Abstract:

This study was conducted in a small watershed of Huodigou, which is a part of natural forest region located at a medium altitude on the south-facing slopes of the Qinling Mountains. Water samples were collected from rainfall, throughfall, litter drainage, subsurface flow, stream water in the tributaries within the watershed, and flow water at the watershed outlet for two years. The samples were analyzed for Cd, Pb, Mn and Zn mass fractions. The effects of different components of the forest ecosystem in the watershed on heavy metals in water during the rainfall-runoff process were studied with comparisons and mechanistic analyses. The results showed that Cd, Pb, Mn and Zn mass fraction decreased by 0.721 μg·kg^-1, 6.482 μg·kg^-1, 0.012 8 mg·kg^-1 and 1.467 4 mg·kg^-1, respectively, and input of Cd, Pb, Mn and Zn through rainfalls was immobilized over 90% by the forest ecosystem. Cd, Pb, Mn and Zn mass fractions decreased stepwise with the rainfalls through out the canopy and the litter. In subsurface flow, Cd, Pb and Mn mass fractions increased while Zn mass fraction decreased. In tributary stream water, Cd, Pb and Mn mass fraction decreased sharply but Zn increased. In flow water at the watershed outlet, Cd, Pb, Mn and Zn mass fraction increased in varying extent. The canopy reduced Cd, Pb, Mn and Zn mass fraction by 0.697 μg·kg^-1, 6.194 μg·kg^-1, 0.008 9 mg·kg^-1 and 1.476 3 mg·kg^-1, respectively, and therefore was a crucially important layer of the forest ecosystem for reducing the heavy metals in water during the rainfall-runoff processes, followed by the litter layer.

Key words: forest ecosystem, rainfall-runoff process, heavy metal, effect