The Multi-Compartment Hg (mercury) Modeling and Analysis Project (MCHgMAP) is an international multimodel research initiative intended to simulate and analyze the geospatial distributions and temporal trends of environmental Hg to inform effectiveness evaluations of two multilateral environmental agreements (MEAs): the Minamata Convention on Mercury (MC) and the Convention on Long-Range Transboundary Air Pollution (LRTAP). This MCHgMAP overview paper presents its science objectives, background, and rationale; experimental design (multimodel ensemble (MME) architecture, inputs and evaluation data, simulations, and reporting framework); and methodologies for the evaluation and analysis of simulated environmental Hg levels. The primary goals of the project are to facilitate detection and attribution of recent (observed) and future (projected) spatial patterns and temporal trends of global environmental Hg levels and identification of key knowledge gaps in Hg science and modeling to improve future effectiveness evaluation cycles of the MEAs. The current advances and challenges of Hg models, emission inventories, and observational data are examined, and an optimized multimodel experimental design is introduced to address the key policy questions of the MEAs. A common set of emissions, environmental conditions, and observation datasets is proposed (where possible) to enhance the MME comparability. A novel harmonized simulation approach between atmospheric, land, oceanic, and multimedia models is proposed to account for the short- and long-term changes in secondary Hg exchanges and to achieve mechanistic consistency of Hg levels across environmental matrices. A comprehensive set of model experiments is proposed and prioritized to ensure systematic analysis and participation of a variety of models from the scientific community.

A study was conducted to determine in situ rates of Fe(II) oxidation and Fe(III) precipitation along a 5.0 m reach of a ferruginous groundwater discharge zone under two distinct conditions; (i) the natural state featuring abundant flocculent mats of bacteriogenic iron oxides (BIOS) produced by Fe(II)-oxidizing bacteria, and (ii) after a manual washout of the streambed to remove the microbial mat. Examination of mat samples by differential interference contrast light microscopy revealed tangled meshworks of filamentous Leptothrix sheaths and helical Gallionella stalks intermixed with fine-grained hydrous ferric oxide (HFO) precipitates. The greatest accumulation of BIOS mat was 1.0 m downstream of the groundwater spring. Redox potential (Eh) increased sharply from 200 mV to over 300 mV over the last 2.0 m of the reach. Similarly, dissolved oxygen increased from < 10% saturation to almost 100% saturation over the last 2.0 m of the reach, whereas pH increased from 6.4 to 7.3. Pseudo-first-order rate constants determined on the basis of analytical solutions to sequential partial differential advection-dispersion-reaction equations for the linear Fe(II)→Fe(III)→HFO reaction network yielded in situ Fe(II) oxidation rate constants (k_ox) of 1.70 ± 0.20 min⁻¹ in natural conditions and 0.48 ± 0.14 min⁻¹ after washout. Corresponding Fe(III)-precipitation rates (k_p) before and after washout were 3.45 ± 0.10 min⁻¹ and 0.90 ± 0.01 min⁻¹, respectively. These values for k_ox and k_p are higher than estimates obtained from closed batch microcosm and laboratory experiments, underscoring the crucial dependence of in situ Fe(II) oxidation and Fe(III) precipitation rates on advective and dispersive mass transport. The results also highlight the influence that BIOS microbial mats exert on the reaction kinetics of the multiple heterogeneous reactions contributing not only to Fe(II)/Fe(III) redox transformations in groundwater discharge zones, but also the precipitation of HFO.

Differentiating biotic and abiotic processes in nature remains a persistent challenge, specifically in evaluating microbial contributions to geochemical processes through time. Building on previous work reporting that biologically-influenced systems exhibit stronger long-range correlation than abiotic systems, this study evaluated the relationship between long-range correlation of redox potential and oxidation rates of circumneutral microaerophilic bacterial Fe(II) oxidation using a series of batch microcosms with bacteriogenic iron oxides (BIOS). Initial detrended fluctuation analysis (DFA) scaling exponents of the abiotic microcosms were lower (ca. 1.20) than those of the biotic microcosms (ca. 1.80). As Fe(II) oxidation proceeded, correlation strength decayed as a logistic function of elapsed reaction time, exhibiting direct dependence on the free energy of reaction. Correlation strength for all microcosms decayed sharply from strong correlation to uncorrelated fluctuations. The decay rates are greater for abiotic microcosms than biotic microcosms. The ΔG_m relaxation edges for biotic microcosms were lower, indicating less remaining free energy for Fe(II) oxidation than abiotic systems, with the implication that biologically-catalyzed reactions are likely more energetically efficient than abiotic reactions. These results strengthen the case for employing novel DFA techniques to distinguish in situ microbial metabolic activity from abiotic processes, as well as to potentially differentiate metabolisms among different chemoautotrophs.