FIELD METHOD FOR MEASURING MOBILE IMMOBILE WATER-CONTENT AND SOLUTE TRANSFER RATE COEFFICIENT

Numerous field and laboratory studies have documented the occurrence o f preferential transport of solutes due to a fraction of the soil wate r being immobile and not taking part in the transport process. Domain models have been developed that describe these processes, but before w e can apply them routinely, we need methods for measuring the required model parameters, particularly the fraction of immobile water to tota l water theta(im)/theta and the exchange coefficient between the mobil e and immobile domains, alpha. We developed a field method for measuri ng both theta(im)/theta and alpha. The method uses a sequence of conse rvative anionic tracers consisting of Br-, pentafluorobenzoate, o-trif luoromethylbenzoate, and 2,6-difluorobenzoate infiltrated with time th rough a tension infiltrometer. Previous studies have confirmed that th ese 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 theta(im)/theta fr actions averaged 0.69 and ranged from 0.25 to 0.98, while calculated a lpha values averaged 0.0081 h-1 and ranged from 0.0030 to 0.021 h-1. T hese values compare well with values reported earlier by other investi gators. The method is simple and allows routine measurement of transpo rt properties of field soils. The method can also be used to validate the applicability of domain models to specific soils.