Can treatment wetlands be constructed on drained peatlands for efficient purification of peat extraction runoff?

Abstract

Peat extraction for energy purposes causes major changes in the aquatic and terrestrial environment. According to national strategies for extracting peat in Finland, new peat extraction areas should be established on previously drained peatlands. On such areas it is difficult to find the natural, intact peatland required for treatment wetlands or so-called overland flow areas, which are considered the best available technology for runoff water purification. Therefore treatment wetlands must be constructed on drained peatland. It is known that drainage causes physical, biogeochemical and hydraulic changes in the peat layer, as well as changes in vegetation. It is probable that these changes affect the purification efficiency of wetland treatment systems in many ways. This study evaluated the function and characteristics of drained peatland areas for purification of peat extraction runoff water. Study sites were established on 11 drained wetlands and their purification efficiency was evaluated. Detailed measurements of peat physical properties and hydraulic conductivity, as well as studies on vegetation, were also made in the study areas. The results showed that wetlands constructed on drained peatland areas can purify peat extraction runoff. However, leaching of phosphorus (P) and iron (Fe) was observed in some areas. Leaching is influenced e.g. by pH and the soil P pool. The chemical oxygen demand was also observed to increase in runoff water from the wetland. The results indicated that low (Fe+aluminium (Al)+manganese (Mn))/P ratio (≤25) and quite high P content (>1200mg/kg) in the surface peat characterised those areas where P leaching was observed. The presence of a dense tree stand in a drained peatland area also seemed to indicate release of nutrients from the area after its rewetting and use as a treatment wetland. Thus, potential nutrient release from a drained peatland area intended for use as a treatment wetland can be assessed by studying the characteristics of the peatland area, especially the peat mineral content, and the vegetation, especially tree stand density in the area. Using the findings obtained, a conceptual decision tree was drawn up in order to help to establish and design wetlands in previously drained peatland areas.