Grundvattensänkning till följd av järnvägstunnel genom Hallandsås : miljökonsekvenser relaterade till förändrad kväveomsättning i mark

Abstract: This thesis was carried out to give some background information to a report of SLU (Swedish University of Agricultural Sciences) for the Swedish National Rail Administration (Banverket). The requested information was to provide basic data for an environmental impact assessment of the construction of the tunnel through Hallandsås. The aim with the thesis work was to discuss and quantify the leaching of nitrate and the emission of nitrous oxide from the soil at the groundwater level established through the tunnel project. The nitrogen related processes that could be affected were described and discussed. The different areas where a decreased groundwater level can cause changes in the nitrogen processes are the wet and moist areas. These areas were located by a vegetation inventory together with two independent assessments of the risk for a change in groundwater table. The potential nitrogen mineralisation and nitrification were determined through incubation experiments. Retention of nitrogen and denitrification were also studied. The result showed that 55 - 67 ha of wet or moist land on Hallandsås could be affected by a decline in groundwater table due to the tunnel project. Vegetation types on this land were wet alder forest, wet birch forest and poor fens of flark-type. Among these types of vegetation wet alder forest was dominating. The three types of vegetation had different chemical properties The wet alder forest had the lowest carbon nitrogen ratio and the highest nitrate to total mineral nitrogen ratio while the poor fens of flark-type had the highest carbon nitrogen ratio and the lowest nitrate to total mineral nitrogen ratio. The vegetation types also differed in their response to a declined groundwater table. The most significant effects on leaching of nitrate and emission of nitrous oxide were recorded for the wet alder forest. The soils under this vegetation type can mineralise and perhaps even nitrify around 100 kg nitrogen per ha and year. Leaching of nitrogen from the soil can increase the content of nitrogen in local watercourses, which might cause damage. However, the leaching will be limited in time, since much nitrogen will be utilised by an increased vegetation growth. The effects of nitrogen supply on the see bays, Laholmsbukten and Skälderviken, would be marginal. The flow of nitrogen from the affected areas on Hallandsås contributes less than 0,1 % to the amount of nitrogen received from other surrounding areas. The effects of a lowered ground-water table on nitrous oxide emission would be negligible.