Aqueous geochemical dynamics of metals and rare earth elements in an acid rock drainage-impacted alpine watershed

Numerous mountain watersheds in the Colorado Mineral Belt (CMB) are impacted by acid rock drainage (ARD) and acid mine drainage (AMD), which mobilize metals and rare earth elements (REEs) into surface waters. In the upper Roaring Fork watershed near Independence Pass, natural ARD from a highly mineralized tributary is the primary source of acidity and element loading into Lincoln Creek, with additional contributions from historical mining at the Ruby Mine. Despite known concerns about metal transport, the fate of REEs remains poorly understood in this system and other similar watersheds. This study focuses on the behavior of major ions, trace metals, and REEs in surface waters along a flow path receiving ARD and AMD inputs. The objectives were to (1) evaluate the transport, mixing, and attenuation of major solutes and REEs across stream reaches, and (2) characterize REE geochemical signatures, including their redox behavior and utility as tracers in ARD/AMD impacted systems. Filtered water samples from six main sites were analyzed by Inductively-Coupled Plasma Mass Spectrometry (ICP-MS) and Ion Chromatography (IC) methods, and results were explored through mass balance transport calculations, bivariate plots, shale-normalized REE patterns, and the cerium anomaly. Water chemistry in the Lincoln Creek basin reveals distinct geochemical fingerprints for the Ruby Mine (enriched in Ca, Mg, Mn, and Cd) and the Mineralized Tributary (enriched in SO4, Fe, Al, and Cu), with several metals showing increasing trends in recent years. REE fractionation patterns and Ce anomalies further distinguish source contributions and processes, with the Mineralized Tributary displaying MREE enrichment from natural pyrite weathering and the Ruby Mine exhibiting HREE enrichment tied to mine-derived flows. Most solutes showed a mix of conservative and reactive transport depending on site and season, while certain late-season metal concentrations at the Grizzly Reservoir inlet exceeded aquatic life standards. These results indicate that source composition mainly controls major solute and REE patterns in surface waters, with reactive transport processes selectively influencing the behavior of certain elements. This thesis advances knowledge of metal and REE dynamics in high-elevation, ARD-affected watersheds and supports continued monitoring and management in the Lincoln Creek basin.

科罗拉多矿带（Colorado Mineral Belt, CMB）内众多山地流域均受酸性岩排水（acid rock drainage, ARD）与酸性矿山排水（acid mine drainage, AMD）影响，两类排水会将金属与稀土元素（rare earth elements, REEs）迁移至地表水体中。独立山口附近的罗林福克河上游流域中，一条高矿化支流产生的天然酸性岩排水是林肯溪酸度与元素负荷的主要来源，鲁比矿（Ruby Mine）的历史采矿活动亦带来额外贡献。尽管学界已关注金属迁移问题，但该系统及其他类似流域中稀土元素的归趋仍未得到充分阐释。本研究聚焦于接收ARD与AMD输入的径流路径沿线地表水中主要离子、痕量金属与稀土元素的行为特征。研究目标为：（1）评估各河段内主要溶质与稀土元素的迁移、混合及衰减过程；（2）表征稀土元素的地球化学特征，包括其氧化还原行为以及在受ARD/AMD影响的系统中作为示踪剂的应用潜力。研究人员对6个主要采样点的过滤水样采用电感耦合等离子体质谱法（Inductively-Coupled Plasma Mass Spectrometry, ICP-MS）与离子色谱法（Ion Chromatography, IC）开展分析，并通过质量平衡迁移计算、双变量散点图、页岩标准化稀土元素配分模式与铈异常对结果进行解析。林肯溪流域的水化学特征显示，鲁比矿（富集钙、镁、锰与镉）与高矿化支流（富集硫酸盐、铁、铝与铜）具有独特的地球化学指纹，且近年来多种金属浓度呈上升趋势。稀土元素分馏模式与铈异常可进一步区分来源贡献与过程：高矿化支流因天然黄铁矿风化呈现中稀土元素（middle rare earth elements, MREE）富集特征，而鲁比矿则因矿山径流表现为重稀土元素（heavy rare earth elements, HREE）富集。多数溶质根据采样点位与季节的不同，兼具保守型与反应型迁移特征；而格里兹利水库（Grizzly Reservoir）入口处的季末部分金属浓度超出水生生物水质基准。上述结果表明，源组成主要控制地表水中主要溶质与稀土元素的分布模式，反应型迁移过程会选择性影响部分元素的行为。本论文深化了对高海拔受ARD影响流域中金属与稀土元素动态过程的认知，并可为林肯溪流域的后续监测与管理提供支撑。