CHEMISTRY AND AGE OF GROUND WATER

IN THE SOUTHWESTERN HUECO BOLSON,

NEW MEXICO AND TEXAS

Prepared in cooperation with

EL PASO WATER UTILITIES

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4237
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4237

CHEMISTRY AND AGE OF GROUND WATER

IN THE SOUTHWESTERN HUECO BOLSON,

NEW MEXICO AND TEXAS

Prepared in cooperation with

EL PASO WATER UTILITIES

By Scott K. Anderholm and Charles E. Heywood

Albuquerque, New Mexico

2003
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Denver, CO 80225-0286

District Chief

U.S. Geological Survey

Water Resources Division

5338 Montgomery Blvd. NE, Suite 400

Albuquerque, NM 87109-1311

from:

Information regarding research and data-collection programs of the U.S. Geological Survey is available

on the Internet via the World Wide Web. You may connect to the home page for the New Mexico District

Office using the URL http://nm.water.usgs.gov.
iii

CONTENTS

Page

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

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

Hydrologic setting ....................................................................................................................................................... 1

Acknowledgments ....................................................................................................................................................... 3

Ground-water chemistry ........................................................................................................................................................ 3

Field properties............................................................................................................................................................ 3

Common constituents .................................................................................................................................................. 4

Nutrients and total dissolved carbon............................................................................................................................ 8

Trace elements ............................................................................................................................................................. 8

Stable isotopes ............................................................................................................................................................. 11

Radioactive isotopes .................................................................................................................................................... 12

Age of ground water .............................................................................................................................................................. 12

Summary................................................................................................................................................................................ 15

References ............................................................................................................................................................................. 15

FIGURES

1. Map showing location of wells sampled in the southwestern Hueco Bolson............................................................ 2

 2. Durov plot of ground-water composition in the southwestern Hueco Bolson........................................................... 9

 3. Plots showing relation between chloride concentration and (A) sodium, (B) potassium, (C) calcium,

(D) sulfate, and (E) bicarbonate concentrations and between (F) calcium and sulfate concentrationsin

southwestern Hueco Bolson ................................................................................................................................. 10

4. Plot showing relation between delta oxygen-18 and delta deuterium in ground water and water

from the Rio Grande in the southwestern Hueco Bolson...................................................................................... 13

5. Plot showing relation between chloride concentration and delta deuterium in ground water in the

southwestern Hueco Bolson .................................................................................................................................. 13

TABLES

1. Water-quality data for wells sampled in the southwestern Hueco Bolson, September 1997..................................... 5

2. Measured carbon-13 and carbon-14 composition, calculated partial pressure of carbon dioxide, saturation

index for calcite, and apparent age of water in the southwestern Hueco Bolson.................................................. 11
iv

CONVERSION FACTORS, DATUMS, AND ABBREVIATED WATER-QUALITY UNITS

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

°F = (1.8 × °C) + 32.

Specific conductance is reported in microsiemens per centimeter at 25 °C (μS/cm).

Water-quality data are given in milligrams per liter (mg/L), micrograms per liter (μg/L), and

picoCuries per liter (pCi/L).

Sea level: In this report, “sea level” refers to the National Geodetic Datum of 1929--A geodetic

datum derived from a general adjustment of the first-order level nets of the United States and

Canada, formerly called Sea Level Datum of 1929.

Elevation, as used in this report, refers to distance above sea level.

Multiply By To obtain

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

meter (m) 3.281 foot (ft)
1

ABSTRACT

This report, prepared in cooperation with El Paso

Water Utilities, presents the results of an investigation

to determine the chemistry and age of ground water on

the southwestern side of the Hueco Bolson. The

radioactive isotope carbon-14 was used to estimate the

length of time that water from wells has been isolated

from the atmosphere, which is the modern carbon-14

reservoir.

Nine wells on the southwestern side of the

Hueco Bolson were sampled for analysis of common

constituents, nutrients, total organic carbon, trace

elements, stable isotopes, and radioactive isotopes.

Dissolved-solids concentrations in water from the

wells sampled ranged from 269 to 2,630 milligrams per

liter. Sodium concentrations generally increased

linearly with chloride concentrations, possibly

indicating mixing of dilute recharge water with sodium

chloride brine. Concentrations of nutrients and trace

elements generally were small. The deuterium and

oxygen-18 composition in all samples except those

from wells adjacent to the Rio Grande indicates that

infiltration of precipitation is the main source of water

to these wells and that evaporation has not affected the

isotopic composition of the water. The source of water

from wells adjacent to the Rio Grande is probably not

the same source as the water from wells adjacent to the

Franklin Mountains. The calculated apparent carbon14 ages ranged from 12,100 to 25,500 years.

INTRODUCTION

The Hueco Bolson is a large alluvial basin in

south-central New Mexico and western Texas. The

basin-fill deposits in the southern Hueco Bolson are an

important source of ground water for the city of El

Paso, Texas. Little is known about ground-water flow

rates in the basin-fill aquifer in this area.

This report, prepared in cooperation with El Paso

Water Utilities, presents the results of an investigation

to determine the chemistry and age of ground water on

the southwestern side of the Hueco Bolson. The

radioactive isotope carbon-14 was used to estimate the

length of time that water from wells has been isolated

from the atmosphere, which is the modern carbon-14

reservoir.

In September 1997, water samples were

collected for analysis of common constituents,

nutrients, trace elements, total organic carbon,

deuterium, oxygen-18, carbon-13, tritium, and carbon14 from nine wells on the southwestern side of the

Hueco Bolson near El Paso, Texas (fig. 1). Seven of the

wells were monitoring wells, and two were municipal

supply wells. A portable pump was used to sample six

of the monitoring wells (one of the monitoring wells

had an existing pump). One to three casing volumes of

water were purged from the monitoring wells prior to

collection of water samples. Samples were not

collected until measured values of the field properties

water temperature, dissolved oxygen, specific

conductance, and pH had stabilized. The two municipal

supply wells sampled were pumped continuously for

several days prior to collection of the water samples.

Therefore, stabilization of field properties was not

required before sampling these wells.

Hydrologic Setting

The Franklin Mountains bound the southwestern

side of the Hueco Bolson near El Paso, Texas (fig. 1).

