U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3355

Prepared in cooperation with Llano Estacado Underground Water Conservation District, Sandy Land Underground 

Water Conservation District, and South Plains Underground Water Conservation District

Changes Between Early Development (1930–60)

and Recent (2005–15) Groundwater-Level Altitudes and

Dissolved-Solids and Nitrate Concentrations In and Near 

Gaines, Terry, and Yoakum Counties, Texas
 
Changes Between Early Development 

(1930–60) and Recent (2005–15) 

Groundwater-Level Altitudes and

Dissolved-Solids and Nitrate Concentrations 

In and Near Gaines, Terry, and Yoakum 

Counties, Texas

By Jonathan V. Thomas, Andrew P. Teeple, Jason D. Payne, and Scott Ikard

Prepared in cooperation with Llano Estacado Underground Water Conservation 

District, Sandy Land Underground Water Conservation District, and

South Plains Underground Water Conservation District

Pamphlet to accompany

Scientific Investigations Map 3355

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Thomas, J.V., Teeple, A.P., Payne, J.D., and Ikard, Scott, 2016, Changes between early development (1930–60) and 

recent (2005–15) groundwater-level altitudes and dissolved-solids and nitrate concentrations in and near Gaines, 

Terry, and Yoakum Counties, Texas: U.S. Geological Survey Scientific Investigations Map 3355, 2 sheets, pamphlet, 

http://dx.doi.org/10.3133/sim3355.

ISSN 2329-1311 (print)

ISSN 2329-132X (online)

ISBN 978-1-4113-4041-1
iii

Contents

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

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

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

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

Hydrogeologic Setting .........................................................................................................................5

Data Compilation .......................................................................................................................................6

Data Processing and Interpretation ...........................................................................................................7

Groundwater-Level Altitudes in the Ogallala, Edwards-Trinity, and

Dockum Aquifers from 1930 to 2015 ..............................................................................................8

Ogallala Aquifer.....................................................................................................................................8

Edwards-Trinity Aquifer.......................................................................................................................9

Dockum Aquifer.....................................................................................................................................9

Changes in Dissolved-Solids and Nitrate Concentrations in the

Ogallala, Edwards-Trinity, and Dockum Aquifers from 1930 to 2015......................................10

Ogallala Aquifer...................................................................................................................................10

Edwards-Trinity Aquifer.....................................................................................................................11

Dockum Aquifer...................................................................................................................................11

References Cited..........................................................................................................................................11

Sheets

[Available at http://dx.doi.org/10.3133/sim3355]

1. Figures 4–9

2. Figures 10–18

Figures

1. Map showing the study area composed of the areas of jurisdiction

of Llano Estacado Underground Water Conservation District,

Sandy Land Underground Water Conservation District, and South Plains

Underground Water Conservation District and downdip extent of the

Edwards-Trinity aquifer in and near Gaines, Terry, and Yoakum Counties, Texas..................3

2 Map showing the location of the study area within the full, regional extent

of the major aquifers and physiographic regions included in the study..................................4

3. Chart showing geologic and hydrogeologic units in and near

Gaines, Terry, and Yoakum Counties, Texas..................................................................................5

4. Map showing mean early development period groundwater-levelaltitude grid for the Ogallala aquifer in and near Gaines, Terry, and

Yoakum Counties, Texas ........................................................................................................sheet 1

5. Map showing mean recent period groundwater-level-altitude grid for the

Ogallala aquifer in and near Gaines, Terry, and Yoakum Counties, Texas ....................sheet 1
iv

6. Map showing changes in groundwater-level altitudes in the Ogallala aquifer

between the early development and recent periods and hydrographs of

groundwater-level altitudes measured at selected wells in and near

Gaines, Terry, and Yoakum Counties, Texas.......................................................................sheet 1

7. Map showing mean early development period groundwater-levelaltitude grid for the Edwards-Trinity aquifer in and near Gaines, Terry,

and Yoakum Counties, Texas.................................................................................................sheet 1

8. Map showing mean recent period groundwater-level-altitude

grid for the Edwards-Trinity aquifer in and near Gaines, Terry, and Yoakum

Counties, Texas .......................................................................................................................sheet 1

9. Map showing groundwater-level-altitude changes between the early

development and recent periods for the Edwards-Trinity aquifer and

hydrographs of groundwater-level altitudes measured at selected wells

in and near Gaines, Terry, and Yoakum Counties, Texas, 1930–2015..............................sheet 1

10. Map showing early development period dissolved-solids concentrations

measured in samples collected from wells completed in the Ogallala aquifer

in and near Gaines, Terry, and Yoakum Counties, Texas..................................................sheet 2

11. Map showing recent period dissolved-solids concentrations measured

in samples collected from wells completed in the Ogallala aquifer in and near

Gaines, Terry, and Yoakum Counties, Texas.......................................................................sheet 2

12. Map showing early development period nitrate concentrations measured

in samples collected from wells completed in the Ogallala aquifer in and near

Gaines, Terry, and Yoakum Counties, Texas.......................................................................sheet 2

13. Map showing recent period nitrate concentrations measured

in samples collected from wells completed in the Ogallala aquifer in and near

Gaines, Terry, and Yoakum Counties, Texas.......................................................................sheet 2

14. Map showing well locations and water-quality sampling results of

dissolved-solids and nitrate concentrations measured in samples collected

from wells completed in the Ogallala aquifer in and near Gaines, Terry,

and Yoakum Counties, Texas.................................................................................................sheet 2

15. Map showing early development period dissolved-solids concentrations

measured in samples collected from wells completed in the Edwards-Trinity

aquifer in and near Gaines, Terry, and Yoakum Counties, Texas....................................sheet 2

16. Map showing recent period dissolved-solids concentrations measured

in samples collected from wells completed in the Edwards-Trinity aquifer

in and near Gaines, Terry, and Yoakum Counties, Texas..................................................sheet 2

17. Map showing early development period nitrate concentrations measured in

samples collected from wells completed in the Edwards-Trinity aquifer in

and near Gaines, Terry, and Yoakum Counties, Texas......................................................sheet 2

18. Map showing recent period nitrate concentrations measured in samples

collected from wells completed in the Edwards-Trinity aquifer in and near

Gaines, Terry, and Yoakum Counties, Texas.......................................................................sheet 2
v

Tables

[Available at http://dx.doi.org/10.3133/sim3355]

1. Early development period and recent period groundwater-level-altitude data collected 

from wells completed in the Ogallala aquifer in and near Gaines, Terry, and Yoakum 

Counties, Texas

2. Early development period and recent period groundwater-level-altitude data collected 

from wells completed in the Edwards-Trinity aquifer in and near Gaines, Terry, and 

Yoakum Counties, Texas

3. Early development period and recent period groundwater-level-altitude data collected 

from wells completed in the Dockum aquifer in and near Gaines, Terry, and Yoakum 

Counties, Texas

4. Early development period and recent period dissolved-solids concentrations measured 

in samples collected from wells completed in the Ogallala aquifer in and near Gaines, 

Terry, and Yoakum Counties, Texas

5. Early development period and recent period nitrate concentrations measured in 

samples collected from wells completed in the Ogallala aquifer in and near Gaines, 

Terry, and Yoakum Counties, Texas

6. Early development period and recent period dissolved-solids concentrations measured 

in samples collected from wells completed in the Edwards-Trinity aquifer in and near 

Gaines, Terry, and Yoakum Counties, Texas

7. Early development period and recent period nitrate concentrations measured in 

samples collected from wells completed in the Edwards-Trinity aquifer in and near 

Gaines, Terry, and Yoakum Counties, Texas

8. Early development period and recent period dissolved-solids concentrations measured 

in samples collected from wells completed in the Dockum aquifer in and near Gaines, 

Terry, and Yoakum Counties, Texas

9. Early development period and recent period nitrate concentrations measured in 

samples collected from wells completed in the Dockum aquifer in and near Gaines, 

Terry, and Yoakum Counties, Texas

10. Long-term Ogallala aquifer and Edwards-Trinity aquifer groundwater-level-altitude data 

from selected wells in and near Gaines, Terry, and Yoakum Counties, Texas

11. Long-term dissolved-solids and nitrate concentrations measured in samples collected 

from wells completed in the Ogallala aquifer in and near Gaines, Terry, and Yoakum 

Counties, Texas
vi

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2)

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

°C = (°F – 32) / 1.8.

