Water rock-equilibrated

Type III. Equilibrated systems with a relatively uniform oxygen isotope composition in all lithologies. These systems require a large water/rock ratio, temperatures between 500 and 800°C, and life times around 5 x 10 y. 

The drawback of tracer methodologies that use stable isotope compositions of water is that water-rock interaction will eventually modify the original oxygen isotopic composition of the re-injected water, if it remains in the reservoir for a long time. Moreover, isotopic re-equilibration may take place between the water and the gas species when the injected fluid comes into contact with large quantities of reservoir gas. Finally, the isotopic composition... 

Fissure Adsorption Experiments. To perform fissure sorption experiments, one end of each fissure was connected via small bore tubing to a microliter pipetter as is shown in Figure 2. Rock-equilibrated water, described below, was injected into each fissure and the fissure walls were allowed to equilibrate with the solution for a period of two days before the adsorption experiments were performed. 

Rock-equilibrated water was produced by allowing distilled water to be in contact with granulated gray hornblende schist for a period of several days. The solution produced was filtered through a 0.4 pm NUCLEPORE filter before use. Analysis of the resulting solution showed that rock equilibrated-water contained 48 mg/L total solids (dried at 180°C). The E of the solution was measured to be 0.275 V. The pH of the gray hornblende schist equilibrated water was measured to be 7.6 at the time of the first adsorption experiment. Thereafter the solution pH was maintained at 7.5-7.7. 

Adsorption experiments were performed by removing rock-equilibrated water from the fissures and injecting stock solution which was made by dissolving tracer amounts of americium-241 in rock equilibrated water. The stock solution was allowed to equilibrate within the fissures for different periods of time and was then removed from each fissure. The stock solution was assayed before injection and after removal from the fissures so that the change in americium concentration was determined for a different time in each fissure. Ten adsorption experiments were performed in this manner and the results are presented in table I. Figure 3 is a graphical representation of the initial part of the adsorption curve. 

Equilibrium Partitioning Experiments. The equilibrium partitioning of americium-III between gray hornblende schist and rock equilibrated water was determined in batch partitioning experiments with rectangular blocks of gray hornblende schist ( 5). The surface area sorption coefficient, K, was determined to be 4.5 . 5 mL/cm where... 

Fissure Desorption Experiments. Desorption experiments were performed with two fissures by injecting rock equilibrated water into the fissures immediately after removing the americium-bearing solution from the fissures in adsorption experiments. After injection, the rock-equilibrated water was allowed to react within the fissures for a period of time before removal. The rock-equilibrated water was assayed after removal from the two fissures and the amount of nuclide desorbed as a function of contact time was determined. Between five and seven desorption experiments were performed for each of six time periods between ten seconds and sixty minutes for each of the two fissures. The results of the desorption experiments are presented in table 2 as the percent of 3 (at equilibrium) that desorbed from the fissures in a period of time. 

Before the elution experiments were performed, the "schist"-equilibrated water was pumped through the fissures for two days to allow the fissure walls to interact with the rock-equilibrated water. 

The second set of fissure-elution experiments was performed in the same manner as described for the first set of fissure-elution experiments. The difference in the second set of experiments was that after 0.67 fissure volumes of stock solution were drawn into the fissures at their respective flow rates, the stock solution in the reservoirs was replaced with rock-equilibrated water. The solution metering pumps were not turned off during the exchange. Subsequently, a total of twenty fissure volumes of solution was drawn through each fissure before the metering pumps were turned off. After the metering pumps were stopped, the rock-equilibrated water was removed from the reservoirs and the solution in the fissures was rapidly drawn off. 

Figures 6, 8, and 10 show that the distributions of americium on the fissure surfaces after the addition of 0.67 fissure volumes of stock solution followed by the elution of 20 fissure volumes of "schist -equilibrated water through the fissures at flow rates of 1.13, 2.29, and 4.77 cm/hr. A comparison was made between the americium distributions found on the fissure surfaces after the addition of americium stock solution and that found after elution of the americium by 20 fissure volumes of "schist"-equilibrated water. It was found that after the initial loading of americium into the fissures in the first 0.67 fissure volumes of solution, the peak concentrations of americium that were sorbed at the top of the fissures decreased in their relative concentration. The leading edges of the detectable nuclide concentration extending into the fissures had increased in length and relative concentration with subsequent elution by rock equilibrated water through the fissures. The ARDISC model predicted the same relationships.

