Time Scales for Dissolution of Calcite Fracture Fillings and Implications for Saturated Zone Radionuclide Transport at Y
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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 104 yr for a 2 mm-thick deposit located I m from a flowing fracture, to over 10' 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. INTRODUCTION Yucca Mountain (YM), Nevada is being considered as a potential site for geologic disposal of highlevel radioactive waste (HLW). In the fractured-rock hydrologic system at YM, it is believed that matrix diffusion - defined as diffusive (Fickian) transfer of dissolved species between water flowing through rock fractures and the relatively stagnant pore water stored in adjacent rock matrix - could play an important role as a dilution mechanism for radionuclides that might escape confinement. There is evidence, however, that diffusive fracture-matrix exchange at YM may be limited. This evidence stems from the fact that calcite mineralization, possibly as old as 10 Ma [11, has been observed sporadically in core samples from wells in the saturated tuff aquifer [2], despite the fact that groundwater that flows into these wells is chemically undersaturated with calcite [3, 4]. Occurrence of calcite could be an indication that groundwater flow is channelized and that portions of the aquifer containing calcite are effectively isolated from present water circulation [4]. Murphy [4] points out that an implication of this calcite occurrence is that matrix diffusion may be limited, perhaps over time scales of millions of years. Until now, however, no attempt has been made to estimate time scales for dissolution of calcite minerals through diffusive exchange with water flowing in fractures. In this paper we develop a simple model coupling the dissolution rate of a calcite fracture filling to diffusive solute transport between fract
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