Vertical Gradients in Water Chemistry and 

Age in the Southern High Plains Aquifer, 

Texas, 2002

By P.B. McMahon,1 J.K. Böhlke,

1

 and T.M. Lehman2

1

U.S. Geological Survey

2

Texas Tech University, Lubbock, Texas

U.S. Department of the Interior

U.S. Geological Survey

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Scientific Investigations Report 2004–5053
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

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FOREWORD

The U.S. Geological Survey (USGS) is committed 

to serve the Nation with accurate and timely scientific 

information that helps enhance and protect the overall 

quality of life, and facilitates effective management 

of water, biological, energy, and mineral resources. 

(http://www.usgs.gov/). Information on the quality 

of the Nation’s water resources is of critical interest to 

the USGS because it is so integrally linked to the longterm availability of water that is clean and safe for 

drinking and recreation and that is suitable for industry, irrigation, and habitat for fish and wildlife. Escalating population growth and increasing demands for 

the multiple water uses make water availability, now 

measured in terms of quantity and quality, even more 

critical to the long-term sustainability of our communities and ecosystems.

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

national, regional, and local information needs and 

decisions related to water-quality management and 

policy. (http://water.usgs.gov/nawqa). Shaped by 

and coordinated with ongoing efforts of other Federal, 

State, and local agencies, the NAWQA Program is 

designed to answer: What is the condition of our 

Nation’s streams and ground water? How are the 

conditions changing over time? How do natural features and human activities affect the quality of streams 

and ground water, and where are those effects most 

pronounced? By combining information on water 

chemistry, physical characteristics, stream habitat, and 

aquatic life, the NAWQA Program aims to provide 

science-based insights for current and emerging water 

issues and priorities. NAWQA results can contribute to 

informed decisions that result in practical and effective 

water-resource management and strategies that protect 

and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 

of the Nation’s most important river basins and aquifers, referred to as Study Units. (http://water.usgs.gov/

nawqa/nawqamap.html). Collectively, these Study 

Units account for more than 60 percent of the overall 

water use and population served by public water 

supply and are representative of the Nation’s major 

hydrologic landscapes, priority ecological resources, 

and agricultural, urban, and natural sources of 

contamination.

Each assessment is guided by a nationally consistent study design and methods of sampling and analysis. The assessments thereby build local knowledge 

about water-quality issues and trends in a particular 

stream or aquifer while providing an understanding 

of how and why water quality varies regionally and 

nationally. The consistent, multiscale approach helps 

to determine if certain types of water-quality issues are 

isolated or pervasive and allows direct comparisons of 

how human activities and natural processes affect 

water quality and ecological health in the Nation’s 

diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile organic compounds, trace metals, and aquatic 

ecology are developed at the national scale through 

comparative analysis of the Study Unit findings. 

(http://water.usgs.gov/nawqa/natsyn.html). 

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

science so that the most recent and available knowledge about water resources can be applied in management and policy decisions. We hope this NAWQA 

publication will provide you the needed insights and 

information to meet your needs and thereby foster 

increased awareness and involvement in the protection 

and restoration of our Nation’s waters. 

The NAWQA Program recognizes that a national 

assessment by a single program cannot address all 

water-resource issues of interest. External coordination at all levels is critical for a fully integrated 

understanding of watersheds and for cost-effective 

management, regulation, and conservation of our 

Nation’s water resources. The program, therefore, 

depends extensively on the advice, cooperation, and 

information from other Federal, State, interstate, 

Tribal, and local agencies, nongovernment organizations, industry, academia, and other stakeholder 

groups. The assistance and suggestions of all are 

greatly appreciated.

Robert M. Hirsch

Associate Director for Water

FOREWORD iii
Contents iii

Foreword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii

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

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

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

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

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

Geohydrologic Setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Land and Water Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

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

Vertical Changes in Lithology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Vertical Hydraulic Gradients. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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

Tritium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Dissolved Oxygen and Organic Carbon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Major Ions and Trace Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Nitrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Pesticides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Vertical Gradients in Ground-Water Age. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Mass-Balance Models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Radiocarbon Ages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

References Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Appendix 1—Water-Quality Data from Monitoring Wells Screened in the Southern High Plains Aquifer 

and Dockum Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Appendix 2—Calculated Phase Mass Transfers and Isotope Balances for Selected Pairs of Initial 

and Final Waters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Figures

1–5. Maps showing:

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

the High Plains aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2. Ground-water elevations in the southern High Plains aquifer and locations of monitoring 

wells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

3. Geology underlying the southern High Plains aquifer and locations of monitoring wells. . . . .6

4. Saturated thickness of the southern High Plains aquifer and locations of monitoring wells. .7

5. Land use overlying the southern High Plains aquifer and locations of monitoring wells . . . . .8

6. Natural gamma and resistivity logs for boreholes at the Castro and Hale County sites. . . . . . . . . . . . . . .10

7–13. Graphs showing:

7. Water-level elevations and vertical hydraulic gradients in relation to time in nested 

wells at the Castro and Hale County sites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

8. Concentration of selected dissolved constituents in relation to well-screen depth 

below the water table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

9. (A) Bromide/chloride mole ratios in relation to concentration of dissolved chloride 

and (B) sulfate-sulfur isotopes in relation to concentration of dissolved sulfate. . . . . . . . . . . . 19

10. (A) Concentrations of dissolved sodium in relation to dissolved chloride, 

(B) concentrations of dissolved bicarbonate in relation to dissolved chloride, 

(C) concentrations of dissolved sodium in relation to dissolved bicarbonate, and 

(D) concentrations of dissolved calcium in relation to dissolved bicarbonate. . . . . . . . . . . . . . 20

11. Nitrogen isotope composition of nitrogen gas as a function of argon/nitrogen 

gas ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
iv 

12. Median ground-water age as a function of well-screen depth below the water 

table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

13. Stable isotope values for water from monitoring wells screened in the southern 

High Plains aquifer and Dockum Group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Tables

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

2. Lithologic description of drill cuttings and cores. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3. Mineralogy in the Ogallala Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4. Carbon content of sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

 5. Isotope data from monitoring wells screened in the southern High Plains aquifer and 

Dockum Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

 6. Mineral-saturation indices for water from monitoring wells screened in the southern 

High Plains aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

7. Dissolved-gas and related data from monitoring wells screened in the southern High Plains

 aquifer and Dockum Group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

8. Additional water-quality data used in mass-balance models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

9. Uncertainties assigned to measured isotope variables and isotope values assigned to

unmeasured variables in PHREEQC simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

10. Unadjusted radiocarbon ages of water from selected wells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

11. Adjusted radiocarbon ages of water from selected wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

12. Estimated rates of dissolved-oxygen reduction and denitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

CONVERSION FACTORS, DATUMS, ABBREVIATIONS, AND ACRONYMS

Temperature can be converted to degrees Celsius (°C) or 

degrees Fahrenheit (°F) by the equations:

°C = (°F – 32) / 1.8

°F = 1.8 (°C) + 32

Datums: Vertical coordinate information is referenced to the National Geodetic Vertical 

Datum of 1929 (NGVD 29). Horizontal coordinate information is referenced to the North 

American Datum of 1983 (NAD 83).

Multiply By To obtain

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

acre 0.4047 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

milligrams per liter (mg/L) 1.0 parts per million

micrograms per liter (μg/L) 1.0 parts per billion

milligrams per liter as calcium 24.950 micromoles per liter

milligrams per liter as sodium 43.497 micromoles per liter

milligrams per liter as bicarbonate 16.389 micromoles per liter

milligrams per liter as nitrogen 71.378 micromoles per liter

milligrams per liter as dissolved oxygen 31.250 micromoles per liter
 v

Acronyms and Abbreviations

BP before present o

C degrees Celsius

14

C carbon-14 or radiocarbon

cm

3 cubic centimeter

DIC dissolved inorganic carbon

DOC dissolved organic carbon

ft feet

ft/d feet per day

HPGW High Plains Regional Ground-Water study

IAEA International Atomic Energy Agency

in./yr inches per year

MCL maximum contaminant level

mg/L milligrams per liter

mg/L as N milligrams per liter as nitrogen

mg/L as P milligrams per liter as phosphorus

mi

2 square miles

mmol/kg millimoles per kilogram

NAWQA National Water-Quality Assessment

NBS National Bureau of Standards (now NIST)

NIST National Institute of Standards and Technology

pmc percent modern carbon

PVC polyvinyl chloride

SOC sedimentary organic carbon

SLAP Standard Light Antarctic Precipitation

TU tritium units

USGS U.S. Geological Survey μg/L micrograms per liter μM micromoles per liter

VCDT Vienna Cañon Diablo troilite

VPDB Vienna Peedee belemnite

VSMOW Vienna Standard Mean Ocean Water

XRD X-ray powder diffraction
Vertical Gradients in Water Chemistry and Age in the 

Southern High Plains Aquifer, Texas, 2002

By P.B. McMahon, J.K. Böhlke, and T.M. Lehman

ABSTRACT

The southern High Plains aquifer is the primary source of 

water used for domestic, industrial, and irrigation purposes in 

parts of New Mexico and Texas. Despite the aquifer’s importance to the overall economy of the southern High Plains, 

fundamental ground-water characteristics, such as vertical 

gradients in water chemistry and age, remain poorly defined. As 

part of the U.S. Geological Survey’s National Water-Quality 

Assessment Program, water samples from nested, short-screen 

monitoring wells installed in the southern High Plains aquifer at 

two locations (Castro and Hale Counties, Texas) were analyzed 

for field parameters, major ions, nutrients, trace elements, dissolved organic carbon, pesticides, stable and radioactive isotopes, and dissolved gases to evaluate vertical gradients in water 

chemistry and age in the aquifer. Tritium measurements indicate that recent (post-1953) recharge was present near the water 

table and that deeper water was recharged before 1953. Concentrations of dissolved oxygen were largest (2.6 to 5.6 milligrams 

per liter) at the water table and decreased with depth below 

the water table. The smallest concentrations were less than 

0.5 milligram per liter. The largest major-ion concentrations 

generally were detected at the water table because of the effects 

of overlying agricultural activities, as indicated by postbomb 

tritium concentrations and elevated nitrate and pesticide concentrations at the water table. Below the zone of agricultural 

influence, major-ion concentrations exhibited small increases 

with depth and distance along flow paths because of rock/water 

interactions and mixing with water from the underlying aquifer 

in rocks of Cretaceous age. The concentration increases 

primarily were accounted for by dissolved sodium, bicarbonate, 

chloride, and sulfate. 

Nitrite plus nitrate concentrations at the water table were 

2.0 to 6.1 milligrams per liter as nitrogen, and concentrations 

substantially decreased with depth in the aquifer to a maximum 

concentration of 0.55 milligram per liter as nitrogen. Dissolvedgas and nitrogen-isotope data from the deep wells in Castro 

County indicate that denitrification occurred in the aquifer, 

removing 74 to more than 97 percent of the nitrate originally 

present in recharge. There was no evidence of denitrification in 

the deep part of the aquifer in Hale County. After correcting for 

denitrification effects, the background concentration of nitrate 

in water recharged before 1953 ranged from 0.4 to 3.2 milligrams per liter as nitrogen, with an average of 1.6 milligrams 

per liter as nitrogen. The δ15N composition of background 

nitrate at the time of recharge was estimated to range from 9.6 

to 12.3 per mil. 

Mass-balance models indicate that the decreases in dissolved oxygen and nitrate concentrations and small increases in 

major-ion concentrations along flow paths can be accounted for 

by small amounts of silicate-mineral and calcite dissolution; 

SiO2, goethite, and clay-mineral precipitation; organic-carbon 

and pyrite oxidation; denitrification; and cation exchange. 

Mass-balance models for some wells also required mixing with 

water from the underlying aquifer in rocks of Cretaceous age to 

achieve mole and isotope balances. Carbon mass transfers identified in the models were used to adjust radiocarbon ages of 

water samples recharged before 1953. Adjusted radiocarbon 

ages ranged from less than 1,000 to 9,000 carbon-14 years 

before present. Radiocarbon ages were more sensitive to uncertainties in the carbon-14 content of recharge than uncertainties 

in carbon mass transfers, leading to 1-sigma uncertainties of 

about ±2,000 years in the adjusted ages. Despite these relatively 

large uncertainties in adjusted radiocarbon ages, it appears 

that deep water in the aquifer was considerably older (at least 

1,000 years) than water near the water table.

There was essentially no change in ground-water age with 

depth in deeper parts of the aquifer, indicating that water in that 

part of the aquifer was vertically well mixed. Both sites are 

located in areas of intensive long-term irrigation; therefore, 

local irrigation pumping is the most likely explanation for vertical mixing. The absence of ground-water age gradients in the 

deep aquifer is an indication that pumping is likely to accelerate 

the downward movement of anthropogenic compounds like 

nitrate and pesticides from the water table to deeper parts of the 

aquifer. Denitrification rates for deeper parts of the aquifer estimated on the basis of nonatmospheric dissolved nitrogen gas 

concentrations and radiocarbon ages were slow, averaging 

about 3.5×10–4 milligram per liter per year as nitrogen. 

Considering these slow denitrification rates, this process may 

not attenuate nitrate that is transported deeper into the aquifer 

by processes like pumping.
2 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

INTRODUCTION

In 1991, the USGS began full implementation of the 

National Water-Quality Assessment (NAWQA) Program. The 

long-term goals of the NAWQA Program are to describe the 

status and trends in the quality of the Nation’s surface- and 

ground-water resources and determine the natural and anthropogenic factors affecting water quality (Gilliom and others, 

1995). Understanding the quality of the Nation’s water 

resources is important because of the implications to human and 

aquatic health and because of the significant costs associated 

with decisions involving land and water management, conservation, and remediation. 

The High Plains Regional Ground-Water study, which 

began in June 1998, is one of more than 50 major river basins 

and aquifers being studied by the NAWQA Program. The High 

Plains aquifer is a nationally important water resource that 

underlies about 174,000 mi2 in parts of eight States (fig. 1). In 

1990, about 28 percent of all the ground water used for irrigation in the United States was pumped from this aquifer (Solley 

and others, 1993; McGuire and Sharpe, 1997). Due to the large 

areal extent of the High Plains aquifer, three geographic regions 

have been delineated for logistical purposes: northern High 

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

southern High Plains (29,000 mi2). The southern High Plains 

aquifer is the focus of this report.

The southern High Plains aquifer is the primary source of 

water used for domestic, industrial, and irrigation purposes in 

parts of New Mexico and Texas. Despite the aquifer’s importance to the overall economy of the southern High Plains, fundamental ground-water characteristics, such as vertical gradients in water chemistry and age, remain poorly defined. 

Understanding how water chemistry changes with depth in the 

aquifer is relevant because large water-level declines in some 

areas of the aquifer, primarily due to withdrawals for irrigation 

(McGuire and Sharpe, 1997), have increased the demand for 

water deeper in the aquifer. As of 1995, the saturated thickness 

of the southern High Plains aquifer in Texas had declined by 

27 percent relative to the predevelopment saturated thickness 

(McGuire and Sharpe, 1997). Ground-water age distributions in 

aquifers also are relevant because, when combined with other 

hydrogeologic and geochemical information, they provide 

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

reaction rates.

Several studies have examined water quality in the southern High Plains aquifer (Feder and Krothe, 1981; Nativ and 

Smith, 1987; Hopkins, 1993; Hudak, 2000; Mehta and others, 

2000; Fryar and others, 2001). Most of those studies relied on 

water samples collected from relatively long-screened domestic, irrigation, or municipal wells to characterize water quality. 

Long-screened wells generally are not suitable for describing 

vertical water-chemistry gradients because water samples are a 

mix of water from the entire screened interval. Some studies 

have measured concentrations of age-dating tracers like tritium 

and 14C in water collected from relatively long-screened wells 

and inferred approximate ground-water ages (Nativ and Smith, 

1987; Dutton, 1995). Fryar and others (2001) measured concentrations of tritium and 14C in relatively short-screened wells, but 

most of those wells were screened near the water table or in 

perched-water zones. Thus, information on vertical gradients in 

ground-water age in the southern High Plains aquifer is lacking.

Purpose and Scope

The purpose of this report is to describe vertical gradients 

in water chemistry and age in selected areas of the southern 

High Plains aquifer. In 2001, seven monitoring wells at two 

sites were completed in the aquifer and one monitoring well was 

completed in sediments of Triassic age underlying the aquifer 

(fig. 2 and table 1). The wells had single screens that were 

placed at various depths below the water table. In 2002, water 

samples were collected once from each well and analyzed for 

field parameters; major ions; nutrients; trace elements; dissolved organic carbon (DOC); pesticides; dissolved helium 

(He), methane (CH4), neon (Ne), nitrogen (N2), oxygen (O2), 

and argon (Ar) gases; tritium (3H); stable isotopes of water 

(δ2H[H2O] and δ18O[H2O]); stable nitrogen isotopes of 

dissolved nitrate (δ15N[NO3]) and nitrogen gas (δ15N[N2]); 

stable and radioactive isotopes of dissolved inorganic carbon 

(δ13C[DIC] and 14C[DIC]); stable isotopes of dissolved sulfate 

(δ34S[SO4]); and the dissolved strontium isotope ratio 

87Sr/86Sr.

Acknowledgments

The cooperation of landowners who agreed to the installation of monitoring wells on their property is gratefully acknowledged. Alan Dutton (Texas Bureau of Economic Geology) and 

Lynne Fahlquist (U.S. Geological Survey, Texas) provided colleague reviews of this report.

DESCRIPTION OF STUDY AREA

The study area includes two sites with nested wells in parts 

of Castro and Hale Counties in northwest Texas (fig. 2). The 

area is characterized by generally flat terrain interrupted by 

numerous surface depressions referred to as playa basins. A 

summary of estimates of the number of playa basins in the 

southern High Plains ranged from 20,000 to 30,000 (Haukos 

and Smith, 1994). The study area is semiarid, receiving on average about 16 to 20 in. of precipitation annually (Gutentag and 

others, 1984). Natural vegetation is sparse and consists mostly 

of grasses. More detailed descriptions of the study area are in 

Seni (1980), Gutentag and others (1984), Nativ (1988), and 

Gustavson and others (1991).
DESCRIPTION OF STUDY AREA 3

Colby

Wayne

Odessa

Guymon

Wichita

Midland

Lubbock

Amarillo

Cheyenne

Garden

City

Scottsbluff

North

Platte

108 ̊ 104 ̊ 100 ̊ 96 ̊

Colorado

New

Mexico

Texas

Oklahoma

Kansas

Nebraska

Wyoming South Dakota

Northern

Area

Central

Area

Southern

Area

42 ̊

34 ̊

Loup R

Platte R

Republican R

Elkhorn R

N. Platte R

S. Platte R

Arkansas R

Cimarron R

Canadian R

0 50 100 150 KILOMETERS

0 50 100 150 MILES

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

Albers Equal-Area projection

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

EXPLANATION

Elevation, in

feet above

NGVD29

7,913

1,089

Red R

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

aquifer.
4 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

Geohydrologic Setting

The southern High Plains aquifer consists of sediments of 

the Ogallala Formation and younger sedimentary deposits, primarily the Blackwater Draw Formation. The Ogallala Formation of Miocene-Pliocene age is the largest geologic unit in the 

southern High Plains aquifer. It contains coarse fluvial sands 

and gravels generally overlain by fine-grained eolian siltstone 

and clay (Seni, 1980; Nativ, 1988). A calcite, and sometimes 

silica, cemented zone, called the caprock caliche, lies near the 

top of the Ogallala Formation. Seni (1980) reported maximum 

total Ogallala thicknesses of 300 to 400 ft in the study area. The 

Ogallala Formation is overlain by younger sedimentary deposits, the most widespread of which is the Quaternary Blackwater 

Draw Formation. Sediments of the Blackwater Draw Formation 

were deposited in an eolian environment and range in texture 

from clay to silt to sand (Gustavson and others, 1991). The High 

Plains aquifer consists of the Ogallala Formation at the nestedwell locations.

The southern High Plains aquifer is underlain by rocks 

ranging in age from Triassic to Cretaceous (fig. 3). The nestedwell locations are underlain by the Triassic Dockum Group. 

