In Situ Measurement of Solute Transport Coefficients: Assumptions and Errors
Single-tracer and sequential-tracer techniques for directly measuring the immobile water content, θim, and a sequential-tracer technique for measuring the solute transfer rate coefficient, α, have been proposed. These techniques have not been subjected to a comprehensive analysis of their assumptions, nor of the errors resulting if those assumptions are violated. Here, an analytical solution to the mobile-immobile convection-dispersion equation was used to assess the range of soil and sampling conditions that can lead to accurate estimates of θim and α. Both techniques were shown to result in considerable error in soils possessing high dispersivity (λ) and high α when conductivity (q) was low, or if sampling was done when the cumulative infiltration (I) was too small. For a wide range of conditions, the single-tracer technique resulted in estimates of θim within 15% of the true value if sampling took place when q > 10 mm h-1, I > λ, and sampling time <1.25 h. The value of θm/θ had only a small effect on appropriate sampling conditions. Sampling conditions were more restrictive for the sequential-tracer technique than for the single-tracer technique. When θm/θ ≈ 0.6, if λ ≈ 5 mm and q > 10 mm h-1, and I > 30 mm, acceptable values of θim and α were calculated. Sampling conditions became more restrictive as λ increased. If λ > 20 mm, there seemed little possibility for obtaining reasonable estimates of either θim or α. Sampling conditions varied strongly with θm/θ, becoming more lenient as θm/θ decreased.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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