Organic Matter in Rain: An Overlooked Influence on Mercury Deposition

The importance of Hg emissions for deposition will be scrutinized in the future as new legislation to control emissions of Hg to the atmosphere comes into effect. We show that mercury (Hg) concentrations in rainfall are closely linked to organic matter (OM) with consistent Hg/TOC ratios over large spatial scales decreasing from that in an open field (OF, 1.5 μg g^–1) to that in throughfall (TF, 0.9 μg g^–1). The leaf area index was positively correlated with both TF [Hg] and total organic carbon ([TOC]), but not the Hg/TOC ratio. This study shows that the progression in the Hg/TOC ratio through catchments starts in precipitation with Hg/TOC_bulk dep > Hg/TOC_soil water > Hg/TOC_streamwater. These findings raise an intriguing question about the extent to which it is not just atmospheric [Hg] but also OM that influences [Hg] in precipitation. This question should be resolved to improve the ability to discern the importance of changing global Hg emissions for deposition of Hg at specific sites

Depth profiles of MeHg pore water concentrations sampled 0–10 cm, 10–20 cm, 20–30 cm and 30–40 cm below the groundwater level at the LowS (upper panel) and HighS (lower panel) plots.

<p>The vertical axis refers to the actual peat depths at which the water was sampled. Each bar represents the mean value (±SE) of MeHg from two plots for the particular treatment. For each treatment (LowS or HighS), significant differences (p<0.05) in the average pore water MeHg concentration between the sampling dates are designated by different upper case letters. Different lower case letters indicate significant differences (p<0.05) in MeHg concentrations between depths for each treatment and sampling date.</p

Weakly fluctuations of A: SO₄²⁻ (filled symbols) and pore water MeHg (open symbols) concentrations, and B: the groundwater level between June 16^th and September 22^nd for both HighS plots.

<p>The concentration of SO₄²⁻ in the LowS plots was below the detection limit during the sampling period. The dotted line in A indicates the detection limit for SO₄²⁻ (0.5 mg L⁻¹). The pH in the pore water for the two HighS plots are shown in panel B by the two lines without labels. The arrows in B refer to the date on which the plots were fertilised with SO₄²⁻.</p

Weakly variation of dissolved organic matter (DOC) for HighS plots (filled symbols) and for LowS plots (open symbols).

<p>The arrows refer to the date on which the plots were fertilised with SO₄²⁻.</p

Weakly variations in A: concentrations of MeHg in pore water collected 10 cm below the groundwater surface at the time of sampling from HighS plots (filled symbols) or LowS plots (open symbols), as well as in the main stream draining the mire where the study was conducted (shaded squares).

<p>B: mean groundwater level from the HighS plots (filled symbols) or LowS plots (open symbols). The bars indicate SE. The arrows in A refer to the date on which the plots were fertilised with SO₄²⁻.</p