According to McGowen and others (1977), sediments of the 

Dockum Group were deposited in a variety of terrestrial environments and consist of clay, silt, and sand. Nativ (1988) 

reported that sandstone in the Upper Dockum Group may be in 

contact with basal sand and gravel in the Ogallala Formation in 

parts of Castro County, but hydraulic-head data indicate the 

direction of ground-water movement is from the aquifer to the 

Dockum Group in Castro and Hale Counties. Sediments of Cretaceous age consisting primarily of limestone directly underlie 

the aquifer in parts of southern Hale County (fig. 3). Hydraulichead data reported by Nativ (1988) indicate a potential existed 

3,600

3,400

3,400

3,600

2,800

3,800

4,000

4,000

3,000

3,200

3,800

3,200

3,000

HALE LAMB

KING

HALL

KENT

LYNN

GAINES

FLOYD

TERRY

GARZA

DEAF SMITH

MOTLEY

BAILEY

COTTLE

CASTRO

CROSBY

PARMER

DICKENS

BRISCOE

RANDALL

SWISHER

HOCKLEY LUBBOCK

YOAKUM

COCHRAN

DAWSON BORDEN SCURRY FISHER

COLLING

WORTH

WHEE

OLDHAM POTTER

Hale

Castro

2,800

CENTRAL HIGH PLAINS AQUIFER

TEXAS

NEW MEXICO

Lubbock

Amarillo

Plainview

0 10 20 KILOMETERS

0 10 20 MILES

Extent of High

 Plains aquifer

Line of equal water-

 table elevation, in

 feet. Contour interval

 200 feet, based on

 measurements from

 2000

Nested wells and

site name

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

Albers Equal-Area projection

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

EXPLANATION

SOUTHERN HIGH

PLAINS AQUIFER

Figure 2. Ground-water elevations in the southern High Plains aquifer and locations of 

monitoring wells. Elevations are based on measurements made in 2000 (V.L. McGuire, 

U.S. Geological Survey, written commun., 2002).
DESCRIPTION OF STUDY AREA 5

for ground water to move from the aquifer in rocks of Cretaceous age to the High Plains aquifer.

In 2000, the average depth to water in the southern High 

Plains aquifer was about 125 ft below land surface and the average saturated thickness was about 59 ft (fig. 4) (V.L. McGuire, 

U.S. Geological Survey, written commun., 2002). Decreases in 

saturated thickness of more than 100 ft have occurred in some 

parts of the aquifer since irrigation pumping began in about the 

1950s (Gutentag and others, 1984; McGuire and Sharpe, 1997). 

Hydraulic gradients in the aquifer indicate ground-water movement generally is from northwest to southeast (fig. 2). Nativ 

(1988) estimated ground-water velocities in the aquifer vary 

between about 0.05 and 5 ft/d based on measured values of 

hydraulic conductivity, hydraulic gradient, and porosity in the 

aquifer. The intensity and duration of well pumpage in the aquifer are likely to affect directions and rates of ground-water 

movement.

Recharge to the southern High Plains aquifer may occur by 

infiltration of irrigation water, aerially diffuse infiltration of 

precipitation, infiltration of stormwater runoff through streambeds, and leakage through the bottom of playa basins. Results 

of unsaturated-zone studies by Scanlon and Goldsmith (1997) 

indicate playas in the southern High Plains focused recharge to 

the aquifer, whereas little to no recharge occurred in undisturbed interplaya areas. They reported recharge rates of 2.4 to 

4.7 in./yr beneath playas and rates less than 0.01 in./yr beneath 

interplaya areas. Wood and Sanford (1995) reported a similar 

recharge rate beneath one playa basin in the southern High 

Plains. They also used a mass-balance method based on dissolved chloride concentrations in ground water to estimate a 

composite recharge rate for the aquifer of 0.4 in./yr. 

Discharge from the aquifer occurs through natural springs 

and seeps (especially along the eastern escarpment), leakage to 

underlying formations, evapotranspiration, and pumpage. 

Pumpage is the primary discharge mechanism in the aquifer 

(Luckey and others, 1986). 

Land and Water Use

Land in the southern High Plains primarily was used for 

grazing (rangeland) and the production of crops (irrigated and 

dryland cropland) (fig. 5). In 1997, cotton (5,088,375 acres), 

wheat (2,505,565 acres), and corn (1,561,551 acres) were the 

primary crops grown in the study area in terms of acres planted 

(U.S. Department of Agriculture, 1999). Confined animal-feeding operations are an important industry in the study area, and 

many of the crops grown there are used for animal feed. The top 

five pesticides used in the study area (in terms of pounds 

applied) in 1997 were trifluralin, malathion, atrazine, 2,4-D, 

and pendimethalin (U.S. Department of Agriculture, 1999). Irrigation is the primary use of ground water in the study area. Of 

the approximately 4.8 million acre-ft of water withdrawn from 

the High Plains aquifer in Texas in 1990, 96 percent was used 

for irrigation (McGuire and Sharpe, 1997). In 1990, 11.4 percent of the southern High Plains area was irrigated (Qi and others, 2002). Irrigated corn was the primary crop grown in the 

vicinity of the Castro County site, and irrigated cotton was the 

primary crop grown in the vicinity of the Hale County site.

Table 1. Construction information for monitoring wells. 

[All depths are in feet below land surface. Casing and screen for all wells are made of threaded, flush-joint, Schedule 80 polyvinyl chloride (PVC) with a diameter of 2.5 inches. Screen openings are 0.020 inch wide]

Site

identification

Well

name

Construction 

date

Elevation 

of land 

surface 

(feet 

above 

NGVD29)

Depth to 

base of 

southern 

High 

Plains 

aquifer 

(feet)

Depth to 

water at 

time of 

construction 

(feet)

Total 

depth of 

hole 

(feet)

Total 

depth 

of well 

(feet)

Depth of 

screened 

interval 

(feet)

Depth of 

screen 

midpoint 

below 

water 

table at 

time of 

sampling 

(feet)

Southern High Plains Aquifer

342313102174404 Castro-241 04/28/01 3,716 430 228.14 390 246 231 to 241 5

342313102174403 Castro-330 04/24/01 3,716 430 228.14 497 335 320 to 330 94

342313102174402 Castro-380 04/27/01 3,716 430 228.14 390 385 370 to 380 144

342313102174401 Castro-427 04/23/01 3,716 430 228.14 497 432 417 to 427 191

335028101365004 Hale-255 05/14/01 3,226 410 229.22 409 260 245 to 255 12

335028101365003 Hale-340 05/03/01 3,226 410 229.22 498 345 330 to 340 96

335028101365002 Hale-400 05/13/01 3,226 410 229.22 409 405 390 to 400 156

Dockum Group

335028101365001 Hale-490 05/02/01 3,226 410 229.22 498 495 460 to 490 237
6 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

STUDY METHODS

In 2001, four nested monitoring wells were installed at 

each of the two sites for the collection of water levels and water 

samples for chemical analysis. The wells were located in some 

of the thickest saturated sections of the aquifer (fig. 4) because 

those areas may represent the best long-term ground-water 

supply in the study area. The Castro and Hale County sites are 

located along a regional hydraulic gradient from northwest to 

southeast in the aquifer (fig. 2). Water samples for chemical 

analysis were collected from the wells by the USGS in 2002. 

The following section of the report describes the methods used 

for the installation of monitoring wells, geophysical logging, 

water-level measurements, sediment-sample collection, watersample collection, describing sediment mineralogy, chemical 

analyses of sediment and water, and quality control of waterquality data.

Well installation. In 2001, the USGS used a mud-rotary 

drill rig to drill boreholes used for well installation. Two boreholes were drilled at each site, and two wells were placed in 

each borehole using established well-installation protocols 

(Lapham and others, 1995). Generally, wells were installed by 

placing polyvinyl chloride (PVC) well screen and casing in the 

borehole after drilling to the target depth, placing a sand filter in 

the annular space around the well screen and a bentonite plug 

on top of the sand filter, then sealing the annular space from the 

top of the plug to the bottom of the next screened interval using 

a high-density bentonite grout. Well screen and casing for the 

shallower well then were placed in the borehole, followed by a 

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

sand filter, and bentonite grout from the top of the plug to land 

surface. At each site, the shallowest well was installed at the 

DEAF SMITH RANDALL

Y

HALE LAMB

HALL

LYNN

GAINES

FLOYD

TERRY

GARZA

MOTLEY

BAILEY

COTTLE

CASTRO

CROSBY

PARMER

BRISCOE SWISHER

HOCKLEY LUBBOCK

YOAKUM

COCHRAN

DAWSON BORDEN

CHILDRE

COLLING

WORTH

WHEE

OLDHAM POTTER

Hale

Castro

CENTRAL HIGH PLAINS AQUIFER

TEXAS

NEW MEXICO

Lubbock

Amarillo

EXPLANATION

Geologic units underlying

the southern High Plains

aquifer

Jurassic/Triassic

 rocks

Lower Cretaceous

 rocks

Nested wells and

 site name

Wells in Cretaceous

 rock (Nativ and 

 Smith, 1987)

0 10 20 KILOMETERS

0 10 20 MILES

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

Albers Equal-Area projection

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

SOUTHERN HIGH

PLAINS AQUIFER

Figure 3. Geology underlying the southern High Plains aquifer and locations of monitoring 

wells. Map modified from Gutentag and others (1984). The well installed in bedrock for this 

study is located at the Hale County site. Wells in Cretaceous rock were not installed for this 

study and were sampled by Nativ and Smith (1987).
STUDY METHODS 7

water table. The wells were developed by bailing and pumping 

(water-table wells) or by air lifting and pumping (wells 

screened beneath the water table) immediately after installation 

and again at least 1 month before sample collection. During the 

second well-development period, pH, temperature, specific 

conductance, and turbidity of the water were monitored to 

determine when the well began producing water representative 

of the aquifer. Water was considered representative of the aquifer after a minimum of five casing volumes of water had been 

pumped and the change in field parameter values between three 

successive measurements (taken at 30-minute intervals) was 

less than 5 percent. Well-construction information is listed in 

table 1. The well name includes site and well-depth information. For example, Castro-241 refers to a well at the Castro 

County site with a maximum depth of the screened interval of 

241 ft below land surface.

Geophysical logging. Natural gamma and electrical resistivity logs were obtained for the open boreholes. Only the deepest borehole at each site was logged. The boreholes were logged 

by Century Geophysical, Inc., Tulsa, Okla.

Water-level measurements. Water levels in all wells 

were measured before water-sample collection by using an 

electric tape. In addition, water levels were measured monthly 

for 15 months. The latitude and longitude of the well locations 

were determined by using a Global Positioning System (GPS) 

with a precision of about ±20 ft. The vertical position of the 

measuring point for each well was determined relative to a local 

datum (land surface) using a transit level and rod with an accuracy of ±0.01 ft. The elevation of the land surface was determined from the USGS National Elevation Dataset (U.S. Geological Survey, 1999).

SOUTHERN HIGH

PLAINS AQUIFER

DEAF SMITH RANDALL

HALE LAMB

HALL

LYNN

GAINES

FLOYD

TERRY

GARZA

MOTLEY

BAILEY

COTTLE

CASTRO

CROSBY

PARMER

BRISCOE SWISHER

HOCKLEY LUBBOCK

YOAKUM

COCHRAN

DAWSON BORDEN SCURRY

CHILDRE

COLLINGS

WORTH

WHEEL

OLDHAM POTTER

Hale

Castro

CENTRAL HIGH PLAINS AQUIFER

TEXAS

NEW MEXICO

Lubbock

Amarillo

EXPLANATION

Saturated thickness

 of the southern High

 Plains aquifer, in feet,

 based on measurements

 from 2000

0 - 50

50 - 100

100 - 150

150 - 200

Nested wells and

site name

0 10 20 KILOMETERS

0 10 20 MILES

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

Albers Equal-Area projection

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

Figure 4. Saturated thickness of the southern High Plains aquifer and locations of monitoring 

wells. Thicknesses based on water-level measurements made in 2000 and U.S. Geological 

Survey base-of-aquifer map (V.L. McGuire, U.S. Geological Survey, written commun., 2002; 

Gutentag and others, 1984).
8 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

Sediment-sample collection. Cuttings were collected 

from the drilling-mud return flow every 10 ft by using a wire 

mesh screen. The cuttings were rinsed in clean tap water to 

remove excess drilling mud and stored in sealed plastic bags 

after the lithology was determined. Cores of sediments from the 

Dockum Group were collected from depths of 491 to 497 ft 

below land surface at the Castro County site and from depths of 

495 to 500 ft below land surface at the Hale County site.

Water-sample collection. Ground-water samples were 

collected and processed in a mobile water-quality laboratory 

using methods described by Koterba and others (1995), 

McMahon (2001), and McMahon and others (in press). Water 

was pumped from the wells by using a submersible piston-type 

pump made of stainless steel and Teflon. To minimize the risk 

of sample contamination, sample collection and preservation 

took place in dedicated environmental chambers consisting of 

clear polyethylene bags supported by tubular PVC frames. 

Sampling equipment was thoroughly cleaned after sampling 

each well. Polyethylene bags containing the sample and preservation chambers were replaced after each sampling event. Measurements of field parameters (pH, water temperature, specific 

conductance, turbidity, dissolved oxygen, and alkalinity) were 

made using the techniques described in U.S. Geological Survey 

field manual (1998). Dissolved hydrogen sulfide (H2S) also was 

measured in the field using a portable spectrophotometer and 

the methylene blue method (Hach Company, 1989).

Mineralogy of sediment samples. Mineralogy of selected 

sediment samples was estimated by examination of thin sections under a petrographic microscope and XRD analysis at 

Texas Tech University in Lubbock, Tex. Twenty-two thin

sections were examined and four mudstone samples were used 

for XRD analysis. Mineral percentages in thin sections are 

visual estimates and are not based on point counts. Percentages 

for XRD analyses are based on relative peak area/intensities and 

are semiquantitative.

Chemical analysis of sediment and water. Chemical 

analyses for major ions, nutrients, trace elements, DOC, and 

pesticides were done at the USGS National Water-Quality 

HALE LAMB

LYNN

FLOYD

ERRY

MOTLEY

CASTRO

CROSBY DICKENS

BRISCOE

SWISHER

CKLEY LUBBOCK

Hale

Castro

Lubbock

KILOMETERS

MILES

0 5 10

0 5 10

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

Albers Equal-Area projection

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

EXPLANATION

Generalized land use

overlying the southern

High Plains aquifer

Urban

Agriculture (includes

nonirrigated cropland,

fallow land, and

pasture/hay)

Irrigated cropland

Rangeland

Water

Nested wells and

site name

Figure 5. Land use overlying the southern High Plains aquifer and locations of monitoring 

wells. Land-use data from Feagas and others (1983). The data were collected from about 

1977 to 1982.
VERTICAL CHANGES IN LITHOLOGY 9

Laboratory in Lakewood, Colo. (Fishman, 1993; Zaugg and 

others, 1995). The charge balance of analyses (calculated relative to the sum of the equivalents of cations and anions) ranged 

from –3.2 to 0.8 percent, with an average absolute value of 

1.5 percent. 

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

the USGS Chlorofluorocarbon Laboratory in Reston, Va. 

(Busenberg and others, 1993;

http://water.usgs.gov/lab/dissolved-gas/, 

accessed on 12/3/2003). Analyses of dissolved He and Ne were 

done at the Lamont-Doherty Earth Observatory, Noble Gas 

Laboratory in Palisades, N.Y. (Ludin and others, 1998). 

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

δ15N[N2], and δ34S[SO4] were done at the USGS Stable Isotope 

Laboratory in Reston, Va. (Coplen and others, 1991; Epstein 

and Mayeda, 1953; Böhlke and Denver, 1995; Böhlke and others, 2002; Carmody and others, 1997). Values of δ2H[H2O] and 

δ18O[H2O] are reported in per mil relative to Vienna Standard 

Mean Ocean Water (VSMOW), normalized to VSMOW of 

Standard Light Antarctic Precipitation (SLAP) (Coplen, 1988). 

The 2-sigma uncertainties of hydrogen (H) and oxygen (O) isotopic results are 2 and 0.2 per mil, respectively. Values of 

δ15N[NO3] are reported with respect to atmospheric N2 and 

were calibrated by analyses of aqueous nitrate laboratory standards prepared as samples and normalized to values of +0.4 and 

+180.0 per mil for International Atomic Energy Agency 

(IAEA)-N1 and USGS-32, respectively (Böhlke and Coplen, 

1995). Values of δ15N[N2] are reported with respect to atmospheric N2 and were calibrated by analyses of air (0 per mil). 

The procedures were verified by analyses of air-saturated water 

samples that were collected at 16oC under controlled laboratory 

conditions, prepared as samples, and yielded δ15N[N2] values 

equal to +0.72±0.03 per mil consistent with air-water equilibrium (Klots and Benson, 1963; Hübner, 1986). Values of 

δ34S[SO4] are reported in per mil relative to Vienna Cañon 

Diablo troilite (VCDT), defined by assigning a value of –0.3 per 

mil exactly (Coplen and Krouse, 1998) to IAEA-S-1 Ag2S. 

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

Standards (NBS) 127 BaSO4 reference materials in this laboratory yield +0.5, –34.05, and +21.10 per mil, respectively. The 

2-sigma uncertainty of sulfur (S) isotopic results is 0.4 per mil. 

Analyses of tritium (3

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

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

(TU). Measurements of δ13C[DIC], reported relative to Vienna 

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

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

Measurements of 14C[DIC] were done at the IsoTrace Radiocarbon Laboratory Accelerator Mass Spectrometry facility in 

Toronto, Canada, using accelerator mass spectrometry. 

Reported 14C values (in pmc) are not adjusted for the difference 

between –25 per mil and the δ13C value of the sample. Strontium isotope analyses were done at the USGS Solid-Source 

Mass Spectrometry Laboratory in Menlo Park, Calif. Ratios of 

87Sr/86Sr were analyzed by the positive ion thermal mass spectrometry method with a precision of 0.00002. 

Quality control of water-quality data. Quality of sample 

collection, processing, and analysis was checked by collecting 

quality-control samples. The number of quality-control samples 

was approximately 30 percent of the number of environmental 

samples. Quality-control samples included field blanks and replicates. Field blanks verified that decontamination procedures 

were adequate and that field and laboratory procedures did not 

contaminate samples. Replicate samples assessed the combined 

effects of field and laboratory procedures on measurement variability.

Blank samples were collected for major ions, nutrients, 

trace elements, DOC, and pesticides. The source solution for 

blank samples was specially prepared organic-free or inorganicfree water provided by the USGS. Blank solution was passed 

through all sampling equipment, and then a sample of the blank 

solution was collected using the same procedures as for environmental samples. Replicate samples were collected sequentially for all analyses except pesticides. The quality-control data 

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

datarep/Toc.html (accessed on 06/30/04).

VERTICAL CHANGES IN LITHOLOGY

Lithologic variability in the aquifer may affect water 

chemistry and ground-water-age distributions because of its 

effect on rock/water interactions and rates of ground-water 

movement. Therefore, vertical changes in lithology in the 

southern High Plains aquifer were examined using geophysical 

(gamma and resistivity) and lithologic logs, thin sections, XRD 

analyses of sediments, and elemental carbon analyses. Gamma 

logs record the natural gamma radiation emitted by radioactive 

elements in sediment (Keys, 1990). Shale and clay tend to have 

larger proportions of these radioactive elements than sand; 

therefore, an increase in gamma activity recorded by gamma 

logs generally is attributed to increased proportions of clay in 

the aquifer or changes in clay mineralogy. Resistivity logs 

record the electrical conductivity or resistivity of the rocks and 

water surrounding the borehole. Electrical conductivity and 

resistivity are affected by the porosity, permeability, and clay 

content of the rocks and by the dissolved-solids concentration 

of the water within the rocks. In freshwater aquifers, decreasing 

resistivity readings normally indicate increasing clay content of 

the formation. The presence of mineralized water in sandy 

deposits, however, also would produce small resistivities, possibly leading to the incorrect interpretation that the formation 

consisted of fine-grained material. Mineralized water in the 

Dockum Group may have affected the resistivity log at the Hale 

County site (fig. 6).

