Spatial variability of the effective retardation factor in an unsaturated field soil

A precisely controlled field study was conducted to determine flow and transport of water and bromide through an unsaturated soil. A 50 m×50 m 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 1.85 cm d-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 coefficientʹ). The van Genuchten flow parameters thetar, thetas, alpha, and n were obtained from laboratory measurements on 11 cores taken 1.5 m 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 3 m depth was found to be 5.35 cm2 d-1, with a coefficient of variation of 52%. This resulted in a relatively small mean dispersivity value of 0.64 cm. The average R value was 0.63 with a range of 0.45–1.02 at 3 m (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.