Source and Movement of Ground Water

in the Western Part of the Mojave Desert,

Southern California, USA

By John A. Izbicki 

Sacramento, California

2004

Water-Resources Investigations Report 03-4313

U.S. GEOLOGICAL SURVEY

Prepared in cooperation with the

MOJAVE WATER AGENCY 7002-07
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall, Suite 2012

6000 J Street

Sacramento, CA 95819-6129

http://ca.water.usgs.gov
Contents iii

CONTENTS

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

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

Purpose and Scope ...................................................................................................................................... 1

Background and Previous Studies............................................................................................................... 3

Sample Collection ....................................................................................................................................... 4

Analyses of Delta Oxygen-18 and Delta Deuterium................................................................................... 5

Acknowledgments....................................................................................................................................... 5

Hydrogeology....................................................................................................................................................... 5

Mojave River Ground-Water Basin ............................................................................................................ 6

Morongo Ground-Water Basin ................................................................................................................... 7

Delta Oxygen-18 and Delta Deuterium Composition of Precipitation ................................................................ 7

Delta Oxygen-18 and Delta Deuterium Composition of Water from Wells........................................................ 11

Mojave River Ground-Water Basin ............................................................................................................ 11

Floodplain Aquifer............................................................................................................................. 14

Regional Aquifer................................................................................................................................ 16

Movement of Water between Aquifers.............................................................................................. 16

Section A–A'.............................................................................................................................. 16

Section B–B'.............................................................................................................................. 18

Section C–C' ............................................................................................................................. 18

Sections D–D' ........................................................................................................................... 20

Section E–E'.............................................................................................................................. 22

Morongo Ground-Water Basin ................................................................................................................... 22

Warren Basin...................................................................................................................................... 22

Joshua Tree Area................................................................................................................................ 24

Lucerne Valley................................................................................................................................... 24

Pipes Wash/Surprise Spring Area. ..................................................................................................... 25

Discussion ............................................................................................................................................................ 25

References ............................................................................................................................................................ 26
iv Figures

FIGURES

Figure 1. Map showing location of the Mojave River and the Morongo ground-water basins, the 

floodplain aquifer, selected faults, geologic section lines, and selected precipitationcollection stations .............................................................................................................................. 2

Figure 2. Graphs showing delta oxygen-18 as a function of delta deuterium composition of 

precipitation in the Mojave River and the Morongo ground-water basins, southern

California ........................................................................................................................................... 8

Figure 3. Graphs showing altitude along topographic divide and changes in delta deuterium 

composition of ground water on the eastern slope of the San Gabriel, the San Bernardino, 

and the San Jacinto Mountains, southern California .........................................................................10

Figure 4. Graph showing delta oxygen-18 and delta deuterium composition of water from wells in 

the Mojave River and the Morongo ground-water basins, southern California, 1986–2000 ............ 12

Figure 5. Map showing delta deuterium composition of water from wells in the Mojave River and

the Morongo ground-water basins, southern California, 1986–2000 ................................................ 13

Figure 6. Graphs showing delta oxygen-18 and delta deuterium composition of water from wells

in the floodplain aquifer and the regional aquifer of the Mojave River ground-water 

basin, southern California, 1986–2000 .............................................................................................. 15

Figure 7. Stratigraphic cross section showing delta deuterium composition of water from selected 

observation wells along section A–A', Mojave River ground-water basin, southern 

California).......................................................................................................................................... 17

Figure 8. Stratigraphic cross section showing delta deuterium composition of water from 

selected observation wells along sections B–B' and C–C', Mojave River ground-water 

basin, southern California)................................................................................................................. 19

Figure 9. Stratigraphic cross section showing delta deuterium composition of water from 

selected observation wells along sections D–D' and E–E' near the Helendale and the 

Waterman Faults, respectively, Mojave River ground-water basin, southern California..................21

Figure 10. Graphs showing delta oxygen-18 and delta deuterium composition of water from wells

in the Warren Basin, Joshua Tree, Lucerne Valley and Pipes Wash/Surprise Spring areas

of the Morongo ground-water basin, southern California, 1989–2000 ............................................. 23
Tables v

TABLES

Table 1. Precipitation collection sites for the determination of delta oxygen-18 and delta deuterium 

composition in the western Mojave Desert, southern California....................................................... 9
vi Conversion Factors , Vertical Datum, and Abbreviations

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

CONVERSION FACTORS 

Temperature in degrees Celsius (oC) may be converted to degrees Fahrenheit (oF) as follows:

oF=1.8 oC+32.

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

micrograms per liter (μg/L). 

VERTICAL DATUM

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

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

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

ABBREVIATIONS

CO2 carbon dioxide

2H deuterium

δD delta deuterium

δ18O delta oxygen-18

1H hydrogen

NWIS National Water Information System

16O oxygen-16

18O oxygen-18

o/oo per mil (parts per thousand)

SLAP Standard Light Antarctic Precipitation

VSMOW Vienna Standard Mean Ocean Water

Organizations

USGS U.S. Geological Survey

VVWRA Victor Valley Wastewater Reclamation Authority

Multiply By To obtain

cubic hectometer (hm3) 810.7 acre-foot

kilometer (km) 0.6214 mile 

square kilometer (km2) 247.1 acre

liter (L) 0.2642 gallon

meter (m) 3.281 foot 

meter per year (m/yr) 3.281 foot per year 

millimeter (mm) 0.03937 inch

millimeter per year (mm/yr) 0.03937 inch per year 
Well-Numbering System vii

WELL-NUMBERING SYSTEM

Wells are identified and numbered according to their location in the rectangular system for the subdivision of 

public lands. Identification consists of the township number, north or south; the range number, east or west; and the 

section number. Each section is divided into sixteen 40-acre tracts lettered consecutively (except I and O), 

beginning with "A" in the northeast corner of the section and progressing in a sinusoidal manner to "R" in the 

southeast corner. Within the 40-acre tract, wells are sequentially numbered in the order they are inventoried. The 

final letter refers to the base line and meridian. In California, there are three base lines and meridians; Humboldt 

(H), Mount Diablo (M), and San Bernardino (S). All wells in the study area are referenced to the San Bernardino 

base line and meridian (S) Well numbers consist of 15 characters and follow the format 007N005W023R003. In 

this report, well numbers are abbreviated and written 7N/5W-23R3. Wells in the same township and range are 

referred to only by their section designation, 23R3. The following diagram shows how the number for well 

7N/5W-23R3 is derived.

R5W R4W R3W R2W R1W

T9N

T8N

T7N

T6N

T5N

San Bernardino Meridian

R5W RANGE

TOWNSHIP

T

7

N

D C B A

E F G H

M L K J

N P

SECTION 23

Q

654321

7 8 9 10 12

18 17 13

19 20 21 22 23 24

30 29

31 32 33 34 35 36

11

16 15 14

28 27 26 25

7N/5W-23R3

Well-numbering diagram (Note: maps in this report use abbreviated well numbers such as "23R3")

R

Introduction 1

Source and Movement of Ground Water 

in the Western Part of the Mojave Desert, 

Southern California, USA

By John A. Izbicki

ABSTRACT

Delta oxygen-18 and delta deuterium 

composition of precipitation and water from wells 

in the Mojave River and the Morongo groundwater basins in the western part of the Mojave 

Desert show that ground-water recharge occurs 

primarily from winter precipitation near lowaltitude passes in the San Bernardino and the San 

Gabriel Mountains—as opposed to runoff from 

higher altitudes in the mountains. The resulting 

deuterium composition of the ground water, about 

−64 per mil, contrasts sharply with the isotopic 

composition of water from wells recharged by 

runoff from higher altitudes of the San Gabriel and 

the San Bernardino Mountains, about −84 per mil. 

These differences define the 3-dimensional 

movement of ground water between aquifers 

especially downgradient from faults that act as 

barriers to ground-water flow. Water recharged 

from runoff in the mountains farther to the east in 

the Mojave Desert plots to the right of the 

meteoric water line and after accounting for 

evaporative effects had an isotopic composition 

lighter than present-day precipitation. 

INTRODUCTION

The study area is the Mojave River and the 

Morongo ground-water basins (referred to as the 

Mojave River and the Morongo basins, respectively) in 

the western part of the Mojave Desert in southern 

California (fig.1), about 120 km east of Los Angeles. 

The area is arid with cold winters and hot, dry 

summers. Population of the area is growing rapidly and 

ground water from the floodplain aquifer along the 

Mojave River and the surrounding and underlying 

regional aquifer are the only dependable sources of 

water supply. With increased population, ground-water 

pumping throughout the study area also has increased 

and water levels have declined (Stamos and others, 

2001). As a result of these declines, there is a need to 

reduce pumping and actively manage ground-water 

supplies. Understanding of the source, movement, and 

age of water from wells is a valuable tool for support of 

ground-water management activities in the area.

