Date of Award

5-2010

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Aria Amirbahman

Second Committee Member

Sarah Nelson

Third Committee Member

Amanda Olsen

Abstract

Estuaries act as repositories for river-borne particulate contaminants such as mercury (Hg) and have a high degree of spatial variability in geochemical characteristics and infaunal density. Porewater and sediment chemical and molecular analyses were performed at two mudflats to study Hg dynamics in the Great Bay, New Hampshire, estuary. Squamscott mudflat was situated at the mouth of the Squamscott River and was both tidally and fluvially influenced with a wide range in salinity. Portsmouth mudflat was located closer to the mouth of the estuary in a cove and was tidally dominated with higher salinity and greater infaunal density than Squamscott. Sediment from both sites was vertically mixed, as indicated by the presence of 7Be (half-life 53 d) at depth; however Squamscott was physically mixed by advective currents, while mixing at Portsmouth was due to bioturbation. Sediment inorganic Hg (Hgi) concentrations and profiles were similar at both mudflats. Porewater Hg! concentrations, acid-volatile sulfides (AVS), alkalinity and dissolved organic carbon (DOC) were lower whereas sediment Fe(III) was higher at Portsmouth compared to Squamscott. Sediment-water partitioning of Hgi at Portsmouth was proposed to be controlled by adsorption to freshly precipitated Fe(III) hydroxides. Lower alkalinity and DOC were attributed to removal by bioirrigation at Portsmouth while lower AVS at this site may have been due to oxidation of FeS(s) with oxygen present. Fluorescence analysis of DOC showed that more labile, protein-like DOC concentrations were low at Squamscott whereas higher proportions of labile DOC were present in Portsmouth porewater and may have been introduced by bioirrigation. Microbial DNA analysis showed similar concentrations and distributions of sulfate-reducers and iron-reducers at both mudflats while methanogen and mer-A (a gene responsible for the production of mercuric reductase which reduces Hg(II) to volatile Hg°) concentrations were significantly higher at Squamscott. The low concentration of methanogens at Portsmouth may be due to introduction of oxygen into the sediment by bioturbation. Mer-A concentrations at Squamscott corresponded closely with the porewater Hgi concentrations. Lack of mer-A at Portsmouth may be attributed to the low porewater concentrations of Hgi Porewater Hgi concentrations were higher at Squamscott and peak methylmercury (MeHg) was closer to the SWI. Peak porewater MeHg concentration was higher and greater in depth at Portsmouth which may be due to the introduction of labile DOC deeper in the sediment, indicating greater methylation efficiency with bioirrigation. In previous studies, MeHg production has been attributed to the presence of uncharged Hgi. Uncharged modeled Hg, peaks at Squamscott corresponded to peak measured porewater MeHg concentrations. Porewater and solid-phase MeHg peaks at Squamscott corresponded to one another and were found within the top 4 cm. A solid-phase MeHg peak was not found in the top 10 cm of Portsmouth although a porewater MeHg peak was found at 12 cm. The findings of this study suggest that bioirrigation may affect the methylation efficiency of sediment by delivering more labile DOC in deeper sediments, and that high sediment Fe(III) levels affect the sediment-water partitioning of Hgi, controlling the availability of Hgi for methylation in bioturbated sediments.

Share