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Notes 1771 T 2.1 m mounted arm mounte~~~~a level k- 2.1 m 4 Fig. 2. Design of the water motion simulator. a seaside well in the upper Gulf of California at Puerto Pefiasco, Sonora, Mexico. Brackish water was a 1 : 1 mixture of tapwater and seawater. Salinities by refractometer were 0, 20, and 407~. Solutions were measured for temperature, pH, and electrical conductivity at the start and finish of each run. Samples of each solution were analyzed for cations and anions, including calcium and sulfate, by AOAC methods (Laboratory Consultants, Tempe, Ar- Table 1. Analyses of tapwater, brackish water, and seawater used in clod card calibration experiments. Constituent Tapwater 20%3 40% Calcium 1.46 7.06 13.20 Sulfate 0.85 16.66 33.94 Boron 0.008 * 0.473 Magnesium 0.417 * 61.6 Sodium 2.13 * 534.8 Chloride 1.71 * 630.4 Fluoride 0.056 * 0.078 PH 8.1-8.5 8.1-8.2 8.2-8.3 EC (mmhos cm-‘) 715 34,000 71,500 * Brackish water (20%~) was a I : 1 mixture of tapwater and 40% water. Analyses of elements of limited importance to this study were omitted for this mixture. (mol m-l) izona, and the Soil and Water Testing Laboratory, University of Arizona, Tucson) (Table 1). Solutions in the tank were changed after 2- 3 runs of clod cards to reduce accumulation of calcium sulfate in the water. The ice-chest experiments used an additional solution, 25o/oo salinity, also prepared by dilution of tapwater and seawater. Weight loss of clod cards vs. water motion is expected to be a decreasing rather than a linear function, because the exposed area of the clod diminishes as dissolution proceeds. The rate of dissolution of a solid in an aqueous solution can be expressed as dW - = -kAAC. d6 W is the weight of the solid, 8 is time, k the mass transfer coefficient, A the exposed area of the solid, and AC the concentration difference between the dissolved solid in the liquid at the solid-fluid interface and in the bulk liquid. The dissolution of solids, including the diffusion process across boundary layers, is described in detail in several mass transfer texts
1772 Notes J 1.0 2 E 0.9 L 2 0.6 P < 0.7 t 0.6 0.010 1 10 100 Velocity, cm s-1 Fig. 3. Plot ofclod card weight loss vs. velocity through 32o/oo seawater for data from Glenn and Doty (1992) used in the form of Eq. 3 (their data extend to higher velocities than our work). The slope of the regression line is 0.838kO.042. 0.5 0.4 r I 0 5 10 15 20 25 30 Temperature, "C Fig. 4. Correction factor for clod card data with T,, = 298°K and pref = viscosity of pure water at 25°C. The four lines correspond to salinities of 0, 10, 20, and 40%~ from top to bottom. Calculated with viscosity data from Riley and Skit-row (1975). (e.g., Cussler 1984; Sherwood et al. 1975; Skelland 1974). For solids of sample geometries which dissolve uniformly, the area and weight relationship can be approximated as A (2) Ai and Wi represent the initial area (exposed) and weight of the solid. For clod cards, this approximation is useful until > 50% of the clod has dissolved or longer, depending on the velocity of the fluid. The mass transfer coefficient is also influenced by the change in clod dimensions; however, the effect is usually minor until well into the dissolution process. Equations 1 and 2 can be combined and integrated to give 1 - (?r = ;k($A,. (3) Wr is the final weight of the clod after time 8 has elapsed. Equations 3 is useful for data analysis; the mass transfer coefficient usually varies with the fluid velocity raised to a power (P), so a log-log plot of [l - (WJ/ Wi)“] against velocity should yield a straight line. This is illustrated in Fig. 3 with the data of Glenn and Doty (1992). Equation 3 can be rearranged to give an expression for the overall fraction of weight 10~s (A W/ Wi): Assuming that AC remained constant during dissolution, experimental data at a given temperature and salinity were correlated with the model ( ) l/3 Wf = aVm. I- j7 I The constants a and m can be determined experimentally by means of the rotating arm, and weight loss data from clod cards exposed in the field can then be used in Eq. 5 to determine water motion. The effect of temperature at a given salinity and CaSO, concentration was corrected by using the group (T/T,,) (p,.,&) to normalize data to a reference temperature, where Tis absolute temperature, and p is viscosity. This group (Fig. 4) is typically used to estimate the changes in diffusion coefficients (Reid and Sherwood 1966) with varying solution strength. From Eq. 1 and 3, the dissolution rates are also dependent on the concentration of CaSO, in the bulk water and factors influencing the concentration, such as the ionic strength of the bulk water. To determine the concentration difference, we assume the solution at the gypsum-water interface is saturated with CaSO, and determine AC with the solubility product as Ksp = [Ca2+]i[S042-]i g= 1 - 11 - ;k($)ACO13. (4) or = ([Ca’+], + AC)([S042-]b + AC) (6)
Page 1 and 2: 1768 Notes coming tide over a broad
Page 3: 1770 Notes r=0.15 (typ.) ‘F q- El
Page 7 and 8: 1774 Notes 0.6 A 25.5’ A 30.5. 0
Page 9 and 10: 1776 Notes 1000 500 < 200 p 100 7 f
Page 11 and 12: 1778 Notes Sample Points Along cent

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