1 A SEASONAL STUDY OF THE ARSENIC AND GROUNDWATER GEOCHEMISTRY IN FAIRBANKS, ALASKA S.H. Mueller 1,2 , R.J. Goldfarb 1 ,G.L. Farmer 2 , R. Sanzolone 1 , M. Adams 1 , and P. Theodorakus 1 U.S. Geological Survey, Denver, Colorado USA 1 University of Colorado, Boulder Colorado, USA 2 1. INTRODUCTION Groundwater arsenic concentrations are of increasing environmental concern due to the risk As poses to plant, animal, and human health. High arsenic concentrations are found in local drinking water supplies in Fairbanks, Alaska USA. Elevated groundwater arsenic concentrations were first recorded here in the mid 1970's, and subsequent studies determined the spatial distribution of high As content waters (Harrington et al, 1978; Wilson and Hawkins, 1978; Hawkins et al 1982, McCrum, 1984; Weber 1986). In 1996, workers from the U.S. Geological Survey and University of Colorado began a geochemical investigation of groundwaters in the Fairbanks area with the intent of further assessing the factors controlling their dissolved As contents (Goldfarb et al., 1998; Farmer et al., 1999). In the summer of 1999, a year-long study was begun to evaluate seasonal trends and fluctuations in the groundwater major, minor, and trace element compositions. The preliminary results of this study are presented here. 1.2 Local Geology and Arsenic-Bearing Mineral Occurrences The geology of the Fairbanks area has been described by Robinson et al. (1990), Foster et al. (1994), and Newberry and Bundtzen (1996). The rocks in the Fairbanks area are part of the Yukon-Tanana terrane, a complexly faulted pericratonic block of middle Paleozoic and older rocks that have been poly-deformed and poly-metamorphosed (Foster et al., 1994). The dominant lithologic unit of the terrane exposed in the Fairbanks area is the Fairbanks Schist. This is a heterogeneous unit comprised of mainly quartzite and muscovite-quartz+garnet+biotite+chlorite schist (Foster et al., 1994). Gold-bearing veins are present in the Fairbanks area and were apparently emplaced in conjuction with the intrusion of granitic rocks in the Late Cretaceous. Where hosted by granitic rocks, the auriferous veins are extremely low in sulfide minerals (<0.1 volume percent), but where hosted by schist, the sulfide mineral abundances are relatively high, typically reaching 1-3 percent of the vein material and locally much higher volumes. Arsenopyrite, pyrite (often arsenian pyrite), and stibnite are the most common sulfide minerals in veins throughout the Fairbanks region (Newberry et al. 1996). Hydrothermally-altered metasedimentary country rocks surrounding the veins are commonly characterized by albite, sericite, ankerite and related carbonate minerals, chlorite and extensive silicification, as well as sulfidization. 2. METHODS 2.1 Sampling and Field Techniques Groundwater samples were collected at two-month intervals, beginning in August of 1999 and ending August of 2000, from seventeen domestic water wells that penetrate the Fairbanks Schist along the Goldstream Valley. This area has been previously identified as yielding high As-bearin groundwater (Weddleton et al., 1989), and is located to the immediate east of Ester Dome and the Ryan Lode gold mine. Several additional sites were selected along the edge of and within the lowlands of the Goldstream Valley to evaluate the possibility that upstream placer tailings are contributing to the elevated groundwater arsenic concentrations. All wells in this study range in depth from 90-375 feet.