Mercury Isotope Fractionation in the Subsurface of a Hg(II) Chloride-Contaminated Industrial Legacy Site

Abstract

To understand the transformations of mercury (Hg) species in the subsurface of a HgCl₂-contaminated former industrial site in southwest Germany, Hg isotope analysis was combined with an investigation of Hg forms by a four-step sequential extraction protocol (SEP) and pyrolytic thermodesorption. Data from two soil cores revealed that the initial HgCl₂ was partly reduced to metallic Hg(0) and that Hg forms of different mobility and oxidation state coexist in the subsurface. The most contaminated sample (K2-8, 802 mg kg^-1 Hg) had a bulk δ²⁰²Hg value of around -0.43 ± 0.06‰ (2SD), similar to published average values for industrial Hg sources. Other sample signatures varied significantly with depth and between SEP pools. The most Hg-rich samples contained mixtures of Hg(0) and Hg(II) phases, and the water-extractable, mobile Hg pool exhibited heavy δ²⁰²Hg values of up to +0.18‰. Sequential water extracts revealed slow dissolution kinetics of mobile Hg pools, continuously releasing isotopically heavy Hg into solution. This was further corroborated by heavy δ²⁰²Hg values of groundwater samples. Our results demonstrate that the Hg isotope signature of an industrial contamination source can be significantly altered during the transformations of Hg species in the subsurface, which complicates source tracing applications but offers the possibility of using Hg isotopes as process tracers in contaminated subsurface systems.

Figure 1

Data for soil cores K2 and K3 and groundwater wells B3, B8, and B10. Colored labels represent samples chosen for isotope analysis. Symbols are located at the vertical midpoints of the samples, which span 20 cm each. (a) Bulk Hg concentrations in micrograms per gram for soil samples and micrograms per liter for groundwater samples (∼10 m depth). (b) Bulk δ²⁰²Hg data. (c) Bulk Δ¹⁹⁹Hg data. The gray dashed lines represent the mean of the estimated source signature for industrial Hg (see the text). 2SD values represent the reproducibility of the “ETH Fluka” secondary standard of the respective measurement session.

Figure 2

Pyrolytic thermodesorption (PTD) curves of the samples of cores K2 (blue) and K3 (red). The peak heights were normalized to the highest peak of each sample. The dashed gray lines serve as guides to the eye. Hg release around 100 °C indicates the presence of elemental Hg(0), while Hg release around 200 °C can be explained by different Hg(II) species. See Figure S1 for comparison with reference material curves.

Figure 3

Sequential extraction results of soil cores (a) K2 and (b) K3. In the left panels, the total Hg concentrations of the samples (black lines) are underlaid by bar plots representing the relative distribution of Hg in the sample among extracted fractions F1–F4. Note that the total concentrations are almost a factor 10 higher in panel a than in panel b. The right panels show the δ²⁰²Hg values of all sequentially extracted pools. Black circles are the calculated means of the bulk samples based on the respective pool sizes. 2SD values are based on the “ETH Fluka” secondary standard reproducibility during the measurement session. The gray dotted line represents the mean of the estimated source signature for industrial Hg (see the text).

Figure 4

(a) Hg concentrations and (b) δ²⁰²Hg values of seven consecutive sequential water extracts of sample K2-8 (F1A–F1G). The concentration axes in panel a refer to the extracted solid phase concentration (left, bulk soil total Hg of 802 μg g^–1) and the solution concentration in extract (right) with a Hg(0) solubility of ∼0.06 mg L^–1 indicated as a pink dashed line. The size of the symbols in panel b corresponds to the relative pool sizes of the extracted Hg. F1D had an equilibration time that was longer than those of the other extracts and is colored pink as a potential outlier (see the text). The gray dashed line at −0.43‰ (δ²⁰²Hg) indicates the bulk soil signature of K2-8.

Figure 5

δ²⁰²Hg values of sequential extracts normalized to the bulk soil δ²⁰²Hg values of the respective sample for cores K2 and K3. The 2SD value is propagated from the uncertainty of the individual δ²⁰²Hg values based on the “ETH Fluka” secondary standard reproducibility during the measurement session.