U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Geologic Setting, Geohydrology, and Ground-Water

Quality near the Helendale Fault in the Mojave River

Basin, San Bernardino County, California

Water-Resources Investigations Report 03-4069

Prepared in cooperation with the

MOJAVE WATER AGENCY
Geologic Setting, Geohydrology and GroundWater Quality near the Helendale Fault in the 

Mojave River Basin, San Bernardino County, 

California

By Christina L. Stamos, Brett F. Cox, John A. Izbicki, and Gregory O. Mendez

U.S. GEOLOGICAL SURVEY

Prepared in cooperation with the

MOJAVE WATER AGENCY

Sacramento, California

2003

Water-Resources Investigations Report 03-4069 7208-24
For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall, Suite 2012

6000 J Street

Sacramento, California 95819-6129

http://ca.water.usgs.gov

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.

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director
Contents iii

CONTENTS

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

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

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

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

Data-Collection Techniques........................................................................................................................ 5

Multiple-Well Monitoring Sites......................................................................................................... 5

Water-Level Data............................................................................................................................... 10

Water-Quality Data ............................................................................................................................ 10

Geologic Setting................................................................................................................................................... 11

Alluvial Topography of the Mojave River Valley ...................................................................................... 11

Stratigraphy ................................................................................................................................................. 11

Igneous and Metamorphic Basement Rocks (pTb)............................................................................ 11

Older Fan and Stream Deposits (QTof) ............................................................................................. 14

Playa Deposits (QTp)......................................................................................................................... 15

Older Alluvium of the Ancestral Mojave River (Qoa) ...................................................................... 15

Younger Fan Deposits (Qyf).............................................................................................................. 16

Alluvium of the Mojave River (Qya and Qra)................................................................................... 16

Geologic Structure ...................................................................................................................................... 17

Helendale Fault .................................................................................................................................. 17

Other Faults........................................................................................................................................ 19

Folds................................................................................................................................................... 19

Geologic History ......................................................................................................................................... 20

Geohydrology....................................................................................................................................................... 21

Floodplain Aquifer ...................................................................................................................................... 21

Regional Aquifer......................................................................................................................................... 22

Ground-Water Movement near the Helendale Fault................................................................................... 23

Aquifer Tests............................................................................................................................................... 28

Ground-Water Quality.......................................................................................................................................... 32

Chemical Composition of Water from Wells.............................................................................................. 32

Isotopic Composition of Water from Wells ................................................................................................ 34

Oxygen-18 and Deuterium................................................................................................................. 34

Tritium and Carbon-14....................................................................................................................... 37

Summary .............................................................................................................................................................. 40

References Cited .................................................................................................................................................. 41
iv Figures

FIGURES

Figure 1. Map showing location of study area .................................................................................................. 3

Figure 2. Map showing surface geology, line of cross sections A–A′ and B–B′, and location 

of the multiple-well monitoring sites near Helendale, San Bernardino County, California.............. 8

Figure 3. Geologic cross sections near Helendale, San Bernardino County, California, (A) cross section 

A–A′, and (B) cross section B–B′...................................................................................................... 12

Figure 4. Map showing water-level data and water table near Helendale, San Bernardino County, 

California, 1995. ................................................................................................................................ 24

Figure 5. Graphs showing water levels in wells at (A) multiple-well monitoring sites (1993–2001), 

and (B) selected existing wells, (1990–2001), near Helendale, San Bernardino County, 

California. .......................................................................................................................................... 26

Figure 6. Graph showing changes in water levels at observation wells 8N/4W-20Q7–11, June 6, 1995, 

located 63 feet from pumping well 8N/4W-20Q12 near Helendale, San Bernardino County, 

California. .......................................................................................................................................... 30

Figure 7. Graphs showing concentrations of selected chemical constituents and chemical characteristics 

of water in the floodplain aquifer, regional aquifer, and underlying basement rocks near 

Helendale, San Bernardino County, California, 1990–2000. ............................................................ 33

Figure 8. Graph showing delta oxygen-18 and delta deuterium from wells in the floodplain aquifer, 

regional aquifer, and underlying basement rocks near Helendale, San Bernardino County, 

California, 1994–97. .......................................................................................................................... 35

Figure 9. Graph showing carbon-14 activity of water from wells in the floodplain aquifer, regional 

aquifer, and underlying basement rocks near Helendale, San Bernardino County, 

California, 1994–97. .......................................................................................................................... 39
Tables v

TABLES

Table 1. Well-construction information and type of data collected for selected wells near

Helendale, San Bernardino County, California, 1990–99 ................................................................. 6

Table 2. Well-construction information for multiple-well monitoring sites near 

Helendale, San Bernardino County, California ................................................................................. 10

Table 3. Hydraulic properties of water-bearing materials estimated from aquifer tests 

near Helendale, San Bernardino County, California ......................................................................... 29
vi Conversion Factors and Vertical Datum

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

CONVERSION FACTORS 

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

oC=(oF-32)/1.8.

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. 

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

micrograms per liter (μg/L). 

ABBREVIATIONS

δD delta deuterium

δ18O delta oxygen-18

14C carbon-14

L liter

MCL Maximum Containment Level

min minute

per mil parts per thousand

pmc percent modern carbon

PVC polyvinyl chloride

SMCL Secondary Maximum Containment Level

TU tritium units

Multiply By To obtain

acre-foot (acre-ft) 1,233 cubic meter 

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year

foot (ft) 0.3048 meter

foot per day (ft/d) 0.3048 meter per day

square foot (ft2) 0.09290 square meter

square foot per day (ft2/d) 0.09290 square meter per day

gallons per minute (gal/min) 0.06309 liters per second

inch (in.) 2.54 centimeter

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer 
Conversion Factors and Vertical Datum vii

Organizations

MWA Mojave Water Agency

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VSMOW Vienna Standard Mean Ocean Water

Stratographic Units

Qoa older alluvium of the ancestral Mojave River (Pleistocene)

Qra recent alluvium of the Mojave River (Holocene)

QTof older fan and stream deposits (Pliocene and Pleistocene)

QTp playa deposits (Pliocene or early Pleistocene)

Qya younger alluvium of the Mojave River (Pleistocene and Holocene)

Qyf younger fan deposits (PLeistocene and Holocene)
viii Well-Numbering System

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 008N004W20Q007S. In 

this report, well numbers are abbreviated and written 8N/4W-20Q7. Wells in the same township and range are 

referred to only by their section designation, 20Q7. The following diagram shows how the number for well 

8N/4W-20Q7 is derived.

R5W R4W R3W R2W R1W

T9N

T8N

T7N

T6N

T5N

San Bernardino Meridian

R4W RANGE

TOWNSHIP

T

8

N

D C B A

E F G H

M L K J

N P R

SECTION 20

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

8N/4W-20Q7

Well-numbering diagram (Note: maps in this report use abbreviated well numbers such as "20Q7")
Abstract 1

Geologic Setting, Geohydrology, and Ground-Water 

Quality near the Helendale Fault in the Mojave River 

Basin, San Bernardino County, California

By Christina L. Stamos, Brett F. Cox, John A. Izbicki, and Gregory O. Mendez

ABSTRACT

The proximity of the Mojave River groundwater basin to the highly urbanized Los Angeles 

region has resulted in rapid population growth and, 

consequently, an increase in the demand for water. 

The Mojave River, the primary source of surface 

water for the region, normally is dry—except for 

periods of flow after intense storms; therefore, the 

region relies almost entirely on ground water to 

meet its agricultural and municipal needs. The area 

where the Helendale Fault intersects the Mojave 

River is of particular hydrogeologic interest 

because of its importance as a boundary between 

two water-management subareas of the Mojave 

Water Agency. The fault is the boundary between 

the upper Mojave River Basin (Oeste, Alto, and 

Este subareas) and the lower Mojave River Basin 

(Centro and Baja subareas); specifically, the fault 

is the boundary between the Alto and the Centro 

subareas. To obtain the information necessary to 

help better understand the hydrogeology of the 

area near the fault, multiple-well monitoring sites 

were installed, the surface geology was mapped in 

detail, and water-level and water-quality data were 

collected from wells in the study area.

Detailed surficial geologic maps and waterlevel measurements indicate that the Helendale 

Fault impedes the flow of ground water in the 

deeper regional aquifer, but not in the overlying 

floodplain aquifer. Other faults mapped in the area 

impede the flow of ground water in both aquifers. 

Evidence of flowing water in the Mojave River 

upgradient of the Helendale Fault exists in the 

historical record, suggesting an upward gradient of 

ground-water flow. However, water-level data 

from this study indicate that pumping upstream of 

the Helendale Fault has reversed the vertical 

gradient of ground-water flow since 

predevelopment conditions, and the potential now 

exists for water to flow downward from the 

floodplain aquifer to the regional aquifer.

Sixty-seven ground-water samples were 

analyzed for major ions, nutrients, and stable 

isotopes of oxygen and hydrogen from 34 wells 

within the study area between May 1990 and 

November 1999. Dissolved-solids concentrations 

in water samples from 14 wells in the floodplain 

aquifer ranged from 339 to 2,330 milligrams per 

liter (mg/L) with a median concentration of 

825 mg/L. Concentrations in water from 11 of 

these wells exceeded the U.S. Environmental 

Protection Agency (USEPA) Secondary Maximum 

Contaminant Level (SMCL) of 500 mg/L. 

Dissolved-solids concentrations of water from 

nine wells sampled in the regional aquifer ranged 

from 479 to 946 mg/L with a median 

concentration of 666 mg/L. Concentrations in at 

least one sample of water from each of the wells in 

the regional aquifer exceeded the USEPA SMCL 

for dissolved solids. Arsenic concentrations in 

water from 14 wells in the floodplain aquifer 

ranged from less than the detection limit of 

2 micrograms per liter (μg/L) to a maximum of 

34 μg/L with a median concentration of 6 μg/L. 

Concentrations in water from six of the 14 wells 

exceeded the USEPA Maximum Contaminant 

Level (MCL) for arsenic of 10 μg/L. Arsenic 
2 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

concentrations in water from nine wells in the 

regional aquifer ranged from less than the 

detection limit of 2 to 130 μg/L with a median 

concentration of 11 μg/L. Concentrations in water 

from five of these nine wells exceeded the USEPA 

MCL for arsenic. Dissolved-solids concentrations 

in water from seven wells completed in the 

igneous and metamorphic basement rocks that 

underlie the floodplain and regional aquifers 

ranged from 400 to 3,190 mg/L with a median 

concentration of 1,410 mg/L. Concentrations in 

water from all but one of the seven wells sampled 

exceeded the USEPA SMCL for dissolved solids. 

Concentrations in water from the basement rocks 

exceeded the USEPA SMCL for arsenic of 10 

μg/L in five of the seven wells. The high 

concentrations of arsenic, dissolved solids, and 

other constituents probably occur naturally.

Stable isotopes of oxygen and hydrogen 

indicate that before pumping began in the early 

1900s, the floodplain aquifer near Helendale was 

recharged by infiltration of winter stormflows 

originating in the headwaters of the Mojave River, 

and the regional aquifer was recharged by the 

infiltration of surface flow from small streams near 

the front of the San Gabriel and San Bernardino 

Mountains and (or) by infiltration of runoff from 

local mountains within the southern Mojave 

Desert. 

Tritium, the radioactive isotope of 

hydrogen, was present in water from every well 

sampled in the floodplain aquifer indicating that 

the water was recharged after 1952. Water from 

most wells in the regional aquifer and the 

underlying basement rocks did not contain tritium, 

which indicates that it was recharged prior to 

1952. Carbon-14, a naturally occurring isotope of 

carbon, was used to estimate the approximate age 

of the water from wells in the regional and 

underlying basement rocks. In general, 

uncorrected carbon-14 activities increase with 

depth and are fairly young near and downgradient 

from the Helendale Fault. The uncorrected carbon14 activities indicate that the water in the regional 

aquifer was recharged between about 6,980 and 

17,500 years ago and that the water in the 

basement rocks was recharged between 8,000 to 

10,800 years ago. The great age of the water in the 

regional aquifer reflects the long flow paths and 

times of travel from the recharge areas near the 

northern front of the San Gabriel and San 

Bernardino Mountains. It is also possible that a 

small amount of this recharge may have been from 

infiltration of runoff from streams draining local 

mountains within and surrounding the study area 

during past climatic and hydrologic conditions that 

were different than those that exist in the presentday Mojave Desert. 

INTRODUCTION

The Mojave River Basin is located in the western 

part of the Mojave Desert in southern California (fig. 1) 

and has a desert climate that is characterized by high 

summer temperatures, low humidity, and low 

precipitation. The proximity of the basin to the highly 

urbanized Los Angeles region has led to a rapid growth 

in population, and consequently, to an increase in the 

demand for water. The Mojave River is the primary 

source of surface water in the basin, but it is not 

dependable for water supply because significant flows 

occur only after intense storms. As a result, ground 

water is used for agricultural and municipal needs.

Ground-water withdrawal since the early 1900s 

has resulted in discharge (primarily from pumping 

wells) that exceeds recharge (natural and artificial). 

This reliance on ground water has resulted in overdraft 

conditions since the mid 1940s (Stamos and others, 

2001). For the purposes of this report, overdraft occurs 

when ground-water discharge (natural discharge plus 

pumpage) exceeds recharge, resulting in a net 

reduction in ground water stored in the aquifer system. 
Introduction 3

Mojave

River

Deep Creek

Silverwood

Lake

West Fork

Mojave River

West

Cronese

Lake

Harper

Lake

Coyote

Lake

Troy

Lake

Lucerne

Lake

El Mirage

Lake

Rabbit

Lake

Lockhart

Fault

Helendale

Fault

Calico Fault

San Andreas Fault Zone

Harper Lake-Camp Rock

Fault

395

Barstow

15

15

40

18

18

58

247

247

Los Angeles

CALIFORNIA

San

Francisco

San Diego

Ventura Mojave

Desert

Mojave Water

Agency

management

area

Centro

subarea

Este

subarea

Alto

subarea

Alto

Transition

zone

Baja subarea

Oeste

subarea

The Forks

Iron

Mtn

San Bernardino Mts

San Gabriel Mts

Afton Canyon

Newberry

Springs

Lower

Narrows

Upper

Narrows

San Bernardino

Co

Los Angeles Co Kern Co

San Bernardino Co

Mojave

Desert

EXPLANATION

Ground-water basin Management area Rabbit

Lake

Victorville

Hesperia

35

00'

34

30'

117 30' 117 00' 116 30'

0 20 KILOMETERS

0 20 MILES

Mojave River Basin – Mojave Water Agency boundaries – Dry lake (playa)

Surface-water drainage basin Subarea Selected faults

STUDY

AREA

Helendale

Cajon

Pass

Adelanto

Victor Valley

Wastewater

Reclamation

Authority

Pacific Ocean

Figure 1. Location of study area.
4 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

In 1990, the city of Barstow and a local water 

company filed a complaint in the California Supreme 

court alleging that the cumulative water production 

from upstream communities between 1931 and 1990 

overdrafted the Mojave River ground-water basin; the 

complaint resulted in the adjudication of the basin 

(Mojave Water Agency, 1996). At the core of the 

complaint was the assertion that the ground-water 

supplies of the downstream users were affected by 

withdrawals in the upstream parts of the basin. In 1993, 

a physical solution was submitted to the court that 

required users to initially reduce their pumpage by 20 

percent over 5 years, followed by possible subsequent 

annual reductions in pumpage. The physical solution 

also required that each subarea within the Mojave 

River ground-water basin provide a specific quantity of 

water to the adjoining downstream subarea.

The area where the Helendale Fault crosses the 

Mojave River is of particular hydrogeologic interest 

because of its importance as a boundary between two 

water-management subareas of the Mojave Water 

Agency (MWA). The Helendale Fault is the boundary 

between the upper Mojave River Basin (Oeste, Alto, 

Transition zone of the Alto, and Este subareas) and the 

lower Mojave River Basin (Centro and Baja subareas); 

specifically, it is the boundary between the Alto and the 

Centro subareas (fig. 1). Previously published waterlevel maps by Stamos and Predmore (1995), Mendez 

and Christensen (1997), and Smith and Pimentel 

(2000) show that in most areas of the Mojave River 

Basin, ground-water flow mainly is toward the Mojave 

River, but in the area downgradient of the Helendale 

Fault, ground-water flow is away from the Mojave 

River toward Harper Lake and downstream toward the 

city of Barstow.

This study was done in cooperation with the 

MWA for the purposes of mapping the surficial 

geology near the Helendale Fault, defining the vertical 

and horizontal extent of the aquifers in the area, and 

determining how the Helendale Fault affects groundwater flow and water quality. Water samples were 

analyzed to determine the concentrations of dissolved 

constituents and the approximate ages and source(s) of 

recharge of the water in the alluvial aquifers and 

underlying basement rocks. This report presents the 

data collected for this study and the interpretation of 

field work done near the Helendale Fault. The field 

work included detailed mapping of the geology of the 

area, drilling and installation of multiple-well 

monitoring sites for gathering data from the aquifer 

system at discrete intervals, and collecting water-level 

and water-quality data.

Description of Study Area

The study area is located in the Mojave River 

ground-water basin approximately 100 mi northeast of 

Los Angeles, California (fig. 1). The area is centered 

where the Mojave River intersects the Helendale 

Fault—the westernmost of several long, northwesttrending faults that cross the central and western 

Mojave Desert. The headwaters of the Mojave River lie 

30 to 40 mi to the south, in the high mountains of the 

central Transverse Ranges that were uplifted along the 

San Andreas Fault during the past several million years 

(Meisling and Weldon, 1989; Matti and Morton, 1993). 

The study area covers an area of about 30 mi2 that 

spans from about 3 mi upstream to about 4 mi 

downstream from the Helendale Fault.

For management purposes, the MWA subdivided 

the Mojave River surface-water drainage basin unit into 

several subareas—Oeste, Alto Transition zone of the 

Alto (hereinafter referred to as the Transition zone), 

Este, Centro, and Baja (fig. 1). The study area straddles 

the Helendale Fault, which is the boundary between the 

upper Mojave River Basin (Oeste, Alto, and Este 

subareas) and the lower Mojave River Basin (Centro 

and Baja subareas).

The principal sources of natural recharge to the 

ground-water basin are the Mojave River, and to a 

lesser extent, small ephemeral streams and washes 

(Izbicki and others, 1995; Stamos and others, 2001). 

