The arsenic source term for an in-pit uranium mine tailings facility and its long-term impact on the regional groundwater Brett J. Moldovan a, * , M. Jim Hendry b , Glenn A. Harrington c a Cameco Corporation, 2121-11th Street West, Saskatoon, SK, Canada S7M 1J3 b Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2 c Department of Water, Land and Biodiversity Conservation, Mount Gambier SA5290, Australia Received 2 November 2006; accepted 31 December 2007 Editorial handling by R. Fuge Available online 13 February 2008 Abstract Detailed field sampling and analyses and laboratory-based diffusion-cell experiments were used in conjunction with 3-D reactive transport modeling (MODFLOW and MT3D99) to quantify the fate and long-term (10 ka) transport of As in the Rabbit Lake In-pit Tailings Management Facility (RLITMF), northern Saskatchewan, Canada. The RLITMF (300 m  425 m  90 m thick) was engineered to ensure solute transport within the RLITMF is dominated by diffusion. Con- centrations of As in the tailings pore fluids ranged from 0.24 to 140 mg/L (n = 43). Arsenic speciation analyses indicate 90% of this arsenic exists as As 5+ . This observation is supported by pH–Eh measurements of pore fluids (n = 135). Geochemical analyses yielded a strong inverse correlation between the Fe/As molar ratio in the tailings solids and the corresponding con- centration of dissolved As, which is attributed to the adsorption of As to secondary 2-line ferrihydrite present in the tailings. Diffusion-cell testing yielded values for the effective diffusion coefficient, sorption coefficient, and effective porosity of As in the tailings of 4.5  10 10 m 2 /s, 2–4 cm 3 /g and 0.36, respectively. Reactive transport simulations using the field and labora- tory data show adsorption of As to the tailings and diffusive transport of dissolved As in the tailings should reduce the source term concentration of As to between 40% and 70% of the initial concentrations over the 10 ka simulation period. Based on these simulations, the As concentrations in the regional groundwater, 50 m down gradient of the tailings facility, should be maintained at background concentrations of 0.001 mg/L over the 10 ka period. These findings suggest the engineered in-pit disposal of U mine tailings can provide long-term protection for the local groundwater regime from As contamination. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction As of 2004, about 1/3 of the world’s annual U production (11,597 t) was obtained from the Ath- abasca Basin, northern Saskatchewan, Canada (Natural Resources Canada, 2006). This mining has resulted in the production of more than 36 mil- lion t of tailings (Natural Resources Canada, 2004). Because these tailings can contain elevated concen- trations of long-lived (up to tens of thousands of years) transuranic elements (i.e., Pu isotopes and 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2007.12.037 * Corresponding author. Fax: +1 306 966-8593. E-mail address: bjm328@mail.usask.ca (B.J. Moldovan). Available online at www.sciencedirect.com Applied Geochemistry 23 (2008) 1437–1450 www.elsevier.com/locate/apgeochem Applied Geochemistry