Understanding the hydraulic behaviour of subsurface flow bioremediation systems is a pre-requisite for characterising their biogeochemical functioning, yet it is often overlooked. Using multiple tracers in parallel, considerable hydraulic insight can be gained. A quantitative multi-tracer test was conducted at a passive coal mine water bioremediation system in the UK (Bowden Close Colliery, County Durham) to assess its hydraulic behaviour. Four tracers were used: bromide (Br-), uranine (Na-fluorescein), lithium (Li+) and NaCl. The system comprises two parallel treatment streams: one receiving 30-50 L min(-1) of moderately acidic adit drainage and the other one 90-110 L min(-1) of strongly acidic spoil leachate. Each of these treatment streams has a separate 'RAPS unit' (Reducing and Alkalinity Producing System) and their effluents are eventually mixed in a single aerobic wetland. The RAPS units are downward-flow porous media with mixed substrates of limestone gravels and compost; RAPS I has a surface area of 1511 m(2) and RAPS II 1124 m(2). The aerobic wetland (990 m2) is a basin of mineral soil planted with Typha latifolia and a shallow (15-50 cm) water level. For the two RAPS units, residence times of 4-5 d and effective velocities of 0.7-0.9 m h(-1) were deduced. In terms of tracer performance, in contrast to earlier findings, bromide and Na-fluorescein tracers were applied successfully, while NaCl and lithium were found to be least useful, particularly during dilution events caused by intense rainfall. (C) 2016 Elsevier B.V. All rights reserved.