USGS

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

 

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4069

Sacramento, California 2003


Prepared in cooperation with the Mojave Water Agency
Text of report (1.8 MB PDF)

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Abstract

The proximity of the Mojave River ground-water 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 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. 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 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 carbon-14 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 present-day Mojave Desert.

 

CONTENTS

Abstract

Introduction

Description of Study Area

Acknowledgments

Data-Collection Techniques

Multiple-Well Monitoring Sites

Water-Level Data

Water-Quality Data

Geologic Setting

Alluvial Topography of the Mojave River Valley

Stratigraphy

Igneous and Metamorphic Basement Rocks (pTb)

Older Fan and Stream Deposits (QTof)

Playa Deposits (QTp)

Older Alluvium of the Ancestral Mojave River (Qoa)

Younger Fan Deposits (Qyf)

Alluvium of the Mojave River (Qya and Qra)

Geologic Structure

Helendale Fault

Other Faults

Folds

Geologic History

Geohydrology

Floodplain Aquifer

Regional Aquifer

Ground-Water Movement near the Helendale Fault

Aquifer Tests

Ground-Water Quality

Chemical Composition of Water from Wells

Isotopic Composition of Water from Wells

Oxygen-18 and Deuterium

Tritium and Carbon-14

Summary

References Cited


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Water Resources of California



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