MERCURY CONTAMINATION AND FLOODPLAIN SEDIMENTATION FROM FORMER GOLD-MINES IN NORTH GEORGIA

Gold was discovered in Georgia in 1829 and mined until about 1940 in the Dahlonega Gold Belt of the north Georgia Piedmont. Streams there are characterized by gravel beds and fine sandy to silty banks. Historical mining-related alluvium is clearly distinguished from prehistoric alluvium because it is contaminated with mercury (Hg), which was used by miners to amalgamate gold. Mercury concentrations in historical floodplain sediments range from 0.04 to 4.0 mp kg(-1), exceeding background (0.04 mg kg(-1)) by as much as two orders of magnitude near the core of the mining district and decreasing in the downstream direction. Law levels (< 0.1 mg kg(-1)) of Hg are established within about 10-15 km from the source mines. The mercury-contaminated sediment exceeds sediment quality guidelines set by many agencies, and is a significant nonpoint source for mercury pollution. Hydraulic mining of saprolite, which began in 1868, and cutting of forests associated with mining and settlement caused unusually rapid sedimentation (1-3 cm yr(-1)) and floodplain aggradation in the region. After mining ceased, streams adjusted by downcutting and forming an historical-age terrace. A new floodplain is currently being formed as streams migrate laterally and erode the mining-related sediment of the historical terrace. High magnitude floods are contained within the confines of the historical terrace, thus limiting quantities of overbank sedimentation, causing channel bank erosion, and transmitting high sediment yields to reservoirs in the region.