Model-based evaluation of reduction strategies for point and nonpoint source Cd pollution in a large river system

Cadmium (Cd) is a toxic trace element that threatens ecosystem and human health worldwide. Quantitative understanding of land-to-river Cd fluxes and riverine Cd loads in response to various watershed management measures is essential for developing effective mitigation strategies for large river systems. However, detailed analyses of watershed Cd dynamics under different management scenarios are lacking. Here, we investigated the effects of four management scenarios by combining point and nonpoint source control measures with a previously developed watershed Cd model that was validated with site-specific measurements. The Soil and Water Assessment Tool-Heavy Metal (SWAT-HM) model was applied to simulate the Xiang River Basin's (XRB, similar to 90,000 km(2)) baseline hydrology, soil erosion, and Cd transport processes in China. Using scenario simulations, we found that smelting emissions reduction was the most influential measure for controlling dissolved Cd (DCd) and particulate Cd (PCd) loads at the basin scale. Elimination of 50% emissions from the smelting sector could significantly (p < 0.05) decrease the monthly mean loads of DCd from 940 to 720 kg and of PCd from 2150 to 1760 kg at the XRB outlet. In contrast, reduction in mining emissions had no influence on the Cd load at the XRB outlet because most mining Cd emissions occurred upstream and midstream of the XRB, and the natural attenuation processes in the river limit the transportation of Cd downstream. The effectiveness of management practices for reducing total Cd (TCd) and DCd loads was not always mutually beneficial. For example, soil erosion control may decrease the PCd flux via erosion but increase the subsurface DCd flux to rivers due to greater lateral flow. In addition, increasing soil pH could be a practical and effective measure to reduce nonpoint DCd and PCd fluxes. Such effects may be caused by the declined upward migration of Cd through soil evaporation owing to the decreased Cd concentration in the soil pore water after pH increases. In conclusion, effective watershed management of Cd pollution in large basins requires an integrated plan that combines multiple mitigation measures; strategic modeling experiments could provide valuable insights into the design of such plans.