RESUMEN
The South River located in the city of Waynesboro, Virginia, contains mercury (Hg) contamination due to historical releases from an industrial facility operating between 1929 and 1950. In 2015, two sampling events were conducted in two of the contaminated bank regions (Constitution Park and North Park) to evaluate non-particulate total mercury (THg) and methylmercury (MeHg) concentrations in bank interstitial waters during river base flows and during bank drainage after flooding events. Porewater THg and MeHg at the bank-water interface were measured using diffusive gradient in thin-film devices (DGTs). The results showed THg mercury concentrations during bank drainage were approximately a factor of 3 higher than during base flow conditions. To have a better understanding of the parameters that control Hg leaching, a series of laboratory experiments were designed using South River sediments. The field and laboratory assessment showed that drainage/inundation cycles can lead to high THg concentration leachate from contaminated sediment due to increased partitioning from solids under oxic bank conditions and mobilization by the drainage waters. The results also demonstrated that methyl mercury concentrations at the bank-water interface are highest under base flow when conditions are more reduced due to the absence of oxic water exchange with the surface water. A remedial approach was implemented involving partial removal of surficial sediments and placement of biochar (to reduce non-particulate THg) and an armoring layer (to reduce erosion). DGT Measurements after bank stabilization showed THg decreased by a factor of ~200 and MeHg concentration by a factor of more than 20.
RESUMEN
Particle-specific properties, including size and chemical speciation, affect the reactivity of mercury (Hg) in natural systems (e.g., dissolution or methylation). Here, terrestrial, river, and marine sediments were size-fractionated and characterized to correlate particle-specific properties of Hg-bearing solids with their bioavailability potential and measured biomethylation. Marine sediments contained â¼20-50% of the total Hg in the <0.5 µm size fraction, compared to only 0.5 and 3.0% in this size fraction for terrestrial and river sediments, respectively. X-ray absorption spectroscopy (XAS) analysis indicated that metacinnabar (ß-HgS) was the main mercury species in a marine sediment, whereas organic Hg-thiol (Hg(SR)2) was the main mercury species in a terrestrial sediment. Single-particle inductively coupled plasma time-of-flight mass spectrometry analysis of the marine sediment suggests that half of the Hg in the <0.5 µm size fraction existed as individual nanoparticles, which were ß-HgS based on XAS analyses. Glutathione-extractable mercury was higher for samples containing Hg(SR)2 species than ß-HgS species and correlated well with the amount of Hg biomethylation. This particle-scale understanding of how Hg speciation and particle size affect mercury bioavailability potential helps explain the heterogeneity in Hg methylation in natural sediments.
