Mercury speciation in drainage from the New Idria mercury mine, California

Little is known about the amount and form of mercury released from inoperative mercury mines in the western United States. To address this, we measured mercury concentrations and speciation in water impacted by the New Idria mine. Total unfiltered mercury concentrations (UHgT) in acid mine drainage (AMD) (5.2 to 41 ng/L) were comparable to concentrations upstream from the mine (4.2 to 13 ng/L). We measured substantially higher UHgT concentrations (2900 to 12 400 ng/L) in water 1.2 km downstream from the AMD input and estimate that the creek transports a baseline flux of 1.5 kg of Hg/yr from the mine site. We hypothesize that tailings are the primary source of mercury to this creek. We attribute the decrease in UHgT along a downstream transect to Hg(ll) scavenging by iron oxyhydroxide particles that precipitate and settle out of the water. Likewise, dissolved gaseous mercury concentrations decreased with increasing distance from the mine (2.8 ng/L at 1.2 km; 0.56 ng/L at 7.5 km). We regard abiotic atmospheric evasion as the primary mechanism driving this loss. The released mercury is biologically available, evidenced by high (1.1 to 1.7 ng/L) unfiltered monomethylmercury concentrations downstream from the mine. We attribute the relatively uniform downstream monomethylmercury concentrations to a balance between microbial methylation and demethylation.