HEAVY METAL CONTAMINATION OF BOTTOM SEDIMENTS FROM PAST MINING OPERATIONS AT COOKS POND, MADISON, NEW HAMPSHIRE

Heavy metals (Pb, Zn, Hg) from operations associated with the former Madison Lead Mine have contaminated sediments in Cooks Pond (14.3 ha) and led to high concentrations of mercury (Hg) and potentially lead (Pb) in piscivorous fish. The mine opened in 1826 and closed in 1923, with the most intensive mining occurring from 1901 to 1923. Lead, Ag and Zn were extracted from galena and sphalerite associated with a highly fractured hydrothermal deposit of Jurassic age. Tailings (0.6 ha) from an ore processing facility on the pond shore entered the water and contaminated pond bottom sediments.

Five cores were collected from Cooks Pond using a Uwitec gravity corer with secondary hammer action. Sample sites were determined based on distance from the tailings fan and pond bathymetry. Cores (90mm diameter) were collected in water depths ranging from 4.5m to 7.5m and were 60-100cm in length. Sediment from a subset of the cores was sampled from the core barrel at 1cm intervals. In these samples, Pb and Zn were extracted using EPA method 3050A and analyzed by Inductively Coupled Plasma (ICP-OES). Mercury was measured directly by cold vapor atomic adsorption using a Teledyne Leeman Labs Hydra IIC Hg Analyzer. A second set of cores was split and scanned using an ITRAX core scanner. Fish were collected by seine net and hand line, and fillets were analyzed for Hg.

Samples show a sharp increase in Hg, Zn and Pb within a high density layer in the top third of the cores reaching concentrations as high as: 460 ppb Hg, 12,000 ppm Zn, and 7,600 ppm Pb. Metal concentrations also decrease with distance away from the processing plant. Analysis of piscivorous fish within the pond show Hg concentrations as high as 1200 ppb.

Washing of mine tailings into the pond explains the increase in Zn and Pb in the sediment. This material added high density mineral matter to the normally low density organic rich sediment and flooded the ecosystem with heavy metals. Mercury, typically associated with organic material, was regionally deposited from atmospheric deposition during the Industrial Revolution. The high Hg concentration within the high density portion of the core is somewhat of a conundrum. It might be expected that the influx of tailings would have diluted the atmospheric Hg. It is possible that Hg was used in the processing of the ore, but there is no historical record of its use.