Simulation of mercury methylation and demethylation coupled to

Simulation of mercury methylation and demethylation coupled to oxidation-reduction reactions in sediments of Delta tributaries Stefanie Helmrich – University of California, Merced Dimitri Vlassopoulos – Anchor QEA LLC Charles Alpers – U. S. Geological Survey Peggy A. O‘Day – University of California, Merced This material is based upon work supported by the Delta Stewardship Council Delta Science Program. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Delta Stewardship Council.

Mercury Cycle Alpers C. N. , Hunerlach M. P. , May J. T. and Hothem R. L. (2005) Mercury Contamination from Historical Gold Mining in California. Publ. US Geol. Surv. 61, 7.

Conceptual Model for Hg Reaction Model Shown are selected processes Software: PHREEQC (Version 3), U. S. Geolological Survey Predecessor of model published in: Bessinger B. a. , Vlassopoulos D. , Serrano S. and O’Day P. a. (2012) Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury. Aquat. Geochemistry 18, 297– 326.

Conceptual Model for Hg Reaction Model Shown are selected processes Software: PHREEQC (Version 3), U. S. Geolological Survey Degradation of organic matter Predecessor of model published in: Bessinger B. a. , Vlassopoulos D. , Serrano S. and O’Day P. a. (2012) Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury. Aquat. Geochemistry 18, 297– 326.

Degredation of organic matter via bacteria Reactions involving organic matter degradation Paraska, D. , Hipsey, M. , Salmon, U. . 2014, EMS,

Conceptual Model for Hg Reaction Model Shown are selected processes Software: PHREEQC (Version 3), U. S. Geolological Survey Reactions involving minerals Predecessor of model published in: Bessinger B. a. , Vlassopoulos D. , Serrano S. and O’Day P. a. (2012) Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury. Aquat. Geochemistry 18, 297– 326.

Reactions involving minerals Dissolution and precipitation of minerals Paraska, D. , Hipsey, M. , Salmon, U. . 2014, EMS,

Conceptual Model for Hg Reaction Model Shown are selected processes Software: PHREEQC (Version 3), U. S. Geolological Survey Reactions involving mercury Predecessor of model published in: Bessinger B. a. , Vlassopoulos D. , Serrano S. and O’Day P. a. (2012) Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury. Aquat. Geochemistry 18, 297– 326.

Reactions involving and influencing mercury

Coupling of mercury transformation to reductionoxidation reactions Model for organic matter degredation via reduction-oxidation reactions Van Cappellen P. and Wang Y. (1996) , Canavan R. W. et. al. (2006) Coupling of OM degradation and mercury in our model Process Overall organic matter degredation Sulfate reduction Mercury methylation Methylmercury demethylation Rate Law in Model

Simulations Objective: test model components and model parameter values Simulated system • Time-series experiments under controlled conditions • Sediments collected in wetlands • Batch experiment • Brought to reducing conditions prior to time series • Addition of sulfate

Coupling of mercury and oxidation-reduction reactions Process Overall organic matter degredation Sulfate reduction Mercury methylation Methylmercury demethylation Observed and simulated Sulfate Rate Law in Model

Coupling of mercury and oxidation-reduction reactions Process Overall organic matter degredation Sulfate reduction Mercury methylation Methylmercury demethylation Observed and simulated Sulfate Rate Law in Model

Simulation of whole system Aqueous species, p. H, minerals simulated by model The more chemical parameters match the better!

Hg methylation and Me. Hg demethylation Process Overall organic matter degredation Sulfate reduction Mercury methylation Methylmercury demethylation Rate Law in Model

Addition of Hg mineral

Summary Model need • • Improves our capability to quantify methylmercury production Tool to deduce underlying mechanisms Can help to improve transferability of laboratory data to field Tool forecasting • Limitations due to large number or parameters and uncertainties connected with them Next steps • • Sensitivity analysis Test model for sites with different environmental conditions Simulation of remediation measures Inclusion of transport to water column

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Parameterization and uncertainties Number of reviewed studies 38 0. 0003 to 6 31 0. 001 - 63

Parameterization and uncertainties Number of reviewed studies 38 0. 0003 to 6 31 0. 001 - 63 Uncertainties

Parameterization and uncertainties Type of parameter Parameter Independent Thermodynamic constants for aqueous reactions and minerals Constrained Rate constants for: methylation, demethylation, mineral dissolution and precipitation, total organic matter degradation via primary and secondary redox reactions or separate constants for primary redox reactions if possible, Limiting concentrations for redox reactions Initial conditions Temperature at time 0, p. H at time 0, Eh at time 0, Fe(II) at time 0, Moles of Minerals (e. g. AVS), Type of parameter: Independent parameters: Reactive value determined based experimental field 0, data that OM at time 0, on Reactive Hg atortime are independent of the data used for. Me. Hg the actual simulations. Reactive at time 0, Sulfate at time 0, Sulfide Constrained parameters: at range data determined based on experimental or field data timeof 0, that are independent of the data used for the actual simulations. Initial conditions: based on field data

Parameterization and uncertainties Hg methylation (forward transformation) Me. Hg demethylation (backward transformation)

Coupling of mercury and oxidation-reduction reactions Process Overall organic matter degredation Sulfate reduction Mercury methylation Methylmercury demethylation Additional decisions to make: • Which minerals to include • Speed of mineral reactions • Limiting concentrations for reactions Rate Law in Model

Step 4: Literature review of (de)methylation rate constants Number of studies using isotope tracers to calculate rate constants Methylation 36 Demethylation 20 Influence of incubation time on rate constants

Review of methylation rate constants

Steps in mercury model develpoment A: Batch reaction model (no transport) Model development: 1: Iron and sulfur chemistry (minerals) 2: Pore water evolution driven by organic matter degradation, and organic matter fractionation 3: Rates of inorganic and organic Hg transformation 4: Inorganic and organic Hg available for transformation B: 1 -dimensional reaction transport model