The mountains, which rise to 3,000 ft above the

relatively flat bolson floor, are composed of igneous,

metamorphic, and sedimentary rocks. The Rio Grande,

which is the major surface-water feature, has eroded a

valley through the basin-fill deposits in the Hueco

Bolson. This valley is about 200 to 250 ft below the

surface of the bolson floor (White and others, 1997).

Mean annual precipitation in El Paso is about 10.2 in.

(U.S. Department of Commerce, 2001).

Basin-fill deposits of Tertiary age compose the

main aquifer in the southwestern Hueco Bolson.

Ground water also occurs in shallow alluvium of

Quaternary age near the Rio Grande (Knowles and

Kennedy, 1958). The principal source of recharge to

CHEMISTRY AND AGE OF GROUND WATER IN THE SOUTHWESTERN

HUECO BOLSON, NEW MEXICO AND TEXAS

By Scott K. Anderholm and Charles E. Heywood
2

0

0

3

3 6 MILES

6 KILOMETERS

NEW MEXICO

TEXAS

NEW

MEXICO

MEXICO

TEXAS

FRAN

KLI

N

M

OU

NTAINS

Approximate area

of El Paso

HUECO BOLSON

RIO

GRANDE

106 ̊30 ́ 106 ̊22 ́30 ́ ́

32 ̊

07 ́

30 ́ ́

32 ̊

00 ́

31 ̊

52 ́

30 ́ ́

31 ̊

45 ́

05-323

05-318

05-504

05-901 06-702,

06-703

13-506

21-318, 21-322

1120

(3675)

1125

(3691)

1130

(3708)

1135

(3724)

1145

(3757)

1140

(3740)

Base map and contours modified from Heywood and Yager, 2003

Figure 1. Location of wells sampled in the southwestern Hueco Bolson.

RIO GRANDE

TEXAS

NEW MEXICO

MEXICO

Study

area

Hueco

Bolson

1120

(3675)

05-323

LINE OF EQUAL PREDEVELOPMENT

WATER LEVEL--Number is elevation,

in meters above sea level. Interval 5

meters. Elevation, in feet, in parentheses

WELL--Number is well number (table 1)

EXPLANATION
3

the ground-water system in this area is infiltration of

precipitation and runoff along the base of the Franklin

Mountains (Knowles and Kennedy, 1958, p. 18).

Ground water moves eastward and southward from the

Franklin Mountains (Knowles and Kennedy, 1958, p.

18). Before large withdrawals of ground water from the

basin-fill deposits, ground water moved from the

recharge areas southeastward toward the Rio Grande

Valley where potentiometric heads were greater than

the elevation of the river (Knowles and Kennedy, 1958,

p. 18 and 19). Ground water probably discharged to the

Rio Grande Valley prior to ground-water pumpage in

the valley area. Part of this water discharged to the Rio

Grande and part of the water was used by native

vegetation in the valley (White and others, 1997).

Saline water occurs in deeper parts of the basin-fill

aquifer and in the shallow alluvium adjacent to the Rio

Grande (Knowles and Kennedy, 1958, p. 19 and 26).

Ground-water pumpage from the basin-fill

deposits has resulted in changes in predevelopment

ground-water flow directions. White and others (1997)

indicated that saline ground water from deeper parts of

the aquifer is moving upward. They also indicated that

saline water from the shallow alluvium adjacent to the

Rio Grande Valley as well as water infiltrating from the

Rio Grande has been moving downward since the

1940’s.

Acknowledgments

Access and sampling of the wells would not have

been possible without the cooperation of personnel

from El Paso Water Utilities, Fort Bliss Military

Reservation, and Biggs Army Airfield. Roger Sperka

of El Paso Water Utilities assisted in the collection of

water samples and freely shared his vast knowledge of

the geology and hydrology of the area. Roger’s

assistance and input are greatly appreciated. Tyler

Coplen of the U.S. Geological Survey supplied the

stable isotope data for the Rio Grande.

GROUND-WATER CHEMISTRY

The wells sampled were located in two general

areas in the southwestern part of the Hueco Bolson.

Seven of the wells sampled were east of the Franklin

Mountains (fig. 1). Water from the wells east of the

Franklin Mountains probably represents water that

recharged the basin-fill aquifer along the Franklin

Mountains and moved southeastward and mixed with

water moving south. Two monitoring wells (21-318

and 21-322) are adjacent to the Rio Grande southeast of

the Franklin Mountains. The area where water from

these wells recharges the aquifer is not clear; however,

some water from these wells probably infiltrates from

the Rio Grande.

The casing material of the seven wells sampled

adjacent to the Franklin Mountains was steel, and the

casing material of the wells sampled adjacent to the Rio

Grande was polyvinylchloride (PVC). The length of

screened interval in four monitoring wells (06-703,

06-702, 13-506, and 05-318) sampled near the Franklin

Mountains ranged from 20 to 50 ft (table 1) and in well

05-323 was 200 feet. The length of screened interval in

the municipal supply wells was 379 (05-901) and 671

(05-504) ft. The top of the screened interval in five of

the seven wells adjacent to the Franklin Mountains was

at or within 20 ft of the water level in the well and in

the other two wells (06-703 and 13-506) was about 180

and 400 ft below the water level, respectively. Water

levels in the wells near the Franklin Mountains ranged

from 316 to 378 ft below land surface. The length of

screened interval in the two monitoring wells adjacent

to the Rio Grande was 10 ft. The top of the screened

interval in well 21-318 was 348 ft below land surface

and in well 21-322 was 659 ft below land surface. The

water level in these wells was about 100 ft below land

surface. The relatively short screened interval (less than

50 ft) in many of the wells allowed sample collection

from discrete parts of the aquifer, usually near the top

of the zone of saturation. Samples from wells with

large screened intervals represent mixtures of water

from larger parts of the aquifer.

Field Properties

Specific conductance, pH, temperature, and

dissolved oxygen were measured in the field. The

specific conductance of water from the wells ranged

from 431 to 4,350 μS/cm (table 1) and from all but

three of the wells was less than or equal to 742 μS/cm.

