Noble Gas Studies of Groundwater Recharge and Residence Time in Semi-Arid Region: Contrasting the Western Mojave River Basin with Central Australia and the Kalahari Desert

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

0001133 Hilton In this proposal, we will conduct a noble gas study of a series of groundwaters from three well-studied semi-arid regions. The aims are twofold: first, we plan to assess the potential of the 4He accumulation method to accurately estimate groundwater residence times over a range of ages from 100s to >500,000 years. The approach will involve inter-calibration with a number of different groundwater dating techniques (tritium, 14C and 36Cl, and integration of the total database into previouslydeveloped groundwater flow models for the individual basins. An important feature will be ascertaining the nature of the 'crustal He flux' and its quantification in the three studied areas, particularly as one, the West Mojave River Basin, is situated on tectonically active crust while the other two, Central Australia and Botswanna, lie on stable cratonic platforms. We will concentrate on deriving long-term, paleo-recharge rates for the three regions-a parameter of vital importance to current resource management strategies-but other hydraulic quantities (e.g., aquifer permeability can be also assessed using the 4He technique. The second aim is to utilize the noble gas abundances (concentrations of Ne, Ar, Kr and Xe) to derive both temperatures and environmental conditions at the time of recharge. In each of the studied areas, we will select waters, dated by 4He and other techniques, to focus on the last glacial period. Through the relationship between noble gases and stable isotopes d18O and d2H, we plan to determine (paleo) temperature variations and other localized effects (e.g., rising water tables) characteristic of past recharge regions. Through collaboration with colleagues from the Geological Surveys of the respective regions (U.S., Australia and Botswanna), we will have unprecedented access to a wide range of geochemical and geophysical data on the regions of study, logistical support for new sample collection, training opportunities on state-of-the-art groundwater software, and localized expertise on real-world resource and groundwater management issues.