public-supply well in York, Nebraska: U.S. Geological 

Survey Scientific Investigations Report 2008–5050, 149 p.
iii

FOREWORD

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

to enhance and protect the overall quality of life and that facilitates effective management of water, biological, energy, 

and mineral resources (http://www.usgs.gov/). Information on the Nation’s water resources is critical to ensuring 

long-term availability of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish 

and wildlife. Population growth and increasing demands for water make the availability of that water, now measured in 

terms of quantity and quality, even more essential to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to support national, 

regional, State, and local information needs and decisions related to water-quality management and policy (http://

water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the condition of our Nation’s streams 

and ground water? How are conditions changing over time? How do natural features and human activities affect the 

quality of streams and ground water, and where are those effects most pronounced? By combining information on water 

chemistry, physical characteristics, stream habitat, and aquatic life, the NAWQA Program aims to provide science-based 

insights for current and emerging water issues and priorities. From 1991-2001, the NAWQA Program completed interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 of the Nation’s river 

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

Multiple national and regional assessments are ongoing in the second decade (2001–2012) of the NAWQA Program as 

42 of the 51 Study Units are reassessed. These assessments extend the findings in the Study Units by determining status and trends at sites that have been consistently monitored for more than a decade, and filling critical gaps in characterizing the quality of surface water and ground water. For example, increased emphasis has been placed on assessing 

the quality of source water and finished water associated with many of the Nation’s largest community water systems. 

During the second decade, NAWQA is addressing five national priority topics that build an understanding of how natural 

features and human activities affect water quality, and establish links between sources of contaminants, the transport 

of those contaminants through the hydrologic system, and the potential effects of contaminants on humans and aquatic 

ecosystems. Included are topics on the fate of agricultural chemicals, effects of urbanization on stream ecosystems, 

bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on aquatic ecosystems, and transport 

of contaminants to public-supply wells. These topical studies are conducted in those Study Units most affected by these 

issues; they comprise a set of multi-Study-Unit designs for systematic national assessment. In addition, national syntheses of information on pesticides, volatile organic compounds (VOCs), nutrients, selected trace elements, and aquatic 

ecology are continuing. 

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

water-resource management and strategies that protect and restore water quality. We hope this NAWQA publication 

will provide you with insights and information to meet your needs, and will foster increased citizen awareness and 

involvement in the protection and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all water-resource issues of 

interest. External coordination at all levels is critical for cost-effective management, regulation, and conservation of our 

Nation’s water resources. The NAWQA Program, therefore, depends on advice and information from other agencies—

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

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

Matthew C. Larsen

Acting Associate Director for Water
This page has been intentionally left blank.
v

Contents

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

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

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

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

Previous Investigations........................................................................................................................8

Methods...........................................................................................................................................................8

Design of Well Network.......................................................................................................................8

Drilling, Well Installation, Coring, and Core Analysis ..................................................................11

Collection and Analysis of Water Samples ....................................................................................12

Collecting Depth-Dependent Flow and Chemistry Data...............................................................14

Quality Assurance...............................................................................................................................15

Blanks. .........................................................................................................................................15

Replicates....................................................................................................................................16

Spikes. .........................................................................................................................................17

Surrogates...................................................................................................................................17

Statistical Methods.............................................................................................................................17

Estimation of Aquifer Properties ......................................................................................................18

Hydrogeology................................................................................................................................................18

Geology, Mineralogy, and Sediment Chemistry.............................................................................18

Ground-Water Occurrence and Movement...................................................................................21

Aquifer Hydraulic Properties ............................................................................................................22

Water Budget ......................................................................................................................................25

Well Classification .......................................................................................................................................27

Chemical Characteristics ...........................................................................................................................27

Stable Isotopes....................................................................................................................................28

Precipitation and Return Flow Fractions in the Shallow Unconfined Aquifer.................30

Fractions of Water from the Unconfined Aquifer in Wells with Mixed Waters 

in the Confined Aquifers..............................................................................................31

Isotopic Differences between Unconfined and Confined Aquifers ..................................31

Oxidation-Reduction and pH Conditions.........................................................................................32

Volatile Organic Compounds.............................................................................................................37

Pesticides.............................................................................................................................................37

Ground-Water Age Tracers ..............................................................................................................40

Nutrients...............................................................................................................................................44

Major Elements ...................................................................................................................................48

Uranium ................................................................................................................................................57

Arsenic..................................................................................................................................................62

Depth-Dependent Sampling..............................................................................................................65

Flow and Chemistry Profile Checks ........................................................................................65

Depth Intervals Contributing Solutes to the Study Public-Supply Well............................69
vi

Processes Affecting Transport of Anthropogenic and Natural Contaminants to 

Public-Supply Wells.......................................................................................................................70

Sources of Water and Solutes..........................................................................................................70

Movement of Water and Solutes to Supply Wells ........................................................................72

Evidence for Well-Bore Leakage ...........................................................................................73

Effects of Attenuation Reactions ............................................................................................78

Supply-Well Effects on Transport of Water and Solutes..............................................................79

Summary........................................................................................................................................................79

Acknowledgements.....................................................................................................................................81

References....................................................................................................................................................81

Tables. ...........................................................................................................................................................91

Figures

Figure 1. Map showing topographic and hydrogeologic features of the regional- and 

local-scale TANC study areas near York, Nebraska, and principal geologic units 

of the High Plains aquifer .................................................................. 5

Figure 2. Conceptual model of ground-water flow in the local-scale TANC study area 

near York, Nebraska ........................................................................ 6

Figure 3. Map showing location of wells installed for the local-scale TANC study 

near York, Nebraska ........................................................................ 7

Figure 4. Distribution of potential contaminant sources compiled from various sources 

in the local-scale TANC study area near York, Nebraska .............................. 9

Figure 5. Cross sections showing hydrogeologic units and geophysical logs from selected 

test holes drilled for the local-scale TANC study near York, Nebraska ............ 19

Figure 6. Graphs showing water-level elevations in selected monitorng wells in the 

local-scale TANC study area near York, Nebraska, 2003–2005 ........................ 23

Figure 7. Oxygen and hydrogen isotope values for ground water in the local-scale 

TANC study area near York, Nebraska ................................................... 29

Figure 8. Graphs showing relation of oxidation-reduction indicators to depth of 

top of screen below the water table in the local-scale TANC study area 

near York, Nebraska ........................................................................ 33

Figure 9. Graph showing sum of dissolved oxygen and nitrate (as nitrogen) and sum of 

manganese and iron in wells in the local-scale TANC study area near York, 

Nebraska .................................................................................... 34

Figure 10. Graphs showing concentrations of selected constituents for unconfined 

shallow wells and depth to water below land surface for the local-scale 

TANC study area near York, Nebraska ................................................... 35

Figure 11. Graph showing relation of pH to depth of top of screen below the water table

 in the local-scale TANC study area near York, Nebraska.............................. 36

Figure 12. Graphs showing relation of depth of top of screen below water table to 

concentrations of selected volatile organic chemicals in the local-scale 

TANC study area near York, Nebraska ................................................... 38

Figure 13. Cross section showing sums of concentrations of pesticides in ground water 

in the local-scale TANC study area near York, Nebraska .............................. 39
vii

Figure 14. Graphs showing concentrations of age-dating tracers in samples and historical 

atmospheric curves corrected for radioactive decay for piston, exponential, and 

exponential binary mixing models, local-scale TANC study area near York, 

Nebraska .................................................................................... 41

Figure 15. Graphs showing relations in ground water in the local-scale TANC study area 

near York, Nebraska: A. measured nitrogen and oxygen isotopic values of nitrate, 

B. measured nitrogen isotopic values of nitrate and concentrations of nitrate, 

C. calculated initial nitrogen isotopic composition of nitrate and concentrations of 

nitrate prior to denitrification, and D. calculated initial nitrogen isotopic composition 

of nitrate prior to denitrification and concentration of orthophosphate .................. 46

Figure 16. Piper diagram showing general types of ground water in the local-scale 

TANC study area near York, Nebraska ................................................... 49

Figure 17. Graphs showing relations between chloride and A. initial δ15N-nitrate, 

B. potassium, and C. orthophosphate and between sulfate and D. calculated 

initial δ15N-nitrate and E. boron in ground water from the local-scale TANC 

study area near York, Nebraska ......................................................... 51

Figure 18. Graph showing chloride and chloride/bromide in ground-water samples 

from the local-scale TANC study area near York, Nebraska ........................... 53

Figure 19. Graphs showing relations between tracers in ground-water samples from the 

local-scale TANC study area near York, Nebraska: δ18O and A. chloride, B. sulfate; 

chloride and C. sulfate, D. bromide, and E. PCE (tetrachloroethylene) .................. 54

Figure 20. Graphs showing relations in ground-water samples from the TANC local-scale 

study area near York, Nebraska: A. pH and bicarbonate, B. chloride and bicarbonate, 

C. chloride and calcium. ..................................................................... 56

Figure 21. Graphs showing the relation of uranium to A. depth below water table and 

B. uranium isotope ratios for samples collected from the local-scale TANC 

study area in York, Nebraska............................................................... 58

Figure 22. Graphs showing relation of concentrations of uranium to A. bicarbonate, 

B. chloride, C. δ15N-nitrate, D. orthophosphate, E. calcium, and, F. iron in samples

from the local-scale TANC study area near York, Nebraska............................ 59

Figure 23. Graphs showing arsenic speciation in ground water in the local-scale 

TANC study area near York, Nebraska ................................................... 63

Figure 24. Graphs showing arsenic and A. chloride and B. orthophosphate, and C. arsenate 

and iron concentrations in ground water in the local-scale TANC study near 

York, Nebraska .............................................................................. 64

Figure 25. Graphs showing the vertical distribution of flow and concentrations of selected 

constituents in samples from different depths in the study supply well and 

adjacent monitoring wells in the local-scale TANC study area near York, 

Nebraska .................................................................................... 66

Figure 26. Graphs showing the vertical distribution of unconfined mixing fractions in samples 

from different depths in the confined layer in the local-scale TANC study area near 

York, Nebraska .............................................................................. 74

Figure 27. Scenarios for movement of unconfined water into the confined aquifer in the 

local-scale TANC study area near York, Nebraska .................................... 75
viii

Tables 

Table 1. Well construction information for monitoring wells installed for the local-scale 

TANC study near York, Nebraska ......................................................... 92

Table 2. Analytes and methods of analyses for water samples collected from wells 

during October 2003–April 2005 for the local-scale TANC study near York, 

Nebraska .................................................................................... 94

Table 3. Quality-control summary for constituents detected in field blanks and groundwater samples collected for the local-scale TANC study near York, Nebraska ... 96

Table 4. Quality-control summary of replicate analytes with relative standard deviations 

greater than zero, collected for the local-scale TANC study near York, 

Nebraska .................................................................................... 97

Table 5A. Quality-control summary of matrix spike recoveries for volatile organic 

compounds and gasoline additives and oxygenates in samples collected 

for the local-scale TANC study near York, Nebraska ................................. 98

Table 5B. Quality-control summary of matrix spike recoveries for pesticides and 

pesticide degradates in samples collected for the local-scale TANC study 

near York, Nebraska ........................................................................100

Table 6. Hydrogeologic units in the local-scale TANC study area near York, Nebraska......102

Table 7. Grain-size analyses of selected core samples collected for the local-scale 

TANC study near York, Nebraska. .........................................................103

Table 8. Mineralogy of selected core samples collected for the local-scale TANC study 

near York, Nebraska ........................................................................104

Table 9. Elemental and isotopic analyses of selected core samples collected for the 

local-scale TANC study near York, Nebraska ..........................................106

Table 10. Values of hydraulic conductivity determined from slug tests in monitoring 

wells and from historical aquifer tests in the local-scale TANC study area 

near York, Nebraska ........................................................................109

Table 11. Wells classified by degree of confinement, depth, land use, well type, and 

mixing, in and near York, Nebraska ......................................................111

Table 12. Hydrogen and oxygen stable isotopic values and mixing calculations for the 

local-scale TANC study near York, Nebraska ..........................................112

Table 13. Concentrations of constituents used to classify oxidation–reduction (redox) 

conditions in ground-water samples collected for the local-scale TANC study 

near York, Nebraska ........................................................................115

Table 14. Results of analyses of volatile organic compounds and gasoline oxygenates in 

samples, and analytes detected in ground-water samples during October 2003 

through April 2005 in the local-scale TANC study area near York, Nebraska ......119

Table 15. Results of analyses of pesticides and pesticide degradates in ground-water 

samples in which analytes were detected during October 2003 through 

April 2005 in the local-scale TANC study area near York, Nebraska ..................125 

Table 16. Age tracer and supporting data for ground-water samples collected from the 

local-scale TANC study area near York, Nebraska ....................................126

Table 17. Age and mixing interpretations for ground-water samples from the local-scale 

TANC study area in York, Nebraska ......................................................128

Table 18. Results of analyses of nutrients in ground-water samples collected during 

October 2003 through April 2005 in the local-scale TANC study area near 

York, Nebraska ..............................................................................130
ix

Table 19. Results of analyses of selected isotopes of nitrogen, sulfur, uranium, and radon 

in ground-water samples collected during October 2003 through April 2005 in the 

local-scale TANC study area near York, Nebraska ....................................132

Table 20. Results of analyses of selected field parameters and major and minor elements 

in ground-water samples collected during October 2003 through April 2005 

in the local-scale TANC study area near York, Nebraska ..............................134

Table 21. Results of analyses of trace elements in ground-water samples collected during 

October 2003 through April 2005 in the local-scale TANC study area near 

York, Nebraska ..............................................................................141
x

Conversion Factors, Notes, Acronyms, and Abbreviations

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2

) 0.0002471 acre 

hectare (ha) 2.471 acre

square kilometer (km2

) 247.1 acre

square centimeter (cm2) 0.001076 square foot (ft2)

square meter (m2) 10.76 square foot (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 0.2642 gallon (gal)

cubic meter (m3

) 264.2 gallon (gal) 

cubic centimeter (cm3

) 0.06102 cubic inch (in3) 

liter (L) 61.02 cubic inch (in3

) 

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per second (m3

/s) 70.07 acre-foot per day (acre-ft/d) 

cubic meter per year (m3/yr) 0.000811 acre-foot per year (acre-ft/yr) 

meter per second (m/s) 3.281 foot per second (ft/s) 

meter per day (m/d) 3.281 foot per day (ft/d)

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

cubic meter per day (m3

/d) 35.31 cubic foot per day (ft3/d) 

liter per second (L/s) 15.85 gallon per minute (gal/min) 

cubic meter per day (m3/d) 264.2 gallon per day (gal/d) 

cubic meter per second (m3/s) 22.83 million gallons per day (Mgal/d) 

millimeter per year (mm/yr) 0.03937 inch per year (in/yr) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

megagram per year (Mg/yr) 1.102 ton per year (ton/yr)

metric ton per year 1.102 ton per year (ton/yr) 

Pressure

kilopascal (kPa) 0.2961 inch of mercury at 60°F (in Hg)

kilopascal (kPa) 0.1450 pound per square inch (lb/ft2

) 

Density

gram per cubic centimeter (g/cm3

) 62.4220 pound per cubic foot (lb/ft3) 

Radioactivity

becquerel per liter (Bq/L) 27.027 picocurie per liter (pCi/L) 

Specific capacity

liter per second per meter [(L/s)/m] 4.831 gallon per minute per foot [(gal/min)/ft]

Hydraulic conductivity

meter per day (m/d) 3.281 foot per day (ft/d) 

Hydraulic gradient

meter per kilometer (m/km) 5.27983 foot per mile (ft/mi) 

Application rate

kilograms per hectare per year [(kg/ha)/yr] 0.8921 pounds per acre per year [(lb/acre)/yr]
xi

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 

(NAD 83).

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

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

(μS/cm at 25°C).

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

or micrograms per liter (μg/L).

Acronyms and Abbreviations

bls below land surface 

BMM binary mixing model

CFC chlorofluorocarbon

CFC-11 trichlorofluoromethane

CFC-12 dichlorodifluoromethane

CFC-113 1,1,2-trichlorotrifluoroethane

CRA contributing recharge areas

CrCl2 chromium chloride

CSI Contaminant Source Inventory

DO dissolved oxygen

DOC dissolved organic carbon

δ13C carbon-13-to-carbon-12 isotopic ratio or delta carbon-13 of dissolved inorganic carbon

δD hydrogen-2-to-hydrogen-1 isotopic ratio or delta deuterium of water

δiE the ratio of a heavier isotope of an element to the more common lighter isotope of that 

element, relative to a standard reference material

δ15N-nitrate delta nitrogen-15-to-nitrogen-14 isotopic ratio or delta nitrogen-15 of dissolved nitrate

δ18O oxygen-18-to-oxygen-16 isotopic ratio or delta oxygen-18 of water

δ18O-nitrate oxygen-18-to-oxygen-16 isotopic ratio or delta oxygen-18 of dissolved nitrate

δ34S sulfur-34-to-sulfur-32 isotopic ratio or delta sulfate-34 of sulfate

ESA ethane sulfonic acid

GPM gallons per minute 3

H tritium 3

H/3He ratio of 3H and degradation product helium-3

HA hydroxylamine

HCl hydrochloric acid 3

He helium-3

HPGW High Plains Ground Water regional study unit

HNO3 nitric acid
xii

H2S hydrogen sulfide

ICP-MS inductively-coupled plasma/mass spectrometry

ICP-OES inductively-coupled plasma/optical emission spectroscopy

Kh horizontal hydraulic conductivity

Kv vertical hydraulic conductivity

LRL laboratory reporting level

LT-MDL long-term method detection level

M molar

MCL Maximum Contaminant Level

MWL Meteoric Water Line

N normal 

NaOH sodium oxide

NAWQA National Water Quality Assessment

nitrate-N nitrate as nitrogen

NWQL National Water Quality Laboratory

PCE tetrachloroethylene

PFM piston flow model

ppb parts per billion

PSW public-supply well

PVC polyvinyl chloride

r the Pearson or linear correlation coefficient

r2 coefficient of determination

RCRA Resource Conservation and Recovery Act

RSD relative standard deviation

s second

SF6 sulfur hexafluoride

SnCl2 stannous (tin) chloride

SWQA Source Water Quality Assessment 

σ standard deviation

TANC Transport of Anthropogenic and Natural Contaminants

TCE trichloroethylene

TDS total dissolved solids

TEAP Terminal Electron Acceptor Process

UBBNRD Upper Big Blue Natural Resources District

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VCDT Vienna Canyon Diablo Troilite

VOC volatile organic compound

VPDB Vienna Pee Dee belemnite

VSMOW Vienna Standard Mean Ocean Water

ZOC zones of contribution

> greater than

≥ greater than or equal to

< less than 

% percent
Abstract 

In 2001, the U.S. Geological Survey, as part of the 

National Water Quality Assessment (NAWQA) Program, 

initiated a topical study of Transport of Anthropogenic and 

Natural Contaminants (TANC) to PSW (public-supply wells). 

Local-scale and regional-scale TANC study areas were 

delineated within selected NAWQA study units for intensive 

study of processes effecting transport of contaminants to 

PSWs. This report describes results from a local-scale TANC 

study area at York, Nebraska, within the High Plains aquifer, 

including the hydrogeology and geochemistry of a 108-squarekilometer study area that contains the zone of contribution to 

a PSW selected for study (study PSW), and describes factors 

controlling the transport of selected anthropogenic and natural 

contaminants to PSWs.

Within the local-scale TANC study area, the High Plains 

aquifer is approximately 75 m (meter) thick, and includes an 

unconfined aquifer, an upper confining unit, an upper confined 

aquifer, and a lower confining unit with lower confined sand 

lenses (units below the upper confining unit are referred to 

as confined aquifers) in unconsolidated alluvial and glacial 

deposits overlain by loess and underlain by Cretaceous shale. 

From northwest to southeast, land use in the local-scale TANC 

study area changes from predominantly irrigated agricultural 

land to residential and commercial land in the small 

community of York (population approximately 8,100). 

For the purposes of comparing water chemistry, wells 

were classified by degree of aquifer confinement (unconfined

and confined), depth in the unconfined aquifer (shallow

and deep), land use (urban and agricultural), and extent of 

mixing in wells in the confined aquifer with water from the 

unconfined aquifer (mixed and unmixed). Oxygen (δ18O) and 

hydrogen (δD) stable isotopic values indicated a clear isotopic 

contrast between shallow wells in the unconfined aquifer 

(hereinafter, unconfined shallow wells) and most monitoring 

wells in the confined aquifers (hereinafter, confined unmixed

wells). δ18O and δD values for a minority of wells in the 

confined aquifers were intermediate between those for the 

unconfined shallow wells and those for the confined unmixed

wells. These intermediate values were consistent with mixing 

of water from unconfined and confined aquifers (hereinafter, 

confined mixed wells). Oxidation-reduction conditions were 

primarily oxic in the unconfined aquifer and variably reducing 

in the confined aquifers. 

Trace amounts of volatile organic compounds (VOC), 

particularly tetrachloroethylene (PCE) and trichloroethylene 

(TCE), were widely detected in unconfined shallow urban

wells and indicated the presence of young urban recharge 

waters in most confined mixed wells. The presence of 

degradation products of agricultural pesticides (acetochlor 

and alachlor) in some confined mixed wells suggests that 

some fraction of the water in these wells also was the result 

of recharge in agricultural areas. In the unconfined aquifer, 

age-tracer data (chlorofluorocarbon and sulfur hexafluoride 

data, and tritium to helium-3 ratios) fit a piston-flow model, 

with apparent recharge ages ranging from 7 to 48 years and 

generally increasing with depth. Age-tracer data for the 

confined aquifers were consistent with mixing of “old” water, 

not containing modern tracers recharged in the last 60 years, 

and exponentially-mixed “young” water with modern tracers. 

Confined unmixed wells contained less than (<) 3 percent 

(%) young water mixed with a much larger fraction greater 

than or equal to (≥) 97% of old water. Confined mixed wells 

contained >30% young water and mean ages ranged from 12 

to 14 years. Median concentrations of nitrate (as nitrogen, 

hereinafter, nitrate-N) were 17.3 and 16.0 mg/L (milligram 

per liter) in unconfined shallow urban and agricultural wells, 

respectively, indicating a range of likely nitrate sources. Septic 

systems are most numerous near the edge of the urban area 

and appear to be a major anthropogenic source of solutes—

including nitrate-N, orthophosphate, chloride, sulfate, calcium, 

potassium, and boron—to unconfined shallow urban wells. 

Hydrogeology, Chemical Characteristics, and Transport 

Processes in the Zone of Contribution of a Public-Supply 

Well in York, Nebraska 

By Matthew K. Landon, Brian R. Clark, Peter B. McMahon, Virginia L. McGuire, and Michael J. Turco
2 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

In oxic unconfined shallow urban and agricultural wells 

with uranium concentrations of 5 to 40 μg/L (microgram 

per liter), the predominant source of uranium was probably 

desorption or dissolution from sediment under oxic conditions 

and the formation of soluble complexes of uranium with 

calcium and bicarbonate. Although septic systems are not 

likely to be a source of uranium, concentrations of uranium 

in oxic unconfined shallow urban wells were greatest where 

tracers of septic-system effects occurred, perhaps because 

of greater calcium concentrations than in wells not affected 

by septic systems. The highest uranium concentrations (44 

to 184 μg/L) in the study area were where water from the 

unconfined aquifer, inferred to have leaked downward in 

multi-layer wells that penetrate the upper confining unit, 

mixed with iron-reducing water in the upper confined aquifer. 

The geochemical mechanism producing these high uranium 

concentrations is not known but may involve dissolution of 

iron oxides transported with colloids from the unconfined 

aquifer by well-bore leakage under iron-reducing conditions 

in the upper confined aquifer, releasing uranium. Relatively 

uniform arsenic concentrations of 2 to 9 μg/L coupled with 

changes in arsenic speciation with increasing depth suggest 

that multiple processes influence arsenic concentrations, 

including competitive desorption of arsenate from sediments 

in the unconfined aquifer in the presence of orthophosphate 

derived from septic-system effluent, long-term reductive 

dissolution and oxidation-reduction reactions in the upper 

confined aquifer and lower confined lenses, and mixing of 

waters from the unconfined and confined aquifers.

Samples collected from the surface discharge of 

the study PSW, which integrated water from the entire 

screened interval in the upper confined aquifer, contained 

concentrations of PCE, TCE, and uranium that were below 

drinking-water standards but of concern as indicators of 

low-level contamination. However, these contaminants were 

not detected in monitoring wells <30 m from the PSW that 

were screened in the same upper confined aquifer. Depthdependent samples were collected from the PSW under typical 

pumping conditions at five depths in the 18-m long screen; 

the depths were selected on the basis of flow profiling using 

the tracer pulse method. The samples from the bottom half 

of the screen had δ18O and δD values and concentrations of 

PCE, TCE, major ions, excess nitrogen gas, and uranium that 

were consistent with those in water derived from the shallow 

unconfined aquifer in the urban area mixed with water from 

the upper confined aquifer. The presence of the water from 

the unconfined aquifer only at the bottom of the study PSW 

screen implies that well-bore leakage in the PSW itself was 

not the pathway for vertical movement. Similar mixtures 

of unconfined and confined aquifer water signatures were 

detected in a few monitoring wells screened in the confined 

aquifers. This nonuniform distribution of mixed waters implies 

that there were preferential flow paths permitting water and 

contaminants from the unconfined aquifer to move through 

the upper and lower confining units. The primary pathway was 

probably downward leakage of water through well bores or 

annular spaces of irrigation, commercial, or older supply wells 

that penetrated the upper and lower confining units. 

In the PSW studied, concentrations of constituents of 

concern were primarily limited by dilution with “old” water 

from the upper confined aquifer (all constituents, particularly 

uranium), and chemical transformation (denitrification of 

nitrate, reductive dechlorination of PCE and TCE). Historical 

data indicate that nitrate concentrations in some PSWs in the 

regional-scale TANC study area screened in both the upper 

confined aquifer and lower confined lenses were sometimes 

higher than drinking-water standards, indicating that shortcircuit pathways and pumping stress can overcome dilution 

and reaction processes under some circumstances. It is likely 

that concentrations of nitrate and other constituents moving 

with water from the unconfined aquifer to PSWs were 

influenced by pumping stress and the number and distance of 

well-bore leakage points from the PSWs. 

Introduction

Contamination of public-supply wells (PSW) is a 

potential threat to the health of people consuming the water, 

and has had negative economic effects on communities that 

need to treat contaminated water or find alternative water 

supplies. Investigations at multiple spatial scales across 

the United States have indicated that shallow ground-water 

quality is affected by land-use activities in many locations, 

but that water quality is less frequently affected at the greater 

depths where PSWs typically are screened (Nolan and 

Stoner, 2000; Gilliom and others, 2006; Zogorski and others, 

2006). In addition, constituents such as arsenic and uranium 

that are often derived from natural sources (hereinafter, 

natural contaminants) are becoming of increasing concern as 

measurements of these constituents have become increasingly 

sensitive, harmful health effects have been identified, and 

drinking-water standards have been lowered (Focazio and 

others, 1999; Welch and others, 2000). Concentrations of 

anthropogenic or natural constituents can change along 

ground-water flow paths from recharge or source areas to 

PSWs as a result of many processes. These processes can be 

broadly grouped into those concerning sources, including 

contaminant loading or mobilization; transport, including 

migration and reaction; and receptors (wells), including 

PSW operations (Franke and others, 1998; Focazio and 

others, 2002). The U.S. Geological Survey National Water 

Quality Assessment (NAWQA) Program began a topical 

study program in 2001 (Wilber and Couch, 2002), to address 

questions regarding factors controlling vulnerability of PSWs 

to anthropogenic and natural contaminants, by using consistent 

and systematic data collected in a variety of principal aquifer 

settings in the United States. The topical program is referred 

to as the study of Transport of Anthropogenic and Natural 

Contaminants (TANC) to PSWs.
Introduction 3

The goal of the TANC program is to understand the 

source, transport, and receptor factors that control the 

movement of contaminants to PSWs in representative aquifers 

of the United States (Eberts and others, 2005a). Studies were 

conducted at regional scales of up to thousands of square 

kilometers (km2) in eight TANC study areas around the United 

States during 2002–2003 and at local-scales of less than (<) 

200 km2 in four of the TANC study areas during 2003–2006. 

One of the TANC study areas where local-scale studies were 

done is at York, Nebraska, within the High Plains Ground 

Water (HPGW) regional study unit of the NAWQA program 

(Dennehy, 2000). 

As part of the regional-scale TANC investigations, 

existing information on hydrogeology, land use, and water 

chemistry was compiled, and existing or new ground-water 

flow models were used to simulate zones of contribution 

(ZOC) and contributing recharge areas (CRA) to PSWs. The 

ZOC is the three-dimensional volumetric part of the aquifer 

through which ground water flows from the CRA to the 

discharging well (Morrissey, 1989). The CRA is defined as the 

surface area on the three-dimensional boundary of the groundwater system that delineates the location of water entering the 

ground-water system that eventually flows to the well and is 

discharged from the aquifer (Reilly and Pollock, 1993). This 

regional-scale analysis was done to identify statistical relations 

between explanatory hydrogeologic and land-use variables and 

water quality in PSWs at regional scales using mostly existing 

information (Eberts and others, 2005a). The multi-scale design 

of the overall study was designed to facilitate extrapolation 

of results from detailed local-scale TANC studies to larger 

areas. Regional-scale TANC study areas are described by 

Paschke (2007). The hydrogeologic setting, water chemistry, 

and simulation of ground-water flow in the regional-scale 

HPGW TANC study area of 388.5 km2

 is described by Landon 

and Turco (2007). This report describes the hydrogeology and 

geochemistry of the local-scale HPGW TANC study area of 

108 km2 and discusses factors controlling transport of selected 

anthropogenic and natural contaminants to PSWs. 

York, Nebraska, was selected for the HPGW TANC 

study for several reasons. Historical data indicated that 

PSWs in York were vulnerable to contamination from 

several sources. The possible sources of contamination and 

the layered hydrogeologic conditions at York are typical of 

conditions in many communities across the Midwest and 

other predominantly rural areas in the Nation. Potential 

contamination threats include chlorinated organic solvents 

from historical chemical use at small commercial facilities 

typically found in Midwestern communities, nitrate and 

pesticides from intensive irrigated cultivated agricultural land 

use, arsenic from natural deposits of shale and glacial till, 

and uranium from alluvial sediments. Movement of these 

contaminants through the aquifer system may be enhanced by 

increased ground-water flow velocities caused by intensive 

ground-water pumping for irrigation and increased recharge 

from irrigation return flows and cultivation. Moreover, the 

common practice of screening irrigation and supply wells 

across large intervals of an aquifer to improve well yields can 

contribute to more rapid vertical movement of contaminants 

than would naturally occur, as a result of penetration of 

confining units by well bores. Because these factors affect 

many other places, a topical study of the transport and fate of 

contaminants at York, Nebraska, was considered to be broadly 

transferable to many communities, particularly in the High 

Plains, and Midwest.

Purpose and Scope

The purposes of this report are to document the 

hydrogeologic setting, and occurrence and distribution 

of selected chemical characteristics, and discuss factors 

controlling transport of selected anthropogenic and natural 

contaminants to PSWs in the local-scale TANC study area 

at York, Nebraska. This report includes a brief description 

of the design of the local-scale monitoring well network and 

methods of data collection and analysis. The hydrogeologic 

setting of the local-scale study area is described, including 

the geology as determined from drilling and geophysical 

logging for the study, the results of tests of aquifer hydraulic 

properties, and historical and seasonal ground-water levels 

and their implications for ground-water flow patterns. A brief 

description of the water budget is given, but a more detailed 

analysis of the water budget and a model constructed and 

calibrated to simulate ground-water flow and age distribution 

in the local-scale TANC study area is described in a separate 

report by Clark and others (2008). 

This report describes the interpretative steps in estimating 

ground-water ages using atmospheric tracers, and describes 

the distribution of the interpreted ground-water ages. General 

water-chemistry characteristics such as oxidation-reduction 

conditions, pH, and water types are described. The occurrence 

and distribution of selected constituents that are useful for 

developing interpretations of patterns and pathways of groundwater recharge and discharge are described. Isotopic and 

geochemical data used in interpreting processes influencing 

nitrate, VOC, uranium, and arsenic distributions are described. 

Factors influencing transport of these anthropogenic and 

natural contaminants to PSWs in the local-scale TANC 

study area are discussed. This report is intended to serve as 

a foundation for analyses comparing results of studies in 

the local-scale TANC study area and others in California, 

Connecticut, and Florida. 
4 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Description of Study Area

The local-scale TANC study area at York, Nebraska, 

was nested within a larger regional-scale TANC study area. 

Although this report focuses on the results of the local-scale 

TANC study, a summary of characteristics of the regionalscale TANC study area is presented here to illustrate the 

features of the system that are most relevant to understanding 

movement of water and solutes along ground-water flow paths 

to PSWs at the local scale. Characteristics of the regionalscale TANC study area around York have been previously 

described by Landon and Turco (2007) (fig. 1). Characteristics 

of the High Plains aquifer influencing water quality have been 

described in publications from the HPGW NAWQA study unit 

(Dennehy and others, 2002). 

The regional-scale TANC study area encompasses 

388.5 km2. The orientation of the rectangular study area 

coincides with regional ground-water flow from northwest to 

southeast. The regional-scale TANC study area was selected to 

facilitate ground-water-flow modeling of the region upgradient 

of and around York. The local-scale TANC study area (fig. 1) 

is a smaller rectangular area of 108 km2 selected for study 

of processes influencing contaminant distribution along an 

apparent ground-water flow path to a PSW in southeastern 

York. 

The regional- and local-scale TANC study areas are 

located in east-central Nebraska within the High Plains 

aquifer, which consists of Quaternary-age alluvial deposits in 

eastern Nebraska and Kansas (fig. 1) (Gutentag and others, 

1984; Dennehy and others, 2002). In the local-scale TANC 

study area, the layered sediments are approximately 75 m 

thick and consist of (with increasing depth) unsaturated 

loess overlying a heterogeneous unconfined coarse sand 

and gravel (hereinafter, unconfined aquifer), a clayey silt 

glacial till (hereinafter, upper confining unit), a confined 

fine sand (hereinafter, upper confined aquifer), and deeper 

clayey silt (hereinafter, lower confining unit) intermixed 

with discontinuous fine sand (hereinafter, lower confined 

lenses) (Keech and others, 1967; Gottula and Link, 1992). 

Depth to water ranges from 3 to 24 m across the local-scale 

TANC study area. The water table is usually near the top of 

the unconfined aquifer but in a few locations is located in the 

overlying loess. A conceptual model of the local-scale TANC 

study area is shown in figure 2. 

York is located within the Central Loess Plains 

Ecoregion, a mostly flat-lying region of windblown silt with 

relatively little dissection by streams (fig. 1) (Bailey, 1980). 

Most of the soils in the area are classified as well drained 

(Verstraeten and others, 1998). 

Mean annual precipitation is 71.1 cm/yr (centimeter 

per year) (High Plains Climate Center, 2003); most of the 

precipitation falls during thunderstorms in the spring and 

fall (Verstraeten and others, 1998). There are no naturally 

perennial streams in the study area other than the lower 

reaches of Beaver Creek near the southeastern edge of the 

regional-scale TANC study area (fig. 1) (Landon and Turco, 

2007). Flows in Beaver Creek through and just to the east of 

York are maintained by discharges from the York wastewater 

plant and industrial return flows. Consequently, Beaver Creek 

loses water to the aquifer downstream from York to near the 

southeastern edge of the regional-scale TANC study area 

(Landon and Turco, 2007).

 Irrigated agriculture is the primary land use. Irrigated 

corn, soybeans, sorghum, alfalfa, and small grains account for 

54 percent (%) of the land area in the local-scale TANC study 

area (Center for Advanced Land Management Information 

Technologies, 2000). Other minor land uses include dryland 

cropland (20%), urban (13%), rangeland (9%), and natural 

land uses (woodland, wetland, open water, 4%). The 

population of York is approximately 8,100 (U.S. Census 

Bureau, 2003). Within York, urban land use is roughly equally 

divided between commercial/transportation and low density 

residential/urban and recreational grass areas (U.S. Geological 

Survey, 1999-2000). 

Ground water is the sole source of irrigation water. 

Irrigation-well density in the regional-scale TANC study 

area was 2.0 wells per km2

. Ground water from the High 

Plains aquifer supplies 100% of the drinking water to York. 

There are no alternative drinking-water sources in the region. 

The Cretaceous-age Dakota sandstone, located more than a 

hundred m below the High Plains aquifer beneath Cretaceous 

shale, is generally too saline for public supply. 

Unconfined and confined ground-water conditions 

occur in the local-scale TANC study area. Many irrigation, 

commercial, and some older PSWs are screened in the 

unconfined aquifer, the upper confined aquifer, and the lower 

confined lenses (fig. 2). Most York PSWs are screened in the 

upper confined aquifer and the lower confined lenses.

The local-scale TANC study area differs from the 

regional-scale TANC study area in that it is focused on an 

apparent ground-water flow path to a PSW in southeastern 

York selected for study, and therefore, has a larger fraction of 

urban area (13%) than the regional-scale TANC study area 

(4%) (fig. 1]). The local-scale TANC study area extends from 

mostly irrigated agricultural areas to the west and northwest of 

York, through a transitional area from rural to urban land use 

along the western edge of York, to residential and commercial 

areas of York (fig. 3). 
Introduction 5

540

530

520

510

500

490

480

Land surface altitude, in meters.

Vertical datum is NAVD 88

York public-supply well

Beaver Creek perennial reach

York city limit

EXPLANATION

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

Base from U.S. Geological Survey

digital data, Albers equal-area projection,

standard parallels 29 30’ North and

45 30’ North, central meridian 97 30’

West, North American Datum of 1983 

106 104 102 100 98 96

40

42

44

38

36

34

32

97

35'

97

40' 97

45'

41 55'

41 50'

41

97

30'

Local-scale

study boundary

Area of figure 3

Regional-scale

study boundary

Texas

Quaternary units

Undivided

Tertiary units

Brule Formation

Arikaree Group

Ogallala Formation

EXPLANATION

TANC

Study Area

0

0

300 MILES

300 KILOMETERS

Nebraska

Kansas

Oklahoma

New

Mexico

NEBRASKA Colorado

North Platte

York

Lincoln

Omaha

Study area

YORK

COUNTY

Figure 1. Topographic and hydrogeologic features of the regional- and local-scale TANC study areas near York, Nebraska, and 

principal geologic units of the High Plains aquifer (modified from Gutentag and others, 1984; Landon and Turco, 2007).
6 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Figure 2. Conceptual model of ground-water flow in the local-scale TANC study area near York, Nebraska.

Irrigation

well

Inactive

well

Public-supply

well

Recharge

Generalized flow path

Water table

Supply well

EXPLANATION

Discharge

Urban area

Loess (Unsaturated)

Sand and gravel (Unconfined aquifer)

Clayey silt (Upper confining unit)

Clayey silt (Lower

Sand

 confining unit)

 (Upper

 confined aquifer)

Sand and silt

(Lower confined

 lenses)Shale

NORTHWEST SOUTHEAST
Introduction 7

Beaver

Creek

Lincoln Creek

490

485

480

475

470

475

Upper confined aquifer

Local-scale study area and ground-water flow model boundary

 (From Clark and others, 2008)

475

485

485

480

490

Simulated hydraulic head contour-Shows altitude of simulated

 hydraulic head April 2003. Contour interval 5 feet. Datum is NAVD 88.

 Closed contours with diameter of less than 175 meters are not shown.

 (From Clark and others, 2008)

Unconfined aquifer

97 41’ 97 35’

41

51’

41

54’

53’

40’ 39’ 38’ 37’ 36’

52’

T

11

N

T

10

N

R 3 W R 2 W

81

81

34

AWT2-71

34

AWT3-73

EXPLANATION

Multiple monitoring well site-7 total sites

Study public-supply well (Study PSW)

Other well, not used in this study, primarily

used for irrigation, commercial, or 

public-supply purposes

Line of geologic section (shown in figure 5)

AWT1-83

FP5

FP5

NWT1-39

FP4

UWT3-34

UWT2-23

UWT4-85

OFPS

OFPN

FP2

FP3

FP1

Single monitoring well-Screened near

 water table (8 total sites)

York

UWT2-23

UWT1-53

2 MILES

2 KILOMETERS

0

0

Figure 3. Location of wells installed for the local-scale TANC study near York, Nebraska.
8 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Additional information on land-use activity potentially 

relevant to ground-water quality was obtained from State and 

local sources. Information on the distribution of potential 

locations where hazardous chemicals may be used or 

generated was collected as part of a Contaminant Source 

Inventory (CSI) obtained from the Nebraska Department of 

Environmental Quality (Deanna Kelley, Nebraska Department 

of Environmental Quality, written commun., April 10, 2003). 

The CSI is a qualitative inventory of potential sites where 

hazardous materials are used, stored, or generated, or where 

regulatory investigations of toxic releases to air, water, or soil 

have been conducted (Resource Conservation and Recovery 

Act (RCRA) or Superfund sites). These locations are plotted 

on figure 4. The CSI data indicate a clustering of potential 

urban contaminant sources along a north-south strip along 

Highway 81 in York. Most of these sites are fuel storage tanks. 

There also are a few storage tanks or RCRA sites in western 

York near the apparent ground-water flow path investigated 

(figs. 3 and 4). 

The approximate distribution of known septic systems 

within or near the boundaries of York was obtained 

from the city of York (Orville Davidson, Public Utilities 

Superintendent, York, Nebraska, written. commun., February 

15, 2003) and is shown on figure 4. This information is 

intended to illustrate approximately where septic systems are 

relatively numerous and could therefore be potential sources 

of solutes to shallow ground water. Most of the septic systems 

are in east-west swaths along major roads or in neighborhoods 

along the western and southwestern edge of York. 

A map of the approximate distribution of current or 

historical livestock facilities or livestock waste disposal was 

obtained from the Upper Big Blue Natural Resources District 

(UBBNRD) (Rod DeBuhr, Water Department Manager, 

written commun., September 1, 2005) and the city of York 

(Orville Davidson, Public Utilities Superintendent, York, 

Nebraska, oral commun., October 3, 2005). Several livestock 

areas were to the west of York (fig. 4).

Previous Investigations

The hydrogeology of York County was described 

by Keech and others (1967). Regional hydrogeologic 

investigations including the study area were conducted by 

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

Several ground-water modeling investigations of study areas 

ranging from three to several thousand square kilometers 

around York have contributed to the understanding of the 

ground-water hydrology in the area (Emery, 1966; Huntoon, 

1974; Cady and Ginsburg, 1979; Nebraska Natural Resources 

Commission, 1983; Luckey and others, 1986; Alley and 

Emery, 1986; Argonne National Laboratory, 1995a,b; Upper 

Big Blue Natural Resources District, 1999). The Nebraska 

Cooperative Hydrology Study Group (2001) constructed and 

calibrated a regional transient multi-layer ground-water flow 

model of an area that included York County. 

Gottula and Link (1992) described the hydrogeology and 

ground-water quality of the western portion of the UBBNRD, 

which encompasses the study area. The spatial and temporal 

distribution of water quality in the High Plains aquifer in the 

UBBNRD was described by Verstraeten and others (1998). 

Relations between nitrate and atrazine concentrations in 

ground water and explanatory variables in York County 

were investigated by Druliner and others (1996). Herbicide 

transport and degradation in the aquifer about 3 km west 

of York were studied by Ma (1996) and Ma and Spalding 

(1997) to evaluate the effects of local artificial recharge with 

impounded surface water on ground-water quality. Argonne 

National Laboratory (1993, 1994, 1995a,b) investigated a 

ground-water carbon tetrachloride plume from a former grain 

storage facility in northern York. Landon and Turco (2007) 

described the hydrogeologic setting, the ambient geochemical 

characteristics, and simulations of ground-water flow, 

including ZOCs and CRAs for York PSW in the regional-scale 

TANC study area. 

Methods

Methods used for the local-scale TANC study at York, 

Nebraska, are described in the section below, and include the 

design of the well network, drilling, well installation, coring, 

core analysis, collection and analysis of water samples, 

collection of depth-dependent flow and chemistry data, 

quality-assurance of water-quality data, statistical analysis of 

water-quality data, and estimation of aquifer properties. 

Design of Well Network

The well network was designed so that an existing 

PSW would be the focus of the study, and additional wells 

were installed to allow monitoring of ground water from 

potential source areas along a likely flow path to the PSW. A 

PSW in York (hereinafter, study PSW) was selected for the 

study because of constituents of concern in the well, a ZOC 

that included both urban and agricultural land-use areas, its 

importance as a heavily-utilized PSW for water supply, and its 

distance from other pumping wells that would minimize the 

effect of pumpage from these wells on the ground-water flow 

paths to the study PSW. These selection criteria are explained 

further in the following paragraph.
Methods 9

Figure 4. Distribution of potential contaminant sources compiled from various sources in the local-scale TANC study area near York, 

Nebraska.

Beaver

Creek

NWT1

FP4 UWT3

UWT2

UWT1

UWT4

OFPS

OFPN

FP2

FP3

FP1

Beaver

Creek

97 37’ 97 36’ 97 35’

41 52’

41 51’

R 3 W R 2 W

T 11 N

T 10 N

1 MILE

1 KILOMETER

EXPLANATION

Single well and designation-Screened near

water table (8 total)

Multiple well and designation (7 total)

0

0

Potential urban contaminant sources (from contaminant

 source inventory classified by regulatory category)

Leaking storage tank

National Pollution

Discharge Elimination

System

Resource Conservation

and Recovery Act

Superfund Amendment

and Reauthorization

Act Title III

Wastewater

Superfund

Approximate location of septic systems

81

UWT2

FP1

Approximate location of current or historical

 livestock facility or livestock waste disposal.

Primarily unsewered area with septic systems

81

Study public-supply well (Study PSW)
10 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Samples were collected from the study PSW and from 

seven other York PSWs for the NAWQA Source Water Quality 

Assessment (SWQA) program in October-December 2002. At 

that time, trichloroethylene (TCE), tetrachloroethylene (PCE), 

and their degradation products were detected in the study 

PSW, along with uranium concentrations that were below 

drinking-water standards but of concern (17 μg/L [microgram 

per liter]), and moderate arsenic concentrations of about 

6 μg/L. The concentration of nitrate (as nitrogen, hereinafter, 

nitrate-N) was low (0.09 mg/L [milligram per liter]), but 

reconnaissance sampling indicated nitrogen gas concentrations 

of 1–2 mg/L larger (hereinafter, excess nitrogen gas) than 

concentrations that would be expected from atmospheric 

equilibrium at the recharge temperature. The excess nitrogen 

gas was consistent with denitrification of nitrate-N, that is, 

more nitrate-N could reach the well if denitrification was 

not occurring. Thus, a variety of anthropogenic and natural 

contaminants of interest could be investigated at the study 

PSW. The simulated ZOC of the study PSW (Landon and 

Turco, 2007) indicated that urban and agricultural land use 

could influence ground water captured by the well. Most of 

the seven other wells sampled for the SWQA program and 

considered as candidate study PSWs had smaller areas of 

upgradient urban land use because they were located closer 

to the western, or upgradient, edge of town, or were used less 

frequently and had smaller withdrawal rates. As the second 

most heavily pumped York PSW during 2000–2004, the study 

PSW provided opportunities to investigate receptor (pumping 

well) effects on contaminant movement. Most of the other 

candidate wells considered were closer to other pumping wells 

than the study PSW, and in some cases, the simulated ZOC 

for the other wells was very complicated (Landon and Turco, 

2007) because of withdrawals from nearby supply wells. Such 

flow fields would have made attempts to monitor an apparent 

ground-water flow path to a PSW complex and uncertain. 

In comparison, the simulated ZOC for the study PSW was 

relatively simple, making it possible to install monitoring 

wells along an apparent ground-water flow path leading to the 

well.

The study PSW was installed in 1977. The diameter of 

the well casing is 40.6 centimeters (cm). The layered geology 

at the study PSW is characteristic of that throughout the study 

area (fig. 2). The study PSW was screened only in the upper 

confined aquifer, with an 18.3-m-long screen from 42.7 to 

61.0 m below land surface (bls). The study PSW originally 

also had a deeper screen in a sand lens (lower confined lenses) 

from 72.8 to 75.9 m. However, the well bore was filled with 

concrete below the screen in the upper confined aquifer in 

the 1990s to reduce sand inflow (Orville Davidson, Public 

Utilities Director, city of York, Nebraska, oral commun, 

September 10, 2003). 

A network of short-screened monitoring wells was 

installed in or near the simulated ZOC of the study PSW 

(fig. 3; table 1 [see back of report]) to allow ground-water 

flow and geochemistry data to be collected along apparent 

ground-water flow paths from locations within the estimated 

CRAs to the study PSW. A total of 36 wells were installed in 

two phases; 18 wells were installed during April-May 2003, 

and an additional 18 wells were installed during April 2004. 

In the first phase, wells were installed in 5 well nests (FP1, 

FP3, FP4, OFPS [2 of 4 wells], and OFPN) along or adjacent 

to the estimated main axis of ground-water flow to the study 

PSW. Well nest FP1 was installed less than 30 m from the 

study PSW to characterize ground-water hydraulic heads, and 

to compare water chemistry in the study PSW with that in the 

adjacent unconfined aquifer (one well), upper confined aquifer 

(two wells, where the study PSW was screened), and lower 

confined lenses (one well). Well nests FP3 and FP4 were 

installed along the axis of highest probability of being within 

the ZOC to the study PSW, determined by using a regionalscale TANC steady-state model (Landon and Turco, 2007; 

Jeffrey Starn, Hydrologist, U.S. Geological Survey, written 

commun., March 15, 2003). Well nests OFPS and OFPN were 

installed to the south and north, respectively, of the main axis 

of the estimated ZOC to bracket the studied ground-waterflow path to the study PSW. Subsequently, local-scale transient 

ground-water flow modeling and particle tracking indicated 

that the OFPS and OFPN well nests also were located within 

the ZOC to the study PSW (Clark and others, 2008). In the 

second phase, two additional well nests were installed—FP2, 

south of the main axis of the ZOC, and FP5, extending the 

flow path farther upgradient into the agricultural area (fig. 3). 

In addition, eight single-completion wells with depths to the 

bottom of the screen ranging from 7 to 27 m were installed 

near the water table to characterize ground-water quality in 

potential recharge source areas of the unconfined aquifer. 

Refined simulations of the ZOC were used to select the 

locations of these wells. Of the eight water-table wells, 

four were installed in urban areas (UWT1-53, UWT2-23, 

UWT3-34, UWT4-85), three were installed in cultivated 

agricultural areas (AWT1-83, AWT2-71, AWT3-73), and one 

was installed at the downgradient end of one of the largest 

pasture areas that could be located within the local-scale 

TANC study area (NWT1-39). Since, in addition to these 

eight single wells, each of the seven well nests also included 

a monitoring well screened near the water table, there were a 

total of fifteen near-water table wells. 
Methods 11

Drilling, Well Installation, Coring, and 

Core Analysis 

All drilling and well installation was done by the U.S. 

Geological Survey (USGS) Rocky Mountain Drilling Unit 

(Denver, Colorado) using techniques that conformed to USGS 

NAWQA protocols (Lapham and others, 1995; Lapham and 

others, 1996). Hollow-stem augering methods were used to 

drill all 19 of the wells installed in the unconfined aquifer and 

five of the 10 wells installed in the upper confined aquifer 

(table 1). The diameter of the resulting boreholes was16.5 cm. 

Mud-rotary methods were used to install five of the 10 wells 

completed in the upper confined aquifer, all six of the wells 

completed in the lower confined lenses, and one well installed 

in the underlying Carlile Shale (table 1). The diameter of these 

boreholes was 22.2 cm. All drilling equipment was steamcleaned between drill sites to prevent cross-contamination of 

boreholes. 

Nearly all monitoring wells were constructed of threaded 

5.1-cm-diameter, schedule 40 polyvinyl chloride (PVC) with 

1.5-m long screens. One 5.1-cm-diameter monitoring well 

had a 3-m-long screen (table 1). A sand pack consisting of 

clean silica sand typically extended from 3 m or less below the 

bottom of the screen to 1.5 m above the top of the screen. The 

wells were sealed by filling the borehole above the sand pack 

with a bentonite slurry and a cement cap above the bentonite 

near the land surface. Most wells were completed at grade 

with the land surface. Protective enclosures and locking caps 

were installed. 

Water levels in all 36 monitoring wells were measured 

approximately monthly. Hourly water-levels were recorded 

using Design Analysis DH21 pressure transducers and data 

loggers in 11 monitoring wells and Beaver Creek during 

April 2003 through September 2005 and in two additional 

monitoring wells during August 2004 through September 

2005. 

One multilayer well, FP5-LS, was constructed of 

10-cm-diameter, schedule 80 PVC, and had a 18.3-m long 

screen across the entire unconfined aquifer and a 21.3-m 

long screen across the entire upper confined aquifer; the 

screens were separated by a 15.2-m-long cased interval across 

the upper confining unit (table 1). The multilayer well was 

installed to directly measure the effects of downward leakage 

of water from the unconfined aquifer to the upper confined 

aquifer through the well bore since well-bore leakage was 

hypothesized to be an important process influencing water 

chemistry in the upper confined aquifer and lower confined 

lenses. This temporary well was similar to many irrigation 

wells in the local-scale TANC study area that have long 

screens in the unconfined aquifer, upper confined aquifer, 

and lower confined lenses to maximize well yields. Water 

chemistry and hydraulic heads in FP5-LS were compared with 

those in adjacent short-screened monitoring wells screened 

in the shallow unconfined aquifer, near the bottom of the 

unconfined aquifer, and the upper confined aquifer. The 

adjacent monitoring wells were located 15 to 30 m upgradient 

of the multilayer well. This temporary well was abandoned in 

August 2005. 

Lithology was determined using drill cuttings collected, 

described, and archived over 1.5-m intervals. Geophysical logs 

were collected in the deepest borehole drilled at each well nest 

site; the deepest borehole was drilled first and the geophysical 

logs were used to plan the placement of all monitoring well 

screens in each nest. Then additional boreholes located 

about 3 m apart were drilled to the target depths, and a single 

monitoring well was installed in each borehole. Geophysical 

logs collected included natural gamma, resistivity, and 

spontaneous potential at all sites; temperature, specific 

conductance, caliper, neutron, and electromagnetic induction 

logs were collected at some sites. 

Core samples were collected and analyzed to characterize 

the chemical, mineralogical, and physical properties of 

the major vertical layers in the hydrogeologic system. In 

April–May 2003, four cores and one drill-cuttings sample 

were collected and analyzed. In April 2004, eight cores were 

collected and analyzed. Four cores from above the water table 

were collected with the hollow-stem auger using a 1-m-long 

7.6-cm-diameter wire-line piston split-barrel sampler lined 

with an acrylic sleeve. Eight cores were collected from the 

bottom of the unconfined aquifer to the shale underlying the 

High Plains aquifer using the mud-rotary drilling rig and a 

variety of coring devices. In fine-textured confining units, 

a 3-m-long core tube was driven into the sediments below 

the bottom of the mud-rotary borehole. This method was 

unsuccessful for recovering coarse-textured sediments. For 

cores of sand near the bottom of the unconfined aquifer, upper 

confined aquifer or lower confined lenses, coring was begun 

in the sand using a 1.5-m-long core barrel, and the barrel 

was driven into an underlying clay to seal the end of the core 

barrel, allowing recovery of the overlying sands. Core samples 

for laboratory analysis were immediately transferred to a 

nitrogen-filled processing chamber, subsampled, and sealed in 

jars or core tubes to minimize exposure to the atmosphere. 

Selected core samples were analyzed by USGS 

laboratories for mineralogy by x-ray diffraction, heavy mineral 

separation, and mass spectrometry. Selected core samples also 

were analyzed for percent organic carbon (loss on ignition), 

total organic carbon, carbon-13/carbon-12 isotopic ratios 

(δ13C) for total carbon, organic carbon, and calcite (USGS 

National Water Quality Laboratory [NWQL]). Particle-size 

distribution of core subsamples ranging from 0.04 micron to 

2 mm (millimeter) was analyzed using a commercially built 

apparatus (K.S. Perkins, USGS, written commun. 2005).

The abundance of elements in grain coatings or other 

mineral phases from core samples was evaluated in USGS 

laboratories using different acids and reagents for extraction, 

including (1) 10% nitric acid (HNO3

), (2) 6-normal (N) 

hydrochloric acid (HCl), (3) 0.5-N HCl-hydroxylamine 

(HA)-extractable iron, and (4) acid-volatile and chromiumreducible sulfides (Craig Brown, Hydrologist, U.S. Geological 

Survey, written commun., July 6, 2006). All extractions were 

performed on an “as received” wet basis, but the moisture 
12 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

content was determined from a separate subsample and used to 

correct the analytical concentrations to dry weight. 

The coating extraction (HCl and nitric acid) methods 

were primarily intended to target ferric oxyhydroxides and 

their associated trace elements, but they can also extract 

various amounts of other oxides and clay, carbonate, and 

sulfide phases. The sample-to-solution ratio was 5 ± 1 g 

(gram) wet sediment to 15 ml (milliliter) of acid solution 

for the 6N HCl and 10% HNO3 extractions and 4–10 g 

wet sediment to 75 ml of reagent for the 0.5-N HCl-HA 

extractions. The sediment-acid aliquots were set on a shaker 

table for 6 hours, then centrifuged, filtered with 0.45 micronpore filters, and analyzed for several minor elements. Leachate 

derived from the 10% HNO3 extractions was analyzed using 

inductively-coupled plasma/optical emission spectroscopy 

(ICP-OES). Further dilution to 5% HNO3 was required 

for analysis of arsenic, cadmium, lead, and uranium by 

inductively-coupled plasma/mass spectrometry (ICP-MS). 

Analysis of the 6-N HCl extracts required dilution to 

1.6-N HCl before analysis by ICP-MS. The 0.5-N HCl-HA 

extractions were done following methods described by Lovley 

and Phillips (1987) and analyzed by ICP-MS and the ferrozine 

method with a Hach DR/2400 Spectrometer (Hach Company, 

2002). 

Sulfide phases in sediments or rock were extracted using 

the method described by Allen and others (1991, 1993) and 

summarized in EPA Method 9030B. The method was modified 

(to avoid oxidizing hydrogen sulfide by soluble ferric iron) 

by adding SnCl2

 (Michelle Tuttle, USGS, Research Chemist, 

personal commun., 2003) and further modified by adding 

CrCl2 (Canfield and others, 1986, Bowles and others, 2003) to 

include pyrite-bound sulfide. Sulfide phases that are extracted 

by this method include pyrite, elemental sulfur, and acid 

volatile monosulfides. Samples of 5–20 g of wet sediment 

were transferred to a 250-ml round-bottomed flask (under 

nitrogen) with 10 g SnCl2. The flask was attached to a nitrogen 

stream on a heating mantle and 75 ml of deaerated 6N HCl 

and 50 ml of CrCl2 were added. The samples were heated and 

kept at a sub-boiling temperature for 1 hour. Hydrogen sulfide 

(H2

S) was collected in 2 sequential aliquots of 100 ml of 

0.5-molar (M) NaOH. Sulfide was analyzed colorimetrically 

using a Hach Spectrometer DR/2400 using the methylene blue 

method specified by the manufacturer (Hach Company, 2002).

Collection and Analysis of Water Samples

Water samples were collected from wells during October 

2003 through April 2005; analytes from different groups 

of wells varied slightly. Samples were collected during five 

sampling periods: October–November 2003, June 2004, 

July–August 2004, October 2004, and March–April 2005. 

For sampling purposes, monitoring wells were classified into 

3 groups: flow-path wells, off-path wells, and source wells 

(table 1). Flow-path wells were deeper than wells screened 

near the water table in well nests along the main axis of the 

apparent ground-water flow path to and less than 3 km from 

the study PSW (FP1, FP3, OFPS, and FP4 well nests [fig. 3]). 

Off-path wells were deeper than wells screened near the water 

table in well nests located in or near the ZOC to the study 

PSW, but not along the main axis of the apparent ground-water 

flow path to the study PSW (OFPN, FP2) or more than 3 km 

from the study PSW (FP5). These wells were sampled for 

fewer analytes than flow-path wells. Source wells included 

the shallowest monitoring wells in each nest (seven wells) and 

eight single completion wells screened near the water table 

in various land use settings. Source wells were sampled once 

for an extensive list of analytes. The analytes in this sampling 

depended upon whether the well was part of a well nest that 

was flow-path (FP1, FP3, OFPS, FP4) or off-path (OFPN, 

FP2, FP5). Source wells also were sampled 2 or 3 more times 

for a more limited number of analytes. The surface discharge 

of the study PSW was sampled according to a source-well 

sampling plan. “Surface-discharge sample,” the terminology 

of Izbicki and others (2005a,b), is used hereinafter to describe 

samples collected from the flow of the entire PSW at the 

wellhead near land surface, integrating flow from all screened 

depths. Shallow wells were sampled more frequently than 

wells screened deeper in the system because shallow water 

chemistry is typically more temporally variable than water 

chemistry deeper in the system. The surface discharge of the 

study PSW was sampled four times to detect temporal changes 

in water chemistry. Analytes measured for each well category 

are listed in table 2 (see back of report). 

Ground-water samples were collected from wells using 

procedures described by Koterba and others (1995) and 

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

variously dated) (table 2). Water samples were processed 

on-site in a mobile laboratory using methods designed to 

minimize changes to the water-sample chemistry. Water was 

pumped from wells using a submersible pump and delivered 

to the mobile laboratory through polytetrafluoroethylene 

(PTFE, hereinafter referred to as Teflon) tubing with stainlesssteel connections to minimize sample contamination. Before 

samples were collected, stagnant water was flushed from 

the well by purging at least three casing volumes. While 

the well was being purged, water temperature, specific 

conductance, pH, dissolved oxygen, and turbidity were 

measured in a chamber filled with flowing ground water 

until readings were stable. Once readings had stabilized, 

water samples were collected in precleaned bottles within a 

plastic-enclosed space to prevent sample contamination by 

airborne particles. To prevent degradation of water samples 

and maintain the initial concentration of compounds between 

the time of sample collection and laboratory analyses, 

sample bottles were preserved according to the requirements 

of the various laboratory methods. For some analytes, 

samples were filtered and the resulting laboratory-reported 

values only represent concentrations of dissolved species 

(table 2). For other analytes, samples were not filtered and 

the concentrations represent those in whole water (including 

particulate fractions); however, most of these unfiltered 
Methods 13

samples were collected for analytes present as dissolved 

gases or isotopic ratios of oxygen and hydrogen in water 

molecules, so concentrations associated with particulate 

phases were expected to be minor. Preservation practices differ 

among analytes and may include chilling, filtration, and (or) 

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

Sampling equipment was cleaned after samples were collected 

at each well using standard NAWQA procedures to prevent 

cross-contamination between wells (U.S. Geological Survey, 

variously dated). 

Samples were collected and analyzed for a broad suite of 

analytes, including water temperature, specific conductance, 

pH, dissolved oxygen, major ions, trace elements, arsenic 

species, nutrients, 85 volatile organic compounds (VOC), eight 

gasoline oxygenates, 64 pesticides and degradation products, 

22 acetamide herbicides and degradation products, dissolved 

organic carbon (DOC), dissolved gases, radium isotopes, 

radon, gross alpha and beta radiation, and isotopes of oxygen, 

hydrogen, nitrogen, sulfur, carbon, and uranium. Selected 

ground-water samples were analyzed for age-dating tracers 

including chlorofluorocarbons (CFC), sulfur hexafluoride 

(SF6), tritium (3H), and ratios of 3H and degradation product 

helium-3 (3He) (hereinafter referred to as 3H/3He technique). 

The 3H/3He technique includes analysis of additional noble 

gases (4He and neon). Dissolved hydrogen gas was measured 

in selected wells having dissolved oxygen concentrations 

below 1 mg/L following the method developed by Chapelle 

and Lovely (1990). A list of constituent groups analyses and 

references describing the methods is provided in table 2. 

Additional details of nutrient, isotopic, and age-tracer 

techniques are described below. 

The USGS NWQL uses the laboratory reporting level 

(LRL) as a threshold for reporting analytical results. The 

LRL is set to minimize the reporting of false negatives (not 

detecting a compound when it is actually present in a sample) 

to less than 1% (Childress, and others, 1999). The LRL is 

usually set at two-times the long-term method detection level 

(LT-MDL), which is the average (long-term) MDL calculated 

from multiple analytical measurements (more than 50). The 

MDL is the minimum concentration of a substance that can 

be measured and reported with 99% confidence that the 

concentration is greater than zero (at the MDL there is less 

than 1% chance of a false positive) (U.S. Environmental 

Protection Agency, 2002). Concentrations below the LRL are 

reported as estimated concentrations (designated with an “E” 

before the value). 

Nitrogen species analyzed included ammonia, nitrite, and 

nitrite plus nitrate. Concentrations were reported in mg/L as 

nitrogen. 

Stable isotopes of oxygen, hydrogen, sulfur, carbon, 

and nitrogen were analyzed in an attempt to improve 

understanding of sources of water and reactions affecting 

the chemical composition of ground water (Clark and Fritz, 

1997). Samples for stable isotopes of hydrogen, oxygen, 

sulfur and nitrogen were analyzed by the USGS Stable Isotope 

Laboratory in Reston, Virginia. Isotope data are presented in 

delta (δ) notation as the ratio of the heavy to the light isotope, 

in units of per mil (‰), normalized to a standard (equation 1). 

δ

i

E per mil

where

 R R 1 1, 

R

sample standard

samp

= 

( )− 

 

 / 

× 000

le standard and R are the ratios of the heavy to the 

light isotope in the sample and in the

standard respectiv , ely.

(1)

Positive values of δiE indicate enrichment of the heavier 

isotope, and negative values indicate depletion of the heavier 

isotope, compared to the ratios observed in the standard 

reference material. (δi

E is the ratio of a heavier isotope of 

an element [iE] to the more common lighter isotope of that 

element, relative to a standard reference material.) Delta 

deuterium (δD) and delta oxygen-18 (δ18O) were analyzed 

using an isotope ratio-mass spectrometer by hydrogen gas/

water equilibration and carbon dioxide gas/water equilibration 

techniques respectively and reported relative to Vienna 

Standard Mean Ocean Water (VSMOW) (Coplen and others, 

1991; Epstein and Mayeda, 1953). Dissolved sulfate was 

analyzed for delta sulfur-34 (δ34S-sulfate) using methods of 

Carmody and others (1997) and reported relative to the Vienna 

Canyon Diablo Troilite (VCDT) standard. Isotopes of nitrogen 

and oxygen of nitrate (hereinafter, δ15N-nitrate and δ18Onitrate, respectively) were analyzed by bacterial conversion of 

nitrate to nitrous oxide and reported relative to nitrogen gas 

in air and VSMOW, respectively (Sigman and others, 2001; 

Casciotti and others, 2002). Water samples were analyzed for 

delta carbon-13 (δ13C) using mass-spectrometry techniques at 

the University of Waterloo Environmental Isotope Laboratory 

and reported relative to the Vienna Pee Dee belemnite (VPDB) 

standard (Coplen, 1993).

A Rayleigh distillation calculation of expected changes 

in δ18O and δD in water while it underwent evaporation during 

evaporation of water was made using the methods of Clark and 

Fritz (1997, p. 55); the results were compared with groundwater values. An equation by Gonfiantini (1986) was used 

to calculate the kinetic fractionation factor, which includes 

kinetic effects on the isotopic fractionation during evaporation 

as a function of humidity. The equilibrium fractionation 

factor as a function of temperature was calculated using the 

coefficients of Kakiuchi and Matsuo (1979). The average 

annual atmospheric relative humidity of 70% and temperature 

of 11°C for York, Nebraska, were used in the calculations 

(High Plains Climate Center, 2003). 
14 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

The CFC and SF6 age-tracer methods are based upon 

the increase in concentrations of these trace gases in the 

atmosphere since the 1940s and 1950s as a result of industrial 

use of these compounds (Busenberg and Plummer, 1992, 

2000). Concentrations of these gases in the atmosphere 

increased from the 1940s to the mid-1990s. Significant 

production of SF6 began in the 1960s for use in high voltage 

electrical switches, and atmospheric concentrations of SF6

have continued to increase. The concentrations of CFCs and 

SF6 in the atmosphere are proportional to their concentrations 

in recharge water entering an aquifer at that time; the 

proportionality is determined by the recharge temperature. 

