Long-term persistence of sedimentary copper contamination in Lake Orta: Potential environmental risks 20 years after liming

Abstract

Lake Orta, northern Italy, has suffered from severe copper pollution and human-induced acidification between the 1920s and the 1990s because of discharges from a rayon factory and electroplating industries located in its drainage basin. Following liming operations in the late 1980s, the chemical quality of the water column has been restored and signs of, still ongoing, biological recovery observed. Two sediment cores (07/1A and 07/1B) were collected close to the main historical Cu discharge (in the southern part of the lake) and one (07/2A) in the central part of the lake. Cores 07/1A and 07/2A were analyzed for Cu content, Loss on Ignition (LOI) and water content. Diatom profiles (cores 07/1B and 07/2A) were used to confirm sediment dating and to identify alterations in the sediment sequence because, among other taxonomic groups, diatom assemblages were strongly affected by lake pollution and recovery. Copper concentrations in the uppermost layers of sediment cores (2007 AD) were above 1,000 mg kg–1 and around 500 mg kg–1 in cores 07/1A and 07/2A, respectively. These values were lower than the corresponding maximum ones of approx. 3500 and 4500 mg kg–1 recorded at the end of the 1950s at both locations, but remained 10 to 40-fold higher than the pre-industrial levels of about 40 mg kg–1 measured in both cores. Diatom profiles revealed a continuous sedimentation pattern in core 07/1A, but highlighted interruptions in sediment deposition in core 07/2A. Copper levels in 2007 were still comparable to or higher than concentrations reported to cause adverse effect on the survival, growth and reproduction of sediment-ingesting organisms. A critical comparison of measured concentrations with relevant ecotoxicological values suggests that particular attention should be given to understand the effects of dietary ingestion of sedimentary Cu which, unlike in previous ecotoxicological studies, may now represent the main route of exposure to Cu for sediment-ingesting benthic organisms.