The southern High Plains aquifer at the Castro and Hale 

County sites is underlain by sediments of the Dockum Group 

(fig. 6 and table 2). Sediments of the Dockum Group generally 

consisted of mudstone near the contact with the Ogallala Formation and graded downward into very fine- to mediumgrained sandstone. The presence of mudstone at the top of the 
10 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

Dockum Group resulted in poor hydraulic connection of waterbearing units in the Dockum Group with the High Plains aquifer 

(see the section “Vertical Hydraulic Gradients”).

The aquifer at the Castro and Hale County sites primarily 

consisted of medium- to coarse-grained sandstone of the Ogallala Formation, occasionally interbedded with thin mudstone 

layers (fig. 6 and table 2). The absence of thick, confining mudstone layers in the aquifer at these locations indicates that watertable drawdowns caused by pumping large-capacity wells 

(McGuire and Sharpe, 1997) could draw anthropogenic contaminants near the water table deeper into the aquifer. Bruce 

and Oelsner (2001) proposed this mechanism to explain the 

more frequent occurrence of recent recharge and pesticides in 

water from large-capacity public supply wells compared to 

domestic wells in the central High Plains aquifer. 

The Ogallala Formation above the water table generally 

consisted of fine- to medium-grained sandstone capped by a 

caliche layer that was about 10 ft thick (table 2). A portion of 

this sediment probably was saturated prior to the onset of widespread irrigation pumping in the area. The generally fining 

upward sedimentary sequence in the Ogallala Formation at the 

Castro and Hale County sites is indicative of their locations 

along the channel axis of a major wet alluvial-fan depositional 

system (Seni, 1980). At both locations, the Ogallala Formation 

was overlain by sediments of the Blackwater Draw Formation. 

These sediments consisted of very fine- to fine-grained sandstone with some caliche (table 2).

Quartz was the primary detrital mineral in the aquifer 

(Ogallala Formation) at the Castro and Hale County sites 

(table 3). Quartz is a relatively stable mineral phase that is not 

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

20 40 60 80 100 10 20 30 40 50 50 30 60 90 120 100 150 200 0

500

400

300

200

100

0 Castro County site Hale County site

DG

OF

OF

OF

BDF

OF

OF

OF

OF

OF

DG

DG

BDF

BDF

Water table

at time of

drilling

Water table

at time of

drilling

Castro-241

Castro-330

Castro-380

Castro-427

Hale-255

Hale-340

Hale-400

Hale-490

NATURAL GAMMA 

LOG, IN COUNTS 

PER SECOND

DEPTH BELOW LAND SURFACE, IN FEET

Screened interval and well name

Dockum Group

Ogallala Formation

Blackwater Draw Formation

EXPLANATION

RESISITIVITY LOG,

IN OHM-METERS

NATURAL GAMMA

LOG, IN COUNTS

PER SECOND

RESISITIVITY LOG,

IN OHM-METERS

Figure 6. Natural gamma and resistivity logs for boreholes at the Castro and Hale County sites.
VERTICAL CHANGES IN LITHOLOGY 11

Table 2. Lithologic description of drill cuttings and cores.

Geologic unit

Depth, 

in feet 

below 

land 

surface

Color Lithology

Castro County Site

Surface deposits 0–5 Dark yellowish 

brown

Sandy mud/muddy, very fine sand.

Blackwater Draw 

Formation

5–10 Pale brown Very fine quartzose sand, small soft caliche nodules.

10–20 Very pale brown Very fine sandy caliche.

Ogallala Formation 20–30 Moderate brown Hard sandy caliche in very fine/fine sandstone.

30–50 Light brown Fine/medium-grained sandstone (weakly cemented with calcite, sublithic, 

some sandy caliche).

50–240 Light brown Fine/medium-grained sandstone (weakly cemented with calcite, sublithic).

240–280 Pale brown Fine/medium-grained sublithic sandstone (weakly indurated) with sparse 

granule gravel (clasts are primarily chert, quartzite, sandstone).

280–310 Pale brown Medium/coarse-grained sublithic sandstone (well indurated with calcite).

310–320 Pale brown As above, but including cuttings of reddish brown mudstone - probably thin 

mudstone beds in this interval.

320–330 Pale brown Fine/medium-grained sublithic sandstone (weakly indurated) with sparse 

granule gravel (clasts are primarily chert, quartzite, sandstone).

330–410 Pale brown Medium/coarse-grained sublithic sandstone and granule gravel (clasts are 

primarily chert, quartzite, sandstone, abundant limestone, shells, some 

igneous and metamorphic rocks).

410–430 Pale brown As above, but with some coarser (small pebble) gravel.

Dockum Group 430–440 Pale yellowish 

brown

Mudstone.

440–450 Pale yellowish 

brown

Mudstone with mottles of moderate brown.

450–491 Moderate brown Unstructured mudstone.

491–494 Moderate brown Massive slickensided mudstone with micaceous laminated siltstone.

494–495 Grayish green Micaceous siltstone/very fine-grained sandstone (lithic, calcite cemented).

Hale County Site

Surface deposits 0–5 Pale yellowish 

brown

Sandy mud/muddy, very fine/fine sand with small caliche nodules and ironoxide nodules.

5–10 Very pale 

yellowish brown

Muddy, very fine/fine sand with small soft caliche nodules.

Blackwater Draw 

Formation

10–15 Light brown Very fine/fine sand (quartzose).

15–20 Light brown Very fine/fine sand with small soft caliche nodules and clay/iron oxide grain 

coatings.

20–40 Light brown Soft sandy caliche in very fine/fine sand.

Ogallala Formation 40–50 Light brown Hard laminated and massive caliche.

Hale County Site—Continued

Ogallala Formation—

Continued

50–60 Moderate reddish 

brown

Massive silty claystone with manganese-oxide coatings on joint surfaces and 

small caliche nodules.

60–100 Moderate reddish 

brown

Weakly indurated siltstone/very fine sandstone with some claystone beds.
12 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

100–140 Moderate to light 

brown

Fine/medium-grained sandstone (weakly indurated, sublithic).

140–160 Moderate to light 

brown

Medium-grained sandstone (weakly indurated, sublithic).

160–190 Light brown Medium/very coarse-grained sublithic sandstone with granule gravel (clasts 

primarily chert, quartzite).

190–210 Light brown Medium/very coarse-grained sublithic sandstone with granule/small pebble 

gravel (clasts primarily chert, quartzite with some schist, gneiss, porphyritic igneous rocks.

210–240 Pale brown Medium-grained sublithic sandstone (well indurated with calcite) and sparse 

granule/small pebble gravel (clasts primarily chert, quartzite, sandstone, 

some limestone).

240–280 Pale brown Medium-grained sublithic sandstone (well indurated with calcite) and sparse 

granule/small pebble gravel (clasts primarily chert, quartzite, sandstone, 

abundant limestone).

280–360 Pale brown Medium/coarse-grained sublithic sandstone (well indurated with calcite) and 

sparse granule/small pebble gravel (clasts primarily chert, quartzite, sandstone, abundant limestone).

360–380 Pale brown As above, but including cuttings of reddish brown sandy mudstone—

probably thin mudstone beds in this interval.

380–410 Pale brown Medium/coarse-grained sublithic sandstone (well indurated with calcite) and 

granule/small pebble gravel (clasts primarily limestone, chert, quartzite, 

sandstone), cuttings of reddish brown sandy mudstone.

Dockum Group 410–450 Moderate brown Unstructured mudstone.

450–490 Moderate brown As above, but with greenish gray reduction spots.

490–495 Greenish gray Unstructured mudstone.

495–500 Greenish gray Crudely bedded micaceous lithic medium-grained sandstone (weakly 

cemented with calcite).

Table 2. Lithologic description of drill cuttings and cores.—Continued

Geologic unit

Depth, 

in feet 

below 

land 

surface

Color Lithology
VERTICAL CHANGES IN LITHOLOGY 13

Table 3. Mineralogy in the Ogallala Formation.

Lithology Description

GRAVEL/CONGLOMERATE QUARTZITE - sutured polycrystalline quartz grains with some included muscovite and 

microcline crystals.

SANDSTONE - reworked fragments of quartzose sandstone, at least three types: (1) with 

calcite cement, (2) with iron-oxide cement, and (3) with siderite cement.

LIMESTONE - reworked fragments of limestone, all composed primarily of calcite (no 

dolomite), at least four types: (1) microcrystalline caliche, some with spar-filled 

cracks; (2) coarse skeletal limestone with sparry calcite cement and minor authigenic 

pyrite (mostly altered to iron oxides); (3) foliated calcite shell fragments (mostly 

ostreid bivalves); and (4) sandy phosphatic limestone with quartz silt/sand grains.

MICROCLINE - large monocrystalline and polycrystalline grains.

VOLCANIC/PLUTONIC ROCKS - fragments of fine-textured igneous rocks, some with 

orthoclase phenocrysts; groundmass of altered microcrystalline feldspar, quartz, and 

iron oxides.

CHERT - microcrystalline quartz with tiny amorphous iron-oxide inclusions.

SHALE - laminated cherty phosphatic shale, possibly with sponge spicules.

SAND/SANDSTONE 

(detrital mineralogy)

QUARTZ (80–90 percent) - mostly monocrystalline plutonic and metamorphic quartz 

types; many grains are reworked sedimentary grains with abraded quartz overgrowths, 

minor number of chert grains.

FELDSPAR (5–10 percent) - mostly microcline (75 percent), minor plagioclase 

(15 percent), and orthoclase (10 percent); most grains are relatively fresh in appearance, except many orthoclase grains are partly sericitized (possibly during predepositional hydrothermal alteration).

ALTERED VOLCANIC ROCK FRAGMENTS (1–5 percent) - altered grains of groundmass from fine-textured volcanic or shallow plutonic rocks, silicic to intermediate 

composition; grains consist of microcrystalline mixtures of quartz, feldspar, and iron 

oxides (little or no remaining unaltered glass).

CALCITE (1–5 percent) - grains of caliche (microcrystalline calcite), limestone, 

reworked shell fragments and foraminifera, abraded spar crystals.

ACCESSORY MINERALS (less than 1 percent) - very small amounts of iron oxides 

(altered and pseudomorphic replacement of magnetite/ilmenite and(or) pyrite), a few 

grains of muscovite, biotite, hornblende, zircon, tourmaline, apatite.

SAND/SANDSTONE

(authigenic mineralogy)

CALCITE - cementing agent is exclusively calcite, mostly microcrystalline, with some 

coarsely crystalline sparry calcite in lower part of the Ogallala Formation (Castro 

County, 290–300 feet and Hale County, 280–290 feet); in well-indurated sandstone 

the grains exhibit “super loose” packing and calcite cement comprises as much as 

30–40 percent of the rock volume (an indication of early cementation at shallow depth 

- probably as pedogenic caliche).

MUDSTONE

(paired Ogallala and Dockum 

mudrock samples)

COARSE FRACTION - 35–50 percent quartz, 2–10 percent plagioclase, 1–8 percent 

calcite, 0–1 percent potassium feldspar.

CLAY MINERALS - 30–40 percent smectite, 5–12 percent illite (+ mica), 5–10 percent 

kaolinite.
14 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

substantial source of dissolved solids in ground water. Calcite 

and feldspars were less abundant than quartz in the aquifer, but 

both are relatively reactive mineral phases. Dissolution of these 

minerals can be a source of dissolved calcium, potassium, 

sodium, bicarbonate, and silica. Detrital calcite occurred in 

various forms, including pore-filling cements in sandstone rock 

fragments, reworked limestone and shell fragments, and caliche. Calcite was the only authigenic cementing agent observed 

in thin sections. Smectite was the primary clay mineral detected 

in mudstone samples from near the Ogallala/Dockum contact 

(table 3). Illite and kaolinite also were detected in lesser 

amounts in the mudstone. Fryar and others (2001) reported 

similar relative abundances of these clay minerals in the High 

Plains aquifer near Amarillo, Tex. Smectite has a relatively 

large cation exchange capacity, and cation exchange can be an 

important process affecting ground-water chemistry. 

The Ogallala Formation at the Hale County site contained 

very little sedimentary organic carbon (table 4). The small 

amounts of organic carbon that were present mostly were in 

mudstone. The largest measured concentration of organic carbon was in a sample of the Randall Clay recovered from a playa 

floor near Lubbock, Tex. Most recharge to the aquifer is thought 

to occur through the bottom of playa basins (Wood and Sanford, 

1995; Scanlon and Goldsmith, 1997), so it is possible that 

organic-carbon-enriched playa sediments could be a source of 

organic carbon to the aquifer. 

VERTICAL HYDRAULIC GRADIENTS

Water-level data were collected from the nested wells at 

the Castro and Hale County sites on about a monthly basis from 

April 2001 through September 2002 (figs. 7A and 7B). Water 

levels at both sites declined during the 2001and 2002 irrigation 

seasons that extended from about April through September. For 

example, water levels declined by about 3 ft from April to September 2002 at the Castro County site. During the same time 

period at the Hale County site, the water-level decline was 

about 11 ft. Water levels dropped below the bottom of the 

screen in the water-table well (Hale-255) at the Hale County site 

during the 2001 and 2002 irrigation seasons; however, water 

levels rose above the bottom of the screen during the intervening winter months. 

The water-level response during the winter of 2001–02 

was very different at the two sites. At the Castro County site, 

wintertime water levels did not recover to levels measured at the 

start of the 2001 irrigation season (fig. 7A). In fact, water levels 

declined by about 0.4 ft from September to April. Wintertime 

water levels at the Hale County site recovered about 15 to 20 ft 

(fig. 7B) but did not recover to levels measured at the start of the 

2001 irrigation season. These data indicate a net water-level 

decline occurred at both sites during the period of measurement.

Vertical hydraulic gradients indicate the direction of water 

movement in the aquifer. Downward gradients indicate the 

potential for contaminants that enter the aquifer from land surface to move deeper into the aquifer. Upward gradients likewise 

indicate the potential for contaminants that enter the aquifer 

from underlying geologic units to move upward into the aquifer. 

Almost all of the measured vertical gradients between adjacent 

wells in the aquifer at the Castro County site were downward 

(fig. 7C), and the median gradient at that site was 0.0013. In 

contrast, more than one-half the measured vertical gradients 

between adjacent wells in the aquifer at the Hale County site 

were upward (fig. 7D), and the median gradient at that site was

Table 4. Carbon content of sediments.

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

organic carbon; depths are in feet below land surface; g/kg, grams of carbon per kilogram of dry sediment; <, less than]

Site Depth

(feet) Geologic unit Lithology

Total

carbon

(g/kg)

Inorganic 

carbon

(g/kg)

Organic

carbon

(g/kg)

Hale County 10–15 Blackwater Draw 

Formation

Red/brown very fine clayey 

sand.

23.21 21.30 1.91

40–50 Ogallala Formation Caliche. 72.64 71.90 .74

50–60 Ogallala Formation Upper fine-grained interval, 

red/brown mudstone.

41.67 39.70 1.97

90–100 Ogallala Fomation Fine-grained sandstone. 36.41 36.40 < .20

140–150 Ogallala Formation Medium-grained sandstone. 5.28 5.50 < .20

410–420 Ogallala/Dockum contact Dockum mudstone and 

Ogallala gravel.

48.12 46.60 1.52

470–480 Dockum Group Red mudstone. 11.70 11.80 < .20

490–480 Dockum Group Green mudstone. 11.98 11.90 < .20

Playa lake near 

Lubbock, Texas

surface Randall Clay soil Clay. 4.17 < .20 4.14
VERTICAL HYDRAULIC GRADIENTS 15

–0.0022. The more frequent occurrence of upward gradients at 

the Hale County site compared to the Castro County site may be 

due, in part, to the proximity of the Hale County site to the 

regional discharge area along the eastern aquifer boundary 

(fig. 2).

The median gradient at the Castro County site (0.0013) 

was about one-half the value of the regional horizontal gradient 

(0.0028) reported by Nativ (1988). These values indicate that 

there was a greater potential for horizontal water movement in 

the aquifer at that site than vertical movement, which is 

expected in a semiarid environment in which recharge rates are 

slow. The actual horizontal and vertical components of water 

flux depend on the horizontal and vertical hydraulic conductivities of the sediment as well as the gradients. The median gradient at the Hale County site (–0.0022) was similar to that of the 

regional horizontal gradient, but opposite in sign, indicating 

approximately equal potentials for horizontal and upward water 

movement existed at that site.

The vertical hydraulic gradient between Hale-400 and 

Hale-490 was downward with a median gradient (0.16) at least 

10 times larger than most other measured gradients, indicating 

a strong potential for water in the aquifer to move into the 

underlying Dockum Group. This finding is important because 

water in the Dockum Group at this location was relatively mineralized compared to water in the aquifer, and the strong downward gradient indicates that little potential existed for that mineralized water to enter the High Plains aquifer. If the net waterlevel decline in the aquifer continues, however, it is possible 

that the hydraulic gradient between the aquifer and Dockum 

Group would be reversed, resulting in movement of mineralized 

water from the Dockum Group to the aquifer. A gradient 

reversal is most likely to occur during the irrigation season 

when water levels in the aquifer are low (fig. 7B).

Dry Dry

C D

A B

3,491

3,490

3,489

3,488

3,487

3,486

3,485

3,484

3,483

3,482

3,481

0.010

0.008

0.006

0.002

0.004

0

–0.002

–0.004

–0.006

–0.008

0.020

0.015

0.010

0

0.005

–0.005

–0.010

–0.015

–0.020

2,980

2,975

2,970

2,965

2,960

2,955

2,950

2,945

VERTICAL HYDRAULIC GRADIENT

WATER-LEVEL ELEVATION,

IN FEET ABOVE NGVD29

2/1/2001 6/1/2001 10/1/2001 2/1/2002 6/1/2002 10/1/2002 2/1/2001 6/1/2001 10/1/2001 2/1/2002 6/1/2002 10/1/2002

DATE DATE

Castro-241

Castro-330

Castro-380

Castro-427

Castro-241/Castro-330

Castro-330/Castro-380

Castro-380/Castro-427

Hale-255

Hale-340

Hale-255/Hale-340

Hale-340/Hale-400

Hale-400

Hale-490

Upward gradient

Dockum Group

Upward gradient

Downward gradient

Downward gradient

Regional

horizontal

gradient

Nativ (1988)

Regional

horizontal

gradient

Nativ (1988)

Figure 7. Water-level elevations and vertical hydraulic gradients in relation to time in nested wells at the Castro and 

Hale County sites.
16 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

VERTICAL GRADIENTS IN WATER 

CHEMISTRY

Concentrations of dissolved constituents varied with depth 

in the southern High Plains aquifer. Some of those variations 

were the result of inputs from sources at the land surface, and 

possibly underlying aquifers, and others were the result of 

chemical reactions along flow paths in the aquifer. This section 

presents chemical data and discusses processes that might 

account for the observed vertical gradients in water chemistry.

Tritium

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

water molecule. Because tritium is radioactive, its concentration in ground water decreases over time due to radioactive 

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

weapons in 1953 (prebomb), the tritium content of precipitation 

in the study area probably was about 8 TU (Thatcher, 1962). As 

a result of radioactive decay, ground water derived from precipitation that fell before 1953 contained less than 0.5 TU in 2002. 

The tritium content of precipitation increased substantially after 

the onset of atmospheric nuclear weapons testing but has slowly 

decreased from its peak in the early 1960s. Even with the variability in tritium content of precipitation over time, ground 

water totally derived from precipitation that fell since 1953 

(postbomb) contained more than 0.5 TU in 2002. Water from 

the water-table wells (Castro-241 and Hale-255) contained 

more than 0.5 TU tritium at the time of sampling in 2002 

(table 5), indicating that at least some of the water from those 

wells was derived from post-1953 recharge. The tritium concentrations in water from all the deeper wells were less than 

0.3 TU, indicating that water from those wells was recharged 

before 1953.

Dissolved Oxygen and Organic Carbon

Some chemicals of environmental concern in the aquifer 

may be reduced to less oxidized forms by naturally occurring 

microorganisms under anoxic conditions, such as nitrate, selenate (the primary form of selenium in oxygenated ground 

water), uranium, and sulfate. Microbial reduction of these

Table 5. Isotope data from monitoring wells screened in the southern High Plains aquifer and Dockum Group.

