They may, however, contain residues of cyanide or arsenic that can harm fish and other aquatic life. Since the cyanide rapidly breaks down into non-toxic products when exposed to sunlight, the danger to the environment can be averted by storing the effluent in a holding pond until oxidation is complete. Tailings that contain arsenic are disposed of in clay pits and do not pose a threat to the aquatic environment. More than 40 years ago some gold mines used mercury to extract gold from the ore leaving tailings contaminated with small quantities of mercury. Sometimes the mercury is slowly released into the aquatic system where it is taken up by fish in amounts that exceed the limit safe for human consumption.
All mines also produce large quantities of waste rock that are usually piled on the surface.
As in the case of tailings, the waste rock may contain sulphides from which acid can be generated. This is usually minor, since the sulphide content of the waste rock is relatively low, but if the sulphide content is high, or the amount of waste rock extensive, the acid can become a serious problem. During the operation of some mines, a portion of the waste rock is used as backfill in the mine, leaving less surface waste. After an open pit mine closes, a large open excavation is left behind. Often partially filled with water, the open pit may represent a danger to people.