CADMIUM DEPOSITION AND MOBILITY IN THE SEDIMENTS OF AN ACIDIC OLIGOTROPHIC LAKE

Abstract

A dated core from the profoundal zone in a pristine oligotrophic acidic lake was analyzed for Cd as well as for Al, Ca, Fe, Mg, Mn, Pb, Ti and total carbon and nitrogen. Overlying water and porewater samples were also obtained on six occasions at the same site, and yielded vertical profiles of pH and dissolved Cd, Ca, Fe, Mg, Mn, sulfide, SO4−2, organic and inorganic carbon concentrations. These extensive porewater and sediment geochemical data were used, together with information on infaunal benthos, to decipher the sedimentary record of Cd contamination. Depth variation of sediment Ca concentrations indicate that the lake suffered from progressive acidification starting about 1950. The present-day accumulation rate of Cd (JaccCd = 5.4 ± 0.4 × 10−11 mol cm−2 yr−1) in the sediments is the sum of the flux of Cd deposited with settling particles (JSCd = 3.3 ± 0.2 × 10−11 mol cm−2 yr−1) and the fluxes of dissolved Cd across the sediment-water interface due to molecular diffusion (JDCd = 1.8 ± 0.3 × 10−11 mol cm−2 yr−1), bioturbation (JBCd = 1.1 ± 0.2 × 10−14 mol cm−2 yr−1) and bioirrigation (JICd = 0.27 ± 0.05 × 10−11 mol cm−2 yr−1). Biological mixing of the sediments was negligible. The shape of the vertical profile of total Cd concentration with depth in the sediment appears to be determined more by its input history than by post-depositional mobilization and redistribution in the sediment column.