2 nd International Salinity Forum Salinity, water and society–global issues, local action 1 Geochemical implications of salinity mitigation drainage engineering options—a global overview SL Rogers 1 , BP Degens 2 , RW Fitzpatrick 1,3 , GB Douglas 1,4 , RJ George 5 , P Shand 1,3 , A Lillicrap 6 , D Gray 1,7 1 Co-operative Research Centre for Landscape and Environments, and Mineral Exploration, Bentley, WA 2 Department of Water WA, Perth, WA 3 CSIRO Land and Water, Glen Osmond, SA 4 CSIRO Land & Water 5 WA Department of Agriculture and Food, Bunbury, WA 6 WA Department of Agriculture and Food, Albany,WA 7 CSIRO Exploration and Mining, Bentley WA Introduction Management of dryland salinity, and the disposal of excess irrigation waters through the adoption of engineered drainage solutions to control groundwater levels, can be an effective management tool in landscapes where the hydraulic properties of the regolith are favourable. The planning and assessment of drainage investment involves hydrological (drain flows, level of water table drawdown, extent of hydrological effectiveness) and engineering (depth, bunding, soil stability, disposal options etc) design considerations, aimed at maximising recovery of productive land or asset protection, while minimising impacts on receiving environments. However, there are a number of examples from around the world where the installation of drains to ameliorate salinity and waterlogging, has had unintended geochemical impacts on drain discharge and regolith materials. The key issue linking these examples is the potential for mineral dissolution and transport of trace elements (including heavy metals and metalloids) as a consequence of changing redox conditions within and adjacent to drains and disposal basins. A review of the literature demonstrates that drains continue to be constructed to control salinity that fail to take into account local and regional bedrock, regolith and groundwater geochemistry. Geochemical risk assessments, and prediction of potential changes in regolith and groundwater chemistry, as a result of drain construction, for instance changing red-ox conditions in groundwater exposed to the atmosphere, are not routinely undertaken. In fact the extent of the potential geochemical risks of drainage is unknown, as in general they only become evident after drain construction. Mobilisation and deposition of ‘naturally occurring’ trace elements in groundwater Drains can be considered low temperature geochemical laboratories, in which elements interact with oxygen, organic matter and sediments. Subsurface irrigation drainage waters can also comprise a complex effluent chemical profile that varies both spatially and temporally within landscapes. One of the first reported examples of unpredicted geochemical consequences of salinity mitigation drainage, was the discovery of elevated selenium (>1000µg L-1) in alkaline, oxidised, sub-surface drainage waters in the Kesterson irrigation disposal basin, San Joaquin Valley California in the early 1980’s. Selenium was derived from Cretaceous marine sedimentary rocks, in the California Coast Ranges. Selenium, as selenate, was ultimately found weathered with sulfur from marine sources in soluble sodium and magnesium sulfate salts, which were concentrated by evaporation on farmland soils, and deposited in irrigation disposal basins. The elevated concentrations of selenate had a significant impact of wildlife and waterfowl in the Kesterson basin (Presser, 1994). Subsequent studies by the US Geological Survey, demonstrated that the situation was not unique. Other regions in the western US have similar geological and landscape settings, leading to selenium mobilization where sub-surface drainage waters collected and discharged to wetlands (Ohlendorf, 1999). In Western Australia, more than 3000Km of salinity mitigation drains have already been constructed, many of which intercept ‘naturally’ acidic (pH<4.5), hyper-saline trace element rich ground waters (Gray 2001). Acid groundwater, that dissolves clays and minerals, has