The influence of a large anthropogenic sediment source on the fluvial geomorphology of the Knabeهna-Kvina rivers, Norway

Molybdenum mining at Knaben discharged more than eight million tonnes of tailings into two lakes in the upstream end of the Knabeåna river, southern Norway, during the period 1918–1973. A dam was built downstream of the lakes in 1976 to stop the fluvial dispersion of tailings. Bulk samples were collected from the tailings pond (n = 30), natural sediment sources (n = 7), sandbars (n = 98), and overbank sediments (0–25 cm depth, n = 79) along the 55-km river reach downstream of the mining area. In addition, overbank sediment samples of 1 cm thickness were collected at depth intervals of 1–5 cm in 19 vertical profiles (n = 497). Sedimentological properties were recorded in 17 of the profiles. Chemical analysis (ICP-AES after hot nitric acid or aqua regia extraction) has shown that a minimum of 420000 tonnes of tailings with a median molybdenum content of 80 ppm are presently stored in the fluvial sediments. The major volumes are present in sandbars in a 6 km long, low-gradient reach that acts as a bedload trap under normal flood conditions. However, bedload may be flushed into the steeper reaches by repeated major events. On the floodplains, 14C-dating has shown that the average sedimentation rate in one profile has increased from 0.5 to 4.3 mm/year due to deposition of tailings. Since lateral channel instability has only been detected in a few locations, the floodplains are regarded as semi-permanent sinks for tailings. However, extreme floods that strip off vegetation or undercut the floodplains may increase the availability of tailings from these deposits. The results from the Knabeåna-Kvina rivers indicate that construction of weirs may stop the dispersion of bedload-sized contaminated sediments, while construction of dams may be necessary to trap contaminated sediments transported in suspension. In each case the benefit and disadvantages of such constructions must be evaluated according to the local conditions.