Purpose and Scope

The purpose of this study was to evaluate the 

source and movement of ground water in the Mojave 

River and the Morongo ground-water basins. The 

stable isotopes of oxygen and hydrogen in precipitation 

were compared with the isotopic composition of water 

from wells to evaluate the source of ground-water 

recharge and the movement of water between 

aquifers—especially near faults that act as barriers to 

ground-water flow.
2 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

R8W R1W R1E R9E

T

1

N

T

9

N

T

32

S

R41E R 45 E

Lucerne

Valley

Warren

Basin

Surprise

Spring

Basin

Surprise

Spring Joshua

Tree

Area

40

15

15

15

215

A A'

B B'

D-D'

C'

C

E E'

Troy

Lake

(Dry)

Lucerne

Lake

(Dry)

Coyote Lake

Harper (Dry)

Lake

(Dry)

El Mirage

Lake

(Dry)

Deadman

Lake

(Dry)

Mesquite

Lake

(Dry)

Victorville

Barstow

Yucca

Valley

Mojave River

Afton Canyon

San Gabriel Mts

C

Pass

ajon

San Bernardino Mts

Helendal e Fault

S an Andreas Fault

Water man

F ault

Surprise Spring Fault

Mojave River

ground-water

basin

Morongo

ground-water

basin

Mojave

Valley Camp

Cady Hinkley

Valley

Summit

Valley

Rock

Springs

Pipes

BB Wash

TP

SC

BV

OH

AQ

SF

DG

T M

Sheep

Creek

VVWRA

Upper

Narrows

Lower

Narrows

11700' 11600'

34

30'

35

00'

C

A

L

I F O R N I A

STUDY

AREA

Los

Angeles

Pacific Ocean

Mojave

Desert

EXPLANATION

Dry lake

Selected major fault

Geologic section line

Victor Valley Wastewater Reclaimation Authority

Areal extent of the

floodplain aquifer

Precipitation stations and locator

Friedman and others, 1992

Izbicki and others, 2000b

D–D'

0 20

0 20

KILOMETERS

MILES

DG

SC

VVWRA

Figure 1. Location of the Mojave River and the Morongo ground-water basins, the floodplain aquifer, selected faults, geologic section lines, and 

selected precipitation-collection stations.
Introduction 3

Background and Previous Studies

Atoms of oxygen-18 (18O) and deuterium (2H) 

have more neutrons and a greater atomic mass than do 

the more common isotopes, oxygen-16 (16O) and 

hydrogen (1H). The difference in weight results in 

differences in the physical and chemical behavior of 

the heavier, less abundant isotopes. As a result of these 

differences, the δ18O and δD composition of a water 

sample can provide a record of the source and 

evaporative history of water and can be used as a tracer 

of the movement of the water. In recent years, the ratios 

of the stable isotopes of oxygen and hydrogen have 

become standard hydrologic tools for the analysis of 

ground-water recharge sources and movement 

(International Atomic Energy Agency, 1981).

Most precipitation throughout the world 

originates from the evaporation of seawater. The δ18O 

and δD composition of precipitation throughout the 

world is linearly correlated and plots along a line 

known as the meteoric water line (Craig, 1961). 

Differences in the isotopic composition of precipitation 

occur along this line if water vapor originated from 

evaporation of cooler or warmer seawater, and as moist 

air masses move across continents and heavier isotopes 

are preferentially removed by precipitation. In a given 

location, the isotopic composition of water vapor in 

different storms trends to an average value over time, 

and the isotopic composition of precipitation is 

determined by local differences in the temperature of 

condensation of water vapor. For example, water that 

condensed at cooler temperatures (often associated 

with higher altitudes, cooler climatic regimes, or higher 

latitudes) is lighter (has less of the heavier isotopes and 

is more negative) than water that condensed at warmer 

temperatures (often associated with lower altitudes, 

warmer climatic regimes, or lower latitudes). Water 

that has been partly evaporated is enriched in the 

heavier isotopes, relative to its original composition. 

These values plot to the right of the meteoric water line 

along a line known as the evaporative trend line. 

There have been several regional-scale isotopic 

studies of precipitation and ground water in the western 

part of the Mojave Desert and southern California. 

Freidman and Smith (1970, 1972) used the δD content 

of snow in the Sierra Nevada to the north of the Mojave 

Desert as an index of winter climate. Friedman and 

others (1992) evaluated the δ18O and δD composition 

of present-day precipitation and concluded that air 

mass trajectory and partial evaporation of rainfall 

controlled the isotopic composition of winter and 

summer precipitation. Smith and others (1992) 

evaluated the stable isotopic composition of ground 

water and its relation to precipitation sampled by 

Friedman and others (1992) and observed that many 

ground-water samples were significantly lighter (more 

negative) than present-day precipitation. Using the δD 

composition of ground water as an index to climatic 

conditions at the time of recharge, Smith and others 

(1992) suggested that the ground water had great age 

and was recharged at a time when winters were cooler 

and evaporation rates were lower.

Ingraham and Taylor (1991) evaluated 

continental and altitude effects on δD composition of 

precipitation, surface water, and ground water in 

California and Nevada with increasing distance from 

the ocean and identified large evaporation effects and 

terrestrial recycling of water. Smith and others (1992) 

and Gleason and others (1994) surveyed the δD 

composition of water from wells and perennial springs 

in southeastern California to identify areas where 

ground water δD composition was different from that 

of present-day precipitation and suggested that greater 

understanding of the climatic and hydrologic processes 

that lead to ground-water recharge was needed. 

Friedman and others (1992), Gleason and others 

(1994), and Williams and Rodoni (1997) identified 

anomalously heavy δD composition in water from 

wells near the Mojave River. These values were related 

by Izbicki and others (1995, 2002) to the isotopic 

composition of precipitation near Cajon Pass and 

subsequent ground-water recharge from infiltration of 

stormflow in the Mojave River. 
4 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Most regional-scale studies of the isotopic 

composition of ground water in the Mojave Desert, the 

southwestern United States, and elsewhere are limited 

to 2-dimensional surveys of isotopic composition of 

water from wells and springs. For example, the studies 

by Smith and others (1992) and Gleason and others 

(1994) of the δD composition of water from wells in 

the Mojave Desert relied primarily on data collected 

from long-screened production wells and did not 

provide information of the δD composition of ground 

water with depth. In some studies, data from different 

aquifer systems, such as alluvial aquifers and 

underlying limestone aquifers each having complex 

and poorly understood flow systems, have been 

combined for regional interpretation (Davisson and 

others, 1999). This approach has been strongly 

criticized (Thomas, 1999) because these studies do not 

permit an evaluation of the 3-dimensional nature of 

ground-water flow systems, or of changes in isotopic 

composition of ground water with depth, or allow for a 

complete understanding of ground-water movement 

through aquifer systems. 

Sample Collection

Precipitation data were collected at five sites 

between December 1994 and November 1997 (Izbicki 

and others, 2000b). Collectors were based on a design 

by Friedman and others (1992) and consisted of a 

75-mm straight-sided Buchner funnel supported on a 

stake about 1 m above the ground. The funnel was 

connected to 1-L plastic bottles placed below the 

ground surface using copper tubing. The bottles 

contained a thin layer of mineral oil, which prevented 

evaporation of water. In most years, precipitation 

samples were collected twice a year, in October prior to 

the start of the rainy season and in April at the end of 

the rainy season. During wet years, precipitation 

samples were collected more frequently to prevent 

sample bottles from overfilling. Precipitation amounts 

were calculated from the volume of water in the 

container. Freidman and others (1992) determined that 

precipitation amounts from these collectors were 

comparable to amounts from standard rain gages. 

Precipitation data collected as part of this study 

were supplemented with data collected by Friedman 

and others (1992) between 1982 and 1988. They 

collected data at 32 sites in the mountain and desert 

areas of southern California. Four of those sites were 

located in the study area, and one site was located near 

Mount San Jacinto to the south of the study area. Data 

from these sites are discussed as part of this paper 

(fig.1). Most samples collected by Friedman and others 

(1992) were originally analyzed only for deuterium. 

Selected samples collected as part of that study were 

archived and later analyzed as part of this study for 

δ18O. Selected samples also were analyzed for δD and 

compared with the original results to ensure that the 

results were not affected by sample storage or differing 

analytical methods. Friedman and others (1992) 

determined that their sample-collection period was 

long enough to dampen yearly variations in the isotopic 

composition of precipitation and reflect average 

conditions at the sample sites. Similar analyses on data 

collected as part of this study produced the same result. 

Therefore, it is possible to neglect differences resulting 

from different sample-collection periods, and the 

combination of data from these studies provide a 

picture of the seasonal and regional variations in the 

δ18O and δD composition of precipitation in the study 

area.

Almost 700 water samples were collected from 

500 wells between 1994 and 2000 (a small number of 

samples collected from U.S. Geological Survey 

monitoring wells between 1986 and 1994 were 

included as part of this study) and analyzed for their 

oxygen-18 and deuterium composition. Samples from 

domestic and production wells were collected using 

existing pumps, and samples from observation wells 

were collected using portable, positive-displacement 

pumps. At least three sample-volumes were purged 

from wells prior to sample collection. During purging, 

pH and specific conductance were monitored and 

additional water was purged from the well if these 

constituents were not stable after three casing volumes 

were removed. In addition to samples for δ18O and δD, 

samples for major ions, nutrients, and selected trace 

elements also were collected and analyzed. Data are 

available in the U.S. Geological Survey's computerized 

database WATSTORE. 

Of the 500 wells sampled, 178 were installed at 

84 sites by the U.S. Geological Survey (Huff and 

others, 2002). Many of these sites contain three to five 

wells installed in the same borehole and separated by 

low-permeability bentonite grout. These wells provide 

depth-dependent data used to determine ground-water 

level and the chemical and isotopic composition of 

ground water with depth. 
Hydrogeology 5

Analyses of Delta Oxygen-18 and Delta Deuterium

In general, isotopic ratios can be measured more 

precisely than can absolute abundance. The ratio of 

oxygen-18 (18O) to the more abundant isotope, 

oxygen-16 (16O), was measured using the CO2

equilibration technique at 25oC (Epstein and Mayeda, 

1953). The ratio of deuterium (2H) to the more 

common isotope hydrogen (1H) was measured using a 

hydrogen equilibration technique at 30oC (Coplen and 

others, 1991). Both techniques measure activity. In 

high-ionic strength brines, values of activity are more 

positive than values of concentration, and this 

difference is proportional to molalities of the major 

dissolved solids (Sofer and Gat, 1972, 1975). Almost 

all water sampled as part of this study was fresh, and 

isotopic activity is presumed equal to concentration. 