Data from in-situ leach mining and restoration of roll-front uranium deposits also provide information on the potential mobility of the waste if oxidizing ground water should enter the repository. Uranium solids probably will be initially very soluble in carbonate ground water however, as reducing conditions are re-established through water/rock interactions, the uranium will reprecipitate and the amount of uranium in solution will again equilibrate with the reduced uranium minerals ... 

Cement-equilibrated water derived from the repository will react with the surrounding rock and form an alkaline-disturbed zone (ADZ) around the repository. The main reactions that are expected to occur within the ADZ are dissolution of primary silicates, and precipitation of hydrated calcium silicates (CSH phases) and possibly zeolites (Rochelle et al. 1992). As such water-rock interaction proceeds, the pH of the repository-derived water will be buffered toward lower values and will eventually reach the nearneutral values typical of unperturbed far-field groundwaters. It is likely that the hydrogeological and radionuclide retardation properties of the ADZ will be different from those of the unperturbed geosphere. The NSARP therefore includes work to evaluate the size of the ADZ and the extent to which its properties will be perturbed. A summary of the ADZ sorption programme is presented below. 

Figure 1.8 Closed-system equilibration between biotite, rock and water for oxygen and hydrogen isotopes (from Taylor, 1978, modified). Data from Tablel.9. 518Orock—518OwaIet = + 2 and 5Dbiol]te — 8Dwater = —40.

Thus the "stability sequence" is equivalent to increasing the intensity of weathering or to completing the process of equilibration under conditions of high water content, low concentration of alkali and silica ions and oxidation of iron in a soil profile. In fact the weathering process in a soil horizon sequence is much the same for pelitic rocks in all environments the dominant sequence is repeated with minor variations due to local conditions of pH or efficiency of the oxidation process. 

Procedure. The rock wafers were gently cleaned three successive times with methanol in an ultrasonic bath, dried and placed in the inert atmosphere box. Two wafers of each rock type were inserted horizontally in stainless steel holders and placed in linear polyethylene containers holding 150 ml of the appropriate prepared water. The wafers were allowed to equilibrate with the water for three days and then 5 X of tracer solution was added. The resultant solutions contained V/ 5x10 M of each element. The pH of each solution was measured before and after the addition of the tracer and did not change sign-... 

Figure 3. Porewater sulfate profiles at 5-meter sites in the north (acidified) and south (control) basins of Little Rock Lake during September, 1986, and January, 1987. Profiles were determined by porewater equilibrators placed in the lake for three weeks. Both sites are gyttja with > 90% water content.

Strontium isotopes have also been used to identify allochthonous sources for saline waters in crystalline rock. Sr/ Sr ratios for deep, saline waters of the Vienne granites (France) show a value that is consistent with Jurassic seawater and not consistent with values expected from equilibration with the rock. A mixing model between Jurassic seawater and crustal end-members can explain the origin of these deep saline ground-waters (Casanova et al., 2001 Negrel et al., 2001). 

When a surfactant-water or surfactant-brine mixture is carefully contacted with oil in the absence of flow, bulk diffusion and, in some cases, adsorption-desorption or phase transformation kinetics dictate the way in which the equilibrium state is approached and the time required to reach it. Nonequilibrium behavior in such systems is of interest in connection with certain enhanced oil recovery processes where surfactant-brine mixtures are injected into underground formations to diplace globules of oil trapped in the porous rock structure. Indications exist that recovery efficiency can be affected by the extent of equilibration between phases and by the type of nonequilibrium phenomena which occur (J ). In detergency also, the rate and manner of oily soil removal by solubilization and "complexing" or "emulsification" mechanisms are controlled by diffusion and phase transformation kinetics (2-2). 

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