Datum

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

(NAVD 88).

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

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

Supplemental Information 

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).
following criteria, in order of priority: (1) the screened or open 

interval(s) of the well, (2) the total depth of the well, or (3) the 

completed aquifer reported for a given well by the data source.

Potentiometric-surface maps were created to depict 

changes in groundwater-level altitudes for the Ogallala 

and Edwards-Trinity aquifers. In addition to comparing 

groundwater-level altitudes and water quality from the early 

development and recent periods, hydrographs of groundwaterlevel altitudes were created, and changes in water quality 

for various periods between 1930 and 2015 were evaluated. 

Variance maps for each groundwater-level-altitude grid were 

used to evaluate the spatial data coverage and to identify 

areas with higher uncertainty because of spatially limited data 

availability for some of the aquifers.

For this report, existing dissolved-solids and nitrate 

concentration data were compiled and assessed for evidence 

of spatial patterns and changes over time. These data were 

compiled for samples collected from wells completed in the 

Ogallala, Edwards-Trinity, or Dockum aquifer during the early 

development period (1930–60) or the recent period (2005–15); 

temporal and spatial variations were assessed from depictions 

of the measured concentration values. Dissolved-solids and 

nitrate concentrations measured in samples from three wells 

completed in the Ogallala aquifer (well identifiers 11524, 

11824, and 11825) for which long-term monitoring was 

done for various periods between 1950 and 2015 were also 

compiled and analyzed.

Groundwater-level altitudes of the Ogallala aquifer are 

generally higher in the northwestern part of the study area 

and lower in the southeastern part of the study area, varying 

by as much as 800 feet. Groundwater flow paths for the 

early development period generally trend from northwest to 

southeast across the study area. Compared to those for the 

early development period, local features in the potentiometric 

surface for the recent period are more pronounced, likely as 

a result of additional data coverage, increased groundwater 

withdrawals, and local flow paths that are more variable. 

Abstract

Llano Estacado Underground Water Conservation 

District, Sandy Land Underground Water Conservation 

District, and South Plains Underground Water Conservation 

District manage groundwater resources in a part of west Texas 

near the Texas-New Mexico State line. Declining groundwater 

levels have raised concerns about the amount of available 

groundwater in the study area and the potential for waterquality changes resulting from dewatering and increased 

vertical groundwater movement between adjacent waterbearing units. 

In 2014, the U.S. Geological Survey, in cooperation with 

Llano Estacado Underground Water Conservation District, 

Sandy Land Underground Water District, and South Plains 

Underground Water Conservation District, began a multiphase 

project to develop a regional conceptual model of the 

hydrogeologic framework and geochemistry of the Ogallala, 

Edwards-Trinity, and Dockum aquifers. The Ogallala aquifer 

is the shallowest aquifer in the study area and is the primary 

source of water for agriculture and municipal supply in the 

area. This report describes the results of the first phase of the 

study, during which groundwater-level-altitude and selected 

water-quality data from wells in and near Gaines, Terry, 

and Yoakum Counties were compiled and evaluated for the 

Ogallala, Edwards-Trinity, and Dockum aquifers.

Readily available digital groundwater data for the study 

area (geologic, well-construction, groundwater-level-altitude, 

and selected water-quality data) were compiled to assess 

temporal and spatial changes in groundwater resources from 

early development (1930–60) to recent (2005–15) periods. 

Pertinent data were compiled from available sources for the 

study area and for a 5-mile buffer area around the study area 

to prevent gridding errors near the boundary. Geologic and 

well-construction data were used to determine or verify the 

aquifer in which each well was completed. Depending on 

the available data, the aquifer assignment (aquifer in which a 

given well was completed) was determined on the basis of the 

Changes Between Early Development (1930–60) and 

Recent (2005–15) Groundwater-Level Altitudes and 

Dissolved-Solids and Nitrate Concentrations In and Near 

Gaines, Terry, and Yoakum Counties, Texas 

By Jonathan V. Thomas, Andrew P. Teeple, Jason D. Payne, and Scott Ikard
2

Early Development and Recent Groundwater-Level and Selected Water-Quality Data, Gaines, Terry, and Yoakum Counties, Texas

Declining groundwater levels have raised concerns about 

the amount of available groundwater in the study area and the 

potential for water-quality changes resulting from dewatering 

and increased vertical groundwater movement between 

adjacent water-bearing units. Large groundwater withdrawals 

in localized areas can lower hydraulic heads and cause 

vertical flow in hydraulically connected aquifer units because 

groundwater moves from regions of high hydraulic head to 

low hydraulic head (Pischel and Gannett, 2015). Because 

groundwater quality generally decreases with depth, the upward 

groundwater movement within a given aquifer or between 

hydraulically connected aquifers can degrade water quality 

(Bartolino and Cunningham, 2003). Changes in water quality 

in the Ogallala aquifer and in the underlying Edwards-Trinity 

aquifer can occur as a result of the upward movement of deeper, 

relatively more saline groundwater.

In 2014, the U.S. Geological Survey (USGS), in 

cooperation with the UWCDs, began a multiphase project to 

develop a regional conceptual model of the hydrogeologic 

framework and geochemistry of the Ogallala, Edwards-Trinity, 

and Dockum aquifers. This report describes the results of 

the first phase of the study, during which groundwater-level 

altitudes and selected water-quality data from in and near 

the study area were compiled and evaluated for the Ogallala, 

Edwards-Trinity, and Dockum aquifers.

Purpose and Scope

The primary purpose of this report is to document 

differences between early development (1930–60) and 

recent (2005–15) groundwater-level altitudes and selected 

water-quality constituents (dissolved-solids and nitrate 

concentrations) for the Ogallala, Edwards-Trinity, and Dockum 

aquifers. Potentiometric-surface maps are used to depict 

changes in groundwater-level altitudes for the Ogallala and 

Edwards-Trinity aquifers. Water-quality changes and spatial 

depictions of dissolved-solids and nitrate concentrations of 

the early development and recent periods are evaluated and 

described in this report. In addition to comparing groundwaterlevel altitudes and water quality from the early development 

and recent periods, hydrographs of groundwater-level altitudes 

and discussions of changes in water quality for various periods 

between 1930 and 2015 are provided. Few data were available 

for the Dockum aquifer, so observations regarding changes in 

groundwater-level altitudes and water quality pertaining to this 

aquifer were cursory compared to observations pertaining to the 

Ogallala and Edwards-Trinity aquifers.