[Multiple entries indicate replicate samples or measurements; TU, tritium units; pmc, percent modern carbon; <, less than; --, no data]

 Isotope

Well name and sampling date

Castro241Castro330Castro380Castro427 Hale-255 Hale-340 Hale-400 Hale-490

04–07–02 04–08–02 04–08–02 04–09–02 04–04–02 04–05–02 04–05–02 04–06–02

Tritium, in TU 1.3 < 0.3 < 0.3 < 0.3 5.7 < 0.3 < 0.3 < 0.3

δ18O[H2O], 

in per mil

–6.04 –7.65 –8.16 –8.22 –5.04 –6.71 –6.77 –7.96

δ2H[H2O], 

in per mil

–44.2 –52.9 –57.3 –58.0 –35.5 –48.1 –45.8 –54.6

δ13C[DIC], 

in per mil

–9.47 –6.26

–4.25

–6.41 –6.16

–6.39

–7.30 –6.49 –6.15 –12.07

–12.14

14C[DIC], 

in pmc

67.03 21.98

22.36

22.05 20.36

21.42

50.54 34.55 31.38 48.08

47.68

δ15N[NO3], 

–in per mil

12.98 -- -- 14.02

14.30

11.26 -- -- --

δ15N[N2], 

in per mil

0.68

0.80

1.51

1.54

1.53

1.61

1.75

1.65

1.75

1.72

1.75

0.55

0.50

0.96 0.93

0.89

1.02

1.58

1.05

1.40

δ34S[SO4], 

in per mil

 3.58 1.95 0.43 0.16 5.45 2.84 3.05 9.84

9.87

87Sr/86Sr 0.70900 0.70902 0.70898 0.70894

0.70893

0.70870 0.70893 0.70890 0.70902

0.70900
VERTICAL GRADIENTS IN WATER CHEMISTRY 17

chemicals is beneficial because it may convert the chemicals 

into less soluble or less toxic forms. Reducing conditions, however, also may lead to the mobilization of toxic elements like 

arsenic. In oxygenated ground water, microorganisms may oxidize some chemicals, such as atrazine, ultimately to carbon 

dioxide (CO2) (McMahon and others, 1992). The role of dissolved O2 in microbial processes makes measuring its distribution in ground water important if the fate of potentially harmful 

chemicals is to be understood.

Concentrations of O2 in the aquifer at the two sites ranged 

from less than 0.5 to 5.6 mg/L (Appendix 1). The largest concentrations occurred in water from the wells screened closest to 

the water table. At the Castro County site, concentrations of O2

in water from the three deepest wells were less than 0.5 mg/L. 

There are two possible explanations for the small O2 concentrations deeper in the aquifer at the Castro County site. The first 

possibility is that water from those wells was recharged with 

small O2 concentrations. The second possibility is that the water 

from those wells was recharged with O2 but subsequently lost 

the O2 through biogeochemical reactions along flow paths in 

the aquifer. Determining which of these possibilities is correct 

has some importance with respect to interpreting other chemical 

data from the wells. Given that water from both water-table 

wells sampled in this study and 27 of 29 water-table wells sampled in another study of the southern High Plains aquifer (Lynne 

Fahlquist, U.S. Geological Survey, written commun., 2003) had 

O2 concentrations larger than 2.5 mg/L, it is likely that water 

from deeper wells at the Castro County site contained O2 at the 

time of recharge but subsequently lost the O2 through biogeochemical reactions along flow paths in the aquifer. Concentrations of O2 in water from the Hale County wells also 

decreased with depth below the water table, although O2 concentrations in water from the deep Hale County wells were 

larger than the concentrations in water from the deep Castro 

County wells. The concentration of O2 in water from the well 

screened in the Dockum Group (Hale-490) was less than 

0.5 mg/L (Appendix 1).

Dissolved O2 is readily reduced by microorganisms if suitable electron donors are available. Typical electron donors in 

aquifers are DOC, sedimentary organic carbon (SOC), and sulfide minerals. Concentrations of DOC in the aquifer were relatively small compared with DOC concentrations in the Dockum 

Group at the Hale County site (Appendix 1). The downward 

hydraulic gradient at the Hale County site precluded advective 

transport of DOC from the Dockum Group to the aquifer. However, it is possible that diffusive transport of DOC from the 

Dockum Group to the base of the aquifer occurred. Upward diffusive transport of DOC could have enhanced O2 reduction near 

the base of the aquifer. Reduction of O2 in the aquifer farther 

above its base may have been focused near contacts with finegrained sediments that contained larger SOC concentrations 

(table 4) or at other locations in the aquifer containing electron 

donors such as sulfide minerals. Pyrite, mostly altered to iron 

oxides, was observed in gravel-sized rock fragments and in 

sand-sized grains in the aquifer (table 3).

Major Ions and Trace Elements

Water samples collected from the southern High Plains 

aquifer were of a mixed cation-bicarbonate type, indicating that 

no single major cation accounted for more than 50 percent of 

the cations and that bicarbonate accounted for more than 50 percent of the anions in the water. This water type is in agreement 

with the water types reported for this part of the aquifer by 

Nativ and Smith (1987). The sole exception to the mixed cationbicarbonate water type was the water from Hale-255, which was 

of a mixed cation-mixed anion type. The water from Hale-490, 

screened in the Dockum Group, was of a sodium-chloride type.

Dissolved-solids concentrations in undisturbed aquifers 

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

progress. In this study, the largest concentrations of dissolved 

solids measured in the aquifer occurred at the water table 

(fig. 8A), which is a condition that has been observed in aquifers 

affected by agricultural recharge (Böhlke, 2002). Major ions 

that consistently exhibited elevated concentrations at the water 

table relative to deeper water included magnesium, sodium, 

chloride, and sulfate (Appendix 1 and fig. 8). The concentration 

of bicarbonate was larger at the water table at the Castro County 

site but not at the Hale County site. The opposite was true for 

potassium. Böhlke (2002) described several processes that can 

result in elevated concentrations of inorganic constituents in 

recharge under agricultural areas. Nitrogen in the fertilizer and 

manure applied to fields commonly is in a reduced form such as 

ammonium. Microbial oxidation of ammonium to nitrate produces acidity (eq. 1) that can result in dissolution of calcite (eq. 

2) that is abundant in the unsaturated zone in the southern High 

Plains.

NH4

+ + 2O2 = NO3–

 + 2H+ + H2O (1)

CaCO3 + H+ = Ca2+ + HCO3

– (2)

Dissolved calcium produced by equation 2 can exchange 

with sodium adsorbed to clay minerals, resulting in a net loss of 

dissolved calcium and a net gain in dissolved sodium (eq. 3).

Ca2+ + 2Na-clay = Ca-clay + 2Na+ (3)

Fryar and others (2001) concluded that cation exchange 

was an important process affecting the chemical composition of 

recharge in the southern High Plains aquifer. They also noted 

that recently recharged water in the aquifer near Amarillo, Tex., 

contained elevated chloride concentrations and attributed the 

chloride to human activities. Potassium chloride applied to 

fields as fertilizer might contribute to the elevated chloride concentrations at the water table, and calcium sulfate applied to 

fields as a soil amendment might contribute to elevated sulfate 

concentrations. Other evidence for the occurrence of recent 

recharge at the water table included postbomb tritium concentrations and pesticide detections (table 5 and Appendix 1).
18 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

Prebomb tritium concentrations in water from the deeper wells 

indicate that those waters were recharged prior to the early 

1950s and large-scale development of irrigated agriculture in 

the southern High Plains. Thus, the major-ion chemistry of 

water from the deeper wells could not have evolved from the 

water chemistry measured at the water table in 2002. The maximum measured dissolved-solids concentration in water from 

the deeper High Plains aquifer wells was 398 mg/L in Hale-340, 

indicating that deep water in the aquifer was dilute relative to 

the water table. In the deeper wells, the small increases in 

dissolved-solids concentrations with depth and distance along 

the regional hydraulic gradient from the Castro to Hale County 

sites primarily can be attributed to increases in concentrations 

of dissolved sodium, bicarbonate, chloride, and sulfate (fig. 8). 

The small increase in chloride concentrations with depth at the 

Castro County site (about 2.5 mg/L) may reflect temporal or 

spatial variability in chloride concentrations in recharge to the 

three deeper wells because there are no apparent chloride 

sources in the aquifer in that area. Chloride concentrations 

exhibited a larger increase (about 4.7 to 10.1 mg/L) along the 

regional hydraulic gradient from the Castro to Hale County sites 

and could result from upward discharge of water from the 

underlying aquifer in rocks of Cretaceous age in addition to 

chemical variability in recharge. Nativ and Smith (1987) 

reported that the aquifer in rocks of Cretaceous age subcropping 

between the Castro and Hale County sites (fig. 3) discharged to 

the southern High Plains aquifer. Discharge from that aquifer 

could be lateral as well as vertical because of relief on Cretaceous erosional remnants (Seni, 1980). A cross plot of bromide/

chloride mole ratios and chloride concentrations (fig. 9A) shows 

that water from the Hale County wells generally plot along mixing lines between water from the Castro County wells and from 

wells screened in the aquifer in rocks of Cretaceous age (Nativ 

and Smith, 1987). Water samples from the Hale County wells 

could contain 10 to 20 percent water from the aquifer in rocks 

of Cretaceous age based on their positions along the mixing 

lines. Data from the water-table wells were excluded from the 

cross plots because they are not considered to be upgradient 

equivalents of water from the deeper wells. Data from Hale-490 

also were excluded from the cross plots because the Dockum le

-

ABC

DEF

0

25

50

75

100

125

150

175

200

225

0

25

50

75

100

125

150

175

200

225

0

25

50

75

100

125

150

175

200

225

0

25

50

75

100

125

150

175

200

200 250 300 350 400

300 350 400 450 500 550 600 30 35 40 40 45 50 20 60 80 100

0 20 40 60 80 100 0 20 40 60 80 100 120 140

225

0

25

50

75

100

125

150

175

200

225

0

25

50

75

100

125

150

175

200

225

DEPTH OF WELL SCREEN BELOW WATER TABLE, IN FEET

CONCENTRATION OF DISSOLVED 

BICARBONATE, 

IN MILLIGRAMS PER LITER

CONCENTRATION OF DISSOLVED 

CHLORIDE, IN

MILLIGRAMS PER LITER

CONCENTRATION OF DISSOLVED 

SULFATE, IN MILLIGRAMS, 

PER LITER

CONCENTRATION OF DISSOLVED 

SOLIDS, IN MILLIGRAMS

PER LITER

CONCENTRATION OF DISSOLVED 

CALCIUM, IN

MILLIGRAMS PER LITER

CONCENTRATION OF DISSOLVED 

SODIUM, IN MILLIGRAMS

PER LITER

Castro 

County Hale

County

Castro 

County

Castro 

County

Castro 

County Castro 

County

Castro 

County

Hale

County Hale

County

Hale

County Hale

Hale County

County

Hale-490: 7,687 mg/L Hale-490: 474 mg/L Hale-490: 1,888 mg/L

Hale-490 Hale-490: 4,155 mg/L Hale-490: 446 mg/L

Figure 8. Concentration of selected dissolved constituents in relation to well-screen depth below the water table.
VERTICAL GRADIENTS IN WATER CHEMISTRY 19

Group in which that well is screened is considered to be part of 

a separate flow system. 

The increase in sulfate concentrations with depth at the 

Castro County site was inversely related to the δ34S[SO4] values (fig. 9B). This trend indicates that the sulfate-concentration 

increase might be related to oxidation of small amounts of 

pyrite, a source of 34S-depleted sulfur, in the aquifer. Pyrite, 

mostly oxidized to iron oxides, was observed in thin sections of 

sediments from this site (table 3). Sulfate concentrations also 

increased along the regional hydraulic gradient from the Castro 

to Hale County sites, but this concentration increase was 

directly related to δ34S[SO4] values (fig. 9B). This trend might 

be explained by 34S-enriched dissolved sulfate from the underlying aquifer in rocks of Cretaceous age (fig. 9B) entering the 

southern High Plains aquifer between the Castro and Hale 

County sites. The presence of an upward hydraulic gradient 

between the southern High Plains aquifer and the aquifer in 

rocks of Cretaceous age (Nativ and Smith, 1987) indicates that 

the potential existed for water to move from that aquifer at least 

to the base of the southern High Plains aquifer. Water at the 

base of the southern High Plains aquifer might be vertically 

mixed with shallower water in the aquifer by irrigation-well

pumping in this area of intensive irrigation (fig. 5). Vertical 

mixing is discussed further in the section “Radiocarbon Ages.”

Cross plots of chloride concentrations compared to sodium 

and bicarbonate concentrations indicate that mixing with water 

from the aquifer in rocks of Cretaceous age cannot fully account 

for observed increases in concentrations of these constituents 

with depth and distance along the regional hydraulic gradient 

(figs. 10A and 10B). Data from the southern High Plains aquifer 

plot above the general trend lines for mixing, indicating that the 

sodium and bicarbonate concentration increases are in excess of 

what could be produced by mixing. Cross plots of sodium and 

bicarbonate and calcium and bicarbonate concentrations indicate that the coupled processes of calcite dissolution and cation 

exchange also might contribute to observed sodium and bicarbonate increases in the aquifer (eqs. 2 and 3; fig. 10C). The net 

result of equations 2 and 3 would be an increase in sodium and 

bicarbonate concentrations in a 2:1 ratio. A line with a slope of 

2.2 was obtained from a linear regression least-squares fit to 

the data in figure 10C. Pyrite oxidation might be one source of 

acidity to cause calcite dissolution. Carbon dioxide produced 

from the oxidation of organic matter coupled to O2 reduction 

(discussed in the section “Dissolved Oxygen and Organic 

Carbon”) and denitrification (discussed in the section “Nitrate”) 

also could dissolve calcite, as shown in equation 4.

CaCO3 + CO2 + H2O = Ca2+ + 2HCO3

–

 (4)

The net result of equations 3 and 4 would be an increase in 

sodium and bicarbonate concentrations in a 1:1 ratio. The measured slope of 2.2 in figure 10C is more indicative of equations 

2 and 3; however, the data in figure 10C also could reflect variable recharge compositions as well as mixing with water from 

the aquifer in rocks of Cretaceous age. The apparent state of 

equilibrium between ground water and calcite based on saturation indices (table 6) indicates that oxidation-reduction reactions and cation exchange were required to cause small amounts 

of calcite dissolution in the southern High Plains aquifer.

Concentrations of dissolved trace elements in the southern 

High Plains aquifer generally were small (Appendix 1) and did 

not exceed primary drinking-water standards (U.S. Environmental Protection Agency, 2003b). Manganese concentrations 

in water from three wells at the Castro County site did, however, exceed the secondary drinking-water standard of 50 μg/L 

(U.S. Environmental Protection Agency, 2003b). Two of these 

relatively large manganese concentrations occurred in anoxic 

water and probably are the products of microbially mediated 

manganese reduction. The largest measured manganese and 

iron concentrations, 471 and 1,062 μg/L, were in water from 

Hale-490, screened in the Dockum Group. These large concentrations indicate more extensive manganese and iron reduction 

occurred in the Dockum Group sediments than in the aquifer, 

probably supported by the large DOC concentrations in water 

from the Dockum Group (Appendix 1). Other evidence of

B

A

0

150 200 250 300

CONCENTRATION OF DISSOLVED SULFATE,

IN MICROMOLES PER LITER

CONCENTRATION OF DISSOLVED CHLORIDE,

IN MICROMOLES PER LITER

BROMIDE/CHLORIDE MOLE 

RATIO δ34S[SO4], IN PER MIL

350 400 450 500

0 500 1,000 1,500 2,000 2,500 3,000

1

2

3

4

5

6

Castro-330

Hale-400

Hale-400

Hale-340

Hale-340

Castro-330

Castro-380

Castro-380

Aquifer in rocks

of Cretaceous age

Percentages of water

from aquifer in rocks 

of Cretaceous age

Aquifer in rocks

of Cretaceous age

Pyrite oxidation

Castro-427

Castro-427

0.001

0.002

0.003

0.004

0.005

0.006

0.007

Mixing

25%

50% 75%

Figure 9. (A) Bromide/chloride mole ratios in relation to concentration of dissolved chloride and (B) sulfate-sulfur isotopes in 

relation to concentration of dissolved sulfate. Chemical and isotope data for water from the aquifer in rocks of Cretaceous age

from Nativ and Smith, 1987. Dashed lines indicate approximate 

trends.
20 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

extensive microbial respiration in the Dockum Group includes 

measurable sulfide (Appendix 1), large methane concentrations 

(table 7), and more negative δ13C[DIC] values in the Dockum 

Group compared to the aquifer (table 5). Microbially mediated 

manganese, iron, and sulfate reduction are coupled to the oxidation of organic carbon that typically has δ13C values ranging 

from –25 to –20 per mil. Other trace elements in water from the 

Dockum Group that were present in relatively large concentrations compared to waters from the aquifer include barium, 

boron, and strontium.

Arsenic adsorbed to surfaces of manganese- and ironoxide minerals can be released to solution as a result of manganese and iron reduction in anoxic environments. The largest 

measured arsenic concentration, 12 μg/L (which exceeds the 

10-μg/L drinking-water standard) (U.S. Environmental Protection Agency, 2003b), was in water from Hale-490 (Appendix 

1), which also contained the largest concentrations of dissolved 

manganese and iron. Concentrations of dissolved arsenic in 

anoxic waters from the aquifer at the Castro County site, however, were not elevated relative to the arsenic concentrations in 

oxic water from the aquifer (for example, Hale-340). 

Nitrate

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

0

5,000 5,200 5,400 5,600 5,800 5,000 5,200 5,400 5,600 5,800

500 1,000 1,500 2,000 2,500 3,000 0 500 1,000 1,500 2,000 2,500 3,000

800

800

900

1,000

1,100

1,200

1,200

1,600

2,000

2,400

5,000

5,200

5,400

5,600

5,800

2,800

3,200

3,000

2,500

2,000

1,500

1,000

CONCENTRATION OF DISSOLVED 

CHLORIDE, IN MICROMOLES 

PER LITER

CONCENTRATION OF DISSOLVED 

SODIUM, IN MICROMOLES

PER LITER

CONCENTRATION OF DISSOLVED 

BICARBONATE, IN MICROMOLES

PER LITER

CONCENTRATION OF DISSOLVED 

SODIUM, IN MICROMOLES

PER LITER

CONCENTRATION OF DISSOLVED 

CALCIUM, IN MICROMOLES

PER LITER

CONCENTRATION OF DISSOLVED 

CHLORIDE, IN MICROMOLES 

PER LITER

CONCENTRATION OF DISSOLVED 

BICARBONATE, IN MICROMOLES 

PER LITER

CONCENTRATION OF DISSOLVED 

BICARBONATE, IN MICROMOLES 

PER LITER

C D

A B

Hale-340

Hale-340

Hale-400 Hale-400

Castro-427

Castro-427

Castro-380 Castro-380

Castro-330

Castro-330

Mixing

Aquifer in rocks

of Cretaceous age

(Nativ and Smith, 1987)

Aquifer in rocks

of Cretaceous age

(Nativ and Smith, 1987)

Hale-340

Hale-400

Castro-427

Castro-380

Castro-330

Hale-340

Hale-400

Castro-427

Castro-380

Castro-330

2.2:1

r2 = 0.91

p = 0.01

Figure 10. (A) Concentrations of dissolved sodium in relation to dissolved chloride, (B) concentrations of dissolved bicarbonate in relation to dissolved chloride, (C) concentrations of dissolved sodium in relation to dissolved bicarbonate, and (D) concentrations of dissolved calcium in relation to dissolved bicarbonate. Chemical 

data for the aquifer in rocks of Cretaceous age from Nativ and Smith, 1987. Dashed lines indicate approximate 

trends.
VERTICAL GRADIENTS IN WATER CHEMISTRY 21

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

also is present in relatively large concentrations in animal 

manure and fertilizers applied to agricultural fields and in 

treated sewage effluent. Nitrate, the most commonly occurring 

form of nitrogen in ground water, is of environmental concern 

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

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

USEPA has established a primary drinking-water standard for 

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

Agency, 2003a).