Recharge temperatures were estimated from concentrations 

of dissolved nitrogen and argon gas (Busenberg and others, 

1993). If there are no other sources or sinks for CFCs and SF6 

in the aquifer and the effects of dispersion are minimal, the 

concentrations of these compounds in the ground-water at the 

time of recharge are preserved and indicate the approximate 

date that the water entered the aquifer. The three individual 

CFC compounds (12, 11, 113), each having a different 

historical atmospheric concentration trend, provide three 

independent age estimates. Because each age tracer technique 

has uncertainties and is affected in different ways by aquifer 

processes, ages estimated using these techniques are referred 

to as “apparent ages.”

The 3

H/3He method is based upon the increase in 3H 

concentrations in precipitation after atmospheric hydrogen 

bomb testing in the 1950s and 1960s (Schlosser and others, 

1988, 1989; Solomon and others, 1992). 3H concentrations 

peaked in 1963 and have declined substantially since then. 3H 

has a half-life of 12.43 years and radioactively decays to form 

3

He. By determining the ratio of 3H to 3He resulting from the 

decay of 3H, the time that the water has resided in the aquifer 

can be calculated. Helium can be produced naturally also by 

sediments; the amount of 3He derived from 3H decay rather 

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

neon gas concentrations. 

Water samples for CFCs, SF6, noble gases, and other 

dissolved gases were collected directly from copper tubing 

attached to the submersible pump and did not flow through 

Teflon sample lines; these samples were prevented from 

coming into contact with the atmosphere, in accordance 

with protocols for sampling for these trace gases (Busenberg 

and Plummer, 1992, 2000; Schlosser and others, 1989). 

Concentrations of two tracers of modern waters (CFCs, SF6

, 

or 3H/3He) and 3H were collected in 13 wells in October–

November 2003, and the surface discharge and four depths in 

the well screen of the study PSW in June 2004. The wells for 

which ground-water ages were calculated were located along 

an apparent flow path to the study PSW (flow-path wells; 

table 2). Concentrations of 3H were measured in all wells in 

which other age tracers were analyzed. 

Collecting Depth-Dependent Flow and 

Chemistry Data

Depth-dependent flow information was collected in the 

study PSW under pumping conditions using the “tracer pulse” 

method of Izbicki and others (1999). The method involves 

use of a high-pressure hose equipped with valves for dye 

injection. The hose is mounted on a trailer mounted reel. The 

apparatus used for this study was designed and constructed 

by Allen H. Christensen, USGS, San Diego, California. The 

approach is suitable for use in supply wells having limited 

access; it can be used where clearance between the pump 

bowls and well casing is 2.5 cm or more. To reduce the 

chances that down-hole equipment would become stuck or 

come in contact with moving pump parts, a 3.17-cm insidediameter PVC access pipe extending from the wellhead about 

0.6 m above land surface to a depth of 41.8 m bls was installed 

by a city contractor. The access pipe ended just above the 

top of the screen at 42.7 m bls but below the pump intake at 

about 39.6 m bls. The access pipe was installed by lifting the 

wellhead and installing the pipe between the casing and pump 

bowls and attaching the pipe to a threaded access port already 

in the casing. 

Measurements were made on June 15, 2004, under typical 

pumping conditions of about 1,940 liters per minute (L/min) 

(510 gallons per minute [GPM]). The well ran continuously 

during the measurement period. To obtain vertical profiles of 

flow in the supply well, the 1.27-cm diameter hose was filled 

with rhodamine WT dye, and the end of the hose was lowered 

to a known depth in the well (Izbicki and others, 1999). A 

pulse of the dye tracer was injected into the water column. The 

maximum instantaneous concentration of rhodamine WT dye 

in the surface discharge of the study PSW during any onesecond measurement interval was 6.5 parts per billion (ppb), 

less than the U.S. Environmental Protection Agency (USEPA) 

maximum of 10 ppb for rhodamine WT for water entering 

a drinking water plant (before treatment and distribution) 

over periods of hours to days. Average concentrations in 

the surface discharge of the study PSW during the injection 

tests were <0.1 ppb, a factor of less than one-hundredth of 

the regulatory limit. The water from the study PSW was then 

mixed with water from wells elsewhere in the distribution 

system so that concentrations in water delivered to customers 

would have been diluted to concentrations lower than those 

in the surface discharge of the study PSW. Dye was never 

visible in the water in the surface discharge of the study PSW, 

and so would not have affected the aesthetic properties of the 

water delivered. The timing of the injection was controlled 

and recorded electronically by an IZ-Mobile 2 injection 

timer. The fluorescence of the surface discharge of the well 

was measured using an inline fluorometer. The calibrated 

fluorometer transferred dye concentration data in ppb to a 

laptop computer in real-time. The travel-time of the tracer 

from the point of injection to the fluorometer was recorded. 

Either the travel time of the first arrival or the center of mass 

of the tracer pulse from the injection point to the fluorometer 
Methods 15

can be used to calculate the velocity, as long as the same 

approach is employed at all depths. For this study, the first 

arrival was always used to calculate velocity. Multiple tests 

were conducted at each depth. The hose was then lowered to 

the next measurement depth and another series of tracer pulse 

tests were conducted. Measurements were made every 3.0 m. 

On the basis of the estimated velocity of the water and the fact 

that the timing of the injection was only known to the nearest 

second, measurements at intervals of <3.0 m were considered 

unwarranted because of resulting uncertainties. Using an 

equation of Izbicki and others (1999), the flow rate (Q) for a 

known well radius (r) was calculated, assuming piston flow:

Q V rw

2 = ( ) π , (2)

where

V d d t t

Q flow rate m /s;

V velocity of 

2 1 2 1

3

= − ( ) ( ) −

=

=

/

,

where

water in measurement interval m/s;

r well radius m;

d de

w

,

= ,

= pth of measurement 1-top of interval 

2-bottom of interva

, ,

l m; and

t travel time of dye pulse from injection point 

,

=

to fluorometer,s.

(3)

Three to eight tracer pulse tests were done every 

3 m along the study PSW screen from 42.7 to 61.0 m bls. 

Two partial profiles were measured: profile 1 consisted of 

measurements at 42.7, 45.7, 48.8, 51.8, 54.9, 57.9, and 61.0 

m bls; profile 2 consisted of measurements at 44.2, 47.2, 50.3, 

53.3, 56.4, and 59.4 m bls. The pump intake was situated 

above the well screen so that all water within the screened 

interval would flow vertically upward during pumping, 

and samples collected at selected depths were presumed to 

represent a composite of all of the water entering the well 

below each sampling point.

A 50.8-cm-long by 2.22-cm-diameter Solinist Model 

407 Integra stainless-steel portable bladder pump was lowered 

down the access pipe to collect water samples. Sampling 

depths were selected on the basis of the flow profile, which 

indicated approximately 75% of the flow came from the upper 

half of the screen. In order to collect more samples from the 

intervals where most of the flow occurred, five samples were 

collected from depths of 42.7, 45.7, 48.8, 51.8, and 61.0 m bls, 

along the screen, which extended from 42.7 to 61.0 m bls. 

Flow rates with the bladder pump were 200 to 400 mL/min. 

The low flow rate permitted the extensive list of analytes 

to be collected from a maximum of only 2 depths per day. 

The bladder pump was cleaned after samples were collected 

at each depth. The pump was removed from the well and a 

peristaltic pump was used to push cleaning solutions through 

the pump and tubing. The same approach was used to drive 

nitrogen-purged universal blank water through the system to 

collect field blanks. Otherwise, the same sampling procedures 

and analytical methods were used to collect the depthdependent samples as were used to collect the monitoring-well 

samples. A sample was collected from the surface discharge of 

the study PSW also on June 18, 2004, to compare with depthdependent samples; the surface-discharge sample integrates 

all of the water flowing into the well from the range of depths 

along the screened interval.

Quality Assurance

Additional samples were collected to evaluate the 

reliability of sample collection and analysis procedures. 

About 14% of the total samples analyzed were qualitycontrol samples; these included field blanks, source-solution 

blanks, sequential replicates, and spikes. In addition, 100%

of the samples for organic constituents were analyzed 

using surrogate compounds to monitor laboratory method 

performance. Additional laboratory quality-control analyses 

were done by the analytical laboratories, as specified by the 

respective analytical methods, and are not described here. The 

quality-management system for the USGS NWQL is described 

in Maloney, T.J., ed. (2005). 

Blanks

Field blanks were collected at about 10% of the wells to 

determine if a sample was biased when collected, transported, 

or analyzed. All field blanks were collected using nitrogenpurged blank water that was certified by NWQL to be free of 

environmental sample analytes above their reporting levels. 

Field blanks were processed using the same procedures and 

equipment as those used for the environmental samples. Field 

blanks were analyzed for major ions, trace elements, nutrients, 

DOC, pesticides, VOCs, and gasoline oxygenates. It is not 

possible to obtain meaningful blank solutions for dissolved 

gases or isotopic species and no blanks for age tracers, 

dissolved gases, radon, or isotopic species were collected. 

Source-solution blanks were collected to verify that 

the blank water used for the associated field blanks had no 

detectable concentrations of VOCs or gasoline oxygenates. 

Source-solution blanks were collected at the sampling 

site by pouring blank water (source solution) into sample 

containers that were then stored, shipped, and analyzed in the 

same manner as the field blank and ground-water samples. 

If both the field blank and its associated source-solution 

blank contained a constituent, the source-solution water was 

assumed to be the origin of the detected analyte in the field 

blanks. The only constituent for which this was the case 

was acetone (table 3 [see back of report]). However, acetone 

was not detected in environmental samples, so this limited 

contamination of the source-solution water had no effect on 

the data interpreted for the study. 
16 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Sample bias with respect to an analyte was of potential 

concern on the basis of data from blanks if the following 

criteria were met: (1) a constituent was detected in one or 

more field blanks and in ground-water samples, (2) the 

concentration detected in the field blank was greater than the 

concentration in the associated source-solution blank, and 

(3) the minimum concentration detected in ground-water 

samples was less than the sum of the maximum concentration 

detected in field blanks and the average long-term method 

detection level (LT-MDL), or half of the LRL, (hereinafter, this 

sum is referred to as the censoring threshold). The LT-MDL 

was used as a measure of analytical uncertainty, which is 

appropriate for concentrations within a factor of less than 

about 10 of the LT-MDL. Nearly all concentrations detected 

in blanks were less than 10 times the LT-MDL. This method 

of censoring follows that of Wright and others (2005), and 

is based on the assumption that the amount of contamination 

in the field blank and the associated ground-water sample 

are comparable. If the results for a constituent met the above 

criteria, the chronological pattern of detections in blanks and 

ground-water samples for each sampling period was evaluated. 

If a constituent was detected in at least one associated blank 

and ground-water sample at similar concentrations during a 

sampling period, all detections in ground-water samples at 

concentrations below the censoring threshold were removed 

from the environmental sample data set for the sampling 

period and flagged as being potentially biased by sample 

collection and analysis procedures (hereinafter, referred to as 

censored). If a constituent was detected in the field blanks, 

but not in the associated ground-water samples, the groundwater data were not censored. If a constituent was detected 

in ground-water samples below the censoring threshold for a 

sampling period different than that for which the constituent 

was detected in field blanks and the constituent was detected 

in fewer than 50% of blanks, ground-water values were not 

censored. In addition, when constituents were detected in 50%

or more of field blanks, the possibility that environmental 

concentrations were higher than they would have been because 

of biased sample collection was assessed by determining the 

number of ground-water samples in which the maximum 

concentration was more than 10% of environmental 

concentrations. The 50% blank detection threshold was used 

to separate constituents suspected to be affected by systematic 

bias of the sampling from constituents affected by random bias 

from the sampling. Constituents for which the maximum blank 

concentration was <10% of the environmental concentration 

were considered to be unaffected by systematic bias. 

Table 3 lists the constituents that had detections in field 

blanks. For 10 of the 22 constituents detected in field blanks, 

no environmental data were censored because environmental 

concentrations were greater than the censoring threshold. One 

constituent, acetone, was detected in 2 or 7 field blanks and 

in associated source-solution blanks but was not detected in 

ground-water samples; consequently, no data were censored. 

For 3 constituents—carbon disulfide, tetrahydrofuran, and 

vanadium—minimum concentrations detected in ground-water 

samples were less than the censoring threshold but no groundwater data were censored because (1) the detection in a single 

field blank occurred during a sampling period different than 

when ground-water sample concentrations below the censoring 

threshold occurred, and (2) field blanks collected during the 

sampling periods when ground-water concentrations were 

below the censoring threshold showed no detection. For eight 

constituents, the concentrations for at least 1 environmental 

sample were censored. Constituents were censored within only 

a certain sampling period if the analysis of blank data from 

only that sampling period showed contamination. This was 

the case for the VOCs chloroform and 1,2,4-trimethylbenzene, 

which were only detected in blanks during the first one or two 

sampling periods of the study. Although 23 environmental 

concentrations for iron were censored, this did not affect 

the interpretative results of the study because only low 

concentrations that were not significant for interpretation of 

oxidation-reduction conditions needed to be censored. The low 

iron concentrations that were censored were in oxic ground 

waters, where low concentrations were expected and iron 

concentrations have little interpretative use. Concentrations 

of chromium, copper, manganese, and zinc were censored 

in 1 to 33 environmental samples per constituent, out of 

56 samples collected. Detections of trace concentrations 

of these constituents in blanks were noted in other waterquality investigations (Apodaca and others, 2006). Only two 

constituents, copper and nickel, were detected in more than 

50% of the blanks and maximum concentrations in blanks that 

were more than 10% of concentrations in some environmental 

samples. For copper, 23 samples met these criteria; for 

nickel, 36 samples met these criteria. For these samples and 

constituents, environmental concentrations may have been 

biased by sampling processes and the data were not used in 

the analyses. The primary trace elements of interpretative 

interest for this study were uranium and arsenic, which were 

not censored; therefore, censoring low concentrations of 

chromium, manganese, and zinc and a substantial number of 

copper and nickel analyses had no effect on the interpretations 

of this study. Concentrations of DOC at or near the LRL 

of 0.3 mg/L were censored in 7 environmental samples, 

indicating low DOC concentrations in some environmental 

samples could not be distinguished from DOC concentrations 

in blanks. 

Replicates

Replicate samples were collected to assess the variability 

resulting from collecting and analyzing samples. All replicates 

were sequential; the replicate sample was collected after 

the ground-water sample and both were analyzed using the 

same method. The relative standard deviation (RSD) was 

used to express differences between replicate pairs. The RSD 

is defined as the standard deviation divided by the mean 

concentration for each replicate pair of samples, multiplied 

by 100. If one value for a sample was reported as a nondetect and the value for the replicate was reported below the 
Methods 17

reporting level, the RSD was set to zero because the values are 

analytically identical. If one value for a sample was reported 

as a non-detect and the replicate value was greater than the 

LRL, the non-detection value was set equal to one-quarter of 

the LRL, and the RSD was calculated. Values of RSD <20%

are considered acceptable in this study. 

Generally, the replicate results indicated low variability of 

the analytical results. The median relative standard deviation 

was <5% for all constituents (table 4 [see back of report]). 

The maximum relative standard deviation was <20% for all 

constituents except manganese. Concentrations of manganese 

measured in one replicate pair were 1.5 and 2.9 μg/L, resulting 

in an RSD of 46%. These concentrations were about 10 

times greater than the laboratory reporting level of 0.2 μg/L 

for manganese. However, these manganese concentrations 

were <10% of the value of manganese used as an indicator 

of manganese reducing conditions of 50 μg/L; manganese 

was primarily used in this study as one of the indicators of 

oxidation-reduction conditions that affect many constituents. 

Thus, uncertainty in analytical concentrations of manganese 

at concentrations of <about 5 μg/L was of no practical 

consequence to the interpretations of this study. 

Replicates of all dissolved gas, CFC, and SF6

 samples 

were analyzed. Most of these analytes had a RSD <5% (not 

shown). 

Spikes

Laboratory matrix spikes are quality-control samples 

used to evaluate the bias and variability of analytical results 

caused by interferences from the chemistry of the ground 

water sampled (matrix interferences). Two spike samples 

each were prepared and analyzed for VOCs (86 analytes), 

gasoline oxygenates (8 analytes), and pesticides (62 analytes). 

Laboratory matrix spikes were prepared by adding solutions 

containing known concentrations of target analytes to replicate 

ground-water samples before sample preparation and analysis. 

The constituents added were the same as those analyzed 

in the environmental samples. Acceptable ranges for spike 

recoveries for this study (after subtracting any detections in 

the corresponding environmental sample) were between 70 

and 130% for VOCs and gasoline oxygenates (Connor and 

others, 1998; Rose and Sandstrom, 2003) and 60 to 120% for 

pesticides (Zaugg and others, 1995; Sandstrom and others, 

2001). 

Recoveries for spiked VOCs and gasoline oxygenates 

were within the acceptable range for all compounds except 

for tetrachloroethylene (PCE) (166%), trichlorofluoromethane 

(CFC-11) (131%), and vinyl chloride (135%), which had one 

spike each outside the acceptable range (table 5A [see back 

of report]). PCE was the most frequently detected VOC in 

ground water, and the slightly higher than optimal recoveries 

may imply that concentrations in ground-water samples were 

slightly over-estimated. The high spike recoveries for CFC-11 

and vinyl chloride were inconsequential, as these analytes 

were not detected in ground-water samples. 

Of the 62 pesticides in spiked samples, 23 had 

recoveries <60% in one or both spikes (table 5B [see 

back of report]). Included in these pesticides having low 

recoveries was 2-chloro-4-isopropylamino-6-amino-s-triazine 

(de-ethylatrazine), which is a pesticide degradation product of 

atrazine that is frequently detected in ground-water and was 

detected in this study. De-ethylatrazine had spike recoveries of 

47 to 58%. Such a low recovery suggests that de-ethylatrazine 

may be more prevalent than was detected and (or) at higher 

concentrations than reported. It also is possible that the 

number of detections and the concentrations of the other 22 

pesticides having at least one spike recovery <60% may be 

underestimated. The only other pesticide detected in groundwater samples for which spikes were collected, atrazine, had 

acceptable recoveries. 

Surrogates

At the NWQL, surrogate compounds were added to all 

ground-water, surface-water, and quality-control samples that 

were analyzed for pesticides, VOCs, and gasoline oxygenates. 

Surrogate compounds are constituents that behave like 

pesticide or VOC analytes but are not normally found in the 

environment. Surrogates are added to the sample just prior 

to analysis. Surrogates are used to evaluate the ability of the 

sampling and analysis methods to detect like compounds in 

each sample and to determine whether the chemistry of each 

sample would interfere with the pesticide or VOC analysis 

(referred to as matrix interference). A 70 to 130% recovery is 

generally considered acceptable, and values outside this range 

indicate possible problems with the processing and analysis of 

samples. 

The surrogate recoveries were nearly always within 

the acceptable range. Two surrogates, α-HCH-d6 and 

diazinon-d10, were added to each of the 21 pesticide samples 

collected for this study. The mean recovery was 91% (standard 

deviation (σ) = 1.3%) for α-HCH-d6 and 95% (σ = 1.6%) 

for diazinon-d10. Three surrogates were added to 86 VOC 

samples at the NWQL and one surrogate was added to 43 

gasoline oxygenate samples. Mean VOC surrogate recoveries 

for 1,2-dichloroethane-d4, 1-bromo-4-fluorobenzene, and 

toluene-d8 were 112% (σ = 7.1%), 95% (σ = 5.6%), and 

99% (σ = 2.7%) respectively. The mean gasoline oxygenate 

recovery for isobutyl alcohol-d6 was 108% (σ = 24.2%). 

Statistical Methods

Nonparametric methods were used to identify 

statistically significant correlations between potential 

explanatory variables and concentrations of constituents 

of particular interest. Potential explanatory variables 

included concentrations of other constituents or physical 

parameters such as depth to water or depth below water 

table. Nonparametric statistics were used for the analyses; 

these are robust techniques that are generally not affected by 
18 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

outliers and do not require that the data follow any particular 

distribution (Helsel and Hirsch, 1992). Use of nonparametric 

analyses was necessary as many of the data sets investigated 

did not appear to follow a normal distribution, as would be 

required for parametric techniques to be used. The significance 

level (p) used for hypothesis testing for this report was 

compared to a threshold value (α) of 5% (α = 0.05) to evaluate 

whether the relation was statistically significant (p < α). 

Correlations were investigated using Spearman’s method to 

calculate the rank-order correlation coefficient (ρ). Values 

of ρ range from +1.0 (perfect positive correlation), through 

0.0 (no correlation), to −1.0 (perfect negative correlation). In 

limited cases when the relation between two variables was 

clearly linear, the Pearson or linear correlation coefficient 

(r) was calculated using the data values rather than the data 

ranks. The value of the coefficient of determination (r2) also 

was calculated in these cases and represents the fraction of the 

variance explained by the linear regression (Helsel and Hirsch, 

1992). All statistical analysis was done using S-PLUS for 

Windows, version 6.1, Professional Edition. 

Estimation of Aquifer Properties

Slug tests were completed in 33 monitoring wells to 

determine horizontal hydraulic conductivity (Kh) of the aquifer 

near the screened interval. Slug tests were conducted using 

pneumatic (air) slugs in 29 wells, solid slugs in 3 wells, and a 

combination of slug types in 1 well. The water-level responses 

were recorded using a Druck pressure transducer attached 

to a Campbell Scientific CR10X data logger. Normalized 

(recorded water-level – static)/initial displacement [y0]) plots 

of the water-level response data were used to determine 

whether the response depended on y0; if so, the response from 

the smallest displacement was analyzed. The normalized 

plots were visually reviewed to determine if the water-level 

response was over-damped, critically damped, or under 

damped (Butler, 1998). For overdamped responses, an Excel 

workbook template of Halford and Kuniansky (2002) applying 

the Bouwer and Rice (1976) method was used to calculate 

Kh

. For underdamped and critically damped responses, a 

spreadsheet template of Butler and others (2003) that is based 

on models described by Springer and Gelhar (1991) and Butler 

(1998) was used that solves the damped spring equation to 

determine the damping factor, and calculates Kh using the 

Bouwer and Rice shape factor (Butler and others, 2003). For 

each of the slug tests conducted for this study, there was a 

substantial linear segment in the response curves, indicating 

that the theoretical conditions of the Bouwer and Rice (1976) 

method were generally met for the tests. Recommended field 

guidelines were followed, including initiating the tests very 

rapidly relative to the formation response, doing a series of 

tests at more than one y0

 to determine if the response depends 

on y0, and ensuring that the pressure transducer was as close to 

the static-water level as possible (Butler and others, 2003).

The saturated hydraulic conductivity (vertical) of core 

samples were analyzed using the methods of Nimmo and 

Mello (1991) and Nimmo and others (2002) in a USGS 

laboratory in Menlo Park, California (Kim Perkins, U.S. 

Geological Survey, written, commun., October 20, 2004). One 

core of unsaturated loess, two cores of the upper confining 

clayey silt till, and one core of a lower confining clayey silt 

were submitted for analysis. 

Hydrogeology

The hydrogeologic setting of the local-scale TANC study 

area is described below. Particular emphasis was placed upon 

the new understanding of the hydrogeology of the localscale TANC study area gained from lithologic, geophysical, 

ground-water level, and aquifer property data collected for this 

investigation. 

Geology, Mineralogy, and Sediment Chemistry

The local-scale TANC study area lies within the 

High Plains aquifer, which consists of Quaternary-age 

unconsolidated deposits in eastern Nebraska (fig. 1) (Gutentag 

and others, 1984). The sediments consist of sand, gravel, silt, 

and clay that form a layered sequence of aquifers separated 

by intervening aquitards (fig. 5). Landon and Turco (2007) 

interpreted available lithologic logs to conceptualize the 

geology of the regional-scale TANC study area as a sixlayer system. This conceptualization was slightly refined to 

describe geologic heterogeneity influencing water and solute 

transport within the 108-km2 local-scale TANC study area 

using lithologic descriptions and geophysical logging of 

seven test holes drilled through the aquifer into the underlying 

shale. The additional test holes indicated the presence of the 

upper four units or aquifers at all sites with some additional 

heterogeneity identified within the units. The hydrogeologic 

units in the local-scale TANC study area are described below 

and summarized in table 6 (see back of report). 

The High Plains aquifer in the local-scale TANC study 

area is underlain by the Carlile Shale of Late Cretaceous age 

(Keech and others, 1967). Unweathered Carlile Shale is black 

or dark-grey, fossil-rich consolidated rock with traces of sand 

and white limestone or calcite fragments. At some locations, 

the upper few meters of the shale has been weathered and 

consists of light brown and light gray silty clay with increasing 

amounts of more consolidated black shale with depth. 

Generally, the thickness of the upper four hydrogeologic 

units does not vary greatly across the study area (fig. 5). Total 

thickness of the aquifer is primarily controlled by variations 

in the bedrock surface. The depth to the top of the shale and 

the thickness of the lower confining unit and lower confined 

lenses vary more than the thickness of the units in the upper 

part of the aquifer. 
Hydrogeology 19

Figure 5. Cross sections showing hydrogeologic units and geophysical logs from selected test holes drilled for the local-scale TANC 

study near York, Nebraska. 

FP2-43

FP2-78

FP2-152

FP2-250

Monitoring well screen

and well name

Well cluster name

Locations of sections

shown on figure 3

Water table

Geophysical logResistivity in ohm-meters

EXPLANATION

FP5

FP5-45

FP5-73

FP5-175

METERS

NORTHWEST SOUTHEAST

SOUTHWEST NORTHEAST

FP4

FP4-28

FP4-83

FP4-168

FP4-231

FP4-231

RESISTIVITY

Ohm-meters

RESISTIVITY

Ohm-meters RESISTIVITY

Ohm-meters

RESISTIVITY

Ohm-meters

RESISTIVITY

Ohm-meters

FP3 OFPS

OFPS-277

OFPS-157

OFPS-38

OFPS-225

FP1

FP1-63

FP1-147

FP1-185

FP1-247

FP3-33

FP3-83

FP3-130

FP3-162

FP3-218

FP4

FP3-33

FP3-83

FP3-130

FP3-162

FP3-218

OFPN-276

OFPN-167

OFPN-88

FP2

Loess (Unsaturated)

METERS

FP3

OFPN

Section below

intersects here

Section above

intersects here

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

Coarse-medium sand and

gravel (Unconfined aquifer)

Clayey silt (Upper confining unit)

Fine sand (Upper confined aquifer)

Carlile Shale

Hydrogeologc unit

Fine sand (Lower confined lenses)

Clayey silt (Lower confining unit)

0 15075

0 15075

0 15075

0 15075 0 100 400

390

410

420

430

440

450

460

470

480

490

500

1 MILE

0 1 KILOMETER

0

Vertical exaggeration greater than X40

0 15075

0 200100

400

390

410

420

430

440

450

460

470

480

490

500

Vertical exaggeration greater than X40

0 0.2 MILE

0 0.3 KILOMETER

Ohm-meters RESISTIVITY RESISTIVITY

Ohm-meters

0 15075

RESISTIVITY

Ohm-meters

VERTICAL DATUM IS NAVD 88 VERTICAL DATUM IS NAVD 88
20 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

The silt, clay, sand, and gravel that make up the High 

Plains aquifer were deposited during the Pleistocene epoch 

(0.01 to 2 million years ago). Sediments of the lower confining 

unit and lower confined lenses, the upper confined aquifer, and 

the upper confining unit (table 6) were deposited during the 

early part of the Pleistocene, when continental glaciers moved 

into the study area from the north (Gottula and Link, 1992). 

The advance and retreat of continental glaciers in the region 

created barriers to western streams and source sediments. 

Glacial drainage resulted in deposition of silt, clay, and sand 

in stream valleys. The uppermost glacially derived sediment 

is the upper confining unit, a mixture of silt, clay, and stones 

derived from source materials to the north. During glacial 

retreat to the north, sand and gravel from streams draining 

from the west was deposited with interbedded silt and clay of 

eolian and fluvial origin. These deposits form the unconfined 

aquifer (table 6). The source of the sediments may have 

included both reworked Ogallala Formation, the principal 

hydrogeologic unit of the High Plains aquifer west of the study 

area, and materials from the Rocky Mountains (Swinehart and 

others, 1994; Snow and Spalding, 1994). Finally, wind-blown 

silt, or loess, was deposited across the landscape during the 

late Pleistocene. 

The uppermost geologic layer is loess, which is 

continuous across the local-scale TANC study area and 

unsaturated in most locations. In a few locations, the water 

table extends into the bottom part of the loess, particularly 

where the depth-to-water is shallow, such as along Beaver 

Creek (fig. 1) or Lincoln Creek (topographic valley north 

of York shown on figure 1). Sediment particle-size analysis 

of a sample of unsaturated loess indicated mostly silt-sized 

particles (80.3%), with lesser amounts of clay (17.3%; table 7

[see back of report]). The loess is the parent material for the 

fertile silt and clay loam soils that cover most of the study 

area. Heterogeneities such as cracks, root casts, and animal 

burrows can allow water to move more rapidly through the 

loess under some circumstances.

Deposits of sand and gravel with discontinuous silt and 

clay lenses underlie the loess and form the unconfined aquifer, 

which is continuous across the local-scale TANC study area. 

The water table is near the top of the unconfined aquifer at 

most locations. Lithologic descriptions and geophysical logs 

(induction and (or) resistivity) indicated sediment texture 

generally became finer downward through the unconfined 

aquifer, with the gravel and coarse sand grading into more 

fine-textured sand, grading into heterogeneous thinly 

interbedded sand, silt, and clay (fig. 5; tables 6 and 7 ). Crosssections by Keech and others (1967) indicate that thin silt 

layers at the bottom of the unconfined aquifer are common.

 A clayey silt that forms the upper confining unit 

underlies the unconfined layer and is continuous across 

the local-scale TANC study area. The upper confining unit 

contains stones of different sizes and is poorly sorted. These 

sediments have been interpreted as a glacial till deposited 

by continental glaciers that advanced southward into eastern 

Nebraska; the western extent of the glacial till is about 20 km 

west of the regional-scale study area (Swinehart and others, 

1994). A sediment particle-size analysis of a core sample of 

the upper confining unit indicated that it consists mostly of 

silt-sized particles and lesser amounts of clay (table 7). 

A moderately well-sorted fine-to-medium sand underlies 

the upper confining unit and forms the upper confined aquifer. 

The upper confined aquifer contains only minor amounts of 

gravel, silt, and clay (table 7) and is more homogeneous than 

the unconfined aquifer. Nearly all PSWs and many irrigation 

and commercial wells are fully screened across the upper 

confined aquifer, which is continuous across the local-scale 

TANC study area and is the principal water-supply unit.

Underlying the upper confined aquifer is a heterogeneous 

mixture of silt, clay, and fine-to-medium sand. Silt and clay 

are more abundant than the poorly-sorted fine-to-medium 

sand lenses (table 6). Sand lenses, when present, are 1- to 

5-m thick and are discontinuous between test holes. Some 

PSWs and irrigation wells have multiple screens that intersect 

the thin sand lenses. Landon and Turco (2007) divided the 

silt, clay, and sand deposits in the lower part of the confined 

aquifer system into two layers for the regional-scale TANC 

study: a lower confining unit consisting of more silt and clay 

than sand and an underlying lower confined layer consisting 

of more sand than silt and clay. The drilling and geophysical 

logging done for the local-scale TANC study indicated 

that the deposits in the bottom part of the confined aquifer 

system were heterogeneous and did not always conform 

with the regional-scale TANC conceptualization. Rather, the 

sediments in the lower part of the confined aquifer system 

mostly consisted of silt and clay deposits, described as the 

lower confining unit in this report, and discontinuous thin sand 

lenses within silt and clay, described in this report as the lower 

confined lenses. Sediment particle-size analysis of a core 

sample from the lower confining unit indicated that the sample 

consisted mostly of silt-sized particles with lesser amounts of 

clay (table 7). 

Quartz is the most abundant mineral in core samples 

from all hydrogeologic units, followed by plagioclase and 

potassium feldspar in most units (table 8 [see back of report]). 

Although the x-ray diffraction (XRD) results are qualitative, 

they indicate the relative abundance of different minerals in 

the solid phase. Calcite was detected in most core samples, 

except those from the loess and upper confining unit. Small 

quantities of gypsum and halite were detected in the shale 

underlying the High Plains aquifer. Gypsum and halite also 

were detected in a sample from FP3 (depth 38.1 to 41.1 m) 

in the upper confined aquifer; however, these detections were 

considered spurious because the water chemistry was not 

consistent with the presence of these minerals. Furthermore, 

these minerals were not detected in other aquifer samples. This 

sample was collected by sieving drill cuttings from mud-rotary 

drilling and washing the drilling mud off with de-ionized 

water. It is suspected that the traces of gypsum and halite in 

this sample represent contamination from the drilling mud. 
Hydrogeology 21

Pyrite was identified in heavy mineral separates in 3 

samples from near the water table in the unconfined aquifer, 

in the upper confined aquifer, and in the lower confining unit 

(table 8). Fractions of heavy mineral separates comprised 

of pyrite were greatest in the upper confined aquifer and the 

lower confining unit samples. Although the pyrite was present 

only in very small amounts (unquantifiable since XRD and 

heavy mineral separates results are qualitative), the presence 

of pyrite indicates that it is potentially available as a reactant 

affecting ground-water iron, sulfate, and oxidation-reduction 

patterns. Sulfide concentrations measured using the acidvolatile and chromium-reducible sulfides extraction process 

ranged from 0.18 to 3.2 mg/kg in the loess, unconfined 

aquifer, upper confining unit, and upper part of the upper 

confined aquifer (table 9 [see back of report]). Sulfide 

concentrations were 1 to 3 orders of magnitude greater than 

these values near the bottom of the upper confined aquifer 

(75 mg/kg) and in the lower confining units (1,800 mg/kg), 

consistent with larger fractions of pyrite in heavy mineral 

separates.

Results of sequential extractions of core samples 

indicate that iron, manganese, uranium, and arsenic were 

present in the aquifer sediments and underlying shale, and 

concentrations of these elements in ground water were not 

limited by source sediments. Total iron concentrations from 

0.5-N HCl-HA extractions represented poorly crystalline, 

microbially reducible fractions of iron oxides in the sediment 

phase (Craig Brown, U.S. Geological Survey, written 

commun., July 5, 2006). Total iron concentrations in these 

extractions were greatest in core samples from the loess, 

unconfined aquifer near the water table, and lower confining 

unit (table 9). No sample of shale was analyzed using the 

0.5-N HCl-HA extraction; however, extractions using stronger 

acids mobilizing more of the total solid-phase chemistry 

indicated that the shale core had the highest concentration 

of iron (table 9). Extractions using 6-N HCl and 10% HNO3

indicated that manganese concentrations were (1) higher in 

fine-textured than in coarse-textured sediments, (2) detected in 

all but one core sample, (3) lowest in the unconfined aquifer, 

and (4) present in much lower concentrations than iron. 

Uranium was detected in all samples analyzed, but the highest 

concentrations were in samples from the lower confining 

unit and shale (table 9). Arsenic was detected in all samples 

analyzed from 10%-HNO3

 extractions, but were highest in 

fine-textured upper confining and lower confining units and 

the Carlile Shale. 

Organic carbon contents in fine-textured sediments 

including the Carlile Shale (3.2%), lower confining unit 

(0.07 and 0.26%), and loess (0.13%) were greater than those 

in core samples from the unconfined and upper confined 

aquifers, which ranged from 0.03 to 0.05% (table 9). The 

upper confining unit not analyzed for organic carbon. The 

similarity of total carbon and organic carbon content indicate 

that total carbon consisted nearly entirely of organic carbon 

in the loess and unconfined aquifer, and Carlile Shale. In the 

upper confined aquifer and lower confining unit, the difference 

between total carbon and organic carbon suggested that most 

of the carbon is present in inorganic forms, most likely as 

calcite. This observation is supported by δ13C values of total 

carbon, organic carbon, and calcite. Total carbon δ13C values 

approached those of organic carbon for the loess, unconfined 

aquifer, and Carlile Shale, where organic carbon dominates. 

Values of δ13C for total carbon in 3 samples from the upper 

confined aquifer and lower confining unit approached those of 

calcite, indicating that inorganic carbon is the dominant form 

in these units. These results suggest that calcite is abundant 

in and below the upper confined aquifer. The small offset 

between total carbon and organic carbon δ13C values and XRD 

results for relative abundances of calcite described earlier 

imply that calcite is present only in small quantities in the 

loess, unconfined aquifer, and upper confining unit. However, 

the presence of even a small amount of calcite in these units 

can have a great effect on inorganic ground-water chemistry 

(see Major Elements).

Ground-Water Occurrence and Movement

Both unconfined and confined conditions exist within the 

TANC local-scale study area. Ground water predominantly 

flows from the northwest and west to the southeast and east 

(fig. 3); the average gradient was about 0.001326 in the 

unconfined aquifer (Keech and others, 1967). Regional watertable maps were available for several periods, including the 

late 1950s (Johnson and Keech, 1959), the mid-1960s (Keech 

and others, 1967), 1979 (Nebraska Conservation and Survey 

Division, 1980), 1995 (Dreeszen, 2001), and 1996 (Verstraeten 

and others, 1998). The maps indicate consistent regional flow 

directions and gradients, even as heads in the entire aquifer 

have risen and fallen over the last 5 decades. Ground water 

passing beneath York that is not withdrawn by pumping 

probably discharges into the West Fork of the Big Blue River 

about 24 to 32 km southeast of York (Landon and Turco, 

2007). 

There are no naturally perennial streams in the immediate 

vicinity of York. Flow in Beaver Creek is primarily maintained 

by return flow of ground water pumped for commercial 

purposes or by discharge of treated wastewater effluent 

(Landon and Turco, 2007). Leakage from Beaver Creek to 

the aquifer over the stream reach extending downstream from 

near the southwestern edge of York to the edge of the regionalscale study area (fig. 1) was estimated to be about 0.042 m3

/s; 

this estimate was based on average return flows and low-flow 

streamflows measured on Beaver Creek downstream of York 

during the late 1980s and early 1990s (Landon and Turco, 

2007). 

The depth to water ranged from 3 to 24 m below land 

surface in 15 shallow monitoring wells installed for this study. 

Depth to water was less within the Beaver Creek alluvial 

valley and in the eastern part of the local-scale TANC study 

area than in the rest of the study area. 
22 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Water levels decreased from 1 to 2 m in wells in the 

shallow unconfined aquifer during April 2003–September 

2005 in response to drought conditions during 2003–2004. 

Ground-water levels in the shallow unconfined aquifer 

fluctuated slowly over weeks to months and did not change 

quickly in response to individual precipitation events 

or seasons. Rather, the thickness and properties of the 

unsaturated loess and the storage properties of the unconfined 

aquifer apparently serve to smooth out wetting fronts from 

precipitation and irrigation events such that recharge events do 

not produce a short-term rise in the water table. 

There are large downward vertical hydraulic head 

gradients from the unconfined aquifer to the upper confined 

aquifer and lower confined lenses, particularly during the 

summer irrigation season (fig. 6). Large hydraulic head 

differences occur across the upper confining unit at all sites. 

Hydraulic heads in the unconfined aquifer were greater than 

those in the upper confined aquifer by 1 to 3 m during the fall, 

winter, and spring when irrigation wells were not pumping; 

during the summer irrigation season, hydraulic heads in the 

upper confined aquifer declined 7 to 13 m in response to 

withdrawals while hydraulic heads in the unconfined aquifer 

declined by less than 1 m. Hydraulic heads in the upper 

confined aquifer decrease more because of the much lower 

storage of water per volume of aquifer in confined than in 

unconfined aquifers (Freeze and Cherry, 1979). Downward 

vertical hydraulic head gradients across the upper confining 

unit during the summer were as much as 0.75 m/m. Hydraulic 

head gradients within the unconfined aquifer were generally 

downward but small (0.001 m/m). Hydraulic head gradients 

within the upper confined aquifer at FP3, away from pumping 

wells, were small and not consistent in direction. At FP1, 

<30 m from the study PSW, the hydraulic head in the upper 

part of the upper confined aquifer (FP1-147) was 1 to 3 m 

lower than the hydraulic head in the lower part (FP1-185). The 

probable cause of the upward hydraulic head gradient is that 

the water intake for the study PSW pump is above the top of 

the upper confined aquifer. At most locations and times, the 

downward hydraulic head gradients of 0.03 to 0.33 m/m from 

the upper confined aquifer to the lower confined lenses were 

relatively small (fig. 6). In the lower confined lenses, hydraulic 

heads were closest to, or sometimes greater than, those in 

the upper confined aquifer during June–July, likely because 

hydraulic heads declined more rapidly in the upper confined 

aquifer at the beginning of the irrigation season. Thereafter, 

hydraulic heads in the lower confined lenses dropped below 

those in the upper confined aquifer for the rest of the irrigation 

and non-irrigation periods.

Hydraulic heads observed in well nest FP1, within 30 m 

of the study PSW, showed more short-term variability than 

those at other sites because of the on and off cycles of the 

supply well pump. Hydraulic heads in the unconfined aquifer 

(FP1-63) did not change in response to study PSW pumping 

cycles, indicating that pumping stress in the upper confined 

aquifer did not propagate into the unconfined aquifer. This 

lack of a response is consistent with the absence of local 

short-circuit pathways across the upper confining unit at 

this location. Withdrawals from the study PSW in the upper 

confined aquifer caused hydraulic heads in the lower confined 

lenses (FP1-247) to decline by <2 m, considerably less than 

the 6-m decline in the upper confined aquifer. 

Exchange of water between the High Plains aquifer 

and the underlying Cretaceous Carlile Shale is likely to be 

volumetrically negligible compared to other aquifer fluxes 

because the permeability of the shale is much lower than that 

of the High Plains aquifer (Luckey and others, 1986). Limited 

data indicate that there have been small upward hydraulichead gradients from the shale to the lower confined lenses, 

implying that small upward fluxes of water into the aquifer 

may occur. McMahon (2001a) found upward hydraulic head 

gradients from underlying bedrock layers in the central High 

Plains aquifer in response to withdrawals from the aquifer. In 

a monitoring well completed in the Carlile Shale (OFPS-277), 

hydraulic heads during the spring and early summer in 2004 

and winter in 2004 and 2005 were consistently slightly higher 

than those in the lower confined layer (OFPS-225). Hydraulic 

head data for well OFPS-277 are more uncertain than data for 

other wells because this well could not be properly developed 

due to very low yield from the well. Because the well had 

a long time to equilibrate to hydraulic heads in the shale, 

measurements during the periods listed above probably reflect 

actual hydraulic heads in the shale even though the well could 

never be fully developed. Hydraulic heads during the late 

summer and early fall in 2004 may have been affected by 

attempts to develop the well. 

Aquifer Hydraulic Properties

Slug tests were done to determine Kh at many locations 

and depths in the aquifer for the purpose of developing a 

conceptual model for the spatial distribution of Kh that could 

be used as initial input into a numerical model (Clark and 

others, 2008). Previous studies often found that effective Kh

is greater for large-volume tests such as aquifer or tracer tests 

than for relatively small-volume tests like slug tests (discussed 

by Butler, 1998). However, it was not possible to conduct 

large-volume aquifer tests at enough locations to characterize 

spatial variability in Kh. 
Hydrogeology 23

Figure 6. Water-level elevations in selected monitorng wells in the local-scale TANC study area near York, Nebraska, 2003–2005.

460

465

470

475

480

485

460

465

470

475

480

485

460

465

470

475

480

485

Unconfined aquifer, deep (FP4-83)

Unconfined aquifer, shallow (FP3-33)

Recorded

Measured

Unconfined aquifer, shallow (FP4-28)

Recorded

Measured

Beaver Creek

EXPLANATION GRAPH A

EXPLANATION GRAPH B

EXPLANATION GRAPH C

Recorded

Measured

Measured

Measured

Upper confined aquifer (OFPS-157)

Recorded

Measured

Unconfined aquifer (OFPS-38)

Recorded

Measured

Measured

Upper confined aquifer (FP4-168)

Recorded

Measured

Lower confined lenses (FP4-231)

Recorded

Measured

Recorded

Measured

Measured

Measured

Measured

Unconfined aquifer, deep (FP3-83)

Upper confined aquifer, near top (FP3-130)

Upper confined aquifer, near bottom (FP3-162)

Lower confined lenses (FP3-218)

Lower Confined lenses (OFPS-225)

Shale (OFPS-277)

C

B

A

ALTITUDE ABOVE SEA LEVEL, IN METERS (DATUM IS NAVD 88)

Apr July Oct Jan JulyApr Oct AprJan OctJuly

2003 2004 2005
24 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

455

460

465

470

475

480

455

460

465

470

475

480

Apr July Oct Jan JulyApr Oct AprJan OctJuly

465

470

475

480

485

490

2003 2004 2005

ALTITUDE ABOVE SEA LEVEL, IN METERS (DATUM IS NAVD 88)

Unconfined aquifer (FP1-63)

Recorded

Recorded

Measured

Recorded

Measured

Measured

Unconfined aquifer (OFPN-88)

EXPLANATION GRAPH E

EXPLANATION GRAPH F

EXPLANATION GRAPH D

Recorded

Measured

Unconfined aquifer, shallow (FP5-45)

Measured

Measured

Upper confined aquifer (FP5-175)

Recorded

Measured

Unconfined aquifer, deep (FP5-73)

Recorded

Measured

Upper confined aquifer (OFPN-167)

Measured

Lower confined lenses (OFPN-276)

Measured

Measured

Upper confined aquifer, near top (FP1-147)

Upper confined aquifer, near bottom (FP1-185)

Lower confined lenses (FP1-247)

Multilayer, unconfined and upper confined aquifers (FP5-LS)

F

E

D

Figure 6.—Continued.
Hydrogeology 25

Results of slug tests in 34 monitoring wells indicated 

vertical variations in Kh consistent with lithologic and 

geophysical data. Kh was highest in the upper and middle 

depths of the unconfined aquifer where the layers have the 

coarsest texture (table 10 [see back of report]). Values of Kh

varied more in the unconfined aquifer than in the deeper units, 

probably reflecting the poor sorting and heterogeneity of the 

deposits, and ranged from 3 to 32 m/d. Kh near the bottom of 

the unconfined aquifer was generally lower than at shallower 

depths, reflecting the finer texture of the sediments in the 

lower part of the unconfined aquifer. Kh in the upper confined 

aquifer was less than Kh in the unconfined aquifer, reflecting 

the generally finer texture of the upper confined aquifer. There 

did not appear to be any consistent vertical differences in Kh

with depth through the upper confined aquifer. Kh in the lower 

confined lenses was mostly <3 m/d and smaller than Kh in the 

upper confined aquifer; most of the monitoring wells in the 

lower confined lenses were screened in silty sand, sandy silt, 

or clayey silt. 

A few aquifer tests to measure Kh values of larger 

volumes have been conducted in the past (table 10) and results 

generally support the conceptual model of greater Kh in the 

unconfined aquifer than in the upper confined aquifer or lower 

confined sand lenses. Two aquifer tests in the unconfined 

aquifer yielded values of Kh that ranged from 41 to 120 m/d 

(Argonne National Laboratory, 1995a; Ma, 1996). One aquifer 

test in the upper confined aquifer yielded a Kh of 20 m/d and 

two tests in the lower confined lenses yielded 4.8 and 6.9 m/d 

(Layne Geosciences, written commun., 1997). 

Laboratory tests of saturated vertical hydraulic 

conductivity (Kv

) of core samples indicated that three samples 

from confining units (two from the upper confining unit 

and one from the lower confining unit) had Kv ranging from 

6.4 × 10-6 to 1.5 × 10-5 m/d; one sample of loess had a Kv of 

1.5 × 10-5 m/d (table 10). These laboratory values derived from 

measuring small core samples probably represent minimum 

values because the small volume measured did not incorporate 

preferential flow paths present in larger volumes that could 

facilitate more rapid movement of water than implied by these 

laboratory Kv values. Nevertheless, these laboratory values 

provide minimum bounding estimates of the Kv of the loess 

and confining units.

Water Budget

A steady-state conceptual and simulated water budget 

for the regional-scale TANC study area for 1997–2001 was 

described by Landon and Turco (2007). More comprehensive 

analyses of the transient simulated water budget of the 

local-scale TANC study area, changes in ground-water-levels 

since the 1950s, and interpretations of historical changes in 

the water balance are described by Clark and others (2008). 

Water-budget information is presented in this report for the 

purpose of characterizing the hydrogeologic setting.

Ground-water withdrawals for irrigation were the largest 

outflow, accounting for 73% of total outflows; withdrawals 

for commercial and municipal supply accounted for 3 and 2%, 

respectively (Landon and Turco, 2007). Withdrawals for selfsupplied domestic or livestock purposes were not quantified 

because they are probably negligible compared with other 

withdrawals (Upper Big Blue Natural Resources District, 

1999). The remaining 22% of outflow from the regional-scale 

TANC study area was ground-water flow out of the study area 

at the southeastern boundary.

460

465

470

475

480

485

ALTITUDE ABOVE SEA LEVEL, IN METERS (DATUM IS NAVD88)

Apr July Oct Jan JulyApr Oct AprJan OctJuly

2003 2004 2005

Unconfined aquifer, shallow (FP2-43)

EXPLANATION GRAPH G

Recorded

Measured

Measured

Measured

Unconfined aquifer, deep (FP2-78)

Upper confined aquifer (FP2-152)

Lower confined lenses (FP2-250)

G

Figure 6.—Continued.
26 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Most ground-water inflows were from recharge, 

accounting for 89% of inflows (Landon and Turco, 2007). 

Regional ground-water inflows from the northwestern 

boundary accounted for 7% of inflows and seepage of water 

from Beaver Creek into the aquifer accounted for 4% of the 

inflows.

Average withdrawal rates for irrigation were estimated 

at 25 cm/yr for 1998 through 2002 on the basis of metered 

pumpage in 50 to 150 wells per year reported to the UBBNRD 

(Rod DeBuhr, UBBNRD, Water Department Manager, written 

commun., April 15, 2003). These average values included 

both sprinkler and gravity irrigation systems. Sprinkler 

irrigated lands accounted for about 46% of the irrigated area 

in the regional-scale TANC study area; gravity irrigated lands 

account for about 54%. Metered pumpage data for earlier 

periods and attempts to estimate historical metered pumpage 

are discussed by Clark and others (2008). 

Initial values of recharge rates for the regional-scale 

TANC study were estimated from the difference between other 

sources of inflow and outflow in the conceptual ground-water 

budget. Estimates of lateral boundary fluxes and groundwater withdrawals were considered to be better constrained 

than ground-water recharge, which can vary greatly because 

of many hydrologic, geologic, and land-use factors. Total 

recharge of about 2.3 m3

/s was needed to satisfy the water 

balance. Recharge from precipitation was initially estimated as 

14.2 cm/yr, 20% of average annual precipitation. Recharge on 

irrigated land was initially estimated as 20.6 cm/yr, the sum of 

recharge from precipitation plus 6.4 cm/yr, representing 25%

return flow from irrigation (25.4 cm/yr). These initial values 

were slightly adjusted during calibration of the regional-scale 

model so that final recharge values were 

Land Use Recharge rate (cm/yr) % of annual average

Non-irrigated 

non-urban

17.1 24.1% of precipitation

Sprinkler irrigated 20.6 (17.1 from 

precipitation, 3.5 from 

irrigation return flow)

Irrigation return flow 

recharge is 13.8% of 

applied irrigation of 

25.4 cm/yr

Gravity irrigated 22.8 (17.1 from 

precipitation, 5.7 from 

irrigation return flow)

Irrigation return flow 

recharge is 22.4% of 

applied irrigation of 

25.4 cm/yr

The irrigation-return flow percentages are within 

historical ranges of values reported for irrigation systems 

within the High Plains region (Mustick and Stewart, 1992). 

Annual withdrawals for public supply from the city 

of York averaged 2.07 × 106 m3 during 1981–2004. Annual 

withdrawals varied from year to year and were within 

about 20% of the average during 1981–2004. However, 

average annual withdrawals gradually increased and were 

2.35 × 106 m3 during 2000–2004, about 17% greater than 

during 1981–84. Withdrawals are projected to increase by 

about 12% in the next 20 years (Kirkam Michael Consulting 

Engineers, 2003). Withdrawals vary seasonally; May–

September withdrawals during 1999–2004 were about 67%

greater than October-April withdrawals, probably primarily 

reflecting outdoor water use.

Annual withdrawal data since 1981 show that 

withdrawals from the study PSW were intermediate and 

variable from the time of installation in 1977 through the 

1980s, relatively low during the 1990s, and greatly increased 

during 2000–2004. During 2001–2004, the study PSW had 

the highest annual average withdrawal rate of any York supply 

well in the system. Monthly withdrawals from the study PSW 

during 1999–2004 varied considerably, reflecting the operation 

of the York supply system, which cycled specific wells on 

and off in response to demand and maintenance schedules. 

Average withdrawals from the study PSW during May–

September were only 29% greater than during October–April 

during 1999–2004, which is less than the average seasonal 

variation in withdrawals for the entire system. 

Higher hydraulic heads in Beaver Creek than in adjacent 

shallow unconfined well OFPS-38 during 2003–2005 were 

consistent with loss of water from Beaver Creek to the 

aquifer, but fluxes may be small. There was a consistently 

downward vertical hydraulic head gradient from Beaver Creek 

to the unconfined aquifer (fig. 6C). However, hydraulic head 

fluctuations in OFPS-38 did not mimic those in the creek, 

except during a large runoff event in the spring of 2005 

that produced a 2-m rise in Beaver Creek and a 50-cm rise 

in OFPS-38. The lack of a close relation between surfacewater and ground-water hydraulic heads may be caused by 

the sediments beneath Beaver Creek not being completely 

saturated. The hydraulic head in OFPS-38 was usually about 

1.5 m below that in Beaver Creek (fig. 6C) although this well 

was located about 10 m from the stream. A lack of saturation, 

except during high water-levels during large storm events, 

would limit the temporal response of ground-water hydraulic 

heads to stream hydraulic heads. In addition, the stream 

sediments are comprised of organic-rich clay and silt and are 

expected to have low permeability. These observations suggest 

that fluxes from Beaver Creek are limited but uncertain. 
Chemical Characteristics 27

Well Classification

In the sections that follow, water chemistry is compared 

among different groups of wells. Monitoring wells were 

initially classified according to their degree of confinement 

(unconfined or confined), depth in the unconfined aquifer 

(shallow or deep, defined below), and land-use setting (urban

or agricultural) (table 11 [see back of report]). Samples 

from the study PSW, screened in the upper confined aquifer, 

including samples of surface discharge collected at 3 different 

times and depth-dependent samples collected from 5 depths 

during June 2004, were lumped into one category, study PSW, 

for plotting purposes (table 11). Initial analysis indicated 

that waters from most monitoring wells screened in the 

upper confined aquifer and lower confined lenses had similar 

chemistry; therefore, most of these wells were lumped under 

the combined category “confined aquifers” (used hereinafter 

in the report), which are compared to wells in the unconfined 

aquifer. In the unconfined aquifer, wells having a depth to 

the top of the screen of <7.5 m below the water table were 

operationally defined as “shallow.” Wells in the unconfined 

aquifer having a depth to the top of the screen greater than (>) 

7.5 m (up to 20 m) below the water table were operationally 

defined as “deep.” Land-use was classified as predominantly 

agricultural or urban for wells based upon observations at the 

well, aerial photographs, and land-use coverages (Center for 

Advanced Land Management Information Technologies, 2000; 

U.S. Geological Survey, 1999–2000). 

Initial analysis with this physical hydrogeologic 

classification indicated that (1) most monitoring wells 

screened in the confined aquifers had oxygen (δ18O) and 

hydrogen (δD) stable-isotopic values that were very similar, 

and (2) some samples from the study PSW and some 

monitoring wells screened in the confined aquifers had δ18O 

and δD values consistent with mixtures of water from the 

unconfined aquifer and the confined aquifers. Values of δ18O 

and δD are particularly useful for identifying waters from 

different recharge sources and mixing because they are tracers 

of the water molecules themselves and are not affected by 

anthropogenic or natural sources or reactions that influence 

concentrations of many dissolved solutes. Mixing water from 

the unconfined aquifer in the confined aquifers was expected 

to affect concentrations of most dissolved solutes in the 

affected wells. Consequently, it was essential to incorporate 

the extent of mixing in wells in the confined aquifers into the 

well classification scheme to relate chemical characteristics to 

explanatory processes. Wells in the confined aquifer that were 

mixtures of water from the unconfined aquifer and confined 

aquifers on the basis of δ18O and δD values were classified 

as confined urban or agricultural mixed wells; wells in the 

confined aquifer with the isotopically uniform δ18O and δD 

values consistent with those of most confined waters were 

classified as confined urban or agricultural unmixed wells. 

Because δ18O and δD data were useful for distinguishing 

mixed waters, which was critical for refining the classification 

of wells, the first section of “Chemical Characteristics,” which 

follows this section, describes interpretations of δ18O and δD 

results. Then, the physical/isotopic scheme used for classifying 

the wells described above is used for the remainder of the 

report to interpret the distribution of oxidation-reduction and 

pH conditions, VOCs, pesticides, ground-water age, nutrients, 

major elements, uranium, and arsenic within a framework 

for well classification that includes degree of confinement, 

depth, land use, well type, and mixing of unconfined and 

confined waters. The well classification scheme results in 9 

categories, shown in table 11 (also in table 1), and are used 

hereinafter in this report. The categories are (number of 

wells shown in parentheses) unconfined shallow urban (9), 

unconfined shallow agricultural (6), unconfined deep urban

(2), unconfined deep agricultural (2), confined urban unmixed

(8), confined urban mixed (4), confined agricultural unmixed

(2), and confined agricultural mixed (2) monitoring wells, and 

the study PSW (1). 

Chemical Characteristics

Chemical characteristics of the ground water within 

the local-scale TANC study area are best understood within 

the context of recharge and mixing patterns, as described 

in “Stable Isotopes.” Following the discussion of “Stable 

Isotopes,” constituent groups are presented in an order 

whereby the most complex subjects, most dependent on 

earlier topics, are presented last: “Oxidation-Reduction and 

pH Conditions” are presented first, as these geochemical 

characteristics affect many constituents. Thereafter, sections 

on “Volatile Organic Compounds” and “Pesticides” are 

presented, as these constituent groups were primarily used 

in this study as tracers of urban and agricultural land uses, 

respectively. Some of the pesticides also serve as approximate 

indicators of ground-water age, based on history of use. 

Following these sections, “Ground-Water Age Tracers” and 

interpreted age distributions are presented. These first five 

sections provide information on sources and travel times of 

water, as well as geochemical characteristics of the aquifer 

system. Thereafter, “Nutrients” are discussed, including 

nitrate, an anthropogenic constituent of principal interest 

to all local-scale TANC study units. As the nitrate and 

associated isotopic data are useful for interpreting major 

element distributions, the “Major Elements” section follows 

“Nutrients.” Major elements are valuable tracers of water 

sources and pathways, which influence “Uranium” and 

“Arsenic,” the next constituent groups presented; uranium 

and arsenic are natural constituents of interest to all localscale TANC study units. Finally, results from all constituent 

groups are discussed in the final section, “Depth-Dependent 

Sampling,” to illuminate the processes and factors affecting 

the water quality of the study PSW.
28 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Temporal variations in water chemistry were generally 

minor in water at most wells. Consequently, temporal 

variations are not discussed except in those few cases when 

temporal variations indicated important processes occurring 

in the aquifer. The emphasis of the report is on describing 

spatial variations in water chemistry that indicate processes 

controlling water quality in different parts of the aquifer.

Interpretations that consider all constituents and tracers 

will be discussed in “Processes Affecting Transport of 

Anthropogenic and Natural Contaminants to Public-Supply 

Wells.” that follows this section of the report. The data will 

be discussed to highlight chemical signatures of waters in 

different portions of the aquifer and source zones, pathways 

for different waters, and possible effects of pumping wells on 

chemical transport. The predominant controlling processes for 

the primary constituents of concern will be discussed based on 

these interpretations.

Stable Isotopes

Values of δ18O and δD differed between the unconfined 

and confined aquifers and between agricultural and urban 

land-use areas in the shallow unconfined aquifer (table 12 [see 

back of report]). Values of δ18O and δD have been widely used 

to identify ground-water recharge sources and areas (Darling 

and Bath, 1988; Stewart and McDonnell, 1991; Coplen, 

1993; Mathieu and Bariac, 1996). In the local-scale TANC 

study area, wells were assigned to four groups on the basis 

of δ18O and δD values (fig. 7), as described in the following 

paragraphs. 

Group 1 includes most wells in the confined aquifers 

(confined urban unmixed and confined agricultural unmixed

wells, referred to as confined unmixed wells for the remainder 

of this section, table 12), which had the most negative isotopic 

values (lightest). Values of δ18O and δD in confined unmixed

wells (group 1) were very similar and varied little in individual 

wells through time (fig. 7; table 12). 

Group 2 includes unconfined shallow agricultural wells, 

which had the least negative isotopic values (heaviest). 

Group 3 includes unconfined shallow urban wells, which 

had more negative values than unconfined shallow agricultural

wells, apart from the similarity of values between the most 

negative unconfined shallow agricultural well, FP4-28, and 

the least negative unconfined shallow urban wells, UWT3-34 

and UWT4-85 (table 12). Isotopic values for unconfined 

shallow urban and agricultural wells varied more spatially 

and through time than values for confined unmixed wells.

Group 4 includes wells having intermediate values 

of δ18O and δD between unconfined shallow urban and 

agricultural wells and confined unmixed wells. Included 

in this group are a minority of monitoring wells (FP5-175, 

FP5-LS, FP3-162, OFPS-157, FP3-218, FP2-250; table 12) 

screened in the confined aquifers (including confined urban 

mixed and confined agricultural mixed wells, referred to 

as confined mixed wells for the remainder of this section; 

table 11). These intermediate δ18O and δD values are 

consistent with these wells containing mixtures of water from 

the shallow unconfined aquifer and water from the confined 

aquifers. Calculations of mixing fractions are described later 

in this section. Similarly, depth-dependent samples from the 

study PSW, particularly those from 61.0- and 51.8-m bls, had 

δ18O and δD values intermediate between those of unconfined 

shallow urban and agricultural wells and confined unmixed

wells. Isotopic values of samples collected from the study 

PSW surface discharge were similar to those of confined 

unmixed wells. 

Finally, unconfined deep urban and unconfined deep 

agricultural wells (referred to as unconfined deep wells for 

the remainder of this section, table 11) also had intermediate 

isotopic values and are included in group 4. In the case of 

confined mixed well OFPS-157, δ18O and δD values were more 

variable through time than in any other well, with the δ18O 

decreasing from −8.84 to −9.56‰ between November 2003 

and April 2005 (table 12). Comparing ground-water δ18O and 

δD values with the Meteoric Water Line (MWL) can indicate 

whether samples primarily follow a distribution reflecting 

variations in the isotopic composition of precipitation or 

whether secondary processes such as evaporation or mixing 

have shifted ground-water sample values off of the MWL 

(Coplen, 1993). Globally, meteoric waters plot along a MWL 

(δD = 8.0 δ18O + 10, shown on fig. 7), which is often used in 

the absence of locally measured MWLs; less negative values 

reflect higher temperature environments and more negative 

values reflect lower temperature environments as affected 

by latitude, elevation, or season (Craig, 1961). Local MWLs 

provide additional information on input waters for groundwater recharge because the slope and intercept of local MWLs 

vary by geographic position and climatic factors (Kendall 

and Coplen, 2001). The isotopic values for local precipitation 

in York were not known, but published local MWLs for two 

nearby cities were used to estimate the MWL for York. There 

is a gradient in the isotopic composition of precipitation 

across Nebraska as a result of changes in moisture sources 

and precipitation (Harvey, 2001). Consequently, MWLs for 

Mead, Nebraska (δD = 7.40 δ18O + 7.32; Harvey, 2001), 

approximately 100 km east of York, and North Platte, 

Nebraska (δD = 7.66 δ18O + 4.66; Harvey and Welker, 2000), 

approximately 270 km west of York, have slightly different 

slopes and substantially different intercepts with the global 

MLW. The MWL of York (not plotted), given its intermediate 

geographic location, should lie between those of Mead and 

North Platte but should be closer to that of Mead because of 

the closer proximity and greater climatic similarity (annual 

precipitation: Mead, 70.4 cm, York, 71.1 cm, North Platte 

50.5 cm). However, both local MWLs and the global MWL 

are shown on fig. 7 for reference because the position of the 

York MWL cannot be known with certainty. 
Chemical Characteristics 29

Ground-water samples from the local-scale TANC 

study area generally plot near the MWLs shown on figure 7, 

indicating that most of the isotopic variation in ground water 

can be attributed to variation in the isotopic composition of 

precipitation. δ18O and δD in precipitation vary seasonally in 

Nebraska as a result of seasonal changes in moisture sources 

and temperature (Harvey, 2001). Isotopic values are the most 

negative in the winter. These seasonal changes in precipitation 

δ18O and δD can result in isotopic values of winter, spring, 

or fall recharge being more negative than those of summer 

recharge. Samples from confined unmixed wells (group 1) plot 

to the right of the local and global MWLs (fig. 7). Evaporation 

shifts the isotopic composition of residual water to the right 

of a MWL along a line that typically has a slope between 

3 and 6.5 (Coplen, 1993). The evaporation line shown on 

fig. 7 was calculated using a Rayleigh distillation equation 

for the average annual humidity and temperature at York (see 

Methods, Collection and Analysis of Water Samples) and 

has a slope of 6.1. The position of confined water samples to 

the right of the MWLs implies that these samples may have 

been affected by evaporation. Samples from unconfined wells 

plotted closer to the MWLs, which indicates that the water in 

these wells has been affected less by evaporation than water 

from the confined wells. These results imply that evaporation 

modified the composition of “old” recharge that resides in the 

confined aquifers more than that of “young” recharge residing 

in shallow depths of the unconfined aquifer. 

The variations in δ18O and δD for ground water at 

different depths and beneath different land-use areas in the 

local-scale TANC study area reflect several factors. First, 

the differences between unconfined shallow agricultural and 

unconfined shallow urban wells, presumably representing 

recently recharged ground water, primarily reflect differing 

proportions of recharge derived from precipitation and 

recharge derived from return flow consisting of groundwater pumped for irrigation or urban supply. Second, values 

of confined mixed wells and the study PSW reflect mixing 

of waters from the unconfined aquifer with water from the 

confined aquifers. Third, the isotopic differences between 

unconfined shallow urban and agricultural wells and 

confined unmixed wells reflect historical changes in recharge 

compositions. These factors are discussed below. 