The Mojave River bisects the study area and, when 

sufficient surface water is present, recharges the aquifer 

system. However, significant recharge occurs only 

during episodic stormflows, usually in the winter; 

during the rest of the year, most of the river is usually 

dry. The floodplain aquifer near Helendale is recharged 

primarily by the infiltration of winter stormflows from 

the Mojave River. Since 1981, the discharge of treated 

municipal wastewater by the Victor Valley Wastewater 

Reclamation Authority (fig. 1) has maintained 

perennial flow in the Mojave River upstream from the 

study area and, as a result, has increased the amount of 

recharge going into the floodplain aquifer in the 
Introduction 5

Transition zone. Discharge from Victor Valley 

Wastewater Reclamation Authority ranged from about 

260 acre-ft in 1981 to about 7,300 acre-ft in 1995 

(Stamos and others, 2001).

Because of the limited availability of surface 

water, water supply in the area is derived entirely from 

ground water. Within the study area, some ground 

water is pumped for agriculture, but most is pumped 

for municipal supply. Upstream of the Helendale Fault, 

about 5,400 acre-ft/yr is pumped for municipal supply 

at Helendale (including the community of Silver 

Lakes) (fig. 1). 

Acknowledgments

Appreciation is expressed to the San Bernardino 

County Flood Control District and Lockheed Martin 

Corporation for allowing access to their property for 

test drilling. In addition, appreciation is expressed to 

local land owners who allowed access to their property 

for water-level measurements and water-quality sample 

collection. The authors also thank the following U.S. 

Geological Survey personnel who assisted with data 

collection: Lowell F.W. Duell, Charles Lamb, Dennis 

Clark, Jill N. Densmore, Gregory Smith, Steven 

Crawford, and John C. Tinsley

Data-Collection Techniques

Several field investigations performed 

specifically for this study aided in the understanding of 

the subsurface hydrogeology. The conclusions in this 

report are a blend of the knowledge gained through 

detailed geologic mapping, the installation of multiplewell monitoring sites, the collection and analysis of 

aquifer-test data, water-level data, and water-quality 

data. A summary of well location and construction 

information and the type of data collected from 58 

wells in and near the study area is presented in table 1. 

To download a file containing site identification 

numbers for these wells, click here; for the retrieval of 

water-level and water-quality data from the U.S. 

Geological Survey (USGS) National data base, go to 

http://waterdata.usgs.gov/ca/nwis/. The drilling 

techniques and procedures followed for the collection 

of the water-level and water-quality data are discussed 

in the following sections.

Multiple-Well Monitoring Sites

 Prior to this study, water-level measurements 

and water samples for geochemical analysis were 

obtained exclusively from irrigation or domestic wells, 

the depths of which were uncertain in many cases. 

Multiple-well monitoring sites were installed for this 

study to measure water levels and to take water-quality 

samples from specific depths within the aquifer system 

in the Helendale area. During the drilling of the 

boreholes for the monitoring wells, a detailed log of the 

subsurface geology was recorded and used to interpret 

the hydrology of the aquifer system and the geology of 

the area.

During 1993 and 1994, four multiple-well 

monitoring sites containing a total of 18 individual 

wells were completed by mud rotary drilling in the 

study area—3 upstream (west) of the Helendale Fault 

and 1 downstream (east) (fig. 2). The monitoring sites 

consist of four to five wells with 2-inch diameter 

polyvinyl chloride (PVC) casings, except for one well 

with a 4.5-inch diameter casing. The well casings were 

installed at different depths in the same borehole at 

specific intervals, generally with 20 ft perforated 

intervals at or near the bottom of the well; a filter pack 

of Monterey #3 sand was placed around each screened 

interval. Each well was isolated from the others in the 

same borehole by low-permeability bentonite grout. 

The construction of these wells enables the collection 

of depth-specific water-level, water-quality, and 

aquifer-property data. Well-construction information 

for the 18 monitoring wells is provided in table 2.

A suite of geophysical logs was obtained after 

the boreholes were completed. The suite of logs 

include caliper, 64- and 16-inch normal resistivity, 

lateral resistivity, and natural gamma. For detailed 

well-construction diagrams, lithologic descriptions, 

and geophysical logs, the reader is referred to Huff and 

others (2002).
6 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California Table 1. Well-construction information and type of data collected for selected wells near Helendale, San Bernardino County, California, 1990–99—Continued USGS identification No. State well No. Altitude of land surface Original depth

Top of 

perforations

Bottom of 

perforations

Multiplewell

site

Type of data collected

Water 

level1

(1995)

Water 

quality Isotopes Tritium C-14

344334117202701 7N/4W-6F5 2,450 99 89 99

— s s —

—

344334117203001 7N/4W-6F6 2,450 20 10 20

— s s —

—

344036117215201 7N/5W-23R1 2,725 740 700 740 x —

— s s s

344036117215202 7N/5W-23R2 2,725 510 490 510 x — s s — s344036117215203 7N/5W-23R3 2,725 306 295 315 x — s s s s344028117210601 7N/5W-24R5 2,505 550 510 550 x — s s s s344028117210602 7N/5W-24R6 2,505 265 265 285 x — s s s s344820117164901 8N/4W-3Q1 2,440

—

—

—

— 2s s —

—

344834117175401 8N/4W-4K1 2,470

—

—

—

— 2s s —

—

344834117175301 8N/4W-4K2 2,470

—

—

—

— s s s s

344817117180201 8N/4W-9B1 2,464 300 100 200 s s s s s344733117173801 8N/4W-9R1 2,367 200

—

— s s s s

344816117163501 8N/4W-10A2 2,398

—

—

— s —

—

—

—

344754117164901 8N/4W-10G1 2,357 85.7

—

— s —

—

—

—

344727117171501 8N/4W-10P1 2,365

—

—

— s —

—

—

—

344728117165001 8N/4W-10Q1 2,355 62 14 59 s —

—

—

—

344807117163001 8N/4W-11D6 2,375

—

—

—

— 2s s —

—

344818117151501 8N/4W-12C1 2,350 150 90 150 s —

—

—

—

344728117145601 8N/4W-12Q1 2,329 49.1

—

— 3s —

—

—

—

344726117145501 8N/4W-13B1 2,330 175 65 175

— s s —

—

344638117162301 8N/4W-14N1 2,460 333 53 

193

153 333

— s s —

—

344714117172001 8N/4W-15C1 2,365 131 55 131

— s s —

—

344712117164301 8N/4W-15H2 2,357 65 15 63 s —

—

—

—

344701117173801 8N/4W-16J1 2,372

—

—

— s —

—

—

—

344659117181901 8N/4W-16L4 2,381 53.2

—

— s —

—

—

—

344702117184201 8N/4W-17H1 2,392.6 47

—

— s —

—

—

—

344645117190401 8N/4W-17Q3 2,400 72.5

—

— s —

—

—

—

344611117200801 8N/4W-19G1 2,458 316.4 295 315 x s s s s s344611117200802 8N/4W-19G2 2,458 239.4 220 240 x s s s s s344611117200803 8N/4W-19G3 2,458 170.6 150 170 x s s s s s344611117200804 8N/4W-19G4 2,458 101.1 80 100 x s s s s s

Table 1. Well-construction information and type of data collected for selected wells near Helendale, San Bernardino County, California, 1990–99[U.S. Geological Survey (USGS) site identification number is the latitude, longitude, and sequence number of the site; State well No.: see well-numbering diagram in the table of contents; altitude of land surface in feet above sea level; original depth, top and bottom of perforations in feet below land surface; C-14, carbon 14; x, denotes that well is a multiple-well monitoring well; s, sample collected; —, no sample collected]
Introduction

7

344628117190601 8N/4W-20B1 2,400 82 — — s — — — —344625117193001 8N/4W-20D1 2,412.2 195 60 195 s — — — —344634117194201 8N/4W-20D2 2,443.9 — — — s — — — —344612117193001 8N/4W-20E1 2,400.6 — — — s — — — —344613117194501 8N/4W-20E2 2,405.5 — — — s — — — —344549117192201 8N/4W-20P2 2,404.7 115 — — s — — — —344546117190101 8N/4W-20Q7 2,397.3 452 440 460 x s s s s s344546117190102 8N/4W-20Q8 2,397.3 350.6 330 350 x s s s s s344546117190103 8N/4W-20Q9 2,397.3 270.8 250 270 x s s s — —344546117190104 8N/4W-20Q10 2,397.3 160.2 140 160 x s s s — —344546117190105 8N/4W-20Q11 2,397.3 51.9 30 50 x s s s — —344546117185901 8N/4W-20Q12 2,397.31 139.1 99.5 139.5 s s s — —344631117182201 8N/4W-21C1 2,385 — — — s — — — —344624117181901 8N/4W-21C2 2,381 140 100 140 — s s — —344609117182901 8N/4W-21M1 2,388.96 370.4 350 370 x s s s s s344609117182902 8N/4W-21M2 2,388.96 228.9 210 230 x s s s s s344609117182903 8N/4W-21M3 2,388.96 140.7 120 140 x s s s — —344609117182904 8N/4W-21M4 2,388.96 40.6 30 40 x s s s — —344544117154601 8N/4W-23Q1 2,489 253 153 253 — s s — —344557117143701 8N/4W-24J2 2,478 360 — — — 2s s — —344524117193401 8N/4W-29E3 2,409.49 309.1 289 309 x s s s s s344524117193402 8N/4W-29E4 2,409.49 210.4 190 210 x s s s s s344524117193403 8N/4W-29E5 2,409.49 131.3 110 130 x s s s s s344524117193404 8N/4W-29E6 2,409.49 41.6 30 40 x s s s s s344401117194701 8N/4W-31R1 2,449 59 — — 3s — — — —344959117173101 9N/4W-34D1 2,483 620 — — 4s s s — s344933117173001 9N/4W-34M1 2,482 605 — — — s s s s

Table 1. Well-construction information and type of data collected for selected wells near Helendale, San Bernardino County, California, 1990–99—Continued

USGS 

identification No.

State 

well No.

Altitude of 

land surface

Original 

depth

Top of 

perforations

Bottom of 

perforations

Multiplewell

site

Type of data collected

Water 

level1

(1995)

Water 

quality Isotopes Tritium C-14

.

1Water-level measurement from the shallowest well at each multiple-well monitoring site was used to construct the water-table map (fig. 4).2Sampled for selected anions only.3March 1995 water-level measurement.4April 1996 water-level measurement.
8 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

13

HELENDALE

IRON

MOUNTAIN

FAULT

FAULT

ZONE

3 12

6

10

1

5

1

National Trails Highway

19G1–4

20Q7–11

20Q12

21M1–4

29E3–6

pTb

pTb

pTb

pTb

pTb

pTb

pTb

pTb

pTb

pTb

QTof

QTof

QTof

QTof

QTof

QTof

QTof

QTof

QTof

QTof

QTof

QTof

Qoa

Qoa

Qoa

Qoa

Qoa

Qoa

QTp

QTp

QTp

Qp

QTp

Qra

Qyf

Qyf

Qyf

Qyf Qyf

Qyf

Qyf

Qyf

Qyf

Qyf

Qyf

Qyf

Qya

Qya

Qra

Qra

pTb

pTb

QTof

QTof

QTof

Mojave

River

Helendale

Qoa

Brisbane

Valley

T

9

N

T

8

N

T

8

N

R5W R4W R4W

T

7

N

34°

45'

34°

50'

117°20' 117°15'

A'

B

B'

A

Point

of Rocks

Wild

Crossing

Buckthorn Wash

Unnamed Wash

Fremont Wash

Unnamed Wash

Iron Mountain

36

1

36

1

31

6

36

1

31

6

36

1

0 3 MILES

0 3 KILOMETERS

Modern

Channel

Figure 2. Surface geology, line of cross sections A–A’ and B–B’, and location of the multiple-well monitoring sites near Helendale, San Bernardino County, 

California.
Introduction 9

EXPLANATION

CORRELATION OF MAP UNITS

DESCRIPTION OF MAP UNITS

QTof

Qra

Qya

Qya

Qra

Qyf

pTb

Holocene

Pleistocene Quaternary

Pliocene

Qoa

Qoa

Line of geologic section

shown in figure 3

Strike and dip of inclined

strata

Horizontal strata

Fault—Dashed where

approximate or inferred;

dotted where concealed;

ball and bar on downthrown

side; arrows indicate

sense of strike slip

Anticline—Single arrow

shows direction of plunge

Unconformity

Unconformity

Unconformity

Tertiary

Pre-Tertiary

QTp

QTp

QTof

Qyf

pTb Igneous and metamorphic

basement rocks

Older fan and

stream deposits

Younger fan deposits

Older alluvium of the

ancestral Mojave River

Playa deposits

Recent alluvium of

the Mojave River

Younger alluvium of

the Mojave River

Floodplain

aquifer

Regional

aquifer

Contact

Single well number

U.S. Geological Survey

multiple-well site

(Sequence number is

from deepest to shallowest)

20Q12

19G1–4

Well and abbreviated

well number—

A A'

Monocline—Arrows indicate

direction of dip within

inclined limb

6

Figure 2.—Continued.
10 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Water-Level Data

Ground-water levels were measured at the 

multiple-well completion sites and selected wells 

within the study area. Water levels were measured to an 

accuracy of 0.01 ft using a calibrated steel tape or a 

calibrated electric tape.

Water-Quality Data

Water-quality sampling was done by USGS 

personnel; all samples were collected, handled, and 

preserved following USGS field procedures (Marc 

Sylvester, U.S. Geological Survey, written commun., 

1990). Purge logs, field measurements, and other 

information related to sample collection are on file at 

the USGS office in San Diego, California. Analyses of 

the major ions, nutrients, stable isotopes, carbon-14 

(

14C), and tritium were done at USGS laboratories. 

Tritium was measured by liquid scintillation 

(Thatcher and others, 1997) with electrolytic 

enrichment (Ostlund and Warner, 1962) in 1-liter 

unfiltered samples collected in glass containers. 

Sample bottles were filled from the bottom and allowed 

to overflow several times the bottle volume. The 2-

sigma detection limit for tritium is 0.2 tritium units. 

The detection limit is a function of the statistics 

associated with the liquid scintillation counter. In 

general, the longer the counting time, the better the 

precision and the lower the detection limit. 

Prior to 1994, 14C samples were collected in a 

96-liter nitrogen-filled stainless-steel vessel. The pH of 

an unfiltered sample water was adjusted to a pH greater 

than 11 using carbon-dioxide-free sodium hydroxide, 

and bicarbonate was precipitated as strontium 

carbonate by super saturating the sample with 

strontium chloride. The strontium carbonate precipitate 

was allowed to settle into a 1-liter polyethylene bottle 

at the bottom of the stainless-steel vessel. The 1-liter 

bottle was shipped to a laboratory where 14C analyses 

were done by scintillation spectrophotometry 

(Thatcher and others, 1997). After 1994, the 14C 

samples were filtered and collected in 1-liter glass 

bottles in the same manner as tritium, and then were 

analyzed for 14C by accelerator mass spectrometry 

(Beukins, 1992).

Table 2. Well-construction information for multiple-well monitoring sites near Helendale, San Bernardino County, California

[State well No.: See well-numbering diagram in the table of contents; altitude of land surface in feet above sea level; perforated interval and original depth in 

feet below land surface]

1All wells are 2 inch in diameter, except 8N/4W-20Q12, which is 4.5 inches.

State well No. Altitude of land surface Aquifer or unit Perforated interval Original depth

8N/4W-19G1 2,458 Basement rocks (pTb) 295–315 315

8N/4W-19G2 2,458 Regional (QTof) 220–240 240

8N/4W-19G3 2,458 Floodplain (Qya) 150–170 171

8N/4W-19G4 2,458 Floodplain (Qya) 80–100 101

8N/4W-20Q7 2,397 Basement rocks (pTb) 440–460 460

8N/4W-20Q8 2,397 Basement rocks (pTb) 330–350 351

8N/4W-20Q9 2,397 Basement rocks (pTb) 250–270 271

8N/4W-20Q10 2,397 Floodplain (Qya) 140–160 160

8N/4W-20Q11 2,397 Floodplain (Qra) 30–50 52

8N/4W-20Q121 2,397 Floodplain (Qya) 80–140 140

8N/4W-21M1 2,389 Basement rocks (pTb) 350–370 370

8N/4W-21M2 2,389 Regional (QTof) 210–230 230

8N/4W-21M3 2,389 Floodplain (Qya) 120–140 141

8N/4W-21M4 2,389 Floodplain (Qra) 30–40 41

8N/4W-29E3 2,409 Basement rocks (pTb) 289–309 309

8N/4W-29E4 2,409 Basement rocks (pTb) 190–210 210

8N/4W-29E5 2,409 Floodplain (Qya) 110–130 131

8N/4W-29E6 2,409 Floodplain (Qra) 30–40 42
Geologic Setting 11

GEOLOGIC SETTING

Geologic features near the intersection of the 

Mojave River and Helendale Fault were mapped during 

previous studies by Bowen (1954), Dibblee (1960a,b,c; 

1967), California Department of Water Resources 

(1967), and Bortugno and Spittler (1986). To address 

geohydrologic questions raised during the present 

study, a new surficial geologic map of the study area 

was prepared (fig. 2). Data for the map were obtained, 

in part, by a field survey of the river bluffs and tributary 

ravines along the Mojave River between Helendale and 

Iron Mountain. Information for other parts of the map 

was obtained from reconnaissance field observations, 

inspection of aerial photographs, and evaluation of the 

cited geologic maps. Data obtained from the boreholes 

and from previous regional geophysical investigations 

were combined with the surficial geology to define the 

three-dimensional geologic framework of the 

Helendale area. Descriptions of the main geologic 

features in the study area, interpretations of their 

origin, and evaluations of how local landforms, 

stratigraphy, and geologic structure may affect groundwater flow are presented in the following sections.

