Historical and contemporary metal budgets for a boreal shield lake

Metal concentrations in sediment cores are widely used to reconstruct metal deposition histories, but rarely have metal budgets based on measured inputs (atmospheric deposition and inflows) and lake outflows been compared with metal fluxes estimated from lake sediment cores. In this study, budgets for six metals (As, Cd, Co, Cu, Ni and Pb) were estimated by measuring inputs in bulk deposition, inputs in the major inflow and export in the lake outflow for one hydrologic year (2002-2003) at Plastic Lake, Ontario, Canada. Inputs in bulk deposition were between 0.03mgm^-2y^-1 (Co) and 0.69mgm^-2y^-1 (Cu), which represented between 2.5 and 80.7% of total metal inputs to the lake. The estimated budgets for Co, Cd and Ni, which exhibit similar geochemical behavior in the major inflow, were comparable to budgets estimated from the upper section of a sediment core taken in 2002, taking into account previously published correction factors for sediment focusing. For example, mass budgets for Co, Cd and Ni were 1.24, 0.28 and 1.89mgm^-2y^-1, compared with sediment budgets estimated to be 0.90, 0.19 and 4.72mgm^-2y^-1, respectively. In contrast, measured budgets for As, Cu and Pb, which also behave similarly in inflows (and different to Co, Cd and Ni), were between 3 (As) and 40 times lower than estimates from the upper sediment core. A possible explanation for the discrepancy is that sediment focusing transfers sediment from shallow to deep areas, which for metals like Pb, which have strong affinities for organic matter and where atmospheric deposition has decreased 15 fold since 1978 (4.31mgm^-2y^-1 in 1978 to 0.28mgm^-2y^-1 in 2013), leads to the transfer and accumulation of pollution metals to deeper parts of the lake long after a decrease in atmospheric deposition.