Concentrations of dissolved nitrite plus nitrate in the aquifer ranged from less than 0.05 to 6.1 mg/L as N. Water samples 

from wells screened near the water table were the only ones 

with nitrite plus nitrate concentrations larger than 1 mg/L as N 

(Appendix 1). In all cases, nitrate concentrations were larger 

than nitrite concentrations. The relatively large nitrate concentrations, pesticide detections, and postbomb tritium concentrations in water from the wells screened near the water table compared to water from deeper parts of the aquifer indicate that 

recent recharge affected by agricultural activities was present at 

the top of the aquifer. The consistently small concentrations of 

nitrate in water farther below the water table might be attributed 

either to a lack of nitrate in recharge to those wells (tritium data 

indicate those waters were recharged prior to the start of modern 

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

aquifer.

Denitrification is a microbially mediated process that 

reduces nitrate to N2 gas (eq. 5) in suboxic environments.

4NO3

– + 5C + 3H2O = 2N2 + 5HCO3–

 + H+ (5)

In equation 5, reduction of nitrate is coupled to the oxidation of organic carbon, which produces N2 and bicarbonate. In 

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

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

which produces N2 and dissolved sulfate (eq. 6). 

6NO3

– + 2FeS2 + 2H2O = 3N2 + 2FeOOH + 

4SO4

2– + 2H+ (6)

Small amounts of organic carbon and pyrite were detected 

in the aquifer (tables 3 and 4). 

One way to evaluate whether denitrification has occurred 

in the aquifer is to calculate the amount of N2 in ground water 

from denitrification, referred to here as N2-nonatmospheric 

(eq. 7).

[N2-nonatmospheric] = [N2-total] – [N2-air saturation] – 

[N2-excess air] (7)

In equation 7, N2-total represents the measured concentration of N2 dissolved in water (table 7), N2-air saturation represents the amount of N2 dissolved in water as a result of air-water 

equilibration at recharge conditions, and N2-excess air represents the amount of N2 in water from dissolution of excess air 

(air bubbles) in water. The value of N2-air saturation is dependent on the temperature and elevation of the water table in the 

recharge area at the time of recharge. Recharge temperatures 

Table 6. Mineral-saturation indices for water from monitoring wells screened in the southern High Plains aquifer.

[A positive value indicates the water was supersaturated with respect to the indicated mineral phase and that the potential existed for that mineral to precipitate. 

A negative value indicates the water was undersaturated with respect to the indicated mineral phase and that the potential existed for that mineral to dissolve. For 

aluminosilicate minerals, an aluminum concentration of 0.0001 or 0.0005 (in parentheses) milligram per liter was assumed if the measured concentration was 

below the 0.001 milligram per liter detection level.]

Mineral

Well name and sampling date

Castro-241 Castro-330 Castro-380 Castro-427 Hale-255 Hale-340 Hale-400

04-07-02 04-08-02 04-08-02 04-09-02 04-04-02 04-05-02 04-05-02

Quartz 1.131 0.893 0.847 0.794 0.853 0.933 0.931

SiO2 

(amorphous)

–0.222 –0.425 –0.476 –0.528 –0.472 –0.384 –0.386

Kaolinite 3.950 0.586 (1.984) 0.573 (1.971) 0.132 (1.530) 1.983 0.352 (1.750) 0.385 (1.783)

Albite 0.075 –2.596 (–1.897) –2.685 (–1.987) –2.619 (–1.920) –0.277 –2.119 (–1.420) –2.107 (–1.408)

Potassium 

feldspar

1.603 –0.697 (0.002) –0.854 (–0.155) –0.976 (–0.277) 1.339 –0.538 (0.161) –0.548 (0.151)

Anorthite –3.268 –6.626 (–5.228) –6.692 (–5.294) –6.835 (–5.437) –3.168 –6.654 (–5.256) –6.689 (–5.291)

Calcium 

montmorillonite

3.432 –0.786 (0.843) –0.874 (0.754) –1.409 (0.220) 1.125 –0.970 (0.658) –0.946 (0.683)

Illite 2.915 –1.394 (0.214) –1.507 (0.101) –1.886 (–0.278) 1.595 –1.420 (0.188) –1.415 (0.193)

Calcite 0.188 –0.048 –0.018 0.095 0.719 0.045 0.007

Anhydrite –2.449 –2.782 –2.742 –2.671 –2.145 –2.644 –2.629
22 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

Table 7. Dissolved-gas and related data from monitoring wells screened in the southern High Plains aquifer and Dockum 

Group.

[Multiple entries indicate replicate samples or measurements. Reported 1-sigma uncertainties are based on the reproducibility of the measured gas concentrations 

and isotope values; μM, micromoles per liter; ±, plus or minus 1 standard deviation; cm3

 STP/L, cubic centimeters per liter at standard temperature and pressure; 

δ15N[NO3], nitrogen isotope composition of nitrate; δ15N[N2-nonatmospheric], nitrogen isotope composition of nonatmospheric nitrogen gas; --, no data; <, less 

than]

 Parameter

Well name and sampling date

Castro-241 Castro-330 Castro-380 Castro-427 Hale-255 Hale-340 Hale-400 Hale-490

04–07–02 04–08–02 04–08–02 04–09–02 04–04–02 04–05–02 04–05–02 04–06–02

Estimated

elevation of 

water table 

in recharge 

area, in feet

3,490 3,570 3,620 3,655 2,995 3,150 3,400 3,480

Dissolved 

oxygen,

in μM

139

139

5

6

5

7

1

2

11

12

15

11

177

181

111

112

121

120

< 1

< 1

< 1

< 1

Dissolved 

methane, in 

μM

9.74

10.6

0.4

0.4

0.4

0.4

8.7

8.5

0.3

0.3

0.3

0.3

< 0.1

<0 .1

<0 .1

<0 .1

<0 .1

<0 .1

649

646

653

661

Dissolved 

nitrogen 

(N2–total), 

in μM

547

547

612

616

618

613

617

606

600

590

590

592

659

661

611

614

604

609

719

716

734

710

δ15N[N2–

total], 

in per mil

0.68

0.80

1.51

1.54

1.53

1.61

1.75

1.60

1.72

1.75

1.75

0.55

0.50

0.96 0.93

0.89

1.02

1.05

1.40

1.58

Dissolved 

argon (Ar–

total), 

in μM

13.80

13.81

14.39

14.49

14.54

14.40

14.22

14.04

13.86

13.74

13.68

13.73

15.00

15.05

14.94

14.97

14.73

14.85

14.77

14.78

15.00

14.62

Dissolved 

neon, in μM 

(x10–3)

 7.79±0.05 8.22±0.03 -- 8.20±0.03

8.19±0.05

8.21±0.03

8.07±0.03

8.81±0.05 9.10±0.06 9.07±0.05 --

Recharge 

temperature, in 

degrees 

Celsius

14.4±0.1 12.7±0.2 14.0±1.0a 15.2±0.3 12.6±0.1

(16.0±1.0)b

13.1±0.1 13.3±0.3 14.0±1.0a

Excess air, in 

cm3

STP/L

0.23±0.06 0.64±0.04 0.88±0.6 0.81±0.09 1.2±0.1

(3.6±0.6)c

1.6±0.1 1.7±0.1 2.3±0.7

Dissolved 

nitrogen 

from air 

saturation 

(N2–air 

saturation), 

in μM

519±1 536±3 521±9 508±3 549±1

(512±8)c

539±1 532±3 523±9
VERTICAL GRADIENTS IN WATER CHEMISTRY 23

Dissolved 

nitrogen 

from excess 

air 

(N2–excess 

air), in μM

8.1±2.2 22±1 32±21 28±3 40±2

(125±17)c

56±3 59±2 80±24

Dissolved 

nitrogen 

from nonatmospheric 

sources 

(N2–nonatmospheric), 

in μM

20±1 57±3 60±12 56±5 71±2

(23±16)c

17±2 16±3 116±13

δ15N[N2–

nonatmospheric], 

in per mil

--d 9.6±0.4 11.1±2.0 11.6±1.1 --d --d --d 4.6±1.6

Nitrate, 

measured, 

in μM

143 < 3.6 10 39 436 26 27 < 3.6

δ15N[NO3], 

measured, 

in per mil

12.98 -- -- 14.02

14.30

11.26 -- -- --

Nitrate, initial, 

in μM

143d 114±6 130±24 151±8 436d 26d 27d 232±26

δ15N[NO3], 

initial, 

in per mil

12.98d 9.6±0.4 11.1±2.0e 12.3±0.7 11.26d -- -- 4.6±1.6

(Nitrate, 

measured)/

(nitrate, 

initial)

1.0 <0 .03 0.08±0.01 0.26±0.01 1.0 1.0 1.0 < .03

a

Recharge temperature estimated from nitrogen and argon data. b

Recharge temperature equal to mean annual air temperature plus 1 degree Celsius (see “Nitrate” section). c

Calculation assumes recharge temperature equal to mean annual air temperature plus 1 degree Celsius. dAssumes sample has no nonatmospheric nitrogen gas.

e

Minimum value.

Table 7. Dissolved-gas and related data from monitoring wells screened in the southern High Plains aquifer and Dockum 

Group.—Continued

[Multiple entries indicate replicate samples or measurements. Reported 1-sigma uncertainties are based on the reproducibility of the measured gas concentrations 

and isotope values; μM, micromoles per liter; ±, plus or minus 1 standard deviation; cm3 STP/L, cubic centimeters per liter at standard temperature and pressure; 

δ15N[NO3], nitrogen isotope composition of nitrate; δ15N[N2-nonatmospheric], nitrogen isotope composition of nonatmospheric nitrogen gas; --, no data; <, less 

than]

 Parameter

Well name and sampling date

Castro-241 Castro-330 Castro-380 Castro-427 Hale-255 Hale-340 Hale-400 Hale-490

04–07–02 04–08–02 04–08–02 04–09–02 04–04–02 04–05–02 04–05–02 04–06–02
24 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

were calculated using measured concentrations of dissolved Ne 

and Ar (Stute and Schlosser, 1993) and assuming complete dissolution of excess air during recharge. The elevation of the 

water table in the recharge area of a well depends on several factors, including depth of the well screen below the water table, 

location of the well in the regional flow system, and recharge 

rate (Cook and Böhlke, 2000). In general, the elevation of the 

water table in the recharge area of a well increases relative to the 

local water-table elevation as the depth of the well screen below 

the water table increases. Coupled ground-water flow and particle-tracking simulations were used to estimate the elevation of 

the water table in the recharge area of wells in the central High 

Plains aquifer that were constructed the same way as those used 

in this study (McMahon and others, in press). Results of those 

simulations also were used to estimate the elevation of the water 

table in the recharge area of the southern High Plains wells by 

taking the difference between recharge elevation and local 

water-table elevation from central High Plains wells and adding 

the difference to local water-table elevations of southern High 

Plains wells with similar screen depths below the water table. 

The estimated water-table elevations in the recharge area are 

listed in table 7. Uncertainty in the amount of nonatmospheric 

N2 resulting from these estimates is discussed later in this 

section.

The value of N2-excess air was calculated using 

equation 8:

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

[Ar-air saturation]} (8)

where [N2/Ar]-air is the ratio of N2 and Ar mole fractions in air 

(83.6), Ar-total represents the measured concentration of Ar 

dissolved in water (table 7), and Ar-air saturation represents the 

amount of Ar dissolved in water as a result of air-water equilibration, as determined from the Ne and Ar data. Equation 8 

assumes that the gas composition of the dissolved air bubbles 

was identical to that of air. 

Excluding Hale-255, recharge temperatures calculated 

using the Ne and Ar data ranged from 12.7±0.2 to 15.2±0.3o

C 

(table 7). A recharge temperature of 14.0±1.0o

C was estimated 

from N2 and Ar data for two wells that did not have Ne analyses. 

As will be discussed, the recharge temperature calculated for 

Hale-255 using the Ne and Ar data is believed to have been 

affected by fractionation of the excess air, so a recharge temperature of 16.0±1.0o

C was assigned to that well. This temperature 

is equal to the mean annual (1890 to 2000; CO2 Science Magazine, 2003) air temperature at Plainview, Tex., plus 1o

C. Stute 

and others (1995) proposed that noble-gas recharge temperatures are approximately equal to the mean annual air temperature at the time of recharge plus 0o

 to 2oC. The amount of nonatmospheric N2 in each sample (table 7) can be calculated once 

recharge temperature is determined. 

Concentrations of N2-nonatmospheric in the water samples ranged from 16±3 to 116±13 μM (table 7). In a plot of 

δ15N[N2] as a function of Ar/N2 mole ratios, three groups of 

samples were identified with distinct O2 concentrations and 

sample depths (fig. 11). Two groups of samples contained more 

than 60 μM O2 and less than 25 μM N2-nonatmospheric and 

one sample group contained less than 60 μM O2 and 56 to 

116 μM N2-nonatmospheric (table 7 and fig. 11). The shallow 

samples containing more than 60 μM O2 are from the watertable wells and have values near those expected for air-saturated 

water or along a mixing line between air and air-saturated 

water, indicating that they contained little or no N2-nonatmospheric. In contrast, the samples containing less than 60 μM O2

have values farthest from those expected for air-saturated water, 

indicating the presence of substantial N2-nonatmospheric in 

those samples. The deep samples containing more than 60 μM 

O2 plotted between the other two sample groups and might represent a mixture or transition between the water types represented by those groups. In general, the pattern illustrated in 

figure 11 is consistent with denitrification in deeper parts of the 

aquifer where sufficient amounts of O2 had been consumed 

prior to denitrification.

A small degree of uncertainty in the amount of N2-nonatmospheric in the samples resulted from uncertainty in the elevation of the water table in the recharge area of the wells. The 

maximum possible elevation of the water table in the recharge 

areas, represented by the water-table elevation at the upgradient 

edge of the aquifer, was about 1,000 ft higher than the water 

table at either site. If this 1,000-ft elevation difference was used 

to calculate the amount of N2-nonatmospheric in the water sample from Castro-427, which contained a relatively large amount 

of N2-nonatmospheric, instead of the 165-ft difference that was 

actually used, the amount of N2-nonatmospheric in the sample 

would decrease by about 5 percent. For the sample from Hale400, which contained a relatively small amount of N2-nonatmospheric, the amount would decrease by about 12 percent.

The small apparent concentrations of N2-nonatmospheric 

in the shallow oxic samples might indicate there were small 

biases in the gas ratios caused by either recharge processes (for 

example, fractionation of excess air) or analytical inconsistencies (Ne and Ar samples were collected and analyzed using different procedures). At Hale-255, the low noble-gas recharge 

temperature and large amount of N2-nonatmospheric calculated 

on the basis of that recharge temperature (table 7) might be 

accounted for by diffusive loss of the light noble gas Ne from 

solution or by preferential dissolution of the heavy noble gas 

Ar. Water from Hale-255 might have been susceptible to fractionation of excess air because of the large (20 ft) water-level 

fluctuations and seasonally dry conditions in that well (fig. 7B), 

which would reduce the confinement of excess air below the 

water table. More rigorous adjustments of recharge temperatures and nonatmospheric N2 might be possible with additional 

noble-gas analyses (for example, Aeschbach-Hertig and others, 

2000).

The calculations of N2-nonatmospheric indicate that small 

nitrate concentrations in suboxic waters were the result of denitrification. The fraction of nitrate remaining in the suboxic 

water samples at the time of collection was less than 3 to 26 percent of the nitrate concentration in water at the time of recharge 

(table 7). In other words, denitrification removed 74 to more
VERTICAL GRADIENTS IN WATER CHEMISTRY 25

than 97 percent of the nitrate originally present in recharge. The 

amount of nitrate in the water samples at the time of recharge 

was calculated, or reconstructed, by summing the amount of 

nitrate in the sample at the time of collection and the amount of 

nitrogen in N2-nonatmospheric. For water samples not affected 

by modern agricultural activity (the three deepest wells at each 

site), nitrate concentrations at the time of recharge ranged from 

26 to 232 μM (0.4 to 3.2 mg/L as N) (table 7), with an average 

of 113 μM (1.6 mg/L as N). This average nitrate concentration 

is considered the average background nitrate concentration at 

these sites and is about 40 percent smaller than the average 

background nitrate concentration of 193 μM (2.7 mg/L) determined from samples collected along a transect in the central 

High Plains aquifer (McMahon and others, in press). 

The reconstructed nitrogen isotope composition 

(δ15N[NO3]) of background nitrate in the aquifer ranged from 

9.6 to 12.3 per mil and was enriched in 15N by 1.4 to 6.0 per mil 

relative to background nitrate along the central High Plains 

transect (McMahon and others, in press). The smaller average 

concentration and larger δ15N values for background nitrate in 

the southern High Plains aquifer relative to the central High 

Plains aquifer are consistent with increased nitrogen loss during 

recharge in the southern High Plains either by volatilization of 

ammonia or by denitrification (Böhlke, 2002); however, more 

work is required to fully evaluate these possibilities. The isotope composition of background nitrate in the southern High 

Plains aquifer was similar to the isotope composition of nitrate 

in water from the water-table wells that may have been affected 

Hale-255

Hale-340

Hale-400

Hale-490

Air-saturated water 900–1,100 

meters above NGVD of 1929

12–15 degrees Celsius

Castro-241

Castro-330

Castro-380

Castro-427

EXPLANATION

0

0.010 0.015 0.020 0.025 0.030

0.5

1.0

1.5

2.0

Ar/N2 mole ratio

δ15N[N2] (PER MIL)

air

O2 > 60 μM - shallow

O2 > 60 μM - deep

O2 < 60 μM - deep

Figure 11. Nitrogen isotope composition of nitrogen gas as a function of argon/nitrogen 

gas ratios.
26 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

by agriculture (table 7). This similarity indicates that nitrate in 

both groups of wells was derived from sources having similar 

isotope compositions or that the nitrogen from different sources 

was subjected to fractionation processes during recharge that 

resulted in similar isotope compositions. The reconstructed isotope composition of nitrate in water from the Dockum Group 

(Hale-490) was depleted in 15N by at least 5 per mil relative to 

the aquifer samples; however, it is known that exceptionally 

large CH4 concentrations (like those measured in water from 

Hale-490) cause problems with the δ15N[N2] analysis, so the 

reconstructed initial isotope composition of nitrate in that sample is questionable.

Pesticides

The two water-table wells (Castro-241 and Hale-255) were 

the only wells to produce water that contained detectable concentrations of pesticides or pesticide degradation products 

(Appendix 1). Atrazine and cyanazine, triazine herbicides 

largely used to control annual grass and broadleaf weeds in 

corn, were detected in water from Castro-241 at concentrations 

of 1.04 μg/L and 0.0526 μg/L. Both of these concentrations are 

less than the drinking-water standards or health advisory levels 

established for these chemicals (U.S. Environmental Protection 

Agency, 2003a,b). Deethylatrazine, a degradation product of 

atrazine, was detected in water from Castro-241 and Hale-255 

at estimated concentrations less than 0.2 μg/L. 

Tritium data indicate that the lack of pesticide detections 

in water from the three deepest wells at each site probably is due 

to that water having been recharged prior to the widespread use 

of pesticides in the study area. Even though irrigation pumping 

has resulted in water-level declines and downward hydraulic 

gradients in some areas (fig. 7), those disturbances did not result 

in the detection of pesticides deeper in the aquifer.

VERTICAL GRADIENTS IN 

GROUND-WATER AGE

As discussed in the “Tritium” section, water from the 

water-table wells was recharged after the early 1950s, whereas 

water from the three deeper wells at each site was recharged 

before the early 1950s. These data indicate vertical gradients in 

ground-water age were present at the Castro and Hale County 

sites. The magnitude of the gradient, however, cannot be determined using tritium data alone. Measurements of 14C[DIC] 

have been used by several researchers to estimate ground-water 

ages greater than about 1,000 years (Plummer and others, 1990, 

2001; Aravena and others, 1995). Applying 14C[DIC] measurements to ground-water age-dating problems requires that those 

measurements be adjusted for the addition or removal of dissolved inorganic carbon along flow paths leading to the sampled wells. Construction of mass-balance models to account for 

changes in concentrations of dissolved constituents between 

initial and final waters along flow paths is one approach for 

making those adjustments (Plummer and others, 1994).

