Pore-geometry effects on solute dispersion in aggregated soils and evaluation of a predictive model
Year: 1974
Type: dissertation
Abstract: Breakthrough curves (BTCs) obtained by miscible displacement of either a solute zone (zonal) or a solute front (frontal) through a saturated column were used to investigate the influence of pore geometry and adsorption on solute dispersion in some Hawaiian aggregated soils.A frontal BTC obtained from a tracer solution displacing tracer-free solution in the column is defined as a Tracer-In BTC, while the opposite technique yields the Tracer-Out BTC.Much more rewarding interpretations of BTC data can be made by plotting the mirror-image of the Tracer-Out BTC along with the Tracer-In BTC and evaluating each curve independently.A study of six principal characteristics of frontal BTCs were found useful in understanding the behavior of the solute in the soil column during displacement.These properties are: (i) shape, (ii) location, (iii) area above the BTC, (iv) area under the ETC, (v) relative locations of the Tracer-In and the Tracer-Out BTCs, and (vi) slope.Equations for quantification of the fraction of soil mass in equilibri.umwith the solute (EQUIL) during the In displacement v and the fraction of the solute desorbed from the soil column (DESOR) during the Out displacement are presented.Qualitative and quantitative techniques developed were useful in the analysis of BTC data obtained by miscible displacement of several adsorbed solutes through saturated columns of several soils.Usefulness and certain limitations of the techniqup.sproposed are also discussed.A simple quantitative technique was employed to determine the nonsolvent fraction of soil water that would be excluded from flow processes in Hilo (Typic Hydrandept) and Wahiawa (Tropeptic Eustrustox) subsoils.Data from equilibrium and miscible displacement studies revealed that all of the soil water in Wahiawa subsoil participated in solute transport, while about 10% of the native soil water in Hilo soil was e][cluded from all solutes other than tritiated water.Both zonal and frontal BTCs obtained by miscible displacement of tritiated water through a saturated column of highly aggregated Molokai (Typic Torrox) subsoil were utilized in the evaluation of a capillary b~ndle model for prediction of solute dispersion in soils.The predictive model is based on partitioning the soil into several poresize increments, each with a characteristic pore-velocity and dispersion coefficient.The solution of the differential equation, subject to appropriate initial and boundary conditions, for each pore-size increment is summed in a linear manner to yield the total solution.The pore-size distributio~l of the Mo10kai soil column determined by both the methods of miscible and iwmiscible fluids (denoted as MMF and MIF, respectively) wer.e strikingly different.Pore-velocity distributions were computed by trle Poiseuille equation from these pore-size distributions and used vi as input data in the capillary bundle model.The BTCs predicted either by using a pore-velocity distribution from the ~W or by employing an average pore-velocity to represent the whole soil approximated the measured dispersion profiles well.Howev"er, when the pore-velocity distribution from the MIF was used as input data in the model, the predicted BTCs failed to describe the measured curves.The assumption of independent flow in all capillaries was not met in the experiments, and mixing between adjacent flow paths resulted in much less dispersion than anticipated from the model.The measurement and mathematical description of accessibility and inter-connectedness of pores appear to be essential for quantitative prediction of the effects of pore geometry on solute dispersion in aggregated soils.
Author: Palakurthi Suresh Chandra Rao
Language: English
Topic: Soil and Unsaturated Flow
Subfield: Civil and Structural Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W62451913