Oxygen and hydrogen isotopic results are reported in 

delta notation (δ) as per mil (‰) differences relative to 

VSMOW (Vienna Standard Mean Ocean Water) 

according to the following:

where Rsample and Rstandard refer to the ratio in the 

sample and the standard, respectively. By convention, 

the value of VSMOW is 0 per mil. Values were 

normalized on scales such that the oxygen and 

hydrogen isotopic values of SLAP (Standard Light 

Antarctic Precipitation) are −55.5 per mil and −428 per 

mil, respectively (Gonfiantini, 1984; Hut, 1987; 

Coplen, 1988 and 1994). The 2-sigma uncertainty of 

oxygen and hydrogen isotopic results is 0.2 per mil and 

2 per mil, respectively (Tyler Coplen, U.S. Geological 

Survey, written commun., 2001).

Acknowledgments

This study was funded cooperatively by the 

Mojave Water Agency of Apple Valley, California, and 

the U.S. Geological Survey (USGS); additional 

funding was provided by the USGS’s Southwestern 

Ground-Water Resources Initiative. The authors thank 

James Ball and James Borchers for assistance with the 

preparation and review of this paper.

HYDROGEOLOGY

The study area and surrounding uplands include 

about 13,000 km2 of the Mojave Desert east of the San 

Gabriel and the San Bernardino Mountains near Los 

Angeles, California. Most of the population is 

concentrated in the upper part of the Mojave River 

basin near Victorville, within commuting distance of 

the Los Angeles metropolitan area. Other population 

centers include Barstow, in the Mojave River basin 

farther downstream from Victorville; and Yucca Valley 

in the Morongo basin. Population in the area has 

increased rapidly since the 1980s—with increased 

population comes increased demand for water, 

increased ground-water pumping, and subsequent 

declines in water levels. 

With the exception of the higher altitudes in the 

San Gabriel and the San Bernardino Mountains, the 

climate of the study area is characterized by low 

precipitation and low humidity. The temperature is hot 

during the summer and cold during the winter. In the 

northern and western parts of the study area most 

precipitation falls during the “winter” rainy season 

(November–March); in the southeastern part of the 

Morongo basin more precipitation falls during the 

summer months (July–September) than during the 

winter rainy season (Freidman and others, 1992). 

Although average annual precipitation in the higher 

altitudes of the mountains may exceed 1 m/yr, in most 

of the study area average annual precipitation is less 

than 150 mm/yr. 

δ

18O or ⋅ ⋅ δD

Rsample Rs dard tan –

Rs dard tan

= --------------------------------------------- × 1000
6 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Mojave River Ground-Water Basin

The Mojave River, the largest stream in the area, 

rises near Cajon Pass—a low-elevation gap between 

the San Bernardino and the San Gabriel Mountains. 

Moist air from the Pacific Ocean can enter the Mojave 

Desert through the pass and produce precipitation 

without passing over the higher altitudes of the San 

Bernardino and the San Gabriel Mountains. With the 

exception of small streams in the San Gabriel and the 

San Bernardino Mountains and short reaches of the 

Mojave River, there are no perennial streams in the 

study area. Prior to ground-water development, the 

Mojave River flowed at a series of narrows near 

Victorville, at Camp Cady, at Afton Canyon, and at a 

other areas where faults cause ground water to 

discharge at land surface, such as near the Helendale or 

the Waterman Faults (Thompson, 1929). The location 

of selected faults is shown in figure 1. Under presentday conditions the Mojave River does not flow 

perennially except at the narrows near Victorville, 

downstream from the Victorville municipal wastewater 

treatment plant (an area known locally as the 

“transition zone”), and near Afton Canyon.

During occasional winter stormflows, the 

Mojave River flows east past Barstow, into the Mojave 

Valley, and if flows are large enough through Afton 

Canyon and out of the study area (Lines, 1996). On the 

basis of streamflow data, the frequency of large flows 

reaching the Mojave Valley and areas farther 

downstream has decreased in recent years (Lines, 

1996). Stamos and others (2001) attributed this 

decrease to increased pumping and a subsequent 

increase in the infiltration of streamflow along the river 

upstream from the Mojave Valley.

The study area contains an unconsolidated 

alluvial aquifer along the Mojave River known as the 

floodplain aquifer. This aquifer is surrounded and 

underlain by unconsolidated alluvial deposits that 

constitute an aquifer known locally as the regional 

aquifer. The regional aquifer includes interconnected 

alluvial basins that drain toward the Mojave River. The 

regional aquifer also includes topographically closed 

basins that drain toward the El Mirage, Harper, Coyote, 

and Troy Lakes (dry). In the geologic past, flow in the 

Mojave River may have reached Harper Lake, 

maintaining surface water in the lake (Meek, 1999). 

Coyote and Troy Lakes are remnants of Lake Mannix, 

which covered much of the Mojave Valley during the 

Pleistocene (Meek, 1999).

The floodplain aquifer along the Mojave River is 

composed of sand and gravel weathered from granitic 

rocks in the San Gabriel and the San Bernardino 

Mountains. This highly permeable aquifer can yield 

large quantities of water to wells. In most areas, the 

floodplain aquifer is less than 80 m thick and less than 

2.5 km wide (Stamos and others, 2001). However, this 

aquifer in the Mojave Valley is more extensive and 

present as much as 10 km from the present-day course 

of the river. Prior to recent population growth most 

ground water pumped in the study area was from the 

floodplain aquifer along the Mojave River. The 

floodplain aquifer is readily recharged by infiltration of 

surface flows from the Mojave River during the winter 

rainy season. Recharge is greater near the mountain 

front where surface flows are more frequent and 

recharge is less farther from the mountain front where 

surface flows are less frequent. Stamos and others 

(2001) estimated that average annual recharge to the 

floodplain aquifer from the Mojave River was about 

56.5 hm3 although the quantity of recharge varies 

greatly from year to year.

The regional aquifer is composed of alluvial fan 

and basin fill deposits (Cox and Hillhouse, 2001). In 

some areas, deposits from the ancestral Mojave River 

are present. Near the front of the San Bernardino and 

the San Gabriel Mountains, alluvial fan and basin fill 

material is composed of material weathered from the 

San Bernardino and the San Gabriel Mountains. In the 

eastern part of the basin, alluvial fan and basin fill 

material is composed of material weathered from 

volcanic and other rocks that compose the desert 

mountains. Lacustrine deposits are present throughout 

the area near present-day playas near Victorville or in 

Mojave Valley, where large lakes were present during 

the Pleistocene (Cox and Hillhouse, 2001). Where 

deposits are coarse-grained and not cemented, they 

may yield large quantities of water to wells. The 

ancestral deposits of the Mojave River are coarser 

grained and more permeable than the alluvium, basinfill, and lacustrine deposits that compose the remainder 

of the regional aquifer. (Hardt, 1971; Stamos and 

others, 2001). 
Delta Oxygen-18 and Delta Deuterium Composition of Precipitation 7

Average annual recharge to the regional aquifer 

was estimated to be about 14.3 hm3 (Stamos and 

others, 2001) and is small in comparison with recharge 

to the floodplain aquifer from infiltration of surface 

flow from the Mojave River (Lines, 1996; Stamos and 

others, 2001). The recharge that does occur to the 

regional aquifer results primarily as infiltration from 

small streams near the flanks of the San Bernardino 

and the San Gabriel Mountains that flow as a result of 

winter stormflows and snowmelt runoff (Hardt, 1971; 

Lines, 1996; Izbicki and others, 1998b, 2002; Stamos 

and others, 20001). Smaller amounts of recharge also 

may occur as infiltration of runoff from streams that 

drain the mountains farther to the east. However, in 

these areas, ground water from the floodplain aquifer 

recharges parts of the regional aquifer. 

As in most of the desert southwest, recharge 

from the direct infiltration of precipitation does not 

typically occur in the study area, and large 

accumulations of chloride and other soluble salts are 

present near the top of the unsaturated zone (Izbicki, 

and others, 1998b; 2000a,b, 2002). Some ground-water 

recharge may occur in arid areas as infiltration from 

surface depressions, intermittent streams, and certain 

playas that lie above the regional water table. Recharge 

also may occur in other areas where surface water 

accumulates for brief periods after storms.

Under predevelopment conditions, the direction 

of ground-water movement in much of the regional 

aquifer was from sources of recharge to the floodplain 

aquifer along the Mojave River that served as a drain 

for the ground-water flow system (Hardt, 1971; Stamos 

and others, 2001). Smaller amounts of discharge 

occurred as ground-water movement toward dry lakes 

where water discharged by evaporation (Stamos and 

others, 2001). Under present-day conditions pumping 

is the largest source of ground-water discharge in the 

study area (Stamos and others, 2001). In 1990, groundwater pumping in the Mojave River ground water basin 

was estimated to be about 296 hm3, but since that time 

pumping has declined to about 203 hm3 as a result of 

the adjudication of the ground water basin (Stamos and 

others, 2001). Despite the reduction in pumping, water 

levels in some areas are declining at rates exceeding

0.3 m/yr (Mendez and Christensen, 1997). In some 

areas, especially near the front of the San Bernardino 

and the San Gabriel Mountains, ground-water pumping 

is limited by the depth to water, which commonly 

approaches 100 m and may exceed 300 m. 

Morongo Ground-Water Basin

The Morongo ground-water basin includes a 

number of small alluvial basins that maintain separate 

ground-water flow systems typically terminating in dry 

lakes scattered throughout the area (Lewis, 1972; 

Mendez and Christensen, 1997). These smaller alluvial 

basins are separated by faults and bedrock outcrops. 

Even the larger intermittent streams in the study area, 

such as Pipes Wash, do not maintain perennial flow. 

Under predevelopment conditions, perennial springs, 

such as Surprise Spring, discharged ground water at 

land surface as a result of barriers to flow, such as 

faults, in alluvial aquifers (Londquist and Martin, 

1991). Perennial springs also were present near the 

margins of some dry lakes where water discharged by 

evaporation. Ground water near many of the dry lakes 

in the Morongo basin (and the Mojave River basin) is 

typically highly saline. Unconsolidated deposits in the 

Morongo basin are similar to those found in the Mojave 

River basin and are weathered from similar materials—

with the exception of deposits weathered from 

limestone found along the eastern edge of the San 

Bernardino Mountains near Lucerne Valley. In some 

areas, alluvial aquifers in the Morongo basin have been 

divided into upper and lower aquifer systems having 

different water levels and distinct hydrologic properties 

(Londquist and Martin, 1991).

