BIOGEOCHEMICAL AND HYDROLOGICAL CONTROLS ON THE FLUX OF MERCURY AND METHYLMERCURY IN FIRST ORDER COASTAL PLAIN WATERSHEDS OF THE CHESAPEAKE BAY

Over the past 7 years we made use of the long-term research site at the Smithsonian Environmental Research Center (SERC) in central Maryland to study the deposition of mercury (Hg) and fluxes of Hg and methylmercury (MeHg) from three small first-order mid-Atlantic coastal plain watersheds. One watershed is entirely forested, one watershed is primarily agriculture with a forested stream buffer, and one watershed is mixed land use but contains a beaver produced wetland pond. Our initial goals were to assess watershed Hg yields in the mid-Atlantic and to establish a baseline prior to implementation of Hg emissions controls. All three studied watersheds produced relatively high yields of Hg, with the greatest yield coming from the forested watershed. Our initial evaluation of three watersheds showed that MeHg production and flux could also be high, but varied dramatically among watersheds, and across years and seasons.

Each year we observed episodic MeHg production in the spring and during prolonged high-flow storm events in the fall. The observed spring maxima of MeHg release coincided with development of anoxia in riparian groundwater. MeHg accumulation began once nitrate was depleted from groundwater, and either iron accumulation or sulfate depletion began. This suggests the presence of nitrate was modulating MeHg production through the suppression of sulfate and iron reducers and perhaps methanogens. As sulfate is not limiting in any of the watersheds owing to the sediments marine origin, we hypothesize the depletion of nitrate allows sulfate reducing bacteria to now utilize available carbon. Although wetlands are generally thought of as the primary zones of MeHg production in watersheds, shallow riparian groundwaters very close to the stream appear to play that role in SERC Coastal Plain watersheds.

At the onset of the study we hypothesized that a decrease in Hg deposition would translate into a decrease in Hg and MeHg export. However, the balance between nitrate, sulfate and other microbial electron acceptors in watersheds is also a major control on MeHg production, and we hypothesize that changes in land-use that influences this balance will impact watershed MeHg production. Land management practices that change nitrate load and carbon quality will undoubtedly impact the cycling of Hg.