The pH of water samples ranged from 7.34 to 8.30. The

pH of water from the two wells adjacent to the Rio

Grande generally was higher than the pH in wells

adjacent to the Franklin Mountains. Water

temperatures ranged from 23.1 to 28.7 °C. Water from

the wells adjacent to the Rio Grande generally had the

lowest temperature, and water from the deepest well

(05-504) (a well with 671 ft of screened interval) had

the highest temperature. Dissolved-oxygen

concentrations ranged from 0.05 to 5.0 mg/L and were
4

largest in the municipal wells. Oxygen from the

unsaturated zone may be drawn into water pumped

through these wells because the water levels in these

wells were below the top of the screened interval.

Common Constituents

The sampled water can be grouped into two

types on the basis of dissolved-solids concentration and

percentage of chloride relative to the sum of chloride,

sulfate, and bicarbonate (in milliequivalents) (table 1

and fig. 2). The differences in water types are probably

related to the composition of recharge water and the

dominant factors affecting geochemical evolution of

the ground water. Processes that could be affecting the

composition of water include dissolution and (or)

precipitation of gases and minerals, ion exchange, and

mixing of dilute recharge water with more saline water

such as a sodium chloride brine.

The dissolved-solids concentration in water from

three wells (06-703, 21-318, and 21-322) ranged from

662 to 2,630 mg/L and was larger than concentrations

in water from any other wells sampled (listed as

“Solids residue at 180 deg C, dissolved” in table 1).

The percentage of chloride relative to the sum of

chloride, sulfate, and bicarbonate was also larger in

water from these three wells (greater than 70 percent)

(fig. 2). Two of these wells (21-318 and 21-322) are

adjacent to the Rio Grande. Well 06-703 is located

farther east of the Franklin Mountains than most of the

other wells sampled (fig. 1). Sodium and chloride are

the dominant ions in water from these three wells

(fig. 2). The relatively large dissolved-solids

concentration and large sodium and chloride

concentrations in water from wells 21-318, 21-322, and

06-703 could be due to larger concentrations of sodium

and chloride in water recharging the aquifer near these

wells or to dilute recharge water mixing with a sodium

chloride brine as water moves through the aquifer

toward these wells.

The dissolved-solids concentration in water from

the other six wells (06-702, 13-506, 05-323, 05-318,

05-504, and 05-901), which are adjacent to the

Franklin Mountains, ranged from 269 to 455 mg/L

(table 1). The percentage of chloride relative to the sum

of chloride, sulfate, and bicarbonate was less than 50 in

water from these wells (fig. 2). Sodium plus potassium,

bicarbonate, and chloride generally are the dominant

ions in water from these wells (fig. 2). Although the

concentrations of individual cations vary among the

samples, the percentage of sodium plus potassium,

calcium, and magnesium relative to the sum of these

ions is relatively constant in water from these wells.

The percentage of sulfate relative to the sum of

chloride, sulfate, and bicarbonate is also relatively

constant, whereas the percentage of bicarbonate in

water decreases as the percentage of chloride increases

(fig. 2).

Although wells 06-702, 13-506, 05-323, 05-318,

05-504, and 05-901 are not along the same precise flow

path, differences in the composition of water from

these wells can be used to infer processes that might be

important during the chemical evolution of water from

these wells. Processes that could be affecting the

composition of water recharging the basin-fill deposits

along the Franklin Mountains include

evapotranspiration of precipitation during infiltration,

dissolution and (or) precipitation of gases and

minerals, ion exchange, and mixing of dilute water and

brine. Water from well 05-504, which is the well

closest to the Franklin Mountains, is probably the most

representative sample of the composition of recharge

water in the area. Water from this well has small

concentrations of sodium, potassium, and chloride and

generally larger concentrations of calcium and

bicarbonate than water from the other sampled wells

(fig. 3 and table 1). Sodium concentrations generally

increase linearly with increasing chloride

concentrations in water from the sampled wells,

possibly indicating that dilute recharge water mixes

with sodium chloride brine as the water moves away

from the recharge area (fig. 3A). The relation between

potassium, calcium, sulfate, and bicarbonate with

chloride (fig. 3B, C, D, and E) is not as clear as the

relation between sodium and chloride, indicating

processes in addition to mixing that affect potassium,

calcium, sulfate, and bicarbonate concentrations. Trend

lines (linear regression) show that potassium and

sulfate (fig. 3B and D) generally increase with

increasing chloride, which could mean that mixing is

an important process affecting the concentrations of

these ions. Calcium and bicarbonate concentrations are

larger in water from well 05-504 than in water from the

other wells adjacent to the Franklin Mountains (fig. 3C

and E), which are farther from the recharge area.

Bicarbonate generally decreases with increasing

chloride concentration (fig. 3E), which is consistent

with calcite precipitation as ground water moves

through the aquifer. With the exception of water from

well 05-504, calcium concentrations are positively

correlated with sulfate concentrations (fig. 3F),

indicating dissolution of gypsum in the water.

Dissolution of gypsum causes calcium concentrations

to increase, possibly causing calcite to precipitate.
5

Table 1. Water-quality data for wells sampled in the southwestern Hueco Bolson, September 1997

[--, no data; μS/cm, microsiemens per centimeter at 25 degrees Celsius (deg C);

mm, millimeters; mg/L, milligrams per liter; water whole, unfiltered sample; dis, dissolved;

pCi/L, picocuries per liter; it, incremental titration; gran t. field, field gran titration

of filtered sample; <, less than]

Station number Date Time

Well

number

(fig. 1)

Elevation

of land

surface

(feet

above sea

level)

Depth of

well, total

(feet)

Depth to

top of

screened

interval

(feet)

Depthto

bottom

of

screened

interval

(feet)

Water

level

(feet

below

land

surface)

315452106203202 08-22-97 1200 06-703 3,914 550 510 550 329.2

315452106203201 08-27-97 1900 06-702 3,914 360 320 360 326.0

314831106260001 08-20-97 1900 13-506 3,884 736 716 736 316.0

315932106245101 08-19-97 1330 05-323 4,038 500 300 500 --

315816106243201 08-21-97 1620 05-318 4,015 500 450 500 378.4

315543106263201 08-27-97 0900 05-504 4,120 1,152 481 1,152 --

315448106242401 08-27-97 1120 05-901 3,940 727 348 727 --

314421106233403 08-26-97 1930 21-318 3,686 378 348 358 91.0

314421106233407 08-26-97 1100 21-322 3,686 678 659 669 107.6

Well

number

(fig. 1)

