Arsenic speciation and mobility in surface water at Lucky Shot Gold Mine, Alaska

Historical mining in Alaska has created a legacy of approximately 6,830 abandoned mine sites which include adits, tailing piles and contaminated land that continue to impact surface and groundwater quality through run-off and leaching of potentially toxic metals, especially arsenic (As). One such site is the Lucky Shot Gold Mine in Hatcher Pass, south-central Alaska, which operated from 1920 until 1942, mining gold-bearing quartz veins hosted in a Cretaceous tonalite intrusion. Arsenopyrite (FeAsS) and pyrite (FeS2) present in the quartz veins contribute to elevated As levels in water draining, abandoned mine adits. As future underground mining at Lucky Shot may further adversely impact water quality, baseline geochemical studies were undertaken to assess As mobility in the vicinity of the mine adits. Water samples were collected from streams, adits and boreholes around the mine and analysed 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 inorganic As(III)/As(V) using anion-exchange chromatography, following established methods. It was determined that water draining the adits had elevated levels of As roughly seventy times the United States Environmental Protection Agency Drinking Water Standard of 10 mu g L-1, although this was rapidly diluted downstream in Craigie Creek to < 2 mu g L-1. Adit and surface water pH was circum-neutral and displayed no characteristics of acid mine drainage. Despite being well oxygenated, As(III) is the dominant As species in adit water, accounting for close to 100 % of total As. The proportion of As(V) increases downstream of the adits, as some As(III) is oxidized, but the speciation enhances arsenic mobility at the site. The delta O-18 measurements indicate 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) in adit water results from the lack of thermodynamical equilibrium being attained and preferential absorbance of As(V).