水資源枯渇の主因が気候ではなく人間活動であることを発見（Study finds humans outweigh climate in depleting Arizona’s water supply）

2025-10-16 アリゾナ大学

＜関連情報＞

• https://news.arizona.edu/news/study-finds-humans-outweigh-climate-depleting-arizonas-water-supply
• https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024WR037606

半乾燥沖積盆地における完新世を通じた地下水涵養と地下水位の変動 Variations in Groundwater Recharge and Water Table Elevations Across the Holocene in a Semi-Arid Alluvial Basin

Chandler Noyes, Grant Ferguson, Alan Seltzer, Jessica Ng, Kenneth C. Carroll, Rebecca Tyne, Katherine Markovich …
Water Resources Research  Published: 22 August 2025
DOI:https://doi.org/10.1029/2024WR037606

Abstract

Many regional aquifer systems undergoing depletion contain “fossil” groundwater recharged under different climate conditions, raising questions on groundwater renewability. We examine the spatial distribution in groundwater ages estimated with multiple tracers covering different timescales, along with differences in paleoclimate and water table depths (WTD) in the Tucson Basin, Arizona, USA, to constrain recharge history. The presence of ³H and ⁸⁵Kr in upgradient wells indicates modern recharge, while ³⁹Ar, ¹⁴C, and ⁴He values show components of water several hundred to tens of thousands of years old in downgradient wells. In some wells, apparent ages on the orders of decades, centuries, and millennia are simultaneously indicated by ⁸⁵Kr, ³⁹Ar, and ¹⁴C activities, indicating that the flow system cannot be explained by a simple piston-flow model. To disentangle mixing and recharge history signals, we apply an inverse modeling technique that couples lumped parameter modeling of residence times with (stable) noble gas reconstructions of recharge temperature and WTD. A mid-Holocene maximum in WTD, reconstructed from Kr and Xe isotopes, suggests enhanced aridity associated with a reduction of wintertime precipitation, inferred from a shift in δ¹⁸O and δD. Colder temperatures were present during the Last Glacial Maximum, reflecting both climatic cooling and cold mountain front recharge. Recharge to the Tucson Basin has been continuous since the LGM, albeit with a likely reduction during the mid-Holocene. Natural fluctuations in groundwater recharge have been small compared to those associated with groundwater extraction, indicating that pumping poses a larger threat to groundwater resources than climate change.

Key Points

• Groundwater age distributions indicatse somewhat continuous recharge from Last Glacial Maximum to present in semi-arid alluvial basin
• Isotopic and noble gas tracers reveal maximum water table depths and reduction in wintertime precipitation during Mid-Holocene arid period
• Natural fluctuations in groundwater recharge have been small compared to pumping-induced declines in regional water table