Sediment Pollution Impacts in The Coastal Zone of Cartagena Bay, Colombia: Monitoring, Modelling and Management

The coastal zone of Cartagena, Colombia, provides important ecosystem services to the surrounding population, such as tourism and fishing. However, Cartagena Bay is impacted by land-based sources of pollution, including continental runoff, industrial effluents and domestic wastewater. Continental runoff from the Magdalena River via Dique Canal into the south of the bay produce seasonal estuarine conditions and deliver sediments to the sea that impact coral reefs, fish and human health. This study monitored suspended sediment concentration (TSS) in coastal waters monthly and metal concentration in the bay’s bottom sediments every three months. Results showed that TSS was consistently above a coral reef ecosystem threshold of 10 mg/l. Concentrations of mercury, cadmium, chromium, copper and nickel were found in excess of Threshold Effects Levels used to indicate potential impacts on marine life. An integrated assessment of pollution sources estimated pollutant loads with confidence intervals for each load value, showing that continental runoff from the Dique Canal is the principal source of sediments and phosphorus in the area, as well as a significant source of nitrogen and organic matter, while further studies of metal contamination are underway to compare fluvial and industrial sources. To mitigate offshore coral reef turbidity impacts, a calibrated 3D hydrodynamic-water quality model (MOHID) was used to calculate a target load for the Dique Canal outlet to inform policy and planning. Results showed that ecosystem TSS thresholds could be maintained within the extent of Cartagena Bay by reducing the canal’s current TSS loads by 80-90% to target loads of 500-700 t/d. The substantial reductions that are needed reflect ongoing issues in the Magdalena watershed which has experienced severe erosional conditions and intense deforestation over the past four decades.