Groundwater Grande Ronde

Based on unpublished analyses of Grande Ronde groundwater by T. E. Jones. 

Eh-pH Conditions. Measured pH values for Grande Ronde groundwater are found to be 9.5 0.9. Hydrothermal experiments on the system basalt-groundwater (11) have shown that during the thermal period, pH could be depressed as much as 1 to 2 pH units. However, since significantly elevated temperatures will not extend far into the host rock (12), this study treats only ambient conditions and is applicable to the far-field or the near-field after the thermal period. 

Two considerations (1) kinetic limitations in aqueous sulfate/ sulfide species equilibration and (2) observed lateral and vertical differences in Grande Ronde groundwater chemistry, were evaluated for their effect on calculated solubilities and associated aqueous speciation. 

Table I. Reference Grande Ronde Groundwater Composition at 25°C ...

A complete summary of the results of the solubility computations for all radionuclides considered is found in Table n. The solubility values listed assume no sulfur speciation and are for a reference Eh of -0.3 V. This value is approximately midway between estimates of Eh from basalt-mineral equilibria and direct measurements of Grande Ronde groundwaters. 

Table II. Solubilities of Radionuclides in Grande Ronde Groundwater at Eh = -.0.3 V...

Solids controlling Dominant species in solution Range in Grande Ronde groundwaters (mol/L) Reference Grande Ronde groundwaters (mol/L) experiments and natural waters (mol/L)... 

The formulation of the synthetic Grande Ronde groundwater used as starting solution in the dissolved oxygen experiments is given in Jones (9). Table I provides an analysis of the starting solution. 

A solution of 125ug/LAs(V) in deionized water was prepared by serial dilution of a 1,000 mg/L As(V) stock solution prepared from As205. This solution and the synthetic Grande Ronde groundwater were air-saturated. 

Table IV. Dissolved Oxygen Data from 150°C Basalt + Synthetic Grande Ronde Groundwater Experiment (Run D2-16)...

Figure 1. Dissolved oxygen vs time data. The experiments were basalt + synthetic Grande Ronde groundwater (B+SW) and synthetic Grande Ronde groundwater (SW) at 300 bars. Determination of uncertainties for B+SW data points is discussed in Table IV. Uncertainties for SW data were derived from replicable tests.

Sufficient DO data were not obtained from basalt-synthetic Grande Ronde groundwater experiments to allow determination of a definitive rate law. A first order kinetic model with respect to DO concentration was assumed. Rate control by diffusion kinetics and by surface-reaction mechanisms result in solution composition cnanges with different surface area and time dependencies (32,39). Therefore, by varying reactant surface area, determination of the proper functional form of the integrated rate equation for basalt-water redox reactions is possible. 

A basalt plus groundwater oxygen buffering experiment was conducted at Arizona State University using Umtanum basalt plus synthetic Grande Ronde groundwater at 200°C in an autoclave equipped with the Teflon membrane previously described. The gas diffusing across the membrane initially rose to a maximum pressure of 10.62 bars after 242 hr. This was followed by a pressure decrease to a minimum of 4.4 bars after 358 hr. Thereafter the pressure steadily increased at a rate of 4.5 x 10 3 bars/hr. A possible explanation of the decrease in pressure from 10.62 to 4.4 bars is the reduction of sulfate ... 

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