Alluvial Topography of the Mojave River Valley

The study area is located within the alluviumfilled valley of the Mojave River near its intersection 

with the Helendale Fault (fig. 2). This segment of the 

river valley is 1.25 to 2.0 mi wide and is arcuate, 

trending north-northeast past Helendale to Point of 

Rocks, and from there heading northeast to Iron 

Mountain. The maximum topographic relief between 

the valley floor and nearby ridges is approximately 750 

feet. Fremont Wash, Buckthorn Wash, and two other 

large unnamed washes (fig. 2) drain into the west side 

of the valley. Alluvial fans deposited at the mouths of 

these washes and neighboring smaller ravines coalesce 

to form a broad, gently sloping apron of sand and 

gravel (unit Qyf) along the northwest side of the valley 

(fig. 2). By contrast, a relatively narrow, discontinuous 

fringe of alluvial fans deposited by minor tributaries 

lines the southeast wall of the valley. The alluvial fans 

terminate down gradient at the nearly flat floor of the 

valley, which is 0.6 to 1.3 mi wide and contains 

Holocene alluvium (unit Qra) deposited in the channel 

and floodplain of the Mojave River (fig. 2). The 

floodplain consists of a low alluvial terrace that is 5 to 

10 ft above the river channel. Older, late Pleistoceneage, river deposits (unit Qoa) are perched on the bluffs 

on either side of the river (fig. 2). An alluvial terrace 

elevated 130 to 220 ft above the valley floor caps these 

older river deposits. The high alluvial terrace is 

strongly dissected by erosion, particularly southwest of 

the Helendale Fault, where it rises to its greatest 

heights above the adjacent valley.

Stratigraphy

Seven stratigraphic units are exposed at the 

ground surface (fig. 2), and six are penetrated by 

boreholes in the study area (fig. 3): (1) pTb, basement 

complex of igneous and metamorphic rocks (preTertiary), (2) QTof, older fan and stream deposits 

(Pliocene and Pleistocene), (3) QTp, playa deposits 

(Pliocene or early Pleistocene), (4) Qoa, older alluvium 

of the ancestral Mojave River (Pleistocene), (5) Qyf, 

younger fan deposits (Pleistocene and Holocene), (6) 

Qya, younger alluvium of the Mojave River 

(Pleistocene and Holocene), and (7) Qra, recent 

alluvium of the Mojave River (Holocene).

Igneous and Metamorphic Basement Rocks (pTb)

Hills and ridges of igneous and metamorphic 

rocks (unit pTb) locally protrude through a thick 

blanket of alluvial deposits that covers most of the 

study area (fig. 2). Basement ridges near the southern 

margin of the study area consist of silicic metavolcanic 

rocks of Jurassic age, including ash-flow tuff and tuff 

breccia, lava flows, and shallow intrusive bodies 

(Bowen, 1954; Dibblee, 1960b; Schermer, 1993). The 

northernmost of these outcrops forms Point of Rocks, a 

low headland of metarhyolite at the southeast edge of 

the Mojave River. Boreholes drilled in the Mojave 

River valley north and west of Point of Rocks 

penetrated similar metavolcanic rocks buried beneath 

about 175 to 275 ft of Quaternary alluvial deposits 

(figs. 2 and 3, well sites 8N/4W-19G1–4, -20Q7–11, 

-21M1–4, and -29E3–6). At Iron Mountain, near the 

northeast corner of the study area (fig. 2), granitic rocks 

of Late Jurassic or Cretaceous age intrude intensely 

deformed, schistose metavolcanic and 

metasedimentary rocks of Jurassic age (Bowen, 1954; 

Dibblee, 1960a,c; Boettcher and Walker, 1993). 

Granitic rocks also form two small hills near Wild 

Crossing.
12 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California Section B–B' 8N/4W-29E3– 6 (Projected) 8N/4W-19G1–4

2,600

2,500

2,400

2,300

2,200

2,100

2,000

ALTITUDE, IN FEET ABOVE SEA LEVEL

A

Northwest

2,600

2,500

2,400

2,300

2,200

2,100

2,000

A'

Southeast

QTof

Qyf

Qyf Qyf

Land Surface

Modern

Channel

Vertical exaggeration X10

pTb

pTb

QTof

Qya

Qra

8,100

<50

<50 <50

<50

10,400

8,000

10,600

8,100

EXPLANATION

QTof

Qya

Qra

Qyf

pTb Igneous and metamorphic

basement rocks

Older fan and stream deposits

Younger fan deposits

Contact - Dashed where uncertain

Water table - 1992

Data - Uncorrected carbon-14 age, in

years. If tritium was detected, age is

given as less than (<) 50

Multiple-well monitoring site and

number - Rectangles indicate

screened interval (Sequence number

is from deepest to shallowest)

Floodplain

aquifer

Regional

aquifer

29E3–6

A

0

0 1,500 METERS

5,000 FEET

Recent alluvium of

the Mojave River

Younger alluvium of

the Mojave River

Figure 3. Geologic cross sections near Helendale, San Bernardino County, California, (A) cross section A–A’, and (B) cross section B–B ’.
Geologic Setting 13

A T

A T

Section A–A'

8N/4W-29E3–6

8N/4W-20Q7–11

8N/4W-21M1–4

Qra

Qya

pTb

pTb

2,500

2,400

2,300

2,200

2,100

2,000

1,900

ALTITUDE, IN FEET ABOVE SEA LEVEL

2,500

2,400

2,300

2,200

2,100

2,000

1,900

Land Surface

B B'

Southwest Northeast

QTof

QTof

<50

—

9,500

3,000

<50 <50

<50 <50

10,400

<50

4,400

5,200

10,600

5,200

EXPLANATION

Qra

Qya

pTb Igneous and metamorphic

basement rocks

Older fan and

stream deposits

Contact - Dashed where uncertain

Inferred Fault - Arrows indicate sense of

dip slip; letters indicate sense of strike slip -

T, towards reader; A, away from reader

Water table - 1992

Data - Uncorrected carbon-14 age, in years.

If tritium was detected, age is given as

less than (<) 50. — indicates no data

Multiple-well monitoring site and

number - Rectangles indicate

screened interval (Sequence number

is from deepest to shallowest)

Recent alluvium of

Floodplain the Mojave River

aquifer Younger alluvium of

the Mojave River

Regional

aquifer 29E3–6

A

T

Vertical exaggeration X10 0

0 1,500 METERS

5,000 FEET

B

QTof

Figure 3—Continued.
14 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Older Fan and Stream Deposits (QTof)

Alluvial-fan and braided-stream deposits of 

Pliocene and early Pleistocene age (unit QTof) overlie 

the pre-Tertiary basement rocks (figs. 2 and 3). These 

deposits mainly consist of unconsolidated to weakly 

indurated sand and pebble-cobble gravel. The deposits 

are hundreds of feet thick in the western part of the 

study area, but they thin and lap onto an irregular 

basement surface in the eastern part of the area. The 

deposits are gently folded, and in some areas, layers 

dip as steeply as 10 to 15 degrees. Deep erosion has 

destroyed all vestiges of the original depositional 

topography at the top of unit QTof. However, clast 

assemblages and sedimentary structures indicate that 

the deposits were laid down by a southward-flowing 

regional stream and by tributary alluvial fans (Cox and 

others, 1998; Cox and Hillhouse, 2000; Cox and 

others, 2003); therefore, the unit predates the origin of 

the present northeast-flowing surface drainage in the 

Helendale area.

The texture and detrital composition of the older 

fan and stream deposits vary laterally and vertically, 

reflecting diverse fluvial and alluvial-fan environments, 

derivation of rock debris from multiple local source 

areas, and varying degrees of weathering and 

diagenetic alteration. The deposits were derived from 

granitic, metamorphic, and volcanic rocks similar to 

those presently exposed in the upland areas of the 

southern and central Mojave Desert. Well-stratified 

sand and gravel deposited by a braided stream are 

exposed in road cuts of the National Trails Highway 

near Helendale, whereas poorly stratified silty sand and 

gravel of alluvial-fan origin crop out extensively in 

hills along the west side of the study area. Subsurface 

deposits penetrated by boreholes consist of a similarly 

diverse assemblage of sand, gravel, silt, and clay. A 

thick layer of reddish-brown clay 180 to 280 ft below 

the ground surface in borehole 8N/4W-19G (fig. 2) is 

absent from the boreholes along section line B–B′, 

located about 1.0 to 1.5 mi east and southeast of 

borehole 8N/4W-19G. This stratigraphic discontinuity 

may indicate that the clay was deposited in a lake or 

playa that did not extend eastward to the area of section 

B–B′ or that a former eastern extension of the clay layer 

may have been uplifted and eroded before the younger 

alluvium of the Mojave River (unit Qya) was 

deposited. A thick succession of calcareous playa 

sediments (QTp) intercalated near the top of the older 

fan and stream deposits in the central and northern 

parts of the study area (fig. 2) is described separately in 

this report.

Regional gravity data suggest that the older fan 

and stream deposits, and possibly the underlying older 

units, fill deep depressions in the basement surface near 

the western and northern margins of the area mapped in 

figure 2. These depressions are part of a chain of north–

south trending basins that extends southward for about 

35 mi from Harper Lake to Victorville (Mabey, 1960; 

Biehler and others, 1988; Subsurface Surveys, 1990). 

The gravity data suggest that the basement surface is 

deeper than 3,000 ft at the centers of the basins, and 

therefore well below sea level (Subsurface Surveys, 

1990). This implies that tectonic subsidence played a 

role in basin development. The basins possibly are 

remnants of a sinuous, southward-draining Miocene 

stream valley that was buried by alluvial deposits and 

then was deformed by northwest-trending faults and 

folds. Alternatively, the basins may consist of a series 

of structural depressions that subsided in response to 

east–west extension of the earth’s crust.
Geologic Setting 15

Playa Deposits (QTp)

A unit of playa deposits (QTp) as much as 70 ft 

thick crops out on either side of the Mojave River in the 

central part of the study area (fig. 2). This unit consists 

of weakly consolidated calcareous silt, clay, and fine 

sand interlayered with indurated beds of limestone, 

calcareous tufa, and sparse opalline chert. Fossilized 

plant stems are present in some of the limestone beds. 

The playa deposits interfinger with coarse-grained 

alluvial sand and gravel near the top of unit QTof. At 

one locality, on the southeast side of the Mojave River 

and approximately 2 mi northeast of Helendale, a 

discontinuous thin layer of volcanic ash overlies the 

playa deposits. The ash bed is 1 to 2 ft thick and is 

exposed in the river bluffs at about 2,525 ft above sea 

level. Based on a geochemical correlation, the ash 

apparently was deposited between about 0.9 and 1.2 

million years ago (A. Sarna–Wojcicki, U.S. Geological 

Survey, written commun., 1998; Cox and others, 2003). 

This implies that the underlying playa deposits are 

early Pleistocene or older. They apparently 

accumulated in a small basin that was dammed on its 

southwest side by faulting or arching along the 

Helendale Fault (Cox and others, 2003).

Older Alluvium of the Ancestral Mojave River (Qoa)

Older alluvium of the ancestral Mojave River 

(unit Qoa) unconformably overlies the older fan and 

stream deposits and associated playa deposits (units 

QTof and QTp) on elevated benches that flank the 

valley of the modern Mojave River (fig. 2). Deposits as 

much as 80 ft thick underlie broad areas west of 

Brisbane Valley, west of Iron Mountain, and near the 

southwest corner of the study area. In addition, several 

small erosional remnants of the unit cap ridges south 

and east of Helendale. The older alluvium unit consists 

of essentially unweathered, loose to weakly 

consolidated sand and pebble-cobble gravel that 

accumulated in braided channels of the Pleistocene 

Mojave River. The upper half of the unit mainly 

consists of granitic debris eroded from the San 

Bernardino Mountains. The lower half is more 

heterogeneous in composition, containing granitic 

debris from the San Bernardino Mountains, mixed with 

detritus of granitic, volcanic, metavolcanic, and 

metasedimentary rocks derived from nearby sources in 

the southern Mojave Desert. Although the unit is 

deeply eroded, topographic summits commonly 

preserve remnants of a planar alluvial surface that 

developed on a broad, late Pleistocene floodplain of the 

ancestral Mojave River (“George surface” of Cox and 

Tinsley, 1999). In the study area, the old floodplain 

diverged significantly from the path of the modern 

Mojave River. For example, the extensive deposits of 

older alluvium located west of Iron Mountain represent 

a large distributary channel of the ancestral river that 

drained northward to Harper Lake (Cox and Tinsley, 

1999; Cox and Hillhouse, 2000). The early river also 

developed alternate routes to Harper Lake on the east 

side of Iron Mountain.

Basal deposits of the older alluvium exposed in 

the river bluffs about l mi northeast of Point of Rocks 

overlie a thin layer of fine-grained volcanic ash. The 

chemical composition of the ash matches that of the 

PICO-40A ash near Ventura, California (about 100 mi 

west of study area), which was erupted between about 

0.9 and 1.2 million years ago (A. Sarna–Wojcicki, U.S. 

Geological Survey, written commun., 1998). Thus, the 

deposits of the older alluvium unit probably began 

accumulating in the study area near the end of the early 

Pleistocene. Deposits from boreholes near Victorville, 

about 10 to 15 mi south of Helendale (fig. 1), indicate 

that deposition began somewhat earlier, during the 

Pliocene to early Pleistocene, in more southerly areas 

approaching the headwaters of the Mojave River (Cox 

and Hillhouse, 2000; Cox and others, 2003). The unit 

ceased accumulating between Victorville and 

Helendale about 60,000 to 70,000 years ago, when the 

Mojave River began incising its modern valley into the 

George surface (Cox and Hillhouse, 2000; Cox and 

others, 2003). The older alluvium generally is very 

porous and permeable; however, the unit has limited 

hydrologic significance because it lies above the water 

table throughout the study area.
16 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Younger Fan Deposits (Qyf)

A widespread unit of locally derived younger 

alluvial-fan deposits (Qyf) flanks the Mojave River 

valley, extends up tributary ravines, and forms broad 

alluvial slopes in Brisbane Valley and west of Iron 

Mountain (fig. 2). These deposits include a belt of 

coalescing alluvial fans lying west of the Mojave River 

between Helendale and Iron Mountain. This tract is 

somewhat unusual; along most deeply incised 

segments of the Mojave River, periodic floods in the 

main river channel effectively curtail the growth of 

marginal alluvial fans. Within the study area, however, 

voluminous sedimentary detritus supplied by several 

large tributary ravines apparently has kept pace with 

river erosion. The main tributaries are Fremont Wash, 

Buckthorn Wash, and two unnamed washes farther 

north. The ephemeral streams in these washes drain a 

broad upland area extending northeastward from El 

Mirage Lake to the Helendale Fault (fig. 1). This 

watershed includes extensive hills of easily eroded, 

unconsolidated older fan and stream deposits (unit 

QTof) that probably are the main source of detritus 

within the young alluvial fans. The younger fan 

deposits mainly consist of sand and pebble–cobble 

gravel deposited by ephemeral streams and mudflows. 

They generally have silty, poorly sorted textures and 

consequently have low permeability. Deposits confined 

within large tributary drainages tend to be better sorted 

and probably are moderately permeable. Fan deposits 

confined within the Mojave River valley and tributary 

ravines likely range from latest Pleistocene to 

Holocene in age. However, the alluvial-fan deposits in 

Brisbane Valley locally intertongue with the older 

alluvium (Qoa) and, therefore, may be as old as early 

or middle Pleistocene.

Alluvium of the Mojave River (Qya and Qra)

The sedimentary fill of the Mojave River valley 

consists of unconsolidated sandy alluvium deposited 

during the late Pleistocene and Holocene. These 

sediments constitute the floodplain aquifer defined by 

Stamos and others (2001), and they are confined within 

a 350-foot-deep trough that formed when the river 

eroded into the older alluvium (Qoa) and older fan and 

stream deposits (QTof) during the late Pleistocene 

(Cox and others, 2003). Separate units of younger 

alluvium (Qya) and recent alluvium (Qra) are 

delineated in figures 2 and 3. Both units include 

abundant medium-grained to very coarse-grained 

arkosic sand, interlayered with lesser amounts of silt, 

fine sand, and pebble-cobble gravel.

Inspection of borehole cuttings and geophysical 

logs revealed that the alluvial deposits in both the Qya 

and Qra units are quite variable in texture and 

composition, comprising layers of poorly sorted silty 

sand and gravel, alternating with layers of moderately 

sorted, clean sand and gravel. The silty sediments 

consist of angular rock detritus derived from a diverse 

assortment of plutonic and metamorphic rocks. These 

sediments are similar in texture and composition to 

those in the younger fan deposits (Qyf), and they 

probably were deposited in much the same manner—

by ephemeral floods and mudflows emitted from the 

mouths of nearby tributary ravines. The unit of older 

fan and stream deposits (QTof), which is abundant in 

the watershed of the tributaries in the study area, 

probably was the main source of the silty sand and 

gravel. In contrast, the interstratified layers of clean 

sand and gravel in units Qra and Qya consist of more 

conspicuously abraded, compositionally homogeneous 

granitic debris. This material apparently was mainly 

derived from distant mountain headwaters of the 

Mojave River in the Transverse Ranges. Major floods 

likely deposited these cleaner granitic sediments in the 

main river channel and on the adjacent floodplain. The 

large amounts of locally derived silty sediments 

intercalated within the valley fill may significantly 

reduce the average permeability of the alluvium along 

this stretch of the Mojave River, compared with more 

typical areas where the fill mainly consists of the 

cleaner, far-traveled river deposits.
Geologic Setting 17

The younger alluvium (unit Qya) is as much as 

175 ft thick in the boreholes and forms the bulk of the 

alluvial fill beneath the Mojave River floodplain 

(fig. 3). The unit is largely concealed beneath recent 

alluvium of the Mojave River (Qra) and younger fan 

deposits (Qyf), but it crops out near the western margin 

of the river valley, north of Buckthorn Wash (figs. 2 and 

3A). The age of the deposits probably range from late 

Pleistocene to Holocene. Deposition followed the 

incision of the Mojave River valley, which began about 

60,000 to 70,000 years ago (Cox and Hillhouse, 2000; 

Cox and others, 2003). Deposition was completed 

before about 6,000 to 7,000 years ago, judging from 

radiocarbon ages obtained from alluvial terraces that 

are inset within the younger alluvium near Victorville 

(Rector and others, 1983; Rector, 1999) and east of 

Barstow (Reynolds and Reynolds, 1985, 1991; 

Densmore and others, 1997).

The recent alluvium (unit Qra) is about 50 ft 

thick in the study area, only partly filling a broad, 

100-ft-deep trough that formed when the Mojave River 

eroded into the younger alluvium unit (fig. 3A). 

Surficial deposits of the recent alluvium form the 

modern channel and adjacent floodplain of the river. 

The unit was deposited during the middle to late 

Holocene, following an episode of river incision that 

evidently began prior to 6,000 to 7,000 years ago.

