Comparative plant growth and metal removal in two adjoining, cognate wetlands (Shilbottle, UK), one receiving acid mine spoil leachates, the other alkaline surface runoff from revegetated spoil

The generation of contaminated waters from abandoned mine sites is a widespread problem and continues to adversely affect many water courses in the UK and throughout the world. The abandoned mine of Shilbottle Colliery Northumberland is an example of acidic spoil heap discharge that contains elevated levels of many metals including Fe, Mn and Al. Aerobic wetlands planted with the common reed, Phragmites australis were constructed at the site to treat surface runoff from the spoil heap. However the presence of a perched water table within the spoil heap resulted in one of the wetlands (lower) receiving acidic metal contaminated water from within the spoil heap. The other wetland (upper) received alkaline, uncontaminated surface runoff from the spoil heap which had been mulched with paper waste This unique situation enabled the comparison of metal uptake and growth of plants used in treatment schemes in two cognate wetlands.Results from 18 months of monitoring indicated a significant difference in plant growth between the two wetlands in terms of shoot height and seed production. The inhibition of growth in the lower wetland was attributed to one or more of three possibilities which are discussed in detail:- i) the toxic effects of high levels of metals, particularly iron in shoot tissues, ii) the inhibition of Ca (an essential nutrient) uptake by the presence of metals and H⁺ ions, and iii) low concentrations of bioavailable nitrogen sources in the lower wetland resulting in nitrogen deficiency in plants.