Dredging and sediment discharge in a river with floodplain. Physicochemical and microbiological effects in Paraná river

Abstract

Alterations caused by human activities in the environment, such as dredging, modify the physicochemical conditions and affect the habitat. Maintenance dredging that allows large vesselsaccess to inland ports is a recurring disruptive action. The study aimed to evaluate, during amaintenance dredging operation in a port area of the Parana´ River, the modifications in thestructure of the river, the presence of contaminants and bacterial organisms. The effects causedduring the operation of a suction dredge in a South America river under two temporal conditionswere analyzed: i) the short and medium-term effects on the physicochemical variables, and ii) theimmediate effects on the physicochemical variables and the abundance of bacterioplankton. Thevariables measured in sediment and water samples were limnological parameters (e.g. conductivity, pH, dissolved oxygen, among others), heavy metal concentration, presence of biocides,hydrocarbon molecules, nutrients, sediment granulometry, Escherichia coli, Enterococcus, totalcoliforms, and heterotrophic bacteria. Some physicochemical variables increased in the watercolumn immediately after the water mass passed through the operating dredge, including sediment resuspension. The parameter changes were transient, as there were no significant increasesin the variables downstream of the dredge during dredging and discharge operations or afterdredging work. Some metal concentrations increased in the sediment and water column. Bacteriaincreased during dredging and more after rainfall events. Then, at the end of the dredging, thebacteria concentrations decreased to previous values. The possible effects of dredging disturbancewere of the same order or less than those of natural ones, i.e. rainfall. Sixty days after the dredgingwork was completed, the system was back to normal both in the dredged and discharge areas.