George, R. and Frantom, P. 1991. Drainage of sandplain seeps for salinity control and stock water supplies. W.A. Journal of Agriculture, Vol. 32, p.88-93.

This report documents the results of several drainage experiments conducted between 1986 and 1989 to determine effective methods of reclaiming saline seeps. The basic approach is to intercept the throughflow of groundwater upstream of a saline seep allowing both the lowering of the water table upstream and at the seep. The intercepted water is used as a sourceof stick water. The trial sites were located in Western Australia at Doodlakine (buried interceptor drains), Bencubbin (open interceptor drains) and Holleton (tube drains).

The sites are located in shallow groundwater systems within the eastern Wheatbelt of WA. The geological profile comprises mainly permeable sandplain soils overlying clayrich hardpan deposits. Groundwater flow is through the more permeable sand aquifer downslope to discharge sites.

Sandplain seeps are derived from a shallow groundwater system which flows from the deep sandplain soils upslope. The seeps result in small areas of salinity and waterlogging, which can be the focus of soil erosion. Sandplain seeps may represent as much as 10 % of the Western Australia's salt problem in the drier agricultural area.
Sandplain soils are more permeable than the underlying clay rich hardpan upon which the aquifer develops. Under these conditions, it is expected that adequate drainage systems could be constructed to reclaim these seeps and provide a valuable supplementary water supply for stock usage.

At Doodlakine and Bencubbin the backhoe built drains were installed on a grade. The surveyed drains were excavated to between 1 and 3 metres in depth onto the hardpan and the water conveyed to existing dams. The drains were located as close as possible to the seep to reduce the depth of excavation required to intercept the hardpan and the water prior to the salts being concentrated by evaporation.
At Doodlakine, the drain was initially lined with 3 to 5 mm diameter coarse stone (bluemetal). Thereafter, one hundred metres of 65 mm diameter slotted pipe was laid and covered with additional bluemetal to a depth of 0.5 metres. The trench was backfilled with soil from the drain and the area was sown to wheat. At Bencubbin, the interceptor drain was left open and have remained open. No bluemetal or slotted piping was used.

Tube drains were constructed at Holleton, using a pipe-laying ripper, attached to a large bulldozer. Sixty-five millimetre diameter slotted and unslotted pipe was laid to a maximum depth of 2 metres below the soil surface and gradually brought to the soil surface at the lowest end of the seep. The drains had a maximum depth of 2 m upslope. The solid pipe was situated in the sandplain seep to prevent the entry of saline water, while the slotted pipe collected fresher groundwater (500 mS/m) immediately upslope, adjacent to the seep. Pipes were located at approximately right angles to the contour. A laser level was utilised to ensure a constant gradient on the pipe to permit gravity flow. The slope of the hardpan governed the gradient of the pipe within the salt affected area. Four drainage lines, each 125 metres in length (50m solid and 75m slotted) were installed, with drain spacing ranging from zero at the outlets to approximately 30 to 50 metres above the seep.

At the trial sites of Doodlakine and Bencubbin, flow began immediately after the interceptor drains were installed in May 1986 and ranged between 0 to 30 kL per day, over the following seven months. The flow had ceased by December 1986, by which time the drains had removed 2,900 kL at Doodlakine and 1,900 kL at Bencubbin. The lower flow rates observed at Bencubbin is believed to be due to its poorer grade control on the hardpan, the low maintenance of the open drains and the inability of the backhoe to cope with saturated sandplain soils deeper than 2.5 metres.

At Doodlakine, water levels in adjacent bores were recorded to decline by up to 1 to 2 metres until dry. Water levels were not monitored at Bencubbin. The water salinities were recorded at less than 100 µS/cm at Doodlakine and less than 1,000 µS/cm at Bencubbin. The water drained from the sandplain groundwater was stored in dams for use by stock the following summer. This practice was successful at Doodlakine due to the low groundwater salinity, however at Bencubbin the more saline waters were concentrated by evaporation to nearly 5,000 mS/m by March each year, making the water unsuitable for stock usage.

At Doodlakine, the area below the drain was cropped to wheat in 1987, the first winter after the drains were installed, with a recorded yield of 0.8 t/ha. A 1988 lupin and 1989 wheat crop both produced 1.8 t/ha. The Doodlakine site is now considered reclaimed of the saline seep.
In comparison, reclamation at the Bencubbin site has been poorer. A three hectare area upslope from the drain, which previously produced poor crops, yielded 0.8 t/ha of wheat in 1987 and in 1988 grass became established on some of the previously bare sandplain seep downslope of the drain.

Based on the monitored results from the trial sites at Doodlakine and Bencubbin, it is concluded that interceptor drains can reclaim saline seeps in one or more years and the water can be used for stock purposes. Lined drains using bluemetal and slotted pipe were found to be more effective than open drains due to the potential for open drains to collapse. It is recommended that brackish waters with a conductivity exceeding 400 mS/m should not be left exposed in open storage's unless used by livestock quickly, or covered to prevent evaporation.

At Holleton, the tube drains discharged water at a flow rate of 5 to 30 kL per day, depending on rainfall. The conductivity of the drain water improved from 900 µS/cm to 500 µS/cm over the two to three month monitoring period. Although longer term monitoring is required, it is estimated that the seep has the potential to supply approximately 1,000 kL per year of stock quality water as a 'one-off' supply or as a perennial supply of 5 to 10 kL per day. The monitoring indicates that the drains could supply 1,000 to 2,000 sheep over the summer without jeopardising water resources.

To effectively lower watertables, and reclaim the seep, tube drains must flow throughout the year. However water with salinity greater than 400 mS/m should not be stored in dams due to the concentration of salinity by evaporation. Finally, tube drains are recommended for use in regions where sandplain groundwaters are more saline than 400 mS/m and where additional stock water supplies are required. The drains may also be adequate to lower the saline watertable, resulting in reduced soil salinities.

This study has demonstrated that interceptor drains (buried and open) and tube drains are capable of reducing waterlogging, sourced from saline seeps, to varying degrees within the sites trialed. As a result of the reduced waterlogging at the trial sites at Doodlakine and Bencubbin, an increase in crop yield has been recorded, giving confirmation of the technical success of the interceptor drains and capability to restore land to productivity.

At the time the article was written, further monitoring was required to determine the effectiveness of the tube drains installed at Holleton.
Although the interceptor drains are proven to be technically feasible, their economic viability requires evaluation prior to considering implementation.
Economic evaluation

The following are key determining factors for the successful implementation of the interceptor and tube drains to control saline seeps 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.