Cases and solutions 937 Environmental Geology 39 (8) June 2000 ´  Springer-Verlag Hydrochemical and stable isotope assessment of tailings pond leakage, Nickel Plate Mine, British Columbia M.M. Ghomshei ´ D.M. Allen Abstract An integrated hydrogeochemical and sta- ble isotope study was undertaken to assess tailings pond leakage at the Nickel Plate Mine site in British Columbia, Canada. The approach used consisted of a hydrochemical mixing model for sulfate in conjunction with stable isotope data (18O and 2H) to estimate possible levels of contam- ination reaching surface waters from leakage through the dam and to determine the relative percentages of "seepage" that can be attributed to leakage through the dam and to natural ground- water flow beneath and around the dam. Results suggest that tailings pond waters are diluted by groundwaters by approximately 50% before their arrival in the collection swamps, and that the leakage from the tailings pond partially by-pass the collection (and pump-back) system at a rate of between 0.09 and 0.12 l/s. Stable isotope data indicate that tailings pond waters and swamp waters are enriched in the heavier isotopes as a result of evaporation, and thus, tend to plot along an evaporation line of slope 3.9 (D/18O). The isotope data indicate that seepage beneath the dam consists of approximately 50% real seepage and 50% meteoric waters (both surface water and groundwaters). Key words Stable isotopes ´ Hydrochemistry ´ Acid rock drainage ´ Tailings pond leakage Background The Nickel Plate gold mine is near the town of Hedley, British Columbia about 50 km west of Penticton (Fig. 1). Mining activities began in 1986 and continued until 1996. The milling operation began in 1997. Mineralization is associated with pyrite, arsenopyrite and pyrrhotite hosted in a wollastonite-rich scarn. Total sulfide in the ore is estimated at 1±3%. Decommissioning of the mine site was initiated in 1995 and included the installation of several monitoring wells in the vicinity of the tailings dam and the initiation of a geochemical sampling pro- gram of the watershed in the vicinity of the mine site. In the Nickel Plate Mine area, seepage from the tailings pond is received mostly by waters of Cahill Creek by way of Nickel Plate Mine Creek (Fig. 2). Cahill Creek drains into the Similkameen River. The flow of water into the creek in the vicinity of the tailings dam can be accounted for both by seepage through the dam and by naturally discharging groundwater. However, it is difficult to deter- mine the percentage of flow coming from each source. Leakage from the tailings pond is presently collected in swamps and is then pumped back into the pond. Despite these efforts, a portion of the leakage by-passes the ªpump-backº system (i.e., the groundwater portion), and creek water has become contaminated. Hydrogeological and/or hydrogeochemical methods are often used to estimate the amount of water that can potentially pass through a dam. When hydrogeological methods are used, the in situ physical parameters, includ- ing the permeability of the tailings dam, and the hydrau- lic gradient through the dam, must first be determined. Often accurate estimates of these parameters are difficult and expensive to determine. Compounding the problem are the possible hydraulic effects of piping, heterogenei- ties in the dam, and difficulties acquiring representative samples of the dam material for off-site permeability measurements, each of which can introduce significant errors in the hydrogeological assessment. The cumulative uncertainty may lead to very speculative flow estimates. Further, the presence of slurry, which affects the hydrau- lic conductivity, can complicate a hydrogeological assess- ment. Hydrogeochemical approaches are typically based on the chemistry of the tailings pond and the receiving waters. Received: 19 April 1999 ´ Accepted: 19 July 1999 M.M. Ghomshei Department of Mining and Mineral Process Engineering, 6350 Stores Rd, University of British Columbia, Vancouver, British Columbia V6 T 1ZA, Canada D.M. Allen ( ) ) Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5 A 1S6, Canada e-mail: dallen@sfu.ca