Specific

conductance, field

(μS/cm)

Specific

conductance, lab

(μS/cm)

pH water

whole,

field

(standard

units)

pH water

whole,

lab

(standard

units)

Temperature

water

(deg C)

Barometric

pressure

(mm of

Hg)

Oxygen,

dissolved

(mg/L)

Calcium,

dissolved

(mg/L

as Ca)

06-703 1,920 1,870 7.34 7.27 24.8 -- 0.05 77

06-702 740 711 7.90 7.97 25.5 -- 0.8 28

13-506 480 463 8.20 8.20 26.0 -- 0.1 22

05-323 431 415 7.78 7.99 26.5 658 2.9 19

05-318 704 677 7.85 8.02 26.4 659 4.0 36

05-504 572 557 7.48 7.70 28.7 658 5.0 41

05-901 742 724 7.61 7.96 26.9 660 4.3 29

21-318 4,350 4,170 8.10 7.74 24.3 669 0.2 180

21-322 1,190 1,140 8.30 8.24 23.1 669 0.8 30
6

Table 1. Water-quality data for wells sampled in the Hueco Bolson, September 1997--Continued

Well

number

(fig. 1)

Magnesium,

dissolved

(mg/L

as Mg)

Sodium,

dissolved

(mg/L

as Na)

Potassium,

dissolved

(mg/L

as K)

Bicarbonate

water dis

it, field

(mg/L as

HCO3)

Alkalinity

water

whole

gran t.,

field

(mg/L as

CaCO3)

Alkalinity,

lab

(mg/L as

CaCO3)

Sulfate,

dissolved

(mg/L

as SO4)

Fluoride,

dissolved

(mg/L

as F)

06-703 18.2 245 9.8 82 68 61 24 0.5

06-702 6.0 109 5.6 155 126 125 52 0.8

13-506 9.4 63 5.3 204 165 159 25 0.6

05-323 4.0 60 4.1 126 101 102 33 0.9

05-318 8.4 91 7.6 187 152 151 74 0.8

05-504 18.0 49 3.9 258 210 209 49 1.6

05-901 9.0 104 9.6 182 147 149 78 1.0

21-318 80.6 560 20 -- -- 132 330 0.4

21-322 4.9 190 3.8 98 86 86 59 0.8

Well

number

(fig. 1)

Chloride,

dissolved

(mg/L

as Cl)

Silica,

dissolved

(mg/L

as SiO2)

Solids,

residue at

180

deg C,

dissolved

(mg/L)

Nitrogen,

nitrite,

dissolved

(mg/L

as N)

Nitrogen,

NO2+NO3,

dissolved

(mg/L

as N)

Nitrogen,

ammonia,

dissolved

(mg/L

as N)

Nitrogen,

ammonia

+organic,

dis

(mg/L

as N)

Phosphorus,

dissolved

(mg/L

as P)

06-703 510 24 1,240 0.06 1.05 0.033 <0.2 <0.01

06-702 110 23 428 0.09 1.71 <0.015 <0.2 0.05

13-506 29 26 297 0.01 1.54 0.032 <0.2 <0.01

05-323 37 29 269 <0.01 1.81 <0.015 <0.2 <0.01

05-318 70 31 433 <0.01 1.82 <0.015 <0.2 <0.01

05-504 19 30 357 <0.01 1.70 <0.015 <0.2 <0.01

05-901 83 30 455 <0.01 1.54 <0.015 <0.2 <0.01

21-318 1,080 30 2,630 <0.01 0.05 0.018 <0.2 <0.01

21-322 270 32 662 <0.01 <0.05 0.036 <0.2 <0.01
7

Table 1. Water-quality data for wells sampled in the Hueco Bolson, September 1997--Continued

Well

number

(fig. 1)

Phosphorus,

ortho,

dissolved

(mg/L

as P)

Carbon,

organic,

total

(mg/L

as C)

Aluminum,

dissolved

(μg/L

as Al)

Antimony,

dissolved

(μg/L

as Sb)

Arsenic,

dissolved

(μg/L

as As)

Barium,

dissolved

(μg/L

as Ba)

Beryllium,

dissolved

(μg/L

as Be)

Cadmium,

dissolved

(μg/L

as Cd)

06-703 <0.01 0.1 4 <1 <1 210 <1 <1

06-702 <0.01 0.1 3 <1 <1 41 <1 <1

13-506 <0.01 1.1 4 <1 4 150 <1 <1

05-323 <0.01 0.3 3 <1 3 47 <1 <1

05-318 <0.01 0.1 3 <1 4 78 <1 <1

05-504 0.014 0.1 4 <1 1 140 <1 <1

05-901 0.012 0.1 4 <1 4 42 <1 <1

21-318 <0.01 1.2 8 <2 2 100 <2 <2

21-322 <0.01 0.2 7 <1 20 67 <1 <1

Well

number

(fig. 1)

Chromium,

dissolved

(μg/L

as Cr)

Cobalt,

dissolved

(μg/L

as Co)

Copper,

dissolved

(μg/L

as Cu)

Iron,

dissolved

(μg/L

as Fe)

Lead,

dissolved

(μg/L

as Pb)

Manganese,

dissolved

(μg/L

as Mn)

Molybdenum,

dissolved

(μg/L

as Mo)

Nickel,

dissolved

(μg/L

as Ni)

Uranium

natural,

dissolved

(μg/L

as U)

06-703 <1 <1 <1 4,100 <1 74 3 1 <1

06-702 1 <1 <1 24 <1 29 6 <1 2

13-506 <1 <1 <1 120 <1 14 6 <1 2

05-323 4 <1 <1 20 <1 23 5 <1 1

05-318 7 <1 <1 4 <1 <1 8 <1 4

05-504 3 <1 12 <3 1 <1 18 <1 6

05-901 7 <1 2 <3 <1 <1 7 <1 5

21-318 <2 <2 <2 <9 <2 61 6 3 16

21-322 <1 <1 <1 <3 <1 6 5 <1 <1
8

Table 1. Water-quality data for wells sampled in the Hueco Bolson, September 1997--Concluded

