Effects of two sorbents applied to mercury-contaminated river sediments on bioaccumulation in and detrital processing by Hyalella azteca

Mercury (Hg) released into the environment often accumulates to high concentrations in sediments, creating a potential risk to aquatic wildlife. The in situ application of sorbents such as activated carbon is one promising option for reducing the bioavailability of sediment-bound Hg. The present study evaluates the influence of contact time of two sorbents (Sedimite(A (R)) and Cowboy(A (R)) biochar) applied to sediment taken downstream and upstream of a historic Hg discharge into the South River (Virginia, USA) on bioaccumulation in and detrital processing by Hyalella azteca. Hg bioaccumulation decreased when sediments were mixed with both sorbents, but their respective efficiency depended on their initial particle size and contact time. Hyalella showed a slight increase in detrital processing and substantial increase in Hg bioaccumulation when exposed to contaminated relative to uncontaminated sediment. Sedimite(A (R)) adversely affected detrital processing but reduced Hg bioaccumulation. In contrast, Cowboy(A (R)) biochar did not impact detrital processing but appeared to decrease bioaccumulation less effectively than Sedimite(A (R)). This difference in sorbent efficacy lessened with duration. It remains unclear whether the Sedimite(A (R))-induced reduction in detrital processing is substantially worse than that associated with natural fines settling on detritus in depositional reaches of rivers. The differences in efficacy of the two materials in reducing bioavailability suggest the need for further studies addressing both the mechanisms causing the reduction in Hg bioavailability as well as associated ecological risks prior to field application of these materials at the larger scale.