TEMPORAL ANALYSIS OF THE FLOW RATE AND CHEMISTRY OF DISCHARGE WATERS FROM 8-LEVEL, LYNX MINE, MYRA FALLS OPERATIONS, VANCOUVER ISLAND, BC, CANADA

The Myra Falls operations of Boliden-Westmin (Canada) Ltd. produce copper and zinc concentrates from several volcanogenic massive sulphide deposits located on a mineral lease completely surrounded by Strathcona Provincial Park, in the mountainous interior of Vancouver Island. In preparation for decommissioning, a detailed investigation of mine drainage from the 8-Level adit of the Lynx mine was undertaken in order to characterize temporal variations in the flow and quality of discharge waters in relation to seasonal recharge events. This was achieved by weekly field sampling of effluent chemistry and by continuous monitoring of discharge rate, pressure head, electrical conductivity and temperature. Annual precipitation at the site averages more than 2500 mm of which 2000 mm occurs as rainfall mainly in October through December. The hydrograph of discharge measured at the portal weir exhibits characteristics typical of low-storage, fracture- dominated groundwater systems, rising steeply with the first autumn rains and from then on fluctuating sharply with recharge from successive weather systems over the winter. The hydrograph of pressure head measured in a packed-off underground borehole quickly reaches a plateau of about 40m in the late fall suggesting that the storage capacity of the groundwater system has been reached and that additional rainfall or snow melt will result in runoff rather than recharge. The electrical conductivity of discharge water is relatively low and constant throughout much of the year. However, values rise sharply with the first autumn rains. Thereafter, for the next two months, values rise and fall as recharge from successive weather fronts gradually flushes solutes from the groundwater system. Similarly, pH is near neutral for much of the year but drops to 2.2 during the first recharge event of the season. Generally, concentrations of sulfate and trace metals increase with the decrease in pH. Concentrations of most species are also highly correlated with electrical conductivity. Using least-squares regression, this makes it possible to estimate continuous chemographs of heavy metal concentration and load in the discharge stream from that of a physical parameter which can be monitored inexpensively.