Assessment of Lead and Beryllium Sorption To Exposed Stream Channel Sediment Under Varying Freshwater Channel Conditions

Abstract

Purpose: Beryllium (Be) and lead (Pb) sorption is important to the utility of the radioisotopes of these elements for sediment fingerprinting. I examined the sorption of Be and Pb to exposed fluvial sediment under varying chemical conditions representative of freshwater streams draining two distinct Critical Zone environments in the eastern United States: the mid-Atlantic piedmont and heavily glaciated Great Lakes region. Materials and Methods: Batch experiments were completed using well-characterized in-stream deposit sediments collected from these two systems and varying solutions to reflect background and elevated levels of iron oxide in the form of goethite across times ranging from 0.25 to 360 h. The mid-Atlantic piedmont sediment had further treatments testing dissolved organic carbon (up to 11.86 mg L-1) and increased sediment to solution ratio (up to 8000 mg: 1 L) effects on Pb and Be sorption. Results and Discussion: Beryllium partition coefficients (Kd) ranged from a log Kd of 1.46 to 3.48 L kg-1 and Pb ranged from 0 to 5.03 L kg-1 across all treatments displaying several noticeable patterns. Two-stage sorption was observed such that sorption increased over time across all treatments and substrates. Goethite additions either enhanced or reduced sorption relative to the base treatment depending on the original sediment and mixing time. Lead sorption with the addition of 100 mg of goethite increased during shorter mixing times before being surpassed by the base treatment at longer mixing times for both the mid-Atlantic piedmont and glaciated Great Lakes sediment. Beryllium sorption was increased with the mid-Atlantic piedmont sediment whereas it was primarily decreased with the glaciated Great Lakes sediment. The 1 mg of goethite generally showed equal to or slightly enhanced sorption relative to the base treatment of both Pb and Be with the exception of Pb sorption to the Great Lakes sediment. The highest DOC concentration that I tested (11.86 mg L-1) retained a greater amount of Be and Pb in solution compared to other treatments after 360 h. Increasing the sediment to solution ratio showed decreased partition coefficients across all analogous times for Be compared to the base treatment whereas Pb sorption surpassed the unaltered treatment after 24 h. Conclusions: It is not recommended to use either 7Be or 210Pb in fluvial systems with high background concentrations of DOC because the DOC was shown to inhibit sorption to sediment surfaces and it could produce erroneous results in sediment fingerprinting studies unless that inhibition or loss is accounted for. The goethite treatments produced mixed results and further research is needed to parse out conditions that enhance or inhibit the sorption of fallout radionuclides in the presence of stream sediments with varying amounts of organic matter occlusion of surface binding sites. Increased sediment to solution ratios increased sorption of Pb suggesting that Pb would be a conservative tracer in fluvial systems with high sediment delivery. These results suggest that fluvial sediment mineralogy, organic matter concentration, and biogeochemical cycling of common stream chemical constituents may play a role in the mobilization or retention of these two trace metals and alter their utility for sediment fingerprinting.