Distribution of inorganic mercury in Sacramento River water and suspended colloidal sediment material

The concentration and distribution of inorganic Hg was measured using cold- vapor atomic fluorescence spectrometry in samples collected at selected sit es on the Sacramento River from below Shasta Dam to Freeport, CA, at six se parate times between 1996 and 1997. Dissolved (ultrafiltered, 0.005 mum equ ivalent pore size) Hg concentrations remained relatively constant throughou t the system, ranging from the detection limit (< 0.4 ng/L) to 2.4 ng/L. To tal Hg (dissolved plus colloidal suspended sediment) concentrations ranged from the detection limit at the site below Shasta Dam in September 1996 to 81 ng/L at the Colusa site in January 1997, demonstrating that colloidal se diment plays an important role in the downriver Hg transport. Sequential ex tractions of colloid concentrates indicate that the greatest amount of Hg a ssociated with sediment was found in the "residual" (mineral) phase with a significant quantity also occurring in the "oxidizable" phase. Only a minor amount of Hg was observed in the "reducible" phase. Dissolved Hg loads rem ained constant or increased slightly in the downstream direction through th e study area, whereas the total inorganic Hg load increased significantly d ownstream especially in the reach of the river between Bend Bridge and Colu sa. Analysis of temporal variations showed that Hg loading was positively c orrelated to discharge.