TAILINGS DISPOSAL AT A MARINE PLACER MINING OPERATION BY WESTGOLD, ALASKA

From 1985 to 1990 WestGold managed a marine placer mining operation wi th substantial documentation concerning its submarine failings disposa l (STD) system. For 5 years the operation involved the use of a floati ng bucket-ladder dredge with an integrated, gravity-based treatment pl ant to extract free particulate gold fi om the seabed gravels. Three-q uarters of the throughput was disposed of at 1.5 m below sea level thr ough two pipes, each of 0.51-m diameter. The system's design resulted from considerable trial and error and modeling studies to achieve regu lated seawater conditions. The information is publicly available as a result of a very open permitting and operating process. The mining per mit specified waste discharge controls and, environmental impact limit s for topics of local concern: the dredge's effluents, the red king cr ab population (an important fishery resource), seawater turbidity, and bioaccumulation of trace metals, especially mercury (a remnant of pri or beach mining). The impact on crab stock was negligible to the point that monitoring was scheduled to be abandoned in 1991 (but mining ope rations ceased in September 1990). An impoverished benthos remained fo r at least 2-3 years after dredging, with sandy areas able to recoloni ze to a highly variable fauna within 3-4 years. Cobble and repeatedly dredged areas recolonize more slowly. An extreme storm in 1992 has als o affected the seabed.