Tucson Chapter

April 2025

Date / Time: April 8, social half hour at 6pm, talk 6:30-7:30pm

Place: Montgomery & Associates, 1550 E Prince Rd 

Cost in-person attendance: $5 for members, $10 for nonmembers, free for student members — please register online in advance.

A Zoom option will also be provided to those who register and want to attend remotely. Zoom will be free for members, $5 for student nonmembers, and $10 for other nonmembers

Abstract: From Mountains to Basins: Tracing variations in groundwater flowpaths, recharge rates, and water table elevations across the Holocene

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. The relatively low hydraulic conductivity of the mountain blocks and overlapping Pantano Formation combined with modest recharge likely favors local flow paths, focusing water through alluvial washes, as opposed to deeper, regional flowpaths required for large diffuse mountain block recharge. The presence of ³H and ⁸⁵Kr in upgradient wells, adjacent to the mountain front and along major washes, 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. These mixed ages in long-screened production wells could not be explained by a simple piston-flow model. Thus, we applied a new inverse modeling technique that coupled lumped parameter modeling of residence times with Kr and Xe reconstructions of recharge temperature and water table depth to disentangle recharge history. Results highlight a maximum in the water table depth during the mid-Holocene, suggesting enhanced aridity associated with a reduction of wintertime precipitation, inferred from a shift in δ¹⁸O and δD values. Colder temperatures were present during the Last Glacial Maximum (LGM), 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.