Geologic Structure

Various deformational structures, including 

faults, monoclinal flexures, and anticlinal folds, are 

present in the study area; several of these features are 

reported here for the first time. The structures are 

manifested by tilted or truncated topographic and 

stratigraphic surfaces observed in the field, and by 

scarps and other linear features visible on aerial 

photographs. The identification and interpretation of 

these structures contributed to a more complete 

understanding of local geologic history. Some of the 

structures also help explain the pattern of ground-water 

distribution and flow in the study area.

Helendale Fault

The Helendale Fault is the westernmost in a 

series of long, northwest-striking Quaternary-age faults 

that traverse the southern Mojave Desert (Dibblee, 

1967; Bortugno, 1986; Bortugno and Spittler, 1986; 

Jennings, 1994). The northeast-flowing Mojave River 

intersects the Helendale Fault at nearly right angles 

within the area of this study (fig. 2). Here, the 

Helendale Fault comprises an irregular zone of 

discontinuous, overlapping fault strands with variable 

west–northwest to north–northwest trends, unlike 

neighboring areas to the southeast where there is a 

single, straight and continuous fault trace.

Linear depressions, low scarps, and lineaments 

visible on aerial photographs mark the individual 

strands within the fault zone. Four fault strands border 

the north side of the large basement ridge southeast of 

the Mojave River (fig. 2). These include two arcuate 

strands that diverge northwestward from the ridge, 

intersect one another, and finally terminate short of the 

Mojave River. About 1 mi to the south of these two 

fault strands, two shorter fault strands border the 

constricted northwest end of the basement ridge. These 

strands also seem to terminate before reaching the 

Mojave River; thus, as was previously concluded by 

Dibblee (1960a,b; 1967), the fault zone evidently does 

not extend across the Mojave River at the land surface. 

The fault zone reemerges northwest of the river, 

however, where two overlapping fault strands describe 

a broad arc concave to the southwest. Three accessory 

fault strands adjoin the southern part of this arc.
18 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

The faults southeast of the Mojave River die out 

northwestward into an apparently unfaulted, northeastfacing monoclinal flexure that is exposed directly east 

of Point of Rocks (fig. 2). The tilted strata in the 

monocline consist of an unconformable succession of 

units QTof, QTp, and Qoa. The beds strike northwest 

parallel to faults of the Helendale zone and dip 

northeastward as steeply as 10 to 15 degrees. Angular 

unconformities between the three units indicate that the 

monocline developed contemporaneously with 

sedimentation. The flexure probably extends 

northwestward beneath the alluvial fill of the Mojave 

River floodplain and presumably is underlain at depth 

by faulted basement rocks; therefore, the fault zone 

probably extends continuously across the Mojave River 

in the subsurface (fig. 3B). Thick masses of intensely 

fractured and brecciated metarhyolite that were 

encountered in outcrops near the north edge of Point of 

Rocks and also 220 to 460 ft beneath the ground 

surface in borehole 8N/4W-20Q (fig. 2) may have been 

produced by shearing or warping along the southwest 

margin of the fault zone. The structural relief across the 

monocline east of Point of Rocks indicates that its 

southwest limb has risen as much as 120 ft relative to 

the northeast limb, which may represent the net vertical 

displacement along this segment of the fault zone since 

the latest Pliocene or early Pleistocene (Cox and 

Hillhouse, 2000; Cox and others, 2003).

Northwest-trending faults of the southern 

Mojave Desert typically show evidence of right-lateral 

strike-slip displacement, which is consistent with the 

northwestward drift of western California along the 

nearby San Andreas Fault (fig. 1) (Dibblee, 1961; 

Dokka and Travis, 1990). However, the discontinuous 

pattern of faulting and warping observed in the study 

area, and supplementary evidence from neighboring 

areas, suggest that the net lateral displacement of the 

Helendale Fault is minimal near the Mojave River. 

Previous investigations inferred that the cumulative 

lateral slip is no greater than about 0.6 to 1.2 mi at two 

sites southeast of the study area (Miller, 1977, 1981; 

Miller and Morton, 1980). During the current study, 

field inspections of areas directly southeast of the 

Mojave River revealed that the southeastern margin of 

unit Qoa is separated right-laterally about 1 mi across 

one strand of the fault (fig. 2). However, as this strand 

apparently terminates abruptly before reaching the 

Mojave River, the lateral separation probably was not 

produced by lateral displacement. It more likely 

resulted from minor vertical displacement that dropped 

unit Qyf down against unit Qoa. Evidence of possible 

strike-slip displacement is found where this fault is 

intersected obliquely by a neighboring, more northerly 

trending fault (fig. 2). If both faults are steeply dipping, 

then the latter may have displaced the former about 500 

ft in a right-lateral sense.

The scarps and lineaments associated with the 

strands of the Helendale Fault in the study area cut 

units Qoa, Qyf, and older deposits, which imply that 

faulting occurred during the Quaternary period. Most 

of the displacement evidently accumulated during the 

Pleistocene epoch. This conclusion is supported by the 

degraded condition of the fault scarps, which suggests 

that the most recent movements occurred during the 

late Pleistocene. There is no conspicuous evidence of 

Holocene movements. Northwest-trending scarps or 

other signs of ground-surface rupture are absent from 

the Holocene and latest Pleistocene-age alluvial 

deposits of units Qyf, Qya, and Qra that blanket the 

floor and margins of the Mojave River valley. 

Furthermore, hydrologic data suggest that the 

Helendale Fault zone does not affect the flow of ground 

water in the subsurface deposits of units Qra and Qya 

that form the floodplain aquifer of the Mojave River 

(Hardt, 1971; Stamos and Predmore, 1995; Stamos and 

others, 2001).

In summary, results of the geologic mapping 

survey and of the borehole drilling done during this 

study suggest that displacement on the Helendale Fault 

within the study area, for the most part, predates the 

alluvial fill of the Mojave River valley and therefore 

should have little, if any, affect on the flow of ground 

water in the floodplain aquifer. However, movements 

along the fault zone have affected pre-Tertiary 

basement rocks (unit pTb) and have disrupted the 

overlying units of older fan and stream deposits (QTof), 

playa deposits (QTp), older alluvium of the ancestral 

Mojave River (Qoa), and some parts of the younger fan 

deposits (Qyf). The resulting deformational structures 

may affect the flow of ground water in the basement 

rocks and in the overlying regional aquifer.
Geologic Setting 19

Other Faults

Field observations and an analysis of aerial 

photographs disclosed several northeast-trending faults 

in the region east of the Helendale Fault (fig. 2). These 

include a 4-mile-long zone of overlapping faults at the 

north edge of the Mojave River valley (referred to as 

the “Iron Mountain fault” in Stamos and others, 2001) 

and a 2-mile-long fault in Brisbane Valley. Several 

shorter northeast-trending faults flank the southwest 

end of the Iron Mountain Fault. Geomorphic and 

stratigraphic evidence indicate that the southeast sides 

of most of the northeast-trending faults have dropped 

downward relative to the northwest sides. Strata near 

the middle of the Iron Mountain Fault are dropped 

about 10 ft on the south side of the zone; the other 

northeast-trending faults probably have similarly small 

net displacements.

The northeast-trending faults cut the younger fan 

deposits (Qyf), which implies that these faults moved 

as recently as the late Pleistocene or the Holocene. The 

scarps in Brisbane Valley are strongly eroded, which 

suggests the most recent faulting in that area may have 

occurred during the late Pleistocene. However, 

lineaments at the southwest end of the Iron Mountain 

Fault cut deposits that appear to be Holocene in age. 

The Iron Mountain Fault may be partly responsible for 

a ground-water barrier that was previously deduced 

from a steep northward inclination of the water table 

northwest of the Mojave River (Stamos and Predmore, 

1995).

A previously unrecognized northwest-trending 

fault may underlie the axis of Buckthorn Wash, which 

has eroded into deposits of older fan and stream 

deposits (QTof) in the southwestern part of the study 

area (fig. 2). A fault is proposed to explain the linear 

configuration of this narrow tributary valley, which 

extends northwestward about 10 mi from its confluence 

with the Mojave River. The proposed fault conceivably 

continues southeastward beneath the Mojave River, but 

there is no obvious geomorphic or structural evidence 

indicating a further extension east of the river. The age 

of the fault is uncertain. By analogy with nearby 

parallel strands of the Helendale Fault, it presumably 

was most active during the Pleistocene.

Folds

Two northwest-trending fold structures are 

associated with the Helendale Fault in the study area 

(fig. 2). One of these is the northeast-facing monoclinal 

flexure southeast of the Mojave River that was 

previously described with the Helendale Fault. The 

other is an anticline within unit QTof northwest of the 

river. The anticline lies near the northwest corner of the 

area shown in figure 2 and was originally mapped by 

Bowen (1954), and subsequently by Dibblee (1960a, 

1967). It consists of a low oval dome about 4 to 5 mi 

long that plunges very gently to the northwest and 

southeast. The crest of the dome stands about 200 to 

400 ft above the adjacent land surface to the southwest 

and northeast, respectively. Alluvial strata exposed on 

its flanks dip very gently, about 5 degrees or less 

(Dibblee, 1960a, 1967). Overlapping strands of the 

Helendale Fault cut the dome high on its southwest 

flank. Much of the dome trends northwest parallel to 

the fault zone, but its southeast end veers to the east. 

Other less prominent anticlinal folds may coincide with 

long northwest-trending ridges on either side of 

Buckthorn Wash, as was previously suggested by 

Bowen (1954).

The age of the anticlinal folding can be estimated 

from stratigraphic relations. The dome northwest of the 

Mojave River postdates the deposition of unit QTof and 

predates deposition of unit Qyf. Thus, it probably 

developed chiefly during the Pleistocene, 

contemporaneously with the nearby monoclinal flexure 

and the overlapping strands of the Helendale Fault. The 

folds and faults mainly developed where a thick 

blanket of ductile alluvial deposits covers brittle 

basement rocks. A more continuous, linear zone of 

faulting—comparable to segments of the Helendale 

Fault southeast of the study area—presumably cuts the 

basement rocks in the deep subsurface. Faulting 

evidently propagated irregularly upward through the 

thick alluvial cover to produce the complex pattern of 

folds and faults observed at the land surface. 
20 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Geologic History

Geologic evidence from this and previous 

investigations shows how a sequence of late Cenozoic 

depositional and tectonic events produced the Mojave 

River ground-water basin. The Mojave Desert 

apparently drained to the Pacific Ocean about 18 to 10 

million years ago, as indicated by southward-flowing 

paleocurrents in the Miocene stream deposits near 

Cajon Pass (fig. 1) (Woodburne and Golz, 1972; Foster, 

1980; Meisling and Weldon, 1989). After about 10 

million years ago, north–south directed tectonic 

compression progressively buckled the crust of the 

Mojave Desert and the surrounding areas (Bartley and 

others, 1990), resulting in east–west-trending ridges 

and basins that altered regional patterns of drainage 

and sedimentation (Cox and others, 2003).

During the Pliocene, about 5 to 2 million years 

ago, streams flowing southward across the Mojave 

Desert terminated in an oblong east–west-trending 

sedimentary basin lying between Victorville and Cajon 

Pass (fig. 1) (Meisling and Weldon, 1989; Weldon and 

others, 1993). The axis of the basin passed through the 

area where the city of Hesperia is now located, and 

extended eastward beyond the Helendale Fault (fig. 1). 

As the basin filled with sediments, streams on its 

northern flank aggraded their valleys; thus, a 

southward-flowing stream and tributary alluvial fans 

deposited the unit of older fan and stream deposits 

(QTof) in the study area (Cox and others, 2003). This 

stratigraphic unit now constitutes much of the regional 

aquifer (described in the following section) of the 

Mojave River ground-water basin. Also during this 

period, basement rocks and overlying Cenozoic 

sediments in the southern Mojave Desert were gently 

folded above a horizontal fault zone, or décollement, 

lying 5 to 8 mi beneath the land surface. This process 

generated a regional arch trending west–northwest for 

about 150 mi across the southern Mojave Desert 

(Howard and Miller, 1992; Howard and Cox, 2001; 

Cox and others, 2003). The western part of the arch 

bounded the northern side of the sedimentary basin; its 

crest lay about 4 mi north of Adelanto (fig. 1).

During the late Pliocene and early Pleistocene, 

about 2.0 to 0.78 million years ago, the northern San 

Bernardino Mountains were warped and faulted up 

along the southern Mojave Desert, and the San Andreas 

Fault shifted high peaks of the central San Gabriel 

Mountains to a position directly west of the San 

Bernardino Mountains (fig. 1) (Meisling and Weldon, 

1989). These tectonic movements produced a lofty 

mountain belt that shed increased amounts of water and 

sediment northward into the adjacent basin at the 

southern edge of the Mojave Desert. The ancestral 

Mojave River transported much of the rock detritus that 

was eroded from the San Bernardino Mountains (Cox 

and others, 2003). The regional arch in the southern 

Mojave Desert continued rising and became a 

significant drainage divide during this period. Detritus 

transported northward by the ancestral Mojave River 

and other streams was therefore barred from areas 

north of the arch, including the study area of this 

report. Also during this period, local folding and 

faulting along or near the Helendale Fault created a 

closed depression in the study area that was filled with 

the fine-grained playa sediments of unit QTp (Cox and 

Hillhouse, 2000; Cox and others, 2003).

The basin near Hesperia had completely filled 

with alluvium by the middle Pleistocene, about 0.5 

million years ago. This occurred when the alluvium 

built up to a low point on the crest of the regional arch 

located due north of Adelanto. Thereafter, the ancestral 

Mojave River rapidly advanced northward beyond the 

arch and deposited sand and gravel (unit Qoa) in the 

study area (Cox and Hillhouse, 2000; Cox and others, 

2003). Throughout the remainder of the Pleistocene, 

the terminus of the Mojave River alternated between 

Harper Lake and basins east of Barstow.

The transition from the ancestral to the modern 

Mojave River occurred during the late Pleistocene, 

about 60,000 to 70,000 years ago, when the river began 

incising its present shallow canyon between the San 

Bernardino Mountains and Barstow (fig. 1) (Cox and 

Hillhouse, 2000; Cox and others, 2003). Once incision 

began, the river was locked into its present 

northeastward course south of Iron Mountain, 

abandoning the alternate northward route west of Iron 

Mountain. Downcutting probably was induced by the 

broad uplift of the southern Mojave Desert (Bowen, 

1954) that occurred in response to renewed north–

south tectonic compression. This inferred late 

Pleistocene pulse of regional compression also may 

account for the latest growth of the monoclinal flexure 

east of Point of Rocks (fig. 2), which lifted the top of 

unit Qoa about 30 ft on the southwest side of the 

Helendale Fault.
Geohydrology 21

The Mojave River initially cut down about 350 ft 

where it crosses the Helendale Fault. However, the 

river and local tributaries filled about half of the depth 

of this early valley with sand and gravel of the younger 

alluvium (Qya) by the early Holocene, before about 

6,000 to 7,000 years ago. By cutting a deep trough into 

the slightly permeable sediments of the older fan and 

stream deposits (QTof) and then partly filling the 

trough with more permeable sand and gravel, favorable 

conditions were established for containment of groundwater flow within the floodplain aquifer (described in 

the following section). The permeability of the 

floodplain aquifer may be lower than expected in some 

areas of the study area, however, because the large 

tributary ravines along the west side of the river 

deposited abundant layers of poorly sorted silty sand 

and gravel that are interstratified among the well 

sorted, more permeable river deposits.

The Mojave River completed a second cycle of 

cutting and filling after about 6,000 to 7,000 years ago. 

During this cycle, the river incised a channel about 

100 ft deep within the younger alluvium (Qya), then 

filled this channel about halfway with coarse sand and 

gravel of the recent alluvium (Qra). These young river 

deposits constitute the most permeable strata of the 

floodplain aquifer.

GEOHYDROLOGY

The unconsolidated alluvial sands and gravels of 

Pliocene and Quaternary age form two aquifers within 

the Mojave River ground-water basin—a floodplain 

aquifer that is composed of the recent and younger 

alluvium (Qra and Qya, respectively) of the Mojave 

River river deposits, and a regional aquifer underlying 

and surrounding the floodplain aquifer that is 

composed of the younger fan deposits (Qyf), older 

alluvium of the ancestral Mojave River (Qoa), older fan 

and stream deposits (QTof), and playa deposits (QTp) 

(figs. 2 and 3). Nonwater-bearing metamorphic and 

granitic rocks (pTb) that underlie the alluvial deposits 

of the basin crop out in the surrounding hills and 

mountains, forming the base of the ground-water 

system. In contrast to the overlying alluvial deposits, 

the consolidated rocks are nonporous and yield only 

small amounts of water to wells. Results of test drilling 

show that metamorphosed basement rocks commonly 

are highly weathered and fractured. These rocks were 

distinguished from the overlying alluvial deposits by 

examination of the borehole cuttings and the high 

resistivity on electric logs.

Floodplain Aquifer

The young, permeable alluvial deposits beneath 

and adjacent to the modern channel and floodplain of 

the Mojave River constitute the floodplain aquifer. 

Results of test drilling during the installation of the 

multiple-well monitoring sites in the study area 

indicate that the floodplain aquifer comprises two 

stratigraphic units deposited by the Mojave River: 

recent alluvium of Holocene age (Qra) and younger 

alluvium of Holocene to Pleistocene age (Qya) 

(fig. 3).The recent river alluvium is thin, typically less 

than 50 ft, and is confined to the shallow canyon of the 

Mojave River. Deposits exposed at land surface consist 

of clean, coarse arkosic sand. The younger alluvium 

underlies the recent alluvium and extends to depths of 

about 100 to 150 ft. Both units were identified by the 

high resistivities on borehole electric logs and by 

borehole cuttings, which include clean, coarse arkosic 

sand. At the westernmost monitoring-well site, 

8N/4W-19G1–4 (figs. 2 and 3A), the younger alluvium 

extends up to about 2,460 ft above sea level, or roughly 

50 ft higher than the active river channel. The younger 

alluvium apparently has been strongly eroded by recent 

downcutting of the river. 
22 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

The floodplain aquifer is more productive than 

the regional aquifer, yielding most of the water pumped 

from the basin. Wells drilled in the river deposits 

typically yield between 100 and 2,000 gal/min (Hardt, 

1971). These deposits receive most, if not all, of the 

recharge from surface-water flows. The floodplain 

aquifer near Helendale is recharged primarily by the 

infiltration of winter stormflows from the Mojave 

River. In recent years, the discharge of treated 

municipal wastewater by the Victor Valley Wastewater 

Reclamation Authority (fig. 1) has maintained 

perennial flow in some areas of the Mojave River 

upstream from the study area and, as a result, has 

increased the amount of recharge going into the 

floodplain aquifer in the Transition zone. During the 

summer and periods when the river is not flowing, 

water levels in the floodplain aquifer decline mainly as 

a result of pumping and transpiration by riparian 

vegetation. Water levels recover in the winter when the 

floodplain aquifer is recharged by stormflow in the 

Mojave River. 

