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USDA-ARS National Soil Tilth Lab., 2105 Pammel Drive, Ames, IA 50011
Dep. of Agronomy, Iowa State Univ., Ames, IA
* Corresponding author ( jaynes{at}nstl.gov).
ABSTRACT
Numerous field and laboratory studies have documented the occurrence of preferential transport of solutes due to a fraction of the soil water being immobile and not taking part in the transport process. Domain models have been developed that describe these processes, but before we can apply them routinely, we need methods for measuring the required model parameters, particularly the fraction of immobile water to total water 
lm/ and the exchange coefficient between the mobile and immobile domains, 
. We developed a field method for measuring both lm/ and . The method uses a sequence of conservative anionic tracers consisting of Br–, pentafluorobenzoate, o-trifluoromethylbenzoate, and 2,6-difluorobenzoate infiltrated with time through a tension infiltrometer. Previous studies have confirmed that these tracers have very similar transport properties in a wide range of soils. The method was applied to an undisturbed loam and a greenhouse soil as an initial test of the approach. Calculated im/ fractions averaged 0.69 and ranged from 0.25 to 0.98, while calculated values averaged 0.0081 h–1 and ranged from 0.0030 to 0.021 h–1. These values compare well with values reported earlier by other investigators. The method is simple and allows routine measurement of transport properties of field soils. The method can also be used to validate the applicability of domain models to specific soils.
Received for publication March 14, 1994.
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