Mass-Balance Models

Mass-balance models require chemical and isotope data 

for an initial (upgradient) and a final (downgradient) water. The 

initial and final waters are from wells assumed to be located on 

the same flow path, and it is further assumed that the chemistry 

of the initial water has not changed over time. In other words, 

the chemistry of the initial water should be representative of the 

chemistry of the final water at the time it was recharged. For 

radiocarbon dating purposes, the samples should be free of postbomb 14C. For the High Plains aquifer, water containing less 

than 0.5 TU tritium is assumed to be free of postbomb 14C. 

McMahon and others (in press) constructed mass-balance models for the central High Plains aquifer by using samples from the 

most upgradient water-table wells in a regional transect as initial waters because simulations of ground-water movement 

indicated those areas were the most likely upgradient sources of 

water to downgradient wells and because they contained less 

than 0.5 TU tritium. 

The selection of appropriate initial waters for massbalance modeling of the southern High Plains transect is complicated by the fact that the chemistry of water from the watertable wells was affected by recent land-use activities, so it is not 

representative of past chemistry. Samples from water-table 

wells also contained more than 0.5 TU tritium. Examination of 

regional water-table maps confirms that the Castro and Hale 

County sites were approximately located on the same flow path, 

but detailed simulations of ground-water movement were not 

done to verify this. As a result, there is uncertainty associated 

with the chemistry of initial waters in the mass-balance models 

described in this report. However, as will be discussed in the 

“Radiocarbon Ages” section, radiocarbon ages were relatively 

insensitive to the uncertainty in carbon mass transfers identified 

by the mass-balance models.

Two different chemical analyses were used to define the 

chemistry of initial waters: an average water composition for 

High Plains recharge (table 8) and the chemical analysis from 

Castro-330 (Appendix 1). High Plains recharge represents the 

average composition of water from eight wells with screens 

placed near the water table in the southern High Plains aquifer 

near Amarillo, Tex. (Fryar and others, 2001). Water from all 

eight wells had tritium concentrations less than 0.5 TU. The 

sample from Castro-330 also was chosen as an initial water 

because that well was the shallowest upgradient well sampled 

in this study that was not affected by recent land-use activities. 

The computer code PHREEQC (Parkhurst and Appelo, 1999) 

was used to determine mass-balance models because it allows 

the user to assign uncertainties to the chemical and isotope data 

for initial and final waters. Initially, an uncertainty of 5 percent 

was assigned to the chemical analysis for each initial and final 

water composition because that was about the maximum 

charge-balance error measured in any of the chemical analyses.
VERTICAL GRADIENTS IN GROUND-WATER AGE 27

Uncertainties assigned to the isotope data are listed in table 

9. Uncertainty in the chemical data for each initial water was 

increased in increments of 2.5 percent if plausible mass-balance 

models could not be obtained with smaller uncertainties. The 

maximum uncertainty used in any model was 12.5 percent for 

the model in which Hale-400 was the final water. PHREEQC 

will determine the number of possible mass-balance models 

that satisfy mole and isotope balance requirements, given specified uncertainties. Models are plausible if they are in agreement 

with calculated mineral saturation indices (table 6), aquifer 

mineralogy, and constraints imposed by isotope data.

Eleven constraints were used in models identified by 

PHREEQC that satisfied the mole and isotope balance requirements: aluminum, calcium, carbon, chloride, iron, magnesium, 

nitrogen, potassium, silica, sodium, and sulfur; however, not all 

constraints were used in each model. The following phases were 

included in the various models: albite, anorthite, calcite, 

Ca-montmorillonite, Ca/Na2 exchange, CO2, goethite, illite, 

kaolinite, K-feldspar, Mg/Na2 exchange, N2, pyrite, SiO2, and 

SOC. Phases were selected on the basis of mineralogic analyses 

from this study (table 3) and the work of Fryar and others 

(2001). Not all phases were used in each model. The mass-balance models were further constrained by isotope data subject to 

uncertainties listed in table 9. The δ13C values assigned to CO2

and calcite are based on measured values for these phases in the 

southern High Plains (Thorstenson and others, 1983; Karlsson 

and others, 1995). The δ13C and δ34S values assigned to SOC 

(–25±5 per mil) and pyrite (–20±5 per mil) are reasonable 

approximations for organic matter and pyrite in terrestrial sediments (Deines, 1980; Krouse and Mayer, 2000).

Table 8. Additional water-quality data used in mass-balance models.

[The chemical analysis for High Plains recharge is the average of the following eight analyses reported 

by Fryar and others (2001): T. Bradshaw, D. Gabel, C. Kirkland, S. McGregor, OM-46, OM-47, OM-48, 

and OM-105. Wells had screens placed near the water table in the southern High Plains aquifer and

tritium concentrations less than 0.5 tritium unit; mg/L, milligrams per liter; N, nitrogen; δ13C[DIC],

stable carbon isotope composition of dissolved inorganic carbon; 14C[DIC], radiocarbon content of 

dissolved inorganic carbon; δ34S[SO4], stable sulfur isotope composition of dissolved sulfate; 

pmc, percent modern carbon; TU, tritium units; --, no data]

Parameter

High Plains 

recharge

(Fryar and others, 

2001)

Aquifer in rocks 

of Cretaceous 

age

(Nativ and Smith, 

1987)

pH, standard units 7.69 8.0

Calcium, dissolved (mg/L) 37.0 64.8

Magnesium, dissolved (mg/L) 27.0 48.3

Potassium, dissolved (mg/L) 6.5 13.6

Sodium, dissolved (mg/L) 21.1 69.5

Bicarbonate, dissolved (mg/L) 264. 326

Chloride, dissolved (mg/L) 8.5 88.0

Sulfate, dissolved (mg/L) 20.9 81.0

Nitrate, dissolved (mg/L as N) 1.2 3.4

Silica, dissolved (mg/L) 41.5 --

Tritium (TU) .2 5.9

δ13C[DIC] (per mil) -7.3 -7.5

14C[DIC] (pmc) 40.4 --

δ34S[SO4] (per mil) -- 5.0

Table 9. Uncertainties assigned to measured isotope 

variables and isotope values assigned to unmeasured 

variables in PHREEQC simulations.

[δ13C, stable carbon isotope composition; δ34S, stable sulfur isotope 

composition; DIC, dissolved inorganic carbon; SO4, dissolved sulfate; 

SOC, sedimentary organic carbon; CO2, carbon dioxide; --, not applicable]

Isotope variable

Uncertainty 

assigned to 

measured 

variable

(±), in per mil

Value 

assigned to 

unmeasured 

variable,

in per mil

δ13C[DIC] - initial water 2.0 --

δ34S[SO4] - initial water 2.0 --

δ13C[DIC] - final water .2 --

δ34S[SO4] - final water .2 --

δ13C[SOC] -- –25±5

δ13C[CO2] -- –20±5

δ13C[calcite] -- –7±2

δ34S[pyrite] -- –20±5
28 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

As previously discussed, some of the increases in concentrations of dissolved constituents along flow paths to the Hale 

County wells may have resulted from mixing with water entering the southern High Plains aquifer from the underlying aquifer in rocks of Cretaceous age. Nativ and Smith (1987) reported 

the chemistry of water from two wells screened in rocks of Cretaceous age in Hale County (fig. 3), and those data were used to 

assign a chemical composition to that aquifer (table 8) for the 

purpose of calculating mixing in the mass-balance models. 

Chloride was used to determine the amount of conservative 

mixing.

Several models were identified for most pairs of initial and 

final waters that satisfied mole- and isotope-balance requirements and were consistent with calculated mineral-saturation 

indices, given the stated uncertainties (Appendix 2). Most models required small amounts of silicate-mineral and calcite dissolution; SiO2, goethite, and clay-mineral precipitation; SOC and 

pyrite oxidation; denitrification; and cation exchange. Models 

in which the Hale County wells were the final waters also 

required mixing of the initial water (83 to 92 percent) with 

water from the aquifer in rocks of Cretaceous age (8 to 

17 percent).

Small increases in concentrations of dissolved sulfate with 

depth at the Castro County site were accompanied by decreases 

in the δ34S[SO4] values (fig. 9B). In the mass-balance models, 

those changes were accounted for by oxidizing small amounts 

of pyrite, with subsequent goethite precipitation. Increases in 

concentrations of dissolved sulfate along the regional hydraulic 

gradient from the Castro to Hale County wells were accompanied by increases in δ34S[SO4] values, which cannot be 

explained by pyrite oxidation. Water from the aquifer in rocks 

of Cretaceous age contained elevated sulfate concentrations and 

δ34S[SO4] values larger than those measured in water from the 

Hale County wells (fig. 9B). Thus, the increase in sulfate concentrations in going from the initial waters to the final Hale 

County waters was accounted for by mixing 8 to 17 percent 

water from the aquifer in rocks of Cretaceous age with the 

initial waters (Appendix 2).

In the mass-balance models, DIC additions were 

accounted for by oxidation of SOC and dissolution of calcite. 

Oxidation of SOC was coupled to O2 reduction and denitrification. The amount of denitrification was assumed to equal the 

amount of nonatmospheric N2-N present in the final waters, as 

previously discussed. Chemical data in Appendix 1 indicate that 

other redox processes like manganese, iron, and sulfate reduction were not important along the flow paths in the aquifer. Of 

the two electron donors, SOC and pyrite, SOC had larger mass 

transfers (was more important in terms of supporting O2 reduction and denitrification) in models of water from the Castro 

County wells (Appendix 2). SOC and pyrite had more similar 

(and relatively small) mass transfers in models of water from 

the Hale County wells in which denitrification did not occur.

Carbon dioxide gas was included as a phase in the models 

to evaluate whether SOC oxidation and calcite dissolution can 

fully account for increases in DIC concentrations along flow 

paths. Carbon dioxide mass transfers will equal zero if SOC 

oxidation and calcite dissolution account for increases in DIC 

concentrations. There are no known sources of CO2 gas (other 

than SOC oxidation) in the aquifer, and the ground-water system below the water table is considered closed to exchange with 

soil-zone CO2; therefore, nonzero CO2 mass transfers probably 

represent uncertainties in (a) selecting the appropriate initial 

water, (b) alkalinity titrations, and (c) the charge balance of 

major-ion analyses. Plummer and Sprinkle (2001) presented 

mass-balance models that required small CO2 mass transfers 

along flow paths in confined parts of the Upper Floridan aquifer, which was considered closed to soil-zone CO2. They suggested that small CO2 additions in the models could indicate 

evolution from recharge waters that had slightly larger DIC 

concentrations than were used in the models. In this study, the 

median CO2 mass transfer was 0.1905 mmol/kg, which is a relatively small amount and represents about 4 percent of the 

bicarbonate present in the initial waters. This relation means 

that radiocarbon ages will be relatively insensitive to adjustments made on the basis of CO2 mass transfers.

Radiocarbon Ages

Adjusted radiocarbon ages of ground water were calculated using the PHREEQC phase mass transfers as input to 

NETPATH (Plummer and others, 1994) and the radioactive 

decay equation:

where ∆t (in radiocarbon years) is the time since ground water 

was isolated from the atmosphere; 5,730 is the half-life of 14C 

(in years); And (in percent modern carbon, pmc) is the 14C[DIC] 

value of the final water adjusted for carbon mass transfers in the 

absence of radioactive decay; and A (pmc) is the measured 

14C[DIC] value of the final water. The value of And is dependent 

on the value of Ao (14C[DIC] value of recharge) and carbon 

mass transfers calculated in the mass-balance models. Sedimentary organic carbon and calcite in the mass-balance models were 

assigned 14C values of 0. Carbon dioxide was assigned values 

of 40 or 65 pmc (refer to discussion below).

In this study, Ao was measured directly in the water-table 

wells Castro-241 and Hale-255 (about 67 to 51 pmc, table 5) 

and also computed as the average of eight measured values 

reported by Fryar and others (2001) (about 40 pmc, table 8), as 

previously discussed. Measurements of Ao must avoid water 

containing a component of 14C derived from atmospheric 

nuclear weapons testing, and water from the eight wells sampled by Fryar did not contain a component of postbomb 14C, as 

indicated by the absence of postbomb tritium in those waters. 

Water from Castro-241 and Hale-255 did contain postbomb tritium, but their 14C[DIC] values were similar to the values in 

water from water-table wells in the central High Plains (65 to 

73 pmc; McMahon and others, in press) that did not contain 

postbomb tritium. Also, Thorstenson and others (1983) reported 

∆t 5 570 ,

ln2 -------------- And

A

-------- ⎝ ⎠ ⎛ ⎞ = ln (9)
VERTICAL GRADIENTS IN GROUND-WATER AGE 29

14C[CO2] values in the deep unsaturated zone of the Texas High 

Plains of about 55 to 60 pmc, even though diffusion models 

indicated that CO2 containing postbomb 14C should have been 

present. The similarity between 14C[DIC] values in southern 

and central High Plains recharge and deep unsaturated-zone 

CO2 may indicate that they are buffered by abundant calcite 

(presumably depleted in 14C) in the deep unsaturated zone.

Unadjusted radiocarbon ages of ground water calculated 

using the approximate range in Ao values from Fryar and others 

(2001) and samples from Castro-241 and Hale-255 are reported 

in table 10. For comparison, values of Ao equal to 100 pmc also 

were used to calculate maximum possible radiocarbon ages. 

Unadjusted ages assume that radioactive decay was the only 

process that reduced the radiocarbon content of DIC along flow 

paths. The radiocarbon ages presented here do not account for 

the possibility that 14C[DIC] of recharge varied over time due 

to variations in the Earth’s geomagnetic field strength and solar 

fluctuations (Mazaud and others, 1991; Bard and others, 1993), 

nor were the ages corrected for the effects of matrix diffusion 

(Bethke and Johnson, 2002). McMahon and others (in press) 

concluded that matrix diffusion could reduce radiocarbon ages 

of water in the central High Plains aquifer by about 20 percent. 

The maximum possible age of ground water at the Castro and 

Hale County sites ranged from 8,800 to 12,900 14C years before 

present (BP), corresponding to the condition Ao=And=100 pmc 

(table 10). Unadjusted radiocarbon ages ranged from 1,200 to 

5,400 14C years BP and from 5,200 to 9,400 14C years BP for 

Ao=And=40 and 65 pmc (table 10).

Radiocarbon ages adjusted for carbon mass transfers 

ranged from less than 1,000 to 9,000 14C years BP (table 11). 

Adjusted radiocarbon ages were much more sensitive to the Ao

value than to carbon mass transfers or the composition of the 

initial water. For example, the difference between unadjusted 

radiocarbon ages (table 10) and median adjusted radiocarbon 

ages (table 11) and ranged from 100 to 1,100 years, with a 

median difference of 600 years. The largest difference in 

median adjusted age between models with the same final water 

and Ao value also was 600 years. In contrast, the minimum difference in median adjusted age between models with the same 

final water and Ao equal to 40 or 65 pmc was 3,400 years. The 

combined 1-sigma uncertainty in radiocarbon ages resulting 

from variability in recharge chemistry and Ao was ±2,000 years. 

Although relatively large uncertainties in the radiocarbon ages 

result from the selected range in Ao values, it appears that deep 

water in the aquifer at these sites was considerably older (at 

least 1,000 years) than water near the water table. 

Ground-water ages are plotted as a function of depth below 

the water table in figure 12. The largest age gradients occurred 

between the water table and next deepest well (fig. 12), indicating a relatively rapid transition from recent recharge near the 

water table to much older water deeper in the aquifer. Age gradient is defined as (change in age)/(change in well depth). The 

age gradients in the upper part of the southern High Plains aquifer are similar to the gradients observed in the upper part of the 

central High Plains aquifer (fig. 12). Age gradients between 

deeper wells at the Castro and Hale County sites were small 

and, considering the large uncertainty in estimated radiocarbon 

ages at each depth, may be close to zero (fig. 12). A negative 

age gradient at depth was observed in one area of the central 

High Plains aquifer and is attributed to pumping of nearby irrigation wells screened in the lower one-half of the aquifer that 

caused vertical mixing of water in the aquifer. Areas of the central High Plains aquifer unaffected by local irrigation pumping 

had positive vertical age gradients throughout the saturated 

thickness (fig. 12). The Castro and Hale County sites are located 

in areas of extensive irrigation (fig. 5); therefore, mixing in the

Table 10. Unadjusted radiocarbon ages of water from selected wells.

[Unadjusted ages assume that the radiocarbon content of dissolved inorganic carbon in recharge 

was 40, 65, or 100 percent modern carbon and that radioactive decay was the only process that 

reduced the radiocarbon content of dissolved inorganic carbon along flow paths; pmc, percent 

modern carbon; 14C[DIC], radiocarbon content of dissolved inorganic carbon; Ao, 14C[DIC] of 

initial water; 14C years BP, radiocarbon years before present]

 Final water

14C[DIC],

final water, 

measured

(pmc)

Unadjusted radiocarbon age of final 

water

(

14C years BP)

Ao=40 

pmc

Ao=65 

pmc

Ao=100 

pmc

Castro-330 22.2 4,900 8,900 12,400

Castro-380 22.0 4,900 9,000 12,500

Castro-427 20.9 5,400 9,400 12,900

Hale-340 34.6 1,200 5,200 8,800

Hale-400 31.4 2,000 6,000 9,600
30 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

Table 11. Adjusted radiocarbon ages of water from selected wells.

[The radiocarbon content of dissolved inorganic carbon in initial water was adjusted for addition or removal of carbon along flow paths according to the phase 

mass transfers calculated with PHREEQC (Parkhurst and Appelo, 1999; Appendix 2). Adjusted radiocarbon ages were calculated assuming two different radiocarbon contents of dissolved inorganic carbon in initial water (Ao = 40 and Ao = 65 percent modern carbon). In PHREEQC, uncertainties were assigned to the 

chemical compositions of initial and final waters, as indicated in parentheses; pmc, percent modern carbon; 14C[DIC], radiocarbon content of dissolved inorganic 

carbon; Ao, 14C[DIC] of initial water; 14C years BP, radiocarbon years before present]

Initial water 1

(uncertainty)

Initial water 2

(uncertainty)

Final water

(uncertainty)

Adjusted radiocarbon age, 

final water

(Ao=40 pmc)

Adjusted radiocarbon age, 

final water

(Ao=65 pmc)

Age (14C years BP) Age (14C years BP)

Minimum Median Maximum Minimum Median Maximum

High Plains recharge

Fryar and others (2001)

(5 percent)

not applicable Castro-330

(5 percent)

4,100 4,100 4,100 8,100 8,100 8,100

Castro-330

(5 percent)

not applicable Castro-330

(5 percent)

4,400 4,500 4,700 8,400 8,600 8,700

High Plains recharge

Fryar and others (2001)

(5 percent)

not applicable Castro-380

(5 percent)

3,700 3,900 4,100 7,800 7,900 8,100

Castro-330

(5 percent)

not applicable Castro-380

(5 percent)

4,200 4,500 4,700 8,200 8,500 8,800

High Plains recharge

Fryar and others (2001)

(10 percent)

not applicable Castro-427

(5 percent)

4,300 4,900 4,900 8,300 8,900 8,900

Castro-330

(10 percent)

not applicable Castro-427

(5 percent)

4,300 4,600 5,000 8,300 8,600 9,000

High Plains recharge

Fryar and others (2001)

(10 percent)

Aquifer in rocks 

of Cretaceous age

Nativ and Smith 

(1987) 

(10 percent)

Hale-340

(5 percent)

< 1,000 < 1,000 1,200 4,400 4,400 5,200

Castro-330

(10 percent)

Aquifer in rocks 

of Cretaceous age

Nativ and Smith 

(1987)

(10 percent)

Hale-340

(5 percent)

< 1,000 < 1,000 1,200 4,500 4,800 5,200

High Plains recharge

Fryar and others (2001)

(12.5 percent)

Aquifer in rocks 

of Cretaceous age

Nativ and Smith 

(1987)

(12.5 percent)

Hale-400

(5 percent)

1,200 1,400 1,500 5,200 5,400 5,600

Castro-330

(12.5 percent)

Aquifer in rocks 

of Cretaceous age

Nativ and Smith 

(1987) 

(12.5 percent)

Hale-400

(5 percent)

1,200 1,900 2,000 5,200 5,900 6,000
VERTICAL GRADIENTS IN GROUND-WATER AGE 31

deep aquifer by irrigation-well pumping is the most likely 

explanation for negligible age gradients in the deep aquifer at 

these sites. Because of possible mixing effects, radiocarbon 

ages reported here should be considered apparent ages. 