Description of the Study Area

The study area (fig. 1) is bounded by the extents of the 

LEUWCD, SLUWCD, and SPUWCD management areas 

and consists of Gaines, Terry, and Yoakum Counties and 

part of Hockley County, Tex. The total study area covers 

approximately 3,227 square miles (mi2

), including 1,525 mi2

For the Edwards-Trinity aquifer potentiometric-surface 

map of the recent period, a general northwest to southeast 

flow gradient was also evident, with some subtle differences 

compared to the early development period. The EdwardsTrinity aquifer water-level-altitude change map between 

the early development and recent periods indicated similar 

spatial trends as in the Ogallala aquifer and indicated that 

groundwater-level altitudes declined over a large amount of 

the area for which sufficient data were available for reliably 

mapping changes. 

During the recent period, median dissolved-solids 

concentrations of less than 1,000 milligrams per liter (mg/L) 

were predominantly measured in the western part of the study 

area, and median concentrations of more than 1,000 mg/L 

were predominantly measured in the eastern part of the study 

area. A general pattern of increasing nitrate concentrations 

from west to the northeast was evident in the study area. 

Nitrate concentrations measured in samples collected from 

16 wells completed in the Ogallala aquifer for the recent 

period were equal to or greater than 10 mg/L, the primary 

drinking water standard for finished drinking water. 

Introduction

Llano Estacado Underground Water Conservation District 

(LEUWCD), Sandy Land Underground Water Conservation 

District (SLUWCD), and South Plains Underground Water 

Conservation District (SPUWCD) (hereafter referred to 

collectively as the “UWCDs”) manage groundwater resources 

in a part of west Texas near the Texas-New Mexico State 

line (fig. 1). The UWCDs are responsible for managing the 

groundwater resources in their respective jurisdictions, which 

are in and near Gaines, Terry, and Yoakum Counties; the study 

area for this report is composed of the areas managed by the 

three UWCDs. An important source of water in the study area 

is the vast High Plains aquifer system, which extends from 

South Dakota to Texas (fig. 2). Historically, the High Plains 

aquifer system was often referred to as the “Ogallala aquifer” 

throughout its extent in reference to the predominant waterbearing unit of the aquifer system, the Ogallala Formation 

(Dugan and others, 1994). Outside of Texas, additional 

geologic units of different ages are part of the aquifer system 

(valley fill, dune sand, the Arikaree Group, and the Brule 

Formation), so use of the more inclusive “High Plains” name 

for the aquifer system became common (Dugan and others, 

1994). The Ogallala Formation is the only geologic unit of 

the High Plains aquifer system in Texas (Dugan and others, 

1994). The Ogallala aquifer is the shallowest aquifer in the 

study area and is the primary source of water for agriculture 

and municipal supply in the areas managed by the UWCDs 

(Rettman and Leggat, 1966). Groundwater withdrawals from 

deeper aquifers (mostly from the Edwards-Trinity aquifer and 

augmented by lesser amounts from the Dockum aquifer) are 

an additional water source in the study area (fig. 1).
Introduction

3

Figure 1. The study area composed of the areas of jurisdiction of Llano Estacado Underground Water Conservation District, Sandy 

Land Underground Water Conservation District, and South Plains Underground Water Conservation District and downdip extent of the 

Edwards-Trinity aquifer in and near Gaines, Terry, and Yoakum Counties, Texas.

laf16-CMSJ00-0739_fig 01A

Brownfield

Denver City

Seagraves

Seminole

Plains

380

385

385

380

62

62

82

180

214

137

83

214

GAINES

COUNTY

HOCKLEY

COUNTY

TERRY

COUNTY

YOAKUM

COUNTY

102°20' 102°40' 103°00'

33°20'

33°00'

32°40'

EXPLANATION

Edwards-Trinity aquifer downdip extent

Study area boundary

Underground water conservation district boundary

Llano Estacado Underground Water Conservation District

Sandy Land Underground Water Conservation District

South Plains Underground Water Conservation District

0 5 10 15 MILES

0 5 10 15 KILOMETERS

Base modified from U.S. Geological Survey 1:250,000-scale to 1:2,000,000-scale digital data,

World Geodetic System of 1984, Universal Transverse Mercator (UTM) Zone 13 TEXAS

NEW MEXICO
4

Early Development and Recent Groundwater-Level and Selected Water-Quality Data, Gaines, Terry, and Yoakum Counties, Texas

laf16-0739_fig01b

90°

100°

110°

40°

30°

COLORADO

IOWA

KANSAS

MISSOURI

NEBRASKA

NEW

MEXICO

OKLAHOMA

MONTANA NORTH

DAKOTA

SOUTH

DAKOTA

WYOMING

TEXAS LOUISIANA

WISCONSIN

MINNESOTA

Base modified from U.S. Geological Survey 1:250,000-scale to 1:2,000,000-scale digital data, 

Texas Water Development Board digital data, Commission for Environmental Cooperation 

1:10,000,000-scale digital data,

Albers Equal-Area Conic Projection, Texas State Mapping System

North American Datum of 1983

0 100 200 300 400 MILES

0 100 200 300 400 KILOMETERS

EXPLANATION

High Plains physiographic region (Fenneman, 1931; Wermund, 1996)

Great Plains physiographic region (Fenneman, 1931; Wermund, 1996)

Study area boundary

Ogallala aquifer extent (George and others, 2011)

Edwards-Trinity aquifer downdip extent (George and others, 2011)

Dockum aquifer downdip extent (George and others, 2011)

Figure 2. The location of the study area within the full, regional extent of the major aquifers and physiographic regions 

included in the study.
Introduction

5

Figure 3. Geologic and hydrogeologic units in and near Gaines, Terry, and Yoakum Counties, Texas. (Series and groups 

modified from Knowles and others, 1984; hydrogeologic units modified from Clark and others, 2014.)

 

 

 

 

Geologic and hydrogeologic units within the spatial extent of the Edwards-Trinity aquifer

Geologic and hydrogeologic units outside the spatial extent of the Edwards-Trinity aquifer

Cenozoic

Mesozoic

Tertiary

Cretaceous

Triassic

Late Miocene 

to Pliocene

Fredericksburg Group

Trinity Group

Dockum Group

Ogallala aquifer

Edwards-Trinity aquifer

Dockum aquifer

Cenozoic

Mesozoic Triassic Dockum Group

Ogallala aquifer

Dockum aquifer

Tertiary Late Miocene 

to Pliocene

Era Period Series or group Hydrogeologic unit

Era Period Series or group Hydrogeologic unit

Hydrogeologic Setting

The Ogallala aquifer is the largest aquifer in the United 

States and is composed primarily of poorly sorted gravel, 

sand, silt, and clay deposits (Texas Water Development Board, 

2015a). The late Miocene- to early Pliocene-age Ogallala 

aquifer consists of heterogeneous sequences of gravel and 

coarse-grained sand in the lower portion and grades upward 

into sand, silt, and clay (Peckham and Ashworth, 1993). 

