Doherty,J. and Stallman, A. 1992. Land Management Options for a Salt- Affected Catchment on the Darling Downs. Dept of Primary Industries Brisbane, Project Report QO92010

The area is located in a salt affected 60ha catchment near Brymaroo on the Eastern Darling Downs in Queensland. There area comprises a self contained catchment with slopes to the north towards Cain Creek. Average Annual rainfall is 670mm, mostly falling between November and March. Evaporation (class A pan) averages 1900mm/yr.

The dominant land use within the catchment is grain cropping, however, there are perennial pastures throughout the alluvial flats and the uplands.
There are two areas of waterlogging and/or severe salting which have been removed from agricultural production. The wetter zones are overgrown with grass and weeds, while further downstream from the wet areas there is a salt crust at the surface.

The catchment geology comprises basement rocks of the Walloon Coal Measures overlain by basalts of the Main Range Volcanics, mainly on the hilly country to the south and west, and by alluvial deposits in the low-lying areas to the north. The Coal Measure sediments are only known susbsurface from drilling data. There is a thin lithosol developed on the basalt in the upland areas, but in lower topographic areas of basalt there are dark grey to black self mulching clays. On the alluvials there are red-brown loams.
The groundwater flow direction is generally northward from the upland recharge areas to the low-lying areas, although there are local variations. Seeps occur in the upland areas. In the downgradient lower areas groundwater levels are above ground in the wet season.

This case study has looked at possible options for management of the dryland salinity in the low-lying areas. A range of options was proposed and evaluated using a MODFLOW groundwater model.
The modelling indicated that the planting of trees alone as a salinity mitigation mechanism was not likely to be viable because of the large extent of plantings required and associated high cost. In addition, there may be a requirement for dewatering prior to planting. Long term dewatering using bores (single well or multiple bores using windmills) appeared to be more cost effective than recharge reduction, and there is the potential re-use of water for irrigation elsewhere, provided suitable water quality can be maintained. Drainage in the discharge area was not considered likely to be feasible because of the low soil hydraulic conductivity. However the use of a drain to intercept water upstream of the discharge area suggested that some drawdown could perhaps occur.

This case study provides a sensible approach to evaluation of an intermediate to local scale groundwater discharge problem. It has outlined the need for an understanding of the groundwater systemto be able to evaluate technically succesful and economically sustainable solutions. It has also indicated that site specific conditions can vary and that implementation of engineering works need to be strategically located.