Background/Objectives: Mercury (Hg) is present as contaminants in sediment at numerous sites around the world. Monitored natural recovery (MNR) processes have the potential to reduce bioavailability and potential risks associated with Hg in sediment.
MNR relies significantly on the continued deposition of new sediment to reduce the bioavailability of contaminants, but sequestration processes can also play an important role in reducing bioavailability. MNR processes may be sufficient to control potential risks, however, in other areas, it may be necessary to enhance natural recovery processes through the use of amendments or thin-layer capping to achieve a greater level of risk reduction or to achieve this risk reduction in a shorter time frame than would be possible by relying on natural processes alone. Gaining acceptance for implementation of this type of enhanced monitored natural recovery (EMNR) remedy requires that the physical, biological and chemical processes that reduce the bioavailability of contaminants be understood and demonstrated for conditions representative of those at a particular site.
Approach/Activities: Laboratory treatability testing was conducted at Geosyntec’s SiREM Laboratory to demonstrate the potential for a variety of in situ amendments to reduce concentrations of Hg and methylmercury (MeHg) in water in contact with sediment from a contaminated sediment site. The concentrations of Hg and MeHg in the water in contact with the sediment in the lab are a surrogate for porewater concentrations and are expected to be an indicator of bioavailability. Sediment containing Hg and MeHg was mixed with surface water to produce a homogenous slurry that was added to identical one-liter glass jars which were then placed in an anaerobic glove box. Ten different amendments or combinations of amendments were added to the microcosm jars. Controls were also prepared to provide a comparison with the amended microcosms. The jars were mixed over a period of eight weeks and then the concentrations of PCB, Hg and MeHg in the aqueous phase were measured and the concentrations in the treated microcosms were compared with the concentrations in the controls. Additional work was conducted at the University of Guelph to evaluate the bioavailability of mercury that had been sequestered using similar amendment combinations based on a simulated fish gut extraction assay.
Results/Lessons Learned: The results of the testing demonstrated that significant reductions in the concentration of Hg and MeHg in water in contact with the sediment could be obtained with specific the amendments evaluated in the test. Iron based amendments when used along with activated carbon were successful in reducing the concentrations of Hg and MeHg relative to controls up to 91% and 94% respectively. Bioavailability testing also confirmed reductions in bioavailability of mercury using these amendment combinations.
The results of this treatability testing demonstrated the site specific capability of a combination of activated carbon and iron based amendments in reducing porewater concentrations and bioavailability of Hg and MeHg in sediment.