The US Department of Energy (DOE) Office of Legacy Management monitors groundwater contamination resulting from ore processing operations conducted between 1953 and 1982 at a former uranium mill site on property that is now part of the Bluewater, New Mexico, Disposal Site. The site is located in the Grants Mineral Belt in northwest New Mexico at the base of the Zuni Mountains. It is estimated that prior to mill tailings encapsulation in 1995, approximately 21.6 billion liters (5.7 billion gallons) of tailings fluid seeped from the main tailings impoundment into the underlying aquifers. Today, there is remnant groundwater contamination from the mill in two aquifers: the San Andres-Glorieta (SAG) bedrock aquifer and the Ancestral Rio San Jose alluvial aquifer. Groundwater contamination in the SAG aquifer is of particular concern, as the aquifer is the primary water source in the region for municipal, agricultural, and industrial use. Municipal wells for the region are located as close as 6.4 kilometers (4 miles) southeast of the Main Tailings Disposal Cell. The SAG uranium plume extends east for more than 4.8 kilometers (3 miles) beyond the site's eastern boundary, along the direction of groundwater flow. The plume intersects the Homestake Mining Company site, another former uranium mill site undergoing active remediation. The SAG aquifer contains two formations, the Lower Permian San Andres Limestone and the Glorieta Sandstone. Most of the groundwater moving through the SAG aquifer is in ancient karst features consisting of open conduits where the limestone has dissolved, resulting in flow through solution cavities, solution channels, cavernous zones, and enlarged fracture openings. These features make the San Andres Limestone extremely permeable and capable of rapidly transporting groundwater. In addition, the karst characteristics make the aquifer difficult to evaluate and the uranium plume difficult to characterize. Numerous faults in the area, including two directly under the Main Tailings Disposal Cell, further complicate characterization by impacting groundwater flow both onsite and in the vicinity. A DOE study of the SAG aquifer uranium plume at the Bluewater site, conducted in 2014, indicated that the plume had moved offsite soon after the start of milling and effectively stabilized in its present location prior to the 1980s. An updated study completed in 2018 confirmed previous information regarding general plume stability and provided insight regarding potential localized trends east and south of the Main Tailings Disposal Cell. One such trend may be decreasing uranium concentrations east of the Main Tailings Disposal Cell, potentially attributable to regional groundwater elevation decreases. Both the 2013 and 2017 plumes suggest that the dissolved uranium contamination is derived from the leaching of a secondary mineralized zone derived from seeped tailings fluids in the geologic formations and fault zones below the Main Tailings Disposal Cell at the Bluewater site. In developing the 2013 and 2017 SAG aquifer uranium plume depictions, DOE had to address challenges associated with data quality and comparability. Analytical results used to determine the plume extent were compiled from multiple sources, including the state of New Mexico, Homestake Mining Company, and DOE. Each source used different well identifiers and coordinate systems, and well information between sources was inconsistent. Also, the integrity of some wells was questionable and necessitated review; casing integrity was found to be compromised in multiple wells. Given these uncertainties, some of these wells were excluded from the characterization. Another source of uncertainty is the paucity of wells, particularly monitoring wells, near or immediately downgradient of the leading edge of the plume. Therefore, the exact extent of the SAG aquifer uranium plume is not known at this time. There is, however, no evidence or indication of milling-related cont
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