Theme: 08. Metals

Topic Area: 8b. Mining and Uranium Site Restoration

Formatted Title:

Multiple Lines of Evidence: Using Mining Forensics to Characterize Abandoned Surface Uranium Mines

Background/Objectives:

Background. Uranium deposits are a common geological feature across many areas of the Colorado Plateau in the United States. Regional geological processes including sedimentation, lithification, roll-front and humate-type uranium mineralization, and subsequent erosion from the Colorado and Little Colorado River basins resulted in the development of an economic source of surface and near-surface uranium deposits that were extensively mined during the Cold War. It is estimated that there are over 4,000 abandoned uranium mines (AUMs) in the United States, with approximately 500 in the Navajo Nation alone, many of which were surface (open-pit) mines. Many of these AUMs are subject to ongoing reclamation and remedial site investigations. For AUMs located in surficial uranium deposits, distinguishing between chemically identical undisturbed naturally occurring radioactive material (NORM) and mechanically disturbed technologically enhanced naturally occurring radioactive material (TENORM) is important for site characterization. Where many ‘traditional’ remedial investigations may rely on the chemical identification of constituents of concern through the collection of samples, a constituent-focused approach alone cannot be used to accurately distinguish between NORM and TENORM in areas where surficial uranium mineralization is abundant. Therefore, characterizing mining disturbances in these surface deposits requires the adoption of a multiple lines of evidence (MLE) approach to fully characterize site boundaries, assess potential offsite transport pathways, characterize background conditions, and perform risk assessments.

Approach/Activities:

Approach. Mining-related activities occurred over three phases and are extensively documented in the literature and government records. First an exploration phase was focused on locating economically viable ore deposits involving systematic radiological surveys, bulldozing, and drilling programs. This was followed by a production phase that extracted and transported ore to nearby mills for further processing. The third and most recent phase consisted of initial reclamation efforts to reduce the radiological and physical hazards present at AUMs. The approach we use includes a detailed review of historical mining and reclamation records, historical and current aerial imagery, geological/geomorphological and mining-related disturbance mapping, the collection of high-resolution gamma data by walkover scans using sodium iodide scintillation detectors, and statistical evaluations of the resulting datasets. Historical records and site observations interpreted in context with onsite geological processes is key to characterizing features onsite.

Results/Lessons Learned:

Results. The integration of multiple lines of evidence has enabled the lateral delineation of mining and non-mining features on and near mine sites. In some cases, mining resulted in an overall decrease in radioactive material in areas where the most mining activity occurred (such as through the removal of isolated, uranium-mineralized fossil logs). In other cases, exploration and production mining activities resulted in disturbances that may alter and even potentially enhance exposure to harmful constituents, including uranium, radium-226, and associated metals. From this, it is possible to distinguish undisturbed naturally radioactive areas (NORM) from radioactive areas disturbed by mining-related activities (TENORM). This approach allows for a robust, data-based assessment of mining-related impacts and sets the framework for future evaluations of potential site risk and developing remedial actions for these AUMs.

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Madelyn Weaverling