Mining companies closing down precious metals heap-leach operations may already own the solution to one of their most pressing problems - handling cyanide-contaminated wastes. Scientists at the U.S. Bureau of Mines have identified bacteria that eat cyanide. These organisms already live in water at the mines where they're needed. "The problem is that there aren't enough bacteria present to make a difference," explained USBM scientist Richard Lien. And, the difference is vital when heap leach operators want to close their mines. The heap leach process uses solutions containing small amounts of cyanide to dissolve gold and silver from large piles of low-grade ore. The process is a closed loop in which the cyanide solutions are recovered and reused. Mining companies take precautions to keep cyanide out of the environment, but the long-term fate of cyanide remaining in spent heaps and leftover processing solutions is a concern when the operation is no longer profitable. USBM scientists discovered that the bacteria Pseudomonas pseudo alcaligenes, found in processing waters and tailings ponds, feed on cyanide and break it down into harmless compounds. By adding nutrients to the water, researchers can increase the bacterial population and speed up cyanide destruction. At first, scientists worked with nutrients such as bacteriological-grade yeast extract and soy-peptone products. Recent studies, however, show that brewer's yeast extract is a more effective and less expensive food. Nutrients represent the major cost involved in the bacterial treatment process. The USBM has already used the new treatment to lower the cyanide content of processing solutions at USMX Inc.'s Green Springs Mine near Ely, Nev., and at a heap leach operation in central Nevada. At both sites, activated carbon tanks (which were used to recover gold from leach solutions) were inoculated with bacteria. Processing solutions were repeatedly circulated through the tanks, and cyanide was oxidized. The test at the Green Springs Mine helped the company meet closure requirements. Bacterial treatment at the other operation, where work will continue this summer helped reduce weak acid dissociable (WAD) cyanide concentrations from 3 parts per million (ppm) to 1 ppm. Tests at two additional Nevada mines - a Santa Fe Gold Inc. operation near Hawthorne and AMAX Gold's Wind Mountain Mine - will begin this summer. Tests at the Santa Fe Gold site will be similar to those conducted at the Green Springs Mine. Researchers are waiting for the WAD cyanide concentrations in the rinse solutions to level off; those concentrations are now between 10 and 20 ppm. At the Wind Mountain Mine, scientists will take their work into a new area, using the bacteria on a spent heap for the first time. Researchers believe the treatment will also prove effective for these heap-leaching wastes.