Characterisation of hydraulic and hydrogeochemical processes in a reducing and alkalinity-producing system (RAPS) treating mine drainage, South Wales, UK

Abstract

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...

A Review of the Performance of Minewater Heating and Cooling Systems

“…MWG systems can facilitate a significant drop in carbon emissions and beneficial economics [14] when compared to hydrocarbon-based heating alternatives [8] and present a sustainable heat supply if managed correctly [13]. [38,39]) temperature is a little higher than annual average air temperature, while the increasing temperatures at Glyncastle (3.6889° W, 51.7119° N) and Morlais (4.0680° W, 51.7004° N) and are believed to be due to increasing depth of minewater derivation. Minewater data provided by Gareth Farr, with thanks, as reported in Farr et al [16].…”

“…Temperatures of minewater in South Wales, presented as daily averages of half-hourly data recorded in surface outflow point, compared with monthly average high/low air temperatures for Neath in the period 1961-1990 (from https://en.wikipedia.org/wiki/Neath (accessed on 18 June 2021), based on Meteorological Office data). Note that the Tany-Garn minewater (3.9849° W 51.7696° N;[38,39]) temperature is a little higher than annual average air temperature, while the increasing temperatures at Glyncastle (3.6889° W, 51.7119° N) and Morlais (4.0680° W, 51.7004° N) and are believed to be due to increasing depth of minewater derivation. Minewater data provided by Gareth Farr, with thanks, as reported in Farr et al[16].…”

The Microbiology of Metal Mine Waste: Bioremediation Applications and Implications for Planetary Health

“…The Tan-y-Garn site in South Wales incorporates a passive water treatment scheme (Figure 5) combining a thick layer of compost and limestone gravel (Reducing Alkalinity Producing System, RAPS) where alkalinity is produced, followed by three settlement ponds and an aerobic wetland (Taylor et al, 2016). The treatment site was built in 2006 to improve the mine water quality that was contaminating a nearby river, treating 7-10 L s −1 (Smail & Thorn, 2016).…”