The geochemical status of the surface water and the sediments in the estuary of the Sangis River, Kalix, Sweden

Abstract: It has been common in forestry and agriculture to drain waterlogged areas through ditching in order to cultivate them. The ditches that drain water are usually connected to some watercourse, such as a river. When the water flow of the river decreases and becomes more still, particles will settle and fall to the bottom of the water body and form sediment. If there is an increasing amount of nutrients and metals transported with the water, it could affect the water quality negatively, since an increased amount of nutrients could result in eutrophication and many metals are toxic in high amounts. In the Sangis River there is an increased amount of sediment deposited at the mouth of the river and inside the estuary, which has resulted in the river and the estuary becoming shallower. The origin of the deposited sediments is believed to partly be from ditching. The residents of the village of Sangis have said that it is difficult to cross the estuary by boat because of the deposited sediments, therefore, their wish is for a channel dredged in the Sangis River and its estuary that they can use. The main aim of this master's thesis was to analyse the geochemical status of the surface water and the sediment in the estuary of the Sangis River and to give recommendations for future actions preventing sedimentation of the river channel. The analyse of the geochemical status of the sediment was achieved by sampling six sediment- cores in the estuary and analysing for different parameters. The analyse of the geochemical status of the surface water was achieved by comparing the water quality regarding metal concentrations in the estuary with other nearby rivers and classifying the concentration of phosphorous in the estuary, to determinate if there was an ongoing eutrophication. The following analyses were conducted for the sediment-cores: Element distribution were analysed with P-XRF, pH and electrical conductivity were measured and loss on ignition was also calculated trough combustion of the sub-samples. The results showed for the P-XRF that the dominating main elements were iron, sulfur, calcium and potassium. Iron and sulfur were correlated to each other in all profiles and had a peak between 20-35 cm in the sediment. This indicated that there could be formation of iron sulfides at that depth. Therefore, sub-sample 4.D was analysed with SEM-EDS instrument and framboidal pyrite (FeS2) was detected in the sample. The results from the pH and conductivity showed that the pH-value varies greatly both with depth and between the sediment cores. However, for the conductivity it could be seen how it peaked at around 20-35 cm in all profiles, with the highest EC-value in profile 4. It could be concluded that the surface water in the estuary contains elevated concentrations of copper and that there is an ongoing eutrophication as well. Due to the eutrophication, it has most likely resulted in an increased amount of aquatic plants during the summer. The increased amount of organic material has probably resulted in oxygen-free bottoms, since all the oxygen has been consumed when the organic material has been decomposed. The formation of framboidal pyrite shows that the redox ladder has reached the two last steps, since framboidal pyrite (FeS2) consists of reduced sulfur and dissolved iron and is formed during anoxic conditions. Reduced sulfur forms when sulfate is reduced in order to oxidize organic matter and dissolved iron forms when iron-oxide hydroxides are reduced in order to oxidize organic matter. If dredging is carried out in the Sangis river and its estuary, oxidation of framboidal pyrite will occur, this could result in formation of acid and leachate of metals that were previously bound to framboidal pyrite. The consequences if it leaches into the river and the estuary is that it creates an acidic environment with elevated metal concentrations, where marine life would find it hard to live. The extent and impact of leached acidity and metals needs to be studied further.