A biogeochemical reactive transport model was developed to predict the transport and fate of arsenic and mercury in sediment cap environments. The model included heterogeneous and homogeneous equilibrium speciation reactions (including organo-metal complexation), as well as, kinetic-based reactions describing biodegradation of organic carbon, redox transformations, mercury methylation, and demethylation. Simulations were performed for both fresh water and salt water sediment scenarios, with discharge of either anaerobic or aerobic ground water. The simulations were designed to (1) examine the evolution of dominant contaminant sequestration mechanisms, (2) assess their impact on ground water contaminant mass flux to surface water over time, and (3) identify geochemical environments where capping is effective.
Model predictions show that a sediment cap-ground water system is a dynamic geochemical environment in which mechanisms responsible for metals attenuation change over time. In addition, sequestration processes strongly depend on the physical and chemical properties of the system, and affect arsenic and mercury differently. Arsenic may initially be adsorbed onto iron oxides phases within the cap, but these minerals partially dissolve over time due to changes in redox state of the cap. In cases where sufficient sulfide is generated in the biologically-active layer, a downward propagating front of iron- and/or arsenic-sulfide minerals can sequester arsenic. Although mercury is also sequestered in this front (as HgS), dissolved mercury and methylmercury concentrations are potentially higher when biological sulfate reduction is greatest.