SEDIMENT DATING FOR POLLUTANT INPUT RECONSTRUCTION AND ECOSYSTEM RECOVERY ASSESSMENT

Sediment Dating using atmospherically delivered radionuclides, either as steady state tracers (e.g., 210Pbxs) or as pulse input tracers (e.g., 137Cs, 239,240Pu) are widely used to reconstruct pollutant input history to a specific location or an ecosystem. In addition, if sufficient sediment cores are collected for ecosystem-wide assessments, it is possible to evaluate recovery rates from natural attenuation. One of such studies was possible for the Penobscot River basin, which was heavily polluted by Mercury (Hg) in the late 1960s by a chlor-alkali production facility, HoltraChem (Santschi et al., 2017. STOTEN, 596–597, 351–359). Sediment accumulation rates were derived using detailed profiles of total Hg concentrations and radionuclide activities (137Cs, 239,240Pu, 210Pb) in 56 sediment cores. It was observed that lateral remobilization by sediment resuspension was the main dispersal mechanism that controlled system-wide recovery and natural attenuation rates over decades, despite rapid localized initial recovery of Hg near the outfall.

Another important factor is a knowledge how chemically mobile or immobile the pollutant is. Examples, besides the one discussed above, include point-source deposition 239,240Pu and dioxins that also indicate strong binding to natural organic matter phases when only pore water diffusion is considered. However, there is evidence for colloidal transport through sediment resuspension for Hg, Pu and dioxins, which would need to be considered as well.