Well

number

(fig. 1)

Selenium,

dissolved

(μg/L

as Se)

Silver,

dissolved

(μg/L

as Ag)

Zinc,

dissolved

(μg/L

as Zn)

H-2/H-1

stable

isotope

ratio per

mil

O-18/

O-16

stable

isotope

ratio per

mil

C-13/

C-12

stable

isotope

ratio per

mil

Tritium,

total

(pCi/L)

Carbon-14

water

filtered

(percent),

modern

Carbon-14

counting

error

water

filtered

(percent)

06-703 1 <1 350 -71.9 -10.17 -7.19 0.38 4.56 0.14

06-702 2 <1 73 -67.4 -9.67 -8.36 0.42 5.66 0.12

13-506 <1 <1 <1 -65.6 -9.33 -9.33 0.74 11.92 0.18

05-323 2 <1 140 -65.8 -9.28 -7.47 1.2 13.18 0.17

05-318 4 <1 83 -66.9 -9.48 -7.92 0.74 18.22 0.21

05-504 1 <1 21 -63.0 -8.96 -8.17 0.83 23.20 0.3

05-901 4 <1 5 -69.8 -9.78 -8.13 0.48 10.80 0.16

21-318 <1 <2 8 -76.8 -9.82 -9.71 25.5 73.13 0.49

21-322 <1 <1 1 -76.4 -10.43 -10.51 1.1 6.60 0.13

Most samples are at saturation or are supersaturated

with respect to calcite (table 2), so calcite would tend to

precipitate, especially if calcium was added to solution

because of dissolution of gypsum as the water moves

through the aquifer.

 Silica concentrations in the six samples from

wells near the Franklin Mountains ranged from 23 to 31

mg/L (table 1). The silica concentration in water from

well 05-504 was 30 mg/L, indicating that most of the

silica dissolves in water during recharge or near the

recharge area. The lack of variation in silica

concentrations in the water indicates that silica is not

dissolving appreciably after water recharges the

ground-water flow system.

Nutrients and Total Dissolved Carbon

Nutrient concentrations generally were small

(less than 0.05 mg/L) with the exception of dissolved

nitrite (NO2) plus nitrate (NO3) (table 1). The nitrite

plus nitrate concentrations in the samples from the two

wells adjacent to the Rio Grande were less than 0.05

and 0.05 mg/L as N. Nitrite plus nitrate concentrations

ranged from 1.05 to 1.82 mg/L as N in the other seven

samples. Nitrite concentrations generally were less than

0.01 mg/L as N, indicating nitrate as the dominant

nitrogen species in the samples. Dissolved phosphorous

concentrations generally were less than 0.01 mg/L.

Dissolved organic carbon concentrations ranged

from 0.1 to 1.2 mg/L as C and were 0.3 mg/L or less in

seven of nine samples. The presence of nitrite plus

nitrate greater than 1 mg/L in seven of nine samples and

dissolved oxygen greater than 0.5 mg/L in five of these

seven samples indicates that ground water is generally

oxidized and that nitrate in recharge water is not

reduced by bacteria to ammonia or nitrogen gas (nitrate

reduction). Nitrate could have been reduced in water

from wells 21-318 and 21-322 (fig. 1), which are both

located near the Rio Grande, because dissolved-oxygen

and nitrate concentrations are small and ammonia was

detected in samples from both wells. The relatively

small organic carbon concentrations (0.3 mg/L or less)

(table 1) and similarity in the partial pressure of carbon

dioxide (table 2) in most of the samples indicate few

chemical reactions involving organic carbon in the

unsaturated or saturated zone.

Trace Elements

Concentrations of most trace elements in the nine

samples were generally small or less than the detection

limit of 1 μg/L and had little variation (table 1).

Aluminum, arsenic, chromium, selenium, and uranium

were detected in most samples at concentrations less

than 5 μg/L. These relatively small concentrations

indicate no significant processes in the area that

contribute to these trace-element concentrations.
9

80

100

60

40

20

0

80

100

60

40

20

0

80

100

60

40

20

0

SODIUM PLUS POTASSIUM

CALCIUM

MAGNESIUM

PERCENTAGE, IN MILLIEQUIVALENTS

100

80

60

40

20

0

80

100

60

40

20

0

80

100

60

40

20

CHLORIDE

SULFATE

BICARBONATE

0

750

1,500

CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

0 2,000 4,000

DISSOLVED-SOLIDS CONCENTRATION,

IN MILLIGRAMS PER LITER

PERCENTAGE, IN MILLIEQUIVALENTS

Figure 2. Durov plot of ground-water composition in the southwestern Hueco Bolson.

21-318

06-703

21-322

06-702

05-318

05-504

13-506

05-901 05-323

0

EXPLANATION

21-322 WELL NUMBER
10

0

200

100

0 20 40 60 80 100 120

Chloride concentration, in milligrams per liter

ation, Sodium concentr

in milligrams per liter

0

80604020

100

0 20 40 60 80 100 120

Chloride concentration, in milligrams per liter

ation, Sulfate concentr

in milligrams per liter

05-504

05-504

13-506

13-506

05-323 05-323

05-318

05-318

05-901

05-901

06-702 06-702

A

D

0

90807060

50403020

10

100

10 15 20 25 30 35 40 45

Calcium concentration, in milligrams per liter

ation, Sulfate concentr

in milligrams per liter

Chloride concentration, in milligrams per liter

8

6

4

2

0

12

10

0 20 40 60 80 100 120

ation, Potassium concentr

in milligrams per liter

0

50

300

250200

150

100

0 20 40 60 80 100 120

Chloride concentration, in milligrams per liter

ation, Bicarbonate concentr

in milligrams per liter

50

50

454035

30

25201510

0 20 40 60 80 100 120

Chloride concentration, in milligrams per liter

ation, Calcium concentr

in milligrams per liter Sample not included in regression

05-504

05-504

05-504

05-504

13-506

13-506

13-506

13-506

05-323

05-323

05-323

05-323

05-318

05-318

05-318

05-318

05-901

05-901

05-901

05-901

06-702

06-702

06-702

06-702

Figure 3. Relation between chloride concentration and (A) sodium, (B) potassium, (C) calcium, (D) sulfate, and (E) bicarbonate concentrations

and between (F) calcium and sulfate concentrations in the southwestern Hueco Bolson.

