Red ochre trapped in reed beds
Reed beds are being used to treat iron-rich minewater from a disused Coal Authority site near Merthyr Tydfil. Jonathan Ralph and Jamie Robinson of Parsons Brinckerhoff report
The Taff Merthyr Colliery was closed in September 1993. During operation minewater had been a continuing problem. As part of the abandonment works the twin 600m deep shafts were filled and capped and the associated colliery buildings demolished. But water continued to fill the shafts and in November 1994 iron-rich minewater began to flow from the two shafts at two main locations via culverts near the top of the main shafts. At the time a small temporary treatment system was installed to treat the water before release into the Bargoed Taff river.
Although some historic data regarding the minewater discharge flows was available from the Environment Agency (EA) for the period between February 1995 and April 1998, V-notch weirs were installed at the source of the discharges from December 1998 and results taken periodically to give flow details.
As the catchment area of the mine is relatively large, flow readings were generally consistent, and not affected by daily rainfall. The maximum flow rate recorded was 121 l/s. Analysis was undertaken to assess the relationship between rainfall and flow. Although minewater increases in the 14 days following heavy rain, there are also patterns over longer time periods. Flow is probably affected by a number of factors.
Drainage within deep abandoned mine workings is commonly oxygen deficient, with reducing conditions often prevalent. As a consequence the iron in the minewater is predominantly in the ferrous (2+) state, although some ferric (3+) iron will also be present. Upon exposure to the surface ferrous iron oxidises to ferric:
Fe2+ +O2 +H+ -> Fe3+ + H2O
(O2 = x0.25, H2O = x0.5)
The pH of minewater can be acidic, mainly due to the presence of sulphate from oxidised sulphides. The rate of iron oxidation is controlled by the pH of the minewater. Below ground and at pH levels of 2-3, the oxidation of the minewater is controlled by bacteria and is relatively slow, the oxidation taking a number of days. At higher pH levels (>pH 5.0) and in the presence of air, the oxidation is not bacterially controlled and can take a matter of minutes.
Reed bed design
Aerobic wetland systems are designed to encourage the oxidation process and are consequently relatively shallow (about 0.3m deep), vegetated and with surface flow predominating. As ferrous iron is converted to ferric iron in the wetland, a hydrolysis reaction occurs which causes the precipitation of ferric hydroxide:
Fe3+ +3H2O -> Fe(OH)3 +3H+ or oxyhydroxide
Fe3+ +2H2O -> FeOOH +3H+
Ferric hydroxide precipitation results in the build up of the characteristic red ochre often observed at mine drainage sites. A consequence of this is a reduction in the pH of the minewater. This can reduce the oxidation rate and cause distress to plants growing in the aerobic cell.
Reeds such as Typha latifolia and Phragmites australis are encouraged to grow as they pass oxygen through their root system causing aeration of the substrate. The discharges from the Taff Merthyr abandoned coal mine are net alkaline - that is the amount of alkalinity present is greater than the acidity. This could be caused by sub-surface sulphate reducing bacteria and/or natural carbonate deposits. Fortunately there was sufficient natural alkalinity to prevent the generation of acidic conditions due to iron precipitation and consequently pH adjustment was not required.
The treatment site, which in total covers around 7ha, now contains over 3ha of wetland treatment area. Part of a national minewater remediation programme by the Coal Authority, it is the largest minewater reedbed treatment project in the UK. Edmund Nuttall was chosen as the main building contractor and is due to complete the project by summer 2001.
There are now four settlement lagoons, 16 reed beds (containing both Typha latifolia and Phragmites australis), and a pumping station compound. There are five streams of tiered reedbeds each preceded by a settlement lagoon. Minewater is collected and directed into a pumping station containing four pumps. From here it is pumped through a rising main, via valve chamber, through a head of approximately 8m. At the outlet the flow is split allowing the flows to be evenly distributed between the settlement lagoons. The large lagoons retain the minewater for 24hrs allowing a large amount of the iron ochre to settle. The reed beds then act as a polishing system to further reduce the iron content.
Building work was hampered by the previous use of the site. The colliery had large foundations which were left underground on the site. Many of these had to be removed to allow construction of the reed beds at the required levels.
Flow in and out of the reed beds is controlled by a concrete weir which acts to evenly collect the water from the previous reed bed/settlement lagoon and distribute the minewater into the next bed. This prevents short-circuiting of the system and maximises the settlement, retention time and efficiency of the reeds. The concrete weir incorporates a cascade and this also oxygenates the minewater, prompting the iron to fall out of solution.
The coal shale abundant on the site was used to construct the majority of the earthworks, however to ensure no loss of minewater occurs through the ground, the beds have been lined with bentonite matting. The matting has been covered with an imported substrate to give the reeds a suitable growing medium. At the maximum expected deposition rates, it is thought that each reed bed will require cleaning and replanting of reeds every 25 years with the settlement lagoons requiring some cleaning every five years. The reed bed's life is extended using a castellated weir system which allows the gradual raising of the water levels within the beds, ensuring the reeds have sufficient water above the build-up of iron ochre.
The minewater discharged into the river now has a considerably reduced iron content. Recent water analysis has shown a 97% drop in iron entering the river from around 20 mg/l total Fe to 0.6mg/l total Fe. This should decrease further with the growth of the reeds next year.