USGS

 

REPORT OF HYDROLOGIC INVESTIGATIONS IN THE THREE SISTERS AREA OF CENTRAL OREGON, SUMMER 2001

Water-Resources Investigations Report 02-4061

By William C. Evans1, Robert H. Mariner1, Steven E. Ingebritsen1, B. Mack Kennedy2, Matthias C. van Soest2, and Mark A. Huebner1

 

1U.S. Geological Survey, Menlo Park, CA

2Lawrence Berkeley National Laboratory, Berkeley, CA

Abstract

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An ongoing episode of crustal uplift centered in the Separation Creek drainage of the Three Sisters area, central Oregon Cascades, may result from a magmatic intrusion that began in 1998. An investigation of springs in this drainage in summer 2001 revealed slightly elevated water temperatures and chloride (Cl-) concentrations of up to about 5°C and 20 milligrams per liter (mg/L), respectively, above background. The total discharge of anomalous Cl- in Separation Creek was 9.2 grams per second, which in combination with the temperature-Cl- relation in the springs results in a total advective heat discharge of 16 MW (megawatts). Comparison with similar findings obtained a decade earlier suggests that total Cl- and heat discharges in the groundwater drainage are unaffected by the current uplift. However, the isotopic composition of the dissolved inorganic carbon (DIC) in the spring waters (delta carbon-13 (sigma13C) = -9.03 to -11.6‰; carbon-14 (14C) <25 pmC) combined with helium-3/helium-4 (3He/4He) ratios near 8 RA and C/3He ratios <1010 in two of the springs are indicative of a magmatic source. The high 3He/4He ratios indicate that the magmatic gas is derived from a relatively recent, if not ongoing, intrusion. The concentration of magmatic carbon is low, a few millimoles per liter (mmol/L) at most, with an average value of 1.53 mmol/L for all the springs sampled in the drainage. Combining this average with the late-summer water flow in Separation Creek suggests a discharge of 21 tonnes/day of magmatic carbon dioxide (CO2). The presence of magmatic carbon in the shallow groundwater system, and the fact that DIC is uncorrelated with Cl-, suggests that some magmatic gas could escape diffusely through the soils.

Table of Contents

Abstract

Introduction

Background

Procedures

Results

Comparisons with Past Data

Chloride/Bromide Ratios

Alkalinity and Dissolved Inorganic Carbon

Carbon: Isotopes and Sources

Gas Results

Discussion

Effects of Intrusion?

Heat and Carbon Discharge

New Data and the Conceptual Mode

The Need for Cold Spring Studies

Conclusions

Acknowledgements

References

 


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