SOIL MERCURY ACCUMULATION IN O HORIZONS FROM THE AVERY BROOK WATERSHED, WEST WHATELY, MASSACHUSETTS

Since the industrial revolution, increased anthropogenic emissions of mercury have led to regional and global scale increases in atmospheric deposition. The objective of this study is to evaluate the size and distribution of mercury pools within the organic horizon of a small forested watershed located in western Massachusetts. The Avery Brook watershed ranges in elevation from 220 m to 450 m and has a forest canopy that is 33 percent coniferous, 23 percent deciduous and 44 percent mixed. Small open areas occur associated with beaver ponds and wetlands within the upper part of the catchment. The podzolic soils have organic horizons that range from 5 – 10 cm thick. Replicate soil samples were collected from a total of 7 sites distributed throughout the various canopy types. Samples of Oi, Oe, and Oa horizons were collected from 3 locations at each site. Samples were dried at 105˚C and analyzed for total Hg (THg) using a Teledyne Leeman Labs Hydra IIC Mercury Analyzer. Triplicate analyses were done on each sample and results are actually averages of these 3 replicates.

Organic horizons at the coniferous site had the highest average THg concentrations (272 ng/g), while the average concentration at the deciduous site was only 189 ng/g. Within the coniferous O horizon the surface Oi horizon had the lowest THg (120 ng/g) while the intermediate Oe horizon had the highest THg (391 ng/g). Total Hg in the lowermost Oa horizon (305 ng/g) was only slightly lower than the Oe. In contrast, THg concentrations in the deciduous O horizons progressively increase with depth from 47 ng/g in the Oi horizon to 215 ng/g in the Oe to 306 ng/g in the Oa. The progressive increase in THg with depth is likely the result of leaching of mercury from the decomposing organic material at the top of the profile and its accumulation in the humus of the lower part of the O horizon. The higher THg concentrations observed under coniferous stands are likely the result of more efficient capture of dry deposition by the coniferous canopy. This could be due to the nature of the surface of the coniferous needles or more likely it is due to the fact that coniferous needles are present throughout the year compared to deciduous leaves that are present only during the growing season.