History of mercury loading in the Upper Mississippi River reconstructed from the sediments of Lake Pepin

An array of sediment cores was analyzed to determine historical trends in m ercury (Hg) accumulation in Lake Pepin, a natural lake on the Upper Mississ ippi River. Whole-basin Hg accumulation rates increased from 3 kg/yr before European settlement (ca. 1830) to a maximum of 357 kg/yr in the 1960s. The recent Hg accumulation rate (110 kg/yr, 1990-1996) is experimentally indis tinguishable from measured Hg loadings in the river entering the lake, indi cating that accumulation rates in Lake Pepin correspond quantitatively to r iver loadings. The modern accumulation rate represents a decline of almost 70% from the peak value, reflecting large decreases in Hg inputs to the Mis sissippi River from industrial and municipal point sources in the metropoli tan Minneapolis/St.Paul area upstream. A total of 18.1 t of Hg has been dep osited in Lake Pepin since 1800; half of that (9.0 t) was deposited between 1940 and 1970, when regional growth accelerated rapidly but pollution cont rol mechanisms were inadequate. Point sources accounted for approximately 6 0% of the Hg accumulating in Lake Pepin in the 1960s, but these inputs have been virtually eliminated since then.