Regional Aquifer

Within the study area, the floodplain aquifer is 

surrounded and underlain by unconsolidated alluvium 

of the regional aquifer (Qyf, Qoa, QTp, and QTof). The 

regional aquifer is composed of younger alluvial fan 

deposits of Holocene to Pleistocene age (Qyf), older 

alluvium of the ancestral Mojave River of Pleistocene 

age (Qoa), and older fan and stream deposits of 

Pleistocene to Pliocene age (QTof) (fig. 2). Unit QTof 

is the most important of these deposits because it is 

thick and extends well below the water table, whereas 

the other units are much thinner and mainly lie above 

the water table in the study area (fig. 3A). The test 

drilling results, combined with the exposures on the 

margins of the river valley, indicate that deposits of unit 

QTof are as much as 350 ft thick in the study area 

(fig. 3A). However, similar alluvial deposits are more 

than 1,000 ft thick in other parts of the Mojave River 

ground-water basin (Stamos and others, 2001).

The younger fan deposits (Qyf) consist of sand 

and gravel from the older fan and stream deposits that 

were eroded and redeposited by tributary streams along 

the margins of the Mojave River. These deposits are 

poorly to moderately sorted and slightly weathered; 

they are less permeable than the recent alluvium (Qra) 

and younger alluvium (Qya) of the floodplain aquifer, 

but more permeable than the older fan and stream 

deposits (QTof). In many areas along the Mojave River, 

including the Helendale area, the recent and younger 

alluvium of the floodplain aquifer are separated at the 

ground surface from neighboring older alluvial 

deposits of the regional aquifer by wedge-shaped 

bodies of younger fan deposits (Qyf)

(figs. 2 and 3A). Within the study area, the younger fan 

deposits are above the water table and are unsaturated. 

The younger fan deposits were distinguished from the 

younger alluvium by lower electrical resistivity on the 

borehole electric logs and by the texture and 

composition of the borehole cuttings. The older 

alluvium (Qoa) consists of sandy river deposits that are 

about 25 to 80 ft thick; the deposits are similar to the 

recent and younger alluvium of the Mojave River, but 

are slightly more consolidated. The older fan deposits 

(QTof) contain more interstitial silt and clay, are more 

consolidated than the deposits of the floodplain aquifer, 

and are also distinguished by their low resistivity. The 

playa deposits (QTp) interfinger with the alluvial 

sediments of the older fan deposits (QTof). The 

permeability of the playa sediments is controlled by 

fine-grained textures and abundant cemented layers. 

The QTp unit probably is not an important aquitard 

within the regional aquifer because it has a limited 

areal extent, and it is largely above the water table in 

the study area.
Geohydrology 23

The regional aquifer is recharged by infiltration 

of stormflow in ephemeral washes along the southern 

boundary of the Mojave River ground-water basin 

(Izbicki and others, 1995). In the Helendale area, 

infiltration of runoff from local desert mountains also 

may be an important source of recharge. In comparison 

to the floodplain aquifer, recharge to the regional 

aquifer is small. On the basis of 14C data from water 

sampled from the regional aquifer near Victorville, 

Izbicki and others (1995) determined that water in the 

regional aquifer was recharged as early as 20,000 years 

ago, during climatic and hydrologic regimes that were 

different than those of the present-day Mojave Desert. 

Although the regional aquifer contains a substantial 

amount of ground water in storage, the low 

permeability and fine-grained texture of the sediments 

in this aquifer results in well yields lower than those of 

the floodplain aquifer, and the water generally is of 

poor quality (high dissolved-solids concentrations) in 

most areas of the Mojave River Basin.

Ground-Water Movement near the Helendale 

Fault

Water levels measured in 26 wells were used to 

construct the water-table map shown in figure 4. Water 

levels from the shallowest wells at the multiple-well 

monitoring sites were used because they are the best 

representation of the water table. All but three of the 

water levels were collected in July 1995. The waterlevel measurements from wells 8N/4W-31R1 and 

8N/4W-12Q1 were collected in March 1995 to 

determine the direction of ground-water flow 

upgradient and downgradient of the Helendale Fault, 

respectively. Water levels also were collected at well 

9N/4W-34D1 in April 1996 to determine the waterlevel altitude in the northern part of the study area. Ten 

of the wells measured in July 1995 are located 

upgradient (west) of the Helendale Fault and 16 of 

them are located downgradient (east) of the fault. 

Depth to water in the wells ranged from less than 5 ft 

below land surface in wells completed in the floodplain 

aquifer near the Mojave River to almost 200 ft below 

land surface in wells completed in the regional aquifer 

on the bluffs overlooking the river.

The regional pattern of ground-water movement 

in the Mojave River Basin is affected by right-lateral, 

strike-slip faults that trend predominately northwest to 

southeast and act as barriers to ground-water flow 

(Stamos and Predmore, 1995). In the area near the 

Helendale Fault, the movement of ground water in the 

floodplain aquifer is from the southwest to the 

northeast—in the same direction as the intermittent 

surface-water flows of the Mojave River. The study 

area contains several previously unmapped faults that 

may restrict subsurface flow (fig. 4). On the basis of 

water-level data from this and previous studies (Hardt, 

1971; Stamos and Predmore, 1995), the Helendale 

Fault was not considered to be a barrier to groundwater flow in the floodplain aquifer. However, waterlevel data collected from wells at the multiple-well 

monitoring sites indicate that the fault does restrict 

flow in the regional aquifer and therefore resulted in 

upward flow of ground water in the past. The historical 

record describes many places along the Mojave River, 

including the study area, that had a shallow water table 

or perennially flowing surface water where active faults 

acted as barriers to ground-water flow. Evidence of 

flowing surface water at the Helendale Fault was 

reported in the 1800s by early explorers and trappers 

who established a watering and resting spot at Point of 

Rocks (fig. 4) (Lines, 1996). Samples of deuterium (a 

stable isotope of hydrogen) taken from the multiplewell monitoring sites upgradient of the Helendale Fault 

indicate that the natural (predevelopment) gradient of 

vertical ground-water flow was upward (discussed later 

in the “Isotopic Composition of Water from Wells” 

section), which implies that ground water discharged to 

the river before ground-water pumping started. This 

conclusion is also supported by the 14C data (discussed 

later in the “Tritium and Carbon-14” section), which 

indicate that water in the floodplain aquifer at well 

8N/4W-19G3 (fig. 2) originated as discharge from the 

regional aquifer. In comparison to the amount of 

recharge that the floodplain aquifer receives from the 

Mojave River, the amount of seepage from the regional 

aquifer during predevelopment conditions likely was 

small.
24 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

HELENDALE

FAULT

ZONE

National Trails Highway

2,250

2,200

2,100

?

?

?

?

?

?

2,310

2,320

2,330

2,340

2,350

2,360

2,370

2,380

2,390

2,390

2,400

2,410

2,420

2,390

?

?

?

?

?

?

?

?

?

?

19G4

2,387

20D2

2,382 20D1

2,379 20E2

2,386

20Q11

2,391

20B1

2,379

20E1

2,382

20Q12

2,390

21C1

2,373

21M4

2,381

29E6

2,396

6F6 6F5

11D6

9R1

3Q1

34M1

4K1,2

13B1

15C1

14N1

23Q1

24J2

17H1

17Q3 2,372

2378

20P2

2,380

9B1

2,268

10P1

2,348

10A2

2,334

12C1

2,317

12Q1

2,315

10G1

2,344

10Q1

2,345

15H2

2,347

16J1

2,355 16L4

2,368

31R1

2,425

34D1

2,087

Mojave

River

Helendale

Brisbane

Valley

T

9

N

T

8

N

T

8

N

R5W R4W R4W

T

7

N

34°

45'

34°

50'

117°20' 117°15'

Point

of Rocks

Wild

Crossing

Iron

Mountain

36

1

36

1

31

6

36

1

31

6

36

1

0 3 MILES

0 3 KILOMETERS

Modern

Channel

IRON MOUNTAIN

FAULT

Figure 4. Water-level data and water table near Helendale, San Bernardino County, California, 1995.
Geohydrology 25

Water-level data measured during this study at 

two of the multiple-well monitoring sites 

(8N/4W-19G1–4 and 8N/4W-29E3–6) indicate that 

hydraulic heads are now higher in the floodplain 

aquifer than in the underlying regional aquifer and 

basement rocks upgradient from the Helendale Fault 

(fig. 5). The hydraulic head data imply that the gradient 

is now downward, from the floodplain aquifer to the 

regional aquifer, and that there is now the potential for 

water to move downward through the floodplain 

aquifer into the regional aquifer. This reversal in the 

vertical gradient since predevelopment conditions 

suggests that water levels have been affected, at least in 

part, by ground-water pumping for the community of 

Helendale (fig. 4). In a 2-year period (1993 and 1994) 

pumping in the study area averaged about 6,200 acreft/yr—95 percent of which was from wells that were 

located upgradient of the Helendale Fault (Valerie 

Wiegenstein, Mojave Water Agency, written commun., 

1999). 

EXPLANATION

DESCRIPTION OF MAP UNITS

Qya

Qra

Qoa

QTp

QTof

Qyf

pTb Igneous and metamorphic

basement rocks

Older fan and

stream deposits

Younger fan deposits

Older alluvium of the

ancestral Mojave River

Playa deposits

Recent alluvium of

the Mojave River

Younger alluvium of

the Mojave River

Floodplain

aquifer

Regional

aquifer

Well with water-level measurement

only. Bottom number is water-level

altitude, measured 1995

Well with water-level measurement

and water-quality sample. Bottom

number is water-level altitude,

measured 1995

Well sampled for water quality

only

Well and abbreviated well

number—

Fault—Dashed where

approximate or inferred;

dotted where concealed;

ball and bar on downthrown

side; arrows indicate

sense of strike slip

Anticline—Single arrow

shows direction of plunge

Contact

Monocline—Arrows indicate

direction of dip within inclined limb

Water-table contour—Dashed

where inferred; queried where

uncertain. Hachures indicate

depression. Contour interval is

10 feet, 50 feet, and 100 feet.

Datum is sea level

?

10P1

2,348

3Q1

19G4

2,387

2,200

Direction of ground-water flow

Figure 4.—Continued.
26 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

2,395

2,390

2,380

2,375

2,385

8N/4W-19G1–4

2,400

2,395

2,385

2,380

2,390

2,400

2,395

2,385

2,380

1993 1994 1995 1996 1997 1998 1999 2000 2001

2,390

2,390

2,385

2,375

2,370

2,380

UPGRADIENT OF HELENDALE FAULT DOWNGRADIENT OF HELENDALE FAULT

YEAR

1993 1994 1995 1996 1997 1998 1999 2000 2001

YEAR

EXPLANATION

A

19G1

19G2

19G3

19G4

BR

RA

FA

FA

29E3

29E4

29E5

29E6

BR

BR

FA

FA

21M1

21M2

21M3

21M4

BR

RA

FA

FA

20Q7

20Q8

BR

BR

20Q9 20Q11 FA

20Q10

BR

FA

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

LSD = 2,458'

8N/4W-20Q7–11 LSD = 2,397'

Land-surface altitude

at the well site, in feet

above sea level

8N/4W-29E3–6 LSD = 2,409'

LSD

8N/4W-21M1–4

LSD = 2,389'

Floodplain aquifer

AQUIFER TYPE

Regional aquifer

Basement rocks

FA

RA

BR

Figure 5. Water levels in wells at (A) multiple-well monitoring sites (1993–2001), and (B) selected existing wells, (1990–2001), near Helendale, San Bernardino 

County, California.
Geohydrology 27

B

2,285

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

YEAR

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

YEAR

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

YEAR

2,400

2,395

2,385

2,380

2,390

2,400

2,395

2,385

2,380

2,390

2,350

2,340

2,320

2,310

2,330

2,325

2,315

2,295

2,305

2,290

2,280

2,260

2,250

2,270

2,340

2,330

2,310

2,300

2,320

8N/4W-20P2

8N/4W-31R1 8N/4W-12C1

8N/4W-9B1

8N/4W-23Q1

8N/4W-10Q1

FLOODPLAIN AQUIFER

REGIONAL AQUIFER

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

WATER-LEVEL ALTITUDE, IN FEET ABOVE SEA LEVEL

LSD = 2,405'

LSD = 2,449'

LSD = 2,355'

LSD = 2,350'

LSD = 2,464'

LSD = 2,489'

DOWNGRADIENT OF HELENDALE FAULT

DOWNGRADIENT OF HELENDALE FAULT

UPGRADIENT OF HELENDALE FAULT

Land-surface altitude

at the well site, in feet

above sea level

LSD

EXPLANATION

Figure 5.—Continued.
28 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

At multiple-well monitoring site 

8N/4W-29E3–6 (figs. 2 and 5), there are significant 

differences in water levels between the upper (Qra) and 

lower (Qya) units of the floodplain aquifer. These 

differences suggest that water levels upgradient of the 

Helendale Fault are affected by recharge from the 

Victor Valley Wastewater Reclamation Authority

(fig. 1). As discussed in the “Description of Study 

Area” section, the discharge of treated municipal 

wastewater by the Victor Valley Wastewater 

Reclamation Authority has maintained perennial flow 

in the Mojave River upstream from the study area since 

1981.

Ground water flows across the Helendale Fault, 

which is the boundary between the Transition zone and 

the Centro subareas, through the floodplain aquifer and 

continues downstream toward Barstow and northward 

to Harper Lake (figs. 1 and 4). Flow across the 

Helendale Fault was estimated for several time periods 

using the ground-water flow model of the Mojave River 

ground-water basin, which spanned 69 years (Stamos 

and others, 2001). Flow across the fault was about 

2,444 acre-ft/yr in 1930 and 720 acre-ft/yr in 1994; the 

average annual flow was 1,566 acre-ft for 1931–90. 

Water levels from the multiple-well monitoring site 

8N/4W-21M1–4 indicate that ground water moves 

downward from the floodplain aquifer to the regional 

aquifer downgradient of the Helendale Fault (fig. 5). 

Unlike the upgradient side of the fault, the historical 

movement of water has been from the floodplain 

aquifer to the regional aquifer and underlying rocks, 

and therefore no reversal of gradient has taken place. 

Further evidence of downward ground-water 

movement is supported by the deuterium data from 

these wells, which is discussed later in the “Oxygen-18 

and Deuterium” section of this report. 

Water-level contours for the area between the 

Helendale Fault and Iron Mountain (fig. 4) indicate that 

some ground water flows northward from the 

floodplain to the regional aquifer. The existence of 

faults in this area may account for the steep water-level 

gradients shown in figure 4. Further evidence of this 

faulting can be seen by a precipitous drop in water 

levels (160 ft) between wells 9N/4W-34D1 and 

9N/4W-34M1 (fig. 4) measured in 2000, which are 

about 0.5 mi apart in (Smith and others, 2003).

The recharge from large stormflows in the 

Mojave River affects water levels in the floodplain 

aquifer on both sides of the Helendale Fault but has 

little effect in the regional aquifer. During water year 

1993 (October 1992 to September 1993), recharge to 

the floodplain aquifer between the Lower Narrows and 

Barstow (fig. 1) was about 190,000 acre-ft, about 

487 percent of average for that reach during water 

years 1931–94 (Lines, 1996). Water levels near the 

Helendale Fault illustrate how quickly the stormflow 

from the winter of 1992–93 recharged the floodplain 

aquifer and how that recharge has continued to affect 

water levels for many years after it occurred, especially 

downgradient from the fault where depth to water is 

generally greater. Wells 8N/4W-10Q1 and 8N/4W12C1 show increases in water levels of more than 20 ft 

immediately after the winter of 1992–93, and water 

levels had not declined to pre-1992–93 levels by 2000 

(fig. 5B). Other stormflows may have helped maintain 

the water levels in these wells since the winter of 1992–

93; however, with the exception of two smaller 

stormflows that reached the USGS gaging station on 

the Mojave River at Barstow in 1995 and 1998, 

streamflow records at the Lower Narrows (fig. 1) show 

a decline in streamflow between the winter of 1992–93 

and the end of 2000 (Julia A. Huff, U.S. Geological 

Survey, written commun., 2003).

Aquifer Tests 

Hydraulic conductivity, transmissivity, and 

storage coefficient values for the floodplain aquifer, 

regional aquifer, and basement rocks were estimated 

from aquifer-test data collected in June and July 1995. 

Interpretations of the hydraulic properties from a 

multiple-well aquifer test (pumping test) and singlewell aquifer tests (slug tests) conducted at the multiplewell monitoring sites are summarized in table 3. The 

data from all the aquifer tests are on file at the USGS 

office in San Diego, California. Because of the 

disparity between the slug-test results and estimates of 

aquifer properties from previously published work 

(Hardt, 1971; Stamos and others, 2001), the aquifer 

properties estimated from the slug tests should be used 

only to compare the relative hydraulic conductivities 

and transmissivities of the wells within the study area.
Geohydrology 29

A multiple-well aquifer test (pumping test) was 

done at well 8N/4W-20Q12 on June 6, 1995, for 

6 hours and 42 minutes at 30 gal/min. The water-level 

responses to pumping were measured in the 2-inch 

monitoring wells 8N/4W-20Q7–11, located 63 ft away 

(figs. 4 and 6). Well 8N/4W-20Q12 is a 4.5-inch 

diameter well that is screened from 99 to 139 ft below 

land surface in the younger alluvial deposits (Qya) of 

the floodplain aquifer (tables 1 and 3). The data from 

the pumping test were analyzed using the method 

developed by Hantush (1960).

