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Time Scales for Dissolution of Calcite Fracture Fillings and Implications for Saturated Zone Radionuclide Transport at Yucca Mountain, Nevada

Published online by Cambridge University Press:  10 February 2011

James R. Winterle  and
William M. Murphy
James R. Winterle
Affiliation:
Center for Nuclear Waste Regulatory Analyses, 6220 Culebra Road, San Antonio, TX 78238
William M. Murphy
Affiliation:
Center for Nuclear Waste Regulatory Analyses, 6220 Culebra Road, San Antonio, TX 78238
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Abstract

An analysis was performed to estimate time scales for dissolution of calcite fracture fillings in the fractured tuff aquifer that underlies Yucca Mountain (YM), Nevada, where groundwater is chemically undersaturated with respect to calcite. The impetus for this analysis originates from speculation that undissolved calcite in the saturated zone is evidence for limited diffusive exchange between fracture and matrix waters. Assuming that matrix diffusion is the rate-limiting process, the time scale for dissolution of calcite fracture fillings depends on the amount of calcite initially deposited, the distance between flowing fractures, the degree of chemical disequilibrium, and the rate of diffusion. Assuming geochemistry of J-13 well water in free-flowing fractures, estimated time scales for complete dissolution of matrix-entrapped calcite range from about 104yr for a 2 mm-thick deposit located I m from a flowing fracture, to over 107 yr for a 2 cm-thick deposit located 100 m from a flowing fracture. We conclude that, given the geochemical and hydrologic characteristics observed at YM, the persistence of calcite minerals over geologic time scales in aquifers where flowing water is under-saturated with calcite does not necessarily preclude matrix diffusion as a dilution mechanism. However, our model suggests that the effective spacing between flowing fractures may be large enough to diminish the overall benefit of matrix diffusion to proposed high-level waste repository performance.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 556: Symposium QQ – Scientific Basis for Nuclear Waste Management XXII , 1999 , 713
Copyright
Copyright © Materials Research Society 1999

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