Constructed wetlands for the treatment of British mine drainage waters : a biogeochemical approach
The problem of (acid) mine drainage (AMD) in Britain is widespread and inherent to the mining industry. Ferruginous discharges from abandoned mines contaminate the waterways, through their high metal concentrations and ferric deposits. During this research, three main mine drainage sites have been studied: Ynysarwed, South Wales; Wheal Jane, South England; Mona, North Wales. These sites have similar ferrous iron concentrations (#250-300mg 1-1), although differing pHs - -6.3, -3.4 and -2.9 respectively. Shake flasks were set up to investigate the factors affecting the rate of iron oxidation (and hence precipitation) as an iron removal mechanism in Ynysarwed and Wheal Jane waters. It was found that the rate limiting factor in both these waters was primarily biological and their biomass was limited by the availability of inorganic nutrients, in particular phosphate. The concentration of phosphate shown experimentally to be most beneficial, was in the order of < 0.01M. Experimental microcosms were used to investigate the effect of soil type and planting on the remediation of Ynysarwed and Mona waters. The results showed, that of the soils tested, mushroom compost was the most beneficial soil in terms of metal removal and alkalinity generation, with an optimal water residence time of four days. Planting was shown to have a detrimental effect on metal removal. However, in terms of sulphate reduction and pH maintenance, plants were beneficial, particularly in peat soil. These soils were shown to have the same remediative effect on several British mine waters, irrespective of their origin and a pilot system was proposed. The mechanisms occurring within planted and unplanted soils were investigated. It was found that competition may be occurring between the plants and microflora for resources such as inorganic nutrients, thus impairing microbial iron removal. Furthermore, the iron formed soluble complexes with the plant derived dissolved organic carbon, preventing removal through precipitation processes.