Dissolved-solids concentrations in the Ogallala aquifer 

vary from less than 600 to more than 6,000 milligrams 

per liter (mg/L) within Gaines County (Llano Estacado 

Underground Water Conservation District, 2015). In many 

areas, groundwater moves vertically between the Ogallala 

Formation and the underlying Cretaceous- and Triassicage formations (fig. 3) (Ashworth and Hopkins, 1995). The 

Cretaceous-age Edwards-Trinity aquifer is a minor aquifer 

(George and others, 2011) that underlies the Ogallala aquifer 

(fig. 3) except in the southern part of the study area. Waterbearing units in the Edwards-Trinity aquifer include the 

sandstone of the Trinity Group and limestone of the overlying 

Edwards Group (fig. 3). Groundwater from the EdwardsTrinity aquifer is typically slightly saline (1,000–3,000 mg/L) 

and contains more dissolved solids than does groundwater 

from the overlying Ogallala aquifer (George and others, 

in LEUWCD, 798 mi2

 in SLUWCD, and 902 mi2 in SPUWCD 

(Llano Estacado Underground Water Conservation District, 

2015; Sandy Land Underground Water Conservation District, 

2015; South Plains Underground Water Conservation District, 

2015). The study area is in the Great Plains physiographic 

region (fig. 2) and consists of an elevated and relatively 

undissected plain (U.S. Geological Survey, 2015a). The 

population of the study area is slightly more than 38,000 

(U.S. Census Bureau, 2015). The combination of minimal 

topographic relief, availability of groundwater for irrigation, 

and excellent soils makes this an important agricultural region 

in Texas (U.S. Geological Survey, 2015a).

The climate of the study area is semiarid (Larkin and 

Bomar, 1983). Precipitation in the study area averages 

approximately 18.8 inches each year, mostly in the form 

of rain; about 5.2 inches of snow falls each year (National 

Oceanic and Atmospheric Administration, 2015). The 

potential evapotranspiration greatly exceeds average annual 

precipitation; Larkin and Bomar (1983) reported average 

annual gross lake surface evaporation rates ranging from 75 to 

79 inches for the study area. The average temperature for the 

study area is approximately 60.5 degrees Fahrenheit (°F), with 

the warmest average monthly temperature in July (79.1 °F) and 

the coolest average monthly temperature in January (40.0 °F) 

(National Oceanic and Atmospheric Administration, 2015). 
6

Early Development and Recent Groundwater-Level and Selected Water-Quality Data, Gaines, Terry, and Yoakum Counties, Texas

as N) by multiplying NO3 values by 0.2259. These values were 

converted so that the TWDB data could be directly compared 

with other compiled nitrate data and because the maximum 

contaminant level for nitrate in finished drinking water is 

for nitrate as N (concentrations of constituents measured 

in untreated groundwater samples are often compared to 

maximum contaminant levels [MCLs] for drinking water as 

a point of reference; U.S. Environmental Protection Agency, 

2016). Additional information on TWDB data, metadata, and 

quality-control procedures can be found at http://www.twdb.

texas.gov/groundwater/faq/faqgwdb.asp. Methods of analysis 

for data in the TWDB groundwater database are not indicated 

in the datasets; however, analyzing and collecting agencies 

are identified, as well as whether the ion balance was good 

or poor (when a complete chemical analysis of major ions is 

conducted, good ion balance is an indicator that analytical 

methods have been performed satisfactorily [Hem, 1985]). 

Since their establishments, the UWCDs have collected 

various types of groundwater data to monitor changes in 

groundwater levels and water quality for the primary aquifers 

in their jurisdictions. These data were provided electronically 

by each district for their respective jurisdictions to the USGS 

for inclusion in this study. Data were analyzed by various 

analytical chemistry methods including colorimetric and 

spectrophotometric methods (SLUWCD, written commun., 

2015). These data are reviewed and maintained in machinereadable format by the UWCDs. Nitrate concentration values 

provided by the UWCDs were reported as nitrate as N.

Because the data were collected and processed during a 

period of many years (1930–2015) by various agencies using 

several different methods and standards, limitations of the 

data are not well known. Data values flagged by the reporting 

agency as containing possible errors were removed from the 

dataset and not included in interpretations; for example, any 

data downloaded from the TWDB flagged as “unsatisfactory” 

were removed. 

To help identify and evaluate outliers, data from nearby 

wells were compared with each other. When an outlier was 

identified, the value was reviewed; that is, the original data 

source was checked to confirm that no typographical errors 

occurred and that the well location was verified. Outliers 

were removed if review of the original data source indicated 

that errors occurred during collection or analysis or if the 

value varied from previous data collected at the same well 

or nearby wells by more than an order of magnitude. For 

example, a maximum nitrate concentration value of 314 mg/L 

reported for a sample collected in 2008 in Terry County was 

an extreme outlier; previous nitrate concentrations measured 

in 1996 and 2004 in samples collected from the same well 

were 21.4 and 28.4 mg/L, respectively. The original laboratory 

report was obtained and reviewed to ensure that there was not 

a typographical error and that the correct value was used from 

the laboratory analysis. Because this value was an extreme 

outlier and more than an order of magnitude greater than 

previously reported data, it was removed from the dataset and 

not used in interpretations.

2011). Most of the dissolved-solids concentrations are 

between 1,000 and 2,000 mg/L (George and others, 2011). 

The Dockum aquifer is composed of the Triassic-age Dockum 

Group (fig. 3) and underlies the Ogallala and Edwards-Trinity 

aquifers throughout much of the High Plains physiographic 

region (fig. 2). The Dockum Group is composed of sandstone, 

siltstone, mudstone, and shale that were originally deposited 

in fluvial and lacustrine environments (McGowen and others, 

1979). Groundwater in the Dockum aquifer is characterized 

by decreasing quality with depth, mixed geochemistry, 

high concentrations of dissolved solids and other 

constituents that exceed secondary drinking water standards 

(U.S. Environmental Protection Agency, 2016), and high 

concentrations of sodium that may negatively affect irrigated 

land (Bradley and Kalaswad, 2003). 

Data Compilation 

Readily available digital groundwater data (geologic, 

well-construction, groundwater-level-altitude, and selected 

water-quality data) for the study area were compiled to assess 

temporal and spatial changes in groundwater resources in and 

near Gaines, Terry, and Yoakum Counties from 1930 to 2015. 

Pertinent data were compiled from available sources for the 

study area and for a 5-mile buffer area around the study area

to prevent gridding errors near the boundary.

Groundwater-level-altitude and water-quality data 

were compiled from various agencies. More than 19,000 

groundwater-level-altitude measurements collected from more 

than 1,800 wells were obtained from databases maintained by 

the USGS (U.S. Geological Survey, 2015b), the Texas Water 

Development Board (TWDB) (Texas Water Development 

Board, 2015b), and the three UWCDs (LEUWCD, written 

commun., 2015; SPUWCD, written commun., 2015; 

SLUWCD, written commun., 2015) (tables 1–3). More than 

4,400 dissolved-solids and nitrate concentration samples 

from 1,500 wells were also compiled from the three UWCDs 

(LEUWCD, written commun., 2015; SPUWCD, written 

commun., 2015; SLUWCD, written commun., 2015), the 

National Water Information System (NWIS) (U.S. Geological 

Survey, 2015b), and the TWDB (Texas Water Development 

Board, 2015b) (tables 4–9). 

