Technical Article Hydrogeochemistry of the Getchell Underground Mine--Part 1: Mine Water Chemistry R.J.Bowell SRK Consulting, Summit House, 9 Windsor Place, Cardiff CF10 3RS, Wales; E-mail: rbowell@srk.co.uk Abstract. The Getchell underground operations in Northern Nevada intersect groundwater associated with marble and hornfel lithologies and a sulfide bearing ore hosted within a 30-km long shear zone system. The deposit is classified as Carlin- type gold mineralization. A distinct feature of the mineralization is the high proportion of arsenic sulfides present in the ore and associated altered wallrock. This results in an intense arsenic enrichment, with some zones containing as much as 30% arsenic, and 0.5-2% arsenic throughout the mineralized envelope. Most of the groundwater in the area is well buffered by the calcareous host rocks and show a macrochemistry of Ca-Na-HCO 3 . Along the shear zone and in zones within the hornfel host rock, the waters are less alkaline and more saline, and have a chemistry of Na-Mg-Ca-SO 4 -HCO 3 . This latter water type occurs in sulfide-bearing zones. Arsenic speciation analysis and theoretical predictions demonstrate that higher arsenic concentrations are associated with reducing conditions, with higher Na/Ca ratios, and with low concentrations of Fe. In these waters, As occurs as arsenite, along with trace concentrations of mono-methyl arsonic acid and di-methyl arsinic acid. Natural attenuation of As appears to occur along groundwater flow paths due to co- precipitation and adsorption onto hydrous ferric oxide particles. However, elevated As is still a notable feature of groundwater quality throughout the Kelly Creek basin. This elevated As occurs in bedrock groundwater during underground mine development, rather than in near-surface alluvium groundwater. Due to this and the protracted history of mining, it is not possible to define a true background value for water quality in the area other than acknowledging that bedrock groundwater is mineralized and has little association with seasonal recharge and water quality in the alluvium cover. Key words: Arsenic, Getchell Mine, groundwater, hydrogeochemistry, Nevada. Introduction The Getchell underground mine, owned by the Getchell Gold Company, is located in northwestern Nevada along the eastern flank of the Osgood Mountains, approximately 70 km northeast of Winnemucca. Mining has occurred on the site during several intervals since 1934. The property has primarily produced gold and arsenic, though minor amounts of copper and tungsten were produced early in the mining history of the area. The geology and hydrogeology of the site is complex due to a myriad of faults that offset geological units on site and juxtapose permeable and impermeable units. As a consequence, distinct hydrogeological units can be identified based on both groundwater elevations and geochemistry. Groundwater quality can vary considerably over relatively short distances, as has been demonstrated in a number of recent studies. In an assessment of hydrogeochemical exploration techniques in the local watershed (the Kelly Creek Basin), the concentrations of Mn, Fe, As, Sb, W, and Au were observed to be higher in the vicinity of known mineralization than in wells sampled from unmineralized bedrock (Grimes et al. 1995). Reducing conditions favoured the mobilization of some elements such as Mn, Fe, Co, As, and Sb. A study by Bennett and Tempel (2000) found that groundwater evolves from CaSO 4 -type to Na-K-HCO 3 type and, due to natural attenuation, was inferred not to present a significant impact to water quality in the Kelly Creek basin. More recently, an enlarged database using historical mine data as well as that of Grimes et al. (1995) produced hydrogeochemical maps of the