MERCURY STORAGE, RELEASE AND TRANSPORT IN THE WATERSHEDS OF SEVEN MAINE LAKES

Seven lakes in the towns of Bucksport, Dedham, Holden and Orrington, Maine, provide an opportunity to investigate temporal changes in the total Hg (Hg_T) and anthropogenic Hg (Hg_A) deposition rates since the late 1800s. The lake sediment was cored using a stationary piston corer during 1999 and 2000. Sediment cores were analyzed for Hg and were dated using ²¹⁰Pb, in order to determine changing Hg deposition rates through time. In principle, Hg_T consists of three components. The background Hg component (Hg_B) is taken to be the Hg accumulation rate in the lake before 1900. A variable Hg component (Hg_V) results from changes in deposition of sediment derived from the watershed, which is assumed to possess a constant, background Hg value. An anthropogenic Hg component, Hg_A, is deposited directly on the lake surface or reaches the lake from the terrestrial portion of the catchment. A series of calculations allows for the separation of Hg_A for each lake. In this study, Hg_B values range from 1 to 2 ng Hg/cm²/year, Hg_T values range from 2 to 20 ng Hg/cm²/year, and Hg_A values range from 0 to 10 ng Hg/cm²/year.

Six of the seven lakes exhibit similar trends in Hg_A, rising above background values near 1900 and steadily climbing to present values (4 to 8 ng Hg/cm²/year) along a relatively smooth profile. Williams Pond has a markedly different trend, with Hg_A rising above background values in the 1880s and rapidly climbing to modern values by 1950. Watershed processes involving the storage, release and transport of Hg may contribute to differences among profiles. Transport of Hg from watersheds is associated with dissolved and particulate organic matter derived from soils and wetlands. This dynamic coupling of watershed processes with atmospheric Hg deposition and sedimentation enables the development of dose-response models for Hg storage and transport in different watersheds.