DELTA OXYGEN-18 AND DELTA 

DEUTERIUM COMPOSITION OF 

PRECIPITATION

The δ18O and δD composition of 120 samples of 

precipitation collected at nine sites in the study area 

ranged from 0.4 to −15.1 and −16.1 to −109 per mil, 

respectively (fig. 2). To make the scales in figure 2

comparable with other graphs presented in the paper, 

one sample having a positive δ18O value and plotting to 

the extreme right of the meteoric water line is not 

shown in figure 2. The altitudes of, and collection 

periods of data from, the precipitation collection sites 

are given in table 1. Locations of collection sites are 

shown in figure 1. 
8 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

–120

–100

–80

–60

–40

–20

0

–120

–100

–80

–60

–40

–20

0

–16 –12 –8 –4 0

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

Meteoric water line

Winter precipitation

Summer precipitation

–16 –12 –8 –4 0

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

Desert sites

Mountain sites

Near Cajon Pass

WINTER AND SUMMER

PRECIPITATION

WINTER PRECIPITATION

AT SELECTED SITES

Meteoric

water line

Evaporative

trend line

A

B

Figure 2. Delta oxygen-18 as a function of delta deuterium composition of precipitation in the Mojave River and the Morongo ground-water basins, 

southern California.(Data modified from Friedman and others, 1992, and Izbicki and others, 2000b.)
Delta Oxygen-18 and Delta Deuterium Composition of Precipitation 9

In general, summer precipitation was heavier 

(less negative) than winter precipitation and many 

values for summer precipitation plot along an 

evaporative trend line to the right of the meteoric water 

line as a result of partial evaporation before the 

precipitation reached the ground (fig. 2A). This is not 

uncommon in desert areas (International Atomic 

Energy Agency, 1981). The volume-weighted average 

δ18O and δD composition of summer precipitation 

collected at the lower altitude desert sites (AQ, BV, 

DG, SF, SC, and TP; fig. 1, table 1) was −4.8 and −34.2 

per mil, respectively. In contrast, the volume-weighted 

average δ18O and δD composition of summer 

precipitation collected at the higher altitude mountain 

sites (BB and TM and at the site near Mount San 

Jacinto to the south of the study area; fig. 1, table 1) 

was much lighter, −9.9 and −64.8 per mil, respectively. 

Summer precipitation collected at the higher altitude 

mountain sites plotted along the meteoric water line 

and showed few effects of evaporation.

 During the winter rainy season, the volumeweighted δ18O and δD composition of precipitation 

collected at the desert sites was −10.9 and −77.4 per 

mil, respectively, and the volume-weighted δ18O and 

δD composition of precipitation collected at the higher 

altitude mountain sites was −11.5 and −79 per mil, 

respectively. Thus, the difference in the δ18O and δD 

composition of precipitation between the desert and the 

mountain sites was less during the winter rainy season 

(0.6 and 1.6 per mil, respectively) (fig. 2A) than during 

the summer (5.1 and 30.6 per mil, respectively). This 

may occur because most of the precipitation falling at 

the desert sites actually condensed over the high 

mountains as winter storms moved inland from the 

Pacific Ocean (Friedman and others, 1992; Izbicki and 

others, 2002).

Some of the air mass and subsequent 

precipitation associated with winter storms enters the 

western Mojave Desert through the lower altitudes of 

Cajon Pass without uplift over the nearby mountains. 

As a result, precipitation that falls near Cajon Pass at 

the Oak Hill (OH; fig. 1; table 1) site has not condensed 

at the higher altitudes of the nearby mountains and has 

a volume-weighted δ18O and δD composition of −9.1 

and −63 per mil, respectively. These values are heavier 

than winter precipitation elsewhere in the study area. 

The precipitation collection site near Cajon Pass is at 

an altitude of nearly 1,200 m, and the collection sites at 

Table Mountain and Big Bear are at altitudes of 2,280 

and 2,053 m, respectively (fig. 3). The average 

difference in the volume-weighted δD composition of 

precipitation near Cajon Pass and the higher altitude 

mountain sites is about −16 per mil per kilometer in 

altitude. This value is greater in magnitude than the 

value of −10 per mil per kilometer of altitude estimated 

by Williams and Rodoni (1997) for altitude-related 

deuterium depletion in southern California—perhaps 

reflecting the extreme relief near the pass. However, 

this value is within the range of −12 to −40 per mil 

estimated for altitude effects by Gat (1980) and smaller 

in magnitude than altitude effects of −40 per mil per 

kilometer of altitude measured in the Sierra Nevada 

(Freidman and Smith, 1970, 1972). 

Table 1. Precipitation collection sites for the determination of delta oxygen-18 and delta deuterium composition in the western Mojave Desert, southern 

California

[Site locations shown in figure 1 by abbreviation; latitude and longitude in degrees, o; minutes, '; seconds, "; m, meters]

1/ Data from Friedman and others, 1992. All other data from Izbicki and others, 2000b.

Station name Abbreviation Latitude Longitude Altitude

(m) Period of record

Aqueduct AQ 34o21'48" 117o19'04" 1,043 12/12/94 – 10/20/98

Bear Valley BV 34o28'12" 117o21'33" 979 12/12/94 – 10/20/98

Big Bear BB 34o15'48" 116o51'12" 2,053 1/10/17/82 – 04/11/89

Daggett DG 34o51'18" 116o47'12" 585 1/10/17/82 – 04/11/89

Oak Hill OH 34o23'14" 117o24'42" 1,172 12/12/94 – 10/20/98

Santa Fe SF 34o28'05" 117o16'46" 911 12/12/94 – 10/20/98

Sheep Creek SC 34o30'38" 117o34'17" 1,021 12/12/94 – 10/20/98

Table Mountain TM 34o22'30" 117o40'30" 2,280 1/10/17/82 – 04/11/89

Twenty Nine Palms TP 34o07'42" 115o56'42" 570 1/10/17/82 – 04/11/89
10 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

70 60 50 40 30 20 10 0

70 60 50 40 30 20 10 0

DISTANCE, IN MILES

ALTITUDE, IN METERS

North South

San Gorgonio

Pass

Cajon Pass

–100

–90

–80

–70

–60

–50

DELTA DEUTERIUM,

IN PER MIL

Summit Valley San Gorgonio Basin

Floodplain aquifer

(Mojave River)

Regional aquifer

(west of Mojave River) Lucerne Valley

Regional aquifer

(east of Mojave River)

Miscellaneous basins

Pipes Wash/Surprise Spring

Warren Basin

Big Bear

Oak Hill

Mount

San

Jacinto

Table Mountain

Oak Hill

Table

Mountain

Big Bear

Mount

San

Jacinto

San Jacinto

Mountains

San Bernardino

Mountains

San Gabriel

Mountains

Figure 3. Altitude along topographic divide and changes in delta deuterium composition of ground water on the eastern slope of the San Gabriel, the San 

Bernardino, and the San Jacinto Mountains, southern California. (Median delta deuterium composition of ground water and extent along mountain front 

shown as gray bars)
Delta Oxygen-18 and Delta Deuterium Composition of Water from Wells 11

The effect of Cajon Pass on the isotopic 

composition of precipitation decreases rapidly with 

increasing distance from the pass. For example, the 

δ18O and δD composition of winter precipitation at the 

Bear Valley (BV) site 10 km east of the Oak Hill (OH) 

site ranged from −9.0 to −12.7 and −62.8 to −94.7 per 

mil, respectively; with a volume-weighted mean 

composition of −10.9 and −76.8 per mil, respectively. 

These values are more similar to the δ18O and δD 

composition of winter precipitation collected at the 

higher altitude sites in the mountains than to the 

composition of precipitation collected near Cajon Pass. 

As previously discussed, this is because precipitation 

that fell at the Bear Valley site probably condensed as 

winter storms moved over the high mountains. 

Although the effect of Cajon Pass on the δ18O and δD 

composition of precipitation is small in areal extent, the 

relatively large amount of precipitation that falls near 

the pass gives rise to winter stormflows in the Mojave 

River. Infiltration of these stormflows creates a strong 

isotopic signal extending along the course of the river 

throughout the study area into the Mojave Desert. 

DELTA OXYGEN-18 AND DELTA 

DEUTERIUM COMPOSITION OF WATER 

FROM WELLS

The isotopic composition of 685 water samples 

from wells in the Mojave River and the Morongo 

ground-water basins ranged from −3.99 to −13.12 per 

mil for δ18O and from −41.1 to −101 per mil for δD 

(fig. 4). These values are in the range of values for 

winter precipitation and are lighter (more negative) 

than values for summer precipitation (fig. 2). These 

data suggest that summer storms do not contribute 

significantly to ground-water recharge—even in the 

southeastern part of the study area where precipitation 

is greater during the summer months. 

The spatial distribution of δD composition of 

water from wells defines an area of isotopically heavier 

water near the floodplain aquifer along the Mojave 

River and a smaller area of isotopically heavier water in 

the southwestern part of the Morongo basin (fig. 5). 

The heavy δD values along the Mojave River in the 

floodplain aquifer were mapped previously, although in 

less detail, by Freidman and others (1992), Gleason, 

and others (1994), Izbicki and others (1995) and 

Williams and Rodoni (1997). The heavy δD values in 

the southern part of the Morongo basin were not 

previously mapped. Comparison of precipitation data 

and topography shows that isotopically heavy δD 

ground water near the floodplain aquifer along the 

Mojave River and in the southwestern part of the 

Morongo basin is related to the movement of air 

masses through the low-altitude passes in the coastal 

mountain ranges (fig. 3). These low-altitude passes 

allow cool moist air to enter the Mojave Desert and 

precipitate without fractionation caused by uplift over 

the high mountains. 

