LEAD BIOGEOCHEMISTRY IN A CENTRAL ONTARIO FORESTED WATERSHED

Abstract

To determine the sources and sinks of atmospherically deposited Pb at a forested watershed (Plastic Lake) in central Ontario, Canada, Pb pools and fluxes through upland, wetland and lake compartments were measured during 2002/2003 and compared with previous measurements taken between 1989 and 1991. In 2002/2003, annual bulk deposition of Pb was 0.49 mg m−2 compared with 1.90–1.30 mg m−2 in 1989–1991. Annual Pb concentrations in stream water draining the upland part of the catchment were very low (0.04 μg l−1) and were approximately half those measured in 1989–1991 (0.11–0.08 μg l−1). Leaching losses in stream water were small and mass balance estimates indicate almost complete retention (>95%) of atmospherically deposited Pb in upland soils. In contrast, annual Pb concentrations in stream water draining a wetland were between 0.38 and 0.77 μg l−1, with the highest concentration occurring in 2002/2003 and mass balance calculations indicate that the wetland is a net source of Pb in all measured years. Lead concentrations in the lake outflow were low and the average Pb concentration measured in 2002/2003 (0.09 μg l−1) was approximately half the value recorded in 1989–1991 (0.19 μg l−1 both years). Annual mass balance estimates indicate that the lake retained between 2.47 mg m−2 (1989/1990) and 1.42 mg m−2 (2002/2003) and that in 2002/2003 68% of the Pb input to the lake is derived from the terrestrial catchment. These estimates are higher than sediment core records, which indicate around 18 mg m−2 Pb was retained in sediment during the 1990s. Nevertheless, Pb concentrations decrease with sediment depth and 206Pb/207Pb concentrations increase with depth, a pattern also observed in mineral soils that reflects the substantial contribution of anthropogenic Pb to the watershed. Lead isotope data from soil and sediment indicate a recent anthropogenic Pb signal (206Pb/207Pb ~ 1.185) in upper soils and sediments and an older anthropogenic signal (206Pb/207Pb ~ 1.20) in deeper soil and sediment. Lead isotope data in sediment and vegetation indicate that practically all the Pb cycled in the forest at Plastic Lake is anthropogenic in origin.