SEDIMENT-WATER DISTRIBUTION COEFFICIENTS AND SPECIATION OF CADMIUM INA CANADIAN-SHIELD LAKE

Sediment-water distribution coefficients for Cd were strongly pH depen dent at pH > 6.5 for three surficial bottom sediments and suspended pa rticles from a Canadian Shield lake (Lake 382). A single exponential e quation in terms of pH fitted the distribution coefficients of all sed iments. In the range 6.5 less than or equal to pH less than or equal t o 8 the distribution coefficients were 109 x 10(-3) to 1579 x 10(-3) L /mg for suspended particulate matter and 0.6 x 10(-3) to 129 x 10(-3) L/mg for the bottom sediments. Speciation calculations performed using the computer program MACS80 and measured Cd fractions associated with suspended particles agreed well down to 7 m. In deeper water the meas ured traction exceeded the calculated fraction, hypothetically because of CdS formation, differing sediment compositions, or nonequilibrium conditions. In September, H2S was 100-300 mu g/L in the lower hypolimn ion. Even at only 5 mu g dissolved sulfide/L, some of the added Cd wou ld precipitate as CdS at pH > 6, and the dissolved Cd, under anoxic co nditions, would be largely Cd(HS)(+) and Cd(HS)(2). Under aerobic cond itions at pH greater than or equal to 7.5, approximately 85% of the Cd was on suspended particles and complexed with dissolved organic matte r. The solids Cd(OH)(2), CdCO3, and Cd-3(PO4)(2) played no role in the control of dissolved Cd.