A combined chemical and thermal tracer test has been attempted, using iced water and sodium chloride, in the compost and limestone Tan-y-Garn Reducing and Alkalinity Producing System (RAPS), designed to treat iron rich net acidic mine water in South Wales, UK. The applied thermal signal was, however, inadequate to produce a temperature anomaly in the output water, being overwhelmed by the summer solar/atmospheric daytime heat input to the lagoon. Nevertheless, natural diurnal temperature signals could be discerned in the outlet water, being retarded relative to the input signal. Thermal retardation could be calculated as approximately 2.3 to 2.9, and various estimates of the RAPS bulk volumetric heat capacity tend to fall between 2 and 3 MJ m(-3) K-1. The natural diurnal thermal signal potentially provides a low cost substitute for artificial tracers in estimating RAPS retention times. (C) 2016 Elsevier B.V. All rights reserved.