George, P.R. and Nulsen, R.A. 1985. Saltland drainage: case studies. Western Australia Journal of Agriculture. Vol. 4. p115-118.

This study focuses on the experimental work undertaken by the Department of Agriculture on two sub-surface drainage engineering options; tube drainage and groundwater pumping, at five trial sites in Western Australia (Esperance, Dalwallinu, Namban, Dalwallinu and Frankland).

Land management issues

The two sites trialed at Esperance Downs Research Station typically comprised sand underlain by gravelly clay sandplain soils. Hillside seepage is present at Site 1 which contains granite bedrock two to three metres below the surface. Site 2 is a broad flat depression with bedrock situated approximately 10 metres below surface.
The trial sites at Dalwallinu comprise typically salt affected valley soil, consisting of red-brown sandy loam underlain by poorly permeable clay sub-soils.

The trial site at Namban is 7 ha in area and contains surface loamy sand soil underlain by a sandy sub-soil. An impermeable, hard clay layer approximately 10 cm in thickness is present on the top of the sub-soil and acts to reduce rainfall infiltration. During the summer season, the saline watertable is approximately 0.8 metres below the soil surface.
At 'Westfield', Franklin, dam water becoming more saline through the input of salts derived from an upslope seep. Preliminary investigations have revealed that the cause of the seep - a narrowing of the aquifer above a dolerite dyke.

The following specifications are known for the engineering options applied at each of the trial sites:

• Esperance Downs Research Station ? Site 1: 65 mm tube drains were installed in April 1981 to a depth of 1.5 to 1.7 metres, with a spacing of 40 metres and a grade of 0.1 to 0.5 %.
• Esperance Downs Research Station ? Site 2: 65 and 100 mm tube drains were installed in April 1984 to a depth of 1.2 to 1.3 metres, with a spacing of 75 metres and a grade of 0.1 to 0.3%.
• Dalwallinu ? Site 1: 65 mm tube drains were installed in April 1983, by the trenchless method, with spacings of 15, 30, 60 and 120 metres. The tube drains discharge into a 2.5 metre deep open collector drain and drainage water is then lifted to shallow surface drain by a windmill equipped with a 150 mm diameter pump.

• Dalwallinu ? Site 2: a groundwater pumping bore was installed 35 metres to bedrock, lined with slotted 100 mm diameter PVC, gravel packed and developed. Preliminary tests indicated the bore could yield 300 m3/d. The bore was equipped with a stainless steel submersible pump and long term continuous pumping started at a rate of 260 to 280 m3/day.
• Namban: 65 mm tube drains were installed in March 1984, by the trenchless method, with spacings of 25 and 50 metres. The tube drains were installed at 1.7 to 2.0 metres in depth into the sandy sub-soil horizon at a grade of 0.15 %. The drains discharge into an open collector drain which has capabilities to receive excess surface water through piped inlets.

• Frankin: a groundwater pumping bore was drilled 18 metres to the bedrock on the upper edge of the saline seep, screened over the depth interval of the aquifer and fitted with a jet pump. The initial pumping rate of 90m3/day declined to 50 m3/day after 25 days and the effluent water was disposed downstream of the dam.

The outcomes of the engineering options applied at each trial site was reported as follows:

• Esperance Downs Research Station ? Site 1: the total drainage water discharged between 1981 and 1984 varied between 1,300 m3/y (1982/83) to 3300 m3/y (1984). The total salt removed from between 1981 and 1984 varied from 21 t/y (1982/83) to 45 t/y (1984), within a 6 ha area.
• Esperance Downs Research Station ? Site 2: the total drainage water discharged in 1984 was 9,500 m3 and contained 18,000 mg/L TDS (equivalent to 170 tonnes of salt removed from 18 ha).

• Dalwallinu ? Site 1: discharge volumes from the tube drains were relatively very low due to the low permeability of the soils. The drains appeared to only flow following rainfall events, indicating that little upward seepage occurred at the site. Barley sown across the site in 1983 and 1984 yielded poorly, however slow reclamation is evident at the trial site by the marginal reduction in soil salinity recorded over the 15 and 30 metre drain spacing.
• Dalwallinu ? Site 2: after 70 days of groundwater pumping, projections indicated that water levels were influenced 800 metres from the bore, with significant effects detected to 200 to 250 metres.

• Namban: the 50 metre spacing drains were recorded to control the watertable effectively, with flow recorded at 430 m3/ha and a salt removal of 5.5 t/ha. Water levels were recorded to decline by 0.5 metres within 20 days of being installed.
• Franklin: water levels reduced by up to 250 metres from the pumping bore. The upward flow of water at the seep was observed to cease and dried out. Furthermore, in 1985, the water quality in the dam had improved to appropriate quality for irrigation use. Trees were planted in the recharge area of the seep are expected to maintain control of the seep, and as a result groundwater pumping may no longer be required.

The engineering options applied at these trial sites in Western Australia demonstrate that the success of each option is largely site specific. Both tube drains and groundwater pumping has shown to decrease water levels and in some cases increased pasture productivity and control saline seeps, however their economic viability requires evaluation prior to considering implementation.

The following are key determining factors for the successful implementation of tube drains to water levels and soil salinity in dryland areas:

• drain spacing adequate to maintain the water table at a particular depth below the ground surface;
• a level of economic return from the land subject to the drains;
• a level of economic value of the re-used water;
• ability to re-use the drained water; and
• a suitable drainage disposal or storage strategy.

The following are key determining factors for the successful implementation of groundwater pumps in dryland areas:

• adequate transmissivity and groundwater pumping system to maintain a particular drawdown of the watertable over a particular area;
• a suitable disposal strategy;
• a level of economic return from the land subject to the influence of the groundwater pump; and / or
• a perceived or actualbenefit from the protection of areas other than dryland.