The Distribution, Residence Time, and Fate of Hg and Sediment in Fine-Grained Channel Margin (FGCM) Deposits within a Steep, Gravel-Bed River

We have mapped deposits of fine sand, silt, and clay trapped by large woody debris (LWD) within the channel of a steep, gravel-bed river with a historic mercury contamination. These “fine-grained channel margin” (FGCM) deposits were dated using bomb radiocarbon, Pb210, and Cs137 and characterized with respect to grain size, organic composition, and Hg concentration. By volume the deposits store about 25% of the annual load. The volume of sediment reworked through erosion and deposition every year is equivalent to approximately 5% of the annual load. We can use the dates to represent the age distribution for the entire population of FGCM deposits and therefore determine the residence time and transit time distributions for sediment in these features. The residence time distribution is comprised of two components: a short turnover time (approximately 5 years) that represents the fraction of sediment which is frequently reworked, and a long turnover time (about 130 years) that represents the fraction that is reworked very infrequently. The distribution of ages and Hg concentrations suggest that approximately 10% of the sediment in the deposits is from the release period (1929-1950) which accounts for 75% of the Hg mass. This observation is significant because much of the Hg is redistributed through erosion of the banks in this system, but most of the Hg in FGCM deposits was transported directly from the plant without intervening storage on the floodplain. Assuming the sediment stored in the deposits is a representative sample of the suspended load of the river, we can create a model that specified the history of accumulation and reworking of Hg through time in FGCM deposits. This model allows us to recreate the loading history Hg from the plant and predict that it will require centuries to remove this material.