Discrete water-quality data were compiled for the study 

area from databases maintained by the TWDB, LEUWCD, 

SLUWCD, and SPUWCD. Data obtained from the TWDB 

were downloaded for each county in the study area and

for a 5-mile buffer area surrounding the study area from the 

TWDB groundwater database (https://www.twdb.texas.gov/

groundwater/data/gwdbrpt.asp). Groundwater-level-altitude 

and dissolved-solids and nitrate concentration data were 

evaluated by the authors and are presented in tables in

this report. 

Nitrate values reported by the TWDB as nitrate (nitrate 

as NO3

) were converted to nitrate values as nitrogen (nitrate 
Data Processing and Interpretation

7

are typically lower compared to the rest of year because most 

groundwater is withdrawn for irrigation purposes during the 

growing season in the study area). Groundwater-level-altitude 

data collected during the dormant season were likely relatively 

unaffected by drawdown and cones of depression (Heath, 

1983). The mean dormant season groundwater-level altitude 

for each well during 2005–15 (hereinafter referred to as the 

“recent period”) was used to make a potentiometric-surface 

map representing the recent period groundwater-level altitudes 

in the study area. Using mean data for relatively long time 

periods helped to ensure sufficient data coverage and that the 

data are not biased by a single-year event such as extreme 

drought or wet period in the study area.

Because groundwater levels are measured as depth to 

water below land surface and because hand-held Global 

Positioning System devices (commonly used for reported 

elevations) typically have poor vertical accuracy, groundwaterlevel altitudes were calculated by subtracting the depth to water 

below land surface from a digital elevation model in order 

to provide consistency and improve accuracy. The Advanced 

Spaceborne Thermal Emission and Reflection Radiometer 

(ASTER) Global Digital Elevation Model Version 2 (National 

Aeronautics and Space Administration, 2015)

was used to determine the land-surface altitude across the 

study area.

Groundwater-level-altitude data consist of x and y 

values (latitude and longitude coordinates) and one or more z 

values (measured groundwater-level altitudes). The process of 

gridding interpolates x, y, and z data spatially. The resultant 

dataset is referred to as a “grid” (Geosoft, 2015). To help 

evaluate the accuracy of groundwater-level-altitude data, 

histograms were produced of the groundwater-level altitudes 

to identify anomalous data that could indicate inaccurate 

well information, data collection issues, or incorrect aquifer 

assignment for the completed interval of a well. A few 

groundwater-level-altitude values were excluded either because 

they were flagged as potentially erroneous by the reporting 

agency or because they differed greatly from groundwaterlevel-altitude values obtained from nearby wells completed in 

the same aquifer. Potentiometric-surface maps for the Ogallala 

and Edwards-Trinity aquifers were created by using Oasis 

montaj (Geosoft, 2015). There were insufficient data to create 

potentiometric-surface maps for the Dockum aquifer. 

Kriging algorithms in Oasis montaj were used to 

determine the most probable value at each grid node

(2,000 meters [m] by 2,000 m) on the basis of a statistical 

analysis of the entire dataset for each grid (Isaaks and 

Srivastava, 1989). This kriging method was chosen in part 

because of its utility for assessing clustered data such as well 

fields or multiple measurements for a single well. Variance 

maps automatically developed during the kriging process 

were used to evaluate the uncertainty in the potentiometricsurface contours. Generally, as the distance between data 

points becomes greater, correlation between points lessens, 

and uncertainty in areas between points increases (Isaaks and 

Srivastava, 1989). These variance maps were used to clip 

Some compiled results from SLUWCD were presented 

as a measured concentration of 0.0 mg/L. Because a 

spectrophotometer instrument was used to analyze these data, 

0.0 is a possible answer, as spectrophotometers do not have an 

analytically derived reporting limit. Spectrophotometers have 

an optimal detection level that should not be confused with a 

reporting limit. The spectrophotometric method used in this 

instance had an optimal range of 0.23–13.5 mg/L of nitrate 

as N (Amber Blount, SLUWCD, written commun., 2015). A 

review by the USGS indicated that the data seemed reasonable 

(within an acceptable range and comparable to nearby data) 

and could not be classified as erroneous; therefore, 0.0 mg/L 

concentrations should be considered a nondetect result.

Data Processing and Interpretation

Geologic and well-construction data (compiled along 

with the groundwater data) were used to determine or verify 

the aquifer in which each well was completed. Depending on 

the available data, the aquifer assignment (aquifer in which a 

given well was completed) was determined on the basis of the 

following criteria, in order of priority: (1) the screened or open 

interval(s) of the well, (2) the total depth of the well, or (3) the 

completed aquifer reported for a given well by the data source. 

Screened or open intervals and total depths of the wells were 

compared to the geologic information for areas near the well 

to determine or verify the correct aquifer assignment for each 

well. For wells with screened or open interval information, the 

aquifer was assigned on the basis of the geologic unit in which 

the screened or open interval was completed. If the well had 

multiple screened or open intervals, the top of the uppermost 

screened or open interval and the bottom of the lowermost 

screened or open interval were used to determine the correct 

aquifer assignment. Wells with screened or open intervals 

completed in multiple aquifers were flagged and not included 

in the report. If no screened or open interval was reported for 

a well, then the total depth of the well was used to determine 

the aquifer assignment, with the assumption that the well was 

cased to the bottom depth of the well and the opening of the 

well was at or near the base of the well. For wells without 

screened interval, open interval, or total depth information, 

the aquifer assignment reported by the data source was used. 

Groundwater data were compared with data obtained from 

nearby wells that had the same aquifer assignment to ensure 

that they were representative of the assigned aquifer.

Potentiometric-surface maps of groundwater-level 

altitudes and maps showing representations of water quality 

were created to help assess temporal and spatial changes in 

groundwater across the study area and within each aquifer. To 

help evaluate water-level data, the mean groundwater-level 

altitude during 1930–60 (hereinafter referred to as the “early 

development period”) for each well was determined for the 

dormant season, defined as November through April in this 

report. (The dormant season is when groundwater withdrawals 
8

Early Development and Recent Groundwater-Level and Selected Water-Quality Data, Gaines, Terry, and Yoakum Counties, Texas

All data were reviewed to identify any obvious typographical 

errors. The remaining suspect data were not used in the 

development of the water-quality maps. Early development 

and recent period water-quality maps for the Ogallala and 

Edwards-Trinity aquifers were compared to evaluate changes 

in groundwater quality from the early development period to 

the recent period.

Groundwater-Level Altitudes in the 

Ogallala, Edwards-Trinity, and Dockum 

Aquifers from 1930 to 2015

Potentiometric-surface maps can be used to evaluate 

groundwater-level-altitude changes spatially and temporally, 

identify regional groundwater flow paths and gradients, and 

compute vertical gradients between adjacent aquifer units. 

Groundwater-level-altitude data were compiled for the 

Ogallala and Edwards-Trinity aquifers within the study area 

and gridded to evaluate groundwater-level changes for each 

aquifer. Variance maps for each groundwater-level-altitude 

grid were used to evaluate the spatial data coverage and to 

identify and remove areas with higher uncertainty caused

by spatially limited data availability for some aquifers

(tables 1–3). 

