Assessing the impact of historical metal mining on upland landscapes: a nested sediment budget approach

Abstract

Historical metal mining has caused extensive landscape disturbance and the delivery of vast quantities of contaminated waste sediments to fluvial systems. The higher erosion rates typical of former mining landscapes also means that the geomorphological legacy of historical mining extends well beyond the active period of extraction. Although channel responses to sediment loading from mining are often well-documented, the scale and distribution of associated catchment disturbance and coupling with the fluvial system are poorly understood and rarely quantified. This study therefore assesses variability in landscape disturbance and sediment production resulting from historical lead mining in the North Pennines, UK. Regional trends in the distribution and geomorphic significance of historical sediment production were reconstructed for the period 1700-1948. Post-abandonment erosion (1868-2014) was quantified in detail at Whitesike and Bentyfield mines, using nested scales of geomorphic change detection and a sediment budget approach.
The results show that ~7.2 Mt of waste sediment were produced by historical mining in the study area but only ~2.3 Mt of this waste is still stored in depositional landforms; resulting in a net sediment output of >4.8 Mt. Waste sediment output from hushing (~4.6 Mt) was significantly higher than that from subterranean mining (~2.6 Mt). Considerable complexity in spatial and temporal patterns of historical mining was also recorded. The intensity of mine sediment production was highest within the Nent (~79000 t km-2), compared with the South Tyne (~20000 t km-2), Upper Tees (~19000 t km-2) and Black Burn (~4000 t km-2). Modelled sediment connectivity was also highest for the Nent, making this the catchment with the most significant level of disturbance and the most efficient slope-channel coupling.
Analysis of post-abandonment change at Whitesike and Bentyfield indicates that fluvial processes are the key drivers of erosion at historical mines, including gullying (187 t a-1) and bank erosion (93 t a-1). Pronounced temporal variation in erosion rates was also recorded, with 76% of the total net change to the main gully system occurring within one monthly interval (May 2013) and associated with a high intensity storm. Large-scale changes at abandoned mines are driven by high magnitude, low frequency rainfall events, although frequent low magnitude changes play an important preparatory role in determining overall sediment yields. This research emphasises the complex interactions between geomorphology and archaeology within intensively mined upland landscapes and the need for interdisciplinary approaches to their understanding and management.