B

C

EF

LINE OF LINEAR REGRESSION--Least squares methodWELL NUMBER 06-702

EXPLANATION
11

The concentrations of barium, iron, manganese,

molybdenum, and zinc were greater than 5 μg/L in

many of the samples and varied considerably (table 1).

This indicates that factors affecting the concentrations

of these trace elements vary in different areas of the

Hueco Bolson. The variations in concentrations of

these trace elements could be related to variations in

the composition of recharge water, spatial variations in

the abundance of minerals containing trace elements in

the aquifer, differences in oxidation/reduction

conditions in the aquifer, or mixing of recharge water

with sodium chloride brine that contains trace

elements.

Barium was detected in the samples at

concentrations ranging from 41 to 210 μg/L (table 1).

Because larger barium concentrations were not related

to a particular area of the Hueco Bolson, the variation

in barium concentrations is probably not related to

variations in the abundance of barium-containing

minerals in particular areas of the bolson. What is

affecting the barium concentrations in ground water in

the area is not clear.

Iron and manganese concentrations varied

considerably in the samples. In general, the largest iron

and manganese concentrations were in samples with

smaller dissolved-oxygen concentrations, indicating

that iron and manganese concentrations are affected by

oxidation/reduction conditions. Water from the well

with the largest iron and manganese concentrations,

well 06-703, also had the smallest measured dissolvedoxygen concentration (0.05 mg/L); water from this

well indicates reducing conditions with respect to iron

and manganese.

Molybdenum concentrations ranged from about

5 to 7 μg/L in eight of the samples; water from well

05-504 had a molybdenum concentration of 18 μg/L.

Why the molybdenum concentration in water from this

well was larger than those in the other samples is not

clear. Zinc concentrations ranged from less than 1 to

350 μg/L (table 1). The reason for the large range is not

known. Some iron, manganese, and zinc detected in the

samples could be due to leaching from steel well

casings.

Stable Isotopes

The delta deuterium and delta oxygen-18

composition in all but the two samples from the wells

adjacent to the Rio Grande (21-318 and 21-322) trends

along the Mean Global Meteoric Water Line (MWL)

(Craig, 1961) (fig. 4), a line that represents the

composition of precipitation throughout the world.

Samples that trend along the MWL represent

precipitation (meteoric water) that has infiltrated and

recharged the ground-water flow system. Evaporation,

Table 2. Measured carbon-13 and carbon-14 composition, calculated partial pressure of carbon dioxide,

saturation index for calcite, and apparent age of water in the southwestern Hueco Bolson

Well

number

(fig. 1)

Carbon-13/

carbon-12

ratio, in per

mil

Carbon-14,

in percent

modern

Partial

pressure of

carbon

dioxide, in

atmospheres

Saturation

index,

calcite

Apparent

age, in years

06-703 -7.19 4.56 -2.46 0.41 25,500

06-702 -8.36 5.66 -2.7 0.06 23,700

13-506 -9.33 11.92 -2.9 0.40 17,600

05-323 -7.47 13.18 -2.7 -0.26 16,800

05-318 -7.92 18.22 -2.6 0.20 14,100

05-504 -8.17 23.20 -2.1 0.07 12,100

05-901 -8.13 10.80 -2.4 -0.13 18,400

21-322 -10.51 6.60 -3.3 0.24 22,500
12

mixing with non-meteoric water, or isotopic exchange

at high temperatures can explain sample compositions

that deviate from the MWL. Samples from the wells

adjacent to the Rio Grande (21-318 and 21-322) plot

significantly below the MWL. The type of

displacement from the MWL shown in these two

samples could be the result of evaporation but not

evaporation of water sampled from the area adjacent to

the Franklin Mountains. The source of recharge water

to these two wells is probably different from the source

of recharge to the other seven wells sampled because

these two samples are isotopically lighter than samples

from the area adjacent to the Franklin Mountains. On

the basis of isotopic composition of modern Rio

Grande water (fig. 4), these two samples have not

evolved from modern Rio Grande water. The source of

recharge water to the wells near the Rio Grande is not

known.

A plot of delta deuterium and chloride

concentration indicates that delta deuterium decreases

with increasing chloride concentration in most of the

samples (fig. 5). This could mean that dilute recharge

water with relatively heavy delta deuterium is mixing

with a sodium/chloride-rich brine that has a relatively

light delta deuterium. The plot of sodium and chloride

concentration (fig. 3A) also indicates that dilute

recharge water is mixing with a sodium/chloride-rich

brine as the water moves through the ground-water

flow system.

The delta carbon-13 composition of the samples

ranges from -10.51 to -7.19 per mil (table 1). The

samples from the wells adjacent to the Rio Grande

(21-318 and 21-322) are generally lighter (more

negative) than the other samples. Delta carbon-13 is

useful for determining how the carbon-14 composition

in the samples has been affected by chemical reactions

because both carbon-13 and carbon-14 compositions

are affected by chemical reactions during recharge and

as water moves through the aquifer. The relatively

small variation in delta carbon-13 composition in the

samples from wells adjacent to the Franklin Mountains

(-9.33 to -7.19) indicates that similar processes have

affected the carbon composition in water from these

wells during recharge or ground-water movement

through the basin-fill aquifer. The lighter delta

carbon-13 in samples from the wells adjacent to the Rio

Grande relative to the other samples indicates

differences in reactions occurring or different delta

carbon-13 composition of recharge water in the

different areas.

Radioactive Isotopes

Because tritium activity generally was small in

most of the samples (table 1), the ground water

probably has been isolated from the atmosphere for

more than 50 years. The tritium activity of 25.5 pCi/L

in the sample from the 378-ft-deep well adjacent to the

Rio Grande (21-318) indicates that water from this well

has a significant amount of relatively modern recharge

water (water less than 50 years old). Water from the

678-ft-deep well adjacent to the Rio Grande (21-322)

had a tritium activity of 1.1 pCi/L, and water from the

500-ft-deep well adjacent to the Franklin Mountains

(05-323) had a tritium activity of 1.2 pCi/L, indicating

a small amount of relatively modern water in these

wells or the introduction of small amounts of tritium to

the samples during sample collection. The rest of the

samples had tritium activities between 0.38 and 0.83

pCi/L, indicating little or no modern recharge water

mixing with these samples.

