Laserfiche WebLink
discharging less than 2 gallons per hour, presumably through the emergency spillway. Monthly <br />inspections since 2003 have reported the pond as dry in most months, and when it contains water <br />the level is more than 5 feet below the emergency spillway. The embankment of the pond appears <br />stable. <br />Risks to Public Health, Safety and Environment <br />A potential risk to public health, safety, and environment at the Coal Gulch site would be a <br />worst-case scenario in which runoff during a prolonged wet period or a period of large snowmelt <br />rapidly fills the site's sediment/treatment pond with water and causes a catastrophic failure of the <br />embankment, resulting in a sudden release of impounded water toward U.S. Highway 160, <br />located 200 feet downslope from the pond. The risk of this scenario occurring can be considered <br />remote because the pond's emergency spillway was designed for safely passing runoff from a 25- <br />year, 24-hour storm and the spillway appears to be in a functioning condition. Discharge from the <br />spillway would make its way to the culverts underneath Highway 160. <br />Another potential risk to public health, safety, and the environment is the risk of water pollution <br />from the discharge of acidic and salty underground mine water from the backfilled pre-law mine <br />portal, north of the pond. This discharge does not appear to represent a significant pollution risk <br />because of the observed relatively small rate of discharge of mine water. The maximum <br />observed discharge in recent years has been reported as a trickle (possibly a few gallons per <br />minute). The discharge has been observed to occur only when there has been recent surface <br />runoff from rain or snowmelt. Apparently, surface runoff feeds the sumped water in the old mine <br />workings, periodically causing the water level in the mine to rise up high enough to discharge out <br />of the pipe that extends from the reclaimed portal toward the pond. From the portal pipe, the <br />mine water discharge collects in the pond and could pollute other waters if the pond discharged <br />through its emergency spillway and if the mine water discharge volume comprised a large portion <br />of the pond discharge. The mine water discharge rate appears too small, however, to overcome <br />the combined dilutional capacity of the water in the pond and the relatively large amount of <br />runoff that would be necessary to cause the pond to discharge. During the Division's April 2001 <br />inspection, the mine water discharge was reported as a trickle and had pH of 3.6 and <br />electroconductivity of 2600 mmhos/cm. The discharge flowed to the pond which contained a <br />small pool of water and had pH of 5.1 and electroconductivity of 2000 mmhos/cm. <br />The surface depression in the northwestern part of the site occasionally impounds surface runoff, <br />resulting in a pool a few feet deep. This impoundment does not have an embankment and does <br />not represent a safety or environmental hazard. <br />Impacts to Alluvial Valley Floors (AVFs) <br />There are no alluvial valley floors within the area of influence of the Coal Gulch Mine. The Coal <br />Gulch drainage does not meet the regulatory definition of an alluvial valley floor as it lacks sufficient <br />water for flood irrigation and sufficient alluvium for subirrigation. <br />9