Mercury in surficial waters of rural Wisconsin lakes

As part of an ongoing program to investigate mechanisms regulating the aquatic biogeochemistry of Hg, unfiltered surface waters of eight rural lakes in northcentral Wisconsin were collected and analyzed for reactive and total Hg. Samples were collected during autumn mixis using ultra-clean, trace-metal-free protocols which have been applied successfully in the marine environment. Results indicate that Hg concentrations are considerably lower than previously reported, and approach levels observed in remote ocean waters. Concentrations of reactive Hg ranged from 0.7 to 2.9 pM in the eight lakes studied. Total Hg, determined following strong oxidation using BrCl, ranged from 4.7 to 9.7 p M in four of the lakes. Although the data set is limited, these concentrations are 20–100 times lower than previous estimates for lakes in the region. The results reemphasize the importance of uncompromised clean laboratory protocols in the collection and analysis of trace constituents in natural waters. They also imply a reevaluation of ideas regarding the sources and distribution of Hg in lakes. A simple mass balance for Little Rock Lake clearly shows the potential importance of both atmospheric Hg deposition and sedimentary remobilization to the geochemical cycling and bioaccumulation of Hg.