Optimization of DGT methyl mercury recovery, bank leaching assessment and evaluation of stabilization efforts on mercury fate and transport in freshwater systems

Bioavailable mercury (Hg) in the environment is methylated by bacteria to form methyl mercury (MeHg) a bioaccumulative acute neurotoxin. The ability to quantify Hg and MeHg in sediment pore water may allow for better understanding of mercury mobility, bioavailability and toxicity in the environment. Flooding events in South River, VA have been associated with leaching of pore water total mercury from the contaminated river banks, creating a potentially significant source of mercury to the system. In this research the mobility and availability of Hg in these river banks is assessed through diffusion gradient in thin film (DGT) devices to measure pore water Hg and MeHg.

This research had four main specific objectives. The first objective was to improve the recovery of MeHg from the DGT devices to ensure quantitative recovery. The second objective was to apply DGTs to measure Hg from the pore water leaching from river banks in the South River during inundation/drainage cycles associated with storm events and the associated potential for methylation by assessing redox conditions and MeHg during these cycles. The third objective was assessment of the ability of stabilization and capping of the river bank to reduce Hg flux. The cap was composed of layers of biochar as a Hg sorbent as well as sand and armoring material. The final objective was to predict long term effects of storm events and bank leaching with and without the stabilization and capping.

Studies were done to improve the recovery of MeHg from the DGT resin, resulting in method for extraction of MeHg that improves currently used poorly reproducible extraction recovery in 1-56% range to a reproducible recovery of 91±9%.

Field sampling was done at the Constitution Park and North Park in 2015 during baseline conditions as well as during bank drainage after inundation by a storm event. The results demonstrated that storm event associated leaching introduced an order of magnitude increase in pore water total mercury due to drainage from contaminated banks. Stabilization of the bank and placement of a composite cap led to reduction of pore water concentrations and likely Hg fluxes by 1-2 orders of magnitude, depending on the initial level of contamination at different locations.

Lastly, the inundation and drainage cycle was simulated using a commercial finite element package, COMSOL®. The simulations demonstrated that leaching predominately (90%) occurs near the bank-water interface and allowed estimation of leaching/seepage fluxes. These results were used to simulate long term chemical containment performance of the composite cap during regular 3 and 6 ft flood events for the next 100 years using CapSim®, a modeling environment designed to simulate contaminant transport at the sediment-water interface. The composite cap was predicted to be effective in reducing the pore water concentration and Hg flux at the cap-water interface by more than 93.5% compared to that estimated without a cap layer in place. The maximum Hg flux associated with the flood events was approximately 0.6 µg/cm^2/yr in the period of 30 to 100 years after the cap implementation.