Comparison of precipitation data (fig.2) with the 

spatial distribution of δD values in water from wells 

(fig. 5) shows that large parts of the study area within 

the Morongo basin and parts of the Mojave River basin 

in the regional aquifer farther from the Mojave River 

and the front of the San Gabriel and the San Bernardino 

Mountains contain ground water lighter than the 

volume-weighted composition of precipitation—and 

lighter than all but a few samples of precipitation 

collected as part of this study. This suggests that much 

of the ground water in the study area was recharged at a 

time when the climate was cooler than under the 

present-day climatic conditions that prevail in the 

higher altitudes of the San Gabriel and the San 

Bernardino Mountains even during the winter months. 

Similar results were obtained by Freidman and others 

(1992) and Izbicki and others (1995) in the Mojave 

Desert and other researchers in the arid southwest 

(Phillips and others, 1989; Zhu and others, 1998). 

Mojave River Ground-Water Basin

The δ18O and δD compositions of water from 

wells in the floodplain and regional aquifers within the 

Mojave River ground-water basin are different because 

of orographic effects related to their different sources 

of recharge. The floodplain aquifer is recharged 

primarily by infiltration of stormflows in the Mojave 

River. The regional aquifer (under present-day climatic 

conditions) is recharged by infiltration of runoff from 

the higher altitudes of the San Gabriel and the San 

Bernardino Mountains, with smaller amounts of 

recharge also occurring as infiltration of runoff from 

mountains in the desert and by infiltration of water 

from intermittent streams near Cajon Pass.
12 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

–120

–100

–80

–60

–40

–20

0

–16 –12 –8 –4 0

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

Meteoric

water line

Mojave River ground-water basin

Morongo ground-water basin

Figure 4. Delta oxygen-18 and delta deuterium composition of water from wells in the Mojave River and the Morongo ground-water basins, southern 

California, 1986–2000. 
Delta Oxygen-18 and Delta Deuterium Composition of Water from Wells 13

–60

–60

–50

–100

–60

Victorville

Barstow

Yucca

Valley

San Gabriel Mts

San Bernardino Mts

Sheep

Creek

Pipes

Wash

Morongo

ground-water

basin

Mojave

River

ground-water

basin

Mojave River

Dry Lake delta Deuterium, in per mil, Dashed where uncertain

Less than –90

–80 to –90

–70 to –80

–60 to –70

Greater than –60

EXPLANATION

Selected major faults

Wells -

Multiple-well site

Sampled well

0 20

0 20

KILOMETERS

MILES

11700' 11600'

34

30'

35

00'

R9W R1W R1E R9E

T

1

N

T

9

N

Figure 5. Delta deuterium composition of water from wells in the Mojave River and the Morongo ground-water basins, southern California, 1986–2000.
14 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Floodplain Aquifer

The δ18O and δD composition of water from 260 

samples from wells in the floodplain aquifer along the 

Mojave River ranged from −4.0 to −10.8 and −41 to

−78 per mil, respectively (fig. 6). The median δ18O and 

δD composition of −8.8 and −62 per mil is similar to 

the volume-weighted isotopic composition of −9.1 and 

−63 per mil measured for precipitation at the Oak Hill 

site near Cajon Pass (fig. 3). Precipitation near Cajon 

Pass that has not been lifted over the higher altitudes in 

the surrounding mountains is isotopically heavier than 

precipitation that falls in the higher altitudes. 

The heaviest δ18O and δD values in the 

floodplain aquifer are in water from shallow wells in 

agricultural areas in Mojave Valley, Hinkley Valley, 

and other agricultural areas throughout the floodplain 

aquifer (fig. 5). These samples have been partly 

evaporated and plot to the right of the meteoric water 

line along an evaporative trend line having a slope of 4 

(fig. 6). Water from shallow wells in discharge areas 

near Camp Cady do not show evaporative effects—

suggesting that ground-water discharge in this area is 

not maintained by irrigation return.

The evaporative trend line shown in figure 6

intersects the meteoric water line at a δ18O and δD 

composition of about −8.8 and −60.5 per mil. This 

value is slightly heavier than the median δ18O and δD 

composition of water from other wells in the floodplain 

aquifer, and slightly heavier than the volume weighted 

δ18O and δD composition of precipitation measured 

near Cajon Pass. Water from wells in the Hinkley and 

the Mojave Valley areas that plot near the meteoric 

water line and do not show evaporative effects also 

have heavier δ18O and δD compositions than does 

water from wells in the floodplain aquifer closer to the 

mountain front and precipitation near Cajon Pass.

The heavier δ18O and δD values with increasing 

distance downstream along the Mojave River may 

reflect the isotopic composition of larger storms, and 

subsequent stormflows, that occasionally recharge the 

floodplain aquifer in the Mojave Valley. As a result of a 

series of El Nino storms during the 1992–93 rainy 

season, the river flowed through Mojave Valley to 

Afton Canyon (Lines, 1996). These were the largest 

flows, and the first significant ground-water recharge, 

since 1983 (Lines, 1996). Water vapor in such storms is 

derived primarily from the warm water of the south 

Pacific and is presumably heavier isotopically than 

most precipitation that falls in the study area (Friedman 

and others, 1992). Samples of Mojave River water 

collected at two sites in the Mojave Valley during the 

1992–93 stormflows had a δD composition of −59 and 

−60 per mil, respectively. These samples were heavier 

than samples from smaller stormflows collected closer 

to the mountain front during the same rainy season that 

had δD compositions ranging from −62 to −67, per mil. 

Additional data need to be collected to determine if 

infrequent recharge along downstream reaches of the 

Mojave River from infiltration as a result of larger 

stormflows has resulted in the heavier isotopic 

composition of ground water in the Mojave Valley. 

The lightest δ18O and δD values in the 

floodplain aquifer were in water from wells at the 

“upper narrows” near Victorville. Ground water from 

the floodplain aquifer discharges at land surface and 

maintains perennial flow in the Mojave River at this 

location. Some ground water discharge from the 

regional aquifer also occurs at this location and may 

explain light δ18O and δD values. In other areas, light 

δ18O and δD values occur in the floodplain aquifer 

because water pumped for public supply from wells in 

the regional aquifer is treated and discharged to the 

floodplain aquifer. Similarly, leakage of water from 

sewage lines observed by Lines (1996) also may 

explain light δ18O and δD values at the “upper 

narrows” near Victorville.

 During the study, imported water from northern 

California was used to recharge the floodplain aquifer 

near Rock Springs, upstream from Victorville. Water 

imported from northern California has a lighter δ18O 

and δD composition than does water in many parts of 

southern California and this difference in composition 

has been used in several studies to trace the movement 

of artificially recharged water (Woolfenden, 1994; 

Izbicki and Martin, 1998; Izbicki and others, 1998a,b). 

Samples collected downgradient from Rock Springs in 

1994, after imported water was released, are lighter 

than most samples from the floodplain aquifer and plot 

slightly to the right of the meteoric water line—

reflecting partial evaporation of imported water during 

storage in reservoirs and transport to southern 

California in open canals. Williams and Rodoni (1997) 

also observed evaporative effects on the isotopic 

composition of imported water transported from 

northern California.
Delta Oxygen-18 and Delta Deuterium Composition of Water from Wells 15

–120

–100

–80

–60

–40

–20

0

–16 –12 –8 –4 0

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

Meteoric

water line

Meteoric

water line

Floodplain aquifer

Shallow wells in Hinkley

and Mojave Valleys

FLOODPLAIN AQUIFER

–120

–100

–80

–60

–40

–20

0

–16 –12 –8 –4 0

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

All other regional

aquifer data

Recharge from the San Bernardino

and the San Gabriel Mountains

REGIONAL AQUIFER

Imported water

Water from Upper Narrows

and some treated municipal

wastewater

Evaporative

trend line

Figure 6. Delta oxygen-18 and delta deuterium composition of water from wells in the floodplain aquifer and the regional aquifer of the Mojave River 

ground-water basin, southern California, 1986–2000.
16 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Regional Aquifer

The δ18O and δD composition of water from 

222 wells in the regional aquifer in the Mojave River 

basin ranged from −8.2 to −13.1 and −57 to −98 per 

mil, with a median composition of −10.5 and −78 per 

mil, respectively (fig. 6). In general, these values are 

lighter than the δ18O and δD composition of water 

from wells in the floodplain aquifer. The δ18O and δD 

composition of water from most wells in the floodplain 

aquifer is within the range of present-day precipitation 

in the higher altitudes of the San Bernardino and the 

San Gabriel Mountains (fig. 3), but many values are 

significantly lighter than the volume-weighted average 

composition of precipitation. 

The lightest δ18O and δD values are from deep 

wells at the downgradient ends of long flow paths 

through the regional aquifer. Water from these wells 

has great age and on the basis of carbon-14 data 

(Izbicki and others, 1995) and helium-4 data 

(Kulongoski and others, 2003) was recharged several 

thousand years before present. The difference between 

the δ18O and δD composition of water from these wells 

and the volume-weighted composition of present-day 

precipitation suggests that the climate was wetter and 

cooler when this old water was recharged. 

Near Cajon Pass to the west of the Mojave River 

(fig. 5), the δ18O and δD composition of water from 

wells in the regional aquifer is similar to the 

composition of water from wells in the floodplain 

aquifer. In this area the regional aquifer is recharged by 

infiltration of water from streams that flow for short 

periods as a result of precipitation near the Cajon Pass 

(Izbicki and others, 1998b, 2000a, 2002) and from the 

movement of water from Summit Valley into the 

regional aquifer (Izbicki and others, 1995; Stamos and 

others, 2001). Although the quantity of water recharged 

from these sources is small relative to recharge from 

the Mojave River and relative to recharge from streams 

that drain the mountains, recharge to the regional 

aquifer near Cajon Pass is locally important (Stamos 

and others, 2001). In addition to these processes, 

isotopically heavy water may have recharged the 

regional aquifer as a result of ground-water movement 

from the floodplain aquifer. These aquifers are 

hydraulically connected and movement of water 

between the two aquifers may have increased in recent 

years as a result of increased pumping in the regional 

aquifer and subsequent water-level declines.