Dutton (1995) concluded that water in the High Plains 

aquifer generally was less than 1,000 years old and locally 

younger than 50 years. Inspection of the data in figure 12 

supports the idea that recent recharge occurs locally in the aquifer (primarily near the water table). Data in figure 12, however, 

indicate that water in some parts of the southern High Plains 

aquifer is considerably older than 1,000 years, and water in the 

central High Plains aquifer could represent recharge from the 

early Holocene or late Pleistocene. Recharge rates in parts of 

the aquifer studied here probably were sufficiently fast to 

remove any remnants of recharge from the last glacial maximum (about 18,000 years BP), whereas recharge from the last 

glacial maximum was still present in formations directly 

beneath the southern and central High Plains aquifers (Dutton, 

1995; McMahon and others, in press).

In areas of the central High Plains aquifer that were unaffected by local pumping, systematic decreases in the δ2H and 

δ18O values and recharge temperatures of water were observed 

with depth below the water table (McMahon and others, in 

press). The decreases were attributed to colder climatic conditions during the early Holocene (represented by water at the 

aquifer base) compared to modern recharge at the water table. 

Systematic decreases in recharge temperature with depth were 

not observed in the southern High Plains aquifer (table 7), probably because of vertical mixing caused by local pumping. The 

average recharge temperature of the five deep samples 

(13.7±1.0oC) plotted in figure 12, however, was colder than the 

average recharge temperature of the two water-table samples 

(15.2±1.1oC). About 30 percent of the 1.5oC temperature difference can be accounted for by the 240-ft difference in average 

water-table elevation between the two sample groups (table 7), 

assuming a –0.18oC/100 ft air temperature-elevation gradient 

(McMahon and others, in press). The small remaining temperature difference (about 1.1oC) might be the result of climate 

change during the Holocene. Castro and Goblet (2003) reported 

450

400

350

300

250

200

150

100

50

0

0 3,000 6,000 9,000 12,000 15,000

EXPLANATION

Castro County site

Hale County site

Central High Plains aquifer

Central High Plains aquifer

TRITIUM OR RADIOCARBON AGE,

IN YEARS BEFORE PRESENT

WELL-SCREEN DEPTH BELOW WATER 

TABLE, IN FEET

Affected by

local well

pumping

Unaffected by

local well

pumping

Figure 12. Median ground-water age as a function of well-screen depth below the 

water table. Horizontal bars represent the 1-sigma uncertainty in estimated ages. 

Data for the central High Plains aquifer are from McMahon and others (in press).
32 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

a larger, 3.4oC, increase in recharge temperatures in the Carrizo 

aquifer in southwest Texas starting about 900 years ago. 

McMahon and others (in press) also reported a larger, 2.4oC, 

increase in recharge temperatures beginning after about 9,000 

14C years BP in the central High Plains aquifer. Apparently, any 

climatically driven shift in recharge temperatures in the southern High Plains aquifer that may have been recorded by noble 

gases has been at least partially erased by vertical mixing.

δ2H and δ18O values for the water samples generally plot 

along the local meteoric water line and show no evidence of 

evaporation (fig. 13). Samples from the aquifer group by age, 

with the youngest samples from the water table having the least 

negative values, the oldest samples (Castro County) having the 

most negative values, and the intermediate-aged samples (Hale 

County) having isotope values between the other two groups. 

Small isotope values for water in the southern High Plains aquifer have been attributed to colder recharge temperatures in the 

past (Dutton, 1995); therefore, the δ2

H and δ18O data also 

appear to preserve a climate record that indicates warming 

occurred during the Holocene. Other environmental factors, 

–25

–30

–35

–40

–45

–50

–55

–60

–65

–9.0 –8.5 –8.0 –7.5 –7.0 –6.5 –6.0 –5.5 –5.0 –4.5 –4.0

Castro-330

Castro-380

Castro-427

Castro-241

Hale-400

Hale-340

Hale-255

Hale-490

Global meteoric water line (Craig, 1961)

Local meteoric water line (precipitation samples from Nativ, 1988)

Less than 25 feet below water table

More than 85 feet below water table

Southern High Plains aquifer

Southern High Plains aquifer

Dockum Group

EXPLANATION

δ18O[H2O], IN PER MIL

δ2H[H2O], IN PER MIL

Figure 13. Stable isotope values for water from monitoring wells screened in the southern High Plains 

aquifer and Dockum Group. Local Meteoric Water Line based on the isotopic composition of precipitation 

samples collected in the southern High Plains (Nativ, 1988).
VERTICAL GRADIENTS IN GROUND-WATER AGE 33

however, such as elevation differences in the recharge areas and 

temporal variability in air-mass sources for precipitation also 

could affect the isotope data. Water samples from parts of the 

aquifer unaffected by local well pumping are needed to construct a more reliable record of paleorecharge conditions in the 

southern High Plains aquifer.

Apparent rates of O2 reduction and denitrification were 

estimated on the basis of decreases in O2 concentrations and 

increases in concentrations of nonatmospheric N2 and groundwater ages along flow paths from the water table to the deep 

wells. All five of the radiocarbon-dated samples had some 

O2 uptake assuming that they were recharged with at least 

4.1 mg/L (128 μM) O2, which was the average concentration in 

water from Castro-241 and Hale-255. Only the radiocarbondated Castro County samples contained nonatmospheric N2. 

For O2, calculations assume reduction began at the water table 

and was constant along flow paths to the wells. For nitrate, calculations assume denitrification began at a distance downgradient from the water table that would result from a traveltime 

equal to half the adjusted radiocarbon age of water from Castro330 (2,050 to 4,350 14C years, table 11). Results of the rate 

calculations are listed in table 12. Median O2 reduction rates 

ranged from 6×10–5 to 6×10–4 mg/L/yr and were fastest at the 

Castro County sites where measured O2 concentrations were 

smallest. Median denitrification rates ranged from 3×10–4 to 

4×10–4 mg/L/yr as N and also were fastest at the Castro County 

sites. The denitrification rates estimated here are about 4 to 5 

times faster than estimated rates in the central High Plains aquifer (McMahon and others, in press) but are several orders of 

magnitude slower than rates in several shallow aquifers containing recent recharge (Korom, 1992; Böhlke and others, 

2002).

Large vertical age gradients in areas of the High Plains 

aquifer unaffected by local pumping indicate that several thousand years might be required for agricultural nitrate at the water 

table to move deeper in the aquifer, which might be sufficient 

time for denitrification (even at very slow rates) to reduce 

nitrate to background concentrations. The small age gradients at 

depth at the Castro and Hale County sites, however, indicate 

that those long traveltimes can be greatly reduced by local 

pumping of large-capacity wells. Bruce and Oelsner (2001) proposed that pumping rate and duration were the primary reasons 

why water from municipal water-supply wells in the central 

High Plains aquifer contained younger water and more pesticide detections than water from domestic wells. Considering the 

slow denitrification rates estimated for the aquifer, this process 

may not attenuate nitrate movement that is accelerated by processes like pumping.

Table 12. Estimated rates of dissolved-oxygen reduction and denitrification.

[The change in dissolved-oxygen concentration was calculated as the difference between the dissolved-oxygen concentration in recharge (assumed to be 

4.1 milligrams per liter) and the concentration measured in water from the well. A dissolved-oxygen concentration of 0.25 milligram per liter was assumed for 

samples that had less than 0.5 milligram per liter of dissolved oxygen. A radiocarbon age of 500 years was assumed for samples with ages less than 

1,000 years; ∆O2, change in dissolved-oxygen concentration; mg/L, milligrams per liter; N, nitrogen; mg/L/yr, milligrams per liter per year; BP, before present; 

--, not applicable]

Well name

Castro-330 Castro-380 Castro-427 Hale-340 Hale-400

∆O2, in mg/L 3.85 3.85 3.85 0.4 0.2

Nonatmospheric N2, in 

mg/L as N

1.60 1.68 1.57 0 0

Minimum radiocarbon 

age, in 14C years BP

4,100 3,700 4,300 < 1,000 1,200

Median radiocarbon 

age, in 14C years BP

6,300 6,200 6,700 2,500 3,600

Maximum radiocarbon 

age, in 14C years BP

8,700 8,800 9,000 5,200 6,000

Median and range in 

O2 reduction rates, 

in mg/L/yr

6×10–4

4×10–4 to 9×10–4

6×10–4

4×10–4 to 1×10–3

6×10–4

4×10–4 to 9×10–4

2×10–4

8×10–5 to 8×10–4

6×10–5

3×10–5 to 2×10–4

Median and range in 

denitrification rates, 

in mg/L/yr 

as N

4×10–4

2×10–4 to 8×10–4

4×10–4

2×10–4 to 1×10–3

3×10–4

2×10–4 to 7×10–4

0

0

0

0
34 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

SUMMARY AND CONCLUSIONS

In 1991, the USGS began full implementation of the 

National Water-Quality Assessment (NAWQA) Program. The 

long-term goals of the NAWQA Program are to describe the 

status and trends in the quality of the Nation’s surface- and 

ground-water resources and determine the natural and anthropogenic factors affecting water quality. The High Plains 

Regional Ground-Water study, which began in June 1998, is 

one of more than 50 major river basins and aquifers being studied by the NAWQA Program. The High Plains aquifer is a 

nationally important water resource that underlies about 

174,000 mi2

 in parts of eight States. Due to the large areal extent 

of the High Plains aquifer, three geographic regions have been 

delineated for logistical purposes: northern High Plains, central 

High Plains, and southern High Plains. The southern High 

Plains aquifer is the focus of this investigation.

Chemical data and tritium and radiocarbon ages show that 

water in the southern High Plains aquifer was chemically and 

temporally stratified in the study area, with a layer of recently 

(less than 50 years) recharged water near the water table containing major ions, pesticides, and nitrate above background 

concentrations overlying a thicker layer of much older (by at 

least 1,000 years) and more dilute water. Elevated chemical 

concentrations near the water table were attributed to agricultural activity on the overlying land surface. Below the zone of 

agricultural influence, changes in chemical concentrations with 

depth and along flow paths were attributed to small amounts of 

silicate-mineral and calcite dissolution; SiO2, goethite, and 

clay-mineral precipitation; organic-carbon and pyrite oxidation; 

denitrification; cation exchange; and mixing with water from 

the underlying aquifer in rocks of Cretaceous age. Dissolvedgas and nitrogen-isotope data from the deep wells at the Castro 

County site indicate that denitrification removed 74 to more 

than 97 percent of the nitrate originally present in recharge. 

Carbon mass transfers along flow paths in the aquifer were 

estimated using the geochemical model PHREEQC. Radiocarbon ages in the deep aquifer, adjusted for carbon mass transfers, ranged from less than 1,000 to 9,000 14C years before 

present. Radiocarbon ages were much more sensitive to uncertainties in the 14C content of recharge than uncertainties in carbon mass transfers, leading to 1-sigma uncertainties in adjusted 

ages of about ±2,000 years. The largest age gradients occurred 

between the water table and next deepest wells. Within the deep 

aquifer, vertical age gradients were relatively small. The small 

age gradients at depth indicate water in the aquifer was vertically mixed, most likely by local irrigation-well pumping. The 

small age gradients at depth also indicate that pumping has the 

potential to accelerate the movement of contaminants like 

nitrate and pesticides at the water table deeper in the aquifer. 

Estimated denitrification rates in the aquifer were slow, averaging about 3.5×10–4 mg/L/yr as N. Considering these slow denitrification rates, this process may not attenuate nitrate that is 

transported deeper into the aquifer by processes like pumping.

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38 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002
Appendixes
40 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002
APPENDIXES 41

Appendix 1

Water-Quality Data from Monitoring Wells Screened 

in the Southern High Plains Aquifer and Dockum 

Group
 42
APPENDIXES 43

Appendix 1. Water-quality data from monitoring wells screened in the southern High Plains aquifer and Dockum Group.

[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; 

P, phosphorus; NTU, nephelometric turbidity units; μg/L, micrograms per liter; --, no data; E, estimated]

 Parameter

Well name and sampling date

Castro-241 Castro-330 Castro-380 Castro-427 Hale-255 Hale-340 Hale-400 Hale-490

04–07–02 04–08–02 04–08–02 04–09–02 04–04–02 04–05–02 04–05–02 04–06–02

Turbidity 

(NTU)

98. 7. 0.6 0.7 0.2 0.4 8. 27.

Oxygen, dissolved 

(mg/L)

2.6 < 0.5 < 0.5 < 0.5 5.6 3.7 3.9 < 0.5

pH, field 

(standard units)

7.69 7.41 7.41 7.60 8.60 7.57 7.54 7.05

Specific 

conductance 

(μS/cm)

713. 518. 537. 564. 891. 616. 631. 12,130.

Water 

temperature (°C)

12.6 17.7 17.0 16.9 16.5 17.8 17.8 17.8

Sulfide, field, total 

(mg/L)

< 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 0.2

Calcium, dissolved 

(mg/L)

40.1 42.5 44.6 37.8 31.2 33.2 32.6 474.

Magnesium, 

dissolved (mg/L)

32.4 27.0 28.8 22.6 32.4 27.0 26.8 306.

Potassium, 

dissolved (mg/L)

6.9 7.8 6.9 7.2 14. 8.5 8.6 25.

Sodium, dissolved 

(mg/L)

64.4 26.5 28.4 45.4 94.5 59.8 64.0 1,888.

Bicarbonate, 

dissolved (mg/L)

379. 306. 316. 321. 210. 349. 349. 223.

Bromide, dissolved 

(mg/L)

.4208 0.1007 0.1118 0.0897 0.6804 0.1281 0.1343 1.849

Chloride, dissolved 

(mg/L)

17.3 7.26 8.84 9.79 95.8 14.5 17.4 4,155.

Fluoride, dissolved 

(mg/L)

3.7 1.9 1.5 1.4 2.1 3.0 3.4 0.40

Silica, dissolved 

(mg/L as SiO2)

54.9 38.1 33.4 29.6 34.8 42.0 41.8 16.4

Sulfate, dissolved 

(mg/L)

44.5 17.7 18.9 25.5 127. 32.3 34.1 446.

Solids, residue at 

180o

C, dissolved 

(mg/L)

444. 312. 331. 349. 550. 398. 396. 7,687.

Nitrogen, 

ammonia, 

dissolved 

(mg/L as N)

0.08 .41 < 0.04 < 0.04 0.49 < 0.04 < 0.04 0.64

Nitrogen, ammonia 

+ organic, 

dissolved 

(mg/L as N)

0.2 .4 < 0.1 < 0.1 0.7 < 0.1 < 0.1 0.4

Nitrogen, nitrite + 

nitrate, dissolved 

(mg/L as N)

2.0 < 0.05 0.14 0.55 6.1 0.36 0.38 < 0.05
44 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

Nitrogen, nitrite, 

dissolved (mg/L 

as N)

0.040 < 0.008 < 0.008 < 0.008 0.43 < 0.008 < 0.008 < 0.008

Phosphorus, 

dissolved (mg/L 

as P)

0.029 0.009 0.006 0.007 E0.002 0.006 0.006 0.008

Phosphorus, ortho, 

dissolved (mg/L 

as P)

0.026 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02

Carbon, organic, 

dissolved (mg/L)

0.93 1.2 1.3 1.3 -- 0.87 1.9 77.

Aluminum, 

dissolved (μg/L)

3. < 1. < 1. < 1. 7. < 1. < 1. < 6.

Antimony, 

dissolved (μg/L)

0.57 < 0.05 < 0.05 0.06 0.41 0.13 0.12 0.38

Arsenic, dissolved 

(μg/L)

7.4 5.0 3.7 2.5 6.6 6.6 6.6 12.

Barium, dissolved 

(μg/L)

81. 118. 177. 95. 97. 78. 77. 1,037.

Beryllium, 

dissolved (μg/L)

< 0.06 < 0.06 < 0.06 < 0.06 < 0.06 < 0.06 < 0.06 < 0.4

Boron, dissolved 

(μg/L)

136. 122. 151. 168. 389. 352. 371. 1,327.

Cadmium, 

dissolved (μg/L)

< 0.04 < 0.04 < 0.04 < 0.04 0.11 < 0.04 < 0.04 E0.21

Chromium, 

dissolved (μg/L)

< 0.8 < 0.8 < 0.8 < 0.8 32. 1.0 1.1 < 1.6

Cobalt, dissolved 

(μg/L)

0.30 0.24 0.24 0.12 0.65 0.08 0.07 0.78

Copper, dissolved 

(μg/L)

1.0 .23 0.26 0.25 2.5 0.50 0.52 3.2

Iron, dissolved 

(μg/L)

< 10. 79. 135. < 10. < 10. 23. < 10. 1,062.

Lead, dissolved 

(μg/L)

0.11 < 0.08 < 0.08 < 0.08 < 0.08 < 0.08 < 0.08 < 0.5

Manganese, 

dissolved (μg/L)

106. 81. 223. 17. 5.6 0.73 0.24 471.

Molybdenum, 

dissolved (μg/L)

7.5 3.7 2.7 4.5 60. 9.1 8.2 1.7

Nickel, dissolved 

(μg/L)

21. 0.50 0.33 5.2 2.6 0.30 0.39 < 0.4

Selenium, dissolved 

(μg/L)

4.0 0.55 0.60 1.3 10. 4.9 6.1 3.9

Silver, dissolved 

(μg/L)

< 1. < 1. < 1. < 1. < 1. < 1. < 1. < 6.

Strontium, 

dissolved (μg/L)

962. 900. 878. 781. 1,060. 936. 922. 14,308.

Thallium, dissolved 

(μg/L)

< 0.04 < 0.04 < 0.04 < 0.04 < 0.04 < 0.04 < 0.04 < 0.2

Appendix 1. Water-quality data from monitoring wells screened in the southern High Plains aquifer and Dockum Group.—Continued

[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; 

P, phosphorus; NTU, nephelometric turbidity units; μg/L, micrograms per liter; --, no data; E, estimated]

 Parameter

Well name and sampling date

Castro-241 Castro-330 Castro-380 Castro-427 Hale-255 Hale-340 Hale-400 Hale-490

04–07–02 04–08–02 04–08–02 04–09–02 04–04–02 04–05–02 04–05–02 04–06–02
APPENDIXES 45

Uranium, dissolved 

(μg/L)

11. 6.3 5.8 5.4 12. 10. 10. 1.6

Vanadium, 

dissolved (μg/L)

19. 11. 4.4 17. 24. 28. 29. < 1.3

Zinc, dissolved 

(μg/L)

5. < 1. < 1. 2. < 1. < 1. < 1. < 6.

