Geochemical signatures of discharge waters, Macraes mine flotation tailings, east Otago, New Zealand

The stream catchment containing the tailings dam complex at the Macraes min e, east Otago, New Zealand has three chemically distinct water types. (1) N atural groundwater has pH beween 6 and 8, and sulphate levels locally eleva ted to 30 ppm. The total carbonate/calcium (Ca) ratio is c. 2, consistent w ith carbonic acid dissolution of basement schist calcite. Sodium (Na) is st rongly elevated over chloride, and these levels are dominated by water-rock interaction. (2) Decant pond waters lie on two tailings dams, one of which was inactive between 1993 and 1998. Their pH is strongly alkaline (8-9). S ulphate, Na, chloride, and Ca contents are higher than groundwater and thes e levels have risen steadily over time. Carbonate content is lower than gro undwater. Arsenic (As) and iron (Fe) contents are high and variable; arseni c in the active tailings pile is commonly 5-15 ppm. (3) Mixtures of tailing s water and natural groundwater(c. 1:1) form from seepage from the upper pa rt of the main tailings pile only and mix beneath the main tailings dam to emerge down stream of the dam. The mixture pH (6.3) is slightly lower than that of the groundwater (mainly >7) whereas the sulphate content is high (> 1500 ppm) because of the water component of the tailings. There is no resol vable time lag on the 1-month scale between discharge of tailings water int o the dam and emergence at the foot of the dam. Two distinct water types ar e identifiable in this setting. Chimney drain discharge is collected in pip es and discharged from those pipes. The tailings component of this water is chemically little different from that which left the tailings. Subsurface flow travels unconfined beneath the tailings dam and interacts chemically w ith the dam aggregate. Attenuation of Na and chloride, and addition of Ca, carbonate, and magnesium (Mg) occurs. Nearly all dissolved As and copper (C u) was extracted from the subsurface flow during passage through the dam, w hereas some As and all Cu is extracted from the chimney drain discharge. A small volume of a mixture of tailings water and groundwater has continued d own stream of the tailings dam complex for c. 50 m in the basement schist a s a contaminant plume which took c. 2 years to travel that distance in frac tures within the schist. This contaminated water is chemically similar to t hat of the subsurface flow, and has high sulphate content but no detectable As or Cu.