Describe the breakthrough curve you would expect for transport of a non-reactive

Question

Describe the breakthrough curve you would expect for transport of a non-reactive solute injected as a finite pulse of duration 1 hour in a designed steady state Darcy flux of 0.5 m/hr arising from a head gradient of 0.1 in a soil column of porosity 0.25 and length 2 m, under the following conditions (ignore diffusion): a. no dispersion b. some dispersion but mostly advection After the experiment in part a, a fracture occurs, running the full length of the column from x=0 to x=2m, and the fracture conductivity is 1/10 of the conductivity of the column before the fracture occurs. You don't know about the fracture. c. Describe the breakthrough curve resulting from your attempt to duplicate case a. d. As in case c, but with a solute that undergoes equilibrium sorption reaction with bulk density = 1.5 g/cm3 and Kd of 1/3 cm3/g.

Explanation / Answer

The parameters of the van Genuchten–Mualem functions, ?(h) and K(?), for the loamy sand and sandy loam soils were estimated with the Hydrus-1D software (Simunek et al., 1998), using a specific particle-size distribution (Table 1) and soil densities (Table 2 ). The water content ?(h) and the hydraulic conductivity K(?)functions shown in Fig. 1 were used to simulate the water flow in these soils. For the initial water (pressure) potential head of -1000 cm H2O, the initial water content ?i in the loamy sand and sandy loam soils was 0.0748 and 0.1579 m3 m-3, respectively. As h increased, the water content increased, but at any given water pressure, the water content in the loamy sand soil was lower than that in the sandy loam soil. Note that in the water (pressure) potential head range from -1000 to -40 cm, the hydraulic conductivity of the loamy sand soil was less than that of the sandy loam soil, whereas the opposite was true for h > -40 cm. View Full Table | Close Full ViewTable 2. Summary of parameters used in simulations. Soil Batch data† Column data‡ Hydraulic parameters(Eq. [6] and [7])§ b m k ? ? ? ?s ?d a n K s mg kg-1 L mg-1 cm-1 h-1 g cm-3 cm3 cm-3 cm-1 cm h-1 Loamy sand 15.44 ± 1.86 0.0440 ± 0.018 0.25 ± 0.11 0.25 ± 0.16 1.40 0.41 0.042 0.039 1.66 5.65 Sandy loam 32.40 ± 2.32 0.0489 ± 0.011 0.30 ± 0.08 0.41 ± 0.09 1.39 0.41 0.057 0.015 1.46 1.08 † Maximum adsorption b m and adsorption k coefficient were obtained from the B adsorption isotherm (Fig. 2). ‡ Pore-scale dispersivity ? was obtained from the Br- breakthrough curves, rate coefficient ? was obtained from the high-velocity B breakthrough curves (Fig. 3), and volumetric pore-water content ?s and soil bulk density ? were obtained from column data § Air-dry water content ?d, fitting parameters a and n, and saturated hydraulic conductivity K s were derived with the Hydrus-1D software using a specific particle-size distribution (Table 1) and ?. Fig. 1. Hydraulic properties of the loamy sand and sandy loam soils used in the simulations. Parameters of the hydraulic functions ? = ?(h) and K = K(h) are given in Table 2; ? = volumetric water content, K = hydraulic conductivity, and h = soil water pressure. The parameters for the NE model were determined from the above-mentioned batch and column experiments. The coefficients b m and k were obtained from the B adsorption isotherms for the loamy sand and sandy loam soils, and the ? values were obtained from the Br- BTCs. Despite the differences in soil texture, the Br- BTCs (not shown) for the loamy sand soil were almost the same as those for the sandy loam soil, and their shape was independent of pore-water velocity. The Br- BTCs were unaffected by the flow interruption, indicating that any nonreactive solute transport in these two soils is ideal, which is consistent with the findings of Communar and Keren (2006) for similar soils. The Br- BTCs were analyzed using the classical CDE with a retardation factor R = 1 + ?K L/?, where K L (L mg-1) is the distribution coefficient for a linear adsorption). The CDE with the retardation factor R of 1 gave good matches (r 2 = 0.97) to the measured Br- BTCs for both soils. The optimized value of the pore-scale dispersivity for the loamy sand soil (? = 0.25 ± 0.11cm) was very close to that obtained for the sandy loam soil (? = 0.3 ± 0.08 cm). The rate coefficients ? were determined by fitting the NE model to the B BTCs obtained from the fast-velocity experiments using the optimized b m, k, and ? values. The flow interruption was simulated by using the NE model at q = 0 and D e = D m Molecular diffusion coefficients, D m, of 0.075 cm2 h-1 (Cussler, 1984) and 0.036 cm2 h-1 (Boudreau, 1997) were used for Br- and B, respectively. The effective molecular diffusion coefficient for unsaturated flow conditions was calculated by using the relationship of Millington and Quirk (1961):

Related Questions

Navigate

• Browse (All)
• Browse D
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.