Water levels declined in wells 8N/4W-20Q10 

and 8N/4W-20Q11 at the multiple-well monitoring site 

in response to the pumping test (fig. 6). Well 

8N/4W-20Q10 is perforated in the younger alluvium 

(Qya) of the floodplain aquifer—the same as the 

pumping well—and had a water-level decline of about 

10 ft during the test. Well 8N/4W-20Q11 is perforated 

in the recent alluvium (Qra) of the floodplain aquifer 

and had about a 0.5 ft water-level decline. Interestingly, 

wells 8N/4W-20Q7–9, located in the fractured 

basement rocks, actually had water-level increases 

between about 0.5 ft and almost 2 ft. This could be the 

result from ‘unloading’ in the overlying floodplain 

aquifer as water was being removed during the test. 

Additional long-term pumping tests and water-level 

monitoring are needed to explain the water-level 

increases more definitively.

Table 3. Hydraulic properties of water-bearing materials estimated from aquifer tests near Helendale, San Bernardino County, California

[State well No.: See well-numbering diagram in the table of contents; ft/d, foot per day; ft2/d, foot squared per day; —, no data; <, actual value is less than 

value shown]

1Transmissivities were estimated from single-well aquifer tests (slug tests) using type curve solutions from Cooper and others (1967), unless otherwise 

noted.2Storage coefficient is equal to the specific storage (10-6) multiplied by the screened interval of the well (see table 2). 3This zone does not respond rapidly enough to calculate hydraulic values; a transmissivity of <1 ft2/d can be assumed. 4 Hydrologic properties estimated from multple-well aquifer test (pumping well 8N/4W-20Q12) using Hantush (1960). 

State 

well No. Aquifer or unit

Hydraulic 

conductivity

(ft/d)

Transmissivity1

(ft2/d) Storage coefficient

8N/4W-19G1 Basement rocks (pTb) 7 137 3 × 10-2

8N/4W-19G2 Regional (QTof) — 3<1 —

8N/4W-19G3 Floodplain (Qya) 23 465 2 × 10-3

8N/4W-19G4 Floodplain (Qya) 20 378 (

2)

8N/4W-20Q7 Basement rocks (pTb) <1 <1 (

2)

8N/4W-20Q8 Basement rocks (pTb) 2 40 (

2)

8N/4W-20Q9 Basement rocks (pTb) <1 10 (

2)

8N/4W-20Q10 Floodplain (Qya) 4 77 (

2)

8N/4W-20Q10 Floodplain (Qya) 49 4180 47 × 10-4

8N/4W-20Q11 Floodplain (Qra) 9 184 1.3 × 10-1

8N/4W-21M1 Basement rocks (pTb) 3 58 (

2)

8N/4W-21M2 Regional (QTof) <1 2 (

2)

8N/4W-21M3 Floodplain (Qya) 3 68 2 × 10-2

8N/4W-21M4 Floodplain (Qra) 13 128 (

2)

8N/4W-29E3 Basement rocks (pTb) 9 176 (

2)

8N/4W-29E4 Basement rocks (pTb) <1 6 (

2)

8N/4W-29E5 Floodplain (Qya) — — —

8N/4W-29E6 Floodplain (Qra) 11 113 9 × 10-4
30 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

The hydraulic conductivity, transmissivity, and 

storage coefficient values for the floodplain aquifer 

calculated from the pumping test at well 

8N/4W-20Q10 were about 9 ft/d, 180 ft2/d, and 

7 × 10-4, respectively (table 3). The value of 9 ft/d for 

hydraulic conductivity is similar to the values for clean 

sand reported by Freeze and Cherry (1979) but is 

significantly less than the hydraulic conductivity value 

of about 240 ft/d estimated from aquifer-test data 

collected in the floodplain aquifer near Adelanto, about 

10 mi south the study area (Steven Crawford, U.S. 

Geological Survey, written commun., 1995). However, 

the hydraulic conductivity value from the pumping test 

at well 8N/4W-20Q10 is consistent with the poorly 

sorted texture of the younger alluvium (Qya) in the 

study area, and is expected to be lower than the 

hydraulic conductivity values from areas upstream 

where coarser sediments are present. Similarly, the 

transmissivity value of 180 ft2/d from the pumping test 

is much lower than the transmissivity values from the 

calibrated ground-water flow model (Stamos and 

others, 2001). Simulated transmissivity values from 

Stamos and others (2001) were between 1,000 to 

20,000 ft2/d. The storage coefficient from the pumping 

test and the calibrated flow model are essentially the 

same.

01 23 45 67

ELAPSED TIME, IN HOURS

CHANGE IN WATER LEVEL, IN FEET

Q7 (440–460 feet)

Q8 (330–350 feet)

Q9 (250–270 feet)

Q10 (140–160 feet)

Q11 (30–50 feet)

BR

BR

BR

FA

FA

12

10

8

6

4

2

0

–2

Floodplain aquifer

AQUIFER TYPE

Basement rocks

FA

BR

EXPLANATION

Increase Decrease

Multiple-well monitoring site

8N/4W-20Q7–11

Figure 6. Changes in water levels at observation wells 8N/4W-20Q7–11, June 6, 1995, located 63 feet from pumping well 8N/4W-20Q12 near Helendale, 

San Bernardino County, California.
Geohydrology 31

Single-well aquifer tests (slug tests) were used 

primarily to compare the relative hydraulic 

conductivities and transmissivities of the wells within 

the study area. These data helped confirm that coarsegrained, unconsolidated deposits encountered at depth 

near wells 8N/4W-19G3 and 19G4 are part of the 

floodplain aquifer (fig. 3). Slug tests measure aquifer 

response to the addition, or removal, of a slug of known 

volume. The slug used in this study was a 52-inch-long, 

1-inch-diameter polyvinyl chloride pipe filled with 

sand and capped on both ends. Typical water-level 

displacements produced by the slug were between 1 

and 2 ft—actual water-level displacement depends on 

how fast the slug is inserted or withdrawn from the 

water column within the well. Changes in water-level 

in response to the addition or removal of the slug were 

measured with a 10-pounds per square inch pressure 

transducer and recorded on a data logger. 

Computations of transmissivity were analyzed in the 

same manner as the pumping-test data using the 

method developed by Cooper and others (1967). The 

method of Cooper and others (1967) assumes that the 

aquifer is confined, homogeneous, and isotropic, and is 

of uniform thickness, and that flow is horizontal and 

radially symmetric. It further assumes that the aquifer 

is screened throughout its entire thickness. The values 

in table 3 were obtained by assuming that the aquifer 

thickness is equal to the length of the screened interval 

of the monitoring well and that the response is 

influenced over entire screened interval. These 

assumptions may account for some of the variability in 

the values from the slug tests. As pointed out by 

Cooper and others (1967), estimates of storage 

coefficient (S) from slug-test data are problematic 

because the determined value of S is extremely 

sensitive to the choice of the matching type curve.

Transmissivity values estimated from the slug 

tests for wells completed in the floodplain aquifer 

ranged from about 68 to 595 ft2/d (table 3). The 

hydraulic conductivity values, which were derived by 

dividing the transmissivity values by the length of the 

perforated interval at each well, ranged from about 3 to 

30 ft/d. Both the lowest and highest values of hydraulic 

conductivity in the floodplain aquifer were from wells 

completed in the younger alluvium (Qya) of the 

Mojave River. Low hydraulic conductivity values from 

these sediments are consistent with their poorly sorted 

texture, which was determined from the lithologic 

cuttings from the borehole. The higher hydraulic 

conductivity values suggests that sorting varies in the 

younger alluvium and that well-sorted zones have 

significant ability to transmit water. These values are an 

order of magnitude lower than those estimated from a 

pumping test done at a well in the floodplain aquifer 

near Adelanto and from the simulated transmissivity 

values used by Stamos and others (2001).

The value of transmissivity from well 8N/4W21M2, completed in the older fan and stream deposits 

(QTof) of the regional aquifer, is about 13 ft2/d 

(table 3); the corresponding value of hydraulic 

conductivity is less than 1 ft/d. This value of hydraulic 

conductivity is similar to values for silty sand reported 

by Freeze and Cherry (1979). However, similar to the 

results for the floodplain aquifer, the transmissivity 

values are about two orders of magnitude lower than 

the simulated transmissivity values used by Stamos and 

others (2001).

Compared with the hydraulic conductivity values 

from the pumping test and from the ground-water flow 

model (Stamos and others, 2001), the estimated values 

of hydraulic conductivity from the slug tests probably 

represent the lower range of values for the floodplain 

aquifer. This could be attributed, at least in part, to the 

poorly sorted nature of the sediments. The average 

permeability of the aquifer may be anomalously low in 

the study area because the large tributary ravines along 

the west side of the river supplied copious amounts of 

poorly sorted silty sand and gravel that are 

interstratified amongst the cleaner, more permeable 

river deposits. The assumed thickness of the aquifer 

material also contributes to the low values of hydraulic 

conductivity. It was assumed that the interval for which 

a slug tests was being performed—10 or 20 ft in the 

wells in this study—is representative of the entire 

aquifer. In some parts of the study area, the aquifers are 

considerably thicker, especially the regional aquifer. 

The lower values from the slug tests can also be 

attributed to localized but significant differences in the 

hydraulic properties of the aquifer material around a 

well, or to the localized affects that the drilling of a 

well may have on the sediments adjacent to the well. 
32 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Hydraulic conductivity values for seven wells 

completed in the underlying basement rocks ranged 

from less than 1 to 9 ft/d. These are similar to values 

for fractured igneous and metamorphic rocks reported 

by Freeze and Cherry (1979). The extent of fracturing 

of the rocks, which was observed during test drilling 

near the Helendale Fault, is consistent with the 

condition of the basement rock exposed southeast of 

the river at Point of Rocks (fig. 4) and may be related to 

deformation along the Helendale Fault. 

GROUND-WATER QUALITY

Ground-water chemistry data were used to 

evaluate the water in the study area for compliance 

with the U.S. Environmental Protection Agency’s 

maximum and secondary maximum contaminant 

levels. Sixty-seven ground-water samples were 

analyzed for major ions, nutrients, and stable isotopes 

of oxygen and hydrogen from 34 wells (table 1). A 

subset of these wells was sampled for tritium, a 

naturally occurring radioactive isotope of hydrogen, 

and 14C, a naturally occurring radioactive isotope of 

carbon. Samples collected from selected wells between 

1994 and 1997 were supplemented with data from an 

ongoing USGS regional monitoring network between 

1990 and 2000. Some wells were sampled several 

times during this period; for the purposes of this 

discussion, results from wells having multiple analyses 

were averaged and the average value was used to 

compute the 25th quartile, median and 75th quartile 

values discussed in the following section. For the wells 

with multiple analyses, “maximum” and “minimum” 

values are the highest and lowest values sampled. See 

“Data Collection Techniques” section to download a 

file containing the site identification numbers for these 

wells. Complete analyses for all the samples can be 

retrieved from the USGS National database: 

http://waterdata.usgs.gov/ca/nwis/.

Chemical Composition of Water from Wells

Dissolved-solids concentrations in water from 14 

wells sampled in the floodplain aquifer between May 

1990 and November 1999 ranged from 339 to 2,330 

milligrams per liter (mg/L) with a median 

concentration of 825 mg/L (fig. 7). Water from 11 wells 

exceeded the U.S. Environmental Protection Agency 

(USEPA) Secondary Maximum Contaminant Level 

(SMCL) of 500 mg/L (U.S. Environmental Protection 

Agency, 2002). The highest dissolved-solids 

concentration was in water from well 8N/4W-19G4. 

This well is screened near the water table and underlies 

agricultural fields irrigated with treated municipal 

wastewater from the community of Helendale. Water 

from this well had a nitrate concentration of 4.5 mg/L 

(NO2+NO3 was analyzed, but because NO2

concentrations were low, NO2+NO3 was assumed to be 

in the form of NO3, as nitrogen for all samples), the 

second highest nitrate concentration in water sampled 

from wells in the floodplain aquifer. The highest nitrate 

concentration was 15 mg/L (fig. 7) in water from well 

8N/4W-13B1. This concentration exceeded the USEPA 

Maximum Contaminant Level (MCL) for nitrate of 10 

mg/L (U.S. Environmental Protection Agency, 2002). 

Arsenic concentrations in water from 14 wells in 

the floodplain aquifer ranged from less than the 

detection limit of 2 to 34 micrograms per liter (μg/L) 

with a median concentration of 6 μg/L (fig. 7). In 2006, 

the USEPA MCL for arsenic will be 10 μg/L. Water 

from six of the wells exceeded the USEPA 2006 MCL 

for arsenic (U.S. Environmental Protection Agency, 

2002).

Dissolved-solids concentrations in water from 

nine wells sampled in the regional aquifer ranged from 

479 to 946 mg/L with a median concentration of 

666 mg/L. With the exception of well 8N/4W-21M2, at 

least one sample of water from all wells in the regional 

aquifer exceeded the USEPA SMCL for dissolved 

solids; however, the median dissolved-solids 

concentration of water from wells in the regional 

aquifer was lower than the median concentration of 

water from wells in the floodplain aquifer (fig. 7). The 

higher median concentration in the floodplain aquifer 

may occur, in part, because irrigated fields (and 

subsequent irrigation returns with high dissolved-solids 

concentrations) generally are located near the Mojave 

River, which overlies the floodplain aquifer. The 

highest nitrate concentration from the regional aquifer, 

5.3 mg/L, was from well 8N/4W-9R1, located near the 

floodplain aquifer. Nitrogen concentrations in water 

from other wells in the regional aquifer did not exceed 

1.4 mg/L and typically were less than 0.6 mg/L.
Ground-Water Quality 33

6

7

8

9

FA RA BR

pH,

STANDARD UNITS

0

1,000

2,000

3,000

FA RA BR

DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER

0

1,000

2,000

3,000

FA RA BR

CHLORIDE,

IN MILLIGRAMS PER LITER

0

1

2

3

FA RA BR

FLUORIDE,

IN MILLIGRAMS PER LITER

0

5

10

15

FA RA BR

NITRITE PLUS NITRATE AS NITROGEN,

IN MILLIGRAMS PER LITER

0

50

100

150

FA RA BR

ARSENIC,

IN MICROGRAMS PER LITER

EXPLANATION

14/33

8/16

7/13

14/33

15/34

14/33

9/16

12/19

8/15

7/12

[3,190]

7/13

7/12

15/34

14/32

[15]

12/19

8/15

7/13

7/12

7/12

[3,190] Maximum

75th quartile

Median

25th quartile

Minimum

First number is

number of wells;

second number is

number of analyses.

Number in brackets,

if given, is maximum value

Floodplain aquifer

Regional aquifer

FA

RA

BR

EPA

MCL

(10)

EPA

SMCL

(250)

EPA

SMCL

(500)

EPA

SMCL

(2)

EPA

MCL

(10*)

EPA

SMCL

(6.5 -

8.5)

Igneous and

metamorphic

basement rocks

EPA—U.S. Environmental Protection Agency

SMCL—Secondary maximum contaminant level

MCL—Maximum contaminant level

(*) Limit for arsenic will be implemented in 2006

Figure 7. Concentrations of selected chemical constituents and chemical characteristics of water in the floodplain aquifer, regional aquifer, and underlying 

basement rocks near Helendale, San Bernardino County, California, 1990–2000.
34 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Arsenic concentrations in water from eight wells 

in the regional aquifer ranged from less than the 

detection limit of 2 to 130 μg/L with a median 

concentration of 11μg/L. Water samples from five of 

the wells exceeded the USEPA MCL for arsenic. The 

median pH of water from the regional aquifer was 

higher than that of water from the floodplain aquifer 

(fig. 7) and as high as 8.6 in water from well 9N/4W34D1. Increases in pH in the regional aquifer have been 

attributed to slow geochemical reactions such as 

silicate weathering (Izbicki and others, 1995). No 

water sample in the regional or floodplain aquifer 

exceeded the USEPA SMCL of 2 mg/L for fluoride 

(U.S. Environmental Protection Agency, 2002).

Dissolved-solids concentrations in water from 

seven wells completed in the basement rocks that 

underlie the floodplain and regional aquifers ranged 

from 400 to 3,190 mg/L with a median concentration 

of 1,410 mg/L. Water from all but one well sampled 

exceeded the USEPA SMCL for dissolved solids. 

Dissolved-solids concentrations were lowest in water 

from well 8N/4W-21M1, completed in basement rocks 

downgradient from the Helendale Fault. The stable 

isotopes of oxygen and hydrogen, discussed in the next 

section, show that the water from this well initially was 

recharged to the overlying floodplain and regional 

aquifers by infiltration of stormflow in the Mojave 

River and then flowed into the deeper deposits. Nitrate 

concentrations were low in the water samples from the 

basement rocks, typically less than 0.2 mg/L. With the 

exception of two wells (8N/4W-20Q7 and 

8N/4W-29E3), water from the basement rocks 

exceeded the USEPA SMCL for arsenic of 10 μg/L. 

Water from most wells also contained high 

concentrations of chloride, sodium, calcium, sulfate, 

fluoride, and many other constituents. The high 

concentrations of the dissolved constituents are 

probably naturally occurring.

Isotopic Composition of Water from Wells

Samples collected from 34 of the wells in the 

study area between May 1993 and October 1997 were 

analyzed for the stable isotopes of oxygen (oxygen-18) 

and hydrogen (deuterium) to determine the source of 

water to wells and to evaluate the movement of water 

between aquifers. Stable-isotope data collected from 

five wells located about 3 mi upstream of the study area 

were used to supplement the data collected for this 

study. Selected samples were analyzed for the 

radioactive isotopes of hydrogen (tritium) and carbon 

(carbon-14) to determine the age, or time since 

recharge, of the ground water.

Oxygen-18 and Deuterium

Oxygen-18 and deuterium are naturally 

occurring isotopes of oxygen and hydrogen. Oxygen18 and deuterium abundances are expressed as ratios in 

delta notation (δ) as a per mil (parts per thousand) 

difference relative to the standard Vienna Standard 

Mean Ocean Water (VSMOW) (Gonfiantini, 1978). By 

convention, the value of VSMOW is 0 per mil.

