Drainage discharge impacts on hydrology and water quality of receiving streams in the wheatbelt of Western Australia

The use of surface and subsurface drainage to manage waterlogging and salinity in dryland (rainfed) and irrigated agricultural systems is common throughout the world. The drainage systems often discharge into natural streams. The same is true for the wheatbelt drainage systems in south-western Australia, where 11,000 km (ABS 2003) of artificial drains have been constructed within the last two decades. Prior to this study, the likely impacts of this discharge on the streambed chemistry and water quality of receiving streams were largely unknown. The study evaluated these impacts in creeks receiving the drainage discharge from engineering options in four river systems in south-western Australia. This study clearly showed elevated levels of metals ions, EC and pH in the stream water at treated sites relative to their levels at untreated sites. At most sites, impacts of drainage discharge were observed on the streambed electrical conductivity (EC) and pH (both in 1:5 extract) in the receiving streams; however, there was little evidence of impact on metal ion content in the streambed soil. The study found no clear differences in the dynamics of the watertable adjacent to streams whether they received drainage discharge or not, irrespective of the size of the artificial drainage systems.