A BENCHMARKING STUDY ON HEAVY METAL CYCLING IN LAKE SEDIMENTS

Upstream mining activities are known to degrade water quality on regional and global scales. The sediments of Lake Coeur d’Alene, Idaho have been similarly contaminated by iron and other heavy metals. This benchmark study is developed after Sengör et al. (2007), where a reactive-diffusive model of metal transport in the lake sediments was presented. Key processes affecting the biogeochemical cycling of heavy metals in lake sediments include microbial reductive dissolution of iron hydroxides (e.g. ferrihydrite), the release of sorbed metals into lake water, reaction of these metals with biogenic sulfide to form sulfide minerals, and sedimentation driving the burial of ferrihydrite and other minerals.

Four scenarios of the benchmark problem are considered. The base-case simulation consists of the development of a multicomponent biotic reaction network with multiple terminal electron acceptors, Fickian diffusive transport, mineral precipitation and dissolution, and aqueous and surface complexation (electrostatic double-layer model) for a 1D vertical sediment column. Variations of the benchmark problem consist of adding sedimentation (2 and 4 cm/yr) with and without compaction. Sedimentation is approximated in the model by advection of solids and pore water downward from the top of the modeled sediment column. Compaction is approximated by decreasing the advection rate with depth. Another problem variation consists of replacing microbial Fe(III) reduction with abiotic Fe(III) reduction by biogenically produced sulfide. This benchmark study uses four different reactive transport modeling codes - TOUGHREACT, CrunchFlow, PHREEQC and PHT3D- to simulate the four scenarios.

For the base-case and the abiotic Fe(III) reduction scenarios, the four codes produce identical results for alkalinity, pH, and electron acceptor concentrations with depth. In particular, an increase in alkalinity and heavy metal concentrations are observed correspondingly with a sequential decrease in the terminal electron acceptor concentrations with depth. Results from both biotic and abiotic cases with different sedimentation velocities (2 and 4 cm/yr) without compaction are also well captured. Differences in the results are observed with the compaction scenario, as it is implemented differently by the different codes.