Submitted to: Moscow University Soil Science Bulletin
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/14/2004
Publication Date: 7/1/2004
Citation: Pachepsky, Y.A., Korsunskaia, L.P., Shein, E.V. 2004. Concurrent transport of chloride, sodium, and calcium ions in undisturbed soil columns. Moscow University Soil Science Bulletin. Interpretive Summary: Contaminant transport in soils continues to be a subject of extensive studies. Transport of conservative tracers is affected mostly by soil structure causing differences in mobility of different parts of soil solution. Transport of reactive contaminants is affected both by soil structure and interactions at surfaces of soil particles. Flow velocity should affect transport of reactive contaminants because high velocities may leave no time for slow adsorption to develop. The objective of this study was to observe effects of soil structure and flow velocity on transport of reactive ions of calcium and sodium. We monitored concurrent chloride, calcium and sodium transport in slow-flow and fast-flow breakthrough experiments with undisturbed southern chernozem soil and chestnut soil columns. Data inspection showed that the pore space could be conceptualized as having a relatively mobile zone and a relatively immobile water flow zones. Compared with slow-flow experiments, fast flow resulted in a decrease of the mobile zone providing the initial breakthrough and an increase of the immobile zone affecting late stages of the breakthrough; still, less water had to infiltrate to provide breakthrough in the fast-flow experiments. Flow rate effects on the transport were more evident in the compacted chestnut soil that had poorer structure. Transport of reactive ions was affected by the flow rates in the same way as the transport of chloride ion. Fast flows provided fast breakthroughs of the reactive ions. Results support the hypothesis that soil physical heterogeneity is the primary factor of the contaminant breakthrough through soils, especially at high infiltration flow rates.
Technical Abstract: Determining aggregate size distribution is a common way of characterizing soil structure. Information about soil structure can also be derived from examining the aggregate mass or density on aggregate size. There were several reports that density-size relationships in air-dry aggregates follow predictions of a model assuming aggregates to be mass fractals. Recently it was demonstrated that such model is applicable to wet aggregates if parameters of this model are assumed to be linear functions of gravimetric water contents. The objective of this work was to evaluate sensitivity of fractal parameters to soil compaction caused by wheel traffic. Irrigated ad non-irrigated plots were laid out at silty clay Greyzem under fallow, and treatments of one tractor pass and three tractor passes were applied. Volume of individual aggregates for four depths in the plow layer was measured with kerosene method at air-dry water content, at two intermediate water contents between saturation and air-dry, and at saturation. The mass fractal model fitted data in a satisfactory manner within the range of water contents from air dry to saturation with R2 of 0.999. Both the slope and the intercept of the dependence of fractal dimension and reference aggregate mass on water content were more sensitive to compaction than soil bulk density and aggregate size distributions. Parameters of fractal scaling showed a promise to diagnose compaction in studied soil.