Ensero Solutions (Ensero) is working with Colorado Legacy Land LLC (CLL) on the remediation of the Schwartzwalder Mine in Golden, Colorado. The Schwartzwalder Mine is an extensive inactive underground uranium mine previously owned by Cotter Corporation (Cotter). The Schwartzwalder Mine was continuously mined and dewatered from the 1950s until 2002. The site contains natural uranium ores and ore concentrates, mine dewatering solution, and water treatment residual and sludge. The site is regulated under a mining reclamation permit issued by Colorado Division of Reclamation Mining and Safety (DRMS). Cotter had previously terminated uranium mining operations at the mine and decommissioned all ore production related facilities. In addition, the site is currently authorized for treatment of uranium contaminated mine water under a Radioactive Materials License (RML) issued by the Colorado Department of Public Health and Environment (CDPHE) under the Agreement State authority. The RML authorizes possession and storage of source material (i.e., the water treatment resin). In March 2018, the site, the DRMS permit, and the RML were transferred from Cotter to CLL.
Excavation of alluvial fill materials that may be contaminated with natural uranium is ongoing and mine pool water and alluvial groundwater is being treated by reverse osmosis (RO). The permeate (clean water) from the RO system is discharged to Ralston Creek. The reject (brine or waste) from the RO units and alluvial groundwater from a sump system are mixed with barium chloride before being injected back into the mine pool. The barium chloride reacts with the mine water to form an insoluble barium sulfate (BaSO4) precipitate and removes radium through co-precipitation. Surface water is also diverted around the mine site to avoid contamination and then discharged into the natural drainage of Ralston Creek.
Ensero, as ordered by the State of Colorado (State), dewaters the mine to a depth of 150 feet below the required elevation. Consistent with State requirements, in situ treatment of the mine pool is being conducted to stabilize the mine pool. In situ treatment is designed to create a sulfate-reducing environment for the reduction of soluble uranium species to insoluble uranium species, e.g., from U(VI) to U(IV), to precipitate and immobilize uranium and dissolved heavy metals. The reducing environment is essential for the reduction of U(VI) to U(IV), and to encapsulate dissolved metals with sufficient iron sulfide (FeS) to inhibit re-oxidation. In situ treatment consists of the injection of carbon sources to stimulate the activity of sulfate-reducing bacteria. The existing microorganisms can couple the oxidation of organic carbon to the reduction of sulfate, producing soluble sulfide that reacts with chalcophile metals (e.g., zinc, cadmium, lead, copper) to precipitate low solubility metal sulfide phases. Previous in situ treatments have achieved an 87% reduction in uranium, a 94% reduction in molybdenum, and a >98.5% reduction in selenium.