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A series of tracer tests has been carried out in the compost and limestoneTan-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 . Electrical conductivity and major ion chemistry were monitored for a 170 hour period.Sodium exhibited a retardation of 1.15 to 1.2 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. 2 KeywordsTracer test, alkalinity, mine water, artificial wetland, matrix diffusion, ion exchange 3 Introduction What is a Reducing and Alkalinity-Producing System (RAPS)?It is common and cost-effective to treat iron-rich mine waters (Banks and Banks, 2001; PIRAMID Consortium, 2003;Sapsford and Watson, 2011) by passive, aerobic methods, such as aerobic sedimentation basins or wetlands.These function by the oxidation of ferrous to ferric iron, followed by the hydrolysis and precipitation of ferric iron from the water. This net process can be described by equation (1) The overall oxidation and hydrolysis reaction is thus acid-producing (2 moles of protons released for every mole of ferrous iron oxidised and precipitated).However, the hydrolysis and precipitation reaction is strongly pH-dependent: if the pH becomes too low, the hydroxide precipitate will not form. Thus, it is important that sufficient alkalinity is available in the water to absorb the protons produced and to buffer the pH at a reasonably high level (Hedin et al., 1994a;Younger et al., 2002). Alkalinity can be added actively, by dosing the influent mine water with hydroxide or carbonate. However, it can also be released passively by allowing the mine water to flow through limestone drains before entering the aerobic treatment lagoons or wetlands (PIRAMID Consortium, 2003). The limestone slowly dissolves, releasing bicarbonate alkalinity to the water. If the water contains oxygen, however, ferric hydroxide may precipitate out in the limestone drains, "armouring" the limestone clasts and inhibiting further dissolution (Hedin et al., 1994b;Cravotta and Trahan, 1999;Watzlaf et al., 2000a;Bernier et al., 2001). Thus, it is important to strip oxygen from the water before it enters the limestone drain, ensuring that iron remains in its ferrous form (Equation 2). This is most effectively done by a layer with a high organic content and thus a high oxygen demand -for example, a compost layer (represented by CH 2 O in equation (2)). On exiting a RAPS, the mine water will typically:(i) pass through a re-aeration system, such as an aeration cascade, before entering(ii) a system of aerobic treatment lagoons and/or wetlands, where the iron conte...

I. (2016) Heat as a natural, low-cost tracer in mine water systems: the attenuation and retardation of thermal signals in a reducing and alkalinity producing treatment system (RAPS).International Journal of Coal Geology, 164, pp. 48-57. (doi:10.1016/j.coal.2016 This is the author's final accepted version.There may be differences between this version and the published version. You are advised to consult the publisher's version if you wish to cite from it.http://eprints.gla.ac.uk/117907/ AbstractA combined chemical and thermal tracer test has been attempted, using iced water and sodium chloride, in the compost and limestone Tan-y-GarnReducing 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. KeywordsTracer test, mine water, RAPS, thermal retardation, heat transport 2 Introduction IntroductionTan-y-Garn was a small drift colliery at Garnswllt, Ammanford, by Strahan et al. (1907). The local dip of strata is between 10° and 17° S to SW according to BGS (1977). water has typically has a relatively constant temperature of approx. 11 °C.After intense rainfall episodes, however, sharp deviations ('spikes') in both temperature and electrical conductivity can be observed in the mine water.These are believed by the UK Coal Authority (who manage the scheme) to represent occasional entry of surface run-off, during rainfall events, to the otherwise groundwater-dominated mine water discharge. The system is described in full in the accompanying paper by Taylor et al. (in press).The iron-rich (pH 6.2, 40-50 mg L -1 iron) water flowing from the mine is treated before it is allowed to discharge to the Afon Cathan. This is achieved by means of a Reducing and Alkalinity-Producing system (RAPS), which passively adds bicarbonate alkalinity to the water via dissolution of limestone clasts in a reducing organic matrix. It is intended that the reducing environment should maintain iron in its dissolved ferrous (Fe II ) state and prevent armouring of the clasts by ferric oxyhydroxides (in reality, in the Tany-Garn system, iron and sulphate concentrations decrease through the RAPS, suggesting removal of iron as an oxyhydroxide and/or sulphide precipitateTaylor et al., in press). On exiting the RAPS, the water flows down a sequence of aeration cascades and settlement ponds, where ferrous iron is allowed to oxidise, hydrolyse and precipitate as ferric oxyh...