Movement of Water between Aquifers

Movement of water between the floodplain and 

the regional aquifers has been evaluated for this study 

on the basis of changes in the delta deuterium 

composition along a series of sections (A–A', B–B', and 

C–C', fig. 1) in different hydrologic settings. The 

movement of water between aquifers and underlying 

consolidated deposits may increase near faults and has 

been evaluated along two sections near the Helendale 

and the Waterman Faults (Sections D–D' and E–E',

fig. 1). Data from selected observation wells drilled as 

part of this study were used to prepare the study 

sections. Data from nearby production wells having 

long-screened intervals that yield a mixture of water 

from different aquifers are not shown.

Section A–A' 

The floodplain aquifer along section A–A' is 

incised into the deposits that compose the regional 

aquifer (fig. 7). In this area, the regional aquifer is 

divided into two units composed of older alluvium of 

the ancestral Mojave River and undifferentiated 

alluvium, which includes alluvial fan deposits from the 

surrounding mountains, basin-fill deposits, and 

lacustrine deposits (Stamos and others, 2001). 

Although the floodplain and the regional aquifers have 

different hydraulic properties, they are hydraulically 

connected and water can move between the aquifers. 

However, a fault east of the Mojave River restricts 

movement of water in that area (Stamos and others, 

2001). 

On the basis of predevelopment water-level maps 

(Hardt, 1971), water from the regional aquifer 

discharged to the floodplain aquifer along this reach. In 

recent years, pumping has lowered water levels in 

much of the regional aquifer (Mendez and Christensen, 

1997), and at the time of this study (1994–2000) water 

levels were higher in the floodplain aquifer than in 

many wells in the underlying regional aquifer. 

However, water levels in the deepest wells in the 

regional aquifer were higher than water levels in the 

floodplain aquifer because there is very little pumping 

from these depths. 
Delta Oxygen-18 and Delta Deuterium Composition of Water from Wells 17

Qa

QTu

EXPLANATION

QToa

Mojave River alluvium

Water table

Floodplain aquifer

Regional aquifer Well and number—

Undifferentiated alluvium

Older alluvium of ancestral

Mojave River

Igneous, metamorphic,

or volcanic rocks

Fault—

Dashed where projected.

Sequence numbers are

from deepest to shallowest.

White box represents

screened interval.

Dashed where

uncertain.

delta Deuterium, in per mil

Less than –80

–70 to –80

Greater than –70

4N/4W-1C2

–5 Vertical scale greatly exaggerated

WEST

A

1,100

1,000

900

800

700

600

500

400

ALTITUDE, IN METERS ABOVE SEA LEVEL

0 5 MILES

5 KILOMETERS

A'

Regional aquifer

Floodplain

aquifer

0

EAST

5N/6W-22E1

–3

4N/3W-12A1

–3

5N/3W-27E3

–6

4N/4W-1C2

–5

4N/4W-3A2

–5

4N/5W-1M1

QToa QToa

QTu QTu

QTu QTu

QTu

Mojave River

Qa

Figure 7. Delta deuterium composition of water from selected observation wells along section A–A', Mojave River ground-water basin, southern 

California. See figure 1 for location of sections. (Modified from Stamos and others, 2001.)
18 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

The δD composition of water from observation 

wells along section A–A' ranged from −96 to −61 per 

mil. The lighter values were in water from deep 

observation well 22E1 in the regional aquifer almost 20 

km west of the Mojave River. At this site δD values 

decreased from −82 to −96 with increasing depth 

although the water was presumably recharged from the 

same source in the San Gabriel Mountains. Depthdependent samples at this location preserve a history of 

climate change reflected in the isotopic composition of 

the recharge water. Heavier δD values, greater than 

−65 per mil, were present in water from wells in both 

the floodplain and the regional aquifers. As previously 

discussed, this occurs because the regional aquifer in 

this area receives recharge as infiltration from 

intermittent streams near Cajon Pass and as groundwater movement from Summit Valley (Izbicki and 

others, 1995). Movement of water from the floodplain 

aquifer into the regional aquifer has occurred naturally 

upstream from section A–A' and has probably increased 

as a result of ground-water pumping and water-level 

declines in the regional aquifer (Izbicki and others, 

1995). The change in δD values with depth is greater 

where the floodplain aquifer overlies the regional 

aquifer because of their different sources of recharge. 

Section B–B' 

Section B–B' is located downstream from section 

A–A', in an area known locally as the “transition zone.” 

The floodplain aquifer along section B–B' also is 

incised into the undifferentiated alluvial deposits that 

compose the regional aquifer (fig. 8). The aquifers are 

hydraulically connected and there is movement of 

water between the two aquifers along this reach 

(Stamos and others, 2001). However, the permeability 

contrast between the floodplain and the regional 

aquifers in this area is greater than along section A–A'. 

On the basis of predevelopment water-level maps 

(Hardt, 1971), water from the regional aquifer 

discharged to the floodplain aquifer in this area. In 

recent years, the Mojave River has flowed perennially 

along this reach as a result of discharge from a regional 

wastewater-treatment plant about 6 km upstream 

(Lines, 1996; Stamos and others, 2001). This discharge 

is a mixture of ground water pumped for water supply 

from both the floodplain and regional aquifers. Waterlevel measurements throughout most of the transition 

zone (downstream from the narrows at Victorville to 

the Helendale Fault) were higher in the floodplain 

aquifer than in the regional aquifer, as a result of 

increased recharge to the floodplain aquifer from 

infiltration of treated municipal wastewater.

The δD composition of ground water in 

observation wells along section B–B' ranged from 

−94 to −67. There were large differences in the δD 

composition of water from wells in the floodplain and 

the regional aquifers along this section. The contrast 

was especially sharp along the west side of section B–

B' as a result of predevelopment discharge of ground 

water from the regional aquifer to the floodplain 

aquifer. Movement of water from the floodplain aquifer 

into the regional aquifer in response to present-day 

water levels may have altered the δD composition of 

the water-table well (7N/5W-23R3) in the regional 

aquifer to the west of the river. 

The isotopic composition of water in the regional 

aquifer east of the Mojave River along section B–B' is 

similar to that of the floodplain aquifer. The reason for 

this is unclear. Water levels in the regional aquifer in 

this area are higher than in the floodplain aquifer 

indicating that water is moving from the east into the 

floodplain aquifer along this section. It is possible that 

water from the floodplain aquifer entered the regional 

aquifer some distance upgradient and is discharging 

back to the floodplain aquifer along section B–B'. 

Section C–C' 

Section C–C' is farthest downstream in the 

Mojave Valley. The Mojave River is incised in the 

alluvial deposits that compose the floodplain aquifer in 

this area (fig. 8). The regional aquifer in this area is 

composed of the older alluvium of the ancestral 

Mojave River underlain by undifferentiated alluvium 

weathered from the local mountain ranges. 
Delta Oxygen-18 and Delta Deuterium Composition of Water from Wells 19

0

0

2 MILES

2 KILOMETERS

C C'

NORTH SOUTH Mojave River

ALTITUDE, IN METERS ABOVE SEA LEVEL

350

400

500

575

9N/3E-22R4–7

10N/3E-27J1–5

QToa

QTu

QTu

Qa

0 Vertical scale greatly exaggerated

0

2 MILES

2 KILOMETERS

ALTITUDE, IN METERS ABOVE SEA LEVEL

B B'

WEST EAST

600

700

800

850

7N/5W-23R1–3

7N/5W-24R6–8

7N/4W-19Q5–7

QTu

QTu

Qa

Mojave River

QToa

Qa

QTu

EXPLANATION

QToa

Mojave River alluvium

Water table

Floodplain aquifer

Regional aquifer Well and number—

Undifferentiated alluvium

Older alluvium of ancestral

Mojave River

Igneous, metamorphic,

or volcanic rocks

Fault—

Dashed where projected.

Sequence numbers are

from deepest to shallowest.

White box represents

screened interval.

Dashed where

uncertain.

delta Deuterium, in per mil

Less than –80

–70 to –80

Greater than –70

7N/4W-19Q5–7

Figure 8. Delta deuterium composition of water from selected observation wells along sections B–B' and C–C', Mojave River ground-water basin, 

southern California. See figure 7 for explanation and figure 1 for location of sections. (Modified from Stamos and others, 2001.)
20 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Predevelopment water-level contours show that 

ground water discharged to the Mojave River 

downstream from section C–C' and supported riparian 

habitat near Camp Cady. As a result of ground-water 

pumping since the 1950s, water levels in the Mojave 

Valley have declined. Although, water levels in the 

regional aquifer remain higher than water levels in the 

floodplain aquifer they are below the bed of the Mojave 

River along section C–C'. Surface discharge 

downstream near Camp Cady has ceased and riparian 

growth in this area has declined in recent years (Lines 

and Billhorn, 1996).

The δD composition of water from observation 

wells along section C–C' in the Mojave Valley ranged 

from −77 to −58 per mil (fig. 5). The heavier values are 

near the water table and are affected by partial 

evaporation of irrigation return. The lightest values are 

in water from wells completed in volcanic rocks 

beneath the regional aquifer. The isotopic composition 

of water from these wells plot to the right of the 

meteoric water line. The isotopic composition of this 

water is not as light as water from wells in the regional 

aquifer closer to recharge areas along the San Gabriel 

and the San Bernardino Mountains, possibly as a result 

of partial evaporation prior to recharge. This is 

consistent with recharge as infiltration of runoff from 

the surrounding desert mountains. 

