Laserfiche WebLink
.~ <br />Mr. Thomas M. Robertson 7 June 17, 1981 <br /> <br /> <br />r <br /> <br />I <br />High concentrations of free cyanide are present at all sampling points <br />downstream of the dam except for the nest of piezometers a[ the toe of <br />the dam (sampling points 3, 4, 16 which are founded in the soil above <br />bedrock). The presence of high concentrations of cyanide indicates that <br />pond seepage from the Lower Tailings pond is present and has not been <br />diluted to a significant extent by natural groundwater. Concentrations <br />of free cyanide at the toe of the dam in the soil and the top of rock are <br />minimal as compared to all other sampling points. Water quality data <br />from these piezometers indicate that significant amounts of tailings <br />water are not seeping along the stream channel at the base of the Lower <br />Tailings dam. The concentration of free cyanide in the piezometer on the <br />crest of [he dam (installed in rock, sampling point 17) is minimal, <br />indicating that in this vicinity, significant seepage of tailings water <br />is not occurring in the top of rock in the west abutment. <br />The concentrations of free cyanide in the wells and seeps downstream of <br />the Lower Tailings dam range from 30 mg/1 to 60 mg/1. The highest <br />concentration of free cyanide is present in the seeps which have formed <br />on the east bank of the topographic ravine (sampling points 11 and 12). <br />The concentration of free cyanide is greater at all sampling points in <br />[he east abutment as compared to the west abutment. The higher concen- <br />trations indicate a shorter, more direct seepage path and/or greater <br />quantities of tailings water seeping through the east abutment. <br />The measured concentrations of copper and zinc fluctuate with time <br />of sampling. Higher concentrations of total copper and total zinc <br />are noted in the water sampled on the east abutment than in the tailings <br />pond water. Since these data were obtained from samples which were not <br />filtered and/or treated with acid, they are difficult to interpret. <br />Variations in the quantity and mineralogical composition of suspended <br />matter in the samples lead to fluctuations in the measured concentration. <br />However, the association of copper and zinc with cyanide in the tailings <br />pond and [he presence of elevated concentrations of these species in [he <br />water sampled downstream of the tailings pond indicate that tailings <br />water is present in the water sampled in the piezometers. <br />The chemical characteristics of tailings water sometimes cause reactions <br />with the soil which may increase the permeability of the soil. Ttiis <br />has been observed at other sites where acidic tailings waters have <br />reacted with and dissolved certain natural calcium complexes in founda- <br />tion soils. The tailings water effluent at the Lower Tailings Dam is <br />alkaline with a pH of 9.4. With this value of pH, [he dissolution of the <br />mineral phases in contact with [he tailings water is not considered <br />likely. Sampling points downstream from the pond indicate lower values <br />of pH, approximately 8. The decrease in pH may be caused by several <br />reactions: <br />• Precipitation of calcite. <br />• Precipitation of metal hydroxides. <br />• Uptake of hydroxide by clays or hydrous metal oxides. <br />~~L~1/l~l 1i~~ <br />