-80

-11 -10 -9 -8 -7

-70

-60

-50

-40

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

Local Mead, Nebraska MWL (Harvey, 2001)

Local North Platte, Nebraska MWL 

(Harvey and Welker, 2000)

Global MWL (Craig, 1961)

Mead annual weighted

North Platte annual weighted

Urban shallow

Unconfined wells

Meteoric Water Lines (MWL’s)

EXPLANATION

Confined wells

Mean precipitation

Urban deep

Urban unmixed

Urban mixed

Study public supply well (Study PSW)

Agricultural shallow

Agricultural deep

Agricultural mixed

Agricultural unmixed

Evaporation line (slope = 6.1) calculated

from Rayleigh distillation at average annual

humidity and temperature 

FP3-33

NWTI-39

Group 1

Group 4

Group 3

Group 2

Confined-unconfined mixing line

Figure 7. Oxygen and hydrogen isotope values for ground water in the local-scale TANC study area near York, Nebraska.
30 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Precipitation and Return Flow Fractions in the 

Shallow Unconfined Aquifer

Isotopic values for unconfined shallow urban and 

agricultural wells reflected different proportions of recharge 

from precipitation and recharge from return flow of ground 

water pumped for irrigation or urban supply (hereinafter 

referred to as “return flow”). A well representing an unmixed 

water source is referred to in this report as an “end-member”; 

mixing two end-members produces waters that plot along a 

mixing line between the two end-member positions on plots 

comparing two or more constituents. The identification of 

end-members and mixing calculations are described below.

The end-member well for precipitation recharge was 

NWT1-39, located at the downgradient edge of a pasture 

that extends about 400 m northwest, a site deliberately 

selected to be as far from irrigated lands as possible. 

Samples from NWT1-39 had the least negative isotopic 

values of any ground-water samples (fig. 7; table 12). The 

annual weighted-mean-isotopic value of Mead precipitation 

(δD = −48‰) was only slightly less negative than that of 

NWT1-39 (δD = −50.8 ‰) (fig. 7); the δD of precipitation in 

York was expected to be more negative than precipitation at 

Mead because of east-west isotopic gradients. Ground water 

recharged by precipitation often has the value of the annualmean-weighted isotopic value of precipitation (Clark and 

Fritz, 1997). The similarity between NWT1-39 and Mead 

annual mean-weighted precipitation values is consistent with 

the interpretation that recharge at NWT1-39 is primarily or 

entirely derived from precipitation. 

In contrast to NWT1-39, the most negative isotopic 

values in unconfined shallow urban wells were in wells 

suspected of being influenced by recharge of return flow. The 

most negative isotopic values for unconfined shallow wells 

were for FP3-33 (table 12), which has a relatively shallow 

depth to water of about 6 m and underlies a heavily-irrigated 

urban lawn. Wells having the next most negative isotopic 

values were OFPN-88, OFPS-38, FP2-43, and UWT2-23 

(table 12). Well OFPN-88 is located at the downgradient 

edge of a well-landscaped and watered city park. Well 

OFPS-38 is located adjacent to Beaver Creek, whose flow 

was primarily from ground water pumped for cooling 

purposes and discharged to the Creek. FP2-43 is in an area 

just downgradient of a neighborhood served by septic systems 

(fig. 4). Because ground water was the source of domestic 

and municipal supply in the local-scale TANC study area, 

the water source to septic systems would have been ground 

water. Likewise, well UWT2-23 had chemical characteristics 

suggesting that it was influenced by septic-system discharge 

(see Major Elements). 

The fractions of precipitation recharge and return-flow 

recharge were calculated for δD and δ18O. Mixing fractions 

based on values of δD were used as the primary estimate 

because values of δ18O can be more strongly affected by 

evaporation (Clark and Fritz, 1997), but both sets of values 

are shown in table 12 for comparison. The fraction of samples 

from unconfined shallow urban and agricultural wells 

consisting of precipitation recharge rather than return-flow 

recharge was calculated as 

F = [( D )/ ( D D )] 100

where

F is fractio

pr s rf pr rf

pr

δ δ D − − δ δ ×

n of precipitation recharge, percent;

D is the D of the s s δ δ ample, per mil;

D is the D of the return flow recharge

e

rf δ δ

nd-member, per mil; and

D is the D of the precipitation pr δ δ recharge

end-member, per mil.

(4)

For unconfined shallow urban wells, the return flow 

end-member was assumed to be the study PSW surface 

discharge, which during the time of this study represented the 

mix of water from unconfined and confined aquifers in the 

city’s municipal water supply as a whole. The annual average 

pumping rates from 2000–04 and simulated fractions of water 

from the unconfined aquifer and confined aquifers withdrawn 

from individual city supply wells represented in the local-scale 

TANC ground-water flow model (Clark and others, 2008) 

were used to estimate the fraction of the municipal supply 

withdrawn from the confined aquifers at 89%; this fraction 

was very close to the fraction of water in the study PSW 

derived from the upper confined aquifer (88%) calculated 

using δD data (table 12). For unconfined shallow agricultural

wells, the amount of irrigation water from the unconfined 

aquifer and confined aquifers applied to upgradient fields was 

expected to vary. Given field-scale uncertainties and mixing 

of irrigation waters from different sources, an average fraction 

of irrigation water derived from the unconfined aquifer was 

calculated for the entire local-scale TANC study area using 

model results of Clark and others (2008). These calculations 

indicated that an average of 65% of irrigation water was 

derived from the unconfined aquifer and 35% derived from the 

confined aquifers (table 12). The composition of unconfined 

aquifer water in agricultural areas was calculated as the 

average of compositions from 2 vertical well profiles through 

the unconfined aquifer at FP4 and FP5 (table 12).

The results of the mixing calculations are shown in 

table 12. Fractions of precipitation recharge were found 

to vary from 47% for FP3-33 to 78% for UWT3-34 for 

unconfined shallow urban wells. For unconfined shallow 

agricultural wells, fractions of precipitation recharge were 

found to range from 65 to 100%.
Chemical Characteristics 31

Fractions of Water from the Unconfined Aquifer 

in Wells with Mixed Waters in the Confined 

Aquifers

Samples from confined mixed and unconfined deep

wells, and depth-dependent samples from the study PSW, 

had intermediate isotopic values between unconfined shallow

urban and agricultural wells and confined unmixed wells 

(table 12), and mostly plotted along an apparent mixing line 

(fig. 7). Mixing fractions of unconfined shallow water in 

the confined mixed wells having intermediate values were 

calculated and are shown in table 12. The unconfined shallow

end-member composition was different for confined mixed

wells in urban and agricultural areas, as shown near the bottom 

of table 12. 

Using δD values, calculated fractions of unconfined 

shallow water in confined mixed wells ranged from 41 to 94%

(table 12). In depth-dependent samples from the study PSW, 

unconfined shallow mixing fractions ranged from 15 to 46%. 

In the study PSW surface discharge, the unconfined shallow

mixing fraction was 12% in 3 samples collected at different 

times during October 2003 to April 2005. Unconfined shallow

mixing fractions in confined unmixed wells ranged from 

0 to 15%. However, the mixing values probably represent 

maximum fractions of unconfined shallow water, as the 

minimum δD value for confined unmixed wells was used as 

the end-member. If the minimum value was an outlier rather 

than the best representation of confined unmixed water, then 

unconfined shallow mixing fractions would be smaller. The 

unconfined shallow mixing fractions can probably only be 

estimated with confidence within a margin of ± 10%. 

The introduction of substantial amounts (41 to 94%) of 

water from the unconfined aquifer to the confined aquifers 

in a minority (6 of 16) of monitoring wells screened in 

the confined aquifers suggests that the mixing is the result 

of movement of water from the unconfined aquifer to the 

confined aquifers along preferential flow paths that are 

only present locally. The most likely preferential flow paths 

through the upper confining unit and lower confining unit are 

multi-layer wells screened in the unconfined and confined 

aquifers, which might permit water to move downward 

through well bores due to the substantial downward hydraulic 

head gradients present. Physical, chemical, and modeling 

results (Clark and others, 2008) that support the interpretation 

of well-bore leakage as a dominant transport process are 

discussed in “Movement of Water and Solutes to Supply 

Wells.” 

Isotopic Differences between Unconfined and 

Confined Aquifers

The differences between the isotopic values of samples 

from unconfined shallow and confined unmixed wells 

indicate that historical changes in the isotopic composition 

of recharge have occurred. Shallow unconfined waters have 

primarily recharged in the last decade, whereas confined 

waters recharged 60 years to several thousand years ago (see 

“Ground-Water Age Tracers”). The change in the isotopic 

composition of ground-water recharge could not be explained 

on the basis of available information but may result from the 

mechanisms listed below or some combination of them. More 

negative δ18O and δD values for “old” confined unmixed wells 

compared with those for “young” unconfined shallow wells 

could indicate 

(1) a predominance of winter recharge during the 

pre-settlement period when the land cover was primarily 

grassland compared to recent decades when the land cover was 

predominantly cropland. Darling and Bath (1988) observed 

more negative isotopic values for recharge under grassland 

than under cropland in a humid temperate setting, consistent 

with winter recharge comprising a larger fraction of total 

recharge under grassland than under cropland. Water use by 

continuous cover grassland could diminish deep percolation 

and recharge from summer rainfall more than crops such 

as corn and soybeans; the lower vegetation density of these 

crops could allow deep percolation from summer rains on 

bare soils between plants. Consequently, a greater fraction of 

total annual recharge would occur during winter on grasslands 

when water use by grass was low and deep percolation could 

occur; 

(2) changes in the isotopic composition of recharge 

reflecting climate change. Paleowaters recharged under 

cooler climatic conditions more than 9,000 years before 

present having more negative isotopic values than modern 

recharge have been reported in the Central and Southern High 

Plains Aquifer (Dutton, 1995; McMahon and others, 2004). 

Uncorrected carbon-14 ages of ground water from the bottom 

of the confined aquifer in the local-scale TANC study area are 

<7,500 years before present; corrected ages, having substantial 

uncertainties, indicate ages of <2,000 years before present (see 

“Ground-Water Age Tracers”). If confined unmixed waters 

are at the young end of this range, they probably do not span 

a long enough time period to indicate a change in climate. 

However, the substantial uncertainties in the age of confined 

unmixed ground water allow for the possibility that these 

waters reflect recharge under a cooler paleoclimatic condition; 

(3) recharge areas for confined unmixed waters located 

far to the west of the local-scale TANC study area. Areas 

currently likely to have modern ground-water recharge 

that has δD values of –74‰ (the least negative value for 

confined unmixed wells, table 12) or lower (because confined 

unmixed waters may have been modified by evaporation) lie 

several hundred kilometers to the west in western and central 

Nebraska (Kendall and Coplen, 2001). Regional ground-water 

flow patterns indicate that these areas are unlikely to be the 

recharge areas for ground water in the confined aquifers near 

York (Luckey and others, 1986). However, simulations and 

understanding of the pre-ground-water development condition 

of the High Plains aquifer are poorly constrained, so it is 

possible that confined unmixed waters may have recharged 

far to the west where isotopic values of recharge are currently 

more negative than in the local-scale TANC study area.
32 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

For the purpose of this study, the most important result 

of the δ18O and δD analyses is that there is an isotopic contrast 

between unconfined shallow and confined unmixed waters that 

can be used to trace the downward movement of water from 

the unconfined aquifer into parts of the confined aquifers. The 

causes of the isotopic contrast between unconfined shallow 

and confined unmixed waters are of interest but less important 

than the fact that the isotopic contrast exists and that δ18O and 

δD values can be used as tracers of unconfined shallow waters. 

Oxidation-Reduction and pH Conditions

Oxidation-reduction (redox) conditions influence the 

transport of many organic and inorganic constituents (Chapelle 

and others, 1995). Redox conditions along ground-water flow 

paths commonly change because of biological utilization of 

dissolved oxygen (DO), nitrate, manganese, iron, and sulfate, 

as electron acceptors in microbially mediated reactions 

with organic carbon or reduced sulfide commonly serving 

as electron donors (Champ and others, 1979; Chapelle and 

others, 1995; Chapelle, 2001). These redox conditions proceed 

along a well-documented sequence of Terminal Electron 

Acceptor Processes (TEAP), in which a single TEAP typically 

dominates at a particular time and aquifer location (Chapelle 

and others, 1995). The predominant TEAPs, from more 

oxidizing to reducing are oxygen-reducing (also referred to 

as oxic in this report), nitrate-reducing, manganese-reducing, 

iron-reducing, sulfate-reducing, and methanogenic. 

Threshold values for classifying the redox conditions 

and interpreted redox conditions for the study area are listed 

in table 13 (see back of report). This classification scheme 

developed for the TANC study is described by Paschke 

(2007). The presence of redox species indicating more than 

one TEAP may indicate mixing of waters from different 

redox zones upgradient of the well, that the well is screened 

across more than one redox zone, or spatial heterogeneity in 

microbial activity in the aquifer. In addition, different redox 

couples often are not consistent, indicating electrochemical 

disequilibrium in ground water (Lindburg and Runnels, 1984). 

Thus, the redox classification was used as a general set of 

guidelines for interpreting redox condition. Interpretations 

were sometimes complicated by mixing and (or) 

disequilibrium and did not always indicate an unambiguous 

redox state for every well. 

Water in most of the unconfined aquifer was oxic, as 

indicated by DO values of 2.5 to 9.0 mg/L (fig. 8A). Water 

in most of the confined aquifers was nitrate-reducing to 

methanogenic. The generally reducing conditions in the 

confined aquifers were indicated by DO and nitrate-N 

concentrations mostly <0.5 mg/L (fig. 8A,B), manganese 

concentrations ≥50 μg/L (fig. 8C) and iron concentrations 

≥100 μg/L (fig. 8D). Some wells in confined aquifers also 

had ≥0.005 mg/L hydrogen sulfide, consistent with sulfatereducing conditions (fig. 8E), and detectable methane, 

consistent with methanogenic conditions (fig. 8F). Zones 

having both high iron and detectable hydrogen sulfide were 

classified as having redox conditions ranging from iron- to 

sulfate-reducing (table 13); these waters may be mixtures of 

waters from these redox zones or zones where both processes 

occur simultaneously. Of the 16 wells screened in the confined 

aquifers, 8 had redox indicators spanning a range from less 

reducing conditions such as nitrate-, manganese-, or ironreducing conditions to more highly reducing conditions such 

as sulfate-reducing and methanogenic conditions. Such a 

range of conditions may indicate mixing of different waters 

from different redox zones or disequilibrium between redox 

indicators. Generally, DO and nitrate-N decreased with depth 

from the unconfined aquifer to the confined aquifers, and 

manganese, iron, hydrogen sulfide, and methane increased. 

Exceptions to the generally oxic conditions in the 

unconfined aquifer occurred in monitoring wells UWT2-23, 

UWT1-53, FP3-33, and OFPS-38 (table 13; figs. 8 and 9), 

which were variably reducing. All of these wells had DO 

<1.0 mg/L (fig. 10A). UWT1-53, FP3-33, and OFPS-38 

had manganese >400 μg/L (fig. 8C), and OFPS-38 had iron 

>650 μg/L, detectable hydrogen sulfide, and the highest 

methane concentration measured in the local-scale TANC 

study area, 297 μg/L. Redox conditions at OFPS-38 may 

represent a mixture of nitrate-reducing, iron-reducing, sulfatereducing, and methanogenic waters because of its proximity 

to Beaver Creek sediments, which are likely organic-rich. 

The different redox zones may narrow near Beaver Creek, 

and water from these zones could mix within the 1.52-m-long 

screen of OFPS-38. Wells UWT1-53 and FP3-33 were 

mixtures of nitrate- and manganese-reducing waters, although 

traces of methane may imply mixing with minor amounts 

of water that had been methanogenic. All of these wells had 

depths to water of less than about 7 m (fig 10). 
Chemical Characteristics 33

-10

0

0 0.10 1

DISSOLVED OXYGEN,

IN MILLIGRAMS PER LITER

NWT1-39 FP3-33

OFPS-38

OFPS-38

OFPS-38 OFPS-38

UWT1-53

10 0 0.1 10 0.01 0.1

0.1

1 100 1 100 1,000

1,000 10,000

10

101 100

10,000

10

20

30

40

50

60

70

DEPTH OF TOP OF SCREEN

BELOW WATER TABLE, IN METERS

-10

0

10

20

30

40

50

60

70

DEPTH OF TOP OF SCREEN

BELOW WATER TABLE, IN METERS

MANGANESE,

IN MICROGRAMS PER LITER

IRON,

IN MICROGRAMS PER LITER

0.1 1 10 100 1,000

HYDROGEN SULFIDE,

IN MICROGRAMS PER LITER

0.1 1 10 100 1,000

METHANE,

IN MICROGRAMS PER LITER

NITRATE PLUS NITRITE-NITROGEN,

IN MILLIGRAMS PER LITER

Urban shallow

Unconfined wells Confined wells

Urban deep

Urban unmixed

Urban mixed

Agricultural shallow Study public supply well (Study PSW)

Agricultural deep

Agricultural mixed

Agricultural unmixed

EXPLANATION

A B C

D E F

Figure 8. Relation of oxidation-reduction indicators to depth of top of screen below the water table in the local-scale TANC 

study area near York, Nebraska.
34 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Ground water in the shallow parts of the unconfined 

aquifer is probably more reducing where depth to water is 

small because dissolved organic carbon (DOC) leached from 

natural organic matter in soils is more likely to reach the water 

table where the depth to water is small (Starr and Gillham, 

1993). Concentrations of DO in unconfined shallow urban

and agricultural wells were strongly correlated (ρ = 0.80; 

p ≤ 0.001) with depth of the water table below land surface. 

Under shallow water-table conditions, the shallowest groundwater flow paths can interact with newly generated DOC 

from soil processes before the microbially labile component 

has been fully consumed, permitting more DOC to reach 

ground water and serve as electron donors for redox reactions 

(Starr and Gilham, 1993; Eberts and others, 2005b). DOC 

concentrations in reducing unconfined shallow urban wells 

OFPS-38, UWT2-23, and FP3-33 were generally ≥2.5 mg/L, 

more than DOC concentrations in most oxic unconfined 

shallow urban and agricultural wells (fig. 10B). Exceptions 

to this relation occurred in oxic wells FP1-63 and NWT1-39, 

which appear to be influenced by anthropogenic sources 

(see “Volatile Organic Compounds,” and “Nutrients”). The 

leaching of DOC from soils is likely to be strongly affected 

by variations in recharge rates, soil properties, soil organic 

matter content (Starr and Gilham, 1993), and depth of the 

screen below the water table. Two other unconfined shallow 

urban (UWT3-34) and agricultural (FP4-28) wells had depths 

to water of 6.8 to 7.3 m, similar to UWT1-53 and FP3-33, but 

were oxic, indicating that factors other than depth to water 

may contribute to DOC, DO, and redox conditions in the 

shallow unconfined aquifer. Some of the unconfined shallow 

urban wells having reducing conditions (OFPS-38, UWT2-23, 

and UWT1-53) also may be influenced by the proximity to 

stream sediments in Beaver Creek. 

In general, the upper confined aquifer can be classified 

as manganese- to iron-reducing. Sulfate-reducing and mixed 

methanogenic conditions occur locally, but their distribution 

is varied and cannot be reliably mapped. Water from 8 of the 

9 monitoring wells screened in the upper confined aquifer had 

≥50 μg/L of manganese. Water from 3 of the wells had ≥100 

μg/L of iron and water from 2 had detectable (≥0.01 mg/L) 

hydrogen sulfide. Traces of methane were detected in 5 of the 

wells. H2

 concentrations measured in a few wells ranged from 

about 0.3 to 0.8 nM (nanomole), consistent with iron-reducing 

conditions (Chapelle and others, 1995). 

 The lower confined lenses can be classified as iron- to 

sulfate-reducing. Iron concentrations were ≥100 μg/L in 3 

of the 6 wells screened in the lower confined lenses, and 

hydrogen sulfide was detected (≥0.01 mg/L) in all 6 wells. 

However, hydrogen sulfide was detected only in 1 of 3 

samples collected from FP1-247. Methane was detected in 4 

of the 6 wells, possibly indicating mixing of water affected by 

methanogenic conditions with less reducing waters.

Figure 9. Sum of dissolved oxygen and nitrate (as nitrogen) and sum of manganese and iron in wells in the localscale TANC study area near York, Nebraska.

Urban shallow

Unconfined wells

Confined wells

Urban deep

Urban unmixed

Urban mixed

Study public supply well (Study PSW)

Agricultural shallow

Agricultural deep

Agricultural mixed

Agricultural unmixed

EXPLANATION

Confined

reduced

Depthdependent

samples

Oxic

unconfined

OFPS-157

FP3-33

FP5-LS

FP4-231

FP3-130

FP3-162

UWT1-53

OFPS-38

OFPS-157

Nov 2003

Oct 2004,

Apr 2005

0.1

0.01 0.1 1 10 100

1

10

100

1,000

10,000

DISSOLVED OXYGEN PLUS NITRATE-NITROGEN,

IN MILLIGRAMS PER LITER

MANGANESE PLUS IRON (Mn + Fe),

IN MICROGRAMS PER LITER
Chemical Characteristics 35

Figure 10. Concentrations of selected constituents for unconfined shallow wells and depth to water below land surface 

for the local-scale TANC study area near York, Nebraska.

Confined mixed wells were sometimes less reducing 

than confined unmixed wells (figs. 8 and 9; table 13). 

Confined mixed wells FP3-162, OFPS-157, and FP5-LS 

had higher nitrate-N and lower manganese and iron than 

confined unmixed wells and were generally classified as 

nitrate-reducing as compared with confined unmixed wells,, 

which were generally manganese- to sulfate-reducing

(table 13). Redox conditions at well OFPS-157 changed 

from November 2003 to October 2004 from less reducing 

to more reducing conditions, with decreases in nitrate-N 

and increases in manganese and iron concentrations (fig. 9). 

Redox conditions at this well were similar in October 2004 

and April 2005. Nearly all other wells had consistent redox 

conditions through time. Not all confined mixed wells had less 

reducing water than confined unmixed wells; well FP3-218 

had sulfate-reducing conditions similar to those in confined 

unmixed wells. Because chemistry in these confined mixed

wells is interpreted as being influenced by well-bore leakage 

(see “Movement of Water and Solutes to Supply Wells”) and 

the distribution of this leakage is inherently uncertain, the 

distribution of nitrate-reducing waters within the confined 

aquifers is unknown. The study PSW had constant redox 

conditions, classified as iron-reducing conditions, throughout 

the study period (figs. 8, 9; table 13). 

OFPS-38 OFPS-38,

DISSOLVED ORGANIC CARBON, IN MILLIGRAMS PER LITER 

0

5

10

15

20

25

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER 

0 1 2 3 4 5 6 7 8 9

DEPTH TO WATER BELOW LAND SURFACE, IN METERS DEPTH TO WATER BELOW LAND SURFACE, IN METERS

5.5 6.0 6.5 7.0 7.5

pH, IN STANDARD pH UNITS

Urban shallow

Unconfined wells

Agricultural shallow

EXPLANATION

FP3-33

and FP3-33

FP1-63

FP3-33

FP4-28

and

UWT1-53

UWT1-53

UWT3-

34

FP4-28

NWT1-39

NWT1-39

UWT1-53

UWT3-34

UWT2-23 UWT2-23,

UWT2-23 C

A B
36 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Generally, pH ranged from about 6.3 to 7.6 for most 

samples but increased with depth (fig. 11). Values of pH 

were lower in the unconfined aquifer, mostly 6.3 to 7.2, than 

confined unmixed wells, mostly 7.3 to 7.6. Exceptions to the 

general trend of increasing pH with depth are discussed in the 

paragraphs below.

Most unconfined shallow urban and agricultural

wells with depths to water less than about 7 m had pH 

<6.6 (fig. 10C). The lowest pH values, 5.8 to 5.9, were 

for well FP3-33. These are the same wells that also had 

nitrate-reducing to methanogenic conditions rather than the 

oxic conditions in most of the unconfined aquifer. The pH 

of unconfined shallow urban and agricultural wells was 

correlated with depth to water below land surface (ρ = 0.59; 

ρ < 0.001) and DO (ρ = 0.79; ρ < 0.001). The low DO and pH 

in areas with depth to water less than about 7 m below land 

surface probably resulted from oxidation of DOC leaching to 

the water table. 

Most confined mixed wells had pHs from 6.7 to 7.4, 

lower than confined unmixed wells, which generally had pHs 

of 7.0 to 7.6 (fig. 11). The distribution of these relatively 

low-pH confined mixed waters is likely complex and 

temporally variable because these mixed waters are interpreted 

to be primarily related to leakage of water from the unconfined 

aquifer down well bores crossing the confining units (see 

“Movement of Water and Solutes to Supply Wells”).

Two confined unmixed wells, FP3-130 and FP4-231, had 

pHs of 8.5 and 8.7. Because waters having a high pH were 

unusual, the distribution and processes producing these waters 

are not well understood. 

0

-10

5 6 7 8 9

10

20

30

40

50

60

70

pH, IN STANDARD pH UNITS

DEPTH OF TOP OF SCREEN BELOW WATER TABLE, IN METERS

Urban shallow

Unconfined wells

Urban deep

Urban unmixed

Urban mixed

Study public supply well

(Study PSW)

Confined wells

Agricultural shallow

Agricultural deep

Agricultural unmixed

Agricultural mixed

FP3-130

FP3-33

FP4-231

Figure 11. Relation of pH to depth of top of screen below the water table in the local-scale TANC study 

area near York, Nebraska.
Chemical Characteristics 37

Volatile Organic Compounds

VOCs were detected in most unconfined shallow urban

wells, some confined mixed urban wells, and in the study PSW

(fig. 12). Because VOCs were detected in most unconfined 

shallow urban wells but detected in very few unconfined 

shallow agricultural wells, the presence of VOCs in confined 

urban mixed wells indicates that some fraction of water was 

affected by urban land use. 

Twenty-five VOCs were detected in ground-water 

samples (table 14 [see back of report]). Fourteen of these 

VOCs were detected in only one well each. Seven of these 

VOCs detected in a single well were detected within a single 

sample from well FP2-250. Most VOCs were detected at low 

concentrations within a factor of 10 of the laboratory reporting 

level and in fewer than 10% of wells. VOCs were detected 

in 21 of 36 wells sampled and in 7 of 9 unconfined shallow 

urban wells. The number of VOCs detected in these wells 

ranged from one to eight.

Concentrations of VOCs were larger than those allowed 

by the U.S. Environmental Protection Agency (USEPA) 

Maximum Contaminant Level (MCL) in 3 wells. Two

unconfined shallow urban wells, had tetrachloroethylene 

(PCE) concentrations approximately equal to the MCL of 

5 μg/L or higher: well FP1-63, about 6 to 12 μg/L and well 

OFPN-88, about 5 μg/L (table 14). In addition, concentrations 

of 1,2-dichloroethane (1,2-DCA) of 5.0 to 5.8 μg/L in 

confined urban mixed well FP3-162 were greater than or 

approximately equal to the USEPA MCL of 5 μg/L. None 

of the monitoring wells containing concentrations of VOCs 

larger than MCLs are used as a drinking-water source. The 

MCL pertains to drinking-water supplies only. Nonetheless, 

the MCLs provide a benchmark for comparison with measured 

concentrations in ground water. 

The most frequently detected VOCs were the 

chlorinated solvents PCE, trichloroethylene (TCE), cis-1,2-

dichlorethylene (cis-1,2 DCE), and trans-1,2-dichlorethylene 

(trans-1,2 DCE) (table 14). PCE and TCE were detected in 

four of nine unconfined shallow urban wells and in the study 

PSW. In three of nine unconfined shallow urban wells, PCE 

was detected but TCE was not. The remaining two unconfined 

shallow urban wells (FP3-33, FP2-43) had no detections of 

PCE or TCE. Trichloromethane (chloroform) was detected in 

10 of 82 samples and seven wells, although only three of the 

samples and wells had concentrations greater than the LRL 

of 0.024 μg/L. Twenty other VOCs besides PCE, TCE, their 

degradation products cis- and trans-1,2 DCE, and chloroform 

were detected; other compounds were detected less frequently, 

usually in only a few wells, and usually in relatively low 

concentrations (table 14). 

There are multiple possible sources for the VOCs in 

unconfined shallow urban wells. Because the seven unconfined 

shallow urban wells where PCE was detected (FP1-63, 

OFPN-88, OFPS-38, UWT1-53, UWT2-23, UWT3-34, and 

UWT4-85) were likely not along the same ground-water flow 

path (fig. 4), it is likely that there were widespread, low-level, 

inputs of PCE to the aquifer from multiple sources in the 

urban area. In addition, different urban unconfined wells had 

different VOCs (table 14), implying different sources. 

PCE, TCE, and cis- and trans-1,2 DCE were detected 

in the surface discharge and the depth-dependent samples 

collected from the study PSW (table 14). Other VOCs were 

detected in the confined urban mixed well FP3-162, about 

1 km upgradient of the study PSW. As VOCs are derived 

from anthropogenic activity at the land surface, primarily 

if not entirely in the urban area, the presence of VOCs in 

confined mixed urban wells indicates that some water from 

the unconfined aquifer in the urban area reached these wells. 

Water from these confined mixed urban wells and the study 

PSW, particularly depth-dependent samples, containing VOCs 

also had stable isotopic compositions indicating that these 

wells contained a mixture of water from the unconfined and 

the confined aquifers. 

The iron-reducing conditions present in the study 

PSW may permit anaerobic degradation of PCE and 

TCE. Constituents cis-1,2 DCE and trans-1,2 DCE are 

degradation products from reductive dechlorination of PCE 

and TCE, a reaction facilitated by microorganisms under 

reducing conditions (Vogel and McCarty, 1985; Bradley, 

2000). In addition, TCE can be formed from degradation 

of PCE. Concentrations of cis-1,2 DCE were greater than 

concentrations of trans-1,2 DCE, consistent with expected 

abundances of these degradation products from reductive 

dechlorination (Vogel and McCarty, 1985). Complicating 

interpretations of the importance of reductive dechlorination 

is the fact that cis-1,2 DCE and sometimes trans-1,2 DCE 

also were detected in unconfined shallow urban wells FP1-63, 

OFPN-88, and UWT1-53 (table 14). The presence of cis- and 

trans-1,2 DCE in unconfined shallow urban wells could be the 

result of reductive dechlorination of PCE and TCE in reducing 

parts of the unconfined aquifer upgradient of the monitoring 

wells where these compounds are detected. 

Pesticides

Selected pesticides were analyzed in four well nests 

(FP1, OFPS, FP3, and FP4; fig. 3) located along the main well 

transect and the study PSW in October 2003 and in depthdependent samples from the study PSW in June 2004 (fig. 13; 

table 15 [see back of report]). Pesticides or degradates were 

detected in 7 of 16 wells. Pesticides were detected in 3 of 4 

unconfined shallow urban and agricultural wells sampled. 

Pesticides were detected in all 3 confined urban mixed wells 

(FP3-162, FP3-218, OFPS-157) sampled for pesticides. A 

pesticide degradation product, acetochlor ESA, was detected 

at a concentration of 0.02 μg/L in a depth-dependent sample 

collected from the top of the screen in the study PSW; the 

study PSW also shows evidence of mixing with unconfined 

waters based on stable isotopes, VOCs, and major elements. 

Pesticides were not detected in 6 confined unmixed wells.
38 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Urban shallow

Unconfined wells

EXPLANATION

Confined wells

Urban deep

Urban unmixed

Urban mixed

Study public supply well

 (Study PSW)

Agricultural shallow

Agricultural deep

Agricultural mixed

Agricultural unmixed

-10

0

10

20

30

40

50

60

70

0.10 1 10 100

TOTAL VOLATILE ORGANIC

COMPOUNDS (VOC),

IN MICROGRAMS PER LITER

0.1 1 10 100

TETRACHLOROETHYLENE (PCE), 

IN MICROGRAMS PER LITER

No

Detection

No

Detection

No

Detection

No

Detection

0.1 1 10

TRICHLOROETHYLENE (TCE),

IN MICROGRAMS PER LITER

-10

0

10

20

30

40

50

60

70

DEPTH OF TOP OF SCREEN BELOW

WATER TABLE, IN METERS

DEPTH OF TOP OF SCREEN BELOW

WATER TABLE, IN METERS

0.1 1 10

cis1,2-DICHLOROETHYLENE,

IN MICROGRAMS PER LITER

A B

C D

Figure 12. Relation of depth of top of screen below water table to concentrations 

of selected volatile organic chemicals in the local-scale TANC study area near York, 

Nebraska.
Chemical Characteristics 39

400

390

380

410

420

430

440

450

460

470

480

490

500

METERS

0.16

ND

ND

ND

ND

ND

ND

ND

ND

ATR

Sum of pesticide concentrations-

 In micrograms per liter

No detection

Alachlor ethane sulfonic acid

Alachlor oxanilic acid

Atrazine

De-ethylatrazine

Metolachlor ethane sulfonic acid 

Metolachlor oxanilic acid

DEA

0.12

0.12

ATR

ATR

DEA

ALA ESA

0.09

ALA ESA

ALA OA

ALA OA

ALA ESA

MET ESA

0.39

ATR

DEA

DEA

0.09

0.02

ALA ESA

ALA ESA

MET OA

ACE ESA Acetochlor ethane sulfonic acid

MET ESA

MET ESA

0.01

ATR

DEA

ACE

ESA

ALA ESA

ALA OA

MET ESA

MET OA

Prometon

Monitoring well screen

Depth-dependent sample

EXPLANATION

ND

ND

ND

FP3 Well cluster name

FP3 FP4 OFPS FP1

Study public

 supply well

surface discharge

ND

0.5 MILE

0 1 KILOMETER

0

Vertical exaggeration greater than X40

NORTHWEST SOUTHEAST

Clayey silt (Lower confining unit)

Loess (Unsaturated)

Coarse-medium sand and

gravel (Unconfined aquifer)

Clayey silt (Upper confining unit)

Shale

Fine sand (Upper confined aquifer)

Fine sand (Lower confined lenses)

Hydrogeologc unit

VERTICAL DATUM IS NAVD 88

Figure 13. Cross section showing sums of concentrations of pesticides in ground water in the local-scale TANC study area near York, 

Nebraska.
Atrazine was detected in 4 wells at concentrations 

of 0.006 to 0.062 μg/L (fig. 13; table 15). The atrazine 

degradation product 2-chloro-4-isopropylamino-6-aminos-triazine (de-ethylatrazine) was detected in 3 unconfined 

shallow urban and agricultural wells at concentrations of 

0.005 to 0.042; de-ethylatrazine-to-atrazine ratios ranged 

from 0.7 to 0.8. The alachlor degradation product alachlor 

ethane sulfonic acid (ESA) was detected in 5 wells at 

concentrations of 0.02 to 0.11 μg/L. The alachlor degradation 

product alachlor oxanilic acid (OA) was detected in 2 wells 

at concentrations of 0.02 to 0.09 μg/L. The metolachlor 

degradation product metolachlor ESA was detected in 3 

wells in concentrations of 0.03 to 0.04 μg/L. There were no 

detections of the parent herbicides alachlor, acetochlor, or 

metolachlor. 

The presence of pesticides can be used to constrain 

the age of the young fraction of ground-water mixtures 

(Plummer and others, 1993) and to indicate contributions from 

agricultural recharge areas of pesticides used exclusively on 

agricultural crops. The estimated age of the young fraction 

was constrained by the presence of pesticides in confined 

mixed wells showing evidence of mixing with water from 

the unconfined aquifer. Acetochlor has been registered for 

use on corn since 1994 (U.S. Environmental Protection 

Agency, 2006). The detection of acetochlor ESA in the 

depth-dependent sample at the top of the screen of the study 

PSW indicated that a fraction of the sample was recharged 

after 1994 from agricultural land on which corn was grown. 

Metolachlor was first registered in 1976 and is used on 

turfgrass and crops such as corn, sorghum, and soybeans (U.S. 

Environmental Protection Agency, 1995). The presence of 

metolachlor ESA in confined urban mixed well OFPS-157 

(fig. 13; table 15) implies a fraction of the water was 

recharged after 1976. Alachlor was first registered in 1969, 

and is used for weed control in corn, soybean, and sorghum 

fields (U.S. Environmental Protection Agency, 1998). The 

detection of alachlor ESA in 3 confined urban mixed wells 

(FP3-162, FP3-218, and OFPS-157) implies that a fraction 

of the water in these wells was recharged after 1969 from 

agricultural land. Atrazine was first registered for use in the 

United States in 1958, and is used for weed control in corn 

and sorghum fields and for non-agricultural weed control 

(U.S. Environmental Protection Agency, 2003). The presence 

of atrazine or de-ethylatrazine in confined urban mixed well 

FP3-162 indicated that some fraction of the water sample was 

recharged after 1958. Although alachlor and acetochlor are 

used exclusively on agricultural crops, it is possible that spills 

during storage or distribution could have resulted in these 

chemicals entering ground water in non-agricultural areas. 

Ground-Water Age Tracers 

Ground-water age tracers analyzed in ground-water 

samples collected for this study included CFC-12, CFC-11, 

CFC-113, SF6, 3H/3He, and 3H. Interpretations of ground-water 

age tracers were complicated by mixing of waters of different 

ages, possible natural (3He, SF6) and anthropogenic (CFCs) 

contamination of ground water, and degradation of CFCs 

under reducing conditions in a few samples. The effect of 

these processes was to add uncertainty to age interpretations. 

Two age-tracer methods, in addition to 3H, were used 

in each well sampled for ground-water age to facilitate 

comparison of results between tracers (see “Collection and 

Analysis of Water Samples”). Because of the relatively 

high cost of the age tracer methods, only two of the three 

available methods were used in each well. Results of 

reconnaissance sampling of five wells in September 2003 

for CFCs suggested that contamination of the ground-water 

system with CFCs from local sources was minimal. CFCs 

are easy to collect and provide three independent age tracers 

(CFC-12, CFC-11, and CFC-113). Thus, all ground-water 

age samples were analyzed for CFCs. Samples from five 

monitoring wells in the unconfined aquifer were collected and 

analyzed for concentrations of CFCs and 3

H/3He. Samples 

from seven monitoring wells and the study PSW in the upper 

confined aquifer and lower confined lenses were analyzed 

for concentrations of CFCs and SF6. Samples from different 

depths in the screen and the surface discharge of the study 

PSW collected in June 2004 were analyzed for 3H/3He and 

SF6. Depth-dependent samples could not be collected for 

CFCs since the submersible bladder pump used to collect 

these samples had Teflon tubing, which can contaminate CFC 

samples for age analysis (table 16 [see back of report]).

Because the same tracers were not analyzed in all wells 

and because of the effects of contamination, mixing, and 

degradation on tracer concentrations in waters collected from 

different wells, no single tracer could be used to interpret 

apparent age in all wells. Consequently, a diverse set of tracers 

and lumped parameter models was used in age interpretations 

(tables 16 and 17 [see back of report]). Multiple interpretations 

are possible using different models; the primary tracers and 

models considered are shown in table 17, although other age 

tracer data also were interpreted. 

Tracer and 3H concentrations were compared with results 

expected from different lumped parameter models including 

(1) a piston flow model (PFM), (2) an exponential mixing 

model (EMM), (3) binary mixing of “old” water containing 

no modern tracers with “young” water containing tracers 

whose distribution is defined by a PFM (binary mixing model, 

BMM), and (4) binary mixing of “old” and “young” water 

whose distribution is defined by an EMM (exponential binary 

mixing model, EBMM) (Cook and Böhlke, 2000). “Old” 

water was considered to be more than 40 to 60 years old, 

before the period when modern tracers CFCs, SF6

, and 3H 

were in the atmosphere and precipitation. Excel workbooks 

for calculation and presentation of environmental tracer data 

for simple groundwater mixtures (Böhlke, 2006) were used to 

analyze the data. Example plots are shown in figure 14, and 

tracer data are given in table 16. Measured concentrations 

of particular tracers also were compared with those of other 

tracers to determine if the ratios, sometimes characteristic 

40 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska
Chemical Characteristics 41

A B

C D

E

Piston flow model

EXPLANATION

Exponential mixing model

Binary mixing lines between

 exponentially mixed and

 old water

Sample

Study public supply well

0

10

0 100 200 300 400 500 600

0 5040302010 90807060 100 0 1 2 3 4 5 6

20

30

40

0

10

20

30

40

0 0.50.40.30.20.1 0.90.80.70.6 1

0

10

20

30

40

CHLOROFLUOROCARBON (CFC)-12,

PARTS PER TRILLION BY VOLUME

CHLOROFLUOROCARBON (CFC)-113,

PARTS PER TRILLION BY VOLUME

SULFUR HEXAFLUORIDE,

PARTS PER TRILLION BY VOLUME

CHLOROFLUOROCARBON (CFC)-11,

PARTS PER TRILLION BY VOLUME

0 50 100 150 200 250 300

TRITIUM, IN TRITIUM UNITS TRITIUM, IN TRITIUM UNITS TRITIUM, IN TRITIUM UNITS

OFPS-38

OFPS38

OFPS157

FP4-83 FP4-83

FP3-83

FP3-33

FP3-162

FP3-33 OFPS-38

FP1-63

FP3-

33

FP3-

162

FP4-168

FP1-63 FP3-33

SPSW

FP4-28

FP4-168

OFPS-38

FP3-162

FP1-147 FP3-130

FP1-185

OFPS-157

TRITIUM TO INITIAL TRITIUM RATIO

SPSW

depth-dependent

samples

SPSW

SPSW

FP1-147, FP1-185, 

FP4-168, FP3-130

Figure 14. Concentrations of age-dating tracers in samples and historical atmospheric curves corrected for radioactive decay for 

piston, exponential, and exponential binary mixing models, local-scale TANC study area near York, Nebraska.
42 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

of a particular time period, constrained the age of the young 

fraction of the water (Plummer and others, 1998). In nearly 

all cases, the use of tracer ratios did not better constrain the 

interpreted age of the young fraction, perhaps because some of 

the tracers in a given sample were affected by contamination 

and (or) degradation. This interpretive process was iterative, 

and refinements were made on the basis of multiple tracers 

and samples. Historical concentrations of tracers were 

obtained from the USGS Chlorofluorocarbon Laboratory. 

Monthly concentrations of tritium in precipitation in the study 

area since 1953 were estimated using a program that calculates 

tritium concentrations for a specified latitude-longitude 

by extrapolating results from stations where tritium in 

precipitation had been measured (Robert L. Michel, Research 

Hydrologist, U.S. Geological Survey, written commun., 

August 31, 2004). Tritium concentration data were then 

corrected for radioactive decay to the time of sampling in 2003 

and 2004.

For wells in the unconfined aquifer (FP1-63, FP3-33, 

FP3-83, FP4-28, FP4-83) sampled for age tracers, 

concentrations generally usually plotted on or near PFM 

curves (fig. 14) and PFM apparent ages using different tracers 

generally corresponded reasonably well (table 17), indicating 

that ground-water ages can be reasonably interpreted using a 

PFM. Apparent ages increased with depth below water table, 

from 7 to 10 years at depths <2.5 m, 15 to 20 years at depths 

of 5 to 7 m, and 44 to 48 years at depths of 17 to 18 m, near 

the bottom of the unconfined aquifer (table 17). The PFM 

presumes that water entering the aquifer moves through 

the aquifer system without mixing and retains the original 

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

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

can be a reasonable approximation in some simple aquifer 

systems for samples from wells with small screened intervals. 

For unconfined shallow urban and agricultural wells, the 

PFM fit CFC-12 at FP3-33 (fig. 14A), CFC-113 at FP1-63 

(fig. 14C), SF6 at OFPS-38 (fig. 14D), and 3H/3He at FP3-33, 

FP1-63, and FP4-28 (fig. 14E) particularly well. 

Age-tracer data for samples from the confined aquifers 

were consistent with mixing of “old” water, not containing 

modern tracers, and “young” water; the “young” water fraction 

was interpreted to be exponentially mixed. In the confined 

aquifers, results for different tracers often diverged and did not 

plot along PFM or EMM curves (fig. 14; table 17). However, 

samples plotting below these curves are along a linear mixing 

line drawn through the samples from the origin of the graph 

(zero concentration, old water recharge before tracers were in 

the atmosphere) to where this mixing line intersects the PFM 

or EMM curves. The mixing fraction is calculated from the 

distance that a sample plots along the mixing line between 

the two end-members. Binary (BMM) and exponential-binary 

(EBMM) mixing models were used to interpret the fraction 

of the sample consisting of young water having a particular 

mean age. With a BMM, the sample is a mixture of two subparcels, one which is young and contains measurable tracer 

and one which is too old to have measurable tracer. With an 

EMM, the sample consists of a number of sub-parcels, each 

having its own tracer concentration and age, which have an 

exponential age distribution and an overall mean age. The 

mean age will affect the weight that new recharge water 

mixing with existing ground water has on the composition of 

the mixture as a whole. The EMM represents a highly mixed 

sample consisting of waters having many different ages along 

a distribution. The EBMM model was a binary mixture of 

(1) a subparcel of water mixed according to the EMM and 

having a particular mean age, and (2) a subparcel too old to 

have measurable tracer. The primary mechanism for water 

from the unconfined aquifer to get into the confined aquifers 

was interpreted to be leakage down well bores connecting the 

aquifers. Because of the large hydraulic head gradient between 

the unconfined and confined aquifers, the leaky well bores 

were expected to result in mixing, within the well bores, of 

waters from the entire unconfined aquifer, which subsequently 

mixes with old water in the confined aquifer, an interpretation 

supported by simulation results (Clark and others, 2008). Of 

the lumped parameter models, the EMM most closely fit the 

expected highly mixed age distribution of well bore leakage 

waters from the unconfined aquifer. When these waters 

from the unconfined aquifer mix with water in the confined 

aquifers, the EBMM was the most appropriate lumped 

parameter model for interpreting the results. The BMM also 

yielded reasonable results for certain mixing scenarios. The 

mean ages and mixing fractions for these two models were 

fairly similar and both are presented in table 17. Implicit in the 

mixing calculations is that the remainder of the sample that 

is not young water consists of water that is more than 40 to 

60 years old and contains no detectable quantities of modern 

atmospheric tracers.

Tracer results and mixing calculations indicated that 

confined unmixed wells (FP1-147, FP1-185, FP3-130, and 

FP4-168) contained <3% young water mixed with a much 

larger fraction (≥97%) of old water with no modern tracers 

(table 17). Fractions of water from the unconfined aquifer in 

these same confined unmixed wells calculated using δD data 

ranged from 5 to 11% (table 12). Mixing fractions calculated 

using stable isotopic and age tracers could differ because of 

the sensitivity of the calculations to small changes in measured 

values and assumed end-members, simplifying assumptions 

made in the mixing calculations, and differences in the timing 

of historical changes in stable isotopic values compared 

to concentrations of atmospheric tracers. Considering 

uncertainties, the δD and CFC-12 mixing calculations 

compare reasonably well and both indicate only small 

quantities of young water (<3 to 11%) from the unconfined 

aquifer in confined unmixed wells. 
Chemical Characteristics 43

Mixing calculations for confined mixed wells FP3-162 

and OFPS-157 indicated substantial young fractions of 70 

and 32%, respectively, and mean ages of 12 and 14 years. 

Corresponding mixing fractions calculated from δD values 

were 89 and 70%, respectively (table 12). Although there 

were substantial differences in mixing fractions indicated by 

SF6 and δD at OFPS-157, both sets of mixing calculations 

indicated substantially larger fractions of young water in these 

monitoring wells than in confined unmixed wells.

The lumped parameter models and binary mixing 

calculations are inherently simplistic, considering the physical 

situation in which young water from the unconfined aquifer 

moves into the confined aquifers through leakage down well 

bores, which results in mixing of waters of different ages from 

the unconfined aquifer with tracer-free water in the confined 

aquifers. Because recharge and discharge have increased 

over several decades (Landon and Turco, 2007; Clark and 

others, 2008), the age distribution of waters in the unconfined 

aquifer was expected to become younger through time. This 

interpretation is supported by transient model simulations of 

ground-water age in the local-scale TANC study area (Clark 

and others, 2008), which indicate decreases in ground-water 

age from the 1940s into the 1980s. Moreover, water from 

the unconfined aquifer leaking down well bores could have a 

considerable range of ages, since this water would have been 

leaking continuously over decades. Given this situation, binary 

mixing calculations that assume a single mean age of a young 

fraction greatly simplify a complex problem. It is not possible 

to quantify more-complex mixing of more than two bodies of 

water having different ages, using lumped parameter models. 

Mixing calculations of greater complexity are implemented 

with the numerical flow and solute transport model described 

by Clark and others (2008).

Interpretation of ages was complicated by contamination 

and degradation of different tracers to different degrees. 

The CFC laboratory reported sample contamination when 

concentrations of CFCs and (or) SF6

 were greater than could 

be derived from atmospheric input, indicating that there were 

local sources of a tracer that precluded its use as an age tracer 

(hereinafter, referred to as “contaminated for age dating 

purposes”). Potential degradation of CFCs was identified 

by (1) comparison of a single-tracer result with those for 

other tracers that are less susceptible to degradation or do 

not degrade and (2) literature review of the redox conditions 

under which different tracers degrade. CFC-12 is the most 

stable (least readily degraded) of the CFCs and is preferred 

for age interpretations if available (Busenberg and Plummer, 

1992). However, CFC-12 data could be interpreted to indicate 

age in only 7 of 13 samples; the other 6 samples had CFC-12 

concentrations that indicated contamination for age- dating 

purposes. CFC-12 appeared to have degraded at well OFPS-38 

on the basis of (1) the observation that the apparent PFM 

age for CFC-12 (1971) was older than the apparent age 

for SF6 (1984) and (2) CFC-12 has sometimes been found 

to degrade under sulfate-reducing to methanogenic redox 

conditions (Oster and others, 1996; Plummer and Busenberg, 

2000), such as those at OFPS-38. On the basis of these 

observations, the interpretation of ground-water age was based 

on SF6 at OFPS-38, as SF6 is not susceptible to degradation 

under reducing conditions (Busenberg and Plummer, 2000). 

Likewise, CFC-113 could only be used for age interpretations 

in 7 of 13 samples, because it was present in concentrations 

that indicated CFC-113 contamination for age-dating 

purposes in one well, was degraded in two wells (described 

in paragraph below), and was not detected in three additional 

wells (table 16). The most widespread available tracer was 

CFC-11; it was used to interpret age in 8 of 13 wells. CFC-11 

concentrations indicated contamination for age-dating 

purposes in three wells and degradation in two wells. 

For two wells, OFPS-38 and FP3-33, concentrations 

of CFC-11 and CFC-113 were substantially lower than 

expected in comparison with other tracers, probably indicating 

degradation of CFC-11 and -113 in these reducing waters. 

Concentrations of different tracers were compared with each 

other (for example, CFC-12 against CFC-11, not shown) 

and with curves from lumped parameter models on plots of 

tracers versus 3

H (fig. 14). On both kinds of plots, where 

concentrations of a particular tracer were lower than expected 

compared with the other tracer or lumped parameter curves, 

those concentrations were flagged as possibly being affected 

by processes that would affect one tracer but not another, 

such as degradation or contamination from non-atmospheric 

sources. CFC contamination from non-atmospheric sources 

usually causes concentrations to be in excess of those 

that could be derived from the atmosphere and is clearly 

identifiable from the reported concentrations (Busenberg 

and Plummer, 2000). The redox conditions for wells where 

concentrations of different CFCs were inconsistent were 

assessed to evaluate whether selective degradation of CFCs 

was consistent with the results. Concentrations of SF6

 were 

slightly higher, or CFC-12 concentrations were slightly 

lower, for OPFS-38 than was expected according to lumped 

parameter models. Because it is reasonable that CFC-12 

degraded in this sample because of methanogenic conditions, 

SF6

 was considered to provide a better estimate of the 

minimum age of the young fraction at OFPS-38. CFC-13 

(not detected) and CFC-11 were similarly interpreted to be 

degraded at OFPS-38. Conditions at FP3-33 were nitrate- to 

manganese-reducing (but methane also was detected, so 

some methanogenic conditions also could occur). CFC-11 

and CFC-113 concentrations were less than expected on 

the basis of CFC-12 and 3H/3He values, implying that these 

tracers degraded. On the basis of the reasonable agreement of 

apparent ages indicated by CFC-12 and 3

H/3He, there was no 

evidence of CFC-12 degradation in water from FP3-33.
44 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

CFC-11 concentrations were compared with other 

tracers in other wells. The results, particularly for wells in the 

confined aquifer having nitrate- to iron-reducing conditions, 

indicated that CFC-11 concentrations were similar to what 

would be expected from lumped parameter models if the 

CFC-11 wasn’t degraded. CFC degradation is usually not 

detected under conditions less reducing than sulfate-reducing

conditions (Plummer and Busenberg, 2000). If CFC-11 was 

degrading in the confined aquifer, CFC-12 dates would have 

been consistently younger than CFC-11 dates. However, 

CFC-11-based age results were generally similar to those 

for CFC-12. The very low tritium concentrations in most 

wells in the confined aquifers was consistent with the very 

low CFC-12 and CFC-11 concentrations, implying that low 

CFC concentrations in the confined aquifers indicated old 

water with very small fractions of young water rather than 

degradation of CFCs. 

Concentrations of SF6

 were greater than expected in 

comparison with apparent ages and mixing fractions indicated 

by CFCs in four confined unmixed wells (FP1-147, FP1-185, 

FP3-130, and FP4-168. Granitic rocks can contain SF6 in 

amounts significant enough to preclude using SF6 as a dating 

tool (Busenberg and Plummer, 2000). As sediments in the 

local-scale TANC study area were derived from granitic rocks 

of the Rocky Mountains, it is possible that SF6 concentrations 

could be biased high by geologic sources of SF6 in the 

confined aquifers. Whatever the source, ground-water ages 

based on SF6 are more uncertain because they may be affected 

by non-atmospheric sources. 

Attempts to determine age using 3H/3He data were 

successful for four unconfined shallow urban and agricultural

wells but were unsuccessful for one unconfined deep 

agricultural well (FP4-83) and for depth-dependent and 

surface-discharge samples collected from the study PSW

because of the effects of elevated terrigenic helium (tables 16

and 17). Terrigenic He may be in aquifers where the rocks are 

enriched in uranium or thorium, or in ground-water samples in 

which young water mixed with relatively old water containing 

terrigenic, and in some cases, mantle He (Schlosser and others, 

1989). This helium derived from geologic sources complicates 

interpretation of the helium isotope ratios. When combined 

with air contamination of some of these samples, 3

H/3He could 

not be used to interpret ground-water age in these samples.

Age determinations in the study PSW were problematic. 

Calculations using SF6 EBMM indicated that the study PSW 

was a 7% mixture of water having a mean age of 1995, 

and older, tracer-free water (table 17). Calculations using 

CFC-11 EBMM indicated a 14% mixture of water having a 

mean age of 1990, and older, tracer-free water. These mixing 

fractions were approximately consistent with mixing fractions 

of unconfined layer water on the basis of δD of about 12%

(table 12). Attempts to determine the age distribution of depthdependent samples from the study PSW collected in June 

2004 were unsuccessful. Depth-dependent 3He samples were 

compromised by high terrigenic helium and air contamination. 

Values of SF6 in three depth-dependent samples from the 

upper half of the screen (42.7-, 45.7-, and 48.8-m bls samples) 

were nearly uniform along the vertical profile (table 16).

Samples for carbon-14 were collected from three 

monitoring wells screened near the bottom of the aquifer and 

two depth-dependent samples in the lower part of the screen of 

the study PSW (table 16). The results were not interpretable in 

three of the five samples (well FP3-218 and 2 depth-dependent 

samples from the study PSW) because substantial amounts of 

modern carbon and tritium indicated that the water contained 

too much young water to be dated using the carbon-14 

technique. In wells FP1-247 and FP4-231, the carbon-14 

method indicated uncorrected ages of 6,568 and 7,456 years 

before present, respectively. However, the uncorrected ages 

are too old—they do not consider the effects of carbonate 

dissolution on the initial carbon-14 activity of the groundwater at the time of recharge (Kalin, 2000). Simple corrections 

applied to account for these factors using the method of 

Tamers (1967) resulted in corrected ages of 1,368 and 

1,767 years before present for wells FP1-247 and FP4-231, 

respectively. Although these corrected ages are considered 

to be more reasonable than uncorrected ages, uncertainties 

in the estimated initial carbon-14 activity of recharge water 

still cause uncertainties of hundreds to thousands of years in 

estimating the age of in these samples. The modern recharge 

composition is completely different from that which existed 

several thousand years ago because of changes in land use and 

water fluxes. Therefore, paleorecharge carbon-14 activities 

could not be measured directly and the carbon-14 dates should 

be regarded as poorly constrained estimates of the ages of old 

water near the bottom of the aquifer. 

Nutrients

Concentrations of nitrite were negligible. Therefore, 

concentrations of nitrite plus nitrate were presumed to 

indicate concentrations of nitrate, measured as N (hereinafter, 

nitrate-N). Concentrations of nitrate-N were highest in 

unconfined shallow urban and agricultural wells, having 

median values of 17.3 and 16.0 mg/L, respectively (fig. 8B). 

Concentrations of nitrate-N were <0.2 mg/L in confined 

unmixed wells, were 2.2 to 7.4 mg/L in unconfined deep urban

and agricultural wells, and ranged from less than the reporting 

level of 0.06 to 13.1 mg/L in confined mixed wells(fig. 8B; 

table 18 [see back of report]), indicating a general decrease 

in nitrate-N concentrations with increasing depth except in 

confined mixed wells. The decrease in nitrate-N concentrations 

with depth corresponds with increasingly reducing conditions 

(fig. 8). The lowest nitrate-N concentrations in unconfined 

shallow urban wells, <5 mg/L, were in wells OFPS-38 

and FP3-33, which had nitrate-reducing to methanogenic 

conditions. Redox conditions more reducing than oxic

conditions are hereinafter referred to as reducing conditions.
Chemical Characteristics 45

Values of δ15N-nitrate and δ18O-nitrate plot along a 

trend consistent with denitrification of nitrate in reducing

ground water in the unconfined aquifer and confined aquifers 

(fig. 15A). Denitrification of nitrate to dissolved nitrogen 

gas and intermediate products in reducing ground water has 

been identified in numerous studies (review by Kendall, 

1998). As denitrification occurs, the δ15N and δ18O values 

of the residual nitrate increase. Across a range of field and 

laboratory studies, denitrification has been shown to enrich 

δ15N and δ18O of nitrate; resulting values plot along a line 

having a slope between about 0.5 and 1.0 on a plot of δ15N 

versus δ18O (Böttcher and others, 1990; Mengis and others, 

1999). A linear best-fit line of the data shown in figure 15A

has a slope of 0.50 (r2 = 0.94), consistent with that expected 

for denitrification. The extent of denitrification was greater for 

confined urban unmixed wells and study PSW samples, having 

δ15N-nitrate values >30‰ and δ18O-nitrate values >15‰, than 

for unconfined shallow urban wells and confined urban mixed

wells having reducing conditions, which had intermediate 

values indicating partial denitrification (fig. 15A). Nitrate-N 

concentrations were ≥10 mg/L only when δ15N-nitrate values 

were less than about 13‰ (fig. 15B). Nitrate-N concentrations 

were smaller in samples when δ15N-nitrate values were larger, 

consistent with expected decreases in nitrate-N concentrations 

with denitrification. 

Excess nitrogen gas (N2) (table 16) in reducing water was 

consistent with the occurrence of denitrification. The excess 

N2 produced by denitrification was estimated by comparing 

measured concentrations of argon and N2 with those expected 

in water in equilibrium with the atmosphere (Vogel and others, 

1981; Böhlke, 2002). Uncertainties in calculating the amount 

of excess N2 on the basis of argon and N2 concentrations 

include the recharge temperature and the amount of excess 

air. Therefore, the amount of excess N2 from denitrification 

cannot be uniquely determined from measuring Ar and N2

alone. Using a spreadsheet provided by the USGS Dissolved 

Gas laboratory, amounts of excess N2 were estimated by 

adjusting the values until the calculated recharge temperatures 

were close to the long-term mean annual air temperature 

at York, about 11°C (High Plains Climate Center, 2003). 

These analyses indicated that 1 to 7.5 mg/L of excess N2

 was 

in reducing ground water, where denitrification would be 

suspected (table 16). 

Additional calculations of the degree of reaction progress 

and initial nitrate-N concentrations and isotopic values, were 

determined from mass-balance calculations based on δ15N 

values of excess N2 and residual nitrate-N, and concentrations 

of nitrate-N and N2 using methods described by Böhlke 

(2002). Detailed description of these analyses is beyond 

the scope of this report, and is part of a synthesis effort to 

understand processes influencing nitrate-N concentrations in 

all TANC study areas (McMahon and others, 2008). Initial 

δ15N-nitrate values and concentrations of nitrate-N (at the 

time of ground-water recharge and before denitrification), 

corrected for denitrification effects on measured values, are 

reported here for the purpose of characterizing nitrate sources 

and comparing those results to other constituents to determine 

solute sources. 

Initial δ15N-nitrate values for unconfined shallow urban

and agricultural wells from the analyses described above 

suggested that there are multiple sources of nitrate, categorized 

as follows (fig. 15C; table 19 [see back of report]):

(1) 4.8 to 8.1‰ in unconfined shallow agricultural

wells other than NWT1-39 and FP4-28;

(2) 11 to 19‰ in unconfined shallow urban wells other 

than UWT3-34;

(3) 10.5‰ for well NWT1-39; and 

(4) 8.6 and 9.3‰ for wells FP4-28 and UWT3-34, 

respectively. 

The initial δ15N-nitrate values of most unconfined shallow 

agricultural wells (group 1 above) were consistent with 

fertilizer being an important source of ground-water nitrate-N. 

Nitrogen fertilizer has been the primary nitrogen source 

applied to crops for the last several decades; manure was not 

widely used as fertilizer in the study area (Rod DeBuhr, Water 

Department Manager, Upper Big Blue Natural Resources 

District, oral commun., March 15, 2003). Nitrogen fertilizer 

applied to crops in York County was estimated from countylevel data on historical fertilizer application rates (Alexander 

and Smith, 1990; David Lorenz, Hydrologist, USGS, written 

commun., 1999) and averaged about 100 kg nitrogen/ha for 

1994–2003; the amount of manure applied averaged about 

20 kg nitrogen/ha, according to estimates derived from data 

compiled from Census of Agriculture data for the same period 

(Sharon Qi, U.S. Geological Survey, Hydrologist, written 

commun., March 22, 2006). Values for δ15N-nitrate in ground 

water beneath fertilized fields are commonly 2 to 6‰ and 

are often slightly larger than fertilizer values and soil-water 

values because of nitrogen mixing, volatilization of ammonia 

fertilizer, or denitrification of nitrate in the unsaturated zone 

(Fogg and others, 1998; Kendall, 1998; Böhlke, 2003). Values 

of δ15N-nitrate for 4 soil-water samples from beneath irrigated 

fertilized fields (FP4 and AWT2 sites) were 2.6 to 9.8‰, 

similar to values for unconfined shallow agricultural wells. 

Values for δ15N-nitrate in most unconfined shallow urban

wells were higher than for unconfined shallow agricultural

wells and consistent with being derived from a animal waste 

source. Values for δ15N-nitrate in ground water affected by 

manure are typically 10 to 25 ‰ (Fogg and others, 1998; 

Kendall, 1998; Böhlke, 2003), almost bracketing the range of 

unconfined shallow urban wells. Relatively high and variable 

δ15N-nitrate values derived from waste sources can result from 

isotopic fractionation during variable amounts of volatilization 

of ammonia from waste and other interactions with soils 

(Kreitler, 1975). At well FP3-33, a high initial δ15N-nitrate 

of 19‰ and a relatively low initial nitrate-N concentration of 

about 4.6 mg/L are consistent with this sample being more 

strongly affected by ammonia volatilization than samples from 

other unconfined shallow urban wells. Generally, different 

animal waste sources cannot be distinguished using δ15N or 

δ18O of nitrate (Kendall, 1998). Within the local-scale TANC 

study area, possible animal waste sources that could contribute 
46 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Figure 15. Relations in ground water in the local-scale TANC study area near York, Nebraska: A. measured nitrogen and oxygen isotopic values of nitrate, 

B. measured nitrogen isotopic values of nitrate and concentrations of nitrate, C. calculated initial nitrogen isotopic composition of nitrate and concentrations of nitrate prior to denitrification, and 

D. calculated initial nitrogen isotopic composition of nitrate prior to denitrification and concentration of orthophosphate.

0

0.5

1

1.5

2

50 10 20 30 40 0

EXPLANATION

Unconfined wellsUrban shallowUrban deepAgricultural shallowAgricultural deepConfined wellsUrban unmixedUrban mixedStudy Public supply well (Study PSW)Agricultural unmixedAgricultural mixed

B

D

DELTA NITRATE-15, OF NITRATE, IN PER MIL

DELTA OXYGEN-18, OF NITRATE, IN PER MIL

DELTA NITRATE-15, OF NITRATE, IN PER MIL

CALCULATED INITIAL DELTA NITRATE-15,

OF NITRATE, IN PER MIL

CALCULATED INITIAL DELTA NITRATE-15,

OF NITRATE, IN PER MIL

-5

0 10 20 30 40

50

201510

AC

0

8070605040302010

NITRATE AS NITROGEN,

IN MILLIGRAMS PER LITER

8070605040302010

0

INITIAL NITRATE AS NITROGEN, CALCULATED 

IN MILLIGRAMS PER LITER

ORTHOPHOSPHATE AS PHOSPHORUS,

IN MILLIGRAMS PER LITER

0 5 10 15 20 25 0 5 10 15 20 25

NWT1-39

FP4-28

UWT3-34

FP3-33

FP4-28

NWT1-39

UWT3-34

Urban

shallow

unconfined

(reduced

waters)

Denitrification

Urban

shallow

unconfined

(oxic

waters) Urban confined

mixed

Linear regression

line (Slope = 0.5)

Urban confined

and supply well

Agricultural shallow and deep

unconfined and confined 
Chemical Characteristics 47

nitrate-N to shallow ground water are septic systems, leakage 

from the municipal wastewater sewer system, and livestock 

waste. Based upon the distribution of sources upgradient 

of the unconfined shallow urban wells and other chemistry 

(see Major Elements), septic systems or wastewater leakage 

sources were thought to be the primary sources of nitrate-N 

within the urban area. 

Well NWT1-39 had the highest concentrations of 

nitrate-N, 69.5 to 80.3 mg/L, measured in this study (fig. 8B; 

table 18). Well NWT1-39 is located at the downgradient end 

of a pasture (figs. 3 and 4) that was used for livestock manure 

disposal at times during the 1980s and 1990s. Leaching of the 

manure may have resulted in the high nitrate-N concentrations 

observed at this site. The δ15N-nitrate value of NWT1-39 

was 10.5‰ (table 19), within the range of δ15N-nitrate values 

expected for ground water affected by a manure source of 

nitrogen. 

Intermediate δ15N values of 8.6 to 9.3‰ at FP4-28 and 

UWT3-34 may indicate mixed sources of nitrate. These values 

were intermediate between lower ground-water values affected 

by fertilizer and higher values affected by animal waste. Well 

FP4-28 was near the western edge of York and would be 

primarily influenced by agricultural land use upgradient of the 

well. However, a garbage processing facility, a landfill, and 

residences with septic systems were identified upgradient of 

the well (fig. 4), so urban sources may have influenced this 

well. Well FP4-28 also was downgradient of several current 

or former livestock waste disposal areas and may have been 

affected by these sources (figs. 3 and 4), which would lead to 

a higher δ15N value. Well UWT3-34 was located about 625 m 

east-by-southeast of FP4-28, about 400 m east of the western 

edge of York (fig. 4) and could have been affected by some of 

the same upgradient mixed sources as FP4-28. 

 Concentrations of orthophosphate as phosphorus 

(hereinafter orthophosphate) were largest in unconfined 

shallow urban wells, and ranged from 0.29 to 2.1 mg/L 

(table 18); the median was 0.46 mg/L. Concentrations of 

orthophosphate in confined mixed wells were 0.021 to 

0.320 mg/L compared with values of 0.027 to 0.186 mg/L in 

confined unmixed wells (table 18). 

Elevated orthophosphate (operationally defined as 

≥0.4 mg/L) in unconfined shallow urban wells and FP4-28 

may be associated with ground water affected by septic 

systems or leakage from the urban sanitary sewer system 

in addition to being affected by reducing conditions in the 

unconfined aquifer. Seven of nine unconfined shallow urban

wells had elevated orthophosphate concentrations (fig. 15D). 

