Flash floods controlling Cu, Pb, As and Hg variations in fluvial sediments of a river impacted by metal mining in the Atacama Desert

Solid mine wastes can be dispersed in river systems by fluvial processes contaminating distant areas. This process has been previously reported in rivers of the Atacama Desert impacted by mining. The removal of waste in these rivers is a complex environmental scenario that involves (i) mining operation plans, (ii) the spatio-temporal variability of precipitation events and, (iii) the sediment-transport pathways within the drainage network. This work presents how flash floods triggered by extreme storm events determine the redistribution and the variation of elemental concentrations of the suspended load fraction in a river with several historical and actual mine wastes situated in the Atacama Desert. The concentrations of Cu, Pb, As and Hg measured in two sampling stations of a mining segment of the Copiapó River after flash floods are lower compared to the concentrations measured in the period before these hydrometeorological events. However, the concentrations of these metals and metalloids increased in areas distant from the mine clusters, where the values observed before the flash floods were lower. The redistributions of the element concentrations can be explained by the occurrence of two major flood events triggered by the extreme storm events of March 2015 and May 2017. The combination of the element concentrations with calculated geo-accumulation indexes shows the potential source of geochemical anomalies for these elements. Cu and Hg anomalies are induced by historical mining activities in the Copiapó River while Pb and As concentrations agree with the regional background. Some Pb and As anomalies would be associated with the geology of the zone where the main cluster of mine exploitations are present. The fact that flash floods result in Cu, Pb, As and Hg concentrations in sediment load more in line with the local background, suggests that this type of hydrometeorological events probably represent the natural processes of geochemical recovery of arid river impacted by metal mining.