Ferguson et al. (2026) Renewability of fossil groundwaters affected by present-day climate conditions

Identification

• Journal: Nature Geoscience
• Year: 2026
• Date: 2026-02-05
• Authors: Grant Ferguson, Mark O Cuthbert, Scott Jasechko, Michael Manga, Jeffrey J. McDonnell, Jennifer C. McIntosh, Chander E. Noyes, Barbara Sherwood Lollar, Richard G. Taylor
• DOI: 10.1038/s41561-026-01923-4

Research Groups

• Department of Civil, Geological and Environmental Engineering, University of Saskatchewan, Saskatoon, Canada
• Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, USA
• Global Institute for Water Security, University of Saskatchewan, Saskatoon, Canada
• School of Environment and Sustainability, University of Saskatchewan, Saskatoon, Canada
• School of Earth and Environmental Sciences, Cardiff University, Cardiff, UK
• Bren School of Environmental Science and Management, University of California Santa Barbara, Santa Barbara, USA
• Department of Earth and Planetary Science, University of California Berkeley, Berkeley, CA, USA
• College of Resources and Environmental Engineering, Ludong University, Yantai, China
• North China University of Water Resources and Electric Power, Zhengzhou, China
• School of Geography, Earth & Environmental Sciences, University of Birmingham, Birmingham, UK
• Waite-Heindel Environmental Management, Burlington, USA
• Department of Earth Sciences, University of Toronto, Toronto, Canada
• Department of Geography, University College London, London, UK

Short Summary

This study compares hydraulic response times (HRT) and aquifer residence times (ART) across 31 major aquifer systems globally, revealing that the presence of fossil groundwater does not necessarily mean current water levels are controlled by past climates, as HRT often dictates the system's adjustment to modern conditions. It concludes that HRT is crucial for assessing groundwater renewability and sustainable management, even in aquifers containing very old water.

Objective

• To clarify the relationship between aquifer residence times (ART) and hydraulic response times (HRT) and their implications for understanding groundwater system behavior.
• To determine whether the presence of fossil groundwater necessarily indicates that current groundwater levels are affected by past climate conditions, thereby informing sustainable groundwater management strategies.

Study Configuration

• Spatial Scale: 31 major aquifer systems distributed across all continents except Antarctica.
• Temporal Scale: Aquifer residence times ranged from 0.7 years to 10 million years (Ma); hydraulic response times varied from 1.6 years to 6.9 million years (Ma). The study also considers Holocene (approximately 12,000 years) and Pleistocene Epoch timescales.

Methodology and Data

• Models used: Piston flow model and piston-exponential model for groundwater age interpretation. Theoretical equations for hydraulic response time (tNE) for confined (Equation S2), unconfined (Equation S4), and mixed (Equation S5) aquifers.
• Data sources: Groundwater ages, transmissivity, storativity, and hydraulic gradients were compiled from published values for 31 regional aquifer systems. Data included regional potentiometric surface maps, O and H isotopes, and groundwater age determinations using environmental tracers such as Carbon-14 (14C), Chlorine-36 (36Cl), Krypton-81 (81Kr), and Helium-4 (4He).

Main Results

• Hydraulic response times (HRTs) were found to be less than aquifer residence times (ARTs) in 21 of the 31 aquifer systems examined.
• ARTs ranged from 0.7 years to 10 Ma, while HRTs ranged from 1.6 years to 6.9 Ma.
• Neither ART nor HRT showed a significant correlation with flow system length (L) or L^2, attributed to high variability in hydraulic gradients (3 × 10^-5 to 9 × 10^-2) and storage coefficients (10^-6 to 0.39).
• Many aquifers containing fossil groundwater (ART > 12,000 years) have HRTs short enough (e.g., centuries to a few millennia) that their water levels are largely in equilibrium with modern climate conditions (e.g., 7 systems reached 80% equilibration with Holocene conditions despite ART > 20,000 years).
• In other instances, aquifers with fossil groundwater have HRTs long enough that past climates still significantly affect current hydraulic conditions (e.g., the Nubian aquifer, which is estimated to be ~94% readjusted from a climate shift at 360,000 years ago).
• A wide range of HRTs are possible for similar ARTs, highlighting that the presence of fossil groundwater does not automatically mean groundwater levels are affected by past climates.

Contributions

• Demonstrates that the presence of fossil groundwater does not necessarily indicate that groundwater levels are controlled by past climates, challenging a common assumption in hydrogeology.
• Emphasizes the critical importance of estimating hydraulic response times (HRT) in addition to aquifer residence times (ART) for accurately assessing groundwater renewability and informing sustainable groundwater management.
• Provides a comprehensive global compilation and comparison of HRT and ART for 31 major aquifer systems, spanning seven orders of magnitude, offering new insights into the complex interplay of these two metrics.

Funding

• NSERC Discovery Grant (to Grant Ferguson)
• UK Natural Environment Research Council (Independent Research Fellowship NE/P017819/1 to Mark O. Cuthbert)
• CIFAR Catalyst workshop
• CIFAR Earth 4D: Subsurface Science and Exploration program (Manga, McDonnell, McIntosh, Sherwood Lollar, Taylor)
• ARCS Foundation scholarship (to Chander E. Noyes)

Citation

@article{Ferguson2026Renewability,
  author = {Ferguson, Grant and Cuthbert, Mark O and Jasechko, Scott and Manga, Michael and McDonnell, Jeffrey J. and McIntosh, Jennifer C. and Noyes, Chander E. and Lollar, Barbara Sherwood and Taylor, Richard G.},
  title = {Renewability of fossil groundwaters affected by present-day climate conditions},
  journal = {Nature Geoscience},
  year = {2026},
  doi = {10.1038/s41561-026-01923-4},
  url = {https://doi.org/10.1038/s41561-026-01923-4}
}