Distribution of environmental chloride in relation to subsurface hydrology

Abstract

Chloride distributions in 32 deeply weathered profiles up to 39 m deep were used to investigate the subsurface hydrology typical of the saline landscapes of the south-west of Western Australia. Studies took place in two forested areas of differing rainfall and landform to compare the processes of water movement. A steady-state model of water and chloride movement which included the effects of diffusion was used to estimate vertical soil water flux density q w from observed concentrations of chloride. These patterns of water flow were compared to the physical properties of the profiles. A great deal of variability was found between profiles and there was a marked difference in the magnitude of q w between the areas of different rainfall. However, consistent patterns of subsurface water movement can be identified. Thin coarse-textured surface soils are well-leached by rainfall but in the clayey unsaturated zone below, vertical soil-water flux densities are very small due to the uptake of water by the eucalyptus forest. Where rainfall was 800 mm yr −1, q w was typically of the order 0.4 mm yr −1 and in the area of 1150 mm yr −1 rainfall, q w was 2–5 mm yr −1. Rates of recharge to groundwater estimated from chloride concentration in the saturated zone were as much as two orders of magnitude greater than q w above the water table. It is concluded that preferred water flow in structural and textural heterogeneities within the regolith is the dominant mechanism of recharge. The preferred flow paths are distributed throughout the landscape but only constitute a small proportion of it.