Most of the world’s precipitation originates from 

the evaporation of seawater, and the delta oxygen-18 

(δ18O) and delta deuterium (δD) composition of 

precipitation throughout the world is linearly correlated 

and distributed along a line known as the Global 

Meteoric Water Line (fig. 8) (Craig, 1961). The δ18O 

and δD composition of precipitation relative to the 

global meteoric water line and relative to the isotopic 

composition of water from other sources, provides 

evidence of the source of the water. For example, water 

from a given air mass that condensed at higher altitudes 

and cooler temperatures has more of the lighter 

isotopes of oxygen and hydrogen and plots at a 

different position along the meteoric water line than 

water that condensed from the same air mass at lower 

altitudes and warmer temperatures. Conversely, 

evaporation preferentially removes the lighter isotopes, 

and water that has been partly evaporated plots to the 

right of the meteoric water line along a line known as 

the evaporative trend line (fig. 8) (Gat and Gonfiantini, 

1981). In some areas, the processes that occur during 

condensation, precipitation, or prior to ground-water 

recharge may create a slightly different distribution of 

δ18O and δD data along a local meteoric water line. 

Information about the source and evaporative history of 

water can be used to evaluate the movement of water 

between aquifers. Because ground water moves slowly, 

isotopic data often preserve a record of ground-water 

recharge and movement under predevelopment 

conditions. This is especially useful in areas where 

traditional hydrologic data, such as water levels, have 

been altered by pumping, changes in recharge and 

discharge, or other changes that have occurred as a 

result of human activities. 
Ground-Water Quality 35

-120

-100

-80

-60

-40

-14 -12 -10 -8 -6

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

Floodplain aquifer

Regional aquifer

Regional aquifer about 3 miles upstream

from study area. (Wells 7N/5W-23R1–3

and 7N/5W-24R5,6)

Igneous and metamorphic

basement rocks

EXPLANATION

Global Meteoric Water Line

(Craig, 1961)

Local Meteoric Water Line

Evaporative

trend line

14N1

20Q9

21M2

24J2

23Q1

29E3

19G3

29E4

21M1

20Q8

19G4

34D1

Well—Number given if discussed in text

Figure 8. Delta oxygen-18 and delta deuterium from wells in the floodplain aquifer, regional aquifer, and underlying basement 

rocks near Helendale, San Bernardino County, California, 1994–97.
36 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Under predevelopment conditions, the floodplain 

aquifer near Helendale mainly was recharged by 

infiltration of winter stormflows from the Mojave 

River. Izbicki and others (1995) showed that winter 

stormflow in the Mojave River was the result of 

precipitation at low altitudes near Cajon Pass (fig. 1) 

which funnels storms passing northward across the San 

Gabriel and San Bernardino Mountains. Cool, moist air 

masses associated with winter storms enter the Mojave 

Desert through Cajon Pass and precipitate without 

orographic uplift and subsequent cooling over the 

higher elevations of the surrounding mountains. As a 

result, precipitation that falls near Cajon Pass has δD 

values that are as much as 20 per mil heavier than 

precipitation that falls in the surrounding mountains 

(Izbicki and others, 2002). The resulting isotopic 

composition of the recharge and subsequent ground 

water has been identified by Freidman and others 

(1992), Gleason and others (1994), Izbicki and others 

(1995), and Williams and Rodini (1997) as far as Afton 

Canyon, almost 100 mi downstream from the 

headwaters of the Mojave River.

The water sampled from 15 wells in the 

floodplain aquifer had δ18O compositions that ranged 

from −8.0 to −11.0 per mil (fig. 8), with a median value 

of −8.8 per mil. The δD composition ranged from 

−59 to −82 per mil, with a median value of −61.6 per 

mil. These values are consistent with the δ18O and δD 

composition of water from wells in the floodplain 

aquifer upstream from Helendale (Izbicki and others, 

1995) and the δD composition of water from wells 

along the Mojave River (Freidman and others, 1992; 

Gleason and others, 1994). The heaviest (least 

negative) δ18O and δD values plot along an evaporative 

trend line (fig. 8) and are from wells having higher 

dissolved-solids concentrations. The lightest (most 

negative) δ18O and δD values in the floodplain aquifer 

were in water from wells 8N/4W-19G3 and 8N/4W19G4, which are near the contact of the regional 

aquifer (fig. 3A). Water from these wells was not 

recharged by infiltration of surface flow in the Mojave 

River but was discharged upward from the regional 

aquifer to the floodplain aquifer upstream from the 

Helendale Fault. As discussed earlier in the 

“Ground-Water Movement near the Helendale Fault” 

section, discharge from the regional aquifer to the 

floodplain aquifer upgradient of the Helendale Fault is 

not apparent on the basis of present-day water-level 

data. Water from well 8N/4W-19G4 has high dissolved 

solids and plots to the right of the meteoric water line 

as a result of evaporation.

Under predevelopment conditions, the regional 

aquifer was recharged largely by infiltration of surface 

flow from small streams near the front of the San 

Gabriel and San Bernardino Mountains. These streams 

flow intermittently as a result of runoff from winter 

storms and snowmelt. Due to orographic uplift and 

subsequent cooling, the δ18O and δD composition of 

precipitation and resulting runoff from small streams is 

lighter (more negative) than the composition of 

precipitation that falls near Cajon Pass (Izbicki and 

others, 2002).

The water from 12 wells sampled in the regional 

aquifer had δ18O compositions that ranged from

−8.5 to −12.4 per mil, with a median value of −11.4 per 

mil (fig. 8). The δD composition ranged from

−60 to −98 per mil, with a median value of −86 per mil. 

These values are consistent with the δ18O and δD 

composition of water from wells in the regional aquifer 

located about 3 mi upstream of the study area (shown 

in blue in figure 8)—except that the wells in the study 

area plot farther to the right of the global meteoric 

water line than water from the upstream wells.

The heaviest δ18O and δD values in the regional 

aquifer were in water from well 8N/4W-14N1. Water 

from this well had an isotopic (and chemical) 

composition similar to water from wells in the 

floodplain aquifer. This well is located in the regional 

aquifer downgradient from the Helendale Fault. Water 

from well 8N/4W-21M2, also completed in the 

regional aquifer downgradient from the Helendale 

Fault, was similarly affected, although to a lesser 

extent. Present-day water-level data collected from 

multiple-well monitoring site 8N/4W-21M1–4 are 

consistent with the δ18O and δD data that show a 

downward hydraulic gradient in this area, indicating 

ground-water flow from the floodplain to the regional 

aquifer.
Ground-Water Quality 37

δ18O and δD data do not indicate significant 

movement of water from the floodplain aquifer toward 

Harper Lake (dry) west of Iron Mountain. This 

suggests that the fault in that area (Iron Mountain 

Fault) (fig. 2) is an effective barrier to ground-water 

flow. The isotopic data are consistent with the results 

from the regional ground-water flow model (Stamos 

and others, 2001) which simulated this fault as a barrier 

to ground-water flow.

There is only one well (8N/4W-19G2) completed 

in the regional aquifer upgradient of the Helendale 

Fault in the study area (fig. 3A). Data from wells at two 

multiple-well monitoring sites (7N/5W-23R1–3 and 

7N/5W-24R5 and 24R6) located about 3 mi upstream 

of the study area were used to supplement δ18O and δD 

data near the Helendale Fault and to provide 

information on the isotopic composition of water from 

the regional aquifer upgradient from the study area. 

The δ18O and δD composition of water from the 

upstream wells was similar to the composition of water 

from most of the wells in the study area—except that 

the wells located upstream plot closer to the meteoric 

water line. In contrast, water from three wells in the 

regional aquifer downgradient from the Helendale 

Fault (9N/4W-34D1, 8N/4W-23Q1 and 8N/4W-24J2) 

(fig. 4) plot farther to the right of the meteoric water 

line than water from wells elsewhere in the study area. 

Along with water from some wells in the underlying 

basement rocks, the data from these three wells define 

a local meteoric water line parallel to, and below, the 

global meteoric water line (fig. 8). The isotopic 

composition of water from these three wells is similar 

to the composition of water from wells in the Mojave 

Desert to the east of the study area that were recharged 

by infiltration of runoff from the local mountains. 

Because of the location of these wells, it is possible 

that they had not been recharged by infiltration of water 

near the front of the San Gabriel and San Bernardino 

Mountains but instead had been recharged by 

infiltration of runoff from local mountains. The shift in 

the meteoric water line probably is due to partial 

evaporation prior to recharge in the dry desert climate.

The water from seven wells sampled in the 

basement rocks that underlie the floodplain and 

regional aquifer in the study area had δ18O 

compositions that ranged from −9.3 to −10.6 per mil, 

with a median value of −10.3 per mil (fig. 8). The δD 

composition ranged from −67 to −85 per mil, with a 

median value of −76 per mil. These values are between 

the composition of the water in the floodplain and 

regional aquifers. The heaviest values were in water 

from wells 8N/4W-20Q8 and 20Q9, which are 

upgradient of the Helendale Fault, and 

8N/4W-21M1, which is downgradient from the fault; 

the water from these wells is similar in composition to 

the water from the wells in the floodplain aquifer. The 

presence of heavy δD values in the regional aquifer 

supports the conclusions drawn from the water-level 

data, which show a downward gradient of ground water 

from the floodplain to the regional aquifer in this area. 

The lightest values were in water from wells 8N/4W29E3 and 29E4 upgradient of the Helendale Fault. The 

data for these wells plot along the local meteoric water 

line and are similar in composition to water from wells 

9N/4W-34D1, 8N/4W-23Q1, and 8N/4W-24J1 in the 

regional aquifer. As previously discussed, this water 

may have been recharged as infiltration of runoff from 

the local mountains. Carbon-14 data, presented in the 

next section, show that water from these wells is very 

old and that recharge from this local source may no 

longer occur under present-day climatic conditions.

Tritium and Carbon-14

Tritium is a naturally occurring radioactive 

isotope of hydrogen having a half-life of 12.4 years. 

The concentration of tritium is measured in tritium 

units (TU); each tritium unit equals one atom of tritium 

in 1018 atoms of hydrogen. Approximately 800 

kilograms of tritium was released as a result of the 

atmospheric testing of nuclear weapons between 1952 

and 1962 (Michel, 1976). As a result, tritium 

concentrations in precipitation and ground-water 

recharge increased during that time. Because tritium is 

part of the water molecule, tritium concentrations are 

not affected significantly by chemical reactions other 

than radioactive decay. Therefore, tritium is an 

excellent tracer of the movement and relative age of 

water on timescales ranging from recent to about 50 

years before present (post 1952). In this report, ground 

water that has measurable tritium is interpreted as 

water recharged after 1952.
38 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Tritium concentrations in water from 16 wells 

ranged from 6.2 TU to less than the detection limit of 

0.2 TU. All the wells sampled in the floodplain aquifer 

had measurable tritium and therefore at least some 

water from these wells is believed to have been 

recharged after 1952. Water samples containing tritium 

were used in conjuction with the samples from the 

aquifer materials obtained while drilling the multiplewell monitoring sites to confirm the depth and lateral 

extent of the floodplain aquifer (fig. 3). Water from 

only one well (8N/4W-9R1) in the regional aquifer had 

measurable tritium (0.9 TU). Because this well is near 

the floodplain aquifer (fig. 4), the relatively low tritium 

concentration may be due to the movement of a small 

amount of water from the floodplain aquifer owing to 

pumping. Water from most of the wells in the regional 

aquifer and the underlying basement rocks did not 

contain tritium and therefore was recharged prior to 

1952.

Carbon-14 (14C) is a naturally occurring 

radioactive isotope of carbon that has a half-life of 

about 5,730 years (Mook, 1980). 14C data are 

expressed as percent modern carbon (pmc) by 

comparing 14C activities to the specific activity of 

National Bureau of Standards oxalic acid: 12.88 

disintegrations per minute per gram of carbon in the 

year 1950 equals 100 percent modern carbon. 14C was 

produced, as was tritium, by the atmospheric testing of 

nuclear weapons (Mook, 1980). As a result, 14C 

activities may exceed 100 pmc in areas where ground 

water contains tritium. 14C activities are used to 

determine the age of a ground-water sample on 

timescales ranging from recent to more than 20,000 

years before present. Because 14C is not part of the 

water molecule, its activity in water is affected by 

chemical reactions that may remove carbon from 

solution or add carbon to solution. In addition, 14C 

activities are affected by the mixing of younger water 

that has high 14C activity with older water that has low 

14C activity. 14C ages presented in this paper do not 

account for changes in 14C activity resulting from 

chemical reactions or mixing and therefore are 

considered uncorrected ages. In general, uncorrected 

14C ages are older than the actual age of the associated 

water. Izbicki and others (1995) estimated that 

uncorrected 14C ages were as much as 30 percent older 

than actual ages for ground water in the regional 

aquifer near Victorville, upstream of the study area. 

The uncorrected 14C ages for ground water from the 

regional aquifer and the underlying basement rocks at 

the multiple-well monitoring sites are shown in 

figure 3. In general, uncorrected 14C ages increase with 

depth and are younger near and downgradient from the 

Helendale Fault.

The 14C activity in water from 17 wells ranged 

from 117 to 12 pmc (fig. 9). In general, 14C activities 

were higher in water from wells in the floodplain 

aquifer and lower in water from wells in the regional 

aquifer and underlying basement rocks (fig. 9). There 

are exceptions to this general rule; 14C activities may 

be lower in areas where water has moved between 

aquifers or is a mixture of water from different sources. 

For example, water from well 8N/4W-19G3 has a 

comparatively low 14C activity for the floodplain 

aquifer—δ18O and δD data indicate that water from 

this well originated as discharge from the regional 

aquifer upgradient of the Helendale Fault. In contrast, 

water from well 8N/4W-9R1 has a comparatively high 

14C activity for water from wells in the regional 

aquifer—tritium data indicate that water in this well 

contains some recently recharged water from the 

floodplain aquifer. Similarly, water from two wells in 

the underlying basement rocks, 8N/4W-20Q8 

(upgradient of the Helendale Fault) and 8N/4W-21M1 

(downgradient from the fault), and one well in the 

regional aquifer, 8N/4W-21M2 (also downgradient 

from the fault), have comparatively high 14C activities. 

The δ18O and δD data for these three wells indicate 

that they contain water from the floodplain aquifer.
Ground-Water Quality 39

0

20

40

60

80

100

120

SAMPLES, IN RANK ORDER

CARBON-14 ACTIVITY, IN PERCENT MODERN CARBON

19G3

9R1

34D1

20Q8

21M1

21M2

Floodplain aquifer

Regional aquifer

Igneous and metamorphic

basement rocks

EXPLANATION

Well—Number given if discussed in text

Figure 9. Carbon-14 activity of water from wells in the floodplain aquifer, regional aquifer, and underlying 

basement rocks near Helendale, San Bernardino County, California, 1994–97.
40 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

When the water from wells of mixed sources is 

excluded, the 14C activity of water from wells in the 

floodplain aquifer, regional aquifer, and the underlying 

basement rocks ranged from 117 to 88 pmc, 43 to 

12 pmc, and 38 to 27 pmc, respectively. These 14C 

activities correspond to uncorrected ground-water ages 

which range from recent for the floodplain aquifer, 

from 6,980 to 17,500 years before present for the 

regional aquifer, and from 8,000 to 10,800 years before 

present for the underlying basement rocks. The great 

age of the water in the regional aquifer reflects the long 

flow paths and times of travel from the recharge areas 

near the front of the San Gabriel and San Bernardino 

Mountains. However, the δ18O and δD data from well 

9N/4W-34D1, which had the lowest 14C activity and 

presumably the oldest ground water, indicate that this 

well was not recharged by infiltration from intermittent 

streams near the front of the San Gabriel and San 

Bernardino Mountains, but rather by infiltration of 

runoff from the local mountains near and within the 

study area. Because presumed flow paths from the local 

recharge areas are short, the age of this water suggests 

that recharge from any local sources is small and may 

not be occurring under present-day climatic conditions. 

This is consistent with the distribution of mountainfront recharge used in the ground-water flow model of 

the Mojave River ground-water basin by Stamos and 

others (2001), which did not simulate any local 

recharge in this area.

SUMMARY

The Mojave River Basin is located in the western 

part of the Mojave Desert in southern California. The 

proximity to the Los Angeles area has led to a rapid 

growth in population and, consequently, to an increase 

in the demand for water. The Mojave River, the 

primary source of surface water for the region, 

normally is dry—except during periods of flow after 

intense storms; therefore, the region relies almost 

entirely on ground water to meet its agricultural and 

municipal needs. The area around the Helendale Fault 

is of particular hydrogeologic interest because of its 

importance as a boundary between two watermanagement subareas of the Mojave Water Agency. 

The Helendale Fault is the boundary between the upper 

Mojave River Basin (Oeste, Alto, and Este subareas) 

and the lower Mojave River Basin (Centro and Baja 

subareas); specifically, it is the boundary of the 

Transition zone of the Alto and the Centro subareas. 

The purpose of this study was to map the surficial 

geology of the area near the Helendale Fault, to define 

the vertical and horizontal extent of the aquifers in this 

area, and to determine what effects the Helendale Fault 

has on ground-water flow and water quality. Selected 

wells in the study area were sampled and analyzed to 

determine the concentration of dissolved constituents 

and to approximate the age and source(s) of recharge of 

the water in the alluvial aquifers and underlying 

basement rocks. 

Data from four multiple-well monitoring sites 

installed as part of this study were used to help 

determine the extent of the floodplain aquifer, the 

regional aquifer, and the underlying basement rocks. 

The insights gained by the detailed mapping of the 

surficial geology, stratigraphy, and effects of the 

Helendale Fault and other faults and folds were used in 

conjuction with borehole data from the multiple-well 

monitoring sites to reveal information about the extent 

of the individual aquifers and underlying basement 

rocks. The younger fan deposits, older alluvium of the 

ancestral Mojave River, older fan and stream deposits, 

and playa deposits constitute the regional aquifer, 

which underlies and surrounds the recent and younger 

alluvium of the floodplain aquifer. Aquifer-test data 

show that hydraulic conductivity and transmissivity 

values from the floodplain aquifer are one to two orders 

of magnitude higher than those for the regional aquifer.

