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Research Article

Base of fresh water, groundwater salinity, and well distribution across California

View ORCID ProfileMary Kang, Debra Perrone, View ORCID ProfileZiming Wang, Scott Jasechko, and View ORCID ProfileMelissa M. Rohde
PNAS December 22, 2020 117 (51) 32302-32307; first published December 9, 2020; https://doi.org/10.1073/pnas.2015784117
Mary Kang
aCivil Engineering, McGill University, Montreal, QC H3A 0C3, Canada;
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  • ORCID record for Mary Kang
  • For correspondence: mary.kang@mcgill.ca perrone@ucsb.edu
Debra Perrone
bEnvironmental Studies Program, University of California, Santa Barbara, CA 93106;
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  • For correspondence: mary.kang@mcgill.ca perrone@ucsb.edu
Ziming Wang
aCivil Engineering, McGill University, Montreal, QC H3A 0C3, Canada;
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  • ORCID record for Ziming Wang
Scott Jasechko
cBren School of Environmental Science and Management, University of California, Santa Barbara, CA 93106;
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Melissa M. Rohde
dThe Nature Conservancy, San Francisco, CA 94105
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  • ORCID record for Melissa M. Rohde
  1. Edited by William A. Jury, University of California, Riverside, CA, and approved November 9, 2020 (received for review July 26, 2020)

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Significance

To sustainably manage groundwater, water managers and regulators currently rely upon understanding the depths at which groundwater transitions from fresh (shallower) to more saline (deeper). This base of fresh water, while important, remains poorly understood. Here we show that, in California, 1) available base-of-fresh-water data do not represent actual base of fresh water, as exemplified by the widespread occurrence of fresh water deeper than the defined bases of fresh water, and 2) wells are already penetrating or encroaching on the defined bases of fresh water. We conclude that using current base-of-fresh-water data may limit efforts to manage deep groundwater effectively.

Abstract

The depth at which groundwaters transition from fresh to more saline—the “base of fresh water”—is frequently used to determine the stringency and types of measures put in place to manage groundwater and protect it from contamination. Therefore, it is important to understand salinity distributions and compare defined bases of fresh water with salinity distributions and groundwater well depths. Here we analyze two distinct datasets: 1) a large set of total dissolved solids concentration (TDS) measurements (n = 216,754) and 2) groundwater well locations and depths (n = 399,454) across California. We find that 19 to 56% of the groundwater TDS measurements made at depths deeper than defined bases of fresh water pump fresh groundwater (TDS < 2,000 mg/L). Because fresh groundwater is found at depths deeper than the base of fresh water, current policies informed by base of fresh water assessments may not be managing and protecting large volumes of deep fresh groundwater. Furthermore, we find that nearly 4% of existing groundwater wells penetrate defined bases of fresh water, and nearly 16% of wells overlie it by no more than 100 m, evidencing widespread encroachment on the base of fresh water by groundwater users. Consequently, our analysis suggests that groundwater sustainability in California may be poorly safeguarded in some places and that the base-of-fresh-water concept needs to be reconsidered as a means to define and manage groundwater.

  • groundwater
  • base of fresh water
  • wells
  • salinity
  • California

Footnotes

  • ↵1To whom correspondence may be addressed. Email: mary.kang{at}mcgill.ca or perrone{at}ucsb.edu.
  • Author contributions: M.K., D.P., S.J., and M.M.R. designed research; M.K., D.P., Z.W., S.J., and M.M.R. performed research; M.K., D.P., Z.W., S.J., and M.M.R. analyzed data; and M.K., D.P., S.J., and M.M.R. wrote the paper.

  • The authors declare no conflicts of interest.

  • This article is a PNAS Direct Submission.

  • See online for related content such as Commentaries.

  • This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2015784117/-/DCSupplemental.

Data Availability.

Data have been deposited in Open Science Framework (https://osf.io/g42s7) (52). The page provides csv of the TDS-depth data and the shapefiles for the Base of Fresh Water.

Published under the PNAS license.

View Full Text

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Base of fresh water, groundwater salinity, and well distribution across California
Mary Kang, Debra Perrone, Ziming Wang, Scott Jasechko, Melissa M. Rohde
Proceedings of the National Academy of Sciences Dec 2020, 117 (51) 32302-32307; DOI: 10.1073/pnas.2015784117

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Base of fresh water, groundwater salinity, and well distribution across California
Mary Kang, Debra Perrone, Ziming Wang, Scott Jasechko, Melissa M. Rohde
Proceedings of the National Academy of Sciences Dec 2020, 117 (51) 32302-32307; DOI: 10.1073/pnas.2015784117
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