HR: 1340h
AN: H53F-1490    [Abstracts]
TI: Solute Diffusivity of Repacked Volcanic Ash Soil: Effect of Changes in Pore Size Distribution due to Soil Compaction
AU: * Perera, M S
EM: saminthaanne@yahoo.com
AF: Graduate School of Science and Engineering, Saitama University, 225 Shimo-okubo, Sakura-ku, Saitama, Saitama, 338-8570, Japan
AU: Resurreccion, A C
EM: acresurrecci@up.edu.ph
AF: Graduate School of Science and Engineering, Saitama University, 225 Shimo-okubo, Sakura-ku, Saitama, Saitama, 338-8570, Japan
AU: Kawamoto, K
EM: kawamoto@post.saitama-u.ac.jp
AF: Graduate School of Science and Engineering, Saitama University, 225 Shimo-okubo, Sakura-ku, Saitama, Saitama, 338-8570, Japan
AU: Komatsu, T
EM: komatsu@post.saitama-u.ac.jp
AF: Graduate School of Science and Engineering, Saitama University, 225 Shimo-okubo, Sakura-ku, Saitama, Saitama, 338-8570, Japan
AU: Moldrup, P
EM: pm@bio.aau.dk
AF: Dept. of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, Sohngaardsholmsvej 57, Aalborg, DK-9000, Denmark
AB: Diffusion is the dominant spreading mechanism of contaminants dissolved in soil-water in the absence of soil- water flow. Solute diffusion coefficient, D_s, is a key parameter in investigating the fate and transport of contaminants from a polluted soil site. However, only a few studies on quantifying D_s as a function of soil- water content were done, especially for aggregated soils with a dual pore system such as volcanic ash soils (Andisols). In this study, we investigated the effect of bulk density on pore size distribution, and, consequently, on solute diffusivity (D_s/D_o, where D_o is the solute diffusion coefficient in pure water) in repacked volcanic ash soil taken at 5-10 cm depth at a pasture site in Nishi-Tokyo, Japan. Measurements of D_s were done on sieved and repacked soil at three bulk densities (0.62 g cm^-3 , 0.7 g cm^-3, and 0.8 g cm^-3 ) and at three soil moisture conditions at pF (= log (-ψ; soil-water matric potential in cm H₂O)) 1.8, 2, and 3 for each bulk density. Half-cell method was used to measure Ds where the source and sink half cells (each cell of 10-cm length and 4.9 cm in diameter) were joined together and the concentration profile was analyzed after a substantial time to determine D_s. Results showed that at a particular bulk density, D_s decreased with decreasing degree of saturation. This is expected since as the soil becomes drier, water films become disconnected resulting in a decrease in D_s. On the other hand, at a particular degree of saturation, the magnitude of D_s considerably decreases with increasing dry bulk density. As soil is compacted (and thus the increase in bulk density), the observed pore size distribution obtained from soil-water retention curve changes where the mainly inter-aggregate large pores become smaller and soil particles become closer to each other. This reduction in inter-aggregate pore size likely increases the liquid-phase tortuosity resulting in the decrease in D_s/D_o at soil-water content at pF < 3. The soil-water retention point at pF 3 was observed to be the separation between the inter- and intra-aggregate pore space regions, where the inter- aggregate pore space was completely drained. Thus at pF close to 3, the difference of D_s/D_o among three bulk densities becomes smaller, probably due to high possibility to ensure continuous water pathways among intra-aggregate pores caused by inter-connection of aggregate. Although volcanic ash soils are distributed across around 0.84% of the earth's land surface, only a limited number of studies about solute diffusion for Andisols are available as compared to numerous studies on water permeability (liquid-phase convection parameter). Therefore, this study contributes to a valuable data set of solute diffusion coefficient for volcanic ash soils.
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2007 Fall Meeting