GEOCHEMICAL STUDIES OF SOILS, STREAM SEDIMENTS, WATER, AND BIOTA IN A SMALL WATERSHED IMPACTED BY MERCURY AMALGAMATION AT AN ABANDONED GOLD MINE IN NORTHERN VIRGINIA

Historic gold mining has left a legacy of mercury contamination due to the use of amalgamation. At the abandoned Greenwood mine, located at the headwaters of the small, forested Quantico Creek watershed, mercury concentrations in soils within 40 m of two water-filled shafts reach a maximum of 692 ppm. Mercury concentrations in stream sediments reach 3.67 ppm at the mine site and decrease to 0.09 ppm 11 km downstream. Concentrations and speciation of mercury in the waters of the shafts, which are the likely sites of methylation, are anomalous; total mercury concentrations (unfiltered) reach 92 ng/L and methylmercury comprises up to 66 % of the total. Total mercury concentrations of nearby, downgradient, shallow ground waters reach 257 ng/L. Total mercury concentrations in the stream at the site reach 124 ng/L, and decrease to 5.7 ng/L 11 km downstream; background surface water concentrations are < 3.3 ng/L.

Mercury concentrations and d¹⁵N, d¹³C, and d³⁴S values of muscle tissue from frogs (bull, green, and pickerel; n = 7) and fish (bluegill, sunfish, and white suckers, among others; n = 18) document relationships among trophic level, other behavioral characteristics, and the bioaccumulation of mercury. Stable isotope signatures of frogs (d¹⁵N = 3.6 to 6.3 ‰; d¹³C = -27.6 to -23.7 ‰; d³⁴S = 2.4 to 4.1 ‰) indicate that they occupy the lowest trophic level sampled, which is consistent with their comparatively low mercury concentrations (14.6 to 87.5 ng/g). The highest concentrations (> 34.2 ng/g) were found near the Greenwood site. Stable isotope compositions of fish (d¹⁵N = 7.5 to 9.6 ‰; d¹³C = -31.7 to -25.9 ‰; d³⁴S = 3.0 to 6.2 ‰) lack significant variation on the basis of species and reflect a higher trophic level than the frogs. Concentrations of mercury in muscle tissue of fish are highest at the mine site (743 to 1,130 ng/g) and decrease downstream (11 km: 89 to 178 ng/g) compared to background values (172 to 244 ng/g). Relative to other fish, mercury and isotopic compositions (19.2 ng/g Hg; d¹⁵N = 12.9 ‰; d¹³C = -22.7 ‰; d³⁴S = 7.9 ‰) of a juvenile American eel are anomalous and reflect its anadromous behavior. Trophic level is important; however, proximity to the source of contamination and reliance on aquatic food sources have a greater effect on determining the bioaccumulation of mercury by fish and frogs in the watershed.