Ogallala Aquifer

Groundwater-level altitudes for the Ogallala aquifer are 

generally higher in the northwestern part of the study area 

and lower in the southeastern part of the study area, varying 

by as much as 800 feet (ft). For the early development and 

recent periods, the highest groundwater-level altitudes (about 

3,750 ft above the North American Vertical Datum of 1988 

[NAVD 88]) were in the northwestern corner of the study 

area (figs. 4 and 5, sheet 1). Groundwater flow paths for the 

early development period generally trend from northwest to 

southeast across the study area. 

In the Ogallala aquifer, patterns in groundwater-level 

altitudes and flow paths for the recent period are diverse 

relative to the early development period. Compared to 

those for the early development period, local features in 

the potentiometric surface for the recent period are more 

pronounced (figs. 4 and 5, sheet 1), likely as a result of 

additional data coverage, increased groundwater withdrawals, 

and local flow paths that are more variable. As shown on 

the recent period map (fig. 5, sheet 1), groundwater in the 

northwestern corner of the study area flows east or southeast 

until near the border of Terry and Yoakum Counties, where 

groundwater then either flows east into Terry County or 

southeast. Groundwater near the border of Terry and Yoakum 

Counties changes direction and flows south near the town of 

Brownfield, Tex. Northwest of the town of Denver City, Tex., 

groundwater flows southeast, whereas groundwater to the 

(that is, exclude beyond a certain spatial extent) the final 

potentiometric-surface grids to certain areas such that only 

those data with acceptable uncertainty values were retained. 

Groundwater-level-altitude grids were reviewed 

for anomalies. In areas where anomalies were identified, 

groundwater-level altitudes were evaluated for consistency 

with nearby wells completed in the same aquifer. After the 

initial grids were completed, a few additional data determined 

to be anomalous were also excluded from the revised grids. 

To generate temporal groundwater-level-change grids for the 

Ogallala and Edwards-Trinity aquifers, the gridded surfaces 

for the early development period were subtracted from the 

gridded surfaces for the recent period. 

Over time, the quality of groundwater can change 

as a result of chemical reactions among the constituents 

dissolved in the water, reactions between these constituents 

and dissolvable minerals in the rocks through which the 

water flows (water-rock interaction), and mixing of water 

from different sources with varying chemistry (Elango and 

Kannan, 2007; Bumgarner and others, 2012). The distribution 

of dissolved solids and nitrate concentrations are useful 

indicators of groundwater quality in west Texas (Reeves and 

Miller, 1978). When multiple samples from a given well 

were collected and analyzed for dissolved-solids and nitrate 

concentrations during early development or recent periods, 

median concentrations were calculated. Dissolved-solids and 

nitrate concentration values were plotted spatially to depict 

how the concentrations of these constituents varied throughout 

the study period during a given period (early development 

or recent periods) and to depict changes in concentrations 

between these two periods. Dissolved-solids and nitrate 

concentrations can change over time as a direct result of 

processes related to agriculture, such as evaporation of 

irrigation water and the application of fertilizers that introduce 

minerals in the near-surface soil (Hudak and others, 2000). 

Because there is a strong and positive correlation between 

dissolved-solids concentrations and salinity, the concentrations 

of dissolved solids are used to evaluate the salinity of water 

(Winslow and Kister, 1956). Dissolved solids were of 

particular interest because of the extensive agriculture in the 

study area; elevated salinity can inhibit or prevent the growth 

of crops (Fipps, 2003). Nitrates, generally found in fertilizers 

and manure, can be a health concern in potable water supplies 

(Dozier and others, 2008), and excess nitrates not absorbed by 

the crops or vegetation can migrate in excess irrigation water 

down to the water table, increasing nitrate concentrations in 

groundwater (Hudak and others, 2000).

To avoid the possibility of introducing a short-term 

bias associated with droughts or excessively wet periods and 

to ensure sufficient data coverage, median concentrations 

for water-quality data were calculated for both the early 

development and recent periods. Additionally, before median 

concentrations were calculated, data were spatially evaluated 

to identify anomalous areas that could indicate inaccurate 

well information, errors in data collection, or data records in 

which the incorrect aquifer was assigned for a given well. 
Groundwater-Level Altitudes in the Ogallala, Edwards-Trinity, and Dockum Aquifers from 1930 to 2015

9

compared to the early development period (figs. 7 and 8, sheet 

1). For example, north of Plains, the groundwater flows east 

and then southeast in eastern Terry County. West of Plains, the 

groundwater generally flows south and then flows southeast in 

Gaines County. Near Denver City, groundwater flows to the 

southeast before flowing slightly more to the south, east

of Seminole.

Because of the limited dataset for the early development 

and recent periods, the potentiometric-surface change is 

shown for limited areas (those for which adequate data were 

available) and is generalized. The map showing EdwardsTrinity aquifer water-level-altitude change between the early 

development and recent periods (fig. 9, sheet 1) indicated 

similar spatial trends as those in the Ogallala aquifer; 

groundwater-level altitudes declined over a large amount of 

the area for which sufficient data were available. Water levels 

in these areas declined a maximum of 61 ft, with the greatest 

decline occurring southwest of Denver City. Some increases 

in the groundwater-level altitudes were mapped north and east 

of Plains in Yoakum County, in northeastern and southeastern 

parts of Terry County, and in localized parts of far eastern 

Gaines County (fig. 9, sheet 1). 

Hydrographs of selected wells across the study area 

showed varying temporal groundwater-level altitudes. The 

hydrographs from wells 10223, 10621, and 10741 depict an 

overall decline in groundwater-level altitudes from around 

1960 to 2015, whereas the hydrograph from well 10071 

depicts a slight increase in groundwater-level altitudes from 

1968 to 2015 (fig. 9, sheet 1; table 10). The hydrograph 

from well 10621, west of Denver City, shows a relatively 

consistent general pattern of declining groundwater-level 

altitudes, whereas the other three hydrographs show greater 

variations in groundwater-level altitudes during the period 

of record (fig. 9, sheet 1). The hydrograph from well 10223, 

west of Brownfield, depicts the most pronounced variation 

in groundwater-level-change patterns, with declining 

groundwater-level altitudes from early development to the 

late 1960s, increasing altitudes into the late 1980s, and then 

declining altitudes through recent conditions (fig. 9, sheet 1).

Dockum Aquifer

Groundwater-level-altitude data were available from only 

a few wells completed in the Dockum aquifer. The available 

data were used to provide additional generalized insights 

regarding groundwater flow gradients and temporal and spatial 

changes in the Ogallala and Edwards-Trinity aquifers (table 3).

Similar to those for the Ogallala and Edwards-Trinity 

aquifers, groundwater-level altitudes for the Dockum aquifer 

were highest to the northwest of the data extent, at more 

than 3,500 ft, and lowest to the southeastern part of Gaines 

County, at about 3,000 ft. This gradient indicates a regional 

groundwater flow path for the Dockum aquifer, from the 

northwest to the southeast of the study area. Compiled early 

development and recent groundwater-level altitude data were 

southwest of Denver City generally flows south from Denver 

City and then east-southeast near the town of Seminole, Tex. 

Groundwater flow in the southwestern corner of the study area 

is generally to the east and then to the southeast (fig. 5, sheet 1).

Groundwater-level-altitude changes of the Ogallala 

aquifer between the early development and recent periods (fig. 

