Helium isotope studies in the Mojave Desert, California: implications for groundwater chronology and regional seismicity

doi:10.1016/j.chemgeo.2003.07.002

Chemical Geology

Volume 202, Issues 1–2, 15 December 2003, Pages 95-113

https://doi.org/10.1016/j.chemgeo.2003.07.002 Get rights and content

Abstract

We report helium isotope and concentration results for groundwaters from the western Mojave River Basin (MRB), 130 km east of Los Angeles, CA. The basin lies adjacent to the NW–SE trending San Andreas Fault (SAF) system. Samples were collected along two groundwater flowpaths that originate in the San Gabriel Mountains and discharge to the Mojave River located ∼32 km to the northeast. Additional groundwater samples were collected from Mojave River Deposits underlying the Mojave River. The primary objective of this study is to identify and quantify crustal and mantle helium contributions to the regional groundwater system.

A total of 27 groundwaters, sampled previously for chemistry and isotope systematics (including ¹⁴C activity) have measured helium concentrations that increase along flowpaths from 9.9×10⁻⁸ to 1.0×10⁻⁴ cm³ STP g⁻¹ H₂O. Concomitantly, ³He/⁴He ratios decrease from 0.84R_A to 0.11R_A (R_A equals the ³He/⁴He ratio in air=1.4×10⁻⁶). We did not record ³He/⁴He ratios equivalent to crustal-production values (∼0.02R_A) in any sample.

Dissolved helium concentrations were resolved into components associated with solubility equilibration, air entrainment, mantle-derivation, in-situ production within the aquifer, and extraneous crustal fluxes. All samples contained the first four components, but only older samples had the superimposed effects of helium derived from a crustal flux. The radiogenic He component has chronological significance, and good concordance between ⁴He and ¹⁴C ages for younger groundwaters (<25,000 year) demonstrates the integrity of the ⁴He-chronometer in this setting. Helium-rich waters could also be dated with the ⁴He technique, but only by first isolating the whole crustal flux (3–10×10⁻⁶ cm³ STP cm⁻² year⁻¹). Mantle-derived ³He (³He_m) is present in all MRB samples irrespective of distance from the SAF. However, regional-aquifer groundwaters near the terminus of the flowpath have a significantly greater content of mantle-derived ³He in comparison with more modern samples. We propose that faults in the basin other than the SAF may be an additional source of mantle-derived helium. The large range in ³He_m concentrations may be related to fault activity; however, groundwaters with lower and more constant ³He_m contents may indicate that seismic activity along the SAF has been relatively constant for the past 30,000 years, demonstrating that ancient groundwaters may serve as an archive for paleo-seismic events.

Introduction

The inert nature of the noble gases, coupled with their distinctive isotopic and solubility characteristics, makes them ideal tracers in groundwater-related studies. Their applicability extends across topics as diverse as groundwater dating, paleoclimatology, mechanism(s) of recharge, and mantle and seismic studies (see reviews by Ballentine et al., 2002; Kipfer et al., 2002). Helium has proven particularly useful owing to its low solubility in water, making it less susceptible to air contamination, large subsurface production rates (particularly of the ⁴He isotope), and large and diagnostic variations in its isotopic composition (³He/⁴He ratios), which impart critical information on fluid provenance and subsequent flow history. These attributes have led to the widespread exploitation of helium isotopes in groundwater-related studies, often independently of the other noble gases (e.g. Davis and DeWiest, 1966).

In this study, we present helium isotope and concentration results for groundwaters from the Mojave River Basin (MRB) in the high desert region of southern California. This groundwater system is well studied (e.g. Izbicki et al., 1995, Lines, 1996) owing to the pressing need to delineate groundwater resources in this area of rapid population growth. As such, the application of helium studies, particularly their ability to quantify groundwater chronologies, is a powerful addition to the array of techniques employed in the region aimed at quantifying flow history. Significantly, however, the MRB is also located immediately adjacent to the San Andreas Fault (SAF) system, imparting a tectonic component to the study. Prior work has shown that mantle-derived helium, identified by high ³He/⁴He ratios, is closely associated with fluid movement in and around the SAF system (Kennedy et al., 1997). In this study, therefore, we have the opportunity to combine two distinct facets of helium isotope geochemistry, i.e. assessing the applicability of the He geochronometer in this area of recent tectonics as well as evaluating the magnitude and extent of mantle-derived volatile fluxing into the regional groundwater system. In this latter respect, an important aim of this study is to determine the viability of ancient groundwater as an archive for paleo-seismic events in the region.

