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Aqueous Geochemistry of Waters and Hydrogeology of Alluvial Deposits, Pinnacles National Park, California

Open-File Report 2022-1026
Prepared in cooperation with the National Park Service
By: , , and 
https://doi.org/10.3133/ofr20221026

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Abstract

A cooperative study between the National Park Service (NPS) and the U.S. Geological Survey (USGS) characterized groundwater quality and hydrogeology in parts of Pinnacles National Park. The water-quality investigation assessed the geochemistry of springs, wells, surface water, and precipitation and analyzed geochemistry of rock formations that affect the water chemistry through water-rock interaction. The hydrogeology investigation used geophysical and groundwater level data to characterize groundwater-flow processes in the alluvial deposits of Bear Valley and the Chalone Creek watershed.

Analysis of aqueous geochemical parameters in water samples from perennial springs, water-supply wells, and surface waters was conducted for samples collected after the dry season (autumnal) and after the wet season (vernal) to assess changes in geochemistry due to changes in groundwater levels or flow resulting from precipitation. The chemistry of bulk precipitation collected during the wet season was also analyzed. Bedrock samples were analyzed for geochemical parameters to help constrain groundwater sources, flow paths, and weathering. The geochemical investigations show a correspondence between the source rock and the spring-water chemistry that can be attributed to the mineralogy of the source rock. The narrow range of strontium isotopes in water samples, sourced in geochemically and mineralogically disparate rocks, indicates that the bedrock groundwater is relatively old and has reached quasi-steady state with respect to weathering of susceptible minerals.

Groundwater-level monitoring indicated that the water table is shallow—from 0 to 10 meters (m) below land surface. In southern Bear Valley and in the Chalone Creek alluvium, water levels rose and declined by several meters over each annual cycle of this study. In northern Bear Valley, water levels rose modestly over two wet seasons but declined during a third wet season. In Bear Valley, groundwater/surface-water interaction occurs along the perennial reach of Sandy Creek. Groundwater discharges to the upstream part of the reach, becomes surface water and is partly consumed by evapotranspiration, and infiltrates farther downstream. In the Chalone Creek alluvium, runoff-generated surface-water flow in intermittent stream reaches is a major component of groundwater recharge. After the onset of significant streamflow, creek water rapidly recharges groundwater until water levels rise to nearly the creek level. Groundwater levels generally remain high throughout the wet season, then gradually decline after the creek becomes dry.

Suggested Citation

Scheiderich, K., Tiedeman, C.R., Hsieh, P.A., 2022, Aqueous geochemistry of waters and hydrogeology of alluvial deposits, Pinnacles National Park, California: U.S. Geological Survey Open-File Report 2022-1026, 39 p., https://doi.org/10.3133/ofr20221026.

ISSN: 2331-1258 (online)

Study Area

Table of Contents

  • Acknowledgments  
  • Abstract  
  • Introduction  
  • Description of Study Area  
  • Geochemistry  
  • Hydrogeology of Bear Valley Alluvium and Chalone Creek Alluvium  
  • Summary  
  • Reference Cited  
  • Appendix 1. Photographs of Selected Springs  
  • Appendix 2. Constituents of Concern in Wells, Springs, and Surface Water  
  • Appendix 3. Seismic Velocities
Publication type Report
Publication Subtype USGS Numbered Series
Title Aqueous geochemistry of waters and hydrogeology of alluvial deposits, Pinnacles National Park, California
Series title Open-File Report
Series number 2022-1026
DOI 10.3133/ofr20221026
Year Published 2022
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) WMA - Laboratory & Analytical Services Division
Description Report: viii, 39 p.; 3 Data Releases
Country United States
State California
Other Geospatial Pinnacles National Park
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
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