6, sheet 1) indicate that groundwater-level altitudes declined 

primarily in western Gaines County, southwestern Yoakum 

County, central Terry County, and in the central part of the 

study area near the city of Seagraves, Tex., with the greatest 

declines occurring in northwestern Gaines County. Areas with 

increased groundwater-level altitudes were primarily observed 

in eastern Gaines County, eastern Yoakum County, and parts 

of both eastern and western Terry County (fig. 6, sheet 1). 

The greatest increases in groundwater-level altitudes occurred 

near the border of Yoakum and Terry Counties northeast of 

Plains, Tex. Within the areas with reliable data coverage, 

the groundwater-level-altitude change ranged from a 174-ft 

decline in northwestern Gaines County to a 66-ft rise northeast 

of Plains (fig. 6, sheet 1).

Hydrographs of selected wells across the study area (fig. 

6, sheet 1; table 10) show groundwater-level altitudes varying 

temporally in the Ogallala aquifer. The hydrograph from 

well 11392, located in the southeastern part of the study area, 

shows increasing groundwater-level altitudes from around 

1979 to early 1987 but then relatively stable groundwater-level 

altitudes through 2012 (fig. 6, sheet 1). The hydrograph from 

well 10128, located in the northeastern part of Terry County, 

shows relatively stable groundwater-level altitudes from 

1960 to 2013 (fig. 6, sheet 1). Hydrographs of wells 10577 

and 11171, in western Gaines County, show varying rates of 

generally declining groundwater-level altitudes from the early 

1960s to 2015.

Edwards-Trinity Aquifer

Evaluation of the variance maps for the early 

development and recent periods indicated that the data 

coverage across the study area was poor for the EdwardsTrinity aquifer (figs. 7 and 8, sheet 1) compared to the data 

coverage for the Ogallala aquifer. Because of the smaller 

dataset, the discussion of the potentiometric-surface maps 

prepared for the Edwards-Trinity aquifer is more generalized 

compared to the discussion of those for the Ogallala aquifer. 

For the early development and recent periods, the highest 

groundwater-level altitudes for the Edwards-Trinity aquifer 

were about 3,800 ft in the northwestern part of the study area 

(figs. 7 and 8, sheet 1), and the lowest altitudes were about 

3,000 ft along the southeastern extent of the aquifer within 

the study area. Groundwater-level altitudes declined to the 

southeast for the Edwards-Trinity aquifer, indicating that 

groundwater flows from northwest to southeast. 

For the Edwards-Trinity potentiometric-surface map 

of the recent period, a general northwest to southeast flow 

gradient was also evident, with some subtle differences 
10

Early Development and Recent Groundwater-Level and Selected Water-Quality Data, Gaines, Terry, and Yoakum Counties, Texas

ranged from 73 mg/L in northeastern Yoakum County, 

northeast of Plains, to about 8,000 mg/L in southeastern 

Gaines County, southeast of Seminole (table 4). All samples 

with median dissolved-solids concentrations higher than 

3,000 mg/L were collected from wells in the far eastern 

part of the study area (fig. 11, sheet 2). A dissolved-solids 

concentration between 3,000 and 10,000 mg/L represents 

moderately saline water (Winslow and Kister, 1956); water 

with a dissolved-solids concentration greater than 3,000 mg/L 

is generally considered too salty to drink (Alley, 2003) and 

can reduce yields from salt-sensitive crops (Fipps, 2003).

Nitrate concentrations measured in 21 samples collected 

from 19 wells completed in the Ogallala aquifer were 

compiled for the early development period (table 5). Nitrate 

concentrations during this period ranged from 0.63 mg/L 

in northern Gaines County, near Seagraves, to 9.71 mg/L 

in southern Gaines County, southwest of Seminole; nitrate 

concentrations of more than 2.5 mg/L were measured in wells 

in the eastern part of the study area (fig. 12, sheet 2).

For the recent period, nitrate concentrations compiled 

from 1,271 samples collected from 345 wells completed in 

the Ogallala aquifer (fig. 13, sheet 2) ranged from 0.0 mg/L 

in Yoakum County to 23.0 mg/L in northern Gaines County, 

northeast of Seminole (table 5). A few elevated nitrate 

concentrations of more than 10 mg/L were widely distributed 

throughout the study area (fig. 13, sheet 2). Elevated nitrate 

concentrations were measured (Amber Blunt, SLUWCD, 

written commun., 2015) primarily in the east-central part 

of the study area, but there were not any spatial patterns 

evident for nitrate concentrations. The nitrate concentrations 

measured in samples collected from 16 wells completed in 

the Ogallala aquifer for the recent period were equal to or 

greater than 10 mg/L, the primary drinking water standard 

for finished drinking water (U.S. Environmental Protection 

Agency, 2016). This report documents nitrate concentrations 

in raw ambient groundwater prior to drinking water 

treatment; the quality of finished drinking water distributed 

through municipal water-supply systems can differ greatly 

from the quality of raw drinking water. 

To gain a better understanding of temporal changes 

in water quality, data were compiled from three wells 

completed within the Ogallala aquifer with long-term waterquality monitoring (more than 25 years of dissolved-solids 

and nitrate concentration data) (fig. 14, sheet 2; table 11). 

Dissolved-solids concentration data for these selected wells 

indicate various water-quality changes. Relatively stable 

dissolved-solids concentrations ranging from about 680 to 

about 790 mg/L were measured in the samples from well 

11524 (the median concentration was 707 mg/L). Dissolvedsolids concentrations measured in samples from well 11824 

were variable, ranging from about 470 to about 1,340 mg/L 

(fig. 14, sheet 2). In the samples collected from well 11825, 

dissolved-solids concentrations increased from about 420 

mg/L in 1975 to 1,030 mg/L in 2008.

In general, the data obtained from these three selected 

wells with long-term monitoring records indicate that nitrate 

primarily from wells in Gaines County in the southern part of 

the study area, with only a few measurements in the northern 

part of the study area. Generalized grids of groundwaterlevel-altitude changes indicate that a decline of about 100 ft 

occurred from the early development to recent periods.

Changes in Dissolved-Solids and 

Nitrate Concentrations in the Ogallala, 

Edwards-Trinity, and Dockum Aquifers 

from 1930 to 2015

For this report, existing dissolved-solids and nitrate 

concentration data were compiled and assessed for evidence 

of spatial patterns and changes over time (tables 4–9). 

These data were compiled for samples collected from wells 

completed in the Ogallala, Edwards-Trinity, or Dockum 

aquifers during the early development period or the recent 

period; temporal and spatial variations were assessed from 

depictions of the measured concentration values. Often, more 

than one dissolved-solids or nitrate sample was available for 

a given well during the early development or recent period, 

so median concentrations were calculated for each well 

with multiple samples per time period. Dissolved-solids and 

nitrate concentrations measured in samples from three wells 

completed in the Ogallala aquifer (wells 11524, 11824, and 

11825) where long-term monitoring was done for various 

periods between 1962 and 2014 were also compiled and 

analyzed (table 11).

Ogallala Aquifer

Dissolved-solids concentration data representing the 

early development period were available for only 24 samples 

collected from 22 wells completed in the Ogallala aquifer 

(table 4). Dissolved-solids concentrations measured in these 

23 samples ranged from 433 mg/L in southwestern Gaines 

County, southwest of Seminole, to about 2,380 mg/L in eastern 

Terry County, southeast of Brownfield (fig. 10, sheet 2). In 

general, dissolved-solids concentrations appeared to increase 

from west to east.

