Document Type

Honors Thesis

Publication Date

Spring 5-8-2026

Abstract

The occurrence and geochemical behavior of rare earth elements (REEs) in mine-impacted and natural waters of Montana are evaluated across a range of environments, including hard rock mining-impacted systems, coal-associated waters, and naturally occurring acid rock drainage (NARD). Understanding the abundance of REEs in secondary sources such as mine-impacted waters is critical for both environmental assessment and resource recovery. A comprehensive dataset of dissolved and total recoverable REE concentrations was analyzed to assess the influence of pH, total dissolved solids, and geologic setting on REE mobility and distribution. Results demonstrate a strong inverse relationship between pH and REE concentrations, with the average ΣYREE concentrations of ~2800 μg/L observed under acidic conditions (pH < 5) and average ΣYREE concentrations of ~23 μg/L at higher pH ranges (pH>6). This behavior reflects enhanced mineral dissolution and aqueous complexation under low pH conditions and removal through adsorption and co-precipitation with iron and aluminum oxyhydroxides as pH increases. REE fractionation patterns are consistent across all sample types, characterized by enrichment in the middle rare earth elements, indicating that fractionation is controlled by aqueous complexation and adsorption processes rather than pH alone. Significant variability in REE concentrations is observed across Montana systems. Hard rock mining environments exhibit the greatest range, including highly acidic waters with elevated REE concentrations and near-neutral systems with minimal REE mobility. Coal-associated waters show similar behavior, with bituminous coal producing acidic, REE-rich waters, and sub-bituminous coal associated with higher pH and lower REE concentrations. NARD systems display moderate concentrations and relatively consistent geochemical behavior. Water treatment systems demonstrate effective removal of REEs through pH-driven processes, with concentrations decreasing substantially between influent and effluent waters. These systems also concentrate REEs into solid phases, suggesting potential for recovery from treatment residuals. This study further reinforces the role of pH as a primary control on REE mobility and highlights the potential for REE recovery from mine-impacted and natural water systems, particularly through water treatment processes.

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