Anthropogenic legacy sediment budgets in heavily disturbed historical mining catchments

Abstract

Unregulated metal mining mobilises high volumes of waste sediments to river systems within impacted catchments, increasing sediment yields, contaminating floodplain stores, and profoundly altering long-term patterns of channel planform evolution. However, quantifying the actual scale of historical mine sediment production and dispersal remains a significant challenge, due largely to the variable quality, character and availability of disparate datasets relating to past mining operations. In this study, we reconstruct anthropogenic sediment production associated with historical lead mining between 1700 and 1948 for a globally significant orefield in the North Pennines, UK, that includes the headwater catchments of two major river systems: the River South Tyne and the River Tees. Using a range of interdisciplinary methods including digital terrain analyses and ore-to-waste sediment scaling ratios, we find that mining produced 4.4 × 10⁵ t of lead ore during the study period, but also mobilised an estimated 7.2 × 10⁶ t of associated waste sediment. Approximately 67 % of this waste sediment cannot be accounted for within extant anthropogenic sediment storage landforms within the catchment areas. Surface working using managed water supply (hushing) was the key sediment production process, with 64 % of waste sediment originating from surficial hushing but only 36 % from subterranean mining. The high connectivity of hushes with river channels resulted in minimal (<1 %) long-term sediment storage in the form of hush outwash fans. We find pronounced spatial and temporal variability in legacy sediment production and storage, which has important implications for understanding reach-scale patterns of channel response to historic mining operations.