Contaminated sediment flux from eroding abandoned historical metal mines: Spatial and temporal variability in geomorphological drivers

Abstract

Abandoned historical metal mines represent significant long-term sediment source locations contributing highly contaminated anthropogenic legacy sediments to river systems. Despite this, our understanding of spatial and temporal variability in the rates and geomorphological drivers of specific contaminated sediment source locations across abandoned mines remains poorly constrained. In this study, sediment flux from two abandoned historical lead mines in the North Pennines, UK, was monitored over an 18 month period using repeat terrestrial laser scanning, enabling the spatial and temporal significance of several common geomorphological processes to be quantified for the first time. A novel contaminated sediment budget approach is used to integrate topographical change data with a pXRF survey of surface sediment metal concentrations. Approximately 434 t (289 t a−1) of eroded sediment entered the stream from a source area of 0.023 km2. The majority of the erosion was driven by two dominant processes, with gullying accounting for 60% and bank erosion contributing 30%. Redeposition of eroded material within the survey area was minimal (3%), indicating very high levels of coupling between source locations and the stream network and the export of the vast majority of eroded sediments (97%) from the mined area. Rates for all erosive processes were highly episodic and primarily driven by high magnitude, low frequency storm events. Metal concentrations in surface sediments exhibited considerable spatial variability, with notable hotspots around the former ore processing areas and on the tailings heaps. However, 84% of all sediments sampled were in excess of available soil guideline values for Pb and 65% in excess of equivalent guideline values for Zn, indicating that abandoned mine sites still have the potential to be significant sources of contaminant metals and pose a risk to the wider environment. In total, an estimated 4.59 t of Pb and 2.14 t of Zn entered the stream network in a sediment-associated form during the 18 month monitoring period. Although these overall contaminated sediment inputs are high, they are restricted to particular geomorphological processes, are spatially variable in terms of the magnitude of specific source locations, and are delivered in fairly discrete events. This provides invaluable information for the future management of other abandoned mines and targeted mitigation of their potential legacy effects.