Metal(loid) Partitioning and Transport in the Jinsha River, China: From Upper Natural Reaches to Lower Cascade Reservoirs-Regulated Reaches

Riverine contamination of metals and metalloids is a growing global concern. However, our understanding of how cascade reservoirs influence the fate of these pollutants remains limited. In this study, we focused on the Jinsha River, a typical large river polluted with metal(loid)s in Southwest China, to investigate the partitioning and transport of these substances from the upper natural reaches to the lower cascade reservoirs-regulated reaches during both flood and dry seasons. Results revealed that the rapid urbanization and industrial development following the construction of reservoirs led to significant copper (Cu), nickel (Ni), lead (Pb), chromium (Cr), and cadmium (Cd) pollution. Through various analytical techniques such as correlation analysis, principal component analysis, redundancy analysis, and structural equation modeling, we determined that reservoirs had a profound impact on solid-associated parameters, including particulate organic matter (POM), suspended solid concentration (SSC), and median particle diameter (D50). These parameters, in turn, affected the fate of metal(loid)s through processes like sorption-desorption and dissolution-precipitation. High POM concentrations and low SSC and D50 values resulted in the precipitation of most particulate metal(loid)s, thereby reducing their transport across reservoirs. As river water moved downstream across the reservoirs, the percentage of particulate metal(loid)s decreased by 6.1% for arsenic (As), 38.3% for Cu, 74.9% for Ni, 36.8% for Cr, 17.8% for Cd, and 41.9% for Pb, respectively. Furthermore, the cascade reservoirs had cumulative effects on the deposition of particulate metal(loid)s, particularly concerning Cr, Cd, and Pb in the final reservoirs. Human health risk assessments indicated that dissolved As and Pb posed potential non-carcinogenic threats to residents. Despite the fact that reservoirs captured most particulate metal(loid)s, the transport of dissolved As and Pb across the reservoirs still impacted the safety of downstream drinking water. Therefore, in cascade reservoir systems, the concern regarding dissolved metal(loid) pollution remains a priority for future reservoir management.