MERCURY IN LAKE SEDIMENTS AND POREWATERS: MULTIPLE EVIDENCE FOR REMOBILIZATION

The cause of depth profiles of Hg and the degree that downcore trends represent a primary signal is hotly debated. To add to this debate new methods were used to examine the Hg biogeochemistry of two immediately adjacent lakes in Kejimkujik National Park, Nova Scotia, Canada – Big Dam West Lake (BDW) and Big Dam East Lake (BDE). Several lines of evidence illustrate that Hg remobilization occurs in the sediments: (1) Acoustic subbottom profiling was used determined the total volume and mass of lake sediments and Hg contained in them and how it varied through time since the inception of the lake after deglaciation some 13,000 years ago. The rate of accumulation in sediment traps over 1 year is higher than that in the sediments over 20 years, which in turn is higher than that in the sediments over 200 years and so on up to 13,000 years. Simply by mass balance, the excess that is missing from the older sediments must be recycled upwards back into the lake. (2) A comparison between lakes shows that BDE has roughly half the sedimentation rate of BDW, a water residence time 8 times longer than BDW’s, and a higher particle sinking velocity. Yet unless remobilization is occurring, the flux of Hg to the sediments is counter-intuitively less by half. (3) Porewater concentrations of Hg have a decreasing upward gradient. Upward diffusion from low to high concentration must therefore occur. (4) A porewater spiking experiment empirically shows that the direction of aqueous Hg diffusion is upwards and that diffusion occurs for the natural gradients observed. (5) The presence and distribution of methane gas bubbles in the sediments is imaged acoustically demonstrating that old organic matter in the lakes is decaying by methanogenesis. Upward moving bubbles provide an advection pathway for dissolved gaseous mercury (DGM) to ascend to the SWI. (6) Thermodynamic modelling of the stability of Hg species for the Eh and pH conditions observed in this study show that at relatively shallow depths, DGM becomes the stable species. DGM strongly partitions into methane bubbles therefore facilitating upward advection. If remobilization of Hg were not occurring in the sediments of BDW and BDE, it becomes difficult to explain the differences in Hg concentration and accumulation in these two different but immediately adjacent lakes.