Tracing the river-water versus waste-water sources of trace elements using rare earth elements in the Nakdong River estuarine waters

The concentrations of dissolved rare earth elements (REEs) and trace elements (Al, V, Mn, Fe, Co, Ni, Cu, Zn, Mo, Cd, and Pb) were measured at multiple sites along Nakdong River Estuary, Korea. This estuary receives 15.7 × 10⁹ m³ of fresh water annually from the river, which is obstructed by a dam upstream. Thus, there are external sources of trace elements from intermittent river-water inputs and perhaps from waste water treatment plant (WWTP) which is located in the eastern part of estuary. In this study, we attempted to utilize REEs, which are relatively stable in estuarine timescales, as tracer of river versus WWTP sources of trace elements for the first time. In general, REEs concentrations decreased toward the sea with strong inverse correlations with salinities, except for the sampling sites close to WWTP where about two orders of magnitude higher concentrations were observed. The Post-Archean Australian shale (PAAS) normalized patterns clearly differentiated the WWTP sources from river and seawater sources along the estuary. Based on this water mass mixing ratio, we conclude that the main sources of trace metals can be attributed to the seawater for Mo and Cd, the seawater and WWTP for V, river and WWTP for Ni and Cu, and WWTP for Al, Mn, Fe, Co, Zn, and Pb. Our results suggest that REEs can be a powerful tracer for WWTP sources, particularly in coastal waters where various trace metal sources are present.