Dissolved Rare Earth Element (REE) fractionation mechanisms in freshwater environments have been parameterized by laboratory experiments and measured in major world rivers. To investigate the relationship between such observations and small-scale natural systems, we analyze REE contents in water samples from groundwater springs and streams on San Cristóbal Island, Galápagos. The island is comprised of a single lithology (Ocean Island Basalt) and exhibits a steep precipitation gradient on the windward slope over a short spatial extent, which results in a wide range of physicochemical properties and regolith weathering intensities. This allows us to investigate primary controls of REE fractionation without the influence from varying lithology. Samples show a wide range of measured pH, as well as concentrations of Mn and Fe. Local basalt-normalized REE patterns exhibit a wide range ([Nd/Yb]_b = 0.19–1.21 and [Ce/Ce*]_b = 0.65–2.22) for the small areal extent of the watersheds (~13 km²). Comparisons between data from 0.45 μm to 0.02 μm filtered samples demonstrate the importance of colloidal-sized (0.02–0.45 μm) metal species in aqueous solution. Our results suggest that two mechanisms are responsible for the observed REE fractionation in this catchment: pH-driven changes in aqueous complex stability by organic or anionic complexes and the preferential adsorption of heavy rare earth elements (HREE) onto Fe- and Mn- oxyhydroxides at low pH. This contrasts with moderate enrichment of light rare earth elements (LREE) observed at higher pH. These results suggest that several proposed REE fractionation mechanisms established previously in laboratory experiments can be observed in a natural watershed system, despite being obscured in larger world rivers. The homogenous lithology and strong climate gradient of San Cristóbal island allow for the observation of REE fractionation mechanisms which are usually obscured in larger world rivers despite being predicted in laboratory experiments, such as a positive Ce anomaly associated with HREE enrichment in low pH waters.

淡水环境中的溶解稀土元素 (REE) 分馏机制已通过实验室实验进行参数化，并在世界主要河流中进行了测量。为了研究此类观测与小规模自然系统之间的关系，我们分析了加拉帕戈斯岛圣克里斯托瓦尔岛地下水泉和溪流水样中的稀土元素含量。该岛由单一岩性（大洋岛玄武岩）组成，在短空间范围内在迎风坡上表现出陡峭的降水梯度，这导致了广泛的物理化学性质和风化层风化强度。这使我们能够在不受不同岩性影响的情况下研究 REE 分馏的主要控制。样品显示了广泛的测量 pH 值以及 Mn 和 Fe 的浓度。_b  = 0.19–1.21 和 [Ce/Ce*] _b  = 0.65–2.22）对于流域的小面积范围（~13 km ²）。从 0.45 μm 到 0.02 μm 过滤样品的数据之间的比较证明了水溶液中胶体大小 (0.02–0.45 μm) 金属物质的重要性。我们的研究结果表明，在该流域中观察到的 REE 分馏有两种机制：有机或阴离子复合物引起的水复合物稳定性的 pH 驱动变化和重稀土元素 (HREE) 在低pH值。这与在较高 pH 值下观察到的轻稀土元素 (LREE) 的适度富集形成对比。这些结果表明，尽管在更大的世界河流中被掩盖，但可以在自然流域系统中观察到先前在实验室实验中建立的几种提议的 REE 分馏机制。