Heavy metal mining and flood plain response in the upper Clyde basin, Scotland

The Lowther Hills in the Southern Uplands of Scotland have a long history of metalliferous mining spanning over 700 years. Mining and ore processing generated significant quantities of metalliferous waste, but in the Clyde basin the geomorphological impact was largely restricted to headwater systems. In the mining district channel metamorphosis and flood plain aggradation resulted in active transformation of valley floors. With the cessation of mining, these headwater systems returned to laterally unstable single-thread gravel bed rivers. The pollution legacy remains acute with sediment-associated Pb values in excess of 150 000 mg kg^-1, surpassing UK contaminated land action trigger values by two orders of magnitude. Channel bank erosion rates of 30-50 mm year^-1 are responsible for remobilizing metal-rich sediment from flood plain storage and now represents the dominant source of metals to downstream reaches. The historical and on-going significance of this process was evaluated 40 km downstream on the main flood plain of the River Clyde. At this site mining-derived sediments were incorporated by passive dispersal. A fingerprinting analysis indicated that the mining epoch contributed only a minor flux of sediment to the flood plain, nevertheless peak Pb values exceed 1 500 mg kg^-1 illustrating more pernicious geochemical impacts.