Field Method for Measuring Mobile/Immobile Water Content and Solute Transfer Rate Coefficient

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 θ lm /θ and the exchange coefficient between the mobile and immobile domains, α. We developed a field method for measuring both θ lm /θ and α. 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 θ im /θ fractions averaged 0.69 and ranged from 0.25 to 0.98, while calculated α 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. Disciplines

A review of model applications for structured soils: a) Water flow and tracer transport

“…Field techniques to estimate two-region model parameters Jaynes et al (1995) and Clothier et al (1995) suggested field methods for estimating MIM parameters. Using disk infiltrometers, several tracers were infiltrated into the soil and their distributions below the disk were then measured in soil samples taken after infiltration.…”

“…Al-Jabri et al (2002) suggested a variant of this method for simultaneous measurement at multiple locations based on point-source dripper lines. They used the Wooding's (1968) analytical solution for estimating K and the capillary length, λ c , while MIM transport parameters were estimated according to Jaynes et al (1995). In an application with several dripping rates, the saturated hydraulic conductivity, K s , the ratio of the immobile and the saturated water content, θ im /θ s , and λ c were similar, while α was different compared to corresponding values derived using the disk infiltrometer (Al-Jabri et al, 2002).…”

Modelling the effect of aggregates on N<sub>2</sub>O emission from denitrification in an agricultural peat soil

“…In a methane simulation model a similar parameter is used to define the immobile pore fraction for gaseous diffusion in peat, with a default value of 0.5 for non-tropical peatland (Walter and Heimann, 2000). For the immobile soil fraction in mineral soils, values in literature range from 0.04-0.98 (Jaynes et al, 1995).…”

Determining Soil Hydraulic Properties

“…While ponded infiltration is used to determine the saturated hydraulic conductivity, tension infiltrometry also provides an opportunity to estimate unsaturated hydraulic properties. TI associated with conservative tracers was also used for inferring parameters describing the water-borne transport of chemicals and other parameters such as mobile/immobile water content fraction or exchange coefficient (Clothier et al, 1992;Jaynes et al, 1995). A number of challenges still remain unresolved for both theory and practice for tension disc infiltrometers.…”