AN ECOSYSTEM ANALYSIS OF LEGACY HEAVY METAL CONTAMINATION IN CENTRAL NEW HAMPSHIRE: REGIONAL ATMOSPHERIC DEPOSITION VERSUS LOCALIZED IMPACTS FROM HISTORIC MINING

In New Hampshire, historic mining operations associated with the former Madison Lead Mine have contaminated sediments in Cooks Pond (14.3 ha) with Pb, Zn, and Hg. The mine operated between 1826 and 1923, with the most intense activity after 1901. Lead, Ag and Zn were extracted from galena and sphalerite that occurred within a highly fractured hydrothermal deposit of Mesozoic age. Mine tailings from an ore processing facility were dumped directly into the pond leading to contamination of pond bottom sediments. A nearby pond, Durgin Pond, is unaffected by the mining operations and is used to assess the differences in local mineral-bound and regional atmospheric sources of mercury.

Sediment cores were collected from Cooks Pond and Durgin Pond using a Uwitec gravity corer with secondary hammer action. Selected cores were sampled at 1cm intervals and subsamples were extracted by microwave digestion using EPA method 3050A and analyzed for Pb, Zn and Se by ICP-OES and GFAAS methods. A second split was directly analyzed for Hg using a CVAAS Hg analyzer. Fish were collected and fillets were analyzed for Hg, Pb and Se using the same methods as for sediments.

The impact of the ore processing is clearly seen in the cores from Cooks Pond with peak concentrations of Pb (8,600 ppm), Zn (16,900 ppm) and Hg (460 ppb) at 28cm below the sediment water interface. This compares with the much lower maximum concentrations of 110 ppm Pb, 170 ppm Zn, and 340 ppb Hg measured at the Durgin Pond control site. The difference in total amount of metals between the two sites is even greater when considering the higher density of the clastic tailings in Cooks Pond compared with the organic rich sediments in Durgin Pond.

A close correlation was found between Zn and Hg concentrations in Cooks Pond sediments that is consistent with the discovery of a high Hg bearing variety of sphalerite at the Madison mine. Thus, Hg in the Cooks Pond ecosystem comes both from atmospheric deposition and from the mine tailings. Although it was expected that fish from Cooks Pond would have significantly higher Hg concentrations, it was found that Durgin Pond fish had much more Hg. This likely reflects the importance of the extensive wetlands that surround Durgin Pond. These wetlands are likely much more efficient at methylating atmospherically derived Hg compared to the mineral bound Hg in Cooks Pond.