Metal Fluxes from Soils to Surface Waters at the Catchment Scale

Abstract

This article reports the contribution of four rain events to the metal load [aluminum (Al), iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn)] reaching a small stream draining an agroforestry catchment in northwestern Spain. During events and at base flow, the mean total metal contents present in the following order of abundance: Fe > Al > Mn > Zn > Cu. In general, the mean total metal concentrations in the stream were always higher during the rain events than during base flow. The contribution of dissolved metals to the total metal concentrations during events was less than 10% for Al, Fe, and Mn, whereas for Cu, and especially for Zn, its contribution was much higher and for Zn exceeded 50% in some events. Particulate fractions of Al, Mn, and Fe during rainfall events showed significant positive correlations with flow rate and suspended solids. Overall, it can be concluded that the amounts of Al, Fe, and Mn reaching the stream during rainfall events were related to the amount of soil erosion occurring in the catchment during rainfall events and not to leaching of these elements in solution. To a lesser extent this was also the case with Cu and Zn. Although the metal loss from the catchment during the rainfall events studied was relatively low, it was observed that one single rain event can contribute in an important way to the annual metal load in the small stream. The total metal loads in the stream differed between events.