SPATIAL VARIABILITY OF THE EFFECTIVE RETARDATION FACTOR IN AN UNSATURATED FIELD SOIL

Abstract

A precisely controlled field study was conducted to determine flow and transport of water and bromide through an unsaturated soil. A 50mx50m plot was instrumented with neutron probe access tubes, tensiometers, and solution samplers. Water containing bromide part of the time was applied at a steady flux of 1.85cmd-1 for 24 days. The average degree of water saturation during water application was about 56%. The Hydrus 1-D model was used to optimize the saturated hydraulic conductivity parameter Ks, and the transport parameters D (dispersion coefficient) and R (used here as a 'bulk retardation coefficient'). The van Genuchten flow parameters θr, θs, α, and n were obtained from laboratory measurements on 11 cores taken 1.5m below the soil surface along a transect through the plot. The estimated field Ks value increased with depth due to higher sand and gravel contents at depth. The mean dispersion coefficient for 13 locations at the 3m depth was found to be 5.35cm2d-1, with a coefficient of variation of 52%. This resulted in a relatively small mean dispersivity value of 0.64cm. The average R value was 0.63 with a range of 0.45-1.02 at 3m (CV=28%). The low R value is indicative of anion exclusion, immobile water, or some other phenomenon difficult to identify from field data. These field data indicate that if a transport model with a bulk retardation factor is used for predicting bromide transport through unsaturated soil a range of retardation values may need to be used. For our soil, the highest R value needed to be at least twice its lowest value.