reuse of slag. As of September 2011, most of the cleanup had been completed so current risk of exposure is low, although pregnant women, nursing mothers and young children are still encouraged to have their blood-lead levels checked. Drainage tunnels As in many mining districts, as the mines extended deeper, keeping the water pumped out of the workings became a major expense. To more economically drain the mines, two tunnels were driven to allow the water to drain by gravity. Water from both tunnels ultimately flows into the Arkansas River. Yak tunnel The Yak tunnel, long and built between 1895 and 1923 to drain the southern part of the district, has its outlet in California Gulch east of the town of Leadville. The tunnel became part of the California Gulch Superfund site in 1983. In October 1985, a large surge of water from the Yak tunnel reached the Arkansas River, and elevated the dissolved metals content of the river for tens of miles downstream. Water flowing from the tunnel has been treated by its owner, ASARCO, since June 1991, to remove metals. Leadville tunnel The Leadville tunnel was started in 1943 by the US Bureau of Mines to drain the mines of the northern part of the district, and so increase metal production. The tunnel has its outlet north of the town of Leadville, on the East Fork of the Arkansas River. In 1959 the US Bureau of Reclamation bought the tunnel for $1.00, as a source of irrigation water. Since March 1992, the Bureau of Reclamation has treated the water flowing out of the tunnel, to remove dissolved metals and bring the water quality into compliance with the Clean Water Act. Collapses within the tunnel that began in 1995 partially blocked flow, and have created a large reservoir of an estimated of water within the tunnel behind the collapse. In February 2008, concerns became public that if the collapse dam should suddenly fail, as has happened in other mine drainage tunnels in Colorado (such as the Yak tunnel, the Argo Tunnel and the 2015 Gold King Mine waste water spill north of Silverton, Colorado), a large slug of contaminated water would suddenly flow out of the tunnel, overwhelm the treatment facilities, and flow into the Arkansas River. The rise in water level inside the tunnel has caused water with high concentrations of dissolved metals to leak out to the