Carbon-14 compositions ranged from 4.56 to

73.13 percent modern (table 1). The sample from the

378-ft-deep well adjacent to the river (21-318) had a

carbon-14 composition of 73.13 percent modern,

indicating a significant amount of relatively modern

water. The use of carbon-14 to calculate the apparent

age of water from well 21-318 is not possible because

of the large amount of modern water in the sample. The

other eight samples had carbon-14 compositions

between 4.56 and 23.20 percent modern, and apparent

ages were calculated.

AGE OF GROUND WATER

The time since water from a well has been

isolated from the atmosphere (apparent age) can be

estimated using its carbon-14 composition. In general,

water is isolated from the atmosphere (the modern

carbon-14 reservoir) when water moves from the

unsaturated zone to the ground-water flow system. The

carbon-14 composition of ground water is the result of

two main factors: (1) chemical reactions that affect the

inorganic carbon concentrations and carbon-14

composition in the water as the water infiltrates through

the unsaturated zone or moves through the aquifer and

(2) radioactive decay of carbon-14 in the water.

Two common chemical reactions affecting the

inorganic carbon concentrations, carbon-13

composition, and carbon-14 composition in ground

water include dissolution of carbon dioxide and
13

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

DELTA DEUTERIUM, IN PER MIL

0 200 400 600 800 1,000 1,200

21-322

06-703

21-318

05-901

06-702

05-318

05-323

13-506

05-504

EXPLANATION

06-703

WELL NUMBER

Figure 5. Relation between chloride concentration and delta deuterium in ground water

in the southwestern Hueco Bolson.

Figure 4. Relation between delta oxygen-18 and delta deuterium in ground water and water

from the Rio Grande in the southwestern Hueco Bolson.

DELTA OXYGEN-18, IN PER MIL

-10.5 -10.0 -9.5 -9.0 -8.5 -8.0 -7.5 -7.0 -6.5

21-322

21-318

EXPLANATION

WELL NUMBER

RIO GRANDE

21-318

06-703

05-901

06-702

05-318

13-506

05-323

05-504

Mean

Global Meteroic Water Line (Craig, 1961)

DELTA DEUTERIUM, IN PER MIL

-62

-64

-66

-68

-70

-72

-74

-76

-78

-62

-64

-66

-68

-70

-72

-74

-76

-78
14

dissolution of carbonate minerals. These reactions

could occur in the unsaturated zone during infiltration

of water or as ground water moves through the aquifer.

Dissolution of carbon dioxide gas, most of which

generally occurs in the unsaturated zone by infiltrating

water, is an important reaction that affects the carbon14 composition of ground water. Infiltrating water and

dissolving carbon dioxide gas in the unsaturated zone

would have a carbon-14 composition of about 100

percent modern because carbon dioxide gas diffuses

from the atmosphere and because plants respire carbon

dioxide gas to the soil zone or unsaturated zone that is

100 percent modern. Water that has dissolved carbon

dioxide gas and is infiltrating through the unsaturated

zone or moving through the aquifer can also dissolve

carbonate minerals, which increases dissolved

inorganic carbon concentrations and can affect the

carbon-14 composition of the water. The carbon-14

composition of water generally decreases during

dissolution of carbonate minerals because the carbon14 composition of carbonate minerals is small relative

to carbon dioxide gas. The dissolution of carbonate

minerals can reduce the carbon-14 composition of

ground water significantly. Although other chemical

reactions can also affect the carbon-14 composition of

ground water, these two reactions are probably the

most important reactions affecting inorganic carbon/

carbon-14 in ground water in the Hueco Bolson.

The carbon-14 composition of water decreases

with time because of radioactive decay of the carbon14. The half-life of carbon-14 is about 5,730 years;

therefore, water with 100 percent modern carbon-14

would have a carbon-14 composition of 50 percent

modern after 5,730 years as the result of radioactive

decay.

Before determining how long ground water has

been isolated from the atmosphere or from the modern

carbon-14 reservoir, the effect of chemical reactions on

the carbon-14 composition of ground water needs to be

determined. After the carbon-14 composition of

ground water has been corrected for changes due to

chemical reactions (Ao), age can be estimated on the

basis of radioactive decay of carbon-14. Investigators

have developed several models to adjust or estimate the

carbon-14 composition of water resulting from

processes in the unsaturated zone and aquifer (Ao)

(Mook, 1980). These models range from simple

models that require little data to complex models that

require much information about the carbon isotopic

composition (carbon-13 and carbon-14) of gas in the

unsaturated zone, carbon isotopic composition of

carbonate minerals in the unsaturated zone and aquifer,

and reactions that occur as water moves through the

aquifer. Much of the data needed for these models have

not been collected and are not available for the Hueco

Bolson. These data were collected in the Middle Rio

Grande Basin, however, and are considered applicable

to the Hueco Bolson.

The Middle Rio Grande Basin, which is about

200 mi north of the Hueco Bolson, is an alluvial basin

similar to the Hueco Bolson. Rock types in the

mountains adjacent to the basin and bolson are similar

igneous and metamorphic rock of Precambrian age to

sedimentary rock of Paleozoic age. The basin-fill

deposits of Tertiary age (principal aquifer) in the

Middle Rio Grande Basin are similar to the basin-fill

deposits in the Hueco Bolson (sand, silt, and clay

fluvial and alluvial-fan deposits). Recharge to the basin

and bolson is along the margins as the result of

infiltration of precipitation, and depths to water in the

recharge areas along the margins are similar (generally

greater than 300 ft).

In the Middle Rio Grande Basin delta carbon-13

compositions in ground water generally range from

-10.4 to -6.0 per mil (values similar to those for the

Hueco Bolson), and the carbon-14 composition of the

water before radioactive decay and after chemical

reactions (Ao) was estimated to be about 100 percent

modern (Neil Plummer, U.S. Geological Survey, oral

commun., 2001). Because of the similarity of the basinfill deposits, hydrology, and carbon-13 composition of

ground water in the basin and bolson, an Ao of 100

percent modern was assumed for the Hueco Bolson.

