Cohassett basalt was reacted with synthetic Hanford groundwater in a Dickson rocking autoclave at 300°C and 30 MPa for ca. 7600 hours. During the experiment fresh solution was twice reinjected into the reaction cell, raising the nominal water:rock mass ratio from 50:1 to 140:1. The reinjection experiment is intermediate between closed-system Dickson experiments and flow-through tests in that the fluid and solids can remain in contact for extended periods of time before the solution is replaced. This allows more time for equilibrium to be approached. Data from this experiment suggest:
(1) After reinjection many solution parameters quickly (hrs-days) returned to near preinjection values. (2) The redox buffer capacity of the basalt was not exceeded, i.e. fO2 values remained near magnetite-hematite values, although the nominal water:rock mass ratio was raised to ca. 140 during the experiment. (3) After reinjection the stable high-temperature pH value was only slightly less than the initial pH value. (4) The silica concentration stabilized near apparent quartz saturation rather than the cristobalite saturation value found in closed-system experiments. (5) Short-term relationships between cations appear to be controlled by ionexchange between the solutions and clay minerals.
Reaction products identified from the experiment include: Fesmectite, illite, hematite, minor cristobalite and possibly Ti-maghemite.