Factors Controlling the Metal Levels in Headwater Stream Draining an Agroforestry Catchment (Galicia, NW Spain)

Abstract

Concentrations of Al, Fe, Mn, Cu and Zn (dissolved and particulate), suspended sediment, dissolved organic carbon, and pH and discharge were determined during a 3-year period in a stream water of an agroforestry catchment in Galicia (NW Spain). The objective of this study is to investigate the role of these variables in the control of dissolved and particulate concentrations of each metal in the stream water. The soils in the catchment are developed on metamorphic schist. Stream water samples were collected at the catchment outlet every 15 days and more frequently with increased stream flows. Metal concentrations were measured by ICP- MS. Pearson correlation coefficients were calculated to examine the possible associations between metals and the four variables under consideration. Stepwise multiple regressions were applied to determine which predictor variables have the strongest influences on controlling concentrations of each metal in the stream. The results showed that metal concentrations were relatively low (Fe > Al > Mn > Zn > Cu), but particulate metals were predominant over those dissolved. Stepwise multiple regression analysis revealed that the most important variable to explain dissolved concentrations for Al, Fe and Cu was the dissolved organic carbon, whereas the suspended sediment was for dissolved Zn and the discharge was for dissolved Mn. The suspended sediment was also a good predictor of particulate metal levels.