Choosing a smaller Ao would cause the calculated

apparent age to be younger (less).

The following equation was used to estimate the

apparent age of ground water in the Hueco Bolson:

t = (5,730/ln 2) ln (Ao/As) (1)

where t = apparent age, in years;

Ao = carbon-14 composition of water before

radioactive decay and after chemical

reactions, in percent modern; and

As = carbon-14 composition measured in the

sample, in percent modern.

The calculated apparent age of the water

sampled ranged from about 12,100 to 25,500 years

(table 2). If the water was isolated from the atmosphere
15

when it recharged the ground-water system, the

carbon-14 ages are estimates of the time water takes to

move through the aquifer from the recharge area to the

well or sampling point. There is some indication that

recharge water mixes with a sodium chloride brine as

water moves through the ground-water flow system. If

the brine was much older than the recharge water, the

calculated apparent ages would indicate travel times

greater than actual travel times. Water from well 05-

504, which is the closest well to the Franklin

Mountains (a possible recharge area), had the youngest

apparent age (12,100 years). Considering the apparent

age of water from this well and its proximity to the

possible recharge area, flow rates could be slow in this

area, possibly because of small rates of recharge to the

aquifer in this area. Water from well 06-703, which has

a screened interval of 500 to 550 ft below land surface,

had the oldest apparent age (25,500 years). Water from

well 06-702, which is next to well 06-703 and has a

screened interval about 200 ft closer to land surface

than that of well 06-703, had an apparent age of 23,700

years, indicating that ground-water age increases with

depth near these wells or that mixing with relatively old

sodium chloride brine has affected the age of water

from the deeper well. Water from the well near the Rio

Grande (21-322) had an age of 22,500 years. The small

amount of tritium in this sample (table 1) could mean a

small amount of modern water in the sample, which

would cause the calculated apparent age of the water to

be younger than that of other water in this area that has

not mixed with modern water.

SUMMARY

This report, prepared in cooperation with El Paso

Water Utilities, presents the results of an investigation

to determine the chemistry and age of ground water on

the southwestern side of the Hueco Bolson. The

radioactive isotope carbon-14 was used to estimate the

length of time that water from wells has been isolated

from the atmosphere or modern carbon-14 reservoir.

Basin-fill deposits, which compose the main aquifer in

the southern Hueco Bolson, are an important source of

ground water for the city of El Paso, Texas. Infiltration

of precipitation and runoff from the Franklin

Mountains and leakage from the Rio Grande Valley are

important sources of recharge to these basin-fill

deposits.

Seven wells adjacent to the Franklin Mountains

and two wells adjacent to the Rio Grande were sampled

for analysis of common constituents, nutrients, total

organic carbon, trace elements, stable isotopes, and

radioactive isotopes. Dissolved-solids concentrations

ranged from 269 to 2,630 mg/L. The dissolved-solids

concentrations in water from one well adjacent to the

Franklin Mountains (1,240 mg/L) and the two wells

adjacent to the Rio Grande (662 and 2,630 mg/L) were

larger than those in the other six wells sampled, which

ranged from 269 to 455 mg/L. Sodium and chloride are

the dominant ions in water with larger dissolved-solids

concentrations, and sodium, chloride, and bicarbonate

are the dominant ions in water with smaller dissolvedsolids concentrations. Sodium concentrations

generally increased linearly with chloride

concentrations, possible indicating mixing of dilute

recharge water with sodium chloride brine.

The delta deuterium and delta oxygen-18

composition in all samples except those from the two

wells adjacent to the Rio Grande indicates that

infiltration of precipitation is the main source of water

to these wells and that evaporation has not affected the

isotopic composition of the water. The source of water

from wells adjacent to the Rio Grande is probably not

the same as the source of water from wells adjacent to

the Franklin Mountains. Delta carbon-13 ranges from

-9.33 to -7.19 per mil in samples adjacent to the

Franklin Mountains, indicating that similar chemical

reactions have affected the carbon composition in this

water.

Calculating the age of water from one well

located near the Rio Grande was not possible because

water from this well contained a significant amount of

modern recharge water based on the tritium

concentration of 25.5 pCi/L. The calculated apparent

carbon-14 ages of eight of the samples with tritium

concentrations equal to or less than 1.2 pCi/L ranged

from 12,100 to 25,500 years. Water from the well

closest to the Franklin Mountains was the youngest;

considering the proximity of this well to the recharge

area, however, flow rates could be slow in this area.

REFERENCES

Craig, H., 1961, Isotope variations in meteoric waters:

Science, v. 133, p. 1702-1703.

Heywood, C.E., and Yager, R.M., 2003, Simulated groundwater flow in the Hueco Bolson, an alluvial-basin

aquifer system near El Paso, Texas: U.S. Geological

Survey Water-Resources Investigations Report 02-

4108, 73 p.
16

Knowles, D.B., and Kennedy, R.A., 1958, Ground-water

resources of the Hueco Bolson northeast of El Paso

Texas: U.S. Geological Survey Water-Supply Paper

1426, 185 p.

Mook, W.G., 1980, Carbon-14 in hydrogeological studies, in

Fritz, P., and Fontes, J. Ch., eds., Handbook of

environmental isotope geochemistry, Volume 1--The

terrestrial environment, A, chap. 2: New York, Elsevier

Scientific Publishing Co., p. 49-74.

U.S. Department of Commerce, 2001, National Oceanic and

Atmospheric Administration, National Weather Service

El Paso Area website at http://www.srh.noaa.gov/elp/

climat, accessed December 10, 2001.

White, D.E., Baker, E.T., Jr., and Sperka, Roger, 1997,

Hydrology of the shallow aquifer and uppermost

semiconfined aquifer near El Paso, Texas: U.S.

Geological Survey Water-Resources Investigations

Report 97-4263, 37 p.
S.K. Anderholm and C.E. Heywood—CHEMISTRY AND AGE OF GROUND WATER IN THE SOUTHWESTERN HUECO BOLSON,

NEW MEXICO AND TEXAS—U.S. Geological Survey Water-Resources Investigations Report 02-4237

U.S. Department of the Interior

U.S. Geological Survey, WRD

5338 Montgomery Blvd. NE, Suite 400

Albuquerque, NM 87109-1311

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