Dispersion of tailings in the Knabena—Kvina drainage basin, Norway, 1: Evaluation of overbank sediments as sampling medium for regional geochemical mapping

Molybdenum mining in the Knabena—Kvina drainage basin (1918–1973) left more than eight million tons of tailings in two small lakes in the headwater area of the Knabena river. The piles, that reach above the water surface, were freely eroded until a dam was built to reduce the dispersion in 1976. Sampling of tailings and fluvial sediments took place almost 20 years later. Sampling media were natural sediment sources, 1-cm-thick slices of overbank sediments of various depths, material from the tailings pond, sandbars, stream sediments, fjord sediments, and integrated samples of floodplain surfaces (0–25 cm). In total 734 samples were collected. Chemical analysis (ICP-AES after aqua regia or HNO₃ extraction) showed that overbank sediments at a certain depth represent the pre-industrial trace metal concentrations within the drainage basin. The tailings and recent fluvial sediments were enriched in approximately the same element suite. The highest enrichment factors were obtained for Cu (8–53) and Mo (22–57). Fluvial processes in the tailings pond have probably selectively eroded fine-grained, low-density particles. Thus, coarse chalcopyrite may have been left behind, while molybdate associated with fine-grained particles may have been selectively entrained causing dilution of Cu and enrichment of Mo in the downstream fluvial sediments. In the sandbars, the highest Cu and Mo concentrations were found in fine-grained sediments downstream of a low-gradient reach that act as a bedload trap. On the floodplains, it is seen that the first areas to be inundated in a flood situation (proximal to the river and in depressions) have the highest metal concentrations. For regional geochemical mapping it is suggested that overbank sediment profiles along river reaches with a laterally stable or slowly migrating channel, should be sampled. In such floodplains, pre-industrial overbank sediments are usually preserved at depth. In case of laterally unstable reaches upstream of the sampling point, polluted and unpolluted sediments may be interlayered or mixed. Therefore, samples should be collected from various depths or sedimentary units in such profiles. A similar sampling strategy should probably be adopted to detect vertical migration of elements especially in areas with acid rain and low bedrock buffer capacity. To obtain high contrasts between polluted and unpolluted drainage basins, the overbank sediment profiles should be within the proximal part of the floodplain.