In contrast to data collected along section B–B', 

the δD compositions of water from wells in the 

floodplain and the regional aquifers along section C–C'

are similar, and both aquifers are recharged by 

infiltration of surface flows in the Mojave River. 

Infiltration from the Mojave River near the head of the 

Mojave Valley, combined with scant local recharge, 

allowed water movement from the floodplain aquifer 

into the surrounding and underlying regional aquifer 

even under predevelopment conditions. 

Sections D–D' 

Section D–D' is located near the Helendale 

Faults. The Helendale Fault is a barriers to groundwater flow, and ground-water discharge near the faults 

maintained high water levels and phreatophyte growth 

on the upgradient side of the fault under 

predevelopment conditions (Thompson, 1929). Three 

multiple-level monitoring sites were installed along 

section D–D', upgradient and downgradient from the 

Helendale Fault, to evaluate the effect of the fault on 

ground-water movement (fig. 9). The deposits that 

compose the regional aquifer along this section are thin 

and volcanic rocks underlie the floodplain aquifer 

along much of this section.

The δD compositions of water from wells in the 

floodplain aquifer along section D–D' ranged from 

−60 to −67 per mil and are typical of water recharged 

as infiltration from the Mojave River. The heavier 

values plot slightly to the right of the meteoric water 

line as a result of evaporation. Water-level data show a 

downward hydraulic gradient along this entire 

section—possibly resulting from the discharge of 

municipal wastewater farther upstream or from local 

pumping. The water-level data indicate that water 

should be moving from the floodplain aquifer into the 

underlying deposits; however, a record of the 

predevelopment movement of water is preserved by the 

isotopic composition of water from wells along this 

section.

Water in deeper wells completed in volcanic 

rocks underlying the floodplain aquifer upgradient 

from the Helendale Fault is lighter than water in the 

floodplain aquifer and has δ18O values near −10.5 per 

mil and δD values near −85 per mil. On the basis of its 

isotopic composition, this water was not recharged by 

infiltration of stormflows from the Mojave River. The 

isotopic composition of water from these wells plot 

slightly to the right of the meteoric water line, and the 

water is slightly saline with dissolved-solids 

concentrations as high as 3,190 mg/L. Downgradient 

from the Helendale Fault, water in the regional aquifer 

and underlying rocks has δ18O and δD values similar to 

values in water from wells in the floodplain aquifer. 

Dissolved-solids concentrations are as low as 400 mg/L 

in the deepest well at the downgradient site. These data 

are consistent with movement of water from the 

floodplain aquifer into the regional aquifer and into the 

underlying rocks downgradient from the Helendale 

Fault. 
Delta Oxygen-18 and Delta Deuterium Composition of Water from Wells 21

8N/4W-29E3–6

8N/4W-20Q7–11

8N/4W-21M1–4

Qa

D D'

SW NE

QTu

QTu

ALTITUDE, IN METERS ABOVE SEA LEVEL

575

600

700

750

0

0

2 MILES

2 KILOMETERS Helendale Fault

FAULT B

FAULT C

FAULT A

FAULT E

FAULT D

? ? ??

?

WATERMAN FAULT 400

500

600

650

ALTITUDE, IN METERS ABOVE SEA LEVEL

WEST

5 MILES

0 5 KILOMETERS

0 Vertical scale

greatly exaggerated

QTu

QToa

Qa

Qa

9N/1W-10J12–15

9N/1W-11K12–15

9N/1W-12N4–7

9N/1E-16F1–4

9N/1E-10Q2–4

9N/1E-10R1

Bend in section

EAST

E E'

Qa

QTu

EXPLANATION

QToa

Mojave River alluvium

Water table

Floodplain aquifer

Regional aquifer Well and number—

Undifferentiated alluvium

Older alluvium of ancestral

Mojave River

Igneous, metamorphic,

or volcanic rocks

Fault—

Dashed where projected.

Sequence numbers are

from deepest to shallowest.

White box represents

screened interval.

Dashed where

uncertain.

delta Deuterium, in per mil

Less than –80

–70 to –80

Greater than –70

9N/1E-16F1–4

Figure 9. Delta deuterium composition of water from selected observation wells along sections D−D' and E−E' near the Helendale and the Waterman 

Faults, respectively, Mojave River ground-water basin, southern California. See figure 7 for explanation and figure 1 for location of sections. (Section D−D' 

modified from Stamos and others, 2001 and section E−E' modified from Densmore and others, 1997.)
22 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Section E–E' 

Section E–E' is near the Waterman Fault near the 

head of the Mojave Valley. The Waterman Fault has a 

number of traces, identified as faults A through E in 

figure 9 and is a barrier to ground-water movement in 

both the regional and the floodplain aquifers 

(Densmore and others, 1997; Stamos and others, 2001). 

Similar to the Helendale Fault, ground water forced to 

the surface on the upgradient side of the Waterman 

Fault maintained riparian vegetation under 

predevelopment conditions (Thompson, 1929). There 

are large changes in the water-table altitude across the 

fault, and surface flow occurred for an extended period 

in this area after the 1992–93 El Nino stormflows as a 

result of ground-water discharge near the fault. 

Infiltration of treated wastewater discharged by the city 

of Barstow and nearby military facilities also helps 

maintain high water levels along this section. Seven 

multiple-level monitoring sites were installed as part of 

this study; five of these wells are located along section 

E–E', upgradient and downgradient from the Waterman 

Fault and near the head of the Mojave Valley to 

evaluate the effect of the fault on ground-water 

movement. In general, the hydraulic head in these wells 

increase with depth on the upgradient side of the fault 

and decreases with depth on the downgradient side of 

the fault.

The δD composition of water from wells in the 

floodplain aquifer along section E–E' ranged from

−55 to −62 per mil and is typical of water recharged by 

infiltration from the Mojave River. The heavier values 

plot to the right of the meteoric water line are 

characteristic of water that has been affected by 

irrigation return or partial evaporation of treated 

municipal wastewater discharged along this reach of 

the river. The δD composition of water from deep wells 

in the regional aquifer upgradient from the fault ranged 

from −59 to −91 per mil. The isotopic composition of 

water from many of these wells plot to the right of the 

meteoric water line and may have been partly 

evaporated prior to recharge. The δD compositions of 

water from wells in the regional aquifer downgradient 

from fault E ranged from −58 to −61 per mil and are 

consistent with values in the floodplain aquifer. These 

data are consistent with water movement from the 

floodplain aquifer into the regional aquifer 

downgradient from the fault. 

Morongo Ground-Water Basin 

The δ18O and δD composition of water from 203 

samples of water from wells in the Morongo groundwater basin ranged from −7.8 to −12.8 and −68 to −101 

per mil, respectively (fig. 4). These values are within 

the range of δ18O and δD values for water from wells 

in the regional aquifer in the Mojave River basin—

although many values plot farther to the right of the 

meteoric water line and as a group have been affected 

to a greater extent by evaporation prior to recharge. As 

previously discussed, water in the southwestern part of 

the Morongo basin is isotopically heavier than water in 

other parts of the Morongo basin because of its 

proximity to San Gorgornio Pass which is south of the 

study area. However, nowhere in the Morongo basin 

are δ18O and δD values as heavy as in water from wells 

in the floodplain aquifer along the Mojave River. For 

this discussion, the Morongo basin is subdivided into 

the Warren Basin, the Joshua Tree area, Lucerne 

Valley, and the Pipes Wash/Surprise Spring area (fig. 

1).

Warren Basin. 

The δ18O and δD values in 61 water samples 

from wells in the Warren Basin ranged from −9.1 

to −11.2 and −68 to −80 per mil, respectively. The 

Warren Basin is at the southeastern end of the Morongo 

basin and is close to San Gorgornio Pass south of the 

study area. The altitude of the mountains is lower in 

this area and cool moist air from the Pacific Ocean can 

enter the Mojave Desert through the pass without uplift 

over the high mountains. Although the effect is not as 

large as in the Mojave River basin near Cajon Pass, the 

δ18O and δD composition of water from wells in the 

Warren Basin is heavier than in water from most wells 

in the Morongo basin (fig. 10).
Delta Oxygen-18 and Delta Deuterium Composition of Water from Wells 23

–120

–100

–80

–60

–40

–20

0

–16 –12 –8 –4 0

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

All other Morongo

ground-water basin data

Warren Basin data

Imported water

–120

–100

–80

–60

–40

–20

0

–16 –12 –8 –4 0

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

All other Morongo

ground-water basin data

Lucerne Valley

North side

LUCERNE VALLEY

–120

–100

–80

–60

–40

–20

0

–16 –12 –8 –4 0

DELTA OXYGEN-18, IN PER MIL

All other Morongo

ground-water basin data

Pipes Wash/Surprise Spring

Deadman and Mainside

PIPES WASH/

SURPRISE SPRING

–120

–100

–80

–60

–40

–20

0

–16 –12 –8 –4 0

DELTA OXYGEN-18, IN PER MIL

All other Morongo

ground-water basin data

Joshua Tree data

WARREN BASIN JOSHUA TREE DELTA DEUTERIUM, IN PER MIL DELTA DEUTERIUM, IN PER MIL

Meteoric

water line

Evaporative

trend line

Meteoric

water line

Evaporative

trend line

Evaporative

trend line

Meteoric

water line

Meteoric

water line

Deep

observation

wells

Figure 10. Delta oxygen-18 and delta deuterium composition of water from wells in the Warren Basin, Joshua Tree, Lucerne Valley and Pipes 

Wash/Surprise Spring areas of the Morongo ground-water basin, southern California, 1989–2000.
24 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

The Warren Basin includes the community of 

Yucca Valley and is urbanized and water levels were 

declining as a result of pumping. To mitigate waterlevel declines, beginning in 1995 water was imported 

from northern California into the Warren Basin. As a 

result of the importation of water, water levels in wells 

near the recharge ponds have risen as much as 60 m 

and imported water is present in formerly dry 

observation wells completed above the pre-recharge 

water table. Ground-water samples affected by 

imported recharge water plot to the right of the 

meteoric water line and may have been partly 

evaporated in reservoirs and aqueducts during transport 

to southern California or in surface ponds prior to 

ground-water recharge. Unlike in the floodplain aquifer 

of the Mojave River basin, the δ18O and δD 

composition of imported water recharged in the Warren 

Basin is not greatly different from the composition of 

native water and imported water is difficult to identify 

solely on the basis of its δ18O and δD composition.

