Drought-induced metal release from a wetland at Plastic Lake, central Ontario

Abstract

With climate change, droughts may become more frequent in southern Ontario, which could release metals from peat and degrade downstream water quality. Monthly volume-weighted metal (Al, Ba, Be, Cd, Co, Mn, Ni, Pb, Sr, and Zn) concentrations and fluxes in streams and bulk deposition at Plastic Lake were monitored over 20 months in 2002–2003, during which there was a summer drought. Monthly concentrations in the outflow from the wetland (PC1) were variable, with very high concentrations following the drought. With the exception of Pb, statistically significant models of metal concentrations with SO42– and dissolved organic carbon concentrations were developed, and these relationships were used to estimate monthly metal exports between 1980 and 2000. Model predictions for Cd and Zn in PC1 agreed well (p < 0.001) with concentrations measured between 1989 and 1991. Model predictions suggesting peaks in metal concentrations are common in years with pronounced summer droughts. In contrast to ombrotrophic bogs, the PC1 wetland receives the majority of its metal input from the terrestrial catchment, and mass balance approximations indicate no substantial depletion of metal reserves in peat. Drought-induced metal peaks may persist for many decades, potentially contributing to the delayed recovery of surface waters at Plastic Lake, despite declining S deposition.