INTERACTIONS BETWEEN PERCHED AND SAPROLITE AQUIFERS ON A SMALL, SALT-AFFECTED AND DEEPLY WEATHERED HILLSLOPE

A small hillslope was chosen to investigate the role of throughflow as a mechanism responsible for the movement of soil water and solutes towards a saline seep and as a source of recharge to a permanent, regional aquifer at depth. The hydraulic properties, chemical characteristics and physical responses of both systems were studied on a deeply weathered, salt-affected hillslope. Additional data were also obtained from other sites in south-western Australia. Regional groundwater flow occurred in a variably textured, deeply weathered material in which the hydraulic conductivity varied from < 0.001 to 0-14 m day-1. Perched groundwater flow (throughflow) occurred in the higher permeability (approximately 1.15 m day-1), near-surface soil materials. Throughflow occurred throughout winter, contributing approximately 530 m3 of fresh (approximately 160 mg l-1 Cl) water to a saline seep. By contrast, the deep aquifer discharged approximately 1100 m3 of waters with salt concentrations of 2000-6000 mg l-1 Cl. Recharge and discharge rates to and from the deep aquifer, were estimated to be of the order of 5 20 mm a-1 and 50-300 mm a-1 respectively. Saturated conditions existed throughout winter within the seep and the immediately adjacent non-saline area, with up to 60 per cent of the hillslope soils becoming saturated after major rainfall events ( > 20 mm day-1). In the mid-slopes, in particular along a central depression, saturation of the shallow soils caused macropore channel recharge to take place through the clay-textured subsoils. Water-level responses suggest that approximately 25-30 per cent of annual recharge occurred from one storm studied in September 1984. Recharge through macropore channels is a significant mechanism in the concave slope areas on the hillslope. Throughflow was found to be a major source of water, but not salt, contributing to the saline seep. In general, the contribution of throughflow was found to decrease further inland at other sites studied. However, at inland sites where perennial, perched aquifers have developed in deep sands, saline areas have been caused by throughflow and not by deep aquifer discharge.