A series of tracer tests has been carried out in the compost and limestone Tan-y-Garn Reducing and Alkalinity Producing System (RAPS), designed to treat iron-rich net acidic mine water (mean pH 6.18, Fe = 47 mg L-1, alkalinity 1.70 meq L-1 and mineral acidity 1.82 meq L-1) in South Wales, UK. Conservative tracer breakthrough time in the RAPS basal effluent is approximately inversely related to throughflow rate. Repeat tracer tests indicate a long term decrease in hydraulic conductivity, but not in total porosity. A specific sodium chloride tracer test from June 2008 is reported, when 15 kg salt was added to a raw mine water inflow rate of 0.87 L s(-1). Electrical conductivity and major ion chemistry were monitored for a 170 h period. Sodium exhibited a retardation of 1.15 to 12 in the RAPS medium relative to chloride, due to cation exchange. Simple 1-D advection-diffusion analytical modelling succeeded in simulating the early portion of tracer breakthrough in the RAPS effluent. More complex analytical modelling, accounting for (i) mixing and dilution effects in the supernatant water input signature and (ii) matrix diffusion effects, was found to be required to adequately simulate the later-stage tail of the breakthrough curve in the RAPS effluent. (C) 2016 Elsevier B.V. All rights reserved.