Factors affecting uranium removal from brackish groundwater using a direct solar powered ultrafiltration-nanofiltration/reverse osmosis membrane system were investigated during a field trial in the Australian outback. The key variables were uranium speciation (as a function of pH), groundwater type as well as energy variation over the course of a day. It was found that uranium was retained by the membranes over the pH range 3-11, but strongly adsorbed to membranes at pH 4-7. The speciation of uranium at pH 4-7 explained the adsorption to the membrane. The presence of other inorganic species, in particular calcium, was a likely cause of uranium co-precipitation at pH 10-11. During solar energy experiments, it was found that the specific energy consumption increased over the course of the day. This indicated fouling through precipitation on the membranes which caused reduced retention of uranium towards the end of the solar day. (C) 2009 Elsevier B.V. All rights reserved.