Quantifying system disturbance and recovery from historical mining-derived metal contamination at Brotherswater, northwest England

Abstract

Metal ore extraction in historical times has left a legacy of severe contamination in aquatic ecosystems around the world. In the UK, there are ongoing nationwide surveys of present-day pollution discharged from abandoned mines but few assessments of the magnitude of contamination and impacts that arose during historical metal mining have been made. We report one of the first multi-centennial records of lead (Pb), zinc (Zn) and copper (Cu) fluxes into a lake (Brotherswater, northwest England) from point-sources in its catchment (Hartsop Hall Mine and Hogget Gill processing plant) and calculate basin-scale inventories of those metals. The pre-mining baseline for metal contamination has been established using sediment cores spanning the past 1,500 years and contemporary material obtained through sediment trapping. These data enabled the impact of 250 years of local, small-scale mining (1696 – 1942) to be quantified and an assessment of the trajectory towards system recovery to be made. The geochemical stratigraphy displayed in twelve sediment cores show strong correspondence to the documented history of metal mining and processing in the catchment. The initial onset in 1696 was detected, peak Pb concentrations (>10,000 µg g-1) and flux (39.4 g m-2 y-1) corresponded to the most intensive mining episode (1863-1871) and 20th century technological enhancements were reflected as a more muted sedimentary imprint. After careful evaluation, we used these markers to augment a Bayesian age-depth model of the independent geochronology obtained using radioisotope dating (14C, 210Pb, 137Cs and 241Am). Total inventories of Pb, Zn and Cu for the lake basin during the period of active mining were 15,415 kg, 5,897 kg and 363 kg, respectively. The post-mining trajectories for Pb and Zn project a return to pre-mining levels within 54-128 years for Pb and 75-187 years for Zn, although future remobilisation of metal-enriched catchment soils and floodplain sediments could perturb this recovery. We present a transferable paleolimnological approach that highlights flux-based assessments are vital to accurately establish the baseline, impact and trajectory of mining-derived contamination for a lake catchment.