Human Impact on Tucson’s Groundwater: A Historical Perspective
A groundbreaking study highlights the significant impact of human activity on Tucson’s groundwater resources over the past decades, far surpassing natural climate impacts. This research marks the first attempt to place human influence on groundwater in a long-term context, offering a multi-millennial perspective.
Jennifer McIntosh, the senior author and Thomas Meixner Endowed Chair of Hydrology and Atmospheric Sciences at the University of Arizona, emphasized the study’s importance by stating, “This is the first time we’ve been able to get a record of the water table through time.”
Findings reveal that since the Last Glacial Maximum, approximately 20,000 years ago, the aquifer beneath Tucson has continuously been recharged by precipitation. However, during drier periods, the water table fell by up to 105 feet (32 meters). In contrast, human-induced groundwater pumping since the mid-20th century has led to a water table drop twice as deep as those caused by natural climate variations.
Chandler Noyes, the study’s first author, remarked, “It really underscores the impact that humans can have on the environment in a short time.”
Prior to this research, there was limited data regarding recharge rates between the Last Glacial Maximum and about 6,000 years ago. Researchers addressed this gap by analyzing the Tucson Basin’s groundwater mixture, which combines fossil water, recharged over 12,000 years ago, with more recent water. This blend provides a comprehensive view of the region’s hydrological history.
Chandler Noyes, first author and recent University of Arizona doctoral graduate, collects a groundwater sample for analysis of chemical and isotopic markers. By taking multiple samples from different depths in 12 wells and applying comprehensive computer modeling, the researchers reconstructed water table changes over thousands of years.
Jennifer McIntosh/Department of Hydrology and Atmospheric Sciences
The research team utilized chemical and isotopic markers, which naturally occur in aquifer sediments, to trace water sources and ages. Mathematical models were employed to decode these overlapping markers, enabling the reconstruction of groundwater ages, recharge rates, and water table depths over millennia.
McIntosh noted, “The water we drink carries this record of what happened with climate in the Tucson Basin,” suggesting that these methodologies could be applied to aquifers globally.
Noyes believes the study’s findings will aid water resource managers in forecasting aquifer responses to future changes, whether natural or human-induced. “These methods provide more complete groundwater ages and an understanding of how groundwater levels respond to climate change,” he explained.
Heavy groundwater pumping in Tucson began in the 1940s to accommodate agricultural needs and a growing population, as detailed in a 2021 U of A report. The 1980 Groundwater Management Act and subsequent sustainability measures helped curb these effects, while the 1992 Central Arizona Project introduced Colorado River water to the city.
Currently, Tucson’s water supply is evenly split between local groundwater and Colorado River sources. Despite local conservation efforts and statewide measures, McIntosh warns that recharge remains slow and climate remains a critical factor in long-term water availability.
McIntosh asserts, “Even if we were to go back to the end of the last ice age, when it was much colder and wetter in the Tucson Basin, we could not recover the amount of groundwater that we’ve removed.” This underscores the irreversible nature of the depletion caused by human activity.
In a positive development, Tucson’s recent adoption of the One Water 2100 plan aims to enhance aquifer recharge through treated effluent and other measures. McIntosh suggests, “We can enhance recharge by adding more water,” highlighting the potential of strategic water management to mitigate past impacts.
This study received support from the U.S. Geological Survey, the Natural Sciences and Engineering Research Council of Canada, the University of Arizona Graduate and Professional Student Council, and the Geological Society of America. Tucson Water contributed with logistical support and data sharing. McIntosh’s work was supported by the Thomas Meixner Endowed Chair, Department of Hydrology and Atmospheric Sciences, University of Arizona, and the CIFAR Earth4D Subsurface Science and Exploration Program.
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