The detailed surficial geologic mapping and the 

water-level measurements indicate that the Helendale 

Fault impedes the flow of ground water in the deeper 

regional aquifer, but not in the overlying floodplain 

aquifer. Other faults mapped in the area impede the 

flow of ground water in both aquifers. The historical 

record contains evidence of upward flow of water on 

the upgradient side of the Helendale Fault, but data 

from this study indicate that water levels in the 

floodplain aquifer are higher than those in the 

underlying regional aquifer and basement rocks. Waterlevel and isotopic data, primarily sampled from the 

multiple-well monitoring sites installed as part of this 

study, indicate that pumping upstream of the Helendale 

Fault has reversed the vertical gradient of ground-water 

flow since predevelopment conditions, and the 

potential now exists for water to flow downward from 

the floodplain aquifer to the regional aquifer. 
References Cited 41

Dissolved-solids concentrations in water from 

14 wells sampled in floodplain aquifer ranged from 339 

to 2,330 mg/L with a median concentration of 825 

mg/L. Water from 11 wells exceeded the USEPA 

SMCL of 500 mg/L. Dissolved-solids concentrations 

of water from nine wells sampled in the regional 

aquifer ranged from 479 to 946 mg/L with a median 

concentration of 666 mg/L. With the exception of water 

from one well, at least one sample of water from all 

wells in the regional aquifer exceeded the USEPA 

SMCL for dissolved solids. Arsenic concentrations in 

water from 14 wells in the floodplain aquifer ranged 

from less than the detection limit of 2 to 34 μg/L with a 

median concentration of 6 μg/L. Water from six wells 

exceeded the USEPA MCL for arsenic of 10 μg/L. 

Arsenic concentrations in water from nine wells in the 

regional aquifer ranged from less than the detection 

limit of 2 to 130 μg/L with a median concentration of 

11 μg/L. Water from six of these wells exceeded the 

USEPA MCL for arsenic. Dissolved-solids 

concentrations of water from seven wells completed in 

the basement rocks that underlie the floodplain and 

regional aquifers ranged from 400 to 3,190 mg/L with a 

median concentration of 1,410 mg/L. Water from all 

but one well exceeded the USEPA SMCL for dissolved 

solids. With the exception of two wells, water from the 

basement rocks exceeded the USEPA SMCL for 

arsenic of 10 μg/L. The high concentrations of arsenic 

and other constituents probably are naturally occurring.

Stable isotopes of oxygen and hydrogen indicate 

that under predevelopment conditions, the floodplain 

aquifer near Helendale was recharged by infiltration of 

winter stormflows from the Mojave River and that the 

regional aquifer was recharged by the infiltration of 

surface flow from small streams near the front of the 

San Gabriel and San Bernardino Mountains, locally 

supplemented by infiltration of runoff from local 

mountains. Tritium, the radioactive isotope of 

hydrogen, was present in every well tested in the 

floodplain aquifer, indicating that at least some of the 

water was recharged after 1952. Water from most wells 

in the regional aquifer and underlying basement rocks 

did not contain tritium and thus was recharged prior to 

1952. Carbon-14, a naturally occurring radioactive 

isotope of carbon, was used to estimate the relative age 

of the water from wells in the regional aquifer and the 

underlying basement rocks. In general, uncorrected 

carbon-14 activities increase with depth and are 

relatively younger near and downgradient of the 

Helendale Fault. The uncorrected carbon-14 activities 

correspond to uncorrected ground-water ages which 

range from recent for the floodplain aquifer, from 6,980 

to 17,500 years before present for the regional aquifer, 

and from 8,000 to 10,800 years before present for the 

underlying basement rocks. The great age of the water 

in the regional aquifer reflects the long flow paths and 

times of travel from the recharge areas near the front of 

the San Gabriel and San Bernardino Mountains. It is 

also possible that a small amount of recharge may have 

occurred from streams draining local mountains near 

and within the study area during past climatic and 

hydrologic conditions that were different than those of 

the present-day Mojave Desert. 

REFERENCES CITED

Bartley, J.M., Glazner, A.F., and Schermer, E.R., 1990, 

North–south contraction of the Mojave block and 

strike-slip tectonics in southern California: Science, 

v. 248, p. 1398–1401.

Beukins, R.P., 1992, Radiometric accelerator mass 

spectrometry: Background, precision, and accuracy, in

Taylor, R.E., A. Long, and R.S., Kra, eds., Radiocarbon 

after four decades: Springer-Verlag, New York,

p. 230–239.

Biehler, Shawn, Tang, R.W., Ponce, D.A., and Oliver, H.W., 

compilers, 1988, Bouguer gravity map of the San 

Bernardino quadrangle, California: California Division 

of Mines and Geology Regional Geologic Map Series, 

map no. 3B, scale 1:250,000.

Boettcher, S.S., and Walker, J.D., 1993, Geologic evolution 

of Iron Mountain, central Mojave Desert, California: 

Tectonics, v. 12, p. 372–386.

Bortugno, E.J., compiler, 1986, Map showing recency of 

faulting, San Bernardino quadrangle, California: 

California Division of Mines and Geology, Regional 

Geologic Map Series, map no. 3A (geology), 

sheet 5 of 5, scale 1:250,000.

Bortugno, E.J., and Spittler, T.E., compilers, 1986, Geologic 

map of the San Bernardino quadrangle, California: 

California Division of Mines and Geology, Regional 

Geologic Map Series, map no. 3A (geology), 

sheet 1 of 5, scale 1:250,000.

Bowen, O.E., Jr., 1954, Geology and mineral deposits of the 

Barstow quadrangle, San Bernardino County, 

California: California Division of Mines, Bulletin 165, 

185 p.

California Department of Water Resources, 1967, Mojave 

River ground water basins investigation: Bulletin 84, 

151 p.
42 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Cooper, H.H., Jr., Bredehoeft, J.D., and Papadopolous, I.S., 

1967, Response of a finite diameter well to an 

instantaneous charge of water: Water Resources 

Research, v. 3, no. 1, p. 263–269.

Cox, B.F., and Hillhouse, J.W., 2000, Pliocene and 

Pleistocene evolution of the Mojave River, and 

associated tectonic evolution of the Transverse Ranges 

and Mojave Desert, based on borehole stratigraphy 

studies near Victorville, California: U.S. Geological 

Survey Open-File Report 00-147, 66 p.

Cox, B.F., Hillhouse, J.W., and Owen, L.A., 2003, Pliocene 

and Pleistocene evolution of the Mojave River, and 

associated tectonic development of the Transverse 

Ranges and Mojave Desert, based on borehole 

stratigraphy studies and mapping of landforms and 

sediments near Victorville, California, in Enzel, Y, 

Wells, S.G., and Lancaster, N., eds., Paleoenvironments 

and Paleohydrology of the Mojave and Southern Great 

Basin Deserts, Geological Society of America Special 

Paper 368, p. 1–42.

Cox, B.F., Hillhouse, J.W., Sarna–Wojcicki, A.M., and 

Tinsley, J.C., III, 1998, Pliocene–Pleistocene 

depositional history along the Mojave River north of 

Cajon Pass, California–Regional tilting and drainage 

reversal during uplift of the central Transverse Ranges: 

Geological Society of America, Abstracts with 

Programs, v. 30, no. 5.

Cox, B.F., and Tinsley, J.C., III, 1999, Origin of the late 

Pliocene and Pleistocene Mojave River between Cajon 

Pass and Barstow, California, in Reynolds, R.E., and 

Reynolds, Jennifer, Tracks along the Mojave: A field 

guide from Cajon Pass to the Calico Mountains and 

Coyote Lake: San Bernardino County Museum 

Association Quarterly, v. 46, no. 3, p. 49–54.

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

Science 133, 1702–1703.

Densmore, J.N., Cox, B.F., and Crawford, S.M., 1997, 

Geohydrology and water quality of Marine Corps 

Logistics Base, Nebo and Yermo Annexes, near 

Barstow, California: U.S. Geological Survey WaterResources Investigations Report 96-4301, 116 p.

Dibblee, T.W., Jr., 1960a, Geologic map of the Hawes 

quadrangle, San Bernardino County, California: U.S. 

Geological Survey Mineral Investigations Field Studies 

Map MF-226, scale 1:62,500.

______1960b, Preliminary geologic map of the Victorville 

quadrangle, California: U.S. Geological Survey Mineral 

Investigations Field Studies Map MF-229, scale 

1:62,500.

______1960c, Geologic map of the Barstow quadrangle, San 

Bernardino County, California: U.S. Geological Survey 

Mineral Investigations Field Studies Map MF-233, 

scale 1:62,500.

______1961, Evidence of strike-slip faulting along 

northwest-trending faults in the Mojave Desert: U.S. 

Geological Survey Professional Paper 424-B, 

p. B197–B199.

______1967, Areal geology of the western Mojave Desert, 

California: U.S. Geological Survey Professional Paper 

522, 153 p., map scale 1:125,000.

Dokka, R.K., and Travis, C.J., 1990, Late Cenozoic strikeslip faulting in the Mojave Desert, California: 

Tectonics, v. 9, no. 2, p. 311–340.

Foster, J. H., 1980, Late Cenezoic tectonic evolution of 

Cajon Valley, southern California [Ph.D. dissertation]: 

Riverside, University of California, 243 p.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: 

Englewood Cliffs, New Jersey, Prentice-Hall, Inc., 

604 p.

Freidman, Irvine, Smith, G.I., Gleason, J.D., Warden, A., and 

Harris, J.M., 1992, Stable isotope composition of 

waters in southeastern California 1. Modern 

precipitation: Journal of Geophysical Research, v. 97, 

p. 5795–5812.

Gat, J.R., and Gonfiantini, R., 1981, Stable isotope 

hydrology, deuterium and oxygen-18 in the water cycle: 

International Atomic Energy Agency, Technical Reports 

Series No. 210, 339 p.

Gleason, J.D., Veronda, Guida., Smith, G.I., and Friedman, 

Irving, 1994, Deuterium content of water from wells 

and perennial springs, southeastern California: U.S. 

Geological Survey Hydrologic Investigations Atlas 

HA-727, 1 map.

Gonfiantini, R., 1978, Standards for stable isotope 

measurements in natural compounds: Nature, v. 271, 

p. 534–536.

Hantush, M.S., 1960, Modification of the theory of leaky 

aquifers, Journal of Geophysics Research, v. 65, 

p. 3713–3725.

Hardt, W.F., 1971, Hydrologic analysis of Mojave River 

basin, California, using electric analog model: U.S. 

Geological Survey Open-File Report, 84 p.

Howard, K.A., and Cox, B.F., 2001, Geologic strain 

partitioning in the eastern California shear zone: riding 

crustal waves in the Mojave Desert: Geological Society 

of America Abstracts with Programs, v. 33, no. 3, 

p. 73–74.

Howard, K.A., and Miller, D.M., 1992, Late Cenozoic 

faulting at the boundary between the Mojave and 

Sonoran blocks: Bristol Lake area, California, in

Richard, S.M., ed., Deformation associated with the 

Neogene Eastern California Shear Zone, southwestern 

Arizona and southeastern California: Redlands, 

California, San Bernardino County Museum Special 

Publication, p. 37–47.
References Cited 43

Huff, J.A., Clark, D.A., and Martin, Peter, 2002, Lithologic 

and ground-water data for monitoring sites in the 

Mojave Water Agency Management area, San 

Bernardino County, California, 1992-98: U.S. 

Geological Survey Open-File Report 02-354, 415 p.

Izbicki, J.A., Martin, Peter, and Michel, R.L., 1995, Source, 

movement, and age of groundwater in the upper part of 

the Mojave River Basin, California, USA, in Adar, 

E.M., and Leibundgut, Christian, eds., Application of 

tracers in arid zone hydrology: International 

Association of Hydrological Sciences, no. 232, 

p. 43–56.

Izbicki, J.A., Radyk, J., and Michel, R.L., 2002, Movement 

of water through the thick unsaturated zone underlying 

Oro Grande and Sheep Creek Washes in the western 

Mojave Desert, USA: Hydrogeology Journal, v. 10, p. 

409–427.

Jennings, C.W., compiler, 1994, Fault activity map of 

California and adjacent areas, with locations and ages 

of recent volcanic eruptions: California Division of 

Mines and Geology, Geologic Data Map no. 6, scale 

1:750,000.

Lines, G.C., 1996, Ground-water and surface-water relations 

along the Mojave River, Southern California: U.S. 

Geological Survey Water-Resources Investigations 

Report 95-4189, 43 p.

Mabey, D.R., 1960, Gravity survey of the western Mojave 

Desert, California: U.S. Geological Survey Professional 

Paper 316-D, p. 51–73.

Matti, J.C., and Morton, D.M., 1993, Paleogeographic 

evolution of the San Andreas Fault in southern 

California: A reconstruction based on a new cross-fault 

correlation, in Powell, R.E., Weldon, R.J., II, and Matti, 

J.C., eds., The San Andreas Fault System: 

Displacement, Palinspastic Reconstruction, and 

Geologic Evolution: Boulder, Colorado, Geological 

Society of America Memoir 178, p. 107–159.

Meisling, K.E., and Weldon, R.J., 1989, Late Cenozoic 

tectonics of the northwestern San Bernardino 

Mountains, southern California: Geological Society of 

America Bulletin, v. 101, p. 106–128.

Mendez, G.O., and Christensen, A.H., 1997, Regional water 

table (1996) and water-level changes in the Mojave 

River, the Morongo, and the Fort Irwin ground-water 

basins, San Bernardino County, California: U.S. 

Geological Survey Water-Resources Investigations 

Report 97-4160, 34 p., 1 pl.

Michel, R.L., 1976, Tritium inventories of the worlds oceans 

and their implications: Nature, v. 263, p. 103-106.

Miller, E.L., 1977, Geology of the Victorville region, 

California [Ph.D. dissertation]: Houston, Texas, Rice 

University, 226 p.

______1981, Geology of the Victorville region, California: 

Geological Society of America Bulletin, Part II, v. 92, p. 

554–608.

Miller, F.K., and Morton, D.M., 1980, Potassium-argon 

geochronology of the eastern Transverse Ranges and 

southern Mojave Desert, southern California: U.S. 

Geological Survey Professional Paper 1152, 30 p.

Mojave Water Agency, 1996, Second annual report of the 

Mojave Basin area watermaster: Apple Valley, 

California, 133 p.

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

in: Fritz, P., and Fontes, J.C. (eds.), Handbook of 

environmental isotope geochemistry, volume 1 The 

terrestrial environment: New York, Elsevier Scientific 

Publishing Company, p. 49–74.

Ostlund, H.G., and Warner, E., 1962, Electrolytic enrichment 

of tritium and deuterium for natural tritium 

measurements. Tritium in the physical and biological 

sciences: Vienna, Austria, International Atomic Energy 

Agency, vol. 1, p. 96–104.

Rector, C.H., Swenson, J.D., and Wilke, P.J., 1983, 

Archaeological studies at Oro Grande, Mojave Desert, 

California: Redlands, California, San Bernardino 

County Museum Association, 181 p.

Rector, Carol, 1999, Human and animal trackway at Oro 

Grande, in Reynolds, R.E., ed., Fossil footprints: 

Redlands, California, San Bernardino County Museum 

Association, 1999 Desert Research Symposium, 

Abstracts from proceedings, p. 53–55.

Reynolds, R.E., and Reynolds, R.L., 1985, Late Pleistocene 

faunas from Daggett and Yermo, San Bernardino 

County, California, in Reynolds, R.E., compiler, 

geologic investigations along Interstate 15, Cajon Pass 

to Manix Lake, California: San Bernardino County 

Museum, p. 175–191.

______1991, Structural implications of late Pleistocene 

faunas from the Mojave River Valley, California, in

Woodburne, M.O., Reynolds, R.E., and Whistler, D.P., 

eds., Inland southern California: The last 70 million 

years: San Bernardino County Museum Association 

Quarterly, v. 38, nos. 3 and 4, p. 100–105.

Schermer, E.R., 1993, Mesozoic structural evolution of the 

west-central Mojave Desert, in Dunn, G., and 

McDougall, K., eds., Mesozoic paleogeography of the 

western United States-II: Pacific Section SEPM, book 

71, p. 307–322.

Smith, G.A., 2003, Regional water table (2000) and groundwater-level changes in the Mojave River and the 

Morongo ground-water basins, southwestern Mojave 

Desert, California: U.S. Geological Survey WaterResources Investigations Report 02-4277, 37 p.
44 Geologic Setting, Geohydrology, Ground-Water Quality, Helendale Fault, Mojave River Basin, San Bernardino County, California

Smith, G.A., and Pimentel, M.I., 2000, Regional water table 

(1998) and ground-water-level changes in the Mojave 

River and the Morongo ground-water basins, San 

Bernardino County, California: U.S. Geological Survey 

Water-Resources Investigations Report 00-4090, 107 p. 

Stamos, C.L., Martin, Peter, Nishikawa, Tracy, and Cox, 

B.F., 2001, Simulation of ground-water flow in the 

Mojave River ground-water basin, California: U.S. 

Geological Survey Water-Resources Investigations 

Report 01-4002, 129 p.

Stamos, C.L., and Predmore, S.K., 1995, Data and watertable map of the Mojave River ground-water basin, San 

Bernardino County, California, November 1992: U.S. 

Geological Survey Water-Resources Investigations 

Report 95-4148, scale 1:125,000.

Subsurface Surveys, Inc., 1990, Inventory of ground water 

stored in the Mojave River basins: unpublished report, 

prepared for Mojave Water Agency, Apple Valley, 

California, 47 p.

Thatcher, L.L., Janzer, V.J., and Edwards, K.W., 1977, 

Methods for determination of radioactive substances in 

water and fluvial sediments: Techniques of WaterResources Investigations of the U.S. Geological Survey, 

chap. A-5, 95 p.

U.S. Environmental Protection Agency, 2002, 2002 Edition 

of the drinking water standards and health advisories: 

EPA 822-R-02-038, Office of Water, U.S., 

Environmental Protection Agency, Washington, DC, 

accessed December 3, 2002 at URL 

http://epa.gov/waterscience/drinking/standards/.

Walker, C.J., 1986, Back door to California--the story of the 

Mojave River Trail: Barstow, California, Mojave River 

Valley Museum Association, 337 p.

Weldon, R.J., II, Meisling, K.E., and Alexander, J., 1993, A 

speculative history of the San Andreas fault in the 

central Transverse Ranges, California, in Powell, R.E., 

Weldon, R.J., II, and Matti, J.C., eds., The San Andreas 

Fault System: Displacement, Palinspastic 

Reconstruction, and Geologic Evolution: Boulder, 

Colorado, Geological Society of America Memoir 178, 

p. 161–198.

Williams, A.E., and Rodini, D.P., 1997, Regional isotope 

effects and application to hydrologic investigations in 

southwestern California: Water Resources Research, v. 

33, p. 1721–1729.

Woodburne M.O., and Golz, D.J., 1972, Stratigraphy of the 

Punchbowl Formation, Cajon Valley, southern 

California: University of California Publications in 

Geological Sciences, v. 92, 73 p.