Negus, T. 1985. Seepage interceptor drains and topsoil salinity. Journal of Agriculture - Western Australia. Vol. 26. No. 4, p. 44-49.

Seepage interceptor drains were trialed at 11 sites in the Narrogin District in 1972 to evaluate the impacts to topsoil salinity. The properties chosen for the study were: G.J. Ward, C.E. Pauley, D.L. Bond and W.B. Woodward.

Investigations during deep drilling in the region confirmed that the source of the salts was groundwater under pressure in the deeply weathered pallid zone, rather than shallow moving groundwater perched on the clay sub-soil at 30 to 60 cm depth.

Soil profiles in the region are typically duplex, consisting of a loaming sand topsoil underlain by mottled clay subsoil.

The following drain specifications for each site are provided in the report:

The topsoil salinity decreased significantly at only one site on the property of G.J. Ward. At six of the sites there was no significant change, while topsoil salinity increased significantly at four of the sites. The drains were however found to be effective in reducing the waterlogging of crops caused by shallow moving perched watertables. This is expected to enable farmers to grow reasonable cereal crops on land with mildly saline topsoil.

Whilst the impact to soil salinity is questionable at these trial sites, the seepage interceptor drains are proven to have reduced waterlogging, which may or may not translate into increased crop/pasture productivity. It is likely that the proficiency of the seepage interceptor drains will be largely site specific and should be evaluated, both in a technical and economic sense prior to considering implementation.

The following are key determining factors for the successful implementation of seepage interceptor drains to control waterlogging 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.