Graduate Thesis Or Dissertation
 

Cation retention and solute transport related to porosity of pumiceous soils

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  • Previous studies of pumiceous soils in Oregon indicated that their physical and chemical properties were strongly related to the porous nature of the pumice matrix. Soils developed on air-fall pumice from the eruption of Mt. Mazama (Crater Lake, Oregon) had low thermal conductivities and low volumetric heat capacities. Values for cation exchange capacities were considered to be high. It was thought that values reflected index solution trapping in pores rather than electrostatic exchange capacity. The presence of 2:1 phyllosilicates in pumice soils was at variance with data on volcanic soils from Japan. Effective retention of soil solution in pumice pores was suggested as a mechanism by which products of silicate hydrolysis might attain concentrations required for 2:1 phyllosilicate synthesis in weakly developed pumice soils. Since the porous nature of a pumice soil is so important to its physical and chemical properties, this study attempted to quantify some of the relationships. Porosity in Newberry and Mazama pumice was related to cation exchange capacity, ionic diffusion through pumice particles, mass transport of solutes by displacement and equilibration between internal pores and external solutions. An index ion depletion technique was developed to determine electrostatic cation exchange capacities of pumice soils. This technique was successful in separating electrostatic exchange from the previously reported phenomenon of index solution trapping. CEC values for all pumice samples ranged between 0.31 and 4.56 meq/ 100 g. The values increased slightly with particle size and with degree of weathering. The younger (2000 B. P. ) Newberry pumice had lower CEC values than the older (6600 B. P.) Mazama pumice. Porous system diffusion coefficients were determined through pumice particles and related to internal porosity. Observed porous system diffusion coefficients were smaller than the bulk solution coefficients. When the porous system diffusion coefficients were corrected for volumetric moisture content, the resulting effective diffusivities were in the range of those for the bulk solutions. Miscible displacement methods were used to correlate the location of breakthrough curves with particle size and internal pore volume for pumice from different sources. Increased translation of solute breakthrough curves toward smaller effluent volumes was observed with increased particle size. This volumetric translation was not highly correlated with measurements of internal pore volume. The leftward translations were strongly correlated (r = 0. 93) with the midrange diameter of particles in the columns and appeared to be a function of increased mixing between the resident and displacing solutions. Under saturated conditions involving a hydraulic gradient, pumice soils should be considered as highly leached. Static equilibration studies demonstrated that the amount of solute retained by pumice samples after equilibration is controlled by the degree of moisture saturation prior to introduction into the index solution. Air trapped in internal pores by capillary rise was only slowly replaced by index solution. Partial and complete drying after solution equilibration reduced the rate and total recovery of index ion. Air entered the pumice blocking access to index solution in the internal pores. A drying treatment following solution saturation was the only way salt trapping could be induced. From a management perspective, the behavior of solutes during a wetting and drying cycle could be quite important. The annual precipitation pattern would result in saturated conditions with intense leaching during the winter and unsaturated conditions with salt trapping during the summer. Management practices for the coarse pumice soils of central Oregon should reflect these soil chemical and physical properties .
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