CHEMICAL CONTROLS ON MERCURY CONCENTRATIONS IN SURFACE WATERS FROM A WESTERN MASSACHUSETTS FORESTED WATERSHED

Increased atmospheric deposition of mercury, associated with the industrial revolution, has been implicated in observations of high concentrations of mercury in fish across wide areas of the northeastern U.S. This study examines the chemistry of surface waters in the Avery Brook Watershed (7.6 km²) located in West Whately and Conway Massachusetts, in order to determine ambient Hg concentrations as well as to determine where and when these concentrations likely reach their highest values.

Prior studies have found that high concentrations of total mercury (THg) in surface waters generally occur in waters with low pH, high Dissolved Organic Carbon (DOC), high ultraviolet absorbance at 254 nm (UV254), and high Specific Ultraviolet Absorption (SUVA). In the Avery Brook watershed, these conditions generally occur within beaver ponds that are found in the headwaters region of the catchment. Eight ponds were sampled and found to have DOC values ranging from <3.0 mg/L during non-summer months to 10.4 mg/L in August. Analyses of THg from these ponds failed to detect any THg greater than 0.5 ng/L. This is lower than what would have been predicted based on THg - DOC relationships developed in other regions. Analysis of THg in watershed soils shows significant THg accumulations in O horizons (150 ng/g), therefore it is unlikely that the lack of THg can be explained by a lack of THg in the soils. A more likely explanation is that the relatively high pH of the surface waters in the area (> 6.8) inhibits THg mobilization. One beaver pond was recently found to have anomalously high DOC (16.5 mg/L) as measured in mid November at a time when all other ponds had DOC < 3 mg/L. This is a relatively newly formed pond that showed evidence of high nutrient levels (algae and duckweed) that probably reflect release from recently flooded soil. Since THg levels likely reach maximum concentrations in August when DOC reaches its maximum levels, it is expected that the THg in this new pond will be significantly greater than the 0.7 ng/L measured in November. It is likely that THg levels are generally higher in new beaver ponds due to the release of mercury from the freshly flooded soil.