The transport of U- and Th-series nuclides in sandy confined aquifers
- Creators
- Reynolds, B. C.
-
Wasserburg, G. J.
- Baskaran, M.
Abstract
Abundances of ^(238)U, ^(234)U, ^(232)Th, ^(226)Ra, ^(228)Ra, ^(224)Ra, and ^(222)Rn were measured in groundwaters of the Ojo Alamo aquifer in northwest New Mexico. This is an arid area with annual precipitation of ∼22 cm. The purpose was to investigate the transport of U-Th series nuclides and their daughter products in an old, slow-moving groundwater mass as a means of understanding water-rock interactions and to compare the results with a temperate zone aquifer. It was found that ^(232)Th is approximately at saturation and supports the view of Tricca et al. (2001) that Th is precipitated irreversibly upon weathering, leaving surface coatings of ^(232)Th and ^(230)Th on aquifer grains. Uranium in the aquifer waters has very high [^(234)U/^(238)U] ∼ 9 and low ^(238)U concentrations. These levels can be explained by low weathering rates in the aquifer (w_(238U) ∼ 2 × 10^(−18) to 2 × 10^(−17)s^(−1)) using a continuous flow, water-rock interaction model. The Ra isotopes are roughly in secular equilibrium despite their very different mean lifetimes. The ^(222)Rn and ^(228)Ra isotopes in the aquifer correspond to ∼10% of the net production rate of the bulk rock. This is interpreted to reflect an earlier formed irreversible surface coating of Th that provides Ra and Rn to the aquifer waters. The surface waters that appear to be feeding the aquifer have low [^(234)U/^(238)U] and high ^(238)U concentrations. The flow model shows that it is not possible to obtain the high [^(234)U/^(238)U] and low [^(238)U] values in the aquifer from a source like the present vadose zone input. It follows that the old aquifer waters studied cannot be fed by the present vadose zone input unless they are greatly diluted with waters with very low U concentrations. If the present sampling of vadose zone sources is representative of the present input, then this requires that there was a major change in water input with much larger rainfall some several thousand years ago. This may represent a climatic change in the Southwest.
Additional Information
© 2003 Elsevier Science Ltd. Received June 6, 2002; revised 13 November 2002; accepted in revised form November 13, 2002. This work was supported by DOE DE-FG03-88ER13851. Caltech Division Contribution No. 8775(1086). Instruction in the arts of U-Th chemistry and mass spectrometry was kindly given by Dr. J. Chen. We acknowledge the gracious aid in initial field support from Fred Phillips and Doug McGhee (New Mexico Institute of Technology) and Dale Worth from the Bureau of Land Management, Farmington. In particular, Fred Phillips made critical contributions to our understanding and to this report. Comments by the referees and the associate editor on both substance and style were very helpful. For kindly providing access and support, we thank members of the Navajo Agricultural Products Industry (NAPI) Feedyard, The Carson School, the Navajo Indian Irrigation Project (NIIP), and especially the Navajo Tribal Utility Authority (Shiprock). Field assistance by Sarah Trimble of Wayne State University was of great help. Associate editor: G. HelzAdditional details
- Eprint ID
- 42549
- DOI
- 10.1016/S0016-7037(02)01341-8
- Resolver ID
- CaltechAUTHORS:20131119-082121630
- DE-FG03-88ER13851
- Department of Energy (DOE)
- Created
-
2013-11-19Created from EPrint's datestamp field
- Updated
-
2021-11-10Created from EPrint's last_modified field
- Other Numbering System Name
- Lunatic Asylum Lab
- Other Numbering System Identifier
- 1086