IMPACT OF LOGGING ON SOIL SOLUTION MERCURY IN THE AVERY BROOK WATERSHED, WEST WHATELY, MASSACHUSETTS

Anthropogenic emissions of mercury (Hg) over the past 100 years have led to anomalously high Hg concentrations in organic soil horizons over much of the Northeastern US. Recent work in the Avery Brook Watershed has found Hg concentrations as high as 270ng/g in organic horizons under coniferous forest stands. Any factors that increase organic decomposition rates can potentially release Hg from the soil to surface waters. While increasing air temperature associated with climate change may be important in the long-term, logging and pest infestation could have more immediate effects. Logging and forest dieback opens up the forest canopy allowing more sunlight to reach the forest floor which in turn increases organic decomposition rates.

To determine the impact of logging on Hg release, two sites in a coniferous stand (logged and undisturbed) were instrumented with tension and zero tension lysimeters, installed to collect soil water primarily from the organic horizons. Samples were collected from the lysimeters and from the nearby Avery Brook gage station during precipitation events. Samples for Hg analysis were filtered and acidified before being analyzed by Cold Vapor Atomic Fluorescence using a Teledyne Leeman Labs Hydra AF Gold Mercury Analyzer. Samples were also analyzed for Dissolved Organic Carbon (DOC), and ultraviolet absorbance at 254nm (UV254).

Water collected from tension lysimeters in the logged site had lower concentrations of DOC (8.2mg/L) and THg (6.8ng/L) than the controls (DOC=22.3mg/L; Hg=20.7ng/L). The zero tension lysimeters had similar concentrations between the two sites (DOC=15.27mg/L; Hg=19.4 ng/L). UV254 values for tension lysimeters in the logged site were significantly lower than the control site (0.25 verses 0.71). The Hg/DOC ratio was high in both sets of lysimeters (zero tension=1.28; tension=0.98) compared to that observed in the stream (0.56). The impact of logging was to reduce the concentration of both Hg and DOC in soil solutions but did not affect the Hg/DOC ratio. Since the logging occurred in the winter of 2014 and soil solutions were not collected until the early fall of 2014, it is possible that the expected increase in DOC and Hg occurred during the spring, before the samples were collected.