ARSENIC SPECIATION IN SURFACE WATER AT LUCKY SHOT GOLD MINE, ALASKA

Historical gold mining in Alaska has created a legacy of abandoned mine shafts, contaminated land and tailing piles, which continue to impact surface and groundwater quality through runoff and leaching of potentially toxic metals, especially arsenic. The Lucky Shot gold mine in Hatcher Pass, South-central Alaska, operated from 1920 until 1942. Mining of gold-bearing quartz veins hosted in a Cretaceous quartz diorite intrusion was done by excavation through horizontal adits. Lucky Shot is being assessed for future development, driven by the high price of gold, currently around $1,600 per ounce (July, 2011).

Arsenopyrite (FeAsS) and pyrite (Fe₂S) present in the quartz veins contribute to elevated arsenic levels in water draining from the adits; up to 700µgL^-1 or roughly seventy times the United States Environmental Protection Agency Drinking Water Standard of 10µgL^-1, although this is rapidly diluted downstream of the site to < 2µgL^-1.

Water samples were collected from streams, adits and boreholes around the mine and analyzed for major and minor elements using inductively coupled plasma mass spectrometry (ICP-MS) and for anions by ion-chromatography (IC). Arsenic species separation was performed in the field to determine the ratio of As(III)/As(V) using anion-exchange chromatography, following established methods. It was determined that in the vicinity of the adits, surface water had circum-neutral pH’s and displayed no characteristics of acid mine drainage. Despite being well-oxygenated, As(III) is the dominant arsenic species in the system, accounting for 75% of total arsenic. The δ¹⁸O signature of the waters indicates that the water in the system has a short residence time as it is very similar to meteoric water, supporting the observation that the predominance of As(III) results from the lack of thermodynamical equilibrium being attained and preferential absorbance of As(V).