Stratigraphy and historic accumulation of mercury in recent depositional sediments in the Sudbury River, Massachusetts, USA

The distribution and deposition of sedimentary mercury in the Sudbury River were linked to an industrial complex (Nyanza site) that operated from 1917 through 1978. In two reservoirs just downstream from the Nyanza site, esti mated rates of mercury accumulation increased markedly in the 1920s and 193 0s, were greatest during 1976-1982, decreased within 5 years after industri al operations ceased, and have decreased further since capping of contamina ted soil at the Nyanza site was completed in 1991. The most contaminated se diments were typically buried, yet the 0- to 1-cm stratum remained substant ially contaminated in all cores. Mercury accumulating in the surficial, res ervoir sediments was probably from continuing, albeit much lesser, inputs f rom the Nyanza site, whereas recent inputs to downstream wetland areas were attributed to recycling of sedimentary mercury or to mercury from unidenti fied local sources. In the reservoirs, burial of highly contaminated sedime nts is gradually decreasing the amount of sedimentary mercury available for methylation. In downstream wetlands, however, sedimentary mercury seemed t o be more available than that in the reservoirs for physical transport and biogeochemical cycling.