Fluxes and budgets of Cd, Zn, Cu, Cr and Ni in a remote forested catchment in Germany

The input of heavy metals by atmospheric deposition to forested watersheds substantially decreased during the last decades in many areas. The goal of our study was to identify the present sinks and sources of metals and factors influencing metal mobility at the catchment and soil profile scale. We determined concentrations and fluxes of Cd, Zn, Cu, Cr and Ni in precipitation, litterfall, soil solutions (Oi, Oe, Oa horizon percolates, 20 and 90 cm soil depth) and runoff in a forest ecosystem in NE-Bavaria, Germany for 1 year. The metal concentrations in solutions were mostly < 10 mu g l(-1) beside Zn (< 1200 mu g l(-1)). The present total deposition was estimated at 1.0, 560, 30, 1.2 and 10.4 g ha(-1) year(-1) for Cd, Zn, Cu, Cr and Ni, respectively. The mass balance (total deposition minus runoff) at the catchment scale indicated actual retention of Zn, Cu and Ni, but an almost balanced budget for Cr and Cd. Considering the soil profile scale, the Oi horizon still acted as a sink, whereas the Oe and Oa horizons were presently sources for all metals. The solid-solution partitioning coefficients indicated higher mobility of Cd and Zn than of Cu, Cr and Ni in forest soils. In the mineral soil horizons, K(d) values derived from field measurements were substantially larger than those predicted with empirical regression equations from Sauv, et al. (Environ Sci Technol 34:1125-1131, 2000; Environ Sci Technol 37:5191-5196, 2003). The mineral soil acted as a sink for all metals beside Cd. Dissolved organic C and pH influenced the metal mobility, as indicated by significant correlations to metal concentrations in Oa percolates and runoff. The solid-solution partitioning coefficients indicated higher mobility of Cd and Zn than of Cu, Cr and Ni in forest soils. Overall, the decreased deposition rates have obviously induced a source function of the Oe and Oa horizon for metals. Consequently, mobilization of metals from forest floor during heavy rain events and near surface flow conditions may lead to elevated concentrations in runoff.