CRYSTAL MINE REMEDIAL INVESTIGATION: FRACTURED ROCK GROUNDWATER CHARACTERIZATION

The alpine wetland area that lies above the Crystal Mine was originally interpreted to be a source of recharge into the underground workings of the mine, where water infiltrates downward through the fractured rock aquifer, flows through the mine workings, and eventually discharges as Acid Mine Drainage (AMD). This AMD significantly impacts water quality and ecological receptors. As such, capturing and diverting the surface water and shallow groundwater in the wetland area could potentially reduce recharge into the mine workings; and could prove to be an effective remediation measure to control the amount of AMD.

Prior to investing in a costly diversion design, a geologic site investigation was conducted to characterize the hydraulic properties of the fractured bedrock aquifer that underlies the wetland, assess whether this area recharges into the Crystal Mine, or conversely, if the wetland represents an area of groundwater discharge from a distant recharge zone. The investigation consisted of test pits excavated into weathered granitic bedrock, and shallow piezometers and deep monitoring wells installed in saturated fractured zones in the bedrock. The test pit data showed that the groundwater discharges upward through fractures at the bedrock surface to the wetland. The nested piezometers also indicated artesian conditions and an upward vertical groundwater gradient in the upper 140 feet. However, 300-foot deep monitoring wells and aquifer testing showed that deeper groundwater has a downward vertical gradient, and the bedrock aquifer at depth appears to consist of a limited number of discrete fractures that transmit little groundwater. Thus, the shallow groundwater discharge through the fractured rock sustains the wetland, rather than infiltrating downward into the mine workings. In addition, the actual path of deeper subsurface flow into the mine workings could not be pinpointed.

Based on this data, we concluded that diverting surface water and intercepting shallow groundwater would not significantly reduce the source of water infiltrating and migrating downward into the mine. Consequently, this potential remedy was eliminated from further consideration, saving up to $1 million in design and construction costs.