Interactions between perched and saprolite aquifers on a small, salt‐affected and deeply weathered hillslope

Abstract

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 0.14m day-'. Perched groundwater flow (throughflow) occurred in the higher permeability (-1.15 m day-' ), near-surface soil materials. Throughflow occurred throughout winter, contributing approximately 530 m3 of fresh (-160 mgl-' C1) water to a saline seep. By contrast, the deep aquifer discharged approximately 1100 m3 of waters with salt concentrations of 20006000 mgl-' C1. Recharge and discharge rates to and from the deep aquifer, were estimated to be of the order of 5-20 mm a -' and 5&300 mm a-' 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-'). 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.

Modelling subsurface flow conditions in a salinized catchment in south-western Australia, with a view to improving management practices

“…Speci®cally, the latter were used as follows. Figure 1 in George and Conacher (1993a) shows the limit of regular winter saturation of the A horizon occurring near a break in slope. This information, as well as measured piezometric heads, was used to help ®ne tune the hydraulic conductivities of the B horizon and of those zones aected by the dyke.…”

“…Much of this information was available from George (1991) and George and Conacher (1993a). Modelling using these parameter values could shed light on the consistency of the published values.…”

“…The site chosen for study was the one near Narrogin, West Australia, described in detail by George (1991) and George and Conacher (1993a). The site was intensively monitored from 1984 to 1989.…”

“…It was decided that the central pro®le along the`C' series of piezometers described and studied in detail by George and Conacher (1993a) would be modelled (Figure 1).…”

“…The main purpose of this study was to investigate the nature of the relationship between the unsaturated ow component and the underlying saturated groundwater¯ow component at the site reported on by George and Conacher (1993a) by using a ®nite element modelling approach. In so doing, a comparison is made between the modelled results and some of the information on runo mechanisms previously reported (George and Conacher, 1993b).…”