Section snippets

Western Mojave River Basin, California

The Mojave River Basin (MRB) is located ∼130 km northeast of Los Angeles in the Mojave Desert of southern California (Fig. 1). The basin occupies an area of 3626 km² and receives drainage from the San Gabriel and San Bernardino Mountains. It lies in a semi-arid region with low annual rainfall (<150 mm year⁻¹), low humidity, and high summer temperatures. Infrequent transient storms cause intermittent flow in the Mojave River, which represents the only surface drainage of the area. In the high

Hydrochemical background (δ¹⁸O, δD, ³H, and ¹⁴C)

Orographic effects control the stable isotope characteristics of groundwaters in the region. In turn, the stable isotopes provide a means of distinguishing groundwaters recharged by different mechanisms in different parts of the MRB (Fig. 3). Friedman et al. (1992) showed that the majority of precipitation events in the area occur as winter storms, originating over the central or northern Pacific Ocean before passing over high mountains to reach the Mojave Desert. Winter precipitation at Big

Analytical methods

Of the 25 boreholes sampled for this study (21 in May 2000 and 4 in June 2001), 15 are 24-cm ID production wells equipped with electric turbine pumps, and 10 are 6-cm ID multi-level observation wells (4N/4W-3A(2–5), 4N/4W-1C(2–5), 5N/6W-22E(1–2)) sampled using a Bennet submersible pump. The multi-level wells represent spatially restricted arrays of wells that penetrate to different depths in the aquifer. In this way, multi-level wells 4N/4W-3A(2–5) and 4N/4W-1C(2–5) sample both the MRD and the

Results

In Table 1, we report helium isotope and abundance results for 27 samples (including two replicate analyses) from the MRB. Results are reported as sample ⁴He and Ne concentrations (⁴He_s and Ne_s) and sample ³He/⁴He ratios (reported as R_s/R_A, where R_s=sample ³He/⁴He, and R_A is the air ³He/⁴He ratio=1.4×10⁻⁶). Note that sample 4N/7W-33J1 forms part of flowpath A–A′ and A–B′. Also included in Table 1 are borehole details including interpreted ¹⁴C ages and tritium contents of the groundwaters (see

Resolution of helium components

Measured sample concentrations (³He_s and ⁴He_s) represent the sum of several helium components, which, upon resolution can provide insight into the provenance of helium fluxes into the groundwater system. These sources of helium (³He and ⁴He) include in situ production (He_is) from the decay of Li, U and Th in the aquifer matrix, a mantle flux (He_m), a deep crustal flux (He_dc), air-equilibrated helium (He_eq), dissolved-air bubbles (He_a), and tritiogenic helium-3 (³He_t). The helium-balance

Concluding remarks

Groundwaters from the Mojave River Basin have a number of sources contributing to their total helium inventory. These components may be isolated using standard modeling techniques, and reasonable assumptions of endmember compositions (R_dc=R_is=0.02R_A, R_m=8 R_A). In addition to atmosphere-derived helium, all samples (from both the regional aquifer and MRD) have mantle-derived ³He and in-situ produced radiogenic helium; however, several of the older samples also possess an extraneous radiogenic

Acknowledgements

This work was supported by the Hydrologic Sciences Program of the National Science Foundation (award EAR-0001133) and by the United States Geological Survey Water Resources Division (San Diego). Chris Ballentine and two anonymous reviewers provided constructive remarks on the manuscript. [LW]

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Helium isotope studies in the Mojave Desert, California: implications for groundwater chronology and regional seismicity

Abstract

Introduction

Section snippets

Western Mojave River Basin, California

Hydrochemical background (δ18O, δD, 3H, and 14C)

Analytical methods

Results

Resolution of helium components

Concluding remarks

Acknowledgements

Chem. Geol.

J. Hydrol.

J. Geochem. Exploration

Appl. Geochem.

Chem. Geol.

J. Hydrol.

Chem. Geol.

Geochim. Cosmochim. Acta

J. Geochem. Explor.

Geochim. Cosmochim. Acta

Production of noble gases in the continental crust

Rev. Mineral. Geochem.

Tracing fluid origin, transport and interaction in the curst

Rev. Mineral. Geochem.

Groundwater flow and the 4He distribution in the Great Artesian Basin of Australia

J. Geophys. Res.

Mojave River ground water basin investigation

Bull. Calif. Dept. Water Res.

Noble gases as natural tracers of water circulation in the Paris Basin. 1. Measurements and discussion of their origin and mechanisms of vertical transport in the basin

Water Resour. Res.

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Water Resour. Res.

Helium-3 and mantle volatiles in the ocean and oceanic crust

Dating groundwater, a short review

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Stable isotope composition of waters in Southeastern California. 1. Modern precipitation

J. Geophys. Res.-Atmos.

Hydrochemical background (δ¹⁸O, δD, ³H, and ¹⁴C)

Groundwater flow and the ⁴He distribution in the Great Artesian Basin of Australia