The city of Rio Grande, located on the right bank of the Patos Estuary, has been severely contaminated by mercury (Hg) due to anthropogenic activities that chiefly began in the eighteenth century. To investigate the natural mercury distribution along the salinity gradient in the estuary, three sediment cores were collected from a region of the estuary that has experienced less anthropogenic impacts, namely its left bank. Our study demonstrates that accumulation of Hg and formation of metal sulfide minerals take place in fine grain sediment horizons within the sampled sediment cores. Mercury immobilization in these sediments occurs via binding to organic matter coatings on fine grain sediment particles, as well as by incorporation into and/or co-precipitation with iron sulfide minerals. The grain size controls over Hg accumulation and sulfide mineral formation were statistically demonstrated using principal component analysis. Different fine particulate sediment deposition patterns occurred at each sampling location, which is attributed to the consequence of hydrological changes in the estuary resulting from navigation infrastructure reforms performed over the past 200 years in the local port (e.g., dredging) and its surroundings. We suggest that the port building and maintenance activities have influenced Hg distributions in the estuarine sediments.
Keywords: Estuary; Hydrological changes; Mercury distribution; Navigation infrastructure reforms; Sulfide formation.