Of these seven wells, six had initial δ15N-nitrate values ≥10‰; 

the exception, UWT3-34, had initial δ15N-nitrate of 9.3‰

and may be affected by mixed nitrate sources. Likewise, 

the one agricultural well having elevated orthophosphate 

concentrations, FP4-28, had an intermediate δ15N-nitrate, 

8.6‰, and may be affected by mixed nitrate sources. Of 

the 7 unconfined shallow urban wells and FP4-28 having 

elevated orthophosphate concentrations and initial δ15Nnitrate ≥8.6‰, 5 of these wells had oxic conditions and 3 had 

reducing conditions. Under reducing (anoxic) conditions, 

phosphorus is released from sediments (House, 2003). The 

highest orthophosphate concentrations in unconfined shallow 

urban wells were in wells OFPS-38 and UWT2-23, which 

have reducing ground water (table 13). In unconfined shallow 

urban wells having reducing conditions, it is likely that 

orthophosphate concentrations ≥0.4 mg/L primarily reflect the 

reducing redox conditions. In unconfined shallow wells having 

oxic conditions, the association of elevated orthophosphate 

with initial δ15N-nitrate values ≥10‰ in most unconfined 

shallow urban wells or ≥8.6‰ in FP4-28 and UWT3-34, 

indicating mixed sources (fig. 15D), suggests that the 

orthophosphate was associated with ground water affected by 

a manure source. This association is suggestive but does not 

prove a common source for the relatively high initial 

δ15N-nitrate and elevated orthophosphate concentrations. 

Other possible sources of phosphorus to ground water 

are considered to be minor. Orthophosphate was relatively 

low, 0.3 to 0.4 mg/L, in unconfined shallow agricultural

wells, suggesting that phosphorus applied to agricultural 

crops did not result in elevated concentrations in ground 

water. A 1996 survey of fertilizer use in the Blue River Basin, 

which includes the local-scale TANC study area, showed that 

phosphate fertilizers were applied to about 75% of corn crops 

at an average rate of about 28 kg/ha (Franti and Dorn, 1998). 

Leaching of phosphorus from fertilizer is typically minimal, 

as phosphorus is readily utilized by vegetation and affected 

by sorption and precipitation reactions in the soil (Sinaj and 

others, 2002). For these reasons, urban phosphorus fertilizer 

application was not considered to be a likely source of the 

relatively high orthophosphate concentrations in unconfined 

shallow urban wells. 

Elevated concentrations of orthophosphate are commonly 

in wastewater and in ground water affected by septic systems 

in aquifer systems that are oxidizing and have calcareous 

sands that produce neutral pH (Robertson and others, 1998; 

Reay, 2004; McCray and others, 2005). In the local-scale 

TANC study area, near-neutral pHs, relatively high alkalinities 

and calcium concentrations, equilibrium of ground water 

with calcite (see “Major Elements”), and calcite in an 

unsaturated sand core samples suggest that calcite dissolution 

influences ground-water chemistry. Elevated orthophosphate 

concentrations in ground water affected by septic-system 

discharges would thus be expected. Therefore, it is likely that 

the elevated orthophosphate detected in unconfined shallow 

urban wells was associated with the same manure source 

that produced the δ15N-nitrate values ≥10‰. It cannot be 

determined whether the orthophosphate in unconfined shallow 

urban wells was derived from septic-systems or sanitarysewer system leakage, because the processes and geochemical 

signatures are similar. 
48 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

 Concentrations of dissolved organic carbon (DOC) 

were generally greatest in unconfined shallow urban and 

agricultural wells. The highest concentrations of DOC of 

6.8 to 7.6 mg/L were in well NWT1-39, interpreted as being 

affected by leaching of livestock manure (fig. 10). The 

relatively high DOC concentrations were consistent with 

leaching of organic material in the manure. Well FP1-63 

had DOC of 3.4 mg/L (fig. 10), relatively high nitrate-N 

(25.6–28.8 mg/L as N [table 18]), a δ15N-nitrate value of 

11.8‰ (table 19), consistent with a manure source, and the 

highest VOC concentrations of any well in the local-scale 

TANC study area (table 14). These characteristics suggest 

that well FP1-63 may have been affected by an anthropogenic 

point source contributing nutrients and VOCs to ground water. 

Relatively high DOC values are reasonable for a site affected 

by anthropogenic sources such as septic systems, wastewater 

spills, landfills, or other sources. 

The presence of relatively high nitrate-N, little or no 

ammonia, and relatively low DOC in water affected by 

septic systems suggests that oxidation of DOC and ammonia 

is mostly occurring in the unsaturated zone. Ground water 

beneath and downgradient of septic systems may be either oxic

or reducing (Robertson and others, 1998). Oxic conditions 

facilitate conversion of ammonia to nitrate and DOC to 

carbon dioxide (Robertson and others, 1991). The absence of 

ammonia, even in UWT2-23, which had reducing conditions, 

was located near areas having septic systems (fig. 4), and had 

indicators of septic-system effects (initial δ15N-nitrate, and 

orthophosphate) (see “Major Elements”), suggests that septicsystem effluents were oxidized primarily in the unsaturated 

rather than the saturated zone in the local-scale TANC study 

area. 

Major Elements

Major element concentrations in unconfined shallow 

urban and agricultural wells in the local-scale TANC 

study area were affected by anthropogenic sources, such 

as septic systems, sanitary sewers, and livestock waste, 

and geochemical reactions, such as dissolution and redox 

reactions. These processes are discussed in the sections below, 

beginning with description of water types, interpretations of 

anthropogenic solute sources effecting on shallow groundwater chemistry, sources of water chemistry signatures in the 

study PSW, and geochemical processes affecting selected 

major elements. 

The relative abundance of major cations and anions 

is shown on the trilinear or piper diagram (Piper, 1953) in 

figure 16. The diagram shows the relative percentages of total 

milliequivalents per liter attributed to individual or groups of 

cations and anions, and can be used to infer major water types. 

In most wells, calcium accounted for more than 60% of the 

total cations, and bicarbonate accounted for more than 60% of 

the total anions; these samples were calcium-bicarbonate type 

waters. There were also many wells that were mixed 

cation-bicarbonate type waters, indicating that no single 

cation accounted for more than 60% of the total cations. 

Calcium was still the most abundant cation in most of these 

mixed cation-bicarbonate waters. The relative proportions of 

cations varied less between different categories of wells than 

the proportions of anions (fig. 16). 

Most samples from confined unmixed wells were 

calcium-bicarbonate type waters (fig. 16). Cation proportions 

varied slightly more than anion proportions; proportions of 

sodium usually increased and calcium usually decreased as 

well depth increased. This pattern of increasing sodium and 

decreasing calcium with depth may indicate cation-exchange 

in the deeper parts of the confined aquifers, where dissolved 

calcium is adsorbed preferentially onto clays compared with 

sodium, which is displaced from clay surfaces by calcium and 

goes into solution (Appello and Postma, 1997). 

Samples from unconfined shallow agricultural wells 

were primarily mixed cation-bicarbonate type waters, except 

for well NWT1-39 (fig. 16). Nitrate-N was the predominant 

anion at NWT1-39; this site was at the downgradient edge 

of a pasture used in the past for livestock waste disposal and 

was considered to primarily have a livestock-waste-disposal 

signature. 

Proportions of major-ions varied most in samples from

unconfined shallow urban wells; these samples were primarily 

mixed cation-mixed anion water types (fig. 16). Unconfined 

shallow urban wells generally had greater proportions of 

sulfate and chloride plus nitrate-N than other types of wells 

(fluoride concentrations were minor and are disregarded in this 

discussion). Bicarbonate was the most abundant anion in most 

of these wells. Cation proportions were relatively uniform 

with calcium as the most abundant cation and roughly equal 

amounts of sodium plus potassium and magnesium. 

Confined mixed well FP3-162 and depth-dependent 

samples from the study PSW had anion proportions 

intermediate between some unconfined shallow urban and 

confined unmixed samples (fig. 16). These intermediate 

proportions are consistent with these samples being mixtures 

of water from the unconfined and confined aquifers.

Associations between concentrations of chloride, initial 

δ15N-nitrate, potassium, orthophosphate, sulfate, and boron 

suggest that the primary source of relatively high amounts 

of these constituents and isotopic signatures in unconfined 

shallow urban wells was septic system and (or) sanitary sewer 

leakage. These two sources cannot be distinguished on the 

basis of available information since both sources would be 

expected to have similar geochemical signatures. In wells 

that were upgradient of the sewer system but downgradient of 

septic systems (UWT2-23, FP4-28), geochemical signals did 

not show a difference between the effects of septic-system and 

sewer leakage. Therefore, these sources are lumped together 

in the following discussion. Septic systems have been shown 

to cause elevated nitrate, orthophosphate, chloride, sodium, 

calcium, potassium, DOC, and depressed pH and DO in 

shallow ground water (Robertson and others, 1991, 1998).
Chemical Characteristics 49

Figure 16. Piper diagram showing general types of ground water in the local-scale TANC study area near York, Nebraska.

PERCENT

CALCIUM

100 

80 

60 

40 

20 

0 

0 

20 

40 

60 

80 

100 

MAGNESIUM

0 

20 

40 

60 

80 

100 

SODIUM PLUS POTASSIUM

CHLORIDE, FLUORIDE, NITRITE PLUS NITRATE

0 

20 

40 

60 

80 

100 

100 

80 

60 

40 

20 

0 

BICARBONATE

100 

80 

60 

40 

20 

0 

SULFATE

0 

20 

40 

60 

80 

100 

SULFATE PLUS CHLORIDE

0 

20 

40 

60 

80 

100 

CALCIUM PLUS MAGNESIUM

100 

80 

60 

40 

20 

0 

100 

80 

60 

40 

20 

0 

PERCENT

PERCENT

FP3-33

UWT3-34

UWT3-34

FP4-28

UWT1-53

OFPN-88

FP2-250

FP4-28

FP3-162

FP2-250

FP2-43

FP1-63

FP3-33

OFPN-276

OFPN-276

OFP3-38

SPSW surface discharge

FP4-231

FP4-231

FP3-162

NWT1-39

NWT1-39

NWT1-39

UWT2-23

UWT4-85

UWT3-34

SPSW

 depth 

dependent

Urban shallow

Urban mixed

Unconfined wells Confined wells

Urban deep

Urban unmixed

Agricultural shallow Study public supply well (Study PSW)

Agricultural deep

Agricultural mixed

Agricultural unmixed

EXPLANATION
50 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Chloride concentrations in ground water varied 

substantially from 6.6 to 187 mg/L (table 20 [see back 

of report]); concentrations ≥30 mg/L were in unconfined 

shallow urban wells, or confined urban mixed wells and the 

study PSW, indicating mixing of water from the unconfined 

aquifer in the urban area with water from the confined 

aquifer (fig. 17A,B). Concentrations of chloride ≥30 mg/L in 

unconfined shallow urban wells were associated with initial 

δ15N-nitrate values ≥8.6‰, implying that chloride has been 

affected by septic systems or wastewater leakage (fig. 17A). 

Chloride is considered to be a conservative constituent 

unaffected by geochemical reactions under most conditions. 

Chloride concentrations were largest in well UWT2-23 

(138–187 mg/L, table 20; fig. 17A,B), which was located 

southeast of an area having many septic systems (fig. 4). 

Unconfined shallow agricultural well FP4-28, possibly 

affected by mixture of fertilizer and animal waste sources (see 

"Nutrients"), had chloride concentrations of 31 to 67 mg/L and 

an initial δ15N-nitrate value of 8.6‰. Well NWT1-39, possibly 

affected by livestock manure disposal (see “Nutrients”), 

had chloride concentrations of 12 to 13 mg/L, suggesting 

that livestock manure was not a source of elevated chloride. 

Consequently, the elevated chloride at FP4-28 most likely was 

affected by upgradient septic systems. Wastewater leakage was 

not expected to affect the rural location at FP4-28, since the 

western boundary of the sanitary sewer system did not extend 

upgradient of FP4-28. Chloride concentrations were <13 mg/L 

in unconfined shallow agricultural wells other than FP4-28. 

The low chloride concentrations in most unconfined shallow 

agricultural wells indicate that application of potash (KCl) 

fertilizers on agricultural land was not an important source of 

chloride to ground water. A 1996 survey of fertilizer use in 

the region indicated that potash was applied to only about 5%

of corn acres at an average rate of about 8 kg/ha (Franti and 

Dorn, 1998). 

 Concentrations of chloride and potassium were strongly 

correlated (ρ = 0.37; p = 0.009) in unconfined shallow 

urban and agricultural wells, suggesting a common source 

for relatively high concentrations of these constituents 

(fig. 17B). The highest potassium and chloride concentrations 

in unconfined shallow urban wells were in UWT2-23, 

where effects of septic systems were considered likely (see 

“Nutrients”), and well FP1-63. Concentrations of potassium 

were <9.5 mg/L in unconfined shallow agricultural wells 

(table 20). In contrast to chloride, potassium is affected by 

cation-exchange and sorption reactions and is not conservative 

(Ceazan and others, 1989; Appelo and Postma, 1999). As 

travel distance downgradient from septic-system or sewer 

leakage increases, concentrations of potassium were expected 

to decrease as a result of these geochemical processes and 

return to values representing ground water unaffected by these 

sources. It is possible that the high potassium concentrations 

(as well as chloride values) at UWT2-23 and FP1-63 may be 

related to proximity to septic-system or wastewater sources or 

particularly high source concentrations. Relatively high values 

of chloride and potassium were also in confined urban mixed 

well FP2-250. This well had unique water chemistry and may 

have been affected by upward movement of water from the 

underlying shale; lines of evidence for this interpretation, 

based on multiple constituents, are discussed later in the report 

in the section “Sources of Water and Solutes.” 

The positive relation of orthophosphate and chloride 

concentrations supports the interpretation that the most likely 

source of orthophosphate was septic-system or wastewater 

leakage. Concentrations of chloride and orthophosphate were 

correlated (ρ = 0.67; p = <0.001) in unconfined shallow urban

and agricultural wells (fig. 17C). High concentrations of 

orthophosphate at OFPS-38 may reflect reducing conditions 

and proximity to organic-rich stream sediments of Beaver 

Creek. Among unconfined shallow urban and agricultural 

wells other than OFPS-38, the largest concentrations of 

chloride and orthophosphate were in wells FP1-63 and 

UWT2-23 (fig. 17C). Both orthophosphate and chloride are 

commonly associated with septic-system effluents (Robertson 

and others, 1991, 1998). 

Additional solutes that appeared to be elevated in 

unconfined shallow urban wells because of septic-system or 

wastewater discharge were sulfate and boron. Concentrations 

of sulfate in all except one of these wells and FP4-28 were 90 

to 220 mg/L; initial δ15N-nitrate values were ≥8.6‰ (fig. 17D). 

Well OFPS-38 was an exception; the sulfate concentration 

was about 50 mg/L. However, this well had the largest H2S 

values measured in the study (fig. 8E), possibly indicating 

sulfate-reducing conditions, which would decrease sulfate 

concentrations. Generally, values of sulfate are not always a 

good tracer of the effects of wastewater (Barrett and others, 

1999). However, in the local-scale TANC study area, variable 

but high concentrations of sulfate associated with initial 

δ15N-nitrate ≥8.6‰ indicate that sulfate in unconfined shallow 

urban wells may be related to septic-system or wastewater 

sources. Concentrations of sulfate in confined unmixed wells 

were less than concentrations in unconfined shallow urban

and agricultural wells (fig. 17E) suggesting that geologic 

sources of sulfate in the aquifer were absent or had little effect. 

Sulfate-reducing conditions were present in a few places in 

the confined aquifers (fig. 8E) but do not appear to have had a 

substantial effect on sulfate concentrations, which were similar 

in confined unmixed wells regardless of whether sulfate 

reducing conditions were present. Samples from selected wells 

were analyzed for sulfur isotopic values of sulfate (table 19) 

but the data did not clearly indicate particular sources or 

processes affecting sulfate concentrations. 

Boron appeared to be elevated in some unconfined 

shallow urban wells (fig. 17E), possibly indicating septicsystem or wastewater leakage, but not in others, perhaps 

indicating variability in septic-system sources. Boron is 

typically found in wastewater because it is a component 

of many, but not all, detergents (Barrett and others, 1999). 

Concentrations of boron and sulfate were positively correlated 

(ρ = 0.94; p <0.001).
Chemical Characteristics 51

Figure 17. Graphs showing relations between chloride and A. initial δ15N-nitrate, B. potassium, and C. orthophosphate and between 

sulfate and D. calculated initial δ15N-nitrate and E. boron in ground water from the local-scale TANC study area near York, Nebraska. 

Measured values of initial δ15N-nitrate corrected for denitrification in some samples.

OFPN-88

FP1-63

FP4-28

NWT1-39

UWT2-23

FP2-250 UWT3-34

(Off scale) 

Sulfate = 1,060

 Boron = 480

OFPN-276

Evaporation

FP3-162

OFPS-38 OFPN-88

FP1-63

FP3-33

FP4-28

NWT1-39

UWT1-53

UWT3-34

FP3-162

FP1-63

UWT2-23

OFPS-38

Evaporation

FP1-63

FP4-28

UWT2-23

FP2-250

OFPN-276

FP3-33

OFPS-38

OFPN88

FP1-63

FP2-43

FP4-28

UWT1-53

NWT1-39 UWT2-23 UWT4-85

UWT3-34

FP3-162

A B

C D

E

0

5

10

15

20

0 50 100 150 200

CHLORIDE, IN MILLIGRAMS PER LITER CHLORIDE, IN MILLIGRAMS PER LITER

INITIAL DELTA NITROGEN-15, IN PER MIL

INITIAL DELTA NITRATE-15, IN PER MIL

0

10

20

30

0 50 100 150 200

POTASSIUM, IN MILLIGRAMS PER LITER

5

10

15

20

25

SULFATE, IN MILLIGRAMS PER LITER

0

0

50

100

150

200

250

300

0

BORON, IN MICROGRAMS PER LITER

50 100 150 200 250

0 50 100 150 200 250

SULFATE, IN MILLIGRAMS PER LITER

10

1

0.1

0.01

0 50 100 150 200

CHLORIDE, IN MILLIGRAMS PER LITER

ORTHOPHOSPHATE AS PHOSPHORUS,

IN MILLIGRAMS PER LITER

Evaporation

Evaporation

Denitrification

Denitrification

Mixing of shallow urban and confined

Evaporation

Urban shallow

Unconfined wells

Confined wells

Urban deep

Urban unmixed

Urban mixed

Study public supply well (Study PSW)

Agricultural shallow

Agricultural deep

Agricultural mixed

Agricultural unmixed

EXPLANATION
52 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Chloride/bromide have been used to identify the sources 

of elevated chloride concentrations in ground water (Vengosh 

and Pankratov, 1998; Davis and others, 1998; Panno and 

others, 2006). Figure 18 shows minimum, median, and 

maximum values of chloride and chloride/bromide from 

literature sources for sewage, septic-system effluent, and 

animal waste source materials and ground-water affected 

by septic systems, animal waste, road salt, agricultural 

chemical application, and landfills, and background groundwater unaffected by any of these sources. The literature 

values provide constraints on likely chloride source material 

compositions. However, because the chloride and chloride/

bromide of source materials and affected ground-water can 

vary considerably (Davis and others, 1998; Panno and others, 

2006), interpretations based comparing chloride/bromide 

ratios in local-scale TANC study-area ground water with 

those in possible sources based on literature values need to be 

supported by independent evidence. 

Chloride/bromide are consistent with elevated chloride 

in unconfined shallow urban wells and unconfined shallow 

agricultural well FP4-28 being derived from septicsystem sources rather than a road salt application (fig. 18). 

Unconfined shallow urban wells and FP4-28 generally plot 

in a cluster having relatively high chloride concentrations of 

33 to 187 mg/L and intermediate to high chloride/bromide 

ranging from 96 to 778 (fig. 18). These values plot close to 

curves for source materials and ground-water affected by 

septic systems and animal waste (Vengosh and Pankratov, 

1998; Panno and others, 2006). Because unconfined shallow 

urban wells were closer to septic systems than to livestock 

waste areas, and because chemical composition in NWT1-39, 

affected by animal waste disposal, and unconfined shallow 

urban wells were substantially different, septic systems were 

likely a greater source of elevated chloride in the urban area 

than livestock waste areas.

Samples from well NWT1-39, affected by livestockwaste disposal, had chloride and chloride/bromide lower 

than the range of values in ground water affected by animal 

waste determined by Panno and others (2006). It is likely 

that livestock waste values vary as a function of local factors. 

Samples from unconfined shallow urban wells FP3-33 and 

FP1-63 plot slightly above the range of values indicated by 

Panno and others (2006) for ground-water affected by landfill 

leachate (fig. 18). However, there were no known landfills 

west or northwest of FP3-33 that likely affected that well. Well 

FP1-63 could have been affected by a former rubble wastedisposal site to the northwest (near OFPN-88, fig. 4), although 

there are many other land uses that could have affected this 

well. 

Chloride concentrations in some confined mixed wells 

and the study PSW have intermediate values, consistent with 

representing mixtures, between unconfined shallow urban

wells and confined unmixed wells (fig. 18). The relative 

uniformity of chloride concentrations in confined unmixed

and unconfined shallow agricultural wells probably reflects 

long-term average evaporative concentration of chloride from 

natural precipitation. Wells FP2-250 and OFPN-276, screened 

in lower confined lenses (table 1), have relatively high 

chloride/bromide and plot close to the road-salt affected curve 

(fig. 18); however, these wells may be affected by interactions 

with water or solutes in the underlying shale (see “Sources of 

Water and Solutes”). 

Different combinations of major elements that are useful 

tracers of unconfined shallow urban waters, combined with 

isotopic and VOC data described earlier, were plotted against 

each other in an attempt to use geochemical signatures to 

identify mixing of water from the unconfined aquifer in the 

confined aquifers based on geochemical signatures (fig. 19). 

The plots shown and discussed below were the most useful for 

interpretations of water and solute sources. 

Analysis of δ18O and chloride, tracers expected behave 

conservatively, showed a clear contrast between unconfined 

shallow urban, unconfined shallow agricultural, confined 

unmixed, and confined mixed wells that supported the 

interpretation that confined mixed wells contained water from 

the unconfined aquifer (fig. 19A). Most confined unmixed

wells plotted in a tight cluster with low δ18O and chloride 

values. Most unconfined shallow agricultural wells had high 

δ18O and low chloride concentrations. Unconfined shallow 

agricultural wells plot along an evaporative trend such that 

water in these wells could have been derived from evaporative 

enrichment of confined unmixed water. Such a pattern could 

result from withdrawal and application of confined unmixed

water as a source of irrigation, subsequent evaporative 

enrichment, and recharge of evaporatively enriched water to 

unconfined shallow agricultural wells. A third end-member 

group consisted of unconfined shallow urban wells (and 

including unconfined shallow agricultural well FP4-28) had 

intermediate δ18O and high chloride values. Confined urban 

mixed wells had intermediate values of δ18O and chloride 

along an apparent mixing line between unconfined shallow 

urban and confined unmixed wells. Confined urban mixed well 

FP3-162 overlapped with unconfined shallow urban wells. 

Depth-dependent samples from the study PSW also plotted 

between unconfined shallow urban and confined unmixed

wells but along a mixing line different from that of confined 

urban mixed and unconfined deep urban wells (fig. 19A). 

These observations lead to two conclusions: (1) the water from 

the unconfined aquifer mixing with water from the confined 

aquifer in the study PSW came from an unconfined shallow 

urban source area and (2) the confined mixed wells and the 

study PSW did not have common unconfined shallow urban 

source areas, implying that there were multiple pathways from 

the unconfined aquifer source areas to zones of mixing in the 

confined aquifer. 
Chemical Characteristics 53

Evaporation

Addition 

of 

Bromide

Addition of Chloride

10

0.1 1 10 100 1,000 10,000

100

1,000

10,000

CHLORIDE, IN MILLIGRAMS PER LITER 

CHLORIDE-TO-BROMIDE RATIO

Urban shallow

Unconfined wells Sewage (no median)

Minimum, median, and maximum shown

Confined wells

Urban deep

Urban unmixed

Urban mixed

Study public supply

 well (Study PSW)

Agriculture shallow

Agriculture deep

Agriculture unmixed

Agriculture mixed

Domestic

Open reservoir

Septic effluent

Panno and others

(2006) 

Vengosh and

Pankratov (1998)

Min Median Max

Animal waste

FP4-28

UWT2-23

NWT1-39

OFPN-276

FP2-250

FP1-63

FP3-33

Ground-water affected by

Septic

Animal waste

Road salt

Agricultural Chemicals

Unaffected ground-water

Landfill

Midwestern precipitation

Figure 18. Chloride and chloride/bromide in ground-water samples from the local-scale TANC 

study area near York, Nebraska.
54 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Unconfined wells Urban shallow EXPLANATION Urban deep Agricultural shallow Agricultural deep Confined wells Urban unmixed Urban mixed Study public supply well (Study PSW) Agricultural unmixed Agricultural mixed -7 -8 -9 -10 -11 50

0

200150100

CHLORIDE, MILLIGRAMS PER LITER

A

-7 -8 -9 -10 -11

 DELTA OXYGEN-18, IN PER MIL DELTA OXYGEN-18, IN PER MIL

50

0

250200150100

SULFATE, MILLIGRAMS PER LITER

B

C

0 0 40 80 120 140 200

250200150100

50

SULFATE, IN MILLIGRAMS PER LITER

CHLORIDE, IN MILLIGRAMS PER LITER 

D

0 40 80 120 140 200

BROMIDE, IN MILLIGRAMS PER LITER

CHLORIDE, IN MILLIGRAMS PER LITER 

0

0.80.70.60.50.40.30.20.1

E

OFPN-88

UWT4-85

UWT2-23 

FP3-33 

FP1-63

OFPN-276

FP2-250

FP2-250

OFPN-276

FP4-28

FP3-162

FP4-28

FP5-175

NWT1-39

FP2-250

(Off scale) 

Sulfate = 1,060

 = -9.0 18O

OFPN-88 

FP3-33

OFPS-38

FP4-28

UWT4-85

FP2-43

UWT1-53

FP3-162 NWT1

UWT3-34

FP1-63 

UWT2-23 

FP2-250

(Off scale) 

Sulfate = 1,060

 Chloride = 135

Evaporation

Evaporation

Evaporation

Evaporation

Confined

OFPN-276

Urban

unconfined

shallow

Confined

supply well

(includes

depthdependent)

Urban

confined

mixed

Agricultural

unconfined and

confined mixed

Urban

unconfined

shallow

Urban

confined

mixed

Supply well

(includes

depthdependent)

Confined

Agricultural

unconfined

Mixing line of unconfined urban shallowand confined urban unmixed in study publicsupply well

Addition of chloride

Addition 

of sulfate

Addition 

of 

bromide

Addition of chloride

86420

1210

PCE, IN MICROGRAMS PER LITER

0 40 80 120 140 200

CHLORIDE, IN MILLIGRAMS PER LITER 

FP1-63 

OFPN-88 

UWT2-23 

Depth-dependent samples line

UWT4-85

Figure 19. Relations between tracers in ground-water samples from the local-scale TANC study area near York, Nebraska: δ18O and A. chloride, B. sulfate; chloride and 

C. sulfate, D. bromide, and E. PCE (tetrachloroethylene).
Chemical Characteristics 55

Similar interpretations were evident from a plot of δ18O 

versus sulfate (fig. 19B). Confined urban mixed wells plotted 

between unconfined shallow urban and confined unmixed

wells, implying that the sulfate that was higher in the confined 

urban mixed wells than in the confined unmixed wells was 

derived from mixing with unconfined shallow urban wells, 

FP4-28, and (or) NWT1-39, rather than most unconfined 

shallow agricultural wells, which have lower sulfate. FP4-28 

plotted similar to unconfined shallow urban wells, as with 

other inorganic constituents discussed earlier, and NWT1-39, 

interpreted to be affected by a manure source, had higher 

sulfate than most other unconfined shallow agricultural

wells. Confined agricultural mixed well FP5-175 had higher 

sulfate than unconfined shallow agricultural wells other than 

FP4-28 and NWT1-39, which were east (downgradient) of 

FP5-175. The relatively high sulfate in FP5-175 was similar 

to concentrations in several unconfined shallow urban

wells and may indicate that the sulfate was derived from a 

common source, either septic systems or wastewater, or from 

reactions of well-bore leakage with sulfur minerals in the 

upper confined unit, or it may indicate effects of an upgradient 

livestock manure source having a composition similar to that 

of NWT1-39. 

A plot of chloride and sulfate (fig. 19C) showed low 

concentrations of chloride and sulfate in confined unmixed

wells, causing them to plot in a tight cluster, but large 

variability of values among unconfined shallow urban wells, 

and in wells FP4-28 and NWT1-39. The wide variability of 

chloride and sulfate concentrations in unconfined shallow 

urban wells probably reflected wide variations in manure, 

septic system, or wastewater-source compositions and in 

amounts of mixing with unaffected waters. 

Factors influencing calcium and bicarbonate 

concentrations were of particular importance because uranium 

concentrations were closely tied to the distribution of these 

constituents (see “Uranium”). Concentrations of calcium and 

bicarbonate were affected by equilibrium with calcite, depth to 

water, and possibly septic-system discharge. 

Saturation indices for calcite indicated approximate 

thermodynamic equilibrium between the mineral calcite and 

all ground-water samples except those from the unconfined 

shallow urban wells described below. PHREEQC Interactive 

(version 2.12.5 [Parkhurst and Appelo, 1999]) was used to 

determine the saturation indices of selected mineral and gas 

phases, which indicate the thermodynamic state of a mineral 

or gas phase relative to the aqueous phase. Calcite was present 

in most cores analyzed for mineralogy (table 8).

Concentrations of bicarbonate were relatively low 

(fig. 20A) and calcite was undersaturated in unconfined 

shallow urban wells with pH less than about 6.7. The 

relatively low pH values occurred in unconfined shallow urban

wells having relatively high DOC; oxidation of organic matter 

produces acidity, which lowers pH (Appelo and Postma, 

1997; Robertson and others, 1998). When pH is low, more of 

the dissolved inorganic carbon exists as dissolved carbonic 

acid or carbon dioxide instead of bicarbonate (Appelo and 

Postma, 1997). The high concentrations of DOC and low pH 

in these wells were suspected to result from a combination of 

factors, including shallow depths to water (FP3-33, OFPS-38, 

UWT1-53, UWT2-23), close proximity to stream sediments 

suspected to contain abundant organic matter (OFPS-38, 

UWT2-23, FP5-45), and livestock waste spread on overlying 

land (NWT1-39). 

Calcium and bicarbonate concentrations in some 

unconfined shallow urban and confined mixed wells may have 

increased because of septic-system effects. Acidity generated 

by oxidation of organic matter and ammonia in septicsystem effluent, when reacted with calcite in the sediment, 

should result in increased concentrations of bicarbonate and 

calcium more than waters not affected by septic systems 

(Robertson and others, 1991). Concentrations of calcium 

and chloride in unconfined shallow urban and agricultural

wells were correlated (ρ = 0.32, p = 0.025 [fig. 20C]), 

consistent with increased dissolution of calcite in waters 

affected by septic systems. Concentrations of bicarbonate 

and chloride in unconfined shallow urban wells were not 

correlated (fig. 20B). However, bicarbonate concentrations 

can be affected by inorganic carbon speciation as a function 

of pH and soil processes generating carbon dioxide; these 

processes complicate interpretation of the effects of septic 

systems on dissolved inorganic carbon. Relatively high 

bicarbonate concentrations, greater than about 300 mg/L, were 

in unconfined shallow urban wells that were oxic. Calcium 

and bicarbonate concentrations were greater in confined mixed

wells and the study PSW depth-dependent samples than in 

confined unmixed wells (figs. 20B,C), probably reflecting the 

effects of mixing of waters from the unconfined aquifer. 

Constituent concentrations calculated to result from 

evaporative concentration of confined unmixed waters were 

plotted on figs 17, 18, 19, and 20 for reference. Evaporation 

lines usually did not uniquely fit the distribution of constituent 

concentrations in unconfined shallow urban and agricultural

wells. The comparison of sample values to evaporation 

lines was inconsistent from plot to plot, making clear 

evaporative effects difficult to identify. Moreover, some of 

the constituents plotted are not conservative. These results 

suggest that evaporative concentration is likely to be a minor 

factor affecting concentrations of solutes in the unconfined 

aquifer. As discussed in “Stable Isotopes,” unconfined 

aquifer δ18O and δD values were near MWLs, suggesting 

that isotopic values have been modified very little or not at 

all by evaporation. However, values of δ18O and δD are not 

modified by transpiration, which is a non-fractionating process 

(Zimmerman, 1967). Therefore, transpirative concentration 

could still occur and not be detectable with δ18O and δD 

values. As most of the landscape in the subhumid local-scale 

TANC study area has vegetation cover during the growing 

season, it is reasonable to consider that more water is lost 

through transpiration than through evaporation. Increased 

concentrations of dissolved constituents in unconfined 

shallow wells compared with confined unmixed wells could 

have resulted from direct evapotranspirative loss from the 
56 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

water table or from seasonal or event-driven recharge of 

constituents that accumulated in the unsaturated zone during 

the growing season. The depth to water table ≥6 m in all but 

two wells, OFPS-38 and UWT2-23, (fig. 10) was greater than 

the maximum depth to water likely be directly affected by 

evapotranspirative loss on the basis of literature values for 

grass land cover and clay loam soil (Shah and others, 2007). 

Thus, direct evaporative loss from the water table is unlikely 

or minor. Evapotranspiration probably affects ground-water 

quality through evapotranspirative concentration of solutes 

from ground-water irrigation in the unsaturated zone. Ground 

water used as the source of irrigation has total dissolved 

solids (TDS) more than an order of magnitude higher than 

that in precipitation (from the nearest precipitation chemistry 

station at Mead, Nebraska, data from National Atmospheric 

Deposition Program, 2005). These processes in the unsaturated 

zone may affect constituent concentrations in some unconfined 

shallow wells, but the effects are likely to be highly variable 

from site to site and would be complicated by adding solutes 

from anthropogenic sources.

Urban shallow

Unconfined wells

EXPLANATION

Confined wells

Urban deep

Urban unmixed

Urban mixed

Study public supply well (Study PSW)

Agricultural shallow

Agricultural deep

Agricultural mixed

Evaporation line

Agricultural unmixed

Mixing line of unconfined

urban shallow and confined urban

unmixed in study public supply well

100

200

300

400

500

600

5 6 7 8 9

pH 

BICARBONATE (HCO3), IN MILLIGRAMS PER LITER

100

200

300

400

500

600

0 50 100 150 200

CHLORIDE (Cl), IN MILLIGRAMS PER LITER 

0 50 100 150 200

CHLORIDE (Cl), IN MILLIGRAMS PER LITER 

BICARBONATE (HCO3), IN MILLIGRAMS PER LITER

0

50

100

150

200

250

300

350

CALCIUM (Ca), IN MILLIGRAMS PER LITER

A B

C

FP3-33

UWT4-85

UWT4-85

UWT3-34

OFPN-88

OFPS-38 FP2-250

UWT2-23 

UWT2-23 

FP1-63

FP1-63

FP3-33

FP4-28

OFPN-88

FP2-250

FP3-

162

Figure 20. Relations in ground-water samples from the TANC local-scale study area near York, Nebraska: A. pH and 

bicarbonate, B. chloride and bicarbonate, C. chloride and calcium.
Chemical Characteristics 57

Uranium

Uranium concentrations in ground water are of concern 

in the High Plains aquifer of Nebraska, as values above the 

U.S. EPA MCL of 30 μg/L for drinking water have been 

detected at several locations (Snow and Spalding, 1994). 

Uranium concentrations in the unconfined shallow urban

and agricultural wells in the TANC local-scale study area 

ranged from 0.22 to 40 μg/L and were generally greater 

than concentrations in confined unmixed wells (fig. 21A). 

Uranium has several oxidation states but is only soluble in 

the most oxidized +6 state (Langmuir, 1997; Kraemer and 

Genereux, 1998). Uranium will form the soluble uranyl cation 

(UO2

2+) in the pH range of 6 to 9 typical of ground water. 

As conditions become more reducing, uranium is reduced 

to the +5 and +4 states, and is removed from solution as 

precipitating insoluble uranium minerals or by sorbing to 

sediment surfaces. Consequently, uranium concentrations were 

expected to be strongly related to the redox conditions of the 

aquifer. Uranium concentrations in unconfined shallow urban

and agricultural wells that were oxic were 1.5 to 40 μg/L; 

concentrations in the wells that were reducing were lower, 

ranging from 0.22 to 4.24 μg/L (tables 13 and 21 [see back 

of report]). The general decrease in uranium concentrations 

with increasing depth (fig. 21A), apart from relatively high 

concentrations in confined mixed wells discussed later in this 

section, primary reflected the change from oxic to reducing

conditions with increasing depth (fig. 8). However, the wide 

range of uranium concentrations in unconfined shallow urban

and agricultural wells and the relatively high concentrations 

of uranium in confined mixed wells suggest that controls on 

uranium concentrations were more complex than a simple 

dependence on redox conditions. The mechanisms influencing 

uranium concentrations in unconfined shallow urban and 

agricultural wells and confined mixed wells were different and 

are discussed separately below.

The distribution of uranium in unconfined shallow urban

and agricultural wells was likely linked to the distribution of 

calcium and bicarbonate. For unconfined shallow urban and 

agricultural wells, concentrations of uranium were correlated 

with bicarbonate (ρ = 0.70 , p = 0.004 [fig. 22A]) and calcium 

(ρ = 0.77, p = 0.002 [fig. 22E]). Uranium in the +6 state forms 

aqueous complexes with carbonate species and calcium that 

reduce adsorption of uranium to sediments (Langmuir, 1997; 

Curtis and others, 2006). Calculations of uranium speciation 

with PHREEQC using a modified thermodynamic database 

compiled by Lawrence Livermore National Laboratory 

indicated that the most abundant uranium species present was 

a calcium-uranyl-carbonate complex (Ca2

UO2(CO3)

3

0

) (Bryant 

R. Jurgens, U.S. Geological Survey, written commun., July 29, 

2006). This complex accounted for more than 90% of uranium 

species in all samples except one; the exception was FP3-33, 

having pH of 5.9, where calcium-uranyl-carbonate complexes 

accounted for about 68% of uranium species. 

In unconfined shallow urban and agricultural wells, 

uranium concentrations were largest where the depth to water 

exceeded about 7 m. Wells with depth to water of less than 

about 7 m had relatively low DO, pH, (fig. 10) and bicarbonate 

(figs. 20A), likely because of the effects of oxidation on 

soil organic matter; these wells had low concentrations of 

uranium also. Wells with depth to water greater than about 

7 m generally had pH between 6.5 and 7.0, DO between 3 and 

8.5 mg/L, bicarbonate concentrations ≥350 mg/L, and uranium 

concentrations ≥5 μg/L.

Uranium concentrations varied considerably over a 

range from 5 to 40 μg/L in unconfined shallow urban and 

agricultural wells having depth to water ≥7 m, DO ≥3 mg/L, 

and pH ≥6.6; hereinafter, these wells are referred to as high 

U unconfined shallow wells (FP1-63, OFPN-88, UWT3-34, 

UWT4-85, AWT1-83, AWT2-71, AWT3-73, FP4-28). In these 

high U unconfined shallow wells, uranium was not correlated 

with bicarbonate (ρ = −0.14; p = 0.66 [fig. 22A]), suggesting 

that once bicarbonate concentrations were above a threshold, 

they did not further affect uranium concentrations. In these 

high U unconfined shallow wells, uranium was correlated with 

chloride (ρ = 0.92; p = 0.004), potassium (ρ = 0.89; p = 0.005), 

calcium (ρ = 0.84; p = 0.008), δ15N-nitrate (ρ = 0.79; 

p = 0.039) and orthophosphate (ρ = 0.72; p = 0.023) 

(fig. 22B–E), which were described earlier as being indicators 

of septic-system or wastewater influence. 

 The mechanism producing higher uranium 

concentrations in unconfined shallow urban and agricultural

wells affected by septic-system leakage may have been 

enhanced solubility of uranium because of formation of 

complexes with dissolved calcium and bicarbonate in 

ground water affected by septic systems. Uranium can form 

complexes with orthophosphate that increase solubility and 

decrease adsorption of uranium to sediments (Sandino and 

Bruno, 1992). However, results of PHREEQC speciation 

calculations show that uranium-phosphate complexes in high 

U unconfined shallow wells were insignificant because of the 

relatively low concentrations of orthophosphate compared 

with bicarbonate (Bryant R. Jurgens, U.S. Geological 

Survey, written commun., July 29, 2006). The correlation 

of calcium and uranium in high U unconfined shallow

wells was consistent with higher uranium (Ca2

UO2(CO3)

3

0

) 

concentrations in wells significantly affected by septic-system 

drainage caused by calcite dissolution in response to increased 

acidity associated with the septic-system drainage. 
58 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Anthropogenic sources of uranium, such as phosphate 

fertilizers, were considered to be unlikely. Although 

correlations of uranium and orthophosphate have been 

sited as evidence to associate uranium with phosphorus 

fertilizers (Snow and Spalding, 1994), it was unlikely that 

orthophosphate concentrations in ground water in the localscale TANC study area were related to phosphorus fertilizer. 

The association of elevated orthophosphate with 

δ15N-nitrate values greater than 8 per mil and elevated 

chloride concentrations suggested that the orthophosphate 

was associated with ground water affected by a septic-system 

source. Other sources of phosphorus to ground water were 

considered to be minor (see “Nutrients”). Concentrations of 

chloride and orthophosphate were relatively low in unconfined 

shallow agricultural wells, suggesting that phosphorus 

and potash fertilizers, applied to agricultural crops in small 

amounts in the local-scale TANC study area, did not result in 

elevated concentrations in ground water. 

Concentrations of uranium in some confined mixed

wells were larger than would be expected from conservative 

mixing of water from the unconfined aquifer with water 

from confined unmixed wells, implying that there was an 

additional source of uranium to confined mixed wells. A 

plot of chloride versus uranium concentrations shows that 

depth-dependent samples from the study PSW and FP5-175 

had uranium concentrations greater than those in the high U 

unconfined shallow wells (fig. 22B); thus, the concentrations 

of uranium of 21 to 184 μg/L in the study PSW and FP5-175 

could not have been generated from conservative mixing of 

water from the unconfined aquifer and confined unmixed

wells. Welch and Lico (1998) used a similar approach to 

identify subsurface sources of uranium and arsenic in the 

Carson Desert in Nevada. Samples from the study PSW and 

FP5-175 that had concentrations of uranium ≥10 μg/L also had 

iron concentrations ≥100 μg/L, consistent with iron-reducing 

conditions (fig. 22F). Confined mixed wells in the upper 

confined aquifer without iron-reducing conditions (OFPS-157, 

FP3-162; fig. 22F) did not have uranium concentrations 

larger than could be caused by mixing of water from the 

unconfined aquifer with water from confined unmixed wells. 

Conversely, confined mixed wells in the lower confined lenses 

(FP3-218, FP2-250) had iron-reducing conditions but did not 

have concentrations of uranium larger than could be caused 

by possible from mixing with water from the unconfined 

aquifer. These observations indicate that elevated uranium 

concentrations were in the upper confined aquifer where 

waters from the unconfined aquifer were mixing with water 

from the upper confined aquifer (represented by confined 

unmixed wells) under iron-reducing conditions. 

FP4-168

FP4-83

FP4-28 FP1-63

FP1-147

OFPS-157

0

10

20

30

40

50

60

70

DEPTH OF TOP OF SCREEN BELOW 

WATER TABLE, IN METERS

Urban shallow

Unconfined wells

EXPLANATION

Confined wells

Urban deep

Urban unmixed

Urban mixed

Agricultural Study public supply well (Study PSW)

 shallow Agricultural unmixed

Agricultural

 mixed

Agricultural

 deep

A B

0 10 20 30 40 50 60

URANIUM, IN MICROGRAMS PER LITER 

URANIUM-234 TO URANIUM-238 ACTIVITY RATIO

0 10 20 30 40

URANIUM, IN MICROGRAMS PER LITER 

1.0

1.5

2.0

2.5

FP2-250

OFPS-157

FP5-175

Study public supply well

depth-dependent

samples

(off scale) 

184 micrograms per liter

at 36 meters

Change in concentration

upwards along study public

supply well screen

Figure 21. Relation of uranium to A. depth below water table and B. uranium isotope ratios for samples collected from the local-scale 

TANC study area in York, Nebraska.
Chemical Characteristics 59

A B

C

E

D

URANIUM, IN MICROGRAMS PER LITER

URANIUM, IN MICROGRAMS PER LITER URANIUM, IN MICROGRAMS PER LITER

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8

ORTHOPHOSPHATE AS PHOSPHORUS,

IN MILLIGRAMS PER LITER

URANIUM, IN MICROGRAMS PER LITER

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30

0

10

20

30

40

50

60

0 20 40 60 80 100 120 140 160 180 200

CHLORIDE (Cl), IN MILLIGRAMS PER LITER

URANIUM, IN MICROGRAMS PER LITER

0

10

20

30

40

50

60

0 20 40 60 80 100 120 140 160 180 200

CALCIUM (Ca), IN MILLIGRAMS PER LITER

0

10

20

30

40

50

60

100 200 300 400 500 600

BICARBONATE (HCO3), IN MILLIGRAMS PER LITER

FP5-175

Study public

suppy well

depthdependent 

samples 

FP2-250

(Offscale) 

Uranium = 184

 Calcium = 128

Study public

suppy well

surface

discharge

Study public

suppy well

surface

discharge

OFPN-88

OFPN-88

OFPN-88

FP1-63

FP1-63

FP1-63 FP1-63

FP2-250

Oxic, high uranium

unconfined

shallow wells

Oxic, high uranium

unconfined

shallow wells

Oxic, high uranium

unconfined

shallow wells

Oxic, high

uranium

unconfined

shallow wells

FP4-28

FP4-28

FP4-28

FP4-28

FP5-175

FP5-175

AWT1-83

AWT1-83

AWT1-71

AWT1-71

AWT1-71

AWT3-73

AWT3-73

AWT3-73

UWT1-39

UWT2-23

UWT3-34

UWT3-34

UWT4-85 UWT4-85

UWT4-85

UWT4-85

DELTA NITROGEN-15 OF NITRATE, IN PER MIL

Urban shallow

Unconfined wells Confined wells

Urban deep

Urban unmixed

Urban mixed

Study public supply

 well (Study PSW)

Agricultural shallow

Agricultural deep

Evaporation line

Agricultural mixed

Agricultural unmixed

EXPLANATION

Figure 22. Relation of concentrations of uranium to A. bicarbonate, B. chloride, C. δ15N-nitrate, D. orthophosphate, 

E. calcium, and F. iron samples from the local-scale TANC study near York, Nebraska.
60 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Interaction of oxic well-bore leakage water from 

the unconfined aquifer with the reducing upper confined 

aquifer has the potential to mobilize uranium through redox 

reactions. The specific mechanism producing high uranium 

concentrations under these conditions was not confirmed 

with available data and analyses. However, a hypothesis for a 

mechanism is described below. 

Elevated uranium concentrations in confined mixed wells 

with iron-reducing conditions could result from release of 

uranium during reduction (dissolution) of iron oxyhydroxides 

on colloids that moved down well bores from the unconfined 

aquifer into the reducing upper confined aquifer. Colloids 

are suspended particles typically <1 micrometer in diameter 

consisting of organic or inorganic particles, microorganisms, 

metal oxides, carbonates, or clay minerals (van der Lee 

and others, 1992). Colloids can be widespread (Kim, 1994) 

and can enhance transport of non-soluble phases like iron 

oxides (Puls and Powell, 1992) and associated adsorbed trace 

elements like uranium (van der Lee and others, 1992; Artinger 

and others, 2002; Missana and others, 2004), radionuclides 

(Kersting and others, 1999), and metals (Sanudo-Wilhelmy 

and others, 2002) under some circumstances. Laboratory 

column and batch reaction studies have indicated that 

uranium migration, especially under reducing conditions, 

can be substantially increased by associating uranium with 

colloids (Artinger and others, 2002; Missana and others, 

2004). Field studies on the effect of colloidal transport on 

uranium are lacking, but long travel distances of radionuclides 

through aquifers near nuclear test sites have been attributed 

to colloid-driven transport (Kersting and others, 1999). 

Despite increasing research into the importance of colloidal 

transport on trace element transport (Sanudo-Wilhelmy and 

others, 2002), colloidal transport is not well understood. 

For colloidal transport of uranium to be a mechanism 

facilitating high uranium concentrations in confined mixed

waters in the local-scale TANC study area, colloids must be 

present, mobile, stable, and able to adsorb the trace elements 

of interest irreversibly (Missana and others, 2004) in the 

source area—in this case, uranium in the shallow unconfined 

aquifer. If colloids have compositions similar to those of the 

core samples from the unsaturated zone and the unconfined 

aquifer, there would be abundant iron oxides, with associated 

uranium and arsenic (table 9). These colloids could move 

through the unconfined aquifer without losing their relatively 

high sorbed uranium concentrations (irreversible sorption) 

and flow down multi-layer well bores into the upper confined 

aquifer, where iron oxides on colloids would come in contact 

with iron-reducing conditions. The organic carbon electron 

donor necessary to allow iron-reduction (Welch and Lico, 

1998) could be supplied by well-bore leakage water or solidphase organic carbon (table 9) present in colloidal phases. 

Unconfined shallow urban and agricultural wells and confined 

mixed wells had DOC values of 1 to 4 mg/L compared 

with the values 0.3 to 0.8 mg/L in most confined unmixed

wells. If iron reduction was initiated under these favorable 

conditions, iron oxides on colloids would dissolve, releasing 

sorbed uranium. The uranium could then form complexes 

with abundant calcium and bicarbonate in the confined 

mixed waters to form calcium-uranyl-carbonate complexes. 

Bicarbonate and calcium were generally present in higher 

concentrations in confined mixed than in confined unmixed

wells (fig. 22A,E). PHREEQC speciation calculations 

indicate that 99% of the uranium would be in the form of 

Ca2

UO2(CO3)

3

0

 in depth-dependent samples from the study 

PSW and FP5-175 (Bryant R. Jurgens, U.S. Geological 

Survey, written commun., July 29, 2006). Complexed 

uranium has greater solubility, potentially allowing uranium 

to persist in solution even under reducing conditions where 

it typically would precipitate or be adsorbed to sediments. 

Colloid distribution was not measured in this study. Therefore, 

URANIUM, IN MICROGRAMS PER LITER 100

100

10

10

1.0

1.0

0.1

1,000

1,000

IRON, IN MICROGRAMS PER LITER

F

FP5-175

FP3-

162

OFPS-157

OFPS-157

FP3-162

OFPS157

FP3-210

FP2-250

Urban shallow

Unconfined wells Confined wells

Urban deep

Urban unmixed

Urban mixed

Study public supply

 well (Study PSW)

Agricultural shallow

Agricultural deep

Evaporation line Agricultural mixed

Agricultural unmixed

EXPLANATION

Figure 22. Continued.
Chemical Characteristics 61

reporting on the effect of colloidal transport on the distribution 

of uranium in this study was speculative. Although the 

proposed mechanism required many assumptions that have not 

been verified, the presence of high concentrations of uranium 

in mixed waters in the upper confined aquifer with ironreducing conditions is consistent with the essential constraints 

for colloid-enhance uranium transport. 

Another mechanism for the elevated uranium 

concentrations in confined mixed waters with iron-reducing 

conditions considered was uranium mobilization from the 

upper confining unit. A study in the Platte River valley of 

Nebraska, (Spalding and Druliner, 1981) suggested that 

aeration from pumping resulted in oxidation of previously 

immobilized uranium from reduced clays. However, the data 

were not consistent with this mechanism being the source 

of elevated uranium in the local-scale TANC study area. 

Although measurable uranium was in core samples in the 

upper confining unit (table 9), oxidation of reducing sediments 

containing uranium should result in the highest uranium 

concentrations associated with relatively more oxidized 

waters in the upper confined aquifer. Moreover, high uranium 

concentrations in unconfined deep wells would be expected 

in oxic waters at the interface of the unconfined aquifer 

and the upper confining aquifer. Neither of these conditions 

was observed. Rather, the highest uranium concentrations 

were in confined mixed wells with iron-reducing conditions, 

and uranium concentrations near the upper confining unit 

(FP1-147, FP3-130) were <4 μg/L. 

Uranium isotope data (234U, 238U) supported the 

interpretation that relatively high uranium concentrations 

in the unconfined shallow urban and agricultural wells and 

the study PSW result from dissolution of uranium-bearing 

sediments in the unconfined aquifer. Uranium-isotope samples 

were collected from seven wells in March-April 2005. 

When uranium is leached or dissolved from a host rock, the 

uranium added to the water usually has 234U/238U of 1.2 to 1.5, 

close to or slightly greater than typical host rock values of 

about 1.0 (Osmond and Cowart, 2000). Such dissolution of 

uranium-bearing solid phases primarily occurs in oxic waters. 

In contrast, uranium mobilized from a host rock by alpha 

recoil during radioactive decay of 238U typically has 234U/238U 

of greater than 1.5 because 234U is preferentially mobilized 

(Dabous and Osmond, 2001); the preferential mobilization 

can occur because the 234U is propelled across the solidaqueous phase boundary during recoil or because the crystal 

lattice is weakened by recoil and the resulting 234U is more 

vulnerable to leaching than undecayed 238U (Osmond and 

Cowart, 2000). In reducing aquifers, uranium mobilization 

is typically dominated by alpha recoil (Osmond and Cowart, 

2000). As water moves from oxic to reducing zones in an 

aquifer, the 234U/238U remains relatively unchanged even as 

uranium concentrations decrease; therefore, uranium isotope 

values can be good tracers of the source of the uranium 

(Dabous and Osmond, 2001). Samples from unconfined 

shallow urban (FP1-63) and agricultural (FP4-28) wells 

had uranium concentrations ≥20 μg/L and relatively low 234U/238U of about 1.2 to 1.3 (fig. 21B), consistent with values 

expected from dissolution of uranium from a solid-phase 

source. Confined unmixed wells FP1-147 and FP4-168 had 

uranium concentrations <3 μg/L and a 234U/238U of about 

1.6 to 2.4, consistent with values for confined aquifers in 

many other locations (Dabous and Osmond, 2001). The 

relatively high 234U/238U were consistent with 234U entering 

the aqueous phase as a result of alpha recoil processes rather 

than dissolution of uranium-bearing sediments in the oxic 

unconfined aquifer. Confined mixed well OFPS-157, which 

during earlier sampling periods showed greater evidence of 

mixing with water from the unconfined aquifer, showed only 

slight evidence of mixing by April 2005. The relatively high 

234U/238U for OFPS-157 in April 2005 was similar to that for 

FP1-147 and was probably representative of confined unmixed 

wells. Unconfined deep well FP4-83, screened at the bottom 

of the unconfined aquifer, was slightly oxic, but the 234U/238U 

of 1.85 suggests that less dissolution of uranium occurs 

than in shallower parts of the unconfined aquifer. A sample 

of the surface discharge of the study PSW had a uranium 

concentration of about 16 μg/L and a 234U/238U of 1.35, nearly 

the same 234U/238U as unconfined shallow urban (FP1-63) and 

agricultural (FP4-28) wells. The similarity of the 234U/238U for 

unconfined shallow wells and the study PSW was consistent 

with uranium in the study PSW being derived from leaching 

of uranium from the shallow unconfined aquifer. However, as 

described earlier, uranium concentrations in depth-dependent 

samples from the lower part of the screen of the study PSW

were higher than in the unconfined shallow urban and 

agricultural wells; this result implies that the mixed waters 

in the upper confined aquifer were enriched with uranium. 

The 234U/238U in the study PSW, implying dissolved uranium 

derived from leaching, was also consistent with leaching of 

uranium from sediments or colloids dissolving during mixing 

of well-bore leakage and iron-reducing confined waters. 
62 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Arsenic

Arsenic concentrations ranged from about 1.6 to 

9.4 μg/L; the median was 6.0 μg/L, less than the USEPA 

MCL, 10 μg/L (fig. 23A; table 21). Arsenic concentrations did 

not vary systematically with depth, and were about the same 

in the generally oxic unconfined aquifer and the reducing 

confined aquifers. However, the speciation of arsenic differed 

between aquifers. In the unconfined aquifer, arsenate was 

the primary arsenic species; in most confined unmixed wells, 

arsenite was the primary arsenic species (figs. 23B–D). This 

distribution of arsenic species was expected, since the more 

oxidized arsenate is typically stable under oxic conditions 

and the more reduced arsenite is stable under reducing 

conditions (Welch and others, 2000). However, there were 

exceptions to the expected distribution of arsenic species on 

the basis of redox condition. Most confined mixed wells and 

the study PSW, showing evidence of mixing with unconfined 

waters, had more arsenate than arsenite (fig. 23D), despite 

having nitrate- to iron-reducing conditions. In addition, 

unconfined shallow urban wells with nitrate- to sulfatereducing conditions (UWT1-53, UWT2-23, FP3-33, OFPS-38; 

table 21) and a confined urban unmixed well (FP1-247) 

having manganese to sulfate reducing conditions, also had 

more arsenate than arsenite. Disequilibrium of arsenic 

speciation with redox conditions are not uncommon (Welch 

and others, 2000). However, these exceptions may indicate 

that there are factors other than redox condition influencing 

arsenic distribution, most likely differences in arsenic 

source materials. The relative uniformity of total arsenic 

concentrations, in spite of the different redox conditions in 

different aquifers, implies multiple mechanisms of arsenic 

mobilization and transport.

In the unconfined aquifer, mobilization of arsenic may 

be facilitated by two processes. First, competitive sorption 

processes—arsenic adsorbed onto iron- or manganese-oxide 

coatings on fluvial sands and gravels could be displaced by 

more strongly sorbing orthophosphate derived from septic 

systems under oxic conditions. Second, release of arsenic may 

occur during reductive dissolution of iron- and manganeseoxides under reducing conditions near Beaver Creek. 

Septic-system drainage may facilitate higher arsenic 

concentrations in unconfined shallow urban wells. Arsenic 

and orthophosphate (ρ = 0.79; p = 0.001 [fig. 24A]) and 

arsenic and chloride (ρ = 0.55; p = 0.024 [fig. 24B]) were 

correlated in unconfined shallow urban and agricultural wells. 

Competition between orthophosphate and arsenic for sorption 

sites on oxyhydroxide coatings on sediments can result in 

desorption of arsenic when orthophosphate is present (Welch 

and Stollenwerk, 2003; Kent and Fox, 2004). Among oxic 

unconfined shallow urban and agricultural wells, UWT2-23 

and FP1-63 had the highest arsenic concentrations of 5 to 

8 μg/L and orthophosphate concentrations of 0.6 to 0.8 mg/L, 

and may have been influenced by septic systems on the basis 

of elevated chloride, potassium, orthophosphate, and δ15Nnitrate. Concentrations of orthophosphate in other unconfined 

shallow urban and agricultural wells ranged from 0.29 to 

0.51 mg/L; over this relatively narrow range of orthophosphate 

concentrations, arsenic concentrations increased from about 3 

to 6 μg/L (fig. 24A), implying that arsenic concentrations may 

be sensitive to small changes in orthophosphate concentrations 

over this range. Correlation of concentrations of arsenic, 

chloride, and orthophosphate in unconfined shallow urban

and agricultural wells suggests that arsenic is not related to 

historical application of agricultural pesticides containing 

arsenic (Welch and Stollenwerk, 2003). 

Arsenic concentrations in the unconfined aquifer were 

greatest (8.8 μg/L) at well OFPS-38 (fig. 24A). Because 

the redox conditions at this well encompass iron-reducing 

conditions (table 13), reductive dissolution of iron oxides 

containing arsenic could occur. This is a common mobilization 

mechanism for arsenic (Welch and others, 2000; Smedley and 

Kinniburgh, 2002). Solid-phase analyses indicated arsenic 

was present in cores collected from the unconfined aquifer 

(table 9). Well OFPS-38 has substantially higher arsenic 

concentrations than other unconfined shallow urban wells with 

similar chloride concentrations (fig. 24B), suggesting that the 

mechanism of arsenic mobilization at OFPS-38 may differ 

from that in other unconfined shallow urban wells. Because 

the distribution of organic-rich stream sediments along Beaver 

Creek (adjacent to OFPS-38) is limited, this mechanism 

for producing higher than typical arsenic concentrations in 

unconfined shallow urban wells may only exist near stream 

sediments. 

In confined unmixed wells, arsenite was the dominant 

arsenic species present (fig. 23). The glaciofluvial sediments in 

the confining units and confined aquifers were deposited tens 

of thousands to two million years ago. When these sediments 

were subsequently buried and redox conditions became 

manganese- to sulfate-reducing, iron and manganese oxides 

on the sediments, in the presence of organic carbon from 

surficial processes, probably underwent reductive dissolution, 

releasing arsenate with iron and manganese (Welch and Lico, 

1998). Eventually, the arsenate likely was primarily reduced 

to arsenite, and a quasi-equilibrium developed in the confined 

aquifer, with relatively slow-moving ground water during 

predevelopment conditions. Sorption and solubility limited 

arsenic concentrations in solution. Arsenite concentrations in 

confined unmixed waters were probably primarily influenced 

by these sediment/water interactions in relatively old ground 

water in the confined aquifer. 
Chemical Characteristics 63 Figure 23. Arsenic speciation in ground water in the local-scale TANC study area near York, Nebraska.

0

10203040506070

10 8 6 4 2 0 10 8 6 4 2 0

10 8 6 4 2 0

10 8 6 4 2 0 DEPTH OF TOP OF SCREEN BELOW WATER TABLE, IN METERS

Unconfined wellsUrban shallow

EXPLANATION

Confined wells

Urban deep

Urban unmixedUrban mixedStudy public supply well (Study PSW) Agricultural shallow

Agricultural unmixedAgricultural mixed

Agricultural deep

ARSENITE, IN MICROGRAMS PER LITER ARSENIC, IN MICROGRAMS PER LITER ARSENATE, IN MICROGRAMS PER LITER 

C

D

B

A

ARSENATE, IN MICROGRAMS PER LITER 

ARSENITE, IN MICROGRAMS PER LITER

86420

10

OFPS-38 FP1-247
64 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Most confined mixed wells have arsenic concentrations 

similar to that expected from mixing of arsenic concentrations 

in unconfined shallow urban and agricultural wells and 

confined unmixed wells. In confined mixed wells in the 

upper confined aquifer (the study PSW, FP3-162, OFPS-157, 

FP5-175, FP5-LS), arsenic is predominantly arsenate 

(table 21). On a plot of chloride versus arsenic (fig. 24B), 

some confined mixed wells (FP3-162 [2 samples] and 

OFPS-157 [1 sample]) and depth-dependent samples from the 

study PSW (depths 51.8 and 48.8 m,) plot between unconfined 

shallow urban wells and confined unmixed wells such that 

they could indicate mixing of these waters. Mixing fractions 

were not calculated, as arsenate is not necessarily conservative 

and the unconfined and confined end-members concentrations 

are not well-defined. Orthophosphate concentrations were 

<0.3 mg/L in confined mixed wells and <0.2 mg/L in confined 

Figure 24. Arsenic and A. chloride and B. orthophosphate, and C. arsenate and iron concentrations in ground water in the local-scale 

TANC study near York, Nebraska.

0 0.5 1 1.5 2

ARSENIC (As), IN MICROGRAMS PER LITER

0 50 100 150 200

CHLORIDE (Cl),

IN MILLIGRAMS PER LITER

Urban shallow

Unconfined wells

EXPLANATION

Confined wells

Urban deep

Urban unmixed

Urban mixed

Study public supply

 well (Study PSW)

Agricultural shallow

Agricultural deep

Evaporation line

Agricultural mixed

Agricultural unmixed

A

C

B

0

2

4

6

8

10

0

2

4

6

8

10

ORTHOPHOSPHATE AS PHOSPHORUS, 

IN MILLIGRAMS PER LITER 

FP1-63

UWT2-23 UWT2-23

OFPS-38 OFPS-38

1 10 100 1,000

ARSENATE, IN MICROGRAMS PER LITER

0

2

4

6

8

10

IRON (Fe), IN MICROGRAMS PER LITER 

ARSENIC (As), IN MICROGRAMS PER LITER

FP1-63

FP2-250

OFPS-157

SPSW depth

61 m

SPSW depth 51.8 m

FP3-162

OFPS-157

OFPN-276

SPSW depth 

61.0 meters

OFPS-157

OFPS-157

FP1-247

FP1-247

FP3-162

FP5-LS

A B

C
Chemical Characteristics 65

unmixed wells (fig. 24A), indicating that it is unlikely 

that interactions with orthophosphate influenced arsenic 

concentrations in the confined aquifer as apparently occurred 

in the unconfined shallow urban wells.

Interaction of relatively oxidizing well-bore leakage 

water with relatively reducing confined unmixed water has 

the potential to mobilize arsenate, as was hypothesized for 

uranium. The arsenic concentration in the depth-dependent 

sample collected from the bottom of the screen (the study 

PSW 61.0 m bls sample) was 9 μg/L, primarily arsenate 

(fig. 24B). This concentration is greater than was expected 

from conservative mixing of water from unconfined shallow 

urban wells and water from confined unmixed wells (fig. 24B). 

Concentrations of arsenate and iron are significantly correlated 

(ρ = 0.539, p = 0.044) in arsenate-dominated confined mixed

(OFPS-157, FP3-162, FP5-LS) wells and samples from the 

study PSW (fig. 24C). Release of arsenate from reduction of 

iron oxyhydroxides on colloids in water from the unconfined 

aquifer moving down well bores into the confined aquifer may 

be consistent with elevated arsenate in iron reducing confined 

mixed wells (see “Uranium” for a more detailed description of 

the hypothesized mechanism). In laboratory batch and column 

studies, Puls and Powell (1992) found that iron-oxide colloids 

were highly mobile and that the rate of colloid-associated 

arsenate transport was more than 20 times greater than that 

of dissolved arsenate. However, field studies describing the 

role of colloidal transport of arsenate were not identified and 

the importance of colloidal transport for arsenate transport is 

speculative. Variations in arsenate concentrations were much 

more subtle than in uranium concentrations, and variations 

in arsenate concentrations in confined mixed wells and the 

study PSW may simply reflect mixing between unconfined 

aquifer and confined aquifer end-member waters that were not 

sampled. 

The distribution of arsenate- or arsenite-dominance in the 

confined aquifer did not always correspond to classifications 

into confined mixed and confined unmixed wells on the basis 

of stable isotopic, major element, and VOC signatures. For 

example, arsenite was the dominant arsenic species in confined 

mixed wells from the lower confined lenses (FP2-250, 

FP3-218; table 21). Arsenate was the dominant arsenic species 

in confined unmixed well FP1-247, from the lower confined 

lenses (fig. 23D). The deviation of arsenic speciation from that 

expected on the basis of stable isotopic and major element data 

may indicate that variable local sediment/water interactions 

influence arsenic concentrations and speciation more than 

other dissolved constituents. 

Depth-Dependent Sampling

The results of flow measurements and sample collection 

from different depths in the study PSW under typical pumping 

conditions are presented in this section. The depth-dependent 

results for different constituents are discussed together in this 

section to determine the depths where water and solutes enter 

the study PSW.

Flow estimates indicated that 73 to 86% of the total 

flow of water, about 1,930 L/min, entering the study PSW

screen came from the upper half of the screen, above 51.8 m 

bls (fig. 25A). Tracer pulse tests were done in two profiles 

with profile 1 consisting of measurements at 42.7, 45.7, 48.8, 

51.8, 54.9, 57.9, and 61.0 m bls, and profile 2 consisting of 

measurements at 44.2, 47.2, 50.3, 53.3, 56.4, and 59.4 m bls.

