Smith and Liu reported in 2018 that rare earth elements (REEs) signatures in the dissolved loads of rivers serve as excellent monitors of weathering processes within a basin. However, quantifying REE behavior spatially (along a river's profile) and temporally (seasonable variations) is rare. REE concentrations of dissolved river loads were obtained by four sampling seasons at five monitoring stations along the Neuse River, North Carolina.
The areal percentage of the Neuse River's catchment lithology composition, Post Archean Australian Shale (PAAS) normalized REE patterns, inter-REE ratios, and calculated anomalous REE values (i.e. cerium anomaly, gadolinium anomaly) were determined at each site in four seasons over a year. They showed slight differences among inter-REE relationships and anomalous REE values in the dissolved loads along the length of the river. PAAS-normalized REE patterns of dissolved water loads vary slightly spatially and temporally among the five sampling stations, and experience varying values of anomalous REE values. Collected data indicates that both variations in localized REE patterns and local bedrock occur along the river profile. Significant gadolinium (Gd) anomalies varied seasonally and spatially at select sites, suggesting changes in REE behavior along a river result from both natural and anthropogenic processes.
The authors concluded that shale-normalized REE patterns (i.e., [La/Yb]P values increase with total REE concentrations in the dissolved load. This trend in complexation is likely controlled by:
1) preferential weathering of LREEs during higher flow conditions, and
2) uptake of LREE into secondary mineral phases during periods of low flow or a combination of the two.
A more detailed investigation of local carbonate ions, organic ligand content, and other localized aquatic parameters could likely identify the critical factor that systematically controls [La/Yb]P in the Neuse River. While most of the REE behavior in this study is controlled by hydrogeological processes, the observed Gd anomalies are not believed to be naturally occurring and are attributed to anthropogenic sources. Significant spatial and temporal variation in both Gd anomalies and total REEs in the dissolved load, which may have partially developed as a result of anthropogenic inputs of REEs in natural waters.
[For additional research on the occurrence of REEs, see:
https://www.sciencedirect.com/science/article/abs/pii/S0278434311003906 Coastal sediments, Spain
https://www.sciencedirect.com/science/article/abs/pii/S027277141530024X Surface sediment of marine coast, China
https://www.sciencedirect.com/science/article/pii/S000925411300394X Below seafloor in pore water below Bering Sea (aka groundwater)]
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