When
Where
Available in person and via zoom (see email for link)
Abstract
Groundwater in semi-arid alluvial basins primarily comes from mountain-front recharge (MFR) with long subsurface flowpaths, leading to prevalence of ‘fossil’ (>12,000 years old) groundwater recharged in the geologic past under different climate conditions. Questions remain about specific flowpaths of MFR to basin-fill aquifers (i.e., surface vs. subsurface components) and how recharge rates and water table elevations have varied since the Late Pleistocene and may be impacted by climate change and groundwater extraction. In addition, common age tracers used to date ‘modern’ (<~75 years old) to ‘fossil’ groundwater, tritium and radiocarbon, respectively, are unable to date groundwater recharged on intermediate timescales (~100 to ~1,000 years ago). The ‘bomb pulse’ of tritium from nuclear weapons testing in the 1950-1960’s is also decaying away in aquifer systems, requiring use of new ‘modern’ groundwater age tracers.
This talk will synthesize recent results from the Tucson Basin, where we have employed multiple age tracers, including relatively new techniques (argon-39 and krypton-85) for dating ‘intermediate’ to ‘modern’ age waters, respectively, and stable noble gases (Kr, Xe) to constrain variations in water table elevations and recharge rates across the Holocene. Combined with numerical modeling, we quantified the amount of diffuse (subsurface) versus surface MFR to the alluvial basin. Results show that most (>94%) groundwater in the Tucson Basin comes from surface MFR and there has been somewhat continuous recharge since the Last Glacial Maximum (LGM). Colder temperatures during the LGM reflect both climatic cooling and cold MFR. Relatively low water table depths during the mid-Holocene are consistent with other records of enhanced aridity in the region and shifts in δ18O and δD values indicate a reduction in wintertime precipitation. Importantly, these 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.
Bio
Jennifer McIntosh is a Professor, University Distinguished Scholar, Associate Department Head, and the Thomas Meixner Endowed Chair of Hydrology and Atmospheric Sciences at the University of Arizona (UA). She is a fellow of the Geological Society of America and the Canadian Institute for Advanced Research (CIFAR) Earth 4D: Subsurface Science and Exploration Program. Her research focuses on the hydrochemistry of waters, dissolved solutes and gases, and microbial communities to understand their origin, residence times, reactions, and flow paths from the earth’s surface to kilometers-depth with applications for water, energy, and critical mineral resources. She regularly serves as a technical expert for the US EPA, National Academies of Sciences, Nuclear Waste Technical Review Board, UK Royal Society, and International Atomic Energy Agency. The more than 64 students and postdoctoral fellows she has advised have gone on to positions in academia, government agencies, and environmental consulting. In addition, McIntosh is dedicated to education and public outreach, leading a K-12 Earth Science program at Flandrau Science Center and visiting local classrooms to teach hands-on groundwater activities.