Reaction of the Topopah Spring Tuff with J-13 water at 120{sup 0}C

Abstract

This report describes a series of hydrothermal experiments using crushed tuff from the Topopah Spring Member and natural ground water from well J-13. The purpose of these experiments is to define the changes in water chemistry that would result from temperature changes caused by emplacing high-level nuclear waste in a repository in the Topopah Spring tuff. Experiments were conducted at 120{sup 0}C in Teflon-lined reaction vessels at four separate rock-to-water ratios and for reaction times up to 72 days. The composition of evaporite deposits contained in the pores of the surface-outcrop rock material used in these experiments is determined from solution compositions resulting from treatment of the rock before the start of the experiments. Results from the experiments at 120{sup 0}C are compared with previous experimental results from hydrothermal reaction of the Topopah Spring tuff with J-13 water at 90 and 150{sup 0}C. The main conclusion that can be drawn from this work is that changes in the water chemistry due to heating of the rock-water system can be expected to be very minor. There is no significant source of anions (F{sup -}, Cl{sup -}, NO{sub 3}{sup -}, or SO{sub 4}{sup 2-}) in the rock; solution anion compositions after reaction of pretreated rock with J-13 water differ very little from the starting compositions. The major changes in cations are an increase in silica to approximately the level of cristobalite solubility, supersaturation of aluminum followed by slow precipitation, and fairly rapid precipitation of calcium and magnesium due to the retrograde solubility of calcite. These results are in good agreement with those previously reported for reaction of the tuff with J-13 water at 90 and 150{sup 0}C. 7 references, 7 figures, 28 tables.