Compared to those for the early period, considerably 

more dissolved-solids concentration data were available 

for the recent period from wells completed in the Ogallala 

aquifer (1,700 samples collected from 355 wells). During the 

recent period, median dissolved-solids concentrations of less 

than 1,000 mg/L (the upper limit for freshwater indicated by 

Winslow and Kister, 1956) were predominantly measured in 

the western part of the study area, and median dissolved-solids 

concentrations of more than 1,000 mg/L were predominantly 

measured in the eastern part of the study area (fig. 11, sheet 2). 

For samples collected from wells completed in the Ogallala 

aquifer, dissolved-solids concentrations in the recent period 
References Cited

11

Gaines County, just southwest of Denver City, to about 

13,430 mg/L in far northeastern Gaines County, northeast 

of Seminole. Nitrate concentrations measured from wells 

completed in the Dockum aquifer ranged from 0.6 mg/L in 

eastern Terry County, east of Brownfield, to 10.9 mg/L in 

northwestern Gaines County.

References Cited

Alley, W.M., 2003, Desalination of groundwater—Earth 

science perspectives: U.S. Geological Survey Fact Sheet 

075–03, 4 p., accessed January 29, 2016, at http://pubs.usgs.

gov/fs/fs075-03/pdf/AlleyFS.pdf.

Ashworth, J.B., and Hopkins, Janie, 1995, Aquifers of Texas: 

Texas Water Development Board Report 345, 69 p.

Bartolino, J.R., and Cunningham, W.L., 2003, Ground-water 

depletion across the Nation: U.S. Geological Survey Fact 

Sheet 103–03, 4 p., accessed September 5, 2014, at http://

pubs.er.usgs.gov/publication/fs10303.

Bradley, R.G., and Kalaswad, Sanjeev, 2003, The groundwater 

resources of the Dockum aquifer in Texas: Texas Water 

Development Board Report 359, 81 p.

Bumgarner, J.R., Stanton, G.P., Teeple, A.P., Thomas, J.V., 

Houston, N.A., Payne, J.D., and Musgrove, MaryLynn, 

2012, A conceptual model of the hydrogeologic 

framework, geochemistry, and groundwater-flow system 

of the Edwards-Trinity and related aquifers in the Pecos 

County region, Texas: U.S. Geological Survey Scientific 

Investigations Report 2012–5124 (revised July 10, 2012), 

74 p. [Also available at http://pubs.usgs.gov/sir/2012/5124/

pdf/SIR12-5124.pdf.]

Clark, B.R., Bumgarner, J.R., Houston, N.A., and Foster, A.L., 

2014, Simulation of groundwater flow in the EdwardsTrinity and related aquifers in the Pecos County region, 

Texas (ver. 1.1, August 2014): U.S. Geological Survey 

Scientific Investigations Report 2013–5228, 56 p., accessed 

January 18, 2016, at http://dx.doi.org/10.3133/sir20135228.

Dozier, M.C., Melton, R.H., Hare, M.F., Hopkins, Janie, and 

Lesikar, B.J., 2008, Drinking water problems—Nitrates: 

College Station, Texas A&M AgriLife Extension Report 

B–6184, 8 p., accessed March 14, 2016, at http://www.

agrilifebookstore.org/Drinking-Water-Problems-Nitrates-p/

eb-6184.htm.

Dugan, J.T., McGrath, T.S., and Zelt, R.B., 1994, Waterlevel changes in the High Plains aquifer—Predevelopment 

to 1992: U.S. Geological Survey Water-Resources 

Investigations Report 94–4027, 56 p.

concentrations increased during the sampling period (table 

11). The largest increase in nitrate was measured in samples 

from well 11824, from 0.2 mg/L in 1990 to 16.1 mg/L in 

2013. Maximum increases in nitrate concentrations of about 

4.0 mg/L and 7.2 mg/L were measured for the other two wells 

(wells 11825 and 11524, respectively). 

Edwards-Trinity Aquifer

For wells completed in the Edwards-Trinity aquifer, 

few dissolved-solids concentration data were available for 

either the early development period (15 samples from 14 

wells) or the recent period (18 samples from 12 wells) (table 

6). Because of the small number of wells with available 

dissolved-solids concentration data, the only apparent pattern 

in dissolved-solids concentrations was a general increase in 

concentrations from west to east (figs. 15 and 16, sheet 2). 

Median dissolved-solids concentration values measured in 

samples from wells completed in the Edwards-Trinity aquifer 

for the early development period ranged from about 430 mg/L 

in south-central Yoakum County, near Denver City, to about 

2,810 mg/L in central Gaines County, southeast of Seminole 

(fig. 15, sheet 2). Dissolved-solids concentrations for the 

recent period ranged from 535 mg/L in northwestern Gaines 

County, northwest of Seminole, to about 1,900 mg/L in central 

Gaines County, near Seminole (fig. 16, sheet 2).

For wells completed in the Edwards-Trinity aquifer, 

few nitrate concentration data were available for either the 

early development period (13 samples from 12 wells) or the 

recent period (18 samples from 12 wells) (table 7). Because 

of the scant number of data values and relatively consistent 

low concentrations, patterns in nitrate concentrations were 

not readily apparent. Nitrate concentrations measured in 

samples from wells completed in the Edwards-Trinity aquifer 

for the early development period ranged from 0.01 mg/L 

in south-central Yoakum County, near Denver City, to 2.71 

mg/L in south-central Yoakum County (fig. 17, sheet 2). 

Nitrate concentrations for the recent period were in general 

appreciably higher compared to those measured in the early 

development period and ranged from 0.0 mg/L in eastern 

Gaines County, east of Seminole, to 7.16 mg/L in north-central 

Gaines County, near Seminole (fig. 18, sheet 2). 

Dockum Aquifer

Few dissolved-solids concentration data existed for 

either the early development period (8 samples from 4 wells) 

or the recent period (3 samples from 2 wells) for samples 

collected from wells completed in the Dockum aquifer (table 

8). Few nitrate concentration data from wells completed 

in the Dockum aquifer were obtained for either the early 

development period (3 samples from 3 wells) or the recent 

period (3 samples from 2 wells) (tables 8 and 9). Dissolvedsolids concentrations measured from wells completed in 

the Dockum aquifer ranged from 427 mg/L in northwestern 
12

Early Development and Recent Groundwater-Level and Selected Water-Quality Data, Gaines, Terry, and Yoakum Counties, Texas

McGowen, J.H., Granata, G.E., and Seni, S.J., 1979, 

Depositional framework of the lower Dockum Group 

(Triassic), Texas Panhandle: The University of Texas 

at Austin, Bureau of Economic Geology Report of 

Investigations no. 97, 60 p.

National Aeronautics and Space Administration, 2015, 

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14

Early Development and Recent Groundwater-Level and Selected Water-Quality Data, Gaines, Terry, and Yoakum Counties, Texas
ISSN 2329-1311 (print)

ISSN 2329-132X (online)

http://dx.doi.org/10.3133/sim3355

Thomas and others—Early Development and Recent Groundwater-Level and Selected Water-Quality Data, Gaines, Terry, and Yoakum Counties, Texas—SIM 3355

ISBN 978-1-4113-4041-1

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