Evaluation of Trends in Mercury Concentration in Onondaga Lake Biota in Recent Years

Onondaga Lake in Syracuse, NY is a 4.6 mi² lake that has been impacted by a long history of municipal and industrial activity, including two mercury cell chlor-alkali plants.  Mercury concentrations in fish have been elevated since at least the 1970s as a result of mercury discharges to the lake in concert with methylmercury production in profundal sediment and accumulation in the hypolimnion during summer stratification.  The remedy for the lake as documented in the 2005 Record of Decision for the Onondaga Lake Superfund Site includes sediment dredging, capping, and monitored natural recovery, as well as potential addition of oxygen or nitrate to the hypolimnion to reduce methylmercury release from sediment.

 In preparation for remediation slated to start in 2012, a baseline monitoring program was implemented in 2008 to document mercury concentrations in water, zooplankton, and fish.  These data, along with historical data, demonstrate a decline in mercury concentration in water, zooplankton, and fish in recent years.  Since implementation of nitrification (to reduce ammonia discharges) at the Metropolitan Sewage Treatment Plant in 2004, methylmercury concentrations in both the hypolimnion and epilimnion have declined dramatically.  The increased nitrate concentration in the lake serves to postpone sulfate reduction during the summer stratified season and results in a significant reduction in methylmercury release from sediment.  Reductions in surface water mercury concentrations are consistent with the observed declines in mercury concentrations in zooplankton.

 Mercury concentrations in piscivorous fish (smallmouth bass and walleye) have exhibited a more complex pattern, including an increase in mercury concentration for a several year period prior to 2006 (possibly due to changes in fish body size), and a subsequent decrease. Based on stable isotope analysis of tissue samples (zooplankton, benthic macroinvertebrates, fish) in 2009 zooplankton and piscivorous fish have a similar carbon signature indicating a common carbon source and therefore a link between the two types of biota in the bioaccumulation pathway.  This finding underscores the importance of controlling methylmercury concentrations in the water column in order to limit bioaccumulation from water ultimately to piscivorous fish.  Future work includes implementation of a nitrate addition pilot in order to assess the ability of added nitrate to further reduce methylmercury concentrations in lake water.

 Monitoring will continue throughout the design, remedy implementation, and for the long-term.