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Shallow subsurface groundwater flow paths and metal contamination in the Malakoff Diggins hydraulic gold mine pit
Malakoff Diggins, in Nevada County, was once the largest hydraulic gold mine in California. Mine drainage still discharges mercury, sediment, and metals to Humbug Creek, degrading downstream habitat. The goals of this study were to (1) characterize the shallow groundwater levels and flow paths in the hydraulically excavated mine pit, (2) compare the distribution of nonfiltered mercury, filtered mercury, and nonfiltered methylmercury in shallow groundwater and surface water drainage from the Pit between saturated and unsaturated conditions, and (3) compare dissolved organic carbon concentrations between saturated and unsaturated conditions in shallow groundwater and surface water drainage from the Pit in water year 2016. Groundwater levels were measured in eight piezometers from November 7, 2015 to October 28, 2016. Mercury samples were collected in piezometers, a pond, and Hiller Tunnel during saturated conditions on May 4, 2016 and unsaturated conditions on July 22, 2016. Dissolved organic carbon samples were collected monthly at all locations from April 25 to October 17, 2016. During saturated and uns aturated conditions, groundwater flow was in two directions, from east to west and north to south towards Hiller Tunnel. During saturated conditions, the nonfiltered mercury concentrations in piezometer P-1, the Pond, and Hiller Tunnel outlet were higher than concentrations measured during unsaturated conditions. Methylmercury concentrations in P-1 and Hiller Tunnel outlet were higher during saturated conditions but higher in the Pond, P-6 and P-7 during unsaturated conditions. Between saturated and unsaturated conditions, the dissolved organic carbon concentrations increased in the piezometers and the Pond but decreased in Hiller Tunnel outlet. The findings presented here suggest that hydrologic condition and dissolved organic carbon concentrations affect the distribution of mercury at Malakoff Diggins.