Flow measurements determined using the tracer pulse method 

had uncertainties, particularly near the bottom part of the 

screen. Travel times from tracer pulse tests collected at 54.9 m 

bls or below varied considerably. For example, at 54.9 m bls, 

travel times in 6 tests varied from 172 to 181 seconds. This 

range in travel times resulted in flow estimates between 51.8 

and 54.9 m bls ranging from 454 to 549 L/min, an uncertainty 

of about 20%. Below 53.3 m bls, the flow measurements were 

not reproducible enough to be used quantitatively in mixing 

calculations described later in this section. Because most of 

the flow came from the upper half of the well screen, flow 

in the lower parts of the screen may have been more variable 

in response to subtle variations in well operation. Tracer 

pulse travel times were replicated within a few percent at 

53.3 m bls or above. Several additional sources of uncertainty 

potentially affected the flow measurements. The tracer pulse 

method presumes piston movement of dye in the well bore. 

Dye could have been distributed non-uniformly in response 

to turbulence or non-vertical flow components in the well 

bore. Non-piston flow could have affected travel times for dye 

from the injection point to the fluorometer. The calculation 

assumes a constant screen and well diameter; variations in the 

actual diameter could have affected the velocity calculations. 

Because of the relatively moderate depth of the well (<61 m), 

stretching of the hose in the well was probably not a major 

factor in this study. 

Flow and Chemistry Profile Checks

The depth-dependent chemistry data was used to check 

whether the depth-dependent flow estimates were consistent 

with reasonable solute concentrations in aquifer inflow. The 

concentration of a constituent in water that entered the well 

from the aquifer between sample-collection points can be 

calculated, assuming simple mixing within the well, according 

to equation 4 from Izbicki and others (1999, 2005b):

C C Q C Q Q

C and Q are the constituent con

a 2 2 1 1 a

a a

= − [ ]/

where

centration and 

volumetric flux of water entering the wellfrom 

the aquifer between sample-collection depths

C an 1

,

d Q are the constituent concentration and the 

volumetric

1

flux of water in the well at the deeper

sample depth , d an 

C and Q are the constituent concentration and the 

volu

2 2

metric flux of water in the well at the shallower 

sample depth .

(5)
66 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Figure 25. Vertical distribution of flow and concentrations of selected constituents in samples from different depths in the study supply well and adjacent monitoring wells in the local-scale TANC study area near York, Nebraska. Loess Silty clay Confined fine sand Carlile Shale 42.7 m 61.0 m

DEPTH, IN METERS

0 0 50 150 50 100

50

0

1020304060708090

100

CUMULATIVE PERCENT OF TOTAL FLOW

-11 -10 -9 -8 -80 -70 -60 -50

A

B

C

Monitoring well

Surface-discharge sampleDepth-dependent sample

 Aquifer inflow

Upper confined

fine sand

Silty clay till

Unconfined sand

and gravel

Pump intake

Upward flow

Supply well screen 

Monitoring well

INDUCTION, IN MICROMHOS PER METER

Profile 1 (initial estimate)Profile 2 (initial estimate)Profile 1 (from chemistry) Monitoring well

Surface-discharge sampleDepth-dependent sample

 Aquifer inflow

DELTA OXYGEN-18, IN PER MIL (VSMOW) DELTA DEUTERIUM, IN PER MIL (VSMOW)

Water table

39.6 m

FP1-63

FP1-147

FP1-185

FP1-247
Chemical Characteristics 67

D E F

Monitoring well

Surface-discharge sampleDepth-dependent sample

 Aquifer inflow

Loess

Silty clay

Confined fine sand

Carlile Shale

39.6 m

DEPTH, IN METERS

0 150 50 100

0

102030405060708090

Upper confined

fine sand

Silty clay till

Unconfined sand

and gravel

Pump intake

Upward flow

Monitoring well

INDUCTION, IN MICROMHOS PER METER

Water table

42.7 m

61.0 m

Supply well screen 

0 20 40 60 80 100 120

CHLORIDE, IN MILLIGRAMS PER LITER

0 1 2 3 4 5 6 7

PCE, IN MICROGRAMS PER LITER

0 50 100 150 200

URANIUM, IN MICROGRAMS PER LITER

FP1-63

FP1-147

FP1-185

FP1-247

Figure 25.—Continued.
68 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

G

H

I Loess

Silty clay

Confined fine sand

Carlile Shale

DEPTH, IN METERS

0 150 50 100 0

102030405060708090

Upper confined

fine sand

Silty clay till

Unconfined sand

and gravel

Pump intake

Upward flow

Monitoring well

INDUCTION, IN MICROMHOS PER METER

Water table

42.7 m

61.0 m

Supply well screen 

39.6 m

Monitoring well

Surface-discharge sampleDepth-dependent sample

 Aquifer inflow

FP1-63

FP1-147

FP1-185

FP1-247

50 0 100 150 200

CALCIUM, IN MILLIGRAMS PER LITER

0 2 4 6 8 10

ARSENIC, IN MICROGRAMS PER LITER

0 2 4 6 8 10

ARSENSATE, IN MICROGRAMS PER LITER

Figure 25.—Continued.
Chemical Characteristics 69

The concentration in the aquifer is calculated as 

a residual; the calculation assumes that the change in 

concentration or flow over the measurement interval must be 

larger than the measurement error. 

Initial flow estimates calculated in the field from the 

tracer pulse tests in profile 1, which were collected at depths 

corresponding to depth-dependent sample collection, were 

used to calculate aquifer inflow concentrations. However, 

calculated inflow concentrations of many constituents were 

unreasonable (negative or unfeasibly small) in the intervals 

from 51.8 to 48.8 m bls, 48.8 to 45.7 m bls, and 42.7 m bls to 

the surface discharge. Concentrations measured in the surface 

discharge were assumed to represent those at the depth of the 

pump intake (39.6 m bls). All flow in the well was upward 

because the pump intake was located above the screen. 

Between 51.8 and 45.7 m bls, there was substantial inflow, yet 

concentrations of most analytes in depth-dependent samples 

decreased substantially (fig. 25C–G). The unreasonable 

calculated aquifer inflow concentrations implied that the 

initial estimates of inflows of water in the 51.8 to 45.7 m 

interval were unreasonably large or that mixing of water in 

the well bore was incomplete because of the large amount of 

water entering the well over relatively short (3 m) intervals. 

Between 42.7 m bls at the top of the screen and the surface 

discharge, concentrations decreased slightly while flow 

apparently changed very slightly (the initial field calculation 

of flow at the top of the screen was 1,824 L/min, close to 

1,930 L/min, the approximate metered flow of the surface 

discharge). As a result, calculated concentrations in aquifer 

inflow between the top of the screen and the well intake were 

negative or unreasonably small. These results imply that the 

initial estimates of the change in flow between the top of the 

screen and the well intake were too small or that the mixing 

of water entering the well bore at the top of the screen was 

incomplete. In an attempt to reconcile these discrepancies, 

an alternative flow profile was estimated by re-examining 

the tracer pulse data and selecting the longest travel times for 

each of the upper intervals, which resulted in greater estimated 

flow in the upper part of the profile and improved estimated 

aquifer inflow concentrations; however, this approach did not 

eliminate most of the negative or unreasonable values.

For the purposes of constraining estimates of 

concentrations in aquifer inflow and to bound uncertainties 

in the well-bore flow profile, flow values were modified until 

aquifer inflow concentrations in the 51.8 to 45.7 m bls and 

42.7 m bls to well intake intervals were reasonable for most 

constituents, particularly conservative constituents such as 

δ18O, δD, and chloride (fig. 25B–D). These adjusted flow 

values determined from chemical mixing were not unique 

but were constrained by maintaining flow at the pump intake 

within 5% of the metered flow of about 1,930 L/min and by 

maintaining the flow profile as close to the original tracer 

pulse measurements as possible while calculating aquifer 

inflow concentrations that were reasonable compared with 

values for monitoring wells. The resulting estimated flow 

profile based on chemical concentrations is shown in figure 

25A [Profile 1 (from chemistry)] and is not dramatically 

different from the flow profile estimated in the field from the 

tracer pulse measurements [Profile 1 (initial estimate)]. 

Depth Intervals Contributing Solutes to the 

Study Public-Supply Well

Most dissolved solutes entered the well in the bottom half 

of the screen, where a minority of flow enters the well. The 

maximum concentrations of most constituents in the depthdependent samples were from the middle of the screen 

(51.8 m bls [fig. 25B–G]). The second highest concentration 

in depth-dependent samples for most constituents was 

at the bottom of the screen (61.0 m). Consequently, the 

largest calculated concentrations in aquifer inflow were 

for the bottom half of the screen. Lower concentrations of 

constituents in the upper half of the screen indicated that 

water inflow in the upper part of the screen diluted constituent 

concentrations in water from the bottom half of the screen. 

These observations were true for diverse constituents, 

including δ18O, δD, chloride, PCE, uranium, and calcium 

(fig. 25B–G), representing similar patterns observed for most 

stable isotopes, major elements, VOCs, trace elements, and 

radiochemicals. 

Values of conservative tracers δ18O, δD, and chloride 

were consistent with inflow to the bottom half of the screen 

below 51.8 m containing a substantial fraction of unconfined 

shallow urban water and inflow to the top half of the screen 

primarily representing confined unmixed waters (fig. 25B–D). 

Mixing calculations with δ18O and δD values suggest that 

the depth-dependent sample from the middle of the screen 

contained approximately 50% unconfined shallow urban water 

(table 12; fig. 25B–C). 

Minor amounts of water and solutes from the unconfined 

aquifer also may enter the study PSW near the top of the 

screen. There were minor shifts to slightly less negative δ18O 

and δD values in calculated aquifer inflow between 45.7 and 

42.7 m bls (fig. 25B–C). Mixing calculations based on the 

calculated δ18O and δD values for this interval indicate 15 

to 18% unconfined shallow urban and agricultural water. 

Calculated concentrations of PCE in aquifer inflow decreased 

upward above the middle of the screen at 51.8 m bls but were 

still greater than zero throughout the upper half of the screen 

(fig. 25E). A pesticide, acetochlor ESA, also was detected only 

in the depth-dependent sample at 42.7 m bls at the top of the 

screen (table 15; see “Pesticides”). Water from the unconfined 

aquifer may reach the upper parts of the study PSW screen in 

the confined aquifer as a result of drawdown around the well, 

which could induce relatively rapid flow of water through the 

confining unit in response to the larger downward hydraulic 

head gradient near the well than elsewhere. However, the 

amount of water from the unconfined aquifer entering the 

study PSW near the top of the screen was small relative to 

the amount of water from the unconfined aquifer entering 

the well along the bottom half of the screen. Moreover, the 
70 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

unconfined water entering at the top of the screen did not have 

elevated concentrations of solutes like that entering along 

the bottom part of the screen, as concentrations of chloride, 

PCE, uranium, and calcium decreased upward between the 

middle of the screen (51.8 m bls) and the surface discharge. 

These results are consistent with the interpretation that the 

primary mechanism permitting unconfined water to reach 

the bottom part of the study PSW is downward movement of 

unconfined water through multi-layer well bores upgradient of 

the study PSW and that downward movement of unconfined 

water through the upper confining unit near the well, possibly 

in response to very large drawdown induced head gradients, 

has a much smaller effect on study PSW water quality. These 

processes will be discussed further in “Water Sources and 

Pathways.” 

Concentrations of uranium in the three deepest depthdependent samples were ≥30 μg/L (the USEPA MCL for 

drinking water), but decreased to 17 μg/L in surface discharge 

as a result of dilution with aquifer inflow having low uranium 

in the upper part of the screen(fig. 25F). The uranium 

concentration, 184 μg/L, measured in the 51.8 m bls depthdependent sample was more than 3 times greater than the 

next highest concentration measured in the study, including 

concentrations measured in the unconfined shallow urban

wells. The uranium concentration of 44 μg/L in the lowermost 

depth-dependent sample indicated that uranium was present 

at the bottom of the well screen also. Even after adjusting 

the calculated flow profile to produce reasonable aquifer 

inflow concentration estimates for chloride and most other 

constituents, calculated concentrations of uranium in aquifer 

inflow above 51.8 m bls were negative. The unreasonable 

calculated uranium concentrations in aquifer inflow could 

either indicate that some uranium was lost because of reaction 

in the well bore or that the high concentrations of uranium 

entering the bottom part of the screen were incompletely 

mixed within the well bore. The latter is more likely. In 

spite of these uncertainties, the results indicate that uranium 

entering the bottom half of the screen appeared to account for 

all of the uranium entering the supply well. 

Concentrations of calcium, described earlier as being 

correlated with uranium concentrations, also had maximum 

values at the mid-screen 51.8 m bls depth and the second 

highest values at the bottom of the screen (table 20). However, 

calculated aquifer inflow values above the middle of the 

screen were 57 to 67 mg/L, reasonable values similar to those 

measured in samples from most confined unmixed wells, 

including those in the FP1 well nest adjacent to the supply 

well. 

In contrast to profiles of most other constituents, 

depth-dependent profiles for arsenic and arsenate had 

maximum concentrations at the bottom of the screen and only 

moderate changes in concentrations upward through the well 

(fig. 25G–I). Most of the arsenic in the study PSW depthdependent and surface-discharge samples was arsenate. Only 

monitoring wells FP1-147 and FP1-185 had greater arsenite 

than arsenate concentrations. Concentrations of arsenate were 

not greatest at the middle of the screen, like other constituents, 

indicating that arsenate (and arsenic) distributions did not 

follow the distribution of unconfined water as closely as most 

other solutes. 

For most constituents, concentrations in depth-dependent 

samples from the supply well and from short-screened 

monitoring wells located <30 m away in the upper confined 

aquifer were different. The differences probably resulted from 

the three-dimensional convergence of many ground-water flow 

paths on the supply well as a consequence of large withdrawal 

rates, resulting in mixing of many waters in the supply well. 

In contrast, the much lower withdrawal rates and the screened 

intervals in the monitoring wells, even those close to a supply 

well, makes it likely that these wells intersected a much 

smaller number of ground-water flow paths moving from 

one direction toward the supply well. The depth-dependent 

samples from the supply well and the monitoring wells could 

be different because the supply well intersected a groundwater flow path(s) having relatively high concentrations of 

solutes that were not intersected by the monitoring wells. 

Processes Affecting Transport 

of Anthropogenic and Natural 

Contaminants to Public-Supply Wells

In this section, results for all constituents that are useful 

as tracers of water sources and pathways are integrated 

to develop a refined conceptual model of major source, 

movement, and well-effect processes influencing transport 

of selected anthropogenic and natural constituents to PSW 

in the local-scale TANC study area. The “Sources of Water 

and Solutes” section includes a summary of sources of water 

and anthropogenic solutes in the shallow unconfined aquifer 

and sources of uranium and arsenic, natural constituents of 

primary interest, throughout the unconfined and confined 

aquifers. The section “Movement of Water and Solutes 

to Supply Wells” summarizes evidence that the primary 

mechanism permitting contamination-susceptible water from 

the unconfined aquifer to reach the confined aquifers where 

PSWs are screened is well-bore leakage. The section “Supply 

Well Effects on Transport of Water and Solutes” describes the 

effects of pumping wells on water quality in the PSW. 

Sources of Water and Solutes

Precipitation is the primary source of recharge in the 

local-scale TANC study area, followed by return flow from 

irrigation or urban water use, as indicated by values of δD and 

δ18O. The δD and δ18O values for unconfined shallow urban 

and agricultural wells indicate variable mixtures of these 

source waters. Calculated fractions of precipitation recharge 

varied from 47 to 78% for unconfined shallow urban wells and 

66 to 81% for unconfined shallow agricultural wells. 
Processes Affecting Transport of Anthropogenic and Natural Contaminants to Public-Supply Wells 71

Anthropogenic sources influencing unconfined shallow 

urban and agricultural wells may include septic systems, 

leakage from the wastewater system, general industrial or 

commercial use of VOCs, livestock waste disposal, and 

application of fertilizers on agricultural fields. Natural sources 

include desorption or dissolution of uranium and arsenic 

from sediment. The highest concentrations of uranium and 

arsenate were in confined mixed wells in which water from 

the unconfined aquifer mixed with water in the upper confined 

aquifer under iron-reducing conditions. These individual 

sources are described more specifically in the following 

paragraphs. 

Septic systems, having their highest density near the edge 

of the urban area, may be the major anthropogenic source of 

solutes, including nitrate-N, orthophosphate, chloride, sulfate, 

calcium, potassium, and boron, in unconfined shallow urban

wells. These constituents, δ15N-nitrate, and chloride/bromide 

were the primary tracers of septic-system effects. Although 

uranium and arsenic were probably not directly derived from 

septic-system effluents, the association of higher uranium 

and arsenic concentrations with septic-system tracers implies 

that septic-system drainage may enhance mobilization of 

these “natural” contaminants. The distinction between the 

contributions of leakage from the municipal wastewater 

system and from septic-system drainage cannot be resolved 

on the basis of the geochemical data. Elevated values of the 

septic-system tracers listed above were present in samples 

from some wells near the western edge of the urban area 

where the wastewater system was absent. Alternatively, parts 

of the urban area that are far from areas having known or 

suspected septic systems may be influenced by wastewater 

leakage. It is possible for septic systems primarily located 

near the western or upgradient margin of York (fig. 4) to 

have a prevailing influence on ground-water chemistry 

downgradient underneath the York urban area. Mixing of 

septic-system-effected waters with other ground water dilutes 

high concentrations but plumes can persist for a considerable 

distance (Robertson and others, 1991). As the density 

and areal extent of septic systems increase, local dilution 

capacity decreases. Because recharge beneath the York urban 

area is estimated to be less than recharge in the upgradient 

agricultural area (Clark and others, 2008), septic systems 

would represent points of focused recharge at the edge of the 

urban area. 

Sources of VOCs are widely distributed in or near the 

York urban area, as indicated by the widespread occurrence 

of relatively low concentrations of VOCs in unconfined 

shallow urban wells, but not in agricultural wells. The primary 

VOCs detected were the chlorinated solvents PCE and TCE. 

However, 23 other VOCs were detected, including chloroform, 

CFC-113 and CFC-12, benzene, MTBE, and the PCE and 

TCE degradation products cis-1,2 DCE and trans-1,2 DCE. 

Septic systems have been identified as likely sources of 

solvents in ground water (Squillace and others, 2004; Moran 

and others, 2007), so detections of solvents in unconfined 

shallow urban wells are consistent with interpretations 

of septic-system effects on the basis of inorganic tracers. 

However, other sources of VOCs, such as general industrial 

or commercial use of VOCs, could contribute to the 

concentrations of VOCs in unconfined shallow urban wells. 

Livestock waste disposal areas were prevalent upgradient 

of York and may have affected local ground-water chemistry. 

The highest concentrations of nitrate-N measured in the 

study, about 80 mg/L (unconfined shallow agricultural well 

NWT1-39), were measured beneath a pasture historically used 

for manure spreading. Although manure disposal may locally 

impact ground-water quality, the distribution of livestock 

manure areas is limited and the effect on ground-water 

chemistry over larger areas is expected to be minor. 

Application of fertilizers on cultivated fields is probably 

the primary source of nitrate-N influencing unconfined 

shallow agricultural wells. Nitrate is the principal constituent 

of concern in agricultural areas. Nitrate-N concentrations in 

unconfined shallow urban and agricultural wells were similar 

but derived from different sources on the basis of δ15N-nitrate 

values. 

The likely source of uranium in the unconfined aquifer is 

desorption or dissolution from sediment under oxic conditions. 

Sequential extractions of core samples indicated that there was 

generally abundant iron and manganese oxides and uranium 

in aquifer sediments which were sources for uranium to oxic 

ground water. 234U/238U were consistent with uranium in oxic 

unconfined shallow urban and agricultural wells resulting 

from leaching from uranium-bearing sediments, oxides, or 

minerals. Uranium concentrations were mostly 5 μg/L to 

40 μg/L in oxic unconfined shallow urban and agricultural 

wells and and <5 μg/L in reducing wells. Where depth 

to water was less than about 7 m, the unconfined shallow 

urban wells were usually anoxic or more reducing, had pH 

from 5.8 to 6.7, and had low concentrations of bicarbonate. 

These conditions probably resulted from oxidation of soil 

organic matter and percolation of DOC to the water table 

where depth to water was shallow. Uranium concentrations in 

oxic unconfined shallow urban and agricultural wells were 

correlated with several indicators of septic-system influence, 

including chloride, δ15N-nitrate, orthophosphate, potassium, 

and calcium. A direct source of uranium in septic-system 

discharge is unlikely. Rather, higher concentrations of uranium 

in ground water affected by septic systems may result from 

complexes with high calcium and bicarbonate formed in 

response to additional calcite dissolving in ground water 

affected by relatively acidic septic-system discharge.

The highest uranium concentrations in the local-scale 

TANC study area, 44 to 184 μg/L, were in confined mixed

wells that were iron reducing. Uranium concentrations were 

low in confined unmixed wells with similar redox conditions. 

These results suggest that uranium can be mobilized during 

mixing of waters from the unconfined and confined aquifers in 

the upper confined aquifer having iron-reducing conditions. 
72 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Desorption or dissolution of arsenic from sediment, 

enhanced in waters affected by septic-system discharges, 

is a plausible source for arsenic concentrations observed 

in oxic unconfined shallow urban and agricultural wells. 

In oxic unconfined shallow urban and agricultural wells, 

arsenic concentrations were correlated with indicators of 

septic-system effects, including chloride and orthophosphate. 

Direct sources of arsenic in septic-system effluent are not 

likely. Rather it is suspected that concentrations of arsenic 

increase in ground water affected by septic systems as a 

result of competitive desorption of arsenate from sediment in 

the presence of orthophosphate derived from septic-system 

effluent (Welch and Stollenwerk, 2003; Kent and Fox, 2004). 

In confined unmixed wells, arsenic primarily occurred as 

arsenite and may be present in fairly uniform concentrations as 

result of long-term dissolution of iron oxides in glaciofluvial 

sediments and redistribution of iron in the solid phase. In most 

confined mixed wells, arsenic concentrations are consistent 

with mixing of waters from the unconfined aquifer and 

confined aquifers. Some high concentrations of arsenic (as 

arsenate) in confined mixed wells may reflect mobilization 

of arsenic where waters from the unconfined aquifer mix 

with water from the confined aquifer under iron-reducing 

conditions. 

Upward movement of water and solutes from the 

shale underlying the aquifer or interactions with minerals 

in the marine shale may locally affect the water chemistry 

of the lower confined lenses, but are unlikely to affect the 

chemistry of water reaching PSWs. The chemistry of well 

FP2-250 (classified as confined urban mixed well) is unique 

in the local-scale TANC study area, having the highest 

concentrations of total dissolved solids (TDS) (2,030 mg/L), 

sulfate (1,060 mg/L, fig. 19B,C), calcium (309 mg/L, 

fig. 20C), magnesium (71.8 mg/L), sodium (207mg/L), 

potassium (16.2 mg/L, fig. 17B), and boron (480μg/L, 

fig. 17E), making this sample a mixed cation-sulfate type 

water (fig. 16). The TDS in FP2-250 was more than twice that 

of other samples (table 20). Phase equilibrium calculations 

using PHREEQC indicated that FP2-250 was the only well 

in the study area near equilibrium with gypsum (saturation 

index (SI) = −0.4; in all other wells, SI = −1.1 to −2.5). The 

only core sample in which gypsum was detected was from 

the shale (table 8). These results suggest that dissolution 

of gypsum in the shale has affected the chemistry of well 

FP2-250. The high concentrations of cations other than 

calcium could indicate cation exchange. Well FP2-250 

had the lowest δ34S-sulfate (−22.3 per mil) of any sample 

(table 19). This value is in the range expected for sulfate 

derived from oxidation of reduced inorganic sulfur (−30 to 

+5 per mil), but is outside the range of values typical for 

gypsum (+10 to +35 per mil) (Krouse and Mayer, 2000). One 

possible explanation is that the gypsum was derived from the 

weathering of pyrite in the shale; drilling indicated that the 

upper few meters of the shale was weathered. Although the 

shale core was not analyzed for pyrite, pyrite is a common 

trace mineral in marine shale (Krouse and Mayer, 2000), and 

iron concentrations in the core were the highest measured in 

the study (table 9). Under semi-arid to arid conditions, pyrite 

could have been oxidized, and gypsum having δ34S-sulfate 

values similar to those of the original pyrite could have been 

formed. This mechanism is consistent with the low δ34S-sulfate 

values and near saturation with gypsum. Well FP2-250 has 

relatively high chloride/bromide and plots close to literature 

values of waters affected by road salt (fig. 18); this could 

indicate dissolution of geologic halite (rather than halite 

or other chloride minerals applied as road salt), which was 

detected in the shale core (table 8). The chemistry of confined 

urban unmixed well OFPN-276 appeared to be intermediate 

between that of FP2-250 and confined unmixed wells for 

many analytes (figs. 16, 17B,E, 18, 19A,B,D). These results 

suggest that OFPN-276 may be influenced by water affected 

by interactions with the shale mixed with water represented by 

confined unmixed wells. Indicators of interactions of the shale 

with ground water were detected in only 2 of 7 monitoring 

wells (FP2-250, OFPN-276) in the lower confined lenses and 

none of 9 monitoring wells nor the study PSW in the upper 

confined aquifer. 

Movement of Water and Solutes to Supply Wells

Ground water generally moves in a southeasterly 

direction across through the study area (fig. 3) and vertically 

downward due to recharge and withdrawal from pumping 

wells. Because storage is much greater in the unconfined 

aquifer, withdrawals from the confined aquifers inherently 

produce hydraulic head gradients downward from the 

unconfined to the confined aquifers (fig. 6). Because of 

increased recharge and discharge over several decades, 

simulated ground-water ages in the aquifer system became 

younger from the 1950s to the 1980s (Clark and others, 

2008). Historical water-level data (Landon and Turco, 2007; 

Clark and others, 2008), simulation results, and differences 

in δ18O and δD values between unconfined shallow wells and 

confined unmixed wells consistent with changes in seasonal 

recharge proportions all indicate that the ground-water flow 

system has changed substantially accompanying development 

of the aquifer system as a source of irrigation. These changes 

in the aquifer system likely cause shorter travel times and 

greater opportunities for contaminants to move from recharge 

areas to PSWs screened in the confined aquifer compared to 

predevelopment aquifer conditions. However, perturbations in 

the generalized flow pattern, including localized pumping and 

well-bore leakage, can cause mixing and transport of solutes 

in ways that are uncharacteristic of the broader ground-water 

system and further enhance opportunities for contaminant 

transport to PSW, as indicated by detailed model simulations 

of the local-scale TANC study area (Clark and others, 2008). 
Processes Affecting Transport of Anthropogenic and Natural Contaminants to Public-Supply Wells 73

The section summarizes geochemical, physical, and 

simulation evidence that well-bore leakage is the predominant 

process facilitating movement of constituents of concern to 

PSW in the confined aquifers. Alternative hypotheses are 

discussed in comparison with the primary hypothesis of the 

dominant influence of well-bore leakage. In addition, evidence 

for the role of reactions in attenuating movement of selected 

contaminants through the aquifer system are discussed. 

Evidence for Well-Bore Leakage 

Multiple tracers indicate water from the unconfined 

aquifer mixed with water in the confined aquifer at a few 

locations. Confined mixed wells and the study PSW had values 

of δ18O, δD, and chloride, and sometimes nitrate-N, uranium, 

arsenic (arsenate), major elements, VOCs, and young groundwater age tracers intermediate between those from unconfined 

shallow urban and those from confined unmixed wells. These 

wells having intermediate composition appear to represent 

mixtures of waters from the unconfined and confined aquifers. 

Although the chemical signatures of these intermediate waters 

were generally consistent with those of an unconfined shallow 

urban recharge area, the detection of acetochlor ESA, alachlor 

ESA, and alachlor OA—degradation products of herbicides 

used only on agricultural crops—implies that at least a fraction 

of the water from the unconfined aquifer mixed in the confined 

aquifer was also derived from unconfined shallow agricultural

recharge areas. On the basis of δD values, calculated fractions 

of unconfined water in confined mixed wells ranged from 63 

to 82% (table 12; fig. 26). In study PSW depth-dependent 

samples, unconfined mixing fractions ranged from 15 to 46%. 

In the study PSW surface discharge, the unconfined mixing 

fraction was 12% in all 3 samples collected through time 

(table 12). Most monitoring wells in the confined aquifers 

(confined unmixed wells) contained little or no water from the 

unconfined aquifer. Minor differences in unconfined water 

mixing fractions calculated using δD and δ18O indicated 

uncertainties of about ±10%, especially in samples containing 

only small amounts of water from the unconfined aquifer. 

There are potentially several pathways that water from 

the unconfined aquifer could follow to reach the confined 

aquifer. Hypotheses for different pathways are conceptually 

illustrated in figures 27A–C and include (1) leakage down the 

well-bore of the supply well because of an incomplete seal 

of the confining unit or leakage through the confining unit 

adjacent to the well as a result of hydraulic head drawdown 

at the supply well (fig. 27A) (these two mechanisms are 

lumped for this discussion because they would result in a 

similar distribution of water from the unconfined aquifer in 

the confined aquifer), (2) areally uniform leakage through the 

confining unit (fig. 27B), and (3) leakage down well-bores 

in multilayer wells located upgradient of the supply well but 

within its ZOC (fig. 27C). Hypothetical fractions of water 

from the unconfined aquifer in monitoring wells and in a 

depth-dependent sampling profile from the supply well are 

shown in figures 27D,F for these three scenarios. Unconfined 

aquifer water fractions vary conceptually from zero (to the 

left on the fraction of unconfined water graph), expected 

in monitoring wells in the confined aquifer with no mixing 

with water from the unconfined aquifer, to one (to the right 

on the fraction of unconfined water graph) in the unconfined 

aquifer. These hypotheses are described in more detail in the 

paragraphs below.

The presence of water in the confined aquifers from the 

unconfined aquifer (hypothesis 1, figs. 27A,D) is not likely 

a result of downward flow of water between aquifers in or 

adjacent to the supply well itself because (1) the greatest 

abundance of water from the unconfined aquifer would be near 

the top of the screened interval under this scenario; instead, the 

greatest abundance of water from the unconfined aquifer was 

in the bottom half of the screen; (2) water from the unconfined 

aquifer would not be present other monitoring wells in the 

confined aquifers under this scenario; instead, water from the 

unconfined aquifer was observed in five monitoring wells in 

the confined aquifers away from the study PSW. 

Areally uniform leakage through the confining unit 

(hypothesis 2, figs. 27B,E) also is not likely to be the 

mechanism for rapid movement of water from the unconfined 

aquifer to the confined aquifer. Under this scenario, the 

greatest abundance of water from the unconfined aquifer 

would be in monitoring wells near the top of the confined 

aquifer and near the top of the supply well. These patterns 

did not fit the observed data, which indicated highly variable 

fractions of unconfined water in a few monitoring wells in 

the confined aquifer unrelated to vertical position (fig. 26) 

and the greatest amounts of unconfined water in the bottom 

half of the PSW profile rather than the top (fig. 25). A hybrid 

of this scenario is water from the unconfined aquifer moving 

through the confining unit along natural preferential flowpaths 

unrelated to multilayer wells. This mechanism will be 

discussed further later in this section. 

Several observations suggest that water from the 

unconfined aquifer moves directly to the confined aquifer 

within the ZOC of the study PSW through multilayer 

wells open to both aquifers or through unsealed well-bores 

connecting the aquifers (hypothesis 3, figs. 27C,F). The 

evidence includes the observed distribution of tracers of 

water from the unconfined aquifer in the confined aquifer, the 

existence of physical factors that facilitate downward leakage 

through multilayer wells or well-bores, results of model 

simulations of the local-scale TANC study area (Clark and 

others, 2008) consistent with tracer distributions, and previous 

investigations that have identified well-bore leakage through 

multilayer wells as a mechanism capable of influencing water 

and chemical fluxes across confining units. These categories 

of supporting evidence are discussed in the paragraphs below. 
74 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Monitoring well

Surface-discharge sample

Depth-dependent sample

EXPLANATION

0

10

20

30

40

50

60

70

80

90

DEPTH, IN METERS

A B

0

10

20

30

40

50

60

70

80

90

DEPTH, IN METERS

C D

UNCONFINED WATER FRACTION

USING DELTA DEUTERIUM

UNCONFINED WATER FRACTION

USING DELTA DEUTERIUM

0 4020 60 80 100 0 4020 60 80 100

0 4020 60 80 100 0 4020 60 80 100

UNCONFINED WATER FRACTION

USING DELTA OXYGEN-18

UNCONFINED WATER FRACTION

USING DELTA OXYGEN-18

Figure 26. Vertical distribution of unconfined mixing fractions in samples from different depths in the confined layer in the local-scale 

TANC study area near York, Nebraska: A. values from oxygen isotopes in and adjacent to the study public-supply well, B. values from 

oxygen isotopes in other monitoring wells in confined aquifers, C. values from hydrogen isotopes in and adjacent to study public-supply 

well, D. values from hydrogen isotopes in other monitoring wells in confined aquifers. 
Processes Affecting Transport of Anthropogenic and Natural Contaminants to Public-Supply Wells 75 Figure 27. Scenarios for movement of unconfined water into the confined aquifer in the local-scale TANC study area near York, Nebraska. 

MW MW SW MW MW SW MW LW MW SW

MW

SW

LW

Monitoring well cluster Relatively rapid movement of water from the

unconfined to the confined aquifersRelatively slow movement of water from theunconfined to the confined aquifers (matrix flow through the confining unit)

Lateral flow of water in the confined aquifer

Well screen

Cased section of well Supply well

Leaky well (multi-layer)Unconfined aquiferConfining unitConfined aquifer

Confined

Unconfined heads

heads

A B C

EXPLANATION
76 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska MW MW SW MW MW SW MW LW MW SW MWSWLW Monitoring well cluster Relatively rapid movement of water from the unconfined to the confined aquifers Relatively slow movement of water from the unconfined to the confined aquifers (matrix flow through the confining unit) Lateral flow of water through the confined aquifer Upward flow of water in supply well to intake Well screen Fraction of water from unconfined aquifer in adjacent monitoring well Profile of fraction of water from unconfined aquifer in supply well with upward flow to intake above confined aquifer Cased section of well Supply well Leaky well (multi-layer) Unconfined aquifer Confining unit Confined aquifer Confined Unconfined heads 0 1 Fraction of unconfined water 0 1 0 1 Fraction of unconfined water 0 1 Fraction of unconfined water 0 1 0 1 Fraction of unconfined water 0 1 Fraction of unconfined water Fraction of unconfined water Fraction of unconfined water Fraction of unconfined water 0 1 0 1 Fraction of unconfined water heads

D

E

F

EXPLANATION

Figure 27. —Colntinued.
Processes Affecting Transport of Anthropogenic and Natural Contaminants to Public-Supply Wells 77

The presence of water from the unconfined aquifer in 

a few monitoring wells in the confined aquifer is consistent 

with water from the unconfined aquifer flowing down 

multilayer wells or leaky well-bores upgradient of the supply 

well (hypothesis 3, figs. 27C,F). The chemical, isotopic, and 

age-distribution data for confined mixed wells and depthdependent samples from the study PSW are consistent with 

this hypothesis. Because these multilayer wells or leaky 

well-bores represent discrete points where water from the 

unconfined aquifer can move into the confined aquifer, rather 

than an areally distributed source, a heterogeneous distribution 

of mixed waters in a few wells in the confined aquifer would 

be expected, and was observed (fig. 27F). Wells located near 

or downgradient of leaky multilayer wells or well-bores would 

contain mixtures of water from the unconfined and confined 

aquifers. Wells located away from these points would contain 

unmixed water from the confined aquifer. This heterogeneous 

distribution of mixed waters fit the observed pattern, in which 

highly mixed waters were in 5 of 15 monitoring wells in the 

confined aquifer (fig. 26). Subsequent lateral movement of 

the water leaked from the well-bore toward the supply well 

and downward displacement because of matrix flow of old 

water through the confining unit could allow water from the 

unconfined aquifer to flow to the lower part of the supply 

well screen, as was observed. Leakage along multilayer wells, 

if fully penetrating the upper confined aquifer, also could 

facilitate vertical distribution of water from the unconfined 

aquifer throughout the upper confined aquifer. 

Tracer results are consistent with multiple pathways, such 

as multilayer well bores, between water in the unconfined 

aquifer and zones of mixing in the confined aquifer. The study 

PSW and confined mixed wells did not plot along the same 

mixing lines on tracer-tracer plots such as δ18O and chloride 

and δ18O and sulfate (figs. 19A,B). These results imply that 

waters from the unconfined aquifer in confined mixed wells 

and the study PSW represent different sources. 

Data collected at multilayer test well FP5-LS (classified 

as confined agricultural mixed well) confirm that the physical 

conditions of a downward hydraulic head gradient and open 

pipe permit flow of sufficient water such that water from the 

well bore in the confined aquifer has the chemical signature of 

the unconfined aquifer. There were downward hydraulic head 

gradients from the unconfined to the upper confined aquifer 

at the FP5 well nest (fig. 6F). A sample collected near the 

top of the well screen in the upper confined aquifer (sample 

FP5-LS) had chemistry very similar to that of unconfined 

shallow agricultural well FP5-45 (tables 12, 20). In addition, 

profiles of specific conductance measured every 3 m in 

FP5-LS indicated that, in the upper confined aquifer, values 

throughout the well profile were intermediate between those 

of FP5-45 and FP5-175. Specific conductance values were 

closer to that of the FP5-45 October 2004 sample than that 

of FP5-175 but moved closer to FP5-175 with depth. These 

results are consistent with downward movement of water from 

the unconfined aquifer to the confined aquifer down the well. 

Physical conditions permitting vertical leakage through 

wells or well-bores in the local-scale TANC study area include

(1) large vertical downward hydraulic-head gradients 

between the unconfined and confined aquifers that result from 

withdrawals, particularly for irrigation during the summer, 

from the confined aquifers (fig. 6) and 

(2) the presence of irrigation wells, some commercial 

wells, and even some older PSWs screened in both the 

unconfined and confined aquifers. Well construction details 

beyond well depth were unknown for some private wells. 

However, available water-level and drawdown data from 

well registration records (Nebraska Department of Natural 

Resources, 2002) were consistent with many wells with 

bottoms in the confined aquifer also having screens in 

the unconfined aquifer. Wells whose construction records 

indicated they were screened only in the confined aquifers 

had lower hydraulic heads and greater drawdowns than wells 

screened only the unconfined aquifer or multilayered wells 

screened in both the unconfined and confined aquifer. The 

hydraulic behavior of multilayer wells was confirmed by 

results from multilayer test well FP5-LS, which had hydraulic 

heads similar to those of adjacent wells FP5-45 and FP5-73, 

screened only in the unconfined aquifer (fig. 6F). Clark and 

others (2008) show the locations of multilayered wells and 

leaky well-bores where downward leakage was simulated for 

the local-scale TANC study area. Seasonally or perennially 

inactive or little used irrigation, commercial, or supply wells 

may be particularly important as locations of well-bore 

leakage because withdrawals from the confined aquifer would 

be less likely to withdraw vertical well-bore leakage at these 

locations (Clark and others, 2006). 

Results of a numerical ground-water-flow and solutetransport model of the local-scale TANC study area are 

consistent with the interpretation that the movement of 

contamination-susceptible water from the unconfined aquifer 

to the PSWs is primarily controlled by leakage through 

multilayer wells and bore holes (Clark and others, 2006; Clark 

and others, 2008). Well-bore leakage in multilayer wells and 

known abandoned wells and test holes needed to be included 

in the model to obtain a reasonable comparison of simulated 

and observed age distributions and CFC concentrations in the 

confined aquifer (Clark and others, 2008). The simulations 

indicated that about 25% of the water flow through the upper 

confining unit moved through leaky well-bores rather than as 

leakage through the confining layer itself. Although multilayer 

well and bore-hole flow accounted for a minority of the total 

flow from the unconfined to the upper confined aquifer, it 

accounts for all of the flow of relatively young, contaminationsusceptible water to the confined aquifer in the simulations. 

Simulated ages of water moving through the upper confining 

unit are hundreds to thousands of years (Clark and others, 

2008), consistent with apparent carbon-14 dates for confined 

unmixed wells in the lower confined lenses (FP1-247, 

FP4-231; table 16). This relatively old water moving by matrix 

flow through the upper confining unit dilute the younger 

contamination-susceptible water moving from the unconfined 
78 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

aquifer into the confined aquifer through well-bores. These 

simulations were consistent with the tracer data discussed 

in this report, both indicating that well-bore leakage enables 

contamination-susceptible water to reach depths of the PSW. 

Without this well-bore leakage, the confined aquifer would 

have contained relatively old water that would not be expected 

to contain anthropogenic tracers. These old waters are 

represented by confined unmixed wells which contained few or 

no young water tracers and had a relatively uniform chemistry 

consisting of lower concentrations of most constituents 

compared with confined mixed wells.

Previous modeling investigations have identified vertical 

leakage through multilayer wells as a process that can 

influence the water balance of confined aquifers on a regional 

scale (Williamson and others, 1989; Hanson and others, 2004). 

Konikow and Hornberger (2006) ran hypothetical simulations 

of solute transport that considered leakage of water between 

unconfined and confined aquifers through multilayered wells 

that indicated such leakage could have a substantial local 

effect on solute concentrations in the confined aquifer. 

Several points of evidence indicate that well-bore leakage 

is a critical process facilitating transport of contaminationsusceptible water to the confined aquifer. However, it is 

possible that other preferential flow paths through the 

confining unit may exist. Water and solute often move 

through aquitards faster than expected as a result of sediment 

heterogeneity, discontinuous confining layers, cracks, burrows, 

or other anomalous features in sediments. Although the 

confining unit was continuous across the local-scale TANC 

study area according to drilling for this study and historical 

drilling logs, discontinuities that have not been identified 

may be in the confining unit. No cracks, burrows, or other 

heterogeneities likely to penetrate through the 8 to 17 m 

thickness of the confining unit were observed during coring 

of the confining unit; however, given the small number of 

places where cores were collected in the confining unit, it 

cannot be ruled out that heterogeneities that permit water and 

solute movement across the confining unit exist. However, it is 

simpler to consider that rapid interlayer flow occurs in known 

preferential flow paths, multilayer wells crossing the confining 

unit, rather than in unknown natural preferential flow paths. 

Leaky well-bores are a point-source of potential 

contamination to the confined aquifer. Well-bore leakage 

water is expected to mix with water in the confined aquifer 

downgradient of the leaky well-bore. The direction and 

rate of downgradient movement of mixed water from leaky 

well-bores depend upon local aquifer stresses. Changes in 

flow direction resulting from changing stresses can result in 

temporal variability of concentrations in the confined aquifer. 

For example, in confined urban mixed well OFPS-157, δ18O 

and δD (table 12), nitrate-N (table 18), chloride (table 20), 

and redox conditions (fig. 9) varied considerably. Calculated 

unconfined mixing fractions based on δD declined from 71 

to 41% from November 2003 to April 2005 in OFPS-157 

(table 12). Monitoring wells in the confined aquifer such 

as OFPS-157 may have chemistry that varies temporally 

because they are relatively far from a leaky well-bore, where 

considerable mixing with water from the confined aquifer 

would occur. 

Effects of Attenuation Reactions

Reactions in reducing ground-water in the confining unit 

and in the confined aquifer have the potential to attenuate 

transport of anthropogenic nitrate, PCE, and TCE, and 

probably natural uranium and arsenic. Thus the reducing 

conditions in the confined aquifer potentially are a natural 

protection for the water quality of PSWs screened in the 

confined layer. 

Values of δ15N and δ18O of nitrate and excess N2 gas and 

initial δ15N-nitrate values derived from isotopic mass balance 

calculations on the basis of δ15N-N2 values indicated that 

nitrate was denitrified in reducing parts of the unconfined and 

confined aquifers (McMahon and others, 2008). Analysis of 

pore-water chemistry in the upper confining unit indicated that 

nitrate was completely denitrified across the upper 3 to 4 m 

of the aquitard (McMahon and others, 2008). In the confined 

aquifers, confined unmixed wells had little or no nitrate-N or 

excess nitrogen gas from denitrification, indicating that these 

“old” waters never had significant nitrate-N. Confined mixed 

wells had nitrate-N concentrations ranging from 0 to 13 mg/L 

and were partially to completely denitrified.

Reductive dechlorination of PCE and TCE may be 

occurring in the confined aquifer, and perhaps in the reducing 

parts of the unconfined aquifer, as evidenced by the presence 

of the degradation products cis- and trans-1,2 DCE. However, 

these reactions appear to be partial, as PCE and TCE were 

detected in the local-scale study PSW in addition to three 

other PSWs in York sampled as part of another study (SWQA) 

in 2002. 

However, these natural attenuation mechanisms may be 

bypassed by short-circuit pathways in the form of leaky wellbores through the confining layer and pumping in the confined 

aquifers, allowing water and contaminants to move more 

quickly from source areas to PSWs than would otherwise 

occur. The short-circuit pathways may permit contaminants in 

the unconfined aquifer to bypass extended contact with finetextured sediments in the confining unit, where contaminants 

are be expected to degrade because of the reducing conditions 

and low-ground-water-flow velocity through these lowpermeability sediments (McMahon and others, 1999; 

McMahon, 2001b; McMahon and others, 2008). 

Several supply wells in the city of York have been taken 

off line because of nitrate-N concentrations greater than the 

USEPA MCL of 10 mg/L. These results suggest that the 

nitrate-N can move from the unconfined aquifer to PSWs 

quickly enough to prevent denitrification sufficient to reduce 

concentrations below the MCL. 
Summary 79

Supply-Well Effects on Transport of Water and 

Solutes

The withdrawals in the study PSW probably drew 

contamination-susceptible water towards the well, but 

concentrations of constituents of concern (primarily uranium, 

PCE, and TCE) were diluted with unmixed water entering 

the well from the confined aquifer. Mixing calculations 

using δ18O, δD, SF6, and CFC-11 indicated that the study 

PSW contained 7 to 14% water from the shallow unconfined 

aquifer (tables 12 and 16). Thus, most of the water reaching 

the study PSW was unmixed water from the confined aquifer, 

most of which was older than 50 to 60 years. Concentrations 

of uranium above of the EPA MCL of 30 μg/L in depthdependent samples were diluted with unmixed water from the 

confined aquifer so that uranium concentrations in the surface 

discharge were about 17 μg/L. The occurrence of greater 

concentrations of VOCs and uranium in depth-dependent 

and surface-discharge samples from the PSW than in shortscreened monitoring wells located <30 m away in the same 

confined aquifer suggests that contamination-susceptible water 

was drawn to the supply well along particular flow paths that 

converged on the supply well as a result of pumping but were 

not intersected by the monitoring wells. 

Because most of the drawdown in the confined 

aquifer was a result of regional withdrawals for irrigation, 

contamination-susceptible water is expected to leak down 

multi-layer well bores regardless of local pumping stress 

caused by PSW withdrawals. Irrigation withdrawals were 

much greater than withdrawals for public supply and resulted 

in regional seasonal declines in hydraulic heads in the 

confined aquifer. As long as there are multi-layer wells or 

leaky well-bores, rapid downward movement of water from 

the unconfined aquifer to the confined aquifer can occur, 

regardless of local pumping stress from the PSW. Thus, it 

is expected that local pumping stress in the PSW will have 

little effect on the amount of well-bore water leaking from 

the unconfined to the confined aquifer. Pumping stress in the 

PSW, relative to other withdrawal wells, is expected to affect 

the rate and direction of movement of well-bore leakage once 

it is in the confined aquifer. 

Although the depth-dependent results indicate that 

water from the unconfined aquifer primarily reached the 

bottom of half of the PSW screen, drawdown at the PSW 

may also induce movement of a small amount of water from 

the unconfined aquifer through the confining unit into the 

upper confined aquifer, where it enters the PSW near the top 

of the screen. Minor shifts in δ18O and δD values and small 

increases in arsenic, arsenate, bicarbonate, and excess N2

gas near the top of the screen in depth-dependent samples 

were consistent with a small increase in the amount of water 

from the unconfined aquifer near the top of the screen. 

However, movement of water from the unconfined aquifer to 

the study PSW by well-bore leakage upgradient in the ZOC 

is the predominant mechanism for moving contaminationsusceptible water to PSW.

Historical data from the city of York indicates that some 

PSWs screened only in the confined aquifers have periodically 

had nitrate-N concentrations above or near 10 mg/L. Thus, 

nitrate-N was able to reach PSW in some cases without being 

denitrified or diluted. This may result from the combined 

effects of short-circuit flow paths through leaky well bores 

and pumping that increased ground-water velocity sufficiently 

to prevent denitrification or dilution from reducing nitrate-N 

concentrations below 10 mg/L. It is likely that nitrate-N 

concentrations in PSWs were influenced by the number of 

well-bore leakage points in the ZOC, the distance from the 

PSW, and the relative pumping stress imposed by the PSW 

compared with that imposed by other withdrawal wells.

Summary

This report describes the hydrogeology and geochemistry 

of the local-scale TANC (Transport of Anthropogenic and 

Natural Contaminants) study area near York, Nebraska 

and discusses factors that may control transport of selected 

anthropogenic and natural contaminants to a PSW (publicsupply well). 

The study area is 108 square kilometers in east-central 

Nebraska within an area containing Quaternary-age alluvial 

deposits of the High Plains aquifer in eastern Nebraska. 

Irrigated agriculture is the primary land use. Ground water 

is the sole source of irrigation and drinking water. Many 

irrigation wells, commercial wells, and some older PSWs 

are screened in both the unconfined and the confined 

aquifers, which can allow localized transfer of water from the 

unconfined to confined aquifer. Most York municipal wells are 

screened in the confined aquifers only.

A network of short-screened monitoring wells was 

installed throughout the zone of contribution for the selected 

study PSW. Criteria used to select the study PSW included 

representative and well-understood hydrogeologic, land use, 

and operational conditions, and the presence of detectable 

anthropogenic and natural compounds of concern. Samples 

were collected from monitoring wells and the study PSW, 

and were analyzed for a broad suite of analytes, including 

isotopic tracers, indicators of oxidation-reduction condition, 

ground-water age tracers, VOCs (volatile organic compounds), 

pesticides, nutrients, major elements, and trace elements, 

including uranium, arsenic, and species of arsenic. Core 

samples were collected and analyzed to characterize the 

chemical, mineralogical, and physical properties of the 

major hydrogeologic units. Flow and chemistry data were 

collected from different depths in the study PSW under 

pumping conditions to refine understanding of where water 

and contaminants entered the supply well. On the basis 

of hydrogeologic unit, land use, depth, and δ18O (delta 

oxygen-18) and δD (delta deuterium) signatures, wells were 

classified into categories for the purpose of relating water 
80 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

quality to hydrogeologic setting and mixing of waters from the 

unconfined and the confined aquifers.

Values of δ18O and δD in unconfined shallow urban

and agricultural wells reflected differing proportions of 

recharge from precipitation and recharge from return flow 

of ground water pumped for irrigation or urban supply. 

Values for confined mixed and depth-dependent and surfacedischarge samples from the study PSW reflected water from 

the unconfined aquifer mixed with water from the confined 

aquifer (represented by confined unmixed wells). More 

negative δD and δ18O values for confined unmixed wells, 

recharged more than sixty to thousands of years ago, to less 

negative values for unconfined shallow urban and agricultural

wells, recharged in the last few decades, indicate historical 

changes in the composition of recharge water. These isotopic 

changes most likely result from an increase in the fraction 

of recharge derived from spring and summer precipitation 

over irrigated agricultural and urban land during the last few 

decades compared to seasonal recharge proportions before 

ground-water irrigation. 

Water in most of the unconfined aquifer was oxic. There 

was more-reducing ground water in a few unconfined shallow 

urban wells where the depth to water was less than (<) 7 m, 

probably because of dissolved organic carbon leaching 

from natural organic matter in soils. Water in most of the 

confined-aquifer wells was nitrate- to iron- to sulfate-reducing. 

Confined mixed wells were generally less reducing than 

confined unmixed wells. 

Interpretations of apparent ground-water age from tracer 

data, including CFC-12, CFC-11, CFC-113, SF6, 3H/3He, 

and 3H, were complicated by mixing of waters of different 

ages, possible natural (3He, SF6) and anthropogenic (CFCs) 

contamination of ground water, and degradation of CFCs 

under reducing conditions in a few samples. In the unconfined 

aquifer, age-tracer concentrations could be reasonably 

interpreted using a piston-flow model. Apparent recharge 

ages increased from 7 to 10 years old at shallow depths, 15 

to 20 years at intermediate depths, and 44 to 48 years at the 

bottom of the unconfined aquifer. Age-tracer data for the 

confined aquifers were consistent with mixing of “old” water 

(not containing modern tracers, recharged more than 60 years 

before present) with exponentially-mixed young water. 

Tracer results and mixing calculations indicated that confined 

unmixed wells contained <3% young water mixed with a much 

larger fraction (greater than or equal to [≥] 97%) of old water 

with no modern tracers. Mixing calculations for confined 

mixed wells indicated young fractions ranging from 32 to 

70%, with mean ages for the young fractions of 12 to 14 years. 

VOCs, primarily chlorinated solvents PCE and TCE, 

were detected in most unconfined shallow urban wells, some 

confined urban mixed wells, and in the study PSW, but were 

generally not detected in unconfined shallow agricultural

wells. Thus, the presence of VOCs in confined mixed

wells indicated that the water was affected by urban land 

use. Sources of VOCs may have included septic systems, 

commercial and industrial uses, wastewater, and (or) spills 

along transportation corridors. Reductive dechlorination of 

PCE and TCE may be occurring in the confined aquifer, 

and perhaps in reducing parts of the unconfined aquifer, as 

evidenced by the presence of the degradation products cis- and 

trans-1,2 DCE. However, these reactions appear to be partial, 

as PCE and TCE were detected in the local-scale study PSW 

in addition to three other PSWs in York sampled as part of 

another study in 2002. Degradation products of agricultural 

pesticides acetochlor and alachlor detected in some confined 

mixed wells suggests that some fraction of the water in these 

wells was recharged in agricultural areas.

Septic systems to the west of or near the upgradient 

(western) edge of York appear to be a major contributor 

to nitrate-N concentrations, which were 13 to 26 mg/L 

(milligram per liter) in unconfined shallow urban wells, 

but sanitary-sewer leakage in the urban area also could 

contribute to urban nitrate loading. In the agricultural 

areas, the primary source of nitrate in unconfined shallow 

agricultural wells was probably agricultural fertilizer 

application. Locally, nitrate-N concentrations were high (up 

to 80 mg/L as nitrogen) downgradient of livestock wastedisposal areas. Combined, these multiple sources of nitrate 

usually resulted in concentrations of nitrate-N ≥10 mg/L 

(the U.S. Environmental Protection Agency's maximum 

contaminant level) in unconfined shallow wells. Values of 

δ15N and δ18O of nitrate and excess N2

 gas and initial δ15Nnitrate values derived from isotopic mass balance calculations 

on the basis of δ15N-N2 values indicated that nitrate was 

denitrified in reducing parts of the unconfined and confined 

aquifers. Nitrate-N concentrations in the study PSW were 

<0.2 mg/L because of denitrification and mixing with confined 

unmixed waters having little or no nitrate-N. Because of the 

widespread distribution of nitrate sources, man-made points of 

leakage through the confining unit, and high pumping stress 

in the confined aquifer, nitrate is likely to continue to be the 

anthropogenic compound of greatest concern in the study area.

Associations between concentrations of chloride, initial 

δ15N-nitrate, potassium, orthophosphate, sulfate, boron, and 

chloride/bromide suggest that the primary source of relatively 

high values of these constituents and isotopic signatures 

in unconfined shallow urban wells was septic-system and 

(or) sanitary-sewer leakage. These two sources cannot be 

distinguished on the basis of available information and 

geochemical signatures. 

Several inorganic, isotopic, and organic constituents 

showed a contrast between unconfined shallow urban, 

unconfined shallow agricultural, and confined unmixed endmember water compositions and mixtures of these different 

end-members. The analyses indicated that waters from the 

unconfined aquifer mixing with water from the confined 

aquifer in the study PSW were derived from an urban source 

area and that waters in the confined mixed monitoring wells 

and the study PSW represent different sources. These results 

imply that there are multiple pathways for water from the 

unconfined aquifer to zones of mixing in the confined aquifer. 
References 81

The predominant transport process controlling movement 

of potential contaminants to the study PSW was well-bore 

leakage. Redox processes affect distributions of many 

constituents; however, the influence of redox processes may be 

lessened by pumping stress and short-circuit pathways, such as 

well-bore leakage that can allow water to bypass the reducing 

confining unit. 

In oxic unconfined shallow urban and agricultural

wells having concentrations of 5 to 40 μg/L (microgram per 

liter) of uranium, the predominant source of uranium was 

probably desorption or dissolution from sediment. Uranium 

concentrations were highest in oxic conditions in which pH, 

dissolved oxygen, and bicarbonate are not decreased by a 

shallow depth to water and interactions with soil organic 

matter. Solubility of uranium was probably enhanced by 

forming complexes with dissolved calcium and bicarbonate. 

Although septic systems are not likely to be a source of 

uranium, concentrations of uranium in oxic unconfined 

shallow urban wells were highest in those wells more strongly 

affected by septic systems, possibly as a result of forming 

complexes with higher concentrations of calcium in ground 

water affected by septic systems. Uranium concentrations 

in confined unmixed wells were <5 μg/L. However, uranium 

concentrations of 44 to 184 μg/L were detected where 

unconfined water that entered the confined aquifer as wellbore leakage water mixed with iron-reducing water in the 

confined aquifer. The mechanism causing the high uranium 

concentrations in iron-reducing confined mixed waters is not 

known. A possible mechanism is iron-oxides transported with 

fine sediment particles or colloids from the unconfined aquifer 

with well-bore leakage undergoing reductive dissolution 

under iron-reducing conditions in the confined aquifer, 

releasing uranium. The uranium complexes with calcium 

and bicarbonate and is able to remain mobile in the reducing 

confined aquifer. While this hypothesis cannot be tested 

without additional data, it is consistent with the available data. 

In oxic unconfined shallow urban wells, concentrations 

of arsenic (arsenate) were greatest in wells having indicators 

of septic-system effects such as chloride, initial δ15N-nitrate, 

chloride/bromide, potassium, and orthophosphate. The 

association of higher arsenic with tracers of septic-system 

effects could result from competitive desorption of arsenate 

from sediment in the presence of orthophosphate derived 

from septic-system effluent. In confined unmixed wells, 

arsenic primarily occurred as arsenite in fairly uniform 

concentrations of about 3 to 7 μg/L that may result from longterm dissolution of iron oxides deposited with the glaciofluvial 

sediments and redistribution of iron in the solid phase. In 

most confined mixed wells, the arsenic was primarily arsenate. 

Arsenic concentrations in most confined mixed wells are in 

a range where they could reflect mixing of waters from the 

unconfined and confined aquifers. 

Acknowledgements

We wish to thank the land owners, public and private, 

who granted permission for wells to be installed on their 

property. We were greatly assisted in this study by cooperation 

with the city of York and the UBBNRD. We are particularly 

indebted to Orville Davidson and Ken Eckler with the city of 

York and Rod DeBuhr, Jay Bitner, Dan Leininger, and Marie 

Krausnick at the UBBNRD. We are indebted to the many 

members of the TANC study team who contributed material 

and intellectual assistance with this study. 

References

Alexander, R.B., and Smith, R.A., 1990, County-level 

estimates of nitrogen and phosphorus fertilizer use in the 

United States, 1945 to 1985: U.S. Geological Survey OpenFile Report 90-130, available online at http://pubs.usgs.gov/

of/1990/ofr90130/report.html

Allen, H.E., Fu, G., Boothman, W., Di Toro, D.M., and 

Mahony, J.D., 1991, Determination of acid volatile sulfide 

and selected simultaneously extractable metals in sediment: 

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Tables 91

Tables
92 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Well name Site identification 

number

Latitude

NAD (83)

Longitude

NAD (83)

Measuring point Altitude of 

land surface

NAVD (88)

(m)

Depth 

to top of 

screen,

bls

(m)

Length of 

screen

(m)

Depth to 

bottom of 

screen,

bls

(m)

Length 

of sump 

below 

screen,

(m)

Altitude Depth

NAVD (88)

(m)

bls

(m)

FP1-63 405130097344802 40 51 30.0 97 34 48.5 490.43 0.09 490.52 17.80 1.52 19.32 1.65

FP1-147 405130097344803 40 51 29.9 97 34 48.5 490.33 0.08 490.40 43.41 1.52 44.94 1.65

FP1-185 405130097344804 40 51 29.4 97 34 48.3 489.80 0.09 489.89 54.91 1.52 56.43 1.60

FP1-247 405130097344805 40 51 29.7 97 34 48.4 490.05 0.12 490.17 73.85 1.52 75.37 1.60

FP3-33 405146097353001 40 51 45.9 97 35 29.8 485.88 0.09 485.97 8.31 1.52 9.83 1.63

FP3-83 405146097353002 40 51 46.0 97 35 29.9 485.75 0.11 485.86 23.74 1.52 25.27 1.60

FP3-130 405146097353003 40 51 45.7 97 35 29.8 485.90 0.08 485.97 38.13 1.52 39.65 1.65

FP3-162 405146097353004 40 51 45.9 97 35 29.8 485.85 0.09 485.94 47.81 1.52 49.34 1.60

FP3-218 405146097353005 40 51 45.5 97 35 29.8 485.83 0.08 485.90 64.75 1.52 66.27 15.37

FP4-28 405205097361701 40 52 04.6 97 36 16.1 488.00 0.11 488.11 7.01 1.52 8.53 1.60

FP4-83 405205097361702 40 52 04.5 97 36 16.1 487.88 0.14 488.02 23.99 1.52 25.51 1.60

FP4-168 405205097361703 40 52 04.4 97 36 16.0 487.69 0.11 487.80 49.74 1.52 51.27 1.59

FP4-231 405205097361704 40 52 04.7 97 36 16.0 488.04 0.08 488.12 68.44 1.52 69.96 24.46

OFPN-88 405151097350901 40 51 50.8 97 35 08.8 497.70 0.11 497.81 25.30 1.52 26.83 1.60

OFPN-167 405152097351402 40 51 51.9 97 35 13.2 493.54 0.06 493.60 49.44 1.52 50.96 1.60

OFPN-276 405151097350903 40 51 50.7 97 35 08.7 497.89 0.11 497.99 82.64 1.52 84.17 1.68

OFPS-38 405131097351401 40 51 31.8 97 35 14.1 482.38 0.06 482.44 9.98 1.52 11.51 1.60

OFPS-157 405131097351402 40 51 31.7 97 35 14.0 482.41 0.06 482.47 46.28 1.52 47.80 1.68

OFPS-225 405131097351403 40 51 30 97 35 14 484.18 0.08 484.25 66.72 1.52 68.25 2.32

OFPS-277 405131097351404 40 51 30 97 35 14 484.06 0.08 484.13 81.26 3.05 84.31 0.85

FP2-43 405134097355301 40 51 34 97 35 53 488.68 0.09 488.77 11.55 1.52 13.08 1.67

FP2-78 405134097355302 40 51 34 97 35 53 488.69 0.11 488.80 22.24 1.52 23.77 1.37

FP2-152 405134097355303 40 51 34 97 35 53 488.93 0.08 489.00 44.88 1.52 46.40 2.33

FP2-250 405134097355304 40 51 34 97 35 53 488.85 0.08 488.92 74.67 1.52 76.20 2.41

FP5-45 405224097382201 40 51 35.0 97 35 51.2 496.57 -0.79 495.78 12.28 1.52 13.80 1.67

FP5-73 405224097382202 40 52 24 97 38 22 496.74 –0.91 495.83 20.73 1.52 22.25 1.68

FP5-175 405224097382203 40 52 24 97 38 22 496.59 –0.81 495.78 51.78 1.52 53.30 2.33

FP5-LS 405224097382204 40 52 23.5 97 38 19.3 496.40 –0.91 495.48 6.10 18.29 24.38 na

FP5-LS 405224097382204 40 52 23.5 97 38 19.3 496.40 –0.91 495.48 39.62 21.33 60.96 3.05

AWT1-83 405314097373201 40 53 14.5 97 37 30.9 507.76 0.09 507.85 24.13 1.37 25.50 1.68

AWT2-71 405406097405601 40 54 05.9 97 40 54.0 511.38 0.08 511.46 20.31 1.52 21.83 1.67

AWT3-73 405313097390201 40 53 12.7 97 39 00.1 506.98 0.06 507.04 20.87 1.52 22.39 1.67

NWT1-39 405216097364301 40 52 16.5 97 36 41.1 492.35 0.04 492.38 10.29 1.52 11.81 1.36

UWT1-53 405147097354001 40 51 45.8 97 35 36.7 486.32 0.06 486.38 13.30 3.05 16.35 1.65

UWT2-23 405149097361001 40 51 49.4 97 36 06.5 485.31 –0.91 484.40 5.54 1.52 7.06 1.67

UWT3-34 405200097355101 40 52 00.7 97 35 49.0 487.42 0.06 487.48 8.99 1.52 10.51 1.37

UWT4-85 405207097345501 40 52 07 97 34 55 501.86 0.05 501.91 24.49 1.52 26.01 1.67

Table 1. Well construction information for monitoring wells installed for the local-scale TANC study near York, Nebraska.

