A study led by University of Arizona researchers shows that decades of groundwater pumping by humans has depleted Tucson-area aquifers far more than natural climate variation. Published in the journal Water Resources Research, the study provides the first multi-millennial reconstruction for the region that places human impacts on groundwater into long-term context.
"This is the first time we've been able to get a record of the water table through time," said Jennifer McIntosh, senior author and the Thomas Meixner Endowed Chair of Hydrology and Atmospheric Sciences in the College of Science.
Since the climactic period known as the Last Glacial Maximum – about 20,000 years ago – precipitation has continuously recharged the aquifer under Tucson, the study concluded. During dry climate periods, less precipitation seeped back into the aquifer, and the water table dropped by as much as 105 feet (32 meters), compared with the levels in wetter periods. However, modern pumping from the mid-20th century to present day caused twice the drawdown of the water table compared with natural climate fluctuations.
"It really underscores the impact that humans can have on the environment in a short time," said first author and recent U of A doctoral graduate Chandler Noyes.
Prior to this study, information on recharge rates between the Last Glacial Maximum and the mid-Holocene – about 6,000 years ago – was limited, McIntosh said. The researchers filled knowledge gaps by analyzing the blend of water under the Tucson Basin, the desert valley surrounding the city. Fossil water – precipitation that entered an aquifer over 12,000 years ago – is naturally mixed with water that recharged the groundwater hundreds to thousands of years ago and some that dates to recent years.
In order to reconstruct the age and recharge history of water in the Tucson Basin, the research team identified chemical and isotopic markers that entered the water either through the atmosphere or via aquifer sediments. While these naturally occurring substances provide clues to a water sample's source and age, the water is from a mix of different times, and mathematical models are needed to untangle the overlapping markers.
The team used mathematical models integrating multiple markers to reconstruct groundwater ages, recharge rates and water table depths over thousands of years. Combining these markers also provided data to infer past climate conditions, including air temperatures and precipitation patterns.
"The water we drink carries this record of what happened with climate in the Tucson Basin," McIntosh said, adding that these new techniques for linking climate and hydrology could be applied to aquifers around the world.
Noyes said the study's results can help water resource managers anticipate how aquifers might react to future changes, whether natural or driven by humans.
"These methods provide more complete groundwater ages and an understanding of how groundwater levels respond to climate change," he said.
Tucson residents began pumping groundwater heavily around the 1940s to support irrigated farming and a rapidly growing population, according to a U of A report. For decades, signs of overdraft and well failures increased in an environment offering little restriction.
The 1980 Groundwater Management Act established formal controls and introduced sustainability measures. In 1992, the Central Arizona Project began delivering Colorado River water to Tucson.
"Today, about half of the water from our taps comes from local groundwater, and the other half from the Colorado River," McIntosh said.
Local conservation efforts, in combination with the statewide measures, have helped many of Tucson's wells partially rebound from heavy usage during the 20th century. However, McIntosh added, the study's results indicate that while the aquifer is somewhat renewable, recharge is slow and limited, and climate continues to play a key role in the region's long-term water availability.
"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," McIntosh said. "Even a really wet climate wouldn't save us," indicating that no natural return to wetter conditions could restore the volume of water that has been lost to pumping.
One promising trend is the city of Tucson's recent adoption of the One Water 2100 plan, McIntosh said. Among other initiatives, the plan calls for continuing and expanding use of treated effluent, including releasing the recycled wastewater into the Santa Cruz River, where it seeps into the ground and helps recharge the aquifer.
"We can enhance recharge by adding more water," she said. "One way of doing that is releasing our treated effluent in those locations where we know there is modern recharge, providing the highest potential of that water making it to the water table."
This study was supported in part by research grants 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 provided logistical support and data sharing. McIntosh was supported by the Thomas Meixner Endowed Chair, Department of Hydrology and Atmospheric Sciences, University of Arizona, and CIFAR Earth4D Subsurface Science and Exploration Program.
