Demystifying mercury geochemistry in contaminated soil-groundwater systems with complementary mercury stable isotope, concentration, and speciation analyses.

D S Mclagan,H Biester,J G Wiederhold,L Schwab,J Pietrucha,S M Kraemer,L Chen

doi:10.1039/d1em00368b

Abstract

Interpretation of mercury (Hg) geochemistry in environmental systems remains a challenge. This is largely associated with the inability to identify specific Hg transformation processes and species using established analytical methods in Hg geochemistry (total Hg and Hg speciation). In this study, we demonstrate the improved Hg geochemical interpretation, particularly related to process tracing, that can be achieved when Hg stable isotope analyses are complemented by a suite of more established methods and applied to both solid- (soil) and liquid-phases (groundwater) across two Hg2+-chloride (HgCl2) contaminated sites with distinct geological and physicochemical properties. This novel approach allowed us to identify processes such as Hg2+ (i.e., HgCl2) sorption to the solid-phase, Hg2+ speciation changes associated with changes in groundwater level and redox conditions (particularly in the upper aquifer and capillary fringe), Hg2+ reduction to Hg0, and dark abiotic redox equilibration between Hg0 and Hg(II). Hg stable isotope analyses play a critical role in our ability to distinguish, or trace, these in situ processes. While we caution against the non-critical use of Hg isotope data for source tracing in environmental systems, due to potentially variable source signatures and overprinting by transformation processes, our study demonstrates the benefits of combining multiple analytical approaches, including Hg isotope ratios as a process tracer, to obtain an improved picture of the enigmatic geochemical behavior and fate of Hg at contaminated legacy sites.

Highlights

Mercury (Hg) is a deviously elusive element that has and continues to effectively obscure the true nature of its exact environmental speciation, oxidation state, and the processes controlling its biogeochemical cycling from the intrigue of scientists
When examining legacy sites contaminated by Hg, it is essential to identify and characterise the location of point or area source(s) as both the magnitude and speciation of the initial contamination inputs are strongly linked to the stability of Hg and its ability to undergo different transformation processes at the sites.[67]
The most obvious means of identifying source Hg at contaminated sites is via “grey” and peer-reviewed literature on the location of historical Hg usage. This has the potential to detail input locations, historical use and contamination inventories, and composition of Hg bearing compounds used during the active industrial period

Summary

Introduction

Mercury (Hg) is a deviously elusive element that has and continues to effectively obscure the true nature of its exact environmental speciation, oxidation state, and the processes controlling its biogeochemical cycling from the intrigue of scientists. Paper represents its most toxic form.[4,5] Aside from organic species, elemental Hg (Hg0) and inorganic Hg2+ species are commonly encountered in the environment. Inorganic Hg1+ species can occur but are generally believed to play a minor role in natural systems, mainly as shorter-lived intermediates in redox cycling between Hg0 and Hg2+ species.[5,6,7]

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