[bls, below land surface; m, meter; NAVD 88, North American Vertical Datum of 1988; NAD 83, North American Datum of 1983; na, not applicable]
Tables 93

Well name Site identification 

number Unit screened Well classification for interpretative purposes

Drilling 

technique

Well sampling 

analytical/frequency 

classification

FP1-63 405130097344802 Unconfined aquifer Unconfined shallow urban Auger Source well

FP1-147 405130097344803 Upper confined aquifer Confined urban unmixed Auger Flow-path well

FP1-185 405130097344804 Upper confined aquifer Confined urban unmixed Mud rotary Flow-path well

FP1-247 405130097344805 Lower confined lenses Confined urban unmixed Mud rotary Flow-path well

FP3-33 405146097353001 Unconfined aquifer Unconfined shallow urban Auger Source well

FP3-83 405146097353002 Unconfined aquifer Unconfined deep urban Auger Flow-path well

FP3-130 405146097353003 Upper confined aquifer Confined urban unmixed Mud rotary Flow-path well

FP3-162 405146097353004 Upper confined aquifer Confined urban mixed Auger Flow-path well

FP3-218 405146097353005 Lower confined lenses Confined urban mixed Mud rotary Flow-path well

FP4-28 405205097361701 Unconfined aquifer Unconfined shallow agricultural Auger Source well

FP4-83 405205097361702 Unconfined aquifer Unconfined deep agricultural Auger Flow-path well

FP4-168 405205097361703 Upper confined aquifer Confined agricultural unmixed Auger Flow-path well

FP4-231 405205097361704 Lower confined lenses Confined agricultural unmixed Mud rotary Flow-path well

OFPN-88 405151097350901 Unconfined aquifer Unconfined shallow urban Auger Source well

OFPN-167 405152097351402 Upper confined aquifer Confined urban unmixed Auger Off-path well

OFPN-276 405151097350903 Lower confined lenses Confined urban unmixed Mud rotary Off-path well

OFPS-38 405131097351401 Unconfined aquifer Unconfined shallow urban Auger Source well

OFPS-157 405131097351402 Upper confined aquifer Confined urban mixed Auger Flow-path well

OFPS-225 405131097351403 Lower confined lenses Confined urban unmixed Mud rotary Flow-path well

OFPS-277 405131097351404 Shale Not sampled Mud rotary not sampled

FP2-43 405134097355301 Unconfined aquifer Unconfined shallow urban Auger Source well

FP2-78 405134097355302 Unconfined aquifer Unconfined deep urban Auger Off-path well

FP2-152 405134097355303 Upper confined aquifer Confined urban unmixed Mud rotary Off-path well

FP2-250 405134097355304 Lower confined lenses Confined urban mixed Mud rotary Off-path well

FP5-45 405224097382201 Unconfined aquifer Unconfined shallow agricultural Auger Source well

FP5-73 405224097382202 Unconfined aquifer Unconfined deep agricultural Auger Off-path well

FP5-175 405224097382203 Upper confined aquifer Confined agricultural mixed Mud rotary Off-path well

FP5-LS 405224097382204 Unconfined aquifer Not sampled Mud rotary Off-path well

FP5-LS 405224097382204 Upper confined aquifer Confined agricultural mixed Mud rotary Off-path well

AWT1-83 405314097373201 Unconfined aquifer Unconfined shallow agricultural Auger Source well

AWT2-71 405406097405601 Unconfined aquifer Unconfined shallow agricultural Auger Source well

AWT3-73 405313097390201 Unconfined aquifer Unconfined shallow agricultural Auger Source well

NWT1-39 405216097364301 Unconfined aquifer Unconfined shallow agricultural Auger Source well

UWT1-53 405147097354001 Unconfined aquifer Unconfined shallow urban Auger Source well

UWT2-23 405149097361001 Unconfined aquifer Unconfined shallow urban Auger Source well

UWT3-34 405200097355101 Unconfined aquifer Unconfined shallow urban Auger Source well

UWT4-85 405207097345501 Unconfined aquifer Unconfined shallow urban Auger Source well

Table 1. Well construction information for monitoring wells installed for the local-scale TANC study near York, Nebraska.—Continued

[bls, below land surface; m, meter; NAVD 88, North American Vertical Datum of 1988; NAD 83, North American Datum of 1983; na, not applicable]
94 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Analyte group Filtered (dissolved concentration) or unfiltered (whole-water concentration) Analyzing laboratory Analytical code Reference(s) Number of samples per well during study Number of additional depth- dependent samples from study PSW Flow- path wells1 Off- path wells Source wells and study PSW Water-quality field parameters (Temperature, pH, turbidity, DO, SC, alkalinity, hydrogen sulfide) Unfiltered except for alkalinity Field analysis na Koterba and others (1995); U.S. Geological Survey (variously dated)

1 1 3 or 4 5

Volatile organic compounds Unfiltered USGS NWQL Sch 2020 Connor and others (1997) 1 1 3 or 4 5

Gasoline oxygenates Unfiltered USGS NWQL Sch 4024 Rose and Sandstrom (2003) 1 1 1 5

Pesticides Filtered USGS NWQL Sch 2003 Zaugg and others (1995); 

Sandstrom and others (2001); Madsen and others (2003)

1 NC 1 5

Herbicide metabolites Filtered USGS OGRL LCPD Lee and others (2002) 1 NC 1 5

Major inorganic constituents Filtered USGS NWQL Sch 2750 Fishman (1993); Fishman and 

Friedman (1989)

1 1 3 or 4 5

Trace elements Filtered USGS NWQL Sch 2710 Garbarino and others (2006) 1 1 1 5

Nutrients Filtered USGS NWQL Sch 2755 Fishman (1993); Patton and 

Kryskaila (2003)

1 1 3 or 4 5

Arsenic speciation Filtered USGS NWQL Sch 1732 Garbarino and others (2002) 1 1 1 5

Dissolved organic carbon Filtered USGS NWQL LC 2613 Brenton and Arnett (1993) 1 1 3 or 4 5

Ultra-violet absorbance Filtered USGS NWQL LC 2614 American Public Health 

Association (1998)

1 1 3 or 4 5

Nitrogen and oxygen isotopes in dissolved 

nitrate 

Filtered USGS RSIL na Sigman and others (2001); 

Casciotti and others (2002)

1 (wells 

with detectable nitrate)

1 (wells 

with detectable nitrate)

1 (wells 

with detectable nitrate)

5

Oxygen and hydrogen isotopes of water Unfiltered USGS RSIL Sch 1142 Coplen and others (1991) 1 1 1 5

Carbon isotopes Filtered Waterloo Sch 1000 Drimmie and others (1994) Selected 

wells (3)

NC NC 2 deepest 

samples

Sulfur isotopes of dissolved sulfate Filtered USGS RSIL LC 1951 Carmody and others (1997) 1 1 Selected 

wells

5

Uranium isotopes Filtered Eberline Sch 1130 American Society of Testing 

and Materials (2002)

Selected 

wells

NC Selected 

wells

NC

Radium isotopes Filtered Eberline Sch 1262 Environmental Protection 

Agency (1980, USEPA methods 903 and 904)

1 1 Selected 

wells

5

Table 2. Analytes and methods of analyses for water samples collected from wells during October 2003–April 2005 for the local-scale TANC study near York, Nebraska. [Study PSW, study public-supply well; DO, dissolved oxygen; SC, specific conductance; USGS, U.S. Geological Survey; USGS laboratories; NWQL, National Water Quality Laboratory, Denver, Colorado; OGRL, Organic Geochemistry Research Laboratory, Lawrence, Kansas; RSIL, Reston Stable Isotopic Laboratory, Reston, Virginia; CFC, Chlorofluorocarbon Laboratory, Reston, Virginia. Additional laboratories: Waterloo, University of Waterloo, Environmental Isotope Laboratory, Waterloo, Ontario, Canada; Miami, University of Miami, Tritium Laboratory, Miami, Florida; Eberline, Eberline Services, Richmond, California; Lamont-Doherty, Lamont-Doherty Earth Observatory of Columbia University, Noble Gas Laboratory. Sch, analytical schedule number; LCPD, Acetamide Parents and Degradation Products; LC, Laboratory Code; number of wells sampled for analyte in parentheses; na, not applicable; NC, not collected]
Tables 95

Analyte

group

Filtered (dissolved 

concentration) or 

unfiltered (whole-water 

concentration)

Analyzing

laboratory

Analytical

code Reference(s)

Number of samples 

per well during study

Number of 

additional 

depthdependent 

samples 

from study 

PSW

Flowpath 

wells1

Offpath

wells

Source 

wells and 

study PSW

Radon-222 Unfiltered USGS NWQL LC 1369 Prichard and Gesell (1977) 1 1 Selected 

wells

5

Gross-alpha/Gross-beta radiation 

(72-hour/30-day) 

Filtered Eberline Sch 1679 Environmental Protection 

Agency (1980, USEPA method 900.0)

1 1 Selected 

wells

5

Dissolved gases (nitrogen, argon, methane) Unfiltered USGS CFC na U.S. Geological Survey (2005) 1 1 1 5

Tritium Unfiltered Miami LC 1043 Östlund (1987) 1 1 Selected 

wells

2 deepest 

samples

Tritium/Helium-3 Unfiltered Lamont-Doherty na Clark and others (1976); 

Schlosser and others (1989)

Selected 

wells (2)

NC Selected 

wells (4)

3 shallowest 

samples

Helium-4 Unfiltered USGS CFC na Busenberg and others (2000) Selected 

wells (2)

NC Selected 

wells (4)

3 shallowest 

samples

Chlorofluorocarbons Unfiltered USGS CFC na Busenberg and Plummer 

(1992)

Selected 

wells (8)

NC Selected 

wells (5)

NC

Sulfur hexafluoride Unfiltered USGS CFC na Busenberg and Plummer 

(2000)

Selected 

wells (8)

NC Selected 

wells (2)

3 shallowest 

samples

Table 2. Analytes and methods of analyses for water samples collected from wells during October 2003–April 2005 for the local-scale TANC study near York, Nebraska.—Continued[Study PSW, study public-supply well; DO, dissolved oxygen; SC, specific conductance; USGS, U.S. Geological Survey; USGS laboratories; NWQL, National Water Quality Laboratory, Denver, Colorado; OGRL, Organic Geochemistry Research Laboratory, Lawrence, Kansas; RSIL, Reston Stable Isotopic Laboratory, Reston, Virginia; CFC, Chlorofluorocarbon Laboratory, Reston, Virginia. Additional laboratories: Waterloo, University of Waterloo, Environmental Isotope Laboratory, Waterloo, Ontario, Canada; Miami, University of Miami, Tritium Laboratory, Miami, Florida; Eberline, Eberline Services, Richmond, California; Lamont-Doherty, Lamont-Doherty Earth Observatory of Columbia University, Noble Gas Laboratory. Sch, analytical schedule number; LCPD, Acetamide Parents and Degradation Products; LC, Laboratory Code; number of wells sampled for analyte in parentheses; na, not applicable; NC, not collected]

1Selected wells resampled for selected analytes.
96 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Constituent

Number of 

field blank 

detections/

analyses

Maximum 

concentration 

detected in field 

blank samples 

Long-term 

method 

detection level

Censoring 

threshold

Minimum 

concentration 

in ground-water 

samples

Number of ground-water 

samples 

Censored

Systematic 

bias >10% of 

concentration

Volatile organic compounds and gasoline additives and (or) oxygenates (μg/L)

Acetone1 2/7 1.54 3 4.54 — 0 0

Carbon disulfide 1/9 0.12 0.019 0.139 E 0.037 2

0 0

1,2,4-trimethylbenzene 1/9 0.22 0.028 0.248 E 0.019 1 0

Tetrahydrofuran 1/9 1.5 1.1 2.6 E 0.7 2

0 0

Toluene 1/9 0.03 0.03 0.06 0.07 0 0

Trichloromethane 

(chloroform)

2/9 E 0.05 0.012 0.062 E 0.01 4 0

Major and minor ions (mg/L)

Calcium 4/4 0.03 0.01 0.04 15.6 0 0

Sodium 2/4 E0.07 0.1 0.17 17.8 0 0

Silica 4/4 0.06 0.02 0.08 30.1 0 0

Bromide 1/3 E0.01 0.01 0.02 .04 0 0

Trace elements (μg/L)

Barium 1/6 E 0.11 0.1 0.21 17.1 0 0

Chromium 1/6 0.44 0.02 0.46 E 0.46 1 0

Cobalt 1/6 0.04 0.02 0.06 0.07 0 0

Copper 4/6 1.14 0.2 1.34 E 0.22 33 23

Iron 1/4 E3.6 3.2 6.80 E3.2 3

23 0

Manganese 1/6 E 0.17 0.1 0.27 E 0.11 1 0

Molybdenum 1/6 E 0.35 0.2 0.55 E 0.89 0 0

Nickel 4/6 0.21 0.03 0.24 0.47 0 36

Vanadium 1/6 E 0.08 0.05 0.13 0.10 2

0 0

Zinc 2/6 1.04 0.3 1.34 E 0.36 11 0

Nutrients and dissolved organic carbon (mg/L)

Dissolved organic carbon 

(DOC)

2/8 E0.3 0.16 0.46 E 0.3 7 0

Total nitrogen 1/4 0.04 0.03 0.07 0.08 0 0

 1Constituents detected in associated source solution blanks also. 

2

No ground-water samples were censored because detections in field blanks occurred during a different sampling period than ground-water concentrations 

below the quality-control threshold. Ground-water samples and field blanks were collected during each of five sampling periods: October–November 2003, June 

2004, July–August 2004, October 2004, and March–April 2005. Ground-water values were censored when most or all blanks had detections or when blank and 

ground-water detections below the quality-control threshold occurred during the same sampling period

3

Estimated iron concentrations (<6 μg/L) were censored because of a detection in a field blank. This censoring is insignificant from an interpretative 

standpoint as these low concentrations were in oxic waters where they were expected.

Table 3. Quality-control summary for constituents detected in field blanks and ground-water samples collected for the local-scale 

TANC study near York, Nebraska.

[μg/L, microgram per liter; mg/L, milligram per liter; E, estimated value; <, less than; >, greater than; %, percent; —, not detected in ground-water samples]
Tables 97

Constituent

Number of relative standard 

deviations greater than 

20 percent/replicate pairs

Maximum relative 

standard deviation 

(percent)

Median of relative

 standard deviations 

(percent)

Volatile organic compounds and gasoline oxygenates/Additives (Schedules 2020 and 4204)1

1,1,2-Trichlorotrifluoroethane (CFC-113) 0/4 0.6 0.0

cis-1,2-Dichloroethylene 0/4 5.3 0.0

Tetrachloroethylene 0/4 2.8 0.3

trans-1,2-Dichloroethylene 0/4 14 1.7

Trichloroethylene 0/4 6.1 0.0

Pesticides and pesticide degradates (Schedules 2003)1

Atrazine 0/2 4.7 2.4

2-Chloro-4-isopropylamino-6-amino-s-triazine (CIAT or 

De-ethylatrazine)

0/2 10 4.9

Nutrients and dissolved organic carbon

Dissolved organic carbon 0/4 13.3 3.4

Orthophosphate 0/4 1.6 1.1

Total nitrogen 0/4 5.3 3.3

Nitrate plus nitrite 0/4 3.3 0.4

Major and minor ions

Bromide 0/2 2.6 1.7

Calcium 0/4 1.2 0.7

Chloride 0/4 0.9 0.3

Fluoride 0/4 5.4 1.9

Magnesium 0/4 1.2 0.2

Potassium 0/4 1.6 0.4

Silica 0/4 2.7 0.7

Sodium 0/4 1.7 0.5

Sulfate 0/4 1.6 0.2

Trace elements

Antimony 0/4 3.3 1.4

Arsenic 0/4 1.6 0.9

Arsenate 0/2 8.9 0.0

Arsenite 0/2 3.1 0.0

Barium 0/4 1.9 0.5

Boron 0/4 0.9 0.3

Cadmium 0/4 13.3 1.2

Chromium 0/4 13.1 3.8

Cobalt 0/4 3.6 2.7

Copper 0/4 11 2

Lithium 0/4 1.3 0.8

Manganese 1/2

6 46 0.6

Molybdenum 0/4 5.0 1.4

Nickel 0/4 7.8 2.9

Selenium 0/4 3.2 1.0

Strontium 0/4 1.8 0.1

Thallium 0/4 3.4 0.0

Uranium 0/4 1.6 0.9

Vanadium 0/4 2.8 1.6

Zinc 0/4 13 1.3

Isotopes

Radon-222 0/2 3.7 2.3

Deuterium/Protium 0/1 0.1 0.1

Tritium 0/1 2.6 2.6

Oxygen-18/Oxygen-16 0/1 0.5 0.5

Sulfur-34/Sulfur-32 0/1 0.2 0.1

Table 4. Quality-control summary of replicate analytes with relative standard deviations greater than zero, collected for the localscale TANC study near York, Nebraska.

[μg/L, microgram per liter]

1

Only detected constituents are shown.

2

Environmental concentration 1.5 μg/L, replicate concentration 2.9 μg/L, laboratory reporting level 0.2 μg/L.
98 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Constituent (common name)

Number 

of spike 

samples

Minimum 

recovery 

(percent)

Maximum 

recovery 

(percent)

1,1,1,2-Tetrachloroethane 2 99 111

1,1,1-Trichloroethane (TCA)1 2 111 125

1,1,2,2-Tetrachloroethane 2 94 110

1,1,2-Trichloroethane 2 96 106

1,1,2-Trichlorotrifluoroethane (CFC-113)1 2 105 123

1,1-Dichloroethane 2 108 116

1,1-Dichloroethylene (DCE)1 2 113 118

1,1-Dichloropropene 2 108 125

1,2,3,4-Tetramethylbenzene 2 101 108

1,2,3,5-Tetramethylbenzene (isodurene) 2 115 120

1,2,3-Trichlorobenzene 2 88 105

1,2,3-Trichloropropane 2 88 97

1,2,3-Trimethylbenzene1 2 104 110

1,2,4-Trichlorobenzene 2 89 98

1,2,4-Trimethylbenzene1 2 107 108

1,2-Dibromo-3-chloropropane (DBCP) 2 80 92

1,2-Dibromoethane 2 103 106

1,2-Dichlorobenzene 2 101 110

1,2-Dichloroethane1 2 98 114

1,2-Dichloropropane1 2 106 110

1,3,5-Trimethylbenzene 2 101 110

1,3-Dichlorobenzene 2 103 104

1,3-Dichloropropane 2 105 108

1,4-Dichlorobenzene 2 93 102

2-Butanone 2 92 123

2,2-Dichloropropane 2 99 110

2-Chlorotoluene 2 104 110

2-Hexanone 2 93 108

2-methyl-2-butanol 2 110 112

3-Chloropropene 2 75 85

4-Chlorotoluene 2 96 97

4-Isopropyl-1-methylbenzene 2 97 104

4-Methyl-2-pentanone 2 91 103

Acetone 2 91 95

Acrylonitrile 2 92 112

Benzene1 2 114 116

Bromobenzene 2 103 109

Bromochloromethane 2 100 107

Bromodichloromethane1 2 114 124

Bromoethene 2 116 124

Bromoform (tribromomethane) 2 98 113

Bromomethane 2 112 120

Butylbenzene 2 84 98

Carbon disulfide1 2 90 98

Chlorobenzene 2 104 105

Chloroethane 2 104 113

Chloroform (trichloromethane)1 2 111 119

Chloromethane 2 110 112

cis-1,2-Dichloroethylene1 2 115 117

cis-1,3-Dichloropropene 2 96 100

Dibromochloromethane 2 108 110

Table 5A. Quality-control summary of matrix spike recoveries for volatile organic compounds 

(VOC) and gasoline additives and oxygenates in samples collected for the local-scale TANC 

study near York, Nebraska. 

[Acceptable recovery range is between 70 and 130 percent; bold font is used to identify recoveries outside of 

acceptable limits] 
Tables 99

Constituent (common name)

Number 

of spike 

samples

Minimum 

recovery 

(percent)

Maximum 

recovery 

(percent)

Dibromomethane 2 102 107

Dichlorodifluoromethane (CFC-12)1 2 80 112

Dichloromethane (methylene chloride) 2 104 111

Diethyl ether 2 102 110

Diisopropyl ether 2 107 114

Ethyl methacrylate 2 91 105

2-Butanone (Ethyl methyl ketone) 2 100 115

Ethylbenzene1 2 107 111

Hexachlorobutadiene 2 96 99

Hexachloroethane 2 99 105

Isopropylbenzene 2 108 110

m- and p-Xylene1 2 102 106

Methyl acetate 2 92 99

Methyl acrylate 2 96 108

Methyl acrylonitrile 2 85 114

Methyl iodide 2 88 93

Methyl methacrylate 2 83 104

Methyl tert-butyl ether (MTBE)1 2 110 112

Naphthalene 2 83 95

n-Propylbenzene 2 98 99

o-Ethyl toluene1 2 96 104

o-Xylene1 2 107 107

sec-Butylbenzene 2 100 102

Styrene 2 100 102

Methyl tert-pentyl ether 2 100 108

tert-Amyl alcohol1 2 106 110

tert-Butyl alcohol 2 106 110

tert-Butyl ethyl ether (ETBE) 2 108 113

tert-Butylbenzene 2 104 119

Tetrachloroethylene (PCE)1 2 113 166

Tetrachloromethane (carbon tetrachloride)1 2 97 121

Tetrahydrofuran1 2 100 121

Toluene1 2 102 108

trans-1,2-Dichloroethylene1 2 111 111

trans-1,3-Dichloropropene 2 98 105

trans-1,4-Dichloro-2-butene 2 105 129

Trichloroethylene (TCE)1 2 109 124

Trichlorofluoromethane (CFC-11) 2 96 131

Vinyl chloride 2 126 135 1

Constituents detected in ground-water samples.

Table 5A. Quality-control summary of matrix spike recoveries for volatile organic compounds 

(VOC) and gasoline additives and oxygenates in samples collected for the local-scale TANC 

study near York, Nebraska. —Continued

[Acceptable recovery range is between 70 and 130 percent; bold font is used to identify recoveries outside of 

acceptable limits] 
100 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Constituent (common name) Number of spike 

samples

Minimum recovery 

(percent)

Maximum recovery 

(percent)

1-Naphthol 2 21 44

2,6-Diethylaniline 2 92 95

2-Chloro-2′,6′-diethylacetanilide 2 98 102

2-Chloro-4-isopropylamino-6-amino-s-triazine (de-ethylatrazine)1 2 47 58

2-Ethyl-6-methylaniline 2 94 95

3,4-Dichloroaniline 2 79 80

4-Chloro-2-methylphenol 2 52 79

Acetochlor 2 90 98

Alachlor 2 96 101

Atrazine1 2 109 112

Azinphos-methyl 2 79 82

Azinphos-methyl oxygen analog 2 12 53

Benfluralin, water 2 56 64

Carbaryl 2 81 108

Chlorpyrifos 2 87 96

Chlorpyrifos oxygen analog 2 12 19

cis-Permethrin 2 53 58

Cyfluthrin 2 38 48

Cypermethrin 2 39 50

Dacthal (DCPA) 2 103 108

Desulfinyl fipronil 2 87 109

Desulfinylfipronil amide 2 66 92

Diazinon 2 92 95

Diazinon oxygen analog 2 63 79

Dichlorvos 2 23 33

Dicrotophos 2 26 32

Dieldrin 2 89 92

Dimethoate 2 24 27

Ethion 2 55 81

Ethion monoxon 2 62 80

Fenamiphos 2 64 66

Fenamiphos sulfone 2 53 72

Fenamiphos sulfoxide 2 49 49

Fipronil 2 58 97

Fipronil sulfide 2 75 103

Fipronil sulfone 2 65 85

Fonofos 2 84 94

Fonofos oxygen analog 2 57 77

Hexazinone 2 74 78

Iprodione 2 17 56

Isofenphos 2 72 101

Malaoxon 2 63 82

Malathion 2 77 98

Metalaxyl 2 95 106

Methidathion 2 76 91

Methyl paraoxon 2 43 66

Methyl parathion 2 59 77

Metolachlor 2 95 105

Metribuzin 2 70 83

Myclobutanil 2 81 88

Pendimethalin 2 59 74

Phorate 2 64 66

Phorate oxygen analog 2 52 58

Prometon 2 91 102

Table 5B. Quality-control summary of matrix spike recoveries for pesticides and pesticide degradates in samples collected for the 

local-scale TANC study near York, Nebraska.

[Acceptable recovery range is between 60 and 120 percent. Bold font is used to identify recoveries outside of acceptable limits]
Tables 101

Constituent (common name) Number of spike 

samples

Minimum recovery 

(percent)

Maximum recovery 

(percent)

Prometryn 2 92 108

Pronamide 2 81 97

Simazine 2 88 108

Tebuthiuron 2 89 123

Terbufos 2 65 77

Terbufos oxygen analog sulfone 2 58 77

Terbuthylazine 2 101 112

Trifluralin 2 62 66

Table 5B. Quality-control summary of matrix spike recoveries for pesticides and pesticide degradates in samples collected for the 

local-scale TANC study near York, Nebraska.—Continued

[Acceptable recovery range is between 60 and 120 percent. Bold font is used to identify recoveries outside of acceptable limits]

1

Constituents detected in ground-water samples. 
102 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Age Hydrogeologic unit name Description Local-scale thickness (m) Local-scale model layer (Clark and others, 2008) Regional-scale model layer (Landon and Turco, 2007) Quaternary Loess Mostly unsaturated loess (Keech and others, 1967; Swinehart and others, 1994) consisting of brown, grey-black, or reddish-brown clayey silt. The loess is thinnest in the valleys along Beaver and Lincoln Creeks, where a thin veneer of loess and soil overlies sand and gravel. Grades into black-brown organic rich silt loam soils in the upper 2 m. 3.7–9.1, average 5.8 1 1 Quaternary Unconfined aquifer Mostly saturated sand and gravel with discontinuous silt and clay. Generally fining downward from brown, grey, or orange-brown poorly sorted coarse sand and gravel to brown fine to medium sand, to brown or grey finely interbedded heterogeneous sand, silt, and clay. Partially screened by many irrigation, domestic, and some commercial and older municipal wells. 15.5–26.8, average 20.1 2,3,4 2 Quaternary Upper confining unit Light brown (yellow) or light grey clayey silt with stones and very fine sand (glacial till). 8.8–17.4, average 13.8 5,6,7,8,9 3 Quaternary Upper confined aquifer Light grey to brown, moderately sorted, fine to medium sand. Primary aquifer layer screened for municipal supply; also screened by many irrigation and commercial wells. 14.0–20.4, average 18.0 10,11,12 4 Quaternary Lower confining unit and lower confined lenses Silt and clay layers with lesser amounts of mostly thin fine to medium poorly sorted sand. The sand layers are screened by some municipal and irrigation wells. The thickness of silt and clay layers ranges from 8.8 to 29.9 m with an average of 19.8 m; the thickness of sand layers ranges from 0 to 10.7 m with and average of 3.6 m. The number of sand layers ranges from 0 to 3. No lower confined sand layers were found at 3 of the 7 test holes drilled to shale by the HPGW TANC local-scale study. 15.2–33.2, average 23.4 13,14 5,6 Cretaceous Carlile Shale Black or dark grey consolidated fossiliferous shale with limestone or calcite fragments, and trace amounts of very fine sand. Table 6. Hydrogeologic units in the local-scale TANC study area near York, Nebraska. [HPGW, High Plains Ground Water regional study unit; m, meter]
Tables 103

Table 7. Grain-size analyses of selected core samples collected for the local-scale TANC study near York, Nebraska.

[m, meter; mm, millimeter; >, greater than; <, less than]

Well nest

Depth to 

top of 

interval 

analyzed 

(m)

Depth to

 bottom of

interval 

analyzed

(m)

Hydrogeologic 

unit sampled 

(additional notes 

on position in 

parentheses)

Gravel-sized 

particles 

>2 mm in 

diameter,

(percent)

Sand-sized 

particles 

0.0625–2 mm 

in diameter, 

(percent)

Silt-sized 

particles 

0.008–0.0625 

mm in 

diameter, 

(percent)

Clay-sized 

particles 

<0.008 mm 

in diameter, 

(percent)

FP5 4.9 5.3 Loess 0.0 2.5 80.3 17.3

FP5 8.5 8.8 Unconfined aquifer (near 

water table)

53.8 41.2 4.0 1.1

OFPS 19.8 20.1 Unconfined aquifer (near 

bottom)

1.0 90.6 6.8 1.6

FP5 28.4 28.6 Upper confining unit 0.1 8.4 72.5 19.0

FP5 57.9 58.5 Upper confined aquifer 0.2 92.6 5.2 2.1

OFPS 66.7 66.8 Lower confining unit 0.0 0.0 68.2 31.8
104 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Well nest Depth to top of interval (m) Depth to bottom of interval (m) Hydro- geologic unit Relative abundance of bulk minerals (1 highest; 6 lowest)

1 Relative abundance of <1μm clays (1 highest; 5 lowest)

1

Quartz Plagioclase Potassium feldspar Calcite Total clay Goethite Gypsum/ bassanite Halite Hornblende Smectite Illite Kaolinite Chlorite Hornblende Goethite

FP5 4.9 5.3 Loess 1 2 3 — 4 —

—

— 5 1 2 3 4 —

—

FP4 7.0 7.2 Unconfined 

aquifer (near water table)

1 tr tr 2 3 —

—

— tr 1 2 3 4 tr

—

FP5 8.5 8.8 Unconfined 

aquifer (near water table)

1 3 2 4 5 —

—

—

— 1 2 3 4 —

—

OFPS 19.8 20.1 Unconfined 

aquifer (near bottom)

1 3 2 4 5 —

—

—

— 1 2 3 4 5 —

FP5 28.4 28.6 Upper confining 

unit

1 3 2 — 4 —

—

—

— 1 2 3 — 4 —

FP1 30.9 31.0 Upper confining 

unit

1 2 3 — 4 tr

—

— 1 2 3 4 — 5

FP3 38.1 41.1 Upper confined 

aquifer

1 tr tr 3 5 — 2

2 2

4 — 1 2 3 4 tr tr

FP5 57.9 58.5 Upper confined 

aquifer

1 2 3 4 5 —

—

—

— 1 2 3 4 —

—

FP1 59.5 59.6 Lower confining 

unit

1 2 3 tr 4 tr

—

—

— 1 3 2 —

— 4

OFPS 66.1 66.7 Lower confined 

lenses

1 2 5 3 4 —

—

—

— 3 1 2 4 —

—

FP1 84.4 84.6 Shale 1 3 2 5 tr

— 4 6 — 3 2 1 4 — 5

Table 8. Mineralogy of selected core samples collected for the local-scale TANC study near York, Nebraska.[C2, slightly magnetic; C3, non-magnetic. tr, trace amount; m, meter; <, less than; %, percent; μm, micrometer; —, not detected in core sample; NA, not analyzed]
Tables 105

Well 

nest

Depth to top 

of interval

(m)

Depth to bottom 

of interval 

(m)

Hydrogeologic 

unit Mineralogy of heavy mineral separates

FP5 4.9 5.3 Loess NAFP4 7.0 7.2 Unconfined aquifer (near water table) NAFP5 8.5 8.8 Unconfined aquifer (near water table) C3: 40% quartz, 20% zircon, 10% amphibole/pyroxene, 20% sphene, trace pyrite and tourmalineOFPS 19.8 20.1 Unconfined aquifer (near bottom) NAFP5 28.4 28.6 Upper confining unit NAFP1 30.9 31.0 Upper confining unit NAFP3 38.1 41.1 Upper confined aquifer C3: 60% quartz, 30% amphibole/pyroxene, 10% (epidote, hematite, zircon, tourmaline) C2: 40% garnet, 30% 

amphibole/pyroxene, 20% hematite, trace epidote and sphene

FP5 57.9 58.5 Upper confined aquifer C3: 90% quartz, 10% framboydal pyrite, trace tourmaline, rutile, zircon C2: 40% garnet 40% hematite, 20% 

(epidote, amphibole, sphene)

FP1 59.5 59.6 Lower confining unit C3: 90% granular pyrite, 10% quartz, trace sphene and zirconOFPS 66.1 66.7 Lower confined lenses NAFP1 84.4 84.6 Shale NA

1Analyzed using X-ray diffraction.2Detections of gypsum and halite in this sample is interpreted to result from sample contamination by drilling mud.

Table 8. Mineralogy of selected core samples collected for the local-scale TANC study near York, Nebraska.—Continued[C2, slightly magnetic; C3, non-magnetic. tr, trace amount; m, meter; <, less than; %, percent; μm, micrometer; —, not detected in core sample; NA, not analyzed]
106 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Well nest Depth to top of interval analyzed (m) Depth to bottom of interval analyzed (m) Hydro- geologic layer Elemental analysis of 6N-HCL extractions1 Alumi- num(Al) (mg/kg) Calcium (Ca) (mg/kg) Chrom- ium(Cr) (mg/kg) Copper (Cu) (mg/kg) Iron (Fe) (mg/kg) Lead (Pb) (mg/kg) Magne- sium (Mg) (mg/kg) Manga- nese (Mn) (mg/kg) Nickel (Ni) (mg/kg) Potas- sium(K) (mg/kg) Sodium (Na) (mg/kg) Uranium (U) (mg/kg) Vana- dium(V) (mg/kg) Zinc (Zn) (mg/kg) FP5 4.9 5.3 Loess 4,400 3,200 5.8 12 6,800 NA 2,300 180 16 940 68 NA 15 25 FP4 7.0 7.2 Unconfined aquifer (near water table) 298 312 0.66 0.68 532 1.49 131 10 0.67 86 40 0.17 1.09 1.67 FP5 8.5 8.8 Unconfined aquifer (near water table) NA NA NA NA NA NA NA NA NA NA NA NA NA NA OFPS 19.8 20.1 Unconfined aquifer (near bottom) 430 370 < 0.5 2.7 360 0.78 130 < 0.5 2.6 77 46 0.10 1.8 < 0.5 FP5 28.4 28.6 Upper confining unit 3,000 3,600 3.9 5.5 3,500 NA 1,700 190 7.8 700 41 NA 11 11 FP1 30.8 31.1 Upper confining unit 4,141 4,990 4.8 6.3 3,900 7.8 2,226 132 6.2 1,525 58 0.97 13.2 11.6 FP3 38.1 39.6 Upper confined aquifer 494 6,169 1.14 1.05 1,600 1.74 615 43 1.72 167 51 0.25 2.86 3.44 FP5 57.9 58.5 Upper confined aquifer 440 410 1.2 3.6 3,200 1.5 180 36 3.5 99 16 0.70 13 < 0.5 FP1 59.5 59.8 Lower confining unit 4,038 34,031 6.8 13.7 4,800 8.6 4,469 550 11.2 1,421 128 10.80 12.5 33.8 OFPS 66.7 66.8 Lower confining unit NA NA NA NA NA NA NA NA NA NA NA NA NA NA FP1 84.7 84.8 Shale 2,149 18,005 2.8 1.5 10,600 6.0 2,505 80 22 790 237 6.98 20 45 Table 9. Elemental and isotopic analyses of selected core samples collected for the local-scale TANC study near York, Nebraska. [m, meter; mg, milligram; kg, kilogram; O, oxygen; δ13C; delta carbon-13; δ15N, delta nitrogen-15; NA, not analyzed; <, less than; N, normal; HCl, hydrochloric acid; %, percent; HNO3, nitric acid; HA, hydroxylamine; AVCRS, acid-volatile and chromium-reducible sulfides; ICP-MS, inductively-coupled plasma/mass spectrometry]
Tables 107

Well

nest

Depth 

to top of 

interval 

analyzed

(m)

Depth to 

bottom of 

interval 

analyzed

(m)

Hydrogeologic

layer

Major oxide analysis of 6N-HCl extractions1 Elemental analysis of 10% HNO3 extractions2 Total iron 

analysis of 

0.5N-HCl-HA 

extractions3

(mg/kg)

Sulfide 

analysis 

of AVCRS 

extractions4

(mg/kg)

Al O2 3

(mg/kg)

CaO

(mg/kg)

Fe O2 3

(mg/kg)

K O2

(mg/kg)

MgO

(mg/kg)

MnO

(mg/kg)

Na O2

(mg/kg)

Arsenic 

(As)

(mg/kg)

Iron 

(Fe)

(mg/kg)

Manganese

(Mn)

(mg/kg)

Uranium

(U)

(mg/kg)

FP5 4.9 5.3 Loess 8,316 NA 9,722 NA NA 232 NA 0.41 NA NA 1.10 1,300 3.2

FP4 7.0 7.2 Unconfined 

aquifer (near water table)

564 437 760 104 219 13.1 54.5 0.07 350 23 0.13 700 0.18

FP5 8.5 8.8 Unconfined 

aquifer (near water table)

NA NA NA NA NA NA NA 0.33 NA NA 3.50 2,100 0.93

OFPS 19.8 20.1 Unconfined 

aquifer (near bottom)

813 NA 515 NA NA < 1 NA 2.30 NA NA 1.30 180 2.7

FP5 28.4 28.6 Upper confining 

unit

5,670 NA 5,004 NA NA 245 NA 0.09 NA NA 0.90 590 0.64

FP1 30.8 31.1 Upper confining 

unit

7,826 6,980 5,638 1,837 3,713 170 77.7 0.20 1,000 170 0.70 210 1.0

FP3 38.1 39.6 Upper confined 

aquifer

933 8,630 2,337 201 1,026 55 69 0.11 330 70 0.18 81 0.67

FP5 57.9 58.5 Upper confined 

aquifer

832 NA 4,575 NA NA 46 NA 1.60 NA NA 0.41 220 75

FP1 59.5 59.8 Lower confining 

unit

7631 47,610 6,885 1,713 7,454 708 172 3.90 2,900 680 9.30 1,800 1,800

OFPS 66.7 66.8 Lower confining 

unit

NA NA NA NA NA NA NA NA NA NA NA NA NA

FP1 84.7 84.8 Shale 4,062 25,189 15,217 952 4,178 103 319 5.30 7,200 250 6.10 NA NA

Table 9. Elemental and isotopic analyses of selected core samples collected for the local-scale TANC study near York, Nebraska. —Continued[m, meter; mg, milligram; kg, kilogram; O, oxygen; δ13C; delta carbon-13; δ15N, delta nitrogen-15; NA, not analyzed; <, less than; N, normal; HCl, hydrochloric acid; %, percent; HNO3, nitric acid; HA, 

hydroxylamine; AVCRS, acid-volatile and chromium-reducible sulfides; ICP-MS, inductively-coupled plasma/mass spectrometry]
108 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Well nest Depth to top of interval analyzed (m) Depth to bottom of interval analyzed (m) Hydro- geologic layer Total carbon content (percent) Organic carbon content (percent)

d13C of 

total 

carbon

(per mil)

d13C of 

organic 

carbon

(per mil)

d13C of 

calcite

(per mil)

d15N of 

total 

nitrogen (N)

(per mil)

Total 

nitrogen 

content 

(percent)

FP5 4.9 5.3 Loess 0.14 0.13 –23.3 –23.8 NA 3.7 0.03 

FP4 7.0 7.2 Unconfined aquifer (near 

water table)

NA NA NA NA NA NA NA

FP5 8.5 8.8 Unconfined aquifer (near 

water table)

0.05 0.05 –24.6 –24.9 NA NA < 0.01

OFPS 19.8 20.1 Unconfined aquifer (near 

bottom)

0.03 0.03 –26.0 –26.8 NA NA 0.01 

FP5 28.4 28.6 Upper confining unit NA NA NA NA NA NA NAFP1 30.8 31.1 Upper confining unit NA NA NA NA NA NA NAFP3 38.1 39.6 Upper confined aquifer NA NA NA NA NA NA NAFP5 57.9 58.5 Upper confined aquifer 0.18 0.03 –6.4 –26.5 –2.6 NA < 0.01

FP1 59.5 59.8 Lower confining unit 0.88 0.26 –7.9 –26.2 –2.6 3.4 0.03 

OFPS 66.7 66.8 Lower confining unit 0.32 0.07 –8.2 –25.6 –4.3 NA 0.01 

FP1 84.7 84.8 Shale 3.36 3.20 –24.5 –26.1 NA 0.7 0.15 

1 6N-HCl extractions: samples were extracted for 6 hours on a shaker table, centrifuged, filtered (0.45 micron) then diluted to 1.2N-HCl for analysis by ICP-MS.2 10% HNO3 extractions: samples were extracted for 6 hours on a shaker table, centrifuged, filtered (0.45 micron) before analysis by ICP-MS.3 0.5N-HCl-HA extractions: samples were extracted for 6 hours on a shaker table, centrifuged, filtered (0.45 micron) before analysis by ICP-MS.4 AVCRS extractions: samples were extracted for 6 hours on a shaker table, centrifuged, filtered (0.45 micron) before analysis by ICP-MS.

Table 9. Elemental and isotopic analyses of selected core samples collected for the local-scale TANC study near York, Nebraska. —Continued[m, meter; mg, milligram; kg, kilogram; O, oxygen; δ13C; delta carbon-13; δ15N, delta nitrogen-15; NA, not analyzed; <, less than; N, normal; HCl, hydrochloric acid; 

%, percent; HNO3, nitric acid; HA, hydroxylamine; AVCRS, acid-volatile and chromium-reducible sulfides; ICP-MS, inductively-coupled plasma/mass spectrometry]
Tables 109

Well name or

core sample name 

(with depth)

Hydrogeologic unit Part of unit

Slug tests Historical aquifer tests Laboratory 

saturated K

v

 of 

core sample, 

(m/d)2

Kh 1

(m/d) 

Min Kh

(m/d) 

Average Kh

(m/d)

Stand Dev 

Kh

(m/d)

Max Kh

(m/d)

 Kh

(m/d)

Number 

of tests

FP5-73, depth 3.4–4.1 Loess 1.53 × 10–5AWT1-833 Unconfined aquifer Upper 32

OFPN-883 Unconfined aquifer Upper 25UWT4-853 Unconfined aquifer Upper 16UWT3-34 Unconfined aquifer Upper 21

FP5-45 Unconfined aquifer Upper 18

AWT3-733 Unconfined aquifer Upper 21

UWT2-233 Unconfined aquifer Upper 15AWT2-71 Unconfined aquifer Upper 15

FP2-43 Unconfined aquifer Upper 15

NWT1-39 Unconfined aquifer Upper 12

OFPS-38 Unconfined aquifer Upper 9.1

FP4-28 Unconfined aquifer Upper 9.1

FP3-33 Unconfined aquifer Upper 6.1

UWT1-53 Unconfined aquifer Upper 3.0

FP1-63 Unconfined aquifer Middle 21

Summary Unconfined aquifer Upper and middle 3.0 16 7.6 32 4,541–120 2

FP3-83 Unconfined aquifer Lower 2.1

FP2-78 Unconfined aquifer Lower 6.1

FP5-73 Unconfined aquifer Lower 9.1

FP4-83 Unconfined aquifer Lower 0.3

Summary Unconfined aquifer Lower 0.3 4.4 4.0 9.1

OFPS-225, depth 25.3–25.6 m Upper confining unit 7.12 × 10–6

FP5-73, depth 27.9–28.1 m Upper confining unit 1.52 × 10–5FP1-147 Upper confined aquifer Upper 3.0

FP3-130 Upper confined aquifer Upper 3.0

FP4-168 Upper confined aquifer Middle 18

OFPN-167 Upper confined aquifer Middle 15

OFPS-157 Upper confined aquifer Middle 12

FP2-152 Upper confined aquifer Middle 15

FP5-175 Upper confined aquifer Middle 6.1

FP1-185 Upper confined aquifer Lower 6.1

FP3-162 Upper confined aquifer Lower 9.1

Summary Upper confined aquifer All 3.0 9.8 5.7 18 620 1

Table 10. Values of hydraulic conductivity determined from slug tests in monitoring wells and from historical aquifer tests in the local-scale TANC study area near York, Nebraska.[Kh, horizontal hydraulic conductivity; Kv, vertical hydraulic conductivity; m/d, meter per day; Min, minimum; Max, maximum; Stand Dev, Standard Deviation]
110 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Well name or core sample name (with depth) Hydrogeologic unit Part of unit Slug tests Historical aquifer tests Laboratory saturated Kv of core sample, (m/d)2

Kh 1

(m/d) 

Min Kh

(m/d) 

Average Kh

(m/d)

Stand Dev 

Kh

(m/d)

Max Kh

(m/d)

 Kh

(m/d)

Number 

of tests

OFPS-225, 53.8–54.3

m Lower confining unit 6.39 × 10–6

FP3-218 Lower confining unit 0.2

FP4-231 Lower confining unit 0.0

FP2-250 Lower confining unit 9.1

Summary Lower confining unit 0.0 3.1 5.2 9.1

FP1-247 Lower confined lenses 3.0

OFPN-276 Lower confined lenses 3.0

OFPS-225 Lower confined lenses 3.0

Summary Lower confined lenses 3.0 3.0 0.0 3.0 64.8–6.9 2

1 Bouwer and Rice (1976) method used to estimate K from slug tests unless otherwise noted next to well name.2 Laboratory tests done at U.S. Geological Survey Laboratory in Menlo Park, California using methods of Nimmo and Mello (1991) and Nimmo and others (2002 [Kim Perkins, U.S. ogical Survey, written 

commun., October 20, 2004]).

3 Butler and others (2003) method used to estimate K from slug tests.4 Ma (1996).5 Argonne National Laboratory (1995a).6 Layne Geosciences, written commun., 1997.

Table 10. Values of hydraulic conductivity determined from slug tests in monitoring wells and from historical aquifer tests in the local-scale TANC study area near York, Nebraska.—Continued[Kh, horizontal hydraulic conductivity; Kv, vertical hydraulic conductivity; m/d, meter per day; Min, minimum; Max, maximum; Stand Dev, Standard Deviation]
Tables 111

Table 11. Wells classified by degree of confinement, depth, land use, well type, and mixing, in and near York, Nebraska.[“Confined” includes monitoring wells screened in upper confined aquifer and lower confined lenses. Mixing of unconfined/confined waters in confined aquifer based on δ18O/δD. ag, agricultural land use; 

na, not applicable; m, meter; PSW, public-supply well; δ18O, delta oxygen-18 stable isotopic ratio in water; δD, delta deuterium stable isotopic ratio in water]

1. Degree of confinement Unconfined Confined

2. Depth in unconfined aquifer Shallow Deep na

3. Land use Urban Agricultural Urban Agricultural Urban

monitoring

Agricultural

monitoring 4. Well type monitoring monitoring monitoring monitoring PSW

5. Mixing in confined aquifer: na na na na Unmixed Mixed Unmixed Mixed

Well classification category name

Unconfined 

shallow 

urban 

Unconfined 

shallow 

ag 

Unconfined 

deep 

urban 1

Unconfined 

deep 

ag 1

Confined 

urban 

unmixed 2

Confined 

urban 

mixed 3

Study 

PSW

Confined 

ag 

unmixed 2

Confined 

ag 

mixed 3

Wells and samples included: FP1-63 FP4-28 FP3-83 FP4-83 FP1-147 FP3-162 Study PSW4 FP4-168 FP5-175

FP3-33 FP5-45 FP2-78 FP5-73 FP1-185 OFPS-157 Study PSW, depth 42.7 m5 FP4-231 FP5-LS

OFPN-88 AWT1-83 FP3-130 FP3-218 Study PSW, depth 45.7 m5

OFPS-38 AWT2-71 OFPN-167 FP2-250 Study PSW, depth 48.8 m5FP2-43 AWT3-73 FP2-152 Study PSW, depth 51.8 m5UWT1-53 NWT1-39 FP1-247 Study PSW, depth 61.0 m5UWT2-23 OFPN-276UWT3-34 OFPS-225UWT4-85

1For some constituents, there was no contrast between unconfined deep urban and unconfined deep ag wells; in places in the report, all of these wells, including both land use settings, are referred to as 

unconfined deep wells.

2For some constituents, there was no contrast between confined urban unmixed and confined ag unmixed wells; in places in the report, all these wells, including both land use settings, are referred to as 

confined unmixed wells.

3For some constituents, there was no contrast between confined urban mixed and confined ag mixed wells; in places in the report, all these wells, including both land use settings, are referred to as confined

mixed wells.

4Samples of the surface discharge of the study PSW, integrating all the water captured by the well.5Depth-dependent samples collected from the study PSW under pumping conditions, representing an integrated sample of all water below the listed depth.
112 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 12. Hydrogen and oxygen stable isotopic values and mixing calculations for the local-scale TANC study near York, Nebraska.—Continued [Stable isotope ratios are reported in the standard delta notation (δ), the ratio of a heavier isotope to the more common lighter isotope of that element, relative to a standard reference material. δ18O, delta oxygen-18 stable isotopic ratio; δD, delta deuterium stable isotopic ratio. Wells are listed from those having the least negative to those having the most negative δ18O values; for wells sampled more than once, samples are listed in chronological order. Fraction of precipitation recharge is the fraction of the water sample consisting of precipitation recharge, rather than ground-water return flow, on the basis of assumed end-member isotopic values listed at the bottom of the table. Fraction of shallow unconfined water is the fraction of water sample consisting of shallow unconfined water, rather than confined unmixed water, on the basis of assumed end-member isotopic values listed at the bottom of the table. Study PSW, study public-supply well; na, not applicable; m, meter] Date sampled Well or sample name Well classification Isotopic group 

(fig. 7)

δD

(per mil)

δ18O

(per mil)

δD

fraction of 

precipitation 

recharge

(percent)

δ18O

fraction of 

precipitation 

recharge

(percent)

δD

fraction of 

shallow 

water

(percent)

δ18O

fraction of 

shallow 

water

(percent)

10/17/2004 NWT1-39 Unconfined shallow agricultural 2 –51.00 –7.65 99 99 na na

4/1/2005 NWT1-39 Unconfined shallow agricultural 2 –50.80 –7.62 100 100 na na

10/18/2004 FP5-45 Unconfined shallow agricultural 2 –53.60 –7.84 81 85 na na

3/30/2005 FP5-45 Unconfined shallow agricultural 2 –53.80 –7.84 79 85 na na

10/19/2004 AWT2-71 Unconfined shallow agricultural 2 –53.70 –7.87 80 83 na na

3/31/2005 AWT2-71 Unconfined shallow agricultural 2 –54.80 –8.14 73 65 na na

10/19/2004 AWT1-83 Unconfined shallow agricultural 2 –54.00 –8.04 78 72 na na

3/31/2005 AWT1-83 Unconfined shallow agricultural 2 –53.60 –8.03 81 72 na na

10/19/2004 AWT3-73 Unconfined shallow agricultural 2 –55.90 –8.14 65 65 na na

4/1/2005 AWT3-73 Unconfined shallow agricultural 2 –54.30 –8.02 76 73 na na

10/29/2003 FP4-28 Unconfined shallow agricultural 2 –55.84 –8.38 66 49 na na

4/12/2005 FP4-28 Unconfined shallow agricultural 2 –55.60 –8.30 67 54 na na

10/17/2004 UWT3-34 Unconfined shallow urban 3 –56.20 –8.38 78 68 na na

4/5/2005 UWT3-34 Unconfined shallow urban 3 –57.30 –8.35 73 69 na na

10/22/2004 UWT4-85 Unconfined shallow urban 3 –56.40 –8.34 77 69 na na

4/6/2005 UWT4-85 Unconfined shallow urban 3 –56.50 –8.31 76 71 na na

10/27/2003 FP1-63 Unconfined shallow urban 3 –56.51 –8.46 76 64 na na

4/11/2005 FP1-63 Unconfined shallow urban 3 –57.60 –8.50 72 63 na na

10/20/2004 UWT1–53 Unconfined shallow urban 3 –58.00 –8.47 70 64 na na

4/5/2005 UWT1–53 Unconfined shallow urban 3 –59.40 –8.62 64 57 na na

10/20/2004 UWT2-23 Unconfined shallow urban 3 –59.30 –8.80 65 50 na na

4/5/2005 UWT2-23 Unconfined shallow urban 3 –58.40 –8.62 69 57 na na

10/13/2004 FP2-43 Unconfined shallow urban 3 –58.90 –8.71 67 54 na na

3/30/2005 FP2-43 Unconfined shallow urban 3 –59.60 –8.61 64 58 na na

11/11/2003 OFPS-38 Unconfined shallow urban 3 –60.10 –8.62 62 57 na na

4/14/2005 OFPS-38 Unconfined shallow urban 3 –59.80 –8.57 63 60 na na

11/13/2003 OFPN-88 Unconfined shallow urban 3 –60.10 –8.59 62 59 na na

4/6/2005 OFPN-88 Unconfined shallow urban 3 –60.40 –8.64 60 57 na na

10/28/2003 FP3-33 Unconfined shallow urban 3 –61.52 –9.00 56 41 na na

4/4/2005 FP3-33 Unconfined shallow urban 3 –63.60 –9.14 47 35 na na

Table 12. Hydrogen and oxygen stable isotopic values and mixing calculations for the local-scale TANC study near York, Nebraska.[Stable isotope ratios are reported in the standard delta notation (δ), the ratio of a heavier isotope to the more common lighter isotope of that element, relative to a standard reference material. δ18O, 

delta oxygen-18 stable isotopic ratio; δD, delta deuterium stable isotopic ratio. Wells are listed from those having the least negative to those having the most negative δ18O values; for wells sampled 

more than once, samples are listed in chronological order. Fraction of precipitation recharge is the fraction of the water sample consisting of precipitation recharge, rather than ground-water return flow, on the basis of assumed end-member isotopic values listed at the bottom of the table. Fraction of shallow unconfined water is the fraction of water sample consisting of shallow unconfined water, rather than confined unmixed water, on the basis of assumed end-member isotopic values listed at the bottom of the table. Study PSW, study public-supply well; na, not applicable; m, meter] 
Tables 113

Table 12. Hydrogen and oxygen stable isotopic values and mixing calculations for the local-scale TANC study near York, Nebraska.—Continued[Stable isotope ratios are reported in the standard delta notation (δ), the ratio of a heavier isotope to the more common lighter isotope of that element, relative to a standard reference material. δ18O, 

delta oxygen-18 stable isotopic ratio; δD, delta deuterium stable isotopic ratio. Wells are listed from those having the least negative to those having the most negative δ18O values; for wells sampled 

more than once, samples are listed in chronological order. Fraction of precipitation recharge is the fraction of the water sample consisting of precipitation recharge, rather than ground-water return flow, on the basis of assumed end-member isotopic values listed at the bottom of the table. Fraction of shallow unconfined water is the fraction of water sample consisting of shallow unconfined water, rather than confined unmixed water, on the basis of assumed end-member isotopic values listed at the bottom of the table. Study PSW, study public-supply well; na, not applicable; m, meter] 

Date sampled Well or sample name Well classification

Isotopic 

group 

(fig. 7)

δD

(per mil)

δ18O

(per mil)

δD

fraction of 

precipitation 

recharge

(percent)

δ18O

fraction of 

precipitation 

recharge

(percent)

δD

fraction of 

shallow 

water

(percent)

δ18O

fraction of 

shallow 

water

(percent)

10/21/2004 FP5-LS Confined agricultural mixed 4 –55.00 –7.94 na na 94 96

10/21/2004 FP5-175 Confined agricultural mixed 4 –58.10 –8.53 na na 82 75

10/28/2003 FP3-162 Confined urban mixed 4 –59.13 –8.63 na na 89 85

10/16/2004 FP3-162 Confined urban mixed 4 –59.20 –8.60 na na 89 87

11/11/2003 OFPS-157 Confined urban mixed 4 –62.90 –8.84 na na 71 74

10/14/2004 OFPS-157 Confined urban mixed 4 –68.50 –9.41 na na 43 42

4/11/2005 OFPS-157 Confined urban mixed 4 –68.90 –9.56 na na 41 34

10/15/2003 FP3-218 Confined urban mixed 4 –63.40 –9.14 na na 68 57

10/16/2004 FP3-218 Confined urban mixed 4 –63.40 –9.00 na na 68 65

4/4/2005 FP3-218 Confined urban mixed 4 –64.00 –9.05 na na 65 62

10/12/2004 FP2-250 Confined urban mixed 4 –64.50 –9.04 na na 63 63

10/18/2004 FP5-73 Unconfined agricultural deep 4 –59.80 –8.58 na na 75 73

10/30/2003 FP4-83 Unconfined agricultural deep 4 –65.98 –9.46 na na 49 33

4/14/2005 FP4-83 Unconfined agricultural deep 4 –66.90 –9.38 na na 45 37

10/27/2003 FP3-83 Unconfined urban deep 4 –62.28 –8.91 na na 74 70

10/13/2004 FP2-78 Unconfined urban deep 4 –63.50 –9.07 na na 68 61

6/17/2004 Study PSW, depth 51.8 m Study PSW 4 –68.00 –9.27 na na 46 50

6/16/2004 Study PSW, depth 61.0 m Study PSW 4 –69.50 –9.48 na na 39 39

6/17/2004 Study PSW, depth 48.8 m Study PSW 1 –73.90 –9.81 na na 17 21

6/18/2004 Study PSW, depth 42.7 m Study PSW 1 –74.20 –9.95 na na 16 13

6/21/2004 Study PSW, depth 45.7 m Study PSW 1 –74.30 –9.96 na na 15 13

10/20/2003 Study PSW, surface-discharge Study PSW 1 –74.90 –9.97 na na 12 12

6/18/2004 Study PSW, surface-discharge Study PSW 1 –75.00 –9.97 na na 12 12

4/7/2005 Study PSW, surface-discharge Study PSW 1 –75.00 –9.91 na na 12 15

10/16/2003 FP4-231 Confined agricultural unmixed 1 –73.80 –10.12 na na 15 3

10/23/2003 FP4-168 Confined agricultural unmixed 1 –74.87 –10.12 na na 11 3

4/14/2005 FP4-168 Confined agricultural unmixed 1 –75.90 –10.16 na na 6 1

11/12/2003 OFPN-276 Confined urban unmixed 1 –74.80 –9.77 na na 13 23

10/12/2004 FP2-152 Confined urban unmixed 1 –74.90 –10.08 na na 12 6

10/20/2003 FP1-247 Confined urban unmixed 1 –74.95 –10.05 na na 12 8

4/7/2005 FP1-247 Confined urban unmixed 1 –76.50 –10.18 na na 4 1
114 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 12. Hydrogen and oxygen stable isotopic values and mixing calculations for the local-scale TANC study near York, Nebraska.—Continued [Stable isotope ratios are reported in the standard delta notation (δ), the ratio of a heavier isotope to the more common lighter isotope of that element, relative to a standard reference material. δ18O, delta oxygen-18 stable isotopic ratio; δD, delta deuterium stable isotopic ratio. Wells are listed from those having the least negative to those having the most negative δ18O values; for wells sampled more than once, samples are listed in chronological order. Fraction of precipitation recharge is the fraction of the water sample consisting of precipitation recharge, rather than ground-water return flow, on the basis of assumed end-member isotopic values listed at the bottom of the table. Fraction of shallow unconfined water is the fraction of water sample consisting of shallow unconfined water, rather than confined unmixed water, on the basis of assumed end-member isotopic values listed at the bottom of the table. Study PSW, study public-supply well; na, not applicable; m, meter] Date sampled Well or sample name Well classification Isotopic group 

(fig. 7)

δD

(per mil)

δ18O

(per mil)

δD

fraction of 

precipitation 

recharge

(percent)

δ18O

fraction of 

precipitation 

recharge

(percent)

δD

fraction of 

shallow 

water

(percent)

δ18O

fraction of 

shallow 

water

(percent)

10/15/2003 FP3-130 Confined urban unmixed 1 –75.17 –10.19 na na 11 0

10/22/2003 FP1-147 Confined urban unmixed 1 –75.28 –10.19 na na 10 0

04/11/2005 FP1-147 Confined urban unmixed 1 –76.50 –10.18 na na 4 1

10/21/2003 FP1-185 Confined urban unmixed 1 –76.43 –10.18 na na 5 1

10/14/2004 FP1-185 Confined urban unmixed 1 –76.50 –10.18 na na 4 1

10/30/2003 OFPN-167 Confined urban unmixed 1 –76.80 –10.09 na na 3 5

10/14/2004 OFPS-225 Confined urban unmixed 1 –77.40 –10.17 na na 0 1

End-member compositions for mixing calculations

Mixing Fraction of precipitation rechargeA. Precipitation recharge end-member (NWT1-39) –50.80 –7.62B1. Ground-water return flow for unconfined shallow urban wells (study PSW median) –75.00 –9.97B2. Ground-water return flow for unconfined shallow agricultural wells (minimum confined unmixed multiplied by 0.35 plus the average of FP4-28, FP4-83, FP5-45, FP5–73 multiplied by 0.65)

–65.42 –9.11

Fraction of unconfined shallow waterA1. Unconfined shallow water end-member (for all confined urban unmixed, confined urban mixed, and the study PSW; it is the average of all unconfined shallow urban and agricultural wells)

–56.89 –8.35

A2. Unconfined shallow water end-member (for all confined agricultural unmixed wells and FP5-175; it is the average of all unconfined shallow agricultural wells)

–53.91 –7.99

A3. Unconfined shallow water end-member (for FP5-LS, it is FP5-45) –53.60 –7.84B. Confined water end-member (minimum confined agricultural and urban unmixed wells) –77.40 –10.19
Tables 115

Table 13. Concentrations of constituents used to classify oxidation–reduction (redox) conditions in ground-water samples collected for the local-scale TANC study near York, Nebraska.—Continued[redox, oxidation–reduction; mg/L, milligram per liter; μg/L, microgram per liter; nmol/L; nanomole per liter; O2, oxygen reducing, NO3, nitrate reducing; Mn, manganese reducing; Fe, iron reducing; SO4, 

sulfate reducing; FeSO4, iron- to sulfate-reducing; CH4, methanogenic; NC, not collected; na, not applicable; >, greater than; X - Y/Z, mixture (X) of waters with different redox ranging from most oxidized (Y) 

to most reduced (Z); ag, agricultural; —, no detection; study PSW, study public-supply well]

Well or 

sample 

name

Date Well classification

Redox constituents

Redox

classification

Redox 

and well 

classification 

notes

Dissolved

oxygen

(mg/L)

Nitrate 

plus

nitrite 

nitrogen

(mg/L)

Manganese

(μg/L)

Iron

(μg/L)

Sulfate

(mg/L)

Hydrogen

sulfide

(mg/L)

Methane

(μg/L)

Hydrogen

(nmol/L)

Redox threshold value >0.5 >0.5 >50 >100 4 >0.005 >1 naPossible redox if concentration greater than threshold O2 NO3 Mn Fe SO4 CH4

Analysis reporting level 0.01 0.06 0.18 5.0 0.18 0.001 1 na

FP1-63 10/27/2003 Unconfined shallow urban 4.7 25.6 0.4 3.4 186 — — NC O2 Oxic unconfined wells

FP1-63 6/15/2004 Unconfined shallow urban 4.7 27.1 0.1 1.2 142 — NC NC O2 Oxic unconfined wells

FP1-63 10/15/2004 Unconfined shallow urban 5.8 28.8 — 2.9 153 — NC NC O2 Oxic unconfined wells

FP1-63 4/11/2005 Unconfined shallow urban 5.7 28.4 — — 148 — NC NC O2 Oxic unconfined wellsFP2-43 7/28/2004 Unconfined shallow urban 3.4 16.4 0.5 2.4 113 — NC NC O2 Oxic unconfined wellsFP2-43 10/13/2004 Unconfined shallow urban 3.6 17.1 0.5 — 113 — — NC O2 Oxic unconfined wellsFP2-43 3/30/2005 Unconfined shallow urban 3.6 15.9 — — 115 — NC NC O2 Oxic unconfined wellsOFPN-88 11/13/2003 Unconfined shallow urban 3.8 22.4 0.7 14.8 220 — — NC O2 Oxic unconfined wellsOFPN-88 7/27/2004 Unconfined shallow urban 5.2 22.9 0.3 2.4 198 — NC NC O2 Oxic unconfined wellsOFPN-88 10/20/2004 Unconfined shallow urban 5.0 22.1 0.1 2.4 194 — NC NC O2 Oxic unconfined wellsOFPN-88 4/6/2005 Unconfined shallow urban 4.9 22.6 — 5.6 195 — NC NC O2 Oxic unconfined wellsUWT2-23 8/3/2004 Unconfined shallow urban 0.7 15.7 5.2 1.0 137 — NC NC O2 Oxic unconfined wellsUWT2-23 10/20/2004 Unconfined shallow urban 0.8 13.6 2.3 1.3 121 — — NC O2 Oxic unconfined wellsUWT2-23 4/5/2005 Unconfined shallow urban 0.7 15.7 4.2 3.3 128 — NC NC O2 Oxic unconfined wellsUWT3-34 8/4/2004 Unconfined shallow urban 4.2 17.0 0.3 0.7 238 — NC NC O2 Oxic unconfined wellsUWT3-34 10/17/2004 Unconfined shallow urban 6.0 19.0 0.2 3.2 222 — — NC O2 Oxic unconfined wellsUWT3-34 4/5/2005 Unconfined shallow urban 4.5 17.6 — 4.6 249 — NC NC O2 Oxic unconfined wellsUWT4-85 8/4/2004 Unconfined shallow urban 6.2 22.1 0.3 1.3 139 — NC NC O2 Oxic unconfined wellsUWT4-85 10/22/2004 Unconfined shallow urban 6.4 22.0 0.2 — 131 — — NC O2 Oxic unconfined wellsUWT4-85 4/6/2005 Unconfined shallow urban 7.2 23.8 — 4.8 147 — NC NC O2 Oxic unconfined wellsFP3-33 10/28/2003 Unconfined shallow urban 0.2 3.84 496 3.4 90.0 — 1 NC X - NO3/CH4 Reduced unconfined 

wells

FP3-33 7/26/2004 Unconfined shallow urban 0.2 2.41 468 4.4 84.5 — NC NC X - NO3/Mn Reduced unconfined 

wells

FP3-33 10/14/2004 Unconfined shallow urban 0.2 2.56 443 3.4 76.1 — NC NC X - NO3/Mn Reduced unconfined 

wells

FP3-33 4/4/2005 Unconfined shallow urban — 2.51 419 4.2 74.5 — NC NC X - NO3/Mn Reduced unconfined 

wells

OFPS-38 11/11/2003 Unconfined shallow urban 0.2 3.41 3,474 655 57.0 0.026 297 NC X - NO3/CH4 Reduced unconfined 

wells

Table 13. Concentrations of constituents used to classify oxidation–reduction (redox) conditions in ground-water samples collected for the local-scale TANC study near York, Nebraska.[redox, oxidation–reduction; mg/L, milligram per liter; μg/L, microgram per liter; nmol/L; nanomole per liter; O2, oxygen reducing, NO3, nitrate reducing; Mn, manganese reducing; Fe, iron reducing; SO4, 

sulfate reducing; FeSO4, iron- to sulfate-reducing; CH4, methanogenic; NC, not collected; na, not applicable; >, greater than; X - Y/Z, mixture (X) of waters with different redox ranging from most oxidized (Y) 

to most reduced (Z); ag, agricultural; —, no detection; study PSW, study public-supply well]
116 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 13. Concentrations of constituents used to classify oxidation–reduction (redox) conditions in ground-water samples collected for the local-scale TANC study near York, Nebraska.—Continued [redox, oxidation–reduction; mg/L, milligram per liter; μg/L, microgram per liter; nmol/L; nanomole per liter; O2, oxygen reducing, NO3, nitrate reducing; Mn, manganese reducing; Fe, iron reducing; SO4, sulfate reducing; FeSO4, iron- to sulfate-reducing; CH4, methanogenic; NC, not collected; na, not applicable; >, greater than; X - Y/Z, mixture (X) of waters with different redox ranging from most oxidized (Y) to most reduced (Z); ag, agricultural; —, no detection; study PSW, study public-supply well] Well or sample name Date Well classification Redox constituents Redox classifi- cation Redox and well classifi- cation notes Dissolved oxygen (mg/L) Nitrate plus nitrite nitrogen (mg/L) Manga- nese

(μg/L)

Iron

(μg/L)

Sulfate

(mg/L)

Hydrogen

sulfide

(mg/L)

Methane

(μg/L)

Hydrogen

(nmol/L)

Redox threshold value >0.5 >0.5 >50 >100 4 >0.005 >1 naPossible redox if concentration greater than threshold O2 NO3 Mn Fe SO4 CH4

