Partition of heavy metals in a tropical river system impacted by municipal waste.

A research program was established to identify the governing factors for the partition coefficient (K(D)) of heavy metals between suspended particulate and dissolved phases in the Day River system a tropical, highly alluvial aquatic system, in Vietnam. The targeted river system, draining an urbanized-industrialized catchment where discharged wastewater is mostly untreated, could be separated into the least impacted, pristine area, and the most impacted, polluted area. Organic matter degradation was shown to govern the variation of parameters like total organic carbon, biochemical oxygen demand, chemical oxygen demand, nutrients, conductivity, or redox potential. Heavy metals in both dissolved and particulate phases were enriched in severely polluted area because of wastewater inflow that contains concentrated metals and intensification of metal influx from sediment. Results show log K(D) in the order Mn < As < Zn < Hg < Ni < Cu < Cd < Co < Pb < Cr < Fe and As < Zn < Ni < Mn < Cr < Cu < Co < Fe in the polluted zone and the pristine zone, respectively. A decreasing tendency of partition coefficients of 11 heavy metals considered from the pristine to the impacted zones was observed. Three explanations for the difference are: (1) increase of solubility of most heavy metals in low redox potential, (2) competition for the binding sites with major and minor cations, and (3) complexation with dissolved organic matter concentrated in municipal waste impacted water. Apart from domestic waste impact, statistical analysis has contributed to identify the influence of climate condition and hydrological regime to the partition of heavy metals in the area.