MERCURY AND MONOMETHYL MERCURY FLUXES IN AN URBAN STREAM IMPACTED BY COMBINED SEWER OVERFLOWS IN HARTFORD, CONNECTICUT

Mercury (Hg) can easily bioaccumulate in the food chain representing a risk even at low levels due to its high toxicity. The presence of urban activities can substantially alter different mechanisms and processes of the Hg biogeochemical cycle. Urban watersheds are characterized by high imperviousness and some may even be impacted by combined sewer overflows, both being fundamental factors contributing to Hg loading, mobilization, and shifts in bioavailability in urban watersheds. Research is still needed to understand the fate and dynamics of Hg in urban streams. We are collecting stream water and suspended sediment samples in the North Park River watershed in Hartford, CT (USA) during baseflow and precipitation events to assess potential for loading, mobilization, and bioavailability of Hg. We also collect water samples at the Hartford’s waste water treatment plant (WWTP) from the influent, effluent, and the wet-weather lagoon. Water samples are analyzed for total, dissolved, and particulate Hg and methyl Hg (MeHg); major ions (Cl^-, NO₃^-, SO₄^2-, and PO₄^3-); total suspended solids (TSS); and dissolved organic carbon (DOC). Our results show that both total and dissolved Hg concentrations increase in the streams during precipitation events, however the greatest portion of Hg is associated, and consequently transported, with suspended sediments, as evidenced by the high correlation coefficient (R² ~ 0.80) between TSS and total Hg. No significant correlation was observed between dissolved or total Hg and DOC for the stream samples. In the WWTP, concentrations of both Hg and DOC are higher than those from the streams, but no significant correlation between TSS and Hg was observed. Overall, effluent samples exhibit lower Hg concentrations than their corresponding influent samples showing that Hg is removed at some level during the water treatment process. Unfiltered MeHg stream samples exhibited a similar pattern than that of total Hg, that is, an initial increase in concentration up to a maximum followed by a decrease close to the starting point. Dissolved MeHg is mostly below our detection limit for the stream sites. MeHg concentrations are generally higher at the WWTP compared to the stream.