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Title: Constructed Wetlands for Minewater Treatment
Author: Wiseman I
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_1448, Representation ID: 498, Object ID: 2602
Background The Pelenna wetlands were constructed between 1995 and 1999 with an aim of treating the most contaminated minewater discharges affecting the Pelenna valley. The design of the schemes aimed to lower the dissolved iron concentration in the river to below the Environmental Quality Standard (EQS) of 1 mg/l. Prior to the system being constructed information had been gathered on the water quality and ecological status of the catchment. The schemes utilise passive constructed wetland systems. These treat iron by creating suitable conditions for oxididation and reduction processes to occur. The first phase of the Pelenna wetlands was one of the first passive treatment schemes to be constructed for minewater treatment in Europe. Subsequent phases were constructed utilizing a number of novel design features. Objectives The main objective of this R and D project was to assess the contaminant removal performance, sustainability and environmental benefits of the treatment systems. This would provide information on how well the variety of passive wetland treatment systems work. The likely lifespan of the systems and their controlling factors would be identified. Finally the improvements in water quality of the rivers and macroinvertebrate, fish and riverine bird populations would be quantified. Results/Discussion Dissolved iron was the major contaminant of concern in the Pelenna minewaters. The wetland treatment systems were removing between 82 to 96 % of the incoming iron loading. Once all the phases were constructed the water quality at all sample points in the receiving watercourses quickly improved with dissolved iron dropping below the EQS limit. Following on from this, improvements in the abundance of macroinvertebrates were seen below the minewaters in the spring and summer after minewater treatment. Diversification of the macroinvertebrate species followed another year or two later. The delay in diversification was due to the lack of downstream drift from upstream populations that were impoverished due to episodic background acidification events. The trout populations recovered the year after minewater treatment, as the attempted spawning was more successful than in previous years. Improvements in the numbers of riverine birds were seen following treatment. Increased numbers of dippers and evidence of successful breeding were especially good indicators of the improved water quality. During early 2000 and 2002 the Whitworth A treatment system blocked and the minewater discharged effectively untreated into the Nant Gwenffrwd. On both these occasions the water quality exceeded the EQS for at least three months. The macroinvertebrate populations were significantly affected, dropping back to pre-treatment levels in some sample points. The trout populations were similarly reduced with the main effect being seen in the numbers of trout fry. The blockage of the system was partly due to the design of the inlet structure and pipeworks, and this was exacerbated by a lack of a regular maintenance regime. Frequent R and D Technical Report P2-181/TR ix clearing of the inlets and pipework as part of a maintenance programme could have avoided the damaging overflows. Studies of the internal processes occurring within the systems have identified that oxidation of the iron is the dominant removal process in all systems. Many of the treatment system components were not working exactly as they had originally been designed. They were however still removing iron and working effectively when receiving contaminated minewater. No trends were identified in the removal performance characteristics of the treatment systems and this indicates that the current excellent removal rates should continue. The likely lifespans of the systems were identified where possible. The scheme that was identified as being most likely to suffer from a reduction in contaminant removal performance is Whitworth A. It is suggested that the ochre accretion on the Reducing and Alkalinity Producing System (RAPS) cell will overwhelm the cell between 2010 and 2016. Conclusions/Recommendations The R and D project has demonstrated that passive constructed wetland treatment systems can be an effective and low cost means of minewater treatment for both net alkaline and net acidic discharges. When maintained properly the schemes can reduce the dissolved iron concentrations below the EQS limit. When this has been achieved substantial recovery of the aquatic ecology has been demonstrated. The first signs of recovery are seen the first year after minewater treatment with diversification occurring one to two years later. Improvements are achieved right through the food chain making this method of minewater treatment an effective way of restoring biodiversity. However if minewater is allowed to discharge untreated again then the rivers will quickly return to their pre-treatment conditions. The main recommendation from the project is that the inclusion of a routine maintenance programme is an essential part of any future treatment schemes. Constructed wetlands are a low maintenance alternative for minewater treatment; they are not however maintenance free schemes.
Publisher: Environment Agency
Subject Keywords: Minewater; Wetlandsss; Sustainability; Environmental benefits; Pelenna; Treatment performance; Ecological recovery; Macroinvertebrate diversity.
Geographic Keywords: Wales
Extent: 137
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