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Tortuosity, Mean Residence Time, and Deformation of Tritium Breakthroughs from Soil Columns
Author(s) -
Ma L.,
Selim H. M.
Publication year - 1994
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1994.03615995005800040011x
Subject(s) - tortuosity , dimensionless quantity , tracer , tritium , dispersion (optics) , soil water , retardation factor , chemistry , volume (thermodynamics) , moment (physics) , mechanics , porosity , soil science , physics , thermodynamics , geology , nuclear physics , chromatography , optics , organic chemistry , column chromatography , classical mechanics
Tritium breakthrough curves (BTCs) are widely used to estimate the hydrodynamic dispersion coefficient ( D ) of the convection‐dispersion equation (CDE). However, D alone often provides inadequate fit. Therefore, a second parameter is commonly added to describe tritium BTCs. We tested the validity of describing tritium transport using an effective path length ( L e ) or a tortuosity factor (τ) in the CDE. We obtained the dimensionless parameter τ from fitted L e and column length L such that τ = L/L e . Miscible displacement experiments were used to obtain tritium BTCs for several soils and materials in uniformly packed columns under water‐saturated and steady upward flow. Two types of tritium pulses were applied: the first was a small pulse (≈ 0.03 pore volume) and used to determine mean residence time ( t m ) in soil columns. The t m values were obtained from the first time moment of BTCs and were subsequently used to estimate L e and τ. The second type of input pulse was about one pore volume and used to obtain the parameters D and L e using the CDE. Tritium results exhibited double peaks and extensive deformations of BTCs for several soils; these were more distinct with increasing pore water velocity. Using the principle of superposition, bimodal peaks from large pulse inputs were successfully described based on small‐pulse BTCs. In addition, agreements between estimated τ using small pulses and fitted τ from large pulses support the use of the fitting parameters D and L e in the CDE as an alternative method for describing tracer BTCs in soils.