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Tortuosity, Mean Residence Time, and Deformation of Tritium Breakthroughs from Soil Columns

Sturgis Hall, Agronomy Dep., Louisiana State Univ., Baton Rouge, LA 70803

*Corresponding author.

ABSTRACT

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 () in the CDE. We obtained the dimensionless parameter from fitted L_e and column length L such that = 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 . 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 using small pulses and fitted 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.

Contribution from the Louisiana Agric. Exp. Stn., LSU Agric. Center as manuscript no. 93-09-7334. This research was supported in part by CSRS Water Quality grant no. 9100657.

Received for publication July 2, 1993.

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