107 - Methodological Assessment of Mercury Measurements in Foliar Samples from Contaminated Sites

Litterfall represents an important pool of mercury (Hg) in forest ecosystems mainly due to foliar atmospheric Hg uptake through stomatal and non-stomatal pathways. Hg deposition onto leaf surfaces is also an important deposition pathway to terrestrial sinks in the form of throughfall. However, one aspect that is often overlooked is the influence of sample preparation methodology for accurate estimation of Hg concentration and isotopic ratios in foliage samples. Furthermore, Hg species that are deposited onto the leaf surfaces and those that are assimilated within the leaf compartments undergo different physical and chemical fractionation processes leading to different isotopic signatures. Determining Hg isotopic signatures would therefore be contingent on the absence or presence of Hg species on leaf surfaces. We tested the effect of removing Hg deposited on leaf surfaces on the overall outcome of both Hg concentration and its isotopic fingerprints. Foliage samples were collected from different locations on the tree crown in the vicinity of Idrija Hg smelter. One portion of the samples were rinsed while the remaining samples were left untreated. All samples were digested in microwave following freeze drying and analysed for total Hg and stable Hg isotopes. We observed that washed foliage samples displayed δ202Hg of -3.65 ± 0.25 ‰ whereas, unwashed samples had δ202Hg of -2.93 ± 0.31 ‰. The process of washing removed some of the Hg with higher δ202Hg, i.e. enriched in the heavier isotopes. The difference calculated from mean total Hg concentrations of foliar samples (washed: 252ng g-1 ± 3 and unwashed: 345ng g-1 ± 35) gave δ202Hg of -1.04 ‰ which is close to what has been reported for particulate bound mercury (Hgp). It is therefore critical to report the appropriate methodology utilized for sample preparation for the analysis of Hg isotopic ratios in foliar samples.

Authors: Saeed Waqar Ali, David Kocman, Dominik Božič, Marta Jagodic Hudobivnik, Milena Horvat