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

Little is known about the amount and form of mercury released from inoperat ive mercury mines in the western United States. To address this, we measure d 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 es timate that the creek transports a baseline flux of 1.5 kg of Hg/yr from th e mine site. We hypothesize that tailings are the primary source of mercury to this creek. We attribute the decrease in UHgT along a downstream transe ct to Hg(ll) scavenging by iron oxyhydroxide particles that precipitate and settle out of the water. Likewise, dissolved gaseous mercury concentration s decreased with increasing distance from the mine (2.8 ng/L at 1.2 km; 0.5 6 ng/L at 7.5 km). We regard abiotic atmospheric evasion as the primary mec hanism driving this loss. The released mercury is biologically available, e videnced by high (1.1 to 1.7 ng/L) unfiltered monomethylmercury concentrati ons downstream from the mine. We attribute the relatively uniform downstrea m monomethylmercury concentrations to a balance between microbial methylati on and demethylation.