SEDIMENT MERCURY DYNAMICS AT TWO ESTUARINE MUDFLATS IN GREAT BAY, NEW HAMPSHIRE

SEDIMENT MERCURY DYNAMICS AT TWO ESTUARINE MUDFLATS

IN GREAT BAY, NEW HAMPSHIRE

By Lauren E. Brown

Thesis Advisor: Dr. Aria Amirbahman

A Lay Abstract for the Thesis Presented

in Partial Fulfillment of the Requirements for the

Degree of Master of Science

(in Civil Engineering)

May, 2010

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. Estuarine sediments may be an important source of methylmercury (MeHg, the form of Hg that can cause neurological development problems in fetuses and infants) to overlying water and marine food webs. 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 by polychaetes (worms) in the sediment. Because of polychaetes in the sediment at Portsmouth, bioirrigation (the flushing of polychaete-created burrows with water) occurred. Bioirrigation can introduce oxygen and nutrients at greater depths in the sediment. Hg is typically methylated, to form MeHg, by sulfate reducing bacteria. More labile organic carbon, either introduced by bioirrigation or excreted by polychaetes, may have increased sulfate reducing bacteria activity and enhanced methylation at Portsmouth. Although lower MeHg concentrations were found in Portsmouth sediment compared to Squamscott, similar concentrations of MeHg were found in killifish from both sites. This led to the hypothesis that bioirrigation released MeHg, which is produced in porewater, into the overlying water at Portsmouth before it could be incorporated into the sediment. Introduction of oxygen into polychaete burrows led to the formation of iron hydroxides in the sediment. Lower porewater inorganic Hg (Hg_i) concentrations were found at Portsmouth compared to Squamscott, even though solid-phase Hg_i concentrations were greater. Partitioning of Hg_i between the sediment and porewater phases could have been controlled by sorption to iron hydroxides at Portsmouth, influencing the availability of Hg_i for methylation by bacteria.