Joshua Tree Area 

The δ18O and δD values in 17 water samples 

from wells in the Joshua Tree area ranged from −10.8 

to −12.3 and −76 to −90 per mil, respectively (fig. 10). 

More precipitation falls in this area during the summer 

months than during the winter months. However, the 

δ18O and δD composition of water from most sampled 

wells is similar to the composition of winter 

precipitation and similar to the composition of water 

from wells in the adjacent Warren Basin—suggesting 

that little recharge results from infiltration of summer 

precipitation or subsequent runoff. Few samples in the 

Joshua Tree area plot to the right of the meteoric water 

line suggesting that there is little evaporation prior to 

recharge, and that most water was recharged as winter 

runoff from the nearby mountains in the same manner 

as runoff from the San Bernardino and the San Gabriel 

Mountains in the Mojave River basin.

Water from deep observation wells completed in 

partly consolidated rocks that underlie the Joshua Tree 

area is isotopically lighter than water from wells in 

other parts of the basin. These data show that water 

from these deep wells was recharged under wetter and 

cooler climatic conditions than was water from most 

wells in the Joshua Tree area.

Lucerne Valley 

The δ18O and δD values in 38 water samples 

from wells in Lucerne Valley ranged from −9.4 

to −12.8 and −82 to −101 per mil, respectively (fig. 10). 

These values were lighter than values from water from 

wells in the Warren Basin and Joshua Tree areas, and 

lighter than water from most other wells in the 

Morongo basin. The altitude of the San Bernardino 

Mountains is higher near Lucerne Valley than that of 

mountains in the Warren Basin and Joshua Tree areas.

Under predevelopment conditions the direction 

of ground-water movement in Lucerne Valley was from 

the margins of the basin toward discharge areas near 

the dry lake at the center of the basin. Despite pumping 

and subsequent drawdown. the areal distribution of 

δ18O and δD values in ground water in Lucerne Valley 

reflects contributions of predevelopment sources of 

recharge. Water from wells near the front of the San 

Bernardino Mountains plots near, or slightly to the 

right of, the meteoric water line. However, in the 

northern part of the valley, farther from the San 

Bernardino Mountains, the δ18O and δD composition 

of water from wells plots to the right of the meteoric 

water line along a line having a slope of 2.8. This line 

intersects the meteoric water line at a δ18O and δD 

composition of −12.8 and −112 per mil, respectively. 

These values are lighter than present-day precipitation 

or water from wells sampled elsewhere in the Mojave 

or Morongo basins and probably were recharged as 

infiltration from streams draining the mountains in the 

Mojave Desert to the north. Infiltration and recharge 

from these sources probably does not occur under 

present-day climatic conditions.
Discussion 25

Pipes Wash/Surprise Spring Area. 

The δ18O and δD values in 82 water samples 

from wells in the Pipes Wash/Surprise Spring area 

ranged from −7.8 to −12.6 and −73 to −96 per mil, 

respectively (fig. 10). The ground-water flow system 

associated with Pipes Wash extends from recharge 

areas along Pipes Wash near the San Bernardino 

Mountains through the Surprise Spring Basin to 

evaporative-discharge areas along the margins of 

Mesquite Lake (dry) (fig. 1). Ground-water movement 

is through several alluvial aquifers separated by faults. 

Water-level changes across some faults are as great 

100 m. Under predevelopment conditions, ground 

water discharged at land surface along the upgradient 

side of some of these faults, maintaining perennial 

discharge in springs (Londquist and Martin, 1991). 

Water levels declined as a result of ground water 

pumping and many springs no longer discharge at land 

surface (Londquist and Martin, 1991).

Water from wells near the recharge areas along 

Pipes Wash is isotopically heavy and similar in δ18O 

and δD composition to water from wells in the Warren 

Basin and Joshua Tree areas (fig. 10). Water becomes 

isotopically lighter and gradually shifts off the meteoric 

water line with increasing distance downgradient as 

water flows through the Surprise Springs Basin toward 

Mesquite Lake. This may result from either mixing 

with increasing proportions of locally derived recharge 

water or from a gradual warming and drying of the 

climate during last several thousand years. Water from 

deep observation wells underlying the Surprise Springs 

Basin and from wells to the north and south of faults 

within Surprise Springs Basin is isotopically very light. 

Water from these wells was recharged as infiltration of 

runoff from local mountains and is similar in isotopic 

composition to water from the north side of Lucerne 

Valley. To the east of the Surprise Springs Fault in the 

Deadman Lake area, water from wells also is light and 

has a composition similar to that of deep wells in the 

Surprise Springs Basin. The isotopic composition of 

water east of the Surprise Springs Fault becomes 

heavier (less negative) as water flows through the 

ground-water system and shifts to the right of the 

meteoric water line along a line having a slope similar 

to that observed in Lucerne Valley. The heaviest 

samples along this line are from wells near Mesquite 

Lake farthest downgradient. Water from these wells is 

saline and has dissolved-solids concentrations as high 

as 18,400 mg/L.

DISCUSSION

Alluvial aquifers underlying the Mojave River 

and the Morongo ground-water basins are recharged 

from several different sources. Altitude and evaporative 

effects create differences in the δ18O and δD 

composition of water recharged from these sources, 

which allows interpretation of the source of water to 

wells and interpretation of the movement of water 

between aquifers. In many areas the movement of 

water between aquifers increases near faults, and in 

some areas the direction of movement between aquifers 

has changed as a result of ground-water pumping and 

other activities.

The largest differences in the δ18O and δD 

compositions of water from wells in the study area 

result from altitude effects. Cool, moist air from the 

Pacific Ocean enters the Mojave Desert near low areas 

in the coastal mountain ranges and precipitates without 

significant fractionation from cooling associated with 

orographic uplift over the higher mountain peaks. The 

Mojave River exists because of the winter precipitation 

that enters the desert near Cajon Pass. This 

precipitation gives rise to surface flows that are the 

primary source of recharge to the floodplain aquifer 

along the course of the river. This water is as much as 

20 per mil heavier (less negative) than water recharged 

by infiltration of runoff from the higher mountain 

peaks. The amount of water recharged as infiltration of 

stormflow in the Mojave River under present-day 

climatic conditions is greater than the amount 

recharged by infiltration of streamflow from the high 

mountains to the south of the study area.

Altitude effects also control the isotopic 

composition of precipitation, but to a lesser extent, in 

the southwestern part of the Morongo basin near San 

Gorgornio Pass south of the study area and have 

resulted in heavier δ18O and δD ground water in that 

area. In the western part of Antelope Valley to the west 

of the study area, the altitudes of the San Gabriel 

Mountains are lower than the altitudes of the mountains 

to the south of the study area. Delta deuterium data in 

water from wells in the western part of the Antelope 

Valley are heavier (Smith, and others, 1992; Gleason 

and others, 1994) and suggest that winter storms enter 

the Mojave Desert in that area with less fractionation of 

condensing water vapor.
26 Source and Movement of Ground Water in the Western Part of the Mojave Desert, Southern California, USA

Throughout most of the southwestern United 

States, the largest amounts of precipitation, subsequent 

runoff, and ground-water recharge are associated with 

the higher altitudes mountains. The large amount of 

recharge derived from air masses that enter the western 

Mojave Desert from low-altitude passes in the coastal 

mountain ranges results from the proximity of the 

western Mojave Desert to the Pacific Ocean and to the 

increased importance of winter cyclonic precipitation 

as opposed to summer convective precipitation. Local 

topography also plays a roll in that much of the 

precipitation that falls in the higher altitudes of the San 

Bernardino and the San Gabriel Mountains runs off to 

the west into streams that drain to the Pacific Ocean 

rather than into the Mojave Desert.

The areal and cross-sectional distribution of δD 

in ground water in the Mojave River basin preserve a 

record of the predevelopment distribution of recharge 

from different sources. In the upstream parts of the 

Mojave River basin, recharge to the regional aquifer 

occurs as infiltration of water from streams that drain 

the San Gabriel and the San Bernardino Mountains. 

The Mojave River acts as a drain for the regional 

aquifer in these areas and the heavier δD values are 

largely restricted to the floodplain aquifer along the 

river—although some movement of water from the 

floodplain aquifer into the regional aquifer occurs near 

faults. In contrast, farther from the San Gabriel and the 

San Bernardino Mountains recharge as infiltration from 

intermittent streams that drain local mountains is scant 

under present-day conditions and infiltration of 

streamflow from the Mojave River recharges both the 

floodplain and the surrounding regional aquifers.

Evaporative effects shift the δ18O and δD 

composition of water to the right of the meteoric water 

line and are evident in water-table wells underlying 

agricultural areas in the Mojave Valley and in areas 

such as Warren Basin where imported water 

transported to southern California through aqueducts 

has been used to artificially recharge aquifers. 

Evaporative effects also are present where water-level 

data suggest that ground water was recharged as 

infiltration of runoff from desert mountains rather than 

as infiltration of runoff from the San Bernardino and 

the San Gabriel Mountains. In these areas the δ18O and 

δD composition of precipitation (estimated by 

projecting the data back to the meteoric water line 

along an evaporative trend line) is significantly lighter 

than the volume-weighted δ18O and δD composition of 

present-day precipitation. This suggests that either 

recharge in these areas is highly episodic and occurs 

only infrequently during unusually cold, wet winters or 

that this water was recharged at a time when the 

climate of the western Mojave Desert was different 

from the present-day climate

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