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Edited by William A. Jury, University of California, Riverside, CA, and approved November 9, 2020 (received for review July 26, 2020)
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.
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.
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Base of fresh water, groundwater salinity, and well distribution across California
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- Physical Sciences
- Sustainability Science
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