Analysis reporting level 0.01 0.06 0.18 5.0 0.18 0.001 1 na

OFPS-38 7/27/2004 Unconfined shallow urban 0.1 3.57 3,360 1810 57.3

— NC NC X - NO3/Fe Reduced unconfined 

wells

OFPS-38 10/14/2004 Unconfined shallow urban 0.1 2.09 NC 2861 44.6 0.009 NC NC X - NO3/SO4 Reduced unconfined 

wells

OFPS-38 4/14/2005 Unconfined shallow urban 0.2 2.67 3,269 3581 46.0 0.9 NC NC X - NO3/SO4 Reduced unconfined 

wells

UWT1-53 8/3/2004 Unconfined shallow urban 0.1 17.3 1,780 1.6 151

— NC NC X - NO3/Mn Reduced unconfined 

wells

UWT1-53 10/20/2004 Unconfined shallow urban 0.2 18.6 1,823 1.4 151

— 1 NC X - NO3/CH4 Reduced unconfined 

wells

UWT1-53 4/5/2005 Unconfined shallow urban 0.1 18.4 1,810

— 147

— NC NC X - NO3/Mn Reduced unconfined 

wells

AWT1-83 7/29/2004 Unconfined shallow ag 7.1 30.2 0.3 2.1 35.0

— NC NC

O2 Oxic unconfined wells

AWT1-83 10/19/2004 Unconfined shallow ag 7.4 36.6 0.1 4.0 34.1

—

— NC

O2 Oxic unconfined wells

AWT1-83 3/31/2005 Unconfined shallow ag 7.6 36.0

—

— 32.1

— NC NC

O2 Oxic unconfined wells

AWT2-71 7/29/2004 Unconfined shallow ag 6.3 14.3 0.2 0.7 38.0

— NC NC

O2 Oxic unconfined wells

AWT2-71 10/19/2004 Unconfined shallow ag 8.3 23.0 0.3 5.7 31.2

—

— NC

O2 Oxic unconfined wells

AWT2-71 3/31/2005 Unconfined shallow ag 7.6 14.3

— 3.5 37.7

— NC NC

O2 Oxic unconfined wells

AWT3-73 7/30/2004 Unconfined shallow ag 7.5 23.7 1.1 13.6 31.6

— NC NC

O2 Oxic unconfined wells

AWT3-73 10/19/2004 Unconfined shallow ag 8.1 13.8 0.2 3.6 38.6

—

— NC

O2 Oxic unconfined wells

AWT3-73 4/1/2005 Unconfined shallow ag 8.4 23.4

—

— 29.1

— NC NC

O2 Oxic unconfined wells

FP4-28 10/29/2003 Unconfined shallow ag 6.3 15.5 1.5 5.0 99.6

—

— NC

O2 Oxic unconfined wells

FP4-28 7/26/2004 Unconfined shallow ag 5.5 13.8 0.4 2.2 102

— NC NC

O2 Oxic unconfined wells

FP4-28 10/15/2004 Unconfined shallow ag 6.3 16.0

— 3.3 92.9

— NC NC

O2 Oxic unconfined wells

FP4-28 4/12/2005 Unconfined shallow ag 6.2 14.4 1.6

— 89.5

— NC NC

O2 Oxic unconfined wells

FP5-45 7/28/2004 Unconfined shallow ag 5.5 5.21 0.8 1.1 23.4

— NC NC

O2 Oxic unconfined wells

FP5-45 10/18/2004 Unconfined shallow ag 5.8 7.52 0.4 1.7 22.0

—

— NC

O2 Oxic unconfined wells

FP5-45 3/30/2005 Unconfined shallow ag 5.8 5.51

—

— 23.2

— NC NC

O2 Oxic unconfined wells

NWT1-39 7/30/2004 Unconfined shallow ag 4.2 69.5 0.5 3.5 97.4

— NC NC

O2 Oxic unconfined wells

NWT1-39 10/17/2004 Unconfined shallow ag 3.5 77.9 0.5 8.9 96.3

—

— NC

O2 Oxic unconfined wells

NWT1-39 4/1/2005 Unconfined shallow ag 4.5 80.3

—

— 94.2

— NC NC

O2 Oxic unconfined wells
Tables 117

Table 13. Concentrations of constituents used to classify oxidation–reduction (redox) conditions in ground-water samples collected for the local-scale TANC study near York, Nebraska.—Continued[redox, oxidation–reduction; mg/L, milligram per liter; μg/L, microgram per liter; nmol/L; nanomole per liter; O2, oxygen reducing, NO3, nitrate reducing; Mn, manganese reducing; Fe, iron reducing; SO4, 

sulfate reducing; FeSO4, iron- to sulfate-reducing; CH4, methanogenic; NC, not collected; na, not applicable; >, greater than; X - Y/Z, mixture (X) of waters with different redox ranging from most oxidized (Y) 

to most reduced (Z); ag, agricultural; —, no detection; study PSW, study public-supply well]

Well or 

sample 

name

Date Well classification

Redox constituents

Redox

classification

Redox 

and well 

classification 

notes

Dissolved

oxygen

(mg/L)

Nitrate 

plus

nitrite 

nitrogen

(mg/L)

Manganese

(μg/L)

Iron

(μg/L)

Sulfate

(mg/L)

Hydrogen

sulfide

(mg/L)

Methane

(μg/L)

Hydrogen

(nmol/L)

Redox threshold value >0.5 >0.5 >50 >100 4 >0.005 >1 naPossible redox if concentration greater than threshold O2 NO3 Mn Fe SO4 CH4

Analysis reporting level 0.01 0.06 0.18 5.0 0.18 0.001 1 na

FP2-78 10/13/2004 Unconfined deep urban 2.8 3.04 0.4 — 37.0 — 1 NC O2 Oxic unconfined wells

FP3-83 10/27/2003 Unconfined deep urban 2.7 7.03 1.1 3.4 53.9 — — NC O2 Oxic unconfined wellsFP4-83 10/30/2003 Unconfined deep ag 3.1 2.15 0.8 3.4 28.1 — — NC O2 Oxic unconfined wellsFP4-83 4/14/2005 Unconfined deep ag 3.5 NC 0.4 — 28.3 — NC NC O2 Oxic unconfined wellsFP5-73 10/18/2004 Unconfined deep ag 6.8 7.36 0.8 4.9 27.4 — — NC O2 Oxic unconfined wellsFP1-147 10/22/2003 Confined urban unmixed 0.1 0.03 360 143 30.4 — — 0.61 Fe Upper confined aquifer

FP1-147 6/14/2004 Confined urban unmixed 0.4 — 330 115 29.0 — NC NC Fe Upper confined aquifer

FP1-147 4/11/2005 Confined urban unmixed 0.1 — 336 94.0 30.7 — NC NC Mn Upper confined aquifer

FP1-185 10/21/2003 Confined urban unmixed 0.2 0.03 285 73.0 32.1 — 2 0.41 X - Mn/CH4 Upper confined aquifer

FP1-185 6/14/2004 Confined urban unmixed 0.2 — 272 111 32.2 — NC NC Fe Upper confined aquifer

FP2-152 10/12/2004 Confined urban unmixed 0.1 — 368 39.3 31.5 0.012 — NC SO4 Upper confined aquifer

FP3-130 10/15/2003 Confined urban unmixed 0.1 0.07 0.2 4.6 30.0 — 1 NC CH4 Upper confined aquiferOFPN-167 10/30/2003 Confined urban unmixed 0.1 0.03 330 58.0 33.3 — — NC Mn Upper confined aquifer

FP4-168 10/23/03 Confined ag unmixed 0.1 0.17 221 3.4 31.1 — 1 0.81 X - Mn/CH4 Upper confined aquifer

FP4-168 4/14/2005 Confined ag unmixed 0.1 0.08 213 3.4 31.9 — NC NC Mn Upper confined aquifer

FP3-162 10/28/2003 Confined urban mixed 0.1 9.31 74.0 3.4 139 — 3 NC X - NO3/CH4 Upper confined aquifer

FP3-162 10/16/2004 Confined urban mixed 0.1 13.1 18.0 1.8 154 — NC NC NO3 Upper confined aquiferOFPS-157 11/11/2003 Confined urban mixed 0.2 4.47 29.2 6.4 73.1 — — NC NO3 Upper confined aquiferOFPS-157 10/14/2004 Confined urban mixed 0.1 — 276 5.5 52.4 — NC NC Mn Upper confined aquifer

OFPS-157 4/11/2005 Confined urban mixed 0.1 — 434 — 58.9 — NC NC Mn Upper confined aquifer

FP5-175 10/21/2004 Confined ag mixed 0.1 — 464 102 71.8 0.007 — NC FeSO4 Upper confined aquifer

FP5-LS 10/21/2004 Confined ag mixed 4.7 6.37 1.6 5.7 34.0 — — NC O2 Upper confined aquiferStudy PSW 10/20/2003 Study PSW 0.1 0.17 397 159 44.5 — 1 0.34 X - Fe/CH4 Upper confined aquiferStudy PSW 6/18/2004 Study PSW 0.8 0.18 356 163 45.2 — — NC Fe Upper confined aquifer

Study PSW 10/15/2004 Study PSW 0.1 0.20 365 169 47.0 — NC NC Fe Upper confined aquifer

Study PSW 4/7/2005 Study PSW 0.1 0.17 352 182 46.5 — NC NC Fe Upper confined aquifer

Study PSW, 

depth 42.7 m

6/18/2004 Study PSW 0.3 0.16 387 169 48.9 — — NC Fe Upper confined aquifer
118 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 13. Concentrations of constituents used to classify oxidation–reduction (redox) conditions in ground-water samples collected for the local-scale TANC study near York, Nebraska.—Continued [redox, oxidation–reduction; mg/L, milligram per liter; μg/L, microgram per liter; nmol/L; nanomole per liter; O2, oxygen reducing, NO3, nitrate reducing; Mn, manganese reducing; Fe, iron reducing; SO4, sulfate reducing; FeSO4, iron- to sulfate-reducing; CH4, methanogenic; NC, not collected; na, not applicable; >, greater than; X - Y/Z, mixture (X) of waters with different redox ranging from most oxidized (Y) to most reduced (Z); ag, agricultural; —, no detection; study PSW, study public-supply well] Well or sample name Date Well classification Redox constituents Redox classifi- cation Redox and well classifi- cation notes Dissolved oxygen (mg/L) Nitrate plus nitrite nitrogen (mg/L) Manga- nese

(μg/L)

Iron

(μg/L)

Sulfate

(mg/L)

Hydrogen

sulfide

(mg/L)

Methane

(μg/L)

Hydrogen

(nmol/L)

Redox threshold value >0.5 >0.5 >50 >100 4 >0.005 >1 naPossible redox if concentration greater than threshold O2 NO3 Mn Fe SO4 CH4

Analysis reporting level 0.01 0.06 0.18 5.0 0.18 0.001 1 na

Study PSW, 

depth 45.7 m

6/21/2004 Study PSW 0.4 0.15 400 179 53.0

—

— NC Fe Upper confined aquifer

Study PSW, 

depth 48.8 m

6/17/2004 Study PSW 0.7 0.19 419 238 67.4

—

— NC Fe Upper confined aquifer

Study PSW, 

depth 51.8 m

6/17/2004 Study PSW 0.9 0.47 571 277 151

—

— NC Fe Upper confined aquifer

Study PSW, 

depth 61.0 m

6/16/2004 Study PSW 0.6 0.15 474 455 107

—

— NC Fe Upper confined aquifer

FP1-247 10/20/2003 Confined urban unmixed 0.2 0.07 247 8.8 41.3

— 6 0.35 X - Mn/CH4 Lower confined lenses

FP1-247 6/14/2004 Confined urban unmixed 0.2

— 226 8.4 31.8 0.009 NC NC SO4 Lower confined lenses

OFPN-276 11/12/2003 Confined urban unmixed 0.1 0.03 200 16.7 94.1 0.011 1 NC X - SO4/CH4 Lower confined lensesOFPS-225 10/14/2004 Confined urban unmixed 0.1

— 314 138 36.6 0.112 11 NC X - FeSO4/CH4 Lower confined lenses

FP4-231 10/16/2003 Confined ag unmixed 0.1 0.03 28.0 3.4 43.5 0.044

— NC SO4 Lower confined lenses

FP2-250 10/12/2004 Confined urban mixed 0.1

— 869 178 1,057 0.162 4 NC X - FeSO4/CH4 Lower confined lenses

FP3-218 10/15/2003 Confined urban mixed 0.1 0.03 481 491 104 0.116

— NC FeSO4 Lower confined lenses

FP3-218 10/16/2004 Confined urban mixed 0.1

— 538 501 110 0.011 NC NC FeSO4 Lower confined lenses
Tables 119

Table 14. Results of analyses of volatile organic compounds and gasoline oxygenates in samples, and analytes detected in ground-water samples during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued[USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; m, meter; V, censored value possibly affected by contamination during sample collection; study PSW, study public-supply well; VOC, volatile organic compound; —, not detected]

Well or sample 

name 

Date Time Well classification (see table 11)

Tetrachloroethylene 

(PCE)

Trichloroethylene 

(TCE)

cis-1,2-

Dichloroethylene 

(cis-1,2 

DCE)

trans-1,2-

Dichloroethylene 

(trans-1,2 DCE)

Chloroform 

(Trichloromethane)

1,1,2-Trichlorotrifluoroethane 

(CFC-113)

Dichlorodifluoromethane 

(CFC-12)

Benzene

1,1,1-

Trichloroethane 

(TCA)

μg/L μg/L μg/L μg/L μg/L μg/L μg/L μg/L μg/L

Laboratory reporting level (LRL) 0.06 0.04 0.024 0.032 0.024 0.038 0.18 0.021 0.032USGS parameter code 34475 39180 77093 34546 32106 77652 34668 34030 34506FP4-231 10/16/2003 1200 Confined agricultural unmixed — — — — — — — E0.03 —

FP4-83 10/30/2003 1000 Unconfined deep agricultural E0.06 — — — — — — — —

FP4-28 10/29/2003 1100 Unconfined shallow agricultural E0.02 — — — — — — — —

FP4-28 7/26/2004 1000 Unconfined shallow agricultural — — — — V0.04 — — — —

FP4-28 4/12/2005 1200 Unconfined shallow agricultural E0.02 — — — — — — — —

Study PSW 10/20/2003 1500 Study PSW 0.92 0.51 0.27 — — — — — —

Study PSW 6/18/2004 1500 Study PSW 0.80 0.39 0.21 E0.02 — — — — —

Study PSW 10/15/2004 1200 Study PSW 0.80 0.37 0.19 E0.02 — — — — —

Study PSW 4/7/2005 0900 Study PSW 0.79 0.36 0.21 E0.02 — — — — —

Study PSW, depth 

42.7 m

6/18/2004 1100 Study PSW 1.29 0.58 0.31 E0.03 — — — — —

Study PSW, depth 

45.7 m

6/21/2004 1000 Study PSW 1.58 0.69 0.37 E0.03 — — — — —

Study PSW, depth 

48.8 m

6/17/2004 1800 Study PSW 2.38 1.15 0.61 E0.05 — — — — —

Study PSW, depth 

51.8 m

6/17/2004 1300 Study PSW 4.56 3.2 1.81 0.17 E0.01 — E0.04 E0.01 —

Study PSW, depth 

61.0 m

6/16/2004 1300 Study PSW 0.81 1.79 1.32 E0.1 — — — — —

FP1-247 10/20/2003 1200 Confined urban unmixed 0.42 0.27 0.16 — — — — — —

FP3-130 10/15/2003 1100 Confined urban unmixed E0.03 — — — — — — E0.03 —

OFPN-276 11/12/2003 0900 Confined urban unmixed E0.02 — — — — — — E0.01 —

FP2-250 10/12/2004 1000 Confined urban mixed — — — — — — — E0.04 —

FP3-162 10/28/2003 1000 Confined urban mixed — 0.14 E0.03 — V0.02 — — — —

FP3-162 10/16/2004 0900 Confined urban mixed — 0.15 E0.03 — — — — — —

FP2-78 10/13/2004 1000 Unconfined deep urban — — — — 2.22 — — — —

FP3-83 10/27/2003 1500 Unconfined deep urban E0.02 — — — — — — — —

FP1-63 10/27/2003 1100 Unconfined shallow urban 11.84 3.97 3.16 E0.11 V0.02 — E0.22 — E0.02FP1-63 6/15/2004 0800 Unconfined shallow urban 6.18 1.39 0.73 E0.03 E0.01 — E0.13 — —

FP1-63 7/27/2004 1600 Unconfined shallow urban 6.08 1.39 0.79 E0.03 — — E0.14 — —

FP1-63 10/15/2004 1400 Unconfined shallow urban 6.85 1.8 1.18 E0.04 E0.02 — E0.16 — —

Table 14. Results of analyses of volatile organic compounds and gasoline oxygenates in samples, and analytes detected in ground-water samples during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska. [USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; m, meter; V, censored value possibly affected by contamination during sample collection; study PSW, study public-supply well; VOC, volatile organic compound; —, not detected]
120 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 14. Results of analyses of volatile organic compounds and gasoline oxygenates in samples, and analytes detected in ground-water samples during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; m, meter; V, censored value possibly affected by contamination during sample collection; study PSW, study public-supply well; VOC, volatile organic compound; —, not detected] Well or sample name Date Time Well classification (see table 11) Tetrachloro- ethylene (PCE) Trichloro- ethylene (TCE) cis-1,2- Dichloro- ethylene 

(cis-1,2 

DCE)

trans-1,2-

Dichloroethylene 

(trans-1,2 DCE)

Chloroform 

(Trichloromethane)

1,1,2-Trichlorotrifluoroethane 

(CFC-113)

Dichlorodifluoromethane 

(CFC-12)

Benzene

1,1,1-

Trichloroethane 

(TCA)

μg/L μg/L μg/L μg/L μg/L μg/L μg/L μg/L μg/L

Laboratory reporting level (LRL) 0.06 0.04 0.024 0.032 0.024 0.038 0.18 0.021 0.032USGS parameter code 34475 39180 77093 34546 32106 77652 34668 34030 34506FP1-63 4/11/2005 1300 Unconfined shallow urban 7.95 2.28 1.39 E0.04 E0.02

— E0.18

—

—

FP3-33 10/28/2003 1300 Unconfined shallow urban

—

—

—

— E0.06

—

—

—

—

OFPN-88 11/13/2003 1000 Unconfined shallow urban 5.66 2.25 1.43 E0.06

—

—

—

—

—

OFPN-88 7/27/2004 1000 Unconfined shallow urban 3.85 0.88 0.47 E0.02

—

—

—

—

—

OFPN-88 10/20/2004 0900 Unconfined shallow urban 3.74 0.72 0.31

—

—

—

—

—

—

OFPN-88 4/6/2005 1500 Unconfined shallow urban 3.2 0.65 0.35 E0.02 E0.01

—

—

—

—

OFPS-38 11/11/2003 1400 Unconfined shallow urban E0.04

—

—

—

—

—

—

—

—

OFPS-38 7/27/2004 1400 Unconfined shallow urban E0.04

—

—

—

—

—

—

—

—

OFPS-38 4/14/2005 1500 Unconfined shallow urban E0.04 E0.02

—

—

—

—

—

—

—

UWT1-53 8/3/2004 1000 Unconfined shallow urban 0.16 2.4 E0.1

—

—

—

—

— E0.04

UWT1-53 10/20/2004 1200 Unconfined shallow urban 0.19 2.16 E0.08

—

—

—

—

— E0.04

UWT1-53 4/5/2005 1200 Unconfined shallow urban 0.72 1.15 E0.05

— E0.01 E0.06

—

— E0.02

UWT2-23 8/3/2004 1300 Unconfined shallow urban E0.04

—

—

—

—

—

—

—

—

UWT2-23 10/20/2004 1400 Unconfined shallow urban E0.04

—

—

—

—

—

—

—

—

UWT2-23 4/5/2005 1000 Unconfined shallow urban E0.04

—

—

—

—

—

—

—

—

UWT3-34 8/4/2004 1000 Unconfined shallow urban 0.53

—

—

—

— 0.75

—

—

—

UWT3-34 10/17/2004 0900 Unconfined shallow urban 0.43

—

—

—

— 0.57

—

—

—

UWT3-34 4/5/2005 1500 Unconfined shallow urban 0.4

—

—

—

— 0.6

—

—

—

UWT4-85 8/4/2004 1300 Unconfined shallow urban 1.1

—

—

— V0.02 E0.07

—

—

—

UWT4-85 10/22/2004 1000 Unconfined shallow urban 1.16

—

—

— E0.02 E0.06

—

—

—

UWT4-85 4/6/2005 1100 Unconfined shallow urban 0.98

—

—

— E0.03 E0.05

—

— E0.01

Samples with detections 40 25 24 16 10 7 6 5 5

Total samples analyzed1,2,3 86 86 86 86 82 86 86 86 86

Sample detection frequency (percent) 47 29 28 19 12 8 7 6 6

Wells with detections 14 7 6 4 7 3 2 5 3

Total wells analyzed 36 36 36 36 36 36 36 36 36

Well detection frequency (percent) 39 19 17 11 19 8 6 14 8

Maximum concentration 11.84 3.97 3.16 0.17 2.22 0.75 E0.22 E0.04 E0.04
Tables 121

Table 14. Results of analyses of volatile organic compounds and gasoline oxygenates in samples, and analytes detected in ground-water samples during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued[USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; m, meter; V, censored value possibly affected by contamination during sample collection; study PSW, study public-supply well; VOC, volatile organic compound; —, not detected]

Well or sample 

name 

Date Time Well classification (see table 11)

tert-Butyl 

methyl ether 

(MTBE)

Tetrachloromethane 

(carbon 

tetrachloride)

Toluene

1,2-

Dichloroethane 

(1,2 DCA)

1,2-

Dichloropropane

Tetrahydrofuran 1,1- Dichloro- ethane 1,1- Dichloro- ethylene (DCE) Bromo- dichloro- methane

μg/L μg/L μg/L μg/L μg/L μg/L μg/L μg/L μg/L

Laboratory reporting level (LRL) 0.17 0.06 0.05 0.13 0.029 2.2 0.035 0.024 0.028

USGS parameter code 78032 32102 34010 32103 34541 81607 34496 34501 32101

FP4-231 10/16/2003 1200 Confined agricultural unmixed — — E0.07 — — — — — —

FP4-83 10/30/2003 1000 Unconfined deep agricultural — — — — — — — — —

FP4-28 10/29/2003 1100 Unconfined shallow agricultural — — — — — — — — —

FP4-28 7/26/2004 1000 Unconfined shallow agricultural — — — — — — — — —

FP4-28 4/12/2005 1200 Unconfined shallow agricultural — — — — — — — — —

Study PSW 10/20/2003 1500 Study PSW — — — — — — — — —

Study PSW 6/18/2004 1500 Study PSW — — — — — — — — —

Study PSW 10/15/2004 1200 Study PSW — — — — — — — — —

Study PSW 4/7/2005 0900 Study PSW — — — — — — — — —

Study PSW, depth 

42.7 m

6/18/2004 1100 Study PSW — — — — — — — — —

Study PSW, depth 

45.7 m

6/21/2004 1000 Study PSW — — — — — — — — —

Study PSW, depth 

48.8 m

6/17/2004 1800 Study PSW — — — — — — — — —

Study PSW, depth 

51.8 m

6/17/2004 1300 Study PSW — — — — — — — — —

Study PSW, depth 

61.0 m

6/16/2004 1300 Study PSW — — — — — — — — —

FP1-247 10/20/2003 1200 Confined urban unmixed — — — — — — — — —

FP3-130 10/15/2003 1100 Confined urban unmixed — — 0.10 — — — — — —

OFPN-276 11/12/2003 0900 Confined urban unmixed — — — — — — — — —

FP2-250 10/12/2004 1000 Confined urban mixed — — — — — — — — —

FP3-162 10/28/2003 1000 Confined urban mixed — — — 4.96 E0.06 — — — —

FP3-162 10/16/2004 0900 Confined urban mixed — — — 5.81 E0.07 — — — —

FP2-78 10/13/2004 1000 Unconfined deep urban — — — — — — — — 0.19

FP3-83 10/27/2003 1500 Unconfined deep urban — — — — — — — — —

FP1-63 10/27/2003 1100 Unconfined shallow urban — — — — — — — — —

FP1-63 6/15/2004 0800 Unconfined shallow urban — — — — — — — — —

FP1-63 7/27/2004 1600 Unconfined shallow urban — — — — — — — — —
122 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 14. Results of analyses of volatile organic compounds and gasoline oxygenates in samples, and analytes detected in ground-water samples during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; m, meter; V, censored value possibly affected by contamination during sample collection; study PSW, study public-supply well; VOC, volatile organic compound; —, not detected] Well or sample name Date Time Well classification (see table 11) tert-Butyl methyl ether (MTBE) Tetra- chloro- methane (carbon tetra- chloride) Toluene 1,2- Dichloro- ethane (1,2 DCA) 1,2- Dichloro- propane Tetra- hydro- furan 1,1- Dichloro- ethane 1,1- Dichloro- ethylene (DCE) Bromo- dichloro- methane μg/L μg/L μg/L μg/L μg/L μg/L μg/L μg/L μg/L Laboratory reporting level (LRL) 0.17 0.06 0.05 0.13 0.029 2.2 0.035 0.024 0.028 USGS parameter code 78032 32102 34010 32103 34541 81607 34496 34501 32101 FP1-63 10/15/2004 1400 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

FP1-63 4/11/2005 1300 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

FP3-33 10/28/2003 1300 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

OFPN-88 11/13/2003 1000 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

OFPN-88 7/27/2004 1000 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

OFPN-88 10/20/2004 0900 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

OFPN-88 4/6/2005 1500 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

OFPS-38 11/11/2003 1400 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

OFPS-38 7/27/2004 1400 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

OFPS-38 4/14/2005 1500 Unconfined shallow urban

—

—

—

—

— E0.7

—

—

—

UWT1-53 8/3/2004 1000 Unconfined shallow urban

—

—

—

—

—

—

— E0.03

—

UWT1-53 10/20/2004 1200 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

UWT1-53 4/5/2005 1200 Unconfined shallow urban

—

—

—

—

—

— E0.01

—

—

UWT2-23 8/3/2004 1300 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

UWT2-23 10/20/2004 1400 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

UWT2-23 4/5/2005 1000 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

UWT3-34 8/4/2004 1000 Unconfined shallow urban E0.13

—

—

—

—

—

—

—

—

UWT3-34 10/17/2004 0900 Unconfined shallow urban 0.13

—

—

—

—

—

—

—

—

UWT3-34 4/5/2005 1500 Unconfined shallow urban

—

—

—

—

—

—

—

—

—

UWT4-85 8/4/2004 1300 Unconfined shallow urban 5.02 E0.05

—

—

—

—

—

—

—

UWT4-85 10/22/2004 1000 Unconfined shallow urban 4.95 E0.04

—

—

—

—

—

—

—

UWT4-85 4/6/2005 1100 Unconfined shallow urban 3.01 E0.04

—

—

—

—

—

—

—

Samples with detections 5 3 2 2 2 1 1 1 1

Total samples analyzed1,2,3 86 86 86 86 86 86 86 86 86

Sample detection frequency (percent) 6 3 2 2 2 1 1 1 1

Wells with detections 2 1 2 1 1 1 1 1 1

Total wells analyzed 36 36 36 36 36 36 36 36 36

Well detection frequency (percent) 6 3 6 3 3 3 3 3 3

Maximum concentration 5.02 E0.05 0.10 5.81 E0.07 E0.7 E0.01 E0.03 0.19
Tables 123

Table 14. Results of analyses of volatile organic compounds and gasoline oxygenates in samples, and analytes detected in ground-water samples during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued[USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; m, meter; V, censored value possibly affected by contamination during sample collection; study PSW, study publicsupply well; VOC, volatile organic compound; —, not detected]

Well or sample 

name 

Date Time Well classification (see table 11)

1,2,4-

Trimethylbenzene Carbon disulfide Ethyl- benzene m- + p- Xylene o-Xylene 2-Ethyl- toluene 1,2,3- Trimethyl- benzene VOC 

detections 

per sample

μg/L μg/L μg/L μg/L μg/L μg/L μg/L

Laboratory reporting level (LRL) 0.056 0.038 0.030 0.06 0.038 0.06 0.06

USGS parameter code 77222 77041 34371 85795 77135 77220 77221

FP4-231 10/16/2003 1200 Confined agricultural unmixed — — — — — — — 2

FP4-83 10/30/2003 1000 Unconfined deep agricultural — — — — — — — 1

FP4-28 10/29/2003 1100 Unconfined shallow agricultural — — — — — — — 1

FP4-28 7/26/2004 1000 Unconfined shallow agricultural — — — — — — — 1

FP4-28 4/12/2005 1200 Unconfined shallow agricultural — — — — — — — 1

Study PSW 10/20/2003 1500 Study PSW — — — — — — — 3

Study PSW 6/18/2004 1500 Study PSW — — — — — — — 4

Study PSW 10/15/2004 1200 Study PSW — — — — — — — 7

Study PSW 4/7/2005 0900 Study PSW — — — — — — — 4

Study PSW, depth 

42.7 m

6/18/2004 1100 Study PSW — — — — — — — 4

Study PSW, depth 

45.7 m

6/21/2004 1000 Study PSW — — — — — — — 4

Study PSW, depth 

48.8 m

6/17/2004 1800 Study PSW — — — — — — — 4

Study PSW, depth 

51.8 m

6/17/2004 1300 Study PSW V0.02 — — — — — — 8

Study PSW, depth 

61.0 m

6/16/2004 1300 Study PSW — — — — — — — 4

FP1-247 10/20/2003 1200 Confined urban unmixed — — — — — — — 3

FP3-130 10/15/2003 1100 Confined urban unmixed — — — — — — — 3

OFPN-276 11/12/2003 0900 Confined urban unmixed — — — — — — — 2

FP2-250 10/12/2004 1000 Confined urban mixed E0.04 E0.04 E0.07 E0.06 E0.03 E0.03 E0.09 8

FP3-162 10/28/2003 1000 Confined urban mixed — — — — — — — 5

FP3-162 10/16/2004 0900 Confined urban mixed — — — — — — — 6

FP2-78 10/13/2004 1000 Unconfined deep urban — — — — — — — 4

FP3-83 10/27/2003 1500 Unconfined deep urban — — — — — — — 1

FP1-63 10/27/2003 1100 Unconfined shallow urban — — — — — — — 7

FP1-63 6/15/2004 0800 Unconfined shallow urban — — — — — — — 6

FP1-63 7/27/2004 1600 Unconfined shallow urban — — — — — — — 5

FP1-63 10/15/2004 1400 Unconfined shallow urban — — — — — — — 9

FP1-63 4/11/2005 1300 Unconfined shallow urban — — — — — — — 6

FP3-33 10/28/2003 1300 Unconfined shallow urban — — — — — — — 1
124 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 14. Results of analyses of volatile organic compounds and gasoline oxygenates in samples, and analytes detected in ground-water samples during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; m, meter; V, censored value possibly affected by contamination during sample collection; study PSW, study public- supply well; VOC, volatile organic compound; —, not detected] Well or sample name Date Time Well classification (see table 11) 1,2,4- Trimethyl- benzene Carbon disulfide Ethyl- benzene

m- + 

pXylene o-Xylene 2-Ethyl- toluene 1,2,3- Trimethyl- benzene VOC 

detections 

per sample

μg/L μg/L μg/L μg/L μg/L μg/L μg/L

Laboratory reporting level (LRL) 0.056 0.038 0.030 0.06 0.038 0.06 0.06

USGS parameter code 77222 77041 34371 85795 77135 77220 77221

OFPN-88 11/13/2003 1000 Unconfined shallow urban

—

—

—

—

—

—

— 4

OFPN-88 7/27/2004 1000 Unconfined shallow urban

—

—

—

—

—

—

— 4

OFPN-88 10/20/2004 0900 Unconfined shallow urban

—

—

—

—

—

—

— 3

OFPN-88 4/6/2005 1500 Unconfined shallow urban

—

—

—

—

—

—

— 5

OFPS-38 11/11/2003 1400 Unconfined shallow urban

—

—

—

—

—

—

— 1

OFPS-38 7/27/2004 1400 Unconfined shallow urban

—

—

—

—

—

—

— 1

OFPS-38 4/14/2005 1500 Unconfined shallow urban

—

—

—

—

—

—

— 3

UWT1-53 8/3/2004 1000 Unconfined shallow urban

—

—

—

—

—

—

— 5

UWT1-53 10/20/2004 1200 Unconfined shallow urban

—

—

—

—

—

—

— 4

UWT1-53 4/5/2005 1200 Unconfined shallow urban

—

—

—

—

—

—

— 7

UWT2-23 8/3/2004 1300 Unconfined shallow urban

—

—

—

—

—

—

— 1

UWT2-23 10/20/2004 1400 Unconfined shallow urban

—

—

—

—

—

—

— 1

UWT2-23 4/5/2005 1000 Unconfined shallow urban

—

—

—

—

—

—

— 1

UWT3-34 8/4/2004 1000 Unconfined shallow urban

—

—

—

—

—

—

— 3

UWT3-34 10/17/2004 0900 Unconfined shallow urban

—

—

—

—

—

—

— 6

UWT3-34 4/5/2005 1500 Unconfined shallow urban

—

—

—

—

—

—

— 2

UWT4-85 8/4/2004 1300 Unconfined shallow urban

—

—

—

—

—

—

— 5

UWT4-85 10/22/2004 1000 Unconfined shallow urban

—

—

—

—

—

—

— 5

UWT4-85 4/6/2005 1100 Unconfined shallow urban

—

—

—

—

—

—

— 6

Samples with detections 1 1 1 1 1 1 1

Total samples analyzed1,2,3 85 86 86 86 86 86 86

Sample detection frequency (percent) 1 1 1 1 1 1 1

Wells with detections 1 1 1 1 1 1 1

Total wells analyzed 36 36 36 36 36 36 36

Well detection frequency (percent) 3 3 3 3 3 3 3

Maximum concentration E0.04 E0.04 E0.07 E0.06 E0.03 E0.03 E0.09

1Less than 86 for some analytes for which data was censored.2Results not shown for 47 samples having no detections of any of 88 VOC or gasoline oxygenate analytes.3Results not shown for 63 of 88 VOC or gasoline oxygenate analytes that were not detected.
Tables 125

Table 15. Results of analyses of pesticides and pesticide degradates in ground-water samples in which analytes were detected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.[USGS, U.S. Geological Survey; study PSW, study public-supply well. ESA, ethane sulfonic acid; OA, oxanilic acid; NC, not collected; —, not detected; μg/L, microgram per liter; E, estimated value; m, meter]

Well or 

sample 

name

Date 

collected Time

Well classification 

(see table 11)

Atrazine 

(μg/L)

Deethylatrazine

(μg/L)

Prometon

(μg/L)

Acetochlor 

ESA

(μg/L)

Alachlor 

ESA

(μg/L)

Alachlor 

OA

(μg/L)

Metolachlor 

ESA

(μg/L)

Metolachlor 

OA

(μg/L)

Pesticide 

detections 

per well Laboratory Reporting Level (LRL) 0.007 0.006 0.005 0.02 0.02 0.02 0.02 0.02

USGS parameter code 39632 04040 04037 61029 50009 61031 61043 61044

FP4-28 10/29/2003 1100 Unconfined shallow 

agricultural

0.062 E0.042 — — 0.02 — 0.04 — 4

Study 

PSW, depth 42.7 m

6/18/2004 1100 Study PSW — — — 0.02 — — — — 1

FP3-162 10/28/2003 1000 Confined urban mixed 0.007 E0.0056 — — 0.11 — — — 3

FP3-218 10/15/2003 1800 Confined urban mixed — — — — 0.07 0.02 — — 2

OFPS-157 11/11/2003 1000 Confined urban mixed — — — — 0.06 — 0.03 — 2

FP1-63 10/27/2003 1100 Unconfined shallow 

urban

E0.0062 E0.0052 — — — — — — 2

FP1-63 6/15/2004 0800 Unconfined shallow 

urban

0.007 — — NC NC NC NC NC 1

OFPS-38 11/11/2003 1400 Unconfined shallow 

urban

0.018 E0.0071 E0.0017 — 0.11 0.09 0.03 0.03 7

Detections 4 0 0 1 5 2 3 1

Total samples analyzed1,2 26 26 26 22 22 22 22 22

1Results not shown for 18 samples having no detections of any of 83 pesticide and pesticide degradate analytes.2Results not shown for 75 of 83 pesticide and pesticide degradate analytes that were not detected.
126 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Well or sample name Date

Calculated from dissolved gases Calculated atmospheric mixing ratio corrected for 

excess air and recharge temperature

USGS Dissolved Gas Laboratory, 

Reston, Virginia USGS CFC Laboratory, Reston, Virginia

Recharge 

temperature

(°C)

Excess air

(mL/kg)

Excess N2

(mg/L)

CFC-11

(pptv)

CFC-12

(pptv)

CFC-113

(pptv)

SF6

(pptv)

FP1-63 10/27/2003 8.1 6.1 0 contam contam 44.0 NC

FP1-147 10/22/2003 13.0 3.5 0 5.4 12.4 0.0 0.40

FP1-185 10/21/2003 11.2 3.3 0 3.8 3.6 4.6 0.11

OFPS-38 11/11/2003 13.7 2.9 4 12.0 166.7 0.0 1.26

OFPS-157 11/11/2003 11.2 2.3 3.5 contam contam 3.6 1.07

FP3-33 10/28/2003 11.9 5.5 0 31.6 537.9 4.4 NC

FP3-83 10/27/2003 13.3 2.7 0 5.7 contam 0.0 NC

FP3-130 10/15/2003 11.8 2.6 0 19.9 12.0 3.9 1.23

FP3-162 10/28/2003 11.6 5.1 3 156.7 contam 1.0 1.14

FP4-28 10/29/2003 11.8 2.4 0 contam contam contam NC

FP4-83 10/30/2005 11.2 1.1 0 9.3 34.5 contam NC

FP4-168 10/23/2005 11.0 2.8 0 5.2 5.3 33.3 0.63

Study PSW 10/20/2003 11.7 3.7 1 38.1 contam 0.0 0.24

Study PSW 6/18/2004 10.9 6.5 1.5 NC NC NC 0.27

Study PSW depth 42.7 m 6/18/2004 11.9 5.7 2 NC NC NC 0.45

Study PSW depth 45.7 m 6/21/2004 11.5 4.0 1.5 NC NC NC 0.40

Study PSW depth 48.8 m 6/17/2004 11.7 3.8 3 NC NC NC 0.41

FP1-247 10/20/2003 11.1 3.0 1 NC NC NC NC

FP3-218 10/15/2003 11.8 4.3 4 NC NC NC NC

FP4-231 10/16/2003 11.8 3.1 0 NC NC NC NC

Study PSW depth 51.8 m 6/17/2004 11.7 2.6 7.5 NC NC NC NC

Study PSW depth 61.0 m 6/16/2004 11.8 4.8 6 NC NC NC NC

Table 16. Age tracer and supporting data for ground-water samples collected from the local-scale TANC study area near York, 

Nebraska. 

[All apparent ages calculated based on a recharge altitude of 488 meters. He, helium; 3H, tritium; 3He, helium-3 concentration; 4He, helium-4 concentration; 

(3He)tritium, helium-3 concentration from decay of tritium; δ, isotopic ratio of light to heavy isotope relative to standard reference; B.P., before present; 

°C, degree Celsius; CFC, chlorofluorocarbon; cm3STP/g, cubic centimeter of gas at Standard Temperature and Pressure per gram of water; contam, tracer 

concentrations greater than would result from atmospheric sources, indicating contamination for the purposes of age dating; italics, tracer was not detected 

where it was expected to be based on other tracers; m, meter; mL/kg, milliliter per kilogram; mg/L, milligram per liter; N, nitrogen; N2, nitrogen gas; NC, 

not collected; Ne, neon concentration; P, phosphorus; pptv, parts per trillion by volume; SF6, sulfur hexafluoride; terrigenic, percent of 3He from terrigenic 

sources; TU, tritium unit; USGS, U.S. Geological Survey; Study PSW, study public-supply well]
Tables 127

Well or sample 

name

Date

Lamont-Doherty Earth Observatory, Noble Gas Laboratory, 

Palisades, New York Carbon-14 

percent 

modern

Uncorrected 

carbon-14 

age 

(years B.P.)

δ13C 

(per 

mil)

Corrected1

carbon-14 age 

(years B.P.) 3

H 

(TU)

δ3He 

(per 

mil)

4

He 

cm3

STP/g 

x 10-8

Ne 

cm3

STP/g 

x 10-8

(

3

He)

tritium 

(TU)

Terrigenic He 

(percent)

FP1-63 10/27/2003 1.04 0.92 4.38 19.43 –0.16 –3.4 NC NC NC NC

FP1-147 10/22/2003 1.47 –47.07 17.84 26.35 42.24 90.5 NC NC NC NC

FP1-185 10/21/2003 0.4 NC NC NC NC NC 45.2 6,568 –8.5 1,368

OFPS-38 11/11/2003 5.5 NC NC NC NC NC 78.3 Not dateable –8.3 Not dateable

OFPS-157 11/11/2003 0.2 NC NC NC NC NC 40.6 7,456 –8.8 1,767

FP3-33 10/28/2003 4.16 NC NC NC NC NC 80.1 Not dateable –11.7 Not dateable

FP3-83 10/27/2003 2.9 NC NC NC NC NC 74.9 Not dateable –11.1 Not dateable

FP3-130 10/15/2003 0.18 NC NC NC NC NC NC NC NC NC

FP3-162 10/28/2003 7.72 NC NC NC NC NC NC NC NC NC

FP4-28 10/29/2003 6.31 –5.66 6.26 22.88 3.02 13.2 NC NC NC NC

FP4-83 10/30/2005 0.66 –27.19 21.05 40.91 25.23 49.1 NC NC NC NC

FP4-168 10/23/2005 0.01 NC NC NC NC NC NC NC NC NC

Study PSW 10/20/2003 0.60 NC NC NC NC NC NC NC NC NC

Study PSW 6/18/2004 0.61 –54.2 57.01 22.88 110.79 90.3 NC NC NC NC

Study PSW depth 

42.7 m

6/18/2004 0.87 –52.77 48.31 57.97 98.37 90.5 NC NC NC NC

Study PSW depth 

45.7 m

6/21/2004 1.04 0.92 4.38 19.43 -0.16 –3.4 NC NC NC NC

Study PSW depth 

48.8 m

6/17/2004 1.47 –47.07 17.84 26.35 42.24 90.5 NC NC NC NC

FP1-247 10/20/2003 0.4 NC NC NC NC NC 45.2 6,568 –8.5 1,368

FP3-218 10/15/2003 5.5 NC NC NC NC NC 78.3 Not dateable –8.3 Not dateable

FP4-231 10/16/2003 0.2 NC NC NC NC NC 40.6 7,456 –8.8 1,767

Study PSW depth 

51.8 m

6/17/2004 4.16 NC NC NC NC NC 80.1 Not dateable –11.7 Not dateable

Study PSW depth 

61.0 m

6/16/2004 2.9 NC NC NC NC NC 74.9 Not dateable –11.1 Not dateable

1

Ages corrected using Tamers (1967) method assuming δ13C of soil carbon dioxide of –23 per mil and δ13C of carbonates of –4 per mil, and a fractionation 

factor for δ13C between dissolved inorganic carbon and soil carbon dioxide of 10 (Clark and Fritz, 1997, fig. 5.5, table 5.3, pH = 6.8, temperature 15°C). Ages 

are highly sensitive to assumptions. 

Table 16. Age tracer and supporting data for ground-water samples collected from the local-scale TANC study area near York, 

Nebraska.—Continued

[All apparent ages calculated based on a recharge altitude of 488 meters. He, helium; 3H, tritium; 3He, helium-3 concentration; 4He, helium-4 concentration; 

(3He)tritium, helium-3 concentration from decay of tritium; δ, isotopic ratio of light to heavy isotope relative to standard reference; B.P., before present; 

°C, degree Celsius; CFC, chlorofluorocarbon; cm3STP/g, cubic centimeter of gas at Standard Temperature and Pressure per gram of water; contam, tracer 

concentrations greater than would result from atmospheric sources, indicating contamination for the purposes of age dating; italics, tracer was not detected 

where it was expected to be based on other tracers; m, meter; mL/kg, milliliter per kilogram; mg/L, milligram per liter; N, nitrogen; N2, nitrogen gas; NC, not 

collected; Ne, neon concentration; P, phosphorus; pptv, parts per trillion by volume; SF6, sulfur hexafluoride; terrigenic, percent of 3He from terrigenic sources; 

TU, tritium unit; USGS, U.S. Geological Survey; Study PSW, study public-supply well]
128 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 17. Age and mixing interpretations for ground-water samples from the local-scale TANC study area in York, Nebraska.—Continued

[3H/3He, tritium-helium dating method, 

%, percent; <, older than; >, more than; bold date and mixing fraction, best estimate of mean age and young water mixing fraction; CFC-12, chlorofluorocarbon 12 dating 

method; CFC-11, chlorofluorocarbon 11 dating method; CFC-113, chlorofluorocarbon 113 dating method; CFC, group of 3 chlorofluorocarbons analyzed; EBMM, Exponential-Binary Mixing Model; m, meter; NC, not collected; NP, not possible to compute; old, ground water recharged before 3H and CFC-12 were present in atmosphere (older than about 60 years); SF6, sulfur hexafluoride dating method; Study PSW, 

study public-supply well]

Well or 

sample 

name 

Sampling 

date

Hydrogeologic 

unit

Depth of 

top of 

screen 

below 

water 

table

(m)

3H/3He

Piston 

model 

recharge 

date

CFC or SF6 tracers Young fraction Old fraction

Method 

for 

best age

Preferred 

tracer

Piston 

model 

recharge 

date1

Exponentialbinary 

model 

recharge 

date

Exponentialbinary 

model

% young

Binary 

model 

recharge 

date

Binary 

model

% 

young

Fraction

%

Best 

mean 

recharge 

date

Best 

mean 

age

 (years)

Fraction

%

Best 

mean 

recharge 

date

Best 

mean 

age

 (years)

FP1-63 10/27/2003 Unconfined 

aquifer

5.93 1989 CFC-113 1985 NP NP 1986 79 100 1989 15 0 NP NP 3H/3He 

piston

FP1-147 10/22/2003 Upper 

confined aquifer

31.54 NC CFC-12 1951 1993 3 1985 3 3 1993 11 97 <1940 >64 CFC-12 

EBMM

FP1-185 10/21/2003 Upper 

confined aquifer

43.03 NC CFC-12 1947 1993 1 1985 2 1 1993 11 99 <1940 >64 CFC-12 

EBMM

OFPS-38 11/11/2003 Unconfined 

aquifer

6.51 NC SF6 1984 NP NP NP NP 100 1984 20 0 NP NP SF6 piston

OFPS-157 11/11/2003 Upper 

confined aquifer

42.80 NC SF6 1982 1992 32 1995 32 32 1992 12 68 <1960 >44 SF6

EBMM

FP3-33 10/28/2003 Unconfined 

aquifer

2.36 2001 CFC-12 1994 NP NP NP NP 100 1994 10 0 NP NP CFC-12 

piston

FP3-83 10/27/2003 Unconfined 

aquifer

17.80 1951 CFC-11 1956 NP NP NP NP 100 1956 48 0 NP NP CFC-11 

piston

FP3-130 10/15/2003 Upper 

confined aquifer

32.18 NC CFC-12 1952 1993 3 1985 3 3 1993 11 97 <1940 >64 CFC-12 

EBMM

FP3-162 10/28/2003 Upper 

confined aquifer

41.87 NC CFC-11 1978 1990 70 1982 83 70 1990 14 30 <1940 >64 CFC-11 

EBMM

FP4-28 10/29/2003 Unconfined 

aquifer

0.20 1997 NP NP NP NP NP NP 100 1997 7 0 NP NP 3H/3He 

piston

FP4-83 10/30/2005 Unconfined 

aquifer

17.17 NP CFC-12 1960 1993 8 1985 9 100 1960 44 0 NP NP CFC-12 

piston

FP4-168 10/23/2005 Upper 

confined aquifer

42.93 NC CFC-12 1949 1993 1 1985 2 1 1993 11 99 <1940 >64 CFC-12 

EBMM

Table 17. Age and mixing interpretations for ground-water samples from the local-scale TANC study area in York, Nebraska.[

3H/3He, tritium-helium dating method, 

%, percent; <, older than; >, more than; bold date and mixing fraction, best estimate of mean age and young water mixing fraction; CFC-12, chlorofluorocarbon 12 dating 

method; CFC-11, chlorofluorocarbon 11 dating method; CFC-113, chlorofluorocarbon 113 dating method; CFC, group of 3 chlorofluorocarbons analyzed; EBMM, Exponential-Binary Mixing Model; m, meter; NC, not collected; NP, not possible to compute; old, ground water recharged before 3H and CFC-12 were present in atmosphere (older than about 60 years); SF6, sulfur hexafluoride dating method; Study PSW, 

study public-supply well]
Tables 129

Table 17. Age and mixing interpretations for ground-water samples from the local-scale TANC study area in York, Nebraska.—Continued[3H/3He, tritium-helium dating method, %, percent; <, older than; >, more than; bold date and mixing fraction, best estimate of mean age and young water mixing fraction; CFC-12, chlorofluorocarbon 12 dating method; CFC-11, chlorofluorocarbon 11 dating method; CFC-113, chlorofluorocarbon 113 dating method; CFC, group of 3 chlorofluorocarbons analyzed; EBMM, Exponential-Binary Mixing Model; m, meter; NC, not collected; NP, not possible to compute; old, ground water recharged before 3H and CFC-12 were present in atmosphere (older than about 60 years); SF6, sulfur hexafluoride dating method; Study PSW, 

study public-supply well]

Well or 

sample 

name 

Sampling 

date

Hydrogeologic 

unit

Depth of 

top of 

screen 

below 

water 

table

(m)

3H/3He

Piston 

model 

recharge 

date

CFC or SF6 tracers Young fraction Old fraction

Method 

for 

best age

Preferred 

tracer

Piston 

model 

recharge 

date1

Exponentialbinary 

model 

recharge 

date

Exponentialbinary 

model

% young

Binary 

model 

recharge 

date

Binary 

model

% 

young

Fraction

%

Best 

mean 

recharge 

date

Best 

mean 

age

 (years)

Fraction

%

Best 

mean 

recharge 

date

Best 

mean 

age

 (years)

Study 

PSW

10/20/2003 Upper 

confined aquifer

30.80 NC SF6 1970 1995 7 1996 7 7 1995 9 93 <1960 >44 SF6

EBMM

Study 

PSW

6/18/2004 Upper 

confined aquifer

31.53 NP SF6 1972 1995 7 1996 7 7 1995 9 93 <1960 >44 SF6

EBMM

Study 

PSW depth 42.7 m

6/18/2004 Upper 

confined aquifer

31.53 NP SF6 1976 1995 11 1996 11 11 1995 9 89 <1960 >44 SF6

EBMM

Study 

PSW depth 45.7 m

6/21/2004 Upper 

confined aquifer

34.57 NP SF6 1975 1995 11 1996 12 11 1995 9 89 <1960 >44 SF6

EBMM

Study 

PSW depth 48.8 m

6/17/2004 Upper 

confined aquifer

37.62 NP SF6 1975 1995 12 1996 12 12 1995 9 88 <1960 >44 SF6

EBMM

1The piston model does not have a % young column, as the exponential and binary models do, because the piston model implicitly assumes 100% young. 
130 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska

Table 18. Results of analyses of nutrients in ground-water samples collected during October 2003 through April 2005 in the localscale TANC study area near York, Nebraska.—Continued

[USGS, U.S. Geological Survey; mg/L, milligram per liter; E, estimated value; <, less than; m, meter; study PSW, study public-supply well; N, nitrogen; 

P, phosphorus; —, not detected]

Well or 

sample 

name

Date Time Well classification 

(see table 11)

Ammonia

mg/L as N

Nitrite

mg/L as N

Nitrite plus 

nitrate

mg/L as N

Orthophosphate

mg/L as P

Laboratory reporting level (LRL) 0.04 0.008 0.06 0.006

USGS parameter code 00608 00613 00631 00671

FP4-168 10/23/2003 1100 Confined agricultural unmixed 0.09 E0.007 0.17 0.081

FP4-231 10/16/2003 1200 Confined agricultural mixed 0.26 E0.005 — 0.027

FP5-175 10/21/2004 1400 Confined agricultural mixed 0.07 — — 0.029

FP5-LS 10/21/2004 1000 Confined agricultural mixed — — 6.37 0.319

FP4-83 10/30/2003 1000 Unconfined deep agricultural — — 2.15 0.179

FP5-73 10/18/2004 1400 Unconfined deep agricultural — — 7.36 0.347

AWT1-83 7/29/2004 1000 Unconfined shallow agricultural — — 30.2 0.362

AWT1-83 10/19/2004 900 Unconfined shallow agricultural — — 36.6 0.352

AWT1-83 3/31/2005 1000 Unconfined shallow agricultural — — 36.0 0.359

AWT2-71 7/29/2004 1300 Unconfined shallow agricultural — — 14.3 0.387

AWT2-71 10/19/2004 1200 Unconfined shallow agricultural — — 23.0 0.369

AWT2-71 3/31/2005 1600 Unconfined shallow agricultural — — 14.3 0.399

AWT3-73 7/30/2004 1200 Unconfined shallow agricultural — — 23.7 0.359

AWT3-73 10/19/2004 1400 Unconfined shallow agricultural — 0.023 13.8 0.396

AWT3-73 4/1/2005 1300 Unconfined shallow agricultural — — 23.4 0.394

FP4-28 10/29/2003 1100 Unconfined shallow agricultural — — 15.5 0.464

FP4-28 7/26/2004 1000 Unconfined shallow agricultural — — 13.8 0.496

FP4-28 10/15/2004 900 Unconfined shallow agricultural — — 16.0 0.485

FP4-28 4/12/2005 1200 Unconfined shallow agricultural — — 14.4 0.489

FP5-45 7/28/2004 1000 Unconfined shallow agricultural — — 5.21 0.367

FP5-45 10/18/2004 900 Unconfined shallow agricultural — — 7.52 0.379

FP5-45 3/30/2005 1500 Unconfined shallow agricultural — — 5.51 0.384

NWT1-39 7/30/2004 1000 Unconfined shallow agricultural — — 69.5 0.327

NWT1-39 10/17/2004 1200 Unconfined shallow agricultural — — 77.9 0.343

NWT1-39 4/1/2005 1000 Unconfined shallow agricultural — — 80.3 0.361

Study PSW 10/20/2003 1500 Study PSW 0.11 E0.004 0.17 0.099

Study PSW 6/18/2004 1500 Study PSW 0.10 E0.004 0.18 0.123

Study PSW 10/15/2004 1200 Study PSW 0.10 E0.004 0.20 0.117

Study PSW 4/7/2005 900 Study PSW 0.10 — 0.17 0.113

Study PSW, depth 42.7 m 6/18/2004 1100 Study PSW 0.08 E0.005 0.16 0.086

Study PSW, depth 45.7 m 6/21/2004 1000 Study PSW 0.08 E0.006 0.15 0.091

Study PSW, depth 48.8 m 6/17/2004 1800 Study PSW 0.10 — 0.19 0.114

Study PSW, depth 51.8 m 6/17/2004 1300 Study PSW 0.10 — 0.47 0.082

Study PSW, depth 61.0 m 6/16/2004 1300 Study PSW 0.20 — — 0.149

FP1-147 10/22/2003 1200 Confined urban unmixed 0.07 — — 0.069

FP1-147 6/14/2004 1600 Confined urban unmixed 0.08 — — 0.084

FP1-185 10/21/2003 1200 Confined urban unmixed 0.15 — — 0.174

FP1-185 6/14/2004 1200 Confined urban unmixed 0.15 — — 0.186

FP1-185 4/7/2005 1500 Confined urban unmixed 0.15 — — 0.169

FP1-247 10/20/2003 1200 Confined urban unmixed 0.39 — 0.07 0.088

FP1-247 6/14/2004 800 Confined urban unmixed 0.36 — — 0.103

FP1-247 4/7/2005 1300 Confined urban unmixed 0.35 — — 0.097

FP2-152 10/12/2004 1500 Confined urban unmixed 0.19 — — 0.070

FP3-130 10/15/2003 1100 Confined urban unmixed 0.07 0.029 0.07 0.048

Table 18. Results of analyses of nutrients in ground-water samples collected during October 2003 through April 2005 in the local-scale 

TANC study area near York, Nebraska.

[USGS, U.S. Geological Survey; mg/L, milligram per liter; E, estimated value; <, less than; m, meter; study PSW, study public-supply well; N, nitrogen; 

P, phosphorus; —, not detected]
Tables 131

Table 18. Results of analyses of nutrients in ground-water samples collected during October 2003 through April 2005 in the localscale TANC study area near York, Nebraska.—Continued

[USGS, U.S. Geological Survey; mg/L, milligram per liter; E, estimated value; <, less than; m, meter; study PSW, study public-supply well; N, nitrogen; 

P, phosphorus; —, not detected]

Well or 

sample 

name

Date Time Well classification 

(see table 11)

Ammonia

mg/L as N

Nitrite

mg/L as N

Nitrite plus 

nitrate

mg/L as N

Orthophosphate

mg/L as P

Laboratory reporting level (LRL) 0.04 0.008 0.06 0.006

USGS parameter code 00608 00613 00631 00671

OFPN-167 10/30/2003 1400 Confined urban unmixed 0.09 — — 0.067

OFPN-276 11/12/2003 900 Confined urban unmixed 0.82 — — 0.053

OFPS-225 10/14/2004 1000 Confined urban unmixed 0.29 — — 0.093

FP2-250 10/12/2004 1000 Confined urban mixed 2.05 — — 0.021

FP3-162 10/28/2003 1000 Confined urban mixed — 0.414 9.31 0.294

FP3-162 10/16/2004 900 Confined urban mixed E0.024 0.045 13.1 0.320

FP3-218 10/15/2003 1800 Confined urban mixed 0.26 — — 0.130

FP3-218 10/16/2004 1300 Confined urban mixed 0.27 — — 0.132

FP3-218 4/4/2005 1200 Confined urban mixed 0.27 — — 0.136

OFPS-157 11/11/2003 1000 Confined urban mixed — 0.23 4.47 0.192

OFPS-157 10/14/2004 1300 Confined urban mixed 0.19 — — 0.143

OFPS-157 4/11/2005 1600 Confined urban mixed 0.20 — — 0.127

FP2-78 10/13/2004 1000 Unconfined deep urban — — 3.04 0.237

FP3-83 10/27/2003 1500 Unconfined deep urban — 0.022 7.03 0.322

FP1-63 10/27/2003 1100 Unconfined shallow urban — — 25.6 0.621

FP1-63 6/15/2004 800 Unconfined shallow urban — — 27.1 0.627

FP1-63 10/15/2004 1400 Unconfined shallow urban — — 28.8 0.631

FP1-63 4/11/2005 1300 Unconfined shallow urban — — 28.4 0.624

FP2-43 7/28/2004 1300 Unconfined shallow urban — — 16.4 0.455

FP2-43 10/13/2004 1400 Unconfined shallow urban — — 17.1 0.458

FP2-43 3/30/2005 1100 Unconfined shallow urban — — 15.9 0.485

FP3-33 10/28/2003 1300 Unconfined shallow urban — 0.009 3.84 0.358

FP3-33 7/26/2004 1400 Unconfined shallow urban — — 2.41 0.411

FP3-33 10/16/2004 1100 Unconfined shallow urban E0.021 — 2.56 0.430

FP3-33 4/4/2005 1600 Unconfined shallow urban E0.027 — 2.51 0.457

OFPN-88 11/13/2003 1000 Unconfined shallow urban — — 24.5 0.438

OFPN-88 7/27/2004 1000 Unconfined shallow urban — — 22.9 0.457

OFPN-88 10/20/2004 900 Unconfined shallow urban — — 22.1 0.433

OFPN-88 4/6/2005 1500 Unconfined shallow urban — — 22.6 0.410

OFPS-38 11/11/2003 1400 Unconfined shallow urban 0.84 E0.005 3.41 1.58

OFPS-38 7/27/2004 1400 Unconfined shallow urban 1.05 E0.007 3.57 1.65

OFPS-38 10/14/2004 1500 Unconfined shallow urban 1.21 E0.006 2.09 1.92

OFPS-38 4/14/2005 1500 Unconfined shallow urban 1.34 0.012 2.67 2.10

UWT1-53 8/3/2004 1000 Unconfined shallow urban — — 17.3 0.413

UWT1-53 10/20/2004 1200 Unconfined shallow urban — — 18.6 0.420

UWT1-53 4/5/2005 1200 Unconfined shallow urban — — 18.4 0.406

UWT2-23 8/3/2004 1300 Unconfined shallow urban — — 15.7 0.786

UWT2-23 10/20/2004 1400 Unconfined shallow urban — — 13.6 0.798

UWT2-23 4/5/2005 1000 Unconfined shallow urban — — 15.7 0.824

UWT3-34 8/4/2004 1000 Unconfined shallow urban — — 17.0 0.485

UWT3-34 10/17/2004 900 Unconfined shallow urban — — 19.0 0.514

UWT3-34 4/5/2005 1500 Unconfined shallow urban — — 17.6 0.509

UWT4-85 8/4/2004 1300 Unconfined shallow urban — — 22.1 0.293

UWT4-85 10/22/2004 1000 Unconfined shallow urban — — 22.0 0.297

UWT4-85 4/6/2005 1100 Unconfined shallow urban — — 23.8 0.291
132 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 19. Results of analyses of selected isotopes of nitrogen, sulfur, uranium, and radon in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [Stable isotope ratios are reported in the standard delta notation (δ), the ratio of a heavier isotope to the more common lighter isotope of that element, relative to a standard reference material; 

δ15N, nitrogen-15/nitrogen-14; δ18O, oxygen-18/oxygen-16; δ34S, sulfur-34/sulfur-32; study PSW, study public-supply well; pCi/L, picocurie per liter; NC, not collected; na, not applicable]

Well or saple name Date collected Well classification (see table 11)

δ15N of 

nitrate 

(per mil)

δ18O of 

nitrate 

(per mil)

δ15 N of 

nitrogen gas

(per mil)

Calculated 

initial δ15N 

of nitrate1

(per mil)

δ15N of 

ammonia

(per mil)

δ15S of 

sulfate 

(per mil)

Uranium-238 

(pCi/L)

Uranium-234 

(pCi/L)

Uranium-235 

(pCi/L)

Radon-222

(pCi/L)

Laboratory Reporting Level (LRL) na na na na na na 0.1 0.1 0.1 26

USGS parameter code 82690 63041 82698 na 82691 49932 22603 22610 22620 82303

FP4-168 10/23/03 Confined agricultural unmixed 8.01 4.12 0.84 8.01 NC –5.98 NC NC NC 567

FP4-168 04/14/05 Confined agricultural unmixed 9.15 4.09 NC NC NC NC 0.48 1.14 0.04 NC

FP4-231 10/16/03 Confined agricultural unmixed NC NC 0.94 NC 7.26 –3.57 NC NC NC 872

FP5-175 10/21/04 Confined agricultural mixed NC NC NC NC NC –4.99 NC NC NC 756

FP5-LS 10/21/04 Confined agricultural mixed 8.31 3.35 NC 8.31 NC –3.20 NC NC NC 261

FP4-83 10/30/03 Unconfined deep agricultural 10.52 0.73 0.67 10.52 NC –11.25 NC NC NC 832

FP4-83 04/14/05 Unconfined deep agricultural NC NC NC NC NC NC 1.27 2.33 0.06 NC

FP5-73 10/18/04 Unconfined deep agricultural 7.86 2.31 NC 7.86 NC –0.49 NC NC NC 360

AWT1-83 10/19/04 Unconfined shallow 

agricultural

5.65 2.44 NC 5.65 NC NC NC NC NC NC

AWT2-71 10/19/04 Unconfined shallow 

agricultural

4.76 3.17 NC 4.76 NC NC NC NC NC NC

AWT3-73 10/19/04 Unconfined shallow 

agricultural

6.06 2.41 NC 6.06 NC NC NC NC NC NC

FP4-28 10/29/03 Unconfined shallow 

agricultural

8.55 2.01 NC 8.55 NC –9.60 NC NC NC 492

FP4-28 04/12/05 Unconfined shallow 

agricultural

NC NC NC NC NC NC 7.08 9.13 0.37 NC

FP5-45 10/18/04 Unconfined shallow 

agricultural

8.05 3.98 NC 8.05 NC –0.12 NC NC NC 281

NWT1-39 10/17/04 Unconfined shallow 

agricultural

10.45 –0.26 NC 10.45 NC NC NC NC NC NC

Study PSW 10/20/03 Study PSW 35.76 15.53 1.32 14.00 NC –6.76 NC NC NC 629

Study PSW 06/18/04 Study PSW 32.40 15.51 NC 14.00 NC –6.58 NC NC NC 726

Study PSW 04/07/05 Study PSW 37.46 16.40 NC NC NC NC 4.48 5.85 0.19 NC

Study PSW, depth 42.7 m 06/18/04 Study PSW 35.20 15.69 1.40 9.00 NC –6.71 NC NC NC 542

Study PSW, depth 45.7 m 06/21/04 Study PSW 37.15 16.27 1.70 15.00 NC –6.58 NC NC NC 436

Study PSW, depth 48.8 m 06/17/04 Study PSW 35.37 16.40 1.94 10.00 NC –7.02 NC NC NC 696

Study PSW, depth 51.8 m 06/17/04 Study PSW 42.82 18.78 3.11 11.00 NC –9.84 NC NC NC 1340

Study PSW, depth 61.0 m 06/16/04 Study PSW 31.84 16.59 3.20 12.00 NC –10.16 NC NC NC 1240

FP1-147 10/22/03 Confined urban unmixed 1.21 NC 0.88 5.00 NC –7.00 NC NC NC 142

FP1-147 04/11/05 Confined urban unmixed NC NC NC NC NC NC 1.09 1.77 0.07 NC

FP1-185 10/21/03 Confined urban unmixed NC NC 0.84 NC NC –4.31 NC NC NC 1048

Table 19. Results of analyses of selected isotopes of nitrogen, sulfur, uranium, and radon in ground-water samples collected during October 2003 through April 2005 in the localscale TANC study area near York, Nebraska.[Stable isotope ratios are reported in the standard delta notation (δ), the ratio of a heavier isotope to the more common lighter isotope of that element, relative to a standard reference material; δ15N, nitrogen-15/nitrogen-14; δ18O, oxygen-18/oxygen-16; δ34S, sulfur-34/sulfur-32; study PSW, study public-supply well; pCi/L, picocurie per liter; NC, not collected; na, not applicable]
Tables 133

Table 19. Results of analyses of selected isotopes of nitrogen, sulfur, uranium, and radon in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [Stable isotope ratios are reported in the standard delta notation (δ), the ratio of a heavier isotope to the more common lighter isotope of that element, relative to a standard reference material; δ15N, nitrogen-15/nitrogen-14; δ18O, oxygen-18/oxygen-16; δ34S, sulfur-34/sulfur-32; study PSW, study public-supply well; pCi/L, picocurie per liter; NC, not collected; na, not applicable]

Well or saple name Date collected Well classification (see table 11) δ15N of nitrate (per mil) δ18O of nitrate (per mil) δ15 N of nitro- gen gas (per mil) Calculated initial δ15N of nitrate1 (per mil) δ15N of ammonia (per mil) δ15S of sulfate (per mil) Ura- nium-238 (pCi/L) Ura- nium-234 (pCi/L) Ura- nium-235 (pCi/L) Radon-222 (pCi/L)

Laboratory Reporting Level (LRL) na na na na na na 0.1 0.1 0.1 26

USGS parameter code 82690 63041 82698 na 82691 49932 22603 22610 22620 82303

FP1-247 10/20/03 Confined urban unmixed 35.72 17.78 1.29 14.00 NC –4.33 NC NC NC 308

FP2-152 10/12/04 Confined urban unmixed NC NC NC NC 3.50 –6.48 NC NC NC 193

FP3-130 10/15/03 Confined urban unmixed 20.60 –6.10 0.85 20.60 NC –7.00 NC NC NC 136

OFPN-167 10/30/03 Confined urban unmixed NC NC 0.88 NC NC –5.47 NC NC NC 140

OFPN-276 11/12/03 Confined urban unmixed NC NC 0.95 NC 3.54 –9.44 NC NC NC 260

OFPS-225 10/14/04 Confined urban unmixed NC NC NC NC 3.03 –1.88 NC NC NC 285

FP2-250 10/12/04 Confined urban mixed NC NC NC NC 3.73 –22.34 NC NC NC 1012

FP3-162 10/28/03 Confined urban mixed 17.80 8.13 1.35 15.00 NC –2.38 NC NC NC 451

FP3-218 10/15/03 Confined urban mixed NC NC 2.68 12.00 NC –15.02 NC NC NC 452

OFPS-157 11/11/03 Confined urban mixed 17.82 7.89 1.50 12.00 NC –5.52 NC NC NC 620

OFPS-157 04/11/05 Confined urban mixed NC NC NC NC NC NC 0.64 1.06 0.01 NC

FP2-78 10/13/04 Unconfined deep urban 9.92 2.37 NC 9.00 NC –12.86 NC NC NC 893

FP3-83 10/27/03 Unconfined deep urban 14.12 5.03 0.97 13.00 NC –6.39 NC NC NC 451

FP1-63 10/27/03 Unconfined shallow urban 11.77 2.83 0.62 11.77 NC –1.61 NC NC NC 356

FP1-63 04/11/05 Unconfined shallow urban NC NC NC NC NC NC 11.15 14.12 0.60 NC

FP2-43 10/13/04 Unconfined shallow urban 11.58 4.38 NC 11.58 NC –14.08 NC NC NC 258FP3-33 10/28/03 Unconfined shallow urban 23.64 12.53 0.39 19.00 NC 1.68 NC NC NC 273

