Geochemistry of the Lule River, northern Sweden, before and after regulation

Abstract: The Lule River is the largest and most important river for hydropower production in Sweden. This river, situated in the northern part of the country, has been regulated since the beginning of the 20th century and is heavily regulated with 15 power stations. The major aim of this study is to quantify the transport of nutrients and other elements in a reservoir, Stora Lulevatten, since knowledge about processes in upstream reservoirs is limited. Monthly sampling of the filtered (<0.22m) phase of river water was performed at the Porjus and Vietas power stations over a period of 15 months. To quantify the effects of river regulation, a geochemical mass balance study was performed. The results show retention of S, K, Si, NO3-N, Fe, PO4-P, Al, Mn and DOC in the reservoir. The study indicates retention levels of 34-43% for Si, Al, Mn, DOC, PO4-P, NO3-N, and for Fe, as much as 68%. A model comparing pre-regulation transport of elements at Porjus power station with present-day transport is presented. The results agree well with the retention of S, DOC, Fe Si, Mn and Al in the reservoir, and thus decreased transport after regulation. One reason for the changed transport is the changed discharge pattern after regulation. Three sediment cores were sampled in the reservoir, and analysed in details for main elements, P and N, and diatom frustules were counted in the sediments. A significantly higher Si/Al ratio could be observed in all three cores after regulation, which correlates with an increased concentration of diatoms in the sediment. The annual retention of non-detrital Si in the sediments as calculated from sediment geochemistry (2138 tonnes Sind/year) is in relatively good agreement with the retention obtained from the mass balance study (3320 tonnes Si/year). A comparison of the background and present-day accumulation of non-detrital Si suggests that the Si retention has increased by 698 tonnes/year after regulation of the Lule River. This increase may be an effect of an increased diatom production in Stora Lulevatten appearing after the construction of the reservoir. Seasonal variations in the geochemical constituents in the regulated Lule River have been studied during an entire year cycle (May 2000 to June 2001) at the Boden power station, near the mouth of the river. Comparing the Si retention in the reservoir with the total transport of Si at Boden, the increased deposition of 698 tonnes Si/year corresponds to a reduction in the Si transport to the Gulf of Bothnia by 2%. The geochemistry of the regulated Lule River is compared with a pristine river, the Kalix River, for which data from a previous study at Luleå University of Technology are available. The results from this comparison show that the transport of Fe (and probably also of P) was found to be clearly decreased due to regulation. The decreased transport of Fe agrees well with the mass balance study, decreased transport after regulation at Porjus and retention of Fe and P in the sediments in Stora Lulevatten. Stora Lulevatten acts as a sink for Fe, Si and P. Considering that Stora Lulevatten is small compared to upstream reservoirs, the total retention is probably larger than that observed in Stora Lulevatten. A result of changed discharge pattern after regulation is that nutrients and other elements reach the Gulf of Bothnia in changed amounts and during different seasons than what would have been the case for an unregulated river.