2011-11-09_PERMIT FILE - P2011032 (4)

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• • Oyama Tunnel Hydrologic Setting Description The Oyama Tunnel is located near the head of its primary watershed, which is situated near the western edge of the San Juan volcanic field. It is carved into thick units of the Tertiary Silverton Volcanics, a series of lava flows and volcanic sediments that accumulated in the region prior to the collapse of the Silverton Caldera. The watershed is located within the San Juan Caldera, and is adjacent to the eastern rim of the heavily mineralized Silverton Caldera resulting in a very high fracture density. Hydrothermal alteration of the fractured volcanic rocks is prevalent throughout the watershed, and is host to numerous historic exploration sites, including the Oyama Tunnel. The fractures in the volcanic rocks provide conduits for groundwater migration, and when exposed on the surface or underground, these fractures often times will yield water. The Oyama Tunnel intersects a fracture zone that yields a continuous discharge that has been previously permitted with CDPHE on two separate occasions during exploration efforts in the 1980's. This discharge has been found to contain consistent, good water quality that does not contribute any significant contaminants of concern to the receiving surface waters of the nearest perennial stream. The nearest perennial stream begins its course from a large wetland at approximately 12,480 feet asl collecting runoff from the surrounding ridgeline 600 feet above. Approximately 2,700 feet along the course of the stream and 400 feet lower in elevation (average gradient of -6.8 degrees or - 15.1 %) is where the discharge from the Oyama Tunnel eventually forms a diffuse confluence into surface waters of the stream. Above the diffuse confluence, the stream's surface flows are approximately 80 to 95 gallons per minute (gpm) (low flow conditions). After the discharge confluence, the stream flows another 9,500 feet to its confluence with a larger perennial stream which is another 2,120 feet lower in elevation (average gradient of -10.0 degrees or - 22.3 %). The discharge that is the subject of the CDPS renewal application (Attachment F) is from the combination of water accumulating in the underground workings of the Oyama Tunnel and an exploratory borehole at the portal. From the portal, the combined flows range from 15 to 25 gallons per minute (low flow conditions), and are combined into an armored channel that carries the flows away from the top of the waste rock pad, and into a settling pond at the toe of the waste rock pad (Figures A -2, and A -3). The settling pond is 115 feet long, 45 feet wide, and on average is approximately 3 feet deep. The water then trickles over a designed low point in the settling pond's dam and seeps through dense ground vegetation for 180 feet until reaching a diffuse confluence with the nearest perennial stream. Surface activities supporting the exploration would not affect the water quality of the discharge due to the flow pathway of the discharge channel. However, the settling pond is present to allow for sediment to fall out of suspension after being stirred into suspension from underground activities. It is highly unlikely that the exploration efforts would yield soluble metals into solution causing elevated metal concentrations, since the host rock and ambient water quality yields a basic character with a buffering capacity. Thus, metals are likely to be relatively insoluble. This is further evidenced by water quality results from exploration activities in the 1980s under two previously released discharge permits administered by CDPHE. Site - related impacts to water quality will be controlled with the use of the following strategies: Attachment D: Page 2 of 4