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In Situ Measurement of Solute Transport Coefficients: Assumptions and Errors

CSIRO Land and Water, G.P.O. Box 1666, Canberra ACT 2601, Australia

*Corresponding author (val.snow{at}cbr.clw.csiro.au).

ABSTRACT

Single-tracer and sequential-tracer techniques for directly measuring the immobile water content, _im, and a sequential-tracer technique for measuring the solute transfer rate coefficient, , 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 _im and . Both techniques were shown to result in considerable error in soils possessing high dispersivity () and high 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 _im within 15% of the true value if sampling took place when q > 10 mm h^-1, I > , and sampling time <1.25 h. The value of _m/ 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 _m/ 0.6, if 5 mm and q > 10 mm h^-1, and I > 30 mm, acceptable values of _im and were calculated. Sampling conditions became more restrictive as increased. If > 20 mm, there seemed little possibility for obtaining reasonable estimates of either _im or . Sampling conditions varied strongly with _m/, becoming more lenient as _m/ decreased.

Received for publication April 29, 1998.

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