Mercury speciation analyses in HgCl2-contaminated soils and groundwater—Implications for risk assessment and remediation strategies

Since the 19th century, mercury(II)chloride (HgCl2) has been used on wood impregnation sites to prevent wooden poles from decay, leaving behind a legacy of highly contaminated soil/aquifer systems. Little is known about species transformation and mobility of HgCl2 in contaminated soils and groundwater. At such a site the behaviour of HgCl2 in soils and groundwater was investigated to assist in risk assessment and remediation. The soil is low in organic carbon and contains up to 11,000 mg Hg/kg. Mercury (Hg) concentrations in groundwater decrease from 230 to 0.5 μg/l within a distance of 1.3 km. Hg species transformations in soil and aqueous samples were analysed by means of solid-phase Hg pyrolysis and CV-AAS. In aqueous samples, Hg species were distinguished between ionic/reactive Hg and complex-bound Hg. Potential mobility of Hg in soils was studied through batch experiments. Most Hg in the soil is matrix-bound HgCl2, whereas in the aquifer secondary formation to Hg0 could be observed. Aqueous Hg speciation in groundwater and soil solutions shows that an average of 84% of soluble Hg exists as easily reducible, inorganic Hg species (mostly HgCl2). The proportion of complex-bound Hg increases with distance due to the transformation of inorganic HgCl2. The frequent occurrence of Hg0 in the aquifer suggests the formation and degassing of Hg0, which is, in addition to dilution, an important process, lowering Hg concentrations in the groundwater. High percentage of mobile Hg (3–26%) and low seepage fluxes will result in continuous Hg release over centuries requiring long-term groundwater remediation. Results of soluble Hg speciation suggest that filtering materials should be adapted to ionic Hg species, e.g. specific resins or amalgamating metal alloys.