New Insights on Ecosystem Mercury Cycling Revealed by Stable Isotopes of Mercury in Water Flowing from a Headwater Peatland Catchment

Stable isotope compositions of mercury (Hg) were measured in the outlet stream and in soil cores at different landscape positions in a 9.7-ha boreal upland-peatland catchment. An acidic permanganate/persulfate digestion procedure was validated for water samples with high dissolved organic matter (DOM) concentrations through Hg spike addition analysis. We report a relatively large variation in mass-dependent fractionation (delta Hg-202; from -2.12 to -1.32 parts per thousand) and a smaller, but significant, variation of mass independent fractionation (Delta Hg-199; from -0.35 to -0.12 parts per thousand) during two years of sampling with streamflow varying from 0.003 to 7.8 L s(-1). Large variations in delta Hg-202 occurred only during low streamflow (<0.6 L which suggest that under high streamflow conditions a peatland lagg zone between the bog (3.0 ha) and uplands (6.7 ha) becomes the dominant source of Hg in downstream waters. Further, a binary mixing model showed that except for the spring snowmelt period, Hg in streamwater from the catchment was mainly derived from dry deposition of gaseous elemental Hg (73-95%). This study demonstrates the usefulness of Hg isotopes for tracing sources of Hg deposition, which can lead to a better understanding of the biogeochemical cycling and hydrological transport of Hg in headwater catchments.