Environmental impact of peat mining : Development of storm water treatment methods

Abstract: Drainage of peatlands increases the transport of suspended solids (SS) and the leaching of nutrients to downstream receiving waters. Increased settling of peat and eutrophication of lakes has been noted in many studies downstream from peat mines. Treatment alternatives have therefore been developed since the early 1980s to reduce the environmental impacts of peat mining. New, efficient methods such as artificial wetlands, chemical treatment and infiltration techniques have been developed, although, in most cases, they have been either too expensive to be used at all peat mines or they have failed to function during high hydraulic loads when the majority of the SS and nutrients are borne by drainage water. The aim of this series of studies has been to develop methods that function when the pollution load is high and that are economically viable for all peat mines. Sediment transport and nutrient leaching were studied with the purpose of establishing more efficient treatment alternatives. A controlled experiment was set up to measure the erosion of peat from the soil surface and from ditch beds during heavy rainfall and runoff events and to measure the settling characteristics of base soil peat and peat deposited in channels. The study demonstrates the importance of channel bed erosion as the main source of sediments during peak runoff. Sediment transport and nutrient leaching were further observed in the field during 1995 and 1996. The study showed that SS is mainly generated during extreme events, such as flooding, when external water from the surrounding uplands flows into the mine and snowmelt. These high flow events erode the material deposited on the channel bed during low flows. The leaching of nitrogen occurs after large rain events when newly infiltrated water, rich in nitrate, reaches the drainage area outlet. High phosphorus concentrations occur when the water-table is low and the groundwater fed baseflow is older and rich in humus. Treatment alternatives were developed to improve removal of SS and nutrients. Different types of ponds (rectangular, compartmental and circular) were tested in a laboratory down-scaled study. The study showed that the main factor affecting the settling of small peat particles is the depth of the settling basin rather than the type of basin. Therefore, artificial floodplains were suggested as a preferred treatment alternative. A mathematical model study showed that in the case of bare soil erosion, the best treatment alternative would be to store the water in the large drainage network rather than in the sedimentation basins which have a substantially smaller capacity. Different structures suitable for peak runoff control were tested under laboratory and in field conditions. The most important results from the thesis are that the structures developed function well and reduce peak flows from peat mines and, hence, reduce ditch bed erosion and SS load.