FP3-33 04/04/05 Unconfined shallow urban 17.56 6.58 NC NC NC NC NC NC NC NC

OFPN-88 11/13/03 Unconfined shallow urban 12.14 2.97 0.62 12.14 NC –11.89 NC NC NC 110OFPS-38 11/11/03 Unconfined shallow urban 18.51 8.60 2.52 12.00 NC –0.10 NC NC NC 390

UWT1-53 10/20/04 Unconfined shallow urban 12.41 4.94 NC 12.41 NC NC NC NC NC NCUWT2-23 10/20/04 Unconfined shallow urban 13.08 6.10 NC 11.00 NC NC NC NC NC NC

UWT3-34 10/17/04 Unconfined shallow urban 9.30 2.01 NC 9.30 NC NC NC NC NC NCUWT4-85 10/22/04 Unconfined shallow urban 11.19 3.76 NC 11.19 NC NC NC NC NC NC

1Calculated from isotopic mass balance for denitrification using methods of Böhlke (2002).
134 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 20. Results of analyses of selected field parameters and major and minor elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [Parameters and constituents listed from left to right are field parameters, major cations, minor cations, major anions, minor anions, major ions with no charge (silica), and TDS. μg/L, microgram per liter; mg/L, milligram per liter; USGS, U.S. Geological Survey; V, censored value possibly affected by contamination during sample collection; <, less than; m, meter; study PSW, study public- supply well; nu, anomalous laboratory analytical result, data not used; NC, not collected; CaCO3, calcium carbonate; μS/cm, microsiemens per centimeter; °C, degrees Celsius; std, standard; na, not applicable; TDS, total dissolved solids] Well or sample name Date collected Time collected Well classification (see table 11) Alkalinity, incremental titration mg/L CaCO3 pH(std units) Specific conductance at 25°C

(μS/cm)

Water 

temperature

(°C)

Calcium

(mg/L)

Magnesium

(mg/L)

Potassium

(mg/L)

Sodium

(mg/L)

Laboratory Reporting Level (LRL) na na na na 0.02 0.01 0.04 0.20USGS parameter code 39086 00400 00095 00010 00915 00925 00935 00930

FP4-168 10/23/2003 1100 Confined agricultural unmixed 202 7.5 410 13.5 64.1 10.2 5.33 19.9

FP4-168 4/14/2005 1000 Confined agricultural unmixed 191 7.5 455 13.0 62.1 10.5 4.92 18.7FP4-231 10/16/2003 1200 Confined agricultural unmixed 141 8.7 383 14.5 15.6 7.51 5.44 59.5FP5-175 10/21/2004 1400 Confined agricultural mixed 306 7.3 723 13.0 89.4 15.0 7.81 47.4FP5-LS 10/21/2004 1000 Confined agricultural mixed 242 6.8 598 13.0 58.8 13.6 7.48 43.0FP4-83 10/30/2003 1000 Unconfined deep agricultural 238 7.3 561 11.5 76.2 12.8 5.84 18.7FP4-83 4/14/2005 1300 Unconfined deep agricultural 223 7.2 535 13.5 77.2 13.2 5.60 17.8FP5-73 10/18/2004 1400 Unconfined deep agricultural 262 7.0 620 13.5 78.2 14.4 6.03 33.8AWT1-83 7/29/2004 1000 Unconfined shallow agricultural 325 6.8 980 14.0 105 22.4 8.29 64.6

AWT1-83 10/19/2004 900 Unconfined shallow agricultural 339 6.7 993 13.0 111 22.7 8.33 67.4

AWT1-83 3/31/2005 1000 Unconfined shallow agricultural 319 6.7 940 12.5 108 22.8 8.01 63.5AWT2-71 7/29/2004 1300 Unconfined shallow agricultural 280 7.1 762 14.0 86.9 15.6 7.63 48.0AWT2-71 10/19/2004 1200 Unconfined shallow agricultural 371 7.1 925 13.0 95.8 17.4 8.11 83.2AWT2-71 3/31/2005 1600 Unconfined shallow agricultural 303 7.0 773 13.5 93.7 16.3 7.78 51.7AWT3-73 7/30/2004 1200 Unconfined shallow agricultural 363 7.2 944 14.0 99.6 19.0 8.16 84.9AWT3-73 10/19/2004 1400 Unconfined shallow agricultural 306 7.0 766 13.5 93.3 15.8 7.89 52.6AWT3-73 4/1/2005 1300 Unconfined shallow agricultural 363 7.0 943 13.0 101 18.3 8.00 82.6FP4-28 10/29/2003 1100 Unconfined shallow agricultural 343 6.8 1,188 13.0 113 30.7 9.47 85.8

FP4-28 7/26/2004 1000 Unconfined shallow agricultural 335 6.9 1,065 13.5 112 30.0 9.51 82.9FP4-28 10/15/2004 900 Unconfined shallow agricultural 326 6.8 1,066 13.0 110 30.1 9.46 75.8

FP4-28 4/12/2005 1200 Unconfined shallow agricultural 342 6.9 1,005 12.5 107 29.0 8.94 77.2

FP5-45 7/28/2004 1000 Unconfined shallow agricultural 186 6.6 471 13.5 53.5 13.0 6.92 26.5

FP5-45 10/18/2004 900 Unconfined shallow agricultural 200 6.5 499 13.0 55.6 13.6 7.33 27.6FP5-45 3/30/2005 1500 Unconfined shallow agricultural 171 6.6 457 12.5 54.0 12.9 6.79 27.9NWT1-39 7/30/2004 1000 Unconfined shallow agricultural 147 6.4 1,107 13.5 125 38.1 8.99 39.3

NWT1-39 10/17/2004 1200 Unconfined shallow agricultural 149 6.3 1,134 13.5 124 39.0 9.34 40.8NWT1-39 4/1/2005 1000 Unconfined shallow agricultural 148 6.3 1,112 13.0 127 38.6 8.97 39.5Study PSW 10/20/2003 1500 Study PSW 211 7.4 442 13.0 72.7 10.9 5.66 26.7

Study PSW 6/18/2004 1500 Study PSW 205 7.5 520 13.0 70.1 10.9 5.43 26.0Study PSW 10/15/2004 1200 Study PSW 208 7.3 523 12.5 70.1 11.2 5.48 26.4Study PSW 4/7/2005 900 Study PSW 208 7.5 519 12.5 74.5 11.8 5.52 27.1Study PSW, depth 

42.7 m

6/18/2004 1100 Study PSW 212 7.4 547 14.5 74.1 11.7 5.74 26.7

Table 20. Results of analyses of selected field parameters and major and minor elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.[Parameters and constituents listed from left to right are field parameters, major cations, minor cations, major anions, minor anions, major ions with no charge (silica), and TDS. μg/L, microgram per liter; mg/L, milligram per liter; USGS, U.S. Geological Survey; V, censored value possibly affected by contamination during sample collection; <, less than; m, meter; study PSW, study publicsupply well; nu, anomalous laboratory analytical result, data not used; NC, not collected; CaCO3, calcium carbonate; μS/cm, microsiemens per centimeter; °C, degrees Celsius; std, standard; na, not 

applicable; TDS, total dissolved solids]
Tables 135

Table 20. Results of analyses of selected field parameters and major and minor elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued[Parameters and constituents listed from left to right are field parameters, major cations, minor cations, major anions, minor anions, major ions with no charge (silica), and TDS. μg/L, microgram per liter; mg/L, milligram per liter; USGS, U.S. Geological Survey; V, censored value possibly affected by contamination during sample collection; <, less than; m, meter; study PSW, study publicsupply well; nu, anomalous laboratory analytical result, data not used; NC, not collected; CaCO3, calcium carbonate; μS/cm, microsiemens per centimeter; °C, degrees Celsius; std, standard; na, not 

applicable; TDS, total dissolved solids]

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Alkalinity, 

incremental 

titration

mg/L 

CaCO3

pH

(std 

units)

Specific 

conductance

 at 25°C

(μS/cm)

Water 

temperature

(°C)

Calcium

(mg/L)

Magnesium

(mg/L)

Potassium

(mg/L)

Sodium

(mg/L)

Laboratory Reporting Level (LRL) na na na na 0.02 0.01 0.04 0.20USGS parameter code 39086 00400 00095 00010 00915 00925 00935 00930

Study PSW, depth 

45.7 m

6/21/2004 1000 Study PSW 195 7.4 562 16.5 77.4 12.1 5.72 27.9

Study PSW, depth 

48.8 m

6/17/2004 1800 Study PSW 236 7.3 630 19.5 84.0 12.9 6.41 34.4

Study PSW, depth 

51.8 m

6/17/2004 1300 Study PSW 335 7.1 975 18.0 128 20.0 10.7 65.2

Study PSW, depth 

61.0 m

6/16/2004 1300 Study PSW 291 7.3 849 21.0 117 18.7 6.74 51.1

FP1-147 10/22/2003 1200 Confined urban unmixed 192 7.5 409 14.5 64.5 10.1 5.28 20.8FP1-147 6/14/2004 1600 Confined urban unmixed 196 7.5 449 19.0 61.5 8.57 4.76 20.7FP1-147 4/11/2005 1100 Confined urban unmixed 194 7.5 390 13.5 63.7 9.20 20.3 4.8FP1-185 10/21/2003 1200 Confined urban unmixed 192 7.5 440 14.5 60.9 9.03 5.15 25.9FP1-185 6/14/2004 1200 Confined urban unmixed 187 7.5 466 16.0 60.2 8.43 4.71 26.2

FP1-185 4/7/2005 1500 Confined urban unmixed NC 7.5 473 14.0 NC NC NC NC

FP1-247 10/20/2003 1200 Confined urban unmixed 213 7.5 492 16.5 62.9 13.7 6.38 31.8

FP1-247 6/14/2004 800 Confined urban unmixed 205 7.6 491 14.5 55.3 12.4 5.46 28.6FP1-247 4/7/2005 1300 Confined urban unmixed NC 7.6 494 14.0 NC NC NC NC

FP2-152 10/12/2004 1500 Confined urban unmixed 195 7.3 464 14.0 60.5 11.1 5.06 23.6

FP3-130 10/15/2003 1100 Confined urban unmixed 174 8.5 415 14.0 56.0 7.31 6.74 28.1OFPN-167 10/30/2003 1400 Confined urban unmixed 190 7.5 513 15.5 60.3 9.57 5.28 20.7OFPN-276 11/12/2003 900 Confined urban unmixed 218 7.6 838 15.0 57.3 13.7 8.10 72.3OFPS-225 10/14/2004 1000 Confined urban unmixed 199 7.5 477 13.0 59.7 11.5 5.15 26.5FP2-250 10/12/2004 1000 Confined urban mixed 229 7.0 2,740 16.0 309 71.8 16.2 207

FP3-162 10/28/2003 1000 Confined urban mixed 303 6.9 1,088 13.5 145 20.7 10.0 43.6

FP3-162 10/16/2004 900 Confined urban mixed 292 6.8 1,094 13.5 157 24.0 10.7 47.2

FP3-218 10/15/2003 1800 Confined urban mixed 286 7.4 743 15.5 110 18.8 6.51 34.3

FP3-218 10/16/2004 1300 Confined urban mixed 298 7.2 798 15.0 114 19.8 6.63 35.0FP3-218 4/4/2005 1200 Confined urban mixed NC 7.4 791 14.5 NC NC NC NCOFPS-157 11/11/2003 1000 Confined urban mixed 268 7.1 842 13.0 107 16.2 7.70 30.0OFPS-157 10/14/2004 1300 Confined urban mixed 284 7.3 584 13.5 83.1 13.6 5.51 27.8OFPS-157 4/11/2005 1600 Confined urban mixed 244 7.4 467 13.5 85.6 14.3 29.3 5.9FP2-78 10/13/2004 1000 Unconfined deep urban 236 7.1 574 13.5 88.2 13.4 6.87 20.9FP3-83 10/27/2003 1500 Unconfined deep urban 245 7.1 765 14.5 95.0 14.8 7.89 27.8
136 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 20. Results of analyses of selected field parameters and major and minor elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [Parameters and constituents listed from left to right are field parameters, major cations, minor cations, major anions, minor anions, major ions with no charge (silica), and TDS. μg/L, microgram per liter; mg/L, milligram per liter; USGS, U.S. Geological Survey; V, censored value possibly affected by contamination during sample collection; <, less than; m, meter; study PSW, study public- supply well; nu, anomalous laboratory analytical result, data not used; NC, not collected; CaCO3, calcium carbonate; μS/cm, microsiemens per centimeter; °C, degrees Celsius; std, standard; na, not applicable; TDS, total dissolved solids] Well or sample name Date collected Time collected Well classification (see table 11) Alkalinity, incremental titration mg/L CaCO3 pH(std units) Specific conductance at 25°C

(μS/cm)

Water 

temperature

(°C)

Calcium

(mg/L)

Magnesium

(mg/L)

Potassium

(mg/L)

Sodium

(mg/L)

Laboratory Reporting Level (LRL) na na na na 0.02 0.01 0.04 0.20USGS parameter code 39086 00400 00095 00010 00915 00925 00935 00930

FP1-63 10/27/2003 1100 Unconfined shallow urban 337 6.7 1,543 13.0 173 35.9 13.1 87.0

FP1-63 6/15/2004 800 Unconfined shallow urban 337 6.7 1,402 14.5 155 34.2 12.1 87.9FP1-63 7/27/2004 1600 Unconfined shallow urban NC 6.7 1,394 15.5 NC NC NC NC

FP1-63 10/15/2004 1400 Unconfined shallow urban 331 6.7 1,395 14.0 160 34.5 12.6 86.1

FP1-63 4/11/2005 1300 Unconfined shallow urban 327 6.7 1,095 14.0 171 35.4 12.3 89.0FP2-43 7/28/2004 1300 Unconfined shallow urban 200 6.6 897 14.0 104 22.6 8.11 47.3

FP2-43 10/13/2004 1400 Unconfined shallow urban 209 6.5 888 13.0 104 23.2 8.17 49.7FP2-43 3/30/2005 1100 Unconfined shallow urban 210 6.4 872 12.5 105 22.7 8.25 49.3FP3-33 10/28/2003 1300 Unconfined shallow urban 106 5.9 713 15.0 60.5 22.6 7.47 34.3

FP3-33 7/26/2004 1400 Unconfined shallow urban 121 6.0 626 15.5 60.5 22.2 7.38 34.4

FP3-33 10/16/2004 1100 Unconfined shallow urban 116 5.8 622 15.0 55.3 21.3 7.30 32.1FP3-33 4/4/2005 1600 Unconfined shallow urban 123 5.9 598 16.5 53.4 20.4 7.08 35.9OFPN-88 11/13/2003 1000 Unconfined shallow urban 283 6.8 1,582 16.0 132 31.6 10.9 99.8

OFPN-88 7/27/2004 1000 Unconfined shallow urban 256 6.8 1,177 15.5 121 28.5 10.1 81.2

OFPN-88 10/20/2004 900 Unconfined shallow urban 255 6.7 1,162 15.0 125 30.1 80.6 10.9

OFPN-88 4/6/2005 1500 Unconfined shallow urban 261 6.7 1,165 15.0 141 33.7 10.5 75.0

OFPS-38 11/11/2003 1400 Unconfined shallow urban 225 6.6 817 12.5 83.6 21.8 7.00 43.1

OFPS-38 7/27/2004 1400 Unconfined shallow urban 229 6.6 787 13.5 82.7 20.7 6.68 40.1OFPS-38 10/14/2004 1500 Unconfined shallow urban 245 6.5 735 12.5 86.3 21.6 6.60 40.7OFPS-38 4/14/2005 1500 Unconfined shallow urban 238 6.6 755 13.0 83.1 21.3 6.58 40.7

UWT1-53 8/3/2004 1000 Unconfined shallow urban 160 6.3 888 15.5 97.6 22.7 10.4 40.1

UWT1-53 10/20/2004 1200 Unconfined shallow urban 171 6.3 871 14.5 96.4 22.2 40.6 10.5UWT1-53 4/5/2005 1200 Unconfined shallow urban 170 6.4 795 14.5 106 22.7 9.84 44.0UWT2-23 8/3/2004 1300 Unconfined shallow urban 176 6.4 1,243 14.0 116 31.3 27.1 61.9

UWT2-23 10/20/2004 1400 Unconfined shallow urban 192 6.3 1,330 14.0 130 33.1 66.9 29.7UWT2-23 4/5/2005 1000 Unconfined shallow urban 186 6.4 1,176 12.5 132 33.4 26.7 67.7UWT3-34 8/4/2004 1000 Unconfined shallow urban 332 6.9 1,291 15.5 109 28.0 9.90 130UWT3-34 10/17/2004 900 Unconfined shallow urban 333 6.8 1,283 14.5 115 28.3 9.82 130UWT3-34 4/5/2005 1500 Unconfined shallow urban 347 6.9 1,263 15.5 129 31.0 9.77 144UWT4-85 8/4/2004 1300 Unconfined shallow urban 397 6.9 1,288 16.0 148 33.3 9.88 72.6UWT4-85 10/22/2004 1000 Unconfined shallow urban 418 6.8 1,277 16.0 153 33.2 9.84 76.2UWT4-85 4/6/2005 1100 Unconfined shallow urban 426 6.9 1,299 14.5 181 36.7 9.88 84.6
Tables 137

Table 20. Results of analyses of selected field parameters and major and minor elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued[Parameters and constituents listed from left to right are field parameters, major cations, minor cations, major anions, minor anions, major ions with no charge (silica), and TDS. μg/L, microgram per liter; mg/L, milligram per liter; USGS, U.S. Geological Survey; V, censored value possibly affected by contamination during sample collection; <, less than; m, meter; study PSW, study public-supply well; nu, anomalous laboratory analytical result, data not used; NC, not collected; CaCO3, calcium carbonate; μS/cm, microsiemens per 

centimeter; °C, degrees Celsius; std, standard; na, not applicable; TDS, total dissolved solids]

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Bromide

(mg/L)

Chloride

(mg/L)

Fluoride

(mg/L)

Sulfate

(mg/L)

Iron

(μg/L)

Silica

(mg/L)

Residue, 

180°C 

(TDS)

(mg/L)

Laboratory Reporting Level (LRL) 0.02 0.12 0.10 0.18 6.4 0.02 10.00USGS parameter code 71870 00940 00950 00945 01046 00955 70300FP4-168 10/23/2003 1100 Confined agricultural unmixed 0.04 8.13 0.35 31.1 <6 34.2 291FP4-168 4/14/2005 1000 Confined agricultural unmixed nu 7.46 0.39 31.9 V3 36.0 285FP4-231 10/16/2003 1200 Confined agricultural unmixed 0.04 9.91 0.45 43.5 <6 30.4 253FP5-175 10/21/2004 1400 Confined agricultural mixed 0.12 8.04 0.34 71.8 102 31.2 439FP5-LS 10/21/2004 1000 Confined agricultural mixed 0.10 7.25 0.28 34.0 V6 38.5 367FP4-83 10/30/2003 1000 Unconfined deep agricultural 0.08 10.2 0.30 28.1 <6 49.4 347FP4-83 4/14/2005 1300 Unconfined deep agricultural nu 9.70 0.34 28.3 <6 52.7 348FP5-73 10/18/2004 1400 Unconfined deep agricultural 0.11 8.60 0.35 27.4 V5 39.7 393AWT1-83 7/29/2004 1000 Unconfined shallow agricultural 0.14 11.4 0.30 35.0 <6 38.0 573AWT1-83 10/19/2004 900 Unconfined shallow agricultural 0.17 12.4 0.31 34.1 V4 40.6 658AWT1-83 3/31/2005 1000 Unconfined shallow agricultural nu 10.3 0.34 32.1 <6 40.8 631AWT2-71 7/29/2004 1300 Unconfined shallow agricultural 0.12 11.8 0.38 38.0 <6 36.6 451AWT2-71 10/19/2004 1200 Unconfined shallow agricultural 0.13 9.33 0.42 31.2 V6 36.5 600AWT2-71 3/31/2005 1600 Unconfined shallow agricultural nu 11.8 0.40 37.7 V4 39.1 501AWT3-73 7/30/2004 1200 Unconfined shallow agricultural 0.14 9.73 0.39 31.6 14 34.6 588AWT3-73 10/19/2004 1400 Unconfined shallow agricultural 0.14 11.9 0.41 38.6 V4 38.7 488AWT3-73 4/1/2005 1300 Unconfined shallow agricultural nu 9.08 0.47 29.1 <6 37.7 591FP4-28 10/29/2003 1100 Unconfined shallow agricultural 0.18 67.0 0.32 99.6 V5 41.5 729FP4-28 7/26/2004 1000 Unconfined shallow agricultural 0.17 39.7 0.35 102 <6 45.8 644FP4-28 10/15/2004 900 Unconfined shallow agricultural 0.19 55.0 0.35 92.9 V3 43.7 686FP4-28 4/12/2005 1200 Unconfined shallow agricultural nu 31.2 0.36 89.5 <6 46.9 639FP5-45 7/28/2004 1000 Unconfined shallow agricultural 0.11 7.58 0.21 23.4 <6 34.2 303FP5-45 10/18/2004 900 Unconfined shallow agricultural 0.11 6.56 0.20 22.0 <6 35.1 316FP5-45 3/30/2005 1500 Unconfined shallow agricultural nu 7.06 0.21 23.2 <6 37.3 311NWT1-39 7/30/2004 1000 Unconfined shallow agricultural 0.13 12.1 0.18 97.4 V3 46.6 857NWT1-39 10/17/2004 1200 Unconfined shallow agricultural 0.07 12.6 0.20 96.3 9 47.4 758NWT1-39 4/1/2005 1000 Unconfined shallow agricultural nu 12.9 0.20 94.2 <6 50.3 766Study PSW 10/20/2003 1500 Study PSW 0.06 11.7 0.36 44.5 159 34.6 328Study PSW 6/18/2004 1500 Study PSW 0.07 11.9 0.36 45.2 163 34.4 310Study PSW 10/15/2004 1200 Study PSW 0.08 11.5 0.36 47.0 169 33.5 334Study PSW 4/7/2005 900 Study PSW nu 11.4 0.38 46.5 182 37.4 328
138 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 20. Results of analyses of selected field parameters and major and minor elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [Parameters and constituents listed from left to right are field parameters, major cations, minor cations, major anions, minor anions, major ions with no charge (silica), and TDS. 

μg/L, microgram per liter; mg/L, milligram per liter; USGS, U.S. Geological Survey; V, censored value possibly affected by contamination during sample collection; <, less than; m, meter; study PSW, study public-supply well; nu, anomalous laboratory analytical result, data not used; NC, not collected; CaCO3, calcium carbonate; μS/cm, microsiemens per 

centimeter; °C, degrees Celsius; std, standard; na, not applicable; TDS, total dissolved solids]

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Bromide

(mg/L)

Chloride

(mg/L)

Fluoride

(mg/L)

Sulfate

(mg/L)

Iron

(μg/L)

Silica

(mg/L)

Residue, 

180°C 

(TDS)

(mg/L)

Laboratory Reporting Level (LRL) 0.02 0.12 0.10 0.18 6.4 0.02 10.00USGS parameter code 71870 00940 00950 00945 01046 00955 70300Study PSW, depth 

42.7 m

6/18/2004 1100 Study PSW 0.07 13.3 0.35 48.9 169 34.0 277

Study PSW, depth 

45.7 m

6/21/2004 1000 Study PSW 0.08 14.7 0.35 53.0 179 34.3 329

Study PSW, depth 

48.8 m

6/17/2004 1800 Study PSW 0.09 19.3 0.34 67.4 238 33.4 326

Study PSW, depth 

51.8 m

6/17/2004 1300 Study PSW 0.21 39.4 0.25 151 277 30.1 459

Study PSW, depth 

61.0 m

6/16/2004 1300 Study PSW 0.17 30.5 0.32 107 455 35.9 405

FP1-147 10/22/2003 1200 Confined urban unmixed 0.06 7.57 0.35 30.4 143 33.5 283FP1-147 6/14/2004 1600 Confined urban unmixed 0.05 7.67 0.37 29.0 115 33.6 217FP1-147 4/11/2005 1100 Confined urban unmixed nu 7.30 0.39 30.7 94 35.4 284FP1-185 10/21/2003 1200 Confined urban unmixed 0.05 8.75 0.38 32.1 73 33.6 287FP1-185 6/14/2004 1200 Confined urban unmixed 0.06 9.19 0.38 32.2 111 34.4 282FP1-185 4/7/2005 1500 Confined urban unmixed NC NC NC NC NC NC NCFP1-247 10/20/2003 1200 Confined urban unmixed 0.05 13.4 0.37 41.3 9 36.7 333FP1-247 6/14/2004 800 Confined urban unmixed 0.05 12.0 0.36 31.8 8 36.7 266FP1-247 4/7/2005 1300 Confined urban unmixed NC NC NC NC NC NC NCFP2-152 10/12/2004 1500 Confined urban unmixed 0.05 7.61 0.40 31.5 39 37.7 262FP3-130 10/15/2003 1100 Confined urban unmixed 0.05 8.37 0.33 30.0 V5 44.8 285OFPN-167 10/30/2003 1400 Confined urban unmixed 0.04 7.77 0.36 33.3 58 33.9 280OFPN-276 11/12/2003 900 Confined urban unmixed 0.05 32.2 0.58 94.1 17 45.6 450OFPS-225 10/14/2004 1000 Confined urban unmixed 0.05 9.32 0.35 36.6 138 38.7 294FP2-250 10/12/2004 1000 Confined urban mixed 0.08 135 0.67 1,057 178 53.1 2,032FP3-162 10/28/2003 1000 Confined urban mixed 0.23 45.7 0.19 139 <6 52.0 718FP3-162 10/16/2004 900 Confined urban mixed 0.33 55.3 0.20 154 <6 59.8 745FP3-218 10/15/2003 1800 Confined urban mixed 0.10 14.2 0.34 104 491 36.7 496FP3-218 10/16/2004 1300 Confined urban mixed 0.12 14.2 0.32 110 501 36.8 518FP3-218 4/4/2005 1200 Confined urban mixed NC NC NC NC NC NC NC
Tables 139

Table 20. Results of analyses of selected field parameters and major and minor elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued[Parameters and constituents listed from left to right are field parameters, major cations, minor cations, major anions, minor anions, major ions with no charge (silica), and TDS. μg/L, microgram per liter; mg/L, milligram per liter; USGS, U.S. Geological Survey; V, censored value possibly affected by contamination during sample collection; <, less than; m, meter; study PSW, study public-supply well; nu, anomalous laboratory analytical result, data not used; NC, not collected; CaCO3, calcium carbonate; μS/cm, microsiemens per 

centimeter; °C, degrees Celsius; std, standard; na, not applicable; TDS, total dissolved solids]

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Bromide

(mg/L)

Chloride

(mg/L)

Fluoride

(mg/L)

Sulfate

(mg/L)

Iron

(μg/L)

Silica

(mg/L)

Residue, 

180°C 

(TDS)

(mg/L)

Laboratory Reporting Level (LRL) 0.02 0.12 0.10 0.18 6.4 0.02 10.00USGS parameter code 71870 00940 00950 00945 01046 00955 70300OFPS-157 11/11/2003 1000 Confined urban mixed 0.12 26.5 0.28 73.1 V6 37.9 480OFPS-157 10/14/2004 1300 Confined urban mixed 0.10 12.2 0.36 52.4 V6 37.1 367OFPS-157 4/11/2005 1600 Confined urban mixed nu 14.5 0.37 58.9 <6 37.5 383FP2-78 10/13/2004 1000 Unconfined deep urban 0.08 12.0 0.30 37.0 <6 58.8 391FP3-83 10/27/2003 1500 Unconfined deep urban 0.14 21.8 0.27 53.9 <6 52.1 462FP1-63 10/27/2003 1100 Unconfined shallow urban 0.56 98.6 0.24 186 V3 44.2 983FP1-63 6/15/2004 800 Unconfined shallow urban 0.68 102 0.24 142 <6 44.1 794FP1-63 7/27/2004 1600 Unconfined shallow urban NC NC NC NC NC NC NCFP1-63 10/15/2004 1400 Unconfined shallow urban 0.67 99.6 0.27 153 <6 44.7 910FP1-63 4/11/2005 1300 Unconfined shallow urban nu 96.2 0.28 148 <6 45.8 901FP2-43 7/28/2004 1300 Unconfined shallow urban 0.20 53.0 0.22 113 <6 45.8 592FP2-43 10/13/2004 1400 Unconfined shallow urban 0.19 52.2 0.23 113 <6 48.5 596FP2-43 3/30/2005 1100 Unconfined shallow urban nu 45.7 0.23 115 <6 48.8 600FP3-33 10/28/2003 1300 Unconfined shallow urban 0.54 69.9 0.17 90.0 V3 41.9 448FP3-33 7/26/2004 1400 Unconfined shallow urban 0.70 67.5 0.17 84.5 V4 43.7 414FP3-33 10/16/2004 1100 Unconfined shallow urban 0.61 64.1 0.13 76.1 V3 41.9 381FP3-33 4/4/2005 1600 Unconfined shallow urban nu 57.1 0.17 74.5 V4 47.3 385OFPN-88 11/13/2003 1000 Unconfined shallow urban 0.29 57.9 0.34 221 15 47.7 917OFPN-88 7/27/2004 1000 Unconfined shallow urban 0.28 52.9 0.32 198 <6 48.4 811OFPN-88 10/20/2004 900 Unconfined shallow urban 0.27 51.3 0.32 194 2 53.7 815OFPN-88 4/6/2005 1500 Unconfined shallow urban nu 52.7 0.34 195 V6 54.0 814OFPS-38 11/11/2003 1400 Unconfined shallow urban 0.13 71.0 0.28 57.0 655 50.2 496OFPS-38 7/27/2004 1400 Unconfined shallow urban 0.13 72.1 0.27 57.3 1,810 50.6 490OFPS-38 10/14/2004 1500 Unconfined shallow urban 0.20 58.0 0.29 44.6 2,861 55.1 468OFPS-38 4/14/2005 1500 Unconfined shallow urban nu 59.8 0.31 46.0 3,581 55.7 487UWT1-53 8/3/2004 1000 Unconfined shallow urban 0.19 38.0 0.20 151 <6 50.1 603UWT1-53 10/20/2004 1200 Unconfined shallow urban 0.19 37.1 0.23 151 1 54.1 606UWT1-53 4/5/2005 1200 Unconfined shallow urban nu 33.2 0.24 147 <6 53.7 587UWT2-23 8/3/2004 1300 Unconfined shallow urban 0.22 138 0.24 137 <6 45.7 745UWT2-23 10/20/2004 1400 Unconfined shallow urban 0.24 187 0.27 121 1 51.2 777UWT2-23 4/5/2005 1000 Unconfined shallow urban nu 159 0.25 128 V3 52.5 759
140 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 20. Results of analyses of selected field parameters and major and minor elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [Parameters and constituents listed from left to right are field parameters, major cations, minor cations, major anions, minor anions, major ions with no charge (silica), and TDS. 

μg/L, microgram per liter; mg/L, milligram per liter; USGS, U.S. Geological Survey; V, censored value possibly affected by contamination during sample collection; <, less than; m, meter; study PSW, study public-supply well; nu, anomalous laboratory analytical result, data not used; NC, not collected; CaCO3, calcium carbonate; μS/cm, microsiemens per 

centimeter; °C, degrees Celsius; std, standard; na, not applicable; TDS, total dissolved solids]

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Bromide

(mg/L)

Chloride

(mg/L)

Fluoride

(mg/L)

Sulfate

(mg/L)

Iron

(μg/L)

Silica

(mg/L)

Residue, 

180°C 

(TDS)

(mg/L)

Laboratory Reporting Level (LRL) 0.02 0.12 0.10 0.18 6.4 0.02 10.00USGS parameter code 71870 00940 00950 00945 01046 00955 70300UWT3-34 8/4/2004 1000 Unconfined shallow urban 0.21 38.5 0.39 238 <6 42.5 881UWT3-34 10/17/2004 900 Unconfined shallow urban 0.22 44.0 0.42 222 V3 44.4 873UWT3-34 4/5/2005 1500 Unconfined shallow urban nu 34.7 0.45 249 V5 47.2 918UWT4-85 8/4/2004 1300 Unconfined shallow urban 0.25 50.3 0.32 139 <6 36.7 801UWT4-85 10/22/2004 1000 Unconfined shallow urban 0.26 49.7 0.38 131 <6 40.5 854UWT4-85 4/6/2005 1100 Unconfined shallow urban nu 50.9 0.37 147 V5 40.7 890
Tables 141

Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued[USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] 

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Aluminum

μg/L

Antimony

μg/L

Arsenic

μg/L

Arsenite

μg/L 

as As

Arsenate

μg/L 

as As

Barium

μg/L

Beryllium

μg/L

Boron

μg/L

Cadmium

μg/L

Laboratory reporting level (LRL) 1.6 0.06 0.12 1.0 1.2 0.08 0.06 8 0.04

USGS parameter code 01106 01095 01000 62452 62453 01005 01010 01020 01025

FP4-168 10/23/2003 1100 Confined agricultural unmixed — E0.11 5.9 3.5 2.4 94 — 51 0.07FP4-168 4/14/2005 1000 Confined agricultural unmixed — — 6.3 NC NC 95 — 48 E0.04FP4-231 10/16/2003 1200 Confined agricultural unmixed 2.3 E0.11 4.9 3.8 1.4 55 — 58 E0.02FP5-175 10/21/2004 1400 Confined agricultural mixed — 0.48 2.4 — 4.5 104 — 47 —FP5-LS 10/21/2004 1000 Confined agricultural mixed E0.9 0.40 4.3 — 12 214 — 43 0.06FP4-83 10/30/2003 1000 Unconfined deep agricultural — E0.12 4.2 — 3.6 163 — 43 —FP4-83 4/14/2005 1300 Unconfined deep agricultural — E0.13 4.3 NC NC 156 — 43 —FP5-73 10/18/2004 1400 Unconfined deep agricultural — E0.13 4.1 — 3.0 238 — 30 E0.03AWT1-83 7/29/2004 1000 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCAWT1-83 10/19/2004 900 Unconfined shallow agricultural — E0.13 3.4 — 3.3 407 — 37 E0.03AWT1-83 3/31/2005 1000 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCAWT2-71 7/29/2004 1300 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCAWT2-71 10/19/2004 1200 Unconfined shallow agricultural — E0.16 5.0 1.5 E1.0 308 — 38 E0.02AWT2-71 3/31/2005 1600 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCAWT3-73 7/30/2004 1200 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCAWT3-73 10/19/2004 1400 Unconfined shallow agricultural — E0.17 5.9 — 6.9 249 — 34 —AWT3-73 4/1/2005 1300 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCFP4-28 10/29/2003 1100 Unconfined shallow agricultural — E0.16 4.6 — 3.8 74 — 63 —FP4-28 7/26/2004 1000 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCFP4-28 10/15/2004 900 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCFP4-28 4/12/2005 1200 Unconfined shallow agricultural E0.9 0.19 5.3 NC NC 103 — 62 E0.02FP5-45 7/28/2004 1000 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCFP5-45 10/18/2004 900 Unconfined shallow agricultural — E0.14 2.9 — 1.9 262 — 37 0.04FP5-45 3/30/2005 1500 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCNWT1-39 7/30/2004 1000 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCNWT1-39 10/17/2004 1200 Unconfined shallow agricultural E1.1 E0.13 3.1 — 1.9 119 — 76 0.05NWT1-39 4/1/2005 1000 Unconfined shallow agricultural NC NC NC NC NC NC NC NC NCstudy PSW 10/20/2003 1500 Study PSW — — 6.4 — 6.3 95 — 57 —Study PSW 6/18/2004 1500 Study PSW — — 6.3 — 5.8 99 — 49 —Study PSW 10/15/2004 1200 Study PSW NC NC NC NC NC NC NC NC NCStudy PSW 4/7/2005 900 Study PSW — E0.11 6.9 NC NC 105 — 66 E0.02Study PSW, 

depth 42.7 m

6/18/2004 1100 Study PSW — — 6.0 — 5.4 99 — 51 —

Study PSW, 

depth 45.7 m

6/21/2004 1000 Study PSW — — 5.7 — 5.7 98 — 45 —

Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.[USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] 
142 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] Well or sample name Date collected Time collected Well classification (see table 11) Aluminum μg/L Antimony μg/L Arsenic μg/L Arsenite μg/L as As Arsenate μg/L as As Barium μg/L Beryllium μg/L Boron μg/L Cadmium μg/L Laboratory reporting level (LRL) 1.6 0.06 0.12 1.0 1.2 0.08 0.06 8 0.04 USGS parameter code 01106 01095 01000 62452 62453 01005 01010 01020 01025 Study PSW, depth 48.8 m 6/17/2004 1800 Study PSW

— E0.14 6.2

— 5.5 96

— 55

—

Study PSW, 

depth 51.8 m

6/17/2004 1300 Study PSW

— 0.48 6.5

— 5.6 92

— 98 E0.03

Study PSW, 

depth 61.0 m

6/16/2004 1300 Study PSW

— E0.13 9.0 E0.65 8.2 108

— 63

—

FP1-147 10/22/2003 1200 Confined urban unmixed

—

— 6.2 5.7

— 90

— 50

—

FP1-147 6/14/2004 1600 Confined urban unmixed

—

— 6.2 5.6

— 97

— 51

—

FP1-147 4/11/2005 1100 Confined urban unmixed

—

— 6.7 NC NC 96

— 62

—

FP1-185 10/21/2003 1200 Confined urban unmixed E1.0

— 8.0 6.3 1.8 80

— 50

—

FP1-185 6/14/2004 1200 Confined urban unmixed

—

— 7.6 5.1 2.6 97

— 47

—

FP1-247 10/20/2003 1200 Confined urban unmixed

— 0.41 6.9 1.7 5.3 86

— 79

—

FP1-247 6/14/2004 800 Confined urban unmixed

— 0.31 4.4 E0.93 3.4 100

— 69

—

FP2-152 10/12/2004 1500 Confined urban unmixed

—

— 6.4 4.8 E1.1 79

— 63

—

FP3-130 10/15/2003 1100 Confined urban unmixed 5.1

— 1.6 4.5 1.5 182

— 54

—

OFPN-167 10/30/2003 1400 Confined urban unmixed

—

— 6.2 5.3

— 92

— 49

—

OFPN-276 11/12/2003 900 Confined urban unmixed

—

— 7.8 6.4 1.5 74

— 153 E0.03

OFPS-225 10/14/2004 1000 Confined urban unmixed

—

— 2.1 1.6

— 103

— 59

—

FP2-250 10/12/2004 1000 Confined urban mixed E0.9

— 9.1 7.4 E1.1 17

— 480 E0.03

FP3-162 10/28/2003 1000 Confined urban mixed

— 0.37 6.1

— 4.6 132

— 141 E0.03

FP3-162 10/16/2004 900 Confined urban mixed

— 0.29 6.1

— 4.2 147

— 156 E0.03

FP3-218 10/15/2003 1800 Confined urban mixed E1.2

— 6.9 6.0 E0.70 159

— 55

—

FP3-218 10/16/2004 1300 Confined urban mixed

—

— 7.8 5.8

— 174

— 52

—

OFPS-157 11/11/2003 1000 Confined urban mixed

— 0.89 5.1 E0.56 4.5 201

— 50 E0.04

OFPS-157 10/14/2004 1300 Confined urban mixed

— E0.16 7.2

— 5.9 148

— 49 0.07

OFPS-157 4/11/2005 1600 Confined urban mixed

— E0.10 9.4 NC NC 149

— 60 0.05

FP2-78 10/13/2004 1000 Unconfined deep urban

— 0.25 7.0

— 4.9 160

— 33 E0.03

FP3-83 10/27/2003 1500 Unconfined deep urban

— 0.49 6.4

— 5.8 235

— 45 E0.02

FP1-63 10/27/2003 1100 Unconfined shallow urban

— E0.16 5.6

— 4.9 489

— 162 0.04

FP1-63 6/15/2004 800 Unconfined shallow urban

— E0.15 5.2

— 4.8 464

— 115 E0.04

FP1-63 10/15/2004 1400 Unconfined shallow urban NC NC NC NC NC NC NC NC NCFP1-63 4/11/2005 1300 Unconfined shallow urban

— E0.18 7.1 NC NC 489

— 134 0.04

FP2-43 7/28/2004 1300 Unconfined shallow urban NC NC NC NC NC NC NC NC NCFP2-43 10/13/2004 1400 Unconfined shallow urban 2.4 E0.15 4.7

— 3.6 337

— 61 E0.02

FP2-43 3/30/2005 1100 Unconfined shallow urban NC NC NC NC NC NC NC NC NC
Tables 143

Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued[USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] 

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Aluminum

μg/L

Antimony

μg/L

Arsenic

μg/L

Arsenite

μg/L 

as As

Arsenate

μg/L 

as As

Barium

μg/L

Beryllium

μg/L

Boron

μg/L

Cadmium

μg/L

Laboratory reporting level (LRL) 1.6 0.06 0.12 1.0 1.2 0.08 0.06 8 0.04

USGS parameter code 01106 01095 01000 62452 62453 01005 01010 01020 01025

FP3-33 10/28/2003 1300 Unconfined shallow urban E0.9 E0.13 4.6 — 2.5 80 — 114 0.36FP3-33 7/26/2004 1400 Unconfined shallow urban NC NC NC NC NC NC NC NC NCFP3-33 10/16/2004 1100 Unconfined shallow urban NC NC NC NC NC NC NC NC NCFP3-33 4/4/2005 1600 Unconfined shallow urban NC NC NC NC NC NC NC NC NCOFPN-88 11/13/2003 1000 Unconfined shallow urban — E0.17 5.2 — 4.5 68 — 219 E0.04OFPN-88 7/27/2004 1000 Unconfined shallow urban NC NC NC NC NC NC NC NC NCOFPN-88 4/6/2005 1500 Unconfined shallow urban NC NC NC NC NC NC NC NC NCOFPS-38 11/11/2003 1400 Unconfined shallow urban — E0.10 8.8 1.9 6.0 321 — 45 0.08OFPS-38 7/27/2004 1400 Unconfined shallow urban NC NC NC NC NC NC NC NC NCOFPS-38 4/14/2005 1500 Unconfined shallow urban NC NC NC NC NC NC NC NC NCUWT1-53 8/3/2004 1000 Unconfined shallow urban NC NC NC NC NC NC NC NC NCUWT1-53 10/20/2004 1200 Unconfined shallow urban — 0.12 4.4 — 3.3 163 — 167 0.43UWT1-53 4/5/2005 1200 Unconfined shallow urban NC NC NC NC NC NC NC NC NCUWT2-23 8/3/2004 1300 Unconfined shallow urban NC NC NC NC NC NC NC NC NCUWT2-23 10/20/2004 1400 Unconfined shallow urban — 0.26 7.8 — 6.9 93 — 70 0.11UWT2-23 4/5/2005 1000 Unconfined shallow urban NC NC NC NC NC NC NC NC NCUWT3-34 8/4/2004 1000 Unconfined shallow urban NC NC NC NC NC NC NC NC NCUWT3-34 10/17/2004 900 Unconfined shallow urban E0.9 E0.20 5.9 — 3.9 55 — 287 —UWT3-34 4/5/2005 1500 Unconfined shallow urban NC NC NC NC NC NC NC NC NCUWT4-85 8/4/2004 1300 Unconfined shallow urban NC NC NC NC NC NC NC NC NCUWT4-85 10/22/2004 1000 Unconfined shallow urban — E0.14 3.8 — 8.3 266 — 167 E0.03UWT4-85 4/6/2005 1100 Unconfined shallow urban NC NC NC NC NC NC NC NC NC
144 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] Well or sample name Date collected Time collected Well classification (see table 11) Chromium μg/L Cobalt μg/L Copper μg/L Lead μg/L Lithium μg/L Manganese μg/L Molybdenum μg/L Nickel μg/L Selenium μg/L Laboratory reporting level (LRL) 0.80 0.04 0.4 0.08 0.6 0.6 0.12 0.06 0.40 USGS parameter code 01030 01035 01040 01049 01130 01056 01060 01065 01145 FP4-168 10/23/2003 1100 Confined agricultural unmixed

— 0.273 VB1.4

— 19.4 221 3.9 VB1.67

—

FP4-168 4/14/2005 1000 Confined agricultural unmixed

— 0.195 V1.3

— 16.5 213 3.8 VB1.65 E0.25

FP4-231 10/16/2003 1200 Confined agricultural unmixed

— 0.069

— 0.08 23.0 28.0 12.9 VB1.56

—

FP5-175 10/21/2004 1400 Confined agricultural mixed E0.53 0.395 V0.7

— 25.2 465 4.9 VB1.05

—

FP5-LS 10/21/2004 1000 Confined agricultural mixed 2.4 0.165 VB2.5

— 22.7 1.6 3.7 2.59 3.3

FP4-83 10/30/2003 1000 Unconfined deep agricultural 2.6 0.161 VB3.0

— 22.3 0.8 2.7 VB0.88 7.1

FP4-83 4/14/2005 1300 Unconfined deep agricultural 3.0 0.165 V1.2

— 21.8 0.4 3.1 VB1.93 7.2

FP5-73 10/18/2004 1400 Unconfined deep agricultural 4.2 0.187 V0.9

— 17.8 0.8 2.5 VB1.59 4.5

AWT1-83 7/29/2004 1000 Unconfined shallow agricultural NC NC NC NC NC

— NC NC NC

AWT1-83 10/19/2004 900 Unconfined shallow agricultural 1.8 0.275 V1.1

— 20.4 E0.1 1.3 VB1.96 6.0

AWT1-83 3/31/2005 1000 Unconfined shallow agricultural NC NC NC NC NC

— NC NC NC

AWT2-71 7/29/2004 1300 Unconfined shallow agricultural NC NC NC NC NC

— NC NC NC

AWT2-71 10/19/2004 1200 Unconfined shallow agricultural 3.5 0.471 V1.0

— 17.5 0.3 2.9 VB2.28 4.6

AWT2-71 3/31/2005 1600 Unconfined shallow agricultural NC NC NC NC NC

— NC NC NC

AWT3-73 7/30/2004 1200 Unconfined shallow agricultural NC NC NC NC NC 1.1 NC NC NCAWT3-73 10/19/2004 1400 Unconfined shallow agricultural 2.7 0.212 V0.9

— 16.9 E0.2 2.8 VB1.36 7.6

AWT3-73 4/1/2005 1300 Unconfined shallow agricultural NC NC NC NC NC

— NC NC NC

FP4-28 10/29/2003 1100 Unconfined shallow agricultural 2.3 0.409 VB1.5

— 32.8 1.5 2.8 5.49 8.7

FP4-28 7/26/2004 1000 Unconfined shallow agricultural NC NC NC NC NC E0.4 NC NC NCFP4-28 10/15/2004 900 Unconfined shallow agricultural NC NC NC NC NC

— NC NC NC

FP4-28 4/12/2005 1200 Unconfined shallow agricultural 2.7 0.315 VB1.3

— 44.2 1.6 2.5 4.52 9.2

FP5-45 7/28/2004 1000 Unconfined shallow agricultural NC NC NC NC NC 0.8 NC NC NCFP5-45 10/18/2004 900 Unconfined shallow agricultural 1.1 0.149 VB1.2 E0.04 13.9 0.4 1.3 VB2.39 2.8

FP5-45 3/30/2005 1500 Unconfined shallow agricultural NC NC NC NC NC

— NC NC NC

NWT1-39 7/30/2004 1000 Unconfined shallow agricultural NC NC NC NC NC E0.5 NC NC NCNWT1-39 10/17/2004 1200 Unconfined shallow agricultural 1.7 0.394 VB5.2 E0.04 27.5 0.5 1.1 6.46 1.7

NWT1-39 4/1/2005 1000 Unconfined shallow agricultural NC NC NC NC NC

— NC NC NC

study PSW 10/20/2003 1500 Study PSW

— 0.630 V0.7

— 19.5 397 4.2 VB1.78

—

Study PSW 6/18/2004 1500 Study PSW

— 0.608 V0.5

— 16.6 356 4.1 VB1.36

—

Study PSW 10/15/2004 1200 Study PSW NC NC NC NC NC 365 NC NC NC

Study PSW 4/7/2005 900 Study PSW

— 0.501 V0.5

— 19.0 352 4.4 2.55 E0.32

Study PSW, 

depth 42.7 m

6/18/2004 1100 Study PSW

— 0.543 V0.5

— 18.3 387 4.3 VB1.22 E0.26

Study PSW, 

depth 45.7 m

6/21/2004 1000 Study PSW

— 0.503 V0.6

— 15.7 400 4.1 2.57

—
Tables 145

Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued[USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] 

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Chromium

μg/L

Cobalt

μg/L

Copper

μg/L

Lead

μg/L

Lithium

μg/L

Manganese

μg/L

Molybdenum

μg/L

Nickel

μg/L

Selenium

μg/L

Laboratory reporting level (LRL) 0.80 0.04 0.4 0.08 0.6 0.6 0.12 0.06 0.40

USGS parameter code 01030 01035 01040 01049 01130 01056 01060 01065 01145

Study PSW, 

depth 48.8 m

6/17/2004 1800 Study PSW 1.4 0.826 V0.6 — 18.6 419 4.2 VB1.94 —

Study PSW, 

depth 51.8 m

6/17/2004 1300 Study PSW — 1.730 V1.0 — 27.3 571 10.3 5.29 0.44

Study PSW, 

depth 61.0 m

6/16/2004 1300 Study PSW — 0.360 V0.9 — 26.7 474 7.6 VB1.34 E0.26

FP1-147 10/22/2003 1200 Confined urban unmixed — 0.198 V0.6 E0.07 23.7 360 4.6 VB0.96 —FP1-147 6/14/2004 1600 Confined urban unmixed — 0.203 V0.5 — 18.6 330 4.2 VB0.66 —FP1-147 4/11/2005 1100 Confined urban unmixed — 0.176 V0.5 — 31.3 336 4.2 VB2.35 —FP1-185 10/21/2003 1200 Confined urban unmixed — 0.229 V0.7 E0.05 18.5 285 3.3 VB1.05 —

FP1-185 6/14/2004 1200 Confined urban unmixed — 0.197 V0.5 — 16.1 272 3.1 VB0.47 —

FP1-247 10/20/2003 1200 Confined urban unmixed — 0.407 V0.8 E0.06 27.1 247 4.0 VB2.00 —FP1-247 6/14/2004 800 Confined urban unmixed — 0.306 V0.5 — 23.7 226 2.5 VB0.99 —FP2-152 10/12/2004 1500 Confined urban unmixed — 0.318 V0.5 — 24.3 368 4.9 VB0.86 —FP3-130 10/15/2003 1100 Confined urban unmixed — 0.138 V0.2 — 9.6 VE0.2 4.9 VB0.82 3.6OFPN-167 10/30/2003 1400 Confined urban unmixed VE0.48 0.185 V0.3 — 18.1 305 5.2 VB0.91 —

OFPN-276 11/12/2003 900 Confined urban unmixed — 0.199 V0.5 — 41.7 200 17.5 VB0.78 —

OFPS-225 10/14/2004 1000 Confined urban unmixed — 0.174 V0.3 — 23.7 314 2.6 VB1.23 —FP2-250 10/12/2004 1000 Confined urban mixed 1.0 1.747 VB8.3 — 88.4 869 4.1 4.17 E0.29

FP3-162 10/28/2003 1000 Confined urban mixed — 0.474 VB2.4 0.10 30.1 74.0 1.6 4.01 8.6FP3-162 10/16/2004 900 Confined urban mixed — 0.405 VB1.9 0.12 32.5 18.0 0.9 VB1.47 10.3

FP3-218 10/15/2003 1800 Confined urban mixed 1.2 0.329 V0.5 — 23.7 481 5.0 6.95 —

FP3-218 10/16/2004 1300 Confined urban mixed E0.46 0.250 V1.0 — 25.7 538 4.9 VB0.81 E0.34

OFPS-157 11/11/2003 1000 Confined urban mixed — 0.410 VB1.7 — 26.0 29.2 3.9 3.82 3.2

OFPS-157 10/14/2004 1300 Confined urban mixed — 0.701 VB1.7 — 21.0 276 3.7 2.82 —OFPS-157 4/11/2005 1600 Confined urban mixed — 0.971 VB1.4 — 32.0 434 3.3 4.3 —FP2-78 10/13/2004 1000 Unconfined deep urban 2.1 0.231 V0.8 — 19.6 0.4 2.1 VB0.81 27.3

FP3-83 10/27/2003 1500 Unconfined deep urban 2.3 0.261 V1.3 E0.04 24.2 1.1 3.6 VB2.10 6.4FP1-63 10/27/2003 1100 Unconfined shallow urban 1.6 0.505 VB2.8 0.20 27.2 0.4 4.5 5.19 15.1

FP1-63 6/15/2004 800 Unconfined shallow urban 1.8 0.428 VB2.4 E0.05 24.1 E0.1 3.4 VB2.18 12.2

FP1-63 10/15/2004 1400 Unconfined shallow urban NC NC NC NC NC — NC NC NC

FP1-63 4/11/2005 1300 Unconfined shallow urban 2.4 0.445 VB2.2 E0.05 40.8 — 3.4 6.54 16.2

FP2-43 7/28/2004 1300 Unconfined shallow urban NC NC NC NC NC E0.5 NC NC NC

FP2-43 10/13/2004 1400 Unconfined shallow urban 1.2 0.336 2.2 — 24.2 0.5 1.0 VB1.82 9.35

FP2-43 3/30/2005 1100 Unconfined shallow urban NC NC NC NC NC — NC NC NC
146 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the local-scale TANC study area near York, Nebraska.—Continued [USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] Well or sample name Date collected Time collected Well classification (see table 11) Chromium μg/L Cobalt μg/L Copper μg/L Lead μg/L Lithium μg/L Manganese μg/L Molybdenum μg/L Nickel μg/L Selenium μg/L Laboratory reporting level (LRL) 0.80 0.04 0.4 0.08 0.6 0.6 0.12 0.06 0.40 USGS parameter code 01030 01035 01040 01049 01130 01056 01060 01065 01145 FP3-33 10/28/2003 1300 Unconfined shallow urban

— 1.310 VB1.7 E0.05 58.3 496 2.8 11.49 23.8

FP3-33 7/26/2004 1400 Unconfined shallow urban NC NC NC NC NC 468 NC NC NC

FP3-33 10/16/2004 1100 Unconfined shallow urban NC NC NC NC NC 443 NC NC NCFP3-33 4/4/2005 1600 Unconfined shallow urban NC NC NC NC NC 419 NC NC NCOFPN-88 11/13/2003 1000 Unconfined shallow urban 3.2 0.374 VB2.0 E0.07 33.9 0.7 3.4 5.85 17.7

OFPN-88 7/27/2004 1000 Unconfined shallow urban NC NC NC NC NC

— NC NC NC

OFPN-88 4/6/2005 1500 Unconfined shallow urban NC NC NC NC NC

— NC NC NC

OFPS-38 11/11/2003 1400 Unconfined shallow urban

— 1.331 V0.9

— 10.9 3,475 7.5 6.82 3.0

OFPS-38 7/27/2004 1400 Unconfined shallow urban NC NC NC NC NC 3,360 NC NC NCOFPS-38 4/14/2005 1500 Unconfined shallow urban NC NC NC NC NC 3,269 NC NC NCUWT1-53 8/3/2004 1000 Unconfined shallow urban NC NC NC NC NC 1,780 NC NC NCUWT1-53 10/20/2004 1200 Unconfined shallow urban 1.5 0.511 VB1.5

— 29.6 1,823 3.3 22.38 16.1

UWT1-53 4/5/2005 1200 Unconfined shallow urban NC NC NC NC NC 1,810 NC NC NCUWT2-23 8/3/2004 1300 Unconfined shallow urban NC NC NC NC NC 5.2 NC NC NCUWT2-23 10/20/2004 1400 Unconfined shallow urban 0.82 0.406 VB2.5 E0.03 39.7 2.3 2.1 5.11 16.4

UWT2-23 4/5/2005 1000 Unconfined shallow urban NC NC NC NC NC 4.2 NC NC NC

UWT3-34 8/4/2004 1000 Unconfined shallow urban NC NC NC NC NC

— NC NC NC

UWT3-34 10/17/2004 900 Unconfined shallow urban 1.2 0.297 VB2.4 E0.07 29.6 0.2 3.2 VB1.36 9.7

UWT3-34 4/5/2005 1500 Unconfined shallow urban NC NC NC NC NC

— NC NC NC

UWT4-85 8/4/2004 1300 Unconfined shallow urban NC NC NC NC NC

— NC NC NC

UWT4-85 10/22/2004 1000 Unconfined shallow urban 1.5 0.347 VB1.8

— 26.5 E0.2 1.8 VB1.98 10.5

UWT4-85 4/6/2005 1100 Unconfined shallow urban NC NC NC NC NC

— NC NC NC
Tables 147

Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the localscale TANC study area near York, Nebraska.—Continued[USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] 

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Silver

μg/L

Strontium

μg/L

Thallium

μg/L

Uranium

μg/L

Vanadium

μg/L

Zinc

μg/L

Laboratory reporting level (LRL) 0.2 0.4 0.04 0.04 0.0 0.6

USGS parameter code 01075 01080 01057 22703 01085 01090

FP4-168 10/23/2003 1100 Confined agricultural unmixed — 419 0.04 2.09 2.5 E0.4

FP4-168 4/14/2005 1000 Confined agricultural unmixed — 442 E0.02 1.79 1.2 V0.7

FP4-231 10/16/2003 1200 Confined agricultural unmixed — 331 — 3.96 0.5 —

FP5-175 10/21/2004 1400 Confined agricultural mixed — 558 — 51.6 1.5 1.4FP5-LS 10/21/2004 1000 Confined agricultural mixed — 356 E0.03 4.44 7.7 5.7FP4-83 10/30/2003 1000 Unconfined deep agricultural — 464 — 4.37 9.4 E0.4

FP4-83 4/14/2005 1300 Unconfined deep agricultural — 459 — 4.21 10.1 2.2

FP5-73 10/18/2004 1400 Unconfined deep agricultural — 363 — 5.11 9.8 E0.4

AWT1-83 7/29/2004 1000 Unconfined shallow agricultural NC NC NC NC NC NC

AWT1-83 10/19/2004 900 Unconfined shallow agricultural — 503 — 4.56 7.6 E0.5

AWT1-83 3/31/2005 1000 Unconfined shallow agricultural NC NC NC NC NC NC

AWT2-71 7/29/2004 1300 Unconfined shallow agricultural NC NC NC NC NC NCAWT2-71 10/19/2004 1200 Unconfined shallow agricultural — 441 — 11.7 10.1 —AWT2-71 3/31/2005 1600 Unconfined shallow agricultural NC NC NC NC NC NCAWT3-73 7/30/2004 1200 Unconfined shallow agricultural NC NC NC NC NC NCAWT3-73 10/19/2004 1400 Unconfined shallow agricultural — 435 — 10.0 9.0 E0.5

AWT3-73 4/1/2005 1300 Unconfined shallow agricultural NC NC NC NC NC NC

FP4-28 10/29/2003 1100 Unconfined shallow agricultural — 436 — 27.5 11.0 E0.6

FP4-28 7/26/2004 1000 Unconfined shallow agricultural NC NC NC NC NC NC

FP4-28 10/15/2004 900 Unconfined shallow agricultural NC NC NC NC NC NCFP4-28 4/12/2005 1200 Unconfined shallow agricultural — 389 — 23.1 14.1 7.4FP5-45 7/28/2004 1000 Unconfined shallow agricultural NC NC NC NC NC NCFP5-45 10/18/2004 900 Unconfined shallow agricultural — 263 — 1.80 6.2 0.6FP5-45 3/30/2005 1500 Unconfined shallow agricultural NC NC NC NC NC NCNWT1-39 7/30/2004 1000 Unconfined shallow agricultural NC NC NC NC NC NCNWT1-39 10/17/2004 1200 Unconfined shallow agricultural — 577 — 1.55 8.3 1.1NWT1-39 4/1/2005 1000 Unconfined shallow agricultural NC NC NC NC NC NCstudy PSW 10/20/2003 1500 Study PSW — 460 — 18.4 0.2 0.8Study PSW 6/18/2004 1500 Study PSW — 424 E0.02 16.2 0.7 V0.9

Study PSW 10/15/2004 1200 Study PSW NC NC NC NC NC NC

Study PSW 4/7/2005 900 Study PSW — 475 E0.02 15.5 0.5 V0.9

Study PSW, 

depth 42.7 m

6/18/2004 1100 Study PSW — 448 — 21.6 0.5 V0.8

Study PSW, 

depth 45.7 m

6/21/2004 1000 Study PSW — 456 — 29.9 0.3 V0.6

Study PSW, 

depth 48.8 m

6/17/2004 1800 Study PSW — 496 E0.02 48.6 0.7 V0.7
148 Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply Well in York, Nebraska Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the local- scale TANC study area near York, Nebraska.—Continued [USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] Well or sample name Date collected Time collected Well classification (see table 11) Silver μg/L Strontium μg/L Thallium μg/L Uranium μg/L Vanadium μg/L Zinc μg/L Laboratory reporting level (LRL) 0.2 0.4 0.04 0.04 0.0 0.6 USGS parameter code 01075 01080 01057 22703 01085 01090 Study PSW, depth 51.8 m 6/17/2004 1300 Study PSW

— 736 E0.02 184 1.2 V1.1

Study PSW, 

depth 61.0 m

6/16/2004 1300 Study PSW

— 750 E0.03 44.2 2.1 V0.6

FP1-147 10/22/2003 1200 Confined urban unmixed

— 417 E0.03 3.26 0.2

—

FP1-147 6/14/2004 1600 Confined urban unmixed

— 384 E0.03 3.76 0.3 V0.6

FP1-147 4/11/2005 1100 Confined urban unmixed

— 399 E0.03 3.56 1.4 E0.3

FP1-185 10/21/2003 1200 Confined urban unmixed

— 417

— 1.16 0.3 0.6

FP1-185 6/14/2004 1200 Confined urban unmixed

— 401

— 0.95 0.3 V1.0

FP1-247 10/20/2003 1200 Confined urban unmixed

— 785 E0.03 2.03 1.5 1.6

FP1-247 6/14/2004 800 Confined urban unmixed

— 680 E0.02 0.66 1.1 V0.9

FP2-152 10/12/2004 1500 Confined urban unmixed

— 438 0.05 1.75 0.4 E0.5

FP3-130 10/15/2003 1100 Confined urban unmixed

— 555

— 0.99 4.3 0.7

OFPN-167 10/30/2003 1400 Confined urban unmixed

— 417 E0.02 3.96 0.5 2.7

OFPN-276 11/12/2003 900 Confined urban unmixed

— 745

— 0.91 1.6 1.3

OFPS-225 10/14/2004 1000 Confined urban unmixed

— 545

— 0.18 E0.1

—

FP2-250 10/12/2004 1000 Confined urban mixed

— 3776 0.51 8.40 0.4 3.4

FP3-162 10/28/2003 1000 Confined urban mixed

— 607 E0.03 5.03 8.8 0.7

FP3-162 10/16/2004 900 Confined urban mixed

— 642

— 4.01 9.5 1.2

FP3-218 10/15/2003 1800 Confined urban mixed

— 831

— 1.10 0.7 E0.4

FP3-218 10/16/2004 1300 Confined urban mixed

— 884

— 1.21 2.4 E0.5

OFPS-157 11/11/2003 1000 Confined urban mixed

— 529

— 13.7 8.2 E0.5

OFPS-157 10/14/2004 1300 Confined urban mixed

— 592

— 2.64 3.5 1.4

OFPS-157 4/11/2005 1600 Confined urban mixed

— 627

— 2.14 2.6 1.0

FP2-78 10/13/2004 1000 Unconfined deep urban

— 432

— 13.1 9.6 E0.4

FP3-83 10/27/2003 1500 Unconfined deep urban

— 470

— 5.74 11.0

—

FP1-63 10/27/2003 1100 Unconfined shallow urban

— 796

— 37.5 10.2 1.6

FP1-63 6/15/2004 800 Unconfined shallow urban

— 735

— 39.9 9.7 V1.1

FP1-63 10/15/2004 1400 Unconfined shallow urban NC NC NC NC NC NC

FP1-63 4/11/2005 1300 Unconfined shallow urban

— 734

— 35.2 12.4 V1.2

FP2-43 7/28/2004 1300 Unconfined shallow urban NC NC NC NC NC NC

FP2-43 10/13/2004 1400 Unconfined shallow urban

— 464

— 2.07 7.8 1.3

FP2-43 3/30/2005 1100 Unconfined shallow urban NC NC NC NC NC NCFP3-33 10/28/2003 1300 Unconfined shallow urban

— 300 E0.02 0.22 7.9 2.1

FP3-33 7/26/2004 1400 Unconfined shallow urban NC NC NC NC NC NCFP3-33 10/16/2004 1100 Unconfined shallow urban NC NC NC NC NC NC
Tables 149

Table 21. Results of analyses of trace elements in ground-water samples collected during October 2003 through April 2005 in the localscale TANC study area near York, Nebraska.—Continued[USGS, U.S. Geological Survey; μg/L, microgram per liter; E, estimated value; V, censored value possibly affected by contamination during sample collection; VB, censored value possibly affected by bias where maximum blank concentration is greater than 10 percent of environmental concentration for constituents detected in 50 percent or more of field blanks; <, less than; m, meter; study PSW, study public-supply well; NC, not collected; —, not detected] 

Well or sample 

name

Date 

collected

Time 

collected

Well classification 

(see table 11)

Silver

μg/L

Strontium

μg/L

Thallium

μg/L

Uranium

μg/L

Vanadium

μg/L

Zinc

μg/L

Laboratory reporting level (LRL) 0.2 0.4 0.04 0.04 0.0 0.6

USGS parameter code 01075 01080 01057 22703 01085 01090

FP3-33 4/4/2005 1600 Unconfined shallow urban NC NC NC NC NC NC

OFPN-88 11/13/2003 1000 Unconfined shallow urban — 499 — 27.5 12.0 2.2OFPN-88 7/27/2004 1000 Unconfined shallow urban NC NC NC NC NC NCOFPN-88 4/6/2005 1500 Unconfined shallow urban NC NC NC NC NC NCOFPS-38 11/11/2003 1400 Unconfined shallow urban — 416 — 0.65 5.2 1.1OFPS-38 7/27/2004 1400 Unconfined shallow urban NC NC NC NC NC NCOFPS-38 4/14/2005 1500 Unconfined shallow urban NC NC NC NC NC NCUWT1-53 8/3/2004 1000 Unconfined shallow urban NC NC NC NC NC NCUWT1-53 10/20/2004 1200 Unconfined shallow urban — 423 0.05 1.07 7.2 0.9UWT1-53 4/5/2005 1200 Unconfined shallow urban NC NC NC NC NC NC

UWT2-23 8/3/2004 1300 Unconfined shallow urban NC NC NC NC NC NC

UWT2-23 10/20/2004 1400 Unconfined shallow urban — 551 — 4.24 10.2 1.8UWT2-23 4/5/2005 1000 Unconfined shallow urban NC NC NC NC NC NCUWT3-34 8/4/2004 1000 Unconfined shallow urban NC NC NC NC NC NCUWT3-34 10/17/2004 900 Unconfined shallow urban — 399 — 18.0 15.5 1.3UWT3-34 4/5/2005 1500 Unconfined shallow urban NC NC NC NC NC NCUWT4-85 8/4/2004 1300 Unconfined shallow urban NC NC NC NC NC NCUWT4-85 10/22/2004 1000 Unconfined shallow urban — 649 — 26.1 11.9 1.0UWT4-85 4/6/2005 1100 Unconfined shallow urban NC NC NC NC NC NC

Landon and others-Hydrogeology, Chemical-Characteristics, and Transport Processes in the Zone of Contribution of a Public-Supply

Well in York, Nebraska-Scientific Investigations Report 2008-5050

Printed on recycled paper