2,6-Diethyl aniline, 

dissolved (μg/L)

< 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006

Acetochlor, 

dissolved (μg/L)

< 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006

Alachlor, dissolved 

(μg/L)

< 0.0045 < 0.0045 < 0.0045 < 0.0045 < 0.0045 < 0.0045 < 0.0045 < 0.0045

alpha BHC, 

dissolved (μg/L)

< 0.0046 < 0.0046 < 0.0046 < 0.0046 < 0.0046 < 0.0046 < 0.0046 < 0.0046

Atrazine, dissolved 

(μg/L)

1.04 < 0.007 < 0.007 < 0.007 < 0.007 < 0.007 < 0.007 < 0.007

Azinphos-methyl, 

dissolved (μg/L)

< 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05

Benfluralin, 

dissolved (μg/L)

< 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01

Butylate, dissolved 

(μg/L)

< 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002

Carbaryl, dissolved 

(μg/L)

< 0.041 < 0.041 < 0.041 < 0.041 < 0.041 < 0.041 < 0.041 < 0.041

Carbofuran, 

dissolved (μg/L)

< 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02

Chlorpyrifos, 

dissolved (μg/L)

< 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005

Cyanazine, 

dissolved (μg/L)

0.0526 < 0.018 < 0.018 < 0.018 < 0.018 < 0.018 < 0.018 < 0.018

DCPA, dissolved 

(μg/L)

< 0.003 < 0.003 < 0.003 < 0.003 < 0.003 < 0.003 < 0.003 < 0.003

Deethylatrazine, 

dissolved (μg/L)

E0.176 < 0.006 < 0.006 < 0.006 E0.031 < 0.006 < 0.006 < 0.006

Diazinon, dissolved 

(μg/L)

< 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005

Dieldrin, dissolved 

(μg/L)

< 0.0048 < 0.0048 < 0.0048 < 0.0048 < 0.0048 < 0.0048 < 0.0048 < 0.0048

Disulfoton, 

dissolved (μg/L)

< 0.021 < 0.021 < 0.021 < 0.021 < 0.021 < 0.021 < 0.021 < 0.021

EPTC, dissolved 

(μg/L)

< 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002

Ethalfluralin, 

dissolved (μg/L)

< 0.009 < 0.009 < 0.009 < 0.009 < 0.009 < 0.009 < 0.009 < 0.009

Ethoprophos, 

dissolved (μg/L)

< 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005

Fonofox, dissolved 

(μg/L)

< 0.0027 < 0.0027 < 0.0027 < 0.0027 < 0.0027 < 0.0027 < 0.0027 < 0.0027

Lindane, dissolved 

(μg/L)

< 0.004 < 0.004 < 0.004 < 0.004 < 0.004 < 0.004 < 0.004 < 0.004

Appendix 1. Water-quality data from monitoring wells screened in the southern High Plains aquifer and Dockum Group.—Continued

[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; 

P, phosphorus; NTU, nephelometric turbidity units; μg/L, micrograms per liter; --, no data; E, estimated]

 Parameter

Well name and sampling date

Castro-241 Castro-330 Castro-380 Castro-427 Hale-255 Hale-340 Hale-400 Hale-490

04–07–02 04–08–02 04–08–02 04–09–02 04–04–02 04–05–02 04–05–02 04–06–02
46 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002

Linuron, dissolved 

(μg/L)

< 0.035 < 0.035 < 0.035 < 0.035 < 0.035 < 0.035 < 0.035 < 0.035

Malathion, 

dissolved (μg/L)

< 0.027 < 0.027 < 0.027 < 0.027 < 0.027 < 0.027 < 0.027 < 0.027

Metolachlor, 

dissolved (μg/L)

< 0.013 < 0.013 < 0.013 < 0.013 < 0.013 < 0.013 < 0.013 < 0.013

Metribuzin, 

dissolved (μg/L)

< 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006

Molinate, dissolved 

(μg/L)

< 0.0016 < 0.0016 < 0.0016 < 0.0016 < 0.0016 < 0.0016 < 0.0016 < 0.0016

Napropamide, 

dissolved (μg/L)

< 0.007 < 0.007 < 0.007 < 0.007 < 0.007 < 0.007 < 0.007 < 0.007

p,p'-DDE, 

dissolved (μg/L)

< 0.0025 < 0.0025 < 0.0025 < 0.0025 < 0.0025 < 0.0025 < 0.0025 < 0.0025

Parathion, 

dissolved (μg/L)

< 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01

Parathion-methyl, 

dissolved (μg/L)

< 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006

Pebulate, dissolved 

(μg/L)

< 0.0041 < 0.0041 < 0.0041 < 0.0041 < 0.0041 < 0.0041 < 0.0041 < 0.0041

Pendimethalin, 

dissolved (μg/L)

< 0.022 < 0.022 < 0.022 < 0.022 < 0.022 < 0.022 < 0.022 < 0.022

Permethrin, 

dissolved (μg/L)

< 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006 < 0.006

Phorate, dissolved 

(μg/L)

< 0.011 < 0.011 < 0.011 < 0.011 < 0.011 < 0.011 < 0.011 < 0.011

Prometon, 

dissolved (μg/L)

< 0.015 < 0.015 < 0.015 < 0.015 < 0.015 < 0.015 < 0.015 < 0.015

Pronamide, 

dissolved (μg/L)

< 0.0041 < 0.0041 < 0.0041 < 0.0041 < 0.0041 < 0.0041 < 0.0041 < 0.0041

Propachlor, 

dissolved (μg/L)

< 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01

Propanil, dissolved 

(μg/L)

< 0.011 < 0.011 < 0.011 < 0.011 < 0.011 < 0.011 < 0.011 < 0.011

Propargite, 

dissolved (μg/L)

< 0.023 < 0.023 < 0.023 < 0.023 < 0.023 < 0.023 < 0.023 < 0.023

Simazine, dissolved 

(μg/L)

< 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005 < 0.005

Tebuthiuron, 

dissolved (μg/L)

< 0.016 < 0.016 < 0.016 < 0.016 < 0.016 < 0.016 < 0.016 < 0.016

Terbacil, dissolved 

(μg/L)

< 0.034 < 0.034 < 0.034 < 0.034 < 0.034 < 0.034 < 0.034 < 0.034

Terbufos, dissolved 

(μg/L)

< 0.017 < 0.017 < 0.017 < 0.017 < 0.017 < 0.017 < 0.017 < 0.017

Thiobencarb, 

dissolved (μg/L)

< 0.0048 < 0.0048 < 0.0048 < 0.0048 < 0.0048 < 0.0048 < 0.0048 < 0.0048

Tri-allate, dissolved 

(μg/L)

< 0.0023 < 0.0023 < 0.0023 < 0.0023 < 0.0023 < 0.0023 < 0.0023 < 0.0023

Trifluralin, 

dissolved (μg/L)

< 0.009 < 0.009 < 0.009 < 0.009 < 0.009 < 0.009 < 0.009 < 0.009

Appendix 1. Water-quality data from monitoring wells screened in the southern High Plains aquifer and Dockum Group.—Continued

[mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25°C; °C, degrees Celsius; CaCO3, calcium carbonate; <, less than; N, nitrogen; 

P, phosphorus; NTU, nephelometric turbidity units; μg/L, micrograms per liter; --, no data; E, estimated]

 Parameter

Well name and sampling date

Castro-241 Castro-330 Castro-380 Castro-427 Hale-255 Hale-340 Hale-400 Hale-490

04–07–02 04–08–02 04–08–02 04–09–02 04–04–02 04–05–02 04–05–02 04–06–02
APPENDIXES 47

Appendix 2

Calculated Phase Mass Transfers and 

Isotope Balances for Selected Pairs of 

Initial and Final Waters
48 Vertical Gradients in Water Chemistry and Age in the Southern High Plains Aquifer, Texas, 2002
APPENDIXES 49

Appendix 2. Calculated phase mass transfers and isotope balances for selected pairs of initial and final waters.

[All phase mass-transfer calculations were performed with PHREEQC (Parkhurst and Appelo, 1999). The minimum and maximum 

phase mass transfers are based on all plausible models; however, not all phases were required in each model. Positive numbers for 

mineral mass transfer indicate dissolution; negative numbers indicate precipitation. For exchange reactions, negative numbers indicate a 

decrease in calcium and (or) magnesium and an increase in sodium in solution; mmol/kg water, millimoles per kilogram of water; δ13C, 

stable carbon isotope composition; δ34S, stable sulfur isotope composition; DIC, dissolved inorganic carbon, SO4, dissolved sulfate; 

SOC, sedimentary organic carbon, CO2, carbon dioxide; --, phase not included in any models]

Initial water 1

(uncertainty)

High Plains recharge

Fryar and others (2001)

(5 percent)

Castro-330

(5 percent)

Initial water 2

(uncertainty) not applicable not applicable

Final water

(uncertainty)

Castro-330

(5 percent)

Castro-330

(5 percent)

Phase mass transfer

(mmol/kg water) Minimum Maximum Minimum Maximum

SiO2 –0.1010 –0.0883 –0.5582 0.0921

Kaolinite –.0110 –.0110 –.4853 –.0032

Albite -- -- .0089 .1602

K-feldspar .0137 .0220 .0076 .0090

Anorthite .0088 .0088 .0103 .0801

Ca-montmorillonite –.0135 –.0095 –.0038 –.4166

Illite -- -- -- --

SOC .3426 .3426 .3420 .3535

CO2 .0833 .0836 -- --

Calcite .1966 .2042 .0183 .0608

Goethite -- -- –.0002 –.0002

Pyrite -- -- .0002 .0002

Ca/Na2 exchange –.0885 –.0885 –.0278 –.0522

Mg/Na2 exchange -- -- .0089 .0278

Fraction of intial well 1 in mixture 1.0 1.0 1.0 1.0

Fraction of initial well 2 in mixture 0.0 0.0 0.0 0.0

δ13C[SOC] (per mil) –20.0 –20.0 –20.0 –20.0

δ13C[CO2] (per mil) –15.0 –15.0 -- --

δ13C[calcite] (per mil) –6.0 –5.0 –6.0 –6.0

δ34S[pyrite] (per mil) -- -- –25.0 –25.0

δ13C[DIC final well, computed] (per mil) –6.5 –6.5 –6.3 –6.3

δ13C[DIC final well, measured] (per mil) –6.3 –6.3 –6.3 –6.3

δ34S[SO4 final well, computed] (per mil) 1.9 1.9 1.9 1.9

δ34S[SO4 final well, measured] (per mil) 1.9 1.9 1.9 1.9
APPENDIXES 50

Appendix 2. Calculated phase mass transfers and isotope balances for selected pairs of initial and final waters.

—Continued

[All phase mass-transfer calculations were performed with PHREEQC (Parkhurst and Appelo, 1999). The minimum and maximum 

phase mass transfers are based on all plausible models; however, not all phases were required in each model. Positive numbers for 

mineral mass transfer indicate dissolution; negative numbers indicate precipitation. For exchange reactions, negative numbers indicate a 

decrease in calcium and (or) magnesium and an increase in sodium in solution; mmol/kg water, millimoles per kilogram of water; δ13C, 

stable carbon isotope composition; δ34S, stable sulfur isotope composition; DIC, dissolved inorganic carbon, SO4, dissolved sulfate; 

SOC, sedimentary organic carbon, CO2, carbon dioxide; --, phase not included in any models]

Initial water 1

(uncertainty)

High Plains recharge

Fryar and others (2001)

(5 percent)

Castro-330

(5 percent)

Initial water 2

(uncertainty) not applicable not applicable

Final water

(uncertainty)

Castro-380

(5 percent)

Castro-380

(5 percent)

Phase mass transfer

(mmol/kg water) Minimum Maximum Minimum Maximum

SiO2 –0.1477 –0.1348 –0.9395 0.0642

Kaolinite –.0003 –.0003 –.7490 –.0075

Albite -- -- .0060 .2472

K-feldspar .0006 .0090 -- --

Anorthite -- -- .0086 .1236

Ca-montmorillonite –.0006 –.0003 –.1061 .6429

Illite -- -- -- --

SOC .3003 .3132 .3374 .3548

CO2 .2027 .2227 -- --

Calcite .3304 .3381 .0359 .1698

Goethite -- -- –.0063 –.0001

Pyrite -- -- .0001 .0063

Ca/Na2 exchange –.1986 –.1507 –.1171 –.0030

Mg/Na2 exchange .0708 .0708 –.0390 .0757

Fraction of intial well 1 in mixture 1.0 1.0 1.0 1.0

Fraction of initial well 2 in mixture 0.0 0.0 0.0 0.0

δ13C[SOC] (per mil) –20.0 –20.0 –20.0 –20.0

δ13C[CO2] (per mil) –15.0 –15.0 -- --

δ13C[calcite] (per mil) –6.0 –6.0 –6.0 –6.0

δ34S[pyrite] (per mil) -- -- –21.8 –21.8

δ13C[DIC final well, computed] (per mil) –6.6 –6.6 –6.2 –6.2

δ13C[DIC final well, measured] (per mil) –6.4 –6.4 –6.4 –6.4

δ34S[SO4 final well, computed] (per mil) -- -- 0.4 0.5

δ34S[SO4 final well, measured] (per mil) .4 .4 .4 .4
APPENDIXES 51

Appendix 2. Calculated phase mass transfers and isotope balances for selected pairs of initial and final waters.

—Continued

[All phase mass-transfer calculations were performed with PHREEQC (Parkhurst and Appelo, 1999). The minimum and maximum

phase mass transfers are based on all plausible models; however, not all phases were required in each model. Positive numbers for 

mineral mass transfer indicate dissolution; negative numbers indicate precipitation. For exchange reactions, negative numbers indicate a 

decrease in calcium and (or) magnesium and an increase in sodium in solution; mmol/kg water, millimoles per kilogram of water; δ13C, 

stable carbon isotope composition; δ34S, stable sulfur isotope composition; DIC, dissolved inorganic carbon, SO4, dissolved sulfate; 

SOC, sedimentary organic carbon, CO2, carbon dioxide; --, phase not included in any models]

Initial water 1

(uncertainty)

High Plains recharge

Fryar and others (2001)

(10 percent)

Castro-330

(10 percent)

Initial water 2

(uncertainty) not applicable not applicable

Final water

(uncertainty)

Castro-427

(5 percent)

Castro-427

(5 percent)

Phase mass transfer

(mmol/kg water) Minimum Maximum Minimum Maximum

SiO2 –3.841 –0.1981 –1.673 –0.1412

Kaolinite –2.032 –.1254 –1.332 –.1886

Albite .2507 .9583 .3772 .6324

K-feldspar .1425 1.375 -- --

Anorthite .1254 .3353 .1338 .1886

Ca-montmorillonite –.1254 2.693 –.2611 1.143

Illite –2.263 –.2089 -- --

SOC .2315 .3164 .2580 .2580

CO2 .1548 .1877 .0226 .1933

Calcite .2334 .4170 .0643 .2600

Goethite –.0254 –.0130 –.0248 –.0248

Pyrite .0130 .0254 .0248 .0248

Ca/Na2 exchange –.5286 –.1202 –.3700 .0362

Mg/Na2 exchange –.3381 .2382 –.2226 –.0242

Fraction of intial well 1 in mixture 1.0 1.0 1.0 1.0

Fraction of initial well 2 in mixture 0.0 0.0 0.0 0.0

δ13C[SOC] (per mil) –20.0 –20.0 –20.0 –20.0

δ13C[CO2] (per mil) –15.0 –15.0 –15.0 –15.0

δ13C[calcite] (per mil) –6.0 –6.0 –6.0 –6.0

δ34S[pyrite] (per mil) –16.4 –15.0 –15.0 –15.0

δ13C[DIC final well, computed] (per mil) –6.5 –6.3 –6.3 –6.3

δ13C[DIC final well, measured] (per mil) –6.3 –6.3 –6.3 –6.3

δ34S[SO4 final well, computed] (per mil) –.04 .4 .08 .08

δ34S[SO4 final well, measured] (per mil) .2 .2 .2 .2
APPENDIXES 52

Appendix 2. Calculated phase mass transfers and isotope balances for selected pairs of initial and final waters.

—Continued

[All phase mass-transfer calculations were performed with PHREEQC (Parkhurst and Appelo, 1999). The minimum and maximum 

phase mass transfers are based on all plausible models; however, not all phases were required in each model. Positive numbers for 

mineral mass transfer indicate dissolution; negative numbers indicate precipitation. For exchange reactions, negative numbers indicate a 

decrease in calcium and (or) magnesium and an increase in sodium in solution; mmol/kg water, millimoles per kilogram of water; δ13C, 

stable carbon isotope composition; δ34S, stable sulfur isotope composition; DIC, dissolved inorganic carbon, SO4, dissolved sulfate;

SOC, sedimentary organic carbon, CO2, carbon dioxide; --, phase not included in any models]

Initial water 1

(uncertainty)

High Plains recharge

Fryar and others (2001)

(10 percent)

Castro-330

(10 percent)

Initial water 2

(uncertainty)

Aquifer in rocks of 

Cretaceous age

Nativ and Smith (1987)

(10 percent)

Aquifer in rocks of 

Cretaceous age

Nativ and Smith (1987)

(10 percent)

Final water

(uncertainty)

Hale-340

(5 percent)

Hale-340

(5 percent)

Phase mass transfer

(mmol/kg water) Minimum Maximum Minimum Maximum

SiO2 –2.025 0.0177 –5.000 0.2333

Kaolinite –.4036 –.0154 –.9052 –.0006

Albite .0124 .9361 .0296 1.226

K-feldspar .0109 .4350 .0017 2.035

Anorthite .0344 .4399 .0910 .2317

Ca-montmorillonite –.4111 .1504 –.4909 1.947

Illite –.6714 .0586 –3.390 –.0007

SOC .0421 .0570 .0337 .0337

CO2 .6231 .7293 .0092 .8719

Calcite .5244 .6178 .2076 .3893

Goethite –.0145 –.0109 –.0191 –.0166

Pyrite .0109 .0145 .0166 .0191

Ca/Na2 exchange –.7658 –.1902 –.7772 –.0737

Mg/Na2 exchange –.2196 –.0516 –.1820 .6702

Fraction of intial well 1 in mixture .90 .92 .88 .88

Fraction of initial well 2 in mixture .08 .10 .12 .12

δ13C[SOC] (per mil) –20.0 –20.0 –20.0 –20.0

δ13C[CO2] (per mil) –15.0 –15.0 –15.0 –15.0

δ13C[calcite] (per mil) –6.0 –5.0 –7.7 –5

δ34S[pyrite] (per mil) –15.0 –15.0 –15.0 –15.0

δ13C[DIC final well, computed] (per mil) –6.7 –6.5 –6.7 –6.5

δ13C[DIC final well, measured] (per mil) –6.5 –6.5 –6.5 –6.5

δ34S[SO4 final well, computed] (per mil) 2.8 2.8 2.6 2.8

δ34S[SO4 final well, measured] (per mil) 2.8 2.8 2.8 2.8
APPENDIXES 53

Appendix 2. Calculated phase mass transfers and isotope balances for selected pairs of initial and final waters.

—Continued

[All phase mass-transfer calculations were performed with PHREEQC (Parkhurst and Appelo, 1999). The minimum and maximum 

phase mass transfers are based on all plausible models; however, not all phases were required in each model. Positive numbers for

mineral mass transfer indicate dissolution; negative numbers indicate precipitation. For exchange reactions, negative numbers indicate a 

decrease in calcium and (or) magnesium and an increase in sodium in solution; mmol/kg water, millimoles per kilogram of water; δ13C, 

stable carbon isotope composition; δ34S, stable sulfur isotope composition; DIC, dissolved inorganic carbon, SO4, dissolved sulfate; 

SOC, sedimentary organic carbon, CO2, carbon dioxide; --, phase not included in any models]

Initial water 1

(uncertainty)

High Plains recharge

Fryar and others (2001)

(12.5 percent)

Castro-330

(12.5 percent)

Initial water 2

(uncertainty)

Aquifer in rocks of 

Cretaceous age

Nativ and Smith (1987)

(12.5 percent)

Aquifer in rocks of 

Cretaceous age

Nativ and Smith (1987)

(12.5 percent)

Final water

(uncertainty)

Hale-400

(5 percent)

Hale-400

(5 percent)

Phase mass transfer

(mmol/kg water) Minimum Maximum Minimum Maximum

SiO2 –1.323 0.0143 –2.801 0.3641

Kaolinite –.0550 –.0021 –2.147 –.0021

Albite .0043 .0101 .0042 1.399

K-feldspar .0087 .8194 .0042 .4921

Anorthite -- -- .0003 .3641

Ca-montmorillonite –.0486 .9707 –.5625 2.134

Illite –1.340 .0492 –.8221 –.0018

SOC .0330 .1196 .0482 .0587

CO2 .4543 .6571 .0039 1.067

Calcite .4082 .5729 .1488 .4004

Goethite –.0182 –.0100 –.0180 –.0093

Pyrite .0100 .0182 .0093 .0180

Ca/Na2 exchange –.8031 –.6092 –.6988 –.0707

Mg/Na2 exchange –.1861 –.0848 –.2322 .2495

Fraction of intial well 1 in mixture .84 .87 .83 .85

Fraction of initial well 2 in mixture .13 .16 .15 .17

δ13C[SOC] (per mil) –20 –20 –20 –20

δ13C[CO2] (per mil) –15 –15 –15 –15

δ13C[calcite] (per mil) –5 –5 –5 –5

δ34S[pyrite] (per mil) –15 –15 –15 –15

δ13C[DIC final well, computed] (per mil) –6.4 –6.4 –6.2 –6.2

δ13C[DIC final well, measured] (per mil) –6.2 –6.2 –6.2 –6.2

δ34S[SO4 final well, computed] (per mil) 2.8 3.0 2.8 3.0

δ34S[SO4 final well, measured] (per mil) 3.0 3.0 3.0 3.0